Metastable Corundum-Type In₂O₃: Phase Stability, Reduction Properties, and Catalytic Characterization

The phase stability, reduction, and catalytic properties of corundum-type rhombohedral In₂O₃ have been comparatively studied with respect to its thermodynamically more stable cubic In₂O₃ counterpart. Phase stability and transformation were observed to be strongly dependent on the gas environment and the reduction potential of the gas phase. As such, reduction in hydrogen caused both the efficient transformation into the cubic polymorph as well as the formation of metallic In especially at high reduction temperatures between 573 and 673 K. In contrast, reduction in CO suppresses the transformation into cubic In₂O₃ but leads to a larger quantity of In metal at comparable reduction temperatures. This difference is also directly reflected in temperature-dependent conductivity measurements. Catalytic characterization of rh-In₂O₃ reveals activity in both routes of the water–gas shift equilibrium, which gives rise to a diminished CO₂-selectivity of ∼60% in methanol steam reforming. This is in strong contrast to its cubic counterpart where CO₂ selectivities of close to 100% due to the suppressed inverse water–gas shift reaction, have been obtained. Most importantly, rh-In₂O₃ in fact is structurally stable during catalytic characterization and no unwanted phase transformations are triggered. Thus, the results directly reveal the application-relevant physicochemical properties of rh-In₂O₃ that might encourage subsequent studies on other less-common In₂O₃ polymorphs