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@mSiO(2) nanocatalysts via encapsulation of 1-3 nm 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 (5-20 nm) Ru particles resulting from calcination in air favored formation of CH4. DRIFTS study of 1-3 urn Ru@mSiO(2) and 5-20 nm Ru@mSiO(2) catalysts after adsorption of reaction mixture of H-2 and CO2 reveals that different reaction intermediates form on catalyst surface: CO-Run+ on 1-3 nm Ru@mSiO(2) and formate species on 5-20 nm Ru@mSiO(2), which are responsible for the distinctively different selectivity observed on 1-3 nm Ru@mSiO(2) and 5-20 nm Ru@mSiO(2) 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 1-3 nm Ru@mSiO(2) catalyst has been demonstrated to be stable. (C) 2017 Elsevier B.V. All rights reserved.