Adsorption of acetic acid on Pt

Adsorption of acetic acid on Pt(lll) surface was studied the surface concentration data were correlated with voltammetric profiles of the Pt(lll) electrode in perchloric acid electrolyte containing 0.5 mM of CHoCOOH. It is concluded that acetic acid adsorption is associative and occurs without a significant charge transfer across the interface. Instead, the recorded currents are due to adsorption/desorption processes of hydrogen, processes which are much better resolved on Pt(lll) than on polycrystalline platinum. A classification of adsorption processes on catalytic electrodes and atmospheric methods of preparation of single crystal electrodes are discussed. 

Case Studies Adsorption of Acetic Acid on Pt(111) Single Crystal Electrodes. Acetic acid is one of a very few organic compounds which is reversibly adsorbed on platinum at room temperature (20,25). We report below our radiochemical results on adsorption of this compound on Pt(111) and on polycrystalline Pt. 

As shown in Figure 4, the increase in adsorption of acetic acid on Pt(lll) occurs in the potential range of 0.15 to 0.3 V, which roughly coincides with position of the current-potential peak of the voltammogram recorded in the same solution. An interdependence can thus be sought between acetic acid adsorption... 

Acetic acid, the oxidation product of ethanol and acetaldehyde, has a very different adsorption behavior from that of most organic species. It is one of the rare organic species which, at least at not too high concentrations, adsorbs reversibly on Pt and other noble metals. It is an open question what is the structure of the adsorbed species and whether acetate ions or undissociated acetic acid molecules are the surface species in the case of an acidic supporting electrolyte. This question was examined by FTIR study at the basal planes at platinum single-crystal electrodes [Pt(lOO), Pt(llO), and Pt (111)]. The authors concluded that acetic acid, adsorbed from a 0.1 mol dm HCIO4 supporting electrolyte, is in the form of acetate-ion on all crystal faces studied. This view is in conflict with the statements made by other authors. ... 

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. 

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. 

We then designed model studies by adsorbing cinchonidine from CCU solution onto a polycrystalline platinum disk, and then rinsing the platinum surface with a solvent. The fate of the adsorbed cinchonidine was monitored by reflection-absorption infrared spectroscopy (RAIRS) that probes the adsorbed cinchonidine on the surface. By trying 54 different solvents, we are able to identify two broad trends (Figure 17) [66]. For the first trend, the cinchonidine initially adsorbed at the CCR-Pt interface is not easily removed by the second solvent such as cyclohexane, n-pentane, n-hexane, carbon tetrachloride, carbon disulfide, toluene, benzene, ethyl ether, chlorobenzene, and formamide. For the second trend, the initially established adsorption-desorption equilibrium at the CCR-Pt interface is obviously perturbed by flushing the system with another solvent such as dichloromethane, ethyl acetate, methanol, ethanol, and acetic acid. These trends can already explain the above-mentioned observations made by catalysis researchers, in the sense that the perturbation of initially established adsorption-desorption equilibrium is related to the nature of the solvent. 

Spectral sequences during the electrochemical adsorption and oxidation of 0.1 M ethanol in 0.1 M HCIO4 on Pt(lll) and Pt(lOO) are displayed in Fig. 29. In Table 2, the band assignments for the bands marked in the spectra are given. Acetaldehyde, acetic acid, and CO2 have been earlier reported from in situ FTIR results, as products of ethanol oxidation [76]. In addition to these soluble products, a gain of bulk perchlorate ions (band at 1110 cm ) can be observed in the spectra with increasing positive potentials. 

In an extensive series of papers, Tyurin and Fioshin, with various coauthors,have studied the supposed (see below) adsorption of a variety of organic compounds on the oxide films formed at Pt and Rh at high anodic potentials. Compounds such as acetonitrile, thiourea, thiazole, acetic, propionic and butyric acids, benzene, cyclohexane, and 3,4-diphenyl-2-(4-dimethylamino)-benzylidenehydrazono-l,2-dihydro-l,3-thiazole (on Rh were studied. 

On a pure Pf/C catalyst, Rousseau et showed that the electro-oxidation of ethanol at the anode of a DEFC working at 80°C mainly led to the formation of acetaldehyde, acetic acid and carbon dioxide, with chemical yields of 47.5%, 32.5% and 20.0%, respectively. By comparing the mass yield and the faradic yields, they concluded that no other products were formed in a significant amount. This result confirms that Pt is able to break the C-C bond to some extent In situ infrared measurements on ethanol adsorption and electro-oxidation at platinum electrodes have clearly shown that the adsorbed CO species are formed from 0.3 V vs RHE at the platinum surface moreover Iwasita and Pastor found some traces of CH4 at potentials lower than 0.4 V vs RHE. Previous studies showed that the initial steps of ethanol adsorption and oxidation on Pt can follow two different modes ... 

Only the first four reactant adsorption steps are assumed to be quasi-equilibrated (QE) they give H atoms and an acetate (Ac) species on the Pt surface based on the study of Vajo et al. [40], plus adsorbed acetic acid (A) and an acyl (Acy) species on the Xi02 surface. Steps 7, 10 and 13 are reversible and allow for the respective desorption of acetaldehyde, ethanol and ethane as products. Note that it is implicitly assumed that the ensemble of Ti and O atoms constituting the S sites can simultaneously coordinate a hydroxyl group. 

The latest developments in the issue are indicating that the view based on the H adsorption model is subject of some revision. In References 23 and 24 the voltammetric contribution of some specifically adsorbed anions (acetate, oxalate, chloride and bromide) was studied in the case of Pt(lll) electrodes by means of experiments involving the displacement of the adsorbed species by CO in acidic medium. The conclusion of this study was that the usual states correspond to the reversible adsorption/desorption of hydrogen, whereas the so-called unusual states would correspond to the adsorption/ desorption of anions. 

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