Vanadia-supported catalysts

Georgiadou, I. et al.. Preparation and characterization of various titanias (anatase) used as supports for vanadia-supported catalysts, Colloids Surf. A. 98, 155, 1995. 

It should be noted that NMR spectra of the samples treated at 500°C after UHIG procedure are similar to those obtained for vanadia supported catalysts prepared by... 

The replacement of vanadia-based catalysts in the reduction of NOx with ammonia is of interest due to the toxicity of vanadium. Tentative investigations on the use of noble metals in the NO + NH3 reaction have been nicely reviewed by Bosch and Janssen [85], More recently, Seker et al. [86] did not completely succeed on Pt/Al203 with a significant formation of N20 according to the temperature and the water composition. Moreover, 25 ppm S02 has a detrimental effect on the selectivity with selectivity towards the oxidation of NH3 into NO enhanced above 300°C. Supported copper-based catalysts have shown to exhibit excellent activity for NOx abatement. Recently Suarez et al and Blanco et al. [87,88] reported high performances of Cu0/Ni0-Al203 monolithic catalysts with NO/NOz = 1 at low temperature. Different oxidic copper species have been previously identified in those catalytic systems with Cu2+, copper aluminate and CuO species [89], Subsequent additions of Ni2+ in octahedral sites of subsurface layers induce a redistribution of Cu2+ with a surface copper enrichment. Such redistribution... 

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... 

Typical catalysts for SCR include supported vanadia, and iron or copper supported on zeolite. Here the application of a model to the design and understanding of vanadia catalyst systems is presented. Over the vanadia-based catalyst system, a Rideal-Eley approach has been adopted by most workers in the field, in which the first step is ammonia adsorption on the catalyst. This stored ammonia can then either react with NOx or be desorbed. Some important contributions to the SCR modelling literature are Andersson et al. (1994), Lietti and Forzatti (1994), Dumesic et al. (1996), Lietti et al. 

Oyama and Somorjai have studied the oxidation of ethanol and ethane over vanadia supported on silica.33 Ethane oxidation yielded ethylene and acetylene with considerable CO2 for highly dispersed catalysts. Ethanol oxidation to acetaldehyde, on the other hand was shown to be structure insensitive. Conversions and selectivities for reaction of ethanol are shown in Table III.33... 

Quantificahon greatly aids the understanding of the catalytic contributions of different vanadia species during catalytic reactions. For example, our preliminary activity test over these supported catalysts showed that the I.2V/6-AI2O3 sample exhibits better stability than higher loading catalysts for butane dehydrogenation in dilute feed [57]. The explanation is that the monovanadate species (ca 50% on the surface) dilute the polyvanadate species so that the two-dimensional coke species responsible for catalyst deactivation are less likely to form [40, 57]. 

There is growing interest in the partial oxidation of the C5 fraction of the hydrocarbon stream from naphtha steam crackers since there is no real market for them at the present time. Furthermore, the partial oxidation of lower alkanes and alkenes continues to pose challenging problems for catalysis researchers. In the case of C5 hydrocarbon oxidation to form phthalic anhydride, the challenge is even greater since the catalyst needs not only to insert oxygen selectively, but also promote the formation of C-C bonds in an oxidative medium. In recent years, several studies have been reported in the literature, focusing on Cg oxidation using catalysts such as supported vanadia, VPO catalysts, and molybdates [1-11]. ... 

A vaguely defined metal-support interaction has frequently been used to describe modifications of metal properties observed when oxide-supported catalysts are thermally treated. After the original report by Tauster et al. (13) on the SMSI effect in Ti02-supported catalysts, the same authors (32) extended their operational definition to other reducible oxides. Consequently, several investigations were conducted using reducible oxide supports such as vanadia (87, 106, 154-156), niobia (157-162), or ceria (95). In general, the same characteristic features of Ti02 were obtained for these oxides, i.e., suppression in H2 and CO chemisorption capacity, suppression of catalytic activity for several reactions, promotion of the CO/H2 reaction, and reversibility by oxidation. 

