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Influence of substrate structure on activity of alumina supported Pd particles: CO adsorption and oxidation
surface ELSEVIER
science
Surface Science 365 (1996) 69-77
Influence of substrate structure on activity of alumina supported Pd particles: CO adsorption and oxidation I. Starer *, V. Nehasil, V. Matolin Department of Electronics and Vacuum Physics, Charles University, V Holegovi~ktich 2, 180 O0 Prague 8, Czech Republic
Received 2 January 1996; accepted for publication 19 March 1996
Abstract Recently the unexpected effects of partial CO dissociation were reported for small Pd particles deposited on y-alumina, prepared by thermal oxidation of aluminium whilst this behaviour was not observed on Pd/ct-alumina model catalysts. In this study we compared the CO adsorption and oxidation properties of Pd particles deposited on a-alumina substrates of different surface stoichiometry with those of Pd on ~,-alumina. It was shown that the oxygen adsorption capacity as well as the reactivity of a-alumina supported catalysts was lower than those of y-alumina supported catalysts. The CO and oxygen sticking probability measurements indicated the CO and O diffusion over the support that in the case of CO decreased with T. The temperature dependent CO oxidation rate was explained in terms of its limitation by the CO diffusion process which is less important in the case of the aluminium rich alumina surface. Keywords: Aluminium oxide; Carbon monoxide; Clusters; Oxidation; Palladium; Surface chemical reaction; Surface diffusion;Thermal
desorption spectroscopy
1. Introduction The investigation of interactions of molecules with the surfaces of transition metals like Pt, Pd, Ni, Rh, R u and others, is i m p o r t a n t for the understanding of surface processes in catalysis. The active phase used in most industrial catalysts consists of small metal particles ranging in size from 1 to 50 n m supported on p o r o u s oxide supports. The surface m o r p h o l o g y , namely the existence of surface irregularities such as step sites with low co-ordination number, can determine the mechanism of catalytic processes [ 1 ]. O n the other h a n d a n u m b e r of catalytic reactions has been c o m m o n l y * Corresponding author. Fax: +422 688 5095; e-mail: [email protected]
considered to be structure insensitive, a m o n g others the C O oxidation over Pt, P d and Rh [ 2 - 5 ] . The shape of small supported metal particles is influenced by the particle size and the particle interaction with supports and adsorbed species. This so-called size effect in catalysis is very complex and it is difficult to investigate it under real, high pressure catalytic conditions. Clearly, it is not possible with any of the surface analytical tools available at the moment. Fortunately in the case of n u m e r o u s reactions, the bridging of the pressure gap does not introduce changes in reaction mechanisms [ 6 ] and studies can be performed with high sensitivity methods under in situ low pressure reaction conditions. The most frequently investigated model catalysts - single crystals - are very different from the real
0039-6028/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0039-6028 (96) 00689-9
70
I. Starfi et al./Surface Science 365 (1996) 69-77
catalytic systems and it is difficult to use them to investigate the size effects. This has led to the utilisation of more realistic model catalysts exhibiting metal particles deposited on a flat support. This approach allows direct comparison of reaction properties of single-crystal surfaces with those of more realistic supported metal catalysts, and thus allows detailed study of structure sensitivity. The results of such investigations often show different surface properties of bulk single crystals and supported clusters [,7-12]. For example, the CO adsorption on 2 nm Pd particles deposited on 7alumina is partially dissociative [,10,11] which is not the case on large particles and continuous Pd surfaces. A very surprising effect is that the CO dissociation on Pd particles is observed on particles deposited on v-alumina and not on ~-alumina [12]. The CO activation energy of desorption for Pd/alumina particles decreases with decreasing size [ 10,12] (for both the a and V modifications) while for Pd/MgO [8] it is the opposite. In order to systematically study the size effect, it is necessary to simultaneously perform the investigations of particle reactivity and structure on well-defined model systems. These model systems can be prepared by means of molecular beam epitaxy on a single crystalline substrate [-7,13-15] giving populations of particles homogeneous in size and morphology. The partial CO dissociation on v-alumina supported Pd particles reported in our previous study [-10] as well as molecular CO adsorption for Pd on an a-alumina support only [12] raised the question for the reason for this behaviour. It seems that any subtle effect of metal-support interaction could be an origin of this phenomenon. Different types of alumina with various surface structure and/or stoichiometry could influence the adsorption properties and reactivity of deposited metal in different ways (for example via particle structure and morphology). The purpose of this work was to contribute to the understanding of effects of Pd-alumina interaction, that could influence the reactivity of deposited catalyst clusters. We compared the CO adsorption and oxidation on Pd particles deposited on different types of alumina surfaces. We prepared them by the heating of a (0001) a-alumina (corun-
dum) single crystal under air, under vacuum, and by Ar + ions sputtering. The AES (Auger electron spectroscopy) data and EELS (electron energy loss spectroscopy) showed that thermal treatment under vacuum or ion bombardment of (0001) aalumina reduces the oxide surface [16]. It led to the formation of a thin aluminium rich layer giving the excitation of aluminium surface plasmons. The a-alumina crystal heated under atmosphere exhibited the best stoichiometry, i.e. the lowest AES IAltOx)/l o intensity ratio and AI plasmon excitation, v-alumina prepared by thermal oxidation of aluminium, has been found to be nonstoichiometric with lack of oxygen. Similar results were reported in Ref. [ 17]. The results of CO desorption and CO oxidation experiments on Pd/a-alumina were compared with the results of the previous study on Pd/v-alumina catalysts.
2.
Experimental
The experiments were carried out in a vacuum system with a base pressure lower than 3 x 10 -8 Pa. For the TPD studies the system was equipped with a specially designed desorption spectrometer described previously [10]. CO and oxygen exposure were made by means of two molecular beam dosers straight to the surface. For these reasons relatively high exposures of the sample were possible, while the low pressure at the chamber was held. The intensity of the molecular beam was about 5 x 1012 molecules cm -2 s -x at the sample surface. The desorbed/scattered molecules were monitored with a differentially pumped computer controlled quadrupole mass spectrometer. A linear heating rate was achieved with a thermoelectric programmer. The small supported palladium particles were deposited in situ by vapour deposition of Pd onto the alumina support from the micro electron beam evaporation source. During the deposition the support temperature was kept at 320 K and the background pressure in the vacuum chamber was lower than 1 x 10 -6 Pa. The samples used in this study (noted A, B and C) were prepared by Pd deposition to different
I. Star6 et al./Surfaee Science 365 (1996) 69-77
types of (0001) s-alumina (corundum) substrates. The substrate of sample A was prepared by heating of a mechanically and chemically polished crystal in air at 1620 K for two hours. The reflection high energy electron diffraction (RHEED) analysis indicated a perfectly smooth surface without steps. The substrate B was prepared by heating in vacuo (10 - 6 Pa) for 30 min at 1570 K. In this case the RHEED showed a smooth surface, too. Pd of sample C was deposited on an ion sputtered substrate, previously treated as in the case of A, at an ion beam energy of 1 keV at 1/~A. cm -2 for 15 min. At the end of the measurement the particles were investigated by TEM using a method of transfer carbon replica [18]. The parameters of these particles are presented in Table 1 where the relative Pd surface area stands for active metal surface related to unity of catalyst surface.
