Illuminating CO₂ reduction on frustrated Lewis pair surfaces: investigating the role of surface hydroxides and oxygen vacancies on nanocrystalline In₂O_3−x(OH)_y

Abstract

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.

Carrier dynamics and the role of surface defects: Designing a photocatalyst for gas-phase CO 2 reduction

“…We attribute this effect to a decrease in the number of active defect sites, specifically surface hydroxides and oxygen vacancies, within the material (18). In a separate study, which combined theoretical simulations and kinetics experiments, we have confirmed that both hydroxides and oxygen vacancies are needed at the active site to facilitate the reduction of CO 2 to CO (17). Significantly, we were able to demonstrate that there is a 20-kJ/mol reduction in the apparent activation energy for the lightdriven reaction relative to the dark reaction, indicating that light plays an important role in activating this defected In 2 O 3-x (OH) y material to facilitate photocatalysis on its surface.…”

“…4B). Our previous work with these materials has shown that CO 2 binds weakly to the surface of defected indium oxide, generally as carbonate, bicarbonate, and formate species (17,18). These weakly adsorbed species could impact the relaxation dynamics by altering the energetics of the surface.…”

“…Previously, our group has demonstrated that highly defected nanostructured In 2 O 3-x (OH) y functions as an effective gas-phase photocatalyst for the reduction of CO 2 to CO via the reverse water-gas shift reaction (RWGS) (17)(18)(19)(20). Notably, the photocatalytic activity was found to be strongly correlated to the number and the type of defects present in the material.…”

“…It has a relatively high conduction band, and common defects such as oxygen vacancies (O v ) have been shown to cause electron accumulation on its surfaces (15), both of which could aid in facilitating electron transfer to adsorbed molecules such as CO 2 . Additionally, indium oxide is also very stable under illumination, unlike other similarly promising semiconductors such as metal sulfides, which can degrade under illumination, particularly in the presence of water (6, 16).Previously, our group has demonstrated that highly defected nanostructured In 2 O 3-x (OH) y functions as an effective gas-phase photocatalyst for the reduction of CO 2 to CO via the reverse water-gas shift reaction (RWGS) (17)(18)(19)(20). Notably, the photocatalytic activity was found to be strongly correlated to the number and the type of defects present in the material.…”

Tailoring Surface Frustrated Lewis Pairs of In2O3−x(OH)y for Gas‐Phase Heterogeneous Photocatalytic Reduction of CO2 by Isomorphous Substitution of In3+ with Bi3+

“…Sample Bi‐0.5%, however, showed a markedly decreased CO production rate (Table S2, Supporting Information). The presence of fewer hydroxide groups and oxygen vacancy surface defects should have resulted in lower photocatalytic activity, due to a reduced number of active sites 26, 33. However, as shown in the XPS and CO production rate results, the CO production rate increases in accordance with a volcano‐shaped trend despite the decreasing number of defects between Bi‐0% and Bi‐0.1% (Figure 4b).…”