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The role of oxide promoters in the dissociation of CO and its reaction with hydrogen on Pd (111) and Rh (111): A molecular beam study
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All rights reserved.
3339
The role of oxide promoters in the dissociation of CO and its reaction with hydrogen on Pd (111) and Rh (111): A molecular beam study Bernhard Kltitzer and Konrad Hayek Institut ftir Physikalische Chemie, Universit~t Innsbruck, Innrain 52A, A-6020 Innsbruck, Austria The combination of a noble _metal surface and a reducible oxide is well-known for catalytic properties generally ascribed to strong metal-support interaction [1]. For example, the methane formation from CO and hydrogen may be strongly promoted by oxide submonolayers on the metal surface [2]. In order to study the basic kinetics of the reaction of hydrogen and CO on clean and vanadia-modified Rh and Pd surfaces, "inverse supported model catalysts" were prepared by vapour deposition of V on top of the respective single crystal surface in the presence of oxygen. They were investigated in a modified molecular beam experiment both under UHV conditions and under local pressures up to a few millibar. As a first step, the probability of CO dissociation on clean and VOx-covered surfaces was studied. Thereafter, the kinetics of the methanation reaction was measured at reactant pressures between 10-2 and 10 mbar. The local pressures obtained by dosing the reactants from a capillary source were found to be sufficient to drive the reaction near 573 K with measurable amounts of products. The reaction is promoted by submonolayer quantities of vanadia, but the reaction rate depends strongly on the extent and temperature of hydrogen reduction. 1. INTRODUCTION State-of-the-art molecular beam techniques represent a powerful tool for the detailed study of kinetics and dynamics of adsorption processes [3] and of fast surface reactions [4]. For studies of reactions with low turnovers they are less suitable because the limited pumping speed in conventional UHV systems leads to detection problems. Moreover, condensation reactions like the methanation of CO are thermodynamically unfavourable at low pressures and no reaction products can usually be measured under conditions prevailing in molecular beam experiments. A meaningful study of these reactions has to provide experimental conditions as close to ,,real" catalysis as possible, but at the same time to employ experimental techniques applicable to single crystal surfaces under UHV conditions (TDS, LEED, AES, work function measurement etc.). In the present experiments the pressure gap between ,,low flux experiments" (<10 ML/sec) and ,,real catalysis" (fluxes around 106 ML/sec) was in part overcome by applying a high intensity molecular beam generated with a capillary. With this arrangement the partial pressures can be locally increased without changing the pumping speed of the system. It will be shown that the so-obtained local pressures are high enough to maintain a sufficient reactant coverage on the surface to drive the methanation reaction at sample temperatures near 573 K. The aim of this work is to understand the basic kinetics of the reaction of hydrogen and CO on clean and oxide-modified (111) surfaces of Rh and Pd, and in particular the
3340 promoting effect of the metal-support boundary. It is well known that the methanation rates on Rh are influenced by Rh oxide precoverage [5], and also that the formation of methane from CO and hydrogen on most noble metal surfaces is strongly promoted by submonolayers of reducible oxides, e.g. titania [6]. ,,Inverse supported model catalysts" were therefore prepared by deposition of vanadium on top of the single crystal surface in the presence of oxygen. Information about surface composition and oxidation states was obtained from Auger and XPS measurements, whereas the structure and morphology of submonolayers of vanadia on Pd (111) were determined in a parallel STM and LEED study [7]. It is generally agreed that on most noble metals the dissociation of CO is rate determining in the Fischer-Tropsch reaction. On the other hand, measurements by various surface science techniques have shown that CO dissociation is impossible or very slow on clean low-index surfaces such as Rh (111) [8]. A very sensitive method for measurement of surface carbon resulting from CO dissociation was therefore developed. 2. EXPERIMENTAL In order to study the kinetics of dissociative and molecular adsorption of CO and the reaction with hydrogen on clean and oxide-modified Rh (111) we used a modified molecular beam technique [9]. A scheme of the experimental setup is shown in Fig.1. An UHV chamber was equipped with LEED/AES, a Kelvin probe for work function measurements, a vanadium microevaporator and a quartz microbalance (not shown in Fig.l). A shielded and differentially pumped quadrupole mass spectrometer allowed line-of-sight detection of molecules desorbing thermally (TPD) or reactively (TPR) from the central part of the sample surface. V
gs
v
V V
Fig. 1: Experimental setup for parallel measurements of CO molecular and dissociative adsorption and methane formation by molecular beam techniques (s sample, 1 capillary array doser, c capillary doser for high local pressures, m main chamber, q quadrupole mass spectrometers, v valves, t turbo pumps, g gas dosing and mixing system, gs gas supply, p pressure transducer, z LN2 cooled Cu~§ zeolite trap, r rotary pump).
