Chapter 5 Adsorption of Gases On Surfaces Modified By Electronegative Adatoms

Chapter 5

ADSORPTION OF GASES O N SURFACES MODIFIED BY ELECTRONEGATIVE ADATOMS

5.1

5.1.1

CARBON MONOXIDE General Remarks for CO Adsorption on Clean Metal Surfaces

The most exploited model for explaining the CO bonding on transition metal surfaces is the synergic mechanism proposed by Blyholder for the description of the metal - CO bond in transition metal carbonyls where CO is known as a ligand in the transition metal inorganic chemistry [l].According to the model proposed by Blyholder, the metal - CO bonding involves donation from the strongly directed CO - 5u highest occupied molecular orbital (HOMO) to the unoccupied or partly occupied s, p or d, metal orbitals, and back donation from the appropriate metal (most often & occupied states) to the CO 2x lowest unoccupied molecular orbital ( L U M O ) . As a result: (i) CO 50 HOMO forms a bonding orbital, which, according to the simple chemical bond model, should cause an increase of the 5u binding energy (BE);

(ii) the mixing of the unoccupied CO 27~levels with the metal dp electrons

will form a bonding configuration with a large amount of d, character.

The degree of backdonation depends on the availability of suitably oriented metal orbitals and is supposed to be the major contribution to the bonding on transition metal surfaces [2]. The most recent theoretical approaches [3, 41 have shown that electrons from the other orbitals (e.g. s p ) can be backdonated to a lesser extent or hybridization with resonant unoccupied metal states also might contribute t o the bonding. Since the 5u HOMO is essentially nonbonding and the 2~ LUMO is strongly antibonding, the formation of the chemisorption bond results in weakening of the C-0 bond , an increase of the C-0 bond length and a reduction of the C-0 stretching frequency below the free molecule value. A schematic

69

Chapter 5.

70

of the CO molecular orbital perturbations during the formation of the metal - CO chemisorption bond is shown in fig.5.1.

2

co

co 2lT*

ZlT-,

gas phase

co on Metal (no interaction)

Strongly Chemisorbed co on Transition Metal

Fig. 5.1. Schematics of the CO molecular orbitals energy perturbations as a result of the CO interaction with a metal surface Adsorption of CO on transition metal surfaces has turned out to proceed via a precursor channel which ensures a non activated pathway to chemisorption [5]. The values of the reported CO adsorption binding energies for the molecular CO state on the Group VIII transition metals within the limits of low CO coverages (initial heats of adsorption, AH'(C0)) are ranging from 140 to 190 kJ/mole. On metals from Groups VIB and VIIB one should discriminate between molecular and dissociated CO coexisting on the surface, the molecular adsorption energies being within the same range [6, 71. Metals from Group IB exhibit a completely different behaviour, showing a rather weakly bound molecular CO state with adsorption energies of the order of 30-70 kJ/mole. Except for weakly-bound CO on Ag and Au, the CO overlayers form ordered structures on all other single crystal metal surfaces under consideration [5, 71. The orientation of the chemisorbed CO molecule with respect to the substrate surface is determined exclusively by the CO coverage. On most of the single crystal surfaces (up to moderate coverages), the CO molecule is bonded via the C atom with a C-0 axis normal to the surface. Existence of off-normal bonded CO molecules has been reported for compressed CO overlayers (tilt angles 6'-12') and stepped surfaces (tilt angles 18') [5]. Nonnormal bonding configurations (strongly inclined or 'lying down') with an involvement of l ? r and 4u molecular orbitals of CO are observed for some transition metal surfaces (Fe(100) [8], C r ( l l 0 ) [9] and Mo(ll0)) [ l o , 111 at N

5.1. Carbon Monoxide

71

low CO coverages. This abnormally bound state is assumed to be a precursor for CO dissociation. T h e dissociation propensity of CO molecules varies on the different single crystal metal surfaces. Fig. 5.2. illustrates the energetic situation for CO and its constituents - 0 and C on some transition metal surfaces. The energy level of molecular CO is determined on the basis of the measured initial heats of adsorption, AH’. T h e energy level of dissociated CO, Ec,o, is estimated using the relationship:

Ec,o = EM-c+ MM-o - Dco,

(1)

where EM-C and EM-^ are the metal-carbon and the metal oxygen binding energies on the metal surface and DCO is the dissociation energy of CO molecule in gas phase (256 kcal/mole).

250-

-W0

I

-

\ E

3 1500

-rc

d

-

F W

W

-

50 -

.0

c

al

a

DCO

Pd ( 1 1 1 ) P t (Ill) Rucoooi)

Ni ciio Feccio)

w (110)

C

c

0

,

50-

c+o

Fig. 5.2. Potential energy diagram for CO adsorbed on P d ( l l l ) , P t ( l l l ) , Ru(0001), Ni(lll), Fe(ll0) and W(110), presenting the molecular and dissociat,ed CO states and the activation energy, of dissociation, EI

The activation energy of dissociation of chemisorbed CO molecule, E:, is estimated using the relationship derived in ref. [12]:

where the AH’S are the heats of atomic chemisorpt,ion of the corresponding dissociation products.

72

Chapter 5.

Fig.5.2. illustrates quite well that the dissociation propensity of the chemisorbed CO cannot be simply relat.ed to the M-CO adsorption bond strength. Obviously, it depends on the affinity of the substrate to the atomic constituents C and 0. It can be expected that changes in the properties of the surface by introduction of modifiers will affect the surface behaviour with respect t o CO dissociation by influencing the energy position both of the molecule and the products of dissociation.

5.1.2

Modifier Effect on the CO Adsorption Energy and on the Surface Adsorptive Capacity

Information concerning the effect of different electronegative modifiers on the CO adsorption energy and on the capacity of the surface for CO adsorption has been obtained mainly by means of the T P D method. This technique is adequate for metal surfaces modified by C1, S, Se, C, N , P, etc., where the desorption of reagents, such as CO, NO, Hz and hydrocarbons takes place at much lower temperatures than that at which the adsorbed modifier can be removed. As will be illustrated below, the introduction of a modifier induces changes both in the location of the T P D maxima and in t.he shape of the T P D spectra, which reflect the different aspects of the modifier effect.

Fig. 5.3. Effect of different amounts of C1 and S on the CO TPD spectra from Ni(lO0) for saturation CO coverages at 100 I< (from ref. (131) Figs. 5.3 and 5.4. show the effect of increasing amounts of various additives on the CO T D spectra from Ni(100) [13, 141. All these modifiers, as outlined in subsection 4.2., occupy the highest coordinated sites tending t o form ordered structures. The results in figs. 5.3 and 5.4. indicate that the presence of a modifier causes a substantial reduction in the occupation of

73

5.1. Carbon Monoxide

the most tightly bound ,&-GO state with increasing modifier coverage. The lower temperature desorption peaks are depopulated less rapidly, which is accompanied by the appearance of new weakly-bound states, associated with desorption from modifier-induced surface states. It should be pointed out that up to certain modifier coverages the highest temperature &state (resulting of desorption from non affected surface states) is not completely removed. It remains preferentially occupied in cases of low CO coverages, followed by a sequential occupation of the less st
024

L 1 0 200 300 400 500 600 IEMPEAAlURE

(I()

TEMPERATURE (I’

Fig. 5.4. Effect of different amounts of C and N on the CO T P D spectra from Ni(lO0) for saturation CO coverages at 100 K (from ref. [14])

A very similar effect of the electronegative additives as that illustrated in figs. 5.3 and 5.4. has been reported for CO adsorption on other modified fcc(100) and fcc(ll0) surfaces, e.g. S/Ni(100) [15, 161, S/Ni(110) 1171, S/Pd(100) [Is-201, S/Pt(llO) [21], N/Rh( 100) [ 2 2 ] ,C/Ni( 110) [23], C/Ni( 100) & (110) [24], etc. Fig. 5.5. shows the CO TPD spectra from Ni(100) modified with different amounts of P (an additive which does not tend to form ordered surface structures). Obviously, the lack of ordering of the modifier and the tendency of P to form amorphous islands (see Subsection 4.2.) lead to a weaker effect on both the CO bonding strength and the CO coverage. When assessing the relative strength of the modifier effect induced by different electronegative additives, two factors may be considered: (i) the critical modifier coverage at which the most strongly-bonded CO adsorption state (associated with CO adsorption on a nonaffected site) is completely eliminated;

Chapter 5.

74

100

200

400 500 600 TEMPERATURE IKI

300

Fig. 5.5. Effect of different amounts of P adsorbed on Ni(100) on the CO TPD spectra for saturated CO coverages at 100 I< (from ref. [13])

(ii) the modifier induced reduction of the adsorptive capacity of the surface (saturation CO coverage at the given adsorption temperature).

Fig. 5.6. presents the induced changes in the relative population of the most-strongly bonded CO molecular /??-state and in the total CO saturation coverage a t 120 K as a function of the coverage of different modifiers, such as C1, S, N , C, and P on Ni(100). The data in fig. 5.6. demonstrate that the rapid initial decay of the total CO coverage is mainly due to the fast reduction of the &states. This eliminated completely for modifier coverages of 0.25 or less in cases when the additive adatoms form ordered p(2 x 2) structures (Cl, S, N, C). It is obvious that the strongest effect is observed in the presence of C1, when the complete &CO statmeelimination is observed at C1 coverages of 0.15. In the case of P, which does not form ordered overlayers, the modification effect is weaker and the elimination of &CO occurs at P coverages exceeding O.G. The same trend in the strength of the modification effect is preserved also for the total CO coverage. There the maximum CO coverage has turned out to drop from 0.62 for a clean Ni(100) surface to 0.15, 0.18, 0.25 and 0.45 for ordered p(2 x 2 ) overlayers of C1, S , N

-

-

5.1. Carbon Monoxide

75

and C, respectively, and to 0.55 for the disordered P overlayer. For the most severe 'poisons', such as C1 and S, the surface can be considered as completely deactivated at modifier coverages approaching 0.5 when the formation of the c(2 x 2) structure is fully completed.

U,

ec''d

00

1

0

01

02 03 04 05 06 A D D I l I V t C O V t R A G t IMLI

2

01

Fig. 5.6. Dependence of the occupation of the Pz-CO state and the total CO uptake on the additive coverage. (from ref. [13])

A very similar effect is exhibited by electronegative additives on the adsorptive capacity and the relative population of the various molecular adsorption CO states on modified fcc(ll1) and hpc(0001) surfaces. Typical examples are the data reported for CO adsorption on S / P t ( l l l ) [25-271, S/Ni(111) [28, 291, C/Ni(111) [24, 301, O / N i ( l l l ) [31], C/Rh(111) [32], S/Ru(OOOl) [33], etc. An illustration of the S-induced changes in the CO T P D spectra from fcc(ll1) and hpc(0001) metal surfaces is given in fig. 5.7., where with increasing S coverage on P t ( l l l ) , the following effects are observed : (i) the CO peak maximum at low CO coverages shifts from 440 K for a clean surface to lower temperatures (down to 360 K for p(2 x 2) 0.25 S / P t ( l l l ) ) , accompanied by an increase of its half width for the same CO coverages;

Chapter 5 .

76

-

(ii) the maximum CO coverage is reduced from 0.66 for a clean surface to 0.24 at 0.25 S / P t ( l l l ) and drops to 0 at ( Ax A ) R 3 0 ° 0.33 S/Pt( 111).

250

ilo

350

400

4.k

so0

TEMPERATURE (K)

Fig. 5.7. CO T P D spectra for clean and sulfided P t ( l l 1 ) at To= 90 K (from ref.

PI)

Approximations based on the initial reduction in population of the most strongly bound CO-state induced by the introduction of increasing amounts of a modifier have shown that, for additives forming ordered overlayers, one modifier adatom affects 3 CO adsorption states on fcc( 111) and hpc( 0001) surfaces and 4 states on fcc(100) and (110) surfaces. Part of the possible adsorption sites is eliminated whereas another part is converted into new modified adsorption sites where the CO molecule is less strongly bound. It should be noted that at low additive coverages, modified and unmodified adsorption sites coexist. The critical coverages, at which the latter are eliminated, are determined by the effective radii of the modifier influence and by the stability of the initial additive adlayer order in the presence of the reagent. Both factors will be considered in detail in the forthcoming subsections. The influence of the modifier on the adsorption energy of the CO molecular form is very well reflected in the T P D spectra, shown in figs. 5.4, 5.5 and

-

-

5.1. Carbon Monoxide

77

5.7. The elimination of the most favourable adsorption sites, associated with the clean surface is accompanied by the removal of the highest temperature adsorption peak. The clearest picture of the effect of the modifier on t,he strength of the CO adsorption bond in the affected adsorption sites can be obtained by comparing the initial heats of CO adsorption for additive free substrate surfaces and for surfaces covered with an ordered modifier overlayer at which all clean surface sites have been eliminated.

p(2x2) S-Pt(111)

( f i ~ a ) R 3 0 S-Pt(111)

S adatom in three-told hollow site

0 nearest Pt alom blocked for CO adsorption 0 next nearest Pt atom with modified properties with respecl to CO adsorption 1 reduced CO sticking coefiicient 2 reduced P1-CO btnding energy 3 increased ESD efficiency 4 stiffened CO bending vibrations

Fig. 5.8. Structural models of S - modified Pt(ll1) by S coverages 0.25 and 0.33.

Table. 5.1. presents some selected data obtained for the desorption energies, E d , and the pre-exponential factors of desorption, v, of GO adsorbed on several clean and modified substrate surfaces. Some of the data in Table 5.1. are obtained without taking into account the possible variations of the pre-exponential factors in the presence of a modifier (e.g. for Ni(100)). This assumption is not accurate, because the changes in the adsorption bond strength and the presence of coadsorbat,es always affect the degree of localization of the adsorption stmate,which determines the pre-exponential factor. That is why, in most recent studies, both parameters were evaluat,ed. As is evident from the data summarized in table 5.1., there is a compensation effect of a concurrent decrease of both parameters. An accurate investigation on the relative contribution of Ed and v reduction has shown that Ed has a greater effect because the rate of CO desorption increases in the presence of a modifier [33]. Within the framework of the transition state theory [34], the

78

Chapter 5.

Fig. 5.9. A scheme of the possible CO adsorptioii sites for Ni(100) modified with p(2 x 2 ) 0.25 S and c(2 x 2 ) 0.5 S overlayers

compensatiiig effect can be quantified as a natural relatioilship between the activation energies and the initial and the transition state entropies. Summarizing the results in Table 5.1., the general conclusion is that the surface - CO binding energy in the adsorption sites affected by the electronegative adatoms is always reduced. The absolute value of this reduction, which reflects the strength of the modifier influence on the particular adsorption site, varies in different adsorption systems. Let the CO initial heats of adsorption be compared for modified and nonmodified fcc( 111) and fcc( 100) surface, e.g. p(2 x 2) 0.25 S - P t ( l l 1 ) with clean P t ( l l 1 ) and p(2 x 2) 0.25 S - Ni(100) with clean Ni(100). The C O / P t ( l l l ) systems are very appropriate for this comparison, because within the limits of low CO coverages, CO occupies ‘on top’ sites on both ( P t ( l l 1 ) and S - P t ( l l 1 ) ) surfaces. The Pt-CO binding energy for CO residing on the nest-nearest neighbour Pt atom of 0.25 S-Pt(ll1) (the only possible adsorption site on this surface as shown in fig. 5.8.) is reduced by 48 kJ/mol compared with that of CO on S-free P t ( l l 1 ) . Almost the same reduction in the binding energy is observed when the CO coverage on clean P t ( l l 1 ) is increased to 0.4 (see the T P D spectra in fig. 5.7.). Consequently, the S effect on the CO adsorption energy for S-CO separation 3.12 A is of the same order of magnitude as that experienced by CO in the denser CO overlayers on S-free P t ( l l 1 ) where the closest CO-CO

-

-

5.1. Carbon Monoxide

79

distances are 3.8 k. The picture is more complicated in the case of modified fcc(100) surfaces where several distinct S affected sit,es are observed at p(2 x 2) 0.25 S. On Ni(100) the three S affected CO states a.re separated from S at 3.53 A, 2.8 k and 2.5 A, respectively (fig. 5.9.) when the CO binding energy decreases with decreasing separation between the coadsorhates (from 110 down t o 30 kJ/mole). However, the Ed results summarized in Table 5.1., do not show a simple correlation between the modifier electronegativity and the magnit.ude of CO adsorption energy reduction. As will he discussed in the next subsection, this is due t o the fact that, the type of CO a.dsorpt,ion sites on modified surfa.ces depends on the adsorption t>emperatureand the tendency towards reconstruction. The complete deactivation of the fcc(ll1) metal surface with respect to CO adsorption at S coverages of 0.33 (when the (fi x &)R30° S structure is formed) indicates that the surfa,ce sites (on top, bridge and threefold) involving nearest neighbour Pt atoms a.re unfavourable for CO adsorption and can be regarded as S blocked ones. The critical concent~ra~tion of the modifiers causing complete blockage of t.he surface is larger for the fcc( 100) planes. This is due to the fact that the substrat,e surfwe struct,ure det,erniines the ordering of the modifier and the effective distances between the coadsorbates on the surface. According to the da ta summarized in Table 5.1., a wea,kly-bound CO state can be detected even on c(2 x 2) 0.5 S-Ni(100) where t.he maximum SCO distance is 2.5 k (compared wit,li 2.8 k for CO in a. three-fold site on 0.33 S-Pt(ll1)). This iiidica,tes that, apa.rt from the modifier - reagent separation, the modifier surface configuration also plays an important role in the determination of the actual extent of the blocking effect. Thus far systems have been considered in which the coadsorption of CO does not disturb the initia.1 order of the additive overlayer, i.e 110 rearrangement in the modifier overhyer takes place upon int,roduction of CO. A substantial difference in the T P D specha (which reflects the energetic statmeof CO and the capacity of tlie surface) is observed i n some ca.ses when the coadsorption of CO can induce a reconstruction of the modifier overlayer. The CO T P D spectra from p(2 x 2 ) Se-Pt(ll1) i n fig. 5.10. show some peculiar changes in the shape of the CO T P D spectra with increming the CO coverage [35]. These T P D changes are usually associa,ted with a t.ransition from first order to fractiona.1 or quasi-zero order desorption kinetics [36, 371. It should be not,ed that, because of this effect, which indica.tes significant changes in the mechanism of CO desorption from selenided-Pt(ll1) at OCO > 0.06, the corresponding Ed and Vd data in Ta.ble 5.1. a,re calculated for OCO < 0.05, and Oco 0.2. As can be seen in fig. 5.10., at OCO 0.2 the CO desorption is described by a second lower temperature desorpt,ion pea.k which follows first order desorption kinetics. This pea.k a.ppea.rsonly at, adsorption beniperatures below 130 K. At higher adsorption t,enipera.t,ures,tlie CO coverage is saturated at 0.16 and the T P D features resemble a zero order desorption process. When comparing the kinetic data for a p(2 x 2) 0.25 S-Pt(ll1) surface, it can be seen tha,t the ca.pa.cityof p(2 x 2) 0.25 Se-Pt(ll1) for CO adsorpt,ion

-

-

-

Chapter 5.

80

Table 5.1. CO Desorpt,ion Energies, E d , and Pre-exponential Factors, n, for Some Transition Metals Modified by Electronegative Additives SURFACE

MODIFIER

E:

E,'

(kJ/mole)

u:

4

(sec-' )

clean 140 98 1015 P(2 x 2 ) s 110, 90 1013 1012 c(2 x 2 ) s 30 P(2 x 2)O 120 40 1013 [221 p(2 x 2)N -90 93 1013 (2 x 2)p4gC ~241 P d ( 100) clean 160 lox5 I301 p(2 x 2)s 86, GO Ni(111) clean 140 1O l 5 p(2 x 2)s 91 1381 p ( 2 x 2)O 105 10l2 1311 Pt(111) clean 154 96 10l5 1o3 106 82 1013 10" [261 p(2 x 2 ) s 110 82 1013 io1O p(2 x 2)Se [351 Ru(0001) clean 170 1Ol6 MI p(2 x 2 ) s 105 5.1010 E d and V d are the adsorption parameters at very low co coverages (the desorption energy is equal to the initial heat. of adsorption because CO adsorption is a non activated process). E,' and v,’ are the desorption parameters for the corresponding saturation CO coverages for clean and modified surfaces. Ni(100) ~411

-

-

is smaller (- 0.16 compared to 0.25 a t T, > 130'). As will be shown below, this reduced capacity is a result of the CO induced changes in t,he initial Se surface order. Reconstruction effects on the capacity ofsome p(2x 2) X-Ni( 100) surfaces have been reported in ref. [44].

5.1.3

Modifier Effect on the CO Adsorption Kinetics

The influence of the additives on the CO a.dsorption kinetics has usuaJly been characterized by: (i) the changes in the initial sticking coefficient, So,

(ii) the dependence of the sticking coefficient on the adsorption temperature, S(T),

(iii) the variations of the sticking coefficient with CO coverage, S(0).

5.1. Carbon Monoxide

81

es = 0 2 5

%o .22 .20 16

.06

!-

250

300

4-

350

:

400

450

5 0

Fig. 5.10. CO T P D spectrafrom clean P t ( l l 1 ) and Pt(1ll) modified with p ( 2 x 2 ) S - and p(2 x 2) Se overlayers. T, = 220 I<. Dashed TPD spectra in the upper panel are for T, = 90 I< (from ref. [35])

For evaluation of SO, S ( T ) and S(B) usually the CO coverage versus CO exposure plots are used. As a measure of the CO coverage one can also use the appropriate XPS (intensity of the 01s or C l s peaks), AES data (the intensity of the C(KLL) or O(KLL) Auger transitions), etc. Typical CO uptake curves for clean and modified surfaces, where no reordering is induced in the mixed overlayer, are shown in fig. 5.11. A similar effect of the electronegative overlayers on the CO adsorption curves is observed for the other modified single crystal surfaces [15, 16, 28, 311. At low adsorption temperatures (80-120 K.),for all systems under consideration a visible effect of tlie additives on tlie CO initial adsorption rate ( S o ) is observed a t modifier coverages close to or above that corresponding to a p(2 x 2) 0.25 X overlayer [15, 16, 211. The modifier coverages at which SO becomes zero depend 011 the adsorption temperature and the type and surface order of the additive adatoms. At, T, < 100 I<, SObecomes zero when

Chapter 5.

