Niobia-supported catalysts

Coke deactivation on Pt/Al203 catalysts have been studied intensively in the literature. Previous works have focused on the kinetics of catalyst deactivation [7] the influence of additives on coke formation [8] the coke deposition on different morphologic surfaces [9] the structure [10] and chemical composition of coke [11]. Deactivation by coke deposition on niobia supported catalysts, or even on other reducible supports which promote SMSI effect has not been studied. 

This work presents a detailed study of coke deposition on Pt/Nb20s catalysts which were deactivated with n-heptane dehydrogenation. Temperature programmed Oxidation (TPO) allowed to the identification of different coke oxidation zones which are related to the distribution of carbonaceous species on the support and on the metallic sites. Chemical analysis of soluble coke was performed allowing to a better understanding of the deactivation processes on niobia supported catalysts. 

Moreover, the heat of combustion of the total coke and of the insoluble coke on Pt/Nb20s are very similar (Table 5), If insoluble coke indicates more polymerized species in coke, it seems reasonable that polymerization reactions on niobia supported catalysts lead to a lower... 

Watling, T.C. Deo, G. Seshan, K. Wachs, I.E. Lercher, J.A. Oxidative dehydrogenation of propane over niobia supported vanadium oxide catalysts. Catal. Today 1996, 28, 139-145. 

Catalysts containing niobia supported on various oxides have been the subject of considerable recent interest [1-4]. The molecular structures and reactivity of niobium oxides supported on alumina, titania, zirconia and silica have been intensively investigated over the last few years. Niobia supported on silica has been shown to be active for the dehydrogenation and dehydration of alcohols, photo-oxidation of propene and oxidative decomposition of methyl tertiary butyl ether. Titania supported niobia is active for the selective catalytic reduction (SCR) of NO by NH3. 

The presence of different oxidation steps during coke burn can be related to the acidity of the support and the structure of the metallic phase. Barbier [13] has shown that the first oxidation zone at low temperatures corresponds to the carbonaceous materials on the metallic phase, whereas at high temperatures of oxidation are attributed to the coke on the acidic sites of the support. In our case, the total acidity varies in a wide range after calcination at 773 K. In fact, the NH3 uptake on alumina was 750 pmol/g catalyst, whereas on calcined niobia it was only 112 (.imol/g catalyst [5]. Moreover, according to Pittman and Bell [14], the acidic sites on a similar niobia support are basically weak Lewis sites. On the other side, coke burn in the neighboring of the metallic sites can be also associated to the promoting effect of platinum and the platinum-niobia interface. 

All catalysts show roughly a first-order dependence on ethane partial pressure, but the dependence on hydrogen partial pressure varies. The three most interacting niobia-supported samples have a reaction order of v -1, whereas the other samples have values between -1.5 and -1.8. Activation energies of all the samples are close to the value of 170 kJ/mole reported for Ni/Si02 (1 1). The one exception is the 7AAP-773 sample, which has a somewhat higher value. 

Fig. 2. TEM image of the 5% Rh/Nl Os catalyst after the activation treatment, evidencing microporous zones of the niobia support. [1cm = 20nm]...

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. 

Musialska, K., Finocchio, E., Sobczak, ., era/. (2010). Characterization of alumina-and niobia-supported gold catalysts used for oxidation of glycerol, App/.Cora/. A. Gen., 384, pp. 70-77. 

Chary KVR, Lakshmi KS, Rao PVR, et al. Characterization and catalytic properties of niobia supported nickel catalysts in the hydrodechloiination of 1,2,4-trichlorobenzene. J Mol Catal A Chem. 2004 223 353-61. 

Rojas, E., Guerrero-Perez, M. O., and Banares, M. A. Niobia-supported nanoscaled bulk-nio catalysts for the ammoxidation of ethane into acetonitrile. Catal Lett 143, 31 2(2013). 

Pham, H.N., Pagan-Torres, Y.J., Serrano-Ruiz, J.C., Wang, D., Dumesic, J.A., Datye, A.K., 2011. Improved hydrothermal stabihty of niobia-supported Pd catalysts. Apphed Catalysis A General 397 (1-2), 153-162. 

Raman spectroscopy has provided information on catalytically active transition metal oxide species (e. g. V, Nb, Cr, Mo, W, and Re) present on the surface of different oxide supports (e.g. alumina, titania, zirconia, niobia, and silica). The structures of the surface metal oxide species were reflected in the terminal M=0 and bridging M-O-M vibrations. The location of the surface metal oxide species on the oxide supports was determined by monitoring the specific surface hydroxyls of the support that were being titrated. The surface coverage of the metal oxide species on the oxide supports could be quantitatively obtained, because at monolayer coverage all the reactive surface hydroxyls were titrated and additional metal oxide resulted in the formation of crystalline metal oxide particles. The nature of surface Lewis and Bronsted acid sites in supported metal oxide catalysts has been determined by adsorbing probe mole-... 

Niobia has a PZC of about 2.5. It is desired to use niobia as a catalyst promoter for Pt, by supporting the niobia onto an oxide support. It will then be attempted to impregnate Pt only onto the niobia phase. Which common support and what Pt complex should be used and why A sketch of the surface potential vs. pH for niobia and the support will help. 

MTO supported on acidic metal oxides was quickly discovered to form metathesis catalysts that are active without the need for additives, even for functionalised olefins [70]. Standard supports are zeolites and niobia (Nb205), and the activity was reported to be related to the surface acidity [79]. 

A more widely used method of heterogenization has also been applied to MTO, by using a metal oxide support onto which the MTO is applied. MTO has been applied onto niobia (Nb2Os) both via impregnation and sublimation [39] and onto silica (Si02) via reaction of a bipyridine-containing siloxane [40]. In a similar manner, MTO has been immobilized on the mesoporous silica MCM-41 [41]. Zeolite NaY has been used as a support material by the in situ immobilization of the MTO catalyst [42]. 

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