Abstracts Engineering

Rates of the surface reactions in methanol and carbon monoxide electrooxidation: experimental measurements and kinetic modeling

by Suresh Sriramulu

Commercial viability of the direct methanol fuel cell is inhibited by low values of the energy conversion efficiency. Electrocatalytic reactions lie at the heart of the energy conversion process in a fuel cell, and the efficiency is linked to the rates of these reactions; the more facile the reactions, the higher the efficiency. Even the most active electrocatalysts known today do not support a sufficient rate of methanol oxidation at the anode. In this work, the mechanism and kinetics of methanol and CO oxidation were determined from the view point of suggesting suitable criteria for the design of effective anode catalysts.Kinetic measurements show that methanol electrooxidation to CO2 occurs via a dual pathways mechanism: one pathway involving the poisoning intermediate, adsorbed CO, and a pathway parallel to this. To determine the dynamics, a micro-kinetic model was developed based on current understanding of the reaction mechanism. The model provided an excellent fit to time-dependent kinetic data that spanned four orders of magnitude in time from 0.03 to 300 s. The intermediate in the parallel pathway was identified as H:C:O strongly complexed to a water molecule. The rate of H:C:O oxidation was three times faster than CO oxidation at steady state. We present and discuss quantitative estimates of the rates of individual reactions with particular emphasis on the poisoning effect of adsorbed CO. The predictive ability of the model was also tested. For reaction of 5 mM methanol on Pt/Ru surfaces, the model predicted an optimum Ru coverage of 10% in agreement with experimental measurements in the literature. Also, the model qualitatively predicted the experimentally observed changes in the optimum Ru coverage with changes in reaction conditions. In the final part, an experimental procedure to directly determine the rate of CO electrooxidation was developed. Potential and coverage dependence of the oxidation rate of sub-saturation CO adlayers are presented. These results provide insights into the rate determining step in CO oxidation.