Promotional effect of MoOx on Pt/ZrO2 catalysts for low-temperature NOx reduction by H2 in excess of O2

This study investigates the NO x reduction by H 2 in excess of O 2 using Pt/Mo/ZrO 2 catalysts with Mo loads ranging from 0 to 10 wt%. Catalytic evaluation performed in synthetic lean exhaust at a gas hourly space velocity of 150,000 h −1 showed the highest efficiency for the catalyst with 3 wt% Mo achieving a maximum NO x conversion of 86% at 135 °C and N 2 selectivity up to 78%. The samples were thoroughly characterized by powder X-ray diffraction, N 2 physisorption, temperature-programmed desorption of CO and NH 3 , laser Raman spectroscopy, diffuse reflectance UV–Vis spectroscopy, X-ray photoelectron spectroscopy and diffuse reflectance infrared spectroscopy using CO, NO and NH 3 as probe molecules. Structure-activity correlations demonstrated that isolated MoO x species, predominately present at Mo loads below 4 wt%, function as structural promoters by stabilizing the ZrO 2 surface against sintering and enhancing the Pt dispersion, thereby improving both NO x conversion and N 2 selectivity. Moreover, pronounced electronic interactions between surface molybdates species and Pt were identified as key contributors to the enhanced low-temperature activity. Structure-selectivity correlations further indicated that MoO x /ZrO 2 supports with strong Lewis and Brønsted acidity promote the N 2 formation. Mechanistic insights by steady-state isotopic transient kinetic analysis as well as in-situ diffuse reflectance infrared spectroscopic investigations performed with isotopic labeled H 2 and NO revealed that NH 3 and NH 4 + surface species formed on acid support sites act as crucial intermediates directing the NO x reduction toward N 2 .