Photocatalytic product-selective reduction of CO2, CO, and carbonates

Abstract

CO 2 conversion to value-added chemicals is a crucial technology toward carbon-neutral fuels. Photocatalysis using sunlight is an energy-efficient alternative to electrochemical and thermal CO 2 reduction. Photocatalysts usually yield either CO or formate with varying degrees of selectivity. Herein, a lanthanide-based photocatalytic platform producing CO with the highest turnover to date is reported. The catalyst consists of a pendant amine for CO 2 capture and a light-harvesting sensitizer that generates the reactive divalent lanthanide center. CO 2 reduction to CO was possible with high selectivity and reactivity by virtue of a distinct mechanistic pathway involving Sm(II), carbamate, and CO 2 ⋅− as identified intermediates. Attractive (bi)carbonate CO 2 feedstocks were efficiently converted to CO. Depending on the conditions, the selective synthesis of formate, methane, and methanol was also possible, demonstrating the wide utility of the platform. • Lanthanide photocatalyst gives CO or HCO 2 H in a reaction condition-dependent manner • Selectivity for both products is >99%, and turnover number for CO is exceptionally high • (Bi)carbonate feedstocks were reduced without energy-demanding CO 2 release steps • CO could be reduced to either methanol or methane There is a direct link between the rise of climate change-related disasters and CO 2 emissions from fossil fuels. One possible strategy to decrease the amount of emitted CO 2 is the capture and conversion of atmospheric and point source-emitted CO 2 . CO 2 conversion to value-added chemicals, such as carbon monoxide, formic acid, methanol, and methane could furthermore reduce reliance on fossil fuel-derived raw materials. Visible light-driven photocatalytic CO 2 reductions offer decentralized options to achieve these goals using solar energy, a renewable energy source with a small carbon footprint. Complete systems that combine CO 2 capture and utilization are limited by the lack of efficient CO 2 capture, the need for expensive noble metal photosensitizers, and the lack of control over the product distribution of the CO 2 reduction reaction. An integrated homogeneous catalyst based on Earth-abundant samarium combined with a polyamine ligand allows for reactive carbon capture and efficient visible-light-driven CO 2 reduction. This catalyst generates either CO or HCO 2 H in a reaction condition-dependent manner with >99% selectivity for both products and with the highest turnover number for CO generation reported to date for any photocatalytic system, including heterogeneous ones. Attractive carbonate and bicarbonate feedstocks, often considered dead ends in CO 2 reduction reactions, could also be reduced without the need for energy-demanding CO 2 release steps, and CO could be reduced further to either methanol or methane. This unprecedented flexibility, efficiency, and selectivity is due to the novel reaction pathway, accessible to samarium in this ligand, but not for transition metal-based catalysts. These results open up new ways for sustainable fuel development from a variety of feedstocks using solar energy and Earth-abundant metals. A possible strategy to decrease the amount of emitted CO 2 is to capture and convert it to value-added chemicals, e.g., carbon monoxide, formic acid, methanol, and methane. A homogeneous samarium-based catalyst reduced CO 2 or (bi)carbonates upon visible light irradiation selectively to either CO or HCO 2 H. CO could be reduced to either methanol or methane. These results open up new ways for sustainable fuel development from a variety of feedstocks, using solar energy and Earth-abundant metals.