Methanol dissociative adsorption

Figure 6.60 shows the capacitance change with potential. In the presence of methanol, the capacitance due to hydrogen adsorption decreases abruptly at certain potentials (200-350 mV) because the adsorbed methanol blocks hydrogen adsorption. On the other hand, a capacitance increase at high potentials (430-650 mV) indicates dissociative methanol adsorption. 

Another consequence of the changes in Pt electronic structure caused by the presence of Ru (i.e., creation of electron deficiency on Pt by decreasing the Fermi level density of states and reducing the Pt-Pt distance) is the increased rate of dissociative methanol adsorption [92]. Thus, in addition to H2O activation (according to the bifunctional mechanism [93]) Ru plays a significant role with respect also to methanol chemisorption and surface diffusion of COad. 

Furthermore, an interesting aspect of the Ru effect relates to the effect of temperature and the optimum Pt Ru ratio. Gasteiger et al. showed that dissociative methanol adsorption can occur on Ru sites as well, but it is a temperature-activated process [94]. Therefore, at low temperatures (e.g., 298 K) a higher Pt Ru atomic ratio (above 1 1) is required to facilitate the dissociative adsorption and dehydrogenation of methanol preferentially on Pt, whilst at high temperatures (e.g., 333 K and above) a surface richer in Ru is beneficial (e.g., 1 1 at. ratio) since Ru becomes active for chemisorption and the rate determining step switches to flie reaction between COad and OHad [94]. 

The mechanism of the ruthenium effect was first described by Watanabe and Motoo [17], postulating a hijurtctional mechanism in which platinum serves as catalyst for a dissociative methanol adsorption and... 

From these results, a mechanism of methanol electrooxidation at PtRu can be proposed. The first step may consist in the dissociative adsorption of methanol at platinum and formation of an adsorbed CHO species according to the schema presented in Fig. 12. This mechanism of methanol adsorption and dehydrogenation is generally admitted." Then, for the co-reduced catalysts (alloy), the number of involved electrons from methanol stripping as determined by DBMS is higher than 2, then adsorbed CHO and CO species seem to be involved in the mechanism. Moreover, the number of electrons for the oxidation of bulk methanol is greater... 

The values of the rate constants are given in Table 3. Since methanol adsorbs dissociatively only on the Pt atoms present on the alloy surface the difference in the adsorption rates indicates that surface properties of platinum became modified by mthenium. Thus the significant difference in methanol adsorption rates on Pt and Pt/Ru is a clear manifestation of the electronic effect, that is either activation energy for adsorption, and/or the pre-exponential factor became affected by the mthenium modification. 

Methanol adsorption at Mo03(010), (001), and (100) surfaces has been studied in cluster models using the semi-empirical ENHT method [230]. The calculations suggest that both molecular and dissociative adsorption may occur at the different surfaces. The stability of molecular adsorption complexes increases from the (100) surface to (001) and (010) if the latter contains oxygen... 

In conclusion, the combined experimental and theoretical study of methanol adsorbed on MgO films with different defect densities allows for a better identification of the surface sites responsible for the MgO reactivity. On the inert terrace sites only physisorption is observed. Molecular chemisorption, activation, and heterolytic dissociation occur on irregular sites. The low-coordinated Mg-O pairs of ions located at edges and steps can lead to strongly activated and even dissociated methanol molecules. Adsorption of CHsO" and H+ fragments seems to be preferred over dissociation into and OH ... 

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 activity of a Pt site in mixed Pt-M for CH3OH dissociation should vary somewhat with atomic M Pt ratio. With more Sn, the activity at Pt becomes less. Because another effect of Sn is to impede methanol adsorption, the optimal Sn surface composition must be low. In contrast, the CH3OH dissociation on a Ru-rich surface can proceed effectively. A metal with relatively high Ru Pt ratio should benefit for CH3OH oxidation. 

It was suggested that methanol adsorption takes place in several steps, forming different species due to dissociation of the molecule [21] ... 

The catalytic mechanism of PtRu has been interpreted in terms of a so-called bifunctional effect of the surface in which Pt sites adsorb and dissociate methanol-forming CO and Ru atoms adsorb and dissociate water molecules, thus providing, at low potentials, oxygen atoms needed to complete the oxidation of adsorbed CO to CO2 [75]. The facts above, showing an increased rate of adsorption of methanol in the presence of Ru, indicate that the bifunctional mechanism alone does not fully describe the catalytic action of ruthenium. 

In general, to balance the decomposition of methanol to CO and further oxidation to CO2 requires the consideration of some key factors. In Gasteiger s model [5], it is assumed that methanol oxidation proceeds only via adsorbed CO, and that the dissociative adsorption of methanol is the rate-determining step for methanol oxidation on PtRu electrodes. The electronic effect of Ru on methanol adsorption was not considered. In fact, at low potentials, a low Ru content can promote methanol decomposition to form adsorbed CO, and the oxidation of methanol adsorbates could also control the oxidation of methanol on PtRu electrodes. 

The influence of the presence of sulfur adatoms on the adsorption and decomposition of methanol and other alcohols on metal surfaces is in general twofold. It involves reduction of the adsorption rate and the adsorptive capacity of the surface as well as significant modification of the decomposition reaction path. For example, on Ni(100) methanol is adsorbed dissociatively at temperatures as low as -100K and decomposes to CO and hydrogen at temperatures higher than 300 K. As shown in Fig. 2.38 preadsorption of sulfur on Ni(100) inhibits the complete decomposition of adsorbed methanol and favors the production of HCHO in a narrow range of sulfur coverage (between 0.2 and 0.5). 

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 ... 

Platinum is the only acceptable electrocatalyst for most of the primary intermediate steps in the electrooxidation of methanol. It allows the dissociation of the methanol molecule hy breaking the C-H bonds during the adsorption steps. However, as seen earlier, this dissociation leads spontaneously to the formation of CO, which is due to its strong adsorption on Pt this species is a catalyst poison for the subsequent steps in the overall reaction of electrooxidation of CHjOH. The adsorption properties of the platinum surface must be modified to improve the kinetics of the overall reaction and hence to remove the poisoning species. Two different consequences can be envisaged from this modification prevention of the formation of the strongly adsorbed species, or increasing the kinetics of its oxidation. Such a modification will have an effect on the kinetics of steps (23) and (24) instead of step (21) in the first case and of step (26) in the second case. 

The different kinds of adsorbed CO were observed by in situ infrared reflectance spectroscopy. The results showed that using bulk Pt-Ru alloys, the adsorbed CO species formed by dissociation of methanol, or from dissolved CO on the surface of the electrode, are different on R and on Ru. The adsorption of CO occurs on pure Pt and Ru and on alloys of different compositions, but a shift detected in the wave number of the... 

Acid-base reactivity is an important property of oxide catalysts, and its control is of interest in surface chemistry as well as being of importance in industrial applications. The exposed cations and anions on oxide surfaces have long been described as acid-base pairs. The polar planes of ZnO showed dissociative adsorption and subsequent decomposition of methanol and formic acid related with their surface acid-base properties[3]. Further examples related to the topic of acid-base properties have been accumulated to date[ 1,4-6]. 

Vidal F, Busson B, Six C, Tadjeddine A, Dreesen L, Humbert C, Peremans A, Thiry P. 2004. Methanol dissociative adsorption on Pt(lOO) as studied by nonlinear vibrational spectroscopy. J Electroanal Chem 563 9-14. 

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