Mechanochemical preparation of Ag catalysts for the n-octane-SCR de-NOx reaction: Structural and reactivity effects

Abstract Mechanochemical preparation of Ag/Al 2 O 3 catalysts used for the selective catalytic reduction of NO x using hydrocarbons has been shown to substantially increase the activity of the catalyst in comparison with Ag/Al 2 O 3 prepared by wet impregnation. The effect of using different ball-milling experimental parameters on both the structure of the material as well as the catalyst activity has been investigated and the optimum conditions established. A phase transition from γ- to α-alumina was observed milling at high speeds which was found to result in lower catalyst activities. At lower milling speeds both fracturing and agglomeration of the alumina support can be observed depending on the grinding time. However, due to ball-milling, a general enhancement in the NO x reduction activity was observed for all catalysts compared with the conventionally prepared catalysts irrespective of the reductant used. Transient DRIFTS-MS experiments were performed to investigate the effect of H 2 in the absence and presence of water on the SCR reaction over catalysts prepared by both ball milling and wet impregnation. In-situ DRIFTS-MS analysis revealed significant differences in both gas phase and surface species. Most notably, isocyanate species were formed significantly more quickly and at higher surface concentration in the case of the mechanochemically prepared catalyst.