NiO/vermiculite composites prepared for photocatalytic degradation of methanol-water solution and hydrogen generation

Abstract

A novel eco-friendly NiO/Vm clay based photocatalysts were synthesized from two vermiculites (Vm1 and Vm2) and nickel(II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O salt precursor by the chemical precipitation without and with the sodium hydroxide (NaOH) or ammonium hydroxide (NH₄OH, 28% NH₃ in H₂O) as precipitation agents (Synthesis A) in comparison with the solid-state thermal decomposition (Synthesis B) at 600 °C. Structural properties of all specimens were characterized by X-ray fluorescence, scanning electron microscopy, X-ray diffraction, infrared (IR) and Raman spectroscopy. The photocatalytic performance of NiO/Vm composites was evaluated under UV radiation (λ = 254 nm) for decomposition of methanol-water to hydrogen over 4-h and the stable yield of hydrogen over the 24-h periods. The NaOH and NH₄OH affected the NiO crystallite size and therefore the photocatalytic activity during 4 h. Different 2:1 layer charge of Vm1 (0.82 e⁻) and Vm2 (0.40 e⁻) and the specific surface area of Vm1 (about 43 m²/g) and Vm2 (about 34 m²/g) supported H₂ yield of 628.2 μmol/g_cat. and 596.8 μmol/g_cat., close to 657.0 μmol/g_cat. produced in the presence of commercial photocatalyst TiO₂ Evonik P25. Crystalline NiO precipitated and anchored in NiO/Vm composites contained smaller crystallites than those in free NiO. Vermiculite silica surface supports coverage of NiO by hydrogen bonding to Si-OH groups influencing the geometry of the NiO crystal structure (disorder NiO(X)). The heterojunction with Si-O-Ni bonding, at which electrons transfer from Vm to NiO cause enriching electron density in NiO and favoring its photocatalytic activity. Photocatalytic hydrogen generation from methanol–water mixture at the presence of all specimens indicated the main product H₂ and minimum by-products CH₄ and CO. The stable hydrogen production for 24 h was confirmed only in the presence NiO/Vm1–24 while maintaining the NiO(X) in small crystallites. The thermal solid-state procedure provided the gradual dehydration of vermiculites and Ni(NO₃)₂·6H₂O to the same amount and crystallinity of NiO in NiO/Vm1 and NiO/Vm2 composites. The results of this work confirm that vermiculites mixed layer structures with different negative layer charge play a dominant role as semiconductors for anchored NiO. The photocatalytic activity of NiO/vermiculite composites can be harnessed to treat wastewater containing organic contaminants.