Fischer-Tropsch Synthesis over Supported Pt-Mo Catalyst: Toward Bimetallic Catalyst Optimization Sergey N. Rashkeev* ,† and Michael V. Glazoff ‡ † Center for Advanced Modeling & Simulation, Idaho National Laboratory, Idaho Falls, Idaho 83415, United States ‡ Advanced Process & Decision Systems, Idaho National Laboratory, Idaho Falls, Idaho 83415, United States ABSTRACT: The product distribution of the Fischer-Tropsch (FT) process demonstrates a strong dependence upon the choice of catalyst, catalytic support, and reaction temperature. To develop understanding of the factors that underpin catalytic activity, we performed density-functional-theory (DFT)-based first-principles calculations for syngas reaction over bimetallic (Pt-Mo) catalysts including bimetallic surfaces and alloyed nanoparticles (NPs) positioned on a top of γ-Al 2 O 3 substrate. It was found that catalytic activity of the (Pt-Mo) nanoparticles depends upon (i) the selectivity and reactivity of different atomic sites at the surface that may significantly affect the kinetics of different stages of the FT synthesis and (ii) the optimal composition of the NP allowing increasing the methane production at the first stage of the FT synthesis. This work highlights the main mechanisms that govern bimetallic catalyst activity for the FT synthesis. Similar considerations could be developed for any bimetallic catalytic system and any catalytic reactions. The results presented here should help to provide a solid basis for the rational design and/or improvement of many bimetallic catalysts. 1. INTRODUCTION The Fischer-Tropsch synthesis has been well-known for about a century. However, it continues to inspire a significant body of research due to the increase in the price of oil and the abundance of coal and biofeeds that could be used to generate liquid fuel. The FT process involves the catalytic conversion of CO and molecular hydrogen into chain hydrocarbons which can be converted to diesel fuel and other commercially important products. The FT synthesized hydrocarbons are virtually free of sulfur, nitrogen, and metallic contaminants which make them more environmentally friendly. Despite being an established industrial technology since 1926, the complex chemistry of FT synthesis is still not fully understood. 1-5 Product distribution of the FT process demonstrates a strong dependence upon the choice of a catalyst, catalytic support, and reaction temperature. The overall FT reaction consists of a complex sequence of the bond-making and bond-breaking elementary steps. First, adsorbed CO and H 2 are activated upon the catalyst surface; second, carbon-containing surface intermediates get hydro- genated; and finally, carbon species react with each other to form complex chain hydrocarbons. A delicate balance between the rates of these reactions controls the reactivity and selectivity of the process. Advances in current catalyst technology require that a more complete understanding of the elementary atomic level transformations involved in the FT synthesis should be developed. Commercially, the FT process is conducted at temperatures around 250 °C, with syngas (CO + H 2 ) pumped through a reactor containing supported a transition-metal-based catalyst. The choice of a catalyst is critically important for the product distribution. A variety of catalysts can be used for the Fischer- Tropsch process, but the most common are the transition metals such as Co, Fe, and Ru. Nickel can also be used but tends to favor methane formation. Co-based catalysts are highly active, although iron may be more suitable for low-hydrogen- content synthesis gases such as those derived from coal due to its promotion of the water-gas-shift reaction. In addition to the active metal, the catalysts typically contain a number of “promoters”, including potassium and copper. Group I alkali metals, including potassium, are poisons for cobalt catalysts but serve as promoters for iron catalysts. 6 Catalysts are supported on a high-surface-area support (silica, alumina, or zeolites). 7 Cobalt catalysts are more active for the FT synthesis when the feedstock is natural gas while iron catalysts are preferred for lower quality feedstocks such as coal or biomass. Unlike the other metals used for this process (Co, Ni, Ru), which remain in the metallic state during synthesis, iron catalysts tend to form a number of phases, including various oxides and carbides during the reaction. This is not necessarily bad. Recent spin- polarized DFT calculations for the carbon pathways and hydrogenation mechanism for CH 4 formation on Fe 2 C(011), Fe 5 C 2 (010), Fe 3 C(001), and Fe 4 C(100) surfaces showed that with the formation of vacancy sites by C atoms escaping from the Fe x C y surface the CO dissociation barrier decreases largely. 8 As a consequence, the active carburized surface is maintained. However, control of the phase transformations should be important for maintaining catalytic activity and preventing the Received: May 10, 2012 Revised: February 5, 2013 Published: February 14, 2013 Article pubs.acs.org/JPCC © 2013 American Chemical Society 4450 dx.doi.org/10.1021/jp304562p | J. Phys. Chem. C 2013, 117, 4450-4458