Ultra-fine copper(ii) oxide nanoparticles were used for the electrocatalytic reduction of CO2 to formic acid at high Faradaic efficiencies.

Copper oxide modified electrodes were investigated as a function of applied electrode potential using in situ infrared spectroscopy and ex situ Raman and X-ray photoelectron spectroscopy. In deoxygenated KHCO3 electrolyte bicarbonate and carbonate species were found to adsorb to the electrode during reduction and the CuO was reduced to Cu(i) or Cu(0) species. Carbonate was incorporated into the structure and the CuO starting material was not regenerated on cycling to positive potentials. In contrast, in CO2 saturated KHCO3 solution, surface adsorption of bicarbonate and carbonate was not observed and adsorption of a carbonato-species was observed with in situ infrared spectroscopy. This species is believed to be activated, bent CO2. On cycling to negative potentials, larger reduction currents were observed in the presence of CO2; however, less of the charge could be attributed to the reduction of CuO. In the presence of CO2 CuO underwent reduction to Cu2O and potentially Cu, with no incorporation of carbonate. Under these conditions the CuO starting material could be regenerated by cycling to positive potentials.

Highly monodispersed CuO nanoparticles (NPs) were synthesized via continuous hydrothermal flow synthesis (CHFS) and then tested as catalysts for CO 2 hydrogenation. The catalytic behavior of unsupported 11 nm sized nanoparticles from the same batch was characterized by diffuse reflectance infrared fourier transform spectroscopy (DRIFTS), extended X-ray absorption fine structure (EXAFS), X-ray diffraction (XRD), and catalytic testing, under CO 2 /H 2 in the temperature range 25− 500 °C in consistent experimental conditions. This was done to highlight the relationship among structural evolution, surface products, and reaction yields; the experimental results were compared with modeling predictions based on density functional theory (DFT) simulations of the CuO system. In situ DRIFTS revealed the formation of surface formate species at temperatures as low as 70 °C. DFT calculations of CO 2 hydrogenation on the CuO surface suggested that hydrogenation reduced the CuO surface to Cu 2 O, which facilitated the formation of formate. In situ EXAFS supported a strong correlation between the Cu 2 O phase fraction and the formate peak intensity, with the maxima corresponding to where Cu 2 O was the only detectable phase at 170 °C, before the onset of reduction to Cu at 190 °C. The concurrent phase and crystallite size evolution were monitored by in situ XRD, which suggested that the CuO NPs were stable in size before the onset of reduction, with smaller Cu 2 O crystallites being observed from 130 °C. Further reduction to Cu from 190 °C was followed by a rapid decrease of surface formate and the detection of adsorbed CO from 250 °C; these results are in agreement with heterogeneous catalytic tests where surface CO was observed over the same temperature range. Furthermore, CH 4 was detected in correspondence with the decomposition of formate and formation of the Cu phase, with a maximum conversion rate of 2.8% measured at 470 °C (on completely reduced copper), supporting the indication of independent reaction pathways for the conversion of CO 2 to CH 4 and CO that was suggested by catalytic tests. The resulting Cu NPs had a final crystallite size of ca. 44 nm at 500 °C and retained a significantly active surface.

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