Surface properties niobium catalysts

VV Bhat, A. Rougier, L. Aymard, G.A. Nazii, J.-M. Tarascon, High surface area niobium oxides as catalysts for improved hydrogen sorption properties of ball milled MgH, J. Alloys Compd. 460 (2008) 507-512. 

In the present work the synthesis of highly dispersed niobium or titanium containing mesoporous molecular sieves catalyst by direct grafting of different niobium and titanium compounds is reported. Grafting is achieved by anchoring the desired compounds on the surface hydroxyl groups located on the inner and outer surface of siliceous MCM-41 and MCM-48 mesoporous molecular sieves. Catalytic activity was evaluated in the liquid phase epoxidation of a-pinene with hydrogen peroxide as oxidant and the results are compared with widely studied titanium silicalites. The emphasis is directed mainly on catalytic applications of niobium or titanium anchored material to add a more detailed view on their structural physicochemical properties. 

In this work, the activity and selectivity of catalysts based on niobium and vanadium oxides supported on high surface area anatase Ti02 in ethane ODH have been investigated. Specifically, the influence of the cooperation of vanadium and niobium oxides supported phases as components inducing respectively redox and acid properties, together with the effect of the preparation conditions on the catal3dic performances have been studied. 

Corrosion of the plates not only detracts from their mechanical properties but also gives rise to undesirable corrosion products, namely, heavy-metal ions, which, when depositing on the catalysts, strongly depress their activity. The corrosion processes also give rise to superficial oxide films on the metal parts, and these cause contact resistance of the surfaces. For a lower contact resistance, metallic bipolar plates sometimes have a surface layer of a more stable metal. Thus, in the first polymer electrolyte membrane fuel cell, developed by General Electric for the Gemini spacecraft, the bipolar plates consisted of niobium and tantalum coated with a thin layer of gold. A bipolar plate could also be coated with a layer of carbide or nitride. 

A 2000 study from Buffon [69] examined the reaction of Schrock-type, alkoxy-Mo-alkylidenes with the surface OH groups of silica, silica-alumina, and niobium oxide. The specific mode of attachment of the Mo-complex was found to be highly dependent upon the acid-base properties of the support. For silica, it appears to be a Lewis acid-base interaction between the Mo center and surface silanols, whereas in the case of silica-alumina, the attachment appears to occur via the addition of a surface OH group across the Mo-imido bond. In the case of niobium oxide, it is possible that the OH adds across the Mo-alkylidene, deactivating the complex, as the resulting material was completely inactive for metathesis. The activities of both immobilized catalysts were less than the parent homogeneous... 

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