On the enhancing effect of Ce in Pd-MOR catalysts for NOx CH4-SCR: A structure-reactivity study

Abstract The effect of palladium and cerium species on the selective catalytic reduction (SCR) of NO x using methane as reductant (NO x CH 4 -SCR) has been investigated using Pd-HMOR and PdCe-HMOR system. The catalysts have been characterised by H 2 -TPR, DRS UV–Vis, TEM/EDS and FTIR using CO and pyridine as probe molecules. The oxidation of NO and CH 4 -SCR catalytic tests have been conducted using monometallic and bimetallic formulations. Above 0.3 wt.% Pd, the increase in Pd loading leads to a decrease in NO x selectivity towards N 2 , with the formation of N 2 O, and a decrease in the CH 4 selectivity towards SCR, due to CH 4 direct combustion. H 2 -TPR and FTIR-CO studies indicate that palladium is stabilised as Pd 2+ in ion-exchange position, probably in two different sites within the MOR framework. The addition of cerium to Pd-HMOR enhances its catalytic performance for NO x CH 4 -SCR. With 1 wt.% Ce, both NO x conversion into N 2 and CH 4 selectivity towards SCR have increased. Small CeO 2 clusters interacting with palladium are likely to play a major role in this catalytic reaction. The number of such species increases up to Ce loading of ca. 2 wt.%. However, above 3 wt%, NO x conversion values decrease with Ce loading, which is attributed to the formation of bulk CeO 2 species not interacting with palladium.