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CO and O2 adsorption on Rh(110)
A442 Surface Science 253 (1991) 33-43 North-Holland
33
CO and O 2 adsorption on Rh(ll0) Michael Bowker, Quanmin Guo and Richard Joyner Leverhulme Centre for Innovative Catalysis, Department of Chemistry and Surface Science Research Centre, University of Liverpool, Liverpool L69 3BX, UK Received 20 November 1990; accepted for publication 14 February 1991
The adsorption of CO and oxygen on R.h(ll0) has been studied using a thermal molecular beam, thermal desorption a n d LEED. CO adsorbs with an initial sticking coefficient of 0.68 and shows a coverage dependence which is well described by the Kisliuk formalism for precursor kinetics. The desorption shows a main peak at 485 K, which shifts to lower temperature with increasing coverage up to ~ 0.4 monolayers. Above this coverage a shoulder appears at - 425 K and a further shoulder at 390 K above about 0.75 monolayers. These effects are due to repulsive lateral interactions in the adlayer, though the only clear ordered pattern seen in the LEED at 320 K is a (2 × 1)plgl pattern above 0.9 monolayers. For oxygen the initial sticking coefficient is 0.62 ( + 0.01) at 310 K which diminishes relatively slowly with increasing oxygen coverage. We believe that this is due to row pairing in the adlayer which produee, s "stretched" rows of higher reactivity adjacent to them. The row pairing is seen in a wide sequence of L E E D patterns with increasing coverage - (1 × 3), (2 × 2)plgl, (1 x 2)/(1 × 3), (2 × 3)plgl, c(2 x 6), c(2 × 8). Much higher exposures are required for the latter 3 structures since the sticking coefficient has diminished to a low value by then. The apparent "saturation" point in the sticking curves is 0.65 monolayers, but oxygen can continue to adsorb slowly. We propose that the last three structures are due to completely restructured surfaces, possibly with subsurface oxygen, though this requires further work. Desorption from these high coverage states begins at the relatively low temperature of 750 K.
44
Surface Science 253 (1991) 4 4 - 5 8 North-Holland
In-situ FT-IRAS study of the CO oxidation reaction over Ru(001) I. Evidence for an Eley-Rideal mechanism at high pressures? C.H.F. Peden *, D.W. Goodman 1 Sandia National Laboratories * *, P.O. Box 5800, Albuquerque, NM 87185, USA M.D.
Weisel and F.M. Hoffmann
*
Exxon Research and Engineering Company, Corporate Research Science Laboratories, Annandale, NJ 08801, USA Received 14 August 1990; accepted for publication 30 January 1991 Utilizing time-resolved Fourier Transform Reflection Absorption-Infrared Spectroscopy (FT-IRAS), we have investigated the CO oxidation reaction on a Ru(001) surface in-situ at high pressures. The vibrational spectra allow us to characterize the nature of the surface during reaction and qualitatively determine steady-state coverages of CO and oxygen. U n d e r oxidizing conditions ( C O / O 2 ratios < 1) where the reaction rate is highest, the surface is found to be covered with a monolayer (ML) of oxygen [O-(1 × l)-Ru(001)]. Little, if any C O (Oco < 0.01) was found to adsorb at these surfaces under high-pressure steady-state conditions at 500 K. or at 85 K in UHV. From this latter result, we estimate an upper limit of Eads(CO ) < 10 k c a l / m o l , and for high-pressure reaction at 500 K. CO coverages of 0¢o < 10 -5 and residence times of rco < 10 -11 s. U n d e r reducing conditions ( C O / O 2 ratios > 2), the steady-state coverage of oxygen decreases with decreasing oxygen partial pressure concurrent with a large reduction in reaction rates. Vibrational data reveal a steady-state coverage of CO (0¢o =0.11, E ~ ( C O ) -- 25 k c a l / m o l ) adsorbed on an O - ( 2 x l ) - R u ( 0 0 1 ) surface (0 o = 0.5). Severely reducing conditions lead to low steady-state oxygen coverages < ~ ML and island formation of oxygen. The impfieations of the various CO species and of the oxygen island formation are discussed in relation to the reaction mechanisms suggested by our previous kinetic study. In particular, we propose for reaction under oxidizing conditions an Eley-Rideal mechanism involving reaction between gas.phase or weakly adsorbed CO and the 0-(1 x l)-Ru(O01) surface. U n d e r reducing conditions on the 0 - ( 2 × 1) surface the reactions proceed via a L a n g m u i r - H i n s h e l w o o d mechanism between chemisorbed C O and oxygen.
