Low-Temperature NOx Reduction by H2 on Mo-Promoted Pt/ZrO2 Catalysts in Lean Exhaust Gases

Abstract

AbstractThis article aims to improve the low-temperature H2-deNOxperformance of the active Pt/ZrO2catalyst using MoOxas a promoter. For this purpose, a systematic series of Pt/ZrO2samples were prepared with a Pt content of 0.25 wt% and Mo loads from 0 to 10 wt%. The samples were physico-chemically characterized by means of powder X-ray diffraction, N2physisorption, temperature-programmed desorption of CO and NH3, Raman spectroscopy and diffuse reflectance infrared spectroscopy using NH3as probe molecule, while the H2-deNOxefficiency was investigated in a lean synthetic exhaust. The Pt/ZrO2catalyst with a Mo load of 3 wt% showed the best performance, including H2-deNOxbetween 80 °C and 150 °C, a maximum NOxconversion of 90% and N2selectivity up to 78%. Isolated MoOxspecies predominately present at Mo loads below 4 wt% were found to act as structural promoter by stabilizing the BET surface area, while also providing smaller Pt particles and more active Pt sites, respectively. By contrast, the aggregated Mo oxide moieties found at higher Mo loads exhibit a clearly weaker promotional effect. The structure–activity-selectivity correlations also suggest that the promoter additionally enables a SCR-related mechanistic pathway to be followed, including the spill-over of NHxspecies from the Pt sites to strong Lewis acid sites in the case of highly dispersed MoOxentities followed by reaction with NOx.