Titania-supported vanadia catalysts

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)... 

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. 

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). 

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... 

Vanadia-titania ( 5 and other supported vanadia catalysts (9) can also be applied for the production of aromatic nitriles by ammoxidation of toluene and of the three xylene isomers allumina-supported V-Sb-based oxides seem to be the best catalysts (10). Detailed kinetic studies of toluene ammoxidation have been reported recently using different vanadia-titania catalysts ( 77,72). Ammonia inhibits toluene conversion, while benzonitrile yields (up to 80 % near 610 K) are mainly limited by... 

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... 

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. 

J. Keranen, C. Guimon, E. Liskola, A. Auroux, L. Niinisto, Atomic layer deposition and surface characterization of highly dispersed titania/silica-supported vanadia catalysts,... 

It has also been proposed that methanol adsorption and its oxidation to formaldehyde occurs at coordinatively unsaturated sites, possessing four-fold coordination, rather than coordinatively saturated sites, possessing six-fold coordination [19]. Unfortunately, the surface vanadia species predominantly possess four-fold coordination which prevents this issue from being address with the current data. However, supported molybdena catalysts possesses both four-fold and six-fold coordination and their TOFs for methanol oxidation have been measured [27]. It was found that, contrary to above hypothesis, the coordinatively saturated surface molybdena species is approximately four times more active than the coordinatively unsaturated molybdena species for titania supported molybdena catalysts. Thus, methanol oxidation proceeds on both coordinatively saturated and coordinatively unsaturated sites at relatively comparable reaction rates (TOFs). 

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. 

The commonly used catalyst today is a vanadia on a titania support, which is resistant to the high SO2 content. Usually the titania is in the anatase form since it is easier to produce with large surface areas than the rutile form. Several poisons for the catalyst exist, e.g. arsenic and potassium. The latter is a major problem with biomass fuel. In particular, straw, a byproduct from grain production, seems to be an attractive biomass but contains potassium, which is very mobile at reaction tern-... 

Both titania and titania/silica supported vanadia, molybdena, tungsta and chromia have been applied as SCR catalysts. Low-temperature and high temperature catalysts have been developed. The vanadia on titania catalysts have received most attention. 

To conclude this section, there is a need for a better understanding of the unusual CO-H2 synthesis properties of metal/ titania catalysts and related systems such as metal/niobia. The primary question to be answered in this regard concerns the stability of reduced titania in the CO-Hg system. The fact that several reducible oxides (titania, niobia, vanadia, MnO, etc. ) have been found to impart unusual CO-Ho synthesis properties to supported metals suggests that support-reaucibility is an important factor that is not cancelled by the CO-H2 reaction environment. 

Wet impregnation is not feasible for all catalyst types. For instance, vanadia on silica cannot be prepared by means of wet impregnation. With homogeneous deposition precipitation (HDP), however, it is possible to prepare vanadia on silica catalysts [21], The principle of this method is to use a lower valence of the metal, which may be produced by cathodic reduction of the respective metal ion. The reason for using lower valence state metal ions is the lower acidity compared to that of the higher valence state and the higher solubility of the metal ions. This technique was used for the preparation of silica-supported vanadia, titania, and molyb-dena catalysts [22, 23]. 

Kasaoka et al. [102] prepared vanadia catalysts supported with titania, activated carbon, and a mixture of carbon and titania, as supports for the simultaneous removal of SO2 and NOv at temperatures ranging from 400 to 425 K. The vanadia on titania catalyst was most appropriate. SO2 from flue gas is oxidized to SO3 and forms sulfuric acid. Ammonia reacts with sulfuric acid forming (NH4)2S04 and NH4HSO4. The catalysts were regenerated with water after treating the catalysts with gaseous ammonia to neutralize the acid sites on the catalyst. 

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