An attempt at modelling the activity of Pt-Rh/Al 2O 3 three-way catalysts in the CO+NO reaction

Abstract

This study deals with the kinetic behaviour of a Pt-Rh/Al 2O 3 three-way catalyst (TWC) in the CO+NO reaction close to the actual conditions. Practically, it consists in validating a previous rate equation, established at 300°C from a selected mechanism [P. Granger, J.J. Lecomte, L. Leclercq, G. Leclercq, J. Catal . 175 (1998) 194], in a wider range of temperature particularly near 100% NO conversion. Preliminary calculations using a non linear least square method lead to an estimation of pre-exponential factors (for k, the rate constant of the rate limiting step and for λ CO and λ NO, the adsorption equilibrium constants of CO and NO), the activation energy for the dissociation of adsorbed NO molecules, and the enthalpies of CO and NO adsorption (Δ H ads,CO and Δ H ads,NO). The temperature-programmed conversion and selectivity curves obtained in differential conditions using a recycle fixed bed flow reactor have been modelled using these adjusted parameters. Based on CO chemisorption observations it was found that a better fit is obtained by accounting for the coverage dependency of Δ H ads,CO and Δ H ads,NO. Finally, we have attempted to quantify such effects.