Titania-supported vanadia

The reactivity ot the titania supported vanadium oxide catalysts was probed by the methanol oxidation reaction. The oxidation ot methanol over the titania supported vanadia catalysts exclusively yielded tormaldehyde, 95%+, as the reaction product. The titania support in the absence ot surtace vanadia yielded dimethyl ether and trace amounts ot CO2 The almost... 

The influence of the specific oxide support phase upon the structure and reactivity of the surface vanadia species was also recently investigated.54 A series of titania-supported vanadia catalysts were synthesized over a series of Ti02 supports possessing different phases (anatase, rutile, brookite and B). Raman and solid state vanadium-51 characterization studies revealed that the same surface vanadia species were present in all the different V20/ri02 catalysts54. The reactivity of the surface vanadia species on the different oxide supports was probed by methanol oxidation and the TOFs are shown in Figure 6 (all the catalysts contained 1% V205)... 

This trend in the TOF values was found not to correspond with the variations in the strength of the terminal V=0 bond as measured by the respective Raman shifts (Banares, 1999 Wachs et al., 1996). Potassiumdoping of alumina-supported vanadia catalysts resulted in lower V = O frequencies, which indicated a weakened terminal V = O bond (Cortez et al., 2003). However, the propane conversion and the catalyst reducibility decreased. Therefore, it was not considered to be likely that the terminal V=0 bond is the active site for alkane ODH on supported vanadia. The same effect was observed for titania-supported vanadia. DFT calculations described a close interaction of potassium ions with both the supported vanadia and the titania support (Si-Ahmed et al., 2007 Lewandowska et al., 2008). Such an interaction leads to an elongated V=0 bond with a... 

It has been shown that titania-supported vanadia materials comprise a distribution of monomeric vanadyl, polymeric vanadates and crystalline vanadia, the amount of which is dependent on the vanadia content. 

The preparation and characterization of titania-supported vanadia catalysts have been reviewed by Bond and Flamerz Tahir and provide a guide to the literature on preparation, structure and catalytic properties of vanadium oxide monolayer catalysts [5]. Preparative methods such as grafting, heating mechanical mixtures, or coprecipitation are also discussed. 

With Raman spectroscopy it was possible to distinguish between vanadia adsorbed on silica or vanadia adsorbed on titania. It was concluded that a very narrow vv=o stretchings at 1040 cm and at 1030cm could be assigned to silica- and titania-supported vanadia, respectively. During preparation vanadium first covers the titania faces followed by interaction with silica. 

Lietti and Forzatti [41] have shown by means of transient techniques such as TPD, TPSR, TPR and SSR (steady state reaction experiments) that isolated vanadyls and polymeric metavanadate species are present on the surface of vanadia on titania catalysts with V2O5 loadings of up to 3.56 wt%. Polyvanadate species are more reactive than isolated vanadyls due to the presence of more weakly bonded oxygen atoms. It has been shown that titania-supported vanadia materials comprise of a distribution of monomeric vanadyl, polymeric vanadates, and crystalline vanadia, the amount of which is dependent on the vanadia content. 

Selective oxidation of 5-hydroxymethylfurfural to 2,5-furan-dicarboxaldehyde in the presence of titania supported vanadia catalysts... 

Titania-supported vanadia catalysts have been widely used in the selective catalytic reduction (SCR) of nitric oxide by ammonia (1, 2). In an attempt to improve the catalytic performance, many researchers in recent years have used different preparation methods to examine the structure-activity relationship in this system. For example, Ozkan et al (3) used different temperature-programmed methods to obtain vanadia particles exposing different crystal planes to study the effect of crystal morphology. Nickl et al (4) deposited vanadia on titania by the vapor deposition of vanadyl alkoxide instead of the conventional impregnation technique. Other workers have focused on the synthesis of titania by alternative methods in attempts to increase the surface area or improve its porosity. Ciambelli et al (5) used laser-activated pyrolysis to produce non-porous titania powders in the anatase phase with high specific surface area and uniform particle size. Solar et al have stabilized titania by depositing it onto silica (6). In fact, the new SCR catalyst developed by W. R. Grace Co.-Conn., SYNOX , is based on a titania/silica support (7). 

Table II summarizes the SCR data for these four titania-supported vanadia samples. The activities for 15V/T and 20V/T are similar on an overall mass and a vanadia content basis. These two samples contain mostly crystalline...

Figure 1. Raman data for titania-supported vanadia.

Figure 2. Raman data highlighting the vanadia region of titania-supported vanadia.

Effect of Vanadia in Titania versus Vanadia on Titania. Rate data in Table n show that the titania-vanadia aerogel, sample (V-T)10, is intrinsically more active than the titania-supported vanadia, sample lOV/T, on a vanadia content basis. This is despite the fact that in the case of (V-T)10 some of the vanadia probably remains in the bulk after heat treatment, and thus not available for catalysis. This result suggests that for an active SCR vanadia catalyst we may actually need vanadia species in close proximity with, and not necessarily supported on, crystalline titania. 

An early example is the work of Pereira and Beeckman [1989] and of Hegedus and Pereira [1990] who optimized the porous structure, more particularly the micropore diameter and the mean porosity of a titania-supported vanadia catalyst for the reduction of NOx by a mixture of NH3 and O2. Another example is given for hydrodemetallation by Keil and Rieckmann [1994]. The support of such a catalyst is manufactured by pelletizing powder of the support... 

We have previously studied the role played by the support (silica, alumina, magnesia and titania, both anatase and rutile) on the surface properties of vanadia (ref. 6-9), concluding that the interaction between the support and the supported phase, and hence their activity in the above mentioned processes, greatly depends on the difference in their basicities. The aim of the present paper is to insight in this study, analyzing the role played by a usual dopant (sodium) on the properties of alumina- and titania- supported vanadia, using two different methods to incorporate vanadia on the surface of the support, i.e., standard impregnation methods and mechanical mixture of the oxides, as we have observed (ref. 7 and 9) that some of the properties of the final... 

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