Methanol oxidation potential scans

The qualitative voltammetric behavior of methanol oxidation on Pt is very similar to that of formic acid. The voltammetry for the oxidation of methanol on Pt single crystals shows a clear hysteresis between the positive- and negative-going scans due to the accumulation of the poisoning intermediate at low potentials and its oxidation above 0.7 V (vs. RHE) [Lamy et al., 1982]. Additionally, the reaction is also very sensitive to the surface stmcture. The order in the activity of the different low index planes of Pt follows the same order than that observed for formic acid. Thus, the Pt(l 11) electrode has the lowest catalytic activity and the smallest hysteresis, indicating that both paths of the reaction are slow, whereas the Pt( 100) electrode displays a much higher catalytic activity and a fast poisoning reaction. As before, the activity of the Pt(l 10) electrode depends on the pretreatment of the surface (Fig. 6.17). 

The drop of the voltammetric crurent is associated with Pt surface oxidation, and the drop on the negative-going mn is due to Reaction (12.9) (surface poisoning by CO) and the Tafehan kinetics of Reaction (12.8). Further, the shift between curves in Fig. 12.13a and b indicates that in the potential range between 0.5 and 0.6 V, methanol oxidation occms with zero or low level atop CO smface intermediate. The amplitudes on Fig. 12.13 on both scans nearly equal to each other indicate a high level of preferential (111) crystallographic orientation of the poly crystalline Pt surface used for this work, as inferred from data in [Adzic et al., 1982]. 

Figure 13.3 Potentiodynamic electrooxidation of (a) formic acid, (b) formaldehyde, and (c) methanol on a Pt/Vulcan thin-film electrode (7 xgpt cm, geometric area 0.28 cm ) in 0.5 M H2SO4 solution containing 0.1 M HCOOH (a), HCHO (b), or CH3OH (c). The potential scan rate was 10 mV s and the electrolyte flow rate was 5 p-L s at room temperature). The top panels show the faradaic current (solid lines), the partial currents for Ci oxidation to CO2 (dashed lines) and for formic acid formation (dash-dotted line), calculated from the respective ion currents, and the difference between the measured faradaic current and the partial current for CO2 oxidation (formic acid oxidation (a), formaldehyde oxidation (b)), or the difference between faradaic current and the sum of the partial currents for CO2 formation and formic acid oxidation (methanol oxidation, (c)) (dotted line). The solid lines in the lower panels in...

OH/oxide species. At potentials anodic of 1 V, incomplete oxidation of formaldehyde to formic acid is activated, while methanol oxidation is almost completely hindered. This reflects an easier oxidation of the C-H group in the aldehyde than in the alcohol. For the negative-going scan, where the COadouble-peak stmcture in the current efficiency. 

Figure 2.2 shows also the oxidation pattern of adsorbed methanol in the absence of methanol in the solution (dashed curve). The experiment was performed using the flow cell procedure [36], Methanol was adsorbed from a 10 2 M CF[3OH/0.5 M H2S04 at 250 mV RHE for 10 min, then the solution was exchanged with pure base electrolyte under potential control, and a potential scan was applied. 

Methanol was added while the potential of Pt-Mo electrode was held at 50 mV. Then the potential was scanned in the anodic direction. The voltammogram is shown in Fig. 4-23. At potentials lower than 500 mV, the voltammogram is almost the same as that without methanol. At higher than 500 mV, methanol oxidation current is seen but is smaller than that of pure platinum. According to Kita et odP, Pt-Mo enhances the methanol oxidation activities at lower potentials than 500 mV. However, the effect was not clearly seen in this experiment. 

FIGURE 11.5 Positive potential scans of methanol oxidation on different platinum surfaces in 0.5 M methanol +1 M sulfuric acid at 0.01 V s-1 (continuous line) bare polycrystalline platinum (dotted line) platinum ruthenium (dash-dotted line) platinum osmium (dashed line) and ternary compound platinum ruthe-nium osmium (obtained by simultaneous spontaneous deposition after 60 s of immersion in a mixed solution). 

The use of an infrared microscope enables the investigation of the surface of rather small electrodes. The resulting miniaturization of the necessary electrochemical cell allows its operation as a fiow cell in thin layer arrangement [242]. Combined with a rapid-scan FTIR spectrometer, acquisition of infrared spectra during electrode potential scans at a rate of d /dr = 200 mV-s are possible. The time resolution is equivalent to one complete spectrum recorded every 2.6 mV. The formation of various reaction intermediates of methanol oxidation in alkaline solution at a platinum electrode could be assigned to specific electrode potential ranges. 

Figure 5.9 shows the voltammetric behavior of Pt/C (a) and Pd-Co-Pt/C (b) towards ORR in the presence and absence of methanol. As can be also observed, the presence of 0.5 mol methanol causes a negative shift of 50 mV in the halfwave potential, in contrast to Pt/C for which there is a severe loss of activity. The linear scan voltammograms of the methanol oxidation on all the investigated materials in 0.5 mol H2SO4 + 0.5 mol L CH3OH solution, showed that the current densities of the methanol oxidation reaction on Pd-Co-X alloy catalysts (X = Au, Ag, Pt) diminish to values much lower than for Pt/C catalyst, and the onset of methanol oxidation occurs at more positive potentials, demonstrating the lowered MOR activity of the Pd-Co-Pt alloy catalysts. 

Fuel Cell Reactions. Low temperature fuel cells such as proton exchange membrane fuel cells (PEMFC) or direct methanol fuel cells (DMFC) employ large amounts of noble metals such as Pt and Ru. There has been extensive research to replace these expensive metals with more available materials. A few studies considered transition metal nitrides as a potential candidate. In an anode reaction of DMFC, Pt/TiN displayed the electroactivity for methanol oxidation (53). Pt/TiN deposited on stainless steel substrate showed the high CO tolerance in voltammogram performed with a scan rate of 20 mV/s and 0.5 M CH3OH - - 0.5 M H2SO4 electrolyte. The bifunctional effect of Pt and TiN for CO oxidation was mentioned as observed between Pt and Ru in commercial PtRu/C catalysts. 

Regarding the activity of PdA Ox-NT for methanol oxidation in 0.1 M NaOH, preliminary cyclic voltammetry experiments showed oxidation peaks at fairly negative potentials (0.07 V vs. SCE and -0.23 V on the anodic and cathodic scans, respectively), which, coupled with very good long-term stability of over 400 cycles at a rate of 100 mV s show promise for fuel cell applications [288]. 

FIGURE 12.10 Cyclic voltammograms for methanol oxidation using 40 wL% Pt Ru (1 1 atomic ratio) on Vulcan XC-72 with a catalyst loading of l.Omgcm in 1.0moldm methanol and sulfuric acid at different temperatures, as marked on each curve. The anode geometric area was 1.0 cm. Potential scan rate 5 mV s [24]. 

In closing our first view on methanol oxidation in acid media, we should give some results about the influence of surface structure on the electrocatalytic activity. For this purpose, we have plotted, in Fig. 6, the first potential scans (in anodic and cathodic direction) for methanol oxidation at the three main single surfaces of platinum [26-29]. Pt(lll) and Pt(llO) show the highest activity at low potentials. A strong early dehydrogenation obviously occurs on Pt(lOO). For recent details, see T. Iwasita in Ref. [26, 27). The discussion of the current maximum on Pt(lll) will be done with the help of information from FTIRS, mentioned below in Sect. S.2.3.2. 

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