Redox supported metals

At present, the molecular structures of the dehydrated reduced surface metal oxide species present for supported metal oxide catalysts under reactive environments are not well-known and, hopefully, will receive more attention in the coming years. Fortunately, the fully oxidized surface metal oxide species are the predominant species found to be present under typical reaction conditions employed for redox supported metal oxide catalysts. 

It is evident that the supported clusters have a strong affinity for hydride ligands provided by the support. The process by which the support delivers these ligands is referred to in the catalysis literature as reverse hydrogen spillover. The opposite process (spillover), well known for supported metals [36], is shown by the theoretical results to be a redox process in reverse spillover, the support hydroxyl groups oxidize the cluster. 

Catalyst redox properties, oxygen mobility and supported metal characterization... 

The author gratefully acknowledges financial support from the Deutsche Forschungsgemeinschaft within SFB 583 Redox-active Metal Complexes . 

First of all, I would like to express my gratitude to the institutions that facilitated this work. Studies related to the formation, properties, and reactivity of metalloporphyrins were supported by grants from the Alexander von Humboldt Stiftung (fellowship 2005-2006), by the Deutsche Forschungsgemeinschaft through SFB 583, Redox-Active Metal Complexes Control of Reactivity via Molecular Architectures and by a grant of computer time on the Hochstleistungsrechner in Bayern II (HLRB II). 

The authors gratefully acknowledge financial support from the Deutsche Forschungsge-meinschaft through SFB 583 on Redox-Active Metal Complexes. 

Promoters. - Many supported vanadia catalysts also possess secondary metal oxides additives that act as promoters (enhance the reaction rate or improve product selectivity). Some of the typical additives that are found in supported metal oxide catalysts are oxides of W, Nb, Si, P, etc. These secondary metal oxide additives are generally not redox sites and usually possess Lewis and Bronsted acidity.50 Similar to the surface vanadia species, these promoters preferentially anchor to the oxide substrate, below monolayer coverage, to form two-dimensional surface metal oxide species. This is schematically shown in Figure 4. 

Heterogeneous catalysts for liquid phase oxidations can be divided into three different categories (a) supported metals (e.g. Pd/C), (b) supported metal ions (e.g. ion exchange resins, metal ion exchanged zeolites) and (c) supported oxometal (oxidic) catalysts (e.g. Ti1v/SiOg, redox zeolites, redox pillared clays). This division of the various catalyst types will be used as a framework for the ensuing discussion. 

Although this chapter focuses on oxidation reactions involving redox supported vanadium oxide catalysts, similar trends with surface coverage and specific oxide support also apply for other redox supported transition metal oxide catalysts, such as supported M0O3 [51], CrOs [52] and Re207 [53], The redox supported vanadium oxide catalytic system was chosen for this review because of the extensive studies that these catalysts have received in recent years as well as their widespread industrial appHcations. 

Supported Nb205 [54], Ta20s [55] and WO3 [26] catalysts typically possess almost no redox potential and exclusively behave as surface acid sites. Other than their acidic properties, these supported metal oxides possess similar molecular and electronic structural characteristics as the redox surface sites discussed above. 

Supported metal oxide catalysts are a new class of catalytic materials that are excellent oxidation catalysts when redox surface sites are present. They are ideal catalysts for investigating catalytic molecular/electronic structure-activity selectivity relationships for oxidation reactions because (i) the number of catalytic active sites can be systematically controlled, which allows the determination of the number of participating catalytic active sites in the reaction, (ii) the TOP values for oxidation studies can be quantitatively determined since the number of exposed catalytic active sites can be easily determined, (iii) the oxide support can be varied to examine the effect of different types of ligand on the reaction kinetics, (iii) the molecular and electronic structures of the surface MOj, species can be spectroscopically determined under all environmental conditions for structure-activity determination and (iv) the redox surface sites can be combined with surface acid sites to examine the effect of surface Bronsted or Lewis acid sites. Such fundamental structure-activity information can provide insights and also guide the molecular engineering of advanced hydrocarbon oxidation metal oxide catalysts such as supported metal oxides, polyoxo metallates, metal oxide supported zeolites and molecular sieves, bulk mixed metal oxides and metal oxide supported clays. 

To date, there is no generally accepted theory that accounts for the development of AD pathology. The multifactorial basis of the disease makes such a theory unlikely to be possible in the foreseeable future. In addition to the NFTs and amyloid-hypothesis of the disease, oxidative stress, systemic levels of redox active metal ions, cardiovascular disease, the apoEe4 allele and type 2 diabetes are all clear risk factors for development of AD. However, recent research supports the notion that the Ap buildup may be a key event in AD and that other manifestations of the disease, like NFT formation, result from an imbalance between AP production and AP clearance (17). 

Many different techniques have been used to probe the redox state of ceria and related oxide systems both in the presence and in the absence of a supported metal phase EPR (5,46,94,95,189,227,326,327), XPS (132,166,177,179,193,199,224,289,328-330),... 

Reverse influences (effect of Ce on the redox state of the supported metal) have been also observed. In some cases the reports claim that ceria specifically favors the retention of the metal in an oxidized state [88], although less than thoria in the case of Rh/M02/Si02 [126]. In any case, XPS is used regularly to check the metal redox state after catalyst preparation or diverse treatments. Oxidized states have been reported for Pd and Pt on Ce02 catalysts prepared by a combustion method leading mainly to (+2) and mixed (+2/+4) states respectively [161] on the... 

In the first case where metal surfaces provide active oxygen species to the support contact structure is not critical. The second case is often observed when supported metal catalysts are prepared by coprecipitation or sol-gel methods. Noble metals whose oxides are more stable than Pt oxides such as Pd and Ir are more readily buried in the bulk of metal oxide supports, and the metal oxide overlayers of a thickness of about a few monolayers are modified in their electronic and redox properties by underlying noble metal nanoparticles to become active at lower temperatures. 

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