Chemisorption and catalytic oxidation of CO on palladium

Surface Science 64 (1977) 681-697 @ Norm-Holland ~bii~~g Company

~HEMISO~ION

AND CATALYTIC OXIDATION OF CO ON PALLADIUM

C .D. HALSEY Department of Chemistry, Universityof Washington,Seattle, Washington98195, USA Received 6 August 1976;manuscript

received in tinal form 30 December 1976

The adsorption of CO on clean, low index crystai faces of palladium is analyzed in terms of the fixed site and mobile models. Isotherms give evidence of adsorbed molecule interaction or heterogeneity, although in the case of Pd(l11) the Clausius-Clapeyron heats do not, and partial mobility is indicated. The oxidation mechanism in the mixed-Langmuir formulation is analyzed in terms of the absolute rate theory, and in terms of the isolated domain or islartd model for mixed adsorption. The data, which are quaI~~tively similar on ah crystal faces can be adequately accounted for on the basis of a non~qu~ibrium mixed-Langmuir adsorption ,on a surface with long-range heterogeneity.

1. Introduction

The catalytic oxidation of carbon monoxide on palladium surfaces has been extensively studied; Close and White [I] present an interesting study of the reaction on polycrystalline foil that covers a wide range of temperature and pressures; references to earlier work will be found there. In particular, however, Ertl and his collaborators [2,3] have provided an elegant and extensive experimental study on clean, single crystal, faces of pa~adium. Techniques have included LEED, contact potential, reversible isotherm measurements, and low-pressure kinetic measurements of several kinds. We will first present an analysis of the adsorption measurements for CO and then consider the mixed adsorption and oxidation process.

2. Adsorption of carbon monoxide 2.1. CO isothemz measurements Isotherms are obtained by a method developed by Tracy and Palmberg [4,5] and are based on pressure measurements and on the change of contact potential, after the latter has been shown to be effectively linear in the fractional coverage 8. Tracy and Palmberg present isosteric heat data for the (100) face of palladium, and Ertl 681

682

G.D. Halsey / Chetnisorprionand catalyticoxidation of CO on Pd

and his collaborators give similar results on other crystal faces. In each case the isosteric heats are the results of the application of the Clausius-Clapeyron equation to the isotherm data. The heats generally decline with coverage. The data of Ertl and Koch ]6,7] for the Pd(ll1) face are distinct in that the Clausius-Clapeyron plots give a constant heat of adsorption, down to lowest coverage, of 34 kcal per mole. This result implies that the adsorption is taking place on a uniform surface, either by condensation into an island, or by random (Langmuir) adsorption. The isotherms which cover a range of temperature from about 300 to 500 K show no evidence of steps, which would be present in the event of island formation of a uniform surface. In general, the CO isotherms on all faces studied extend over a wide range of pressure amounting to some four cycles of ten in PC* from above IO-’ to lo-’ Torr. 2.2. Dissociative adsorption These isotherms are quite similar in appearance to the hydrogen isotherms on Ni(ll1) from the same laboratory [8], as contrasted to the remarkable step-wise isotherms given in the same paper for the case of hydrogen on Ni(l10). Two selected sets of Ertl and Koch’s data on Pd(l11) derived from their published graphs [6,7], are analyzed according to the Langmuir equation, and the dissociative Langmuir equation, in table I. One is tempted to conclude the adsorption state of CO is dissociative, as it may be on nickel surfaces [9]. However, the chemistry of the system shows that this hypothesis is untenable. Experiments ]3,7] have shown that CO reacts readily at the temperature of the isotherm measurements with preadsorbed oxygen atoms. Thus, the surface should become rapidly carbided (as does Ni[9]) as the CO reacts with itself. The very existence of these equilib~um isotherms is thus incompatible with dissociative adsorption. 2.3. The pressure ratio, r As a convenient rough measure of isotherm shape, we define a ratio of pressures at two coverages, which are chosen as B values of l/4 and 314 of the maximum value. For the Langmuir equation (non-dissociative case) this ratio, r = P(3/4)~P(l/4),

(If

is equal to 9, and for the dissociative case it equals 81. The experimental value for many of the isotherms for CO on palladium are closer to this latter value. We will now consider the rather unlikely case of mobile adsorption, and the value of the ratio r for this case. 2.4. Mobile adsorption If molecules are not effectively confined to independent sites, but are absorbed on a uniform surface with no barrier to translation, the Langmuir equation no

-3.9

104 120 155 280 so0 lSO0

0.08 0.20 0.34 0.48 0.64

0.77

370

0.9 3.0 8.0 26 90

33 20 59 122

0.73 0.64 0.79 0.86

4.8 10

P[f3/(1 - 0)1-r x 108

93.3 22 7s

9

0.26 0.44

10’ (Torr)

0.17 0.7

X

18

3s 22 17 17 17

3.5 3.3 3.9 4.4

3.7 4.0

W[e/(l x 104

- e)]-’

66

IS 52 39 39 37

1.26 1.14 1.73 2.6

1.5 1.4

P[6l(l - e)]-’ x 108 X exp[-4000/RT]

a From refs. [3] and [4]. The e values are twice the structural ones, which have a maximum of l/2.

