The Dehydroxylated Surface

The comparable number of diamagnetic groupings of another type, namely (sSi-0-)2Si<02>Si(-0-Si=)2 (SRs) (see Section 2.1 and Table 7.1), are also stabilized on the surfaces of RSi samples in addition to SCs. The absorption bands at 888 and 908 cm-1 [3,4] in a silica IR transparency window are connected with these groupings. Such centers were registered for the first time on the dehydroxylated surface of silica. Defects of the type considered are characterized by a high reactivity with respect to H20, CH3OH, and NH3 molecules [3,72]. At the same time, reliable information about the reactivity of SRs with respect to another molecule (e.g., H2(D2)) is actually lacking. The first part of this section is concerned with a discussion about physico-chemical properties of SRs. 

However, Agzamkhodzhaev and Zhuravlev21 showed that the dehydroxylated surface of silicas, treated in the range from 673 K to 1373 K, can be completely rehydroxylated by treatment with water at room temperature. 

From the tabulated data, it may be observed that the amount of desorbed molecules varies a little with pretreatment temperature. For the 473 - 673 K pretreated samples about 10% of the total loading is lost during curing. For the dehydroxylated silica (1073 K), a relative loss of 35% is observed. Similarly to the processes reported in the loading step, the behaviour of the dehydroxylated surface clearly differs from the other samples. This will be discussed below. 

As noted above, it is widely adopted that trigonal aluminum is one of the most important chemisorption and catalytic sites in aluminosilicates. Formation of these centers is usually associated with dehydroxylation. In the preceding section this concept was used to discuss different types of BASs formed as a result of water adsorption on the dehydroxylated surface of a model aluminosilicate fragment. The activity of the trigonal aluminum atoms was particularly manifested in the strong activation of the coordinatively bonded water molecules. In the chemical sense, such a site comprises a typical Lewis acid, which is also confirmed by quantum-chemical calculations. 

It is now apparent that isolated silanols have relatively low affinity for water. Thus, the hydrophobic nature of silica is manifested after dehydroxylation when only the siloxane bridges and some isolated silanols (giving an IR band at c. 3750 cm-1) remain. On the dehydroxylated surface the net adsorption enthalpy for water is negative. In this case, the enthalpy of adsorption is lower than the normal enthalpy of condensation. Application of adsorption microcalorimetry has allowed an assessment to be made of the relative extents of the hydrophilic and hydrophobic areas of the surface (Bolis et al., 1991). On the hydrophilic surface, it appears that water is adsorbed via two hydrogen bonds to two silanols - one acting as the hydrogen donor and the other as the acceptor. In the case of the weaker attachment to the isolated OH, the attachment involves one hydrogen bond. 

Aluminas. Again the active exchange sites are the surface hydroxyl groups, which now have a more basic character and will also exchange or react with anions. These surface hydroxyl groups can again be removed by thermal dehydroxylation or be deactivated by anionic replacement by anions of both minerals and organic acids. The dehydroxylated surface is more readily rehydrated and is fairly readily hydrolysed by mineral acids. 

Based on the preceding discussions of water adsorption on dehydroxylated surfaces, the most likely mechanism of rehydration of silicate surfaces dehydroxylated above 450°C is adsorption on acidic silicon sites contained in strained two- and three-membered rings, followed by dissociative chemisorption. Since two-membered rings comprise a small fraction of the silica surface, the rehydration kinetics will initially reflect the rate of dissociative chemisorption of three-membered rings which cover approximately one quarter of the dehydroxylated surface. Subsequent water adsorption occurs preferentially on silanols formed by hydrolysis of three-membered rings. 

In addition to altering the surface character with respect to physisorption, condensation reactions involving neighboring surface OH groups produce a distribution of cyclic species (rings) on the dehydroxylated surface. (See Fig. 3.) Investigations of the reactivity of such surfaces by Morrow and Cody [70-72] and more recently by Bunker and coworkers [29,30,73] and Brinker and coworkers [28,74] have shown that the kinetics of dissociative... 

A completely dehydroxylated surface consists essentially of an array of oxygen atoms the Si-0 linkages are essentially covalent so that the silicon atoms are almost completely screened by the much larger oxygen atoms. Such a surface represents the extreme case and, even on samples ignited at 1100°C, a minute residue of isolated hydroxyl groups will be present. 

