Theoretical calculations of the mechanism and rate of electrochemical CO2 reduction to methane on metal electrodes
1Science Institute and Faculty of Physical Sciences, University of Iceland, Reykjavík, Iceland
2Department of Applied Physics, Aalto University, Espoo, Finland
Theoretical calculations based on density functional theory (DFT) have led to a deeper understanding of catalytic activity and thus helped develop improved heterogeneous catalysts, a task that is vital for the design of processes with higher energy and atom efficiency in the chemical industry. By providing information about the factors controlling the reactivity, such as the energetics of molecule-surface interaction and the kinetics of elementary processes at solid surfaces, trends in the catalytic activity as the chemical composition of the catalyst is varied have been established and predictions made for new and improved catalysts.
If a carbon-neutral economy is to become realized in the future, systems capable of reducing CO2 in a decentralized small-scale device need to be developed [1–4]. Also, due to the increase in anthropogenic CO2 in the atmosphere and global climate change, a particularly appealing approach is to use CO2 as a reactant in the conversion of energy produced in various ways (geothermal, wind, solar etc.) into fuel such as methane or methanol. This is currently done in a two-step process at Carbon Recycling Inc. where hydrogen is first produced electrochemically and then reacted with CO2. A single step process where CO2 is reduced directly in an electrochemical cell could be more efficient. This could open up the possibility of small-scale decentralized production units of fuels . In laboratory studies of various metal electrodes only copper has been found to catalyze CO2 reduction to hydrocarbons, while more reactive metals mainly form H2 gas and less reactive metals either form CO gas or format [6-8]. Until now, this unique catalytic activity of copper has not been fully explained. At the same time, more energy efficient and selective catalysts need to be developed for this important reaction.
We have carried out DFT calculations to determine the mechanistic pathway of this process including activation energy of the elementary steps on various transition metal electrodes . The calculations explain why copper is the only metal that can form hydrocarbons in the electrochemical CO2 reduction. The calculations also explain why methanol is not formed in any significant yields for this electrochemical process whereas a copper catalyst is used in the industrial process where CO2 and H2 gas are converted to methanol at high temperature and high pressure. Our results provide an improved fundamental understanding of the electrochemical reduction of CO2 and can help in the development of an improved catalyst.
 D.W. DeWulf, J. Electrochem. Soc. 136 (1989) 1686.
 E.E. Benson, C.P. Kubiak, A.J. Sathrum, J.M. Smieja, Chem. Soc. Rev. 38 (2009) 89.
 E. B. Cole, P.S. Lakkaraju, D.M. Rampulla, A.J. Morris, E. Abelev, A.B. Bocarsly, J. Am. Chem. Soc. 132 (2010) 11539.
 M. Le, M. Ren, Z. Zhang, P.T. Sprunger, R.L. Kurtz, J.C. Flake, J. Electrochem. Soc. 158 (2011) E45.
 D.T. Whipple, P.J.A. Kenis, J. Phys. Chem. Lett. 1 (2010) 3451.
 Y. Hori, A. Murata, R. Takahashi, J. Chem. Soc. Faraday Trans. 185 (1989) 2309.
 Y. Hori, H. Wakebe, T. Tsukamoto, O. Koga, Electrochim. Acta 39 (1994) 1833.
 Y. Hori, Modern Aspects of Electrochemistry; C.G. Vayenas, R.E. White, M.E. Gamboa-Aldeco, Eds.; (Springer, New York, 2008) 42, p. 89.
 J. Hussain, H. Jónsson, and E. Skúlason, to be submitted (2015)