Kinetics of the CO+NO reaction over rhodium and platinum-rhodium on alumina - II. Effect of Rh incorporation to Pt

The effect of Rh on the kinetics of the CO+NO reaction on Pt/Al2O3 has been investigated using a fixed bed flow reactor at 300 degrees C under atmospheric pressure with initial CO and NO partial pressure ranges of 1.05 x 10(-3) to 14.7 x 10(-3) atm. The kinetic performances of Rh/Al2O3 and Pt-Rh/Al2O3 catalysts have been interpreted on the basis of two kinetic models which assume competitive adsorptions of NO and CO or noncompetitive adsorptions of the reactants. The former model can correctly fit experimental data both on Ph and Pt-Rh. But the model with noncompetitive adsorptions seems preferable for Pt-Rh/Al2O3 for reasons developed in this paper. The equilibrium adsorption constants of NO are similar on Rh/Al2O3 and Pt-Rh/Al2O3 while those of CO are similar on Pt/Al2O3 and Pt-Rh/Al2O3 which shows that NO preferentially adsorbs on Rh and CO on Pt on Pt-Rh/Al2O3 in agreement with previous results of Van Slooten and Nieuwenhuys (16). It has also been found that adsorbed NO on Ph probably dissociates on a Pt site on the bimetallic Pt-Ph catalyst. N2O is the major N-containing product on Rh/Al2O3 and Pt-Rh/Al2O3. In addition, the selectivity for the formation of N2O is similar on these two catalysts; it is also insensitive to reaction conditions (P-NO, P-CO, and temperature). All these observations would emphasize the fact that NO is coordinated to Ph. The selectivity of Rh/Al2O3 and Pt-Rh/Al2O3 is controlled by a bimolecular reaction (NOads+N-ads), yielding either N-2 or N2O. differs from what is observed on Pt/Al2O3 since the rate of the recombination of two adsorbed N atoms cannot be neglected on Pt alone as shown in the previous paper of this series. (C) 1998 Academic Press.