Acetaldehyde electrooxidation

Farias MJS, Camara GA, Tanaka AA, Iwasita T. 2007. Acetaldehyde electrooxidation The influence of concentration on the yields of parallel pathways. J Electroanal Chem 600 236. 

On large 0.8 am diameter PtSn particles ethanol was oxidized mainly to acetic acid [235], Structure sensitivity of ethanol and acetaldehyde electrooxidation was also implied in experiments using Au nanoparticles where the electrocatalytic activity per real surface area increased with an increase in particle diameter [236]. 

Wang H, Jusys Z, Behm RJ. 2006. Electrooxidation of acetaldehyde on carbon-supported Pt, PtRu and Pt3Sn and unsupported PtRuo,2 catalysts A quantitative DEMS study. J Appl Electrochem 36 1187-1198. 

We have recently performed a variety of these and related SPAIRS-voltammetric measurements on platinum and palladium <5c. 12b ), and have concluded that the adsorbed CO formed in most cases acts predominantly as a poison for organic electrooxidation. Interestingly, the potential at which the CO undergoes electrooxidation, and hence where the electrocatalysis commences, can be strongly dependent on the structure of the solution species involved. Thus for acetaldehyde, for example, this process occurs at about 0.3 V lower overpotentials than for benzaldehyde under comparable conditions (5c). 

Ethylene can be oxidized to acetaldehyde in high yields similar to the Wacker-process if electrogenerated palladium(ll) is used as catalyst. In this way the copper(II) catalyzed air oxidation of palladium(O) is replaced by the electrooxidation according to Eq. (40). 

Other electrocatalysts were considered for the electrooxidation of ethanol, such as rhodium, iridium or gold, " " leading to similar results in acid medium. The oxidation of ethanol on rhodium proceeds mainly through the formation of acetic acid and carbon monoxide. Two types of adsorbed CO are formed, i.e., linearly-bonded and bridge-bonded, in a similar amount, at relatively low potentials, then leading rapidly to carbon dioxide when the rhodium surface begins to oxidize, at 0.5-0.7 V/RHE. On gold in acid medium the oxidation reaction leads mainly to the formation of acetaldehyde. " " ... 

In-situ FTIR studies on the electrooxidation of ethanol on polycrystalline Pt [97-99] as well as on single-crystal Pt electrodes [lOO/lOl] have shown the formation of acetaldehyde and acetic acid in addition to cmbon dioxide as soluble products. Figure 29 shows the typical features for thesq/products, which were assigned according to Ihble 1. 

Homogeneous catalysis by redox metals is also known for nonelectro-chemical processes. Thus, ethylene is oxidized to acetaldehyde in the Wacker process in aqueous solutions containing Pd " (504). Apart from complex formation and insertion (505), ionic oxidation and reduction may take place. It is noteworthy that palladium oxidation to form ions that act as homogeneous catalysts has been suggested as an important step in ethylene electrooxidation on solid palladium electrocatalysts 28, 29). 

The selectivity of palladium and gold for alkene oxidation to aldehydes 28,29,170) was attributed initially to adsorption strength. However, electrooxidation in the presence of palladium ions indicates possible homogeneous alkene insertion, similar to the Wacker process 304). Homogeneous reaction is also involved in redox oxidations of hydrocarbons. In this case, the nature of the metal ions is expected to control selectivity. Indeed, toluene yields 20% benzaldehyde in electrolytes containing Ce salts, while oxidation proceeds to benzoic acid with Cr redox catalysts 311). In addition, the concentration of redox catalysts appears to affect yields in nonelectrochemical oxidation of ethylene large amounts of palladium chloride promote butene formation at the expense of acetaldehyde 312). Finally, the role of the electrolyte and solvent should not be ignored. For instance, electrooxidation of ethylene on carbon, in aqueous solution of acetic acid yields acetaldehyde 313) in the... 

Although PtSn/C are the best binary electrocatalysts for the electrooxidation of ethanol, the main reaction products are acetic add (AA) and acetaldehyde (AAL). ... 

The ethanol oxidation on Pd electrocatalysts is dramatically affected by the pH of file aqueous ethanol solution no reaction occurs in acidic solutions, while the reaction is fast in alkaline solutions. Some raticmale for the origin of this pH effect on the ethanol oxidation to acetaldehyde has been provided by DFT calculations [111] (Fig. 8.8). DFT calculations show that in acidic media continued dehydrogenation of ethanol is difficult due to the lack of OH species to instantly remove hydrogen, which inhibits the ethanol electrooxidation. Conversely, both ethanol and sufficient OH can adsorb on Pd in alkaline media, leading to continuous ethanol electrooxidation. DFT calculations show that in acidic media continued... 

