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The mechanism of formate oxidation on Ag(110)
A557 Surface Science 176 (1986) 415-424 North-Holland, A m s t e r d a m THE
MECHANISM
Allen G. SAULT
415
OF FORMATE and Robert
OXIDATION
ON
Ag(ll0)
J. M A D I X
Department of Chemical Engineerin~ Stanford University, Stanford, CA 94305, USA Received 24 February 1986, accepted for publication 23 June 1986 The presence of coadsorbed oxygen atoms results in a 4 k c a l / m o l e decrease in the decomposition stability of formate on Ag(ll0). This destabilization provides an explanation for an observed 500-fold increase in the rate of formate decomposition at 327 K in the presence of 1.0 Torr of O 2 gas. Thus, the effect of ambient O 2 on the stability of surface formate can be explained by alteration of the surface composition in the presence of the ambient gas. This result shows that reactions which occur at high pressures can be observed using ultra-high vacuum techniques provided the surface concentrations obtained at high pressures can be duplicated in vacuum at temperatures which are high enough for reaction to proceed at a measurable rate.
Surface Science 176 (1986) 425-437 North-Holland, A m s t e r d a m ENERGY
TRANSFER
MOLECULE-SOLID Andrew
E. D E P R I S T O
PROCESSES SURFACE * and Lynn
425 IN LINEAR
TRIATOMIC
COLLISIONS C. GEIGER
**
Department of Chemistry, lowa State University, Ames, 1,4 50011, USA Received 23 April 1986; accepted for publication 19 June 1986 The semiclassical stochastic trajectory method is extended to the study of rotational and vibrational transitions for linear triatomic molecules colliding with non-rigid solid surfaces. Rotational and vibrational motion are treated by q u a n t u m mechanics, translational motion by classical mechanics, and surface atom motion by the classical generalized Langevin equation. Self-consistent coupling of all motions is enforced via Ehrenfest's theorem. Calculations of the kinetic energy and gas temperature dependence of trapping probabilities, vibrational relaxation probabilities and final vibrational state distributions are presented for the CO 2 - A g ( l l l ) system at surface temperatures of 0 and 600 K. The trapping probabilities are greatly enhanced by the rotational motion and also vary to some degree with the initial vibrational state of the CO 2. Total vibrationally inelastic probabilities ~ire on the order of 10 -2 for a single collision event with an initial state (00%). For the initial state (0110) these are m u c h larger, - 10-1, due to the nature of bending mode motion. In conjunction with the large trapping probabilities, the mechanism of vibration to vibration, rotation, translation, phonon energy transfer can provide vibration relaxation probabilities in the range of those measured experimentally. A pseudo-selection rule for conservation of vibrational angular m o m e n t u m is found.
MECHANISM
Allen G. SAULT
415
OF FORMATE and Robert
OXIDATION
ON
Ag(ll0)
J. M A D I X
Department of Chemical Engineerin~ Stanford University, Stanford, CA 94305, USA Received 24 February 1986, accepted for publication 23 June 1986 The presence of coadsorbed oxygen atoms results in a 4 k c a l / m o l e decrease in the decomposition stability of formate on Ag(ll0). This destabilization provides an explanation for an observed 500-fold increase in the rate of formate decomposition at 327 K in the presence of 1.0 Torr of O 2 gas. Thus, the effect of ambient O 2 on the stability of surface formate can be explained by alteration of the surface composition in the presence of the ambient gas. This result shows that reactions which occur at high pressures can be observed using ultra-high vacuum techniques provided the surface concentrations obtained at high pressures can be duplicated in vacuum at temperatures which are high enough for reaction to proceed at a measurable rate.
Surface Science 176 (1986) 425-437 North-Holland, A m s t e r d a m ENERGY
TRANSFER
MOLECULE-SOLID Andrew
E. D E P R I S T O
PROCESSES SURFACE * and Lynn
425 IN LINEAR
TRIATOMIC
COLLISIONS C. GEIGER
**
Department of Chemistry, lowa State University, Ames, 1,4 50011, USA Received 23 April 1986; accepted for publication 19 June 1986 The semiclassical stochastic trajectory method is extended to the study of rotational and vibrational transitions for linear triatomic molecules colliding with non-rigid solid surfaces. Rotational and vibrational motion are treated by q u a n t u m mechanics, translational motion by classical mechanics, and surface atom motion by the classical generalized Langevin equation. Self-consistent coupling of all motions is enforced via Ehrenfest's theorem. Calculations of the kinetic energy and gas temperature dependence of trapping probabilities, vibrational relaxation probabilities and final vibrational state distributions are presented for the CO 2 - A g ( l l l ) system at surface temperatures of 0 and 600 K. The trapping probabilities are greatly enhanced by the rotational motion and also vary to some degree with the initial vibrational state of the CO 2. Total vibrationally inelastic probabilities ~ire on the order of 10 -2 for a single collision event with an initial state (00%). For the initial state (0110) these are m u c h larger, - 10-1, due to the nature of bending mode motion. In conjunction with the large trapping probabilities, the mechanism of vibration to vibration, rotation, translation, phonon energy transfer can provide vibration relaxation probabilities in the range of those measured experimentally. A pseudo-selection rule for conservation of vibrational angular m o m e n t u m is found.