3. Results and discussion
71
sticking coefficient of molecules impinging on the substrate area. For this reason si depends on a ratio of metal/substrate surface. In our case the sticking probability of CO on the bare alumina substrate can be considered as zero. In Fig. 1 measured CO initial integral sticking probabilities Sio,co (i.e. zero coverage sticking coefficient) as a function of substrate temperature T are presented for all Pd/~-alumina samples and are compared with the results obtained for the yalumina substrate and the P d ( l l l ) single crystal from Ref. [20]. Using a known value of CO sticking probability on alumina and the real active surface area of Pd particles, known from TEM observation (for simplicity the particles were approximated by half spheres), a theoretical value Sox,co of integral sticking coefficient can be calculated on the assumption that the So,co on Pd particles is 0.96 as on the P d ( l l l ) [21]. The room temperature values of si0,co and SoT,co for different samples are compared in Table 2. It can be seen that for small particles Sio,co is several times larger
3.1. CO adsorption and desorption The sticking coefficient s(O) (0 stands for coverage) can be investigated by continuous monitoring of intensities of scattered molecular fluxes during exposure. The sticking probability s is derived from these curves according to the relationship s(t)= 1 I(t)/Io where I o is the incident beam intensity and I(t) the intensity of scattered molecules in time t. The time dependence can easily be converted into a variation with relative coverage. The details of this method are given in Ref. 1-19]. In the case of supported metal particles the measured sticking coefficient si has an integral character. It consists of a sticking coefficient of gas molecules reaching the metal surface and of a
1,0 t
•
0,8
-
0 0
6
0,6 ,\
...........
~< . . . . . . . . . . . . .
0,4
>.~)< . . . . . .
i~
Ilk
",,
"',
A\
0,2
0 . ',
,
I ' , ',
Table 1 Morphological parameters of Pd particle samples Sample
Pd/y-al. Pd/ct-al.(A) Pd/ct-al.(B) Pd/~-al.(C)
Size (nm)
Density (10 lz cm -2)
Relative Pd surface area
2.5 2.0 1.8 1.9
1.1 1.14 2.7 2.6
0.1 0.07 0.13 0.15
0,0
300
~
~
~
350
400
450
"'~
500
T [K] Fig. 1. Integral initial sticking coefficient of CO versus temperature: ( . - - ) P d ( l l l ) , (X - - ) Pd/~,-alumina, (O . . . . ) Pd/~alumina (A), (A . . . . ) Pd/~-alumina (B), (11 ......) Pd/~-alumina (C).
I. Star~ et al./Surface Science 365 (1996) 69-77
72
Table 2 Initial sticking coefficients and radius of capture zones for C O and oxygen adsorption at room temperature Sample
Sio,co
SOT,CO
Sio/SoT,CO
Rco(nm)
Sio,o
SiO/SOT,O
Ro(nm)
Pd(111) Pd/y-al. Pd/a-al.(A) Pd/a-al.(B) Pd/c~-al.(C)
0.93 0.51 0.71 0.50 0.43
0.96 0.10 0.07 0.13 0.14
-
3.8 4.3 2.2 2.1
0.77 0.18 0.21 0.19 0.14
3.0 5.1 2.2 1.8
2.3 2,4 1.4 1,2
5 10 3.8 3
than S0T,CO as it was reported before 1-20]. For supported Pd clusters, which represent only several percent of the whole sample surface, the real sticking coefficient of CO would be higher than unity which is not possible in principle. This behaviour can be explained by the diffusion of CO molecules on the substrate, a commonly observed phenomenon in the case of discontinuous supported catalysts I20,22-24]. The amount of adsorbed molecules can be considered as composed of two parts: the first part is given by the number of chemisorbed CO molecules after straight collision with Pd clusters, and the second part by the amount of gas molecules captured by the substrate. We can consider that hypothetical circular capture zones of radius Rco, can be drawn around the particles [-23]. The sticking probability inside the capture zone is 1, whilst outside it is zero. If the diffusion length of CO diffusing on the substrate is long, the radius Rco and consequently Sio,co have to be large, too. It should be noted that for high density particles Rco is limited by the overlapping of zones. Rco can easily be calculated from Sio,co, the particle size and density, assuming So,co to be 0.96 for Pd and 1 inside the capture zone. The obtained values of Rco are presented in Table 2. Due to the fact that in all cases Rco values were not limited by the average distance between the nearest neighbour particles, they show that the diffusion is highest for catalyst A and lower for the Pd/yalumina sample. The lowest values were obtained for samples B and C. The substrates of these samples had the reduced surface covered by an aluminium rich overlayer [16]. Thus it seems probable that the effect of CO diffusion depends on the stoichiometry of the alumina surface. The plots of Fig. 1 show that Sio is constant up
to 450 K for P d ( l l l ) . For Pd particles on ~alumina Sio,cO decreases with increasing T above 375 K. For Pd/7-alumina Sio,coexhibits an approximately linear slow decrease with temperature. Variations of Sio,co are probably given by the decrease in the number of CO molecules captured through the diffusion zones that were reduced as long as the mean diffusion length of CO molecules on the alumina support decreased with T. Comparing the curves in Fig. 1 it seems that the diffusion process is more temperature dependent in the case of the ~-alumina substrate than in tb~ case of 7-alumina. The properties of Pd catalysts were investigated by means of temperature programmed desorption (TPD) of CO from small Pd particles for different types of alumina substrates. The desorption spectra for saturation CO coverage of A, B, C and Pd/yalumina are compared with the desorption feature of a Pd(111) single crystal in Fig. 2. It can be seen that the desorption properties vary with type of substrate. The TPD curves from supported Pd particles exhibit 2 peaks - a low energy desorption peak at 375 K of various intensity and a second peak above 450 K. In contrast to the Pd/o~-alumina systems there is well-resolved double-peak structure of desorption profiles on the y-alumina supported Pd particles. On the single crystal the TPD curve is found at somewhat higher temperature than on the supported catalysts, and its shape can be explained by the lateral interaction between the adsorbates [25]. The ~-alumina supported particles have TPD features exhibiting less important low energy desorption peaks, and thus they resemble those of the P d ( l l l ) much more than the Pd/ y-alumina ones. One can advance a hypothesis that the a-alumina supported Pd particles are morphologically closer to the Pd (111) surface. For
I. Star~ et al./Surface Science 365 (1996) 69-77
Pd
73
Pdly
(111)
E = 121 kJ/mol
.
