(18g) Statistics of Adsorption Sites During Oxygen Adsorption On Pt(111)
Chemical processes at heterogeneous metal and metal oxide surfaces occur by the adsorption and reaction of molecules from the gas or liquid phase. Developing quantitative, predictive models of the rate laws governing these processes has a wide range of applications in surface and environmental science, including catalysis. The Langmuir-Hinshelwood (L-H) model is the most common way of quantifying reaction rates. For instance, the rate of O2 dissociation, a common step in catalytic oxidations, would be written as:
O2 + 2* -> 2O*, rate = kPO2θ2*
where θ* is the number of empty reactive sites at the Pt surface. This familiar mean-field expression
hides significant and often unreliable assumptions: (1) that the vacant site locations are uncorrelated and randomly distributed, and (2) that the rate of dissociation k is the same at every type of reactive site, i.e., that k is independent of adsorbate coverage. In this work, we use DFT-parameterized cluster expansions of oxygen adsorption at Pt to test these assumptions. We explicitly model the distribution of adsorption sites as a function of adsorbate coverage and temperature, and combine these results with DFT-calculated site-specific dissociation rates. The approach permits a quantitative analysis of more approximate means of incorporating coverage effects into microkinetic models.