Dimers on the way

J. Am. Chem. Soc. https://doi.org/cmv6 (2018).

Catalytic CO₂ reduction is an important technology for the production of fuels and chemicals. Mechanistic studies have suggested that both electro- and photocatalytic approaches may share a common intermediate, namely a carbon dioxide radical anion (CO₂^–) bound to the catalyst’s surface.

Now, using rapid-scan Fourier-transformed infrared spectroscopy in combination with isotopic labelling, Heinz Frei and co-workers identified a carbon dioxide dimer radical anion (C₂O₄^–) as the crucial surface intermediate during the photocatalytic reduction of CO₂ on copper nanoparticles. Although recent electrochemical investigations have suggested the existence of this one-electron surface intermediate, this study provides its first direct experimental evidence.

In a second set of experiments the authors were also able to show a correlation between the decay of the spectral signals associated with the CO₂ dimer species and the growth of the CO and CO₃^2– signals; those two being disproportionation products of the surface-bound intermediate. In this case, however, they employed a different catalyst, as the copper-nanoparticle-based system does not allow the monitoring of CO and C₂O₄^– on the same timescale due to the limited spectral sensitivity. In fact, the group performed the reaction using cadmium selenide nanoparticles (pictured), providing complementary evidence for the existence of C₂O₄^– on different catalytic materials. As the growth of the carbon dioxide dimer radical anion does not show an induction period on the timescale of seconds, the authors suggest that any CO₂^– species possibly formed could further evolve into C₂O₄^– on a very fast timescale by reaction with a second CO₂ molecule. On the other side, compared to the traditional path for CO₂ reduction, which only implies a surface-bound carbon dioxide radical anion, a direct dimer route is estimated to be energetically more favourable.

This study could have important implications for the design of catalysts with superior performance, as in principle CO₂ reduction efficiency can be improved by enhancing the formation of surface-bound dimers. To this end, one approach could be the combination of the nanoparticles with supports that feature high CO₂ adsorption capacity.