Ethoxide, adsorption

An alternate view regarding selectivity has been proposed for Si02-supported V2Os (20). In this proposal, ethane could be activated in two ways. One was by dissociative adsorption across a V=0 bond to form H—V—OC2H5. a- or -elimination of the ethoxide would result in acetaldehyde or ethene, respectively. The other way was by dissociative adsorption across a V—O— bond to form —OH and V—C2H5. The latter species would lead to combustion products. 

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. 

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). 

With a still stronger base, 0.1 M sodium ethoxide in ethanol, stiU higher adsorption values are obtained. It was shown [37, 42] that a quantity of ethoxyl groups is bound to the surface that is equal to the difference between NaOEt consumption and NaOH uptake (see Table 13.2). The conclusion is that sodium... 

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. 

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