Synergistic effect of microwave rapid heating and weak mineralizer on silica-stabilized tetragonal zirconia nanoparticles for enhanced photoactivity of Bisphenol A

Abstract

Silica-doped tetragonal mesoporous zirconia nanoparticles (SZ) were successfully synthesized with various concentrations of ammonia solution(NH4OH) using microwave-assisted method for photodegradation of Bisphenol A (BPA). The phase of SZ catalyst was pure mesoporous tetragonal ZrO2 with nano in size (5–10 nm). The increasing concentration of NH4OH solution enhanced the hydrolysis and condensation process, as confirmed by formation of additional Zr–O–Zr, Si–O–Si and Si–O–Zr bonds. The formation of oxygen vacancy (Vo) and Zr3+ surface defects (ZSD) also increased proportionally with the concentration of NH4OH solution. Both properties and the Si–O–Zr bond are supposed to play important roles in stabilizing the tetragonal phase of ZrO2. The Vo and ZSD also lowered the band-gap energy (from 4.35 eV to 3.05 eV) and act as electron acceptors. Catalytic testing showed that 8-SZ gave the highest initial rate (0.372× 10−3 mM min−1) of 10 mg L−1 BPA at pH 7 using 0.5 g L−1 catalyst within 4 h while under similar conditions, 5-SZ, 3-SZ, 0-SZ, ZrO2 and TiO2 only gave 0.263 × 10−3, 0.211 × 10−3, 0.145× 10−3, 0.026× 10−3 and 0.305 × 10−3 mM min−1, respectively. These results demonstrate that higher numbers of Si–O–Zr, Vo, and ZSD seemed to be the main factors in the high photoactivity of SZ prepared in 8 M NH4OH solution (8-SZ). A kinetic study demonstrated that the photodegradation followed the pseudo-first-order Langmuir-Hinshelwood model. The mechanism study using scavenging agents proved that photogenerated hole was the main active species, thus led more hydroxyl radicals formed to degrade the BPA. The 8-SZ also exhibited a great potential on degradation of various phenols and dyes in wastewater.