Alkali-Free Synthesis of Formic Acid from CO ₂ Hydrogenation on a Pd/CoAlO _<i>x</i> Catalyst Rich in Intimate Pd ⁰ –CoO–AlO _<i>x</i> Ensembles

CO₂ hydrogenation to formic acid is not only an effective route to achieving carbon neutrality but also a sustainable way to create a recyclable hydrogen energy storage and release system. Typically, a large amount of bases, such as alkali/alkaline earth metal hydroxides or other organics, is added to the reaction system to enhance the reaction, thereby facilitating the formation of formates that are difficult to decompose for recycling CO₂ and releasing H₂. In this context, a Pd/CoAlO_<i>x</i> catalyst with intimate Pd⁰-CoO-AlO_<i>x</i> ensembles was synthesized, and it showed an unprecedentedly high formic acid turnover frequency (TOF) of ∼94 h^-1 at 80 °C and 3.0 MPa in the absence of any alkali additives, which is significantly higher than those of the reported catalysts. Aberration-corrected high-angle annular dark field scanning transmission electron microscopy, high-resolution electron energy loss spectroscopy, quasi in situ X-ray photoelectron spectroscopy, quasi in situ low-energy ion scattering, X-ray absorption near-edge structure/extended X-ray absorption fine structure, in situ CO₂-DRIFTS, H₂-D₂ exchanging experiment, and density functional theory calculation results unraveled that the intimate assembly of CoO and AlO_<i>x</i> species to CoO-AlO_<i>x</i> interfaces can not only provide essential active sites for CO₂ adsorption but also facilitate the atomic dispersion of metallic Pd⁰ species that enhances H₂ dissociation. As a result, Pd⁰-CoO-AlO_<i>x</i> ensembles significantly enhance the catalytic activity of Pd/CoAlO_<i>x</i>(1/1) for the hydrogenation of CO₂ to formic acid.