3341 In one part of the experiments a capillary array doser was used as a molecular beam source for controlled gas adsorption with a small flux gradient across the sample surface [10]. Calibration of absolute beam fluxes was achieved by measuring the absolute pressure decrease in a differentially pumped gas dosing system. A second quadrupole mass spectrometer was used for King and Wells type measurements of net adsorption rates and sticking probabilities [11 ]. As shown in Fig.2, the experimental setup was sensitive to detect less than 10.2 ML of surface carbon, arising from 12CO dissociation, by reaction with oxygen. This technique was further improved by using isotope-labeled 13CO, which strongly reduced the background signal and allowed to discriminate between carbon originating from CO dissociation, from bulk segregation or from the residual gas. In the second part of experiments V metal was vapour deposited on the clean Rh (111) surface, and oxidised to V203 in 10-7 mbar 02 at 673 K [7,13]. The resulting vanadium oxide was reduced under varying hydrogen pressure and substrate temperature. Surface structure and composition were monitored by LEED and Auger electron spectroscopy. The sample was exposed to the capillary dosed high-flux beam (D2/CO ratio 10:1) at pressures in the millibar range for varying periods of time. The reaction products were pumped off by a turbomolecular pump together with the non-reacted species, and were thereafter collected in a CuI+ZSMX zeolite trap held at liquid nitrogen temperature. This adsorbent does not pump D2 at temperatures around 77 K, but strongly binds CO and D20, which start to desorb only above 500 K. By differentially warming the trap the product (deuterated methane) could be desorbed near 300 K and effectively separated from the reactants. The total amount of desorbing CD4 was introduced into a separate small UHV system and was there determined with a quadrupole mass spectrometer. Integral reaction rates could in this way be measured.
"7". 6.0
lil
v
10c<0.01 ML
E
5.0 o~ t-
.c_ 4.0
E
3.0 '
I
90
'
I
'
1O0
I
110
'
I
120
'
I
130
time [sec] Fig.2: Titration of carbon resulting from dissociative 12CO adsorption after exposing a partly vanadia-covered Pd(111) surface to a 3:1 D2+CO reaction mixture (CO flux ca. 5 ML/sec for 10 min, Tsample=600 K)
3342 3. RESULTS AND DISCUSSION As a first step the interaction of the clean Rh (111) surface with CO was studied using the capillary array doser at fluxes around 0.1 ML/sec. The sticking probability of molecular CO was measured as a function of surface coverage and sample temperature. On clean and annealed Rh (111) no CO dissociation was observed, and the sticking coefficient of molecular CO was in good agreement with a recent supersonic beams study [12]. However, using again the low-flux beam source, the influence of surface oxide and of hydrogen on the dissociative CO adsorption could be monitored, and submonolayers of carbon on vanadia-covered Pd (111) resulting from dissociatively adsorbed CO were quantitatively determined. An example is shown in Fig. 2: The vanadia-precovered Pd (111) surface (Ov <0.5 ML) was exposed to a 3:1 D2/CO mixture at 600 K for 10 minutes (CO flux 5 ML/sec). At this temperature molecular CO is completely desorbed and the reactivity of submonolayers of carbon resulting from dissociative CO adsorption is high enough to allow a quantitative oxygen titration. By turning the sample in the oxygen beam and measuring the QMS amu 44 signal the carbon dioxide arising from C oxidation on the Pd(lll)/vanadia surface could be effectively separated from the residual gas background (Fig.2). If higher local fluxes were applied by means of the capillary doser, the clean Rh (111) surface remained inactive towards CO dissociation and reaction up to local pressures of 10-1 mbar and 573 K, and no products were detected. However, small amounts of deuterated methane (12CD4) could be found at the same local pressure and temperature if the surface was partly covered with vanadium oxide. While local pressures up to 0.1 mbar were inefficient to initiate a reaction on clean Rh (111), a very sudden onset of the reaction was observed upon a pressure rise to about 5 mbar (total pressure in the system --10-5 mbar).
