Ethanol electrooxidation on a carbon-supported Pt catalyst: Reaction kinetics and product yields

The ethanol oxidation reaction (EOR) on a carbon-supported Pt nanoparticle catalyst was studied by cyclic voltammetry and potential-step measurements as a function of ethanol concentration and reaction temperature (23-60 degreesC), combining on-line mass spectrometric analysis of the reaction products and electrochemical current measurements. The effect of catalyst loading/electrode roughness was elucidated by comparison with a polycrystalline Pt electrode. Individual, absolute rates for CO2 and acetaldehyde formation were determined via the doubly ionized carbon dioxide molecular ion at m/z = 22 and the CHO+ fragment at m/z = 29, whereas acetic acid yields were calculated as the difference between the Faradaic current (charge) and the sum of the partial currents for oxidation to CO2 and acetaldehyde, calculated from the calibrated mass spectrometric currents. Incomplete ethanol oxidation to acetaldehyde and acetic acid prevails over complete oxidation to CO2 under all conditions, the dominant products being acetic acid at low (1 mM) and acetaldehyde at high (0.5 M) ethanol concentration or low catalyst loading/electrode roughness, i.e., on the smooth Pt electrode, whereas current efficiency and product yield for CO2 formation is on the order of a few percent. The reaction orders for ethanol on Pt/Vulcan are 0.3, 0.6, and 0.9 for CO2, acetic acid, and acetaldehyde formation and 0.6 for the total Faradaic current, respectively. These trends are discussed in terms of increasing readsorption and subsequent oxidation of volatile, desorbing reaction intermediates with increasing catalyst loading/electrode roughness, considering that acetic acid oxidation is kinetically hindered at room temperature, and a rather low rate for C-C bond breaking under these conditions. The temperature dependence in this temperature range results in an apparent activation energy for the total reaction (Faradaic current) of 32 kJ/mol. The respective values for the partial reactions for CO2, acetic acid and acetaldehyde formation are 20, 28, and 43 kJ/mol, respectively.