Methanol adsorption and oxidation

Figure 11.8 (a) SNIFTIR spectra of the species coming from methanol adsorption and oxidation at a Pt/C electrode 0.1 M HCIO4 + 0.1 M CH3OH 25 °C. (h) SNIFTIR spectra of the species coming from ethanol adsorption and oxidation on a Pt/C electrode 0.1 M HCIO4 + 0.1 M C2H5OH 25 °C. 

Figure 11.9 Intensities of the COl ( ) and CO2 ( ) bands as functions of potential, (a, b) From the spectra of the species coming from methanol adsorption and oxidation (0.1 M HCIO4 + O.IM CH3OH, 25 °C) (a) Pt/C electrode (b) Pto.g + RU0.2/C electrode, (c, d) From the spectra of the species coming from ethanol adsorption and oxidation (0.1 M HCIO4 + 0.1 M C2H5OH, 25 °C) (c) Pt/C electrode (d) Pto.pSno.i/C electrode. The dashed curves in (c) and (d) show I E).

Coutanceau C, Hahn F, Waszczuk P, Wieckowski A, Lamy C, Leger J-M. 2002. Radioactive labeling study and FTIR measurements of methanol adsorption and oxidation on fuel cell catalysts. Fuel Cells 2 153-158. 

The electrochemical oxidation of methanol has been extensively studied on pc platinum [33,34] and platinum single crystal surfaces [35,36] in acid media at room temperature. Methanol electrooxidation occurs either as a direct six-electron pathway to carbon dioxide or by several adsorption steps, some of them leading to poisoning species prior to the formation of carbon dioxide as the final product. The most convincing evidence of carbon monoxide as a catalytic poison arises from in situ IR fast Fourier spectroscopy. An understanding of methanol adsorption and oxidation processes on modified platinum electrodes can lead to a deeper insight into the relation between the surface structure and reactivity in electrocatalysis. It is well known that the main impediment in the operation of a methanol fuel cell is the fast depolarization of the anode in the presence of traces of adsorbed carbon monoxide. 

Lamy et al. [37] have done extensive research on methanol adsorption and oxidation on fuel cell through radioactive labeling study and FTIR measurements. Kinetics of formation of surface and bulk products coming from methanol and surface/bulk exchange processes were found to be significantly different on Pt compared to Pt/Ru [38,39]. Modifications of the electronic surface of platinum atoms by ruthenium were clearly seen in the electrode structure. Adsorption of carbon monoxide from methanol... 

To summarize these results, it becomes now clear that EMIR Spectroscopy is particularly well suited to follow the fate of the different small adsorbed organic residues, resulting from the chemisorption of small organic molecules, such as CH3OH. The nature and the quantity of adsorbed species depend strongly on the structure of the catalytic surface, on the concentration of methanol in solution, on the adsorption time, on the applied electrode potential,... All these various experimental conditions lead to a great variety of adsorbed species, and control their surface distribution. According to these spectroscopic data, the reaction oxidation mechanisms of methanol adsorption and oxidation at platinum electrodes... 

In the case of methanol adsorption and oxidation, which has been thoroughly studied since several years, and which was taken as a typical example, apart fiom the poisoning CO species, which are always present at the electrode surface, particularly for high methanol concentration ( 0.1 M), there are many other adsorbed species at the electrode surface, which behave as reactive intermediates, namely methyl, formyl, formate radicals, and even methyl formate. The surface distribution of the different adsorbed species was shown to depend strongly on the electrode structure, electrode potential, concentration of the electroreactive species, temperature and pH of the electrolytic solution. 

Iwasita T, Vielstich W. 1988. New in-situ IR results on adsorption and oxidation of methanol on platinum in acidic solution. J Electroanal Chem 250 451-456. 

The adsorption and oxidation of the Ci molecules methanol, formaldehyde, and formic acid over a carbon-supported Pt/C fuel cell catalyst under continuous electrolyte flow have been investigated in a quantitative, comparative online DBMS study. 

Biegler T, Koch DFA. 1967. Adsorption and oxidation of methanol on a platinum electrode. J Electrochem Soc 114 904-909. 

K.3.1. High-pressure Methanol Oxidation on Pd(l 11). Figures 53a and b show PM-IRAS surface (p—s) and gas-phase (p + s) spectra acquired during methanol exposure and oxidation at mbar pressures. The gas-phase composition, determined by GC and by PM-IRAS, respectively is shown in Figs 53c and d. After exposure of Pd(l 1 1) to 5 mbar of CH3OH at 300 K, PM-IRAS was used to identify adsorbed CO (vco at approximately 1840 cm , typical of approximately 0.3 ML coverage) as well as formaldehyde (pcHj formaldehyde in two different adsorption geometries... 

