Mechanism, configurational degeneracy, and mass action in transition-state rate functions for adsorption-desorption reactions

Abstract

A discussion of classical transition-state theory for adsorption and desorption rates is presented, the purpose of which is to demonstrate through several examples how the reaction mechanism and the configurational degeneracy of the adsorbed layer conspire to determine mass actions in rate functions and the relationship between preexponential factors and molecular degrees of freedom. An approximate cell model for the degeneracy of ordered layers is introduced, and the efficacy of the model is illustrated through analyses of several representative systems: e.g., CO on Ni( 1 10) at low coverages, where independent-molecule lattice gas behavior is identified; CO on Ni(ll0) at high coverages, where desorption converts the overlayer from (2x1) to ~( 4 x 2 ) symmetry; and H2 on (2X 1)-Si( loo), where the rate of recombinative desorption displays first-order kinetics due to site pairing. Finally, the principles of these analyses are used to develop a model that describes the behavior of the hexagonal compression structure that forms for CO on Ru(001) at high coverages.

Thermal chemistry of biacetyl on Si(100)

“…After a 4.0 L dose of biacetyl at 160 K, two desorption features are observed at 185 and 263 K. The cracking ratio, 43 amu/86 amu, corresponds to molecular biacetyl desorption and is the same for both states. 44,15,16, and 86 ͑fragment ions consistent with molecular biacetyl͒ also contain peaks at 185 and 285 K. The peak at 263 K shifts to 285 K and saturates for doses above 12.5 L. We assign the 185 and 285 K desorption states to molecular biacetyl desorbing from the the multilayer and from a chemisorbed state in the monolayer, respectively.…”

Current Status of Transition-State Theory

“…In related work, Anton attempted to include adsorbate coverage dependence in classical TST and tested the method for desorption reactions. Pitt et al argue that this derivation cannot be correct for barrierless adsorption.…”

“…The validity of the model for the lateral interactions and the coverage dependence has been questioned 771,772 and defended. 773,774 In related work, Anton 775 attempted to include adsorbate coverage dependence in classical TST and tested the method for desorption reactions. Pitt et al 765 argue that this derivation cannot be correct for barrierless adsorption.…”

The application of transition state theory to gas–surface reactions in Langmuir systems

“…The discussion of Anton 24 is typical of the attempts 25,26 in the literature to describe regime ͑i͒, i.e., reproduce Sϭ1Ϫ . Anton specifies two models for the transition state; ͑a͒ ''transition state molecules are assumed to translate freely in two dimensions over the vacant sites in the chemisorbed layer;'' ͑b͒ ''transition state molecules are assumed to be localized at individual sites over vacancies in the chemisorbed layer.''…”