Adsorption and coadsorption of carbon monoxide and hydrogen on Pd(111)

61

Surface Science 123 ( 1982) 61-76 NorthHolland Publishing Company

ADSORPTION AND COADSORPTION OF CARBON MONOXIDE AND WYDROGEN ON Pd( 111)

Received

6 May 1982; accepted

for publication

13 September

I982

The adsorption and coadsorption of CO and H, have been studied by means of thermal desorption (TD) and electron stimulated desorption (ESD) at temperatures ranging from 250 to 400 K. Three CO TD states, labelled as pa, fit and & were detected after adsorption at 250 K. The population of & and p, states which are the only ones observed upon adsorption at temperatures higher than 300 K was found to depend on adsorption temperature. The correlation between the binding states in the TD spectra and the ESD 0’ and CO+ ions observed was discussed, Hydrogen is dissociatively adsorbed on Pd( 111)and no ESD H’ signal was recorded following Ha adsorption on a clean Pd surface. The presence of CO was found to cause an appearance of a H” ESD signal, a decrease of hydrogen surface population and an arisement of a broad Ha TD peak at about 450 K. An apparent influence of hydrogen on CO adsorption was detected at high hydrogen precoverages alone, leading to a decrease in the CO sticking coefficient and the relative population of CO & state. The coadsorption results were interpreted assuming mutual interaction between CO and H at low and medium CO caverages, the “cooperative” species being responsible for the Hi ESD signal. Besides, the presence of CO was proved to favour hydrogen penetration into the bulk even at high CO coverage when H atoms were completely displaced from the surface.

1. Introduction The adsorption and coadsorption of CO and H, on metals of the VIII group have been extensively studied because of the recent interest in hydrocarbon synthesis. A mutual interaction of CO and H, coadsorbed on the metal surfaces has been observed using appropriate surface techniques [l-9]. No formation of hydrocarbons occurred under ultrahigh vacuum conditions [lo,1 l] but replacement of the preadsorbed hydrogen by CO and further blocking of the adsorption sites for hydrogen were reported. With some metals (Pt, R.h and Pd) [3,4,7,12] hydrogen penetration into the bulk was favoured in the presence of adsorbed CO. Following our previous studies on adsorption and coadsorption of CO and * Temporary address: institut fur ~renzfl~chenforschun~und Jiilich, Fed. Rep. of Germany

0039-502g/82/0000-OooO/$O2.75

~akuumphysik,

0 1982 North-Holland

KFA Jiilich, D-5170

H, on ~~ly~~sta~line Pd f12,I3j this paper presents some results obtained with single crystal Pd( 1 I I). Although the adsorption of CO [ I5- 191 and W, 124-271 alone on Pd singie crystal surfaces was dealt with in numerous papers, same new information obtained using a combination of thermal desorptian and electron stimulated desporption methods will be reported and related to CO-H, coadsarption studies.

The experiments were performed within a conventional UHV syustem equipped with a cylindrical mirror analyzer and a quadrupoie mass spectrometer (QMS), The ion source of the QMS was supplemented with a simple electron gun and a three-grid ion optic for electron stimulated desorption measurements. The equipment has been described in detail in ref. [ 141. The Pd(ll1) crystal (0.7 x 0.8X 0.03 cm”) used for these studies was oriented with an accuracy of 1” and polished by Al,O,. The cleaning procedure involved heating in 1 X lo-' Torr 0, for X h at 900 K followed by repeated cycles of Ar ion bombardment (400 eV, 5 PA crnpZ) and annealing at tem~~ratnres up to 1100 K. These cleaning procedure was repeated at various times to remove the traces of S and C a~e~aring on the surface. The surface cfeanliness was checked by the Auger spectrometer. The crystal was mounted to a manipulator by two high-purity Ta leads. The sample temperature was determined using a W-~~Re/W-~~~R~ thermocoupte spot welded on the rear face on the crystal. fn the present experiments the crystal temperature was linearly increased by 10 K s-’ by passing a direct current through the sample. High purity gasses (99.999%) H, and CO were introduced into the experimental chamber with leak valves. In order to minimize wall effects, effusion sources connected with the leak valves were used, which ensured an increase of the effective pressure on the sample in front of the effussor of about 10 tintes.

