Vanadia on titania

While vanadia- on titania-based catalysts can be used for both the classes of applications, there are other types of catalysts such as those based on copper [31b], which show good performances in case of mixtures of N0/N02 (nitric acid plants), while performances are worse when applied to emissions from catalytic processes. 

Same, plus Rh, with careful control of oxidising/reducing conditions Vanadia on titania with addition of NH ... 

Selective oxidation materials fall into two broad categories supported systems and bulk systems. The latter are of more practical relevance although one intermediary system, namely vanadia on titania [92,199-201], is of substantial technical relevance. This system is intermediary as titania may not be considered an inert support but rather as a co-catalysts [202] capable of, for example, delivering lattice oxygen to the surface. The bulk systems [100, 121, 135, 203] all consist of structurally complex oxides such as vanadyl phosphates, molybdates with main group components (BiMo), molybdo-vanadates, molybdo-ferrates and heteropolyacids based on Mo and W (sometimes with a broad variation of chemical composition). The reviews mentioned in Table 1.1 deal with many of these material classes. 

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. 

It is well known that the activity of SCR catalysts depends on the amount of vanadia present on the support [61]. Dispersion of vanadia is necessary in order to increase the number of catalytically active species. For instance, four layers of vanadia on titania exhibit an increase of two orders of magnitude in reaction rate with respect to a monolayer. Titania shows a strong interaction with vanadia. In order to decrease the influence of titania, silica is added to the support. It was found that vanadia on silica/titania catalysts are far more active catalysts than vanadia on silica and less active than vanadia on titania materials (see Table 5.5). 

Evidence for polymeric species of the metavanadate type in submonolayer vanadia on titania catalysts is obtained by the appearance of a Vy=o stretching mode at 870 cm [68]. The stretching mode of V=0 for isolated vanadyls is found at 1030 cm ... 

Lietti and Forzatti [69] 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. 

Buzanowski and Yang [76] studied the kinetics over both unpoisoned V2O5 on Ti02 and alkali poison-doped catalysts. The results for two unpoisoned vanadia on titania catalysts are given in Fig. 5.21. It was observed that the higher the vanadia load the higher the conversion. 

The main reaction rate is valid for NH3 and NO concentrations ranging from lO" to 10 mol and for water concentrations ranging from zero to 10 vol.-% [80]. The values for a, b, c and are given in Table 5.6 [80]. The influence of water on the reactivity of vanadia on titania is hardly described. 

Turco et al. [81] studied the influence of water on the kinetics of the SCR reaction over a vanadia on titania catalyst in more detail and found that water inhibits the reaction. The influence of water on the SCR reaction is largest at low temperatures (523-573 K) and low at 623 K. They considered a power rate law... 

Three types of vanadium-containing species are present at the surface of the vanadia on titania catalysts. Monomeric vanadyl, polymeric vanadates, and crystalline vanadia depending on the vanadia loading (see Fig. 5.19). Moreover, Bronsted acid sites and Lewis sites are present at the surface of vanadia on titania catalysts. All species are needed to explain the mechanism of the SCR reaction over this type of catalyst. The active sites are related to previously mentioned vanadia/vanadium species. A variety of active sites were proposed such as two adjacent V =0 groups or co-ordinated vanadyl centres [81,84-86] V =0 and... 

Fig. 5.25. The relative ESR signal intensity of in vanadia on titania catalyst as a function of the gas...

Recently, Schneider et al. [96] characterized the active surface species of vanadia on titania by means of diffuse reflectance FTIR. Their results supported the mechanism proposed by Janssen et al. [86]. From the signal intensities at 1435 and 1660 cm it was concluded that ammonia adsorbs on Bronsted sites [96]. The 1435 cm band was assigned to the n4(F) bending mode of ammonia adsorbed on Bronsted sites. The corresponding n2(E) feature was observed as a shoulder at 1660 cm . ... 

By-products were found over vanadia on titania and over chromia... 

