On the contribution of oxygen from Co3O4 to the Pd-catalyzed methane combustion

The high oxygen storage capacity of exhaust gas treatment catalysts is a desirable feature for stabilizing fuel conversion at lambda variations for stoichiometric engine performance. Pd catalysts supported on Co3O4, Al2O3, CeO2 and ZrO2, as well as Pd-free Co3O4, were evaluated using methane combustion at sub-stoichiometric oxygen-to-fuel ratios at temperatures below 550 °C. The product analysis was performed with an online mass spectrometer calibrated for CH4, O2, CO2, H2O, CO and H2. Only both cobalt catalysts demonstrated the insensitivity of the methane conversion to O2/CH4 variations. The Pd/Co3O4 catalyst was the only catalyst that produced a 40 % increase in exit gas flow above the feed gas mass flow rate at ignition (light-off) between 400 °C and 550 °C, with the same decrease upon extinction. The oxygen from the catalyst participated in the combustion, even while the molecular oxygen supplies lasted. Selected catalysts were analyzed by temperature-programmed desorption, reduction in H2 and surface reaction with CH4. In the absence of O2 in the feed, Pd/Co3O4 supplied a "tithe" of its bulk oxygen at temperatures of 400−550 °C for the formation of CO2 and H2O, while Pd/Al2O3 provided only PdO-associated low oxygen for the CO and H2 formation. Co3O4 surpasses CeO2 in its oxygen-donating properties at low temperatures and at the conditions tested and, thus, is potentially capable of widening the operational lambda window of stoichiometric combustion to a larger extent than ceria.