Catalytic methane reduction

Natural gas powered vehicles are considered to be environmental friendly because of their low emissions and high efficiency. However, the methane slip – the climate-damaging main component of natural gas – is problematic. That is the reason why reliable oxidative methane reduction in catalytic exhaust gas treatment is so tremendously important. Especially catalytic converters' insufficient durability has posed a problem that up to now has never been sufficiently resolved. The project presents new findings that address this. The basis for this is formed using kinetic measurements about methane conversion behaviour on a model catalyser made of Palladium (Pd) and Platinum (Pt) on alumina (Al2O3). The tests showed that methane caused the catalyser to deactivate quickly. Nitrogen oxide, however, led to a reactivation of aged catalyser material as did a reduction through hydrogen at 400 °C. Sulphur dioxide in exhaust gas also determines the behaviour of the catalyser and leads directly to its contamination.

Motivation

Lower CO2 and soot emissions count as one of the key advantages of natural gas powered cars over vehicles powered by diesel or gasoline. But these advantages are countervailed by an increased slip rate of methane. Methane is the main component of natural gas and presents a greenhouse gas potential many times higher than CO2. For this reason catalytic exhaust gas treatments are used in natural gas powered vehicles, the reliable operation of which is technically very challenging. The goal of the project was to systematically examine the factors for making these catalysers more durable.

Methodology

The tests were conducted using a Pd-Pt/Al2O3 model catalyser for methane oxidation. Using a physical-chemical analysis of catalyser samples, the influence of different gas components, the space velocity as well as the methane concentration could be tested. Mixtures of methane, water, oxygen and nitrogen were used as well as simulated real exhaust emissions. Long-term tests provided insights into catalyser aging. Reactivation experiments to test the reversibility of aging concluded the tests. These also served to present measures for improving catalysers' long-term activity.

Result

In operation the activity of catalysers quickly drop when methane is present in gas emissions. Nitrogin oxide is, however, capable of reactivating aged catalysers. Reactivating aged catalyser samples is also possible by hydrogen reduction at 400 °C. Sulphur dioxide is present in part in fuel for gas engines and also decisively influences the behaviour of the catalyser. The catalyser volume regresses immediately and the long-term activity sinks dramatically because of a blockage to the centres of the active precious metal. The findings provide important development impetus for further improving the properties of catalytic converters.

Documentation

Methane Catalytic | Survey of the mechanisms of catalytic methane reduction | Project No. 1134

Methane Catalytic II | Methane oxidation catalysts: Influence of catalyst composition, pressure and gas composition on activity, aging and reactivation | Project No. 1177

Themis