Niobia

Silica often does not show any PZC at all. The results presented here refer to the pH range 2-4. 

Negligible or negative uptake of Na from 0.001-0.1 M NaCl was found in [969]. 

Negligible uptake of the following inert-electrolyte ions has been reported  

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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-... 

As it can be observed in Table 13.1, Ir supported over pure oxides exhibited low acidity, but Ir supported on mixed Nb20s-Si02 displayed an important enhancement in the surface acidity with surface coverage by niobia increases. Binding energies (BE) of core-level electrons and metal surface composition were obtained from XP spectra. The BE values of Si 2p, Ti 2p3/2, Nb 3ds/2 were 103.4, 458.5 and 123 eV respectively, which are exactly the expected values considering the presence of oxides of Si (IV), Ti (IV) and Nb (V). With regard to Ir 4f7/2 core level, a... 

Ninhydrin, 22 101 Ninhydrin-color reaction amino acids, 2 570 Niobates, 27 152-153 24 315 Niobia-phosphate catalytic aerogels, 2 763t Niobic acid, 27 152 Niobic salts, 27 152-153 Niobium (Nb), 27.132-157 24 313, 315. See also Nb-Ti entries Niobium compounds Niobium metal analytical methods for, 27 142-144 dissolution methods for, 27 142 economic aspects of, 27 140-142 effect on stainless steel corrosion resistance, 7 809... 

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. 

The application of the SEA approach to other systems is straightforward, and involves the three steps (PZC determination, uptake-pH survey, and tuned reduction) demonstrated in the earlier sections. It has recently been suggested that electrostatic adsorption over silanol groups is the cause for metal overexchange in low-aluminum zeohtes [61], It is presently being employed to study noble metal uptake on pure oxides of titania, ceria, zirconia, and niobia. 

Extend the model from Problem 5 to simulate the pH shifts for alumina and silica seen in Figure 13. Then model other oxides titania (PZC 6, 8 OH/nm2), niobia (PZC 2.5, 5 OH/nm2), magnesia (PZC 12, 10 OH/nm2). 

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. 

NaX zeolite, carbonylation, 31 40-43, 46 -niobia catalysts, SMSI, 36 227-228 -NMDPP catalyst, 25 89, 90 asymmetric hydrogenation of a,P-... 

Halides (chloride, in particular) also react promptly with surface OH groups, as has been shown for [W(=CCMe3)Cl3(dme)] and several inorganic oxides (silica, alumina, silica-alumina, niobia) [5-7]. The same was observed for the reaction of [V(=0)Cl3] with silica in this case, using a large excess of the vanadium complex, one mole of HCl is released per mole of grafted vanadium [8]. 

It has been shown that the activity of niobia for the oxidative dehydrogenation of propane can be increased by adding vanadium or chromium, while maintaining a high selectivity towards propylene. 

We attempted to improve the V-Mg-O catalyst system by doping it with various elements (S, 11 but these attempts were not successful. We have therefore focused our attention on niobium pentoxide. In previous papers 9 ylO) we have shown that high selectivities towards propylene are possible using pure niobia, but that the conversions were low. 

In this paper we present the results of experiments which show that the activity of niobia can be improved by adding other suitable elements, while maintaining high selectivity. These results bring the proces of oxidative dehydrogenation of propane closer to practical application. 

The other method involved wet impregnation of the vanadium by evaporation of an aqueous NH4VO3 solution in the presence of a niobia support (14.3 m /g) prepared by calcination of hydrated niobia for 5h at 600 °C (VlNb imp.). 

Figure 1 Oxygen conversion and propylene selectivity as a function of temperature for niobia with various additives.

Since little is known about crystal structures of the low-temperature mked oxides of niobia, it is difficult to explain from a theoretical point of view the good results obtained for doping with vanadium or chromium. It appears that the vanadium and chromium are present in the catalyst in a unique environment, which makes... 

Figure 5 Model for the formation of a layer of the active compound on a vanadia-niobia catalyst.

We wish to thank E.M. Kool, P.H.H. in t Panhuis and A. Toebes for carrying out a substantial part of the experimental work. The hydrated niobia and the niobium oxalate used in the preparation of the catalysts were kindly provided by the Niobium Products Company, Inc., USA. One of the authors (R.H.H. Smits) thanks the Dutch Foundation for Chemical Research (SON) for financial assistance. 

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]. 

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