Effect of water vapor on the activity of Pt-Pd/Al2O3 catalysts for methane combustion

A study on the deactivation phenomena affecting the catalysts of small-scale natural gas catalytic combustors was undertaken in this work. The influence of simulated aging pretreatments on the activity of Pt-Pd catalysts supported over gamma alumina for the catalytic combustion of CH4 was assessed under atmospheric conditions and in the 50-950degreesC temperature range. Accelerated deactivation tests consisted of steaming at temperatures between 500-800degreesC. The results showed that the activity of the deactivated catalysts depended on two opposite phenomena. An enhancement of the activity was encountered for low aging temperatures and was addressed to chlorine (Cl) elimination (as detected by chemical analysis), Conversely, at higher temperatures the accelerated aging treatment produced a decrease in the activity, and this was allegedly linked to a decrease in the available metal surface area produced by sintering. During the catalyst lifespan and under real operation conditions, a first period is therefore expected where the catalytic activity improves due to the Cl elimination by the water (1120) produced during combustion. Afterwards, the activity would decrease because of the sintering of the metal phase. Both processes, Cl elimination and sintering, occur simultaneously. Five different catalyst formulations with variable Pt and Pd contents were tried and 0.4%Pt-0.8%Pd was found to be the most active. The results found are discussed in terms of the activity of the monometallic catalysts, the degree of interaction of the Pt and Pd phases and the previously mentioned phenomena of sintering and Cl elimination. (C) 2002 Elsevier Science B.V. All rights reserved.