Anatase, surface groups

The authors assume that the activation of Mg(OH)2+Ti02 (anatase) mixture involves the transition of proton from OH groups on anatase surface to the hydroxyl group of Mg(OH)2 which results in the formation of water and Mg-O-Ti bond, the latter being a nucleus for the formation of MgTiOj phase during consequent calcination and agglomeration. 

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. 

Figure 3. The anatase surface shows zigzag chains of Ti surface octahcdra. Each Ti site carries a =TiOH ligand that may bind adsorbed metal centers. The two surface groups to the right represent a bidentate (above) and a tridentate surface complex (below).

The most commonly employed titania modification in catalysis is anatase, whereas there are fewer applications of rutile. In many reports, mixed ana-tase-rutile samples were investigated. In sum, at least 12 types of OH groups on the anatase surface have been proposed by different authors (115). The most frequently reported hydroxyls are characterized by IR bands around 3720-3715 and 3675 cm (27,28,115,269,272-274,330,357,504,651, 678,679). These two bands are well discemable after evacuation at a temperature of 673 K. A band around 3640 cm is also often reported and characterizes less stable hydroxyls (230,273,356,504,651). For a Degussa P25 sample (containing rutile), an additional band at 3662 cm was reported (273). An increase in the evacuation temperature results in a complex spectrum containing a set of low-intensity bands. 

If efficient active sites shall be created by stabilizing the molecular species at the surface of a well-defined phase, as in the case of V0x/Ti02 - anatase catalysts, the better activity in NO SCR was achieved by cocondensation of the dopant alkoxide with the surface groups of preformed colloidal particles of the matrix precursor, followed by gelation [76]. A similar approach of modifying the matrix precursor sol with dopant molecules yielded well-dispersed tantalum species at the silica surface, efficient in selective oxidation of pyrimidine thioether [77]. 

Further chemical evidence for the existence of hydroxyl groups on anatase was obtained by Herrmann (305). Anatase made by flame hydrolysis of TiCl4 was used. Its surface area was 60 m /gm. The... 

As Smith (300) has shown by infrared spectroscopy, carboxylic acids are adsorbed either by hydrogen bonding of the carboxyl group or by proton transfer to the surface. Carboxylate absorptions were observed in the spectra. Very likely O " or OH ions acted as proton acceptors although no OH absorption bands could be detected after carboxylic acid adsorption. The isoelectric point of pure anatase is near pH 6.6 (305). 

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