Low-temperature selective catalytic reduction (SCR) of NO _x with n-octane using solvent-free mechanochemically prepared Ag/Al₂O ₃ catalysts

Low-temperature (<200 °C) hydrocarbon selective catalytic reduction of NO_x has been achieved for the first time in the absence of hydrogen using a solvent-free mechanochemically prepared Ag/Al₂O ₃ catalyst. Catalysts prepared by this ball-milling method show a remarkable increase in activity for the reduction of nitrogen oxides with octane by lowering the light-off temperature by up to 150 °C compared with a state-of-the-art 2 wt % Ag/Al₂O₃ catalyst prepared by wet impregnation. The best catalyst prepared from silver oxide showed 50% NO _x conversion at 240 °C and 99% at 302 °C. The increased activity is not due to an increased surface area of the support, but may be associated with a change in the defect structure of the alumina surface, leading to the formation of the small silver clusters necessary for the activation of the octane without leading to total combustion. On the other hand, since one possible role of hydrogen is to remove inhibiting species from the silver, we cannot exclude some change in the chemical properties of the silver as a result of the ball-milling treatment.