On the influence of hydrothermal treatment pH on the performance of Bi2WO6 as photocatalyst in the glycerol photoreforming

Solar driven semiconductor-based photoreforming of biomass derivatives, such as glycerol is a sustainable alternative towards green hydrogen evolution concerted with production of chemical feedstocks. In this work, we have investigated the influence of the pH of the hydrothermal treatment on the efficiency of Bi₂WO₆ as photocatalyst in the glycerol photoreforming. Bi₂WO₆ is pointed as a promising material for this application due its adequate band gap and the ability to promote hole transfer directly to glycerol without formation of non-selective ^⋅OH radicals. Samples prepared at neutral to moderate alkaline conditions (pH = 7-9) are highly crystalline, while those prepared in acidic media (pH = 0-2) exhibit higher concentrations of oxygen vacancies. At pH = 13, the non-stoichiometric Bi(III)-rich phase Bi_3.84W_0.16O_6.24 is formed. All samples were fully characterized towards their optical and morphological properties. UV-Vis irradiation of the photocatalysts modified with 1% m/m Pt and in the presence of 5% v/v aqueous glycerol solution leads to H₂ evolution and glycerol oxidation. The sample prepared at pH = 0 exhibited the highest photonic efficiency (ξ) for H₂ evolution (1.4 ± 0.1%) among the investigated samples with 99% selectivity for simultaneous formic acid formation. Similar performance was observed for the non-stoichiometric Bi_3.84W_0.16O_6.24 sample (ξ = 1.2 ± 0.1% and 88% selectivity for formic acid), whereas the more crystalline sample prepared at pH = 9 was less active (ξ = 0.9 ± 0.1%) and leads to multiple oxidation products. The different behaviors were rationalized based on the role of oxygen vacancies as active adsorption and redox sites at the semiconductor surface, stablishing clear relationships between the semiconductor structure and its photocatalytic performance. The present work contributes for the rational development of specific photocatalysts for glycerol photoreforming.

Fabrication, Characterization and Response Surface Method (RSM) Optimization for Tetracycline Photodegration by Bi3.84W0.16O6.24- graphene oxide (BWO-GO)

RSM is a powerful tool for optimizing photocatalytic processes. The BWO-GO photocatalysts have been successfully synthesized via inorganic-salt-assisted hydrothermal method. XRD, TEM, FESEM, HRTEM and STEM are used to characterize BWO-GO heterojunction. UV-vis, PL, ESR and radical scavenger experiments are used to explore the photocatalysis mechanism. The photocatalysts are evaluated by TC photodegradation under visible light irradiation. And the main active species in TC photodegradation is ·O₂⁻. Response surface methodology is used to optimize three key independent operating parameters, namely photocatalyst dosage (X₁), percentages of GO (X₂) and reaction time (X₃), for TC photodegradation. The central composite design (CCD) is used to conduct experiments. The results showed that TC removal is significantly affected by the synergistic effect of linear term of X₁ and X₃. However, the quadratic terms of X₁² and X₃² had an antagonistic effect on T removal. The obtained RSM model (R² = 0.9206) shows a satisfactory correlation between experimental and predicted values of TC removal. The optimized conditions is of 0.3 g photocatalyst dosage, 1.49 wt% GO loaded percentage and 90 min reaction time. Under this condition, theoretical prediction removal is 80.22% and the actual removal is 78.43%.