Water−Gas Shift and Formaldehyde Reforming Activity Determined by Defect Chemistry of Polycrystalline In₂O₃

The interaction of In2O3 with methanol steam reforming reactants (H2O), intermediates (formaldehyde), and products (CO, CO2) as well as (inverse) water−gas shift reaction mixtures is studied by volumetric adsorption, temperature-programmed reaction, electric impedance measurements, and Fourier-transform infrared spectroscopy to clarify the high CO2 selectivity of pure In2O3 in methanol steam reforming. Reduction in dry CO occurs already slightly above 300 K, yielding CO2 by reaction with reactive lattice oxygen. Replenishment of any lattice oxygen species by defect quenching with CO2 is strongly suppressed. Adsorption of dry CO or CO2 leads to formation of weakly (monodentate HCO3) or more strongly bound carbonate species (bidentate or bridged CO3), for CO at least partly via reaction with lattice oxygen to CO2 (gas) and readsorption of CO2 (gas) on the In2O3 surface. Whereas CO2 evolution via reaction of a CO + H2O mixture on In2O3 starts at 430 K and accelerates above 550 K, only trace amounts of CO are formed upon reaction in a CO2 + H2 mixture. Formaldehyde is converted with 95% selectivity to CO2 under typical steam reforming conditions and temperatures of 550 K, i.e., at rates and selectivities comparable to methanol