Hydroxyl groups titanium dioxide

Ganichenko and Kiselev (303) and Ganichenko et al. (304) studied the adsorption of water and determined the corresponding heats of adsorption. They concluded that hydroxyl groups exist on titanium dioxide even after outgassing at high temperatures. 

The chemical nature of the trapped holes has not been clearly clarified yet. Older reports assume that the holes are trapped at the titanium dioxide surface in adsorbed hydroxy groups yielding weakly adsorbed hydroxyl radicals (reaction (7.6)) [14,15]. 

Although the surface models for anatase and rutile, as proposed by different authors, are idealized and differ from each other in details, it can certainly be concluded that coordinatively unsaturated Ti4+cations, O2- ions, and OH groups in widely varying configurations should be exposed on partially hydrated and/or hydroxylated surfaces. Depending on the local environments of these sites, a wide spectrum of possible intermolecular interactions should be the consequence which may render specific adsorption processes possible. Finally, the ease of the surface reduction of titanium dioxides due to hydrocarbon contamination (19) leads to the formation of new types of surface sites and to drastic changes of the surface properties. 

The substantial parameter at the modeling of the electric double layer at metal oxide-electrolyte solution interface is a number of the hydroxyl group per surface unit of the oxide. For the titanium dioxide, although different crystalline faces form the surface [rutile 60% of the surface is formed by the face (110) whereas for anatase by (001)] the same density 12.8 of —OH group/nm2 is assumed [28]. That results from the very similar intersection of the elementary cells of the mentioned face, which have the highest density of the atoms in both oxides. 

Methanol decomposes on titanium dioxide surfaces by mechanisms that are similar to those by which formic acid decomposes. Methanol can reversibly adsorb on single crystal surfaces of titania (reaction 16) in a molecular state, or it may dissociatively adsorb by interaction with surface lattice oxygen anions, forming a surface methoxide (reaction 17). Reaction (18) represents the disproportionation reaction of hydroxyl groups on the surface of the metal oxide. 

The doubly-bonded (bridged) OH groups, formed according to Eq. (2), become thus Bronsted acid centers, while the singly-bonded (terminal) hydroxyls are expected to exhibit a predominantly basic character A large amount of reliable data regarding the interaction of water with titanium dioxide, are provided by infra-red spectroscopic studies associated with temperature-programmed desorption measurements. 

The essential point arising from the above discussion is that the saturation coverage of titanium dioxide with the surface-bonded peroxo species, photogenerated in alkaline or neutral solutions, ranges most likely from 4 to 5 peroxo groups per nm . The latter value is close to the total number of active OH groups present initially on each nm of fully hydroxylated (and unilluminated) anatase surface. 

Herrmann, M. and Boehm, H.-P. (1969). On the chemistry of the titanium dioxide surface — II. Acidic hydroxyl groups on the surface (in German). Z. Anorg. Allg. Chem., 368, 73-86. 

Titanium tetrachloride is a very reactive molecules which will readily hydrolyze to yield titanium dioxide. Its reaction with the hydroxyl groups on silica has been studied extensively and it is generally assumed that it can react monofunctionaUy with single SiOH groups or bifunctionally with pairs of hydrogen bonded SiOH... 

The semiconductor most frequently used is undoubtedly titanium dioxide, produced in large amounts as a low cost pigment. The photocatalytic activity of anatase, rutile and brookite polymorphic modifications of Ti02 is affected by several factors, such as the crystalline structure, the surface area, the particle size distribution and the density of surface hydroxyl groups. Although the positions of valence and conduction bands of both anatase and rutile are positive enough to allow the oxidation of many organic molecules, anatase... 

Titanium dioxide nanoparticles are the most attractive because of the developed surface of titanium dioxide, the formation of surface hydroxyl groups with high reactivity resulted from reacting with electrolytes as crystallite sizes decrease down to lOOA and lower, and a high efficiency of oxidation of virtually any organic substance or many biological objects. 

For the case of the particular grade of titanium dioxide tested, a hydroxyl head group did not bond at all to the filler surface as no change in viscosity was noted compared to the case with no dispersant present (denoted none ). In contrast, carboxylic acid and succinic anhydride head groups were very effective at bonding surfactant to the particles and resulting in 1000-fold drop in viscosity. The best three dispersant head groups are listed for each filler (Table 22.1). 

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