T=493 K r=7S Xl = -4.3

Xl =

T= 448 K r=80

P

Table 1 Selected isotherm data for CO on Pd( 111) (Ertl and Koch) a

52

9s 93 92 111 84

2.20 3.38 1.3 0.26

3.4 4.5

P[@/(l - e)]-t x 108 X exp[-O(1 -@)I

E

8 s 2

P %

p

i :: s y 0’

f .. 8 3 g.

684

G.D. Halsey / Chemisorption and catalytic oxidation of CO on Pd

longer applies. If the molecules behave as hard discs, isosteric heat will still be a constant, however. An approximation to the isotherm is given by the complete Volmer equation [lo], which contains an exponential term in addition to the Langmuir ratio; kvP = [e/(1 - 0)] exp[8/(1

- e)].

(2)

The molecules on the surface need not be free in two dimensions for this equation to be applicable; in fact the equation is strictly true only when the molecules are confined to channels, and free to translate over the surface in one dimension. That is because the isotherm corresponds to the equation of state (spreading pressure C#J and area A) @A/NkT=

1 + 0 + e2 t .. . = l/(1 - e),

(3)

which is equivalent to the one-dimensional equation of state for a hard-disc gas derived by Tonks [ 111. For the complete Volmer equation the value of the ratio of P (0 = 0.75) to P (0 = 0.25) is much larger than that for the Langmuir equation; r = 130 approximately. For motion across a real surface, it is to be expected that at least some periodic variation in potential would be encountered. Hill [ 121 has given a treatment of the transition from lattice-site adsorption to mobile adsorption as temperature is increased on a surface characterized by a sinusoidal variation of adsorption potential. If the difference between the lowest and highest potential is Vc the transition occurs over a temperature range in the vicinity of T = Vo/5R and the adsorbed film is essentially mobile when the temperature is twice this value. It is to be expected that the value of r would change from 9 to 130 in this transition region, and that intermediate values of I would characterize a surface with values of Vo and Tin the transition region. 2.5. Analysis with the Langmuir-Fowler

equation

A crude quantitative treatment of the effect of molecular interaction on the surface was presented by Fowler [13]. His isotherm equation (2322) can be written P=f(T)

(4)

[e/(1 - 0) exp]-BXr/RTl,

where x1 measures the change in isosteric heat from 0 = 0 to unity. Thus qst = -d lnP/d(l/kT)

where x0 is the heat of adsorption at zero coverage. The model is based on fixed lattice sites with nearest-neighbor ratio r=P(3/4)/P(1/4)=

(5)

= x0 + 0x1,

9 exp[-x1/2RTJ.

interaction.

The (6)

685

G.D. Halsey / Chemisorption and catalytic oxidation of CO on Pd

The factor 9 would be the pressure ratio in the absence of interaction and so a rough value of x1 can be calculated from the observed shape of the isotherm as measured by the ratio r. 2.6. Comparison of the Langmuir-Fowler and Clausius-Clapeyron results A rough estimate of r can be made from the isopiests of Tracy and Palmberg [4], and more accurate values can be obtained from the isotherms reported by Koch [7] and by Christmann [ 141. The values of r so obtained are used to calculate x 1 in eq. (6). The value of x0 is set so that the Clausius-Clapeyron and the LangmuirFowler results (eq. (5)) agree at 8 = 0.5. Results are compared in table 2. The ‘results are in three categories; on the (100) face, the results are ‘insubstantial agreement with each other. On (110) and (210) faces, the r values are smaller than the values that would correspond to the Clausius-Claperon results, while for the two results on the (111) face, the r values are larger. We have no explanation for the first discrepancy; the larger values on the (111) faces can be explained if the films are in the mobile state.

2.7. Mobile adsorption on the (111) face The application of the completely mobile model (table 1) overcorrects the data. Agreement becomes better at the higher temperature, as is to be expected in the region of the mobile transition. Such a model is consistent with the fixed-site LEED patterns observed at lower temperatures, and is indeed required by these observations. It has already been demonstrated, by LEED studies at higher coverages [5], that continuous adsorbate compression in one dimension occurs with CO on the (100) face of palladium. A similar compression may take place on the (111) face. Calculations made by Doyen and Ertl [ 151 indicate that the energy profile for CO on Pd(ll1) is the flattest of low-index planes investigated. The energy varies by only about 2 kcal in the (I 0) direction. Hill’s calculations indicate that for a barrier

Table 2 Comparison of isosteric heats on palladium surfaces Face

T(K)

r

Clausius-Clapeyron: 4 &call

Langmuir-Fowler: 4 (kcal)

Ref.

