Assessing the surface modifications following the mechanochemical preparation of a Ag/Al₂O₃ selective catalytic reduction catalyst

The surface modification of a mechanochemically prepared Ag/Al ₂O₃ catalyst compared with catalysts prepared by standard wet impregnated methods has been probed using two-dimensional T ₁-T₂ NMR correlations, H₂O temperature programmed desorption (TPD) and DRIFTS. The catalysts were examined for the selective catalytic reduction of NO_x using n-octane in the presence and absence of H₂. Higher activities were observed for the ball milled catalysts irrespective of whether H₂ was added. This higher activity is thought to be related to the increased affinity of the catalyst surface towards the hydrocarbon relative to water, following mechanochemical preparation, resulting in higher concentrations of the hydrocarbon and lower concentrations of water at the surface. DRIFTS experiments demonstrated that surface isocyanate was formed significantly quicker and had a higher surface concentration in the case of the ball milled catalyst which has been correlated with the stronger interaction of the n-octane with the surface. This increased interaction may also be the cause of the reduced activation barrier measured for this catalyst compared with the wet impregnated system. The decreased interaction of water with the surface on ball milling is thought to reduce the effect of site blocking whilst still providing a sufficiently high surface concentration of water to enable effective hydrolysis of the isocyanate to form ammonia and, thereafter, N₂.