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-deNOx performance of the active Pt/ZrO2 catalyst using MoOx as a promoter. For this purpose, a systematic series of Pt/ZrO2 samples 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, N2 physisorption, temperature-programmed desorption of CO and NH3, Raman spectroscopy and diffuse reflectance infrared spectroscopy using NH3 as probe molecule, while the H2-deNOx efficiency was investigated in a lean synthetic exhaust. The Pt/ZrO2 catalyst with a Mo load of 3 wt% showed the best performance, including H2-deNOx between 80 °C and 150 °C, a maximum NOx conversion of 90% and N2 selectivity up to 78%. Isolated MoOx species 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 NHx species from the Pt sites to strong Lewis acid sites in the case of highly dispersed MoOx entities followed by reaction with NOx.