New insights on how sulphur degrades catalysts

Natural gas, which is mostly made of methane (CH₄), is currently seen as a transitional energy source on the route to sustainable energy scenarios. Since methane contains more hydrogen than carbon compared to many fuels, it produces less carbon dioxide (CO₂) on combustion and fewer harmful pollutants like nitrogen oxides when used to run engines and turbines. Methane can even be made in environmentally friendly ways, such as using renewable electricity to reduce CO2, or by harvesting from biogas. However, if methane escapes without being completely burned, it can trap much more heat in the atmosphere than CO₂—about 20 times more—so it is crucial to have effective systems to clean up engine exhaust when using methane as a fuel.

The problem is ultimately that methane (CH₄) is very stable, which makes it hard to fully transform it into CO₂. Researchers have used palladium catalysts to transform it at lower temperatures, and, whilst effective, the catalysts degrade over time. This is due to the presence of water in the exhaust and sulphur: Water can block the catalyst active sites temporarily, but sulphur can lead to permanent damage.

Sulphur is naturally present in raw natural gas and especially biogas, therefore sulphur poisoning is relevant both to fossil methane as well as sustainable methane. During combustion, sulphur is oxidized and reacts with the catalyst. Over time, these reactions can form stable sulphur compounds that build up and deactivate the catalyst.

Researchers have found ways of minimising this problem, by using materials like alumina to store sulphur or regeneration treatments using hydrogen gas to recover some activity, but at the end, the issue persists.

Now a team led by the Karlsruhe Institute of Technology (KIT), in collaboration with the ESRF, has investigated how sulphur affects palladium-based catalysts over time and how well they recover after hydrogen treatment. The researchers used X-ray holotomography at ESRF’s ID16A in combination with X-ray fluorescence and X-ray absorption spectroscopy at the Swiss Light Source to investigate changes at different scales – from nanoscale to the macroscopic catalyst structure.

The results show that sulphur poisoning causes a much stronger and more persistent loss in catalytic activity compared to water. They revealed sulphur gradients along the catalyst channel, with higher accumulation upstream, some of which remained even after regeneration. They also discovered that the deeper the sulphur goes and the longer it stays, the harder it is to remove—especially because the sulphur changes into a very stable form that does not easily go away, even after treatment with hydrogen. Hydrogen regeneration partially restored activity, but its effectiveness declined with longer sulphur exposure times.

“This new understanding on how sulphur deactivates the catalyst, from the atomic scale up to the full reactor size, shows that multiscale analysis is the way to go to get a full picture of what takes place. This will hopefully help in understanding the effects of sulphur deactivation, and eventually in designing longer-lasting catalysts that resist sulphur damage better”, explains Prof. Jan-Dierk Grunwaldt, who led the study together with Prof. Thomas Sheppard.

Reference:

Delrieux, T., Sharma, S., et al, ACS Catalysis. 2025, 15, 13470–13485 DOI: 10.1021/acscatal.5c02678

TrackAct project page : https://www.trackact.kit.edu

Text by Montserrat Capellas Espuny

Top image:  Volume rendering of the mesoscale catalysis sample and a corresponding 2D tomographic slice.