Silver-alumina catalysts for selective reduction of NO by higher hydrocarbons: structure of active sites and reaction mechanism

Silver-alumina catalysts prepared by sol-gel method (Ag-Al2O3) and impregnation method (Ag/Al2O3) were studied for the selective catalytic reduction (SCR) of NO by higher alkanes (n-hexane and n-octane). UV-VIS and Ag K-edge XAFS results established the structure of the catalysts; below 2 wt.%, highly dispersed Ag+ ions are predominant Ag species, while at higher Ag loading, Ag-n, clusters are predominant. A relationship between the structure of Ag species and their catalytic function for SCR by n-octane was clarified. Ag+ ions are responsible for the selective reduction of NO to N-2, while the Ag-n, clusters are responsible for the hydrocarbon combustion and N2O formation. The mechanism of SCR by n-hexane on Ag-Al2O3, which mainly consists of Ag+ ions, were investigated by in situ FTIR spectroscopy. During NO + n-hexane + O-2 reaction, the acetate produced via the partial oxidation of n-hexane and the nitrates produced via the oxidation of NO were main adspecies in the steady-state condition at 473-623 K. The acetate, which was stable in O-2 or NO, was reactive in NO + O-2. Nitrates, which were relatively stable in n-hexane, were quite reactive toward n-hexane + O-2. The rate of nitrates reaction in n-hexane + O-2 was close to the steady-state rate of NO reduction over wide range of temperature, indicating that the nitrate is a possible intermediate in the SCR. A proposed mechanism, suggesting the reaction of nitrates with partially oxidized hydrocarbon species as a crucial step, explains the steady-state kinetic results. At relatively high NO concentration or at low temperature, nitrates should inhibit the reaction by strongly adsorbing on the catalyst surface. (C) 2001 Elsevier Science B.V. All rights reserved.