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

Abstract

A combined surface science/microreactor approach was applied to examine interface effects in ethylene hydrogenation on carbon-supported Ag, Au, and Cu nanoparticle catalysts. Turnover frequencies (TOFs) were substantially higher for supported catalysts than for (unsupported) polycrystalline metal foils, especially for Ag. Spark ablation of the corresponding metals on highly oriented pyrolytic graphite (HOPG) and carbon-coated grids yielded nanoparticles of around 3 nm size that were well-suited for characterization by X-ray photoelectron spectroscopy (XPS), high-resolution (scanning) transmission electron microscopy (HRTEM/STEM), and energy dispersive X-ray spectroscopy (EDX). Polycrystalline metal foils characterized by scanning electron microscopy (SEM), EDX, electron backscatter diffraction (EBSD), XPS, and low-energy ion scattering (LEIS) served as unsupported references. Employing a UHV-compatible flow microreactor and gas chromatography (GC) allowed us to determine the catalytic performance of the model catalysts in ethylene hydrogenation up to 200 °C under atmospheric pressure. Compared to the pure metal foils, the HOPG-supported metal nanoparticles exhibited not only strongly increased activity but also higher stability (slower deactivation) and differing reaction orders. For the most active Ag catalysts, DFT calculations were carried out to determine the adsorption energies of the reacting species on single-crystal surfaces as well as on carbon-supported and unsupported Ag nanoparticles. Adsorption of molecular hydrogen was very weak on all unsupported Ag surfaces, resulting in hydrocarbon-"poisoned" surfaces. However, when a carbon support was present, the adsorption strength of H₂ on Ag nanoparticles increased on average by -0.5 eV, driven by changes in Ag-Ag distances near the metal-carbon three-phase boundary (whereas subsurface carbon lowers hydrogen bonding). On Cu particles, the interface effect was calculated to be somewhat weaker than for Ag particles. H₂/D₂ scrambling experiments on Ag catalysts then corroborated a facilitated hydrogen activation for carbon-supported metals. Thus, the carbon support effect is attributed to an improved hydrogen availability at the metal-carbon interface, controlling performance.