Kinetic And Mechanistic Studies Of Methanol And Formic Acid Oxidation At Model Platinum Electrode Surfaces

The present work are mainly focused on kinetic and mechanistic studies of methanol and formic acid oxidation at model platinum electrode surfaces. The major results of this thesis are summarized below:1. Preparation of the model Pt electrodes:Electrochemical deposition of silver on stepped Pt electrodes with (100) oriented terraces.Ag mono layers and sub-monolayers were deposited onto Pt(s)[n(100)×(111)] electrodes both in the over potential deposition (OPD) region under diffusion limited conditions and in the under potential deposition (UPD) region. Diffusion limited deposition from Ag+concentrations in the micromolar range was monitored by the suppression of the hydrogen adsorption peaks; selective suppression of the peaks for hydrogen adsorption at (100) terraces sites as opposed to that at (111) step sites demonstrates that Ag is preferentially deposited at terrace sites. Underpotential deposition gives rise to three peaks:Two large peaks correspond to Ag deposition in the first and the second layer. An additional small peak increases linearly with step density and is due to Ag deposition at the (111) step sites. Its peak potential, which is lower than that of Ag deposition at terraces, again demonstrates the lower stability of Ag at step sites as compared to terrace sites. By controlling the upper potential limit for the oxidative removal of deposited Ag, Ag at steps can be selectively removed and reaction sites with one dimensional Pt atoms can be generated.2. DEMS study of methanol electrooxidation at bead Pt single crystals using pulsed voltammetry.Electrooxidation of methanol at a bead Pt(100) and at Pt stepped single crystal vicinal to (100) planes (Pt(s)[n(100)×(111)]) was studied using a new DEMS cell that applicable for small diameter electrodes combined with the mass spectrometer. The new cell is characterized by:(1) working under controlled electrolyte flow keeping the hanging meniscus of the bead single crystals,(2) the delay time of the volatile products formation and their detection by mass spectrometer is about1s. Different electrochemical techniques of CV, potential step and pulsed voltammetry are applied. The current efficiencies with respect to CO2formation during methanol oxidation are calculated and the effect of surface step density of the electrode on the catalytic activity towards methanol oxidation was investigated. Whereas the current density in the CVs decreases with the increasing step density, the average current efficiencies for CO2are hardly changing. While in the pulsed voltammetry experiment, the highest current efficiency for CO2is detected during the oxidation of methanol at Pt(100) and decreases with the reaction time due to the accumulation of the poisoning species at the Pt surface.3. Determination of isotherm for acetate and formate adsorption at Pt(111) electrode by fast scan voltammetry.Fast scan voltammetry is an efficient tool to distinguish oxidative/reductive adsorption/desorption from that for bulk reaction. In this contribution, We provide a methodology that the isotherm of oxidative/reductive adsorption desorption processes at electrode surface can be obtained using just one solution with relatively low reactant concentration, by taking the advantage of varying the potential scan rate (relative of the diffusion rate) to tune the adsorption rate and proper mathematic treatment. The methodology is demonstrated by taking acetate adsorption at Pt(111) in acidic solution as an example. The possibility for extension of this method toward mechanistic studies of complicated electrocatalytic reactions is also given.4. On the mechanism of the direct pathway for formic acid oxidation at Pt electrode.In order to find out whether formate is the reaction intermediate for the direct pathway for formic acid oxidation at Pt electrode, formic acid (HCOOH) oxidation at Pt(111) electrode has been studied by normal and fast scan voltammetry in0.1M HClO4solutions with different HCOOH concentrations. The relationship between HCOOH oxidation current density (jox) and formate coverage (θformate) is quantitatively analyzed. The kinetic simulation reveals that previously proposed formate pathway with decomposition of bridge-bonded formate (HCOOB) as rate determining step (rds) cannot be the main pathway responsible for the majority current for HCOOH oxidation. Instead, a kinetic model based on a mechanism with formic acid adsorption and the simultaneous C-H bond activation) as the rds for the direct pathway, explains well the measured data. In parallel with the relatively slow formic acid oxidation, faster adsorption/desorption of formate always occurs, which competes for the surface sites for formic acid oxidation.