(100)

454 489 448 448

400 110 80 40

35-60 40-8 0 34 38

35-6.9 e 38-4.8 e 36-5.0 e 39-2.7 e

I4951 [71 [71 I141

441

700

35-10 e

34 - 7.6 e

I71

(110) (111) (111) on mica (210)

686

G.D. Halsey / Chemisorption

and catalytic oxtiation of CO on Pd

of this height, the transition to mobile adsorption would take place over the range of 200 to 400 K, in the limit of a dilute layer. The results on the ~pitaxiaily-grown (111) surface on mica are strikingly different, however. The isosteric heats are constant over a wider range than those on the single-crystal (111) face. The smaller r value also suggests a more nearly constant heat of adsorption, and at the same time substantially less mobility over the surface. It is possible, although difficult to believe, that irregularities at the atomic scale in the epitaxial film could destroy the potential for mobility and at the same time preserve almost perfect energetic homogeneity. However, the 15% smaller total change in contact potential at maximum coverage, for the epitaxial film versus the single crystal [7] supports the idea that CO adsorbed on the epitaxial film cannot be compressed as readily as the film on the single crystal. This point of view is supported also by the fact that the marked decline in the heat associated with structural changes is absent in the results on the epitaxial film, but present in the single crystal results on Pdll 11) as well as Pd(lO0). The Yvalues on the latter surface are consistent with site-wise adsorption and the measured Clausius-Clapeyron heat; however the r values on the (11) and (2 10) surfaces are, as noted, even smaller than the immobile values that would agree with the Clausius-Clapeyron slopes. 2.8. ~nte~retari~n as site-wise ads~~tio~~ It will be observed (table 2) that the two estimates of isosteric heats differ at most by about 2 kcal, and that the Yvalues are very sensitive to changes in the slope of the heat curves. Since the stated accuracy of the Clapeyron plots is approximately -tl kcal, any discrepancies between the two heats rest on one or two points at the extreme ends of the measurements alone. Note that if we accept the suggestion that the heat of adsorption of CO is not constant on the Pd(l11) face, then the results on this face are then similar to the declining heats found on other surfaces. It is also noteworthy that the kinetic results on all surfaces are also similar to each other, so that the discussion of CO oxidation that follows, although largely confined to the (111) results, wit1 apply qualitatively to any low-index result. 2.9. Heterogeneity versus iuteraction Although we have analyzed the result in terms of interaction theory above, the results can equally well be explained by a heterogeneous or dirty surface [ 161. Althou~ in every case, efforts have been made to expose a clean, single face the surprising difference in the single crystal (I 11) results and the epitaxially grown mica-supported (Ill) film. The larger value of the heat of adsorption when coupled with its improved constancy and smaller r value on the supported surface argue for an energetically more perfect surface if not a crystallographically more perfect one. Since degrees of heterogeneity can be subtly different on nominally similar sur-

G.D.Halsey/ Chemisorptionand catalyticoxidationof COon Pd

687

faces, and interaction forces on a uniform surface should be the same, at least a partial role of heterogeneity is indicated. Possible further uses of the heterogeneous option will be mentioned later in this paper.

3. Mixed-Langmuir analyses of the reaction 3.1. Oxygen adsorption and the mechanism of oxidation We will summarize some of the experimental results and conclusions that have been demonstrated by previous authors [2,3]. Oxygen is adsorbed as atoms, and does not desorb except at higher temperatures and in an irreversible manner. Thus there are no oxygen isotherm data to compare with the CO data. However, oxygen is readily removed from the surface by reaction from above by CO (Rideal mechanism), and this is the principal mechanism for the oxidation: o* + 2s + 20,,

o,tco+co,.

