Paper SS2-FrM2
A Theoretical Study of Methanol Synthesis from CO₂ Hydrogenation on Metal-doped Cu(111) Surfaces

Friday, November 4, 2011, 8:40 am, Room 109

The synthesis of methanol (CH₃OH) from CO₂ hydrogenation (CO₂ + 3H₂ → CH₃OH + H₂O) has attracted considerable attention in the past decades. It is not only industrially important, but also of great environmental significance due to its application in the conversion of greenhouse gas, CO₂. Commercially, the reaction is performed on a catalyst containing Cu, ZnO and Al₂O₃, on which the conversion of CO₂ to CH₃OH is kinetically limited to 15-25%. To improve the performance of the Cu catalysts, the effect of alloying on CH₃OH synthesis was investigated in this study.

Density functional theory (DFT) calculations and Kinetic Monte Carlo (KMC) simulations were employed to investigate the CH₃OH synthesis reaction from CO₂ hydrogenation on metal-doped Cu(111) surfaces. Both the formate pathway and the reverse water gas shift (RWGS) reaction followed by CO hydrogenation pathway (RWGS + CO-Hydro) were considered. Our calculations showed that the overall CH₃OH yield increased in the sequence: Au/Cu(111) < Cu(111) < Pd/Cu(111) < Rh/Cu(111) < Pt/Cu(111) < Ni/Cu(111). On Au/Cu(111) and Cu(111), the formate pathway dominates the CH₃OH production. Doping Au does not help the CH₃OH synthesis on Cu(111). Pd, Rh, Pt and Ni are able to promote the CH₃OH production on Cu(111), where the conversion via the RWGS + CO-Hydro pathway is much faster than that via the formate pathway. Further kinetic analysis revealed that the CH₃OH yield on Cu(111) was controlled by three factors: the dioxomethylene hydrogenation barrier, the CO binding energy and the CO hydrogenation barrier. Accordingly, two possible descriptors are identified which can be used to describe the catalytic activity of Cu-based catalysts towards CH₃OH synthesis. One is the activation barrier of dioxomethylene hydrogenation; the other is the CO binding energy. An ideal Cu-based catalyst for the CH₃OH synthesis via CO₂ hydrogenation should be able to hydrogenate dioxomethylene easily and bond CO moderately, being strong enough to favor the desired CO hydrogenation rather than CO desorption, but weak enough to prevent CO poisoning. In this way, the CH₃OH production via both the formate and the RWGS+CO-Hydro pathways can be facilitated.