Elettra | APE-HE The direct activation of methane at low temperatures remains a major challenge in catalysis due to the molecule’s high stability and strong C–H bond dissociation energy. Conventional processes require high temperatures and are often non-selective, leading to total oxidation of methane with consequent production of undesired products (carbon dioxide and water). In this context, the development of real catalysts that enable low-temperature methane activation, ideally through scalable and sustainable synthesis routes, is a critical research goal. In fact, at low temperature partial oxidation reaction pathways are possible, leading to the production of precious chemicals such as methanol or formaldehyde. In particular, methanol is considered nowadays fundamental in the context of the energy transition, due to its high potentialities to be employed as hydrogen liquid carrier and for industrial purposes. In this study, we report the synthesis, characterization, and mechanistic investigation of a copper-cerium mixed oxide catalyst (M-CuO/CeO2) prepared by ball milling method, a solvent-free, cheap and straightforward synthetic approach. Copper-Ceria formulations have indeed gained considerable attention through the years for their good catalytic activity in environmental reactions such as the oxidation of Volatile Organic Compounds (VOCs), water gas shift reaction and CO preferential Oxidation. The favorable characteristics of these systems, combined with the lower cost compared to noble metal-based catalysts, pushed scientific research toward identifying the specific catalytic active sites/interfaces responsible for the good catalytic performance. The catalyst synthesized in this work demonstrated activity for methane activation at temperatures as low as 250 °C, significantly lower than those typically required for such reactions, and shows stability over multiple redox cycles. A similar composite (I-CuO/CeO2) synthesized with a conventional impregnation method was also analyzed for comparison, in order to elucidate the origin of this peculiar catalytic activity. The success of this study was empowered by the use of a multi-technique approach comprising forefront spectroscopic techniques, possible thanks to the joint collaboration between different research institutes of Friuli Venezia Giulia (CNRIOM, Elettra Synchrotron, University of Udine, University of Trieste and Area Science Park). The standard ex situ characterization techniques used to investigate the main features of the materials revealed comparable morphological and structural/reduction properties between the ball milled and impregnated composites. However, the combined operando Near Edge X-ray Absorption Fine Structure Spectroscopy (NEXAFS) and in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) investigation probed a considerably distinct surface reactivity for the two composites under actual operating conditions. At a temperature of 250 °C, the impregnated catalyst was indeed unable to activate methane, and resulted to be inactive for partial oxidation. Conversely, the ball-milled composite showed methane activation capabilities. The origin of this behavior was elucidated by NEXAFS spectroscopy (performed at APE-HE beamline) combined with DFT calculations, which indicated a net charge transfer occurring from CeO2 to CuO surfaces following the ball milling process and the thermal activation. The material exhibits specific reactive interfaces that are absent in the conventional impregnated catalyst and whose formation is likely to be favored by the mechanical energy supplied during milling (Fig. 1). The key factor was found to be the high concentration of Ce3+ ions (provided by the use of a high surface area CeO2 as a precursor for the milled composite), aided by the mechanical action provided by milling, effectively enhancing the Ce → Cu charge transfer, as also demonstrated by DFT calculations. The In situ DRIFT investigation showed that methane is adsorbed on the catalyst surface on reactive oxygen atoms, in the form of methane oxidation intermediates CxHyO2 (Fig. 2), leading to the formation of partial and total oxidation products, detected by the micro-GC directly connected to the operando NEXAFS reaction cell during the exposure of the sample to an oxidizing atmosphere. At the same time, during CH4 exposure NEXAFS spectra of Cu reveals the formation of Cu+ sites, a further validation of methane activation on the surface (Fig. 3). The desorption of products promoted by O2 restores the initial electronic configuration of the catalyst surface, allowing more cycles to be carried out without deactivation (Figs. 2,3). The in-depth spectroscopic investigation under real catalytic conditions coupled with DFT calculations has demonstrated to be fundamental in identifying the reaction active sites and in describing the reaction mechanism. These were oxygen atoms located close to the CeO2/CuO interface, able to activate the methane molecule (Fig. 1), which is consequently adsorbed on another O atom of the interface. The oxidizing agent (O2) acts as a promoter for the products desorption, refilling the created oxygen vacancies and thus recovering the initial catalyst surface. These results could help to pave the way to the design of more robust catalytic systems prepared by sustainable mechanical routes and highlight the role of mechanical activation of interfaces offering a valuable tool for catalyst engineering. Operando study reveals reaction pathways for direct methane activation on CuO/CeO2 green catalysts Elettra Highlights 2024–2025 10
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