At lower temperatures, the surface becomes covered with CO and the reaction stops. At about 400 K, dependent on pressures, CO desorbs sufficiently for the reaction to start almost abruptly. The oxygen coverage and the reaction rate peak at about 500 K; the rate then falls off as the temperature increases and bare surface increases. When oxygen is adsorbed at room temperature on a surface partially covered with CO LEED measurements indicate [6] that oxygen and CO are on the surface together, but isolated from each other in separate domains or islands. Thus they do not react to any important extent, unless the temperature is raised to the neighborhood of 400 K. In this region, the transient reaction between adsorbed species has been studied by Ertl and Neumann [ 171. Quantitative examples will be confined to the results on the Pd(ll1) crystal face; results on the other faces when available are remarkably similar to these. 3.2. Absolute rate theory analysis of a simple mechanism The only kinetic mechanisms that can be readily analyzed in a quantitative fashion are those that involve site-wise adsorption on fixed-lattice sites, in the absence of surface interactions or heterogeniety. Although this simple’ model may evidently not apply to the adsorption of CO, we will explore its potential to explain the data in the region covering the onset of the steady-state reaction. The treatment given below can be readily modified to accommodate other versions of’ the mechanism; many of these have been described in detail by Close and White [ 11. 3.3. Steady-state analyses In an earlier paper [ 181, various examples of steady-state mechanisms for catalysis that were based on equilibrium and non-equilibrium applications of the mixed

688

G.D. Halsey 1 Chemisorption and catalyticoxidation of CO on Pd

Langmuir equation were presented. Ertl and his collaborators have presented similar analyses that are applicable to the oxidation of CO on palladium. For example, the set of steady&ate equations (A stands for oxygen, B for CO) d8,ldt

= 2k,P,(l

- eA - e,# - kRPBeA = 0,

(7)

de,ldt = kBPB(l - eA - e,) - kD eB = 0,

(8)

are successful in explaining the observed pressure dependence of the CO oxidation. These equations embody the conditions that the adsorption of 02 is as atoms, and that it leaves the surface only by reaction; and that CO is absorbed molecularly on the same sites, and leaves the surface only by desorption. For the ratio of the oxygen coverage to the square of the CO coverage, one obtains e,/ez, = (2k,P,/k;P&(kL/k&

(9)

We will choose as a standard pressure lop7 Torr. Although relative pressures were known more accurately than the absolute pressures, the pressures of CO and O2 used in the experiments were of this order. With this convention we set the pressure ratio in eq. (6) equal to unity in order to discuss the temperature dependence. 3.4. Application of ART to steady-state isothems The absolute rate theory in a formal version can be applied to non-equilibrium Langmuir-type mixed isotherms [ 111, which are based on various rate constants and concentrations. Each rate is written as ki = (kT/h) exp[--G”/RT] ; the kT/h factors cancel in the steady-state cases. For example, for the equilibrium adsorption of a gas such as carbon monoxide,

kBPB( 1 - e) = ekD

or

0/l - I3= kBPB/kD = exp(-AGt/RT)

P,,

where the total free energy change is composed of the contributions tions involved are

AG#=G&G;=H&H&

T(S&Sg).

(10) from the reac-

(11)

From the information that -Hads = 34 kcal, and that the surface of Pd(ll1) is half covered at the reference pressure of lo-’ Torr at -450 K, we can calculate -AH’ = Hg - Hg = t34, and AS0 = Sg - Sf = 75.6 cal deg-‘. At 450 K and the reference pressure of lop7 Torr, the total entropy of gaseous CO is S” = 95.3 cal deg-’ . This procedure, though correct, is unnecessary for the equilibrium case. For the steady-state, non equilibrium case (eq. (9)) derived above, it provides a useful result, however, AG=G;-2G;+2G$-G;=AH+-TAS+.

(12)

C.D. H&y / Chemisot-ptionand catalytic oxldntion of CO on Pd

689

We thus see that & has an equilibrium component 2(Hf, - Hg), equal in magnitude to twice the heat of adsorption of CO, and non-equilibrium components Hi - H’;f. The latter will be the difference between two relatively smahactivation energies. Likewise, A,!? contains minus twice the equilibrium (S$ - Sg) pIus,S~ - 5’;. Each of these latter quantities involves the standard-state loss of translational freedom for in one case an oxygen molecule, and the other, a CO molecule. Since the gaseous entropy S” of these molecules differs by only about 2 cal deg-‘, this term will be relatively unimportant. 3.5. Onset of reaction Under conditions of fixed pressures of lO_’ Torr, the reaction will reach its highest rate whew8the CO coverage, 8 At approaches zero. The transition region will be located where @A= Bu, and if the further condition of near-saturation is satisfied, both these coverages will be 112, and the ratio eA/ei will be 2, and AG’ will be zero. We have already observed that the terms contributing to Gi -,G$ might be expected to partially cancel. If they do so completely, the only term left is the AC’ value for the equilibrium adsorption of CO, multiplied by the factor minus two. Thus, with complete cancellation of the extra terms, the steady state mechanism for CO oxidation, in the standard state of all pressures equal lo-’ Torr, gives the value 0,/e: = 2 at the same temperature that the equilibrium adsorption of CO finds e/(1 - 8) ,= 1, or -450 K at lo-‘Torr. At a temperature 50°,10wer, the ratio @,/8; would diminish to 10m2, in keeping with the observed rapid decline in reaction rate between these temperatures. 3.6. Fractional coverages at the transition tempemtwe If the pressures in eq. (9) are all unity, then at the transition temperature the coverages are determined by the condition eA = 20: and the oxygen balance (eq. (7)) 2( 1 - IjA - dB)2(kA/kR) = eAe Now, in order to account for the near equality of the transition temperature and the temperature at which; the CO isotherms show half coverage at the reference pressure the quantities AG: and AC; in eq. (12) must nearly cancel; in other words the rate constants for the adsorption of oxygen and for the reaction of CO with adsorbed oxygen must be nearly equal. With exact cancellation of fS2 - Gz in eq. (12), kA/kR is equal to unity, en = (1 - eA - 0,) = 0.366 and &, = 0.268. At higher temperatures, where 8,~ 0 the oxygen coverage approaches 0~ q 0.5. If the reaction constant k;~ is one tenth of the adsorption constant ,jthen at the transition temperature 1 ,-- eA T- eB = 0.14, ee = 0.45 and eA = OAl.‘The upper limit of the oxygen coverage is 0.8. Such a difference in rates could be1caused by

