Titania-metal systems

More recent research has greatly improved our understanding of these systems and has led to new questions as well. Of major importance is the discovery that reduced-titania (usually referred to as TiOx) is surprisingly mobile and can migrate onto the surface of metals at temperatures commonly used for catalytic reaction or pretreatment. This remarkable structural dynamism introduces a new dimension to the interpretation of metal/titania systems. 

TRe signi ficance o TiOx-overlayer formation extends beyond the question of bonding between metals and titania. It suggests that the suppression of H2 and CO chemisorption in metal/ titania systems can be explained as simple site blockage due to the overlayer. Despite the appealing simplicity of this model the question is still open and evidence can be cited that points to a more complicated situation. Experiments have been performed under... 

C02, spillover hydrogen, CO and hydrocarbons. Indeed there is some evidence that the properties associated with anhydrous metal/ titania systems can occur under H20-containing (but net reducing) conditions as well. Suppressed CO chemisorption has been found... 

Although this review has dealt with the interactions of metals with reduced oxide surfaces, metal-support interactions are certainly not limited to these. Evidence for metal-support interaction involving non-reduced surfaces exists even in the metal/ titania system. Enhanced hydrogenolysis activities have been found for low-temperature-reduced Rh/titania (7) and Ru/titania (49). These effects presumably involve interaction with Ti4+ ions. 

Colmenares, J.C., M.A. Aramendia, A. Marinas, J.M. Marinas and F.J. Urbano (2006). Synthesis, characterization and photocatalytic activity of different metal-doped titania systems. Applied Catalysis A-General, 306, 120-127. 

Fig. 8. Interactions in the Ni/titania system as the activation temperature is increased cn, NiO an, bulk Ni metal surface Ni (< 1 nm) in the SMSI state t , Ni in a partially ionized subsurface state c, oxygen anion vacancy (113).

TiO Overlayer Formation in Metal/Titania Model Systems... 

Studies have shown that metal/titania catalysts often show enhanced activity and/or selectivity for the C0-H2 synthesis reaction. This literature will not be reviewed here. Instead we will concentrate on the question of how these reaction features relate to the properties of metal/titania catalysts previously discussed in this review. An important problem is that HoO is a by-product of the C0-H2 reaction, raising the possibility that reduced titania, "TiOx", cannot exist in these systems. Since there is much evidence that TiOx is intrinsically related to the results obtained in H20-free systems, the relationship between the effects observed in these different environments is called into question. 

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 XRD patterns as shown in Fig 1 obtained for all the samples after their reduction in hydrogen indicate the presence of unreduced platinum oxides even after HTR in the supported titania gel and mixed oxide sample. However, complete reduction to metallic platinum was observed on supported commercial titania system. It can be observed that the phase transformation of titania from anatase to rutile does not occur under the experimental... 

Torr of CO, the doublet structure was not observed. This result was also confirmed by Harrod et al. (102) on rhodium films evaporated under ultrahigh vacuum conditions. The difference in these results obtained using different methods of sample preparation can be accounted for either in terms of crystallite size or differences in electronic environment at the metal surface. Blyholder (98) believes that crystallite size also accounts for the differences observed in the infrared spectra of CO adsorbed on the nickel-titania system compared with the nickel-silica and nickel-alumina systems (111) as outlined in Table VI (107). 

In polymerization-induced colloidal aggregation (PICA) processes, a reactive monomer, generally urea formaldehyde, is mixed with a stable, submicrometer diameter metal oxide sol and undergoes an acid-catalyzed polymerization that results in porous, uniformly sized polymer-oxide composite microspheres [24,25], PICA has been applied to a variety of metal oxide systems, primarily silica, but also alumina, titania, zirconia, ferric oxide, and antimony pentoxide [24,25]. The process is affected strongly by solution acidity [26]. At lower pH, polymerization is more rapid and a more porous but mechanically weaker particle is formed. 

Sol-gel routes for binary titanium dioxide, tertiary titanates, and other mbced metal oxide systems not only employ various Ti(lV) alkoxides and modified alk-oxides but also Ti(IV) chlorides, oxychlorides, oxynitrates, and so on. The microstructure of the resulting titania and titanates depends on the morphology and interactions between primary particles (clusters) forming upon hydrolysis-condensation of Ti(IV) precursors. An apparent lack of crystalline order and very small size of primary particles and clusters (<1 nm) is observed in the early stages of reaction. At a more advanced stage, the morphology is determined by interparticle interactions and aggregation mechanisms. 

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

Ordered mesoporous materials of compositions other than silica or silica-alumina are also accessible. Employing the micelle templating route, several oxidic mesostructures have been made. Unfortunately, the pores of many such materials collapse upon template removal by calcination. The oxides in the pore walls are often not very well condensed or suffer from reciystallization of the oxides. In some cases, even changes of the oxidation state of the metals may play a role. Stabilization of the pore walls in post-synthesis results in a material that is rather stable toward calcination. By post-synthetic treatment with phosphoric acid, stable alumina, titania, and zirconia mesophases were obtained (see [27] and references therein). The phosphoric acid results in further condensation of the pore walls and the materials can be calcined with preservation of the pore system. Not only mesoporous oxidic materials but also phosphates, sulfides, and selenides can be obtained by surfactant templating. These materials have pore systems similar to OMS materials. 

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. 

It is important to note that in addition to microporous solids, other chemical systems have been used to template the growth of nanomaterials. For example, emulsions have been used to pattern both the pores in titania [14] and the packing of latex particles [46]. Reversed micelles have also been used as patterning agents. Examples include the syntheses of super-paramagnetic ferrite nanoparticles [15] and BaC03 nanowires [47]. Finally, carbon nanotubules have also been used as templates [16,48,49]. A variety of nanomaterials including metal oxides [16,48,49] and GaN have been synthesized inside such tubules [50]. 

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