Alcohol electro-oxidation

Upper part alcohol electro-oxidation curves bottom part oxygen electro-reduction curves). 

The possible mechanistic pathways involved in alcohol electro-oxidation on Pt which have been proposed are illustrated in Scheme 5. 

However, CO2 is produced in the anode of a DAFC as final product of the alcohol electro-oxidation and, if the CO2 bubbles cannot be efficiently removed from the surface of the DL, they block the access of alcohol to the catalyst layer decreasing the cell efficiency. On the cathode, the formation of water on the catalyst layer and the electro-osmotic drag of water through the membrane can lead to GDL flooding if the excess water cannot be eliminated, reducing the transport of oxygen/air to the catalyst. 

Fig. 4.1 Important aspects for the anode performance towards the alcohol electro-oxidation (i) interaction between the catalyst surface and the alcohol molecules, (ii) interaction between the catalyst surface and the resulting adsorbed fragments from the original alcohol molecules, and (iii) surface oxides formation from the water splitting and reaction products desorption. Glycerol is taken as an example of alcohol molecule M metal atom...

Sao Paulo, at Sao Carlos, Brazil. He is employed at the University of Sao Paulo since 1981 where actually he is Full Professor of Physical Chemistry (2009) at the Institute of Chemistry of Sao Carlos. He made a post-doctorate at the University of Ottawa, Canada (1988-1990) and was a Visiting Professor at the University of Illinois at Urbana-Champaign, USA (1998-1999). He has published more than 130 articles in international scientific journals. He is author of four book chapters and one book. He had supervised 18 Ph.D. thesis and 21 M.Sc. thesis. He has delivered 87 international conferences. The main research areas are electro-catalysis, fuel cells, and alcohol electro-oxidation. He is the Associate Editor of the journal Electrocatalysis, published by Springer. 

Photocatalytic decomposition of alcohol Electro-oxidation of hydrogen Electroreduction of oxygen Ammonia synthesis Carbon monoxide methanation Carbon monoxide methanation Carbon monoxide oxidation Propene hydrogenation Benzene hydrogenation Oxidation of ethylene Coal liquefaction Electroreduction of oxygen Dehydrogenation of butadiene... 

Alcohol electro-oxidation at Pd-based nanocatalysts in alkaline media... 

Sine G, Smida D, Limat M, Foti G, Comninellis C (2007) Microemulsion synthesized Pt/Ru/Sn nanoparticles on BDD for alcohol electro-oxidation. J Electrochem Soc 154 B170-B174... 

Atomic Oxygen Activation Alcohol Electro-Oxidation... 

The path shown by eq. (4) is the main reaction path and assumes the formation of reactive intermediates, weakly bonded to the surface, in contrast to the poisoning intermediates, which are strongly bonded (Moralldn et al. 1995). Tripkovic et al. (2001) proposed the following general mechanism for the alcohol electro-oxidation (eqs. (6)-(9)) on noble metal catalyst in alkaline medium ... 

The electrochemical oxidation of polyhydric alcohols, viz. ethylene glycol, glycerol, meso-erythritol, xilitol, on a platinum electrode show high reactivity in alkaline solutions of KOH and K2C03 [53]. This electro-oxidation shows structural effects, Pt(lll) being the most active orientation. This results from different adsorption interactions of glycerol with the crystal planes [59]. 

The electro-oxidation of organics and more specifically of alcohols and polyols is also possible on silver electrodes in the following activity sequence methanol < ethylene glycol < glycerol [64]. With a bulk silver electrode and with a silver-modified glassy carbon electrode, oxidations proceed only in the area of silver oxide formation. 

The thermodynamic stabilities of phenonium ions have been determined based on bromide-transfer equilibria in the gas phase and, depending on the substituents, the bridged species (1) has been proposed as an intermediate or transition state on the potential-energy surface for the 1,2-aryl rearrangement of triarylvinyl cations (see Scheme 1). Phenonium ion (3) has been presented as an intermediate to account for the fact that lactonization of methyl 4-aryl-5-tosyloxy hexanoate (2) produces y-lactone (4) selectively under thermodynamic conditions, but affords 5-lactone (5) preferentially under kinetic conditions. It has been shown that anodic oxidation of frany-stilbene in alcohols in the presence of KF or BU4NBF4 is accompanied by its electro-oxidative rearrangement into diphenylacetaldehyde acetals. The mechanism outlined in Scheme 2 has been proposed" for the transformation. 

Hydrogen is a secondary fuel and, like electricity, is an energy carrier. It is the most electroactive fuel for fuel cells operating at low and intermediate temperatures. Methanol and ethanol are the most electroactive alcohol fuels, and, when they are electro-oxidized directly at the fuel cell anode (instead of being transformed in a hydrogen-rich gas in a fuel processor), the fuel cell is called a DAFC either a DMFC (with methanol) or a DEFC (with ethanol). 

Methanol is the most electro-reactive organic fuel, and, when it is electro-oxidized directly at the fuel anode (instead of to be transformed by steam reforming in a hydrogen-rich gas), the fuel cell is called a DMFC. More generally if the direct oxidation of a given fuel (alcohols, borohydrides, etc.)... 

