Selective CO2 hydrogenation on the γ-Al2O3 supported bimetallic Co-Cu catalyst

Recycling CO2 to fuel or other useful chemicals will have a positive impact on alleviating the greenhouse effect. In the present study, we analyzed the effect of introducing Cu to a Co-based catalyst on CO2 hydrogenation by examining the pathway and activation barriers of CO2 activation and hydrogenation over the gamma-Al2O3 supported Co-Cu clusters using periodic density functional theory slab calculations. Two reaction channels, CO2 + H. CO + OH and CO2 + H -> HCOO, were followed at different active sites. Introducing Cu to a Co-based cluster resulted in a less oxidized metal cluster. Consequently, the adsorbed CO2 became less activated on Co3Cu than that on Co-4. For the reactions only involving supported metal catalysts, introducing Cu led to a decrease of the activation barrier by 0.36 eV for HCOO formation but left the barrier for CO formation almost unchanged. On the other hand, the surface hydroxyls on the support directly participate in the elementary reactions at the metal-support interface. The adsorbed CO2 at the interface can be easily protonated by the hydroxyl on the support. MD trajectories showed that the protonated CO2 at the Co-4/oxide interface quickly dissociates at 300 K to CO and OH, and the OH further reacts with a surface hydroxyl disproportionately to form H2O at 500 K. At the interfacial site of the supported Co-Cu catalyst, the adsorbed COOH species was found to be the dominant product. The present study clearly demonstrated the bimetallic effect on catalytic activity and selectivity: introducing a second metal, in combination with the hydroxyls on the substrate, alters the reaction pathways, and consequently, the product distribution. (C) 2012 Elsevier B.V. All rights reserved.