Role of Interfacial Hydrogen in Ethylene Hydrogenation\non Graphite-Supported Ag, Au, and Cu Catalysts

A combined surface\nscience/microreactor approach was applied to\nexamine interface effects in ethylene hydrogenation on carbon-supported\nAg, Au, and Cu nanoparticle catalysts. Turnover frequencies (TOFs)\nwere substantially higher for supported catalysts than for (unsupported)\npolycrystalline metal foils, especially for Ag. Spark ablation of\nthe corresponding metals on highly oriented pyrolytic graphite (HOPG)\nand carbon-coated grids yielded nanoparticles of around 3 nm size\nthat were well-suited for characterization by X-ray photoelectron\nspectroscopy (XPS), high-resolution (scanning) transmission electron\nmicroscopy (HRTEM/STEM), and energy dispersive X-ray spectroscopy\n(EDX). Polycrystalline metal foils characterized by scanning electron\nmicroscopy (SEM), EDX, electron backscatter diffraction (EBSD), XPS,\nand low-energy ion scattering (LEIS) served as unsupported references.\nEmploying a UHV-compatible flow microreactor and gas chromatography\n(GC) allowed us to determine the catalytic performance of the model\ncatalysts in ethylene hydrogenation up to 200 °C under atmospheric\npressure. Compared to the pure metal foils, the HOPG-supported metal\nnanoparticles exhibited not only strongly increased activity but also\nhigher stability (slower deactivation) and differing reaction orders.\nFor the most active Ag catalysts, DFT calculations were carried out\nto determine the adsorption energies of the reacting species on single-crystal\nsurfaces as well as on carbon-supported and unsupported Ag nanoparticles.\nAdsorption of molecular hydrogen was very weak on all unsupported\nAg surfaces, resulting in hydrocarbon-“poisoned” surfaces.\nHowever, when a carbon support was present, the adsorption strength\nof H<sub>2</sub> on Ag nanoparticles increased on average by −0.5\neV, driven by changes in Ag–Ag distances near the metal–carbon\nthree-phase boundary (whereas subsurface carbon lowers hydrogen bonding).\nOn Cu particles, the interface effect was calculated to be somewhat\nweaker than for Ag particles. H<sub>2</sub>/D<sub>2</sub> scrambling\nexperiments on Ag catalysts then corroborated a facilitated hydrogen\nactivation for carbon-supported metals. Thus, the carbon support effect\nis attributed to an improved hydrogen availability at the metal–carbon\ninterface, controlling performance.