Reduction Behavior of Cubic In2O3 Nanoparticles by Combined Multiple In Situ Spectroscopy and DFT

Unraveling the molecular and growth mechanism of colloidal black In2O3-x

Black metal oxides with varying concentrations of O-vacancies display enhanced optical and catalytic properties. However, direct solution syntheses of this class of materials have been limited despite being highly advantageous given the different synthetic handles that can be leveraged towards control of the targeted material. Herein, we present an alternate colloidal synthesis of black In2O3-x nanoparticles from the simple reaction between In(acac)3 and oleyl alcohol. Growth studies by PXRD, TEM, and STEM-EDS coupled to mechanistic insights from 1H, 13C NMR revealed the particles form via two paths, one of which involves In0. We also show that variations in the synthesis atmosphere, ligand environment, and indium precursor can inhibit formation of the black In2O3-x. The optical spectrum for the black nanoparticles displayed a significant redshift when compared to pristine In2O3, consistent with the presence of O-vacancies. Raman spectra and surface analysis also supported the presence of surface oxygen vacancies in the as-synthesized black In2O3-x.

Molecular Dynamics Approach to the Physical Mixture of In2O3 and ZrO2: Defect Formation and Ionic Diffusion

Recent research on the use of physical mixtures In₂O₃-ZrO₂ has raised interesting questions as to how their combination enhances catalytic activity and selectivity. Specifically, the relationship between oxygen diffusion and defect formation and the epitaxial tension in the mixture should be further investigated. In this study, we aim to clarify some of these relationships through a molecular dynamics approach. Various potentials for the two oxides are compared and selected to describe the physical mixture of In₂O₃ and ZrO₂. Different configurations of each single crystal and their physical mixture are simulated, and oxygen defect formation and diffusion are measured and compared. Significant oxygen defect formation is found in both crystals. In₂O₃ seems to be stabilized by the mixture, while ZrO₂ is destabilized. Similar results were found for the ZrO₂ doping with In and ln₂O₃ doping with Zr. The results explain the high activity and selectivity catalyst activity of the mixture for the production of isobutylene from ethanol.

Hybrid MOF Template-Directed Construction of Hollow-Structured In2 O3 @ZrO2 Heterostructure for Enhancing Hydrogenation of CO2 to Methanol

Direct hydrogenation of CO₂  to methanol using green hydrogen has emerged as a promising method for carbon neutrality, but qualifying catalysts represent a grand challenge. In₂ O₃ /ZrO₂  catalyst has been extensively applied in methanol synthesis due to its superior activity; however, the electronic effect by strong oxides-support interactions between In₂ O₃  and ZrO₂  at the In₂ O₃ /ZrO₂  interface is poorly understood. In this work, abundant In₂ O₃ /ZrO₂  heterointerfaces are engineered in a hollow-structured In₂ O₃ @ZrO₂  heterostructure through a facile pyrolysis of a hybrid metal-organic framework precursor MIL-68@UiO-66. Owing to well-defined In₂ O₃ /ZrO₂  heterointerfaces, the resultant In₂ O₃ @ZrO₂  exhibits superior activity and stability toward CO₂  hydrogenation to methanol, which can afford a high methanol selectivity of 84.6% at a conversion of 10.4% at 290 °C, and 3.0 MPa with a methanol space-time yield of up to 0.29 g_MeOH  g_cat ^-1  h^-1 . Extensive characterization demonstrates that there is a strong correlation between the strong electronic In₂ O₃ -ZrO₂  interaction and catalytic selectivity. At In₂ O₃ /ZrO₂  heterointerfaces, the electron tends to transfer from ZrO₂  to In₂ O₃  surface, which facilitates H₂  dissociation and the hydrogenation of formate (HCOO*) and methoxy (CH₃ O*) species to methanol. This study provides an insight into the In₂ O₃ -based catalysts and offers appealing opportunities for developing heterostructured CO₂  hydrogenation catalysts with excellent activity.

Elucidating CO2 Hydrogenation over In2 O3 Nanoparticles using Operando UV/Vis and Impedance Spectroscopies

In₂ O₃ has emerged as a promising catalyst for CO₂ activation, but a fundamental understanding of its mode of operation in CO₂ hydrogenation is still missing, as the application of operando vibrational spectroscopy is challenging due to absorption effects. In this mechanistic study, we systematically address the redox processes related to the reverse water-gas shift reaction (rWGSR) over In₂ O₃ nanoparticles, both at the surface and in the bulk. Based on temperature-dependent operando UV/Vis spectra and a novel operando impedance approach for thermal powder catalysts, we propose oxidation by CO₂ as the rate-determining step for the rWGSR. The results are consistent with redox processes, whereby hydrogen-containing surface species are shown to exhibit a promoting effect. Our findings demonstrate that oxygen/hydrogen dynamics, in addition to surface processes, are important for the activity, which is expected to be of relevance not only for In₂ O₃ but also for other reducible oxide catalysts.