REACTIVITY AND EXTENT OF POISONING DURING METHANOL ELECTROOXIDATION ON PLATINUM(100) AND PLATINUM(111) - A COMPARATIVE-STUDY

The reactivity and self-poisoning of platinum in the electrocatalytic oxidation of methanol depends on surface structure and potential. To s tudy these effects, we determined the charge passed, extent of poisoni ng, and yield of complete oxidation product (CO2) from electrocatalysi s of methanol on Pt(100) and Pt(lll) electrodes through chronoamperome try and linear sweep voltammetry. Measurements were performed in a dua l cell apparatus with perchloric acid supporting electrolyte at room t emperature. Methanol was reacted at constant potential within the rang e 0.35-0.7 V-RHE for 60 s. Linear sweep voltammetry in methanol-free e lectrolyte allowed for measurement of adsorbed partial oxidation produ cts. The most significant differences in behavior between Pt(100) and (111) occurred below 0.5 V. In this potential range reaction on Pt(100 ) saturates, producing exclusively CO, while on Pt(lll) reaction is sl ow, but sustained, and produces small quantities of CO2. The extent of poisoning showed a pronounced maximum at 0.6 V for reaction on Pt(111 ), but remained relatively constant on Pt(100) below 0.65 V. Between 0 .5 and 0.7 V the charge passed and CO2 yields compared favorably on bo th surfaces. In this intermediate potential range, the different CO co verages which form from methanol reaction represents the most signific ant structural effect. Although the Pt(100) surface exhibits a greater tendency to dehydrogenate methanol, the Pt(111) surface oxidizes CO m ore efficiently. This balance between the rate of CO formation and oxi dation results in greater poisoning of the (100) surface. We discuss b oth kinetic and mechanistic aspects of the methanol reaction, and pay particular attention to the applicability of a parallel reaction mecha nism for methanol. (C) 1998 Elsevier Science B.V.