Multi-metal-oxide catalysis

Multi-metal-Oxide Catalysis. The ReaxFF potential has also been utilized to study the catalytic properties of complex metal oxides. Che-noweth et al. developed and implemented a V/O/C/H force-field that, when combined with the existing hydrocarbon force-field, can model the interaction between gas-phase hydrocarbons and the vanadium oxide surface. For motivation, the authors cite numerous examples in which V2O5 is used to catalyze industrial processes that selectively oxidize both... 

Another method at the molecular level is the inhibition of oxidation catalysis by alkali and transition metal impurities. In particular, alkali metal oxides in traces serve as effective catalyst with almost ubiquitous presence in technological environments. The mechanism of operation is well described in the literature [64,72-77] despite its complex and multi-pathway behavior. 

A key aspect of metal oxides is that they possess multiple functional properties acid-base, electron transfer and transport, chemisorption by a and 7i-bonding of hydrocarbons, O-insertion and H-abstraction, etc. This multi-functionality allows them to catalyze complex selective multistep transformations of hydrocarbons, as well as other catalytic reactions (NO,c conversion, for example). The control of the catalyst multi-functionality requires the ability to control not only the nanostructure, e.g. the nano-scale environment around the active site, " but also the nano-architecture, e.g. the 3D spatial organization of nano-entities. The active site is not the only relevant aspect for catalysis. The local area around the active site orients or assists the coordination of the reactants, and may induce sterical constrains on the transition state, and influences short-range transport (nano-scale level). Therefore, it plays a critical role in determining the reactivity and selectivity in multiple pathways of transformation. In addition, there are indications pointing out that the dynamics of adsorbed species, e.g. their mobility during the catalytic processes which is also an important factor determining the catalytic performances in complex surface reaction, " is influenced by the nanoarchitecture. 

Native semiconductor surfaces are fairly inactive from a catalysis perspective. Thus, noble metal or metal oxide islands have been implanted on photoelectrode surfaces as electron storage centers to drive multi-electron redox processes such as HER, photo-oxidation of H2O and photo-oxidation of HCl, HBr or HI. Examples of this sort of chemical modification strategy are also given in Table 3. 

Few examples of mono-substituted polyoxometalates, namely [Ru -(H20)(a-XWii039)] (X = Si, Ge) and [(IrCl4)KP2W2o072] , have been also reported as water oxidation catalysts, but their reactivity is significantly lower with the respect of Ru4SiWio and C04PW9, thus confirming the pivotal role played by the multi-metal catalysis. 

Several studies have been conducted aiming ai the separation of steric and electronic effects [71-73]. For a single step process such as an oxidative addition or one-electron change in electrochemical processes this may be useful, but for multi-step reactions as we are dealing with in catalysis, this technique will encounter many problems. There will be different effects on the distinct steps and linear free-energy relationships will be an exception rather than the rule. When for instance both an oxidative addition step and a reductive elimination step are involved volcano curves must be expected for reactivity versus a ligand property, as in a series of metal oxides when... 

The concept presented in Fig. 6 could use also other type of ordered mesoporous membranes, based on silica for example. As discussed before, oxides such as Ti02 provide better multi-functionalities for the design of such a type of nanofactory catalysts. Worth to note is that in the cover picture of the recent US DoE report Catalysis for Energy a very similar concept was reported. This cover picture illustrates the concept, in part speculative, that to selectively convert biomass-derived molecules to fuels and chemicals, it is necessary to insert a tailored sequence of enzyme, metal complexes on metal nanoparticles in a channel of a mesoporous oxide. 

Other routes to MMA start from ethylene, propylene or propyne and involve metal catalysis at some stage of multi-step transformations for example by the hydroformylation of ethylene to intermediate propionaldehyde, oxidation to propionic acid, followed by condensation with formaldehyde. The Pd-catalyzed carbonylation of propyne to MMA is a further method. However only the ethylene route has found some industrial application (see Chapter 4, Section 4.3.1). 

When one thinks about only two-electron reduction of a substrate (A), the reduction and protonation give nine species at different oxidation and protonation states as shown in Scheme 35. Each species can have an interaction with a metal complex (M +-L) and such an interaction can control each redox step. Moreover, the interaction between the ligand L and a substrate has the possibility of controlling not only the reactivity but also the stereoselectivity of the redox reaction. With regard to multi-electron reduction or oxidation of a substrate, the much more redox and protonated or deprotonated states should be considered for the interaction with metal complexes. The scope and the application of catalysis in electron transfer are thereby expected to expand much further in the near future. 

The interaction of nitric oxide (NO) with metal ions in zeolites has been one of the major subjects in catalysis and environmental science and the first topic was concerned with NO adsorbed on zeolites. NO is an odd-electron molecule with one unpaired electron and can be used here as a paramagnetic probe to characterize the catalytic activity. In the first topic focus was on a mono NO-Na" complex formed in a Na -LTA type zeolite. The experimental ESR spectrum was characterized by a large -tensor anisotropy. By means of multi-frequency ESR spectroscopies the g tensor components could be well resolved. The N and Na hyperfine tensor components were accurately evaluated by ENDOR spectroscopy. Based on these experimentally obtained ESR parameters the electronic and geometrical structures of the NO-Na complex were discussed. In addition to the mono NO-Na complex the triplet state (NO)2 bi-radical is formed in the zeolite and dominates the ESR spectrum at higher NO concentration. The structure of the bi-radicai was discussed based on the ESR parameters derived from the X- and Q-band spectra. Furthermore the dynamical ESR studies on nitrogen dioxides (NO2) on various zeolites were briefly presented. 

The second model requires catalysis by adsorbed protons and is applied to silicate minerals with covalent, polymerized structures, like quartz. In this model, protons react quickly with oxide bonds at the surface, accelerate cleavage, and return to solution. As bonds are progressively cleaved, a monomer or small oligomer is released from the surface. The weakness of this model is the enormous difficulty in simultaneously determining rates of dissolution and proton adsorption densities on complicated multi-oxide mineral structures. Protons taken up by leaching alkaline-metal cations from the mineral must be separated from those involved in protonation of bonds in order to assign a value to the rate order. 

J. E. Backvall, Stud. Surf. Sci. Catal, 1988, 41, 105-114. Metal-Catalyzed Oxidations of Unsaturated Hydrocarbons by Molecular Oxygen The Use of Multi-step Catalysis. 

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