j
a
/ "~
Rh
b
Rh
Rh I'''1'''1'''1'''1'''1'''1'''1''
200 220 240 260 280 300 320 340
E n e r g y [eV] Fig 3: Left: AES spectra (a) from the clean Rh(111) surface and (b) after reaction at around 5 mbar total reactant pressure (D2:CO=10:I) at 573 K for 10 min. Right: Corresponding LEED pattern. Both the AES and LEED primary electron beam were directed onto the small region of the sample hit by the impinging reactant beam.
3343 This onset was not exactly reproducible, but appeared to depend on the initial state of the surface (defects, impurities, etc.). Furthermore, it was observed that the successive reaction is always accompanied by increasing carbon deposition, but also by severe structural alterations (roughening) of the exposed crystal surface. Carbon deposition on the surface occurs partly ordered, as seen from the LEED pattern and shown with the corresponding Auger features in Fig.3. The reaction-induced structural changes of the originally clean and well-annealed Rh (111) surface may be strong enough to become optically visible (Fig. 4). The region involved in the surface reaction (impingement area 0.5mm 2) appears black and roughened on a macroscopic scale, and carbon is found deposited in this area and also on the surrounding parts of the surface. The ring-like diffraction features in Fig.3 correspond to a bonding distance of 1.40~0.3/~ indicating the formation of a disordered graphitic adlayer. Subsequent exposure to oxygen (700 K, 5x10 -6 mbar, 5 min) could neither restore the initial fiat, well annealed (111) surface nor remove the black or grey surface area in Fig.4, although AES showed that carbon was removed by oxygen during this procedure. Only at temperatures close to the decomposition temperature of Rh203 at about 1400 K the black deposit vanished, but even heating up to 1700 K was not sufficient to completely remove the surface roughness induced by the reaction experiment. It must be therefore assumed that at higher surface concentrations of CO massive adsorbate-induced restructuring occurs, possibly via mobile subcarbonyls [14]. This restructuring will in turn lead to further enhanced catalytic activity via formation of additional and more active surface area, resulting in an autocatalysed reaction. In a recent STM investigation Wilson et al. [15] observed a similar behaviour during CO hydrogenation on cobalt single crystal surfaces. It may be of interest to apply the same experimental conditions to other Rh surfaces on which CO is known to dissociate, e.g. polycrystalline metal foils, and to comparable the corresponding reaction rates [16].
Fig.4: Photograph of the Rh(111) single crystal surface after exposure to a 5 mbar 10:1 D2+CO beam at 573 K for 10 min.
3344 4. CONCLUSIONS The versatile molecular beam arrangement used in our experiments is well-suited for measurement of small amounts of carbon monoxide dissociated on the surface, but it allows also to detect products and monitor surface alterations arising from reactions at local pressures in the millibar range. Under the latter reaction conditions a sudden onset of the CO hydrogenation on Rh (111) is observed, presumably a result of an autocatalytic process induced by surface restructuring and carbon deposition from CO dissociation. ACKNOWLEDGEMENT This work was supported by the Joint Research Project "Gas Surface Interactions" (S 8105) of the Fonds zur F6rderung der wissenschaftlichen Forschung of Austria.
