Simulation of the transient CO oxidation over Rh0/SiO2 and Rhx+/Ce0.68Zr0.32O2 catalysts

The simulation of temperature transient CO oxidation by molecular oxygen was performed over well-defined catalytic centers (zero-valent (Rh0) or oxidized (Rhx+)). Over both sites, a reliable estimation of CO light-off temperatures could be achieved. For this purpose, detailed kinetic rate equations were established from the proposed catalytic sequences of elementary steps. Over Rh0, the detailed rate equation, approximated to a simplified rate equation which is very close to the global power-rate equation, allowed for a good fitting of the transient CO oxidation up to 60% conversion of CO. Over Rhx+, the preexponential factors and the activation energies of the kinetically significant elementary steps were determined. The simulation of transient CO oxidation with the detailed rate equation allowed for a much better fit of the experimental data (up to 60% conversion) than that of the global power-rate equation (up to 10%). Rhx+ provides an excellent example of the superiority of the detailed kinetic rate equation over that of the global power rate, the evidence of which could not be revealed over Rh0. Finally, for Rhx+, the nature of the kinetically significant elementary steps and the limits of these kinetics are discussed on the basis of the estimated preexponential factors.