An electrochemical and spectroscopic investigation into carbon monoxide surface poisoning
Kardash Richardson, Dawn Jo-Elle
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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.