82

0.8

T, = 9 0 K

CI

J

2 0.6 w

99

0

u

0

0.4

0.2 0.0

0.0

3.0

6.0 9.0 12.0 CO EXPOSURE (mborsec x 106,

15.0

Fig. 5.11. CO coverage vs CO exposure for clean and siilfided P t ( l l 1 ) with different S coverages (from ref. [ 2 6 ] )

the formation of a (& x 4)R30 0.33 S overlayer is completed on fcc(ll1) and hpc(0001) surfaces , whereas on the sulfided fcc(lOO), hcc(100), fcc(ll0) surfaces, SO does not become zero even in the presence of a c(2 x 2) 0.5 S overlayer. The lower adsorption temperature ensures an increase in lifetime of the weakly-bound CO states. That is why the reduction of the adsorption rate is less severe than that observed at higher adsorption temperatures. It is worth pointing out that for clean transition metal surfaces, S " ( C 0 ) is close to unity and remains independent of the adsorption temperatures up to 400 I<, while for modified surfaces, So(C0) is a stronger function of the adsorption temperature. Since the introduction of a modifier affects wit*lia different strength the population of the CO surface states, it is important to find out how the local sticking coefficient of the different GO adsorption states changes with increasing modifier coverage. In real catalysis, the reaction temperatures are always above 300 K. That is why, it is of great interest to explain the variations of So of the most tightly-bound CO states as a function of the additive coverage. An illustration of SOvariations as a function of the additive coverage for CO adsorption at 300 I< is given in fig. 5.12. It is obvious that SOsuffers a pronounced, almost linear reduction with increasing additive coverage, which is most severe in the case of a chlorided surface. The influence of electronegative adatoins on the initial rate of molecular adsorption of reagents, such as CO and NO can be satisfactorily explained within the framework of the precursor state model for nonactivated adsorption

-

5.1. Carbon Monoxide

83

’0

ADDITIVE COVERAGE (ML)

Fig. 5.12. Initial sticking coefficient, So, of the &CO state as a function of the additive coverage, The dashed line represents the theoretical dependence according to the relationship S," = St(1 - 4Bx) (from ref. [?a])

[6, 391 where the sticking coefficient can be described by the relationship S=

Q(t, 0) 1 -k (vd/va) exP[-(Ed - E a ) / h T ]

'

Here, vd and va are the pre-exponentials for chemisorption and desorption from the precursor state, Q is the trapping probability, which is coverage and temperature dependent, and Ea and E d are the activation energies for chemisorption and desorptioii from the precursor state. As pointed out in ref. [5, 71, for CO adsorption on most of the transition metal surfaces under consideration ndexp(-Ed/RT) << v,exp(-E,/RT) and SOZ Q is close to unity. The reason for the reduction of So with the intaroduction of an electronegative additive can be understood if one considers the modifier surface order illustrated in figs. 5.8. and 5.9. and the possible adsorption sites for CO on these surfaces. Within the framework of the description given by eq. ( 3 ) the reduction of So can be ascribed t o a decrease in lifetime of the CO precursor on the modifier affected adsorption sites, wliich results in an increase of the second term in eq. (3), i.e the desorption rate of the precursor cannot be neglected for the modified surface sites. One reason for the reduced lifetime of the precursor can be its reduced migration mobility due to the changes of the potential energy surface for CO diffusion in the presence of the modifier adatoms. The absence of an effect on So at low adsorption temperatures (90-100 K) for S coverages much less than 0.25 on most of the substrates under con-

Chapter 5.

84

sideration [15, 16, 26, 311 can be satisfactorily explained by an non-uniform distribution of the modifiers because of the tendency of most of the electronegative adatoms to form ordered islands even at very low coverages (see subsection 4.2.1.). Thus, up to some critical additive coverages a presence of modifier-free islands is possible where the adsorptive properties of the clean surface are preserved.

0.3 h

J

I

I Pt(ll1)

I I

%(S)

= 0.Z5

S/Pt(lll)

Fig. 5.13. CO coverage versus CO exposure on the sulfided and selenided P t ( l l 1 ) surface at T, = 90 and 220 K (from ref. [35])

For all modified surfaces, where the introduction of reagents does not disturb substantially the initial order of the modifier, the dependence of CO sticking coefficient on the CO coverage resembles that for the adsorption on a clean surface, despite the reduced So value. This S(0) behaviour is illustrated in fig. 5.13. for CO adsorption on p(2 x 2) S-Pt(ll1) and p(2 x 2) SePt(ll1) at 90 K . The initial constancy of the S(B) plots followed by a gradual decrease above certain CO coverages, reflects a precursor mediated process where the molecules occupy identical adsorption sites. A more complicated behaviour with respect to the CO adsorption kinetics is observed in the cases when the coadsorption of CO results in a substantial reordering of the modifier overlayer. The effect of the structural changes in the mixed overlayers on the CO adsorption kinetics is illustrated by the S ( 0 ) plot in fig. 5.14. obtained at an adsorption temperature of 220 K for selenided P t ( l l 1 ) . As indicated in this figure, the stepwise reduction of S at 0 ~ > 0 0.04 is accompanied by structural changes in the overlayer where the new surface order offers less favorable adsorption sites (see fig. 5.19.). Finally, one exception should be mentioned, when a typical electronegative adatom does not cause a reduction of the initial adsorption rate of CO. This

85

5.1. Carbon Monoxide

is the system p(2 x 2) 0.25 O / P t ( l l l ) , where CO preserves the same So as on a clean surface. As discussed in det,ail in ref. [40], the main reason for that is the great affinity of CO and 0 coadsorbed on P t ( l l 1 ) for COa formation. But on other substrates (e.g. Ni(ll1) [31]), 0 behaves as a typical 'poison' with respect t o CO bonding strength, adsorption rate and adsorptive capacity of the surface.

0 .6

0.5

90 K

ie---o,---o

A

\

1

1 ’

Q s e ( ~=) 0.25 0

\

A

i 0,

Fig. 5.14. CO sticking coefficient, vs. CO coverage on sulfided and selenided Pt(ll1) at T, = 90 and 220 I< (from ref. [35])

5.1.4 Modifier Effect on the CO Adsorption Site Occupation

Vibrational spectroscopy data (HREELS and IRAS) as well as some selected ESDIAD data have shown that the presence of electronegative coadsorbates, occupying the highest coordinated sites, always causes changes in the nature of the molecular adsorption sites of the reagents. Comparison between the observed molecular stretching frequencies Me-C and C-0 and the GO TPD spectra gives a good basis for correlating the binding sites with the corresponding adsorption binding energies. Since CO adsorption on various bare metal surfaces exhibits a different sequence of the site occupation, it is impossible to generalize the observed effects induced by the modifier on the CO site occupat,ion. For example, fig. 5.15. shows that, as a result of increasing S coverages on S/Ni(100) [41], the original CO vibrational modes, associated with 'on-top' bonding (at Bco < 0.5) are replaced by: (i) three new modes ascribed to twofold (3 = 1910-1960 cm-l) or fourfold (hl = 1740 cm-') S affected sites in the next nearest position for the p(2 x 2) S-Ni( 100) surface and (ii) one mode ascribed

86

Chapter 5.

t o a fourfold (hz = 2115 cm-l) S affected site for the c(2 x 2) S-Ni(100) surface (see the structural model in fig. 5.9., where b, hl and h2 sites are indicated). The corresponding CO adsorption binding energies are shown in Table 5.1.(110, 90 and 30 kJ/mole, respect,ively).

Fig. 5.15. A set of TPD and HREELS spectra, illustrating the effect of preadsorbed S on CO adsorption states on Ni(100) for an adsorpt,ion temperahre of 90 I< (from ref. [41])

In the case of S/Pd( 100) [19] where on a clean surface CO occupies twofold (bridge) sites, the presence of S up to moderate sulphur coverages does not affect substantially the CO stretching modes, although the T P D spectra show a strong reduction in the population of the highest temperature state. This is not surprising because, as outlined above, the S-affect,ed b state has same bridge coordination (fig. 5.9.). Similar t,o the case of S/Ni(100,) a peak at 2110 cm-I can be distinguished 011 c ( 2 x 2) S/Pd(100) also [19]. The latter is associated with a h2 adsorption site. A very similar behaviour of the CO site occupation w a s observed for C/Pd (100) [19]. The coordination of the modifier-affected CO adsorption sites is different in the case of substrate surfaces with fcc( 111)-like crystallographic symmetry. Detailed studies of CO adsorption on S / N i ( l l l ) [as],O / N i ( l l l ) [31], S and S e / P t ( l l l ) [26, 35, 42 ] have shown that the CO* modifier-affected sites are more likely to be ‘on-top’ sites on the next nearest substrate atom. Thus, the IR studies [28, 311 have shown that the S induced CO adsorption state is characterized by a stretching frequency 2100 cm-’ and its relative concentration grows a t the expense of the original bridge and on top adsorption

-

5.1. Carboil hlonoxide

87

sites (fig. 5.8.). The difference between O / N i ( l l l ) and S/Ni(111) is that the S induced CO* band frequency at 2108 cm-l is invariant with 0s and 0 ~ 0 , whereas the 0 induced CO" band frequency suffers a strong coupling effect with increasing Bco (fig. 5.16.).

I3 : e4 A

a 0

2200

2100

Wavenumber

zoo0

lcm-')

Fig. 5.16. Effect of oxygen (right) and sulphur (left) precoverages on C-0 ing frequencies for CO adsorbed on modified N i ( l l 1 ) (from refs. [28, 311)

stretch-

Regardless of the assignment of the coordination of the most strongly affected CO adsorption site, comparison with the available vibrational data for the C-0 stretching modes shows the following common features: (i) the most strongly perturbed CO' state (the most weakly bound one) always exhibits the highest vibrational frequency mode (of the order of 2110 cm-') which is closer to that of a free CO molecule; (ii) up to certain critical modifier coverages, the original (clean surface) CO bands coexist with the modifier-induced ones, the frequencies of the former being only weakly perturbed by the presence of the additive ad atoms. 5.1.5

Surface Order in Mixed Overlayers

The effect of the electronegative adatoms on the surface order of the coadsorbed CO molecules is studied exclusively by LEED. On all single crystal surfaces, CO tends to form commensurately and incommensurately ordered structures at moderate and high coverages [ 5 , 431. This tendency is preserved

88

Chapter 5.

only in the presence of negligible amounts of additives (less than 0.05) but even then the CO induced spots are fainter and the background is larger than that for CO on a clean surface [15-19, 26, 281. At higher additive coverages CO adsorption does not give rise to ordered structures, which means that CO adsorbs in a disordered manner. The lack of order can be easily understood since the introduction of foreign adatoms prevents long range ordering of the CO molecule, because of hindered occupation of certain surface sites. At moderate and high modifier coverages, when the additive adatoms have formed ordered structures, the possible adsorption sites for CO are restricted and usually, the initial surface order determined by the modifier, is preserved. For an example, CO adsorption on p(2 x 2) S-fcc(ll1) substrates is possible only in tlie "on top" positions on the next nearest substrate surface atom (see fig. 5.8.), which means that at saturation the CO molecules are in the same p(2 x 2) order as the modifier adatoms. In the case of p(2 x 2) - fcc(100) surfaces, the picture is not very clear because, as pointed out in ref. [44], CO-induced reconstruction is possible in the case of some of the modifiers. Most recent studies of mixed electronegative adatoms - CO systems have shown that the CO coadsorption might disturb tcheorder of the electronegative adatoms and, in some cases, can even result in a rearrangement of the modifier. There is a tendency for GO to exert this effect, above certain CO coverages and adsorption temperatures. As illustrated by tlie LEED data in fig. 5.17., at Oco > 0.05 and T, > 240 K CO induces a certain disorder in the initially well ordered p(2 x 2) S overlayers on P t ( l l 1 ) . This disorder is temperature irreversible and the initial well-ordered p(2 x 2) S layers are restored only after CO desorption. More severe structural changes have been observed upon CO coadsorption on p(2 x 2) Se-Pt(ll1). As shown i n figs. 5.18. and 5. 19., the coadsorption of CO causes a phase transformation and an establishment of a new x 8 ) R 1 9 . l 0 order in the mixed CO + Se overiayer. In this new order CO molecule again is adsorbed on the next-nearest on top P t site, but the capacity of the surface for CO adsorption is reduced at the expense of the increased CO-Se separation. Detailed studies have shown that this phase transition proceeds easily at 0.lG > Oco > 0.04 and T > 130 K. As will be discussed below, the observed CO induced disturbance in the initial surface order of the electronegative additives is accompanied by a reduction of the constraint imposed by the modifier on the CO frustrat,ed translational modes parallel to the surface. The observed relation between the structure in the mixed overlayer and the energetics of the CO adsorption site for selenided P t ( l l 1 ) indicate that one of the most important factors which determine the strength of the modifier effect, is the separation (effective distance) between the electronegative adatom and the coadsorbed GO molecule, whereas the coordination number of the former is of less importance. As shown above, the fi order with 6 Se atoms coordinated around CO with a CO-Se separation of 4.22 A, turns out to be energetically more favoured than the p(2 x 2) structure with 3 Se adatoms spaced around CO at a distance of 3.19 A. The rapid decay of the modifier - CO repulsive forces with the increasing

8

(a

5.1. Carbon Monoxide

co + SIPt(ll1)

LEED AK, CO

-

ESOIAD DATA

e,, .

-

210 K '

89

0.15

T,=240

K

F-.

Fig. 5.17. Top: CO' ESDIAD patterns as CO is adsorbed at 90 I< and after annealing at 270 K or upon CO adsorption at 240 I<. Bottom: LEED pattern changes as C O + S / P t ( l l l ) layer is annealed after CO adsorption at 90 K (left); LEED pattern changes as CO adsorption takes place at 340 I< (right.) (from ref.

WI)

coadsorbate effective distance explains satisfactorily the absence of a CO induced phase transition in CO/p(2 x 2) S-Pt(ll1). Because of the difference in the Se and S adatom sizes (covalent radii 1.16 A and 1.04 A, respectively) the CO-S effective distance will be by 0.14 shorter, i.e. the GO-S repulsive strain will be weaker. Consequently, even when assuming the same activation energy for the transformat,ion of the p(2 x 2) S and p(2 x 2) Se overlayers, thermodynamically the reconstruction will be less favourable in the case of sulfided Pt(ll1). It is quite obvious that the different geometrical arrangements of the additive adlayers are an important factor in determining the possible number of the modifier induced new adsorption states. A recent IR study of the temperature effects on the GO site occupation on Ni(100) - p(2 x 2) X (X = C1, S, 0, N , C) surfaces has indicated that the break in the correlation adatom electronegativity - strength of the poisoning effect in some systems is due

A

Chapter 5.

90

CO

+ 8.IpHtlll-

WED AND CO* ESDIAD DATA

Fig. 5.18. CO’ ESDAD patterns (left) and the corresponding LEED structures (right) as CO is adsorbed 011 p(2 x 3 ) Se-Pt(ll1) at 220 I< (from ref. [42])

t o the reconstruction of the additive adlayer induced by CO above certain temperatures [44]. Table 5.2. summarizes the temperature changes in the CO site occupation (as judged by the measured C-0 stretch frequencies) observed for Ni( 100) - p(2 x 2)X surfaces. C-0 frequencies above 2000 cm-’ are attributed to ‘on top’ CO (‘t’), whereas those below 2000 cm-’ are associated with bridge bonded (‘b’) CO [5]. In all mixed systems considered in table. 5.2., the increase of temperature causes partial CO desorption. It is obvious that in the case of C1, S and 0 the bridge bonded CO is removed, while in the case of N and C the ‘on top’ CO states are desorbed preferentially. Another important feature is that both the C-0 stretch frequencies and the adsorption energy of CO remaining on the S, 0 and C1 modified surfaces after heating to 300 K, are very similar to the ones measured for clean Ni(100). These results for a strong temperature dependence of the CO site occupation for Ni(100), modified with p(2 x 2) overlayers of S, 0, and CI have been explained by considering the possibility for CO induced p(2 x 2) to c(2 x 2) transformation of the modifier overlayer. As a result, the surface will consist of patches of

-

5.1. Carbon Monoxide

91

POSTULATED Se AND CO ARRANGEMENT I N THE TWO PHASES p(2x2kSe with disordered CO

($7x$7)R19.1°rnixed Se-GO ordered layer

Fig. 5.19. CO and Se arrangement for the p(2 x 2) and overlayers on Pt(ll1) (from ref. [42])

(8 x fi)R19.1°

mixed

c(2 x 2) adatoms and patches of a clean surface where the CO adsorption sites are least disturbed. This is another example of the possible changes in the structural order of the adlayer when the adatoins and the reagent tend to separate in two phases, instead of forming one mixed surface phase as was the case of CO + Se on P t ( l l 1 ) [42] As outlined in ref. [44], therinodyiiam~callythe CO induced reconstruction of a p(2 x 2) adatom overlayer on Ni(100) will be favoured only in the case when the energy gain for CO moving from the modifier perturbed ‘bridge’ site t o an energetically more favourable ‘on top’ adsorption state is larger than the energy required for the p(2 x 2) to c(2 x 2) transition. Since the estimated energy gain is found to be 40 kJ/mole, the lack of reconstruction in the case of N fits in well with the estimated energy of 80 kJ/mole for the p(2 x 2)N t o c(2 x 2)N transformation (whereas for 0, S and C1 this energy is 40-50 kJ/mole) [44]. Although to date, few studies have been concerned with the temperature and the reordering effects on the C-0 staretchingfrequencies, CO bending vibrations and the overlayer surface order, the available data indicate that the possibility of CO-induced structural transformations cannot be excluded. These are similar to that observed for CO/Ni(100) - p(2 x 2)X [44], CO/Pt(lll) - p(2 x 2) S and C O / P t ( l l l ) - p(2 x 2) Se [35, 421 in the other modified systems under consideration. Such structural changes are more likely to be responsible for the lack of a straightforward correlation be-

-

-

Chapter 5.

92

Table 5.2. C-0 Stretch Frequencies, w , at Different Temperatures for p(2 x 2) Overlayers of Various Modifiers on Ni(100) (data from ref. [44]) ADDITIVE

w (cm-I) at

T = 170 I<

w (cm-I) at

bare Ni(100) p(2 x 2)Cl

1965 (‘b’), 2035 ( 1 ) 1945, 1955 (two ‘b’) and 2036 ( ‘ 2 ’ ) 1946 (‘b’) 1957 (‘b’) 1934 (‘b’) 2046 and 2067 (two 9’) weak 1937 (‘b’) 2081 (‘2’)

2038 (‘1’) 2028 ( ‘ t ’ )

p(2 x 2 ) s p(2 x 2)O p(2 x 2)N p(2 x 2)C

T, > 290 I<

2021 (9’) 2017 ( 1 ) weak at 1940 weak 1937 2060 ( ‘ t ’ ) with a reduced in t,ensity

-

(‘b’) means bridge CO bonding, and ( ‘ t ’ )means terminal CO bonding.

tween the strength of the poisoning effect a.nd the adatom electronegativity which applies in some cases to additives with close atomic sizes.

5.1.6

Modifier Effect on the CO Mobility and Bonding Orientation

Another important characteristic of the modified adsorption site is the effect on the molecule soft ‘bending’ vibrations parallel to the surface. These vibrations are sensitive to the modifier induced changes in the surface potential energy contour and are related to the adsorbate mobility and desorption rate. Information about the relative changes in the amplitudes of tlie translational (‘wagging’) vibrational modes by the introduction of a modifier ( S or Se) has been obtained recently from measuring the widths of the ESDIAD patterns of the excited neutral CO species from sulfided and selenided P t ( l l 1 ) . As described in refs. [45-471, the widths of t,he ESDIAD patterns of the neutral species are determined by the root-niean-square vibrational amplitudes of the Me-C-0 bending modes. The effect of p(2 x 2)S(Se) and x &)R19.l0 Se overlayers on tlie CO frustrated t,ranslational vibrational frequencies are summarized in table 5.3. Comparison of the wt data in table 5.3. and the wt values for a clean surface (48 c1n-l) shows that for both S and Se, the CO vibrational motions are constrained somewhat as compared to the unmodified surface. Considering the data on tlie reduced CO adsorption energies, which implies increased mobility and less hindered translational and rotational degrees of freedom, it is likely that the reason for the observed reverse effect ( a hindrance of the CO surface motions) can be only the existence of substantial modifier - CO repulsive interactions. The trend in the changes of w t ( X ) going from p(2 x 2) to (fi x J?)R19.lo surface order shows that the new surface order established as a result of the CO induced phase transitions offers

(J?

5.1. Carbon hlonovide

93

CO adsorption sites with less hindrance to its vibrational freedom. Indeed, an inspection of the structural models for the arrangement of the coadsorbates in the p(2 x 2) and x a ) 1 9 . l o orders shows that at the expense of the reduced adsorption sites for CO (from 0.25 down to 0.16) the Se-CO separation in the J? order increases by 1.03 A (from 3.19 A for p(2 x 2) to 4.22 for the J? order). It should be noted that the order of Se is stabilized only by the presence of CO arranged in the same structural order. The sharp GO desorption peak froin the fi mixed overlayer reflects a drop in the CO desorption energy, because, as a result of the destruction of the J? configuration the less favourable CO p(2 x 2) adsorption configuration is restored.

(8

J?

Table 5.3. CO Frustrated Translation Vibrational Frequencies, wt,on S and Se Modified P t ( l l l ) , as Estimated on the Basis of the CO ESDIAD HWHM, 0,using the simple relationship w t ( 0 ) / w t ( X ) = O2(X)/0’(0),where: 0 ( 0 )= 6.3', w t ( 0 ) = 48 cm-' are the values for Bco

<

0.15 for clean P t ( l l 1 ) (data from refs. [16, 35,

421)

SU B STRATE p ( l x a ) S-Pt(ll1) p(2 x 2) Se-Pt(ll1) ( a x a ) 1 9 . 1 ' Se-Pt(ll1)

O( X ) (degrees) 4.5-5f 0.5 4.4 f 0.5 5.5 f 0.5

wt(X)

(cm-') 100-80 100 60

The ESDIAD results for CO adsorption on sulphided and selenided P t ( l l 1 ) with normally centered ESDIAD narrow patterns indicate that the CO molecule preserves its orientation with the C-0 axis normal to the surface both for the p(2 x 2) and (8x 8 ) R l g . l " surface orders. Certain slight 'offnormal' tilting of the coadsorbed CO, which causes extreme broadening of the ESDIAD patterns, were detected only in disordered CO Se overlayers [35]and at CO coverages larger than 0.25 on a p(2 x 2 ) O / P t ( l l l ) surface [40]. For transition metal surfaces where at low CO coverages the CO bonding orientation is not normal to the surface, the introduction of an electronegative modifier was found to inhibit the population of t,he "lying down'' adsorption state for CO. Such a modifier induced reorientation of molecularly adsorbed CO is observed for C r ( l l 0 ) [48,49] precovered with atomic oxygen. It should be pointed out that the CO state the intramolecular axis of which is nearly parallel to the substrate surface, is the most favourable adsorption state and serves as a precursor for CO dissociation. Thus, by removing this CO adsorption configuration, the electroiiegative adatoms are expect.ed to inhibit the CO dissociation as well.

+

94

5.1.7

Chapter 5. Effect of the Substrate Surface Orientation on the Range and Strength of the Modifier Effect

Summarizing all data concerning the various effects on the CO adsorption on modified surfaces, it is obvious that there is a significant difference in the effective range of the modifier effect for different substrate crystallographic planes. On the basis of available data, the best examples are the differences in the behaviour of the p(2 x 2) S-fcc(100) and fcc(ll1) planes (see figs. 5.8 and 5.9.), where S occupies fourfold hollow and threefold hollow sites, respectively. According to the assignments of the CO adsorption sites on the p(2 x 2) S modified surfaces, for the fcc(100) plane (fig. 5.9.), all S affected CO sites are bridge sites sharing a substrate surface atom with S, whereas for the fcc(ll1) plane all adsorption sites (bridge or on t.op) involving the nearest substrate surface atom are unfavourable for CO adsorption. Here it should be recalled that the separation of the threefold adsorption sites foq ( Ax fi)R30° S-Ni(ll1) from the S adsorption site is by 0.3 A larger than the separation of the four-fold adsorption site for c(2 x 2) S-Ni(100) (figs. 5.8 and 5.9). This result indicates that the strength and the extent of the effect of the same modifier and reagent cannot be simply correlated only with the separation between the adsorption sites because obviously the actual configuration of the modifier adsorption site is also of importance. Taking into account the present state of knowledge about the interaction of electronegative additives with the transition metal surfaces, described in the Section 4.2., the following explanation can be offered regarding the observed difference in the range of the S effect on the fcc(100) and fcc(ll1) planes. The interaction of S with the substrate atom from the second layer below the hollow is stronger for the fourfold S coordination on the fcc( 100) plane than for the threefold S coordination on the fcc(ll1) plane. As will be discussed in more detail in the forthcoming section 5.1.8., the stronger coupling with subsurface metal atoms will reduce the range of the blocking effect along the surface. This explanation fits in well with the data on the weaker poisoning effect in the case of additives such as C (which has the same electronegativity as S) which are deeply embedded in the substrate surface.

