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 + O2 ↔ 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 PtO, PtNO, and PtNO2 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,BO + PtH,A,BNO ↔ PtH,A,B + PtH,A,BNO2, 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 ΔH†=83 kJ/molΔH†=83 kJ/mol and ΔS†=20 J/(Kmol). 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 NOx chemistry on Pt(100) in that it predicted that the greatest reaction velocities (forward and reverse) occurred via the reaction PtO(atop) + PtNO(bridge) ↔ Pt(atop) + PtNO2(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.