Seriatim ECSTM-DEMS of Cu-catalyzed reduction of CO in alkaline solution: Operando correlation of electrode-surface atomic structure with product selectivity ABSTRACT Copper is the only unalloyed metal that can deliver, in a “one-pot” heterogeneous electrochemical reduction of CO 2 , a remarkable variety of products, up to fifteen hydrocarbons and oxygenates, in different yields. Its overall activity may be substantial, but its selectivity is far from desirable. In the production of liquid fuels, Cu generates only ethanol at nominal efficiencies that depend upon the particular electrode-surface structure. The optimization of ethanol production may be aided by the correlation, under actual reaction conditions, between the atomic structures of the Cu surfaces and their respective product selectivities. Such operando correlation is made possible by the seriatim (sequential) application of electrochemical scanning tunneling microscopy (ECSTM) and differential electrochemical mass spectrometry (DEMS). The present quasi-review paper describes how seriatim ECSTM-DEMS was utilized to show that ethanol is generated exclusively, sans other hydrocarbons and oxygenates, by a stepped Cu(S)-[3(100)×(111)], or Cu(511), surface at appreciably low overvoltages. KEYWORDS: electrochemical scanning tunneling microscopy (ECSTM), Cu-catalyzed electrochemical reduction of CO 2 , differential electrochemical mass spectrometry (DEMS), seriatim ECSTM-DEMS, Cu(S)-[3(100)×(111)] or Cu(511) stepped surfaces. INTRODUCTION The electrochemical reduction of carbon dioxide directly to valued products such as liquid fuels is an exceptionally difficult proposition because the conversion is both thermodynamically and kinetically unfavorable [1-9]. Whereas the power requirements may be mitigated by the use of solar energy, as done in artificial photosynthesis [10-14], the resolution of the torpid reactivity is a much more onerous aspiration since a catalyst is required that significantly enhances both the activity and the selectivity. Copper is the only unalloyed metal known to catalyze the production of a remarkable variety of compounds, albeit at rather different yields [1, 2, 15-23]. The commercial use of Cu electrocatalysts is presently obstructed, however, because the overall energy-conversion efficiency, the ratio of the free energy of the products to that consumed in the reaction, is less than 40%; furthermore, at a benchmark current density of 5 mA cm -2 , the overpotential at Cu remains unacceptably large at ca. -1.1 V [1, 2, 15, 17]. The analytical separation of multiple products also introduces unwelcome analytical challenges, especially when only one product is desired. The simplest of liquid fuels that can be obtained from CO 2 reduction (CO 2 R) are methanol and ethanol. On Cu electrodes, CH 3 OH is produced in Joint Center for Artificial Photosynthesis, 1 Division of Chemistry and Chemical Engineering; 2 Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125, USA. Youn-Geun Kim 1 , Alnald Javier 2 , Jack H. Baricuatro 1 and Manuel P. Soriaga 2, * *Corresponding author: msoriaga@caltech.edu Current Topics in Catalysis Vol. 13, 2017