Kinetics of the anodic oxidation of isopropanol on chromium/titanium—oxide electrodes

Abstract

The anodic oxidation of isopropanol to acetone on ceramic Cr 2O 3, TiO 2/Ti electrodes with a thickness of the porous mixed oxide layer of about 1 μm has been studied in detail by voltammetric measurements. Peak-type current-voltage behaviour has been found throughout in the potential region between 1.7 and 2.2 V vs. SHE in 1 M H 2SO 4. Electrode processes occur on the Cr 2O 3 surface. At more positive potentials the doped TiO 2 interlayer is subject to an increase of thickness. The ascending part of the current-voltage curve gives Tafel lines with a slope of 120 mV decade −1 due to a rate-determining one-electron step in the overall three-electron oxidation of Cr 2O 3 to surface-bound CrO 3. It undergoes two chemical follow-up reactions, one with isopropanol yielding acetone (heterogeneous redox catalysis) and the other with water,in which it is dissolved yielding chromic acid. The current maximum is determined by the chemical kinetics of these follow-up reactions as well as by a passivation process, leading finally to a strong decrease of the current. The chemical rate constant is found to be 3.4 × 10 −4 cm s −1, which is larger by a factor of 150 with regard to the homogeneous chromic acid oxidation due to the more positive activation entropy. The passivation is caused by the formation of a CrO 3 film of high resistivity which absorbs most of the voltage in an ionic space charge. With increasing voltage scan rate v S the peak charge decreases proportionally to the reciprocal of the seventh root of v S, as predicted theoretically. This potentiodynamic passivation mechanism differs from that for galvanostatic long-term experiments, where exhaustion of Cr 2O 3 is the main factor. All findings support our view of a full analogy of the electroorganic process to the chemical chromic acid oxidation in solution.