Methane oxidation hysteresis over Pt/Al2O3

Abstract

With growing, accessible natural gas reserves, there is renewed interest in natural gas engines for the wider automotive market. A potential issue, if legislated, is controlling (reducing) the un-combusted CH4 emissions. The key challenge in catalytic CH4 oxidation is the high temperature required, relative to other hydrocarbon species. In this context, a monolith-supported Pt/Al2O3 catalyst was evaluated for CH4 combustion under fuel lean and fuel rich mixtures using temperature programmed reaction (TPRxn) and step-change temperature and oxygen level experiments. The experiments included performance evaluation during both ignition (increasing temperature) and extinction (decreasing temperature after ignition). Conversion hysteresis was observed, with the conversions during extinction higher than those during ignition under fuel lean or stoichiometric combustion reaction conditions. Results obtained demonstrate that this hysteresis effect can be used to achieve high CH4 oxidation conversions at temperatures lower than that required for ignition, admittedly first through using high temperatures to obtain ignition, then lowering the temperature to take advantage of the hysteresis. Results also suggest that changing O2 levels can lead to similar benefits. With the assumption that lean conditions and lower exhaust temperatures are associated with improved fuel economy, while higher exhaust temperatures and fuel rich conditions lead to higher CH4 oxidation rates over the catalyst, the findings presented clearly demonstrate the potential to achieve both via a cyclic operating approach, with the frequency on the order of 10s of minutes. Data obtained when cycling between temperatures above and below the ignition point and between excess O2 and stoichiometric O2 levels proved this hypothesis.