CHAPTER 11: Electro- and Photocatalytic Reduction of CO₂: The Homogeneous and Heterogeneous Worlds Collide?

This chapter focuses on the use of molecular catalysts and/or electrode materials (electrocatalytic, semiconductor) to sustain the reduction of CO2. It includes a comparison of molecular catalysts for both electrochemical and photochemical systems as well as a review of the progress made in our own laboratories on semiconductor photocatalysts. Only a few molecular catalysts are capable of deeper reduction than the two-electron reduced products of CO2 (such as CO and formic acid) and the generation of value-added reduction products such as methanol and methane are needed. The challenge to overcome is the overpotential for these electrochemical reactions and short-lived one-electron reduced species for the photochemical systems. Incorporation of a chromophore with the real catalyst in either intermolecular or intramolecular photochemical systems has demonstrated the feasibility of CO2 photoreduction. However, photoinduced electron transfer from the chromophore to the catalyst or from the semiconductor to the solution still account for much of the inefficiency in these systems. Semiconductor-based photocatalyst systems (nanoparticles and electrodes) have shown formation of two-electron reduced products as well as deeper reduction pathways although with limited efficiency. It is our hope that this chapter will contribute to further progress and stimulate future generations of scientists to develop new electro-/photocatalyst design paradigms.