An Energy Descriptor To Quantify Methane Selectivity in Fischer-Tropsch Synthesis: A Density Functional Theory Study Jun Cheng, † P. Hu,* ,† Peter Ellis, ‡ Sam French, ‡ Gordon Kelly, § and C. Martin Lok § School of Chemistry and Chemical Engineering, The Queen’s UniVersity of Belfast, Belfast BT9 5AG, United Kingdom, Johnson Matthey Technology Centre, Reading RG4 9NH, United Kingdom, and Johnson Matthey Technology Centre, Billingham CleVeland, TS23 1LB, United Kingdom ReceiVed: February 5, 2009; ReVised Manuscript ReceiVed: April 8, 2009 Selectivity is a fundamental issue in heterogeneous catalysis. In this study, the CH 4 selectivity in Fischer-Tropsch synthesis is chosen to be investigated: CH 4 selectivity on Rh, Co, Ru, Fe, and Re surfaces is computed by first-principles methods. In conjunction with kinetic analyses, we are able to derive the effective barrier difference between methane formation and chain growth (ΔE eff ) to quantify the CH 4 selectivity. By using this energy descriptor, the ranking of methane selectivity predicted from density functional theory (DFT) calculations is consistent with experimental work. Moreover, a linear correlation between ΔE eff and the chemisorption energy of C + 4H (ΔH) is found. This fundamental finding possesses the following significance: (i) it shows that the selectivity, which appears to have kinetic characteristics, is largely determined by thermodynamic properties; and (ii) it suggests that an increase of the binding strength of C + 4H will suppress methane selectivity. 1. Introduction Selectivity is one of the most important issues in heteroge- neous catalysis. Despite its significance, understanding of this issue still falls well short of chemists’ expectations: selectivity in many catalytic systems has not been well understood at a level that many chemists would like. In this work, we use methane selectivity as an example to tackle this fundamental issue. Methane selectivity in Fischer-Tropsch (FT) synthesis was chosen for the following reasons. First, it is one of the most complicated systems in heterogeneous catalysis, and any ap- proaches developed from this one may be extended to other systems. Second, the production of hydrocarbons from synthesis gas (CO + H 2 ) in FT synthesis 1-10 over transition metals is one of the most promising sources of transportation fuels (gasoline and diesel) and chemicals (in particular, 1-alkenes) from non-petroleum-based feedstocks such as natural gas, biomass, and coal. In FT technology, synthesis gas production via steam re-forming typically accounts for 60∼70% of the capital and the running costs of the total plant. Hence, maximum utilization of synthesis gas in the downstream FT reactors is very important. The formation of CH 4 , however, is inevitable in FT synthesis. It is totally wasteful because it reverses the steam re-forming process. Therefore, suppressing CH 4 formation is of paramount importance in FT synthesis. In this work, we investigate CH 4 selectivity over several transition metals (Rh, Co, Fe, Ru, and Re) from density functional theory (DFT) calculations, and we provide insight into how to suppress CH 4 selectivity. Ru, Fe, and Co are the most active catalyst metals for FT synthesis. Of the three metals, Ru has the best activity and selectivity. However, the low availability and high cost of Ru eliminate its use for large-scale applications. Thus, only Fe and Co have been used in industry. Although Co-based catalysts are more expensive than Fe ones, they are more resistant to deactivation. Hence, Co-based catalysts appear to be the optimal choice for synthesis of long-chain hydrocarbons due to its high stability, activity, and selectivity, and huge efforts have been dedicated to Co-based catalysts in the past decade. 11-24 The major disadvantage of Co-based catalysts is that they are too hydrogenating and produce more CH 4 than Fe-based catalysts. Therefore, modification of Co-based catalysts to decrease CH 4 selectivity will lead to huge commercial profits. Vannice 25 observed that the average hydrocarbon molecular weight produced in FT synthesis decreases in the order Ru > Fe > Co > Rh > Ni > Ir > Pt > Pd. This result can also be interpreted as the CH 4 selectivity increases in the order Ru < Fe < Co < Rh < Ni < Ir < Pt < Pd. However, the following questions need to be answered in the field: (i) What is the key factor that leads to this ranking of CH 4 selectivity? (ii) Is there an intrinsic property of metal surfaces controlling the ranking? (iii) If the answer to the second question is yes, how can it be used as a guide to suppress CH 4 selectivity? Aiming to answer these questions, we choose metallic Re, Ru, Fe, Co, and Rh to study CH 4 selectivity in this work. It should be mentioned that it was found experimentally that iron carbide is the active phase for FT reactions in Fe-based catalysts. 26 In the present study, we use only metallic Fe as a model system to understand the intrinsic trend of CH 4 formation and chain growth processes on transition metals. Re is also investigated for the same purpose. The paper is arranged as follows: In the next section, calculation details will be described. Following this, the calculation results of CH 4 formation on the stepped metal surfaces will be presented, and the reaction rate of CH 4 formation and chain growth will be evaluated and thus CH 4 selectivity will be discussed. In the last section, some conclu- sions will be summarized. * Corresponding author. † The Queen’s University of Belfast. ‡ Johnson Matthey Technology Centre, Reading. § Johnson Matthey Technology Centre, Billingham Cleveland. J. Phys. Chem. C 2009, 113, 8858–8863 8858 10.1021/jp901075e CCC: $40.75  2009 American Chemical Society Published on Web 04/29/2009