Acid-base and redox properties

ACID-BASE AND REDOX PROPERTIES OF MODEL SURFACES... 

The oxides represent one of the most important and widely employed classes of solid catalysts, either as active phases or as supports. They are used for both their acid-base and ReDox properties and constitute the largest family of catalysts in heterogeneous catalysis [76]. 

One of the most important factors providing acceleration of enzymatic reactions as compared to chemical reactions is drastic changes of chemical reactivity catalytic groups inside and outside the enzyme protein globule. Drawing the charges of metal ions, carboxylate and protonated residues into the protein interior is accompanied by essential alternation of its acid-base and redox properties. This effect can be illustrated by the reaction of cleavage and formation of an a-C-H bond in enzymatic reactions of racemization, transamination, and isomerization (Ha et al., 2000 and references therein). 

Faujasite type zeolites because of the size of their cavities and apertures were the most frequently used in this purpose. Their well known structure, acid-base and redox properties helped much in selecting those zeolites. 

The effect of Ca loading on the acid-base and redox properties of chromia catalysts supported on alumina has been investigated by microcalorimetry of NH3 adsorption and TPR. This alkaline promoter strongly decreases the acidity of the chromia catalyst, particularly suppressing the medium and strong acid sites. No clear correlations were found between the surface acidic properties and the catalytic behavior of the investigated samples in the oxidative dehydrogenation of isobutene, while clear trends were observed between reducibihty and catalytic activity [52]. 

The role of transition metals as counter-cations in polyoxometaUates used as oxidation catalysts has been reviewed [75]. Transition metals have important and complex effects on textural, acid-base and redox properties of the heteropolyanions, as described in a number of studies. The interaction of the molybdophos-phoric Keggin heteropolyanion with the iron counter-ion has been studied and... 

Mendeleev s early periodic table placed manganese and chlorine in the same group. Discuss the chemical evidence for these placements, focusing on the oxides of the two elements and their acid-base and redox properties. Is there a connection between the electronic structures of their atoms In what ways are the elements different ... 

The dehydration of an alcohol usually competes with its dehydrogenation the proportion of each type of product obtained depends on the particular catalyst and the reaction conditions used. The selectivity of the catalyst is dictated by its acid-base and redox properties [7,8], which are acquired during the synthetic process. In fact, the transformation of alcohols is a test reaction for determining catalyst acidity and basicity. The dehydration is believed to be catalysed by acid sites and the dehydrogenation by both acid and basic sites, via a concerted mechanism [9] which, however, has been questioned [10]. 

The first successful preparation of micro/mesoporous or micro/macroporous molecular sieves as well as mesoporous zeolite single crystals started an intensive search of optimization procedures for their synthesis, to increase their thermal stability and to tailor their acid, base and redox properties for possible applications in heterogeneous catalysis. There is no doubt that mastering of synthesis of these hierarchic materials is an important challenge in the area of porous materials. 

Jovanovic et al. studied the acid-base and redox properties of the flavonoid aroxyl radicals by pulse radiolysis of aqueous solutions [119]. The flavonoid aroxyl radicals were generated by bromide radical ion (Br2 ) induced oxidation of flavonoids (F-OH) (Eq. 19), followed by a rapid loss of a proton to form the neutral flavonoid aroxyl radical (F-O ) (Eq. 20). 

Chemical properties on the surface of oxides, such as acid-base and redox properties, are directly responsible for the catalytic actions, though it is still hard to get details of these properties for various oxides. For the sake of convenience, attempts were made to classify oxides into several groups in terms of their acid-base and redox properties, as shown in Table 1. ... 

The novel large-pore vanadosilicates AM-13 and AM-14 (Aveiro-Manchester, structure number 13 and 14) containing stoichiometric amounts of vanadium (SiA = 10 and 4, respectively), have been synthesised. Characterisation techniques such as bulk chemical analysis (ICP), powder X-ray diffraction (XRD), scanning (SEM) and transmission (TEM) electron microscopy, and N2, -hexane, benzene, tripropylamine and perfluorbutylamine adsorption measurements were used for the structural studies. The acid-base and redox properties of these materials were assessed by the conversion of isopropanol and ethanol oxidation, respectively. Both materials exhibit high selectivity to acetaldehyde indicating that these two novel vanadosilicates are promising redox catalysts. 

The well-known coordination chemistry of molybdenum suggests a distinct relationship between acid-base and redox properties of molybdenum complexes and that a coupled electron-proton transfer (to or from the substrate) maybe mediated by molybdoenzymes. Simple molecular mechanisms... 

Vedrine J.C., Millet J.M.M., Volta J.-C. (1996). Molecular description of active sites in oxidation reactions Acid-base and redox properties, and role of water, Catal Today, 32, pp. 115-123. 

Vedrine, J., Millet, J. and Volta, J. (1996). Molecular Description of Active Sites in Oxidation Reactions Acid-Base and Redox Properties, and Role of Water, Catal. Today,32,. 115-123. Oyama, S. (1996). Factors Affecting Selectivity in Catalytic Partial Oxidation and Combustion Reactions, Heterogeneous Hydrocarbon Oxidation, 638, pp. 2-19. 

Likewise, Park et al. [123,124] have studied the effect of ceria on the structure and catalytic activity of V205/Ti02-Zr02 catalyst for the CO2-ODE reaction. In their previous research, the authors had found that titania-zirconia mixed oxides exhibited excellent catalytic activity and selectivity for the CO2-ODE to styrene because of the high specific surface area, good acid-base and redox properties. 

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... 

Acid-Base and Redox Properties of Mixed Oxides... 

Further work has been reported - with Fe-Mo models for nitro-genase, and a molecular mechanism has been proposed for the action of molybdenum in enzymes. In all reactions catalysed by Mo enzymes, the product and substrate differ by two electrons and two protons (or some multiple thereof). The co-ordination chemistry of Mo suggests that there is a distinct relationship between acid-base and redox properties of Mo complexes, and that a coupled electron-proton transfer (to or from substrate) may be mediated by Mo in enzymes. Each of the molybdenum enzymes (nitrogenase, nitrate reductase, xanthine oxidase, aldehyde oxidase, and sulphite oxidase) is discussed and it is shown that a simple molecular mechanism embodying coupled proton-electron transfer can explain many key experimental observations. 

Organometallic probes of surfaces. So far the chemical means to characterize surfaces at a molecular level have been based mainly on the spectroscopic methods used with molecular probes, including CO, NO, O2, C02 and organic bases. These molecules give information concerning the acid-base and redox properties of the surfaces of oxides and sulfides. They also probe the coordination properties of ensembles of metal atoms at the surfaces of monometallic or bimetallic particles. 

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