Adsorption of methanol

At high alkali coverages (near monolayer coverage), when the adsorbed alkali overlayer shows a metal-like character, alkali-methoxy species are formed. As shown by TPD experiments in the system K/Ru(001) these alkali-methoxy species are more stable than the methoxy species on clean Ru(001) and adsorbed methanol on 0.1K/Ru(001). On metal surfaces inactive for methanol decomposition, e.g. Cu(lll), these alkali-methoxy species are formed even at low alkali coverages, due to the weaker interaction of the alkali atoms with the metal surface. The formation of these species stabilizes the methoxy species on the metal surface and enhances the activity of the metal surface for methanol decomposition. 

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The location and shape of the entire electrocapillary curve are affected if the general nature of the medium is changed. Fawcett and co-workers (see Ref. 126) have used nonaqueous media such as methanol, V-methylformamide, and propylene carbonate. In earlier studies, electrocapillaiy curves were obtained for O.OIA/ hydrochloric acid in mixed water-ethanol media of various compositions [117, 118]. The surface adsorption of methanol, obtained from... 

Adsorption of methanol and ethanol from gas phase on the surfaee of VO, was smdied by isotherms of resistanee (R, Om - t, s). 

From the results obtained with in situ reflectance spectroscopy and on-line analytical methods, investigators at Universite de Poitiers proposed a complete mechanism for the electrooxidation of methanol at a platinum electrode. The first step of the electrooxidation reaction is the dissociative adsorption of methanol, leading to several species according to the following equations ... 

Sinee no oxidation of methanol was observed on pure Ru, it is probable that there is no -( CHO)ads on Ru. However, adsorption of CO was observed on an Ru eleetrode from solution eontaining dissolved CO. The adsorbed CO observed on a R-Ru eleetrode during the adsorption of methanol may arise mainly from the dissoeiation of methanol at Pt sites, followed by the migration of sueh CO speeies toward the neighboring Ru sites. The removal of adsorbed CO by oxidation with adsorbed OH may oeeur by one of two ways (1) at R sites aeeording to reaetions (26) or (27) and (25), and (2) at Ru sites aeeording to reaetion (26 ) ... 

Adsorption due to the operation of chemical forces is called chemisorption. A typical example of chemisorption is also the adsorption of methanol on platinum electrodes, which is accompanied by a deep destruction of the methanol molecule. 

Electrode processes can be retarded (i.e. their overpotential is increased) by the adsorption of the components of the electrolysed solution, of the products of the actual electrode reaction and of other substances formed at the electrode. Figure 5.43 depicts the effect of the adsorption of methanol on the adsorption of hydrogen at a platinum electrode (see page 353). 

The dependence of the relative coverage of the platinum electrode with methanol on its concentration in solution indicates that the adsorption of methanol obeys the Temkin isotherm (4.3.46). 

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. 

A flow cell-procedure was then applied. The experiment consisted of (a) adsorption of methanol (in a solution containing deuterated methanol and light hydrogen base electrolyte), (b) solution exchange with base electrolyte and (c) application of two potential steps, one of short duration to oxidize the adsorbed residue and then a second one in the negative direction to reduce the ions H+ and/or D+ formed. During this time the mass intensity signals for HD, (m/e = 3) and for COz (m/e = 44) were... 

IR investigations of methanol oxidation have been mainly devoted to demonstrate the nature of strongly adsorbed residues produced during the adsorption of methanol [21,59,67,68], Bulk oxidation products were not investigated by in situ IR spectroscopy. 

After adsorption of methanol at 0.4 V and electrolyte exchange with base solution to eliminate bulk methanol (flow cell), a step to potentials between 0.25 V and 0.6 V was applied, then Sn(IV) or Sn(II) was added and the C02 mass intensity signal was monitored. 

Figure 3.9 STM images of same area before and after adsorption of methanol on reduced Ti02(l 1 0) at 300 K (Vt = 1.0 0.3 V and /t = <0.1 nA) (a) bare surface (b) after 80s exposure to methanol (c) after 110 s exposure to methanol (d) taken on (c) after spontaneous tip change (e) after high bias (3.0V) sweep of (c) (f) schematic model of the adsorption...

Table 3.2 The results of Bagotzky and Vassilyev (1967) on the adsorption of methanol at bulk platinum in acid solution at 0.4 V vs. RHE.

