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.