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Structure sensitivity of methanation on nickel
Vacuum/volume 41/numbers 1-3/pages 147 to 149/1990 Printed in Great Britain
0042-207X/9053.00+.00 © 1990 Pergamon Press plc
S t r u c t u r e sensitivity of m e t h a n a t i o n on nickel A B e r k b * , F P C o e n e n a n d H P B o n z e l , Institut for Grenzflachenforschung und Vakuumphysik, Kemforschungsanlage JOlich, Postfach 1913, D-5170 JOlich, FRG
In the present work we demonstrate that the methanation activity of the flat Ni(110), Ni(lO0) and the periodically profiled Ni(110) surfaces increases by 100-200% gradually in the first few hours of the reaction at 500°C. AES measurements and the investigation of CO and H 2 adsorption-desorption after the reaction revealed that neither the surface chemical composition nor the area increase of the surfaces are responsible for the enhanced activity. Immediate evidence for a surface restructuring is available only for the profiled Ni(110) surface, but possibly on all of the surfaces the increase in activity is due to microscopic scale restructuring. It was also detected that, during the early period of the reaction, the segregation of the sulphur is markedly increased. From a catalytical point of view our results reveal that the poison role of surface sulphur may manifest itself not only in decreasing the methanation rate but also in inhibiting the structural transformation.
1. Introduction The apparent structure insensitivity of CO hydrogenation on low area Ni catalysts is well demonstrated 1'2. On the other hand, the so called particle size effect in the case of supported Ni catalysts suggests a measurable structure sensitivity of methanation 3-6. Recently, Wesner et al have found that a periodically profiled Ni(ll0) surface shows a characteristic macroscopic facetting after a methanation run at 500°C 7. On the one hand their experimental data suggested a highly increased surface self diffusion of Ni itself, on the other hand a possible structure sensitivity of this catalytic reaction. In this work we studied the connection between the methanation activity and the induced structural reformation of the profiled Ni(110), fiat Ni(110) and Ni(100) surfaces by AES, TDS, laser diffraction and gas-chromatography. 2. Experimental The measurements were performed in an uhv chamber connected to a catalytic microreactor operating up to 6 bar pressure. The valveless sample transfer system (Leybold-Heraeus, LH) made a relatively fast transfer of the crystal between the two parts possible. In the uhv chamber we used an LH 180° hemispherical electron energy analyser for AES and an LH-quadrupole massspectrometer for TDS measurements. To produce laser diffraction patterns the sample was exposed through the window to laser (He-Ne) light and the intensity of the diffracted beams was measured outside of the uhv chamber by a photodetector. The surface was cleaned by a LH ion gun. The reactant-product gas stream in the 4 cm 3 volume microreactor was directed to a 8 ft Porapak Q column run at 423 K and to a thermal conducting or a flame ionization detector of a Hewlett-Packard 5840 gaschromatograph. A photolithographic technique and electrochemical etching was used to produce periodic surface profiles
* Permanent address: Reaction Kinetics Res Group of Hungarian Academy of Sciences, Attila Jozsef University, Szeged, Pob. 105, 6701 Hungary.
(gratings) with depths < 1 pm and periodicities of 7 #m on a total 7 x 6 mm E area Ni(110) surface. More details in ref 8. The high purity, to an accuracy of 0.1 ° oriented Ni surfaces were cleaned in situ by repeated cycles of a few minutes of argon ion bombardment (500 eV, ~ 1 pAcm -2) at room temp and annealing at 900°C for 10 min. Possibly all of the remaining surface carbon was removed very easily by a short heating of the sample in 1 bar pressure H 2 at 200°C. 3. Experimental results and discussion 3.1. Ar ion bombardment assisted activation of Ni surfaces for methanation. In these experiments we used the same conditions as Wesner et al, who found that the originally sinus-shaped, etched Ni(llO) surface (the profile grooves are parallel to the [OO1] direction) shows a characteristic faceting after a few hours of methanation: HE:CO = 3.5, 10ml min - t flow rate, Ptotal = lbar, sample temperature 500°C. After each 30 min we stopped the reaction for removing of accumulated surface sulphur contamination by Ar ÷ bombardments (0.5 keV, 2 min, perpendicular incidence). The results gained by different methods are presented on Figure 1. At the beginning of the reaction the interference micrograph of the edged and annealed surface indicates a clear sinusoidal profile, which is radically changed after the total 4 h reaction time, in that the hills became strongly faceted (Figure IA). It can be seen in Figure 1C, that the methane turnover number (TON) increases with the reaction time and the number of Ar + bombardment cycles until 3 h total reaction time, then saturates at a TON of 44 CH4 molecules site- 1 s- 1. This means a near 120Vo increase in the activity. The laser diffraction patterns, as fingerprints of the profile shape (Figure 1B), show that the macroscopic shape of the profiles hardly changes until 1.5 h reaction time, but in this period the methanation rate systematically increases. The other observation, that a short annealing (1 min, 800°C) in uhv after more than 4 h reaction time results in a drop of activity down to near the original value, but at the same time causes no change in the diffraction intensity distribution. This behavior suggests that the cause of the increased activity is on the microscopic scale of the surface structure, and is not
147
A Berk6 et al." Methanation on nickel
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Figure I. Characterization of the edged Ni(ll0) surface during the sinus---,faceted transformation in methanation reaction by different methods: A--interference micrographs; B--laser diffraction intensities (0-4 orders); C--methane turnover number measured by gas chromatography; D--relative AES signals as referred to Ni (848 eV) of sulphur, carbon, oxygen before and after the Ar ion bombardments.
