Green synthesis of NiO from watermelon seed shell extract for the evaluation of H₂ production from NaBH₄ hydrolysis and photocatalytic reduction of methylene blue

In the study, NiO nanoparticles were successfully prepared from watermelon seed shell extract as a reducing agent by the green synthesis method. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), transmission electron microscopy (TEM) and ultraviolet–visible (UV–vis) were used for the characterization of NiO nanoparticles to evaluate the structural and morphological identifications. The NiO nanoparticles were applied for the first time as a photocatalyst in the photocatalytic reduction of methylene blue and as a catalyst in H₂ production from NaBH₄ hydrolysis. In the photocatalytic reduction of methylene blue with NaBH₄ as a reducing agent, it was observed that there was no colour change for 24 h without catalyst, and 98 % of methylene blue was reduced in 13 min in the presence of NiO nanoparticles. The photocatalytic reduction kinetics of methylene blue was investigated using the pseudo-first-order model, and the rate constant was found to be 0.382 min⁻¹. H₂ production from NaBH₄ hydrolysis using the NiO nanoparticles was investigated in terms of the effect of microwave radiation ambient and temperature. The kinetics of H₂ production from NaBH₄ hydrolysis was studied by applying the nth-order kinetic model. The H₂ production rate (r_H2), activation energy (Ea) and reaction order (n-value) were found to be 660 mL_H2 min⁻¹ g_cat⁻¹ (at 70 °C), 39.8 kJ mol⁻¹ and 0.59, respectively. This study revealed that NiO nanoparticles prepared by an eco-friendly green synthesis method can be used as an alternative potential catalyst for the photocatalytic reduction of methylene blue and H₂ production from NaBH₄ hydrolysis.