The mitigation of climate change has quickly become the grand challenge of our modern society. Currently, new robust methods of reducing greenhouse gas (GHG) emissions are being globally developed for all sectors. One of the largest contributors to GHG emissions, both globally and in the EU, is transportation, with road transport being the largest contributor, accounting for 71.7% of transport-related emissions in the EU-28 area. The use of natural gas and its bioalternatives, mainly comprising methane, is considered a promising option to mitigate GHG emissions, especially in the sector of heavy-duty vehicles. However, this strategy suffers from certain problems, e.g., the related methane oxidation catalysts are highly susceptible to sulfur poisoning through sulfate formation.
Compared to on-road testing, computational modelling and in-lab reactor studies are powerful and cost-effective methods of probing sulfur–catalytic converter interactions. Herein, periodic density functional theory models are used to describe poisoned oxidation catalysts and then applied to explain the experimental observations concerning methane oxidation on poisoned catalysts. The results suggest that sulfates are formed on the catalyst surface as thin PdSO4 films rather than as bulk deposits. Moreover, the models support the experimental observations that methane oxidation is rather favourable under dry exhaust gas conditions but unfavourable under wet feed conditions.
Lab-scale reactor studies reveal that the stability of the produced sulfates is affected by SO2 concentration and NOx presence, i.e., low SO2 concentration and the absence of NOx promote the formation of PdSO4 over Al2(SO4)3. Poisoned catalytic converters are shown to be regenerated by altering the atmosphere in a suitable fashion during driving. Although the catalyst can be regenerated at modest temperature of 450 °C, the regeneration method should be carefully chosen according to catalyst nature, as O2 concentration strongly affects GHG formation during regeneration.
The obtained results deepen our understanding of sulfur poisoning and facilitate the development of new and more robust catalytic converters for engines operating on natural gas.
The doctoral dissertation of MSc Paavo Auvinen, entitled Sulfur poisoning of modern methane oxidation catalysts - experimental and theoretical studies will be examined at the Faculty of Science and Forestry on the 16th of December online. The opponent in the public examination will be Docent Mika Huuhtanen, University of Oulu and the custos will be Professor Mika Suvanto, University of Eastern Finland. The public examination will be held in Finnish.