As discussed in a previous section, metal oxides represent an important class of materials exhibiting a broad range of properties from insulators to semiconductors and conductors and have found applications as diverse as electronics, cosmetics and catalysts. Metal oxides have been widely used in many valuable heterogeneous catalytic reactions. Typical metal oxide-catalyzed reactions, including alkane oxidation, biodiesel production, methanol adsorption and decomposition, destructive adsorption of chlorocarbons and warfare agents, olefin metathesis and the Claisen-Schmidt condensation will be briefly discussed as examples of metal oxide-catalyzed reactions. 

Therefore more quantitative and selective methods are needed such as DBMS and radiotracer labehng, to investigate the adsorption and oxidation of methanol. 

Differential Electrochemical Mass Spectrometry (OEMS) was also used for methanol stripping experiments, which can give some information on the electrode coverage by species coming from the adsorption and oxidation of methanol. First, it can be seen from the CVs and the MSCVs recorded on a coreduced PtogRuo2/C catalyst as an example (Fig. 19) that the coverage of the electrode is much lower from methanol adsorption (curves 2) than that from CO adsorption (curves 1). 

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... 

One of the most attractive applications of bimetallic surfaces is for the analysis of their performance on the adsorption and oxidation reactions of interest in fuel cells. It is well known that one of the main limitations during the operation of a methanol fuel cell is the fast depolarization of the anode in the presence of traces of the adsorbed carbon monoxide. 

The sensors based on hetero-junction oxide structures show considerable response in alcohol (ethanol, methanol) media. The hetero-junction between oxide and solid solution phases appear to be very active in a course of both adsorption and oxidation of alcohol. 

It has also been proposed that methanol adsorption and its oxidation to formaldehyde occurs at coordinatively unsaturated sites, possessing four-fold coordination, rather than coordinatively saturated sites, possessing six-fold coordination [19]. Unfortunately, the surface vanadia species predominantly possess four-fold coordination which prevents this issue from being address with the current data. However, supported molybdena catalysts possesses both four-fold and six-fold coordination and their TOFs for methanol oxidation have been measured [27]. It was found that, contrary to above hypothesis, the coordinatively saturated surface molybdena species is approximately four times more active than the coordinatively unsaturated molybdena species for titania supported molybdena catalysts. Thus, methanol oxidation proceeds on both coordinatively saturated and coordinatively unsaturated sites at relatively comparable reaction rates (TOFs). 

Further, oxides that are characterized by the possibility of metal ion reduction without oxide state modification have the greatest ability to promote oxidizing dehydrogenation processes. Oxide such as Iu203 is inclined to the changing of the metal ion oxidizing state In(III) In(II), while the oxide phase remains original. Due to this fact, sensors based on heterojunction oxide composites show considerable response to alcohol vapors (methanol, ethanol). The heterojunction between an oxide and solid solution phases appears to be very active in both adsorption and oxidation of alcohol. 

As mentioned above, the alcohol crossover from the anode to the cathode is a important problems to be overcome to improve the DAFC performance. This is due to the fact that the commonly used Pt-based cathode electrocatalysts are also active for the adsorption and oxidation of methanol [1]. So, in addition to the resulting mixed potential at the cathode, there is a decrease in the fuel utilization. Therefore, considering the above exposed reactions for the alcohol electrooxidation, and the features that govern the ORR electrocatalytic activity, as discussed in the Sect. 5.2, it is ready to conclude the importance of the modification of the active ORR electrocatalyst surfaces in order to inhibit the methanol or ethanol oxidative adsorption steps. In the next sections, some recent materials being developed to overcome the problems caused by the alcohol crossover will be presented. 

In the surface science based studies on the methanol oxidation over single crystalline Cu(llO), Wachs and Madix [1, 2] have shown that the active state of the copper surface for the methanol oxidation was a partially oxidized copper surface exhibiting nucleophilic oxygen ad-atoms. The ojQ gen activated the surface for methanol adsorption and removed hydrogen released by water formation on the surface via a low-energy reaction pathway. 

Wieckowski A. Kinetic isotope effects between light and heavy water in HCOOH and CH30F1 adsorption and oxidation on Pt. J Electroanal Cbem 1977 78 229-41. Wieckowski A, Sobkowski J. Comparative study of adsorption and oxidation of formic acid and methanol on platinized electrodes in acidic solution. J Electroanal Cbem 1975 63 365-77. 

Further kinetic studies of methanol selective oxidation to formaldehyde over ferric and ferrous molybdate bismuth, chromium, aluminum, and heteropoly molybdates, demonstrated that a wide range of metal molybdates are active and selective for this reaction and that the same mechanism of methanol adsorption and reaction occurs [19]. 

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