Adsorption of ethanol

Fig. 53 Adsorption of pentane vap>our at 273 K on a sample of nonporous rutile before and after modification of the surface by pre-adsorption of ethanol. Curve (A), unmodified surface curve (B), surface containing 52 pmol of ethanol. (After Parfitt.)...

The electrociiemical oxidation of ethanol has been extensively studied at platinum electrodes [22-34]. The first step is the dissociative adsorption of ethanol, either via an 0-adsorption or a C-adsorption process [25, 26], to form acetaldehyde (AAL) according to the following reaction equations. Indeed, it was shown by Hitmi ef al. [34] that AAL was formed at potentials lower than 0.6 V vs RHE. Thus ... 

Table 1.2 indicates that alloying platinum with tin led to important changes in the product distribution an increase in the AA chemical yield and a decrease in the AAL and CO2 chemical yields. The presence of tin seems to allow, at lower potentials, the activation of water molecules and the oxidation of AAL species into AA. In the same manner, the amount of CO2 decreased, which can be explained by the need for several adjacent platinum atoms (three or four) to realize the dissociative adsorption of ethanol into CO species, via breaking the C-C bond. In the presence of tin, dilution of platinum atoms can limit this reaction. The effect of tin, in addition to the activation of water molecules, may be related to some electronic effects (ligand effects) on the CO oxidation reaction [38]. 

On Pt-Sn, assuming that ethanol adsorbs only on platinum sites, the first step can be the same as for platinum alone. However, as was shown by SNIFTIRS experiments [37], the dissociative adsorption of ethanol on a PtSn catalyst to form adsorbed CO species takes place at lower potentials than on a Pt catalyst, between 0.1 and 0.3 V vs RHE, whereas on a Pt catalyst the dissociative adsorption of ethanol takes place at potentials between 0.3 and 0.4 V vs RHE. Hence it can be stated that the same reactions occur at lower potentials and with relatively rapid kinetics. Once intermediate species such as Pt-(COCH3)adsand Pt-(CO)ads are formed, they can be oxidized at potentials close to 0.3 V vs RHE, as confirmed by CO stripping experiments, because OH species are formed on tin at lower potentials [39, 40] ... 

The enthalpy changes for adsorption of acetaldehyde (step 3), ethanol (step 5), hydrogen (step 6), water (step 8), and acetic acid to form adsorbed acetate (step 9) were adjusted in the reaction kinetics analysis. The initial estimates of the heats of adsorption of acetaldehyde, ethanol, and hydrogen were obtained from the DFT predictions for these species on Cu(211) (Table VIII). The heat of adsorption of water was constrained to be equal to the heat of adsorption of ethanol in these analyses. The steps involving adsorption of ethanol, acetaldehyde, water, and the step in which acetic acid forms the surface acetate species were all assumed to be nonactivated. 

Silvestrelli PL (2004) Adsorption of ethanol on Si(100) from first principles calculations, Surf. Sci. 552 17-26... 

Senchenya et al. (96) have treated the adsorption of ethanol on a structural hydroxyl group (Fig. 14) using a CTP scheme and the CNDO/BW method. The separation of a molecule and cluster with respect to the z axis was optimized, the optimal values being r = 1.19 A and R = 1.28 A The adsorption energy was 23.2 kcal/mol, which was close to the experimental value (97). Note that this was essentially the two-point adsorption involving both acid and base sites. This case is quite similar to the above propylene adsorption (90). There is also no definite trend toward proton transfer from the hydroxyl group of a zeolite to the alcohol molecule. The carbocation state is also predicted to be activated. This, in turn, increases relative efficiency of the synchronous mechanism (with the same recommendation for its experimental examination). The estimation (96) of the energetics of the intermediate structures of the synchronous mechanism showed that such a mechanism is quite realistic. 

In contrast to water adsorption, the adsorption of ethanol is rather independent of the variety of starch.417 Like certain other molecules, ethanol has the effect of crystallizing starch and its components,696 an effect that is not linearly proportional to concentration. The intrinsic viscosity of aqueous... 

On the other hand, the absorption band due to a carbonyl group (at about 1725 cm ) displays, at high potentials, a lower intensity on PtSn (the absorption band located close to 1725 cm remains as a shoulder even at high potentials) than on Pt (Fig. 34b), which indicates that the formation of C2 species (presumably acetaldehyde) resulting from a non-dissociative adsorption of ethanol is lower on a PtSn catalyst. 

