An electrochemical and spectroscopic investigation into carbon monoxide surface poisoning
Kardash Richardson, Dawn Jo-Elle
An electrochemical cell was constructed that allows surface infrared spectroscopy measurements to be made in situ at temperatures relevant to the operation of direct methanol fuel cells (ambient to 80°C). The cell was used to investigate temperature effects on the electrochemistry of water, CO and methanol at bulk Pt and Pt-Ru electrodes in 0.1 M HCIO4. Initially, the surface chemistry of CO on a polycrystalline Pt electrode was studied. An Adiayer of CO at saturation coverage was stable over a period of five hours in the range of 25 °C-50 °C. Above 60 °C, the adiayer became unstable. In the absence of CO in solution, only low CO coverages could be sustained between 60 °C and the high temperature limit of the experiments (75 °C). However, with CO or a source of CO such as methanol in solution, high CO coverages were sustained up to 75°C. In measurements of CO oxidation, the onset potential for the conversion of CO to CO2 decreased by 50 mV when the temperature was increased from 25 °C to 75 °C. In contrast, adsorbed CO formed through the dissociative chemisorption of methanol (1.5 x 10'^-1.0 M) was more oxidation resistant between 50 °C-75 °C. The in situ spectroscopic measurements provide molecular level evidence that the thermal activation of water dissociation can decrease the steady-state coverage of surface poisons and thereby increase the rate of methanol oxidation on Pt electrodes. In final studies, the surface chemistry of 0.1 M methanol on two bulk Pt-Ru alloy electrodes (10 atomic % Ru and 90 atomic % Ru) was investigated at 25 °C - 80 °C. High CO coverages were sustained on both alloys at all temperatures. However, CO2 evolved rapidly from CO covered surfaces above 0.4 V-0.5 V, suggesting that CO formed during methanol oxidation is more reactive and transient on the alloys than on Pt. The experiments reported in the dissertation provide a foundation for the in situ study of fuel cell reactions on new catalyst preparations with FTIR spectroscopy.