Adsorption and oxidation of ethanol on colloid-based Pt/C, PtRu/C and Pt3Sn/C catalysts: In situ FTIR spectroscopy and on-line DEMS studies

Ethanol electrooxidation on the Pt(111) electrode has been studied with computational theory. Using a solvation model and a modified Poison-Boltzmann theory for electrolyte polarization, standard reversible potentials for forming 17 reaction intermediates in solution were calculated with density functional theory. Reversible potentials for adsorbed intermediates were then determined by inputting calculated adsorption energies into a linear Gibbs energy relationship. A path to CO 2 was found where surface potentials were low and close to the calculated 0.004 V reversible potential for the 12 electron oxidation of ethanol. An exception was the 0.49 V potential for forming the OH(ads) from H 2 O(l), this being required for oxidation of CO(ads) and RH(ads) intermediates. The surface potentials show that acetyl, OCCH 3 (ads) forms at small positive potentials and decomposes to CH(ads), CH 3 (ads), and CO(ads), which poison the surface at these potentials. Energy losses due to non-electron transfer reaction steps are small and cause a small shift in the reversible potential for the 12 electron oxidation. Values for adsorption bond strengths over a perfect catalyst were determined. It is concluded that on an ideal catalyst most intermediates will adsorb more weakly and OH more strongly than on Pt(111). Unlike a fossil fuel, ethanol is renewable and can be produced from biomass. Compared to methanol, ethanol is less toxic. The specific energy density of ethanol is high (8.0 kWh/kg). It is liquid, which makes it easy to store and transport. These advantages make the direct ethanol fuel cell (DEFC) a promising green energy source. However, commercialization of DEFC is hindered by the slow inefficient electrooxidation reaction of ethanol on platinum. Platinum is active as the anode electrocatalyst in hydrogen fuel cells but is much less active for ethanol oxidation. Theoretical understanding of the failures of platinum will establish the specific steps during the electrooxidation which present the challenges. As shown in this paper, some of these challenges can be overcome by using electrocatalysts where reaction intermediates have specific adsorption energies to the active site. Designing materials with these properties is the goal for electrocatalyst development.The complete electrochemical oxidation reaction which takes place at the anode surface produces twelve electrons, twelve protons, and carbon dioxide:The 4 Thus, the overpotential for oxidation is about 300 mV-400 mV under typically employed conditions of study.However, the oxidation is not complete, and several final oxidation products have been observed over platinum electrodes. Iwasita's on-line differential electrochemical mass spectroscopy (DEMS) mea- * Electrochemical Society Active Member.z E-mail: aba@po.cwru.edu surements identified acetaldehyde, OCHCH 3 , forming at potentials greater than about 0.3 V and identified CO 2 forming at potentials greater than about 0.5 V.2 Fourier transform infrared (FTIR) spectroscopy provided evidence for the functional g...

Section: Resultsmentioning

confidence: 98%

mentioning

confidence: 99%

Mechanisms for Ethanol Electrooxidation on Pt(111) and Adsorption Bond Strengths Defining an Ideal Catalyst

Asiri

Anderson

2014

“…Five milligrams of the catalyst was dispersed in a mixture of 2 mL water, 2 mL ethanol, and 50 L Nafion solution ͑5 wt %, Aldrich͒ with ultrasonic stirring to form a homogeneous ink. 19 The catalyst layer was prepared by dropping 20 L of the ink onto a GC disk electrode by a microsyringe and drying at room temperature. All potential values in this paper are referred to a reversible hydrogen electrode.…”

Section: Methodsmentioning

confidence: 99%

“…17,18 According to CO stripping voltammetry results, the PtSnO 2 /C catalyst presented a much negative shift in the onset potential compared with the PtRu/C catalyst; however, the oxidation current on the PtRu/C catalyst increased more rapidly with potential than that on the PtSnO 2 /C catalyst, suggesting that Ru is more effective than SnO 2 in promoting CO ads electro-oxidation at relatively high potentials. 18,19 For the ternary PtRuSn/C catalyst previously reported, Sn is mainly present in the oxidized state on the catalyst surface, and the improvement in the catalytic activity for methanol electro-oxidation is usually ascribed to the synergistic effect of SnO 2 and Ru species. 10 However, the catalytic activity of the PtRuSn/C catalyst is also reported to be inferior to that of the PtRu/C catalyst because SnO 2 that covers with Pt sites blocks the adsorption of methanol and is detrimental to the catalytic activity.…”

mentioning

confidence: 99%

Effect of SnO[sub 2] Deposition Sequence in SnO[sub 2]-Modified PtRu/C Catalyst Preparation on Catalytic Activity for Methanol Electro-Oxidation

