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 C1 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)2KFe(OtBu)3}2], was supported on alumina that had been partially dehydroxylated at 500 °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 H2 activation by K[(AlsO)2FeOH], leading to K[(AlsO)2FeH]. CO2 insertion gives hydroxycarbonyl intermediate K[(AlsO)2FeCOOH], followed by liberation of CO to regenerate K[(AlsO)2FeOH].