-

5.1.8

Modifier Effect on the Electronic Structure of the Adsorbed CO Molecule

Undoubtedly, the reduction of the adsorption energy of the CO molecule adsorbed on adsorption sites influenced by the additive adatoms will be accompanied by changes in the core and valence electron energies of the adsorbed CO. Recent ARUPS data on a CO-S/Ni(lOO) adsorption system [50] have revealed that the major effect of the coadsorbed S lies in the change in the energy position of the CO 517 level which moves by 0.7 eV to a lower binding energy, i.e. closer to its gas phase value. Since in the case of strong CO chemisorption (as outlined in subsection 4.1.1.) the stabilization of the 5s level is regarded as being due to 5u-metral bonding interactions, the S induced destabilization of the 5s levels indicates a reduction of the CO 5o-metal coupling in the S affected adsorption sites. This impeded 5a donation to the

-

5.1. Carbon Monoxide

95

metal can be ascribed to the repulsive interactions occurring bet,ween the CO 5a molecular orbitals and the energetically close 3 p orbitals of S . Another indication of the changes in the degree of coupling of the CO molecular orbitals with the metal in the modified adsorption sites is found in the work function results. Table 5.4. 0 1s and C 1s Core Level Binding Energies for CO Coadsorbed with Electronegative Adatoms SURFACE Fe( 100)

[521 c ( 2 x 2) C-Fe(100) p(1 x 1) 0-Fe(100) c(2 x 2) S-Fe(100) Ni(100) [531 0.4 S/Ni(100) Rh(100)

P I

c0 (0 1s)

co ( C 1s)

(eV)

(eV)

530.6, 531.4, 532.2 (01) (02)

-

533.9 531.5 533.7 531.3

(03)

284.3 284.8 284.9 285.4 -

-

0.4 N/Rh(100) Mo( 110)

531.6 531.7

385.0

[541 (1 x 1 ) C-Mo(ll0)

533.1

285.5

~

Fig. 5.20. shows the typical changes induced in the work function data by the presence of electronegative ada.toms [31]. Similar work function results concerning CO adsorption on modified surfaces are reported for other systems where CO adsorption on a clean surfa.ce ca.uses positive changes in the work function (e.g. CO-S/Ni(lOO) [ l G ] , CO-O/Ir(llO) [15], etc.). The positive work function cha.nges induced by CO adsorpt,ion 011 a.11 c1ea.n transitmionmetal surfaces under considera.tion (with the exception of P t ) reflect the direction of the net charge transfer as a result, of the met,al/ CO 2 i ~bonding. It is obvious from fig. 5.20., tha.t on modified surfaces the total work function changes induced by the same amount of adsorbed CO tend to decrea,se with increa.sing modifier covera.ge. For very high modifier coverages, when the CO adsorption sites are strongly perturbed, negligible or even negative work function changes can be associated with the occupation of these sites. These work function results reflect the influence of the electronegative a.dditives on the contribution of the nietal/CO 277 backdona.tion to the CO adsorption bond. The smaller dipole moment of the perturbed CO molecules indicates a reduction of the metal/2i~coupling, i.e. weakening of the CO adsorption bond. Consequently, the ARUPS and the work function results indica.te that the

Chapter 5.

96

reduced metal surface - CO coupling in the affected adsorption sites is the result of modifier effects on both the donor and the acceptor component of the metal - CO bonding.

1.5

a

>,

2 1

1.0

al CI, e 0

z 0

._ 0 . 5 c

U E

a

A 0.10

OH@

LL

x

0

L

0.18 0.25

0.2

Co Coverage,

0.4

0.6

ecc0 (CO/Ni 1

Fig. 5.20. CO induced work function changes during adsorption on N i ( l l 1 ) modified with increasing amounts of oxygen (from ref. [31])

The changes in the metal surface - CO coupling also affect the core electron energies of the CO molecule. As can be expected, in the presence of a modifier both C 1s and 0 1s levels move to higher biiiding energies compared t o that measured for CO on a clean surface for the same adsorption site symmetry. Selected data concerning the effect of different modifiers on the 0 1s and C 1s binding energies for molecularly adsorbed CO are given in Table 5.4. This tendency of an increase of the 0 1s and C 1s binding energies of CO adsorbed in the modified sites is consistent with the general trend of reduced final state relaxation effects for adsorbates less strongly coupled with the surface.

5.1.9

Modifier Effect of the C O Dissociative A d s o r p t i o n

Thus far, the influence of the electronegative modifiers on the molecular adsorption state of CO was dealt with. As noted in subsection 5.1.1., the ten-

5.1. Carbon Monoxide

97

dency of the CO molecule to dissociate on clean transition metal surfaces increases going to the left in the corresponding row of the Periodic table. Under ultra high vacuum fractional CO dissociation at T > 300 can be observed on Fe, Mo, Re, W, Cr and Co surfaces [5, 71. The importance of CO dissociation as an intermediate step in the methanation and Fischer - Tropsch syntheses is well recognized [55]. This is due to the fact that the methane formation and the chain growth of higher hydrocarbons on the catalyst surface are preceded by rupture of the C-0 bond.

@,SO.

e. = .o 9.- .l

8.. .2

8. .a 9,. .I

Fig. 5.21. Changes in the CO TPD spectra on sulphur - covered Fe(100) with increasing sulphur coverage. CO exposure = 12 L (from ref. [59])

Recent HREELS and SEXAFS data [49,56-58] have defined the precursor state for CO dissociation as a tilted (flat-lying) molecular state characterized by an extensive occupation of t,he 27r CO molecular orbihls, compared to that for a normally bonded CO molecular adsorption state. As suggested in ref. [56], the energy positions and the intensities of the CO ?r and (I resonances in the SEXAFS spectra indicate an extreme elongation of the C-0 bond (1.47 A) and a lowering of the C-0 bond order of the precursor CO state. Consequently, in the adsorption state which serves as a precursor for CO dissociation, the CO occupied molecular orbitals are strongly rehybridized in comparison with the way CO is conventionally bonded. CO adsorption data

Chapter 5.

98

for systems where a fraction of the adsorbed molecules can dissociate have shown that the precursor state for dissociation is occupied first and the fraction of the dissociated molecules decreases with increasing CO coverage. The number of dissociated molecules is restricted for the lack of sufficient appropriate (highly coordinated) adsorption sites for the dissociation products. It is worth pointing out that even on a unmodified surface, the dissociation by itself introduces the electronegative adatoms C and 0 which can act as poisons if they do not participate in further reaction steps. Consequently, if the dissociation occurs at the adsorption temperature, the surface is gradually deactivated by the increasing amount of C and 0 adatoms with increasing CO exposure. Actually, under the real conditions of the catalytic reaction of CO hydrogenation when C and 0 do not exceed certain critical coverages, they can be removed from the catalyst surface by the reaction steps: (i) zC (ii) 0

+ yH - hydrocarbons(gas);

+ 2H = HzO(gas)

Since the precursor CO molecular state for dissociation is supposed to be more strongly coupled to the substrate, it should be expected that the presence of electronegative additives will significantly affect this state, leading to the inhibition of CO dissociation. The effect of the increasing sulfur coverages on the CO T P D spectra from Fe(100) is illustrated in fig. 5.21. The clean surface CO peaks designated as a1, Q , and a3 are due to desorption from molecular adsorption states, whereas the high temperature one - P-peak is the result of recombination of the dissociation products C and 0. Thorough studies have proved that only the CO as-state is related to CO dissociation. The dissociation is competing with the a3 molecular desorption and occurs at temperatures higher than 450 K [52, 59, 601. The introduction of S affects the desorption spectra from the CO molecular adsorption states in the usual way (as described above), i.e. by removing the original clean surface states and replacing them with new less-strongly bound states. The major S effect is a reduction of the dissociated fraction and complete inhibitsionof dissociation at the p(2 x 2) 0.25 S layer. A close inspection of the T P D spectra shows that the inhibition of dissociation cannot only be associated with the removal of the a3 ‘precursor’ state. This state with reduced population still exists at 0s = 0.25 but cannot undergo dissociation because S has blocked most of the fourfold sites which are the adsorption sites favoured by the dissociation products C and 0. At low Os, the presence of S also hinders, to a certain extent, the recombination of the dissociation products reflected by the increasing temperature of the /3 peak. The same effects consisting of the removal of the most strongly bound CO molecular states and reduction of the fraction of dissociated CO have been observed during CO interaction with sulfur predosed Re(0001) and Re(1010) surfaces [63]. Fig. 5.22. illustrates how, with increasing S coverage, the capacity of the surface for CO adsorption and the fraction of dissociated GO are reduced. The inhibition effects on CO dissociation of the other electronegative adatoms are quantitatively the same. An illustration of the differences in

5.1. Carbon Monoxide

99

Fig. 5.22. Total amount of adsorbed and the dissociated fraction (as evaluated from the total CO TPD area and the area under the p recombination peak) as a function of sulfur coverage on Re(1010) (from ref. [63])

strength of the effects of different modifiers are the CO TPD spectra show11 in fig. 5.23. [52]. It is obvious that the effect on the energetics and populat,ion of the molecular adsorption states, and on the dissociation propensity of GO becomes less deleterious in the sequence S, 0, C. The modifier induced reduction of the adsorption rate on the S, 0, and C modified Fe( 100) surfaces also decreases in the same direction. The effects of the three electronegative additives on the molecular adsorption bond, the dissociation propensity and the CO sticking coefficient are summarized in table 6.5. It should be noted that in the case of a carbided surface, the fact that the fraction of dissociated CO is small, might be due to the tendency to diffusion of carbon into the bulk at elevated temperatures and this would liberate some C-free areas. The same trend in the strengths of the C and 0 effects on CO dissociation is observed with modified Mo(100) [Gl]. It has been found that, with increasing carbon coverage, the dissociated fraction linearly decreases from 504% for one monolayer of C (Il40( 100)-(1 x 1)C). This observation iudicat.es that the suppression of CO dissociation is due esclusively to blocking of the available fourfold sites. In the presence of oxygen there is complete inhibition of the CO dissociative adsorpt,ion at half a monolayer of oxygen, accompanied by a considerable reduction in the desorption energy of the CO molecular state. Obviously, because of its higher electronegativity, oxygen behaves like a more severe poison than carbon. The oxygen-induced inhibition of CO dissociation cannot be related to blocking of the available fourfold adsorption sites

Chapter 5.

100

l

a

200

400 TEMPERATURE

600 (K)

80 0

Fig. 5.23. Effect of carbon, oxygen and sulfur adlayers on CO desorption from Fe(100): (a) clean surface, 10 L exposure; (b) c(2 x2)C, 10 L exposure; (c) p(1 x 1 ) 0 , 100 L exposure; (d) c(2 x 2)S, 1000 L exposure (from ref. [52])

alone. The main reason for the observed pronounced destabilization of the CO molecular state on a p( 1 x 1) O-Mo( 100) surface is supposed to be the substantially reduced backdonation response of the surface in the presence of oxygen. This enhances the efficiency of oxygen as an inhibitor to the CO dissociation process which surpasses the efficiency expected when considering only the effect of blocking of the fourfold sites. Since the additives C and 0 are also dissociation products, their simultaneous presence a t the surface is unavoidable at the catalytic reaction temperatures. Studies of the effect of increasing equal carbon and oxygen coverages on Table 5.5.

Effect of Electronegative Modifiers on the Average CO Adsorption Binding Energies, E d , the CO Adsorption Rate, SO,and t,he Dissociated Fraction of CO, &,o, on Fe(100) for T, = 150 I< ( d a t a from ref. [52])

SURFACE Fe(100) c(2 x 2) C-Fe(100) (1 x 1) 0-Fe( 100) c(2 x 2) S-Fe(100)

Ed (kJ/mole)

so

95 75 75 45

0.2 lo-?

1

10-4

AC,O(%)

-- 525 0 0

101

5.1. Carbon Monoxide

Fe (100) [G2] and M o ( l l 0 ) [54] on the molecular and dissociative adsorption of CO have shown the following peculiarities:

+

0 on the metal (i) the same total amount of equal concentrations of C surface causes a weaker reduction in the CO adsorption bond strength than the same amount of C (the measured initial heats of CO adsorption on c(2 x 2) 0.5 C-Fe(100) equals 77 kJ/mol, while 95 k.J/mol corresponds to (0.250 0.25C)-Fe( 100) [G2]);

+

(ii) the effect on the dissociation rate is average between that of a surfacemodified one with the same amount of C or 0 adatoms alone. The weaker effect of the mixed C + 0 overlayers than that of C alone is attributed to the possible immobilization of the effect if C and 0 are bound in close proximity on the surface. As regards the CO dissociation rate, it has turned out that the additive effects on the kinetic parameters ( activation energy of dissociation and frequency factor) tend to compensate each other, and the net effect on the dissociation rate is relatively small [54]. Consequently, the major reason for the inhibition of CO dissociation is more likely the enhanced probability of desorption of the molecular CO state, rather than the inhibition of the dissociation rat8e. 5.1.10

Influence of the Chemical State of the Modifier on the Strength of Poisoning

In this subsection the dependence of the modified properties of transition metal surfaces on the chemical state of the modifier will be considered. As outlined in chapter 4.2., one of the factors determining how strong the poisoning is, is the actual chemical state of the electronegative additive 011 the surface. A good example of the relat,ion between the chemical state of the modifier and its effect on the CO molecular adsorption state and dissociation propensity are the recent HREELS studies of CO adsorption on oxygen modified Cr(ll0). Three states of oxygen are studied: the chemisorption state, the subsurface oxide and the oxide phase. Table 5 . 6 . illustrates selected data about, CO site occupation and adsorption rate for oxygen-modified C r ( l l 0 ) [44, 641. From the data in table. 5.6. the following important information can be derived: (i) the chemisorbed atomic oxygen species has a stronger direct site-poisoning effect than that of the intermediate oxidation state of Cr( 110), where oxygen is located in the near-surface region; (ii) the oxidized surface is significantly passivated with regard to CO adsorption and the weak CO features can be at,trihuted to the existence of isolated metal defect sites because of imperfect areas in the oxide layer obtained under mild conditions. It is obvious that the removal of the ‘lying down’ CO-state which serves as a precursor for CO dissociation is favoured only when oxygen is present on the

Chapter ,5.

102

Table 5.6. CO Adsorption State, the Corresponding C-0 Stretching Frequencies, wco, and the Initial Sticking Coefficient, So, for C r ( l l 0 ) Modified with Oxygen (data from ref. [64])

wco (cm-’)

SURFACE

ADSORPTION STATE

Cr(ll0)

‘lying down’ CO 1150, 1330 ‘top’ 1955-1 975 ‘bridge’ 1865 ‘top’ 1975 ‘bridge’ 1865 co3 ? 1500 ‘lying down’ 1150-1 330 ‘top’ 1840-1 955 new stat.e 3035 very weak feat.ures at 1170, and 1350, and 1915

C r ( l l 0 ) with chemisorbed eo 0.15 Cr(11O) sub- o xi de

5

Cr(ll0) ‘oxide’

-

SO 1

3.10-*

surface or a surface oxide phase being formed.The effect of the sub-surface oxygen, despite the fact that its absolute concentration is higher than that of the adsorbed oxygen, exhibits significantfly weaker effects on both the occupation of the molecular adsorption stmatesand the dissociation of CO. The difference in the poisoning effects exhibited by the same modifier present on the surface in various chemical states is well demonstrated by comparing the adsorptive properties of substrates ‘carbide’ and ‘graphite’ overlayers. Thus, as shown in fig. 5.24., surface carbon in the carbide form on N i ( l l 0 ) has weaker effect on both the CO adsorption energy and the capacity of surface for CO adsorption than is the case of surface carbon forming a basal layer of graphite [23]. Since the graphite phase grows in islands, the presence of two CO adsorption states when the surface is not completely covered with a graphite layer, is associated with CO adsorbed at the periphery of the graphite islands (CO adsorption energy 50 kJ/mole) and CO molecules residing on C-free pat,ches (CO adsorption energy 96 kJ/mole, i.e. close to that for high CO coverages on a clean surface) [2G,661. A complete surface deactivation is observed on completion of a monolayer of graphitic carbon, because the CO heat of adsorption for a. graphite surface is as low as 15 kJ/mole [GG]. A similar effect of the growing graphite phase on CO adsorption is observed for carbon modified Pt( 111) [ G S ] .

- -

5.1.11

-

Concludiiig Remarks

As will be illustrated in Chapter 8., the adsorptive properties of modified surfaces determine to a large extent the changes in activity and selectivity of the metal catalysts. Since CO coadsorption with electronegative additives is characteristic, it serves t o explain the poisoning effect with respect to reagent

103

5.1. Carbon Monoxide

1

1

.

1

.

I

150200 250

1

I

I

.

I

.

I

.

I

300 350 400 450 500 I

.

TEMPERATURE ( K )

Fig. 5.24. CO TPD spectra for CO from N i ( l l 0 ) ( 2 x l ) C and N i ( l l 0 ) with carbon in graphite islands (from ref. [23]) molecules which exhibit electron acceptor behaviour. The major changes in the CO adsorption kinetics, energetics, site occupation, surface dynamics and dissociative propensity revealed by means of extensive surface science studies are now summarized. These changes are as follows:

(1) Reduction in the CO adsorption rate due to a decrease of the lifetime of the GO precursor for adsorption on the surface sites affected by the modifier. The magnitude of this reduction depends on the adsorption temperature and modifier coverage.

(2) Reduction in the total adsorptive capability of the surface with respect t o molecular CO adsorption due t,o blocking of the favourable adsorption sites by the modifier.

(3) Sequential elimination of the original CO molecular adsorption states starting with the most tightly bound one. The critical modifier coverages at which the original CO adsorption states are completely eliminated depend on the size, the electronegativity and the surface order of the additive adatoms. (4) Appearance of new less strongly bound CO molecular adsorption states. They are associated wit81ithe occupat,ion of different adsorption sites in the close vicinity of the additive adatoms. CO molecules adsorbed in

Chapter 5.

104

these affected sites exhibit different vibrational modes, electronic structure and reduced adsorption binding energies compared to CO molecules adsorbed on clean surfaces.

(5) Constraints of the CO frustrated translational vibrational motions par-

allel to the surface, which indicates that the modifier induced changes in the surface potential energy cont,our affects the CO mobility as well. As is well known, the adsorbate surface dynamic are an important factor in the surface reactions.

(6) Suppression of the CO dissociation propensity and complete inhibition of GO dissociation above certain additive coverages. This effect on the CO dissociation propensity is due to several factors:

(i) a decrease in the lifetime of the molecular CO adsorption state which is a precursor for dissociation; (ii) blocking of the energetically most favourable adsorption sites for the transition state and dissociation product,s; (iii) a possible increase of the activation barrier for dissociation,provided that the binding energies of the dissociation products C and 0 are affected substantially by the presence of a modifier; (iv) impeded CO surface diffusion, etc. The first three factors determine the magnitude of the poisoning effect on the nearest and next-nearest substrate surface atoms. Similar effects of electronegntive additives on the adsorption of acceptorlike molecules, such as NO, Na, 0 2 , are presented in the forthcoming sections.

5.2 5.2.1

NITRIC OXIDE General Remarks for NO Adsorption

faces

011

Clean Metal Sur-

Extensive studies of NO interaction with single crystal metal surfaces during the last decade are directly related to the fact that NO is one of the most tedious components in the automobile exhaust gases. In looking for the most effective catalyst for NO reduction, fundamental knowledge about its chemisorptive behaviour on the catalyst surface is necessary. Of greatest interest as catalysts for NO reduction are the metals P t , Rh and Pd. The electron structure of the NO molecule is similar to that of CO, the essential difference being caused by an additional electron occupying the antibonding 2n level. The presence of this unpaired electron in the 27r molecular orbital of NO promotes dissociation probability for NO compared with CO (the dissociation energy, D N O , for the NO molecule in gas phase is 717 kJ/mole, compared to 1072 kJ/mole for CO [67]). That is why NO exhibits a more pronounced tendency to dissociative adsorption than CO.

5.2. Nitric Oxide

105

The description of the possible NO adsorption bonding on the metal surfaces is made on the basis of the NO bonding in transition metal - nytrosil complexes [68-701. Similarly to CO, the formation of the adsorption bond is via N , N-5a lone pair and partly filled 27r orbitals being involved in the bonding. Because of the presence of an extra unpaired 27r electron the following three types of NO bonding to the surface are possible: (i) ‘on top’ linear bonding, with donor 5a/metal d, and covalent NO27r/metal d , (Ir-like) components; (ii) ‘on top’ bent bonding, where only the NO 27r-orbital participates in the formation of a covalent s bond as a result of overlap between the unpaired 27r electron and a s electron of the metal. In this bent configuration the 50 orbital remains non bonding; (iii) two-fold bridge bonding, where one of the metal atoms is involved in the formation of a donor (a-like) bond with the 5a orbital and the other, in the formation of a covalent ( d i k e ) bond with the 27 NO orbital.

I

I

100

400

700

1oOo

TEMPERATURE IK)

Fig. 5.25. NO, N2 and (from ref. [SS])

0 2

1 m

TPD spectra for increasing NO coverages on Rh(ll1)

This bonding configuration also has a.n N-0 axis perpendicular to the surface plane. Evidently, the backbonding component of adsorption is largest in case (iii) and smallest in case (ii), i.e. the dissociation of NO will be easiest in the case of bridging NO adsorption configuration. The strongest backbonding component for the bridging configuration is also confirmed by the fact that

106

Chapter 5

the occupation of bridge sites is accompanied by a larger increase of the work function than those observed with adsorption in the ‘on top’ sites. Vibrational spectroscopy data (HREELS and IR) have shown that the twofold bridge bonding is the preferred adsorption configuration of NO especially at low NO coverages [71-801. Besides, a variety of other bonding modes, associated both ‘on top’ configurations [74, 76, 79, 81-83], threefold sites [84] and side-on bonded species [75], have been observed.

