Photocatalytic reduction of Cr(VI) using Nb₂O₅ under UV irradiation: Influence of polymorphs

Abstract This paper discusses the influence of niobium pentoxide (Nb 2 O 5 ) polymorphs on Cr(VI) photoreduction under UV. The tetragonal Nb 2 O 5 (M-Nb 2 O 5 ) and monoclinic Nb 2 O 5 (H-Nb 2 O 5 ) were synthesized by thermal oxidation of Nb powders at 200 - 1000 °C in a tube furnace under the air atmosphere. The morphology and crystal structure of the annealed Nb powders were examined using Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) analysis. Phase percentages of the phase were determined using X’pert Highscore Plus software via the Rietveld refinement method. Cr(VI) concentration was measured colorimetrically using UV-Visible spectroscopy. SEM results showed closed-packed rod-like structures for samples annealed at 400 - 1000 °C. XRD revealed that both tetragonal and monoclinic Nb 2 O 5 structures co-existing at 400 - 1000 °C. Rietveld refinement analysis indicated an increase in H-Nb 2 O 5 and a decrease in M-Nb 2 O 5 with rising temperatures, suggesting a transformation from tetragonal to monoclinic Nb 2 O 5 at higher temperatures. Photocatalytic tests demonstrated that the sample synthesized at 1000 °C achieved the highest Cr(VI) removal rate of 62% after 150 min, attributed to the higher photoactivity of monoclinic Nb 2 O 5 than tetragonal Nb 2 O 5 .