Simultaneous abatement of NO and N2O with CH4 over modified Al2O3 supported Pt,Pd,Rh

Pt, Pd and Rh supported on Al2O3-SiO2 and Al2O3-ZrO2, prepared by adsorbing noble metal ions from salt aqueous solutions on well-mixed supports, were characterized by XRD, N-2 physisorption and FESEM, and studied for the simultaneous abatement of NO and N2O by selective catalytic reduction in the presence of O-2 using CH4 as reductant (SCRsim). To give a better insight into the simultaneous process, the reactions related to SCRsim (SCRNO, SCRN2O, CH4 combustion), as well as the abatements in the absence of O-2 (CRsim, CRNO, CRN2O), and the N2O decomposition by itself and in the presence of O-2 and NO, were investigated. The catalytic measurements were performed in a flow apparatus with GC analysis of reactants and products. Catalytic results showed that Pt,Pd, Rh/Al2O3-SiO2 and Pt,Pd,Rh/Al2O3-ZrO2 are effective catalysts for SCRsim above 400 degrees C from feeds containing O-2/CH4 less than 1, yielding complete NO and N2O conversions and complete selectivity to CO2 and N-2. At lower temperatures, N2O and NO are unconverted and only the competitive CH4 combustion occurs. Compared to the separate NO and N2O abatement reactions, a slight shift in activity towards higher temperatures occurs in SCRsim. Such a shift can be related to the possible formation of strongly adsorbed NOy-like species formed in the presence of NO and competing with the N2O adsorption sites. The poisoning effect of these species disappears above a threshold-like temperature (about 300 degrees C), suggesting that above this temperature the high surface-O mobility, guaranteeing a partial reduction of the noble metal ions by CH4, sustains the reductive reactions of both NO and N2O.