Fig. 5.26. Dissociated NO amount (as a fraction of saturated NO coverage at 100 K ) and the amount of molecularly desorbed NO as a function of t.he initial NO coverage for several single crystal metal surfaces (from ref. [86])

With the exception of P t ( l l l ) , P d ( l l 1 ) and Pd(100)’ NO adsorption on the transition metals (Nil Ru, Rh, Ir, Cu, Pt(100)) which were also studied is complex, with both molecular and dissociative adsorption states [85]. The relative amount of dissociated NO decreases with increasing NO coverage a t the expense of an increasing amount of undissociated NO [85-901. At low

5.2. Nitric Oxide

107

-

NO coverages the decomposition of NO usually occurs at temperatures 200300 K , the decomposition temperature increasing to 400 K for high NO

coverages. The removal of dissociation products by associative desorption as N2 and 0 2 is possible for substrates which do not exhibit a strong tendency to nitride or oxide formation, e.g. on Ni surfaces the dissociation product 0 remains [89, 901, whereas it desorbs as 0 2 from Ru, Rh and P t [71, 86, 871. Figs. 5.25 and 5.26 illustrate the NO, N2 and O2 T P D spectra for increasing NO coverages on R h ( l l 1 ) and the dependence of the fraction of dissociated NO on the initial NO coverages for several substrates. From the T P D spectra in fig. 5.25 it is obvious that the 0 adsorption state on Rh, which is one of the most promising catalysts for NO reduction, is rather stable and can enrich the surface and alter the reaction pathways. Similarly t o CO, NO tends to form various ordered structures on single crystal surfaces determined by the NO coverage and substrate nature [43]. With most transition metals the NO initial sticking coefficient is close to unity and the initial heats of adsorption are in the range 100-120 kJ/mole [85911. At low NO coverages, the activation energy of dissociation for substrates such as Rh, Ru and Ni is of the order of 90 kJ/mole. If NO coverages are high, desorption of part of NO is necessary for the creation of vacant sites for NO dissociation [78]. 5.2.2

Modifier Effect on the N O Molecular A d s o r p t i o n

A. A d s o r p t i o n kinetics and energetics. As outlined above, from all NO/single crystal transition metal systems, studied to date, no NO dissociation occurs in the N O / P t ( l l l ) , N O / P d ( l l l ) and NO/Pd(100) systems [72, 73, 81, 82, 91-95]. That is why some of these systems will be used for illustrating the effect of electronegative modifiers, such as 0, S and Se, on the NO molecular adsorption kinetics and energetics. In contrast to CO [40, 96, 971, no oxidation to NO2 occurs in the NO 0 coadsorption system on P t ( l l 1 ) [82, 981, since thermodynamically the NO2 reduction is the favoured process [99]. Figs. 5.27. and 5.28. present the NO T P D spectra and the adsorption kinetics plot for p(2 x 2) 0.25 O-Pt(ll1). The higher temperature NO T P D peak from O / P t ( l l l ) , located a t 340 K saturates at NO coverage 0.25, whereas the maximum NO coverage on 0.25 O / P t ( l l l ) is 0.38. Compared t o the data for a clean Pt(ll1) surface, both the capacity of the surface to adsorb NO, and the initial heat of adsorption are reduced by the presence of oxygen. Similar results for NO adsorption on 0.25 O / P t ( l l l ) have been observed by M. Bartram et al. [98]. These authors have also worked at a higher oxygen coverage and have shown that in the presence of 0.75 0 the NO saturation coverage and adsorption energy are reduced further. The kinetic curves in fig. 5.28. show that the presence of 0.25 0 does not affect the initial adsorption rate of NO. Figs. 5.29 and 5.30. present the NO T P D spectra for p(2 x 2) 0.25 SPt(ll1) and p(2 x 2) 0.25 S-Pt(ll1) surfaces. In the case of a sulphided surface, NO desorbs in a single peak located a t 270 K . Together with the reduced adsorption capacity, the sulphided surface also exhibits a decrease in

+

-

-

-

Chapter 5

108

I

100

200

300

400

TEMPERATURE [K]

Fig. 5.27. NO TPD spectra for increasing NO coverages on p(2 x 2) 0.25 0P t ( l l 1 ) . Dashed curve shows the NO TPD spectra for saturated NO coverage on Pt'(111). To= 100 I< (from ref. [82])

0.5 a

3 NO FLUX [ 1O+’ 4M0LECULES/crn2]

Fig. 5.28. NO coverage vs.exposure plots for NO adsorption on clean (a) and modified with p(2 x 2) 0.25X overlayers P t ( l l 1 ) . X: (b) - 0; (c) S; (d) Se. To= 100 K (from ref. [82])

5.2. Nitric Oxide

109

-

3

n

1

1OL'

200

300

TEMPERATURE [K]

400

500

Fig. 5.29. NO T P D spectra for increasing NO coverage on ~ ( 2 x 2 0.25 ) S-Pt(ll1).

T h e dashed curve shows the NO T P D spectrum for a saturated NO coverage on clean P t ( l l 1 ) . T, = 100 I< (ref. [82])

dT/dt= 1.5 K/sec

0.04 10

Fig. 5.30. NO T P D spectra for increasing NO coverage on p(2 x 2 ) 0.25 SeP t ( l l 1 ) . T h e dashed curve shows the NO TPD spectrum for a saturated NO coverage on clean P t ( l l 1 ) . T, = 100 I< (ref. [82])

Chapter 5

110

the NO adsorption rate (fig. 5.31.). The NO TPD spectra from a selenided surface are more complicated, consisting of three T P D peaks, located at 280, 245 and 225 K at saturation. The effect of Se on the adsorption capacity and adsorption rate at 100 K is similar to that of S. At elevated adsorption temperatures (- 200 K), the capacity t o NO adsorption on Se becomes much less which, as will be shown in the forthcoming sections, is due to structural changes in the overlayer. The effect of 0, S, and Se overlayers on the NO initial heats of adsorption (equal to the NO desorption energy in the limit of very low NO coverages, E z , ) , saturation coverage and initial sticking coefficient at adsorption temperatures 100 K and 200 (determined on the basis of the NO T P D data) is illustrated in Table 5.7. The reduced capacity of the 0.25 O / P t ( l l l ) at 200 K is obviously due t o the removal of the lower temperature state (see fig. 5.27.). In the case of Se, because of the NO induced phase transformation in the overlayer, the NO desorption spectra resemble a fractional order desorption process and this poses difficulties in accurately estimating Ez. The data summarized in Table 5.7. indicate that the molecular adsorption kinetics and energetics of NO are affected in the same way as for the molecular adsorption of CO. Comparing the electronegativities of the modifiers considered in Table 5.7. (3.5, 2.5 and 2,4) and the covalent radii (0.77, 1.04 and 1.16 A) for 0 , S and Se, respectively, the weakest poisoning effect is exerted by the modifier with the highest electronegativity: oxygen. This indicates that the of the modifier is probably a more important factor. Table 5.7. NO Desorption Energy, Ei (in kJ/mole), NO Saturation Coverage, 6sat for T, = 100 and 200 I< (in ML), and NO Initial Sticking Coefficient, SO,for a Clean and Modified P t ( l l 1 ) Surface. (from ref. [S2]) SURFACE

Ei

Pt(ll1) p(2 X 2) 0.25 0 - P t ( l l 1 ) 0.75 0 - P t ( l l 1 ) [98] p(2 X 2) 0.25 S-Pt(ll1) p(2 x 2) 0.25 Se-Pt(ll1)

99 91 86 79 76

N

6 (100 I<)

6 (200 I<)

So

0.55

0.4

1

0.40 0.15 0.17 0.16

0.25 0.17 0.08

-1

--

-

0.75 0.7

A more detailed study of the effect of different amounts of a modifier on the NO molecular adsorption rate has been proposed in ref. [102], where the S-induced reduction of the NO initial sticking coefficient, SO,is found to obey the relation &(S) = So(1 - 28s) (8s is S coverage in ML). Obviously, the influence of sulphur on the NO adsorption rate is weaker than that expected from the simple consideration of four blocked adsorption sites for adatom residing in a fourfold adsorption site. This is probably due to the fact that, for a precursor mediated adsorption process (as is the case of CO and NO molecular adsorption), the sites unfavourable for chemisorption can still serve as possible residence sites of the precursor state.

5.2. Nitric Oxide

111

I

Fig. 5.31. NO uptake versus NO exposure for clean (dashed line), and modified Pt(ll1): p(2 x 2 ) S (open circles) and p(2 x 2) Se (half filled circles) (from ref. 1821)

Studies of the effect of different modifier coverages on NO adsorption are helpful in establishing the sequence and the effectiveness i n the eliiiiinat 1011 of the NO adsorption states The rapid removal of the most strongly bound NO state is illustrated in Figs. 5.32. and 5.33., as well as the reduction in the total amount of NO adsorbed as a result of increasing S coverage on a Pd( 100) surface. It is obvious that for this system the greatest poisoning effect will be observed to a S coverage limit of 0.1. For this low S coverage range, the initial drop of 0 ~ indicates 0 that 1 S adatom blocks 3 adsorption sites of NO and all sites associated with the 535 I< T P D peaks are removed at 0s 0.1. According to the HREELS data, the effect of S on the adsorbed NO bond order, is rather small when S coverages are low, and the reason for that will be discussed in the next Subsections. Both the T P D and the vibrational data show that at 0s < 0.15, no new S-induced NO adsorption states appear At moderate and high S coverages, the reduction in the adsorptive capacity of the surface becomes less severe (1 NO per S) and new S induced weakly bound NO adsorption states arise. Similar effects, like those described above, on the amount and the desorption energy of molecularly adsorbed NO have also been reported for other modified transition metal surfaces, e.g. S/Rh(100) [loo], S/Pt(100) [ l o l l , S/Ni(100) [102], N , O / N i ( l l l ) [103], O/Rh(111) [78], and S/Pd [104]. B. Site occupation and electronic structure of the coadsorbed N O molecule. HREELS [78, 98, 1051, UPS [98] and ESDIAD [82] data have shown that in the presence of electronegative modifiers, the amount of NO in a two-fold bridge bonding configuration is greatly attenuated and completely

-

-

-

Chapter 5

112

166

2 M

NO/NO t S

42

.35

30

.a 15 .09

03S

31

Fig. 5.32. NO TPD spectra for saturated NO coverages from sulphur predosed Pd(100). T, = 80 I< (from ref. [95])

removed above certain modifier coverages. Instead, on surfaces modified by electronegative adatoms, NO occupies exclusively ‘on top’ sites. Compared with Me-NO and N-0 stretching frequencies for linear ‘on top’ NO on a clean surface the Me-NO stretching frequency decreases, and the N-0 stretching frequency increases with the introduction of the modifier and the increase of the modifier coverage. This effect is attributed to an increase of the NO bond order (i.e. a decrease of the degree of coupling of the NO molecule with the surface in the presence of the modifier). Table 5.8. illustrates the changes in the Me-NO and N-0 stretching frequencies and in the NO 2n, l n , 50 and 4a binding energies induced by the presence of 0.25ML and 0.75ML oxygen on P t ( l l 1 ) . It becomes clear from Table 5.8. that the main differences observed for the ‘on top’ NO species on clean and modified with 0.25 0 - P t ( l l l ) , and the ‘on top’ NO species on 0.75 0-Pt(ll1) are the appearance of a second low stretching frequency mode at 510 cm-’ and the increase of the binding energies of the NO molecular orbitals in the case of 0.75 0-Pt(ll1). Assuming that the presence of an electronegative additive should result in a reduction of the ability of the surface for r-backbonding, the observed changes in the NO vibrational and UPS can be satisfactorily explained with ‘on top’ bent NO bonding on 0.75

5.2. Nitric Oxide

NO/NO+S

Pd (100) Dosed at 8Ou

1-

0 2

113

.P.... '.--.. *...._----. ....._..., .........-........... ....

'.

'1\

-\--

.I

I

Sulfur

I

1 .4

.3

Covercqe (monoloyer)

Fig. 5.33. Effect of sulphur coverage on the total NO coverage (full line) aud the local coverages in the most strongly bound ( a t 535 K (dashed line)) and the less strongly bound ( a t N 440 K ) NO adsorption states on Pd(100) (from ref. [95])

0-Pt(ll1). As described in Section 5.2.1., this NO bonding configuration occurs with a negligible x-backbonding contribution. The same argument of a reduced backdonation ability of the modified surface explains the removal of the bridge bonding configuration, where the r-backbonding contribution is significant. Table 5.8. Pt-NO ( w l ) and N-0 ( w 2 )Stretching Frequencies (in cm-') and NO Z A , I T , 5u and 40 Binding Energies (in eV) for NO Adsorbed on a Clean and

Oxygen-Covered P t ( l l 1 ) Surface (from ref. [98]) SURFACE Pt(ll1) Pt(ll1) 0.250/Pt( 111) 0.750/Pt(lll)

w1

w2

2*

lX+5fJ

two-fold bridge NO 1490 2.1 9.1 'on top' NO 390 1710 3.9 9.4 9.5 280 1740 2.9 Z 10 265 & 510 1775 3 . 3 450

4u 14.9 13.9 13.9 14.3

The same trend of a preferential 'on top' 1inea.rconfiguration on the modifier affected sites has been observed for NO/O-Rh(lll), where on a mod-

Chapter 5

114

ifier - free surface NO adsorbs in a two-fold bridge configuration for the whole coverage range at which T, = 100 K [78]. The presence of oxygen (0 < 00 < 0.8) causes a shift from bridge to an ‘on top’ linear NO configuration, and when oxygen coverages are very high (00 > 0.8), a backward shift to bridge NO takes place. Relative t o the vibrational data, the NO desorption states distinguished in the NO T P D spectra, the following three NO adsorption configurations have been proposed:

-

(i) O-affected bridge NO, which appears when oxygen coverages are very high and desorption takes place at 280 K;

-

(ii) 0 affected ‘on top’ linear NO, which is the preferred adsorption state for moderate oxygen coverages and when desorption takes place at 380 K;

-

(iii) unperturbed bridge NO which exists up to moderate oxygen coverages and desorbs at 430 I<. The N-0 stretching frequency value measured for the oxygen affected ‘on top’ NO is slightly higher than that of the ‘on top’ linear (- 30 cm-’) NO frequency measured for the other P t group metals. Almost the same stretching frequency value was reported for NO on 0-Ru(0001) [log]. The effect of (O+N), produced by NO dissociation, on the NO site occupation on Ru(0001), i.e. moving from bridge to ‘on top’ adsorption site [log], is similar. Modifier-induced changes in the electronic structure of the NO molecule adsorbed on the affected sites are also evidenced by:

+

(i) the intensity and energy changes in the (Is 5a) emission peak in the MQS spectra [103]; (ii) the 0 1s or N 1s core level energy changes of the adsorbed NO molecule [l02], and (iii) the changes in sign of the dipole induced by NO adsorption on clean and modified surfaces [102, 1031.

+

Thus, for NO adsorbed on Ni(ll1) precovered with 0 or a 0 N mixture [103], MQS studies have shown that the (Is 5u) emission peak decreases and shifts t o higher binding energies with increasing amount of 0 or 0 + N present on the surface, whereas the intensity of the 2s emission hardly changes even after the complete disappearance of the ( 1 s 5a) peak (attributed to a change in the spatial extension of the Is and 5a orbitals). For the same systems parallel work function measurements have shown that the NO induced work function changes are in a direction opposite to those for molecular NO adsorption on a clean N i ( l l 1 ) surface, i.e. for a saturated NO coverage on 0.2 O / N i ( l l l ) , the work function drops down t,o -0.2 eV below the clean surface value, whereas a saturated NO coverage on a bare surface causes an increase of +1 eV. These work function results indicate opposite signs of the dipole moments for NO adsorbed on modified and unmodified fcc(ll1) surfaces, which agrees very well with the designation of the modifier-induced changes in the NO bonding configurations described above. The same trend

+

+

-

5.2. Nitric Oxide

115

in the NO induced work function changes reported for the NO/S-Ni(lOO) system, accompanied by a tendency towards an increase of the NO-0 1s binding energy (from 531.2 eV for a S-free surface to 532.2 eV for c(2 x 2) 0.5 S-Ni(100) [102]) is consistent with the decreased metal - NO coupling in the presence of electronegative adatoms. Modifier induced changes in the occupation of surface sites are more complicated in the case of fcc(100) single crystal planes. HREELS data for NO adsorption on S/Pd(100) [95] have shown that the appearance of the new Sinduced NO adsorption states is favoured above certain concentrations of the modifier on the surface. At low S coverages (6s < 0.15), the presence of S: (i) hardly affects the N-0 stretching frequencies of bridge-bonded (1510 cm-1) and ‘on top’ bonded (- 1700 cm-’) NO, and this indicates a negligible disturbance of the NO bond order; (ii) does not change the sequence which adsorption sites are filled (starting with the bridge bonding configuration); but (iii) prevents on top t o bridge conversion, observed on S-free surfaces along with partial ‘on top’ NO desorption.

For higher S coverages (0.15 < 0s 5 0.5), three adsorption NO configurations are observed: one bridge, one ‘on top’ and a third very weakly bound NO (desorption temperature 165 K and stretching frequency 1750 cm-l). The latter was associated with the same h2 four-fold hollow site postulated for GO on c(2 x 2) S-Ni(100) [41, 981 (see fig. 5.9.). Finally, in order to illustrate the effect of the chemical state of the electronegative additives on the NO adsorption configuration, the vibrational data concerning NO adsorbed on (100) faces of NiO [lo61 are worth mentioning. The IR data have shown only linearly ‘on top’ bonded NO molecules with N-0 stretching frequencies 1800 cm-’ for all NO coverages, the adsorption sites being the Ni2+ ions in the oxide surface lattice. Comparison of these data with the vibrational data for NO adsorbed on a fcc(100) metal surface precovered with adsorbed oxygen, reveals that the oxidation of the surface results in a more severe reduction of the r-backbonding component as manifested by a higher N-0 stretching frequency (indicating a larger NO bond order) for linear ‘on top’ NO on a metal oxide surface.

-

5.2.3

Surface Order in Mixed Overlayers

The introduction of a modifier above certain critical coverages prevents the formation of the usual NO ordered structures as is the case for CO. LEED data on the NO/S-Pd(lOO) system have shown that the presence of only 0.03 ML sulphur prevents the formation of the p(4 x 2) NO ordered structure, with NO occupying two-fold bridge sites. The p(4 x 2) structure is readily observed on a S-free surface when by heating of a NO saturated surface to 410 K part of the ‘on top’ NO desorbs and another part converts into bridge bonded NO. The S induced inhibition of the long range order of the bridge bonded NO species agrees well with the observed impediment of the ‘on top’

-

Chapter 5

116

bridge conversion in the presence of S due to the reduced stability of the ‘on top’ NO species. However, the presence of small amounts of S (0s < O.l), does not prevent the formation of a c(2 x 2) NO ordered structure observed at NO coverages of 0.5 on a S-free surface. Since the appearance of c(2 x 2) patterns on a sulphided Pd( 100) surface is observed at NO coverages as low as 0.25 this indicates that up to certain value for S coverages, NO tends to adsorb in a separate phase where the local NO density reaches 0.5 ML. This tendency t o formation of a separate surface adsorption phase clearly explains the negligible influence of S on the N-0 stretching frequencies at 0s < 0.1 (because a few NO molecules are directly coordinated to the S adatoms), as discussed in Subsection 5.2.3. 4

N

b.

0.

c.

d.

I

Fig. 5.34. Changes in the LEED patterns induced by annealing of a p(3 x 2 ) 0.25 Se 0.16 NO mixed layer to different temperatures. For the sake of clarity, the NO TPD curve for a saturated NO coverage on p(2 x 2) Se-Pt(ll1) obtained at 90 K is given. The arrows connected with LEED panels indicate both the annealing temperature and the remaining part of the TPD spectrum (from ref. [82])

+

Studies of the effect of NO adsorption on the initial structural order of the modifier on the surface have been carried out for P t ( l l l ) , precovered with p(2 x 2) overlayers of oxygen, sulphur and selenium [82]. For p(2 x 2) 0.25 O-Pt(ll1) and p(2x 2) 0.25 S-Pt(ll1) no extra LEED spots or disturbance of the initial modifier surface order are detected upon NO adsorption in the temperature range 90-250 I< (at higher adsorption temperatures NO starts t o desorb). The picture is completely different in the case of a selenided P t surface where the initial p(2 x 2) Se order is preserved only when NO adsorption is carried out at temperatures lower than 170 I<. As shown in fig. 5.34., annealing of the mixed NO p(2 x 2) Se with saturated NO coverage

+

-

5.2. Nitric Oxide

117

of 0.16, obtained at 100 I<, to various temperatures causes streaking of the extra spots in the original p(2 x 2) patterns followed by the conversion of the p(2 x 2) pattern into a mixture of p(2 x 2) and (Ax &)R30° patterns at 230 K when part of NO desorbs. With a further increase in the annealing temperature and complete desorption of NO, the initial p(2 x 2) Se order is restored.

-

? 0

n 3

$

v

0

.-x

- 1

dT/dt=

1.5 K/sec

= 40 pA

--

C.

c n I

0

b. /

a.

100

300

200 temperature

400

[K]

Fig. 5.35. NO TPD spectrafrom: (a) asaturated NO coverage (- 0.07) on p(2 x2) NO+(& x &)R30 Se-Pt(ll1); (b) after additional 10 L NO exposure at 90 K on (a); (c) from saturated NO coverage on p(2 x 2) Se-Pt(ll1) obtained at 90 K (from ref. [SZ])

In fig. 5.35., the NO T P D spectra obtained after NO adsorption on p(2 x 2) S e / P t ( l l l ) at 220 K when the mixed p(2 x 2) + ( Ax fi)R3Oo structure is formed are compared with the NO T P D spectra obtained by direct desorption of NO from a p(2 x 2) S e / P t ( l l l ) surface, saturated with NO a t 90 I<. It is worth pointing out that the same NO T P D spectra as those observed after 220 K NO adsorption can be obtained if a ~ ( 2 x 2S) e / P t ( l l l ) surface saturated by NO at 90 K is annealed to 230 K prior desorption. The coverages of the Se and NO constituents in the overlayer with mixed p(2 x 2) + x &)R30 structures are 0.25 (initial) for Se and 0.07 for NO. The new structural order obviously offers less adsorption sites for NO, because the surface with an already formed mixed structure can not adsorb more NO even after a long additional NO exposure at 90 I< (curve b in fig. 5.35.). Considering the adsorptive capacity of the P t ( l l 1 ) surface, the best explanation of the new surface order is an NO-induced phase transition from p(2 x 2) Se to ( A X &)R30° Se, accompanied by the formation of a separate

-

(a

118

Chapter 5.

p(2 x 2) NO phase. This tendency of forming of two-phase, immisibly ordered islands, is a probable development where many coadsorbates with dominant repulsive interactions are concerned [43, 107l.The lack of NO induced restructuring on p 0 and p(2 x 2) S-Pt(ll1) and the decrease of the adsorptive capacity of the surface in the sequence 0, S, Se (see Table 5.7.) indicates the significant contribution of the size of the modifiers to the strength of the poisoning effect, i.e. the steric effect seems tooverweigh the electronic one. Indeed, comparison of the adsorptive capacities of P t(ll1 ) precovered with p(2 x 2) overlayers of oxygen, sulphur or selenium, shows that, in the case of oxygen the fraction of NO molecules beyond 0.25 (corresponding to the low temperature TPD peak in fig. 5.27.), resides on bridge sites, sharing a Pt atom with the adsorbed oxygen. ESDIAD data for NO adsorbed on a modified P t ( l l 1 ) surface [82] indicate that for p(2 x 2) S-Pt(ll1) and a selenided surface (in both structural orders) only ‘on top’ linearly-bonded NO species are present, whereas for p(2 x 2) O-Pt(lll), when the NO coverage exceeds 0.2 a bridge bonded configuration is also possible. The observed structural changes in the NO (C0)-electronegative adatom coadsorbate layers, when each adsorbate tends to form a separate adsorption phase complicate the description of the poisoning effect. However, the experimental data show definitely that, even when forming a separate phase, the molecular adsorption state of the reactants is destabilized (they desorb at a lower temperature than from a clean surface) and the local density that is reached in this separate phase, is far below that which is rea&ed bll a clean surface. Several factors should be considered when trying to explain these effects: (i) for the formation of dense compressed overlayers of CO or NO a longrange order is needed which is constrained by the presence of ordered modifier islands; (ii) the reduced mobility of the coadsorbed molecules which leads to an increase of the pre-exponential factor for desorption, i.e. an increase of the desorption rate; (iii) the possible propagation of the local modifier coadsorbed molecule repulsive interactions through the adlayer via molecule - molecule interactions; (iv) the existence of a long range electronic effect of the modifier which extends beyond the next-nearest neighbours. The relative contribution of each effect is likely to be different for the various coadsorbate systems, and no generally applicable definition can be proposed at this stage. 5.2.4

Modifier Effect on the NO Dissociative Adsorption

A significant fraction of NO readily dissociates on most of the transition metal surfaces at temperatures higher than 200 K. This described already in Section

5.2. Nitric Oxide

119

5.2.1. There are minor exceptions like some planes of P t and Pd. In these systems, where the NO dissociation process is competing with molecular NO desorption, the introduction of an electronegative additive always results in the inhibition of the dissociation, accompanied by the reduction of the surfacemolecular adsorption capacity. The fraction of NO, dissociated on S/Ni( 100) as a function of S coverage, and the corresponding NO and Nz T P D spectra is shown in fig. 5.36. It is obvious that NO dissociation is completely inhibited, for sulphur coverages of 0.25 when a p(2 x 2) S ordered structure is formed and all NO adsorption sites are influenced by the additive. The complex N2 T P D spectra are due to the influence of the other dissociative product (0) which remains on the Ni surface. The data in fig. 5.36. can be summarized as follows: (i) the relative fraction of dissociated NO molecules decreases with increasing S coverage; (ii) the inhibition of the dissociation is stronger than the reduction of the surface adsorption capacity for molecular adsorption; (iii) the sulphur induced reduction in saturated NO coverage is adsorption sites per S adatom, and (iv) the sulphur induced inhibition of NO dissociation is

per S adatom.

