Additive-free CO2 hydrogenation to pure formic acid solution via amine-modified Pd catalyst at room temperature

Abstract

Direct conversion of CO₂ into pure formic acid (FA) solution in the absence of base additives is of great significance, but is challenging due to thermodynamic limitations. Here, we have synthesized ultrafine Pd nano-catalysts modified with three kinds of amine groups, including a primary amine (PA), secondary amine (SA) and tertiary amine (TA), which realized the efficient hydrogenation of CO₂ to pure FA in water at room temperature for the first time. Comprehensive characterizations reveal that all the amine groups are beneficial to the formation of ultrafine and uniform Pd nanoclusters (<2 nm), and that the electron-richness of Pd has a competitive effect with the steric hindrance of the amine groups on the CO₂ hydrogenation activity. The SA-modified catalyst Pd/AC-SA, which has a lower binding energy of N and lower steric hindrance, thus shows the best turnover frequency (TOF) of 29.1 h⁻¹ at 298 K and 4.0 MPa, which is even comparable to that of reported heterogeneous catalysts with base additives under harsh conditions. The DFT calculations further revealed the mechanism of CO₂ hydrogenation; the first H-atom hydrogenation is the rate-determining step, which is in good agreement with the experiments. This work, as a successful example of the direct generation of pure FA solution at room temperature, exhibits great potential for future large-scale hydrogen storage.