Titania-supported Systems

The second major difference between ceria- and titania-supported systems is related to the re-oxidation conditions allowing to recover them from decorated or alloyed states. By analogy with the well established conditions for regenerating M/TiOj catalysts from the SMSl state, re-oxidadon treatments at 773 K, and even lower temperatures, have often been applied to ceria-based systems. However, as shown by the HREM studies reported in section 4.3.3.2, this reference temperature does not allow the reversion of the above effects. Consequently, if the catalyst is recovered from a deactivated state, it should be interpreted as a proof of the absence of significant decoration or alloying in the catalyst. 

Chromium Oxide-Based Catalysts. Chromium oxide-based catalysts were originally developed by Phillips Petroleum Company for the manufacture of HDPE resins subsequendy, they have been modified for ethylene—a-olefin copolymerisation reactions (10). These catalysts use a mixed sihca—titania support containing from 2 to 20 wt % of Ti. After the deposition of chromium species onto the support, the catalyst is first oxidised by an oxygen—air mixture and then reduced at increased temperatures with carbon monoxide. The catalyst systems used for ethylene copolymerisation consist of sohd catalysts and co-catalysts, ie, triaLkylboron or trialkyl aluminum compounds. Ethylene—a-olefin copolymers produced with these catalysts have very broad molecular weight distributions, characterised by M.Jin the 12—35 and MER in the 80—200 range. 

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. 

Evaluation of the suitability was carried out by investigating the catalysts and catalytic performances, putting emphasis on the deactivation behavior. The present paper reports on the properties of titania supported iron oxide catalysts. This system was chosen to extend the concept to other supports, since a priori titania appeared to be a suitable material. Although textural properties were satisfactory, the strong interaction of titania with the applied components resulted in a rapid deactivation. However, the mechanism of deactivation was completely different from systems investigated earlier. 

The data of Figure 1 clearly show that irradiation of the titania pillared clay systems leads to an enhancement in the moles of Cl produced during photodegradation of dichloromethane. Prolonged pre-irradiation leads to lower conversion than shorter periods of pre-Irradlatlon. This is a general result and was found for all supported systems studied here. 

The reversibility is a major characteristic feature of the SMSI effect (300-302). In the case of NM/TiOj, reoxidation at about 773 K, followed by a reduction at low temperature, 473 K, is known to be effective for recovering the catalysts from the SMSI state (300-302,323). Probably by analogy with these earlier studies on titania-supported noble metal systems, similar reoxidation temperatures (773 K) have also been applied to NM/Ce02 catalysts for recovering their chemisorptive and/or catalytic properties from the deactivated state (133,144,221). Data commented below, in which the nanostructural changes of Rh and Pt catalysts in a redox cycle have been followed, prove, nevertheless, that drastic differences are also observed in the reversibility behaviour of ceria based systems, and also that more severe treatments are required to recover this family of catalysts from their corresponding interaction states. 

The titania supported platinum systems showed complete suppression in the hydrogen chemisorption unlike the system involving the mixed oxide support. 

The principal steps in the process proposed by the UKAEA for recovery of uranium from seawater are shown in Fig. 5.20. The titania recovery system consists of 60 beds, 1.3 ft (0.4 m) deep, each with a flow area of 188,000 ft (17,500 m ), filled with hydrous titanium oxide supported on an inert carrier. The inventory of the entire system is 71 million lb (32.2 million kg) of Ti, valued at 71 million in 1966. 

Combustor system using the PdO to Pd transition to control the temperature. Use of metal oxide e.g. ceria, titania) supported PdO and restoring of catalyst activity... 

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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