REFERENCES ,
2. 3. 4. 5. 6. .
.
9. 10. 11. 12. 13. 14. 15. 16.
S. J. Tauster, S.C. Fung, J. Catal. 55 (1978) 29 A.B. Boffa, C. Lin, A. T. Bell, G.A. Somorjai, Catalysis Letters 27 (1994) 243 T. Ali, A.V. Walker, B. K16tzer, D.A. King, Surf. Sci. 414 (1998) 304 A.T. Pasteur, X.-C. Guo, T. Ali, M. Gruyters, D.A. King, Surf. Sci. 366 (1996) 564 D. G. Castner, R.L. Blackadar, G. A. Somorjai, J. Catal. 66 (1980) 275 K.J. Williams, A.B. Boffa, J. Lahtinen, M. Salmeron, A.T. Bell, G.A. Somorjai, Catal. Lett. 5 (1990) 385 F.P. Leisenberger, S. Surnev, L. Vitali, M.G. Ramsey, F.P. Netzer, J. Vac. Sci. Technol. A 17 (1999) 1743. T. Koerts, W.J.J. Welters, R.A. van Santen, J. Catal. 134 (1992) 1 K. Hayek, M. Fuchs, B. K16tzer, W. Reichl, Ct. Rupprechter, Topics in Catalysis 2000, in press. M.J. Bozack, L. Muehlhoff, J.N. Russell Jr., W.J. Choyke, J.T. Yates Jr., J. Vac. Sci. Technol. A5(1) (1987) 1 D.A. King, M.G. Wells, Surf. Sci. 29 (1972) 245 M. Beutl, J. Lesnik, K.D. Rendulic, Surf. Sci. 429 (1999) 71 T. Hartmann, H. Kn6zinger, Z. Phys. Chem. 197 (1996) 113 N. Kruse, A. Gaussmann, J. Catal. 144 (1993) 525 J. Wilson, C. de Groot, J. Phys. Chem. 99 (1995) 7860 W. Reichl, K. Hayek, this conference.
3339
The role of oxide promoters in the dissociation of CO and its reaction with hydrogen on Pd (111) and Rh (111): A molecular beam study Bernhard Kltitzer and Konrad Hayek Institut ftir Physikalische Chemie, Universit~t Innsbruck, Innrain 52A, A-6020 Innsbruck, Austria The combination of a noble _metal surface and a reducible oxide is well-known for catalytic properties generally ascribed to strong metal-support interaction [1]. For example, the methane formation from CO and hydrogen may be strongly promoted by oxide submonolayers on the metal surface [2]. In order to study the basic kinetics of the reaction of hydrogen and CO on clean and vanadia-modified Rh and Pd surfaces, "inverse supported model catalysts" were prepared by vapour deposition of V on top of the respective single crystal surface in the presence of oxygen. They were investigated in a modified molecular beam experiment both under UHV conditions and under local pressures up to a few millibar. As a first step, the probability of CO dissociation on clean and VOx-covered surfaces was studied. Thereafter, the kinetics of the methanation reaction was measured at reactant pressures between 10-2 and 10 mbar. The local pressures obtained by dosing the reactants from a capillary source were found to be sufficient to drive the reaction near 573 K with measurable amounts of products. The reaction is promoted by submonolayer quantities of vanadia, but the reaction rate depends strongly on the extent and temperature of hydrogen reduction. 1. INTRODUCTION State-of-the-art molecular beam techniques represent a powerful tool for the detailed study of kinetics and dynamics of adsorption processes [3] and of fast surface reactions [4]. For studies of reactions with low turnovers they are less suitable because the limited pumping speed in conventional UHV systems leads to detection problems. Moreover, condensation reactions like the methanation of CO are thermodynamically unfavourable at low pressures and no reaction products can usually be measured under conditions prevailing in molecular beam experiments. A meaningful study of these reactions has to provide experimental conditions as close to ,,real" catalysis as possible, but at the same time to employ experimental techniques applicable to single crystal surfaces under UHV conditions (TDS, LEED, AES, work function measurement etc.). In the present experiments the pressure gap between ,,low flux experiments" (<10 ML/sec) and ,,real catalysis" (fluxes around 106 ML/sec) was in part overcome by applying a high intensity molecular beam generated with a capillary. With this arrangement the partial pressures can be locally increased without changing the pumping speed of the system. It will be shown that the so-obtained local pressures are high enough to maintain a sufficient reactant coverage on the surface to drive the methanation reaction at sample temperatures near 573 K. The aim of this work is to understand the basic kinetics of the reaction of hydrogen and CO on clean and oxide-modified (111) surfaces of Rh and Pd, and in particular the
