Titania cation exchange

Rh dispersion appears to be a key factor in the activity of zeolite-supported systems. In studies of the effect of Si/Al ratio in Rh/NaX (faujasite-type zeolite), prepared by cation exchange of NaX with [Rh(NH3)5Cl](OH)2, maximum carbonylation activity (ca. 8 mol/(gRh-h) at 200°C) correlates with maximum Rh dispersion in the prepared catalyst (48). Similarly, cation exchange of NaX with [Rh(NH3)5Cl]Cl2 has been reported to give carbonylation rates of ca. 1 mol/(gRh h) at 150°C (49), and NaY exchanged with rhodium salts (50) gives carbonylation activity of 0.4 mol/(gRh-h) at 170°C. These rates are claimed to be higher than those for Rh impregnated alumina, silica-alumina, silica, or titania (49,50). Optimum Rh dispersion corresponded to approximately two Rh atoms per vmit cell (50). 

Three different procedures were used to introduce vanadium into titania-pillared clays Two of them consisted, as in the case of Al-pillared samples, in cationic exchange of Ti-PILC and/or Ti-mt with vanadyl ions while one, referred to as "co-pillaring , employed a... 

Metameric Colours. Colours that appear to be the same under one type of illuminant but, because they have different spectral reflectivity curves, will not match under a different illuminant. Two white vitreous enamels may be metameric, for example, if one is opacified with titania in the anatase form whereas the other contains titania in the form of rutile. Methuselah. A 9-quart wine bottle. Methylene Blue Index A simple approximate method for assessing the surface area of a clay. O.OIM methylene blue dye is added to aqueous suspension of clay, in successive small amounts, to determine the least amount of dye required to impart colour to the water suspending the clay. Dye up to this amount is absorbed by the clay, so that this is a measure of the clay s cation exchange capacity (see ionic exchange). For most pottery clays this is proportional to the surface area to a good approximation. ASTM C837 specifies a test for clays. 

Although the direct reaction of titanium alkoxide with tetraalkylammonium hydroxide was a route to layered titania, another route has been investigated by Sasaki et al., Watanabe et al. This was an exfoliation of protonic lepidocrocite type layered ti-tanate, H Ti2 jc/4D /404-2H20, where represents a vacancy as shown in Figure 5-7. (Sasaki et al., 1997). The lepidocrocite type titanate was prepared by cation exchange of Cs Ti2 jc/4D /404 with HQ (Sasaki et al., 1995). Tetrabuthylammonium hydroxide... 

There are about 4-5 surface hydroxy groups per square nanometer on anatase and these are of two different types. One type is basic in nature and can be exchanged by F". This type of site is responsible for the adsorption of acidic species onto the surface and initiates the adsorption of base sensitive compounds such as Mo02(acac)2. The second type of surface hydroxide is acidic. It interacts strongly with base and initiates the adsorption of acid sensitive compounds such as Fe(acac)3.2 This amphoteric nature of titania is illustrated by the zeta potential versus pH curve shown in Fig. 9.9. Cationic catalyst precursers are adsorbed at pH values greater than 7-8 and anionic species from solutions having a pH less than about 5. 

Porous titania Anatas Br0nsted and Lewis acid and basic sites, cationic and anionic exchange groups, point of zero charge at pH 5, pJCa of Bronsted add sites 0.2-0.5... 

Ramis et al. [44] studied the effect of dopants and additives on the state of surface vanadyl species of vanadia on titania catalysts by means of FTIR spectroscopy. Additives such as alkali and alkali-earth metal cations (typically Cs, K, Na, Li and Mg), oxoanions (such as sulphates and arsenates), and other species (such as AP+, MoO +, and WO ), influence the position of V=0 stretching frequencies. The position of vy=o for a 3 wt% V2O5 on titania was observed at 1035 Two percent W or Mo did not show any shift of the stretching frequency of V=0, whereas Cs lowered the band position by 45 cm . This was explained in terms of the formation of strong basic sites and the exchange of Ti in 0=V-0-Ti by 0=V-0-Cs. The elements Al, S, and As shift the position of vv=o to higher frequencies. Oxoanions are coordinatively bond to vanadyl centers [44],... 

It should be emphasized that hydroxyls that are not acidic do not participate in ion exchange, contrary to what is sometimes suggested. Indeed, it has been reported that the hydroxyl groups on titania are unaffected after deposition of copper, cobalt, or nickel ions by a procedure similar to that used for ion exchange (563). To explain the observation, it was proposed that the deposited cations are located on coordinatively unsaturated oxygen ions. 

The photocatalytic activity of Ti02-montmorillonite in the reduction of with oxidation of triethanolamine (261) and the oxidation of several aliphatic alcohols have also been reported (259). The photodegradation of dichloro-methane, which is not readily degraded or hydrolyzed in an aquatic environment, to hydrochloric acid and carbon dioxide using titanium-exchanged montmorillonite, titania-pillared montmorillonite, and titanium-aluminum polymeric cation pillared montmorillonite has been reported (261). 

But the acid character can be observed with the ion exchange of the hydroxyl of titania with cations, for example. 

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