-

-

2 NO

4 NO molecules

For the NO-S/Ni( 100) system, what makes it complicated, is that, with the dissociation of the first NO doses 0 and N , are also introduced to the surface and they also inhibit, to a certain extent, further NO dissociation. This self poisoning effect accompanying the NO dissociation on a clean surface is weaker than that induced by S [89]. Comparison with the case of a N or 0 precovered surface has shown that the presence of sulphiir influences twice as many dissociation sites [102], i.e. this further proofs the relevance of the adatom size over the electronegativity with respect to the strength of the modifier. Here it is worth noting that, when the effect of 0 on the adsorptive properties of Ni is studied, the possible 0 induced reconstruction and formation of oxide islands will complicate the picture. As reported in ref.[l06], no NO dissociation occurs on NiO (100) surfaces. Generally, the same trend in the S ‘poisoning’ effect has been reported for NO/p(2 x 2) S-Pt( 100) [loll and NO/S-Rh( 100) [loo]. In the presence of a p(2 x 2) 0.25 S overlayer on Pt(100) the dissociation of NO is completely inhibited and the molecular adsorption state is destabilized (the NO T P D data suppose a reduction of the Pt-NO binding energy by more than 50 k.J/mol for NO adsorbed on p(2 x 2) S-Pt(100) [loll). As a result. of this inhibition of NO dissociation, the p(2 x 2) S-Pt(100) surface is completely deactivated in respect of the reaction CO+NO = COz+N2, which proceeds readily on S-free Pt(100). In the case of NO on sulphided Rh(100) it has been found that at 300 I< the presence of only 0.08 S inhibits Completely NO dissociat,ion which is accompanied by a significant decrease in the amount of molecularly-adsorbed NO.

120

Chapter 5.

05 04

03

ji 1

9

02 01

0 04

03 02 0 : 0

02 01

0

01

02

03

04

05

Fig. 6.36. (Left) NO and Nz TPD spectra from Ni(100) with different sulphur precoverages, 0s (in ML), exposed to various NO doses at 110 K. The NO exposures from bottom to top within each manifold are: 0.2, 0.2, 1.0, 1.5, 2.0 and 10 L. (Right) Dependence of the total NO coverage (a), dissociated NO fraction (b), and molecular NO fraction (c) on the S coverage, 0s. NO exposures from bottom to top in each panel is 0.2, 0.6, 1.0, 1.5, 2.0 and 10 L (from ref. [102])

Other electronegative adatoms, such as 0 and N , also cause a decrease of the NO dissociation propensity but they act as less severe deactivators than S [78, 103, 1091. In the case NO/O R h ( l l l ) , the presence of oxygen causes the following changes in the adsorption behaviour of NO : (i) destabilization of the NO molecular adsorption state and the adsorption state of the dissociative product N, both NO and Nz desorption occurring at lower temperatures in the presence of oxygen; (ii) inhibition of NO dissociation at 00 turbed bridge NO is removed.

> 0.8, i.e. when almost all undis-

Parallel T P D and HREELS studies indicate that no dissociation is likely for 0 affected ‘on top’ and bridge NO [78]. 5.2.5

Differences in the Effect of O x y g e n on the A d s o r p t i v e Properties and Reactivity of P t ( l l 1 ) with respect to CO and NO

In this Subsection is illustrated that, depending on the particular system, the same electronegative additive can serve as a ‘poison’ with respect to one

5.2. Nitric Oxide

121

reactant (NO) or can participate in a catalytic reaction with another (CO), although both molecules are behaving as electron acceptor coadsorbates. As has been outlined in Section 5.1., oxygen on P t ( l l 1 ) does not serve as a typical poisoning additive with respect to CO adsorption and participates as a reactant in the important (from the viewpoint of environmental protection) catalytic reaction of CO oxidation. It has been found that, contrary to the expectations concerning the behaviour of an usual ‘poison’, the increase of the oxygen-modified CO adsorption states resulting in a more facile reaction [96, 1081.

k---------A

-- -

0.0

2.0

C02 product

CO rwnainlng (450K)

4.0 6.0 8.0 10.0 CO EXPOSURE [ r n b a r . ~ e c . l O - ~ ]

12.0

Fig. 5.37. CO coverage as a function of exposure o n clean P t ( l l 1 ) (dashed curve) and p(2 x 2) O-Pt(ll1). T h e amounts of CO and COz desorbing from CO/p(2 x 2) O-Pt(ll1) during heating are also shown (from ref. [40])

The effect of 0 on the adsorption rate and sequence in the adsorption site occupation of NO and CO on p(2 x 2) O-Pt( 111) will now be compared. In the case of NO, the presence of a p(2 x 2) 0 overlayer leads to: (i) a decrease of the NO initial sticking coefficient (see Table 5.7.); (ii) a reverse in the site occupation, with preferential occupation of linear ‘on top’ sites [98], and

-

(iii) an appearance of a weakly bound NO state with a desorption energy 36 kJ/mole at NO coverages exceeding 0.2, when NO starts to occupy bridge sites (see fig. 5.27.) [82, 981. In the case of CO, the presence of a p(2 x 2) 0 overlayer does not affect the initial rate of CO adsorption (see fig. 5.37.). This indicates that the CO molecules are trapped at 90 K with an equa.1 efficiency in the precursor

Chapter 5

122

OXYGEN COMRAGE CO C O V E W E (YL):

= 2 5 O/Pt 1 r.42 CO/W

2=.3* 3=.32

4z.24 5z.14

dT/dt= 1.4 K/D*c I

Fig. 5.38. COz, CO, and ref. [40])

0 2

TPD spectra from CO/p(2 x 2 ) O-Pt(ll1).

(from

state above each surface site, i.e. the presence of oxygen does not affect the lifetime of the adsorption precursor as has been supposed for the case when the additive acts as a ‘poison’. N o CO desorption occurs at temperatures below 280 K , even at a saturation CO coverage of 0 42 on p(2 x 2) O--Pt(lll), i.e. the oxygen induced destabilization effect (provided t,hat it exists) on the CO adsorption state is much weaker than in the case of NO. At temperatures above 280 K (as is illustrated in fig. 5.38.),the oxidation reaction takes place. However, comparison with the CO T P D spectra from a clean P t ( l l 1 ) surface (see the bottom panel in fig. 5.7.) for a CO coverage of the order of 0.40, where CO desorption starts at 320 I<, permits making the assumption that the effect of oxygen on the stability of the CO adsorption state (if there is any) is insignificant. The most interesting behaviour observed for CO adsorption on p(2 x 2) O-Pt( 111) is that there is no preferential occupation of the ‘nonmodified’ (‘on top’ next-nearest P t site in fig. 5.8.) and both ‘on t,op’ sites on the nearest (directly coordinated with 0) and nest nearest Pt are filled together [40]. As the bond orientation of the coadsorbed CO molecules is concerned, the ESDIAD data have shown that CO ‘on top’ bound on the nearest P t atom is slightly tilted (- 5’). This bonding configuration of CO and 0 sharing the same P t atom is supposed to be a precursor for COz formation. The difference in behaviour of the NO + 0 and CO + 0 coadsorbed layers on Pt(ll1) has been discussed recently in ref. [98]. It has been shown that the ready CO oxidation is due not only to the lower activation barrier for CO oxidation (53 kJ/mol, compared to 110 kJ/mole for NO oxidation on Pt) , but

-

5.2. Nitric Oxide

123

also to the lower adsorption binding energy of NO. In the case of NO, sharing P t atoms with the oxygen adatoms (a bonding configuration which is likely to be a precursor for NO2 formation) the NO adsorption bind is strongly reduced. Thus, because of the reduced activation barrier for NO desorption, no NO2 formation is observed in NO 0 coadsorbate overlayers. Consequently, the absence of an oxygen-induced ‘poisoning’ effect with respect to CO adsorption on P t implies that the strong CO-0 affinity leading to the formation of C02 might compensate the repulsive forces between these two electron-acceptor coadsorbates.

+

5.2.6

Differences in the Effects of the N O Dissociation Products 0 and N and the Reactivity of Pt and Rh Surfaces with Respect to CO Oxidation and NO Reduction

Since, as outlined above, P t an Rh are presently the best catalysts used for the effective removal of the troublesome automobile exhaust gases NO and CO, it is of particular interest to know how the presence of the NO dissociative products will affect the adsorptive behaviour of the reagents. According to the T P D data summarized in ref. [86], the rate-limiting step in the CO(a) NO(a) = C02(g) N2(g) reaction on R h ( l l 1 ) is the reaction CO(a)+O(a) = CO,(g). On Rh(ll1) when allsites are affected by N adatoms both NO and CO adsorb in ‘on top’ sites, where the contribution of the 7rbackdonation to the adsorption bond is reduced. On the one hand, this leads to destabilization of the molecular adsorption state, and on the other, to inhibition of NO dissociation which is an intermediate step in the NO reduction process. Thus, because of its reduced adsorption binding energy (by 30-40 kJ/mol), in a close proximity of N , CO desorption occurs at temperatures close to and even lower than that at which CO oxidation on Rh takes place. This also leads to an increase in relative concentration of oxygen on the surface, which, above certain coverages, will start t o act as a deactivator of further NO dissociation. The inhibition of NO dissociation in the presence of additives is more severe in the case of Pt, where NO dissociation is more difficult. Because of the presence of at least four different species on the surface (NO, CO, 0 and N ) the real picture is much more complicated. Thus, one should take into account that, besides the electronegative adatoms (0,N and eventually C) there exists a mutual effect of NO and CO coadsorption. The latter is quite different for coadsorbed layers on Pt and Rh. In the case of R h ( l l l ) , NO acts t o a certain extent as a deactivator with respect t o CO adsorption. This causes (similarly to N) destabilization of the molecular CO adsorption state by inducing a discreet new CO adsorption state with a population increasing with the CO coverage. On the contrary, the NO adsorption behaviour on Rh(l1 l), including the dissociation propensity, are negligibly affected by the presence of CO. The only CO effect concerns the capacity for NO adsorption, which could be expected from a site exclusion view, because NO cannot remove CO from the surface. The absence of a measurable effect of CO on the NO dissociation can be attributed to the fact that CO occupies exclusively ‘on top’ sites. Thus the threefold sites favourable for 0 and N adsorption on fcc(ll1) surfaces are free.

+

-

+

Chapter 5

124

The picture is different for P t ( l l l ) , where the presence of CO causes a continuous reduction in stability and coverage of the molecular NO adsorption state with increasing CO coverage, whereas the effect of NO on the CO adsorption behaviour on P t ( l l 1 ) is much weaker. That is why, under the typical reaction conditions, where the concentratmionof CO in the gas phase is 5-10 times larger than that of NO (both gases adsorb on Rh and Pt with sticking coefficients close to unity), Rh will be a better catalyst for NO reduction, whereas Pt remaiiis the best one for CO oxidation. Finally, it should be noted that the above considerations merely express a very simplified picture. As has been shown in the previous subsections, when different species with the same surface dipole polarity (electron acceptors in this particular case) are coadsorbing, it is likely that they will tend to form separate islands. This tendency is frequently observed with coadsorbate systems, such as 0 CO, CO NO, etc.[43, 1071. Such a separation of the reactants in islands impedes the surface reaction to an additional extent.

-

+

5.3

+

NITROGEN AND OXYGEN

The interest in understanding the effect of electronegative modifiers on the interaction of N2 with transition metal surfaces has arisen because nitrogen is one of the reactants participating in the important catalytic reaction of ammonia synthesis, where iron is the most effective catalyst [110]. Since N2 dissociation is believed to be the rate limiting step in t>hehigh pressure ammonia synthesis from Nz and H z , it is important to find out to what extent the presence of additives such as S and 0 will affect the interaction of Nz with the catalyst surface. 5.3.1

General Reinarks for N? A d s o r p t i o n on Transition Metal Surfaces

The nitrogen molecule is isoelectronic with and structurally similar to the CO molecule. Generally, with both molecules the formation of the molecular adsorption bond is via the combination of a a / d , donor Component, where a orbitals are lone pair orbitals of the adsorbing molecules, and a En*/ dz acceptor component, where the E* are unoccupied antibonding orbitals in the adsorbing inolecules. The difference between the N2 and CO bonding is that the a valence orbitals 2uu and 3ug, shared equally between the two nitrogen atoms in a free molecule, mix upon interaction with a metal surface to form two new a orbitals which show a lone pair character to some extent. The resulting donor bond is weaker than that of CO (formed with the participation of the CO-5a lone pair only). By analogy with CO, the backdonation component in the Me-Nz bonding involves the lrgantibonding orbital [ l l l ] . The backdonation component is expected to be more important in CO than N2 bonding, since the 2r CO orbitals lie at a lower energy (of 0.6 eV) than the l?rg Nz ones [112]. As a result the adsorption binding energy of N2 is rather small rangkg from 20 t o 40 kJ/mol for transition metals, such as P d ( l l 0 ) [114], Re (1120) and Re(OOO1) [115], Fe(ll0) and Fe(100) [116], Ru(0001) [117], Ni(ll0) [118, 1191, etc. [113]. These weakly bound molecular

-

5.3. Nitrogen and Oxygen

125

states occupy usually ‘on top’ sites wit,li the molecular axis perpendicular to the surface plane [ill, 114, 117-1231. It is worth mentioning that studies of the influence of the nitrogen coverage on the Me-N:, and N-N stretching frequencies have shown that the Me-N2 stretching frequency increases (e.g. from 278 to 291 cm-’ for N2 on Ru(0001) [117]), whereas the N-N frequency shifts to lower energies with increasing nitrogen coverage (e.g. from 2252 t o 2198 cm-’ for the same system [117]). This trend is opposite to the one observed in CO/transition metal adsorption systems. The reason for this behaviour has not been completely clarified, but one of the possibilities is the formation of a In, band with a significant dispersion as a result of overlapping between the spatially extended N2 17rg orbitals in denser nitrogen overlayers. It would be reasonable to suppose that, if the In, band broadens sufficiently, it might cross the Fermi level and induce the observed reduction in the N-N stretching frequency. Since the changes on the N-N stretching frequency with increasing Nz coverage are accompanied by a reduction in the NZ desorption energy, the above explanation will be adequate, supposing parallel changes in the donor bonding contribution. Because of the weak adsorption bond, the adsorption rate and the saturation nitrogen coverage for molecular Nz adsorption are strongly dependent on the adsorption temperature. Thus, at room temperature for most of the transition metals under consideration, the initial sticking coefficient is less than lo-’. Actually, at temperatures exceeding the desorption temperatures of the molecular nitrogen, the only stable state on the surface can be nitrogen atoms produced as a result of dissociative nitrogen adsorption [113]. It has been established that the dissociation propensity of N2 is structurally sensitive, e.g. Fe( 111) and polycrystalline Fe are more active with respect t o Nz dissociation and ammonia synthesis than the less open Fe(100) and Fe(ll0) planes [113]. Similarly t o F e ( l l l ) , the open plane of Re(1120) and polycrystalline Re is more active than the close-packed Re(0001) [115, 1241, etc. Recently, HEELS and ARUPS data [125-1281 have shown t,hat there is a second adsorption state of N:, on the open Fe(ll1) surface. The orientation of the N:, molecules in this state and the bonding to the surface differ from the ‘end-on’ linear configuration described above. The second Nz adsorption state det,ected on the open Fe(ll1) plane has an unusually low N-N stretching frequency and is described as a ‘side-on’ r-bonded complex with both N atoms interacting with the metal, with a stronger backdonation contribution compared to ‘end-on’ configuration. The ‘side-on’ Nz cqnfiguration is assumed to be an immediate precursor of Nz dissociation. As described i n ref. [127], the adsorption in this ‘precursor’ state can occur via the more weakly bonded Lend-on’linear state (by interstate conversion) and also by direct adsorption. The direct adsorption process has a rather low initial sticking coefficient (and is the only adsorption channel at higher temperatures and low pressures when the concentration of the ‘end-on’ linear state becomes negligible. The same ‘r-bonded’ N2 species have also been observed on a Cr( 111) surface [ 1291.

Chapter 5

126 5.3.2

Modifier Effect on the sorption

N2

Molecular and Dissociative Ad-

Oxygen, often introduced as an impurity in the reactant mixture is one of the most effective poisons in ammonia synthesis [130]. Studies of thg effect of increasing amounts of oxygen over polycrystalline Fe on the dissociative nitrogen adsorption have shown a linear decrease of the atomic N uptake with increasing oxygen coverage [131]. The effect of preadsorbed oxygen is assumed t o be due t o the fact that both nitrogen and oxygen prefer the same type of adsorption sites. This indicates that the two effects are additive because, as outlined in section 3.2.' both 0 and N cause reconstruction of the Fe surfaces with a strong tendency t o compound oxide and nitride formation. Similar effect on nitrogen dissociative adsorption on Fe is also induced by the presence of sulphur adatoms [113]. The effect of oxygen on the molecular adsorption process on N i ( 110), where dissociation under UHV conditions is negligible consists of a linear displacement of N2 with a negligible influence on the stability of t,he molecular state (slight downward shift of the N 2 T P D peak by 10 I<) and on the Me-N:! and N-N stretching frequencies [119]. The linear decrease of the saturation N2 coverage, e ( N z ) , is found to obey the following relationship:

-

where Omax(N2) is the saturation N:! coverage on a O-free surface, and e(0) and O,(O) are the actual oxygen coverage and the critical oxygen coverage at which O(N2) becomes 0. It has been established that 6,(0) is 0.5 and one oxygen adatom blocks one N2 adsorption site. This result agrees well with the simple blocking model of competitive adsorption. The study of the influence of ordered and disordered 0 overlayers on molecular nitrogen adsorption over Ru(0001) [117] hasbeendone in greater detail. I t has been found that the increase in oxygen coverage from 0 to 0.25 when the p(2 x 2) 0 surface structure is formed, causes a gradual attenuation of the original 94 I< and 117 I< N2 T P D states and an appearance of a new NZ TPD peak at 140 K. The original N2 T P D peaks completely disappear at p(2 x 2) 0.25 0 when only the new TPD feature at 140 K remains. An increase of oxygen coverage beyond 0.25 causes further reduction of the nitrogen coverage [the new O-induced N2 state) until the surface is completely deactivated for nitrogen adsorption at a 0.5 0 p(2 x 1) coverage. An interesting finding is that, irrespective of the reduction of the surface adsorptive capacity (following the same blocking mechanism a5 described by eq.(4)), the oxygen induced Nz molecular state is stabilized compared to a clean surface by 6 kJ/mole. This state is supposed to he 'on top' linear N 2 residing on the only next-nearest Pt atom on a p(2 x 2) 0.25 O / P t ( l l l ) surface (see fig. 5.8.). The Me-N2 stretchingfrequency of this O-affected site is 249 cm-' (by 30 cm-' lower than the lowest value on a clean surface), whereas the N-N stretching frequency is 2763 cm-' (by 15 cm-' higher than the highest 'on top' N 2 value on a clean surface). The vibrational data indicate that, as expected, the presence of oxygen inhibits the ability of the affected site for backdonation, i.e weakens the contribution of l r g / d T backbonding. In order

-

-

5.3. Nitrogen and Oxygen

127

t o explain the fact that nevertheless the N? adsorption state is stabilized, the authors suppose that the increased Lewis acidity of the next nearest Ru atoms enhances the ability of the surface to accept the Nz lone pair, i.e.enhances the contribution of the s donation bonding. Comparison with the effect of the same oxygen overlayers on the CO adsorption behaviour shows that together with the surface adsorptive capacity, the CO adsorption bond is also reduced (by 30 kJ/mole). The opposite effects of oxygen on the stability of the Nz and CO molecular states indicate that the backdonation contribution to the bonding is much weaker in the case of N?, whereas in the case of CO, the 27r backdonation is very significant.

-

5.3.3

Modifier Effect on the

0 2

Adsorption

Generally, the effect of the electronegative a.dditives on the oxygen dissociative adsorption is the same as that observed for CO, NO and N?. Oxygen adsorption on transition metal surfa.ces plays a. central role i n several important catalytic processes, such as the ca.talytic oxidation of CO and ammonia, where the oxidation processes are preceded by oxygen dissociative adsorption. Compared to Nz, NO and CO, oxygen adsorption proceeds dissociatively even at temperatures lower than room temperature on most metals so that the molecular adsorption state can be detected only at rather low adsorption temperatures. This is due t o the lower shbility of the oxygen molecular bond. Thus, the formation of a donor/acceptor adsorption bond causes weakening and rupture of the 0-0 bond as a result of the charge backdonation to the 1 7 molecular ~ ~ orbital. The introduction of electronegative adatoms causes the reduction of the rate of 0 2 dissociation because of blocking of the adsorption sites favourable for oxygen ada.t.oms. An example of the effect of an electronegative additive on the oxygen dissocktion rate is given in fig. 5.39. AS can be seen, a C1 covemge of N 0.25 suppresses the rate of oxygen dissociative adsorption more than ten times. The sa.turation oxygen coverage is observed t o decrease almost linearly with B c ~ falling to zero a.t C1 coverages close to 0.5 [186, 1871. Since the presence of CI does not affect significa.ntly the shape and position of the 0 2 T P D spectra, it has been suggested that the coa.dsorbed C1 and 0 atoms tend to form separate islands. This agrees well with the linear decline of the oxygen covera.ge with increasing Bcl “71. The electronegative additives which are electron acceptors a.ffect mainly the donation ability of the surface atoms, so that the reactants molecular adsorption state where the formation of the adsorption bond involves a significant backdonation contribution should be more severely destabilized. Since, as outlined above, the dissocia.tion of molecules with a donor/accept,or type of adsorption bond, such as CO, NO, Nz and 0 2 proceeds via a precursor with a dominating backbonding contrihut>ion,the presence of electronegative additives will always inhibit the dissociation of these molecules, irrespective of the effect on the sta.bility of the possible molecular adsorption states.

Chapter 5

128

Fig. 5.39. The effect of C1 coverage on the rate of oxygen dissociative adsorption on Ag(ll0) (from ref. [ISS]) 5.4

HYDROGEN

Hydrogen is a main reactant in a number of catalytic reactions, such ils Fischer

- Tropsch synthesis, ammonia synthesis, hydrogenation of hydrocarbons etc.