primarily connected with the macroscopic scale facetting detected by interference microscopy. To check the steady-state chemical composition of the surface by AES (and also to measure the laser diffraction pattern), we stopped the reaction and waited for the sample to cool down to 1 48
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Figure 2. Effect of different frequency ofAr ÷ ion bombardment cycles on the activation of ftat Ni(110) surface.
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60°C (average time 2 min). Then we moved the crystal to the uhv and took the AES spectra after a flash to 200°C in uhv before and after the Ar ion bombardment (Figure ID). The amount of removed sulphur by a 2 min Ar ÷ (0.5 keV) bombardment correlates well with the increase in methanation activity (see later). It is important to note that the amount of surface sulphur which appeared after a 30 min reaction gradually decreases with the total reaction time and that after the last two periods there is no detectable increase of the sulphur contamination after a 30 min reaction cycle. The behavior of the carbon (carbide-type line shape) signal is surprising because in each case the signal increases to nearly the same level after Ar ÷ bombardment• To understand this effect we investigated this behavior in a separate study 9 and concluded that the measured amount of surface carbon after Ar + bombardment is characteristic of the amount of subsurface carbon. In this way the increased carbon signal detected after the argon ion bombardment shows the presence of subsurface carbon formed during the reaction. The appearance of surface oxygen detected on Ni(110) after stopping the reaction is a bit surprising. By comparison, on Ni(100) and N i ( l l l ) surfaces only carbon was detected after the methanation ~°. Our systematic measurements on Ni(ll0) and Ni(100) have shown, that depending on the orientation of the Ni surface and on the reaction temperature, surface oxygen can appear under certain conditions9. The experiments with different characteristic amplitudes of the profiles have shown that this behavior, presented in Figure 1C, is in its main features independent of the profile amplitude• Almost the same activation can be achieved by this method also on flat Ni(110) and Ni(100) surfaces, with the slight difference that the total methanation rate increase is higher on Ni(100). 3.2. The role of segregated sulphur and Ar ion bombardment for the activity increase. Although in the last years the mechanism of
the methanation on Ni single crystal surfaces has been thoroughly characterized L2'9-~3, only little attention seems to have been paid to the subsurface connected phenomena in this process. We found that the sulphur segregation increases during the high temperature methanation on all of the investigated Ni surfaces, relative to the segregation in uhv or in 1 bar H 2 at the
A Berk6 et al: Methanation on nickel
same temperature. The fact, that in 1 bar H 2 there is no measurable increase in the segregation rate, excludes the explanation that the increased S segregation in the H2:CO reaction would be connected with a decreased barrier for S segregation due to adsorbed hydrogen. We also excluded that this effect was caused by some contamination of the gas mixture, because the sulphur appeared only in the early periods of the activation (Figure 1D). A plausible explanation is, that the methanation reaction induces a surface restructuring and as a result of this, that deeper layers (more rich in sulphur) contribute to S segregation. This explanation supposes a relatively high mass transport of Ni itself under the reaction conditions. As suggested above, the stepwise increase of the methanation rate measured after Ar ion bombardment (Figure l) is likely to be due to the removal of surface sulphur contamination. To prove this, we studied the effect of segregated sulphur on methanation rate for the flat Ni(100) and Ni(ll0) surfaces. We obtained a consistent correlation between the surface sulphur coverage and the methanation rate. We found that at low sulphur coverages the AR s = 0.01 increase of the relative Auger signal (R s ----IStlSO)/INItSa8), R s = 0.01 --* ~ 1 x 1013 S atom cm -2) causes 1-2 CH 4 site-1 s-1 T O N (turnover number) decrease at a reaction temperature of 500°C. Nearly the same value ( ~ 4 CH 4 site-1 s-1) was calculated on the basis of the data by Goodman et al, who measured the effect of sulphur decomposed chemically from H2S on a Ni(100) surface 14. Under the very plausible assumption, that the effect of segregated sulphur is the same on the stepped Ni(110) and on flat surfaces, we concluded on the basis of Figure 1, that the main effect of Ar + bombardment from the point of view of the reaction is the removal of the surface sulphur. In Figure 2 we present two activation processes for a fiat Ni(l I0) surface with different reaction cycles (10 and 30 min) but with the same Ar ion bombardments (2 min, 0.5 keV) in between them. The results show, that a more frequent Ar ion bombardment causes a faster transformation of the low activity state into the high activity state with smaller stepwise increases after each bombardment. It means, that the effect of segregated sulphur manifests itself not only in decreasing the methanation rate, but also in inhibiting the surface structural transformation.
