In Situ Electronic Probing of Photoconductive Trap States for the Catalytic Reduction of CO2 by In2O3- xOH y Nanorods

Cooperative Coupling of H2 O2 Production and Organic Synthesis over a Floatable Polystyrene-Sphere-Supported TiO2 /Bi2 O3 S-Scheme Photocatalyst

Cooperative coupling of photocatalytic H₂ O₂ production with organic synthesis has an expansive perspective in converting solar energy into storable chemical energy. However, traditional powder photocatalysts suffer from severe agglomeration, limited light absorption, poor gas reactant accessibility, and reusable difficulty, which greatly hinders their large-scale application. Herein, floatable composite photocatalysts are synthesized by immobilizing hydrophobic TiO₂ and Bi₂ O₃ on lightweight polystyrene (PS) spheres via hydrothermal and photodeposition methods. The floatable photocatalysts are not only solar transparent, but also upgrade the contact between reactants and photocatalysts. Thus, the floatable step-scheme (S-scheme) TiO₂ /Bi₂ O₃ photocatalyst exhibits a drastically enhanced H₂ O₂ yield of 1.15 mm h^-1 and decent furfuryl alcohol conversion to furoic acid synchronously. Furthermore, the S-scheme mechanism and dynamics are systematically investigated by in situ irradiated X-ray photoelectron spectroscopy and femtosecond transient absorption spectrum analyses. In situ Fourier transform infrared spectroscopy and density functional theory calculations reveal the mechanism of furoic acid evolution. The ingenious design of floatable photocatalysts not only furnishes insight into maximizing photocatalytic reaction kinetics but also provides a new route for highly efficient heterogeneous catalysis.

Waveguide photoreactor enhances solar fuels photon utilization towards maximal optoelectronic - photocatalytic synergy

A conventional light management approach on a photo-catalyst is to concentrate photo-intensity to enhance the catalytic rate. We present a counter-intuitive approach where light intensity is distributed below the electronic photo-saturation limit under the principle of light maximization. By operating below the saturation point of the photo-intensity induced hydroxide growth under reactant gaseous H₂+CO₂ atmosphere, a coating of defect engineered In₂O_3-x(OH)_y nanorod Reverse Water Gas Shift solar-fuel catalyst on an optical waveguide outperforms a coated plane by a factor of 2.2. Further, light distribution along the length of the waveguide increases optical pathlengths of the weakly absorptive green and yellow wavelengths, which increases CO product rate by a factor of 8.1-8.7 in the visible. Synergistically pairing with thinly doped silicon on the waveguide enhances the CO production rate by 27% over the visible. In addition, the persistent photoconductivity behavior of the In₂O_3-x(OH)_y system enables CO production at a comparable rate for 2 h after turning off photo-illumination, enhancing yield with 44-62% over thermal only yield. The practical utility of persistent photocatalysis was demonstrated through outdoor solar concentrator tests, which after a day-and-night cycle showed CO yield increase of 19% over a day-light only period.

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

The photocatalytic activity of nanostructured In₂O_3-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 Bi_zIn_2-zO_3-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 Bi_zIn_2-zO_3-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 Bi_zIn_2-zO_3-x(OH)_y enables optimization of the rate of the photochemical RWGS.

X-ray Photospectroscopy and Electronic Studies of Reactor Parameters on Photocatalytic Hydrogenation of Carbon Dioxide by Defect-Laden Indium Oxide Hydroxide Nanorods

In the study reported herein, glovebox-protected X-ray photoelectron spectroscopy (XPS) and in situ Hall charge carrier measurements provide new insights into the surface physical chemistry of gaseous H₂, CO₂, and H₂+CO₂ combined with nanostructured In₂O_(3−x)(OH)_y nanorods, which ensue under photochemical and thermochemical operating conditions. Heterolytic dissociation of H₂ in H₂-only atmosphere appears to occur mainly under dark and ambient temperature conditions, while the greatest amount of OH shoulder expansion in H₂+CO₂ atmosphere appears to mainly occur under photoilluminated conditions. These results correlate with those of the Hall measurements, which show that the prevalence of homolytic over heterolytic dissociation at increasing temperatures leads to a steeper rate of increase in carrier concentrations; and that H₂ adsorption is more prevalent than CO₂ in H₂+CO₂ photoillumination conditions.