Thermochemical and electrochemical CO 2 reduction on octahedral Cu nanocluster: Role of solvent towards product selectivity Kuber Singh Rawat a , Arup Mahata a , Biswarup Pathak a,b,⇑ a Discipline of Chemistry, Indian Institute of Technology (IIT) Indore, Indore, M.P. 453552, India b Discipline of Metallurgy Engineering and Materials Science, Indian Institute of Technology (IIT) Indore, Indore, M.P. 453552, India article info Article history: Received 3 November 2016 Revised 9 February 2017 Accepted 12 March 2017 Keywords: Heterogeneous catalysis Catalytic activity CO 2 reduction reaction Adsorption sites abstract There has been a major interest in the development of efficient catalysts for the electrochemical reduction of CO 2 into useful chemicals and fuels. In this work, the thermochemical and electrochemical CO 2 reduction pathways are systematically studied on the (1 1 1) facet of an octahedral copper (Cu 85 ) nanocluster (NC) using density functional theory (DFT) calculations. An investigation is carried out to understand the catalytic activity of the Cu NC towards CO 2 reduction to explore its activity and product selectivity (CH 3 OH vs. CH 4 ; i.e., six-electron vs. eight-electron reduction reaction). Interestingly, CO 2 adsorbs strongly the (1 1 1) facet of the Cu NC, as opposed to the previous reports on the periodic Cu (1 1 1) surfaces. Furthermore, such NC shows excellent catalytic activity towards direct CAO bond disso- ciation, which is again in contrary to what was reported in the literatures. Besides, the CO 2 hydrogenation reaction has been investigated using different proton transfer mechanisms (surface-hydrogenation, water-assisted, and water solvated) with/without the help of a solvent water molecule. We find that the water-assisted H-shuttling mechanism lowers the activation barrier significantly and thus favours the CO 2 hydrogenation. However, the direct CAO bond dissociation is very competing with respect to indirect CAO bond dissociation. Based on our reaction free energy calculations, activation barrier calcu- lations, and simulated Pourbaix calculations, we find that the Cu NC shows excellent catalytic activity and selectivity over any catalysts studied to date. Besides, the Cu NC requires a lower overpotential (0.53 V) compared to the periodic Cu(1 1 1) surface (0.71 V) and thus can be a promising catalyst for CO 2 reduction reaction. Ó 2017 Elsevier Inc. All rights reserved. 1. Introduction The increasing emission of CO 2 has adverse effects on our envi- ronment and climate changes. On the other hand, CO 2 can be an important precursor for fuels and valuable chemicals [1,2]. The electrochemical reduction of CO 2 has many advantages as it can be converted into CO and various hydrocarbons (CH 3 OH, HCOOH, CH 4 ,C 2 H 4 ) under ambient conditions. Out of all these, CH 3 OH is one of the most desired hydrocarbons owing to its important as a liquid fuel for direct methanol fuel cells (DMFCs) [3]. Besides, methanol is used as an important feedstock for many other processes. In industries, methanol is produced using a mixture of syngas (CO, CO 2 ,H 2 ) over the Cu-ZnO/Al 2 O 3 catalyst at high pres- sures (10–100 bar) [4]. However, Chinchen et al. reported that ZnO does not play any significant role for methanol synthesis as SiO 2 and MgO support also show similar activity for methanol syn- thesis [5]. Earlier studies reported that Cu + is the active catalytic site for CO 2 reduction [6,7], whereas linear scaling of the catalyst suggests that metallic Cu is responsible for improved catalytic activity [5]. Experimental results also support that metallic Cu is the active catalyst for methanol synthesis and the turnover frequencies (TOF) are comparable with respect to the industrial catalyst [8]. Nonetheless, many other bulk metal-based (Ag, Au, Pd, Pt, Ni) catalysts have been studied for CO 2 reduction. However these metals show good catalytic activity for CO and H 2 formation over hydrocarbon formation [9]. Thus, Cu(0) is the most active cat- alyst for methanol synthesis. Several experimental [10–20] and computational [21–26] stud- ies have demonstrated that the hydrocarbon formation is favour- able over Cu electrodes. Mavrikakis and co-workers performed a detail computational study on CO 2 hydrogenation on the periodic Cu(1 1 1) surface and reported that the CO 2 hydrogenation reaction undergoes via CO 2 ? HCOO ? HCOOH ? CH 3 O 2 ? CH 2 O ? CH 3 O ? CH 3 OH pathway [27]. Yang et al. reported metal-doped http://dx.doi.org/10.1016/j.jcat.2017.03.011 0021-9517/Ó 2017 Elsevier Inc. All rights reserved. ⇑ Corresponding author at: Discipline of Chemistry, Indian Institute of Technol- ogy (IIT) Indore, Indore, M.P. 453552, India. E-mail address: biswarup@iiti.ac.in (B. Pathak). Journal of Catalysis 349 (2017) 118–127 Contents lists available at ScienceDirect Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat