Reductive Dispersion of In2O3 into H-CHA and Reversible Formation of Isolated In(I)/In(III) Species under H2/O2 Redox Cycling

Abstract The reductive dispersion of metal oxides into isolated cationic species in zeolites is an important strategy for generating well-defined catalytic sites, yet its kinetics and molecular mechanism remain insufficiently understood. Here, we investigate the H2-induced redispersion of In2O3 physically mixed with H-CHA using in situ/operando UV–vis spectroscopy, In K-edge X-ray absorption near-edge structure (XANES), SEM-EDS mapping, modulation-excitation spectroscopy, H2-TPR, XRF, and density functional theory (DFT) calculations. Time-resolved UV–vis spectroscopy quantitatively tracks the consumption of bulk-like In2O3 under H2 and reveals that the redispersion rate depends on temperature, H2 concentration, In2O3 precursor properties, and zeolite framework. Higher temperatures and H2 concentrations accelerate the process, with the highest reliable initial rate observed at 600 °C under 10% H2. In K-edge XANES indicates selective formation of reduced In species assigned to framework-stabilized In+ rather than metallic In0, demonstrating a partial-reduction pathway in CHA. H2-TPR provides quantitative redox evidence consistent with reductive solid-state ion exchange and reversible reduction of isolated oxygen-containing In species. SEM–EDS mapping shows the disappearance of surface In-rich domains after H2 treatment, consistent with indium redistribution, while XANES provides bulk-sensitive evidence for formation of reduced In species. Operando redox-cycling experiments reveal reversible interconversion between In(I) and oxygen-containing In(III) species under H2/O2 switching. DFT calculations using representative 1Al and 2Al CHA models suggest that local Al environments can influence the relative stability of framework-bound reduced In(I) and oxygen/hydroxyl-containing In(III)-type species. Together, these results support a two-step mechanism in which bulk In2O3 is first reduced to mobile In+ species and then anchored at acid sites within the zeolite framework. This work provides mechanistic insight into reductive redispersion and offers design principles for regenerable indium-based zeolite catalysts.