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, N2 physisorption and FESEM, and studied for the simultaneous abatement of NO and N2O by selective catalytic reduction in the presence of O2 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 O2 (CRsim, CRNO, CRN2O), and the N2O decomposition by itself and in the presence of O2 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 °C from feeds containing O2/CH4 less than 1, yielding complete NO and N2O conversions and complete selectivity to CO2 and N2. 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 °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.