Relating surface defect energetics with reactant gas adsorption during the photo-catalytic reduction of CO2 by partially hydrolyzed In2O3 nanorods.

Photo-Induced Transient Current Spectroscopy (PICTS) is a versatile technique for measurements of defect state energies and densities in photo-active materials. It is suitable for investigating the surface-gas adsorbate behavior and the defect characteristics of defect laden In2O3-x(OH)y nanorods, having oxygen vacancies and hydroxide surface groups, under in situ reactor conditions of dark ambient temperature, dark 150 °C and photo-illuminated 150 °C, for the photo-assisted Reverse Water Gas Shift reaction. From glovebox-protected X-ray Photoelectron Spectroscopy and in situ PICTS measurements we determined that the reduction of CO2 is associated with heterolytic dissociation of H2 into In-H§- and HO-H§+ centres accompanied by an increase in average carrier trap energies; increased carbonate formation in a photo/thermal reactor state of H2 + CO2, and an average trap energy decrease of 0.11 eV from H2 to a CO2 + H2 mixture, which correlates with binding energy shifts of the OH shoulder of the O1s spectra. These results show the reactivity link between the various OH groups, oxygen vacancies and trap energies of In2O3-x(OH)y in the reactant gas atmosphere components.

Synergistic Coupling of Photo and Thermal Conditions for Enhancing CO2 Reduction Rates in the Reverse Water Gas Shift Reaction.

The photocatalytic activity of nanostructured In2O3-x(OH)y for the reverse water gas shift (RWGS) reaction CO2 + H2 → CO + H2O can be greatly enhanced by substitution of Bi(III) for In(III) in the lattice of BizIn2-zO3-x(OH)y. This behavior was hypothesized as the effect of the population and location of Bi(III) on the Lewis acidity and Lewis basicity of proximal hydroxide and coordinately unsaturated metal surface sites in BizIn2-zO3-x(OH)y acting synergistically as a frustrated Lewis acid-base pair reaction. Nonetheless, such photocatalytic activity is usually optimized in a specific batch reactor setup sequence, with H2 as an initial gas input under photo and thermal conditions before introducing CO2. Hence, the chemical interplay between environment parameters such as photoillumination, thermal input, and gas reactant components with the effects of Bi substitution is unclear. Reported herein, glovebox-protected X-ray photoelectron spectroscopy (XPS) interrogates this photochemical RWGS reaction transiting from vacuum state to similar conditions in a photocatalytic reactor, under dark and ambient temperatures, 150°C in dark and 150 °C under photoillumination. Binding energy shifts were used to correlate the material system's Lewis basicity response to these acidic probe gases. In-situ gas electronic sensitivity and in-situ UV-vis-derived band-gap trends confirm the trends shown in the XPS results, hence showing its equivalency with in situ methods. The enhanced photocatalytic reduction rate of CO2 with H2 with a low doped 0.05% a.t Bi system is thus associated with an increased gas sensitivity in H2 + CO2, a greater expansion in the OH shoulder than that of the undoped system under heat and light conditions, as well as a greater thermal stability of dissociated H adatoms. The photoinduced expansion of the OH shoulder and the increased positive binding energy shifts show the important role of photoillumination over that of thermal conditions. The poor catalytic performance of the high doped system can be attributed to a competing H2 reduction of In3+. The results provide new insight into how pairing photo and thermal conditions with the methodical tuning of the Lewis acidity and Lewis basicity of surface frustrated Lewis acid-base pair sites by varying z amount in BizIn2-zO3-x(OH)y enables optimization of the rate of the photochemical RWGS.

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.

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.

Spatial Separation of Charge Carriers in In2O3-x(OH)y Nanocrystal Superstructures for Enhanced Gas-Phase Photocatalytic Activity.

The development of strategies for increasing the lifetime of photoexcited charge carriers in nanostructured metal oxide semiconductors is important for enhancing their photocatalytic activity. Intensive efforts have been made in tailoring the properties of the nanostructured photocatalysts through different ways, mainly including band-structure engineering, doping, catalyst-support interaction, and loading cocatalysts. In liquid-phase photocatalytic dye degradation and water splitting, it was recently found that nanocrystal superstructure based semiconductors exhibited improved spatial separation of photoexcited charge carriers and enhanced photocatalytic performance. Nevertheless, it remains unknown whether this strategy is applicable in gas-phase photocatalysis. Using porous indium oxide nanorods in catalyzing the reverse water-gas shift reaction as a model system, we demonstrate here that assembling semiconductor nanocrystals into superstructures can also promote gas-phase photocatalytic processes. Transient absorption studies prove that the improved activity is a result of prolonged photoexcited charge carrier lifetimes due to the charge transfer within the nanocrystal network comprising the nanorods. Our study reveals that the spatial charge separation within the nanocrystal networks could also benefit gas-phase photocatalysis and sheds light on the design principles of efficient nanocrystal superstructure based photocatalysts.