Direct STM Studies of CO Adsorption and Oxidation on Pt(111) Electrodes: (1) Electrochemical Measurements and STM Observations
Single-crystal beads of Pt were made by crystallization of molten balls formed at the end of Pt wires in a hydrogen–oxygen flame. The (111)-oriented planes were exposed by mechanical polishing. The surfaces were then heat-treated in an infrared imaging furnace and cooled under hydrogen. The electrochemical measurements were carried out in 0.1 M HClO4. EC-STM measurements were carried out using a CO- or N2-saturated solution under a CO atmosphere. For the reference electrode, an RHE was used. Fig. 1(a) shows voltammograms obtained in CO-saturated HClO4 on Pt(111) with (111) steps The CO oxidation started at ca. 0.4 V in the prepeak region. After the electrochemical potential was cycled between 0.05 and 0.95 V 30 times, the prepeak observed at lower potentials completely disappeared.4) By examining CO stripping voltammograms with and without CO in solution (Fig. 1(b)), it was suggested that the origin of this prepeak is the oxidation of bulk CO in solution. Fig. 2 shows the STM images at a (111) step on Pt(111) recorded before and after the potential cycling. The (2x2) structure of the CO adlayer is clearly seen. Before the potential cycling, the step has defects, forming a zigzag line (Fig. 2(a)). After the potential cycling, the step became very straight running in the  direction, with no defects observed (Fig. 2(b)). The structure change at steps should be related to the reactivity. On Pt(20 19 19) with (100) steps, the prepeak was very small, and after the potential cycles disappeared completely. Fig. 3(a) shows an STM image on Pt(20 19 19) before cycling. The step is straight, which might explain the small prepeak on pristine Pt(20 19 19). Fig. 3(b) shows an image after the potential cycles. The (111) microsteps were introduced to the (100) step, and the step became zigzagged. From the results on Pt(111)-related surfaces with different steps, it can be understood that not every defect is active for the electrooxidation of CO in solution.
This research was supported by MEXT and NEDO, Japan.
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