,
300
•
,
350
.
,
400
.
,
450
.
,
500
.
•
550
600
300
,
350
.
,
400
T [K]
.
,
450
•
,
.
,
500
550
•
600
T [K]
B
A
E = 116 kJ/mo
O
o.
I1.
"
300
350
400
450
500
550
600
300
350
400
T [Iq
450
500
-'E
550
600
T [K]
C
-
300
,
350
•
,
400
•
J
•
450
i
500
•
,
550
•
600
T [K]
Fig. 2. T P D curves for saturation CO coverage.
example, the particles could be limited rather by low-index planes without steps (we proposed the existence of surface steps in the case of 7-alumina supported Pd particles in Ref. [ 10]). The activation energy of desorption, determined using Readhead's method [26] for the highest peaks of desorption spectra at saturation coverage, was found to be 116, 118, 120, 121 and 125 kJ mo1-1 for samples B, C, A, Pd/~,-alumina and
Pd(111), respectively. In respect to the inaccuracy of activation energy determination (units of kJ mol-1) it can be seen that the value of activation energy of desorption for supported particles are nearly the same. The most important difference between these catalysts was the CO2 production via a disproportionation reaction 2 C O ~ C + CO2 accompanying the CO desorption from the 7-alumina supported sample. The CO2 pro-
/. Star(t et al./Surface Science 365 (1996) 69-77
74
duction indicated the partial CO dissociation [ 10]. This behaviour was not observed on a-alumina supported catalysts.
3.2. Oxygen adsorption
-
The sticking probability of oxygen adsorption has been measured in the same manner as CO adsorption. After each oxygen scattering experiment the adsorbed oxygen was reacted off by CO titration. The integral sticking coefficient Sio,o as well as the ratio SiO,o/SoT,O are reported in Table 2. The high sticking coefficient of oxygen indicates the oxygen surface diffusion across the substrate as it was reported for )'-alumina supported particles before [27]. The radius of capture zone Ro was calculated assuming the oxygen sticking coefficient on Pd to be 0.6 [28]. Ro is presented in Table 2, too. It can be seen that both Rco and R o vary in the same way for different substrates. In contrast to the CO adsorption results, the oxygen sticking coefficient was found to be temperature independent for all supported Pd samples. This can be explained by assuming the temperature independence of oxygen surface diffusion. The concentration of adsorbed oxygen atoms noaa has been found from the variation of the intensity of scattered oxygen molecules with time for more details about the method see Ref. [27]. The noaa values for 300 and 430 K are compared in Table 3. The difference between the 300 and 430 K values is given by the dependence of oxygen saturation coverage on temperature. It can be seen that on the small supported particles no~d is generally higher than on the Pd single crystal. Namely the Pd particles on ),-alumina exhibited abnormal adsorption of oxygen that has been explained by subsurface oxygen diffusion [29]. Table 3 Oxygen saturation concentrations at 300 and 430 K Sample
no*a(430 K) c m - 2
no.a(300 K) c m - 2
Pd(lll) Pd/y A B C
1.3 x 1014 1.2 x 1015 6.8 X 1014 4.9 x 10 TM 4.2 x 1014
4.0 x 1014 3.0 x 1015 8.7 X 1014 9.1 × 1014 8.3 x 101'*
3.3. CO oxidation All results are reported for the transient experiments in which the surface was predosed with a saturation coverage of oxygen and then was exposed to the molecular beam of CO. The CO molecular beam was switched on at time t = 0 and thus gave a steplike CO2 pressure rise from a background-level. The intensities of the CO2 flux produced by surface reaction, I CO2, were continuously monitored during the CO exposure at different substrate temperatures T. This method was described in detail previously [29]. Assuming the Langmuir-Hinshelwood mechanism of reaction [30], each dissociatively preadsorbed oxygen was removed from the adlayer in the form of a CO2 molecule by the reaction with adsorbed CO molecules. The non-steady-state rates of CO2 formation for 2.5 nm Pd particles on v-alumina and a-alumina (A), respectively, are plotted for several substrate temperatures in Figs. 3 and 4. In each figure the curves are of the same vertical scale but they have been displaced vertically for clarity. By comparing the plots from Figs. 3 and 4, a striking difference between the form of the curves can be seen. The CO2 formation for the Pd/ y-alumina sample exhibits a sharp maximum at the beginning of the reaction whilst in Fig. 4 the reaction rate is nearly constant during the first several tens of seconds. Furthermore the length of this plateau increases with increasing temperature up to 100 s at 500 K. The other samples of Pd/a-alumina (B, C) show qualitatively the same behaviour. In Fig. 5 the CO2 reaction curves of all investigated samples are compared for T = 430 K corresponding to the maximum reaction rate. All curves are of the same vertical scale, except the curve Pd/ )'-alumina that is plotted with half the sensitivity. The most interesting effect which can be seen in Fig. 5 is the difference in the total amount of CO2 produced during the reaction. Due to the stoichiometry of the reaction C O a d " ~ - O a d " ' ~ C O 2 the amount of CO2 produced during the reaction related to the unity of Pd surface should correspond to the initial concentration of adsorbed oxygen atoms noad. noaa has been found from the variation of the intensity of scattered oxygen mole-
I. Star(t et aL/Surface Science 365 (1996) 69-77
7 °n
I
75
beam on
31)0K
350K
e-
P,
E 430 K
O
O
"6 450K
500K
0
!
!
