PNNL

Abstract

By combining first-principles kinetic Monte Carlo (KMC) simulation with a second-order finite difference continuum model, a hybrid computational model is developed to study the effects of heat and mass transfer in the surrounding gas phase on the heterogeneous reaction kinetics. The integrated computational framework consists of a surface phase where catalytic surface reactions occur and a gas-phase boundary layer imposed on the catalyst surface where the temperature and pressure gradients exist. The temperature and pressure gradients in the gas-phase boundary layer are the consequence of thermal and molecular diffusions of reactants and products under reaction conditions. The surface phase domain is modeled using the site-explicit first-principles KMC simulation. The gas-phase boundary layer domain is described using the grid-based Crank-Nicolson method. At each time step, the heat and mass fluxes between two domains are calculated simultaneously until the steady-state reaction condition is reached. At the steady-state reaction condition, the activity, the surface coverages of reaction intermediates, as well as the temperature and pressure gradient profiles in the gas-phase boundary layer are statistically constant with very small fluctuations. To demonstrate the idea that the proper consideration of heat and mass transfer in the reaction environment is essential to accurate measurement of the intrinsic reaction kinetics, we investigated the kinetics of CO oxidation over the RuO2(110) catalysts. By varying the thickness of the RuO2(110) catalyst, the pronounced temperature and pressure gradients are formed in the gas-phase boundary layer. Our simulation results indicate that the temperature and pressure gradients caused by the heat and mass transfer could dramatically affect the observed intrinsic reaction kinetics under presumed nominal operating reaction conditions. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.