Catalytic behavior of Co-based/zeolites catalysts was investigated in NOx reduction by CH4. Optimization of promoter and support was investigated by catalytic tests, and the relationship between catalytic activity and catalyst structure was illustrated by catalyst characterization. Co-Fe/SAPO-34 exhibited the highest activity among various Co-base/zeolites catalysts. The maximum conversion of NOx with 52.7% was obtained on Co-Fe/SAPO-34 at 450℃. The inhibition of activity of Fe/zeolites became severe in the presence of SO2, CO2, and H2O. CO2 exerted virtually no effect on the SCR activity of Co-Fe/zeolites. The inhibition of NOx conversion by H2O was reversible for Co-Fe/zeolites catalysts. Cobalt species were mainly present in CoO and Co(OH)2 states in Co-Fe/SAPO-34. Co3O4and Co(OH)2 were the main cobalt species of Co-Fe/ZSM-5, while CoO, CoAl2O4 and Co3O4 might be present in Co-Fe/Beta. The ratio of Fe2+/Fe3+ in the surface layer of Co-Fe/zeolites decreased in the order of Co-Fe/ZSM-5(3.98) > Co-Fe/SAPO-34(0.52) > Co-Fe/Beta(0.43). The active states of cobalt species and suitable ratio of Fe2+/Fe3+ were important for the activity of Co-Fe/zeolites in CH4-SCR. CH4-SCR over Co-Fe/zeolite catalysts started with the adsorption of NO and CH4 on Brønsted acid sites of the zeolite to produce NO+and carbon-containing species(-C=O and -COO) in the presence of oxygen, respectively. Subsequently, the important intermediates of nitrate species were generated from NO+ at the active sites. Finally, nitrate species reacted with carbon-containing species to form N2 and CO2.
Keywords: CH4; Co-based/zeolites; NOx; catalytic reduction; promoter; support.