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

Bond order conservation Morse-potential (BOCMP) model has been used to stud y theoretically the dependence of CO oxidation rate on the size of Rh and P d metal particles. We assume cubo-octahedral geometry for the metal particl es having 38, 201, 586, 1289 and 2406 total number of atoms and calculate t he 'integral' heat of adsorption of atoms and molecules by a statistical av erage of the heat of adsorption at different sites. The bond order conserva tion model is then used to calculate the activation barrier for CO2 formati on as a function of the particle size. An analysis of the activation barrie rs 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 w ith the experimental observations.