I
1O0
200
300
400
Time [s]
Fig. 3. Non-steady-state rates of CO2 formation versus time for Pd/7-alumina.
cules with time (described above), the CO2 quantity by integrating the CO2 curves. Both values are in good agreement for all samples showing that all adsorbed oxygen is really converted into CO2 by the reaction. The noad values for 430 K are compared in Table 3. It can be seen that on the small supported particles noaa was generally higher than on the Pd single crystal. Namely the Pd particles on y-alumina exhibited abnormal adsorption of oxygen. Consequently the total CO2 production was the highest for this sample. The difference between the shape of reaction curves of v-alumina and ~-alumina supported particles (the sharp maximum is missing for the second ones) shows that the v-alumina supported particles are more reactive. The mentioned difference reports additional evidence of the specific properties of this model catalyst which are: (i) the partial CO dissociation, (ii) high oxygen coverage and (iii) high CO oxidation activity. We are not able to give a satisfying explanation
I
I
I
I
0
100
200
300
400
Time [s]
Fig. 4. Non-steady-state rates of CO2 formation versus time for Pd/ct-alumina (A).
of the high CO oxidation activity effect at the moment. Nevertheless the following hypothesis can be advanced. Supposing that each CO molecule reaching the Pd surface reacts with an O atom, the reaction rate could be limited by the reactant species transport-diffusion of adsorbed CO molecules. As we have mentioned above, the CO diffusion over the ~-alumina substrate decreases rapidly with increasing surface temperature, which is not the case for y-alumina. Therefore the time extended plateau of CO2 production with increasing T (see Figs. 4 and 5) for ~-alumina could be explained by a decrease of the capture zone surface. Consequently the number of CO molecules captured by every particle per unity of time should be lower for or-alumina supported catalysts. The TPD experiments of CO adsorption on yalumina supported particles show the increase of total CO adsorption capacity (nearly two times) after the reaction experiments described above. Therefore, a further phenomenon, the oxygen induced reversible reconstruction or redispersion
76
I. Star& et aL /Surface Science 365 (1996) 69-77
430K
beam on
~...~....~
. . . . .
Pd (111)
Pd/~,
~o
0
100
200
x0 -
300
400
Time [s]
Fig. 5. Non-steady-staterates of CO2 formationversus time for all samples at 430 K. of particles, could contribute to the explanation of the observed effects. Decrease of particle size and simultaneous increase of their density would lead to the enhancement of oxygen adsorption capacity, to the increase of contribution of CO diffusion over the substrate and a decrease in activation energy of CO oxidation [29]. It seems very probable that there is a link between the effects of partial CO dissociation and high oxygen adsorption and high reactivity of ~alumina supported Pd particles. This could be the consequence of particle interaction with the underlying substrate. Actually we are continuing this work in order to determine in situ particle morphology variation during the CO adsorption and oxidation processes.
a-alumina supports were prepared in different ways in order to obtain more or less stoichiometric surfaces. It was shown that the CO and oxygen molecules diffusing across the ~,-or a-alumina substrate increase the flux of molecules captured by particles during the adsorption processes. The radius of capture zones was estimated from the integral sticking probabilities and known average particle size. Both the CO as well as the oxygen diffusion were found to be more important for stoichiometric a-alumina than for aluminium rich a- and ~,alumina surfaces. On the other hand, the integral sticking coefficient of CO depending on the diffusion length decreases more rapidly with increasing Ton a-alumina than on 7-alumina. This behaviour can explain the lower CO oxidation activity of Pd/ a-alumina particles at higher temperatures. The amount of adsorbed oxygen is higher on supported Pd particles than on the single crystal. This effect is more pronounced on Pd/y-alumina particles. Two explanations seem to be probable, the subsurface O diffusion and/or oxygen induced particle reconstruction. CO TPD experiments show the different CO desorption features for different substrates. Only small variations of the high temperature desorption peak position were observed giving slight variations with activation energy of desorption between 116 and 121 kJ m o l - < The CO dissociation, reported previously for the ~,-alumina support, was not observed on Pd/aalumina. So, the choice of support plays a major role in determining the unexpected effects observed on alumina supported Pd model catalysts. The studies permitting to relate the Pd particle adsorption and reaction properties to their size and morphology are in progress.
References 4. Summary In this study we compared the CO adsorption and oxidation properties of Pd(111) single crystal, 7-alumina and a-alumina supported Pd particles.
[1] G.A. Somorjai,Adv. Catal. 26 (1977) 1. [2] J.A. Rodriguez and D.W. Goodman, Surf. Sci. Rep. 14 (1991) 27. I3] G.A. Somorjai, Chemistry in Two Dimensions (Cornell University Press, London, 1981) p. 483. [4] M. Boudart and G. Dj6ga-Mariadassou, Kinetics of
L Star~ et al./Surface Science 365 (1996) 69-77
Heterogeneous Catalytic Reactions (Princeton University Press, Princeton, NJ, 1984) p. 106. [5] G.A. Somorjai and M.A. Van Hove, Progr. in Surf. Sci. 30 (1989) 201. [6] T. Engel and E. Ertl, in: The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 4, Eds. D.A. King and D.P. Woodruff (North-Holland, Amsterdam, 1982) p. 73. [7] D.L. Doering, J.T. Dickinson and H. Poppa, J. Catal. 73 (1982) 104. [8] C.R. Henry, C. Chapon, C. Goyhenex and R. Monot, Surf. Sci. 272 (1992) 283. [9"] V. Matolin, E. Gillet, N. Reed and J. Vickerman, J. Chem. Soc. Faraday Trans. 85(15) (1990) 2749. [10"] I. Star~ and V. Matolin, Surf. Sci. 313 (1994) 99. [11"] V. Matolin and E. Gillet, Surf. Sci. 238 (1990) 75. [12] H. Cordatos, T. Bunluevin and R.J. Gorte, Surf. Sci. 323 (1995) 219. [13] M. Gillet and S. Channakhone, J. Catal. 97 (1986) 427. [14] M. Gillet and V. Matolin, J. Crystal Growth 134 (1993) 75. [15] K. Masek and V. Matolin, Thin Solid Films (1995), to be published.
77
[16] I. Star,i, D. Zeze, V. Matolin, J. Pavluch and B. Gruzza, Appl. Surf. Sci (1995) submitted. [17] B. Ealet, M.H. Elyakhloufi, E. Gillet and M. Ricci, Thin Solid. Films 250 (1994) 92. [18] M.F. Gillet and S. Channakhone, J. Catal. 97 (1986) 427. [19] G. Ertl, Surf. Sci. 89 (1979) 525. [20] I. Star~, E. Tomkov~i and V. Matolin, Czech. J. Phys. 43 (1993) 1023. [21] T. Engel and G. Ertl, Adv. Catal. 28 (1979) 1. [22] V. Matolin and E. Gillet, Surf. Sci. 166 (1986) Ll15. [23] F. Rumpf, H. Poppa and M. Boudart, Langrnuir 4 (1988) 115. [24] C.R. Henry, C. Chapon and C. Duriez, Z. Phys. D - Atoms, Molecules and Clusters 19 (1991) 347. [25] T. Engel and G. Ertl, Adv. Catal. 28 (1979) 14. [26] P.A. Readhead, Vacuum 12 (1962) 203. [27] I. Star~i and V. Matolin, Fizika A4 2 (1995) 163. [28] T. Engel, J. Chem. Phys. 69 (1978) 373. [29] I. Star,t, V. Nehasil and V. Matolin, Surf. Sci. 331-333. (1995) 173. [30] T. Engel and G. Ertl, J. Chem. Phys. 69 (1978) 1267.
