Hollow Mesoporous Organosilica Spheres Encapsulating PdAg Nanoparticles and Poly(Ethyleneimine) as Reusable Catalysts for CO₂ Hydrogenation to Formate

CO₂ hydrogenation to formic acid, a renewable hydrogen storage compound, has been regarded as a key reaction to realize hydrogen energy cycles. However, the development of robust heterogeneous catalysts with high activity and stability has been a challenge. We herein report a synthesis of hollow nanostructured composite consisting of PdAg nanoparticles (NPs) and aminopolymers, poly­(ethyleneimine) (PEI), confined in hollow mesoporous organosilica spheres (HMOSs), which act as an efficient and stable heterogeneous catalyst for hydrogenation of CO₂ to formate. The catalyst exhibits high formate yield with a turnover number (TON) of over 2700 and 13 700 for 22 and 110 h, respectively, under mild reaction conditions (total 2.0 MPa, 100 °C), which outperforms the conventional supported Pd catalysts because of the cooperative action of PEI and PdAg NPs confined in a nanospace to directly capture, activate, and hydrogenate the CO₂ molecule. The catalyst is reusable over multiple cycles with high activity because of the protective effect and alkali-tolerant property of HMOSs. This study offers a strategy for the design and development of an efficient and stable heterogeneous catalyst for CO₂ conversion.