Electrochemical Science for a Sustainable Society by Kohei Uosaki

Author:Kohei Uosaki
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

Conversely, a single peak is always observed for surfaces having (100) terraces, and the presence of the step is catalyzing the oxidation of CO for terraces with n > 4 [32]. The presence of a single peak in this case is probably related to the small difference in the adsorption energies of the step and terraces for the surfaces having (100) terraces [66]. These small differences normally imply that the reactivity of the (100) terraces and steps on those surfaces are very similar, unlike what it is observed for the surfaces with (111) terraces. Thus, processes on both type of sites cannot be resolved. In the surfaces with (111) terraces, the higher difference between the adsorption energies of (111) terraces and steps [74] leads to a clear separation of both processes, as has been experimentally observed.

The second important difference is the usual appearance of an oxidation pre-peak at low potentials for all the surfaces (Fig. 4). These pre-peaks were first occasionally observed in acid solutions and its appearance depends on the CO adsorption conditions [75–79]. In alkaline solutions, the pre-peak is normally very prominent. As happens in acidic solutions, the CO adsorption conditions, essentially, the electrode potential at which CO is adsorbed, are the key element in the appearance of the pre-peak [80]. When CO is adsorbed at 0.1 V versus RHE in acidic solutions and then transferred to an alkaline solution, the pre-peak is absent [81] (Fig. 6). Moreover, CO oxidation peaks are narrower and shift to higher potential values, indicating that the CO adlayer has lower amount of defects. If the electrode potential is reversed after the pre-peak, it can be observed that hydrogen adsorption is still blocked by the presence of the CO layer [70, 81]. Thus it has been proposed that the pre-peak corresponds always to the initial stage of the CO oxidation process to form a less compact adlayer. This oxidation process is triggered by the presence of defects in the adlayer. If the adlayer is formed in acidic solutions, the high mobility of CO on these environments leads to the formation of a more ordered adlayer with a lower amount of defects. Thus, in acid solutions or when the electrode is transferred to alkaline solutions, no pre-peak is observed [81]. On the other hand, when the adlayer is formed in alkaline solutions, the low mobility of CO necessarily implies the formation of CO adlayers with a higher number of defects. Those defects trigger the oxidation of CO at low potential to form a more relaxed adlayer. It should be stressed that the CO coverage attained in acidic and alkaline solutions is very similar [81], which highlights the role of a relatively low number of the defects in the oxidation mechanism.

Fig. 6Voltammetric profile on a Pt(776) electrode for CO stripping in 0.1 M NaOH when CO adsorbed at 0.1 V in 0.5 M H2SO4 (blue line) and when CO adsorbed at 0.1 V in 0.1 M NaOH (red line). The black curve is the blank voltammetry.

Download

Copyright Disclaimer:
This site does not store any files on its server. We only index and link to content provided by other sites. Please contact the content providers to delete copyright contents if any and email us, we'll remove relevant links or contents immediately.