A comparison of the electrooxidation kinetics of p-methoxyphenol and p-nitrophenol on Sb-doped SnO2 surfaces:: Concentration and temperature effects

We discuss the role of the phenol substituent on the kinetics of electrochemical oxidation on metal oxide surfaces. Using UV-vis spectroscopic and high performance liquid chromatographic analyses of solutions during electrolysis on antimony-doped tin oxide (Sb-SnO2) electrodes, we established that the oxidation of p-methoxyphenol (PMP) and p-nitrophenol (PNP) follow different reaction mechanisms. The initial PNP and PMP oxidation rates are well represented as functions of the initial phenol concentrations by the Langmuir-Hinshelwood mechanism. At high concentrations of phenol in solution, the oxidation rates are controlled by surface processes. Under surface saturation conditions the formation of CH surface species required for complete oxidation is hampered, lowering the extent of PNP oxidation. The effect Of temperature on the oxidation of PNP and PMP on Sb-SnO2 electrodes was also studied. UV-vis spectroscopy of solutions during electrolysis show that the oxidation rates of both compounds follow Arrhenius behaviour; however, the fraction of the PMP initially present oxidized after the passage of a given electric charge was not affected by temperature, whilst the fraction of PNP oxidized increased with temperature. The dissimilar behaviour is ascribed to differing adsorption of PMP and PNP on the surface of the SnO2 electrode, as established from the correspondence of kinetic data to the Langmuir-Hinshelwood mechanism. (c) 2006 Elsevier B.V. All rights reserved.