Surface Analogues of Molecular Frustrated Lewis Pairs in Heterogeneous CO₂ Hydrogenation Catalysis

The discovery of homogeneous, solution-based molecular frustrated Lewis pairs, denoted FLPs, comprising main-group elements that can activate H 2 heralded a paradigm shift in chemistry and catalysis. In FLPs, unquenched Lewis base and Lewis acid sites (B···A) are able to polarize and dissociate H 2 heterolytically to form adjacent proton and hydride sites (BH – ···AH + ), which can enable reactions such as CO 2 reduction. In this paper, we draw attention to a relationship between these well-known molecular FLPs and the surface active sites comprised of proximal Lewis base and Lewis acid pairs, which have been reported multiple times in the literature to be responsible for driving various heterogeneous catalytic reactions. From our recent studies that described one such surface site in a nanostructured defect laden indium oxide, capable of activating H 2 and enabling the hydrogenation of CO 2, it was conjectured that these sites are surface FLPs. Significantly, the conversion rate for this hydrogenation reaction is observed to be more rapid in the light than in the dark. Kinetic measurements and density functional theory simulations are consistent with a reaction that proceeds via a surface FLP. It is found that the higher Lewis acidity and Lewis basicity in the excited state, which originates from trapping of the photogenerated hole and electron at the FLP acid and base sites, respectively, is responsible for the higher reactivity in the light in comparison to the dark. With the emerging experimental and theoretical understanding of the chemical and physical principles that underpin the reactivity of FLPs in both homogeneous and heterogeneous systems, it is now possible to rationally conceive and synthetically target heterogeneous FLP materials that bear a compositional and structural connection to homogeneous FLP molecules, and vice versa. This synergistic relationship between FLP molecules and materials could prove beneficial in future efforts aimed at expanding the accrued scientific knowledge on photochemical versus thermochemical activation of CO 2 and thereupon to exploit the perceived technological attributes of both systems in the catalytic conversion of carbon dioxide to value-added chemicals and fuels.

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