KINETICS OF THE CO-RHODIUM ON ALUMINA - II - EFFECT OF RH INCORPORATION TO PT(NO REACTION OVER RHODIUM AND PLATINUM)

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 und er 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 c orrectly fit experimental data both on Ph and Pt-Rh. But the model wit h noncompetitive adsorptions seems preferable for Pt-Rh/Al2O3 for reas ons developed in this paper. The equilibrium adsorption constants of N O are similar on Rh/Al2O3 and Pt-Rh/Al2O3 while those of CO are simila r on Pt/Al2O3 and Pt-Rh/Al2O3 which shows that NO preferentially adsor bs on Rh and CO on Pt on Pt-Rh/Al2O3 in agreement with previous result s of Van Slooten and Nieuwenhuys (16). It has also been found that ads orbed 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 sim ilar on these two catalysts; it is also insensitive to reaction condit ions (P-NO, P-CO, and temperature). All these observations would empha size the fact that NO is coordinated to Ph. The selectivity of Rh/Al2O 3 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/Al2 O3 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.