Abstract
Frustrated Lewis pairs (FLPs) created by sterically hindered Lewis acids and Lewis bases have shown their capacity for capturing and reacting with a variety of small molecules, including H2 and CO2, and thereby creating a new strategy for CO2 reduction. Here, the photocatalytic CO2 reduction behavior of defect‐laden indium oxide (In2O3− x(OH)y) is greatly enhanced through isomorphous substitution of In3+ with Bi3+, providing fundamental insights into the catalytically active surface FLPs (i.e., In—OH···In) and the experimentally observed “volcano” relationship between the CO production rate and Bi3+ substitution level. According to density functional theory calculations at the optimal Bi3+ substitution level, the 6s2 electron pair of Bi3+ hybridizes with the oxygen in the neighboring In—OH Lewis base site, leading to mildly increased Lewis basicity without influencing the Lewis acidity of the nearby In Lewis acid site. Meanwhile, Bi3+ can act as an extra acid site, serving to maximize the heterolytic splitting of reactant H2, and results in a more hydridic hydride for more efficient CO2 reduction. This study demonstrates that isomorphous substitution can effectively optimize the reactivity of surface catalytic active sites in addition to influencing optoelectronic properties, affording a better understanding of the photocatalytic CO2 reduction mechanism.
Highlights
Photocatalytic reduction of carbon dioxide (CO2) has been explored for many years as an attractive strategy for reducing CO2 emissions while producing renewable fuels and chemicals.[1–3] Oxide semiconductors are widely used as CO2 reduction photocatalysts due to their light-harvesting property, high stability, and low cost.[4–7] due to the high thermodynamic stability of CO2 and the limitations of intrinsic semiconductor photocatalysts, such as poor charge separation and weak interaction with reactants, intensive efforts have been made to improve these oxide semiconductor photocatalysts via doping.[8,9] For instance, in fluorinated anatase TiO2 nanosheets prepared via substitution of surface hydroxyl groups with fluoride anions, surface fluorination promoted the formation of Ti3+ defects that helped extend the lifetime of photogenerated electrons and holes, and facilitated the reduction of CO2 to CO2−.[10]
The insight gained from density functional theory (DFT) calculations on this system provides compelling evidence that the In OH···In Frustrated Lewis pairs (FLPs) can be systematically tuned in order to maximize its catalytic activity toward heterogeneous CO2 reduction
High-resolution transmission electron microscopy (TEM) (HRTEM) images (Figure 1b,f) show lattice fringes which are continuously distributed over large areas, with pores between interconnected nanocrystals revealing their crystalline structure
Summary
Photocatalytic reduction of carbon dioxide (CO2) has been explored for many years as an attractive strategy for reducing CO2 emissions while producing renewable fuels and chemicals.[1–3] Oxide semiconductors are widely used as CO2 reduction photocatalysts due to their light-harvesting property, high stability, and low cost.[4–7] due to the high thermodynamic stability of CO2 and the limitations of intrinsic semiconductor photocatalysts, such as poor charge separation and weak interaction with reactants, intensive efforts have been made to improve these oxide semiconductor photocatalysts via doping.[8,9] For instance, in fluorinated anatase TiO2 nanosheets prepared via substitution of surface hydroxyl groups with fluoride anions, surface fluorination promoted the formation of Ti3+ defects that helped extend the lifetime of photogenerated electrons and holes, and facilitated the reduction of CO2 to CO2−.[10]. In the mole cular FLPs system, originally developed by Stephan and Erker, a Lewis acid and Lewis base are sterically prevented from bond formation, and can act cooperatively to capture and react with a variety of small molecules, such as during the heterolytic dissociation of H2 and reaction with CO2.[22,23] Our recent studies have discovered FLPs on the surface of In2O3−x(OH)y nanocrystals, with indium hydroxide groups as the Lewis base and coordinatively unsaturated indium sites as Lewis acid. The insight gained from density functional theory (DFT) calculations on this system provides compelling evidence that the In OH···In FLP can be systematically tuned in order to maximize its catalytic activity toward heterogeneous CO2 reduction
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