The same surface species is obtained at ambient temperature by the reaction of Bu3SnH and the silanol groups, suggesting that the Sn-H bond is more reactive in this case than the Sn-C bond. The surface reaction depends upon the degree of dehydroxylation of the surface of silica. On silica dehydroxylated at 500°C the reaction leads to one well-defined surface complex. On the other hand, on silica dehydroxylated at 200°C, the evolution of alkane is continuous. The difference in the latter case is related to the presence of neighboring OH groups, because the number of the surface vicinal OH groups capable of... 

The effect of the surface area is far from being a simple one. It was shown for titania that when the surface area changes from 110 to 12 m2/g, the average time required for a complete mineralization of organic substrates increased from 40 to 75 and 50 to 75 min for salicylic acid and phenol, respectively [135], These results clearly show that textural properties, particularly the surface area, strongly affect the photoreactivity, although a high-temperature treatment improved their crystallinity [18], Therefore, this phenomenon may be explained only in connection with the catalyst surface dehydroxylation. 

The carbonyl cluster Rh,5(CO)i,5 was initially stable as such on the completely dehydroxylated alumina surface. But as soon as hydroxyl groups were generated (e.g., by adding traces of water) it decomposed to give various surface transformations. First, the cluster structure was dismpted, with breakage of the core cluster frame, into (Al-0-)(Al-0H)Rh (C0)2, Rh > monoatomic species sigma and n-bonded to the oxygens atoms of the alumina surface, with formation of molecular... 

Scheme 1.5 Silica, alumina and titania surface oxygens behaving as ligands in the M.L.H. Green formalism [9] after reaction of r -tris(allyl)rhodium with a partially dehydroxylated surface [39].

Mainly Fe aggregates form when highly dehydrated magnesia is impregnated with Fe3(CO)i2 or Fe(CO)5 and the resulting surface species are thermally decomposed conversely, when MgO is not dehydroxylated the thermal treatment generates Fe and H2 because of the electrophihc attack by surface protons on the carbonyhc surface species [73]. 

On an alumina support, independently of the cobalt carbonyl precursor used, complex cobalt sub-carbonyls compounds, [Co(CO)4] and hydrogencarbonate species formed [143, 149]. However, the reactivity of the alumina surface depends on the degree of hydroxylation highly hydroxylated alumina is more reactive against Co2(CO)g and facilitates decarbonylation, whereas dehydroxylated alumina favors the formation of high nuclearity species like [Co6(CO),5] , which would need higher temperatures than the initial Co2(CO)8 to be decarbonylated [149]. 

Two different approaches have been used to graft molybdenum on alumina, namely, either a two-step process involving gas-phase impregnation and further decomposition at high temperature (GPID) or the direct contact of [Mo(CO),5] vapor with the alumina support placed in a hot zone so as to achieve its decomposition. All of the relevant studies point to the existence of a close relationship between the OH group density on the support and the amount of deposited molybdenum as well as the chemical nature of such deposits. Hence, we successively deal with three types of alumina highly, partially and fully dehydroxylated surfaces. 

Importantly, under CVD conditions the temperature can be sufficiently high to induce dehydroxylation of the alumina surface, so that the mechanism could follow reaction routes other than the one observed when GPID is carried out. This phenomenon can be even more pronounced when experiments are performed under dynamic UHV. 

The tris-neopentyl Mo(VI) nitride, Mo(-CH2- Bu)3(=N) [134], reacts with surface silanols of silica to yield the tris-neopentyl derivative intermediate [(=SiO)Mo (-CH2- Bu)3(=NH)] followed by reductive elimination of neopentane, as indicated by labeling studies from labeled starting organometallic complex, to yield the final imido neopentylideneneopentyl monosiloxy complex [(=SiO)Mo(=CH- Bu)(-CH2 - Bu)(=NH)] [135]. The surface-bound neopentylidene Mo(VI) complex is an active olefin metathesis catalyst [135]. Improved synthesis of the same surface complex with higher catalytic activity by benzene impregnation rather than dichlorometh-ane on silica dehydroxylated at 700 °C has been reported [136],... 

Comparison of NMR data between these molecules (solution spectra) and the surface species obtained by the grafting of [W(=Ar)(=CH Bu)(CH 2Bu)2] on silica dehydroxylated at 200 °C (solid-state spectra) allowed the proposal of a grafting reaction sequence for the organometallic W(VI) precursor on the silica surface (Scheme 14.15). 

In the case of oxide catalysts or alkali metal-doped oxide catalysts, basic surface sites can be generated by decarboxylation of a surface metal carbonate exchange of hydroxyl hydrogen ions by electropositive cations thermal dehydroxylation of the catalyst surface condensation of alkali metal particles on the surface and reaction of an alkali metal with an anion vacancy (AV) to give centers (e.g., Na + AV — Na + e ). 

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