science
Surface Science 365 (1996) 69-77
Influence of substrate structure on activity of alumina supported Pd particles: CO adsorption and oxidation I. Starer *, V. Nehasil, V. Matolin Department of Electronics and Vacuum Physics, Charles University, V Holegovi~ktich 2, 180 O0 Prague 8, Czech Republic
Received 2 January 1996; accepted for publication 19 March 1996
Abstract Recently the unexpected effects of partial CO dissociation were reported for small Pd particles deposited on y-alumina, prepared by thermal oxidation of aluminium whilst this behaviour was not observed on Pd/ct-alumina model catalysts. In this study we compared the CO adsorption and oxidation properties of Pd particles deposited on a-alumina substrates of different surface stoichiometry with those of Pd on ~,-alumina. It was shown that the oxygen adsorption capacity as well as the reactivity of a-alumina supported catalysts was lower than those of y-alumina supported catalysts. The CO and oxygen sticking probability measurements indicated the CO and O diffusion over the support that in the case of CO decreased with T. The temperature dependent CO oxidation rate was explained in terms of its limitation by the CO diffusion process which is less important in the case of the aluminium rich alumina surface. Keywords: Aluminium oxide; Carbon monoxide; Clusters; Oxidation; Palladium; Surface chemical reaction; Surface diffusion;Thermal
desorption spectroscopy
1. Introduction The investigation of interactions of molecules with the surfaces of transition metals like Pt, Pd, Ni, Rh, R u and others, is i m p o r t a n t for the understanding of surface processes in catalysis. The active phase used in most industrial catalysts consists of small metal particles ranging in size from 1 to 50 n m supported on p o r o u s oxide supports. The surface m o r p h o l o g y , namely the existence of surface irregularities such as step sites with low co-ordination number, can determine the mechanism of catalytic processes [ 1 ]. O n the other h a n d a n u m b e r of catalytic reactions has been c o m m o n l y * Corresponding author. Fax: +422 688 5095; e-mail: [email protected]
considered to be structure insensitive, a m o n g others the C O oxidation over Pt, P d and Rh [ 2 - 5 ] . The shape of small supported metal particles is influenced by the particle size and the particle interaction with supports and adsorbed species. This so-called size effect in catalysis is very complex and it is difficult to investigate it under real, high pressure catalytic conditions. Clearly, it is not possible with any of the surface analytical tools available at the moment. Fortunately in the case of n u m e r o u s reactions, the bridging of the pressure gap does not introduce changes in reaction mechanisms [ 6 ] and studies can be performed with high sensitivity methods under in situ low pressure reaction conditions. The most frequently investigated model catalysts - single crystals - are very different from the real
0039-6028/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved PII S0039-6028 (96) 00689-9
70
I. Starfi et al./Surface Science 365 (1996) 69-77
catalytic systems and it is difficult to use them to investigate the size effects. This has led to the utilisation of more realistic model catalysts exhibiting metal particles deposited on a flat support. This approach allows direct comparison of reaction properties of single-crystal surfaces with those of more realistic supported metal catalysts, and thus allows detailed study of structure sensitivity. The results of such investigations often show different surface properties of bulk single crystals and supported clusters [,7-12]. For example, the CO adsorption on 2 nm Pd particles deposited on 7alumina is partially dissociative [,10,11] which is not the case on large particles and continuous Pd surfaces. A very surprising effect is that the CO dissociation on Pd particles is observed on particles deposited on v-alumina and not on ~-alumina [12]. The CO activation energy of desorption for Pd/alumina particles decreases with decreasing size [ 10,12] (for both the a and V modifications) while for Pd/MgO [8] it is the opposite. In order to systematically study the size effect, it is necessary to simultaneously perform the investigations of particle reactivity and structure on well-defined model systems. These model systems can be prepared by means of molecular beam epitaxy on a single crystalline substrate [-7,13-15] giving populations of particles homogeneous in size and morphology. The partial CO dissociation on v-alumina supported Pd particles reported in our previous study [-10] as well as molecular CO adsorption for Pd on an a-alumina support only [12] raised the question for the reason for this behaviour. It seems that any subtle effect of metal-support interaction could be an origin of this phenomenon. Different types of alumina with various surface structure and/or stoichiometry could influence the adsorption properties and reactivity of deposited metal in different ways (for example via particle structure and morphology). The purpose of this work was to contribute to the understanding of effects of Pd-alumina interaction, that could influence the reactivity of deposited catalyst clusters. We compared the CO adsorption and oxidation on Pd particles deposited on different types of alumina surfaces. We prepared them by the heating of a (0001) a-alumina (corun-
dum) single crystal under air, under vacuum, and by Ar + ions sputtering. The AES (Auger electron spectroscopy) data and EELS (electron energy loss spectroscopy) showed that thermal treatment under vacuum or ion bombardment of (0001) aalumina reduces the oxide surface [16]. It led to the formation of a thin aluminium rich layer giving the excitation of aluminium surface plasmons. The a-alumina crystal heated under atmosphere exhibited the best stoichiometry, i.e. the lowest AES IAltOx)/l o intensity ratio and AI plasmon excitation, v-alumina prepared by thermal oxidation of aluminium, has been found to be nonstoichiometric with lack of oxygen. Similar results were reported in Ref. [ 17]. The results of CO desorption and CO oxidation experiments on Pd/a-alumina were compared with the results of the previous study on Pd/v-alumina catalysts.
2.
Experimental
The experiments were carried out in a vacuum system with a base pressure lower than 3 x 10 -8 Pa. For the TPD studies the system was equipped with a specially designed desorption spectrometer described previously [10]. CO and oxygen exposure were made by means of two molecular beam dosers straight to the surface. For these reasons relatively high exposures of the sample were possible, while the low pressure at the chamber was held. The intensity of the molecular beam was about 5 x 1012 molecules cm -2 s -x at the sample surface. The desorbed/scattered molecules were monitored with a differentially pumped computer controlled quadrupole mass spectrometer. A linear heating rate was achieved with a thermoelectric programmer. The small supported palladium particles were deposited in situ by vapour deposition of Pd onto the alumina support from the micro electron beam evaporation source. During the deposition the support temperature was kept at 320 K and the background pressure in the vacuum chamber was lower than 1 x 10 -6 Pa. The samples used in this study (noted A, B and C) were prepared by Pd deposition to different
I. Star6 et al./Surfaee Science 365 (1996) 69-77
types of (0001) s-alumina (corundum) substrates. The substrate of sample A was prepared by heating of a mechanically and chemically polished crystal in air at 1620 K for two hours. The reflection high energy electron diffraction (RHEED) analysis indicated a perfectly smooth surface without steps. The substrate B was prepared by heating in vacuo (10 - 6 Pa) for 30 min at 1570 K. In this case the RHEED showed a smooth surface, too. Pd of sample C was deposited on an ion sputtered substrate, previously treated as in the case of A, at an ion beam energy of 1 keV at 1/~A. cm -2 for 15 min. At the end of the measurement the particles were investigated by TEM using a method of transfer carbon replica [18]. The parameters of these particles are presented in Table 1 where the relative Pd surface area stands for active metal surface related to unity of catalyst surface.