690

C.D. Halsey J ~em~rpl~n

and cat&tic

oxidation of CO on Pd

an activation energy for reaction 2 kcal in excess of that for oxygen adsorption, and should lower the transition temprature about I$‘, In the opposite direction, if the activation energy for oxygen adsorption exceeded that for reaction, so that kA/kR = 1110 (about 2 kcal difference) at the transition temperature aA = 0.095, Bn = 0.218. The upper limit of the oxygen coverage becomes 0.145. The transition temperature for the onset of reaction will now be about 15’ above where the equilibrium CO isotherm shows half coverage at the reference pressure. 3.7. Effect of the pressure ratio on the onset of reaction The choice of reference pressure is arbitrary, and the shift of reactant pressure to a different pressure (pA = Pn) has the physical interpretation of moving the onset temperature to a new value but where as before the equilibrium value of en = l/2 for the new pressure is found. Thus, the onset temperature can be deduced from the CO isotherm data. If the pressure are varied independently (PA f Pn) the location of the onset temperature depends on the ratio PAfPB’+n where for the particular model chosen here n = 2. 3.8. Comparison with e~pe~rne~t~ results If the center of the transition region for reaction is identified with the ratio of = 2 and the pressure ratio P,/Pi in eq. (9) is varied from unity, a corresponding shift in AG’ in eq. (10) must take place. A relatively smali change in temperature will provide a non-zero value of AH* - TAS’ to compensate for the change in the pressure ratio. If conditions are varied from PB = 0.6 and PA = 0.3 to PA = 0.6 and PB = 0.3 (in IO-’ Torr), the transition region is lowered by about 15”) as the pressure ratio changes by a factor of 16 [ 71. This change is consistent with a value of AH* = 2 X 34 kcal, for which a change of 15’ would produce a factor of 14 in the pressure ratio.

e,fe2,

3.9. Langmuir-Hinshelwood (surface-surface) reaction Ertl and Neumann [ 171 have provided a study at a lower pressure {PA = PB = 8 X lo-‘) where the longer residence time of oxygen favors the reaction between adsorbed species over the Rideal (kn) term in eq. (7). They have used laser-induced desorption to evaluate coverages during reaction. At a temperature where equilibrium coverage of CO would be l/2, the measured coverage during reaction is 0.43. This value is fairly close to the value predicted by the simple mixed-~gmuir model of 0.366. However, the transition region is only beginning at this temperature (400 K). In the middle of the transition region (430 K), Bn has decreased to about 0.15. These observations are compatible with a small activation energy excess for oxygen adsorption of about 4 kcal. It is clear from the experimental curves that

G.D.Halsey/ Chemisorptionand catalyticoxidationof COon Pd oxygen adsorption of a surface-surface

691

is hampered or reduced in some way. We will consider the effect reaction between 0 and CO.

3.10. Pure Langmuir-Hinshelwood mechanism If the atomic adsorption of oxygen is considered, and the surface-surface reaction is the only step removing material from the surface, the steady-state equations become dO,/dt = 2(1 - 8, - e,)2k*P* de,/dt

= (1 - eA - en) k,P,

- kreAeB = 0,

(13)

- kreAeB = 0.

