Hybrid catalysts for the selective catalytic reduction of NO by NH3: The influence of component separation on the performance of hybrid systems

The selective catalytic reduction (SCR) of NO by NH3 was investigated over physical mixtures of an oxidation catalyst with an SCR catalyst ("hybrid catalyst") in which the components were arranged in variable distance to each other: pressed in mixed particles after grinding the mixture, mixing particles of individual components, and in two-layer and four-layer beds. Using Fe-ZSM-5 or V2O3 WO3/TiO2 as SCR components and Mn2O3, Mn-Cu and Ce-Zr mixed oxides as oxidation components, it was observed that the pronounced synergies described in literature are observed only when the components are mixed within the particles. The synergy is strongly attenuated in beds of component particles and fades away completely in the layered structures. This observation is at variance with earlier ideas according to which the oxidation component catalyses the formation of NO2 which opens a fast SCR reaction path over the SCR component. Instead, a more labile critical intermediate is more likely, e.g., HNO2, which may be formed over the oxidation component and proceeds to the SCR sites for further reaction along routes discussed in recent literature. The new understanding is in accordance with earlier observations that correlation between NO oxidation activities of oxidation components and performace of the resulting hybrids was observed to be coarse and to include remarkable outliers. (C) 2015 Elsevier B.V. All rights reserved.