ANODIC OXIDATION OF SATURATED HYDROCARBONS . A MECHANISTIC STUDY

Determinations were made of the rate of electrochemical oxidation of propane, n-hexane, cyclohexane, and 2,2-dimethylbutane in concentrated phosphoric acid solutions at 80-150°. Transient values were recorded at fixed potentials upon introducing the reactant to solution. Steady-state rates were measured as a function of propane pressure, the activity of water, and the electric field gradient applied across the surface on which the reaction occurred. Thr Tafel slope is F/RT in the low-rate section of the rate-potential relation. The order of reaction with respect to hydrocarbon is one; that for water is zero. The rate at constant field gradient as a function of time passes through a maximum at about t = 5 sec. At constant potential, the rate of oxidation to CO2 is propane > 2,2-dimethylbutane > n-hexane > cyclohexane. At potentials of more than 0.48 V on the hydrogen scale, the rate of oxidation at 80-130° decreases anomalously with increasing field. For propane, at 135-150° the rate undergoes inhibition at 0.48 V but continues to increase with an increase in field strength. Some 15 partial-mechanism hypotheses (suggested sequences up to and including the rate-determining step) are used to predict the kinetic behavior for which data exist. Large numbers of the sequences are excluded by the coefficient ∂ log (rate)/∂ (electric field strength). Two (partial) mechanisms are consistent with the observations. Of these, that which is most consistent with energy considerations of the bonding involved is the most likely. It is, for a saturated hydrocarbon RH: RH ⇄ Rads + H+ + e- and Rads →rds organic radicals. The rate-determining step for the alternative possibility was also a chemical reaction. The hypothesis is applied to the interpretation of the i-t transient. It yields a value for θC3H7(480 mV) of 0.3. The inhibition occurring above 0.48 V is shown to be consistent with data published elsewhere for the adsorption of H2PO4-.