Redox supported metal oxides

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. 

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. 

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. 

Metal oxides represent one of the most important and widely employed classes of solid catalysts, either as active phases or as supports. Metal oxides are used for both their acid-base and redox properties and constitute the largest family of catalysts in heterogeneous catalysis [1-6]. The three key features of metal oxides, which are essential for their application in catalysis, are (i) coordination environment of the surface atoms, (ii) redox properties of the oxide, and (iii) oxidation state of the surface. Surface coordination environment can be controlled by the choice of crystal plane exposed and by the preparation procedures employed however, specification of redox properties is largely a matter of choice of the oxide. The majority of oxide catalysts correspond to more or less complex transition metal oxides containing cations of variable oxidation state. These cations introduce redox properties and, in addition, acid-base properties. The acid-base properties of the oxides are usually interrelated to their redox behavior. Many attempts were made... 

When supported metal oxide phases are concerned, the kind of surface species that can be present at the surface depends on the support nature, being active a strong or weak metal-support interaction. CuO is an important catalytic phase, easy to disperse on acid supports that can interact with it with strong metal-support bond this interaction can influence the redox properties of CuO. Modified silicas with amount of alumina (SA), titania (ST), and zirconia (SZ) in 12-14 wt.% concentration were used to support CuO (8-9 wt.%) [20] a commercial silica-alumina support was comparatively studied (SAG). On the different supports, the CuO redox properties were controlled by combining TPR and successive TPO experiments. 

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. 

Transition metal oxides, rare earth oxides and various metal complexes deposited on their surface are typical phases of DeNO catalysts that lead to redox properties. For each of these phases, complementary tools exist for a proper characterization of the metal coordination number, oxidation state or nuclearity. Among all the techniques such as EPR [80], UV-vis [81] and IR, Raman, transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS) and NMR, recently reviewed [82] for their application in the study of supported molecular metal complexes, Raman and IR spectroscopies are the only ones we will focus on. The major advantages offered by these spectroscopic techniques are that (1) they can detect XRD inactive amorphous surface metal oxide phases as well as crystalline nanophases and (2) they are able to collect information under various environmental conditions [83], We will describe their contributions to the study of both the support (oxide) and the deposited phase (metal complex). 

The ability of thioether macrocyclic complexes (and especially those of [9]aneS3) to support multi-redox behaviour at the coordinated metal centre is particularly notable. This allows a series of reversible stepwise one-electron oxidation and/or reduction processes, and stabilization of highly unusual transition metal oxidation states e.g. mononuclear [Pd([9]aneS3)2]2+/3+/4+,149 [Au([9]aneS3)2]+/2+/3+,150 [Ni([9]aneS3)2]2+/3+,151 and [Rh([9]aneS3)2]+/2+/3+.152 It appears to be the ability of the crown thioethers to readily adjust their... 

Metal oxides possess multiple functional properties, such as acid-base, redox, electron transfer and transport, chemisorption by a and 71-bonding of hydrocarbons, O-insertion and H-abstract, etc. which make them very suitable in heterogeneous catalysis, particularly in allowing multistep transformations of hydrocarbons1-8 and other catalytic applications (NO, conversion, for example9,10). They are also widely used as supports for other active components (metal particles or other metal oxides), but it is known that they do not act often as a simple supports. Rather, they participate as co-catalysts in the reaction mechanism (in bifunctional catalysts, for example).11,12... 

Influence of Particle Size and Interaction with the Support on Redox and Catalytic Properties of Metals, Metal Oxides, and Metal Complexes... 

Metal oxides are an important elass of heterogeneous catalysts. They find direct application in a variety of reactions, from acid-base to redox reactions, in photocatalytic processes, and as catalysts for environmental protection. In addition, they are widely used as supports for other active components (metal particles or other metal oxides), although often they act not only as a support, but actively participate in the reaction mechanism." ... 

We focus attention here on titania (Ti02) for the following reasons. The first is that titania is a widely used oxide support for both metal particles and metal oxides, and used in some cases also directly as catalyst (Claus reaction, for example). The second is that it possesses multifunctional properties, such as Lewis and Bronsted sites, redox centres, etc. The third is that it has several applications both as a catalyst and an advanced material for coating, sensors, functional films, etc. The fourth is its high photocatalytic activity which make titania unique materials. 

In this paper selectivity in partial oxidation reactions is related to the manner in which hydrocarbon intermediates (R) are bound to surface metal centers on oxides. When the bonding is through oxygen atoms (M-O-R) selective oxidation products are favored, and when the bonding is directly between metal and hydrocarbon (M-R), total oxidation is preferred. Results are presented for two redox systems ethane oxidation on supported vanadium oxide and propylene oxidation on supported molybdenum oxide. The catalysts and adsorbates are stuped by laser Raman spectroscopy, reaction kinetics, and temperature-programmed reaction. Thermochemical calculations confirm that the M-R intermediates are more stable than the M-O-R intermediates. The longer surface residence time of the M-R complexes, coupled to their lack of ready decomposition pathways, is responsible for their total oxidation. 

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