Effects of heat and mass transfer on the kinetics of CO oxidation over RuO 2(1 1 0) catalyst

Abstract

Combining first-principles kinetic Monte Carlo (KMC) simulation with a finite difference continuum model, a hybrid computational model was developed to study the effects of heat and mass transfer 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 fluctuating temperature and pressure gradients exist. The surface phase domain is modeled by the site-explicit first-principles KMC simulation. The gas-phase boundary layer domain is described using the second-order grid-based Crank–Nicolson method. To simplify the model, the flow and gas-phase reactions are excluded. The temperature and pressure gradients in the gas-phase boundary layer are the consequence of thermal and molecular diffusions of reactants and products under nominal reaction conditions. Different from previous hybrid multiscale models, the heat and mass fluxes between two domains are directly coupled by the varying boundary conditions at each simulation timestep from the unsteady state reaction regime to the steady state reaction regime in the present model. At the steady-state reaction regime, the activity, the surface coverages of reaction intermediates, along with the temperature and pressure gradient profiles in the gas-phase boundary layer are statistically constant with very small fluctuations. As an illustration example, we studied the effects of heat and mass transfer on the reaction kinetics of CO oxidation over the RuO 2(1 1 0) catalysts. We assume the heat from CO oxidation is exclusively dissipated into the gas-phase via thermal diffusion. By varying the thickness of RuO 2(1 1 0) catalysts, the surface temperature changes correspondingly with the heat produced by occurring surface reactions, resulting in the pronounced temperature and pressure gradients in the gas-phase boundary layer. Our simulation results indicate that the limitation of heat and mass transfer in the surrounding environment over the catalyst could dramatically affect the observed macroscopic reaction kinetics under presumed operating reaction conditions. To fully elucidate the complex heterogeneous catalytic system, proper physical description of fluid phase that imposed on the catalyst and its effect on the surface kinetics should be integrated with current surface computational models.