Scaling Parameters for Dynamic Diffusion-Reaction over Porous Catalysts

Abstract

The e↵ect of di↵usion resistance in porous solid catalysts on reaction rate during periodic cycling of CO concentration is shown for CO oxidation over Pt/Al 2 O 3 by numerical simulation. At some cycling frequencies, the average reaction rate during cycling is higher than the steady-state rate at the mean CO concentration, as expected for this nonlinear, reactant-inhibited reaction. In order to identify major aspects of dynamic di↵usion-reaction behavior, a simple kinetic mechanism that shows the main features of CO oxidation and other reactions with significant inhibition by reactants is investigated. A single dimensionless parameter group, the dynamic di↵usion coe cient, is added when going from steady-state to unsteady-state di↵usion-reaction equations. In the dynamic di↵usion coe cient, the rate at which gas-phase reactant di↵uses is reduced by the surface adsorption capacity of the catalyst. The frequency at which the peak average rate occurs is controlled by the dynamic di↵usion coe cient.