Hydroxyapatite Supported Gold Nanoparticles Catalyzed Efficient Reduction of Nitroarenes and Degradation of Azo Dyes

Gold nanoparticles supported on hydroxyapatite functions as a very efficient catalyst for the reduction of nitroarenes as well as for the degradation of azo dyes. The reaction takes place in aqueous medium at room temperature, using sodium borohydride as the source of hydrogen. The catalyst was prepared by a deposition–precipitation process using gold (III) chloride trihydrate solution containing hydroxyapatite as the support. The catalyst was thoroughly characterized by a pltehora of analytical techniques viz., TEM, HRTEM, FESEM, powder XRD, EDX and FTIR. The catalyst was then employed after optimization of reaction conditions. No additives or inert atmosphere was required and a very low loading of gold was sufficient enough to promote the reaction. Reaction kinetics studies were performed on the reduction of 4-nitrophenol to 4-aminophenol and a very high apparent rate constant of 1.63 × 10^–2 s⁻¹ was obtained. Reaction kinetics studies have also been demonstrated for the degradation of methyl orange and congo red dyes. Appreciable apparent rate constants namely 8.678 × 10⁻³ and 3.464 × 10⁻³ s⁻¹ were obtained for the degradation of methyl orange and congo red dyes respectively. The catalyst was recoverable by simple centrifugation and can be reused for at least five reaction cycles.