Vanadia-titania systems

Lower rate constant values are obtained on samples treated in air flow. Massucci et al. reported in (6) that V02+ exchanged o-ZrNaHP show redox properties in the temperature range extending from 150 to above 400 C. The treatment in He flow resulted in oxidation of at lower extent in respect to air flow. On the base of these results, and literature informations dealing with vanadia/titania systems (3, 4, 12) the activity of V containing species towards NO reduction could be hypothesized. 

Haber, J., Machej, T., Serwicka, E., et al. (1995). Mechanism of Surface Spreading in Vanadia-titania System, Catal. Lett., 32, pp. 101-114. 

ASPECTS OF CATALYST DEVELOPMENT FOR MOBILE UREA-SCR SYSTEMS - FROM VANADIA-TITANIA CATALYSTS TO METAL-EXCHANGED ZEOLITES... 

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]. 

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. 

Another method of abating tail gas from ammonia-oxidation processes that normally do not exceed 2000 ppm nitrogen oxides is selective catalytic reduction (SCR). The term selective is used because the ammonia fuel only reacts with the nitrogen oxides and not with the oxygen first as in other abatement systems. Catalyst materials include vanadia/ titania and iron-chromium compounds. 

Selective oxidation materials fall into two broad categories supported systems and bulk systems. The latter are of more practical relevance although one intermediary system, namely vanadia on titania [92,199-201], is of substantial technical relevance. This system is intermediary as titania may not be considered an inert support but rather as a co-catalysts [202] capable of, for example, delivering lattice oxygen to the surface. The bulk systems [100, 121, 135, 203] all consist of structurally complex oxides such as vanadyl phosphates, molybdates with main group components (BiMo), molybdo-vanadates, molybdo-ferrates and heteropolyacids based on Mo and W (sometimes with a broad variation of chemical composition). The reviews mentioned in Table 1.1 deal with many of these material classes. 

Polydimethylsiloxane and Zr oxo species have been reacted to give hybrid materials which were characterized by solid-state NMR.149 TEOS/PEG (polyethylene glycol) materials are biphasic systems. The materials were studied by 13C NMR, EPR, and thermal analysis.150 Other systems studied include titania/polyvinylacetate,151 titania/PEG,152 silica/polyacrylates,153 polyimide/ silica,154"156 linseed oil alkyds/titania,157 and PVC/titania and vanadia/sulfonated polyaniline... 

To conclude this section, there is a need for a better understanding of the unusual CO-H2 synthesis properties of metal/ titania catalysts and related systems such as metal/niobia. The primary question to be answered in this regard concerns the stability of reduced titania in the CO-Hg system. The fact that several reducible oxides (titania, niobia, vanadia, MnO, etc. ) have been found to impart unusual CO-Ho synthesis properties to supported metals suggests that support-reaucibility is an important factor that is not cancelled by the CO-H2 reaction environment. 

The titania-vanadia system showed a high reduction in NOx (> 90%) at tenq)eratures between 250°C and 450°C. The SO2 oxidation was nil at plant operating conditions of300°C to... 

Titania-supported vanadia catalysts have been widely used in the selective catalytic reduction (SCR) of nitric oxide by ammonia (1, 2). In an attempt to improve the catalytic performance, many researchers in recent years have used different preparation methods to examine the structure-activity relationship in this system. For example, Ozkan et al (3) used different temperature-programmed methods to obtain vanadia particles exposing different crystal planes to study the effect of crystal morphology. Nickl et al (4) deposited vanadia on titania by the vapor deposition of vanadyl alkoxide instead of the conventional impregnation technique. Other workers have focused on the synthesis of titania by alternative methods in attempts to increase the surface area or improve its porosity. Ciambelli et al (5) used laser-activated pyrolysis to produce non-porous titania powders in the anatase phase with high specific surface area and uniform particle size. Solar et al have stabilized titania by depositing it onto silica (6). In fact, the new SCR catalyst developed by W. R. Grace Co.-Conn., SYNOX , is based on a titania/silica support (7). 

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