3.3. Evidence for structural sensitivity of methanation of single nickel-crystal surfaces. The fact that alternate methanation reaction and Ar + bombardment cycles cause a systematic change in the activity of Ni surfaces, indicates the structure sensitivity of this reaction. To prove that it is not simply the surface area that is changed by Ar + bombardment, we investigated the adsorption and desorption characteristics of CO and H 2 on different treated
and untreated surfaces. For this we had to take into account that surface coverages of S, C, or O also influence the desorption behavior of CO and H2. The evaluation of these measurements has shown, that the area of the surface in the 'low' and 'high' activity state does not change by more than 20~, which is much less than the methanation rate change (100-200~). The different steady state level of surface oxygen and carbon depending on orientation and activity state ('high' or 'low') of the nickel surfaces detected by AES at the end of the reaction will be discussed thoroughly in a later publication9. It also indicates structural sensitivity of the rate of the different elementary steps of methanation on Ni surfaces. 4. Conclusions We detected that the methanation activity of Ni(ll0) and Ni(100) surfaces changes during the early stage, and that this process very possibly is due to a restructuring of the surface. The detected increased sulphur segregation during reaction seems to be connected to this process. Our results have shown that the effect of sulphur is not only to reduce the methanation activity, but also to hinder the restructuring process. The parallel change of the surface structure and the methanation xate clearly indicates the surface structure sensitivity of methanation on Ni surfaces.
Acknowledgement One of the authors (AB) would like to express his gratitude to the Alexander von Humboldt Foundation for financial support.
References D W Goodman, R D Kelley,T E Madey and J T Yates, J Catal, 63, 226 (1990). 2 R D Kelley and D W Goodman, Surface Sci, 123, L743 (1982). 3 R Z C Van Meerten, A H G M Beaumont, P F M T Van Nisselrooij and J W E Coenen, Surface Sci, 135, 565 (1983). '* F J Schepers, E M Van Boekhoven and V Ponce, J Catal, 96, 82 (1985). 5 j T Richardson and R Koveal, J Catal, 98, 559 (1986). 6 C Lee, L D Schmidt, J F Moulder and T W Rusch, J Catal, 99, 472 (1986). D A Wesner, F P Coenen and H P Bonzel, The Structure of Surfaces II, Springer Series in Surface Science (Edited by J F van der Veen and M A van Howe), Vol 11, p 612. Springer, Berlin (1988). 8 H P Bonzel, E Preuss and B Steffen, Appl Phys, A35, 1 (1984). 9 A Berk6 and H P Bonzel, Unpublished results. lo R D Kelley and S Semancik, J Catal, 84, 248 (1983). t~ R S Polizotti and J A Schwarz, J CataL 77, 1 (1982). ~2y Soong, K Krishna and P Biloen, J Catal, 97, 330 (1986). 13 p Winslow and A T Bell, J Catal, 94, 385 (1985). 14 D W Goodman, In Springer Series in Surface Science, Chemistry and Physics of Solid Surfaces, (Edited by G Ertl and R Gomer), Vol VI, p 169. Springer, Berlin (1986).
