Enhanced Base-Free Formic Acid Production from CO₂ on Pd/g-C₃ N₄ by Tuning of the Carrier Defects

CO₂ hydrogenation is attracting increasing attention as a sustainable route to produce formic acid, a commodity and potential energy vector. Here, bifunctional catalysts comprising metal nanoparticles deposited on bulk graphitic carbon nitride were assessed under base-free conditions, identifying supported Pd as the best performer. The catalyst productivity was enhanced by maximizing the edge-defects of the g-C₃ N₄ carrier, amino groups able to activate CO₂ , and by generating welldispersed 5 nm Pd particles, required to split H₂ . Bottom-up synthesis methods, that is, hard-templating and carbon enrichment upon polymerization, and top-down strategies, that is, thermal exfoliation of the as-prepared solid, were explored to boost the defects, the nature and density of which were evaluated by thermal and (in situ) spectroscopic techniques. After optimization of temperature, pressure, and reaction time, a 20 times higher turnover frequency compared with the best Pd/g-C₃ N₄ catalyst reported producing formic acid from CO₂ without base was attained. This activity level was retained upon recycling with intermediate catalyst regeneration at mild temperature.