Heterogeneous redox catalysis with titanium/chromium(III) oxide + TiO 2 composite anodes

Abstract

Titanium/chromic oxide (Ti/Cr 2O 3 + TiO 2) composite electrodes were fabricated by a ceramic method, starting with activation solutions containing CrCl 3·6H 2O and Ti(OBu) 4 with subsequent firing in air at 650°C. Surface α-Cr 2O 3 could be anodically stripped in 1 M H 2SO 4 as H 2CrO 4 at U H ≈ 1.8 V. Conversion of Cr 2O 3 decreased with increasing thickness of porous Cr 2O 3 layer. The electrodes were used for the anodic oxidation of aliphatic alcohols and ethers in 1 M H 2SO 4. Validity of model of heterogeneous redox catalysis is proved by the following results: 1. large amplification of anodic stripping curve in the presence of oxidizable starting materials. 2. coincidence of anodic current voltage curve with basic (stripping) curve at low current densities and 3. reaction limitation at high current densities. Lifetime time (τ) of electrodes, measured galvanostatically, is determined by dissolution of CrO 3, present at the surface of the polarized electrode. A dependency j 1.33 τ = constant was found. Substantial improvement of τ by modification of the solid (SbO x doping) or the electrolyte (cosolvents) was demonstrated. Turn over factors of the surface fixed redox system in excess of 1000 were realized.