Carbon and nitrogen co-doped WO3 as sonocatalyst

Abstract

The calcination of a mixture of ammonium para tungstate and graphitic C3N4 (10:1 by weight) in flowing nitrogen atmosphere resulted in blue colored products. The powder X-ray diffraction patterns of the calcined products at 350 and 450 °C contained diffraction peaks related to hexagonal and orthorhombic WO3, respectively. The microscopy and Raman spectral measurements reaffirmed these observations. The X-ray photoelectron spectral analysis of hexagonal and orthorhombic WO3 samples indicated carbon and nitrogen-doped in them, causing the reduction of W6+ to W5+ to a smaller extent. The concentration of carbon and nitrogen in these samples were also independently verified by combustion-based elemental analysis. The hexagonal WO3 showed a higher optical bandgap (2.77 eV) than the orthorhombic WO3 (2.58 eV). Both the samples were EPR active, originating from the singly ionized oxygen (paramagnetic) vacancies. The concentration of oxygen vacancies was higher in hexagonal WO3 than orthorhombic WO3. While both these samples were mesoporous, hexagonal WO3 exhibited almost double the surface area than orthorhombic WO3. We evaluated these WO3 samples as catalysts to degrade cationic and anionic dye aqueous solutions oxidatively under ultrasonic conditions. The hexagonal WO3, in general, facilitated higher amounts of degradation of both rhodamine B and methyl orange dye solutions under the influence of sonic waves. The efficiency of orthorhombic WO3 to degrade both these dye solutions was substantially lower. There was no change in the structure of catalysts after their use. A possible mechanism operative in the oxidative degradation of dye solutions is outlined. Correlation between the higher concentrations of oxygen vacancies with higher catalytic capacity is deduced. The catalytic reactions were found to follow pseudo-first-order kinetics.