A considerable amount of work has been published dealing with the electro-oxidation of organics at Ni anodes in aqueous base [548-552], These reactions have generally been dehydrogenations, e.g. primary alcohols to aldehydes, secondary alcohols to ketones and primary amines to nitriles. The reactions occur on a relatively thick layer of oxide on the Ni anodes. Pletcher and co-workers [529, 548, 549] observed that most of the oxidizable compounds were found to oxidize at the same potential and this potential coincided with that at which the surface of the Ni became oxidized. A typical cyclic voltammogram recorded at Ni in dilute KOH in the presence and absence of n-propylamine is shown in Fig. 23. It can be seen that addition of n-propylamine results in an oxidation wave being observed which is... 

Fleischmann et al. [549] studied the electro-oxidation of a series of amines and alcohols at Cu, Co, and Ag anodes in conjunction with the previously described work for Ni anodes in base. In cyclic voltammetry experiments, conducted at low to moderate sweep rates, organic oxidation waves were observed superimposed on the peaks associated with the surface transitions, Ni(II) - Ni(III), Co(II) -> Co(III), Ag(I) - Ag(II), and Cu(II) - Cu(III). These observations are in accord with an electrogenerated higher oxide species chemically oxidizing the organic compound in a manner similar to eqns. (112) (114). For alcohol oxidation, the rate constants decreased in the order kCn > km > kAg > kCo. Fleischmann et al. [549] observed that the rate of anodic oxidations increases across the first row of the transition metals series. These authors observed that the products of their electrolysis experiments were essentially identical to those obtained in heterogeneous reactions with the corresponding bulk oxides. 

In the following we will illnstrate, with the help of several examples deahng with the electro-oxidation of alcohols, how the use of coupled experimental techniqnes will be able to elncidate most of the reaction pathways. 

As an electrochemical reaction, Torii and co-workers demonstrated that the facile transformation of alkenes into allylic alcohols and ethers proceeded in the presence of a catalytic amount (10 mol%) of diphenyl diselenide (Scheme 15) [18]. Most of terminal co double bonds of isoprenoids undergo regioselective oxyselenenylation-deselenenylation to give frans-allylic alcohols in aqueous acetonitrile and methyl ethers in methanol. The addition of SOI salts improves chemical yields since SOI salts prevent the conversion of phenylselenenic acid (PhSeOH) into the inert phenylseleninic acid (PhSe02H) by both disproportionation and electro-oxidation. This method was also applied to intramolecular reaction to form -lactone in high yield. 

Besides, it has been shown that palladium is promising for direct alcohol fuel cells applications as it is very active for ethanol electro-oxidation in basic media and that its electroactivity is even higher than that of platinum. Recently, Pd nanoballs and nanowires synthesized by radiolysis in hexagonal mesophases have shown an important elec-trocatalytic activity for ethanol electro-oxidation. ... 

Small metal particles as model systems for electrocatalysts are crucial to unravel the influence of electronic or geometrical structure on the catalytic activity. The effect of metal particle size on electrochemical reactivity has been proposed to exist for the electro-oxidation of alcohols as well as for the reduction of oxygen [7,75,76], both vital processes that require much deeper understanding for the development and... 

Carbon monoxide is a key molecule in the electro-oxidation of Cl compounds and of many alcohols, since it is always produced by the dissociative chemisorption of the molecule, and since it may block the active catalytic sites. Therefore, its electrooxidation on platinum-based metals dispersed in an electron-conducting polymer, such as PAni, was investigated for a long time in our laboratory [8,28,34]. 

Combination of SC CO2 and IL has also been accomplished in the field of electro-organic syntliesis, and electro-oxidation of benzyl alcohol to benzaldehyde was conducted in the CO2/IL system. 

Electro-Organic Oxidation Properties. Table I lists some results for the electro-oxidation of primary alcohols and propylene on leadsubstituted lead ruthenate. Propylene was cleaved with nearly 100% selectivity to acetic acid and CO2. In borate buffer at pH 9 the oxidation of propylene also occurred, and the selectivity to acetate and CO2, based on the amount of carbonate isolated, was also close to 100%. Dissolved ethanol and propanol were both converted with high selectivity to the corresponding carboxylic acid salts in alkaline electrolyte. In contrast, Pt black (also shown in Table I) oxidized ethanol to CO2 and then rapidly deactivated. 

Table II shows results for the electro-oxidation of secondary alcohols and ketones. In alkaline electrolyte, secondary butanol was not oxidized to methyl ethyl ketone but was cleaved to acetate. Similarly methyl ethyl ketone was cleaved to acetate, although some CO2 and propionate formed, indicative of cleavage on the other side of the carbonyl group. Butanediol (2 ) went to acetate yielding less CO2. At pH 9 in borax buffer 2 Trtanol went exclusively to methyl ethyl ketone at 89% conversion, suggesting that enolization in alkali is a necessary part of the cleavage process. Cyclohexanol and cyclohexanone were both converted to adipic acid. Figure 12 summarizes the various types of electro-organic oxidations, thus far discussed, which are observed to occur on lead ruthenate in alkaline electrolyte.

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