3. Results and discussion
71
sticking coefficient of molecules impinging on the substrate area. For this reason si depends on a ratio of metal/substrate surface. In our case the sticking probability of CO on the bare alumina substrate can be considered as zero. In Fig. 1 measured CO initial integral sticking probabilities Sio,co (i.e. zero coverage sticking coefficient) as a function of substrate temperature T are presented for all Pd/~-alumina samples and are compared with the results obtained for the yalumina substrate and the P d ( l l l ) single crystal from Ref. [20]. Using a known value of CO sticking probability on alumina and the real active surface area of Pd particles, known from TEM observation (for simplicity the particles were approximated by half spheres), a theoretical value Sox,co of integral sticking coefficient can be calculated on the assumption that the So,co on Pd particles is 0.96 as on the P d ( l l l ) [21]. The room temperature values of si0,co and SoT,co for different samples are compared in Table 2. It can be seen that for small particles Sio,co is several times larger
3.1. CO adsorption and desorption The sticking coefficient s(O) (0 stands for coverage) can be investigated by continuous monitoring of intensities of scattered molecular fluxes during exposure. The sticking probability s is derived from these curves according to the relationship s(t)= 1 I(t)/Io where I o is the incident beam intensity and I(t) the intensity of scattered molecules in time t. The time dependence can easily be converted into a variation with relative coverage. The details of this method are given in Ref. 1-19]. In the case of supported metal particles the measured sticking coefficient si has an integral character. It consists of a sticking coefficient of gas molecules reaching the metal surface and of a
1,0 t
•
0,8
-
0 0
6
0,6 ,\
...........
~< . . . . . . . . . . . . .
0,4
>.~)< . . . . . .
i~
Ilk
",,
"',
A\
0,2
0 . ',
,
I ' , ',
Table 1 Morphological parameters of Pd particle samples Sample
Pd/y-al. Pd/ct-al.(A) Pd/ct-al.(B) Pd/~-al.(C)
Size (nm)
Density (10 lz cm -2)
Relative Pd surface area
2.5 2.0 1.8 1.9
1.1 1.14 2.7 2.6
0.1 0.07 0.13 0.15
0,0
300
~
~
~
350
400
450
"'~
500
T [K] Fig. 1. Integral initial sticking coefficient of CO versus temperature: ( . - - ) P d ( l l l ) , (X - - ) Pd/~,-alumina, (O . . . . ) Pd/~alumina (A), (A . . . . ) Pd/~-alumina (B), (11 ......) Pd/~-alumina (C).
I. Star~ et al./Surface Science 365 (1996) 69-77
72
Table 2 Initial sticking coefficients and radius of capture zones for C O and oxygen adsorption at room temperature Sample
Sio,co
SOT,CO
Sio/SoT,CO
Rco(nm)
Sio,o
SiO/SOT,O
Ro(nm)
Pd(111) Pd/y-al. Pd/a-al.(A) Pd/a-al.(B) Pd/c~-al.(C)
0.93 0.51 0.71 0.50 0.43
0.96 0.10 0.07 0.13 0.14
-
3.8 4.3 2.2 2.1
0.77 0.18 0.21 0.19 0.14
3.0 5.1 2.2 1.8
2.3 2,4 1.4 1,2
5 10 3.8 3
than S0T,CO as it was reported before 1-20]. For supported Pd clusters, which represent only several percent of the whole sample surface, the real sticking coefficient of CO would be higher than unity which is not possible in principle. This behaviour can be explained by the diffusion of CO molecules on the substrate, a commonly observed phenomenon in the case of discontinuous supported catalysts I20,22-24]. The amount of adsorbed molecules can be considered as composed of two parts: the first part is given by the number of chemisorbed CO molecules after straight collision with Pd clusters, and the second part by the amount of gas molecules captured by the substrate. We can consider that hypothetical circular capture zones of radius Rco, can be drawn around the particles [-23]. The sticking probability inside the capture zone is 1, whilst outside it is zero. If the diffusion length of CO diffusing on the substrate is long, the radius Rco and consequently Sio,co have to be large, too. It should be noted that for high density particles Rco is limited by the overlapping of zones. Rco can easily be calculated from Sio,co, the particle size and density, assuming So,co to be 0.96 for Pd and 1 inside the capture zone. The obtained values of Rco are presented in Table 2. Due to the fact that in all cases Rco values were not limited by the average distance between the nearest neighbour particles, they show that the diffusion is highest for catalyst A and lower for the Pd/yalumina sample. The lowest values were obtained for samples B and C. The substrates of these samples had the reduced surface covered by an aluminium rich overlayer [16]. Thus it seems probable that the effect of CO diffusion depends on the stoichiometry of the alumina surface. The plots of Fig. 1 show that Sio is constant up
to 450 K for P d ( l l l ) . For Pd particles on ~alumina Sio,cO decreases with increasing T above 375 K. For Pd/7-alumina Sio,coexhibits an approximately linear slow decrease with temperature. Variations of Sio,co are probably given by the decrease in the number of CO molecules captured through the diffusion zones that were reduced as long as the mean diffusion length of CO molecules on the alumina support decreased with T. Comparing the curves in Fig. 1 it seems that the diffusion process is more temperature dependent in the case of the ~-alumina substrate than in tb~ case of 7-alumina. The properties of Pd catalysts were investigated by means of temperature programmed desorption (TPD) of CO from small Pd particles for different types of alumina substrates. The desorption spectra for saturation CO coverage of A, B, C and Pd/yalumina are compared with the desorption feature of a Pd(111) single crystal in Fig. 2. It can be seen that the desorption properties vary with type of substrate. The TPD curves from supported Pd particles exhibit 2 peaks - a low energy desorption peak at 375 K of various intensity and a second peak above 450 K. In contrast to the Pd/o~-alumina systems there is well-resolved double-peak structure of desorption profiles on the y-alumina supported Pd particles. On the single crystal the TPD curve is found at somewhat higher temperature than on the supported catalysts, and its shape can be explained by the lateral interaction between the adsorbates [25]. The ~-alumina supported particles have TPD features exhibiting less important low energy desorption peaks, and thus they resemble those of the P d ( l l l ) much more than the Pd/ y-alumina ones. One can advance a hypothesis that the a-alumina supported Pd particles are morphologically closer to the Pd (111) surface. For
I. Star~ et al./Surface Science 365 (1996) 69-77
Pd
73
Pdly
(111)
E = 121 kJ/mol
.
