The selectivity and activity of catalyst for CO hydrogenation to methanol and hydrocarbon: A comparative study on Cu, Co and Ni surfaces

Typical Fischer-Tropsch catalysts display different selectivity and activity in catalyzing CO hydrogenation to diverse products. In this work, the preferable routes for CH₃OH formation on Cu, chain growth on Co and CH₄ formation on Ni are identified guided by the comprehensive reaction network that is mapped out by density function theory calculations. The difference in selectivity among catalysts is controlled delicately by several reactions, including CH₃O + H 虠 CH₃OH, CH₃ + H 虠 CH₄ and CH₂ + CO 虠 CH₂CO. The equilibrium shifts of CH₂O + H 虠 CH₃O and CH₂ + H 虠 CH₃ also make an impact on selectivity. The distinct selectivity can be understood further with the activity of catalysts. Our results show that the ability of surface to absorb species increases in the order Cu < Ni < Co. Generally, Cu catalyzes the association reaction better than Co and Ni, while Co facilitates the dissociation reaction. Two key factors, thermodynamic effect and kinetic effect, are identified in determining the activity of catalyst. We proof that surface with strong binding capability promotes the dissociation reaction, meanwhile impedes the association reaction when the thermodynamic effect is dominant in determining the barrier height. The Brønsted-Evans-Polanyi relation is observed for C-O bond breaking reactions. In addition, kinetic effect also affects the barrier when special transition state exists. The tilt of CO at the transition state for COH formation and chain growth reactions introduces the interaction of atom O with surface. The stronger binding of atom O on Co is crucial to branch the selectivity of Co to chain growth rather than methane. Present study provides a comprehensive picture on the activity and selectivity of catalysts, which is the essential to develop novel catalyst for syngas conversion.