The adsorption of ethanol occurs at platinum by the carbon of the alcohol functional group. This adsorption involves C-H dissociation with the transfer of one electron (step 1). The adsorbed intermediate can desorb and leads to acetaldehyde with the transfer of another electron (step 2). Alternatively, a second hydrogen can be eliminated by a nucleophilic attack of an oxygen from H2O (step 2 ) and the resulting species is an adsorbed acetyl, " which can lead (step 4 ) to a (CO)ads poisoning intermediate and to an adsorbed methyl (CH3)ads, which is a reactive intermediate dtrring the negative sweep, and will desorb as CH4 into the gas phase. ... 

The adsorption of ethanol on Pd(l 11) in UHV was examined by Davis rmd Barteau [80]. At 170K, surface ethoxide (CH3CH2O) was observed. Annealing to 200 K transforms the ethoxide into acetyl, which degrades above 300 K to CO. 

CDPIO-Ag Suggest a rate law and mechanism for the catalytic oxidation of ethanol over tantalum oxide when adsorption of ethanol and oxygen take place on different sites. [2nd ed. P6-17]... 

Primary alcohols are dissociatively adsorbed on rutile TiO lllO) [43], TiO lOll) [44] and UOj(l 11) [45, 46] surfaces at 300 K. Figure 7.6 is an XPS Cls spectra of adsorbed ethoxide (CHjCH O) formed from the dissociative adsorption of ethanol... 

Fig. 7.6 XPS spectra of the Cls region after the adsorption of ethanol on a TiO CllO) single crystal at 300 K. The dots represent raw data while the solid line represents the convoluted curves. The peak at 286.5 eV corresponds to -CH O- group while the 285.0 eV peak corresponds to the -CHg group. The inset indicates the dissociative adsorption of ethanol to an ethoxide (CH CH O-) on a fivefold coordinated TP+ ion black small spheres) while the hydrogen is found on the twofold coordinated O ion gray large spheres)...

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). 

Figure 10,15. Possible mechanisms for the adsorption of ethanol and aniline by Mg -saturated layer silicate clay. Water molecules are depicted as shaded circles. (Adapted from B.K.G. Theng. 1974. The Chemistry of Clay-Organic Reactions. London Adam Hilger.)...

The effect of temperature on equilibrium for liquid adsorption is generally much more complex than that for pure vapor phase adsorption due, partially, to the involvement of a solvent in liquid adsorption systems (22). The amount of an adsorbate adsorbed in liquid adsorption may decrease or increase with increasing temperature, depending mainly on the difference of the heats of immersion for the adsorbate (solute) and solvent (22). In the present study, the saturation adsorption capacity n for ethanol, i-propanol and i-butanol are insensitive to temperature, while the parameter b for the three alcohols decreases as the temperature increases. Bui et al. (4) reported a similar temperature dependence of n and b for the adsorption of ethanol from an aqueous solution in silicalite. 

Hong, J., M. Voloch, M. R. Ladisch, and G. T. Tsao. "Adsorption of Ethanol-Water Mixtures by Biomass Materials," Biotechnology and Bioengineering, vol. 24, September 1981, pp. 725-730. 

The stop-effect, a drastic increase of the reaction rate when the feed concentration of a reactant is switched to zero, was studied for the dehydration of ethanol to ethylene on 7-alumina at 180 and 200°C. Two basic models exist in the literature to describe this phenomenon. They were discriminated on the basis of transient and periodic experiments, coupled with FTIR data of the adsorbed species. The model that best describes these measurements postulates the adsorption of ethanol on two different sites, S and S2, with a free S2 site being necessary for ethylene formation. 

The predicted results of models 1 and 2, calculated with optimum parameters determined on transient experiment, are compared to experimental results for the gas phase and the surface in Figures 5a and 5b, respectively. It must be pointed out that the periods invariance is not obtained simultaneously for ethylene and the surface ethoxides. The response of ethylene becomes reproducible after a different first cycle (not shown here), whereas the infrtu ed signal is not yet stable. Its value still increases during the following periods, due to a parallel slow adsorption of ethanol under a non-reactive form. This adsorption was also observed in the transient experiment, and consequently was included in the determination of a. Therefore the comparison between models and experiment can only be made on the last period in the case of the adsorbed species. 

The adsorption/desorption equilibrium constant for single-site adsorption of ethanol, with units of inverse atmospheres, is Fa = ai/ao-... 

The triethoxysilyl compounds Id,e with a silica specific coreactivity gave covalent coupling and the materials Ila-d were not extractable. The phenone la was found as a residue in the (Si02 + la) extract laut the car)3on value of the extracted silica was not decreased (probably by the adsorption of ethanol at refluxing). The i.r. spectrum of this silica was almost identical with a reference of untreated material Si 60. 

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. 

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