Designing catalytic nanostructures that can thermochemically or photochemically convert gaseous carbon dioxide into carbon based fuels is a significant challenge which requires a keen understanding of the chemistry of reactants, intermediates and products on surfaces. In this context, it has recently been reported that the reverse water gas shift reaction (RWGS), whereby carbon dioxide is reduced to carbon monoxide and water, CO2 + H2 → CO + H2O, can be catalysed by hydroxylated indium oxide nanocrystals, denoted In2O(3-x)(OH)y, more readily in the light than in the dark. The surface hydroxide groups and oxygen vacancies on In2O(3-x)(OH)y were both shown to assist this reaction. While this advance provides a first step toward the rational design and optimization of a single-component gas-phase CO2 reduction catalyst for solar fuels generation, the precise role of the hydroxide groups and oxygen vacancies in facilitating the reaction on In2O(3-x)(OH)y nanocrystals has not been resolved. In the work reported herein, for the first time we present in situ spectroscopic and kinetic observations, complemented by density functional theory analysis, that together provide mechanistic information into the surface reaction chemistry responsible for the thermochemical and photochemical RWGS reaction. Specifically, we demonstrate photochemical CO2 reduction at a rate of 150 μmol gcat(-1) hour(-1), which is four times better than the reduction rate in the dark, and propose a reaction mechanism whereby a surface active site of In2O(3-x)(OH)y, composed of a Lewis base hydroxide adjacent to a Lewis acid indium, together with an oxygen vacancy, assists the adsorption and heterolytic dissociation of H2 that enables the adsorption and reaction of CO2 to form CO and H2O as products. This mechanism, which has its analogue in molecular frustrated Lewis pair (FLP) chemistry and catalysis of CO2 and H2, is supported by preliminary kinetic investigations. The results of this study emphasize the importance of engineering the surfaces of nanostructures to facilitate gas-phase thermochemical and photochemical carbon dioxide reduction reactions to energy rich fuels at technologically significant rates.

In this study we investigated, theoretically and experimentally, the unique photoactive behavior of pristine and defected indium oxide surfaces providing fundamental insights into their excited state properties as well as an explanation for the experimentally observed enhanced activity of defected indium oxide surfaces for the gas-phase reverse water gas shift reaction, CO2 + H2 + hν→ CO + H2O in the light compared to the dark. To this end, a detailed excited-state study of pristine and defected forms of indium oxide (In2O3, In2O3-x, In2O3(OH)y and In2O3-x(OH)y) surfaces was performed using time dependent density functional theory (TDDFT) calculations, the results of which were supported experimentally by transient absorption spectroscopy and photoconductivity measurements. It was found that the surface frustrated Lewis pairs (FLPs) created by a Lewis acidic coordinately unsaturated surface indium site proximal to an oxygen vacancy and a Lewis basic surface hydroxide site in In2O3-x(OH)y become more acidic and basic and hence more active in the ES compared to the GS. This provides a theoretical mechanism responsible for the enhanced activity and reduced activation energy of the photochemical reverse water gas shift reaction observed experimentally for In2O3-x(OH)y compared to the thermochemical reaction. This fundamental insight into the role of photoexcited surface FLPs for catalytic CO2 reduction could lead to improved photocatalysts for solar fuel production.

Silicon constitutes 28% of the earth's mass. Its high abundance, lack of toxicity and low cost coupled with its electrical and optical properties, make silicon unique among the semiconductors for converting sunlight into electricity. In the quest for semiconductors that can make chemicals and fuels from sunlight and carbon dioxide, unfortunately the best performers are invariably made from rare and expensive elements. Here we report the observation that hydride-terminated silicon nanocrystals with average diameter 3.5 nm, denoted ncSi:H, can function as a single component heterogeneous reducing agent for converting gaseous carbon dioxide selectively to carbon monoxide, at a rate of hundreds of μmol h−1 g−1. The large surface area, broadband visible to near infrared light harvesting and reducing power of SiH surface sites of ncSi:H, together play key roles in this conversion. Making use of the reducing power of nanostructured hydrides towards gaseous carbon dioxide is a conceptually distinct and commercially interesting strategy for making fuels directly from sunlight.

The dissociation of water on 2D monolayer molybdenum disulfide (MoS 2 ) edges was studied with density functional theory. The catalytically active sites for H 2 O, H, and OH adsorption on MoS 2 edges with 0% (Mo-edge), 50% (S50-edge), and 100% (S100-edge) sulfur coverage were determined, and the Mo-edge was found to be the most favorable for adsorption of all species. The water dissociation reaction was then simulated on all edges using the climbing image nudged elastic band (CI-NEB) technique. The reaction was found to be endothermic on the S100-edge and exothermic for the S50-and Mo-edges, with the Mo-edge having the lowest activation energy barrier. Water dissociation was then explored on the Mo-edge using metadynamics biased ab initio molecular dynamics (AIMD) methods to explore the reaction mechanism at finite temperature. These simulations revealed that water dissociation can proceed by two mechanisms: the first by splitting into adsorbed OH and H species produced a particularly small activation free energy barrier of 0.06 eV (5.89 kJ/mol), and the second by formation of desorbed H 2 and adsorbed O atom had a higher activation barrier of 0.36 eV (34.74 kJ/mol) which was nevertheless relatively small. These activation barrier results, along with reaction rate calculations, suggest that water dissociation will occur spontaneously at room temperature on the Mo-edge.

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.