Doped and Modified Oxides

Doped metal oxide catalysts are widely used in various catalytic processes. In many cases, the catalytic activity and selectivity of these catalysts may be related to their acidity or basicity. 

Microcalorimetry of ammonia and sulfur dioxide adsorption and the catalytic reachon of 2-propanol conversion have been used to study the effects on the acid-base properhes of adding small amounts of various ions (Ca, Li, Nd, Ni, Zn, SO ) to y-alumina, silica or magnesia surfaces [51]. 

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]. 

Sulfated zirconias and sulfated titanias are interesting solids that were first reported to be superacids. The acidity of such samples has been determined by NH3 adsorption calorimetry [47, 53-56]. Depending on the preparation method, the quantity of sulfate ions, calcination temperature and hydroxylation degree, the initial heats of adsorption of NH3 observed can vary from -120 up to 200kJ moT, but most of these materials display heats of adsorption close to those of H-ZSM-5 zeolite. 

The very low initial enthalpy of adsorphon observed by calorimetry on sulfated titanias suggests the occurrence of an endothermic process (the dissociation of NH3) counterbalancing the exothermic process of adsorption. A plateau of heats around 150kjmoh is then observed, followed by a regular decrease of the heats. 

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Single Oxides, Doped and Modified Oxides, Supported Oxides, Mixed Oxides, and Complex Oxides... 

Doped and modified oxides are a wide family of samples which are synthesized with the aim of modifying some surface property of a given oxide. In particular, when an acid or base solid has to be used as catalyst, it can happen that the average acid/base strength of its surface active sites is not useful for the studied reaction. 

Nonstoichiometry of the oxides can be due to a number of reasons, such as hydration,159 incomplete oxidation,158 and the generation of defects at interfaces.157 An important factor affecting the chemical composition of the oxides is the incorporation of electrolyte species into the growing alumina. There have even been suggestions to use this for impurity doping of oxides and modifying their properties.161 Various kinds of anion distributions and mechanisms of anion incorporation and their influence on oxide properties have been reported. The problems attracting attention are ... 

For the electrochemical capacitors of Carbon-Ni Oxide system with aqueous KOH solution it is expedient to use carbonaceous graphite materials with expanded structure and modified surface. The best results were achieved with carbon surface doped with Boron, which makes this carbon superior than other conductive additives used in this study, due to its... 

An example of recent achievement in this area is a flexible, thin film Cu(In,Ga)Se2 solar cell deposited on a titanium foil, which was combined with a TiC>2 photocatalyst layer and modified by a niobium-doped titanium oxide front electrode to function as a photoelectrochemical tandem cell/membrane for a direct light-driven hydrogen evolution from an aqueous solution [48], Under illumination with UV/vis light, the system produced up to 0.052 pLH2/scm2 (e.g. the hydrogen formation rate was approximately 7,250 pmol/h g relative to the amount of TiC>2 used). Several aspects of the operating principles of the photoelectrochemical devices, the materials requirements, main bottlenecks, and the various device concepts (in relation to H2... 

In addition, one needs not to deal with impure surfaces so much because the reaction takes place in solution and surface layers (oxides for example) may dissolve under appropriate potential conditions. Several procedures for achieving activation are also known for electrocatalysts. Ion implantation may be a convenient method of activating and modifying an electrocatalyst. Activation takes place by the introduction of defects and modification by the production of surface-doped layers of varying composition. 

Transalkylation of alkylbenzenes, polyalkylbenzenes and other arenes can be brought about by a variety of catalysts including Lewis acids, Brpnsted acids and various zeolites and silicates with or without being doped with various transition metals or their oxides. There has been a particularly explosive growth in the volume of literature pertaining to the use of various natural and modified zeolites. Recent developments include the study and applications of shape-selective catalysis by zeolites. Much of the work is patented, and largely applies to industrial processes. 

The chemical modification of oxide surfaces with trialkynylorganotins 1 is not restricted to silica substrates. Indeed, nanoporous and nanocrystalline F-doped and undoped tin dioxide powders, prepared by a sol-gel route, 26 can be chemically modified with organotins la and Ic-d to provide fluorous or dye-sensitized Sn02 nanohybrids.2 27,28 for silica, the amount of deposited organic group progressively rises to a plateau value as the concentration of 1 in the grafting solution is increased (Table 3.2.2).22... 

A promising application of this new functionalization method of metal oxides relates to the photosensitization of semiconducting oxides to convert light into electricity. Thus, using the cavity microelectrode technique, a significant photocurrent is produced under blue light illumination by Ic- and Id-modified, F-doped and undoped Sn02 nanoporous powders, in the presence of an aqueous electrolyte, the... 

Surface treatment has also been used to modify the threshold voltage as well as measured mobility in pentacene and Cgo TFTs [60]. These transistors have a heavily doped silicon/silicon oxide gate dielectric structure where alkyl, aUcylamine, and fluoroalkyl silanes are used to modify the Si02. Evaporated pentacene and Cgo form the active p- and n-type semiconductors. The experimental effect of these monolayer treatments is to alter Vj and effective mobility dramatically (see Table 3.2.3). For pentacene, the mobility decreases from -F, -CH3, untreated, -NH2, with a similar shift in Vj from 17 to -11V. The opposite trend is observed for Cgo, in which mobility is largest for the untreated material and smallest for the fluorinated SAM. In the case of Vji the alkylamine SAM shows the lowest VjOi 5.3 V. The underlying reasons for these trends are not completely understood. What is intriguing is how dramatic... 

Related and Modified Reagents. In addition to the standard reagent, a polymer-supported perruthenate (PSP) compound has been described. i Recent work has also examined the doping of organically modified silicas (ormosils) with TPAP via a sol-gel process, 7 which enhances the general versatility and reusability of TPAP catalysts. The use of supercritical carbon dioxide as a solvent has also been investigated. TPAP has also found use as a convenient source of ruthenium in mthenium-catalyzed hypochlorite oxidations. ... 

In situ generation of metal, metal salt and metal oxide produces materials with distributions of particles that would be difficult to achieve via heterogeneous doping. Surface metal or metal oxide coatings can also be obtained that would not result from conventional heterogeneously modified polymers. The chemistry and response of these microcomposite polymers to process conditions need to be explored further. As the structure and properties of the systems that have been herein described are better understood and new metal/polymer systems are explored, their application in electronic, adhesive and composite systems should emerge. 

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