Effect of RVC porosity on the performance of PbO2 composite coatings with titanate nanotubes for the electrochemical oxidation of azo dyes

Abstract

Reticulated vitreous carbon (RVC) of different porosities (20, 45, 60, 80, and 100 ppi - pores per inch) has been used as a large surface area substrate for preparing 3D-like PbO2 coatings (RVC/PbO2) as well as composite coatings with hydrothermally synthesized titanate nanotubes (RVC/PbO2/TiNT) by galvanostatic electrodeposition from baths containing lead(II) methanesulfonate and methanesulfonic acid. The effect of the RVC porosity on the coating thickness, the electrocatalytic behaviour and the ability to remove the colour and total organic carbon (TOC) from solutions containing the azo dye Methyl Orange has been systematically assessed. As shown from scanning electron micrographs, the greatest thickness (up to 120 μm) was obtained using > 60 ppi, but the β-PbO2 nanocrystallytes mainly grew on the external surface, leaving mostly uncoated inner RVC stripes and ending in planar-like PbO2-based electrodes. In contrast, thinner but perfectly adherent and homogeneous coating of the inner and outer surface was achieved with 20-60 ppi, showing electrodes with an optimal three-dimensionallity. This was especially confirmed by cyclic voltammograms for the composite coatings, as deduced from their highest electroactivity that can be related to enhanced adsorption onto the TiNT clusters and the larger ability to produce active PbO2( OH). The comparative electro-oxidation of 0.25 × 10-3 mol dm-3 Methyl Orange acidic solutions in 0.05 mol dm-3 Na2SO4 at 0.6 A demonstrated that RVC (45 ppi)/PbO2/TiNT was the optimum material. It allowed the quickest decolourisation, reaching 60% in 2.5 min and > 98% at 45 min, and > 55% TOC abatement at 240 min. The anode presented a perfect surface coverage, with no evidence of RVC degradation. The effect of dye concentration and supporting electrolyte nature was studied, revealing a very positive effect of NaCl.