Lai SCS, Kleyn SEF, Rosea V, Koper MTM (2008) Mechanism of the dissociation and electrooxidation of ethanol and acetaldehyde on platinum as studied by SERS. J Phys Chem C 112 19080-19087... 

In addition, electrooxidation of cystine and cysteine at platinum and gold electrodes has been described [158-160]. All a-amino acids have been found oxidizable at solid metal electrodes at approximately the same potentials [161, 162]. This oxidation leads to the formation of an imine intermediate, which is further oxidized to nornitril. At a silver electrode slow hydrolysis of this intermediate to noraldehyde also takes place. The electrochemical oxidation reactions of a- and jS-alanine at a platinum electrode in aqueous solutions produce free radicals accompanied by a second reaction involving loss of CO2 [163]. In the electrooxidation of a-alanine, the adsorbed intermediate species is either hydrolyzed anodically to acetaldehyde and ammonia, or is oxidized to a carbonium ion which is subsequently hydrolyzed to acetaldehyde and ammonia in solution, analoguous to the behaviour of glycine [164]. The mechanism for jS-alanine is similar except carbonium ion formation is accompanied by a hybrid transfer to form acetaldehyde. 

The ethanol electrooxidation mechanism on platinum electrodes in acidic solution has been studied by various techniques and a number of adsorbed intermediates have been identified [16-25]. Carbon dioxide (CO2), acetaldehyde (CH3CHO), and acetic acid (CH3COOH) are the main products of the reaction. The global... 

As the complete electrooxidation of ethanol in an acid medium yields two molecules of CO2 and 12 electrons per ethanol molecule and involves the cleavage of the C-C bond, which requires rather high activation energy, the anodie eflianol electrooxidation on Pt is very sluggish, especially at low temperatures [99]. Despite significant efforts and numerous studies, the mechanism of flie ethanol electrooxidation reaction still remains unclear some studies are even contradictory. Nevertheless, electrooxidation of ethanol often does not proceed to completion, yielding adsorbed intermediates such as acetaldehyde [100,101] ... 

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. 

CO2 is the most common product. Other products and by-products such as acetaldehyde and acetic acid will inevitably decrease flie fuel efficiency. The electrooxidative removal of CO-like intermediates and flie cleavage of the C-C bond are the two main obstacles and rate determining steps. It is elear fliat ethanol eleetrooxidation involves more intermediates and produets than that of methanol, and thus more active electrocatalysts are needed to promote eflianol eleetrooxidation at tower temperatures [102]. Although fliere are some similarities in the oxidation of low molecular weight alcohols on Pt (e.g., CO is produced as intermediate), the best catalyst is not the same for all situations. Contrary to what was found for the oxidation of methanol, the more effective catalyst for flie oxidation of ethanol is not necessarily a Pt-Ru alloy [104]. 

Wu G, Swaidan R, Cui G. Electrooxidations of ethanol, acetaldehyde and acetic acid using PtRuSn/C catalysts prepared by modified alcohol-reduction process. J Power Sources 2007 172 180-8. 

In a review by Lamy et al. (2004) on Pt-Sn catalysts for ethanol electrooxidation, in a paper by Wang et al. (2011), and in a paper by Barroso et al. (2011), conclusions on a possible rupture of C-C bonds and on further oxidation of acetaldehyde and acetic acid were made from an analysis of voltanunetric measurements. 

Remarkably, the scarce pioneering studies of the ethanol electrooxidation reaction already identified acetic acid (AA) and acetaldehyde (AAL) as the major products of the electrooxidation of ethanol in acid medium with a minority production of CO2 [9,10]. In spite of the enormous body of work published during the last years, and of the availability of powerful in situ diffraction and spectroscopy methods coupled to electrochemical techniques (EC-FTIR, OEMS, Raman, X-ray) features such as the actual electrooxidation mechanism, reaction kinetics, the nature of the active site(s) and accurate identification reaction intermediates of the electrooxidation of small organic molecules remain elusive, especially when molecules containing C—C bonds such as ethanol are involved. 

