Ni–N synergy enhanced the synthesis of formic acid via CO2 hydrogenation under mild conditions

Abstract

The direct CO₂ hydrogenation to formic acid is appealing since it is a potential hydrogen storage material and can be a precursor of higher value-added products. Due to thermodynamic limitations, developing an efficient catalyst system for this process is quite a challenge. Herein, a Ni-based N-modified TiO₂-supported catalyst that showed exemplary activity and selectivity toward formic acid synthesis (757 TON), which was reported to be the highest over noble metal-free catalyst system, is designed. The combined impact of Ni and N over TiO₂ as a support was thoroughly investigated in this research for improved activity in the selective conversion of CO₂ to formic acid. X-ray photoelectron spectroscopy and X-ray absorption spectroscopy investigations showed direct coordination between Ni and N, which emphasizes the metal support interaction for significantly higher catalytic activity. In-depth characterization of Ni/N-TiO₂ reveals that CO₂ undergoes insertion reaction with H₂ to yield formic acid on Ni⁰ as an active surface via the formate intermediate. Complementary DFT studies elucidate the pivotal effect of nitrogen doping on the TiO₂ surface, which provides a more feasible pathway than pure TiO₂ by lowering the activation barriers. More significantly, the catalyst demonstrated a steady performance in experiments involving continuous recycling and long-term study.