Synthesis and Catalytic Evaluation of Silver@nickel Oxide and Alginate Biopolymer Nanocomposite Hydrogel Beads

Abstract In this study, silver-nickel oxide/ calcium alginate (Ag@NiO/Alg) hydrogel beads were synthesized. The preparation of Ag@NiO was carried out in two steps: 1) synthesizing the nickel oxide nanoparticle through precipitation method, and 2) adhering of the Ag+ ion from silver nitrate solution and reduction with sodium borohydride. The nanomaterial was mixed with alginate biopolymer aqueous solution and extruded through syringe in a dropwise manner into a CaCl2 aqueous solution for inotropic crosslinking. Through this method, Ag@NiO/Alg beads were formed which were characterized by different instrumental techniques. The catalytic efficiency of Ag@NiO/Alg was evaluated for the reduction of rhodamine B (RhB) and methyl orange (MO), in the presence of sodium borohydride reducing agent. The uncatalyzed RhB and MO reduction reactions did not show a significant decrease in the absorbance value at λmax within the given experimental time. It was due to the large kinetic barrier. A small decrease in the absorbance value for uncatalyzed reaction in the presence of very high concentration of reducing agent was observed, indicating that the reactions are thermodynamically feasible. After applying various kinetic order equations to the experimental data, the reactions were found to follow pseudo-first order kinetics using the Ag@NiO/Alg catalyst. The apparent rate constants (kapp) were determined as 0.33358 min-1 and 0.20882 min-1 for catalytic reduction of RhB and MO, respectively. Furthermore, reduction reactions were studied at varying dye concentration and catalyst dosages. The recovered catalyst was reused up to five cycles without significant drop in the activity.