The complete electrooxidation of ethanol to CO2 releases 12 electrons and two molecules of CO2 per molecule of ethanol. Alas, in aqueous acid medium at room temperature, the partial oxidation of ethanol is the most favorable route, leading to the formation of acetaldehyde and acetic acid releasing of 2 and 4 electrons, respectively (see Figure 3.1). Whereas acetaldehyde can be further oxidized to acetic acid and CO2, acetic acid is a dead-end product of the electrooxidation of ethanol in acid medium. The formation of CO2 implies the scission of the C—C bond, a process which seems to be the bottleneck step for the complete oxidation of ethanol. Many aspects of the electrooxidation of ethanol still remain unclear in particular it not yet understood how the cleaving of the C—C bond proceeds. The nature of the ethanol adsorbate(s) and the intermediate adsorbed species leading to the cleavage of the C—C bond are also still under debate. Some authors propose that C—C scission can happen directly from ethanol whereas others claim that acetaldehyde (or acetyl) species are formed before C—C scission. The nature of the active site for the cleavage of the C—C scission is also under debate. 

Early studies about ethanol electrooxidation detected only acetic acid and acetaldehyde as the reaction products 110,12,131 and concluded that their relative production depended upon the working potential. Rightmire et al. 112] analyzed by gas chromatography the liquid products resulting from the electrolysis of ethanol with platinized Ft electrodes in 0.5 M H2SO4 and identified acetaldehyde as the primary product with small amounts of acetic acid at potentials of 0.75 V. They went further and reported that polarographic scans in acetic acid solutions showed no reaction leading to the conclusion that acetic acid is a final oxidation... 

The Pt (100) surface records the highest current densities for the ethanol electrooxidation reaction however, it is the Pt (111) surface the one that records the lowest onset potential, hence it would be the preferred surface for fuel cell applications. Furthermore, the production of acetic acid is higher on Pt (111) whereas on Pt (100) and (110) acetaldehyde production predominates [28]. Alas, the current density reached by such Pt (111) surface during the ethanol electrooxidation is the lowest of the three basal planes of Pt. This is because of the low ability of this surface for the formation of COad thus indicating that adsorbed CO acts as a poisoning species on Pt. 

A closer inspection of the yields to the different products reveals that carbon dioxide is a minor product, below 3%, of the ethanol electrooxidation. The main product of the ethanol electrooxidation is acetaldehyde with a yield above 60% when using 1M ethanol. Nevertheless, the selectivity to the different products strongly depends on the concentration of ethanol thus the selectivity to acetaldehyde increases as the concentration of ethanol increases from 0.001 to 0.5 M whereas the selectivity to CO2 and to acetic add decreases in this concentration range [47] as shown in Figure 3.6. This observation has been ascribed to the lower probability of readsorption and further oxidation of partially oxidized intermediates due to a higher surface coverage of ethanol. 

So far we have shown that ethanol adsorption/oxidation on Pt is a complex process which leads to the formation of absorbed Ci species, (COad and CH c,ad) at relatively low potentials which by remaining adsorbed on the surface of the Pt electrode impede the ethanol electrooxidation reaction to proceed further. The main products of the electrooxidation of ethanol are acetaldehyde and acetic acid along with a minor amount of CO2. The selectivity to those products depends on the reaction conditions. The next sections describe the adsorption and oxidation of acetic acid and acetaldehyde on Pt. 

Acetaldehyde can be considered either as an intermediate or as a final product of the electrooxidation of ethanol. CO2 is the only product of the electrooxidation of acetaldehyde on Ft in acid medium at concentrations below 0.01 M. As the concentration of acetaldehyde increases, both CO2 and acetic acid are formed. However, the amount of CO2 formed does not vary significantly with the concentration of acetic acid suggesting that its formation is not linked to the production of CO2, i.e., they do not share any common intermediate species. The intermediate species responsible for the formation of CO2 is limited by the amount of the adsorbate formed at low potentials. 

The oxidation of acetaldehyde on Ft is also a structure sensitive reaction as it can be seen from the different voltammograms depicted in Figure 3.7 and recorded during the electrooxidation of acetaldehyde on Ft (111) and Ft (100) in acid medium [45]. 

The cyclic voltammograms for the adsorption/electrooxidation of acetaldehyde on a Pt thin-film electrode are shown Figure 3.8, upper panel. In the positive going scan two oxidative waves are observed at 0.9 and 1.3 V vs. RHE. These peaks are associated with the production of acetic acid rather than CO2. 

The second and subsequent cycles of the oxidation of acetaldehyde are shifted to more positive potentials than the first one. This behavior has been also observed for the electrooxidation of ethanol (see above). The identification of the adsorbates and the products of the electrooxidation of acetaldehyde have been accomplished by different techniques. In the following we shall concentrate on the results obtained by ATR-IR spectra in a flow cell [21]. The set of bands at 2080 and 1880 cm is indicative of the presence of adsorbed CO (pcOl and cOb, respectively). The presence of adsorbed acetate is identified by a band at 1410 cm . The evolution of these species with the applied potential has been followed by... 

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