Catalytically active sites titanium oxide

Niobium oxide and titanium oxide are also reduced in the presence of alcohols or hydrocarbons under UV irradiation (2,13,14,15). The changes in the absorption spectra and in the color of the oxides indicate the formations of Nb and Ti " " ion, respectively (2,14,15). The selective photoreduction described here can be utilized for the preparation of catalytically active sites on catalyst surfaces. 

Bonding modifiers are employed to weaken or strengthen the chemisorption bonds of reactants and products. Strong electron donors (such as potassium) or electron acceptors (such as chlorine) that are coadsorbed on the catalyst surface are often used for this purpose. Alloying may create new active sites (mixed metal sites) that can greatly modify activity and selectivity. New catalytically active sites can also be created at the interface between the metal and the high-surface-area oxide support. In this circumstance the catalyst exhibits the so-called strong metal-support interaction (SMSI). Titanium oxide frequently shows this effect when used as a support for catalysis by transition metals. Often the sites created at the oxide-metal interface are much more active than the sites on the transition metal. 

The presence of the framework titanium sites in the aluminophosphate molecular sieves (TAPO-5, -11, -31 and -36) were proved indirectly by the catalytic activity of these materials in the liquid-phase oxidation and epoxidation reactions by hydrogen peroxide. The incorporation of Ti(IV) centers in mesoporous hexagonal alnminophosphates was determined by catalytic activity in the oxidation of phenols at room temperature, where remarkable paraselectivity was achieved in TiHMA (188) and TAP (189). 

More complex microporous support compositions such as SAPO (a mixture of Si, Al, and P oxides) can also be substituted by metals such as titanium. Using hydrogen peroxide as the oxidant, such Ti-substituted SAPO molecular sieves marked catalytic activity in phenol hydroxylation reaction. However, using the same catalysts, only little catalytic activity was observed in the oxidation of organic substrates of low polarity, such as alkenes [71]. Correlations between the structure and catalytic activity of titanium sites and oxo-titanium intermediates are also described [72]. 

Recently, it is reported that Xi02 particles with metal deposition on the surface is more active than pure Ti02 for photocatalytic reactions in aqueous solution because the deposited metal provides reduction sites which in turn increase the efficiency of the transport of photogenerated electrons (e ) in the conduction band to the external sjistem, and decrease the recombination with positive hole (h ) in the balance band of Xi02, i.e., less defects acting as the recombination center[l,2,3]. Xhe catalytic converter contains precious metals, mainly platinum less than 1 wt%, partially, Pd, Re, Rh, etc. on cordierite supporter. Xhus, in this study, solutions leached out from wasted catalytic converter of automobile were used for precious metallization source of the catalyst. Xhe XiOa were prepared with two different methods i.e., hydrothermal method and a sol-gel method. Xhe prepared titanium oxide and commercial P-25 catalyst (Deagussa) were metallized with leached solution from wasted catalytic converter or pure H2PtCl6 solution for modification of photocatalysts. Xhey were characterized by UV-DRS, BEX surface area analyzer, and XRD[4]. 

Heat-flow calorimetry may be used also to detect the surface modifications which occur very frequently when a freshly prepared catalyst contacts the reaction mixture. Reduction of titanium oxide at 450°C by carbon monoxide for 15 hr, for instance, enhances the catalytic activity of the solid for the oxidation of carbon monoxide at 450°C (84) and creates very active sites with respect to oxygen. The differential heats of adsorption of oxygen at 450°C on the surface of reduced titanium dioxide (anatase) have been measured with a high-temperature Calvet calorimeter (67). The results of two separate experiments on different samples are presented on Fig. 34 in order to show the reproducibility of the determination of differential heats and of the sample preparation. 

Although the identification of tetrahedrally coordinated, tetra- and tripodal Ti4+ ions on the surface of titanosilicates, as the likely active sites in reactions that require Lewis acidity, seems convincing, the structure and role of the sites active in catalytic oxidation, presumably oxo-titanium species, formed by the interaction of H202 (or H2 + 02) with these surface Ti ions, are not clear. In recent years, this problem has been investigated by FTIR (133), Raman (39,40), XANES (46-48), electronic (54-57), and EPR (51-54) spectroscopies. This is one of the areas in which major progress has been made since the reviews of Notari (33) and Vayssilov (34). Zecchina et al. (153) recently summarized some of the salient features of this progress. 

