Methanol dissociated

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. 

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 ... 

In this section we will discuss the role of surface modification to enhance electrocatalytic oxidation of methanol, one of the interesting components for fuel cell technology. Perhaps the most successful promoter of methanol electrooxidation is ruthenium. Pt/Ru catalysts appear to exhibit classical bifunctional behavior, whereas the Pt atoms dissociate methanol and the ruthenium atoms adsorb oxygen-containing species. Both platinmn and ruthenimn atoms are necessary for eomplete oxidation to occur at a significant rate. The bifunctional mechanism can account for a decrease in poisoning from methanol, as observed for Pt/Ru alloys. Indeed, CO oxidation has been attributed to a bifimctional mechanism that reduces the overpotential of this reaction by 0.1 V on the Pt/Ru surface. 

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. 

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 dissociated methanol fuel that is rich in hydro n and CO would be much cleaner than the liquid methanol fuel Lean and complete combustion would ensure low CO and hydrocarbon emission. The formaldehyde emission would be improved. NO emission would be greatly reduced because of lower combustion temperatures. 

Dissociated methanol as an alternative automobile fuel was mentioned earlier (Seet. 1.4.2). Because of limited space in the en e compartment and limited temperatures during cold start, on-board methanol dissoeiation would need eatalysts that are active at low temperatures. The activity and stability are two key points for these eatalysts. Coke formation has been a problem that results in catalyst deactivation [82]. Methanol dissociation on board a vehicle also requires a eompact and efficient heat-exchange reactor to make use of engine waste heat. The reactor should also be resistant to the maximum antieipated exhaust temperature, thermal cycling fatigue, hydrogen embrittlement, and methanol corrosion. Althou a number of catalysts and dissociators have been devised [3139], there are still many opportunities for improvement. 

Methanol dissociation on board a passenger vehiele operates near atmospheric pressure, a condition that thermodynamicalfy strongfy favors the dissociation reaction. However, applying the dissociation to a diesel en e would require operation at such high pressures as 1020 MPa (100200 atm). Exhaust gas temperatures from a diesel engine could vary in a wide ran from as low as 150°C to well over 500°C. Development of an active and stable catalyst and technology to accommodate these harsh conditions is needed to use dissociated methanol... 

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... 

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