(14)

If kAPJk& is less than l/2, the surface becomes completely covered with component B. If the ratio is larger than l/2 the reaction rate must increase to a critical value before steady-state Langmuir-Hinshelwood kinetics begins. For the simple case where kAPA = ICBPB,kr/kBPB must equal eight or more. At the onset of reaction eA = f?u = l/4. As temperature increases, the value of k, increases, the rate and amount of bare surface stays the same, but eA > en_ The rate limiting step has become adsorption on the surface. We are assuming of course that the only important activation energy term is in k,, and that PA = Pg, for these numerical examples. 3.1 I. Model kinetics including two reaction paths If molecular oxygen is replaced by a hypothetical the steady state equations can be written

molecule

0 in the gas phase

de,/dt

= kAPA(l - eA - e,) - k,e,e,

- kRe*PB = 0,

(15)

de,/dt

= k,PB(l

- kDeFj = 0,

(16)

- eA - e,) - k,e,e,

which describe a model reaction that includes a surface-surface reaction as well as the Rideal mechanism. Investigation of the case in which kR = kD = 0 ‘[181 shows that in the case of no desorption, the mixed film is unstable, and switches abruptly from total coverage of A to total coverage of B, when the two adsorption terms are equal: kAP* = kBPB. This corresponds to the physical situation where the pressures of oxygen and CO are equal and the adsorption probability is the same ‘for each. If we assume that PA = PB there are negligible activation energies for any,reaction or adsorption from the gas, we have kA s kB P kR for a very simple and; illustrative case. If we subtract eq. (15) from eq. (16), we find that the first two terms cancel, and Bn/eA = k, PB/k,, = kBPB/k,, . The last expression is also the equilibrium

(17) value of e&l

- en) in the absence of A.

We see then that when the equilibrium coverage is 1j2, the value of Bs equals eA, which is the ‘onset” condition. If we substitute these values in eq. (I 5) we find for the case of no surface-surface reaction that f?~ = 6~ = l/3. The maximum steadystate value of 0, when Bu has become zero is l/2, and the rates of reaction will be in the ratio (l/3& to (l/2)/~. If P&n = k,, eA and 8~ = 0.30, but the rate of reaction must include both terms and is 0.39 PAkR. If the surface-surface reaction predominates fk, = 10 PAkR) the onset turns into a maximum rate, and a peak in the reaction rate results. Whether onset of reaction or peak, however, the simple, component B, adsorption equilibrium locates the event at the temperature where 8~ for the simple isotherm equals l/2. There is no shift of the onset temperature of the kind observed by Ertl and Neumann [I?‘]. The slight peak in reaction rate observed at about 500 K may indicate some participation of the surface-surface reaction mode but the desorption of 0 could also produce this effect. At higher temperature, where the reaction is proceeding rapidly, there must be a large fraction of bare surface to accommodate oxygen adsorption; if the oxygen is distributed at random on an otherwise bare surface, there is very little d~s~nct~on between the reaction of a CO from the gas, perhaps bound in the Van der Wads field of the surface, and the reaction of a CO molecule bound for an instant in an adjacent site for chem~sorpt~on of course, the Rideal mechanism by itself is completely adequate to explain the reaction in this region. If a surface-surface channel is added to the reaction scheme, the oxygen coverage is reduced at the onset candition, but that the reaction velocity is not; clearly adding a channel is not going to reduce the reaction rate. Also, the location of the onset of reaction is still determined by the CO equilibrium constant which contains the largest contribution to AH’. The low-pressure steady-state data show no de% nite evidence for the participation of the surface-surface mode of reaction, but do indicate a small activation energy for oxygen adsorption.

4. Kinetics showing islland formation 4.1. Island formation

LEED results demonstrate the formation of separate domains or islands of CO and of oxygen before reaction takes place, Below the onset temperature of the steady&ate Rideal mechanism, reaction between the adsorbed species takes place, and ErtI and Neumann [ $71 have demonstrated that this process has an a~ti~t~on energy of about 7 kcal and takes place relatively slowly in the vicinity of 4oQ K. The combination of low activation energy and slow rate suggests an overall entropy af activation AS’ that is large and negative, which in turn suggests that the reactants are widely separated. For the reaction between two adjacent adsorbed species, the entropy of activation AS* would be small, and the rate constant should be kr