Sets of CO TD spectra collected after various CO exposures at 250, 30 and 390 K are presented in fig. 1. For exposures higher than 0.5 L (1 L = 1 X 10 -’ Torr s) two distinct peaks are seen in the TD spectra at adsorption temperatures, Za, 250 and 300 K. The higher temperature peak at 490 K was designed as & and the lower temperature one at 430 K as &. The relative intensity of the & and & peaks changes with incr~~s~~~ total CO coverage. For exposures

c-50 550 300

Loo

500

390

500

T(K) Fig. 1. CO TD spectra CO exposure

at different

adsorption

temperatures, T,: (a) 250 K: (b) 300 K; (c) 390 K;

in 10e6 Torr s.

higher than 4 L a small shoulder on the lower temperature side of TD spectra collected upon adsorption at 300 K appears, whereas a distinct peak located at 315 K arises in the TD spectra recorded after CO adsorption at 250 K. The relative CO coverage was determined from the areas under the TD curve accepting the calibration value for CO saturation coverage at 300 K, 8”“’ co = 0.55, as reported by Erley and Wagner 1201. The coverage-exposure relationship for four adsorption temperatures is shown in fig. 2. As is evident, the initial slope of the four plats is the same and gives an initial sticking coefficient value, sO, of about 0.8. The experimentally observed dependence of the sticking coefficient on 19,~ was fitted to the relationship proposed by Clavenna and Schmidt [21] based on the Kisliuk precursor state kinetics model: I

.s ---z&z

.sg

1 -I-kf3/(l

-t?) *

where k = P,/‘p, (Pd is the probability for desorption of the precursor over occupied sites; P, is the probability for chem~so~tion from the precursor state). As is evident from fig. 2b the experimental points obtained at T, is 250, 300, 358 and 390 K can be fitted satisfactory to theoretical (s/s,)(@) curves with k equal to 0.1, 0.2, 0.7 and 1, respectively. It has to be noted that at T, = 390 K the CO adsorption kinetics follow the Langmuir model, where k is equal to 1. The activation energy for desorption of the & state was estimated at low coverages (Bo. G 0.05) using the simple Redhead equation:

E,/RT,

= ~n~~~T~/~~ - 3.64,

(2)

64

M. P. Kiskinovu,

G.M. Biirnakov

/ Carbon

monoxrde

and h.vdrogen on Pd(l I I)

b

’ +

0.2

04

e co

O-6

Fig. 2. (a) CO coverage. &o. as a function of CO exposure at LX,:(A) 250 K: (A) 300 K: (0) 358 K; (0) 390 K. Dashed line represents the CO adsorption kinetics over Pd( 1I I) saturated with hydrogen (19~ = 0.9) at 250 K. (b) Sticking coefficient, s/so. versus 0,o for the same r, as in (a). The solid lines correspond to the theoretical curves according to eq. (I) for four different values of li. The points represent the experimental data.

where v,-, is the pre-exponential factor, p the heating rate and Tp the peak temperature. With the usual assumption for vgtbeing lOI3 s-’ we found Ed to be equal to 31.5 kcal mole-‘. This value is lower than the reported isosteric heat of CO adsorption on Pd( 11 l), 34 kcal mole-’ 1151. In fact, as was discussed in refs. [22] and [23], the actual pre-exponential factors for gas desorption are considerably higher than the vibrational frequency 10” s-’ and this leads to the observed disagreement with the isosteric heats of adsorption. With the system under consideration, agreement with the isosteric heat will be achieved for v0 = 1.5 x 1015 s-'.In order to minimize the influence of the pi peak on the behaviour on the & peak at higher S,.,, TD spectra have been collected upon CO adsorption at higher temperatures. As is shown in fig. lc, the contribution of the /3, state to the spectra is negligible and no shift of the & peak maximum was observed for &o < 0.2, which implies a constancy of & Ed in this coverage range. A very small decrease of Ed by about 1 kcal mole-’

