The overall economics of Fischer-Tropsch technology strongly depends on the performance of the catalyst used. Both iron and cobalt are commercially used, but cobalt is preferred if the raw material syngas is relatively sulfur free and if straight chain paraffin products are preferred. Promotion of cobalt catalysts with noble metals results in a significant increase in Fischer-Tropsch catalytic activity1,2. Promoters increase the dispersion of catalyst, which increases the catalytic activity, as the number of active sites in a well-dispersed catalyst is higher. Commonly used promoters for cobalt catalysts are Rh, Re, Pt, Pd and Ru. Bimetallic catalysts often show properties that are distinctly different from those of the corresponding monometallic catalysts. Activity is inﬂuenced by surface-ligand effect and lattice strain effect3,4. Though the structure sensitivity of Co surfaces has been investigated, the structure sensitivity of the promotional effect has not been accounted. Two types of promotional effects are observed, namely structural promotion and textural promotion. Structural promoters exhibit surface-ligand effect which increases the amount of active sites in promoted catalysts, e.g. by increasing reducibility of metal oxides to metallic states; and textural promoters, which exhibit the lattice strain effect, changes the intrinsic properties of surface sites, mainly by modifying electronic properties of the surface to affect adsorption energies. Co-Pt bimetallics have exhibited structural promotion effect 5.
In this work, the effect of Pt on the reduction of cobalt oxides to metallic cobalt is studied on both planar and stepped surfaces using surface alloy models where the promoter metal is dispersed on the top surface of the catalyst. One of the important steps in the catalyst preparation is the reduction of cobalt oxides to metallic cobalt using hydrogen or CO. It is believed that this reduction is difficult especially for smaller cobalt particles. The promoters are believed to enhance the reducibility of the metal oxides. Our goal is to explore the electronic effect of Pt and the influence of Co-Pt bimetallic bonds on the reduction of CoO to metallic cobalt to gain a better understanding of the enhanced reducibility of bimetallics. For this purpose, the activation barrier for the reaction, O + H --> OH on a Co-Pt bimetallic and on Co monometallic catalyst is calculated. The adsorption energies of O, H and OH are decreased in the bimetallic catalyst relative to their energies on Co monometallic. If Pt enhances the reduction, the activation barrier on Co-Pt bimetallic will be reduced compared to that on the monometallic. In this work, VASP (Vienna Ab Initio Simulation package)6,7 code with Perdew–Burke–Ernzerhof (PBE)8 form of the generalized gradient approximation (GGA)9 functional was used for the exchange and correlation functional. The activation barrier and the transition states were determined using the Climbing Image Nudged Elastic Band (CI-NEB)10 method.
(a) (b) (c)
Figures: (a) Planar surface of CoPt model, stepped surface of CoPt model (b) top view (c) side view
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Acknowledgements: The authors wish to acknowledge funding provided by the Florida Energy Systems Consortium.
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