G.D.Halsey/ Chemisorptionand catalyticoxidationof COon Pd

693

= (KT/h)exp[-AG#/RT] = 10’ see-‘, at 400 K if AGf = AH’ is about 7 kcal. The observed reaction [17) takes place in times of the order of 10 set or more. Since during the transient experiment, the ambient pressure of oxygen is -2 X lo-’ Torr, any appreciable activation energy for the chemisorption 06 O2 might cause this step to be rate determining. On the other hand, since LLED studies have shown oxygen and CO to be stable on the surface together, the rate determining step may be diffusion of the adsorbed species between separate domains. The experiments demonstrate that the surface-surface reaction does not have a large activation energy, comparable to the energy of desorption, and that tlie reaction of adjacent species would be very rapid even at room temperature. Thus, separate domains, demonstrated by LEED results at lower tepperature, are consistent with the transient results which cover a temperature range of from about 350 to 450 K and nearly reach the onset temperature. At highejr temperatures, since CO is almost completely absent from the surface, the question of domain formation does not arise. 4.2. Mechanism of domain formation It is difficult to explain separate domains on the basis of adsorption and desorption constants that are independent of coverage (the mixed Langmuir model). It is clear from the adsorption studies on CO that no attractive forces exist to condense the CO. However, in the absence of oxygen isotherm data, there is no way to rule out negative interaction and condensation for the oxygen film. Ertl et al., have reported a striking case of such interaction for hydrogen on Ni( 110) [ 81. It is not necessary that oxygen show such a two-dimensional condefisation; all that is required is an energetic model for the surface that is unfavorable to the adjacent adsorption of the two species. A strong repulsion between CO and 0 on the surface would destroy the Langmuir model and cause segregation. One must also assume that the activation energy for the adsorption of CO is increased by the presence of adjacent 0 atoms, and vice-versa, so that the integrity of :the islands would be maintained during Rideal reaction of the oxygen and thermal desorption of the CO. One must also assume that this repulsion would impede adsorptiog of CO in the oxygen domains, when vacancy occurs by reaction, and that the oxygen would be similarly prevented from adsorption in the CO regions. 4.3. Reaction along the line of interface With the integrity of the separate islands protected by some mechanism, one island can only grow at the expense of the other along the line or zone ~ofintersection of the two islands. Here, rates of reaction will be different frort$ inside the islands; the direction of movement of the interface will be determinedi by desorption and reaction rates, and the relative rates of adsorption of the, two components.

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For example, the CO island will grow if, first, an oxygen at the boundary is removed by reaction, and second, if it is replaced by the adsorption of a second CO. On the other hand, if two adjacent CO atoms at the boundary are desorbed and then replaced by an oxygen molecule from the gas, the oxygen island will have grown. In order for the boundary to be stable, a whole family of such reactions must be equal in rate:

c

.. . kcoP&

.. . = c

.. . 2kOP0, .,. .

all reactions

There is no reference in this equation to the size of the islands, so for a given set of pressures and a variable temperature, there will be only a sharply defined temperature where the equality will hold. Otherwise, one ialsnd will grow at the expense of the other until it covers the surface. Under steady-state conditions, then one cannot explain the general presence of more than one island on a uniform surface. An approximate and formal condition for the stability of islands can be obtained if the lateral transition region from pure A to pure B is gradual enough so that in a sufficiently wide region of the transition area one can set BA = Bn. This condition locates the line of separation, and is formally given by AC’ = 0 in eq. (12), if the pressures in eq. (9) are set at the reference value. The transition conditions then become formally the same as in the mixed-Langmuir case, but since AC’ can only be zero at one value of temperature, the transition from total coverage with A to total coverage with B is abrupt. The enthalpies and entropies of activation involved are values characteristic of the line interface, and not those of the bulk coverages, and might be different from values derived from the CO isotherm, for example. 4.4. Effect of heterogeneity If the surface is heterogeneous, and that heterogeneity is of the patch-wise, or long range type, the conditions for island formation will be different at different places on the surface, and stable islands can be easily explained over a range of temperatures relatively close to the unique conditions called for on a similar uniform surface. However, mixed-Langmuir adsorption, which is in accord with the rest of the experimental result, also has this property on a heterogeneous surface. In the latter case, only the energy of adsorption of the CO enters the relationship for 0~, and so only the heterogeneity in that parameter is required. The (unknown) energy values for oxygen are not required. Since the energy of adsorption of CO is multiplied by two the effective range of heterogeneity is also doubled to 8 kcal or a Boltzmann factor at 450 K of the order of 10d4 for oxygen coverage between the strongest and weakest sites. Thus if the median sites were half-covered in random Langmuir fashion, the strongest sites would have about 1% oxygen contaminant, and because of the square term in en, about 10% CO contaminant in the oxygen-rich region of the heterogeneous surface, where the CO is bound least

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strongly. Thus, with long-range heterogeneity, it would be possible to have large regions covered almost completely with CO, and other, separate regions under the same conditions, largely composed of 0 atoms. It would appear that in this way the LEED observations could be explained without invoking special stabilizing mechanisms otherwise needed for island formation. 4.5. Effect of mobility We have noted that the mobile-immobile transition may be a factor in the adsorption of CO on Pd(ll1) and on Pd(lOO). There is no simple model like the mixed Langmuir model that treats a mixture of mobile B and immobile B and so we cannot develop the appropriate steady-state kinetics. If on the other hand the species are in separate domains or islands no modification in the discussion given is necessary. The relative rates of (unspecified) events at the line interface would determine which species survived on a uniform surface, and the introduction of long-range heterogeneity would allow the simultaneous presence of A and B domains.