M. P. Kiskinova, C. M. Bliznakov / Carbon monoxide and hydrugen on Pdflll)

65

was

estimated from the position of the fi, peak maximum at saturation pO. coverage 19?$ = 0.26 at T, = 390 K, assuming a constant pre-exponential On the basis of the results in fig. lc we resolved the complex TD spectra recorded at T, is 250 and 300 K in order to characterize the ,G, and & desorption states. Using the Redhead relationship (2) and assuming the pre-exponential JJ to be the same for the PI, & and & states (1.14 X 10” s-‘) we derived Ed for the j3, and & states: 30.0 and 22.0 kcal mole-‘. Upon electron impact (80 eV, 6 PA) of CO covered Pd(ll1) we detected 0’ and CO+ ions, the latter appearing only at high coverages at Ta = 250 K. The O+ and CO+ ion yields as a function of CO coverage are shown in fig. 3. In order to minimize the possible effect of electron bombardment on CO adsorption kinetics, the points in fig. 3 were obtained after exposure of the sample to CO doses far away from the electron gun. Thus the sample precovered with CO was irradiated no more than several seconds during the actual ESD measurements. At adsorption temperatures ranging from 300 to 400 K an O+ signal alone was detected which increases initially with increasing @co, reaching its maximum intensity at L&o about 0.3. At T, is 250 and 300 K the further increase of @co leads to a decline of the O+ yields. As is illustrated in fig. 3 the initial slope of the Of{ 6) plots is almost the same for the four values of Ta but temperatures. At it decreases with 8,o more rapidly for lower adsorption T, = 250 K a measurable CO+ signal was recorded at &co higher than 0.35. It is interesting to inspect the behavior of the O+ and CO+ signals during

0

0.1

02

0.3

Od

0.5

0.6 T (K)

Fig. 3. Plots of the ESD ion yield as a function of CO coverage, &,, for T,: (A) 250 K; (A) 300 K; (0) 358 K; (0) 390 K. Dashed line represents O+ signal for CO adsorption on hydrogen saturated Pd(lll)at250K(BH=0.9). Fig. 4. CO TD spectra and ESD signals recorded during heating after adsorption of CO: (a) T, = 390 K, initial f&., = 0.2; (b) T, = 300 K, initial f&, = 0.24; (cf T, = 250 K, initial f&o = 0 .55 .

66

M.P. Kiskinoua, GM.

Blirnakor; / Carbon monoxide and h_ydrogeen on PdfliI,i

thermal desorption at different ec,, and T,, presented in fig. 4. After adsorption at 390 K (fig. 4a) the Ot signal continuously decreases in the temperature range of the & TD peak for all initial I+,,. At lower 7Y,the behavior of the Oi signal during desorption was found to depend on the initial @co: for 0,.,, < 0.1 the O+(T) plot is the same as in fig. 4a; for 0.1 < SC,, < 0.35 the O+(T) plot consists of two segments (fig. 4b), the temperature range of every segment corresponding to ,f3, and & TD states. respectively. As is evident in fig. 4b, the main reduction of the Ot signal is observed during desorption of the pz state. At tYc, > 0.35, an initial increase of the O+ signal in the temperature region where the p, state is desorbing was detected, followed by a drop of the O+ signal within the & state temperature range (fig. 4~). A reduction of the CO+ signal was observed in the temperature region where the &, state is desorbing. 3.2. H_, adsorption-desorption

behuviour

H, adsorption on Pd( 111) was studied at two adsorption temperatures, 250 and 300 K. As was reported in ref. 1251 at temperatures below 350 K the dissolution of hydrogen is negligible, but a certain dissolution was observed during TD after H, adsorption on Pd ribbon at 300 K [ 131. A penetration of hydrogen in the first few subsurface layers was suggested also by the LEED measurements reported in ref. [26]. Fig. 5a presents H, TD spectra, recorded after adsorption at 250 K. The main higher temperature peak which grows at 350 K shifts to lower temperatures upto medium coverages, 8,, and remains at the same position with further increase of 8,, when a second peak, located at lower temperature arises. The coverage calibration of our TD data was made by accepting the

a

b Hz EXPOSURE (tow sx106)

