In Situ Environmental TEM Observation of Cu/Cu₂O Interface-Modulated Methanol Reaction Dynamics

Cu-based catalysts are the most widely used commercial catalysts in methanol chemistry due to their cost-effectiveness and high reactivity with methanol. Recent studies on Cu catalyst oxidation states indicate that during the Partial Oxidation of Methanol (POM), Cu catalysts are partially oxidized, producing reactivity and selectivity changes in pertinent reactions. These changes dynamically form metal/metal-oxide (M/MO) interfaces featuring short-lived and high-energy sites, which likely contribute to measured reactivity increases near phase boundaries. However, previous studies on these reaction mechanisms have primarily focused on pure Cu or Cu₂O surfaces, rather than investigating impacts on M/MO interfaces.<br/><br/>In this work, using <i>in situ</i> environmental transmission electron microscopy (ETEM) with machine-learning enhanced advanced data analysis and correlated DFT simulations, we investigated the influence of Cu₂O/Cu interfacial structures on methanol reduction dynamics. The atomic-resolution reaction dynamics are observed using <i>in situ </i>ETEM under 1 Pa methanol vapor at 300 C on heteroepitaxial Cu₂O/Cu model catalysts. Our observations reveal two-stage reduction dynamics modulated by the structures of Cu||Cu₂O junctions: when the Cu||Cu₂O interface at a junction is oriented along (100), an anisotropic layer-by-layer reduction occurs at the side facets of the Cu₂O island, followed by a Cu₂O-to-Cu interfacial transformation along (100). When the Cu||Cu₂O interface is oriented along (110), isotropic reduction at both top and side facets of Cu₂O islands is observed. Using machine-learning enhanced advanced data analysis, atomic-level size evolution kinetics are extracted. Stochastic trend analysis on island size kinetics suggests two distinct kinetic stages caused by different reaction mechanisms. Using correlated density functional theory (DFT) simulations of MeOH dissociative absorption, we found that MeOH adsorption energetics favor defect sites at Cu||Cu₂O(100) junctions, as opposed to fully coordinated sites on Cu₂O(110) surface steps or near Cu||Cu₂O(110) junctions. As the Cu||Cu₂O interface changes during the anisotropic-to-isotropic stage transition, active sites correspondingly relocate from Cu||Cu₂O(100) junctions to Cu₂O(110) surface steps. Our results emphasize the importance of M/MO interfacial dynamics during catalytic reactions, and provide new insights towards catalyst design and interface engineering.<br/><br/><b>Acknowledgements:</b><br/>The authors acknowledge funding from National Science Foundation (NSF) grants DMR-1410055, CBET-1264637, DMR-1508417, DMR-1410335, and DMREF CHE-1534630, and support from Hitachi High-Tech. Technical support from the Nanoscale Fabrication and Characterization Facility in the Petersen Institute of Nano Science and Engineering at the University of Pittsburgh is appreciated. This research used the Electron Microscopy facility of the Center for Functional Nanomaterials (CFN), which is a U.S. Department of Energy Office of Science User Facility, at Brookhaven National Laboratory under Contract No. DESC0012704. J. McEver was supported in part by the U.S. Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists (WDTS) under the Science Undergraduate Laboratory Internships Program (SULI).