Figure 3.35 Potential dependence or the ft) integrated band intensity of the linear COad< derived from methanol at 0,4 V vs. RHE in I M CH3OH/0.5M H SO and (2) the methanol electro-oxidation current observed after the adsorption of methanol at 0.4 V. From K. Kunimatsu, Berichte der Bunsen-Ceseiischaft jur Phy-sitcafische Chemie. 1900.94. 1025 1030-...

Figure 3.38 (a) Current-time transient during the adsorption of methanol from solution at 0.330 V vs, Pd/H in 0.01 M CH.,OH + 0.5 M H2SOA. (b> Cyclic voltammograrn showing the oxidation of the adsorbate after replacing the solution by pure supporting electrolyte. 10 mV s, same solution as in (a). From Iwasita et at. (1987). 

The adsorption of methanol as well as methyl iodide was complete at ambient temperature on Fe-Beta-300 (917 m2/g surface area) since radioactive gas was not detected at catalyst outlet. Only a small part of methyl iodide (conversion rate was 10-25 %) while most part of methanol was converted (conversion rate was 60-90 %) up to... 

A dissociative adsorption of methanol forming surface methoxy groups is suggested as the initial step. This is followed by the slow step, the formation of some form of adsorbed formaldehyde species. Evidence.for the bridged species is not available, experiments with °0 labeled methanol are expected to clarify this. Continued surface oxidation leads to a surface formate group and to carbon monoxide. All the byproducts can be obtained by combination of the appropriate surface species. 

As was previously mentioned, PtRu alloys exhibit improved performance over pure Pt alloys.117,118 This is primarily a result of the ability of Ru to dissociate H20 for reaction with CO adsorbed on Pt sites.115,116 That CO oxidation on pure Ru is unfavorable indicates that on the bimetallic surface, CO is oxidized only on the Pt sites.119 Thus, CO is oxidized on Pt sites adjacent to Ru sites, where water is activated.120,121 This is known as a bifunctional mechanism. In addition, the presence of Ru atoms reduces the adsorption energy of CO on neighboring Pt atoms, lowering the activation energy of CO oxidation.122 This effect is purely electronic and is less significant than the bifunctional effect of Ru.123 One significant limitation of PtRu is the weak adsorption of methanol on Ru, particularly at room temperature.117,124 The weak adsorption severely hinders methanol decomposition, which is evident in Fig. 7 by the drop in current density for PtRu electrodes with high Ru composition.125... 

Reaction Steps 8.6 to 8.8 are also relevant to the catalytic reaction of methanol in a flow of CH3OH, although the steps 8.5 and 8.8 cannot be separated anymore. The net reaction for dissociative adsorption of methanol is expressed by ... 

Figure 9.16 The practical examples of comparative plots (a) N2 adsorption on a graphitized carbon black, modified by physical adsorption of methanol (the numbers correspond to the amount of CH3 OH in the fractions of monolayer capacity) [83] (b) the usual types of comparative plots by [3] and (c) N2 isotherms on microporous titanium oxide after various amount of preadsorbed nonane by [53].

The nondissociative adsorption of methanol also decreased the work function, indicating that the molecule adsorbed with the negative end of... 

Potentiodynamic Technique. Adsorption of methanol on Pt in acid solution was studied by Breiter and Gilman (3) using a potentiostatic technique. The anodic sweep, with a sweep rate of 800 V/s, was started at rest potential and extended to 2.0 V with respect to a hydrogen reference electrode in the same solution. As shown in Figure 10.8, the current was recorded as a function of potential (time) in the absence (curve A) and in the presence (curve B) of methanol. The increase in current in curve B is due to oxidation of the adsorbed methanol on the platinum electrode. Thus, shaded area 2 minus shaded area 1 (Fig. 10.8) yields the change 2m (C/cm ) required for oxidation of the adsorbed methanol ... 

A detailed investigation of aniline N-methylation on Cui xZnxFc204 was carried out through in situ FTIR spectroscopy. The reactants (aniline and methanol) and possible products (NMA, DMA and o-toluidine) were adsorbed on the catalysts and analyzed [106,107]. Adsorption of methanol indicated a dissociative chemisorption as methoxy species on catalyst surface at 100°C. As the temperature increased, oxidation of methoxy species to formaldehyde to dioxymethylene to formate species was observed, and above 300°C complete oxidation takes place to CO, CO2 and H2. Indeed methanol alone on Cui xZnxFc204 and Cui.xCoxFc204 behaves in a similar way [79,107]. 

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