Applications to Catalysis

Structural characterisation of catalysts by EXAFS can be largely divided into these two problems outlined above viz either the identification of the ligand donor set at a metal centre or the study of the morphology of metal particles, and the effects of their interactions with adsorbates different classes of catalysts will provide one or both of these situations. 

1 Homogeneous Transition Metal Catalysts. - There have been comparatively few examples of non-enzymic homogeneous catalysts 

More recently, EXAFS has been used to investigate the 

As shown in Fig. 26, molybdenum oxide/Si02 catalysts show a characteristic photoluminescence spectrum having a near 480 nm and an excitation 

When the molybdenum content was increased, the intensity of the photoluminescence increased, passing through a maximum at approximately 1 atom% of Mo 138, 140). The maximum concentration of the tetrahedrally coordinated molybdenum oxide species w as found at the same concentration at which the X-ray-amorphous M0O3 exhibited a maximum concentration. It is therefore most likely that tetrahedrally coordinated molybdenum oxide species are formed on a restricted part of the amorphous M0O3. The photoluminescence intensity of the catalyst decreased at concentrations higher than 2 atom% Mo, suggesting that the coordination state of the molybdenum oxide species changes from a tetrahedral to an octahedral coordination. 

Only the copper-based catalysts which were degassed at high temperatures and showed very weak EPR signals attributed to the Cu-+ ions exhibit a photoluminescence spectrum near 400-500 mn, attributed to the presence of a Cu species of w hich the absorption band could be obseiv ed at about 300 nm (see Section lV.D.2.a) (105-108, 192-197). The Cu(l)ZSM-5 catalyst exhibits a major photoluminescence near 440 nm and has a weak shoulder at about 510 nm. On the other hand, the Cu(l)Si02 and Cu(l)Vycor catalysts show photoluminescence spectra near 520 nm and have very weak shoulders at about 430 nm and the Cu(l)Si02 and Cu(l)PVG catalysts exhibit very similar photoluminescence spectra, with the spectrmn of the former being more intense. 

These results indicate that the isolated copper species on ZSM-5 have an activity for the decomposition reaction of NO different from that of the dimeric or polynuclear copper species, probably because there are different reaction mechanisms (237). The results obtained with the Cu(ll)ZSM-5 catalyst also suggest that Cu ions promote the spontaneous low -tempera-ture dehydroxylation of nearby Brpnsted sites or the elimination of lattice oxygen anions which play a vital role in the decomposition of NO. When the dimeric or polynuclear species of Cu are present, the spontaneous elimination of the lattice oxygen bridging the two Cu + sites does not occur at low temperatures however, this reaction occurs at high temperatures. The activity for the decomposition of NO is nearly zero at about 573 K, but in the presence of O2 a different reaction mechanism is initiated and this results in the enhancement of NO conversion. Moreover, the presence of stronger Brpnsted sites in ZSM-5 can explain why only the CuZSM-5 catalyst exhibits much higher activity for the reduction of NO in NO-NH3-O2 reaction systems. 

Selective Dimerization and Hydrogenation of C2H4 on /r02 with Coordinalively Unsaturated Surface Sites 

2 An Example Screening of a Library of Catalytic Systems for a C-H Insertion Reaction 

Burgess et al. (128) reported the catalyst screening of a 96-member array of catalytic systems L20 on a C-H insertion reaction of substrate 9.77 (Fig. 9.32), a transformation usually catalyzed by rhodium (138) or copper salts (139) in the presence of chiral ligands (140). The stereochemical outcome was measured on the diastereomeric couple 9.79-9.80, obtained following uncatalyzed oxidation of 9.78 (Fig. 9.32), to simplify the determination of the chiral products while evaluating the stereoselectivity of the tricycle formation. The stereoselectivity of the C-H insertion was not significantly influenced by the presence of the (L)-methyl ester (128). 

An increased reliability of the results generated in the primary screening, an increased throughput, and the use of larger sets and libraries of catalytic systems were the main issues emerging from this pivotal contribution. The following examples will include more recent reports that partially address these concerns. 

