Engineering Transition-Metal-Coated Tungsten Carbides for Efficient and Selective Electrochemical Reduction of CO 2 to Methane Sippakorn Wannakao, [a, b] Nongnuch Artrith, [a] Jumras Limtrakul, [b, c] and Alexie M. Kolpak* [a] Introduction Capture and conversion of carbon dioxide (CO 2 ) into valuable chemicals is a critical challenge in the development of environ- mentally friendly, economic, and renewable energy technolo- gies. [1] As a result of the high stability and low reactivity of CO 2 , the identification of efficient catalysts for the net chemical sequestration of CO 2 (or at least the carbon-neutral chemical conversion of CO 2 ) has been very challenging. Although a number of electrocatalysts have been shown to be active for electrochemical CO 2 reduction to CO, [2] methane, [3] methan- ol, [3a, 4] and other products, [5] the low stability [6] and/or large overpotentials [3, 7] required by these catalysts can severely limit their overall effectiveness. For example, copper, one of the most active catalysts for electrocatalytic CO 2 reduction to methane, requires overpotentials of 1 V, [3b] whereas Pt has problems with preferential hydrogen evolution and CO poison- ing. [8] The mechanism of CO 2 reduction to methane is complex and involves eight electron–proton-transfer intermediates. The binding energies of these intermediates on pure transition- metal surfaces are correlated to each other and can be de- scribed by a set of simple scaling relations that illustrate funda- mental limits to the catalytic activity. [9] In particular, the activity of transition-metal catalysts is correlated to the adsorption strengths of the reaction intermediates, which in turn are cor- related with the mean energy level of the transition-metal sur- face d-band projected density of states (pDOS). [10] Recently, transition-metal carbides (TMCs) have been of in- terest as a promising class of catalytic materials that may offer a low-cost alternative to conventional platinum group metal catalysts. [11] Among the TMCs that have been investigated to date, tungsten carbide (WC) has generated the most interest and has proved to be a good candidate in heterogeneous and electrochemical catalysis applications. [11c–e, 12] With recent ad- vances in synthetic processes, [13] the size and composition of metal-coated WC nanoparticles is highly controllable and thus offers numerous possibilities for tuning the catalytic properties. Interestingly, TMCs may transcend the simple scaling relations mentioned above. [14] In general, the d-band model that de- scribes pure transition-metal surfaces so well does not extend to more complex systems such as metal alloys [15] or materials such as TMCs, [14] which have a mix of covalent, ionic, and met- allic bonding characteristics. Although relationships between the transition-metal d-band center and the binding energies of the intermediates have been studied for various TMCs, no simple relationship between the electronic structure of the cat- alyst surface and the binding energies of the reaction inter- mediates has been identified so far. In this work, we demonstrate that, as a result of the pres- ence of directional, covalent-like bonding in the metallic TMCs, trends in TMC-based catalysts are described by the energy levels and pDOS of specific orbital-resolved d-band compo- nents. In particular, by using density functional theory (DFT) to investigate the reaction mechanism of CO 2 to methane on WC and several WC-supported transition-metal films, we show that the orbital components of the metal d states are necessary to explain trends in the adsorption energies, which are sensitive to the binding-site preferences and geometries of the reaction The design of catalysts for CO 2 reduction is challenging be- cause of the fundamental relationships between the binding energies of the reaction intermediates. Metal carbides have shown promise for transcending these relationships and ena- bling low-cost alternatives. Herein, we show that directional bonding arising from the mixed covalent/metallic character plays a critical role in governing the surface chemistry. This be- havior can be described by consideration of individual d-band components. We use this model to predict efficient catalysts based on tungsten carbide with a sub-monolayer of iron ada- toms. Our approach can be used to predict site-preference and binding-energy trends for complex catalyst surfaces. [a] S. Wannakao, Dr. N. Artrith, Prof. A. M. Kolpak Department of Mechanical Engineering Massachusetts Institute of Technology Cambridge, MA 02139 (USA) E-mail: kolpak@mit.edu [b] S. Wannakao, Prof. J. Limtrakul Department of Chemistry and NANOTEC Center for Nanoscale Materials Design for Green Nanotechnology Kasetsart University Bangkok 10900 (Thailand) [c] Prof. J. Limtrakul Department of Materials Science and Engineering Vidyasirimedhi Institute of Science and Technology Rayong 21210 (Thailand) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201500245. ChemSusChem 0000, 00,0–0  0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1 & These are not the final page numbers! ÞÞ These are not the final page numbers! ÞÞ Full Papers DOI: 10.1002/cssc.201500245