5. Discussion The complexity of the CO adsorption data on the various faces of palladium is a warning that simple kinetic arguments are likely to be only approximations to the truth. Further transient measurements, and measurements of the absdute rate of reaction would be helpful in disentangling the effect of surface-surface reaction, diffusion over the surface, and possible activation energies for adsorption. There is no definite evidence that domains are present on the surface during the onset of the steady-state reaction, but the forces responsible for separation at lower temperatures would make any simple one-domain kinetic shceme impossible. In any event, it is clear that the desorption equilibrium of CO seems to be the dominant event in the onset region. It is important to realize that the mixed-Langmuir formulation of surface rates, using overall 0 values for the reactants, is fundamentally based on the assumption of random processes on uniform substrates with common sitewise bonding. Its extension to the mobile case, interaction between adjacent ad-atoms, or to domain formation is incorrect. Extension to patch-wise heterogeneity involves ‘recognition of individual Bi values on each of the i patches. The total 0 = ZZei for each reactant is of no obvious value in kinetic expressions.

6. Summary

of alternatives

Since there are a confusing number of choices to be made in order’to the oxidation mechanism, a list follows:

describe

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(1) Mixed-Langmuir adsorption on a uniform surface. Supported by the steady-state kinetic data, but contradicted by the CO isotherms, transient experiments and LEED. (II) Mixed repulsive adsorption on a uniform surface. No theory exists for this type of kinetics, but activation energy for desorption would be expected to depend on coverage [ 191 and CO-CO interaction would be complicated by the presence of 0 atoms. Contradicted by evidence for island formation. (111) Island formation on a uniform surface. Compatible with LEED, and possibly with CO isotherms at high total coverage, but predicts abrupt onset of reaction at fixed temperature; incompatible with kinetic data. (IV) Mixed-Langmuir adsorption on a non-uniform surface. If the heterogeneity is long-range enough to provide coherent patches of CO and 0, there are no contradictions with the data. (V) Island formation on a non-uniform surface. Also compatible with the data. No kinetic or equilibrium data for the mechanism of island preservation is available however. With the information available, it is not possible to choose between the last two alternatives, which are physically very similar indeed.

Acknowledgment The author is greatly indebted to Professor Sam Fain for suggesting this problem, and for many valuable consultations afterwards. Professor G. Ertl kindly read an earlier draft, and made some valuable suggestions.

References [l] J.S. Close and J.M. White, J. Catalysis 36 (1975) 185. [2] G. Ertl and P. Rau, Surface Sci. 15 (1969) 443. [3] G. Ertl and J. Koch, in: Catalysis, Vol. 2, Ed. J. Hiphtower (North-Holland, Amsterdam, 1973) p. 969. (41 J.C. Tracy and P.W. Palmberg, Surface Sci. 14 (1969) 274. [5] J.C. Tracy and P.W. Palmberg, 1. Chem. Phys. 51 (1969)4852. [6] G. Ertl and J. Koch, in: Adsorption-Desorption Phenomena, Ed. 1:. Ricca (Academic Press, London, 1972) p. 345. (71 1. Koch, Wechselwirkung von Kohlenmonoxid und Sauerstoff mit PalladiumaberflB’chen, Dissertation, Hannover (1972). [8] K. Christmann, 0. Schober, G. Ertl and M. Neumann, J. Chem. Phys. 60 (1974) 4528. 19) H.H. Madden, J. Kuppers and G. Ertl. J. Chcm. Phys. 58 (1973) 3401. [lo] J.P. Stebbins and G.D. Halsey, Jr., J. Phys.Chem. 68 (1964) 3863. [ 1 l] L. Tonks, Phys. Rev. 50 (1936) 955. 1121 T.L. Ilill, Introduction to Statistical Thermodynamics (Addison-Wesley, Reading, Mass., 1960) p. 172.

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[ 131 R.H. Fowler, Statistical Mechanics (Cambridge Univ. Press, Cambridge, England, 1936).

[ 141 K. Christmann and G. Ertl, Surface Sci. 33 (1972) 254. 1151 G. Doyen and G. Ertl,Surface Sci. 43 (1974) 197. [16] G. Halsey and H.S. Taylor, 3. Chem. Phys. 15 (1947) 624. [17] G. Ertl and M. Neumann, Z. Physik. Chem. NF 90 (1974) 127. [ 181 G.D. Halsey, Jr., J. Phys. Chem.67 (1963) 2038. [19] G.D. Halsey, J. Chem.Phys. 65 (1976) 2029.