33
CO and O 2 adsorption on Rh(ll0) Michael Bowker, Quanmin Guo and Richard Joyner Leverhulme Centre for Innovative Catalysis, Department of Chemistry and Surface Science Research Centre, University of Liverpool, Liverpool L69 3BX, UK Received 20 November 1990; accepted for publication 14 February 1991
The adsorption of CO and oxygen on R.h(ll0) has been studied using a thermal molecular beam, thermal desorption a n d LEED. CO adsorbs with an initial sticking coefficient of 0.68 and shows a coverage dependence which is well described by the Kisliuk formalism for precursor kinetics. The desorption shows a main peak at 485 K, which shifts to lower temperature with increasing coverage up to ~ 0.4 monolayers. Above this coverage a shoulder appears at - 425 K and a further shoulder at 390 K above about 0.75 monolayers. These effects are due to repulsive lateral interactions in the adlayer, though the only clear ordered pattern seen in the LEED at 320 K is a (2 × 1)plgl pattern above 0.9 monolayers. For oxygen the initial sticking coefficient is 0.62 ( + 0.01) at 310 K which diminishes relatively slowly with increasing oxygen coverage. We believe that this is due to row pairing in the adlayer which produee, s "stretched" rows of higher reactivity adjacent to them. The row pairing is seen in a wide sequence of L E E D patterns with increasing coverage - (1 × 3), (2 × 2)plgl, (1 x 2)/(1 × 3), (2 × 3)plgl, c(2 x 6), c(2 × 8). Much higher exposures are required for the latter 3 structures since the sticking coefficient has diminished to a low value by then. The apparent "saturation" point in the sticking curves is 0.65 monolayers, but oxygen can continue to adsorb slowly. We propose that the last three structures are due to completely restructured surfaces, possibly with subsurface oxygen, though this requires further work. Desorption from these high coverage states begins at the relatively low temperature of 750 K.
44
Surface Science 253 (1991) 4 4 - 5 8 North-Holland
In-situ FT-IRAS study of the CO oxidation reaction over Ru(001) I. Evidence for an Eley-Rideal mechanism at high pressures? C.H.F. Peden *, D.W. Goodman 1 Sandia National Laboratories * *, P.O. Box 5800, Albuquerque, NM 87185, USA M.D.
Weisel and F.M. Hoffmann
*
Exxon Research and Engineering Company, Corporate Research Science Laboratories, Annandale, NJ 08801, USA Received 14 August 1990; accepted for publication 30 January 1991 Utilizing time-resolved Fourier Transform Reflection Absorption-Infrared Spectroscopy (FT-IRAS), we have investigated the CO oxidation reaction on a Ru(001) surface in-situ at high pressures. The vibrational spectra allow us to characterize the nature of the surface during reaction and qualitatively determine steady-state coverages of CO and oxygen. U n d e r oxidizing conditions ( C O / O 2 ratios < 1) where the reaction rate is highest, the surface is found to be covered with a monolayer (ML) of oxygen [O-(1 × l)-Ru(001)]. Little, if any C O (Oco < 0.01) was found to adsorb at these surfaces under high-pressure steady-state conditions at 500 K. or at 85 K in UHV. From this latter result, we estimate an upper limit of Eads(CO ) < 10 k c a l / m o l , and for high-pressure reaction at 500 K. CO coverages of 0¢o < 10 -5 and residence times of rco < 10 -11 s. U n d e r reducing conditions ( C O / O 2 ratios > 2), the steady-state coverage of oxygen decreases with decreasing oxygen partial pressure concurrent with a large reduction in reaction rates. Vibrational data reveal a steady-state coverage of CO (0¢o =0.11, E ~ ( C O ) -- 25 k c a l / m o l ) adsorbed on an O - ( 2 x l ) - R u ( 0 0 1 ) surface (0 o = 0.5). Severely reducing conditions lead to low steady-state oxygen coverages < ~ ML and island formation of oxygen. The impfieations of the various CO species and of the oxygen island formation are discussed in relation to the reaction mechanisms suggested by our previous kinetic study. In particular, we propose for reaction under oxidizing conditions an Eley-Rideal mechanism involving reaction between gas.phase or weakly adsorbed CO and the 0-(1 x l)-Ru(O01) surface. U n d e r reducing conditions on the 0 - ( 2 × 1) surface the reactions proceed via a L a n g m u i r - H i n s h e l w o o d mechanism between chemisorbed C O and oxygen.