With the greater implementation of intermittent renewable energy sources, such as wind and solar power, efficient energy storage technologies need to be developed. In addition, alternative energy carriers to fossil fuels need to be found in order to decrease emission of CO₂ from the transport sector. A highly promising means of doing so would be to hydrogenate CO₂ via electrolysis into fuels, such as methanol or ethanol; as such, it would constitute a carbon-neutral energy carrier.^[1] Carbon dioxide could be captured from point sources, such as fossil fuel power plants or by burning biomass.^[1] In order to make CO₂ electroreduction feasible for implementation in real devices, an efficient and stable catalyst exhibiting selectivity towards few, preferably liquid, compounds is required. Such a material is yet to be discovered; even so, the field is relatively unexplored. Kanan and coworkers showed that a viable route towards doing so could be reduce CO₂ in two stages: first to CO on nanostructured Au, and then to reduce CO separately on  nanostructured oxide-derived  (OD) Cu. The latter catalyst shows an unprecedented selectivity towards the liquid products acetate and ethanol, along with high activity at low overpotentials.^[4]

In the current study, we find that an additional product, acetaldehyde, is formed on OD-Cu during CO evolution, and that it is a intermediate for ethanol formation. ^[5] It is present at a moderate Faradaic efficiency of ~5% at -0.3 V vs. RHE. Although Hori et al. previously discovered it as a product from CO reduction on polycrystalline copper electrodes,^[6] acetaldehyde has not been reported for this reaction on OD-Cu. The reason that it was previously overlooked is related to product analysis. It is not detectable using routine nuclear magnetic resonance (NMR) spectroscopy, the method-of-choice for many groups in the field. When using headspace-gas chromatography, however, it can be readily identified. We hypothesise that the reason for this discrepancy is that acetaldehyde is unstable in alkaline solutions, such as the 0.1 M KOH (pH 13) used as electrolyte for these measurements. It agglomerates and polymerizes, leading to line broadening in NMR spectra, and precipitates out of solution, with these effects combining to reduce the signal drastically. Our density functional theory calculations support the notion that acetaldehyde is an intermediate.

The knowledge that ethanol is produced through acetaldehyde provides us with valuable mechanistic information.  Moreover, acetaldehyde is a valuable chemical in its own right.  Future work will aim to determine how the catalyst can be engineered to exclusively produce acetaldehyde or ethanol at high kinetic rates with minimal potential losses.

References

[1]        A. Goeppert, M. Czaun, J. P. Jones, G. K. Surya Prakash, G. A. Olah, Chem. Soc. Rev. 2014, 43, 7995–8048.

[2]        W. Tang, A. A. Peterson, A. S. Varela, Z. P. Jovanov, L. Bech, W. J. Durand, S. Dahl, J. K. Nørskov, I. Chorkendorff, Phys. Chem. Chem. Phys. 2012, 14, 76–81.

[3]        C. W. Li, M. W. Kanan, J. Am. Chem. Soc. 2012, 134, 7231–7234.

[4]        C. W. Li, J. Ciston, M. W. Kanan, Nature 2014, 508, 504–507.

[5]        E. Bertheussen, A. Verdaguer-Casadevall, D. Ravasio, J. H. Montoya, C. Roy, D. B. Trimarco, S. Meier, J. Wendland, J. K. Nørskov, I. E. L. Stephens, et al., Angew. Chem. Int. Ed. 2015, Accepted.

[6]        Y. Hori, R. Takahashi, Y. Yoshinami, A. Murata, J. Phys. Chem. B 1997, 101, 7075–7081.