ELECTROCHEMISTRY OF METHANOL AT LOW-INDEX CRYSTAL PLANES OF PLATINUM - AN INTEGRATED VOLTAMMETRIC AND CHRONOAMPEROMETRIC STUDY

We have studied a catalytic decomposition of methanol on low Miller index platinum surfaces, Pt(111), Pt(110), and Pt(100) in perchloric, sulfuric, and phosphoric acids at room temperature. The instantaneous methanol oxidation current is unaffected by the methanolic CO formation (surface poisoning) and depends on platinum surface structure and composition of supporting electrolyte with respect to the anions. The highest oxidation current, 156 mA.cm(-2), is observed with the Pt(110) electrode in perchloric acid solution at 0.200 V vs Ag/AgCl reference. In terms of turnover, this current translates to 163 molecules (Pt site)(-1).s(-1), a high rate exceeding previous expectations in methanol electrode kinetics. Overall, the oxidation current changes by 3 orders of magnitude between the extreme cases examined in this study. Breaking up the total effect into individual components shows that the surface geometry and anionic effects are roughly comparable. Therefore, we have an evidence that anion-platinum interactions are as important in determining the methanol oxidation rate as is the surface geometry of the Pt catalyst. Being encouraged by the magnitude of the oxidation current, especially with the Pt(110) electrode, and by the control of the oxidation process through the structural and electrochemical variables of this research, we also report that the rate of methanolic CO formation follows the same pattern as does the oxidation current. Namely, the CO poisoning is the highest for the Pt(110) electrode in perchloric acid and the slowest with the Pt(111) electrode in phosphoric acid. We conclude that optimizing the structure of clean platinum, and solution composition, is not a sufficient remedy for platinum deactivation and that the CO poisoning process must be addressed with new force in basic research on platinum fuel cell catalysis.