3340 promoting effect of the metal-support boundary. It is well known that the methanation rates on Rh are influenced by Rh oxide precoverage [5], and also that the formation of methane from CO and hydrogen on most noble metal surfaces is strongly promoted by submonolayers of reducible oxides, e.g. titania [6]. ,,Inverse supported model catalysts" were therefore prepared by deposition of vanadium on top of the single crystal surface in the presence of oxygen. Information about surface composition and oxidation states was obtained from Auger and XPS measurements, whereas the structure and morphology of submonolayers of vanadia on Pd (111) were determined in a parallel STM and LEED study [7]. It is generally agreed that on most noble metals the dissociation of CO is rate determining in the Fischer-Tropsch reaction. On the other hand, measurements by various surface science techniques have shown that CO dissociation is impossible or very slow on clean low-index surfaces such as Rh (111) [8]. A very sensitive method for measurement of surface carbon resulting from CO dissociation was therefore developed. 2. EXPERIMENTAL In order to study the kinetics of dissociative and molecular adsorption of CO and the reaction with hydrogen on clean and oxide-modified Rh (111) we used a modified molecular beam technique [9]. A scheme of the experimental setup is shown in Fig.1. An UHV chamber was equipped with LEED/AES, a Kelvin probe for work function measurements, a vanadium microevaporator and a quartz microbalance (not shown in Fig.l). A shielded and differentially pumped quadrupole mass spectrometer allowed line-of-sight detection of molecules desorbing thermally (TPD) or reactively (TPR) from the central part of the sample surface. V
gs
v
V V
Fig. 1: Experimental setup for parallel measurements of CO molecular and dissociative adsorption and methane formation by molecular beam techniques (s sample, 1 capillary array doser, c capillary doser for high local pressures, m main chamber, q quadrupole mass spectrometers, v valves, t turbo pumps, g gas dosing and mixing system, gs gas supply, p pressure transducer, z LN2 cooled Cu~§ zeolite trap, r rotary pump).
3341 In one part of the experiments a capillary array doser was used as a molecular beam source for controlled gas adsorption with a small flux gradient across the sample surface [10]. Calibration of absolute beam fluxes was achieved by measuring the absolute pressure decrease in a differentially pumped gas dosing system. A second quadrupole mass spectrometer was used for King and Wells type measurements of net adsorption rates and sticking probabilities [11 ]. As shown in Fig.2, the experimental setup was sensitive to detect less than 10.2 ML of surface carbon, arising from 12CO dissociation, by reaction with oxygen. This technique was further improved by using isotope-labeled 13CO, which strongly reduced the background signal and allowed to discriminate between carbon originating from CO dissociation, from bulk segregation or from the residual gas. In the second part of experiments V metal was vapour deposited on the clean Rh (111) surface, and oxidised to V203 in 10-7 mbar 02 at 673 K [7,13]. The resulting vanadium oxide was reduced under varying hydrogen pressure and substrate temperature. Surface structure and composition were monitored by LEED and Auger electron spectroscopy. The sample was exposed to the capillary dosed high-flux beam (D2/CO ratio 10:1) at pressures in the millibar range for varying periods of time. The reaction products were pumped off by a turbomolecular pump together with the non-reacted species, and were thereafter collected in a CuI+ZSMX zeolite trap held at liquid nitrogen temperature. This adsorbent does not pump D2 at temperatures around 77 K, but strongly binds CO and D20, which start to desorb only above 500 K. By differentially warming the trap the product (deuterated methane) could be desorbed near 300 K and effectively separated from the reactants. The total amount of desorbing CD4 was introduced into a separate small UHV system and was there determined with a quadrupole mass spectrometer. Integral reaction rates could in this way be measured.
