IrO2-Ta2O5|Ti electrodes prepared by electrodeposition from different Ir:Ta ratios for the degradation of polycyclic aromatic hydrocarbons

Abstract

This work investigates the feasibility of producing IrO2-Ta2O5|Ti electrodes by electrodeposition. Using precursor solutions with Ir:Ta molar ratios from 0:100 to 100:0, followed by thermal treatment, the goal was to find the optimal composition for enhancing the formation of hydroxyl radicals and providing long service lives. Scanning electron microscopy (SEM), coupled with energy dispersive X-ray spectroscopy (EDX), revealed that the production of homogeneous coatings with a good surface coverage and absence of agglomerates was only possible for electrodes with 70% or 100% Ir. The potential for O2 evolution was similar for all the electrodes containing Ir, at about 0.90 V vs Ag|AgCl. However, the ability to produce M(OH) clearly increased with increasing Ir in the Ir:Ta ratios (100:0 > 70:30 > 30:70 > 0:100). This observation was confirmed by the transformation of coumarin to 7-hydroxycoumarin as determined by spectroscopic and chromatographic techniques after treatment. Once manufactured and characterized, the electrodes were tested, as anodes, for the electro-oxidation of polycyclic aromatic hydrocarbons in aqueous solutions at natural pH (i.e., without pH adjustment). The anodes prepared from 70:30 and 100:0 ratios produced the fastest and highest removal, reaching 86% and 93% for phenanthrene and naphthalene, respectively, after 120 min at 50 mA. This was accompanied by a high degree of mineralization, as the result of direct and M(OH)-mediated oxidation, with some refractory intermediates remaining in the final solutions. The interaction between IrO2 and Ta2O5 oxides appeared to be important. The 100:0 anode provided high electrocatalytic effectiveness, whereas the anode with the 70:30 ratio provided improved long-term stability, as confirmed by its service life of about 93 h.