Vanadia-titania

Active heterogeneous catalysts have been obtained. Examples include titania-, vanadia-, silica-, and ceria-based catalysts. A survey of catalytic materials prepared in flames can be found in [20]. Recent advances include nanocrystalline Ti02 [24], one-step synthesis of noble metal Ti02 [25], Ru-doped cobalt-zirconia [26], vanadia-titania [27], Rh-Al203 for chemoselective hydrogenations [28], and alumina-supported noble metal particles via high-throughput experimentation [29]. 

The same mechanism proposed for the combustion catalyst Mg-chromite apply also to catalysts that allow significant yields in acetic acid from n-butane, like vanadia-titania, that accordingly also show a medium-high Brpnsied acidity. Being acetate ions intermediates in the combustion way, it is easily rationalized that the production of acetic acid is favored by the addition of steam in the reactant mixture and by adjusting the reaction conditions. 

Other metal oxide catalysts studied for the SCR-NH3 reaction include iron, copper, chromium and manganese oxides supported on various oxides, introduced into zeolite cavities or added to pillared-type clays. Copper catalysts and copper-nickel catalysts, in particular, show some advantages when NO—N02 mixtures are present in the feed and S02 is absent [31b], such as in the case of nitric acid plant tail emissions. The mechanism of NO reduction over copper- and manganese-based catalysts is different from that over vanadia—titania based catalysts. Scheme 1.1 reports the proposed mechanism of SCR-NH3 over Cu-alumina catalysts [31b],... 

Scheme 4.1. Catalytic cycle for SCR reaction over vanadia—titania catalyst (from ref. [39]).

Hoang-Van, C., Zegaoui, O. and Pichat, P. (1998) Vanadia-titania aerogel dcNOx catalysts, J. Non Cryst. Solids, 225, 157. 

Topspe, N.Y., Anstrom, M. and Dumesic, J.A. (2001) Raman, FTIR and theoretical evidence for dynamic structural rearrangements of vanadia/titania dcNOx catalysts, Catal. Lett., 76, 11. 

Hadjiivanov, K., Concepcion, P. and Knozinger, H. (2000) Analysis of Oxidation States of Vanadium in Vanadia-Titania Catalysts by the IR Spectra of Adsorbed NO, Top. Catal., 11/12, 123. 

ASPECTS OF CATALYST DEVELOPMENT FOR MOBILE UREA-SCR SYSTEMS - FROM VANADIA-TITANIA CATALYSTS TO METAL-EXCHANGED ZEOLITES... 

Alternative to the conventional vanadia/titania-based catalysts for the selective reduction of NO by ammonia... 

Presently the catalytic selective NOx reduction by ammonia is efficient and widespread through the world for stationary sources. The remarkable beneficial effect of 02 for the complete reduction of NO into nitrogen is usually observed between 200 and 400°C. However, such a technology is not applicable for mobile sources due to the toxicity of ammonia and vanadium, which composes the active phase in vanadia-titania-based catalysts. Main drawbacks related to storing and handling of ammonia as well as changes in the load composition with subsequent ammonia slip considerably affect the reliability of such a process. On the other hand, the use of urea for heavy-duty vehicles is of interest with the in situ formation of ammonia. 

Figure 2. Dehydrated Raman spectra of vanadia-titania catalysts as a function of vanadia loading.

Selective and Nonselective Pathways in Oxidation and Ammoxidation of Methyl-Aromatic Compounds over Vanadia—Titania... 

The gas-phase selective oxidation of o-xylene to phthalic anhydride is performed industrially over vanadia-titania-based catalysts ("7-5). The process operates in the temperature range 620-670 K with 60-70 g/Nm of xylene in air and 0.15 to 0.6 sec. contact times. It allows near 80 % yield in phthalic anhydride. The main by-products are maleic anhydride, that is recovered with yields near 4 %, and carbon oxides. Minor by-products are o-tolualdehyde, o-toluic acid, phthalide, benzoic acid, toluene, benzene, citraconic anhydride. The kinetics and the mechanism of this reaction have been theobjectof a number of studies ( 2-7). Reaction schemes have been proposed for the selective pathways, but much less is known about by-product formation. 

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... 

The purpose of the present paper is to offer a contribute to the understanding of the mechanisms of these reactions by using an IR spectroscopic method and well-characterized "monolayer" type vanadia-titania (anatase) as the catalyst. We will focus our paper in particular on the following subjects i) the nature of the activation step of the methyl-aromatic hydrocarbon ii) the mechanism of formation of maleic anhydride as a by-product of o-xylene synthesis iii) the main routes of formation of carbon oxides upon methyl-aromatic oxidation and ammoxidation iv) the nature of the first N-containing intermediates in the ammoxidation routes. 

As a conclusions, we propose the following two-step mechanism for alkyl aromatic activation over vanadia-titania catalysts ... 

Figure 1. FT-IR spectra of the surface species arising from adsorption of toluene (A), o-xylene (B), m-xylene (C) and p-xylene (D) on vanadia-titania. Full lines after outgassing at 350 K.

Oxidation of ortho-xylene. The spectra of the adsorbed species arising from interaction of ortho-xylene with the surface of the vanadia-titania catalyst in the presence of oxygen are shown in Figure 4. The spectra show some parallel features with respect to those discussed above concerning the oxidation of toluene and meta- and para-xylene. Also in this case the o-methyl-benzyl species begins to transform above 373 K, with production of adsorbed o-tolualdehyde (band at 1635 cm 0 and of a quinone derivative (band at 1670 cm. Successively bands likely due to o-toluate species (1530,1420 cm 0 grow first and decrease later with production of CO2 gas. 

In conclusion, we can propose the following reaction scheme for the oxidation of ortho-xylene on the surface of vanadia-titania ... 

The comparison of the bands observed after benzonitrile and benzylamine transformation over vanadia-titania allows us to suggest that benzamide can also be obtained by oxidation of benzylamine. Moreover, the growth of the absorption near 1640 cm and the appearance of bands at 1330 and 1240 cm during benzylamine oxidation suggests that benzaldehyde is also formed. A likely assignment for the band at 1670 cm is to the stretching of a C=N double bond (75), so being likely indicative of the formation of benzaldimine. 

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