Selective CO₂ hydrogenation on the γ-Al₂O₃ supported bimetallic Co–Cu catalyst
2012
Yin, Shuxia | Ge, Qingfeng
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 γ-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 Co₄. For the reactions only involving supported metal catalysts, introducing Cu led to a decrease of the activation barrier by 0.36eV 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 Co₄/oxide interface quickly dissociates at 300K to CO and OH, and the OH further reacts with a surface hydroxyl disproportionately to form H₂O at 500K. 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.
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