0

350

LSO T(K)

Fig. 5. H, TD spectra,

collected

after adsorption

at T,: (a) 250 K; (b) 300 K.

coverage saturation vatue at 250 K, 8 r, equal to 0.9 as reported by Engel et al. [25]. Asuuming that the TD spectra, shown in fig. 5a reflect exclusively hydrogen desorption from the surface, we used the simple Redhead relationship for a second order desorption process to determine Ed and v,,:

From the linear plot tn(fpTz) versus l/7” we obtained & = 20.0 kcal mole- ’ and .v@ = 9 X IO-’ cm’s . The anafysis of TD spectra does not give an answer about the origin of the second peak appearing at 8, z=-0.5. However, as was discussed in ref. f27], the effective repulsive interactions become stronger at 8, > 0.5 which is most probably the reason for the appearance of the second d~s~~tio~ peak at high 8,. The TD spectra recorded upon adsorption at 300 K (fig, 5b) are similar to those reported previously for adsorption on a Pd( 111) surface [24] and Pd ribbon [ 131. The peak shapes with their sloping higher temperature tails, particularly at high f?,, differ much from the theoretical one predicted for the second order desorption process. As in ref. 1133,this result might be explained

68

M. P. Kiskrnova, G. M. Blirnakoo / Carbon monoxide and hydrogen on Pd(1 I I)

by contribution of the hydrogen penetrated into the subsurface layers. As was pointed out in refs. [13] and [25] the desorption kinetics depend on T, because of the different degree to which equilibrium between the chemisorbed and dissolved hydrogen is achieved. Fig. 6 shows the coverage-exposure plots and the dependence of the hydrogen sticking coefficient on OH. The slope of the curves in fig. 6a yields the initial sticking coefficient, sO.. 0.5 at 250 K and 0.36 at 300 K. It has to be noted that these values are not very accurate because of the uncertainties of the pressure gauge calibration. If we assume that H, adsorption proceeds according to the precursor model, proposed in refs. [28-301, the decrease of s,) with T, can be explained satisfactory when the activation energy of migration to the chemisorptive center is lower then the activation energy of desorption from the precursor state. As can be seen in fig. 6b the experimental points of the variation f the relative sticking coefficient s/s,, with or, fit to theoretical curves obtained by using the second order precursor model [21]: s

so=

1 (4)

1+/c/3/(1-e)”

with k equal to 0.7 and 1 for T, is 250 and 300 K, respectively. Under the experimental conditions of the present study (P,, < 1 X lo-’ Torr and T, = 250 K) no H+ ESD signal was detected following hydrogen adsorption on a clean Pd( 111) surface. As will be reported in the next section and in ref. [31] a H+ signal was detected in the presence of adsorbed CO, or in the presence of carbon contaminants on the surface. 3.3. Coadsorption

of CO and H2

Fig. 7a shows the H, TD spectra recorded with a Pd( 111) surface saturated with hydrogen at’ 250 K and subsequently exposed to various CO doses. A visible change in the shape of the H, TD curves was detected for CO exposures higher than 0.5 L (fIc, > 0.05) which is expressed first in broadening of the high temperature side of the peak, followed by the appearance of a second broad peak at about 450 K. The TD data in fig. 7b illustrate H, desorption observed with Pd( 111) covered with different amounts of CO and exposed to desorbed and the fraction H,. The dependence of the total H, amount desorbed in the temperature region where H, TD peaks from an initially clean surface appear on 8co is shown in fig. 8. The results in fig. 8 are obtained from the area under the corresponding H, TD spectra in fig. 7. It has to be noted that whereas the fraction of H, desorbed in the temperature region, corresponding to desorption from initially clean surface, drops rapidly with increasing S,,, the total H, amount desorbed decreaes almost linearly, achieving half of its initial value at saturated CO coverage. Since a noticeable effect of hydrogen on CO adsorption was observed at

M. P. Kiskinoua,

0

co

G.M.

