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

Abstract

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.

Keywords: Biomass valorization; H2 evolution; Photocatalysis; Surface properties.