The electrocatalytic oxidation of small organic molecules on platinum
The major objectives of this thesis research are two fold: (1) to study the mechanisms of the electrocatalytic oxidation of methane, ethane and ethylene and (2) to understand the factors controlling the kinetics of these processes. An understanding of the mechanisms of the electrocatalytic oxidation of these hydrocarbon molecules has been reached by identifying the adsorbed species, intermediates as well as products which are either adsorbed on the electrode surface or dissolved in the electrolyte. Two types of adsorbed species have been identified by means of electrochemical, in situ FTIR and ex situ NMR techniques. Type I species are linearly bonded carbon monoxide (CO L), which can be formed on platinum electrode surfaces from the adsorption of any of the three hydrocarbon molecules investigated in this thesis work. Type II species are alcohol-like (most probably ethylene glycol), which are expected to be formed from the adsorption of C2 molecules (e.g., ethane and ethylene). Carbon monoxide has been considered as a reaction intermediate rather than just a "blocking agent" for the electrochemical oxidation of small hydrocarbon molecules. The alcohol-like type II species adsorbed on the platinum surface with higher bonding strength are expected to be the possible blocking species for the ele ctrocatalytic oxidation of small hydrocarbon molecules with two or more carbon atoms at lower temperatures. On the basis of the electrochemical and spectroscopic investigations, the possible reaction paths have been proposed by the author. At elevated temperatures, the rate-determining step in the oxidation of these hydrocarbons has been considered to be the reaction between adsorbed carbon monoxide and water, which is represented as the following: COad + H2Oad? COOHad + H+ + e-. The temperature effect on the formation and oxidative desorption of the two types of species on platinum has been investigated from room temperature to 180°C. At elevated temperatures (>150°C), the oxidation of these hydrocarbons proceeds through the formation and oxidative desorption of carbon monoxide. The electrocatalytic oxidation of the three selected small organic molecules has been investigated using low index and high index single crystal platinum electrodes at elevated temperatures. Pronounced sensitivities to the structure have been observed. The following electrocatalytic activity order for the adsorption and oxidation of all of the molecules studied has been observed: Pt(100)>Pt(111)>Pt(110)The high activity of Pt(100) has been interpreted on the basis of its special atomic structure, which can enhance the C-C bond rupture and oxygenation processes.
School:Case Western Reserve University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:electrocatalytic oxidation small organic molecules platinum
Date of Publication:01/01/1993