Effect of sodium and aluminum on TS-1. The catalytic activities of aluminum and/or sodium containing TS-1 are depicted in Table IV. The data show that the addition of aluminum during the synthesis of TS-1 yields a material (TAS-1(D)) that has a lower conversion of n-octane oxidation and a smaller IR peak ratio. The existence of the acid sites due to the incorporation of aluminum into the framework of TS-1 may accelerate the decomposition of H2O2 to water and oxygen during the reaction. However, reducing the number of acid sites by exchanging with sodium ions only increases the conversion by 1% (Na/TAS-1(D)). Therefore, the addition of aluminum into the synthesis mixture most likely reduce the amount of titanium present in the sample. 

This species was proposed to play an important role in the catalytic activation of H202 on TS-1. IR spectroscopic investigations showed that a Ti(t 2-OOH) moiety oxidized small alkenes at room temperature in the dark (propene) or upon photoexcitation (ethene). These data appear to be the first direct detection of the active oxidation site in H202-loaded titanium silicalite (Lin and Frei, 2002). 

More promising from an industrial perspective, however, is the separation of the oxidation zone from the aqueous one effected by the catalytic material itself, through the selective adsorption of the reagents. The introduction of Titanium Silicalite-1 (TS-1), in which the hydrophobic properties of the pores protect the active sites from the inhibition of the external aqueous medium, was a demonstration of the concept. The catalyst, the substrate and the aqueous soluhon of hydrogen peroxide can, in this case, be mixed together, with a great simplification of the process and also a reduction of the hazards. Three commercial processes. 

The vanadium silicalites (with MFI and MEL stmcture) are active oxidation catalyst in gas and liquid phase reactions [180]. As for the titanium silicalites, only the ftamework associated vandium exhibits redox properties [181]. For example, in the hydroxylation of phenol, silicalite impregnated with vanadium compounds is catalytically inactive [182]. The catalytically active vanadium species is speculated to be located in non-tetrahedral positions, most probably chemically bound to the framework. Vanadium bound in that way is not extractable from the lattice [ 183]. A proposed stmcture of the vanadium site is schematically shown in Scheme 21. Note that the Si-O-V bonds are longer than the Si-O-Ti bonds and that V seems to be more exposed. The redox properties are affiliated with the changes in the oxidation state of vanadium between +IV and +V. Vanadium silicates with SiA ratios ranging from 40 to 160 have been reported and these high values suggest (in accordance with V MAS-NMR measurements) that the V sites are isolated in the lattice. 

The incorporation of Ti into various framework zeolite structures has been a very active research area, particularly during the last 6 years, because it leads to potentially useful catalysts in the oxidation of various organic substrates with diluted hydrogen peroxide [1-7]. The zeolite structures, where Ti incorporation has been achieved are ZSM-5 (TS-1) [1], ZSM-11 (TS-2) [2] ZSM-48 [3] and beta [4]. Recently, mesoporous titanium silicates Ti-MCM-41 and Ti-HMS have also been reported [5]. TS-1 and TS-2 were found to be highly active and selective catalysts in various oxidation reactions [6,7]. All other Ti-modified zeolites and molecular sieves had limited but interesting catalytic activities. For example, Ti-ZSM-48 was found to be inactive in the hydroxylation of phenol [8]. Ti-MCM-41 and Ti-HMS catalyzed the oxidation of very bulky substrates like 2,6-di-tert-butylphenol, norbomylene and a-terpineol [5], but they were found to be inactive in the oxidation of alkanes [9a], primary amines [9b] and the ammoximation of carbonyl compounds [9a]. As for Ti-P, it was found to be active in the epoxidation of alkenes and the oxidation of alkanes and alcohols [10], even though the conversion of alkanes was very low. Davis et al. [11,12] also reported that Ti-P had limited oxidation and epoxidation activities. In a recent investigation, we found that Ti-P had a turnover number in the oxidation of propyl amine equal to one third that of TS-1 and TS-2 [9b]. As seen, often the difference in catalytic behaviors is not attributable to Ti sites accessibility. 

While titanium substituted for antimony and this had a dramatic effect on catalytic activity as expected, there is a question as to how much of the uranium was converted from the +5 to the +6 oxidation state. The shifts in the infrared bands indicate a shortening of the bond distance and a lengthening of the Sb-0 bond distance which is consistent with an increase in hexavalent character, but the magnetic measurements show that a substantial portion of the uranium remained in +5 state. If the valence of uranium is not changed, then the replacement of Sb" by Ti must generate oxygen vacancies in the USb Oj Q lattice. It is these sites that may be responsible for the high activity of the promoted catalysts. 

In Table 2 the results for the reaction of propylene with hydrogen and oxygen over AuATiOj/Ti-MCM are presented. The most active sample was Ti/Si with a ratio of 1/100. From obtained data it can be concluded that for oxidation of propylene there exists an optimum amount of titanium in the catalyst. This may suggest that the density of the active sites influences the catalytic activity and PO selectivity. 

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