Nowadays, the population of natural-gas powered vehicles (NGVs) is growing exponentially in the automotive market due to great benefits of NGVs in energy economy and air pollutant reductions. Despite of advances in engine technologies, the slip of unburned methane (CH4) gas that has high greenhouse effect (25 times that of CO2) is still a great concern. Lean-CH4 combustion operation with a PGM-based three-way catalytic converter could completely convert CH4 into CO2 and H2O. The water gas at high temperature, and sulfur compounds (SO2/H2S) originating from the fuels, additives and lubricants are severe poisons in the exhaust which significantly impact the efficiency of converters, especially in the real-world practice. Yet current research in this regard is still insufficient to help understand the fundamentals at a microscopic scale. In this presentation, we will show our insights into the deactivation mechanism via a comprehensive study of commercial catalysts, with two aging methods: i.e., bench aging (HTA) and bench aging + engine aging test (HTA+S). A series of characterizations have been carried out to understand the impact from the different aging protocols of the catalysts. Both HTA and HTA+S aging result in degraded performance compared with fresh sample. A redispersion of Pd occurs in both cases that Pd agglomerates decrease in size, probably caused by trapping in the CeO2-ZrO2 (CZ) lattice matrix during aging. The 18O2 isotope experiment shows that CH4 combustion proceeds via the Mars van Krevelen mechanism in which the lattice O (OL) plays a key role. As compared to HTA-aged sample, the sulfation aging for the HTA+S sample promotes the adsorption and exchange of gaseous O2 with surface O possibly by forming Pdδ+-(SO42-)δ- and Ce3+-VO couple at the interface, yet the accumulation of sulfates also blocks the exchange of OL from bulk to surface and across surface. Overall, the promoting effect of sulfate on oxygen mobility is compromised by its blocking effect, resulting in lower performance of HTA+S than that of HTA. The results suggest that both the size of Pd-based metal particles and the redox property of the support play an important role in methane combustion.

Insights into engine-aged deactivation of Pd/CeO2-ZrO2 in CH4 combustion

ACS Fall 2021

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