Do poluente tóxico à energia limpa: óxidos de nióbio(V) modificados ativados por luz visível na rota fotocatalítica

Abstract

Brazil, a world leader in producing niobium(V) oxide semiconductors, strategically applies this material in heterogeneous photocatalysis. This study aimed to overcome challenges such as the low quantum yield and high bandgap of Nb2O5 through controlled syntheses targeting structural and chemical changes, incorporating cobalt and methylene blue species into this oxide. Materials without chemical modifications are characterized by a bandgap of around 3.1 eV of indirect transition. Meanwhile, materials with new chemical species showed a wide range of visible light absorption. The results of the studies indicate promising potential for these chemically modified materials in removing inorganic sulfides from aqueous effluents through heterogeneous photocatalysis using visible light. Cobalt-modified materials, particularly npNb(V)OCoO, demonstrated high performance, with a relative photocatalytic efficiency above 84% and significant reusability. Furthermore, the products generated by these photocatalysts are predominantly compounds of lower toxicity, such as SO32- and SO42-, compared to the initial substrate. The quantity and distribution of Co+3 and Co+2 species in these materials seem to have a direct influence on photocatalytic activity; the high ligand field stabilization, due to the higher amount of Co(III) in npNb(V)OCoO material, emerges as a critical factor in reducing charge carrier recombination. In CO2 photoreduction and H2 production using water molecules, the irradiation source with higher energy seems to favour both competitive reactions, generating both CH4 and H2 as gaseous products. In contrast, visible light irradiation appears more specific in selectivity between the two reductions, producing more H2 in the gas phase, especially in cobalt-modified materials. However, nsNb(V)O and npNb(V)OAM materials highly preferred CO2 photoreduction under UV and visible light, respectively. Although the photosensitized material cannot be classified as robust due to the high leaching rate of the dye molecule, for CO2 photoreduction, this may be a differential for the material's preference for this reaction, as the leached methylene blue starts to act as an electron donor.