Influence of particle size on catalytic activity: CO oxidation on Rh and Pd nanostructures

Abstract

Bond order conservation Morse-potential (BOCMP) model has been used to study theoretically the dependence of CO oxidation rate on the size of Rh and Pd metal particles. We assume cubo-octahedral geometry for the metal particles having 38, 201, 586, 1289 and 2406 total number of atoms and calculate the 'integral' heat of adsorption of atoms and molecules by a statistical average of the heat of adsorption at different sites. The bond order conservation model is then used to calculate the activation barrier for CO₂ formation as a function of the particle size. An analysis of the activation barriers for different steps indicates that for both the metals CO₂ formation is the rate limiting step. The calculated rate of CO₂ formation for both metal particles shows higher activity for smaller particles in total agreement with the experimental observations.