COad is removed

When the potential is high, once oxidative removal of COad begins, sites where COad Is removed turn into Pt-OH immediately and work as the oxygen source for oxidizing COad adsorbing to the next platinam atom. This causes a rapid COad reaction chain until all COad is oxidized. This is the reason that when a high scan rate was employed, COad oxidation peak is very sharp. 

Probably, the little ciurent in the first few cycles is due to the lack of electrocatalytic activity for the oxidation of methanol to COad on the tin-rich surface, although the svuface has the oxidation capability for COad ot lower potentials as shown in the previous section. As more platinum became exposed, the more COad is likely to form, followed by the early oxidation of COad on Pt-Sn surface. Further removal of tin, however,... 

H2O dissociation on pme Pt and on the Pt site in mixed Pt-Ru clusters is difficult. Ru sites more actively dissociate H2O [6,7]. In the present study we also find that Mo, W, and Re activate H2O more effectively than does Ru, partly because these metals (Mo, W, Re) adsorb OH strongly. COads(Pt) is removed by OHads(M), the principal oxidizing agent. However, strong OH adsorption introduces a barrier to CO oxidation. In this section, we examine the energetics of the surface reaction of COads(Pt) with OHads(M)- The scheme for calculating the combination energy (Ce,ads) is... 

To remove COads from the surface it is necessary to generate some oxygenated species that can react with COads to produce CO2 and release some free sites on Pt surfaces for the hydrogen oxidation reaction. The mechanism for the oxidative removal of the COads from platinum anodes has been a topic of intense investigation for the past 40 years. The overall reaction for removing COads is... 

Hydrogen oxidation reaction occurs on the free sites liberated during the time between COads oxidative removal, Equation 16.4, and CO re-adsorption from solution. Equation 16.2. At the low potential (E < 0.6 V) the rate constant of COb re-adsorption is much higher than the rate constant for COads oxidation, and in practical terms only an infinitely small number of platinum sites could be liberated for H2 oxidation. 

It is essential to understand the mechanisms of methanol oxidation including the adsorbate formation and removal. To this purpose, the electrochemical oxidation of adsorbed carbon monoxide (COad) and methanol was studied using electrochemical and two spectroscopic... 

Voltammograms of Ptdll) with and without COad adsorption in 0.5 M perchloric add are shown in Fig. 2-25. The voltammogram without CO was considerably different from those in sulfuric acid. The symmetric features in the range from 600 to 800 mV correspond to the anodic portion of the two split area of hydrogen adsorption-desorption in sulfuric add. Hydrogen adsorption-desorption features did not change after the oxidation peak at 1050 mV and its reduction while the further oxidation removes the feature irreversibly. Therefore the peak at 1050 mV is considered as a formation of a weak interaction with water. 

Figure 4-11 shows the infiiared spectra for Pt- Ru electrode after Ru was stripped at 1100 mV for 30s. Unlike for pure platinum (See Fig. 3—30), no clear COad was found at any potential except a very blunt peaks around 2070 cm at 400 to 450 mV. CO2 starts to appear at 375 mV. which is not very different from the 400 mV for pure platinum. This suggests that the direct oxidation of methanol to CO2 is not enhanced much. The activity enhancement effect of ruthenium is, therefore, considered mainly as the result of a high COad removal rate. 

Raman parent spectrum is obtained simply by adding all the spectra together. However, the ROA spectrum is obtained by either adding or subtracting from the current coadded spectrum depending on the position of quarter-wave plate. After several cycles, the Raman and ROA spectra are saved on the hard disk as one data set. After several data sets, all the data are added and saved into either the Raman parent spectrum or the ROA raw data spectrum and stored for the data manipulation. Occasionally, the raw data contains parental bias or first derivative contributions due to the fluctuation of the laser intensity or the quality of the optics, and these are reduced or removed digitally. 

The removal of oxygen moieties from the surface by reaction with CO is also an essential step in the steam reforming reaction catalyzed by nickel or rhodium (47—52). On these metals, the oxygen removal most likely occurs via reaction of OHads ndth COads this reaction has a low activation barrier, probably less than 60 kj/mol. Coadsorbed oxygen (Oads) can lower the barrier further by accepting the hydrogen atom from OHads-... 

A problem in the context of low-temperature fuel cells is the extreme sensitivity of H2 oxidation on Pt to traces of CO (ppm level reformed fuel always contains some of it). The reason is that CO adsorbs much more strongly on Pt than H2 and is only removed electro-oxidatively at rather high potentials (COad +H2O C02+2H +2e", E > 0.5 V), thus blocking the surface for the desired H2-oxidation reaction. Adding some Sn, Ru, or perhaps Mo, to the Pt alleviates the problem to a certain extent, by promoting partial removal of CO at lower potentials, presumably through activation of water molecules, but does not solve it. Anode catalysts exist that are insensitive to CO, an example being WC, but their intrinsic activity for the main reaction is unfortunately much too low. 

Cu, Zn, Ge, Zr, and Rh have high D> c and E for water dissociation, and thus are unsuitable as secondary metals. The enhanced promoting effect of Re, W, or Mo toward H2O dissociation are due to strong M-OH adsorption, and large Eads(OH) does not promote oxidative removal of CO. For M = Re, the activation energy for the COads + OHads combination reaction is the highest of all, and thus the reaction becomes the rate-determining step in CO removal. 

The following sequence of an oscillatory cycle has been proposed (i) Os b formation takes place only on the Oads-covered palladium surface, accompanied by a decrease of the sticking coefficient for the oxygen adsorption So (ii) the formation of COads layer is a result of the fast reaction COgas + Oads With the formation of CO2 molecules and their desorption (iii) the elevated concentration of the empty active sites appears either due to reverse diffusion process O ub Oads with subsequent removal of Oads in the reaction with COads, or due to slow reaction Osub with COads to form CO2 (iv) the transition to the initial oxygen layer proceeds from S(02) increase due to the decrease of Osub concentration. 

Wieckowski s group has studied formic acid electrooxidation on Pt nanoparticles decorated with controlled amounts of Pd and Pd-l-Ru adatoms [41]. They reported two orders of magnitude increase in the reactivity of the Pd-decorated catalyst compared to pure Pt towards formic acid oxidation. Also, it was concluded that the impact of COads on the Pt/Pd catalyst through the dual pathway mechanism is much lower even though the potential required to remove COads from the surface was the highest. 

To overcome CO deactivation, alloys of Pt with more oxophilic elements have been investigated as methanol electrooxidation catalysts. PtRu bifunctional catalysts are presently the most active for methanol oxidation. It is believed that Ru serves the role of removing COads as CO2 [93] ... 

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