Porous titanium dioxide doped with Ag and Fe prepared by plasma electrolytic oxidation. Photocatalytic enhancement of Cr(VI) reduction

H.D. TRAID; M.L. VERA; G.E. ZAMPIERI; M.I. LITTER

Congreso; WCCE11- 11th world Congress of Chemical Engineering; 2023

To avoid the expensive recovery step of suspended TiO2 in heterogeneous photocatalysis, the immobilization of the photocatalyst is necessary. The anodic oxidation of titanium by plasma electrolytic oxidation (PEO) technique is one of the simplest and cheapest methods of synthesis of TiO2 coatings, leading to porous and crystalline materials, capable of reducing Cr(VI) through the very well-known TiO2 photocatalytic mechanism. Moreover, the doping could improve the photocatalyst efficiency. The present work assesses the influence of Ag and Fe addition to the electrolyte used in the synthesis of porous TiO2 by PEO on the photocatalytic reduction of Cr(VI) in the presence of ethylenediaminetetraacetic acid (EDTA) as a synergetic agent.Commercially pure titanium plates (Grade 2, ASTM B367), 3 × 2 × 0.2 cm3, were polished and used as substrates for anodic oxidation. The anodization was performed at a maximum potential of 120 V, a maximum current density of 1200 A m-2, for 5 min, using a direct current source. H2SO4 (4 M) was used as the base electrolyte (undoped sample), and Ag2(SO)4 and Fe2(SO4)3 were added to obtain solutions of 0.001, 0.003, and 0.01 M, and 0.005, 0.02, and 0.1 M, respectively. The coatings were characterized by scanning electron microscopy (SEM) combined with energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), glancing incidence X-ray diffraction (GI-DRX) and UV-Vis diffuse reflectance spectra (DRS). The photocatalytic activity of the coatings was evaluated through the reduction of Cr(VI) in the presence of EDTA (0.4 mM K2Cr2O7, 1 mM EDTA, pH 2), irradiated during 5 h and monitored by the diphenylcarbazide (DFC) method, measuring spectrophotometrically at 540 nm.The coatings were porous with an average pore diameter of about 120 nm. The doped samples showed particles on the surface. The coatings had an anatase fraction of 0.7 and bandgaps of ≈3 eV, without any influence of the doping. Ti, O, and S were identified in all samples through EDS and XPS analyses, and Fe and Ag were identified in the doped ones.All the coatings yielded a reduction of the Cr(VI) concentration higher than that taking place in the absence of the photocatalyst. Furthermore, all the doped coatings (except Fe 0.1 M) presented a faster Cr(VI) removal than the undoped ones. The maximum enhancements (16 and 14%) were obtained with the Fe- and Ag-doped samples prepared with the lower concentrations of the dopants. The enhancement could be attributed to the change in the oxidation state of the dopants by the electrons of the TiO2 conduction band, which delays the detrimental recombination of the electron-hole pairs. Higher doping concentrations were disadvantageous for the activity because the dopant acts as a charge recombination center.