Influence of Nitrogen-Doped Carbon Dots on H• Radical-Mediated Au–H Formation in the Hydrogenation of 4‑Nitrophenol Using NCDs-Au Nanohybrids

The hydrogenation of 4-nitrophenol using carbon dot-stabilized gold (Au) nanoparticles is well-studied, with Au–H species known to catalyze the reaction. However, the impact of specific nitrogen moieties in nitrogen-doped carbon dots on Au–H formation and catalytic activity remains unexplored. These nitrogen species, acting as surface ligands, may influence the catalytic properties through the generation of Au–H species via H• radicals. In this regard, modulation of the catalytic properties of Au nanoparticles has been explored by conjugating their surface with nitrogen-doped carbon dots (NCDs). Three distinct nanohybrid formulations comprising NCDs and Au nanoparticles (i.e., NCDs-Au) have been prepared, where the NCDs were derived from different carbon sources (e.g., citric acid and l-malic acid) and varying mole ratios of the nitrogen source (i.e., urea). The impact of NCDs on Au nanoparticle-mediated catalysis has been investigated using the model reaction of hydrogenation of 4-nitrophenol (4-NP) in the presence of NaBH4. The fractions of different nitrogen species (such as pyrrolic, pyridinic, and amidic) in the different NCDs-Au nanohybrids were quantified through XPS analysis, and their roles in catalytic performance have been studied. Further, the size, shape, crystallinity, defects, and exposed facets of the NCDs-Au nanohybrids have also been assessed (through XRD, HRTEM, and Raman studies), and their structure–activity relationships have been corroborated. The hydrogenation of 4-NP is proposed to happen through the formation of gold-hydride (Au–H) species facilitated by H• radicals, as confirmed by EPR analysis. The NCDs-Au nanohybrid, synthesized from NCDs derived from a 1:3 molar ratio of l-malic acid and urea (MU13-Au), exhibits superior catalytic efficiency with a rate constant of 1.013 min–1, attributed to its abundant defects and a notably high relative content of catalytically favorable pyridinic nitrogen species compared to other tested nanohybrids.