Non-Thermal Plasma-Assisted CH4–CO2 Co-Conversion: A Reservoir of Surface Species is the Key to Selective Liquid Product Formation

Non-thermal plasma (NTP) catalysis enables the direct co-conversion of CH4 and CO2 into thermodynamically limited liquid products under mild conditions. However, achieving high selectivity toward liquid products and clarifying the role of the catalyst in steering product distribution remain significant challenges. Here we uncover a previously unrecognized mechanism governing selectivity in plasma catalysis: the formation of a reservoir of strongly adsorbed surface species that evolves during reaction and dynamically controls liquid-product formation. We investigated the catalytic performance, catalyst structure, plasma properties, and surface species during time-dependent liquid methanol production—in addition to valuable syngas and C2 hydrocarbons in the gas stream—over an α-Fe2O3 catalyst. Stable catalyst structure and plasma conditions were unable to explain the transient product selectivity towards methanol. Instead, in situ plasma–IR studies revealed that strongly bound C═O and C–H species progressively accumulated with time-on-stream (TOS) and correlated with the observed increase in methanol yield. The strongly bound C═O and C–H species are proposed to enhance methanol selectivity by modifying the surface Brønsted and Lewis acidity, promoting methoxy hydrogenation, and weakening methanol adsorption. Studying the plasma interaction of select reaction species (CH4, CO2, CO, methanol, acetaldehyde, and acetic acid) with the catalyst surface showed that the main liquid product, methanol, was responsible for populating the surface with hydrogenated carbonyl species that boosted its own production. A methanol-plasma pretreatment was envisioned and implemented to preload a reservoir of the C═O and C–H species on the surface of α-Fe2O3, resulting in a significant improvement in catalyst performance for selective methanol production. After 8 h TOS, the treated catalyst exhibited a 30% relative increase in selectivity and a threefold increase in methanol yield compared to the untreated catalyst, offering a promising route toward highly selective liquid product formation. Additionally, the methanol–plasma pretreatment to boost methanol production was extended to other metal oxide catalysts, highlighting the broad versatility and potential of this strategy across a wide range of plasma catalyst systems.