FTIR mechanistic studies on the selective catalytic reduction of NO_x with methane over Ni-containing zeolites:comparison between NiY and Ni-ZSM-5

摘要

Low-temperature CO adsorption on NiNaY leads to formation of Ni~(2+)-CO species (IR band at 2195cm~(-1)).In line with this,mainly mononitrosyls (1892 cm~(-1)) are produced after NO adsorption on the sample.These results show the existence of one coordinative vacancy of the Ni~(2+) ions in NiNaY.The results are compared with those of Ni-H-ZSM-5.In the latter case formation of dicarbonyls,dinitrosyls and mixed carbonyl-nitrosyl complexes of Ni~(2+) have been established.It was found that Ni-H-ZSM-5 is active in the selective catalytic reduction (SCR) of nitrogen oxides with methane,whereas the activity of NiNaY is rather poor.The stable species formed after coadsorption of NO and O2 on both samples are surface nitrates and/or nitrites.However,whereas the monodentate nitrates on Ni-H-ZSM-5 are highly reactive towards methane,the reactivity of the nitratesitrites on NiNaY is significantly lower.The mechanism of SCR with methane is discussed.In particular,it is proposed that the high coordinative unsaturation of the Ni~(2+) cations in Ni-H-ZSM-5 is very important for the proceeding of SCR with methane.