Silica dissociative chemisorption

Surface hydroxyl groups on an oxide can usually be replaced by other organic functional groups, thereby altering the polarity or hydrophobicity of the surface. One simple process involves the dissociative chemisorption of methanol on silica. 

On exposure to water, an anhydrous oxide can become hydrated by physical adsorption of water molecules without dissociation, dissociative chemisorption of water leading to new hydroxy groups, and finally to the formation of superficial oxyhydroxide or hydroxide, such as for MgO [14]. When silica groups are exposed to water for an extended time, their hydroxylation produces polymeric chains of -Si(0H)2-0-Si(0H)2 0H groups which can link up to form three-dimensional silica gel networks. Around 2 nm thick silica gel layers have been observed on silica surfaces prepared by evaporation of silica on mica which were exposed to humid air [70], Thus, it may be postulated that surface groups are present not only in a two-dimensional oxide-liquid interface, but also in a bulk phase of finite thickness extending from the surface into the interior of the solid [71]. 

This is a simple second order equation which usually applies to description of dissociative chemisorption from the gas phase on the homogeneous surface. However, Eq. (43a) describes well only an initial part of kinetic chemisorption isotherm for interaction of trimethylchlorosilane on dehydrated pyrogenic silica [113] and mixed alumina-silica and titania-silica [114] surface. At the same time, whole kinetic chemisorption isotherms are described using equations derived for the heterogeneous surface. Thus, high fractional reaction orders in respect to the silica surface OH groups obtained by Hertl and Hair for chemisorption of silanes and siloxanes [106,107] and in other studies [113,114] may be explained by heterogeneity of the oxides surface. 

Figure 12.10 Steps in the dissociative chemisorption of water at the tip of a silica glass. (a) Tip of crack with approaching water molecule h) chemisorption of water and its alignment (c) the breaking of an Si-O-Si bond and the formation of two Si-OH bonds.

This scheme summarizes the properties of the sites, that is, of a pair of silicon radicals associated with two anomalously reactive oxygen atoms — but provides little information concerning the geometry, apart from the requirement that the two silicon atoms must be closely spaced. Dissociative chemisorption of several molecules — that is, H2O, CH3OH, O2, etc. — takes place on dehydroxylated silica [116] as compared with nonpretreated surfaces, and this can be primarily associated with the presence of Si defects, as depicted by structure R. 

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. 

Reactions A-E which follow have been proposed by Brinker and Haaland [96] as possible schemes for surface nitridation via ammonolysis. Lewis acid adsorption (A) is a possible scheme for electrophilic metals capable of formally increasing their coordination numbers, e.g., trigonally coordinated boron or tetrahedrally coordinated aluminum. Lewis acid adsorption may be followed by dissociative chemisorption as in B. This scheme depends on the Lewis acidity of the metal site but does not necessarily involve a stable Intermediate with a formal Increase in coordination number. As discussed in the previous section, dehydroxylation of the silica surface at temperatures above 250°C progressively creates strained surface silicon species with enhanced Lewis acidity. The Importance of scheme B for silica is therefore expected to increase with the extent of suface dehydroxylation. Reaction C was proposed by Mulfinger [97] to account for dehydroxylation of silica... 

The role of boron in the dissociative chemisorption process (H) may be either to promote adsorption or dissociation (or both). Trigonally coordinated boron is a Lewis acid it can accept the lone pair electrons of nitrogen, promoting NHj adsorption (left-side of scheme H). Low and Ramasubramanian [102] have pointed out that B-O-Si bonds are more easily ruptured than Si-O-Si bonds, thus promoting dissociation (right side of scheme H). The combined effects result in increased nitridation compared to pure silica gels. 

Mahadevan TS, Garofalini SH (2009) Dissociative chemisorption of water onto silica surfaces and formation of hydronium ions. J Phys Chem C 112 1507... 

Chemisorption, consisting of a chemical reaction confined to the solid surface, does involve rearrangement of electrons of both adsorptive molecules and surface atoms, yielding new surface terminations. Adsorptive and adsorbate being chemically different species in dissociative chemisorption, spectroscopic and/or ab initio modeling methods are required to assess the nature of surface species formed upon contact of the adsorptive with the reactive surface atoms [25, 26, 29]. Further, chemisorption is structure-sensitive in that the features of the process depend on the solid crystal structure (see for instance anatase vs. rutile, [56, 101] and amorphous silica vs. crystalline quartz, [15, 85, 102]) and on the crystal faces exposed by the solid material [103],... 

Reactions of organosilicon compounds with active sites of silica surface proceed via SeI mechanism [4,57]. Nevertheless, it has been established that, apart from the occurrence of chemisorption by the Ssi mechanism, dissociation of silica surface siloxane bonds occurs by the Aai mechanism at interaction of such compounds, as (CH3)3SiX (X = N3, NCS and NCO) [58-61]... 

Using Eq. (73) for chemisorption of the (CH3)3SiX compounds on silica surface, the difference in activation energies of the reactions occurring by the Ssi and Abond dissociation energies in the reactants and products... 

The rehydroxylation of a wide-pore silica gel sample calcined in air at 850 °C and held in water at 100 °C for periods covering 1 to 100 h was found to take 5-10 h for complete rehydroxylation. These and other results indicate that rehydroxylation of dehydroxylated silica (calcined at >400 °C) in the presence of water requires considerable energy to activate the process of dissociative adsorption, Ed. Chemisorption of water appears to take place, resulting in the formation of hydroxyl groups bound through... 

Extensive studies of hydrocarbon chemisorption have been made by Eischens and Pliskin (1). In a series of studies on olefins and paraffins chemisorbed on silica-supported nickel they were able to show that both associative and dissociative adsorption could occur, depending on catalyst pretreatment. Associative chemisorption of olefins is observed when hydrogen is left on the nickel surface dissociative absorption, when the hydrogen has been pumped off at an elevated temperature before chemisorption. The associative mechanism is deduced from the fact that the only absorption bands found when ethylene is added to a hydrogen-covered surface are in the C-H stretching region characteristic of saturated hydrocarbons, and that a C-H deformation band at 1447 cm-1 characteristic of two hydrogens on a carbon is also observed. 

It has been shown that hydrogen chemisorption on cobalt is activated and that the extent of activation depends upon the support and metal loading [1,3,41], This is due to an increase in the activation barrier for dissociative adsorption if the temperature is lowered and/or the degree of metal-support interaction is increased. This is in turn controlled by the preparation method and conditions. It is expected that, contrary to supports such as alumina and silica, the presence of zero-valent cobalt in the catalyst will be favorable due to the inertness of the carbon support [5]. It has been observed that the adsorption temperature for maximum hydrogen uptake is 423 K if cobalt (10 wt %) is supported on activated carbon [1]. For these reasons, die H2 chemisorption experiments in the present case were performed above this temperature, i.e., 493 K. 

This section summarizes the investigations dealing with purely physical adsorption of PH3 on adsorbents like activated carbon or silica gel, dissociative adsorption on various adsorbents which can already occur at low temperatures, and irreversible chemisorption involving the decomposition of PH3 on certain surfaces at higher temperatures. The products formed by PH3 reacting with the surface atoms were generally not studied. This section also includes studies dealing with the coadsorption of PH3 with Hg, Dg, O2, HgO, DgO, or CO on various surfaces. 

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