Blirnakov

EXPOSURE

b

/ Carbon

_

350

L50

250 TM)

1

350

and

hydrogen

on Pd(l

69

I I)

%O:

1,

50

monoxide

LSO

I

-x

0.2

0.4

,

0.6

e CO

Fig. 7. H, TD spectra, collected after adsorption at 250 K: (a) H, TD spectra from Pd(lll) initially saturated with hydrogen (13H = 0.9) and exposed to CO; (b) H, TD spectra from Pd( 11 I) with fractional coverages of CO exposed to 10~ 10e6 Torr s H,. Fig. 8. Amount of H, desorbing under the H, TD curves, shown in fig. 7 as a function of f&o: (0) data for Pd( II 1) saturated with H, at 250 K and exposed to CO; (0) data for Pd(ll1) with fractional CO coverages exposed to 10~ 10m6 Torr s Ha. Dashed line represents the decrease of H, amount desorbing under the TD curves, corresponding to desorption from a clean surface.

B,, a detailed study at saturation hydrogen coverages at 250 K (0” = 0.9) was carried out. Several CO TD curves from Pd( 111) saturated with hydrogen at 250 K and exposed to CO are shown in fig. 9 and compared to CO TD curves (dashed lines) recorded after CO adsorption on a clean surface. It is obvious that the total amount of CO adsorbed and the relative population of the p, and & states are somewhat influenced by the presence of hydrogen. The CO adsorption kinetics on Pd( 111) initially saturated with hydrogen is presented by the dashed line in fig. 2a. Its initial slope yields an s0 value of 0.66, which is lower than that found for a clean surface, 0.8. In addition, the increase of the PI/,& ratio in comparison with that for a clean surface is indicated by the dashed line in fig. 12a. The behavior of the O+ ESD signal was also found to be affected by the presence of hydrogen. The dashed line in fig. 3 shows that for Pd(ll1) preexposed to hydrogen, the O+ (&o) plot is characterized by slower initial increase and lower maximum value than that of a clean surface. When hydrogen is coadsorbed with CO, a significant H+ signal was detected. The appearance and increase of the H+ ESD signal after saturation with hydrogen at 250 K and subsequent exposure to CO is shown in fig. 10a. Fig. lob presents the decay of the H+ electron induced current, I,+, with bombardment time. From the observed time dependence of I,+ under high

M. P. Kiskinova, G. M. Bliznakou /

70

Carbon monoxide and hydragen on Pd(l f I)

-.-..---1 b

r

2

,...

t (mi4n)

6

a

k

,,-:

A%*

a

: : ; : \ ;

~ (7,’ I’ ,’ ___-*k

250

350

L50

LB

‘\._ 550

CO

at 250 K on a clean (solid

line).

I

.____I..__._-A

0

T(K) Fig. 9. CO TD spectra after adsorption hydrogen saturated surface (t?H=0.9) 2.0~ 10m6 Torr s.

/

CO

2 (tom s X18)

1

EXPOSURE surface

exposure:

(a)

(dashed line) and on a 1.2~ IO--” Torrs; (h)

Fig. 10. (a) H+ ESD signal as a function of CO exposure from Pd( I1 1) initially saturated with hydrogen at 250 K (0, = 0.9). (b) Electron bombardment induced changes of the H+ current from Pd( I1 1)saturated with hydrogen and exposed to 1.5 X 10e6 Tom s CO. Electron flux density = lOI s-I cm-2,

bombardment we determined the value of the H’ section, q, using the equation described in ref. 1281:

total

desorption

cross

(5) where n, is the electron flux density. From the straight line semilog plot obtained we obtained q equal to 1.5 x lo- I6 cm* at 80 eV. Within error limits (15%) no dependence of the q value on the CO coverage was detected. Heating of the sample causes a decrease of the Ht ion signal within the temperature interval where CO is desorbing. It is interesting to note that at an initial increase of high Bco, when the H’ ion yield is strongly diminished, the H’ signal with heating was detected (fig. 11).

