Enhanced Base‐Free Formic Acid Production from CO2 on Pd/g‐C3N4 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‐C3N4 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‐C3N4 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. Perfect imperfections: Bifunctional Pd on carbon nitride catalysts are designed for the base‐free production of formic acid by CO2 hydrogenation. Careful tuning of the amount of N‐based basic edge defects of the carrier and of the size and distribution of the metal particles, along with the optimization of reaction conditions, leads to a system that greatly exceeds the best Pd/g‐C3N4 material reported in the literature and, in addition, is recyclable.