Methanol surface intermediates

Figure 8.3 Proposed reaction mechanism for methanol synthesis on Pd and comparison with gas-phase mechanism surface intermediates are speculative and associated energies are estimates...

Although the accuracy of this explanation will be discussed later, it is easily understood that the behavior of the electrode is greatly influenced not only by the instantaneous potential of the electrode (potential dependence ) but also by the history of the electrode (time dependence). As this example shows, the electrochemical oxidation of methanol is a series of reactions in which methanol, water, intermediates and surface adsorbates are interacting with each other in various ways, and are yet to be fully understood. 

In the decomposition or oxidation of methanol, methoxide (CH3O) is formed as a surface intermediate on a number of metal surfaces . The... 

V / RHE. Figure 6.32 provides a schematic illustration of the series pathway with CO as intermediate. Methanol adsorbs on Pt forming some H-containing surface intermediate H atoms are abstracted in a sequence of steps, until absorbed CO is formed. Removal of CO requires adsorption of water to yield water-based oxygenated surface species. 

Theoretical investigations indicated641 642 that two main pathways are possible for the first, dehydration step. One proceeds via a methoxy surface intermediate (10) formed as a result of the adsorption of methanol on acidic site of the zeolite (HO-Z) to give dimethyl ether [Eqs (3.63) and (3.64)]. The other, direct route is a reaction between two methanol molecules adsorbed simultaneously on the same active site [Eqs (3.65) and (3.66)] ... 

There is interest in the synthesis of methanol from CO/H2 over Pd, and the reaction has been studied over Pd/Si02 and Li+/Pd/Si02 (245). Only SSITKA experiments have been performed, and it was found that the coverage of surface intermediates is increased for the promoted catalyst. Similar studies have focused on other aspects of the system (246-248). 

Methanol reactions have also been studied on polycrystalline wafers of UO2 [76]. Two parent desorption states existed for methanol adsorbed at 90 K. Molecularly adsorbed methanol desorbed at 110 K, and methanol generated by surface recombination of methoxides and protons desorbed at 180 K. Carbon (Is) XPS demonstrated that methanol dissociatively adsorbed on the urania surface and that methoxide was the only surface intermediate present above 150 K. Primary reaction products were methane and carbon monoxide at 480 K. Oxygen atoms not removed from the surface as CO were incorporated into the oxide surface isotopically labeled U 02 surfaces did not exchange oxygen with methoxide to produce C 0 [76]. 

Selectivity on metal oxide catalysts is ultimately determined by complex intermolecular and surface-adsorbate interactions. Competing reaction channels are facilitated or hindered by the coordination geometry around metal cations, the ease of reduction of the surface, and the resulting stabilization of surface intermediates. The decomposition of relatively simple organic molecules like methanol and formic acid can be surprisingly complex, but attention to a few concepts may help to understand the reaction processes ... 

The presence of solution can dramatically affect dissociative chemisorption. In the vapor phase, most metal-catalyzed reactions are homolyticlike, whereby the intermediates that form are stabilized by interactions with the surface. Protic solvents, on the other hand, can more effectively stabilize charge-separated states and therefore aid in heterolytic activation routes. Heterolytic paths can lead to the formation of surface anions and cations that migrate into solution. This is directly relevant to methanol oxidation over PtRu in the methanol fuel cell. The metal-catalyzed route in the vapor phase would involve the dissociation of methanol into methoxy or hydroxy methyl and hydrogen surface intermediates. Subsequent dehydrogenation eventually leads to formation of CO and hydrogen. In the presence of an aqueous media, however, methanol will more likely decompose heterolytically into hydroxy methyl (—1) and intermediates. 

Mechanism and Kinetics. The most detailed study of the reaction mechanism has been made by Wachs and Madix. They used isotopic tracers and flash desorption to study the species produced when methanol is adsorbed on an oxygen-doped copper (110) single-crystal surface. While the results of such a study are of considerable interest, they are not necessarily representative of a copper catalyst continuously exposed to reaction conditions. From the desorption spectra, methanol shows exchange only of the hydroxy-hydrogen surface methoxide was identified as the most populous surface intermediate. As formaldehyde and hydrogen also appeared to be produced from the same intermediate, the mechanism (21)—(24) was proposed for the selective reaction ... 

The adsorption of alcohols, aldehydes, and carbon oxides on metal electrocatalysts has been extensively studied because of the significance of their oxidation reactions for electrochemical energy generation (7,9,81,195). Particular attention has been payed to the surface intermediates of methanol oxidation on platinum. At least two adsorption states have been assigned to methanol, a weak one possibly associated with physisorption (196) and one or more states arising from dissociative strong adsorption of the reactant (797, 198). Breiter (799) proposed a parallel scheme for methanol oxidation... 

The first paper on methanol electrocatalysis under UHV conditions was published by Attard et al. [139] on the most active surface, Pt(110). Similar results to those on Pt(l 11) were found, that is, carbon monoxide and molecular hydrogen, but with a slightly larger methanol surface coverage of 9 = 0.10. It was the first time that methoxy species were proposed as intermediates and were different from the carbon monoxide or formyl species proposed earlier by Bagotskii et al. [140], However, traces of the formyl species were also detected on reconstructed Pt(l 10) using vibrational spectroscopy, which was able to co-adsorb this species with atomic oxygen [117]. 

