Catalyst Development for Stoichiometric and Lean Bum Natural Gas Engines

The exhaust emissions from stoichiometric gas engines contain CO, Hydrocarbons and NOx besides CO₂ and H₂O. The Hydrocarbon emissions consist mostly of unburnt methane. Conventional Pt and Rh based three-way catalysts show a good performance for the conversion of CO and NOx at stoichiometric exhaust gas composition, but methane passes almost unconverted through the catalytic converter. At rich exhaust gas composition (Lambda = 0.995) higher conversion rates are achieved for methane, but the efficiency is still not satisfactory for instationary applications.

Therefore specific catalyst formulations based on Pd were developed. Model gas reactor and first engine evaluation results are presented. The model gas data indicate that with Pd/Rh catalysts both a wider lambda window and a methane conversion of more than 90% can be achieved.

Furthermore catalysts for lean burn applications were investigated. In this case the NOx emissions play only a minor role, but the CO and methane emission levels are important. CO is easily oxidised even at temperatures below 250°C. For methane oxidation high temperatures (> 500°C) are needed with conventional catalysts, these temperatures usually do not occur in the exhaust gas of lean-burn gas engines. For this application therefore different washcoat compositions were tested, which in combination especially with Pd gave catalysts that show a good performance even at low temperatures.