New photocatalysts: titanium sponges coated with TiO2 nanotubes

A.N. DWOJAK; M.L. VERA; H.D. TRAID; E.C. BUCHARSKY; K.G. SCHELL; C.E. SCHVEZOV; M.I. LITTER

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

TiO2 nanoparticles are one of the most widely used materials in heterogeneous photocatalysis for water decontamination, due to their low cost, low toxicity, and relatively high photocatalytic activity. One of the main limitations of the development of practical systems based on this technology is that they require a costly stage of separation and recovery of the photocatalyst. A feasible solution is the fixation of nanostructures to suitable supports or the synthesis of nanostructured TiO2 coatings. One of the simplest and cheapest TiO2 coating synthesis techniques is the anodic oxidation of titanium (Ti), which allows to obtain oxides of diverse morphologies in different geometries of Ti substrates. To counteract the loss of area due to immobilization, commercial titanium sponges (99.5%) with a high surface area (106 cm2 g-1) were selected as substrates for the anodic synthesis of TiO2 nanotubes [1]. For the surface preparation of the substrates, chemical pickling with HF:HNO3:H2O 1:4:5 was used. The anodic synthesis was carried out at 20 V for 2 h, using solutions with 0.27 M FNH4 as the electrolyte in two organic media: glycerol (G) with 50% v/v water and ethylene glycol (E) with 3.5% water [2,3]. Subsequently, thermal treatments of 2 h at 450 °C were carried out. As a reference, flat Ti plates (2 x 2 cm2) were anodized under the same conditions.Crystalline (anatase) nanotubular coatings were obtained on the plates, with internal diameters of 40 and 50 nm, wall thicknesses of 14 and 10 nm, and lengths of 1 and 1.7 µm, with G and E electrolytes, respectively. Nanotubular coatings were also obtained on the sponges, but with a compact TiO2 surface layer above. The coatings were characterized by scanning electron microscopy, X-ray diffraction, and X-ray microcomputed tomography. The activity of the photocatalysts was evaluated using Cr(VI) in the presence of ethylenediaminetetraacetic acid (EDTA) as a model contaminant ([Cr(VI)] = 0.8 mM; [EDTA] = 1 mM; pH 2), irradiated with UV light (max = 365 nm) during 5 h. Due to the porous geometry of the sponge substrate and considering the irradiated geometric area of the catalysts, sponges were more efficient than plates, with 67% vs. 23% Cr(VI) transformation/cm2 irradiated geometric area. Moreover, in sponges and flat plates, E nanotubes rendered a better performance than G ones due to higher and longer nanostructures.TiO2 photocatalysts synthesized on Ti sponges are promising for application in packed-bed photoreactors.