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

Methanol (MeOH) is an essential feedstock in chemical engineering for hydrogen production via steam reforming and partial oxidation reactions, as well as for production of chemicals such as formaldehyde and acetic acid. 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 the oxidation state of Cu catalyst indicates that during the Partial Oxidation of Methanol (POM), Cu catalysts are partially oxidized, resulting in changes in reactivity and selectivity of the reactions [1]. These dynamically formed metal/metal-oxide (M/MO) interfaces yield short-lived, high-energy sites, which likely contribute to the measured reactivity increase near the phase change. However, previous studies on these reaction mechanisms have primarily focused on pure Cu or Cu2O surfaces, rather than investigating the impact of the M/MO interfaces. This is mainly caused by the buried nature of these interfaces and their dynamic changes during reaction. To address this, we use in situ environmental transmission electron microscopy (ETEM) to investigate the influence of Cu2O/Cu interfaces during MeOH reduction dynamics. The ETEM – a Hitachi H-9500 operating at 300 keV – is equipped with an in-house designed gas delivery system that enables MeOH vapor injection, in addition to O2 and H2 gas feeds. To eliminate impacts of the support material on the reaction dynamics, we used model Cu/Cu2O catalyst formed on Cu thin films, instead of supported Cu/Cu2O nano particles. The model catalysts were synthesized by in situ ETEM oxidation of the single-crystalline Cu thin films, producing epitaxial Cu2O nano-islands with well-characterized surfaces and interfaces [2,3]. The atomic-resolution reaction dynamics are observed using in situ ETEM under 1 Pa methanol vapor at 300 ˚C in real time. Our observations reveal two-stage reduction dynamics modulated by the structure of Cu||Cu2O junction (Figure 1): when the Cu||Cu2O interface at the 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 interface transformation along (100). When the Cu||Cu2O interface is oriented along (110), isotropic reduction at both top and side facets of the Cu2O island is observed. Using machine-learning enhanced advanced data analysis, the atomic-level size evolution kinetics are extracted from the movies. Stochastic trend analysis on the 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 the Cu||Cu2­O(100) junction, as opposed to fully coordinated sites on the Cu2O(110) surface step or near the Cu||Cu2O(110) junction. Thereby, the active site changes from Cu||Cu2O(100) junctions during the anisotropic stage to Cu2O(110) surface steps in the isotropic stage, when the Cu||Cu2O interface changes. Further experiments on Cu catalysts during POM reaction are underway. Our results emphasize the importance of the dynamic M/MO interface during catalytic reactions, and provide new insights towards catalyst design and interface engineering [5]. In situ ETEM observation of two-stage reduction of Cu2O(110) under 1 Pa MeOH vapor at 300°C. (a-b) Anisotropic reduction on Cu2­O side facets. (c-d) Isotropic reduction on Cu2O top and side facets. Insets show proposed atomistic models (Cu is gold, O is red, C is black, and H is white).