149
0042-207X/9053.00+.00 © 1990 Pergamon Press plc
S t r u c t u r e sensitivity of m e t h a n a t i o n on nickel A B e r k b * , F P C o e n e n a n d H P B o n z e l , Institut for Grenzflachenforschung und Vakuumphysik, Kemforschungsanlage JOlich, Postfach 1913, D-5170 JOlich, FRG
In the present work we demonstrate that the methanation activity of the flat Ni(110), Ni(lO0) and the periodically profiled Ni(110) surfaces increases by 100-200% gradually in the first few hours of the reaction at 500°C. AES measurements and the investigation of CO and H 2 adsorption-desorption after the reaction revealed that neither the surface chemical composition nor the area increase of the surfaces are responsible for the enhanced activity. Immediate evidence for a surface restructuring is available only for the profiled Ni(110) surface, but possibly on all of the surfaces the increase in activity is due to microscopic scale restructuring. It was also detected that, during the early period of the reaction, the segregation of the sulphur is markedly increased. From a catalytical point of view our results reveal that the poison role of surface sulphur may manifest itself not only in decreasing the methanation rate but also in inhibiting the structural transformation.
1. Introduction The apparent structure insensitivity of CO hydrogenation on low area Ni catalysts is well demonstrated 1'2. On the other hand, the so called particle size effect in the case of supported Ni catalysts suggests a measurable structure sensitivity of methanation 3-6. Recently, Wesner et al have found that a periodically profiled Ni(ll0) surface shows a characteristic macroscopic facetting after a methanation run at 500°C 7. On the one hand their experimental data suggested a highly increased surface self diffusion of Ni itself, on the other hand a possible structure sensitivity of this catalytic reaction. In this work we studied the connection between the methanation activity and the induced structural reformation of the profiled Ni(110), fiat Ni(110) and Ni(100) surfaces by AES, TDS, laser diffraction and gas-chromatography. 2. Experimental The measurements were performed in an uhv chamber connected to a catalytic microreactor operating up to 6 bar pressure. The valveless sample transfer system (Leybold-Heraeus, LH) made a relatively fast transfer of the crystal between the two parts possible. In the uhv chamber we used an LH 180° hemispherical electron energy analyser for AES and an LH-quadrupole massspectrometer for TDS measurements. To produce laser diffraction patterns the sample was exposed through the window to laser (He-Ne) light and the intensity of the diffracted beams was measured outside of the uhv chamber by a photodetector. The surface was cleaned by a LH ion gun. The reactant-product gas stream in the 4 cm 3 volume microreactor was directed to a 8 ft Porapak Q column run at 423 K and to a thermal conducting or a flame ionization detector of a Hewlett-Packard 5840 gaschromatograph. A photolithographic technique and electrochemical etching was used to produce periodic surface profiles
* Permanent address: Reaction Kinetics Res Group of Hungarian Academy of Sciences, Attila Jozsef University, Szeged, Pob. 105, 6701 Hungary.
(gratings) with depths < 1 pm and periodicities of 7 #m on a total 7 x 6 mm E area Ni(110) surface. More details in ref 8. The high purity, to an accuracy of 0.1 ° oriented Ni surfaces were cleaned in situ by repeated cycles of a few minutes of argon ion bombardment (500 eV, ~ 1 pAcm -2) at room temp and annealing at 900°C for 10 min. Possibly all of the remaining surface carbon was removed very easily by a short heating of the sample in 1 bar pressure H 2 at 200°C. 3. Experimental results and discussion 3.1. Ar ion bombardment assisted activation of Ni surfaces for methanation. In these experiments we used the same conditions as Wesner et al, who found that the originally sinus-shaped, etched Ni(llO) surface (the profile grooves are parallel to the [OO1] direction) shows a characteristic faceting after a few hours of methanation: HE:CO = 3.5, 10ml min - t flow rate, Ptotal = lbar, sample temperature 500°C. After each 30 min we stopped the reaction for removing of accumulated surface sulphur contamination by Ar ÷ bombardments (0.5 keV, 2 min, perpendicular incidence). The results gained by different methods are presented on Figure 1. At the beginning of the reaction the interference micrograph of the edged and annealed surface indicates a clear sinusoidal profile, which is radically changed after the total 4 h reaction time, in that the hills became strongly faceted (Figure IA). It can be seen in Figure 1C, that the methane turnover number (TON) increases with the reaction time and the number of Ar + bombardment cycles until 3 h total reaction time, then saturates at a TON of 44 CH4 molecules site- 1 s- 1. This means a near 120Vo increase in the activity. The laser diffraction patterns, as fingerprints of the profile shape (Figure 1B), show that the macroscopic shape of the profiles hardly changes until 1.5 h reaction time, but in this period the methanation rate systematically increases. The other observation, that a short annealing (1 min, 800°C) in uhv after more than 4 h reaction time results in a drop of activity down to near the original value, but at the same time causes no change in the diffraction intensity distribution. This behavior suggests that the cause of the increased activity is on the microscopic scale of the surface structure, and is not
147
A Berk6 et al." Methanation on nickel
Ni(110) , depth: 0.25~Jm
Profited ~~
:
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Figure I. Characterization of the edged Ni(ll0) surface during the sinus---,faceted transformation in methanation reaction by different methods: A--interference micrographs; B--laser diffraction intensities (0-4 orders); C--methane turnover number measured by gas chromatography; D--relative AES signals as referred to Ni (848 eV) of sulphur, carbon, oxygen before and after the Ar ion bombardments.
