Methane oxidation over a honeycomb Pd-only three-way catalyst under static and periodic operation

Abstract

Natural gas is receiving increasing awareness as fuel for passenger vehicles due to its very low specific CO2 emissions. However, control of CH4 emissions from natural gas combustion in stoichiometric engines requires a specific three-way catalyst (TWC). The understanding of the TWC chemistry of CH4 under the periodic rich/lean reaction conditions is a key issue for targeted catalyst development. A commercial Pd-only TWC was tested under various reaction conditions to characterize the chemical processes and the mode of operation leading to efficient operation. It was demonstrated that periodic rich/lean operation obtained by variation of the mean O2 concentration fed to the catalyst with various amplitudes is highly beneficial for CH4 oxidation. Especially asymmetric oscillations into rich of stoichiometry produced higher CH4 conversion. Compared to operation with gasoline fuel using propene as the model hydrocarbon, substantial differences were observed in static experiments that reflect the different chemistry at work with the two hydrocarbons. In particular, the stoichiometric point (λ=1) did not coincide with maximum CH4 oxidation, which was obtained rather under rich conditions. The shift of the optimum stoichiometric point was associated with the necessity to consume CO and O2 before CH4 can react. Spectroscopic characterization during reaction aimed at rationalizing the role of NO in isothermic experiments when varying stepwise the oxygen concentration from net oxidizing to net reducing reaction conditions. The overall results should provide recommendations for the design of TWC for natural gas operation and for control strategies to improve CH4 emission levels.