Developing Benign Ni/g-C 3 N 4 Catalysts for CO 2 Hydrogenation: Activity and Toxicity Study.

This research discusses the CO 2 valorization via hydrogenation over the non-noble metal clusters of Ni and Cu supported on graphitic carbon nitride (g-C 3 N 4 ). The Ni and Cu catalysts were characterized by conventional techniques including XRD, AFM, ATR, Raman imaging, and TPR and were tested via the hydrogenation of CO 2 at 1 bar. The transition-metal-based catalyst designed with atom-economy principles presents stable activity and good conversions for the studied processes. At 1 bar, the rise in operating temperature during CO 2 hydrogenation increases the CO 2 conversion and the selectivity for CO and decreases the selectivity for methanol on Cu/CN catalysts. For the Ni/CN catalyst, the selectivity to light hydrocarbons, such as CH 4 , also increased with rising temperature. At 623 K, the conversion attained ca. 20%, with CH 4 being the primary product of the reaction (CH 4 yield >80%). Above 700 K, the Ni/CN activity increases, reaching almost equilibrium values, although the Ni loading in Ni/CN is lower by more than 90% compared to the reference NiREF catalyst. The presented data offer a better understanding of the effect of the transition metals' small metal cluster and their coordination and stabilization within g-C 3 N 4 , contributing to the rational hybrid catalyst design with a less-toxic impact on the environment and health. Bare g-C 3 N 4 is shown as a good support candidate for atom-economy-designed catalysts for hydrogenation application. In addition, cytotoxicity to the keratinocyte human HaCaT cell line revealed that low concentrations of catalysts particles (to 6.25 μg mL -1 ) did not cause degenerative changes.