Adsorption of acetic acid

In this study, adsorption of acetic acid under voltammetric conditions was observed by a vibrational technique for the first time. The first work in the field was carried out using FTIR (potential difference infrared spectroscopy, PDIRS) and by radioactive labeling [Corrigan et al., 1988]. Both techniques... 

Corrigan DS, Krauskopf EK, RiceLM, Wieckowski A, Weaver MJ. 1988. Adsorption of acetic acid at platinum and gold electrodes— A combined infrared spectroscopic and radiotracer study. J Phys Chem 92 1596-1601. 

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... 

Adsorption of Acetic Acid onto Activated Charcoal... 

Experiment 3.1 Adsorption of acetic acid on to activated charcoal... 

It is obvious from the above results that adsorption of acetic acid, and, of course, presumably other carboxylic acids, is different in detail from one metal oxide to another and is perhaps also somewhat a function of whether adsorption occurs from gas or solution phase. However, in all cases acetate ions are formed and differences presumably reflect more subtle features of surface structure and chemistry. In general, there seems to be a correspondence between the frequencies reported by IR and IETS for IR active modes although intensity patterns are not similar, as one should expect based on the different mechanisms of vibrational excitation. Further work is obviously needed to define the differences between the two spectroscopies more exactly. 

Another interesting and different type of catalysis is involved in the catalyzed reconstruction of an indium oxide overlayer on indium. This study was alluded to earlier in the discussion of acetate ion species formed on indium oxide by chemisorption from several torr of acetic acid gas. At low partial pressures of acetic acid (<< 0.1 torr) the reversible adsorption of acetic acid catalyzes the reconstruction of a thin ( 10-15A), porous indium oxide overlayer to a defect-free (no pin holes) film as judged by pinhole sensitive tunnel junction measurements. Some clues as to the mechanism were obtained from IR plus Auger and electron loss spectroscopy as well as ellipsometry measurements. The overall process is shown in Fig. 8. This is an example where processes in the substrate themselves can be usefully catalyzed. 

Figure 2.5 Relative occupancy (%) of the intracrystalline volume of a H-BEA zeolite during the transformation of a 2 1 molar anisole - acetic anhydride mixture in a batch reactor, assuming no adsorption of acetic acid and full occupancy of the micropores. Anisole ( ), acetic anhydride (o) and 4-methoxyacetophenone (x). Reprinted from Journal of Catalysis, Vol. 187, Derouane et al., Zeolite catalysts as solid solvents in Fine Chemicals synthesis 1. Catalyst deactivation in the Friedel-Crafts acetylation of anisole, pp. 209-218, copyright (1999), with permission from Elsevier...

The activation energies for the dissociative adsorption of ethyl acetate and acetic acid were adjusted in the analysis. To be consistent with the DFT results shown in Table VIII, the activation energy for the dissociative adsorption of ethyl acetate was constrained to be less that the activation energy for the dissociative adsorption of acetic acid. The standard entropy changes for these steps were constrained such that the activated complexes were immobile species. The standard entropy and enthalpy changes to form the activated complex for step 2 were adjusted. 

The role of gold in the Pd/Au/K acetate catalysts is to stabilize the size of Pd crystallites and avoid sintering. The role of potassium acetate is to maintain the catalyst activity and decrease CO2 selectivity. Potassium acetate favours a strong adsorption of acetic acid on palladium, lowering the barrier to vinyl acetate formation. Gold by itself is inactive in the catalysis of vinyl acetate. Pd only catalysts produce vinyl acetate at much lower rates than the Pd/Au/K catalyst system and their activity decays rapidly. 

The pervaporation of water/acetic acid mixmres was also evaluated with ZSM-5 membranes by the Matsukata group [129] the initial values of the separation factors in a 50 wt% acetic acid aqueous solution were around 10 to 20 due to the preferential adsorption of acetic acid which decreased the amount of water adsorbed. To increase the amount of water adsorbed, a surface modification that consisted of an alkali treatment with NaOH was carried out. After the treatment, the water flux and separation factor increased markedly, reaching values up to 381 and 0.783 kg/m h, respectively. 

Figure 12 The adsorption of acetic acid on Pd(l 11) in the presence of a water solution. The binding energy for the acetate anion on Pd(lll) in the vapor phase is —212kJ/mol. The binding energy for acetate in the presence of a water solution is estimated to be +62kJ/mol. (Adapted from Ref. [77].)...

The dissociative adsorption of acetic acid therefore leads to (CH3COOH),/,q-(CH3COO)-... 

For the conversion of ethanol to acetic acid, a redox mechanism may be proposed [3]. Tin oxide catalyst being basic, activates oxygen strongly for the total oxidation. This may be weakened to a suitable strength by the addition of molybdenum [4]. Similarly, the incorporation of M0O3 into Sn02 modifies the acid-base properties as well as the surface area [5]. It seems that tin contributes to the increase of activity by the adsorption of oxygen and its supply to the surface reaction and molybdenum, contributes to the adsorption of acetic acid in the active form for this reaction. This catalytic activity is not due to a bulk compound but must be ascribed to a surface compoimd of two oxides. 

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