PNNL

Abstract

The kinetics of NO reduction over platinum nanoparticles under lean-burn condition was investigated by first-principles-based kinetic Monte Carlo simulations. Three-dimensional model platinum nanoparticles with diameters ranging from 2.29 to 4.61 nm were represented by a truncated octahedron model consisting of a single (100) facet and eight (111) facets. First-principles density functional theory (DFT) calculations were used to determine the intrinsic kinetic parameters including the binding energies for all of the surface intermediates as well as the activation barriers and reaction energies that comprise the reaction mechanism over the (100) facet, the (111) facet and the (111)/(100) edge sites on the three-dimension nanoparticle. Both intra- and inter- facet diffusion of adsorbates were included to model mass transport over the particle surface. The simulation results show that in the presence of excess oxygen, NO reduction to N2 occurs only on the (100) facets. The oxidation of NO to NO2, while much more favored on the (111) facets, can occur on both (100) and (111) facets. Only small amounts of N2O form over the (100) facet. The apparent activation energies for N2 and NO2 formation over the overall particle are 45 kJ/mol and 42 kJ/mol, respectively. This is in agreement with previous experiments. Particle size effects on the activities of NO reduction and oxidation depend on the atomic fractions of the (100) and the (111) facets exposed on the platinum nanoparticles. For the three-dimensional model platinum nanoparticles examined here, the atomic fraction of the (100) facet is nearly the same while the atomic fraction of the (111) facet increases with the increasing particle size. As a result, the activity of NO reduction is insensitive to the particle size. While the larger particle shows higher activity for NO oxidation thus elucidating previous experimental observations. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.