Vanadia-titania catalysts

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

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... 

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

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... 

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

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. 

The reaction mechanism on a vanadia-titania catalyst has been re-investigated using FTIR spectroscopy [114], although the conclusions were not markedly different from earlier proposals. The interaction of methylpyrazine with the catalyst surface involves a consecutive transformation of co-ordinatively bound methylpyrazine into oxygenated surface compounds, namely an aldehyde-like complex and an asymmetric carboxylate. The main reaction product, amidopyrazine, is formed through the interaction of the surface oxy-intermediates with adsorbed ammonia species. 

H G Liniz, T Turek, Intrinsic kinetics of nitric oxide reduction by ammonia on a vanadia-titania catalyst, Appl Catal, A. General 55.13 (1992). 

The IR bands from vC02 for Cp2V(OOCR)2, where R = H, CC13, CF3, are consistent with the presence of two unidentate carboxylates.354 Toluene partial oxidation on V205 in vanadia-titania catalysts leads to the formation of benzoate species, detected by DRIFTS.355... 

Weng and Lee [85] found that the number of active sites, and of Lewis and Bronsted acid sites as well, increase with increasing amounts of niobium oxide in vanadia/titania catalysts. The highest activity of the promoted catalysts was found at 573 K. At lower and higher temperatures ammonia is oxidized. 

The results of TPR experiments are reported in Table 1. For aU the samples no significant effects of H2 reduction were observed below 330°C. Similar results were obtained by Topsoe et al. [16] on vanadia-titania catalysts. 

Table HI. Examples of Vanadia/titania Catalyst Technology Development...

Selective Catalytic Reduction (SCR) using ammonia as the reductant provides NOx reduction levels of greater than 80%. Three types of catalyst systems have been deployed commercially noble metal, base metal and zeolites. Noble metals are typically washcoated on inert ceramic or metal monoliths and used for particulate-free, low sulfur exhausts. They function at the lower end of the SCR temperature range (460-520°F) and are susceptible to inhibition by SOx [14]. Base metal vanadia-titania catalysts may either be washcoated or extruded into honeycombs [11]. Typically washcoated catalysts are only used for treating particulate-free, clean gas exhausts. Extruded monoliths are used in particulate-laden coal and oil-fired applications. The temperature window for these catalysts is 600-750°F. Zeolites may also be washcoated or extruded into honeycombs. They function at relatively high temperatures of 650-940°F [15]. Zeolites may be loaded with metal cations (such as Fe, Cu) to broaden the temperature window [16]. 

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... 

V.S. Escribano, G. Busca, V. Lorenzelli, Fourier Transform Infrared Spectroscopic Studies of the Reactivity of Vanadia-Titania Catalysts Towards Olefins. 1. Propylene, Journal of Physical Chemistry 94, 8939, 1990. 

Topsoe N-Y, Dumesic JA, Tops0e H (1995) Vanadia-Titania Catalysts for Selective Catalytic Reduction of Nitric-Oxide by Ammonia. 2. Studies of Active-Sites and Formulation of Catalytic Cycles. J Catal 151 (l) 241-252... 

Anastasov, A.I. (2003) An investigation of the kinetic parameters of the o-xylene oxidation process carried out in a fixed bed of high-productive vanadia-titania catalyst Chem. Eng. Sci., 58, 89-98. 

Vanadia-titania catalysts have been the subject of research for a long time due to their outstanding industrial importance in (amm) oxidation reactions and for selective catal3hic reduction of NOx (SCR-DeNOx), and there is an abundance of literature on this system. 

Grzybowska-Swierkosz, B. (1007). Vanadia-titania Catalysts for Oxidation of o-Xylene and Other Hydrocarbons, Ap/>/. Catal. A Gen., 157, pp. 263-310. 

Bulushev, D., Kiwi-Minsker, L. and Renken, A. (2000). Vanadia/Titania Catalysts for Gas Phase Partial Toluene Oxidation. Spectroscopic Characterization and Transient Kinetics Study, Catal. 

Bond, G. and Flamerz, S. (1989). Structure and Reactivity of Titania-supported Oxides. IV. Characterization of Dried Vanadia/Titania Catalyst Precursors, Appl. Catal B Environ., 46, pp. 89-102. 

The reaction is carried out either over bulk Fe2(Mo04)3-Mo03 or over silver catalysts with very high selectivity. Over different catalysts other selective oxidation products or total oxidation can be obtained. Some years ago we focused our attention on vanadia-titania catalysts, where methyl formate can be obtained with high yields. ... 

Ramis, G., Cristiani, C., Forzatti, R, et al (1990). On the consistency of data obtained from different techniques concerning the surface structure of vanadia-titania catalysts, J. Catal., 124, pp. 574-575. 

Busca, G. (1993). Infrared characterization of the hydrocarbon species intermediates in the oxidation of toluene and xylenes over vanadia-titania catalysts, J. Chem. Soc. Faraday Trans. I, 89, pp. 753-755. 

Busca, G. (1993). Selective and non-selective pathways in the oxidation and ammoxidation of methyl-aromatics over vanadia-titania catalysts FTIR studies, in S. Oyama and J. Hightower (eds.), Catalytic Selective Oxidation, American Chemical Society, Washington, DC,... 

Grzybowska-Swierkosz, B. Vanadia-titania catalysts for oxidation of o-xylene and other hydrocarbons, Catal A Gen. 1997,157, 263-310. 

Tops0e, N.-Y., Topspe, H., and Dumesic, J.A. Vanadia/titania catalysts for selective catalytic reduction (SCR) of nitric oxide by ammonia. I. Combined temperature programmed in situ FTIR and on-line mass spectroscopy studies. J. Catal. 1995,151, 226-240. 

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