9 co

T(Kl

Fig. II. H, and CO TD spectra and 0” and III ESD signals recorded during heating adsorption on Pdg I Ii) ~n~~~~l~~ saturated with hydrogen at 250 K. 8,,: (a) 0.2; (b) 0.5. Fig. 12. &/& 390 K. Dashed

hydrogen

after CO

ratioas a functionof t&-Of0r line in (a) represents

diffezent q,: {A) 250 K; {Anf300 K; {U) 358 K; {Of the results for CO adsorption on Pd(ilJ) saturated with

at 250 K.

4, Discussion

The results obtained usingboth TD and Esfp methods give evidence of the existence of two binding states of CO at adsorption temperatures af 3~~-~~ K and a third state population at lower temperatures. Recently, studies with a madulated CO beam [ 171 have shown that at 300 K for S,, r 0.1 the adsorption of CQ on Pd( 111) can be explained on assuming a model with three different species on the surface, the physical difference of which is not clarified. The fR and CEED data indicate that at low coverages the CO molecules most probably accupy three-fold coordinated sites, although some degree of disorder in the overlayer is possible [16,18]. Transitions into two-fold sites take place with increasing f&o, both sites being simultaneously occupied in the intermediate coverage regions. The existence of two adsorption states of CO on Ptf f i I) ftrts been reported in refs. [?2f and $33].

In contrast, however, in the case of Pt( 111) the most strongly bound state was associated with linearly bonded “on top” CO [34] whereas the more weakIy bound one was associated with bridge (two-fold) bonded CO. The difference btween the binding energies of the two states was found to be - 1 kcal mole-’ in agreement with the theoretical predictions of Doyen et al. [35]. Assuming that the pi and flz states in the present TD spectra (fig. 1) correspond to desorbing bridge and linearly (three-fold) bonded CO, respectively, we made an attempt to inspect the changes in the bridge to Iinear coverage ratio, PifJZrY with S,, and 77,. Fig. 12 shows the fli,!& ratio as a function of S,,, For different T,, as obtained using TD and ESD data. The results based on TD spectra (fig. 12a) are collected by approximate deconvolution of the corresponding TD curves. However, the /3,/& ratio obtained by this method is not perfect, because interconvertion between the two states can occur during heating. The evaluation of the ,B,/& ratio from the O’(8) dependence was made assuming a proportionality between the deputation of the surface states and the corresponding ESD signal. We consider that the initial rise of the O+ ion yield (fig. 3) is due to ESD from the & state alone whereas the O+ ion signal at f?,., = 0.5 is produced by the p, state alone. The latter suggestion is based on the structural mode1 for CO adsorption on Pd(I I 1) where c(4 x 2) patterns detected at fi = 0.5 are associated with occupation of the bridge bonded states [IS,1 $1. However, the above considerations do not take into account that at high &, the localized CO adsorption is strongly affected by direct CO-CO interactions. It is evident from fig. 12 that for T, is 250 and 300 K the fit between the /3,/& values obtained by ESD and TD data is imperfect, and the possible reason for the observed discrepancies will be discussed below. The dependence of the PI,/& ratio on the adsorption temperature indicates that the equilibrium between the two states is not achieved at lower adsorption temperatures. The sharper increase in slope of the fi,/& versus_Bc_o plot at f&, > 0.3 might be associated with a compression from a (a x $3 )R30° to a c(Z x 4) LEED structure, i.e. a general movement from three- to two-foId coordinated sites. However, as was discussed in ref. f 181 even at fow a,,, the population of both sites is possible and a strict locahzation in the linear state does not take place. If we appiy the equaltion proposed by Ertl et al. [32] (derived from the CO coverage of a linear adsorption state) to our data we can estimate the desorption energy difference, AE,,, between the & and p, states: S,z/&r - 1 = 2 exp( -AE/RT).