The H:! molecule possesses a n occupied lug and an unoccupied la, orbital, Both orbitals are involved in the formation of bonding with transition metal surfaces. The existence of a high d-electron d a t e density at the Fermi level of the substrate is found to be the major parameter that governs the dissociation process. Thus, on most of the transition metals hydrogen readily dissociates, hydrogen adatoms forming a rather strong bond with the substrate (of the order of 240-300 kJ/mole [132]). Because of its small size, the hydrogen adatom is located close t o the surface, occupying usually the highest coordinated sites. Pertinent to H adsorption is the smaller activation energy for surface diffusion compared t o other adsorbates. The high mobility of hydrogen on the surface is an important property explaining the fact that hydrogen diffusion is seldom rate-limiting in heterogeneous catalysis. The rate of the dissociative hydrogen adsorption on a particular surface depends on the height of the activation barrier which is small or negligible for most of the transition metal surfaces. The selected values of the measured initial sticking coefficients for hydrogen dissociative adsorption on clean single crystal surfaces show that with most of the transition metal surfaces, 5’0 is in the range from 0.5 to unity. Lower So values are likely to be due t o the existence of an activation barrier for adsorption.

5 . 4 . Hydrogen 5.4.1

129

Hydrogen Dissociative Adsorption on Modified Surfaces: Adsorption Kinetics, Energetics and Capacity for Adsorption

All studies up t o date have shown that the presence of electronegative additives results in a strong reduction in the hydrogen dissociative adsorption rate, a decrease of the hydrogen saturation coverage and significa.nt changes in the desorption parameters [13 - 15, 22, 23, 52, 61, 66, 133-1381.

Fig. 5.40. Effect of varying chlorine, sulphur and phosphorus precoverages 011 the Hz TPD spectra from Ni(100). Hz exposure 10 L; T, = 100 I< (from ref. [13])

Figs. 5.40.-5.42. illustrate the effect of various modifiers on the H2 T P D spectra, initial sticking coefficient and saturation coverage. Obviously, the introduction of electronegative adatoms causes a reduction in the hydrogen uptake, a shift of the H? T P D maxima to lower temperatures and a broadening of the T P D spectra. The effect of soiiie modifiers on the desorption parameters of Ha, as calculated from the T P D data, is presented in Table 5.9. It is obvious that the presence of electronegative additives causes a concomitant decrease of the desorption energy, Ed, and the preesponential factor, v. As will be discussed in more detail later on, the observed relatively small shift of the H:! T P D maxima accompanied by a marked broadening of the peaks, despite the relatively large E d and v changes, reflects the constraint on the recombination process due to the modifier induced changes i n the potential energy contour for hydrogen surface diffusion. It is interesting that, compared t o Ed, the effect of the electronegative additives on the Me-H bond is less severe (of the order of 10-30 kJ/mol). This indicates that the modifier influence on the H adsorption sites which are not blocked is, not significaiit In the case of other substrates, where the H? T P D spectra reveal the existence of more than one hydrogen adsorption state, e.g. H 2 / P t ( l l l ) [137], Hz/Mo(100) [61], etc. [132], the reduction in the ability of the surface to chemisorb hydrogen proceeds by a subsequent elimination of the high temperature adsorption states with increasing additive coverage. Thus, as illustrated

130

Chapter 5

Fig. 5.41. Hz TPD spectra for increasing hydrogen coverages on clean P t ( l l l ) , p(2 x 2) 0.25 S-Pt(lll) and ( Ax &)R30° 0.33 S-Pt(ll1). T, = 200 K (from ref. [137])

in fig. 5.41., the increase of the S coverage on P t ( 111) leads to elimination of the p3 peak at 400 K first, followed by a reduction of the less strongly bound pzstate. The saturation hydrogen coverages at 200 I< evaluated from the T P D data in fig. 5.40. are 0.3, 0.24 and 0.04 for clean P t ( l l l ) , p(2 x 2) S / P t ( l l l ) and (fi x fi)R30° S / P t ( l l l ) surfaces, respectively. The adsorptive ability of S-modified P t ( l l 1 ) is more severely affected a t higher adsorption temperatures, e.g. at 300 K it becomes 0 for the ( A X &)R30° S-Pt(ll1) surface and is of the order of 1/10 of the clean surface saturation coverage for p(2 x 2) S / P t ( l l l ) [13G]. As can be judged from the data presented in Table 5.10. and fig. 5.42., the effectiveness of modifiers with respect t o reduction of the adsorption capacity is more sensitive to the size of the additive adatoins, i.e. the larger chlorine and sulphur adatoms exhibit the most severe influence on the hydrogen uptake. The OH versus modifier coverage plots in the case of modifiers which tend t o adsorb in highly coordinated sites and form ordered structures, are very steep in the beginning. This indicates that an isolated additive adatom which competes with hydrogen for the same adsorption sites eliminates not only the site where it resides but also influences nearest-neighbour positions. The number of the latter is varying with the type of the modifier and the substrate crystallographic plane. Here it is worth noting that again in the case of additives which tend to form islands of a separate phase on the surface, the poisoning effect is less severe, because patches of unaffected areas persist until the surface is covered with a complet,e overlayer of the modifier. This is the case of phosphorus Ni(100) in fig. 5.42., where the reduction of hydrogen adsorption is linear. The same behaviour is exhibited by surfaces with a graphite deposit. For an example, in the presence of graphite islands on the Ni( 110) surface, hydrogen desorbing exactly like H 2 from clean Ni(ll0) has been det,ected [23].

5 . 4 . Hydrogen

131

ADOITIVI COVtllACt [ML)

0

'r

d

CI

i

05

Fig. 5.42. (Top) Dependence of the hydrogen saturation coverage, 6 , at 100 K on the additive coverage. (Bottom) T h e initial sticking for dissociative hydrogen adsorption, SO,as a function of the addit.ive coverage. T h e dashed line represents the theoretical dependence according to the relationship SO= So(clean)(l - 46%)' (from ref. [13])

Since graphite (phosphide) surfaces do not adsorb H?, the reduction in the adsorptive capacity depends simply on the fraction of the clean surface islands. The d a t a in Table 5.10. and figs. 5.39.-5.41. show that the introduction of electronegative adatoms induces a severe reduct,ion in the rate of hydrogen dissociative adsorption. The presence of additives affects the dissociative hydrogen adsorption in several ways: (i) by occupying and blocking highly coordinated surface sites favourable for hydrogen adsorption; (ii) by reducing the lifetime of the molecular precursor for dissociation, provided hydrogen dissociative adsorption proceeds via a precursor, and (iii) by creating an activation barrier for dissociation of the precursor The simple blocking effect of the modifier, (i), concerns the available adsorption sites for hydrogen adatoms. The extent of this effect can be established

Chapter 5

132

Table 5.9. Effect of Several Electronegative Adatom Overlayers on the Kinetic Parameters of Hydrogen Desorption, E d (in kJ/mol) and v (in cm2/s), and the MeH Bond Strength, E M ~ - (kJ/mol), H as Estimated from the Relationship E M ~ - H = 1/2(Ed

+

(data from ref. [15, 52, 136, 137bl)

SURFACE ~~

Ni(100) p(2 x 2) S-Ni(100) c(2 x 2) S-Ni(100) Fe(lO0) c(2 x 3) 0-Fe(lO0) Pd( 100) 0.15 S-Pd(100) Pt( 111) p(2 x 2) S-Pt(ll1)

~

Ed

U

102 f 5 84 f 10 48 f 16 87 f 5 60 f 10 85 49 75

5x 4x 10-~

EM+H

~~

-

10-8 .5 x lo-*

10-2.5 10-6.8 10-~

266 f 5 256 f 10 239 f 16 258 f 5 245 f 10 256 240 253 244

studying the adsorptive capacity of the surface exposed directly to atomic hydrogen. As has been reported in ref. [52], there is a significant increase of the amount of adsorbed hydrogen (roughly by a factor of five) upon atomic hydrogen adsorption. Let us suppose that the blocked area around the additive adatom is determined by the repulsive interactions between the modifier and the hydrogen adatoms. Consequently, taking into account the small dimensions of the hydrogen adatom, it should be expected that the effective blocking radius will be of the order of the Van der Waals radius of the modifier atoms when the latter are located above the surface plane. As an example, for S on Ru(0001) the effective blocking radius is found to be 2 (the Van der Waals radius of S is 1.85 A). This means that for S located in a threefold site on Ru(0001), the three nearest-neighbour threefold sites should be excluded as sites for hydrogen adsorption because they are at a distance of 1.56 A. This prediction agrees with the finding that one S adatom blocks four sites for hydrogen adsorption on Ru(0001) [139]. However, the effective blocking radius becomes much smaller in the case of a smaller additive adatom, located deeply in the first layer of the substrate. This is the case of C/Mo( 100) where in the presence of t9c = 0.7 ML the adsorption capacity for atomic hydrogen is 0.3 ML, i.e. because of its small size one C (residing deep down in the fourfold surface sites) eliminates only one adsorption site. Following these considerations about the elimination of hydrogen adatom adsorption sites, it is obvious that a simple site blocking alone cannot explain the observed severe reduction of the initial sticking coefficient for dissociative adsorption, SO,(implying 6-9 sites blocked per one S or C1 adatom for fcc(ll1) [133, 135, 136) or fcc(100) [13] surfaces). Undoubtedly, the contribution of the kinetic factors (ii) and/or (iii) are also significant, especially in the case of additives with larger sizes. The differences in the efficiency of the poisoning effect for ordered additive overlayers and islands of a separate additive phase are very well demonstrated

-

A

133

5.4. Hydrogen

Table 5.10. Saturation Hydrogen Coverage, 8~ (in ML), and the Initial Sticking Coefficient for Hz Dissociative Adsorpt,ion, SO,for Several Clean and Modified with Electronegative Additives Single Crystal Surfaces (from ref.[l3, 15, 23, 52, 1371)

so

SURFACE ~~~~~~~~

~

Ni(100) p(2 x 2) S-Ni(100) c(2 x 2) S-Ni(100) p(2 x 2) C1-Ni(100) 0.5 P/Ni( 100) Ni( 110) (4 x 5) C-Ni(ll0) (2 x I) C-Ni(110) graphite/Ni(llO) Fe(100) c(2 x 2) C-Fe(100) p(1 x 1) 0-Fe(100) c(2 x 2) S-Fe(100) Pd(100) 0.08 S-Pd( 100) 0.15 S-Pd( 100) p(2 x 2) S-Pd(100) Pt(ll1) p(2 x 2) S-Pt(ll1) c(2 x 2) S-Pt(ll1)

~

~

T,

~

0.74 0.12 < 0.01 < 0.001 0.36 0.5

0.005 < 0.0015 < 0.001 0.5 0.1 0.4 0 1.0 0.84 0.42 0.11 0.3 0.2 0.04

120 K 120 I< 120 K < lod4 120 K 120 I< 2.5 x lo-’ -1 200 I< 5x 200 I< 5x 200 I< < 3 x 1 0 - ~ 200 I< 250 I< 3x 10-~ 250 I< 1 0 - ~ 250 I< 0 250 I< 100 I<

6 x lo-’ 7 x 10-2 < lod3

--

-

0.69So 0.31So

0.05So -

100 I<

100 I< 100 I< 200 I< 200 I< 200 1;

by comparing tlie adsorptive properties of a (4 x 5) C-Ni(ll0) surface and a N i ( l l 0 ) surface covered wit,h graphite islands, produced after a carbon phase transition. The latter came about as a result of heating of the (4 x 5) CNi(ll0) layer t o 750 I< (the (4 x 5) C graphite transition which leads to N 40 % of the saturated graphite layer). It turns out that the 0.4 graphite surface adsorbs substantial amounts of H? on tlie remaining patches of unaffected areas, whereas the ( 4 x 5) carbide surface does not chemisorb hydrogen at room temperature [23]. For surfaces where the modifiers are arranged in islands, leaving completely clean patches, the changes in the initial rate of dissociative adsorption are less severe. As illustrated in fig. 5.41., 5’0 decreases linearly with the increase in the fraction of the surface occupied by modifier islands and falls t o zero on complet-ion of the modifier monolayer (since the latter does not adsorb hydrogen at all).

-

5.4.2

Modifier Effect on the Surface Diffusion of Hydrogen Adatoms

The surface mobility of tlie reactants is of great importance in catalytic processes [140]. As outlined a.bove, hydrogen adatom diffusion on clean metal

134

Chapter 5

-

surfaces occurs readily, the activation energy of diffusion being of the order of or less than 25 kJ/mol [132]. Since besides t,he effects on the adsorption kinetic parameters and Me-H bond strength, the introduction of foreign adatoms changes the surface potential contour for diffusion, it is necessary to find out how this is going to affect the mobility of H on the surface. Such fundamental knowledge will provide a valuable insight into the very important phenomena of surface diffusion directly related to catalysis. Fig. 5.43. presents the changes in the hydrogen surface diffusion coeficients induced by various sulphur coverages on Ru(O1). It is obvious that the presence of small amounts of S causes a severe reduction of the hydrogen diffusion coefficient approximately by a factor of 30 for a sulphur coverage 0.2. Since the ratio between the diffusion coefficients measured at 300 and 270 K remains constant, this indicates that the presence of S does not affect the activation barrier for diffusion (19 kJ/mol). Consequently, the presence of S should affect the pre-exponential factor in the hydrogen surface diffusion coefficient [142].

-

-

Sulfur Coverage

BS

(ML)

Fig. 5.43. Hydrogen surface diffusion coefficients on Ru(0001) at 300 and 370 I< versus sulphur coverage. The solid lines are Monte Carlo simulat,ion results for sulphur blocking ten three-fold adsorption sites (from ref. [143])

A similar strong reduction of the hydrogen surface diffusion coefficient has been observed in the presence of C which is also a common surface modifier in many catalytic reactions. The difference with a S additive is that the presence of a C deposit causes a change in the activation energy for hydrogen surface diffusion [141]. The observed differences have been associated with the existence of two possible mechanisms for hindrance of the coadsorbate surface mobility:

5 . 5 . Water

135

(1) A blocking mechanism in the case where the additive adatom residing in a highly coordinated site forces the coadsorbate adatom t o diffuse around this site, the precluded area being determined by the t,ype of the modifier. Obviously, in this case, the activation energy for diffusion is likely to remain constant and the restricted mobilit,y will be determined by the pre-exponential of the surfa.ce diffusion coefficient; (2) A trapping mechanism when the coadsorbate adatoms are permanently or temporarily trapped by the additive adatoms. In this case the formation of a H-additive bond should affect the activation energy for surface diffusion and might also cause a shift of the H? T P D spectra to a higher temperature due to the formation of a hydrogen-modifier bond. A deta.iled exa.mination of the dat8aconcerning the effect of S and C: additives on the hydrogen diffusion coefficient and the €12 TPD data. on Ru(0001) [141, 1421 has led to the conclusion that the site-blocking model is of importance in the case of S, w11erea.s the trapping mecha~nismexplains the effect of C. Actually, there is a great dea.1 of proof provided by HREELS and ESD of H attaching t o C atoms on met,al surfaces [143-1451, which supports the existence of a trapping mechanism. The Monte Carlo simulations performed in order to determine the effective radius of the S influence on hydrogen surface diffusion have shown that the experimental d a t a will fit a.ssuming that one S adatom residing i n a threefold site on the hpc (0001) surface perturbs ten threefold surface sites. They involve the S adsorption site, the three nea.rest neighbour and the six nextnearest neighbour sites. The restricted mobility of the hydrogen adat$omsin the presence of additives explains the observed drama.tic reduction of the 112 desorption preexponential factors (see Table 5.9.). This reduction reflecting a steric hindrance of hydrogen recombination on the surface can be directly related t o the observed severe restriction of the hydrogen surface diffusion in the presence of S adatoms. Undoubtedly, as will be illustrated in Section 8.1., the effect on the hydrogen surface mobility is one of the most import,ant,factors in explaining the S poisoning effect, on the methanat,ion reaction and the Fischer - Tropsch syntheses. 5.5

WATER

Water participates as a reactaiit or a product in numerous het,erogeneous catalytic reactions, such as water - gas shift synthesis, Fischer - Tropsch synthesis, etc. Isolated water molecules possess four doubly occupied orbitals, the two 0-H bonds being located i n t,he y r plane, and the two lone pairs, (3al and l b l ) , in the z z plane. Water adsorption on the single crystal metal surfaces under consideration is nondissociative. T h e bonding to the surface is formed via the oxygen atmomwith a 3u1 partly bonding lone pair and a Ibl non bonding lone pair being involved in bonding. T h e strength of the molecular adsorption bond with metal surfaces is typically of the order of 4065 kJ/mol, which indicates a relatively weak coupling with the metal surface.

136

Chapter 5

The formation of the molecular adsorption bond is accompanied by a decrease of the work function, i.e, with respect to its adsorption behaviour water should be considered as a n electron donor (opposite to CO, NO, Nz and 0 2 ) . The configuration of the water molecule chemisorbed on the surface depends on the actual adsorption site, because there is competition between 3al and I b l orbitals for optimal overlap with suitable surface &orbitals. Because of the non-bonding character of the water lone pairs participating in the formation of the adsorption bond and the relatively weak interactions with the surface, the internal molecule bonds are only slightly perturbed upon adsorption. The relatively weak coupling with the surface enables the formation of hydrogenbonded clusters even at low water coverages (the hydrogen bond strengths are of the order of 15-25 kJ/mol). More details about the water - metal surface interactions are given in the extended review of P. Thiel and T. Madey (14GI. Since, as outlined above, water acts as an electron donor in the bonding formation with the metal surface, the introduction of electronegative additives will be expected t o influence the surface structure and reactivity of water in a way quite different from those observed for GO, NO, N?, 0 2 and H?. The influence of oxygen as an electronegat(ive additive is most widely studied. In order to understand the oxygen effect, it is necessary to consider separately the observed stabilization of the molecular adsorption state and the promotion of dissociation of the water molecule on some oxygen-modified single crystal metal surfaces. The best example for the oxygen effect on the molecular surface structure and bonding of water is the system H?O/ 0Ru(OOO1) [147-1491. It involves: (i) stabilization (increase of the adsorption binding energy) of the water molecular state;

(ii) destruction of the long range ordering due to the formation of H? 0H 2 0 hydrogen bonds on the O-free surface; (iii) changes in the H 2 0 orientation preferred on a clean surface due to direct water - oxygen interactions in the mixed overlayers, which overcome the formation of hydrogen bonds. The same stabilization and orientation effects on the molecular adsorption state is observed for the H20/O-Ni(lll) system [150, 1511. The experimental data [148, 150-1531 show that several water molecules can be influenced by each adsorbed oxygen atom, e.g. the number of affected water molecules reaches 6 to 8 for low oxygen coverages [152]. An excellent illustration of the oxygen-induced orientational changes of the coadsorbed water molecule are the ESDIAD results, shown in fig. 5.44. The change in the Ht ESDIAD pattern with the introduction of oxygen on Ni( 111) indicates that oxygen causes an azimuthal order in the OH bond orientation along [I121 azimuths. In addition, because of interaction of the water molecule with the coadsorbed oxygen via one of the hydrogen atoms (as illustrated by the structural model in fig. 5.44.) the molecule inclines towards the oxygen adatom, so that only the second hydrogen atom is seen in ESDIAD. The origin of the central beam (the only one that remains after heating) from the linear Ni-0-H group,

5.5. Water

137

118

Fig. 5.44. LEED (a), ESDIAD (b-g) patterns and a structural model for fractional monolayers of oxygen and water coadsorbed on N i ( l l 1 ) . Panel (b) shows the Ht ESDIAD pattern of water on clean N i ( l l l ) , and panel (c) shows the H+ ESDIAD

pattern of water with coadsorbed oxygen. Panels (e-g) illustrate the heating induced changes in the H+ ESDIAD pattern when OH, species are formed. (from ref. [150])

formed as a result of oxygen promoted dissociation of water on the surface. Oxygen induced water dissociation and OH formation have been reported for Ag, C u , P t , P d a n d Ni single crystal surfaces [150-1571. It has been supposed t h a t the dissociation of water on the surface proceeds via a hydrogen abstraction reaction, which takes place in the temperature range 130-200 K: 0, HzO, = 20H,. T h e formation of adsorbed OH species on oxygen predosed metal surfaces is found to be very sensitive to t h e actual oxygen coverage, as illustrated in fig. 5.45. T h i s indicates that the dissociation of water via a hydrogen abstraction reaction is favoured only at relatively low oxygen coverages (maximum production of OH, in the 0.1-0.2 ML range), t h e oxygen saturated coverages becoming unreactive. Oxygen coverage being dependent on reactivity is supposed to be due to a geometrical site-blocking effect, because significant coverage-induced changes in the electronic properties of the adsorbed oxygen atoms are unlikely. However, if oxygen tends to form islands at high coverages, t h a n this adds to the reduction of the surface reactivity, because t h e reaction occurs only at the edges of the islands. An additional severe reduction of t h e reactivity towards hydrogen abstraction occurs when a n oxide phase is formed on the surface. As has been shown in ref. [158], both the

+

Chapter 5

138

I

0

u

/

'

.4\

\

I

i

\

\

\

0 2 1

r,

01

02

03

0.

Oxygen Couerage (ML) Fig. 5.45. Dependence of the hydroxyl coverage on t,he initia1 oxygen coverage, 80, on C u ( l l 0 ) (from ref. [152])

clean Ni(ll0) and the heavily oxidized metal surface are completely inert to HzO adsorption at room temperature. As illustrated by the HzO T P D data in fig. 5.46. (where H 2 0 desorption results from OH, recombination), the dissociation of HzO takes place preferentially within a critical oxygen coverage span, where oxygen is in a n adsorption state. Summarizing the results for water coadsorption with electronegative adatoms, it becomes obvious that the effects of the electronegative additives with respect to the molecular adsorption state stability and the dissociative propensity of water are opposite to that observed in the case of CO, NO, N 2 , 0 2 and Hz. This is not surprising because in the case of water adsorption, the metal surface can be considered as an electron acceptor so that it is likely that the introduction of an electronegative modifier will increase the heat of water adsorption. Besides, the presence of additive adatoms with dipoles opposite to that of the coadsorbed water will also contribute change in the bonding energies and in the orientation of H20 via attractive electrostatic interactions. In the case of an oxygen modifier these interactions can lead to hydrogen abstraction and dissociation of the water molecules on the surface. The resulting OH, species are electron acceptors bonded via an 0 atom in an adsorption site which offers a n optimal overlap between OH x and the appropriate metal d orbitals.

139

5.6. Organic Compounds

-g;*

-

,

1

300

-0.65

--.---. 0.6

-

0.1 1

0.06

A ’0.0 400 500 Temperature (K)

Fig. 5.46. Thermal desorption spectra for H 2 0 o n clean and oxygen covered N i ( l 1 ) ) . The formation of an oxide phase is observed at 00 > 0.5 (from ref. [158]) 5.6

ORGANIC COMPOUNDS

There is a great number of hydrocarbon reactions catalyzed by transition metals, e.g. hydrogenation of unsaturated hydrocarbons, dehydrogenation, dehydroisomerisation, dehydro- cyclisation, isomerisation, selective oxidation, etc. T h e complicated hydrocarbon syntheses involve a sequence of several distinct steps, t h e first one being always adsorption of the reactant - hydrocarbon molecule. W i t h the exception of some hydrogenation reactions, several alternative reaction paths are usually possible. They include hydrocarbon dissociation followed by fragmentation, rearrangement of the intermediates a n d secondary reactions within the adsorbed layer. Any single step may be favoured for a given structure and composition of the catalyst surface. Because of t h e complexity of the syntheses based on hydrocarbons, one cannot draw a definite line, whether additives are acting as poisons or promoters T h e reason for t h a t is t h e necessity to introduce a selective poison in order to inhibit the undesired reaction paths or to stabilize some surface chemical s t a t e or structure. There is tremendous variety of hydrocarbons and hydrocarbon interactions with clean a n d modified transition metal surfaces; it, is impossible to deal with these in the present review. T h a t is why several examples of electronegative additive effects on interactions of unsaturated hydrocarbons,

140

Chapter 5

alcohols and R I R ~ C Ocarbonyl compounds have been singled out. The main aspect thereby is the influence of some electronegative additives (0,S, C) on the molecular adsorption state, surface decomposition and secondary reactions in the mixed overlayers.