primarily connected with the macroscopic scale facetting detected by interference microscopy. To check the steady-state chemical composition of the surface by AES (and also to measure the laser diffraction pattern), we stopped the reaction and waited for the sample to cool down to 1 48
i
i
2
Figure 2. Effect of different frequency ofAr ÷ ion bombardment cycles on the activation of ftat Ni(110) surface.
c
t/I
"-ILJ
reaction firne/h
o
o
C ~"u aJ
20
z
60°C (average time 2 min). Then we moved the crystal to the uhv and took the AES spectra after a flash to 200°C in uhv before and after the Ar ion bombardment (Figure ID). The amount of removed sulphur by a 2 min Ar ÷ (0.5 keV) bombardment correlates well with the increase in methanation activity (see later). It is important to note that the amount of surface sulphur which appeared after a 30 min reaction gradually decreases with the total reaction time and that after the last two periods there is no detectable increase of the sulphur contamination after a 30 min reaction cycle. The behavior of the carbon (carbide-type line shape) signal is surprising because in each case the signal increases to nearly the same level after Ar ÷ bombardment• To understand this effect we investigated this behavior in a separate study 9 and concluded that the measured amount of surface carbon after Ar + bombardment is characteristic of the amount of subsurface carbon. In this way the increased carbon signal detected after the argon ion bombardment shows the presence of subsurface carbon formed during the reaction. The appearance of surface oxygen detected on Ni(110) after stopping the reaction is a bit surprising. By comparison, on Ni(100) and N i ( l l l ) surfaces only carbon was detected after the methanation ~°. Our systematic measurements on Ni(ll0) and Ni(100) have shown, that depending on the orientation of the Ni surface and on the reaction temperature, surface oxygen can appear under certain conditions9. The experiments with different characteristic amplitudes of the profiles have shown that this behavior, presented in Figure 1C, is in its main features independent of the profile amplitude• Almost the same activation can be achieved by this method also on flat Ni(110) and Ni(100) surfaces, with the slight difference that the total methanation rate increase is higher on Ni(100). 3.2. The role of segregated sulphur and Ar ion bombardment for the activity increase. Although in the last years the mechanism of
the methanation on Ni single crystal surfaces has been thoroughly characterized L2'9-~3, only little attention seems to have been paid to the subsurface connected phenomena in this process. We found that the sulphur segregation increases during the high temperature methanation on all of the investigated Ni surfaces, relative to the segregation in uhv or in 1 bar H 2 at the
A Berk6 et al: Methanation on nickel
same temperature. The fact, that in 1 bar H 2 there is no measurable increase in the segregation rate, excludes the explanation that the increased S segregation in the H2:CO reaction would be connected with a decreased barrier for S segregation due to adsorbed hydrogen. We also excluded that this effect was caused by some contamination of the gas mixture, because the sulphur appeared only in the early periods of the activation (Figure 1D). A plausible explanation is, that the methanation reaction induces a surface restructuring and as a result of this, that deeper layers (more rich in sulphur) contribute to S segregation. This explanation supposes a relatively high mass transport of Ni itself under the reaction conditions. As suggested above, the stepwise increase of the methanation rate measured after Ar ion bombardment (Figure l) is likely to be due to the removal of surface sulphur contamination. To prove this, we studied the effect of segregated sulphur on methanation rate for the flat Ni(100) and Ni(ll0) surfaces. We obtained a consistent correlation between the surface sulphur coverage and the methanation rate. We found that at low sulphur coverages the AR s = 0.01 increase of the relative Auger signal (R s ----IStlSO)/INItSa8), R s = 0.01 --* ~ 1 x 1013 S atom cm -2) causes 1-2 CH 