In situ FTIR has been used to study the reaction steps and intermediates in vanadia on titania catalysts [91]. The changes in concentration of surface sites and adsorbed species of the working catalyst were also obtained. A relationship between the concentration of the Bronsted acid sites and the NOx conversion was found. Adsorption of ammonia results in two ammonia species. Bands at 3020, 2810, 1670, and 1420 cm are assigned to adsorbed NH4 species, whereas intensities at 3364,3334,3256,3170 and 1600 cm" are responsible for coordinated NH3. A mechanism was suggested in which initially, ammonia is adsorbed on forming an activated species. Then NO reacts with this species forirung V -OH, N2 and H2O. Subsequently the vanadia species is reoxidized by NO or O2 in order to obtain V =0. 

So far, no exclusive reaction mechanism is found for the SCR reaction over vanadia on titania catalysts which can explain all experimental observations. The Eley-Rideal mechanism, however, is favoured by many investigators. Ammonia is adsorbed on the surface on Bronsted or Lewis sites, as NHj ion or as an NH2 species, respectively. Then NO from the gas phase reacts with the ammonia species forming nitrogen and water leaving oxygen vacancies behind. Thus, lattice oxygen is involved in the reaction mechanism. These vacancies are then... 

Vanadia on titania catalysts prepared by wet impregnation (ammonium metavanadate) and monolayer catalysts prepared by grafting using vanadyl acetylacet-onate were compared [20]. It was demonstrated that monolayer catalysts show better activities at similar vanadium loadings than those of commercial catalysts. 

Grafting has been used for the preparation of vanadia on titania catalysts [24]. Vanadyl triisobutoxide was used as precursor. The vanadia species were well dispersed. It was observed by electron microscopy that silica was present as an amorphous phase. Moreover, the amount of titania present in the catalysts influences the specific surface area and the pore volume. Lower titania concentrations give rise to both higher specific surface areas and pore volumes. The most stable catalyst contained equimolar titania-silica or pure titania. 

By means of laser Raman spectroscopy two characteristic band were found at 996 and 285 cm for vanadia on titania catalysts which were assigned to V=0 groups [42]. A band at 1030 cm from a singly grafted catalyst was assigned to small vanadia clusters [25],... 

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],... 

Figure 4. First-order rate constants, A no (m g" s ), as a function of temperature for two vanadia-on-titania catalysts ...

The reaction rate of the reaction of NH3, NO and O2 over vanadia on titania catalysts at 365 K was 1.9x10 (molg s ) and the activation energy 37kJmol [48]. These data were obtained by using NO, a positron emitter, at very low concentrations (5 x 10 ppm). Moreover, it was confirmed that the nitrogen atom of ammonia combines with the nitrogen atom of nitric oxide. 

Figure 5. Measured (points) and predicted (lines) NO conversion rates over a vanadia on titania catalyst [NO], =[NH3], = 1000 ppm, [O2] = 4 vol. %, balance nitrogen (adapted from Ref. 50).

The adsorbed NH group could be hydrolyzed into NH3 and a V=0 group. Byproducts were found over vanadia on titania " N NO, and over chromia N2, Nz, and NzO. 

Square-channelled monolith or plate-type vanadia on titania catalysts are commonly used in SCR units of power plants. A typical monolith block (Fig. 16) has the geometrical parameters given in Tables 7 and 10. 

Beeckman and Hegedus [50] determined the intrinsic kinetics over two commercial vanadia on titania catalysts. A mathematical model was proposed to compute NO and SO2 conversions and the model was validated by experimental values. Slab-shaped cutouts of the monolith and powdered monolith material were used in a differential reactor. The cutouts contained nine channels with a length of 15 cm and with a channel opening and wall thickness of 0.60 and 0.13 cm, respectively. The SCR reaction over a 0.8 wt% V2O5 on titania catalyst was first-order in NO and zero-order in NH3. 

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. 

Hagenmaier and Mittelbach [150] used a vanadia on titania catalyst at 523 K. The activity decreased by 30% after 6000 h on stream. The catalyst was contaminated with Hg, Pb, Cd, As, and sulfates. Additionally, it was found that dioxines and furanes were oxidized over the catalyst (Section 1.2.5). 

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