An Interplay of PdAg Nanoparticles and Aminopolymer Confined in Hollow Silica for Additive-Free CO2 Hydrogenation to Formic Acid

Abstract

Carbon dioxide (CO2) hydrogenation to formic acid (FA) is a promising route for implementing sustainable hydrogen energy cycles. Although the reaction readily proceeds in alkaline aqueous media, with bases facilitating CO2 dissociation and activation, formate salts are formed as products, requiring additional post-treatment to yield pure FA. Under base-free conditions, an inherently high activation barrier is required; however, high temperatures reduce gas solubility and hence lower the activity, making FA synthesis at low temperatures under base-free conditions still challenging. Herein, we report efficient CO2 hydrogenation to FA in pure water by exploiting cooperative catalytic actions between PdAg nanoparticles (NPs) and poly(ethylenimine) (PEI) confined within hollow silica shells. The confined PEI functions as a heterogeneous base that activates water/CO2 molecules and promotes bicarbonate (HCO3–) formation and adsorption near the PdAg NPs, while the PdAg NPs hydrogenate the HCO3– to FA. As a result, the catalyst achieves a turnover number (TON) of 83 over 6 h at 4.0 MPa (CO2/H2 = 1:1) and 40 °C in the absence of any base additives, outperforming other Pd-based catalysts previously reported. Moreover, the hollow architecture suppresses leaching and aggregation of the core materials, imparting enhanced stability and reusability. This work demonstrates a novel strategy for boosting aqueous CO2 hydrogenation by utilizing metal–polymer cooperative catalysis and provides a practical synthetic protocol for robust heterogeneous catalysts for aqueous-phase CO2 conversion.