Silica nanowires encapsulated Ru nanoparticles as stable nanocatalysts for selective hydrogenation of CO2 to CO

Abstract

Hydrogenation of carbon dioxide (CO2) to produce useful chemicals has been identified as a promising strategy for mitigation of greenhouse gas emission. Ruthenium (Ru) based catalysts have been reported to be the most active catalysts for the hydrogenation of CO2 to methane (CH4) which unfortunately is also a greenhouse gas and is difficult to activate. Controlling the hydrogenation selectivity to produce carbon monoxide (CO), a direct precursor for enormous important chemicals, thus becomes desirable. However, achieving high CO selectivity with supported Ru catalysts has remained a challenging task. In this work, we report the synthesis of highly selective and stable Ru@mSiO2 nanocatalysts via encapsulation of 1–3nm Ru nanoparticles within mesoporous silica nanowires for hydrogenation of CO2 to CO. Calcination of the catalyst in nitrogen prevented sintering of the encapsulated Ru nanoparticles, making high CO selectivity of up to 100% possible, while larger (5–20nm) Ru particles resulting from calcination in air favored formation of CH4. DRIFTS study of 1–3nm Ru@mSiO2 and 5–20nm Ru@mSiO2 catalysts after adsorption of reaction mixture of H2 and CO2 reveals that different reaction intermediates form on catalyst surface: CO-Run+ on 1–3nm Ru@mSiO2 and formate species on 5–20nm Ru@mSiO2, which are responsible for the distinctively different selectivity observed on 1–3nm Ru@mSiO2 and 5–20nm Ru@mSiO2 catalysts. Plausible reaction pathways have been proposed for selective hydrogenation of CO2 on the two types of catalysts, respectively. In addition, high CO selectivity of 1–3nm Ru@mSiO2 catalyst has been demonstrated to be stable.