Thermal deactivation mechanism of the commercial Pt/Pd/Rh/CeO2-ZrO2/Al2O3 catalysts aged under different conditions for the aftertreatment of CNG-fueled vehicle exhaust

Abstract

Commercial three-way catalysts with high-activity and long-term durability are critical for the aftertreatment of compressed natural gas vehicle exhausts to meet increasingly stringent policy requirements, but high concentrations of CH4 and H2O in the exhaust gas make the catalyst susceptible to deactivation. In this work, a series of commercially deactivated and aged honeycomb Pt/Pd/Rh/CeO2-ZrO2/Al2O3 catalysts were obtained by dry-aging, hydrothermal-aging and engine bench-aging methods, and the effects of aging conditions on catalytic performance, surface composition and microstructure evolution were investigated. The combinations of H2-TPR, XRD, Raman, HRTEM, XPS and in situ DRIFTS of CO/CH4 characterizations confirmed that the stability of CeO2-ZrO2 solid solution structure is a key factor affecting the catalytic activity and durability. Hydrothermally-aged catalyst has a relatively stable CeO2-ZrO2 solid solution structure, which ensures a high dispersion of precious metal particles in the form of small particles on the catalyst surface. Dry-aged catalyst shows that more CeO2 crystals have been segregated from CeO2-ZrO2 solid solution, resulting in most precious metal particles in the form of agglomerates. For the deactivated catalysts after engine bench aging, the microstructure was significantly changed with a large number of CeO2 crystals detached from CeO2-ZrO2 solid solution, resulting in more active sites of Platinum Group Metal (PGM) being encapsulated. In situ DRIFTS of CH4 experiments showed that the process of generation and depletion of intermediate species (e.g., methoxy, formate and bicarbonate species) is a matching process. For the deactivated catalysts with fewer active sites, the subsequent reaction steps of these intermediate species to CO2 and H2O are hindered by the accumulation of stable formate and bicarbonate intermediate species.