Vanadates polymeric

Fig. 5.18. Monomeric vanadyl (left) and polymeric vanadates (right). Adapted ft-om [65].

It has been shown that titania-supported vanadia materials comprise a distribution of monomeric vanadyl, polymeric vanadates and crystalline vanadia, the amount of which is dependent on the vanadia content. 

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... 

Went et al. [70] used TPD and laser Raman spectroscopy to determine the structure of the catalyst and of the adsorbed species. It was found that the specific activity of polymeric vanadate species was 10 times greater than that of monomeric vanadyls at 500 K. Monomeric species produce N2 both in the presence and in the absence of oxygen whereas polymeric species produce both N2 and N2O. The selectivity towards N2O increases with increasing O2 concentration in the feed. 

During TPR water is formed and desorbs from the surface [35]. It is observed that the hydrogen consumption starts at 350 K, whereas water is formed at temperatures above 425 K. The monomeric vanadyls are restored after one cycle of reduction and oxidation. For polymeric vanadates terminal vanadyl oxygen atoms are removed leaving the structure unchanged. Upon oxidation the polymeric vanadate splits into a dimeric form and a monomeric vanadyl (Fig. 2). Oszkan et al. [36] concluded from their TPD studies that two types of ammonia species were present at the surface. One type was ammonia coordinatively bonded to surface vanadyls (Lewis sites) and a second type of ammonia species formed by the reaction of NH3 and surface hydroxyl groups [36]. 

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. 

Such a catalyst is well known for several reactions, such as o-xylene oxidation to phthalic anhydride and selective catalytic reduction (SCR) of NO by ammonia. The anatase form of TiO appears to be better than the rutile form. Such catalysts with 1 and 8 wt% V20s/anatase was prepared by Rhone-Poulenc (S 10 m g ) for an exercise of characterization by 25 different european laboratories. All results are assembled in one issue of Catalysis Today published in May 1994, vol. 20 n°l. Surface vanadium species were observed to exist in three different forms monomeric V04 species, polymeric vanadate species and V2O5 crystallites [27], the relative amount of which depended on initial wt % V20s/anatase and on the subsequent selective dissolution treatment. 

Of the halogens, only the strongly oxidizing fluorine produces a pentahalide of vanadium, and the other vanadium(V) compounds are based on the oxohalides and the pentoxide. The pentoxide also gives rise to the complicated but characteristic aqueous chemistry of the polymerized vanadates (isopolyvanadates) which anticipates the even more extensive chemistry of the poly molybdates and poly tungstates this is only incompletely mirrored by niobium and tantalum. 

G. Busca, Differentiation of mono-oxo and polyoxo and of monomeric and polymeric vanadate, molybdate and tungstate species in metal oxide catalysts by IR and Raman spectroscopy, J. Raman Spectrosc., 33(5), 348-358, 2002. 

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