Acetic acid, dissociative adsorption

Acetic acid chemisorption has been previously studied using IETS by Lewis, Mosesman and Weinberg for oxide covered aluminum surfaces. Using reflection IR Tompkins and Allara have reported spectra for adsorption on oxidized copper and Hebard, Arthur and Allara S for adsorption on oxidized indium. All these studies demonstrate that chemisorption from the gas phase involves proton dissociation since the observed spectra are those of acetate ion species. 

The standard entropy and enthalpy changes for the dissociative adsorption of ethyl acetate (step 1), acetic acid (step 7), and the hydrogenation of adsorbed acetaldehyde to form ethoxy species (step 4) were constrained to maintain thermodynamic consistency for the appropriate overall gas-phase reactions. 

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 adsorption of formic acid and acetic acid leads to the formation of car-boxylate groups on aluminas (194, 295-299), titanium dioxides, (134, 135b, 176, 194, 300, 301), chromium oxide (134, 302, 303), zinc oxide (298, 304-306), and magnesium oxide (299, 304, 306). The corresponding dissociative chemisorption step most probably takes place on acid-base pair sites of the type... 

Photoreactions of organic compounds over model surfaces of wide band-gap oxide semiconductors have received considerable attention recently [43, 79-82]. The most-studied photocatalytic reactions on rutile TiO lllO) single-crystal surfaces include ethanol [43], acetic acid [78], trimethyl acetic acid [80, 81], and acetone [82]. In this section, we will focus on the photoreaction of ethanol over TiOj(llO). Ethanol is dissociatively adsorbed via its oxygen lone pair on fivefold coordinated Ti atoms to produce adsorbed ethoxide species (Fig. 7.6). STM studies of the adsorption of ethanol on TiO2(110) demonstrated the presence of both alkoxides and surface hydroxyls [83] confirming the adsorption is dissociative. Figure 7.11 is the XPS Cls spectra after the exposure of ethanol (9=0.5 with respect to Ti atoms). 

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

It takes place between 160 and 180°C, at between 0.5 -and 0.8.10 Pa absolute, in the presence of a supported palladium base catalyst, on which ethylene and acetic acid arc activated by dissociative adsorption thereby becoming capable of combining to form vinyl acetate. 

The aqueous solution layer that forms at the metal interface can ultimately provide a medium for the dissolution of Pd ions or oxidized Pd clusters into the supported liquid layer where they can then act as homogeneous catalysts. As was discussed earlier, the acetoxylation of ethylene can be carried out over various Pda,OAcj, clusters where alkali metal acetates are typically used as promoters. DFT calculations were carried out on both the Pd2(OAc)2 and Pd3(OAc)e clusters in order to examine the paths that control the solution-phase chemistry. The Pd3(OAc)e cluster is the most stable structure but is known experimentally to react to form the Pd2(OAc)2 dimer and monomer complexes in the presence of alkali metal acetates. The reaction proceeds by the dissociative adsorption of acetic acid to form acetate ligands. Elthylene subsequently inserts into a Pd-acetate bond. The cation is then reduced by the reaction to form the neutral Pd°. The reaction is analogous to the Wacker reaction in which ethylene is oxidized over Pd + to form acetaldehyde. Pd° is subsequently reoxidized by oxygen to form pd2+[35,36,44]... 

From these results it appears that the addition of tin to platinum greatly favors the formation of acetic acid comparatively to acetaldehyde. This can be explained by the bifunctional mechanism [17] where ethanol is adsorbed dissociatively at platinum sites, either via an 0-adsorption or a C-adsorption process [9], followed by the oxidation of these adsorbed residues by... 

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