To hinder global temperature increase, regulations for greenhouse gas emissions targeting negative emission technologies are becoming more relevant. However, net-zero emissions alone cannot effectively limit increasing temperature.(1) Removal of gases other than CO₂ can have synergistic effects. CH₄ (Methane) removal is a promising process because CH₄ is 25 times more potent than CO₂ at trapping heat in the short term, and the removal of 1 g of CH₄ can be as effective as the removal of 86 g of CO₂ over 20 years.(2) Therefore, developing atmospheric CH₄ removal technology, especially in combination with direct air capture efforts, can offer a new pathway to restore the atmosphere. Atmospheric CH₄ concentration can be restored to pre-industrial levels by removing ~3.2 of the 5.3 Gt of CH₄, which will result in reducing radiative forcing by ~16 %.(3)

Among various methods to remove CH₄ from the atmosphere, thermal catalytic oxidation of CH₄ (CH₄ + 2O₂ → CO₂ + 2H₂O) is being considered as one of the most promising technologies.(4) The savings in greenhouse gas potential are large in CO₂ equivalents because converting 3.2Gt of CH₄ only produces 3.2 Gt of CO₂, much lower than the annual anthropogenic CO₂ emissions (~40 Gt of CO₂) (5) while eliminating a significant amount of total radiative forcing. Therefore, atmospheric CH₄ removal using thermal catalytic oxidation could be a prominent technology and its feasibility needs to be studied. However, the temperature of operation can only be a few degrees above ambient temperature to make it feasible.(6) Current systems for atmospheric methane oxidation operate at temperatures >200 °C and require large amounts of catalyst. Finding cheap and abundant alternatives can be a breakthrough in this field. We found that palladium catalysts can oxidize nearly atmospheric methane streams at <150 °C.

We here propose to develop multimetallic catalysts for low-temperature oxidation of methane at atmospheric concentrations (2 ppm) using very low amounts of palladium and earth-abundant metals in a high-entropy alloy configuration to boost methane activation. We plan to understand and engineer the mechanism of methane activation at the lowest possible temperatures in 2 ppm methane to develop a feasible technology for atmospheric methane removal. We envision this catalyst to be coupled with direct air capture systems or operated alone with lowest energy input. More broadly, the fundamental knowledge of methane activation on earth-abundant materials can promote research in this area to develop catalysts that can achieve very low temperature performance to tackle atmospheric methane removal.

Oxidation of CH₄ requires thermal energy to break strong C-H bonds. To increase the energy efficiency of this reaction, catalysts are needed. Optimizing catalyst design while considering activity (temperature to fully convert CH₄ to CO₂) and stability (to water and poisoning byproducts) is crucial for developing a cost-effective and sustainable CH₄ removal technology. In this project, we aim to develop highly active and stable catalysts based on our experience using palladium.(7) The main hypothesis driving this work is that by using very low loadings of palladium-based multimetallic catalysts (high-entropy alloys with 5 or more metals in a single particle), it is possible to increase the low-temperature activity of palladium while diluting it with cheap and abundant metals. A few scientific developments from our group support this hypothesis:

1. By diluting Pt and Pd with cheap base metals, C-H activation rates increased by an order of magnitude;(8)
2. Encapsulated metal particles within porous oxide layers showed increased activity (9-10) and stability;(11)
3. High-entropy alloys that can decrease the overall amount of palladium while maintaining its high catalytic activity for methane oxidation were prepared by us. 

We therefore propose to develop palladium-based high-entropy alloy particles that are encapsulated within oxide layers for atmospheric methane oxidation. The overarching goal is to decrease the light-off temperature below 100 °C, with the opportunity to demonstrate feasibility for near-room temperature activity.