Crystal Facet-Dependent Reactivity of Cu₂O in Direct Synthesis of Tetraethoxysilane from Silicon and Ethanol

ABSTRACT: Ethanol can react with active metals to form alkoxides and hydrogen gas, but metallic silicon exhibits relatively stable chemical properties and does not react with ethanol significantly under catalyst-free and low-temperature conditions. Previous research on organosilicon synthesis has primarily focused on the Rochow reaction, in which metallic silicon reacts with methyl chloride. The extension of copper-based catalysts from the Rochow reaction to enable the direct reaction of metallic silicon with ethanol, in which silicon replaces the hydroxyl hydrogen of ethanol to form silanes, has attracted widespread interest. In this study, we synthesized Cu₂O with well-defined exposed crystal facets via facet engineering, modulating the surface facet ratios and systematically investigating the effects of these Cu₂O with different morphologies on the direct reaction. Combining advanced spectroscopic techniques, density functional theory calculations, and molecular dynamics simulations, we revealed that Cu₂O (111) exhibits the highest surface oxygen vacancy concentration and hydroxyl density. The oxygen vacancies on the Cu₂O (111) surface facilitated the adsorption of ethanol, while the surface hydroxyl groups effectively promoted ethanol dehydrogenation. These two effects work in concert to render the Cu₂O (111) facet the highest facet-dependent chemical reactivity in the direct reaction of metallic silicon and ethanol relative to those from other facets. Our study provides crucial insights into the cooperative roles of exposed active sites and surface crystal planes that are potentially relevant to heterogeneous catalysis. Furthermore, this work proposes an understanding of product selectivity to guide the development of other copper-based catalysts for the direct synthesis strategy.