Cooper (145) recently reported the use of dye-containing substrates as screening tools for the rapid evaluation of different catalytic systems. Hydrosilylation was chosen as 

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Volume 4 Growth and Properties of Metal Clusters. Applications to Catalysis and the Photographic Process. Proceedings of the 32nd International Meeting of the Society de Chimie Physique, Villeurbanne, September 24-28,1979 edited by J. Bourdon... 

Field Emission Microscopy and Some Applications to Catalysis and Chemisorption Robert Gomer... 

Volume 4 Growth and Properties of Metal Ousters. Applications to Catalysis and the... 

L. H. Germer and A. U. MacRae, A new low electron diffraction technique having possible applications to catalysis, The Robert A. Welch Foundation Research Bulletin, 1961, No. 11. 

The chapters for niobium and tantalum in COMC (1982) and COMC (1995) provide an in-depth discussion concerning complexes of these metals as well as their applications to catalysis.172 173 Alkylcyclopentadienyl complexes of niobium and tantalum have also been reviewed.1 Similarly to vanadium (Section 10.09.4.1.2), the... 

Marcus theory, first developed for electron transfer reactions, then extended to atom transfer, is now being applied to catalytic systems. Successful applications to catalysis by labile metal ions include such reactions as decarboxylation of oxaloacetate, ketonization of enolpyru-vate, and pyruvate dimerization (444). 

Using frameworks to define the meaning of the very modeling constructs themselves— and even to define and encapsulate known inference rules—is very similar to the approach in Larch [Guttag90], Their application to Catalysis modeling constructs, UML stereotype-based extension, and new modeling constructs and notations is described in [D Souza97a],... 

Crooks, M. et al., Dendrimer-encapsulated metal nanoparticles synthesis, characterization, and applications to catalysis, Acc. Chem. Res., 34, 181, 2001. 

This section briefly describes dendrimer-encapsulated metal particles, a new family of composite materials, and their applications to catalysis. 

Transition-metal nanopartides are of fundamental interest and technological importance because of their applications to catalysis [22,104-107]. Synthetic routes to metal nanopartides include evaporation and condensation, and chemical or electrochemical reduction of metal salts in the presence of stabilizers [104,105,108-110]. The purpose of the stabilizers, which include polymers, ligands, and surfactants, is to control particle size and prevent agglomeration. However, stabilizers also passivate cluster surfaces. For some applications, such as catalysis, it is desirable to prepare small, stable, but not-fully-passivated, particles so that substrates can access the encapsulated clusters. Another promising method for preparing clusters and colloids involves the use of templates, such as reverse micelles [111,112] and porous membranes [106,113,114]. However, even this approach results in at least partial passivation and mass transfer limitations unless the template is removed. Unfortunately, removal of the template may re-... 

Well-Defined Surface Rhodium Siloxide Complexes and Their Application to Catalysis... 

I 7 Well-Defined Surface Rhodium Siloxide Complexes and Their Application to Catalysis Table 7.5 Hydrosilylation of 1-hexadecene and allyl ethers by polyhydrosiloxane (7.1) . 

Since 1962 rare earths have been used to stabilize zeolite cracking catalysts for the petroleum industry (1, 2. Until recently this application to catalysis has been the only commercially significant one. Currently, however, a number of new applications of potential commercial significance are appearing. One of the most important of these is the use of cerium in catalysts for automobile exhaust emission control. We will emphasize this application in our review without neglecting other applications. 

Beyond the point resolution, calculations to match experiments are required. In the following section we discuss the progress in HRTEM instrumentation applicable to catalysis. 

Andersson, A. (1982). An oxidized surface state model of vanadium oxides and its application to catalysis. /. Solid State Chem. 42, 263-75. 

In connection with the engineering content of the book, a large number of reactors is analyzed two- and three-phase (slurry) agitated reactors (batch and continuous flow), two-and three-phase fixed beds (fixed beds, trickle beds, and packed bubble beds), three-phase (slurry) bubble columns, and two-phase fluidized beds. All these reactors are applicable to catalysis two-phase fixed and fluidized beds and agitated tank reactors concern adsorption and ion exchange as well. 

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