Mechanism of electro-oxidation of carbon monoxide on stepped platinum electrodes in alkaline media: a chronoamperometric and kinetic modeling study

Abstract

From a detailed analysis of the chronoamperometry of CO stripping on stepped platinum single-crystal electrodes in alkaline solution, in combination with kinetic modeling, a mechanistic and kinetic picture of the CO oxidation mechanism is derived. On Pt(111), CO oxidation starts at defect sites (steps, kinks), following a one-dimensional nucleation-and-growth mechanism, or a Langmuir-Hinshelwood mechanism with no effective competition between CO and OH. The carbonate that is formed in this reaction blocks the active oxidation sites, so that CO adsorbed on terraces further away from the defect sites can be oxidized at defects sites only very slowly. At potentials above ca. 0.75 V vs. RHE this CO is oxidized on the Pt(111) terrace. On stepped Pt electrodes, CO oxidation is also initiated at the kink (step defects) and step sites. We postulate that carbonate partially blocks the active site, but CO from the terrace still prefers to react at the steps over the terrace sites. The oxidation of terrace-bound CO at the step sites follows a competitive Langmuir-Hinshelwood mechanism. The least reactive CO on the surface is the CO that is adsorbed on the step sites, and it is oxidized by OH on terraces. Finally, on the terrace, the Tafel slope for CO stripping is close to 60 mV dec(-1), suggesting an EC mechanism, i.e. reversible OH formation followed by a chemical rate-determining step, presumably the CO + OH combination reaction. At the step site, the Tafel slope is close to 120 mV dec(-1), suggesting an electrochemical rate-determining step, ascribed to slow OH formation at the step site.

Effect of the Surface Structure of Pt(100) and Pt(110) on the Oxidation of Carbon Monoxide in Alkaline Solution: an FTIR and Electrochemical Study

“…Based on experimental data of voltammograms at low scan rates and chronoamperometries of the CO oxidation on Pt(111) and step surfaces in alkaline media [9,10] and Pt(100) in acidic media [25], the authors suggest that the oxidation of CO follow a nucleation and grow behavior. The instantaneous nucleation of OH (responsible of the oxidation) takes place on the steps sites.…”

“…sites with (111), (110) and (100) orientation, as well as kink sites were observed during CO stripping voltammetry [8,9]. The fact that, in alkaline media, the oxidation of CO can be observed on different sites was attributed to the blockage of the reactive step site by absorbed carbonate, which is the product of CO oxidation in alkaline media [8][9][10].…”

CO oxidation on stepped-Pt(111) under electrochemical conditions: insights from theory and experiment

“…Whereas in acidic media, only one oxidation peak is found in the oxidation voltammogram, two or even three peaks appear in basic solutions [27], depending on the different type of sites present on the surface. Thus, the peak corresponding to CO oxidation on (111)-steps in alkaline media appears at lower potentials than on (100)-steps or on (111) terraces [28,29]. This clearly indicates that the reactivity of steps is indeed different depending on their symmetry.…”

On the behavior of the Pt(100) and vicinal surfaces in alkaline media

“…For the surfaces vicinal to the Pt(111) electrode, stripping voltammetry for CO shows peaks corresponding to the oxidation of CO on the terraces, step and kink sites [42][43][44]. However, in this case, a single peak is always observed (aside from the presence of occasional prewaves).…”