Developing multiple-site kinetic models in catalysis simulation : A case study of O2 + 2NO ↔ 2NO2 oxidation-reduction chemistry on Pt(100) catalyst crystal facets

It is generally recognized that developing a kinetic model for a supported catalyst is difficult because of the existence of multiple sites. These sites can arise from a distribution of crystal facets (e.g., (100), (110)) each with its unique intrinsic site types (e.g., atop, bridge, hollow). Additional complexities arise from non-basel plane site types (e.g., defect, edge, corner), the differing lateral interaction energies of which may be coverage-dependent for each of their pairwise interactions. To demonstrate the complexities that develop for even a greatly simplified system, we examine a multiple site kinetic model of the reaction 2NO + O 2 - 2NO2 on an ideal Pt(100) catalyst. A model of the Pt(100) surface is adopted where atop, bridge, and fourfold hollow sites are responsible for O2 , NO, and NO2 chemisorption to form Pt[single bond]O, Pt[single bond]NO, and Pt[single bond]NO2 species. In our kinetic scheme, equilibrium is assumed for O2 , NO, and NO2 chemisorption due to their high sticking coefficients (all >0.1). A single rate-determining step of the Langmuir-Hinshelwood type was chosen to describe the oxidation of NO on platinum via the reaction PtH,A,B [single bond]O + PtH,A,B [single bond]NO - PtH,A,B + PtH,A,B [single bond]NO2 , where H, A, and B represent hollow, atop, and bridge sites. Equal kinetic parameters for all site combinations were assumed to exist and were in part taken from the literature to be H = 83 kJ/mol and S = 20 J/ (K mol). The exercise here is largely hypothetical but offers insight into how more detailed kinetic models may be developed, such as through the use of reaction velocity matrices, a concept introduced here. Specifically for this system, the model yielded insight into NO x chemistry on Pt(100) in that it predicted that the greatest reaction velocities (forward and reverse) occurred via the reaction Pt[single bond]O(atop) + Pt[single bond]NO(bridge) - Pt(atop) + Pt[single bond]NO2 (bridge). We believe that the framework of a site-specific modeling scheme presented here is an important starting point for future site-specific microkinetic modeling. In particular, a definition and description of use of surface coverages, reaction rate coefficients, and computed reaction velocity matrices are presented.