Authors: Meng Li | Matthew T. Curnan | Hao Chi | Stephen D. House | Wissam A. Saidi | Götz Veser | Judith C. Yang
Source: Microscopy and Microanalysis (article, 2023-07-22)
Field: Materials Science — Materials Chemistry

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/metaloxide (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 Cu 2 O 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 Cu 2 O/Cu interfaces during MeOH reduction dynamics.The ETEMa Hitachi H-9500 operating at 300 keVis equipped with an in-house designed gas delivery system that enables MeOH vapor injection, in addition to O 2 and H 2 gas feeds.To eliminate impacts of the support material on the reaction dynamics, we used model Cu/Cu 2 O catalyst formed on Cu thin films, instead of supported Cu/ Cu 2 O nano particles.The model catalysts were synthesized by in situ ETEM oxidation of the single-crystalline Cu thin films, producing epitaxial Cu 2 O 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||Cu 2 O junction (Figure 1): when the Cu||Cu 2 O interface at the junction is oriented along (100), an anisotropic layer-by-layer reduction occurs at the side facets of the Cu 2 O island, followed by a Cu 2 O-to-Cu interface transformation along (100).When the Cu||Cu 2 O interface is oriented along (110), isotropic reduction at both top and side facets of the Cu 2 O 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||Cu 2 O(100) junction, as opposed to fully coordinated sites on the Cu 2 O(110) surface step or near the Cu||Cu 2 O(110) junction.Thereby, the active site changes from Cu||Cu 2 O(100) junctions during the anisotropic stage to Cu 2 O(110) surface steps in the isotropic stage, when the Cu||Cu 2 O 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 ; Materials science ; Interface (matter) ; Methanol ; Dynamics (music) ; Chemical physics ; Chemical engineering ; Chemistry ; Composite material ; Physics ; Engineering ; Organic chemistry ; Acoustics Copper-based nanomaterials and applications ; Electrocatalysts for Energy Conversion ; Electronic and Structural Properties of Oxides