Nickel nanoparticles supported by fishbone-derived hydroxyapatite as a bifunctional catalyst for 4-nitrophenol reduction and methanation of carbon oxides.

Developing low-cost, multifunctional catalysts for aqueous pollutant reduction and gas-phase carbon oxide conversion is important for sustainable environmental technologies. In this study, biogenic HA was extracted from Pangasius bocourti fishbone waste and used as a support for NiNPs. A series of xNi/HA catalysts, where x = 5, 7.5, 10, and 12.5 wt%, was prepared through wet impregnation, calcination, and hydrogen reduction. The best 10Ni/HA(R) catalyst retained the crystalline HA framework and exhibited an accessible mesoporous and interparticle pore structure. Relatively dispersed metallic NiNPs with an average diameter of 15.20 ± 3.95 nm were observed, while H2-TPR indicated the presence of NiO species with different degrees of interaction with the HA support. For aqueous 4-NP reduction, 10Ni/HA(R) achieved complete conversion to 4-AP within 5 minutes at a catalyst dosage of 0.2 g L-1 and a 4-NP/NaBH4 molar ratio of 1/200. The reaction followed apparent pseudo-first-order kinetics under excess-NaBH4 conditions, with k = 1.137 min-1. During successive 4-NP addition, conversion remained over 93% through ten successive 4-NP additions. The subsequent decrease was mainly associated with progressive NaBH4 depletion and reaction-product accumulation in the closed batch system, while post-reaction characterization indicated preservation of the major crystalline phases and a comparable Ni nanoparticle-size range. Under continuous-flow gas-phase methanation conditions at a GHSV of 20 000 h-1, the catalyst achieved nearly complete CO conversion at 300 °C and more than 60% CO2 conversion at 400 °C. These results demonstrate a promising waste-valorization strategy for producing multifunctional Ni/HA catalysts for aqueous 4-NP reduction and gas-phase carbon oxide conversion.