Methanol, adsorbed

Rousseau, R., Dietrich, G., Kruckeberg, S., Lutzenkirchen, K., Marx, D., Schweikhard, L. and Walther, C. (1998) Probing cluster structures with sensor molecules methanol adsorbed onto gold clusters. Chemical Physics Letters, 295, 41-46. 

Beden B, Juanto S, Leger JM, Lamy C. 1987. Infrared spectroscopy of the methanol adsorbates at a platinum electrode Part III. Structural effects and behaviour of the polycrystalline surface. J Electroanal Chem 238 323-331. 

Iwasita T, Nart EC. 1991. Identihcation of methanol adsorbates on platinum An in situ FT-IR investigation. J Electroanal Chem 317 291-298. 

Waszczuk P, Lu GU, Wieckowski A, Lu C, Rice C, Masel MI. 2002. UHV and electrochemical studies of CO and methanol adsorbed at platinum/ruthenium surfaces, and reference to fuel cell catalysis. Electrochim Acta 47 3637-3652. 

Wilhelm S, Vielstich W, Buschmann HW, Iwasita T. 1987. Direct proof of the hydrogen in the methanol adsorbate at platinum—An ECTDMS study. J Electroanal Chem 229 377-384. 

Haase, F., Sauer, J., Hutter, J., 1997, Ab Initio Molecular Dynamics Simulation of Methanol Adsorbed in Chabazite , Chem. Phys. Lett., 266, 397. 

Investigation of Methanol Adsorbate by On-Line Mass Spectroscopy and... 

Interaction of Methanol Adsorbate with Bulk Methanol. 155... 

Fig. 2.2. Cyclic voltammogram of a polished Pt electrode in 1CT2 M CH3OH/O.l M HCI04 solution, (full line) and potentiodynamic oxidation of methanol adsorbate after solution exchange with base electrolyte (dashed line). Sweep rate 60 mV/s, room temperature.

In order to check the survival of methanol adsorbate to the transfer conditions, the following experiment was performed. After adsorption of methanol and solution exchange with base electrolyte, the Pt electrode was transferred to the UHV chamber over a period of ca. 10 min, then back to the cell where it was reimmersed into the pure supporting electrolyte. A voltammogram was run and compared with that of an usual flow cell experiment. The results, (see Fig. 2.5a,b), show that the transfer procedure is valid. The areas under the oxidation curve are the same. As in the case of adsorbed CO on Pt (see Fig. 1.4), the change in the double peak structure indicates that some surface re-distribution may occur. 

The thermal desorption mass spectra of methanol adsorbate obtained from 5 x 10 3M CH3OH + 0.05M H2SC>4 and in 0.5 M CH3OH + 0.5 M H2S04, are... 

Fig. 2.5. Test for the survival of methanol adsorbate in the UHV the potential scan was applied a) after adsorption in 5 x 10 J M C H3OH5 x 10 2 M H2S04 followed by electrolyte exchange with base solution (b) after adsorption, transfer in the UHV chamber and reimmersion in base electrolyte dotted line base voltammogram sweep rate 62.5 mV/s.

Fig. 2.7. Mole fraction of methanol adsorbate obtained from ECTDMS measurements as a function of coverage (see text) 5 x 10-3 M CH3OH, + 0.5 M CH3OH, 0.1 M H2S04.

According to these results methanol adsorbate seems to consist of a mixture of (C, O) and (C, O, H) particles, the actual ratio depending on the concentration and total degree of coverage. This is in good agreement with coulometric determinations of the charge ratio for methanol adsorption, Qad, (see Eqs. 2.1 to 2.3) and adsorbate oxidation, Qox (see Eqs. 2.4 to 2.6) [14,47], These results will be discussed in Section 2.1.4. 

Charge measurements, as mentioned above, were also performed using the porous Pt electrodes required by the on-line MS technique. At low methanol concentrations (10 2 M), the charge ratio QaJQm, near 1 indicates that (C,0, H) must be the predominant adsorbate composition [14,47], This result is in good agreement with that of Heitbaum and co-workers [15] who used Eq. 1.2 to determine the number of electrons, n, per C02 produced from methanol adsorbate. They found for n a value of 3, which would be in agreement with reactions 2.1 or 2.2 for methanol adsorption. 

Fig. 2.9. Current transient and mass signal responses during oxidation of methanol adsorbate in pure base electrolyte (flow cell procedure). Methanol was adsorbed from a 10 2 M CD3OH + 10 4 M HC104 + 0.1 M NaC104. ad = 356 mV tad = 400 s. Potential step to 975 mV vs. Pd-H for 0.5s to produce C02 (m/e = 44) and hydrogen ions, followed by a step to —574 mV vs. Pd-H to detect HD...

Fig. 3.1. Current (a), and mass intensity for 13COz production (b) during the potentiodynamic oxidation of methanol adsorbate (flow cell procedure, ad = 0.2 V RHE, see text). Scan rate 10 mV/s.

Fig. 3.3. Current and mass intensity signal showing the effect of the interaction of bulk 12CO with 13C-methanol adsorbate (flow cell procedure), (a) Current due to the oxidation of methanol and CO adsorbates, (b) Mass intensity for 12C02 due to COad, (c) mass intensity for 13C02 (due to rest of adsorbed methanol.

It has been shown in Section 2.1.4 that methanol adsorbate formed from dilute solutions on a porous Pt surface, consists of COad and COHad in a ratio CO COH of ca. 20-30% [14]. The results of isotopic exchange with bulk CO seem to indicate that only the fraction present as COad can be desorbed and replaced by bulk CO. Probably the same arguments as in the case of pure COad can apply. COHad seems to be more strongly bound to the Pt surface and cannot be desorbed. 

Fig. 4.5. Mass spectroscopic detection of carbon dioxide during methanol adsorbate oxidation and Sn(IV) injection. Porous Pt electrode, real area 12.3 cm2. Procedure after methanol adsorption at 0.4 V from 10 2 M 13CH3OH/0.5 M H2S04, the electrolyte was exchanged with 0.5 M H2S04, then potential step to 0J was applied and Sn(lV) was added. Dashed line no tin added.

Fig. 4.7. Current (a) and mass intensity (b) voltammograms for methanol adsorbate oxidation without tin (dashed line) and with tin (full line). Base electrolyte dotted line. The voltammograms were recorded after applying a potential step to E0, = 0.475 V during 15 min.

In view of the fact that the oxidation of methanol adsorbate requires an additional oxygen atom (see Eqs. 2.4 to 2.6) this theory seems quite plausible to explain the data given above. 

As we have seen in Section 2.1.4, depending upon the concentration, methanol adsorbate seems to consist of variable amounts of COH and CO species. Oxidation to C02 requires the splitting of HzO molecules which could deliver an oxygen atom to form C02. The stability of H20 makes the oxidation process difficult. Its weak adsorption on platinum does not contribute to improve the situation. 

In this way Sn(II) would bring oxygen-containing species to the Pt interface which are stronger adsorbed than HzO and can therefore act as oxygen donors more easily during methanol adsorbate oxidation ... 

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