Mechanism by which three-dimensional ordered mesoporous CeO2 promotes the activity of VOx-MnOx/CeO2 catalysts for NOx abatement: Atomic-scale insight into the catalytic cycle

Abstract

Submonolayer (below one monolayer coverage) vanadium oxide (VOx)-supported catalysts exhibit high activity, which can be attributed to the nature of the active surface VOx species and electronic effects. Here, a catalyst architecture of submonolayer dimeric VOx species and MnOx supported on three-dimensional (3-D) ordered mesoporous CeO2 was successfully synthesized (VOx-MnOx/kit-CeO2) to enhance the selective catalytic reduction (SCR) of NOx with ammonia activity at low temperatures (<300 °C). The VOx-MnOx/kit-CeO2 catalyst performed excellent NH3-SCR activity at 250 °C under a high gas hourly space velocity of 160000 h−1. The larger surface area and 3-D mesoporous channels of the VOx-MnOx/kit-CeO2 catalyst endowed the material with a significantly strong NH3 adsorption capacity, surface reducibility and surface oxygen activity, which were crucial to determine the low-temperature NH3-SCR performance. The highly dispersed submonolayer dimeric VOx sites over the large surface area (>100 m2/g) and 3-D mesoporous channels of VOx-MnOx/kit-CeO2 produced abundant Brønsted and Lewis acid sites, subsequently enhancing the acid cycle of NH3-SCR. The high ratio of Ce3+/(Ce3++Ce4+) and Mn4+/(Mn3++Mn2+) and easy electron transfer sped up the NH3-SCR redox cycle. Ammonia species on both Brønsted and Lewis acid sites rapidly reacted with bridging nitrite species to yield N2 and H2O.