,
300
•
,
350
.
,
400
.
,
450
.
,
500
.
•
550
600
300
,
350
.
,
400
T [K]
.
,
450
•
,
.
,
500
550
•
600
T [K]
B
A
E = 116 kJ/mo
O
o.
I1.
"
300
350
400
450
500
550
600
300
350
400
T [Iq
450
500
-'E
550
600
T [K]
C
-
300
,
350
•
,
400
•
J
•
450
i
500
•
,
550
•
600
T [K]
Fig. 2. T P D curves for saturation CO coverage.
example, the particles could be limited rather by low-index planes without steps (we proposed the existence of surface steps in the case of 7-alumina supported Pd particles in Ref. [ 10]). The activation energy of desorption, determined using Readhead's method [26] for the highest peaks of desorption spectra at saturation coverage, was found to be 116, 118, 120, 121 and 125 kJ mo1-1 for samples B, C, A, Pd/~,-alumina and
Pd(111), respectively. In respect to the inaccuracy of activation energy determination (units of kJ mol-1) it can be seen that the value of activation energy of desorption for supported particles are nearly the same. The most important difference between these catalysts was the CO2 production via a disproportionation reaction 2 C O ~ C + CO2 accompanying the CO desorption from the 7-alumina supported sample. The CO2 pro-
/. Star(t et al./Surface Science 365 (1996) 69-77
74
duction indicated the partial CO dissociation [ 10]. This behaviour was not observed on a-alumina supported catalysts.
3.2. Oxygen adsorption
-
The sticking probability of oxygen adsorption has been measured in the same manner as CO adsorption. After each oxygen scattering experiment the adsorbed oxygen was reacted off by CO titration. The integral sticking coefficient Sio,o as well as the ratio SiO,o/SoT,O are reported in Table 2. The high sticking coefficient of oxygen indicates the oxygen surface diffusion across the substrate as it was reported for )'-alumina supported particles before [27]. The radius of capture zone Ro was calculated assuming the oxygen sticking coefficient on Pd to be 0.6 [28]. Ro is presented in Table 2, too. It can be seen that both Rco and R o vary in the same way for different substrates. In contrast to the CO adsorption results, the oxygen sticking coefficient was found to be temperature independent for all supported Pd samples. This can be explained by assuming the temperature independence of oxygen surface diffusion. The concentration of adsorbed oxygen atoms noaa has been found from the variation of the intensity of scattered oxygen molecules with time for more details about the method see Ref. [27]. The noaa values for 300 and 430 K are compared in Table 3. The difference between the 300 and 430 K values is given by the dependence of oxygen saturation coverage on temperature. It can be seen that on the small supported particles no~d is generally higher than on the Pd single crystal. Namely the Pd particles on ),-alumina exhibited abnormal adsorption of oxygen that has been explained by subsurface oxygen diffusion [29]. Table 3 Oxygen saturation concentrations at 300 and 430 K Sample
no*a(430 K) c m - 2
no.a(300 K) c m - 2
Pd(lll) Pd/y A B C
1.3 x 1014 1.2 x 1015 6.8 X 1014 4.9 x 10 TM 4.2 x 1014
4.0 x 1014 3.0 x 1015 8.7 X 1014 9.1 × 1014 8.3 x 101'*
3.3. CO oxidation All results are reported for the transient experiments in which the surface was predosed with a saturation coverage of oxygen and then was exposed to the molecular beam of CO. The CO molecular beam was switched on at time t = 0 and thus gave a steplike CO2 pressure rise from a background-level. The intensities of the CO2 flux produced by surface reaction, I CO2, were continuously monitored during the CO exposure at different substrate temperatures T. This method was described in detail previously [29]. Assuming the Langmuir-Hinshelwood mechanism of reaction [30], each dissociatively preadsorbed oxygen was removed from the adlayer in the form of a CO2 molecule by the reaction with adsorbed CO molecules. The non-steady-state rates of CO2 formation for 2.5 nm Pd particles on v-alumina and a-alumina (A), respectively, are plotted for several substrate temperatures in Figs. 3 and 4. In each figure the curves are of the same vertical scale but they have been displaced vertically for clarity. By comparing the plots from Figs. 3 and 4, a striking difference between the form of the curves can be seen. The CO2 formation for the Pd/ y-alumina sample exhibits a sharp maximum at the beginning of the reaction whilst in Fig. 4 the reaction rate is nearly constant during the first several tens of seconds. Furthermore the length of this plateau increases with increasing temperature up to 100 s at 500 K. The other samples of Pd/a-alumina (B, C) show qualitatively the same behaviour. In Fig. 5 the CO2 reaction curves of all investigated samples are compared for T = 430 K corresponding to the maximum reaction rate. All curves are of the same vertical scale, except the curve Pd/ )'-alumina that is plotted with half the sensitivity. The most interesting effect which can be seen in Fig. 5 is the difference in the total amount of CO2 produced during the reaction. Due to the stoichiometry of the reaction C O a d " ~ - O a d " ' ~ C O 2 the amount of CO2 produced during the reaction related to the unity of Pd surface should correspond to the initial concentration of adsorbed oxygen atoms noad. noaa has been found from the variation of the intensity of scattered oxygen mole-
I. Star(t et aL/Surface Science 365 (1996) 69-77
7 °n
I
75
beam on
31)0K
350K
e-
P,
E 430 K
O
O
"6 450K
500K
0
!
!
