Ethanol oxidation dissociative mechanism

Indirect evidence for a relatively slow isomerization step that is rate-limiting under some conditions has also been obtained 35). The dissociation velocity constant. A -, for the compound E-NADH is increased threefold in the presence of sodium chloride, but the maximum rate of ethanol oxidation is only slightly increased thus, dissociation of NADH can no longer be the sole rate-determining step. Since the fast hydride transfer step was not affected by sodium chloride, and reasonable evidence that aldehyde dissociation is also relatively fast was obtained. Shore et al. 35) concluded that a new step in the mechanism had been revealed. This could be isomerization of either the ternary product complex or the binary complex E-NADH. Evidence of a similar slow step in the oxidation of ethanol and propanol with APAD as coenzyme 71) was referred to in Section II,E,1. 

The detailed decomposition (P-H ehminahon) mechanism of the hydrido(alkoxo) complexes, mer-crs-[lr(H)(OR)Cl(PR 3)3] (R = Me, Et, Pr R = Me, Et H trans to Cl) (56, 58, 60), forming the dihydrides mer-cis-[lr H)2Cl PR )2] (57, 59) along with the corresponding aldehyde or ketone was examined (Scheme 6-8). The hydrido(ethoxo) as well as the hydrido(isopropoxo) complexes 60 could also be prepared by oxidative addition of ethanol and isopropanol to the phosphine complexes 39 [44]. In the initial stage of the P-H elimination, a pre-equiUbrium is assumed in which an unsaturated pentacoordinated product is generated by an alcohol-assisted dissociation of the chloride. From this intermediate the transition state is reached, and the rate-determining step is an irreversible scission of the P-C-H bond. This process has a low... 

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

The kinetics of the reduction of [Rh(bipy)2Cl2]+ in alkaline aqueous ethanol (under H2) revealed a two stage process an initial induction period, during which time Rh1 species formed, and a faster, autocatalytic region. The kinetics could be fit to the expression d[Rh ]/df = k[Rhm]2[Rh1]. The reaction rate was retarded by the addition of excess bipy, suggesting the suppression of a dissociation equilibrium, and the rate constant varies with [OH- ], but npt with [Cl- ]. A five-step mechanism for the autocatalytic process was proposed, involving the formation (via an unspecified mechanism) of [Rh(bipy)2]+, followed by the oxidative-addition of H2 to [Rh(bipy)2]+, and chloro-bridged dimeric and trimeric intermediates.823... 

From these results it appears that the addition of tin to platinum greatly favors the formation of AA eomparatively to AAL. This can be explained by the bifunctionnal mechanism where ethanol is adsorbed dissociatively at platinnm sites, either via an 0-adsorption or a C-adsorption process followed by the oxidation of these adsorbed residues by oxygenated species formed on Sn at lower potentials giving AA. 

The dissociative adsorption of ethanol occurs at lower potentials on ft-Sn/C electrocatalysts than on Pt/C anodes. OH species are formed on Sn sites at low potentials, leading to the oxidation of (CO)ads into CO2, in agreement with the bifunctional mechanism. 

The role of Ru in the mechanism of ethanol electro-oxidation is similar to that of Sn. The adsorption and decomposition of ethanol and its intermediate reaction products happen on Pt active sites, while the dissociative adsorption of water occurs over Sn or Ru sites, to form oxygen-containing surface species. Antolini et al. [24] have shown that the Ru addition of Ru to PtSn catalysts can enhance the catalytic activity of a certain composition. However, this enhancement is related to the Ru/Sn ratio that is present in the alloy, as well as to the synergetic effect of Ru and Sn oxides. 

Abstract In this chapter, we present new insights in direct alcohol fuel cell-related anode electrocatalysis based on quantitative differential electrochemical mass spectrometry (DEMS) studies. First, we review the history and development of the DEMS technique, as well as the calibration method for quantification. We then discuss some contributions of quantitative DEMS to the study of the mechanism of methanol electrooxidation on Pt and PtRu model catalysts. We also discuss quantitative DEMS studies of the mechanism of dissociative adsorption and electrooxidation of ethanol and acetaldehyde at Pt, Pt3Sn, PtRu, and PtRh nanoparticle catalysts. Finally, the mechanism of dissociative adsorption and electrooxidation of ethylene glycol and its oxidative derivatives on carbon-supported Pt, Pt3Sn, and PtRu nanoparticle catalysts are discussed, based on quantitative DEMS results. 

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