Vanadia supported on silica(with titania) promoted by Fe and CTu oxides was studied for SCR of NOx by Bjorklund et al.28 Both Fe and Cu (as oxides) enhanced the activity however, the resistance to deactivation by SO2 differed as the Fe-promoted catalyst became slightly more active with time on stream and the Cu-promoted catalyst decreased in activity to less than half the initial activity with time on stream. The activity for SCR was related to the concentration of as inferred from the solid electrical conductivity. In this case different promoters were shown to change dramatically the ability of a potential poison to deactivate the SCR catalyst Nikolov, Klissurski, and Hadjiivanov29 also studied the deactivation of vanadia/titania. A combination of ESCA, XRD, and IR were employed to characterize the surface and bulk compositions. They concluded that deactivation involved the transformation of the active anatase titania to inactive rutile. Further, there was a concomitant decrease in total surface area and a loss of phosphate promoters for the selective oxidation of xylenes to phthalic anhydride. 

In recent years, much attention has been devoted to the oxidative dehydrogenation (ODH) of light paraffins [1] and alcohols to aldehydes [2]. Among all the explored catalysts, supported vanadia-based catalysts have been seen promising both in terms of activity and selectivity. A large number of factors may determine their catalytic performances such as (i) the nature,... 

Also for oxidation reactions, the choice of the alumina support mainly depends on two criteria the stability of the phase at the reaction temperature and the reactivity (or better the lack thereof) toward feed components and products. For example, ethylene oxychlorination to ethylenedichloride is performed at approximately 220-250° C and 5-6 atm in the presence of a y-Al203-supported catalyst, which has a surface area of from 100 to 200 m and contains 10wt% CUCI2 and 3 wt% KCl, (423,424). Another example is a process called ammonia selective oxidation (ASO, or also selective catalytic oxidation, SCO), which converts small amounts of NH3 from waste gases to N2 at reaction temperatures of 150—300 °C. The process is used to abate the ammonia sHp after a selective catalytic reduction process with ammonia or urea in diesel-engine-exhaust after-treatment (425). The patented catalyst consists of Y-AI2O3 (60—300 m g ) loaded with 0.5-4 wt% platinum and 0.5—4 wt% vanadia and is coated onto the surface of a ceramic or metallic monoftthic structure (426). 

Finally vanadia catalysts are used extensively for oxidations of aromatic hydrocarbons. With bei zene, the mechanism for ring breakage is not well defined, and the desorption of maleic anhydride itself appears to be ratecontrolling. For the oxidation of o-xylene, the use of supported vanadia-titania catalysts limits ring cleavage. A well-defined major product sequence, o-xyltne o-tolualdehyde o-toluic acid phthalide phthalic anhydride... 

Early catalysts reported to be active for the ODH of butane were based on the molybdates of Mg, Mn, Fe, Co, and Ni. Selectivities of 52% to butadiene at a conversion rate of 27% were achievable with the Mg molybdate. Rung and coworkers (131) also reported that V-Mg-0 catalysts are also useful for this reaction. Vanadia supported upon Mg-Al mixed oxides have similarly received considerable attention (155). 

Some of the most active catalysts used in the Selective Catalytic Reduction (SCR) processes to remove nitrogen oxides (NOx) from exhaust gas streams are those based on vanadia supported on titania [1]. In order to avoid the problems associated with pressure drop and diffusional limitations, encountered with conventional peletted catalysts in forms of cylinders and spheres etc., the supports should ideally be configured as honeycomb monoliths for these reactions which normally take place with high space velocities due to the large volumes of gas to be treated [2]. However, the difficulties encountered in the preparation of monoliths based solely on titania makes the inclusion of binders to both improve the rheological properties of the paste prior to extruding and the soundness of the monolith with subsequent thermal treatment a necessity [3]. 

From these results it may be observed that the inclusion of sepiolite in the monolith composition stabilised the anatase phase. This was important since in practice vanadia supported on titania is more active when the titania is anatase than when it is rutile. Thus, the stabilising effect of the inclusion of sepiolite on the anatase was beneficial for the activity of any vanadia catalyst supported on these mixed composition materiais. 

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