Hierarchical Vanadia Model Catalysts for Ammonia Selective Catalytic Reduction

There is an ongoing debate on the structure of titania supported vanadia used for catalytic partial oxidation reactions as well as for abatement of nitrogen oxide (NO_x) by ammonia selective catalytic reduction (NH₃-SCR). To gain further insight into the essential features of the catalyst composition of the surface we introduce a new class of vanadia model catalysts based on monodispersed silica particles as a platform. The evaluation of the influence of different catalyst components (TiO₂, WO₃) and different vanadia synthesis approaches (incipient wetness, ion-exchange) is facilitated by the hierarchical structure of the catalysts as well as the controlled synthesis approach. Catalysts tested under industrial ammonia SCR conditions show a strong dependence on the catalyst composition. In particular, the presence of crystalline titania and tungsta leads to a strong increase in NO conversion. Detailed catalyst characterization by X-ray diffraction and by visible Raman, UV Raman, UV–Vis and X-ray photoelectron spectroscopies provides insight into the bulk structure, the surface composition as well as the vanadia surface structure of the catalysts. The potential of UV resonance Raman spectroscopy for structural analysis of vanadia model catalysts for SCR is highlighted. Differences in catalytic activity are discussed in light of the structural results and the available literature.