Additive-Free Aqueous Phase Synthesis of Formic Acid by Direct CO₂ Hydrogenation over a PdAg Catalyst on a Hydrophilic N-Doped Polymer–Silica Composite Support with High CO₂ Affinity

A series of N-doped polymer containing silica (NPS) materials were synthesized via condensation reactions of 3-aminopropyltriethoxysilane (APT) and various aldehydes. These materials were assessed as supports for a PdAg alloy catalyst intended to promote the CO2 hydrogenation reaction under additive-free aqueous conditions to produce pure formic acid. A PdAg-supported specimen synthesized from APT and glutaraldehyde (PdAg/NPS(Glut)) exhibited higher catalytic activity than any other catalysts and superior activity compared to previously reported heterogeneous systems, with a turnover number (TON) of 241 over 24 h at 4 MPa using a H2/CO2 volume ratio of 1:1. The electron-rich state of the supported Pd in prepared materials has resulted in this improved performance, based on X-ray photoelectron spectroscopy analyses, which facilitated the hydrogenation of the C atom of CO2. Contact angle measurement for water droplets showed NPS(Glut) material has a hydrophilic nature; although, this material is composed of a polymer-like structure. The higher adsorption capacity and affinity of these materials for CO2 at low temperatures compared to metal oxide supports may also have contributed to the increased activity that was observed in both CO2 adsorption and CO2-TPD analysis.