Cobalt doped CuO nanoparticles as a highly efficient heterogeneous catalyst for reduction of 4-nitrophenol to 4-aminophenol

We present here a simple thermal route for synthesizing 5.1 wt% cobalt doped CuO nanoparticles (NPs) as the optimized composition of the best heterogeneous CuO catalyst for catalytic reduction of toxic 4-nitrophenol to industrially beneficial 4-aminophenol. The reduction is completed in merely 3 min in the presence of 8 mM NaBH4 as reducing agent. The optimized catalyst dose was 2 mg/L for converting 0.12 mM 4-nitrophenol and the corresponding rate constant (k) for reduction reaction was 43.8 × 10−3 s−1 per mg of catalyst. The catalytic reduction reaction was monitored by UV–vis spectroscopy method and by HPLC analysis. The mechanism is discussed in the light of hydride transfer phenomenon facilitated by large surface area and positive surface charge of the cobalt doped CuO nanoparticles. The cobalt doping resulted in: (a) increasing surface area due to decrease in particle size of the CuO NPs to 10 nm, measured by HRTEM; (b) improve stability of the cobalt doped CuO NPs. The cobalt dopant occupied the grain boundaries of CuO to reduce the particle size as derived from positron annihilation lifetime spectroscopy. The X-ray photoelectron spectroscopy analysis of unused catalyst and the spent catalyst revealed occurrence of Co2+ and Co3+ states at the surface. While the X-ray diffraction studies of spent catalyst confirmed the inhibition of reduction of the surface CuO to metallic copper in the presence of NaBH4, attributable to cobalt doping. It was concluded that cobalt doping led to stable and efficient CuO NPs as catalyst for reduction reaction.