Effect of water vapor on the deactivation of Pd-zeolite catalysts for selective catalytic reduction of nitrogen monoxide by methane

Abstract

For the selective reduction of nitrogen oxides (NOx) by methane, Pd-MOR (Pd: 0.47 wt.%) showed stable activity in the absence of water vapor at 450°C for more than 30 h, while it showed a gradual decrease in NOx conversion in the presence of water vapor. Pd-ZSM-5 (Pd: 0.58 wt.%) showed a gradual decrease in NOx conversion even in the absence of water vapor and almost lost activity in 20 h in the presence of water vapor. CO adsorption measurement revealed that Pd-ZSM-5 tested in the presence of water suffered the severest decrease in Pd dispersion. Raman measurement clearly indicated that PdO is formed on the deactivated samples. These results suggested that deactivation of Pd-zeolite is caused by Pd agglomeration to form PdO and water vapor promotes the agglomeration.