Methanol chemisorbate

The third assumption above is that methanol chemisorbs at lower potentials essentially solely to give the chemisorbed intermediate(s), with only a negligible quantity of charge being associated with either any parallel oxidation pathway or with further conversion of the intermediate to C02. This problem was first tackled by Breiter [27-30], who suggested a parallel mechanism rather than a serial one at lower potentials, with the formation of a second active intermediate in a scheme of the form... 

Fig. 24. Infrared spectra of methanol chemisorbed on zinc oxide at 200°C ( — -) background (—) CHjOH ( ) CD3OD. The bands at 2930 and 2830 cm 1 are assigned to the CH3 asymmetric and symmetric stretching vibrations of surface methoxide ions. Bands at 2870, 1571, and 1367 cm-1 are assigned to the C—H stretching, O—C—O antisymmetric stretching, and O—C—O symmetric stretching vibrations of surface formate ions. Further details of assignments are given in Ref. 88. [Reprinted with permission from Trans. Faraday Soc. 67, 3585 (1971). Copyright (1971) Chemical Society, London.]...

Figure 6. Spectra of methanol chemisorbed on Silicalite (after subtraction of original background).

Oxidation catalysts are either metals that chemisorb oxygen readily, such as platinum or silver, or transition metal oxides that are able to give and take oxygen by reason of their having several possible oxidation states. Ethylene oxide is formed with silver, ammonia is oxidized with platinum, and silver or copper in the form of metal screens catalyze the oxidation of methanol to formaldehyde. Cobalt catalysis is used in the following oxidations butane to acetic acid and to butyl-hydroperoxide, cyclohexane to cyclohexylperoxide, acetaldehyde to acetic acid and toluene to benzoic acid. PdCh-CuCb is used for many liquid-phase oxidations and V9O5 combinations for many vapor-phase oxidations. 

Xs are surface fractions (or active centers), free and covered by chemisorbed species of hydrogen, carbon monoxide, and methanol. H, C, and M are activities of hydrogen, carbon monoxide, and methanol. Primes indicate equilibrium values. 

Methanol synthesis over the heterogeneous catalyst is thought to occur by a successive hydrogenation of chemisorbed carbon monoxide. 

R. G. Herman et al. (8) studied these catalyst systems In great detail and suggested a Cu-fl solution In ZnO as active phase where Cu- - non-dlssoclatlvely chemisorbs and activates CO and ZnO activates H2. In the range of 15 to 85Z CuO In the catalyst, up to 16% Cu+1 became dissolved In the ZnO (9) and Cu+1 has been widely accepted as active site (10). Recently, however, Raney Cu-Zn catalysts have been shown to be very active methanol synthesis catalysts (11). The active component for these Raney catalysts was found to be metallic Cu with an activity maximum at 97 wt% Cu (12). 

The synergistic elfect seen in Pt-Rn alloys has aronsed great interest, since it opened perspectives for their nse in efficient methanol fnel cells. Many studies were performed to elucidate the origins of this effect. Some workers believe that it is due to changes in the electron strnctnre of platinnm npon alloy formation with ruthenium. A popular interpretation is the bifunctional mechanism, according to which the organic species are preferentially chemisorbed on platinnm sites while the ruthenium sites facilitate the adsorption of the species needed for oxidation of the orgaiuc species. 

Surface reactions that give rise to the electric crurent measmed depend on the electrode potential range and the type of measurements reported. The dominating reactions contributing to the voltammetric data in Fig. 13a-d are chemisorbed CO oxidation, oxidation of methanol to CO2 (on both scans), and chemisorbed CO formation on the reverse mn ... 

The Pt/Ru catalyst is the material of choice for the direct methanol fuel cell (DMFC) (and hydrogen reformate) fuel cell anodes, and its catalytic function needs to be completely understood. In the hrst approximation, as is now widely acknowledged, methanol decomposes on Pt sites of the Pt/Ru surface, producing chemisorbed CO that is transferred via surface motions to the active Pt/Ru sites to become oxidized to CO2... 

Lewis Bases. A variety of other ligands have been studied, but with only a few of the transition metals. There is still a lot of room for scoping work in this direction. Other reactant systems reported are ammoni a(2e), methanol (3h), and hydrogen sulfide(3b) with iron, and benzene with tungsten (Tf) and plati num(3a). In a qualitative sense all of these reactions appear to occur at, or near gas kinetic rates without distinct size selectivity. The ammonia chemisorbs on each collision with no size selective behavior. These complexes have lower ionization potential indicative of the donor type ligands. Saturation studies have indicated a variety of absorption sites on a single size cluster(51). 

Definitive evidence of the correlation between the decrease in catalytic activity of Pt with respect to methanol oxidation and the appearance of adsorbed CO is to be found in further work by Kunimatsu and Kita (1987). Methanol was chemisorbed at 0.4 V vs. RHE for various times, after which cyclic voltammograms were taken, as shown in Figure 3.36(a). Clearly, as... 

The role of the much discussed "primary reaction" of formation of a C2-hydrocarbon from methanol is then limited to producing a very small amount chemisorbed ethene during the incubation period. This C2 will react easily to C3 via alkylation with methanol. 

Electron-Induced Reactions—HREELS Measurements. Novel LEE-induced chemistry has also been observed in HREEL measurements of molecular solids and molecules physisorbed on the surface of RGS. For example, Lepage et ah, building on the initial observations of Jay-Gerin et al. [141], have employed HREELS to measure in situ, neutral dissociation products arising from the impact of low-energy electrons on thin multilayer films of methanol [37] and acetone [38]. The technique is similar to that developed earlier by Martel et al. [258] for chemisorbed systems, in that the same electron beam is used for both the production and the detection of the neutral fragments. However, in the work of Lepage... 

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