Wang

Takeguchi

Zhang

et al. 2009

SnO 2 -modified PtRu/C catalysts were prepared in a polyol process to investigate the effect of a SnO 2 deposition sequence on the catalytic activity for methanol electro-oxidation. The structure, morphology, and chemical state of the prepared catalysts were characterized by X-ray diffraction, scanning transmission electron microscopy, and X-ray photoelectron spectroscopy. The electrochemical activities were evaluated by cyclic voltammetry, linear sweep voltammetry, and chronoamperometry measurements in combination with in situ IR reflection-absorption spectroscopy ͑IRRAS͒. Compared with those in the PtRu/C catalyst, a simultaneous deposition of PtRu and SnO 2 particles or a deposition of SnO 2 prior to PtRu improved the metal dispersion and decreased the alloyed Ru fraction. The deposition sequence of SnO 2 did not alter the chemical-state distribution of Pt and Ru but resulted in a different surface atomic ratio of Pt, Ru, and SnO 2 . Electrochemical and in situ IRRAS measurements indicated that the SnO 2 -modified PtRu/C catalyst prepared by the deposition of PtRu prior to SnO 2 gave the best catalytic activity for CO ads and methanol electro-oxidation among the PtRu/C and SnO 2 -modified PtRu/C catalysts. The roles of SnO 2 deposited with different sequences in the SnO 2 -modified PtRu/C catalysts were proposed. The electro-oxidation of methanol has attracted considerable interest due to its importance for direct methanol fuel cells ͑DMFCs͒, and the PtRu/C catalyst has been recognized as the most active binary catalyst. 1 The enhanced performance on the PtRu/C catalyst is usually explained by a bifunctional mechanism, 2-4 where methanol is adsorbed and dehydrogenated to produce CO ads on Pt active sites, Ru is more active than Pt to provide OH ads at low potentials by a water-discharge reaction, and CO ads on Pt active sites reacts with the neighboring OH ads to yield CO 2 . A PtRu alloy is also thought to weaken CO adsorption on Pt active sites by the electronic effect. [5][6][7]

“…This means that H ads is expected to block ethanol oxidation when the electrode is at potentials below 0.1 V. Blocking was observed by Behm and coworkers. 18 Therefore, for potentials above 0.1 V, up to about 0.6 V, reactions such as 1a and 2a may be allowed over unblocked sites, provided the activation energies are low enough.…”

Section: R-h Oxidation In the Under-potential Deposited (Upd)mentioning

confidence: 99%

Reaction Energy for an Electrode Surface Atom Inserting into an R-H Bond and Its Dependence on Electrode Potential: Application to Pt(111)

Anderson

Zhao

2015

Formulas are derived for calculating the dissociation energies for adsorbed R-H forming adsorbed R and H on Pt(111) electrodes from the reversible potentials for oxidizing R-H ads to R ads + H + aq . Here R is a radical CH x O y · such as CH 3 · , OH · and HOĊHCH 3 . When the oxidation reversible potentials lie potential range where in the under potential deposited H ads is present on the surface, which is zero to approximately 0.35 V, on the standard hydrogen scale, the R-H bond strengths are predicted to be zero. For adsorbed molecules with reversible potentials above 0.35 V, the dissociation energy increases linearly as the difference between the reversible potential and 0.35 V and for molecules with reversible potentials below zero volts the dissociation energy becomes negative. Applications to reversible potentials for water and ethanol redox steps show that H ads does not play a role in oxygen reduction to water and for only one of the steps in ethanol oxidation is dissociation forming H ads excluded. For the other ethanol oxidation steps there is currently insufficient information to decide whether R-H dissociates prior to oxidation. Electrochemistry is not ordinarily expected to be a tool for measuring bond strengths. In this paper it is shown how to determine R-H bond strengths in adsorbed molecules, such as CH 4ads , H 2 O ads , and HOCH 2 CH 3ads , from standard reversible potentials for oxidation of the bonds to R ads + H + aq + e − . We call this the oxidation into solution reaction. The electrode treated in this paper is platinum, and the close-packed (111) surface is chosen for modeling.The standard reversible potentials employed in this paper were determined previously.1 In Ref. 1 this was done by taking known standard Gibbs reaction energies for solution phase reactions and perturbing them by the adsorption bond strengths of R-H and R to give Gibbs energies for the reactions on the surface. Reversible potentials for the surface reactions were then calculated using these surface reaction Gibbs energies. Except for O-H and HO-H bonds, standard reversible potentials for oxidizing R-H bonds in aqueous molecules treated in this paper are unavailable from experiments. This makes it necessary to calculate the missing values. Because reversible potentials are functions of reaction Gibbs energies, predicting them demands accurate calculations of the Gibbs free energies of the reactants and products. For this, a self-consistent density functional theory (DFT) which includes the electrolyte and its polarization by the aqueous molecules 2 was used in Ref. 1. The details of the DFT will be outlined later in this paper.The R-H oxidation reactions addressed in this paper occur as steps during electrooxidation of ethanol on the anode of a fuel cell. The Pt(111) surface is chosen because it has broad, well-defined under potential deposited (UPD) H ads and double layer potential ranges and because it is well-studied by electrochemists. Knowing the reversible potentials for all the conceivable oxidation into solut...