Mechanistic study of the effect of coexisting H2O on the selective reduction of NO with propene over sol–gel prepared In2O3-Al2O3 catalyst

Abstract The effect of coexisting H 2 O on the catalytic activity of In 2 O 3 -Al 2 O 3 prepared by the sol–gel method for NO reduction by propene in the presence of oxygen was studied. No activity depression due to coexisting H 2 O was observed for In 2 O 3 -Al 2 O 3 with an In 2 O 3 loading of more than 5 wt.%, although the catalysts with a lower In 2 O 3 loading (∼3 wt.%) showed an inhibition effect by H 2 O. X-ray diffraction (XRD) and X-ray absorption spectroscopy (XANES) revealed the presence of small In 2 O 3 crystallites and relatively large In 2 O 3 particles in the catalysts with lower (∼3 wt.%) and higher (>5 wt.%) In 2 O 3 loadings, respectively. A high H 2 O tolerance of In 2 O 3 -Al 2 O 3 with higher In 2 O 3 loadings of more than 5 wt.% was considered to be due to the presence of the relatively large In 2 O 3 particles interacting with Al 2 O 3 . The results of kinetic studies and in situ diffuse reflectance Fourier transform infrared (FT-IR) experiments showed that coexisting H 2 O suppresses the formation of and/or promotes the desorption of formate and NO 3 − surface species. The present effect of coexisting H 2 O was interpreted by the compensation of the following two opposite effects: the removal of the formate species as a spectator species and the promotion of the hydrolysis of the NCO species (positive effect), and the inhibition of the formation of NO 3 − species as the initial intermediate (negative effect).

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