Enhanced catalytic reduction of nitrophenols by sodium borohydride over highly recyclable Au@graphitic carbon nitride nanocomposites

Abstract

In this study, a photochemical green synthesis using thermal exfoliation process is developed to fabricate Au@graphitic carbon nitride (g-C3N4) nanocomposite, highly recyclable and reusable, for the catalytic reduction of nitrophenols by NaBH4. Au nanoparticles (Au NPs) in the diameter of 5-15 nm are deposited onto the surface of g-C3N4 in 3-6 layers of structure. The synthesized Au@g-C3N4 nanocomposites exhibit excellent catalytic activity and stability in the reduction of nitrophenols including 2-nitrophenol, 3-nitrophenol, 4-nitrophenol, 2,4-nitrophenol, and 2,4,6-nitriophenol. The catalytic performance of Au@g-C3N4 is highly dependent on the initial nitrophenol concentration, Au loading, inorganic anions, and pH. The rate constant of 4-nitrophenol reduction over Au@g-C3N4 (2 wt%) is 26.4 times that of pure Au NP in the presence of 7 mM of NaBH4 at pH 5. Moreover, Au@g-C3N4 can be reused for at least 10 consecutive cycles without considerable loss of catalytic activity. The presence of anions (0.1 M) such as H2PO4-, SO42-, HCO3-, and NO3- decreases the rate of 4-nitrophenol reduction by a factor of 1.2-8.8; whereas there is a 1.2-time increase in rate constant upon the addition of Cl- ion. The detection of H-center dot radical adducts indicates that Au NPs adsorbs BH4- ions and forms Au-H species. The porous and conductive g-C3N4 provides large surface area for nitroarene adsorption and subsequent electron transfer from the Au-H species to 4-nitrophenol, which results in accelerating the reduction of 4-nitrophenol. Results clearly demonstrate that Au@g-C3N4 is a promising green catalyst of enormous potential for nitroaromatic reduction, which provides a new venue for tailoring Au-based nanomaterials in elucidation of a wide variety of heterogeneous catalytic reactions.