Reaction pathways of methane abatement in Pd-Rh three-way catalyst in heavy duty applications: A combined approach based on exhaust analysis, model gas reactor and DRIFTS measurements

Methane abatement pathways in Pd/Rh three-way catalysts have been investigated in three scales ranging from a vehicle application size catalyst, a model gas reactor and the catalyst in powder form. A special test rig was designed for the investigation of vehicle size catalysts, allowing sampling along the catalyst at discrete spatial locations, which are subject to different feed compositions. Dependent on the history of chemical environment of the catalyst, significant differences in methane conversion rate at identical feed have been identified. At steady state methane conversion rate was low and the reaction pathway was identified as limited to only direct oxidation by oxygen. Following a rich-to-lean transition, the catalyst exhibited more than 8 times higher methane conversion rates compared to steady state. The high methane conversion rates have been identified and attributed to the activation of methane steam reforming (SR) related to transient reduction of ceria. Methane SR efficiency decreased with time and the conversion rate finally converged to steady state levels. The findings were validated using a model gas reactor enabling analysis under well-defined feed compositions. The deactivation of SR was further analyzed with infrared spectroscopy (DRIFTS). Evidences from DRIFTS measurements showed that the deactivation was linked to the formation of carbonaceous species on the catalyst surface, most likely carbonates. The coherent results from engine exhaust analysis, model gas reactor and DRIFTS study give important insights in the activation and deactivation of methane reaction pathways. The results of this study suggest that catalyst formulation and operation strategies of methane conversion should focus on the stimulation of SR and the maintenance of catalyst activity towards SR through targeted periodic lean/rich transitions.