Effect of Hydrothermal Aging on the Catalytic Performance and Morphology of a Vanadia SCR Catalyst

Titania supported vanadia catalysts have been widely used for the selective catalytic reduction (SCR) of nitrogen oxides (NO_x) in diesel exhaust aftertreatment systems. Vanadia SCR (V-SCR) catalysts are preferred for many applications because they have demonstrated advantages of catalytic activity for NO_x removal and tolerance to sulfur poisoning. The primary shortcoming of V-SCR catalysts is their thermal durability. Degradation in NO_x conversion is also related to aging conditions such as at high temperatures. In this study, the impact that short duration hydrothermal aging has on a state-of-the-art V-SCR catalyst was investigated by aging for 2 hr intervals with progressively increased temperatures from 525 to 700°C. The catalytic performance of this V-SCR catalyst upon aging was evaluated by three different reactions of NH₃ SCR, NH₃ oxidation, and NO oxidation under simulated diesel exhaust conditions from 170 to 500°C. The catalytic activity for SCR was stable when this V-SCR catalyst was aged up to 630°C. Significant SCR reactivity loss was observed when aging at temperatures above 680°C. SEM/EDS and N₂ adsorption measurements were applied to elucidate the structure change during the aging process. Increasing the hydrothermal aging temperature results in not only the decrease of the BET surface area and BJH pore volume of the V-SCR catalyst as measured by N₂ adsorption, but also the crack formation of the washcoat as illustrated by SEM.