5.6.1

Interaction of Hydrocarbons w i t h Modified Metal Surfaces

Most of the catalytic reactions involving unsaturated hydrocarbons contain breaking the C-H and C-C bonds as a reaction step, followed by hydrocarbon adsorption on the surface. The formation of the molecular adsorption bond of olefins with the transition metal surface has been established exclusively on the basis of HREELS, ARUPS, IR and T P D studies of C2H4 adsorption on clean single crystal surface at low adsorption temperatures [163]. Two types of bonding with the metal surface have been identified. The first di-a type of bonding occurs via the two carbon atoms and involves a large amount of backdonation from the ds-hybrid metal orbitals to the antibonding r* orbitals of the adsorbed ethylene. As a result, the C-C bond order decreases, the C hybridization changing from sp2 to sp3. The second r type of bonding retains the sp2 hybridization of the C atoms involved in the double bond and the main contribution to the bonding is via x-donation from the molecule t o the metal ds-hybrid orbitals. The relative amount of the di-a and a-bonded ethylene varies with various transition metals and crystallographic planes, the di-a bonded state being the most favourable at low coverages and the more stable (with a higher adsorption binding energy) one. Since in both bonding configurations the ethylene C-C axis is parallel to the surface plane, one should expect the same steric blocking effect of the additives with respect t o di-a and a-bonded molecules. At elevated temperatures (2 300 I<), the di-a bonded species undergo subsequent dissociation starting with breakage of the C-H bonds and formation of ethylidyne species C-CH3, coordinated to three surface atoms. With a further increase of the adsorption temperature (> 400 K), decomposition to CCH and complete dehydrogenation occur. Oxygen is one of the most usual impurities but it also can participate as a reactant in some catalytic reactions, e.g. epoxidation of ethylene to ethylene oxide on silver catalysts [164]. This has given rise to a great number of recent studies dedicated to describing the different aspects of the influence of oxygen on the adsorption and the decomposition of ethylene and other olefins [165-1741 on single crystal metal surfaces. The introduction of oxygen (as well as other electronegative additives) to the surface generally leads to the reduction of the relative amount of di-a bonded molecular species in favour of dominating x-bonded species. On metal surfaces where no secondary reaction between the hydrocarbon radicals a i d coadsorbed oxygen atoms occurs (e.g. Ru and Fe [167, 1681) it has been found that the presence of oxygen reduces both - the total amount of adsorbed ethylene and the fraction of adsorbed ethylene molecules which decompose at elevat,ed temperatures. Table 5.11. presents the effect of oxygen overlayers on the adsorptive capacity and the decomposition efficiency of the surface. It is obvious that the decomposition of ethylene is completely hindered on Ru(0001) covered with a p(2 x 1) 0.5 0 overlayer. However, close inspection of the experimental data

5.6. Organic Compounds

141

has shown that no decomposition takes place at oxygen coverages exceeding 0.4 [167]. Table 5.11. Saturation Ethylene Coverage, OSat (in ML) at T, = 100 K , the Fraction Decomposed upon Heating, &ins (in ML), and the Type of Bonding, MeC2H4, of the Molecular Adsorption State for Clean and Oxygen Precovered Ru(0001) Surfaces (from ref. [167]) SURFACE

6,,,

Ru(0001) p(2 x 2) 0.25 0-Ru(0001) p(2 x 2) 0.5 0-Ru(0001)

0.3 0.12 0.10

edirs

0.24 0.08 0

Me-CzH4 di-a/

TI .I

HREELS da t a for a clean and oxygen modified Ru(0001) surface show t h a t p(2 x 2) 0.250 and p(2 x 1) 0.5 0 overlayers favour n-bonded molecules where C atoms remain sp2 hybridized, and remove the di-a (C sp3 hybridized) bonded molecules observed on an oxygen-free Ru (0001) surface. This effect of the oxygen adatoms has been ascribed mainly to significant oxygen-induced perturbations of the electronic properties of the surface (increasing of the Lewis acidity of the Ru surface atoms). The contribution of the simple steric blocking of the available adsorption sites is supposed to be less important, because both di-u and n-bonded molecules require almost the same surface space. As a result of this oxygen induced electron deficiency on the surface, the adsorption of x-bonded molecules which act as electron donors become more favourable. The dissociation of molecules in a x-bonding configuration wlll be less easy because the negligible backdonation involved in the formation of the n-bond is not capable of substantially weakening the double C-C bond. Another proof of the domination of oxygen induced electronic perturbations over steric blocking is the much weaker effect, of the p(2 x 2) 0.25 C overlayer on the ethylene bonding configuration, i.e. no removal of the di-a ethylene bonding is observed in the presence of a carbon coverage of 0.25. Since C and 0 have very close covalent radii, the more severe effect of 0 can be satisfactorily explained with the significant difference in the Pauling electronegativity of C and 0: 2.5 vs.3.5 (implying larger electronic perturbations on the surface induced by the adsorbed oxygen). Apart from the change in molecular bonding configuration and the inhibition of ethylene decomposition on the surface, the presence of oxygen on Ru(0001) has been found to cause stabilization of the intermediate products on the surface during the decomposition process. Thus, on an oxygen-free surface the only dissociation product detected is ethylidyne (CCH3) which decomposes readily to C and H at T > 400 I<, no stable intermediates were detected in the vibrational spectra. In the presence of oxygen, the ethylidyne decomposition proceeds via the following reaction steps, the intermediate products being detected in I

Chapter 5

142 the HREELS spectra [lG7]: ethylidene

’2’vinylidene 4 2 K C + methylidene

> 550K

C+H

With the exception of small amounts of GO formed at temperatures above 500 K (as a result of the reaction C,+O, = CO) no other secondary oxidation products (COz or HzO) are detected in ethylene + oxygen coadsorbate layers

on Ru(0001). The lack of secondary oxidation products is also reported for oxygen modified Fe [168]. These results are not surprising since GO and hydrogen oxidation reactions do not take place on Ru and Fe surfaces under UHV conditions because of the high Me-0 bond strength. On the contrary, on oxygen-modified Pt, Pd and Ir surfaces, secondary reactions leading to the formation of COz and H 2 0 readily occur [165, 169172, 1751. Because of the activity of P t , Pd and Ir with respect to CO and hydrogen oxidation reactions, the effect of oxygen on the adsorptive capacity and the dissociation probability of CzH4 and other olefins is negligible at low and moderate oxygen coverages (i.e. the oxygen effect is restricted exclusively t o the removal of di-a in favour of r-bonded molecules). On Pd the presence of oxygen has been found even to lower the temperature for dissociation of the unsaturated hydrocarbons from 500 down to 420 K [170], due to the fact that 0 efficiently withdraws hydrogen and inhibits any possible rehydrogenation. At T 2 400 K, the rate of ethylene dissociative adsorption on an oxygen precovered Pd surface is very high ( the initial sticking coefficient is close to unity) and complete dissociation is favoured because the reaction of water formation is faster than the rehydrogenation reaction and C can be readily removed by complete combustion to COz. At lower reaction temperatures, as a result of the reduced rate of COz formation, the amount of C increases on the surface. This leads to a decrease of the reaction rate of water formation because of the H lateral mobility thereby being reduced (by trapping to C as described the Subsection 5.4.2.) and further inhibition of the ethylene dissociation by blocking the appropriate adsorption sites [169-171]. Thus, a conclusion can be made that for transition metals which are good catalysts for CO oxidation, the effect of the oxygen additive is restricted only to the inhibition of the di-a molecular bonding of et,hylene. However, because of the oxygen-induced electron deficiency on the affected substrate surface atoms, the n-bonding (where the ethylene molecule acts as an electron donor) might be stabilized by the presence of an electronegative additive. More peculiar is the case of ethylene - silver systems, where the extensive HREELS and IR studies have shown that the molecular adsorption of C2H4 and higher olefins occurs only via a P-bonding without rehybridization of the molecule, the adsorption energy of ethylene being of the order of 40 kJ/mole [176, 1771. That is why it is not surprising that the introduction of an electronegative additive, such as oxygen leads to an increase in the amount of n-bonded ethylene with increasing additive coverage [176]. The adsorptive behaviour of ethylene on Ag surfaces is most likely the reason that Ag is the best catalyst for selective oxidation of ethylene to ethylene oxide. Ethylene oxide is a basic chemical for many important syntheses, e.g. production of polyesters, antifreeze etc. [164, 1781. There are several different views about the type of

-

-

5.6. Organic Compounds

143

the oxygen adsorption state participating in the formation of CzH4O. Some of the authors support the mechanism whereby diat,omic oxygen species are active for epoxidation by the following reaction [164, 1791:

CzH4(a) + @ ( a )

-+

CzH40

+ O(a),

whereas the atomic oxygen state exclusively combust.s ethylene by the surface reaction: CzH4(a) 6 0 ( a ) 2C02 2 H z 0 .

+

-

+

Other authors are of the opinion t,Iiat the direct pwticipation of 0 (a) in the formation of C2H4O cannot be excluded [180, 1811. Recently, it has been proposed that depending on the orientatmionof the Ag surface crystallographic planes, the oxygen ada.toms can be i n different adsorption sta.tes, which exhibit different reaction activities [182]. Thus, for the corrugated Ag( 110) surfa.ce, the low coverage fourfold coordinat,ed oxygen state is expected to have a diu type bonding and is supposed t,o lead to ethylene combustion. At high oxygen coverages threefold a.nd/or t.wofold states a.re occupied by species of oxy-radical character, which are a.ssumed t,o be the a.ctive epoxidation species [182]. As outlined above, besides ethylene oxide formation, the second rea.ction path leads to products of total combustion (CO? and HZO), so that much effort has gone into finding appropriate additives for a.chieving masimuni selectivity. It has turned out that typical electronega.t.ive additives, such as C1, S, Se, Br, etc.as well as typical electropositive (I<, Cs) additives are known t o increase the selectivit,y and activit,y of the Ag catalysk for the epoxidation reaction [183, 1841. In order to explain the effect of the electronegative additives on the ethylene oxide forinat,ion, coadsorption experiments involving electronegative adatoms and the reagents ethylene and oxygen have been performed in model systems. Figs. 5.47 and 5.48. present thermal desorption spectra of ethylene from Ag(ll0) modified by increasing a.mount,s of C1 or 0. Obviously, the int,roduction of elect.ronega.tive additives up t.0 covemges of 0.5 leads t o an increa.se in the adsorbed a.mount of ethylene and the appearance of a second higher temperature desorption pea.k due tro stabilization of the ethylene molecular adsorption state on the modifier affected sites. The measured heats of ethylene adsorption on C1 and 0 affected sites are 62 kJ/mol and 53 kJ/mol, respectively, compared to 40 kJ/mol for a clean surface. Irrespective of the increased stability of t.he ethylene molecular state no ethylene decomposition has been observed. At very high additive coverages the reverse trend is observed, which indimtes t1ia.t the adsorption of ethylene can ta.ke place on the affected sites, but not. over t.he sit,es a1rea.dy occupied by the additives. The more pronounced sta.biliza.tion effect and the la.rger relative amount of affected molecules i n the ca.se of CI suggests the prevalence of the size contribution over the electronegativity factor in determining the strength and the extent of the modification effect,. As has been a.lready discussed, the C1- a.nd O-induced electron deficiency in the affected surface Ag a.toms fa.vours the C2H4 a-electron donation which contributes mainly to the strength of C Z H a-bonding ~ on the surface. However, the enhanced affinity and capacity of the surface to a-bonded ethylene

144

Chapter 5

0 ’

AV I

I30

I90

TEMPERATURE (K)

Fig. 5.47. CzH4 TPD spectra from A g ( l l 0 ) surfaces containing various C1 coverages. T, = 134 I<. (from ref. [lSS])

molecular adsorption in the presence of electronegative additives cannot explain the enhanced selectivity for ethylene oxide formation. A satisfactory explanation of the observed inhibition of the complete ethylene combustion (GO:! formation) is the effect of electronegative modifiers on the rate of dissociative oxygen adsorption. As has been already discussed in the previous Subsection, the presence of electronegat
5.6. Organic Compounds

145

Ethylene TPD spectra from A g ( l l 0 ) containing various coverages of atomically adsorbed oxygen (from ref.[186]) Fig. 5.48.

is possible by favouring new reaction pathways [25, 188-1901. Studies of the S and C effect on the reactivity of Mo(100) with respect to several unsaturated hydrocarbons have shown that, while molecules, such as butadienes and butenes, undergo almost complete decomposition in several steps to C(af and H z , both C and S adatoms inhibit the complete decomposition of the adsorbed molecules under consideration. This S- and Cinduced inhibition of the complete hydrocarbon decomposition is accompanied by favouring new reaction paths leading to the formation of hydrocarbons with a higher hydrogen content. As illustrated in figs. 549. and 5.50., for 1, 3butadiene adsorption a hydrogenation reaction leading to the formation of a mixture of 30 % to 70 % 1- and 2-butenes is favoured at sulphur coverages ranging from 0.2-0.4 and C coverages of 0.2-0.8. Above certain S ( w 0.5) and C (- 0.8) coverages, the decomposition process is almost completely eliminated and only molecular butadiene desorption is observed. A similar effect of S and C on the decomposition reactivity of the surface is observed in the case of butenes. However, since even a clean Mo (100) surface is active towards l-butene hydrogenation, the S- or C-induced slight increase in hydrogenation activity at low additlive coverages (2 0.2) is followed by fast

Chapter 5

146

I

1,3-Butadlone/S/Mo( 100)

Sulfur C0v.r.g. (a)

1

8-

Fig. 5.49. 1,d-butadiene, 1,a-butene and Hz production as a function of S coverage (in ML) on Mo(100), as evaluated from the mass 54, mass 56 and mass 2 TPD peak

areas (from ref. [188])

Fig. 5.50. 1,d-butadiene, 1,Z-butene and Hz production as a function of C coverage (in ML) on Mo(100), as evaluated from the mass 54, mass 56 and mass 2 TPD peak areas (from ref. [188])

5.6. Organic Compounds

147

-

reduction of the butane production at higher S or C coverages until complete inhibition a t S (C) coverages 0.4 [la81 has been reached. The TPD data of the molecular adsorption states of butadiene and butene indicate that in the case of S, no new molecular states are created (both from a. clean and from a S modified Mo( 100) surface the molecular desorption peak is at 150 K , i.e. the desorption energy is of the order of 30-40 kJ/inol). In the cases of a stable molecular adsorption state on a clean surface, e.g. benzene on P t ( l l 1 ) [25], the increasing amounts of S lead to continuous removal of the most tightly bound states accompanied by a strong reduction of the amount of benzene desorbing from the first overlayer. In the case of carbided Mo( loo), new higher temperature molecular desorption peaks associated with hydrocarbon adsorption on C illflueliced sites are observed with a desorption energy by 10-30 kJ/mol larger than the clesorptioii energy measured for a clean surface [188]. This stabilized molecular state is supposed to be bound to the nearest on top Mo sites which become more efficient electron acceptors in the presence of a C adatom residing i n a fourfold site. The absence of a similar stabilization effect in the case of S is attributed to the larger size of S which sterically hinders the adsorption 011 the most strongly perturbed site that is the nearest on top one. The enhancement of the molecular desorptioii fraction at the expense of the dissociated one and the promotion of the hydrogenation reaction within a cert,ain S- and C-coverage range indicates that the major effect of S and C is in blocking the favourable adsorption sites for the decomposition products H and C. Obviously, because of its larger size S blocks the sites for decomposition more efficiently and behaves like a more drastic poison than C. Generally, the same effect on the surface activity (blocking of the available adsorption sites and inhibition of the tota.1 dehydrogena.tion on the surface in the presence of S and C additives) is observed for adsorption systems involving saturated hydrocarbons [188, 1901.

-

-

5.6.2

Effect of S and C on the Interaction of Thiophene with Transition Metal Surfaces

The great interest in understanding t,he adsorption and decotiiposit,ion hehaviour of thiophene is directly related to the necessity to reinove the undesired sulphur compounds from the crude oil (generally present as part of a thiophene ring) T h e reason is that. these compounds iiitroduce S onto the catalyst surface which severely poisons catalytic reforming and other iinport,a,nt ca.talytic processes. Molybdenum, tungsten and ruthenium sulphide catalyst are found to be the most effective for the removal of S containing organic compounds by a hydro-desulphurization process. C2H4S adsorption and deconipositioii has been studied on several tra.nsition metal single crystal planes which exhibit different kinds of behaviour with respect to the thiophene molecular bonding configuration and varying efficiencies for decomposition of the adsorbed molecule. When the thiophene adsorption is performed at low temperatures, a molecular state is detected by means of vibrational spectroscopies. At low thiophene coverages on Mo( 100) and Mo(ll0) [188, 192,1931, Ru(0001) [194, 1951, Cu(100) [196], Ni [197,198],

Chapter 5

148

and Pt [199,200] single crystal surfaces, the thiophene ring is oriented parallel to the surface plane, bound through the aromatic ring via a ir-bonding. In this bonding configuration no selectivity in the C-H bonds breaking is observed. At high thiophene coverages in most cases a change in the bonding geometry takes place. The molecule stands up and a a-bonding via the lone electron pair on S is realized. In this bonding configuration, a selectivity in the C-H bond breakage is observed, the a-CH bond scission being more facile. The temperatures at which C-S, C-C and C-H bonds break, vary with the different substrate surfaces and the actual thiophene coverage. Cu(100) [196] is found to be completely inactive for thiophene decomposition under UHV conditions. On P t ( l l l ) , C-S bond breakage occurs at T > 290 K. The desulphurisation of thiophene, tetrahydrothiophene and 2,5-dimethylthiophene leads to H and a variety of hydrocarbon products preceded by metallacycle-like intermediates [199, 2001. On Ni(100) C-S scission takes place at 90 K, forming the C ~ H B metallacycle species which undergoes further dehydrogenation at T > 500 K [197]. On Ru(0001), C-S breakage occurs at 120 K, the formed metallocyclelike intermediates being further dehydrogenated stepwise with Hz desorption at 230, 305 and 450 K [194]. While from all systems described above, a certain fraction of the adsorbed thiophene desorbs molecularly, negligible molecular desorption occurs up t o monolayer thiophene coverages on Mo because Mo is one of the most effective substrates with respect to complete thiophene decomposition. The C-S cleavage is favoured at temperatures 100 K , whereas the break of the C-H bond and the complete decomposition to C, S, and H occurs:

-

(i) at low thiophene coverages (for the parallel ir-bonding configuration) at T < 500 K and (ii) at high thiophene coverages at T

-

650 K.

This coverage dependence on the thiophene decomposition temperature is due t o the change in the bonding configuration which introduces a high energy pathway for P-CH cleavage H2 is the only desorption gas product as a result of the complete thiophene decomposition on Mo. The modification of the transition metal surface with S adatoms causes the following identical changes in the surface activity with respect to thiophene adsorption and decomposition [lSS, 193-195, 198, 200, 2011:

(i) a decrease of the thiophene sticking probability and the saturation coverage;

(ii) an inhibition of the total decomposition on the surface; (iii) an enhanced selectivity towards hydrocarbon formation after desulphurization at low up to moderate (0-0.25) sulphur coverages a t the expense of the reduced fraction of totally decomposed molecules, and (iv) a complete passivation of the surface for decomposition above critical S cover ages.

5.6. Organic Compounds

149

The main reason for the deactivation effects of the preadsorbed S is that sulphur blocks the multiatom ensembles and this prevents decomposition. The fact that thiophene hydrodesulphurisation can be observed up to rather high sulphur coverages, i.e. the amount of the decomposed fraction is decreasing almost linearly with increasing sulphur coverage [201], can be explained by the tendency of S to form islands of ordered structures leaving ensembles of bare Mo atoms. The enhanced selectivity towards hydrocarbon formation a t low and moderate S coverages supposes that the preferential orientation of thiophene in the presence of S should be a perpendicular one. After C-S scission, the hydrocarbon products desorb rather than decompose because, as described in the previous Section, S inhibits hydrocarbon dissociation as well.

SULFUR COVERAGE (ML)

Fig. 5.51. Effect of increasing S coverages on Mo(100) on the amount of desorbed HP and S residue (resulting from thiophene decomposition) (from ref. [20l])

The studied effect of adsorbed sulphur on the activity of Mo single crystal surfaces is a good example of the differences in activities of surfaces with adsorbed additive overlayers and the corresponding metal-additive compounds. It is well known that the layer compound MoSz is an excellent catalyst for the hydrodesulphurization reaction. The established deactivation effect of S on the decomposition of thiophene and related hydrocarbons on transition metal surfaces supports the mechanism according to which the active sites on MoS2 catalysts are the edge sites (anion vacancies) where Mo ions are present in

Chapter 5

150

a number of different oxidation states. These sites are likely to favour the formation of an electron donor s-bonding via the lone pair located at the S atom.

I

Thlophom/C/Yo( 100)

Fig. 5.52. Hz and C2H4S TPD spectra after thiophene adsorption on clean and carbon covered Mo(100) surfaces. (from ref. [188])

Similar but less severe is the effect of C adatoms on thiophene adsorption and decomposition [188, 1951. The only essential difference is that, in contrast to S, the presence of C on the surface creates a new higher temperature desorption state in the thiophene molecular desorption spectra [188, 1951. Fig. 5.52. presents HS and CzH4S T P D spectra from clean and C covered Mo(100) surfaces. It is obvious that C induced reduction of the amount of decomposed thiophene is compensated (to a certain extent) by an increase in the fraction of non dissociated thiophene and stabilization of the molecular adsorption state. That a similar stabilization of the hydrocarbon molecular state by C is not observed in the case of S, has already been discussed in the previous Subsection.