(6)

We used the results obtained at q,, 358 and 390 K for 8,, < 0.2, where almost equilibrium conditions might be assumed, and a AE value of - 1 kcal mole-’ was evaluated. This value is by a factor three smaller than the difference between the corresponding Ed. as determined in section 3.1. This discrepancy is most probably due to the fact that in section 3.1 the evaluation of Ed for the

M. P. Kiskinoua, G. M. Biiznakor; / Curbon monoxide and hydrogen on Pd(fi I)

73

PI and & states was made assuming the same pre-exponential factor, va. However, as was shown by Erley and Wagner [20] the adsorption site plays an important role in the calculation of vO. Thus, if the Ed value for the j3, and & states does not change significantly with &o, the pre-exponential factor of 2.6 x lOI s-l for the p, state will be in reasonable agreement with AE = 1 kcal mole-‘. This higher value of v~, in comparison with 1.5 X lOI s-’ for vp2 implies that the CO molecules adsorbed in the bridge bonded state are less mobile than those adsorbed in the linear ones. Consequently, it is reasonable to attribute the poor fit of the ESD and TD data in fig. 12 to diffusion from p, to & states provoked by the temperature increase during TD experiments. Certain interconversion between the binding states satisfactorily explains the initial increase of the O+ signal during CO TD at Q, > 0.4 (fig. 4~). The better fit between the ESD and TD data in fig. 12 for T, is 358 and 390 K implies that at this temperatures the convertion to the & state is more efficient and the adsorption systems are closer to equilibrium conditions. The & TD peak and the CO+ signal, recorded at T, is 250 K have most probably the same origin as those suggested by Madey et al. [36] and can be associated with occupation of on top adsorption sites. Indeed, as was reported in ref. [37], at B > 0.5 two structures with hexagonal arrangements were formed where “on-top” positions were also occupied.

As was demonstrated in section 3.3, the presence of saturated hydrogen precoverage affected to a certain extent the CO adsorption. The observed decrease of the CO initial sticking coefficient and the changes in the /3,/& ratio imply that both CO and hydrogen compete for the same adsorption sites. The recent theoretical and experimental studies for hydrogen adsorption on Ni, Pt and Pd( 111) surfaces [38-441 deduced that the hydrogen atoms occupy preferentially three-fold coordinated sites. Thus, it was to be expected that the presence of preadsorbed hydrogen atoms would perturb CO adsorption on the three-fold sites. Indeed, the data shown in fig. 10 illustrate a decrease of the relative & CO coverage, when hydrogen was present. However, with increasing CO exposure the competition for adsorption sites leads to replacement of the adsorbed hydrogen, as shown in fig. 8. The most interesting fact is that the decrease of the hydrogen coverage even upon hydrogen adsorption on a surface precovered with CO is not so strong as reported in ref. [7]. The results in the present study (fig. 8) show that one CO molecule precludes the adsorption of one hydrogen atom at 250 K. Let us consider the changes in the H, TD spectra provoked by CO, i.e. the decrease of the H, amount desorbing under the main “clean surface” peak and the appearance of a broad higher temperature peak. The new peak desorbing in the temperature region where CO is desorbing may be due to:

74

M. P. Kiskinooa, G.M. Bliznakov / Carbon monoxide and hydrogen on Pd(i II)

(1) The formation of a new hydrogen adsorption state in the presence of CO, or a cooperative surface complex, and (2) CO induced penetration of the adsorbed hydrogen below the surface. In the second case the fact that hydrogen desorption within the new peak occurs at temperatures where CO is desorbing, implies that the desorption of the absorbed hydrogen requires free surface sites for recombination. Valuable information, concerning the check of the above asumptions was obtained by studying the behavior of the H+ ESD signal. As is shown in fig. 10, the H+ signal appears when CO is present and increases with 8,.,, upto 8co - 0.3. Further increase of 19,~ causes a decline of the H’ signal which almost disappears when the surface is saturated with CO. During heating, the Ht signal attenuated and disappeared within the temperature interval of CO desorption (fig. 11). It has to be noted that the ESD cross section, q, of H’emitted from the coadsorption layer is extremely high (1.5 X IO- I6 at 80 eV) in comparison with pure hydrogen layer where no H+ emission can be detected. A high ESD H” yield in the presence of coadsorbates was repoted in refs. [42] and 1431 for W and Nb. The observed effect of the coadsorbates (CO, 0) was attributed to an increase of the ionization probability and a reduction of the neutralization rate of the emitted Hf. The observations summarized above suggest that the hydrogen responsible for the H+ ESD signal is adsorbed in intimate contact with CO where certain interaction between the CO(ads) and H(ads) is possible. A similar conclusion was made by Conrad et al. [44] and Yates et al. [45] in their CO and H, coadsorption studies. Thus, according to shown in fig. 10, the existence of the “cooperative” the H’ behavior H(ads)-CO(ads) species is favoured upto ti,.,, - 0.3. The decline of the H’ a displacement of H(ads) and signal at higher 6,o reflects most probably consequently a decrease in concentration of the “cooperative” species. It seems reasonable to asume that the H + signal and the new high temperature H, TD peak have the same origin. However, the following arguments arising from the experimental data ruled out any relation between the H+ signal and the new H, TD peak: (1) The H+ signal appeared at much lower i&o than those at which the higher temperature H, TD peak was collected. (2) The H+ signal decreased at 8,., > 0.3, whereas the intensity of the new H, TD peak achieved a maximum at &.o - 0.35 and does not change with further increase of @,o. (3) The decrease of the H-’ signal upon heating did not cover the temperature interval of the new H, TD peak (see fig. 11). Followirrg the above arguments it is unreasonable to seek any correlation between the high temperature Hz desorption state and the “cooperative” CO(ads)-H(ads) species. Indeed, a more detailed study showed that the new broad H, peak is similar to the TD peaks observed in the coadsorption experiments in refs. [ 1,4,7], attributed to desorption of hydrogen dissolved into

the bulk. Such interpretation of the H, TD peak under consideration implies that besides displacement of the adsorbed hydrogen the presence of CO favours hydrogen penetration under the surface. An interpretation of the promoting effect of CO on hydrogen dissolution was proposed by Ratajczykowa f7] but to our knowledge this problem is not yet clarified. As a matter of fact, we would like to point out that the new broad H, peak was detected even after exposing Pd( 111) saturated with CO (&.o = 0.6) to H,. This indicates that the presence of CO does not substantialiy hinder the absorption of hydrogen, which undoubtedly passes through a surface precursor state. In addition the reverse exposure sequence revealed that part of the adsorbed hydrogen penetrated under the surface rather than desorbed. To our knowledge the mechanism of transition between surface and bulk hydrogen is not completely understood, but in recent molecular beam investigations [25] a kinetic scheme was proposed where H, dissolution requires hydrogen dissociation and the desorption of the adsorbed hydrogen can only occur after recombination in the chemisorption layer. However, in the case of H, adsorption on a clean Pd( 111)for pressures as low as 5 X lo-’ Torr, the dissolution of hydrogen at T < 350 K was found to be negligible [25]. Thus the convertion of adsorbed hydrogen into dissolved in the presence of CO at temperatures lower than 350 K, might be ascribed to the lowering of the activation barrier for penetration under the surface, caused by coadsorbed CU.

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M. P. Kiskinma, GM. Bliznakoo / Curhon monoxide and hydrogen on Pd(lll)

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