Selective Catalytic Reduction of CO₂ to CO by a Single-Site Heterobimetallic Iron-Potassium Complex Supported on Alumina

CO2 has attracted much attention as a C-1 feedstock for synthetic fuels via its selective catalytic hydrogenation to liquid hydrocarbons. One strategy is the catalytic reduction of CO2 to CO through the reverse water-gas shift (RWGS) reaction, followed by the hydrogenation of CO. In this work, potassium tris(tert-butoxy)ferrate, [{(THF)(2)KFe(OtBu)(3)}(2)], was supported on alumina that had been partially dehydroxylated at 500 degrees C (Al2O3-500), and the resulting catalyst was investigated in the selective reduction of CO2 to CO. The active site precursor was identified as [(THF)K(AlsO)Fe(OtBu)(2)(OHAl)] (i.e., [(THF)KFe(OtBu)(2)]/Al2O3-500), denoted 2-K, based on elemental analysis, diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, scanning electron microscopy with energy-dispersive X-ray spectroscopy (high-resolution transmission electron microscopy (HRTEM) and EDS), X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy. Under the reaction conditions, the precursor becomes an active, stable, and selective RWGS catalyst (100% selectivity to CO at 22.5% CO2 conversion). The reaction mechanism was studied by operando DRIFT spectroscopy and density functional theory (DFT) modeling. The results are consistent with a mechanism involving H-2 activation by K[(AlsO)(2)FeOH], leading to K[(AlsO)(2)FeH]. CO2 insertion gives hydroxycarbonyl intermediate K[(AlsO)(2)FeCOOH], followed by liberation of CO to regenerate K[(AlsO)(2)FeOH].