5.6.3

Interaction of Alcohols, Aldehydes, Carbonyl Compounds etc. with Modified Metal Suri’aces

Synthesis, decomposition and oxidation of alcohols are catalytic processes of great industrial importance. Since adsorption and decomposition of alcohols on catalyst surfaces is an important step in the aldehyde and carbonyl compound synthesis, a significant number of studies have been concerned with

5.6. Organic Compounds

151

the description of the influence of electronegative modifiers on the interaction of alcohols, aldehydes and carbonyl compounds with single crystal transition metal surfaces. The interaction of alcohols with transition metal surfaces can proceed via several reaction channels. As an example, consider the most extensively studied methanol (CH30H) adsorption and decomposition process. The following four major reaction channels of methanol interaction with metal surfaces are possible:

-

(1) Dissociative adsorption even a t temperatures as low as 100 K leading to the formation of methoxy (CH30,) and hydrogen (H,) species. This first step is observed on many clean surfaces, e.g. Fe(ll0) and (100) [204, 2051, Mo(100) [206], N i ( l l 0 ) [207], Pd(100y [208]', W(l00) [209]', Ru(0001) [210], etc;

(2) Decomposition of the methoxy intermediate to CO and H 2 , which takes place on Fe(100) [211], Mo(100) [206], Ni(ll0) [207], Pd(100) [208], and Ru(0001) [210] at temperatures varying with the different substrates (ranging from 200 to 400 I<); (3) Formation of aldehyde (CHzO) by dehydrogenation of the methoxy intermediate, which is exclusively favoured in the presence of electronegative additives ( 0 ,S, C) [66, 207,209, 211-2211; (4) Recombination of adsorbed methoxy species and H thereby forming CH30H. The effect of S on the adsorption and decomposition of alcohols leads to quite different kinds of behaviour than that observed on a clean surface. In some cases the influence of S is not, simply restricted to the reduction of the adsorption/dissociation rate and the surface adsorptive capacity but also leads to a significant alteration of the decomposition reaction path [66, 2121. Fig. 5.53. presents the sulphur induced changes in the distribution of CO and CHzO products from methanol adsorption layers on a Ni(100) surface [212]. For a S-free surface at T > 300 K CH30H undergoes complete decomposition to H and CO, whereas the molecular desorption takes place at T 200 K. The presence of S leads to the following effects:

-

(i) a decrease of the amount of molecularly adsorbed CHSOH, attributed to the usual induced reduction in the surface adsorptive capacity; (ii) a rapid decrease of the extent of H 2 and CO desorbing from the surface due t o inhibition which prevents complete methanol decomposition, and (iii) a change in the selectivity towards the production of CH20, which is favoured a t 0.2 < 6.5 < 0.5.

As can be seen in fig. 5.53., the decomposition is completely hindered on

-

p(2 x 2) 0.25 S--Ni(100), whereas the maximum in the formaldehyde amount occurs at 6s 0.3. Comparing the T P D spectra of the decomposition products arising from a clean and from a sulphided Ni(100) surface, leads to the conclusion that probably the selectivity changes induced by S are due to

152

Chapter 5

Fig. 5.53. Effect of preadsorbed S on the amount of HzCO and GO formed as a result of C H 3 0 H interaction with Ni(lO0) (from ref. [212])

(i) the increased stability of the intermediate methoxy-species because of the increase in the activation energy for dehydrogenation; (ii) the suppression of the C H 2 0 adsorption bonding configuration favouring further dehydrogenation, and (iii) the effective blocking of the hydrogen adsorption sites. The maximum of the C H z O production should be associated with competition between two S effects: stabilisation of the C H 3 0 , intermediate and blocking of the surface sites allowing C H 3 0 H dissociation to C H 3 0 and H . Thus, the surface becomes completely deactivated as the S coverage reaches 0.5 when no dissociation of C H 3 0 H is detected. An accurate study of the effect of S on the decomposition kinetics of methanol has been recently reported for S-covered Ru(0001) [220]. It has been found that the initial decomposition activation energy increases almost linearly with increasing S coverage from 33 kJ/mol for a S-free surface to 42 kJ/mol for a surface covered with f?s = 0.07. The increase of the activation energy is compensated for by an increase of the pre-exponential factor from 4.2 x 10' for a clean surface to 3.3 x lo7 for 0.07 S/Ru(OOOl). These changes in the kinetic parameters for methanol decomposition together with the dramatic reduction in the methanol initial decomposition rate indicate that the effect of the sulphur additive cannot only be explained by a simple site blocking mechanism. Obviously, perturbations of the surface electronic structure beyond the nearest neighbours should be also considered.

5.6. Organic Compounds

153

The effect of S on the formaldehyde interactions with P t ( l l 1 ) is simiIar [66, 1371. On a clean Pt(ll1) surface, CH2O decomposition causes mainly H2 and CO to be produced and very small amounts of C02, CH3OH and HCOOCH3. The presence of S reduces the amounts both of molecularly adsorbed and dissociated CH2O and changes the decomposition reaction path towards hydrocarbon formation (CH4) at the expense of HCOOCH3 and C02 formation. Carbon overlayers of C in the carbidic adsorption state affect the adsorptive capacity in the same way as S does. It also affects the activity of the surface with respect to the decomposition of alcohols, RlR2CO compounds and formates and in addition it alters the product distribution [66, 2261. However, as has been observed with the other hydrocarbons, the poisoning effect of C is weaker than that of S. A more severe deactivation effect is exhibited by graphitic overlayers, which are completely inert with respect to the decomposition of oxygen-containing hydrocarbons. S and C act exclusively as selective poisons, while the oxygen adatoin serves several different functions, i.e, oxygen can act as a reactant and/or poison. These actions involve the following steps: (i) hydrogen abstraction from the hydroxyle group of the alcohol and formation of adsorbed alkoxides;

(ii) stabilization of the adsorption state of the alkoxides (RCH20,) and prevention of their complete decomposition; (iii) alteration of the preferred adsorption geometry and the thermal stability of the carbonyl compounds obtained a result of partial dehydrogenation of the alkoxide species, and (iv) formation of carboxylate as a result of a nucleophilic attack of the adsorbed oxygen. The last step is not favoured on all 0-modified metals because it depends on the actual oxygen adsorption state, Figs. 5.54. and 5.55. illustrate the oxygen-induced inhibition of complete methanol decomposition at the expense of opening up a new reaction path for partial dehydrogenation resulting in the format,ion of formaldehyde. It is obvious that the maximum activity towards the formation of formaldehyde is achieved at oxygen coverages 0.2 -0 25. At high oxygen coverages (> 0.4), CH30H decomposition is completely inhibited and only molecular CH30H desorption is observed. This complete passivation of the oxygen precovered Fe(100) surface with respect to methanol dissociation is confirmed by the HREELS data which suggest that the molecular methanol is weakly bound via hydroxy hydrogen and surface oxygen [all]. In the case of 0-covered Ru(0001) [213], the presence of oxygen (up to 00 0.25) exhibits the following effects:

-

-

(i) promotion of the formation of methoxy species even at high methanol coverages where, on a clean surface, the methoxy-formation is favoured only at low methanol coverages;

Chapter 5

154

0.44

+-O.X!

0.20

f

II

I

Fig. 5.54. Effect of increasing oxygen coverages on Fe(100) on the H2 and GO TPD spectra. The oxygen induced changes in the CO and Hz TPD area reflect the reduction of the fraction of totally decomposed CH30H. (from ref. [211])

(ii) quenching of the second pathway of methoxy decomposition which leads to the formation of HzO, C and H on a clean surface. The facilitation of methoxy formation can be related to the hydrogen abstraction ability of the adsorbed oxygen. The inhibition of C-0 bond breaking, which occurs on a clean surface, can be related to the poisoning effect of oxygen with respect to the appropriate bonding configuration required for C-0 cleavage (as described below). Similar to the case of 0-covered Fe(lOO), high oxygen coverages on Ru(0001) act as poison with respect to methoxy formation because oxygen blocks the adsorption sites which would otherwise readily adsorb methoxy. Since the selective poisoning effect is very important with respect to selective oxidation of alcohols to desired products, it is worth discussing briefly the mechanism of the electronegative modifier action on the stability of the carbonyl compounds which is observed with all the Group VIII metals. The carbonyl compounds of the type RIRZCO, where R1 and RZ are H or hydrocarbon radicals, exhibit two different coordination geometries in the adsorption state associated with two types of bonding: (0)and ?72(c,o) [223-2251. The q l ( 0 ) bonding results from a donor component involving overlapping of the non-bonding oxygen lone pair orbital with a ds hybrid orbital of the metal surface, where the metal acts as an acceptor. The small metal to anti bonding s*-CO backdonation contributes negligibly to the q l ( 0 ) bonding. The role of the metal in this type of interaction is that of a weak Lewis acid and the

5.7. Conclusive Remarks

1

155

0

+l_jbb-%5

Fig. 5.55. Effect of increasing oxygen coverages on the HzO, HzCO and C H 3 0 H

TPD spectra from oxygen modified Fe(100). The appearance and increase of the intensity of the HzCO TPD spectra at 0.1 < 80 < 0.35 indicate the selective poisoning effect of oxygen (from ref. [211])

resulting q l ( 0 ) bond is rather weak (- 40 kJ/mol). The qz(C,O) bonding configuration results from overlapping of the a-CO bonding orbital with a ds hybrid acceptor metal orbital and an overlap of a metal ds-orbital with a 7r*CO orbital. Since the backdonation from the metal is the major contribution to the qz(C,O) bonding, it causes a decrease in the C-0 bond order. qz(C,O) species are more strongly bound than ql(C) species and exhibit a higher tendency t o further decomposition than the q I ( C ) bound ones. The formation and the preference of the qz(C ,0) bonding configuration depends crucially on the backdonation ability of the surface. That is why the introduction of electronegative additives, which enhances the Lewis acidity of the surface, will suppress the q2(C,O) bonding configuration in favour of the q,(C) one. Since q1 (C) bound carbonyl compounds exhibit a high preference for molecular desorption, the observed inhibition of the total alcohol decomposition in the presence of electronegative additives is explained satisfactorily. 5.7

CONCLUSIVE REMARKS: THE POISONING EFFECT DESCRIBED WITHIN THE FRAMEWORK OF THE POSSIBLE INTERACTIONS IN THE COADSORBED LAYER

Because of the complexity arising from the specificity of the modified systems and the variety of possible interactions in the coadsorbed layer, there is still no unified model for the mechanism of the poison action even for the idealized model systems described in the previous sections. Up to date the

156

Chapter 5

scientific efforts have succeeded in clarifying that the following main factors might contribute t o the poisoning effect, the weight of every one varying with the different modifiers: 1. Steric blocking. This is due to direct repulsive interactions between the modifier and the coadsorbed molecules which excludes the adsorption sites occupied by the modifier and hinders coadsorption over a certain area around the modifier. These interactions can involve direct repulsion between some spatially extended and/or energetically suited coadsorbate and modifier electron orbitals. The steric blocking is a short-range effect, and its effective radii depend exclusively on the modifier’s size and to a lesser extent, on the modifier’s Pauling electronegativity and adsorption site .

2. Substrate mediated electronic effects. These effects are associated with the changes in the surface local density of states near the Fermi level as a result of the formation of the modifier adsorption bond. As will be discussed in more detail in Section 7.2., the presence of electronegative adatoms perturbs substantially the substrate electronic states which can concern the formation of an adsorption bond with coadsorbates. Because of the significant screening effects in the metal substrates, the effective radii of the electronic effect usually do not exceed 5 A.

3. Electrostatic interactions between the modifier and the coadsorbate. The contribution of these interactions depends on the actual effective charge of the interacting species and the corresponding dipole lengths. The range of such interactions does not exceed the screening length of the metal (- 4 A). In the case of most of the electronegative adatoms with prevailing covalent type of bonding and small dipole lengths, this kind of interaction does not contribute substantially to the poisoning effect.

Obviously, all factors summarized above, account for a rather localized modifier action. Considering the complexity due to the different kinds of interactions an attempted explanation will now be given as to the experimentally observed ‘poisoning’ effects on the adsorption properties of some single crystal planes modified by ordered adatom overlayers. Fig. 5.5G. presents schematics of fcc(100) and fcc(ll1) types of surfaces with the modifier adatom residing in the highest coordinated site. The substrate atoms directly bound to the modifier are assigned as nearest neighbours, and the substrate atoms next to the directly bound ones as next-nearest neighbours. Obviously, in the case of the fcc( 111) plane one should distinguish between the three close and six remote next-nearest neighbours. The separation of the various adsorption sites from the modifier site is determined by the lattice constant of the substrate. On the basis of the experimental data for larger electronegative adatoms, such as S, Se, Te and C1, the following may be considered:

(i) the adsorption sites which involve only substrate atoms directly coordinated t o the modifier as blocked and

5.7. Conclusive Remarks

157

0 QIIP

@ 0

Fig. 5.56. Schematics of the possible number of surface atoms which might be affected by the presence of an additive adatom located in the highest coordination site on fcc(ll1) and fcc(100) surfaces.

(ii) the adsorption sites sharing some substrate atoms with the modifier as subst an tially perturbed. This means that for the fcc (100) plane one modifier adatom blocks four on top, four bridge and one fourfold site and perturbs eight bridge, four close and four remote fourfold sites. For the fcc(ll1) plane, the modifier blocks three on top, three bridge and one threefold site and perturbs six close bridge, six remote bridge, three close and nine remote threefold sites. With increasing modifier coverage, because of overlap of the effect, the number of affected sites per modifier adatom decreases. Following this simple model, it is obvious that the coadsorbate's most tightly bound molecular states, associated with unaffected surface sites, should be rapidly removed with increasing modifier coverage. Indeed, as reported in refs. [28, 291 , at low S coverages, one S adatom was found t o eliminate 6 to 9 most tightly bound &CO bridge states on a Ni(ll1) surface, which is close to the sum of the blocked bridge and perturbed six close-bridge sites. In the case of Ni( 100) where CO favours on top sites, one S adatom is found to remove four on top &CO states, i.e. those blocked by the additive (see fig. 5.6.). In addition, as evidenced by HREELS and TPD data [41], CO is pushed to occupy the close-bridge and close fourfold sites, where the coupling with the substrate is reduced. The above simplified picture can describe satisfactorily the modifier effect on the adsorption rate of the coadsorbate and on the adsorptive capacity of the surface. Let molecular adsorption first be considered. It obeys a trapping-

158

Chapter 5

dominated precursor mechanism (see eq.(3) describing the sticking coefficient of non activated molecular adsorption). In the case of CO and NO, the initial sticking coefficient for molecular adsorption on most transition metals is close t o unity and is almost constant up to moderate coverages. This indicates that the elastic and nonelastic back scattering are negligible and all molecules hitting the surface are trapped and chemisorbed. On assuming invariance of the rates of adsorption and desorption of the precursor upon introducing the modifier, the observed strong decrease of So(P2) for the most strongly bound Pz-CO state on Ni(100) (fig. 5.12.) with increasing modifier coverage, Ox, might be described by introducing a factor (1 - aOx), i.e. SX = So(@z).(l fytjx). The coefficient accounts for the number of sites, where the lifetime of the precursor at room temperature is drastically reduced. Thus, assuming that the number of these sites is equal to the coordination number of the modifier fourfold adsorption site, CY can be substituted: a = 4. This means that, at p(2 x 2) adatom overlayers (Ox = 0.25), the Pz-CO adsorption state is completely eliminated. The experimental results in fig. 5.12. show that the effects of S and C1 are even stronger, i.e. a is larger than 4. This implies that the modifier perturbations are extended beyond the nearest neighbours, affecting also the lifetime of the precursor state on these sites. The situation with modified fcc( 111) surfaces (modifier residing in a threefold adsorption site) is similar, there the reduction of the local sticking coefficient for the most tightly bound &CO state in the case of S is stronger than that predicted by a = 3 [28, 291. In general, for most of the electronegative adatoms under consideration, which tend to form ordered overlayers, occupying the highest coordinated surface sites, the blocking efficiency with respect to the originally most tightly bound molecular state of NO and CO is ranging from 3 to 6 per modifier adatom (depending on the substrate and the type of the modifier). The effect of the electronegative modifiers on the total sticking coefficient, SX, and on the total adsorption capacity observed at low adsorption temperatures is much weaker. In most cases SX preserves the clean surface value a t low modifier coverages and is less affected by the presence of a modifier than predicted by a equal to the adatom coordination number. Our studies on CO and NO adsorption on sulphided and selenided P t ( l l 1 ) [26, 35, 821 have shown that, at an adsorption temperature of 90 I< for p(2 x 2) 0.25 S(Se) overlayers SX =- 0.5s0, while according to the factor (1 - 3Ox) it should be 0.25 So. This can be ascribed to: (i) new access enabling the occupation of energetically less favourable (affected) adsorption states, and/or (ii) a certain residence lifetime of the mobile precursor even on surface atoms directly coordinated by the modifier at sufficiently low temperatures. However, the elimination of the most-strongly-bound molecular state is independent of the adsorption temperatures and its removal is usually completed at BX p(2 x 2) 0.25 X on f cc (ll l), hpc(0001), fcc(lOO), fcc(llO), bcc(llO), etc. single crystal planes. The reduction of the unaffected adsorption sites with increasing modifier coverage forces the trapped precursor to pass into the less favoured adsorption sites which are influenced to a different extent by the modifier until the surface is eompletely deactivated at certain critical modifier coverages. For the same coadsorbate, the critical modifier coverages vary with the type

5.7. Conclusive Remarks

159

of modifier, the crystallographic plane and the adsorption temperature. Now consider the effect of the electronegative modifiers on dissociative adsorption, where the molecular adsorption state plays the role of a precursor. As outlined above, the bonding strength of the molecular state and its concentration on the surface are reduced. Obviously, when the molecular desorption energy (which reflects the strength of the molecular adsorption bond) falls far enough below the activation energy of dissociation, the dissociative adsorption will be hindered. In addition, it is quite possible that the presence of electronegative adatoms can also cause an increase of the activation energy for dissociation and will affect the binding energy of the products. Besides these energetic factors, a third steric blocking effect also contributes to the inhibition of the dissociative process because the dissociation products compete with the modifier atoms for the same adsorption sites and two adjacent favourable sites are required by the dissociation products. That is why the reduction of the dissociative adsorption rate induced by the electronegative additives is very severe. For example, the (1 - a&) factor for dissociative Hz adsorption on sulphided Ni(100) [13, 141 and Pt(ll1) [35] is estimated to be of the order of (1 - (8 f l)O,) i.e. within the limits of low sulphur coverages (see fig. 5.42.). A similar reduction factor is also found for 0 2 adsorption on P t ( l l 0 ) [21]. It is worth discussing the possible reasons for the observed differences in the poisoning strength with various elect#ronegativeaddit.ives. For this purpose we several simpler cases are selected:

(1) Additive adatoms with close sizes (covalent radii), localized in adsorption sites with the same symmetry. For such systems it is likely that the adatoms will exhibit approximately the same steric blocking effect (covering exclusively the nearest neighbours). Consequently, the observed variations in the strength of the poisoning effect should be ascribed to differences in the modifier induced perturbations on the next-nearest neighbours. As an example of this case,t,lie deactivation effects of C1, S and P which have similar covalent radii (0.99, 1.02, 1.04 A) but different Pauling electronegativities (3.0, 2.5 and 2.1) can be compared. The increasing strength of the poisoning effect with respect to adsorption of acceptor-type reagents in the sequence P, S, C1, can be satisfactorily explained by the increased ionicity of the substrate - modifier bonding, causing an enhanced net charge movement in the 2 direction. This extends the range of the perturbations due to both the enhanced substrate mediated electronic effects and the increased direct electrostatic interactions, as will be considered in more detail in Chapter 7.2. (2) Additive adatoms with close electronegativities (implying negligible differences in the nature of bonding) but substantially different sizes, occupying identical adsorption sites (e.g. C and S, S and Se). In this case the observed differences in the strength of the poisoning effects can be attributed to the fact that the smaller adatoms are located deeper (and even embedded) in the substrate surface and can even interact with the subsurface substrate atoms. This leads to weakening of both the steric blocking effect (because the effective radii of strong repulsion between

160

Chapter 5 the modifier and the coadsorbate decreases with the modifier’size) and the electronic perturbation effects on the next-nearest neighbours. Thus, in the case of sufficiently small, additive adatoms (e.g. C, 0) which are imbedded in the surface layer, even the substrate atoms directly bound to the modifier can be considered as possible adsorption sites.

The picture becomes more complicated when one should compare the poisoning effect of additive adatoms possessing substantially different electronegativities and sizes. On such an example, the effects induced by 0 and S, where the electronegativity factor (3.5 and 2.5) and the size factor ( r c : 0.77 and 1.02 A) act in opposite directions. The available experimental data have indicated that the poisoning effect on the adsorption process is always weaker in the case of 0 (e.g. CO adsorption on S/Ni(111) [28] and O / N i ( l l l ) [31], NO adsorption on S(Se)/ P t ( l l 1 ) [82] and O / P t ( l l l ) [82], etc.). This supposes that the strength of the modifier induced perturbations and the range of the effect on the adsorptive properties of the surface are determined to a larger extent by the size rather than the electronegativity of the modifier. Most of the current experimental data presented in this Chapter support the above considerations which account for a rather localized action of the modifier, usually not extending beyond the next-nearest neighbours. Indeed, let it be assumed that the introduction of a foreign adatom changes the distribution of the electrons of the whole surface. This should lead to continuous variations of the coadsorbate adsorption energies, stretching frequencies, etc. with increasing modifier concentration and immediate removal of the unaffected adsorption sites, which contradicts the experimental data. However, some authors ascribe the certain influence on the shape and position of the coadsorbate T P D peaks and the stretching frequencies of the coadsorbates residing far beyond the affected sites (low modifier coverages), to possible long range effects. In the author’s opinion, no unambiguous conclusion about the long range effects can be made because, in the case of non uniform distribution of the additive adatoms a t low coverages, tending to form islands of ordered structures, the following factors might influence the position of the T P D peaks and the stretching frequencies of the coadsorbates residing beyond the affected area:

(i) overlap of the desorption peaks of the coadsorbate desorbing from un-

affected a r e a and areas located close to the boundaries of the modifier ordered islands. This explains satisfactorily the observed small shift to lower temperatures and the increase of the CO T P D halfwidths for low CO coverages at 6 s =- 0.1, illustrated in fig. 5.10.;

(ii) at the same coadsorbate coverage there is a more pronounced compression of the coadsorbate overlayer on the modified surface (compared to that on a clean surface) because of the modifier induced reduction in the unaffected surface area. It is well known that both the positions of the T P D maxima and the stretching frequencies are very sensitive to the actual density of the adsorbate overlayer. Thus, the observed small decrease of the temperature maximum ( 5 25 K) and the increase of the stretching frequencies ( 5 15 cm-’) associated with the unaffected

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161

adsorption states of the coadsorbates agree well with the above consider a t ions. Up t o this point the description of the effects of the modifier has been restricted t o systems where the additive adatoms tend to form ordered overlayers. The experimental data have shown that the picture becomes more complicated in cases of:

(i) modifier-induced surface reconstruction; (ii) surface compound formation, and (iii) lack of a tendency to ordering and occupation of definite adsorption sites. Thus, as has been described in Chapter 4.2., rather often, 0, N or C adsorption on some transition metal surfaces can cause a surface reconstruction or formation of surface or bulk compounds (oxides, nitrides or carbides). When true transition metal carbides, oxides or nitrides are formed, then the activity of the surface is determined by both the changes due to the new type of modifier substrate interactions in the compound (which alters completely the electronic structure) and the substantial structural changes. This implies a completely new type of adsorption sites and substantially altered adsorptive properties of the surface. It is obvious that i n such cases the behaviour of the surface changes drastically. It will acquire the properties of the new phase, which does not necessarily mean that the poisoning effect will he more severe than that observed when the modifier is in an adsorption state. The lack of a surface order and the tendency to island formation usually lead to a substantial reduction in the relative number of the perturbed surface sites, because they are restricted to the neigbours of the modifier island boundaries. This is the reason for the substantially weaker poisoning effect of P and C forming phosphide or graphite islands [13, 651. It is worth mentioning that in the case of C one should distinguish between the two possible surface states of C. The first state is ‘carbidic’ carbon where C is in a typical adsorption state forming ordered overlayers and residing in the highest coordinated sites. In this state, C is strongly coupled with the substrate atoms and acts in the way described above for ordered electronegative adatoms. In the second ‘graphitic’ surface state the coupling between the C adatoms and the surface is reduced a t the expense of rather strong attractive C-C interactions being allowed to arise; the latter would lead to the growth of a graphitic phase. That is why, when C tends to form islands of a graphite phase, they coexist on the surface with fractions of a free surface until the first graphite overlayer has completely formed and the poisoning effect is thus much weaker.

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