"7". 6.0
lil
v
10c<0.01 ML
E
5.0 o~ t-
.c_ 4.0
E
3.0 '
I
90
'
I
'
1O0
I
110
'
I
120
'
I
130
time [sec] Fig.2: Titration of carbon resulting from dissociative 12CO adsorption after exposing a partly vanadia-covered Pd(111) surface to a 3:1 D2+CO reaction mixture (CO flux ca. 5 ML/sec for 10 min, Tsample=600 K)
3342 3. RESULTS AND DISCUSSION As a first step the interaction of the clean Rh (111) surface with CO was studied using the capillary array doser at fluxes around 0.1 ML/sec. The sticking probability of molecular CO was measured as a function of surface coverage and sample temperature. On clean and annealed Rh (111) no CO dissociation was observed, and the sticking coefficient of molecular CO was in good agreement with a recent supersonic beams study [12]. However, using again the low-flux beam source, the influence of surface oxide and of hydrogen on the dissociative CO adsorption could be monitored, and submonolayers of carbon on vanadia-covered Pd (111) resulting from dissociatively adsorbed CO were quantitatively determined. An example is shown in Fig. 2: The vanadia-precovered Pd (111) surface (Ov <0.5 ML) was exposed to a 3:1 D2/CO mixture at 600 K for 10 minutes (CO flux 5 ML/sec). At this temperature molecular CO is completely desorbed and the reactivity of submonolayers of carbon resulting from dissociative CO adsorption is high enough to allow a quantitative oxygen titration. By turning the sample in the oxygen beam and measuring the QMS amu 44 signal the carbon dioxide arising from C oxidation on the Pd(lll)/vanadia surface could be effectively separated from the residual gas background (Fig.2). If higher local fluxes were applied by means of the capillary doser, the clean Rh (111) surface remained inactive towards CO dissociation and reaction up to local pressures of 10-1 mbar and 573 K, and no products were detected. However, small amounts of deuterated methane (12CD4) could be found at the same local pressure and temperature if the surface was partly covered with vanadium oxide. While local pressures up to 0.1 mbar were inefficient to initiate a reaction on clean Rh (111), a very sudden onset of the reaction was observed upon a pressure rise to about 5 mbar (total pressure in the system --10-5 mbar).
j
a
/ "~
Rh
b
Rh
Rh I'''1'''1'''1'''1'''1'''1'''1''
200 220 240 260 280 300 320 340
E n e r g y [eV] Fig 3: Left: AES spectra (a) from the clean Rh(111) surface and (b) after reaction at around 5 mbar total reactant pressure (D2:CO=10:I) at 573 K for 10 min. Right: Corresponding LEED pattern. Both the AES and LEED primary electron beam were directed onto the small region of the sample hit by the impinging reactant beam.
