The effect of zeolite structures on the creation of InO + active sites for NO xreduction with methane

Abstract

Catalytic activities of indium oxide (In 2O 3) physically mixed with various H-form zeolites for NO x reduction with methane were investigated, and the effects of zeolite type, acidity, and structure on the catalytic activity are discussed. H-mordenite, H-ZSM-5, and H-beta gave higher activities with In 2O 3 for reduction of NO 2 than in the absence of In 2O 3. This indicates that InO +, formed by the solid state ion exchange between In 2O 3 and protonic acid sites of zeolites, is active for this reaction. The level of solid state ion exchange was well correlated to the strength of zeolitic acidity; the sequence of the catalytic activity was on the order of H-mordenite > H-ZSM-5 > H-beta ≫ H-Y>SiO 2-Al 2O 3. The size of H-ZSM-5 particles made a difference in the catalytic activity for NO 2-CH 4 reaction; the activity decreased with increasing particle size. There exists a particle structure effect on the creation of InO + on the zeolite structure: mesopores are needed for the dispersion of In 2O 3 into ZSM-5 zeolite structures, while the In 2O 3 is not always used for the creation of InO + sites. In/H-ZSM-5 having larger amounts of In on the external surface of ZSM-5 showed lower catalytic activities for NO 2CH 4 reaction in the case that the same ZSM-5 grains are used. The decrease of the In/Si ratio on the outer surface of zeolite corresponds to the decrease in the amount of NH 3 desorbed during TPD measurements, that is, the increase in the number of InO + sites. There might be a diffusion limitation on ln/H-ZSM-5 during NO 2-CH 4 reactions in the reaction conditions studied.