Light-assisted hydrogen evolution from gallic acid aqueous solutions using RuO2-ZnO composites prepared via supercritical CO2 assisted micronization route

A key goal of contemporary chemical engineering is to optimize and enhance the treatment and purification of industrial wastewater. Growing scientific attention is focused on biomass photoreforming processes, which aim to generate hydrogen through the breakdown of organic matter in wastewater. Among these substances, polyphenols are prevalent in wastewater from the agricultural and food sectors, such as the olive oil and wine industries. This research aims to photocatalytically generate hydrogen from aqueous solutions containing gallic acid, utilizing, for the first time, RuO2-ZnO composites synthesized via supercritical CO2 assisted micronization route of precursor salts for photoactive phases. A key innovation lies in the synthesis method, which employed micronized precursors obtained through the supercritical antisolvent (SAS) technique utilizing CO2 as the antisolvent. The prepared samples were characterized by N2 adsorption at -196 degrees C, X-ray diffraction (XRD), Dynamic Light Scattering (DLS), FESEM, Raman spectroscopy, and UV-Vis Diffuse Reflectance spectroscopy. The influence of RuO2 content (in the range of 0.33-1.67 wt%) within RuO2-ZnO composites and photocatalyst dosage (in the range 0.5-2 g/L) on hydrogen generation under UV irradiation was examined. Experimental results revealed that the presence of RuO2, synthesized from micronized Ru-precursor via the SAS method, enhanced the photocatalytic efficiency of SAS-derived ZnO. This enhancement is attributed to the efficient transfer of electrons from the conduction band of ZnO to the conduction band of RuO2, which effectively reduces the recombination of electron-hole pairs. Specifically, a hydrogen production of 4461 mu mol/L was observed after 180 min of UV irradiation, utilizing a photocatalyst containing 1 wt% RuO2 and a photocatalyst dosage of 1 g/L. Additionally, the role of water in the photocatalytic hydrogen evolution reaction was explored by utilizing D2O. The obtained results demonstrated that hydrogen was mainly generated through the reduction of H+ by electrons excited into the ZnO conduction band at the RuO2-ZnO interface.