3343 This onset was not exactly reproducible, but appeared to depend on the initial state of the surface (defects, impurities, etc.). Furthermore, it was observed that the successive reaction is always accompanied by increasing carbon deposition, but also by severe structural alterations (roughening) of the exposed crystal surface. Carbon deposition on the surface occurs partly ordered, as seen from the LEED pattern and shown with the corresponding Auger features in Fig.3. The reaction-induced structural changes of the originally clean and well-annealed Rh (111) surface may be strong enough to become optically visible (Fig. 4). The region involved in the surface reaction (impingement area 0.5mm 2) appears black and roughened on a macroscopic scale, and carbon is found deposited in this area and also on the surrounding parts of the surface. The ring-like diffraction features in Fig.3 correspond to a bonding distance of 1.40~0.3/~ indicating the formation of a disordered graphitic adlayer. Subsequent exposure to oxygen (700 K, 5x10 -6 mbar, 5 min) could neither restore the initial fiat, well annealed (111) surface nor remove the black or grey surface area in Fig.4, although AES showed that carbon was removed by oxygen during this procedure. Only at temperatures close to the decomposition temperature of Rh203 at about 1400 K the black deposit vanished, but even heating up to 1700 K was not sufficient to completely remove the surface roughness induced by the reaction experiment. It must be therefore assumed that at higher surface concentrations of CO massive adsorbate-induced restructuring occurs, possibly via mobile subcarbonyls [14]. This restructuring will in turn lead to further enhanced catalytic activity via formation of additional and more active surface area, resulting in an autocatalysed reaction. In a recent STM investigation Wilson et al. [15] observed a similar behaviour during CO hydrogenation on cobalt single crystal surfaces. It may be of interest to apply the same experimental conditions to other Rh surfaces on which CO is known to dissociate, e.g. polycrystalline metal foils, and to comparable the corresponding reaction rates [16].
Fig.4: Photograph of the Rh(111) single crystal surface after exposure to a 5 mbar 10:1 D2+CO beam at 573 K for 10 min.
3344 4. CONCLUSIONS The versatile molecular beam arrangement used in our experiments is well-suited for measurement of small amounts of carbon monoxide dissociated on the surface, but it allows also to detect products and monitor surface alterations arising from reactions at local pressures in the millibar range. Under the latter reaction conditions a sudden onset of the CO hydrogenation on Rh (111) is observed, presumably a result of an autocatalytic process induced by surface restructuring and carbon deposition from CO dissociation. ACKNOWLEDGEMENT This work was supported by the Joint Research Project "Gas Surface Interactions" (S 8105) of the Fonds zur F6rderung der wissenschaftlichen Forschung of Austria.
REFERENCES ,
2. 3. 4. 5. 6. .
.
9. 10. 11. 12. 13. 14. 15. 16.
S. J. Tauster, S.C. Fung, J. Catal. 55 (1978) 29 A.B. Boffa, C. Lin, A. T. Bell, G.A. Somorjai, Catalysis Letters 27 (1994) 243 T. Ali, A.V. Walker, B. K16tzer, D.A. King, Surf. Sci. 414 (1998) 304 A.T. Pasteur, X.-C. Guo, T. Ali, M. Gruyters, D.A. King, Surf. Sci. 366 (1996) 564 D. G. Castner, R.L. Blackadar, G. A. Somorjai, J. Catal. 66 (1980) 275 K.J. Williams, A.B. Boffa, J. Lahtinen, M. Salmeron, A.T. Bell, G.A. Somorjai, Catal. Lett. 5 (1990) 385 F.P. Leisenberger, S. Surnev, L. Vitali, M.G. Ramsey, F.P. Netzer, J. Vac. Sci. Technol. A 17 (1999) 1743. T. Koerts, W.J.J. Welters, R.A. van Santen, J. Catal. 134 (1992) 1 K. Hayek, M. Fuchs, B. K16tzer, W. Reichl, Ct. Rupprechter, Topics in Catalysis 2000, in press. M.J. Bozack, L. Muehlhoff, J.N. Russell Jr., W.J. Choyke, J.T. Yates Jr., J. Vac. Sci. Technol. A5(1) (1987) 1 D.A. King, M.G. Wells, Surf. Sci. 29 (1972) 245 M. Beutl, J. Lesnik, K.D. Rendulic, Surf. Sci. 429 (1999) 71 T. Hartmann, H. Kn6zinger, Z. Phys. Chem. 197 (1996) 113 N. Kruse, A. Gaussmann, J. Catal. 144 (1993) 525 J. Wilson, C. de Groot, J. Phys. Chem. 99 (1995) 7860 W. Reichl, K. Hayek, this conference.