4 site-1 s-1 T O N (turnover number) decrease at a reaction temperature of 500°C. Nearly the same value ( ~ 4 CH 4 site-1 s-1) was calculated on the basis of the data by Goodman et al, who measured the effect of sulphur decomposed chemically from H2S on a Ni(100) surface 14. Under the very plausible assumption, that the effect of segregated sulphur is the same on the stepped Ni(110) and on flat surfaces, we concluded on the basis of Figure 1, that the main effect of Ar + bombardment from the point of view of the reaction is the removal of the surface sulphur. In Figure 2 we present two activation processes for a fiat Ni(l I0) surface with different reaction cycles (10 and 30 min) but with the same Ar ion bombardments (2 min, 0.5 keV) in between them. The results show, that a more frequent Ar ion bombardment causes a faster transformation of the low activity state into the high activity state with smaller stepwise increases after each bombardment. It means, that the effect of segregated sulphur manifests itself not only in decreasing the methanation rate, but also in inhibiting the surface structural transformation.
3.3. Evidence for structural sensitivity of methanation of single nickel-crystal surfaces. The fact that alternate methanation reaction and Ar + bombardment cycles cause a systematic change in the activity of Ni surfaces, indicates the structure sensitivity of this reaction. To prove that it is not simply the surface area that is changed by Ar + bombardment, we investigated the adsorption and desorption characteristics of CO and H 2 on different treated
and untreated surfaces. For this we had to take into account that surface coverages of S, C, or O also influence the desorption behavior of CO and H2. The evaluation of these measurements has shown, that the area of the surface in the 'low' and 'high' activity state does not change by more than 20~, which is much less than the methanation rate change (100-200~). The different steady state level of surface oxygen and carbon depending on orientation and activity state ('high' or 'low') of the nickel surfaces detected by AES at the end of the reaction will be discussed thoroughly in a later publication9. It also indicates structural sensitivity of the rate of the different elementary steps of methanation on Ni surfaces. 4. Conclusions We detected that the methanation activity of Ni(ll0) and Ni(100) surfaces changes during the early stage, and that this process very possibly is due to a restructuring of the surface. The detected increased sulphur segregation during reaction seems to be connected to this process. Our results have shown that the effect of sulphur is not only to reduce the methanation activity, but also to hinder the restructuring process. The parallel change of the surface structure and the methanation xate clearly indicates the surface structure sensitivity of methanation on Ni surfaces.
Acknowledgement One of the authors (AB) would like to express his gratitude to the Alexander von Humboldt Foundation for financial support.
References D W Goodman, R D Kelley,T E Madey and J T Yates, J Catal, 63, 226 (1990). 2 R D Kelley and D W Goodman, Surface Sci, 123, L743 (1982). 3 R Z C Van Meerten, A H G M Beaumont, P F M T Van Nisselrooij and J W E Coenen, Surface Sci, 135, 565 (1983). '* F J Schepers, E M Van Boekhoven and V Ponce, J Catal, 96, 82 (1985). 5 j T Richardson and R Koveal, J Catal, 98, 559 (1986). 6 C Lee, L D Schmidt, J F Moulder and T W Rusch, J Catal, 99, 472 (1986). D A Wesner, F P Coenen and H P Bonzel, The Structure of Surfaces II, Springer Series in Surface Science (Edited by J F van der Veen and M A van Howe), Vol 11, p 612. Springer, Berlin (1988). 8 H P Bonzel, E Preuss and B Steffen, Appl Phys, A35, 1 (1984). 9 A Berk6 and H P Bonzel, Unpublished results. lo R D Kelley and S Semancik, J Catal, 84, 248 (1983). t~ R S Polizotti and J A Schwarz, J CataL 77, 1 (1982). ~2y Soong, K Krishna and P Biloen, J Catal, 97, 330 (1986). 13 p Winslow and A T Bell, J Catal, 94, 385 (1985). 14 D W Goodman, In Springer Series in Surface Science, Chemistry and Physics of Solid Surfaces, (Edited by G Ertl and R Gomer), Vol VI, p 169. Springer, Berlin (1986).
149