CH4 combustion over a commercial Pd/CeO2-ZrO2 three-way catalyst: Impact of thermal aging and sulfur exposure

Abstract

Thermal aging and sulfur poisoning are major problems influencing the efficiency and lifetime of natural gas three-way catalysts (TWCs) during real-world operation. In the present study, thermal aging and sulfur-induced deactivations on CH4 combustion were investigated over commercial monolithic honeycombs, i.e., Pd/CexZr1-xO2 (CZ)-based TWCs that are thermally aged in the bench (HTA), followed by being operated behind the stoichiometric natural gas engine in high sulfur fuel (HTA + S). Both HTA and HTA + S samples show degraded performance in CH4 combustion relative to the fresh one, which are caused by the reconstruction of Pd species induced by thermal aging and the decreased reducibility of the support. The 18O2 labelling experiments show that in all cases CH4 combustion proceeds via a MvK mechanism in which lattice O (OL) plays a key role. The sulfation aging (HTA + S sample) promotes the activation of CH4 and O2 by forming Pdδ+-(SO42-)δ- and Ce3+-VO couples at the interface as compared to HTA sample. However, such a promotion effect could be compromised by the blocking effect of sulfate on the exchange of OL from the bulk to the surface and across the surface. The fundamental understanding of thermal and S deactivation mechanisms from this study could help to predict a real-world lifetime use curve of natural gas TWCs.