Photochemical Reduction of CO₂ with Visible Light using a Polyoxometalate as Photoreductant

The reduction of CO₂ to a higher energy species such as CO is a key transformation and by and large an important missing link towards the development of carbon-based solar fuels to remediate increasing amount of CO₂ in the atmosphere and replace finite amounts of fossil fuels. Both photochemical and electrochemical pathways are being studied. The present state of the art teaches that the CO₂ to CO reduction by

• a photochemical pathway requires sacrificial tertiary amines as the source of electrons and protons needed for the transformation and either low wavelength light or photosensitizers based on Ir and Ru compounds.¹
• an electrochemical pathway that still requires prohibitively high potentials, typically higher than 1.7 V versus Ag/AgNO₃.²

In order to overcome these two basic deficiencies, we combine a new di-rhenium molecular catalyst active for CO₂ photoreduction that also has a tether to bind a polyoxometalate via a simple acid-base interaction. The polyoxometalate is an electron reservoir that can shuttle electrons from an electrode to the molecular catalyst.

Now, in a cascade of transformations a new photoelectrochemical pathway is presented wherein a polyoxometalate, the commercially available phosphotungstic acid, H₃PW₁₂O₄₀, is electrochemically reduced at low potential (1.3 V versus Ag/AgNO₃), and low intensity visible light (60 W tungsten lamp) is used to transfer electrons from the polyoxometalate to the catalyst that is active for selective reduction of CO₂ to CO.

1. a)Ziessel, R., Hawecker, J., Lehn, J.-M., Chim. Acta, 69, 1990–2012 (1986). (b) Fujita, E., Coord. Chem. Rev., 185-186, 373-384 (1999). (c) Takeda H., Koike K., Inoue H., Ishitani O., J. Am. Chem. Soc., 130, 2023-2031 (2008).
2. Kumar, B., Llorente, M., Froehlich, J., Dang, T., Sathrum, A., Kubiak, C. P., Rev. Phys. Chem., 63, 541-569 (2102).