I
1O0
200
300
400
Time [s]
Fig. 3. Non-steady-state rates of CO2 formation versus time for Pd/7-alumina.
cules with time (described above), the CO2 quantity by integrating the CO2 curves. Both values are in good agreement for all samples showing that all adsorbed oxygen is really converted into CO2 by the reaction. The noad values for 430 K are compared in Table 3. It can be seen that on the small supported particles noaa was generally higher than on the Pd single crystal. Namely the Pd particles on y-alumina exhibited abnormal adsorption of oxygen. Consequently the total CO2 production was the highest for this sample. The difference between the shape of reaction curves of v-alumina and ~-alumina supported particles (the sharp maximum is missing for the second ones) shows that the v-alumina supported particles are more reactive. The mentioned difference reports additional evidence of the specific properties of this model catalyst which are: (i) the partial CO dissociation, (ii) high oxygen coverage and (iii) high CO oxidation activity. We are not able to give a satisfying explanation
I
I
I
I
0
100
200
300
400
Time [s]
Fig. 4. Non-steady-state rates of CO2 formation versus time for Pd/ct-alumina (A).
of the high CO oxidation activity effect at the moment. Nevertheless the following hypothesis can be advanced. Supposing that each CO molecule reaching the Pd surface reacts with an O atom, the reaction rate could be limited by the reactant species transport-diffusion of adsorbed CO molecules. As we have mentioned above, the CO diffusion over the ~-alumina substrate decreases rapidly with increasing surface temperature, which is not the case for y-alumina. Therefore the time extended plateau of CO2 production with increasing T (see Figs. 4 and 5) for ~-alumina could be explained by a decrease of the capture zone surface. Consequently the number of CO molecules captured by every particle per unity of time should be lower for or-alumina supported catalysts. The TPD experiments of CO adsorption on yalumina supported particles show the increase of total CO adsorption capacity (nearly two times) after the reaction experiments described above. Therefore, a further phenomenon, the oxygen induced reversible reconstruction or redispersion
76
I. Star& et aL /Surface Science 365 (1996) 69-77
430K
beam on
~...~....~
. . . . .
Pd (111)
Pd/~,
~o
0
100
200
x0 -
300
400
Time [s]
Fig. 5. Non-steady-staterates of CO2 formationversus time for all samples at 430 K. of particles, could contribute to the explanation of the observed effects. Decrease of particle size and simultaneous increase of their density would lead to the enhancement of oxygen adsorption capacity, to the increase of contribution of CO diffusion over the substrate and a decrease in activation energy of CO oxidation [29]. It seems very probable that there is a link between the effects of partial CO dissociation and high oxygen adsorption and high reactivity of ~alumina supported Pd particles. This could be the consequence of particle interaction with the underlying substrate. Actually we are continuing this work in order to determine in situ particle morphology variation during the CO adsorption and oxidation processes.
a-alumina supports were prepared in different ways in order to obtain more or less stoichiometric surfaces. It was shown that the CO and oxygen molecules diffusing across the ~,-or a-alumina substrate increase the flux of molecules captured by particles during the adsorption processes. The radius of capture zones was estimated from the integral sticking probabilities and known average particle size. Both the CO as well as the oxygen diffusion were found to be more important for stoichiometric a-alumina than for aluminium rich a- and ~,alumina surfaces. On the other hand, the integral sticking coefficient of CO depending on the diffusion length decreases more rapidly with increasing Ton a-alumina than on 7-alumina. This behaviour can explain the lower CO oxidation activity of Pd/ a-alumina particles at higher temperatures. The amount of adsorbed oxygen is higher on supported Pd particles than on the single crystal. This effect is more pronounced on Pd/y-alumina particles. Two explanations seem to be probable, the subsurface O diffusion and/or oxygen induced particle reconstruction. CO TPD experiments show the different CO desorption features for different substrates. Only small variations of the high temperature desorption peak position were observed giving slight variations with activation energy of desorption between 116 and 121 kJ m o l - < The CO dissociation, reported previously for the ~,-alumina support, was not observed on Pd/aalumina. So, the choice of support plays a major role in determining the unexpected effects observed on alumina supported Pd model catalysts. The studies permitting to relate the Pd particle adsorption and reaction properties to their size and morphology are in progress.
References 4. Summary In this study we compared the CO adsorption and oxidation properties of Pd(111) single crystal, 7-alumina and a-alumina supported Pd particles.
[1] G.A. Somorjai,Adv. Catal. 26 (1977) 1. [2] J.A. Rodriguez and D.W. Goodman, Surf. Sci. Rep. 14 (1991) 27. I3] G.A. Somorjai, Chemistry in Two Dimensions (Cornell University Press, London, 1981) p. 483. [4] M. Boudart and G. Dj6ga-Mariadassou, Kinetics of
L Star~ et al./Surface Science 365 (1996) 69-77
Heterogeneous Catalytic Reactions (Princeton University Press, Princeton, NJ, 1984) p. 106. [5] G.A. Somorjai and M.A. Van Hove, Progr. in Surf. Sci. 30 (1989) 201. [6] T. Engel and E. Ertl, in: The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Vol. 4, Eds. D.A. King and D.P. Woodruff (North-Holland, Amsterdam, 1982) p. 73. [7] D.L. Doering, J.T. Dickinson and H. Poppa, J. Catal. 73 (1982) 104. [8] C.R. Henry, C. Chapon, C. Goyhenex and R. Monot, Surf. Sci. 272 (1992) 283. [9"] V. Matolin, E. Gillet, N. Reed and J. Vickerman, J. Chem. Soc. Faraday Trans. 85(15) (1990) 2749. [10"] I. Star~ and V. Matolin, Surf. Sci. 313 (1994) 99. [11"] V. Matolin and E. Gillet, Surf. Sci. 238 (1990) 75. [12] H. Cordatos, T. Bunluevin and R.J. Gorte, Surf. Sci. 323 (1995) 219. [13] M. Gillet and S. Channakhone, J. Catal. 97 (1986) 427. [14] M. Gillet and V. Matolin, J. Crystal Growth 134 (1993) 75. [15] K. Masek and V. Matolin, Thin Solid Films (1995), to be published.
77
[16] I. Star,i, D. Zeze, V. Matolin, J. Pavluch and B. Gruzza, Appl. Surf. Sci (1995) submitted. [17] B. Ealet, M.H. Elyakhloufi, E. Gillet and M. Ricci, Thin Solid. Films 250 (1994) 92. [18] M.F. Gillet and S. Channakhone, J. Catal. 97 (1986) 427. [19] G. Ertl, Surf. Sci. 89 (1979) 525. [20] I. Star~, E. Tomkov~i and V. Matolin, Czech. J. Phys. 43 (1993) 1023. [21] T. Engel and G. Ertl, Adv. Catal. 28 (1979) 1. [22] V. Matolin and E. Gillet, Surf. Sci. 166 (1986) Ll15. [23] F. Rumpf, H. Poppa and M. Boudart, Langrnuir 4 (1988) 115. [24] C.R. Henry, C. Chapon and C. Duriez, Z. Phys. D - Atoms, Molecules and Clusters 19 (1991) 347. [25] T. Engel and G. Ertl, Adv. Catal. 28 (1979) 14. [26] P.A. Readhead, Vacuum 12 (1962) 203. [27] I. Star~i and V. Matolin, Fizika A4 2 (1995) 163. [28] T. Engel, J. Chem. Phys. 69 (1978) 373. [29] I. Star,t, V. Nehasil and V. Matolin, Surf. Sci. 331-333. (1995) 173. [30] T. Engel and G. Ertl, J. Chem. Phys. 69 (1978) 1267.