Fig. 5. Geometries of the possible intermediate fragments involved in methanol dehydrogenation on metal surface intermediates for (I) methanol dehydrogenation, (II) water dissociation, and...

Figure 47). This can be explained by the fact that a certain part of the ionic copper species can be accommodated in the zinc oxide lattice. Another explanation is that the specific activity of metallic and ionic copper species is differ-ent. In addidion, a correlation was found between the alkylation activity and the sum of surface metallic and ionic copper content of the catalyst. Based on this correlation it was suggested that in the alkylation of butylamine with methanol ionic copper species were involved in the rate-determining step of the reaction, i.e. in the dehydrogenation of alcohol into an aldehyde type surface intermediate.

The rate-limiting step of the alkylation of butylamine with methanol on CuO-ZnO-AhOs is the dehydrogenation of the alcohol to an aldehyde type surface intermediate. Literature data indicate that in the dehydrogenation of cyclohexanol to cyclohexanone on CuO-ZnO-AhOs catalysts two kinds of copper active sites, i.e. monovalent copper and metallic copper have been revealed. It was shown that the activity of monovalent copper is 15 times higher than that of metallic copper. Upon increasing the reduction temperature the alkylation activity of the CuO-ZnO-AhOs catalyst decreased, whereas the amount of metallic copper increased. A good correlation was obtained between the sum of the amounts of metallic surface copper atoms and ionic copper atoms and the alkylation activity of the catalyst. Because the reaction rate of butylamine alkylation depends on both the amount of zero- and monovalent copper, it is suggested that the active site is an ensemble of copper metal ion - metal nanocluster . 

Other Oxygenated Hydrocarbons Reductants. Other oxygenated hydrocarbons— 2-propanol, ethanol, methanol, iso-butanol, ethyl ether—were also tested. The inlet carbon atom concentration of these hydrocarbons was calculated to be equal to that of acetone in the tests reported above, e.g., equivalent to 1,300 ppm acetone. The NO conversion at steady state for each reductant is shown in Table II. All reductants showed 100% selectivity to N2. Since the carbon concentration of all reductants was the same, the activity of the reductants can be compared by comparing the NO conversion. The only reductant with activity close to acetone is 2-propanol with 31% conversion. Others showed much less activity than acetone. Specifically, methanol showed negligible NO reduction activity. It is speculated that the NO selective reduction activity is closely related to the ability of hydrocarbons to form oxygenated surface intermediates at these reaction conditions. This is being investigated further. 

Various surface intermediates are formed during methanol electrooxidation. Methanol is mainly decomposed to CO which is then further oxidized to CO2. Other CO-like species are also formed such as COHa s, HCOads. HCOOads [13]. Main by-products are formaldehyde and formic acid. Some of these intermediates are not easily oxidizable and remain strongly adsorbed to the catalyst surface. Consequently, they prevent methanol molecules adsorbing and undergoing further reaction. Thus the electrooxidation of the reaction intermediates reveals to be the rate limiting step. 

As the surfaces were heated, the intermediates that were left adsorbed to the surfaces were investigated for the different single-crystal surfaces. For the C/W (111) and CAV(llO) surfaces, the methoxy was found to react without producing any other surface intermediates. The Pt-modified surfaces reacted at a lower temperature than the pure carbides. The methanol on Pt(lll) mostly desorbed from the surface by 300 K [41]. These conclusions from the single-crystal studies were then applied to study polycrystalline foils which are much more realistic surfaces than the idealized single crystals. 

Several reaction schemes have been proposed to explain tee formation of all byproducts during methanol oxidation over Mo-Fe catalysts. Edwards et al [5] and Machiels [6] have suggested reactional mechanisms for tee formation of formaldehyde and by-products. However intermediate speeies proposed in such mechanisms have not been identift by any spectroscopic or other techniques. More recently Busca [7] on basis of infrared studies of surface intermediates species has proposed a rake-type mechanism for methanol oxidation over oxide catalysts. This mechanism account for the formation of formaldehyde and byproducts. 

Much of the effort on the electrooxidation of ethanol has been devoted mainly to identifying the adsorbed intermediates on the electrode and elucidating the reaction mechanism by means of various techniques, as differential electrochemical mass spectrometry, in situ Fourier transform infrared spectroscopy, and electrochemical thermal desorption mass spectroscopy. The established major products include CO2, acetaldehyde, and acetic acid, and it has been reported that methane and ethane have also been detected. Surface-adsorbed CO is still identified as the leading intermediate in ethanol electrooxidation, as it is in the methanol electrooxidation. Other surface intermediates include various Ci and C2 compounds such as ethoxy and acetyl [102]. There is general agreement that ethanol electrooxidation proceeds via a complex multi-step mechanism, which involves a number of adsorbed intermediates and also leads to different byproducts for incomplete ethanol oxidation, as shown in Figure 1.22. 

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