Characterization of catalytic systems

Characterization of Catalytic Systems Using Nuclear Magnetic Resonance and Vibrational and Electron Spectroscopies... 

Utilization of setups including X-rays for in situ characterization of catalytic systems requires access to synchrotrons, and is therefore rather restricted to groups with the requisite expertise. To overcome this, more intensive collaboration between catalytic scientists and synchrotron specialists should be encouraged. 

Characterization of catalytic phenomena at oxide surfaces includes (1) characterization of established catalyst surfaces to improve the catalytic performance, (2) characterization of new catalysts in comparison with conventional catalysts, (3) characterization of specific model surfaces such as single crystals and epitaxial flat surfaces to transfer the knowledge so obtained to catalytic systems or even to create a new type of catalyst, and (4) characterization of catalysis... 

A number of zinc halide complexes, including bridging halides, have already been mentioned in the context of the other ligands systems discussed in this Chapter. Examples will be presented of catalytic systems and solution speciation, particularly in the presence of other coordinating ligands. There are now also a few zinc fluoride species that have been well characterized. 

As noted above, it should be realized that understanding the activity on a surface is not the only issue needed to fully characterize the catalytic system. There must be an understanding of how the surface reactions produce intermediates that can desorb and initiate gas phase reactions close to the surface. To that extent, the author is developing a catalytic shock tube technique to probe the interface of the catalyst surface and the immediate gas phase layer, bridging the gap between surface mechanisms and gas phase mechanisms. 

In this chapter, some of the essential aspects of the synthesis and characterization of copolymers derived from the coupling of C02 with various monomers, namely, epoxides, oxetanes, and aziridines, have been reviewed. In addition, the use of carbon disulfide as a resource for copolymer production was introduced, and the present understanding of the mechanistic aspects of processes involving cyclic ethers and C02 catalyzed by well-defined metal systems has been emphasized. This knowledge has led to the development of catalytic systems capable of controlling not only the product selectivity but also the regio- and stereoregularities of the resultant copolymers. 

Notwithstanding the relatively limited time resolution of the conventional fast FTIR technique, it will be demonstrated here that the number of catalytic systems that can be characterized successfully in dynamic investigations can be increased by adopting suitable temperature and pressure conditions, which permit the observation of otherwise elusive intermediates. 

The characterization of Rh phases supported on alumina has been carried out by DRS. Although the spectra present less intense bands than those of pure phases, it is possible to observe the bands of load transference of octahedral Mo ( 300 nm) as well as a band of wide transition and lower resolution attributed to Rh(III), around 400 nm. The surface of catalytic systems based on CoMoe/y-AbOs and NiMoe/y-AlzOs, sulfided and untreated, has also been recently studied by XPS and EXAFS [4, 24], and the corresponding RhMo6/7-Al203 study is in course. 

Finally, by definition, catalysis is purely a kinetic phenomenon. Many studies, directed at the elucidation of catalytic phenomena in both homogeneous and heterogeneous systems, emphasize the characterization of such systems and identification of the species present, by structural and spectroscopic methods. It is only to the extent that the results of such studies are related to the rates of the catalytic reactions through appropriate kinetic measurements that they are relevant to the catalytic process. This point is illustrated by... 

Raman spectroscopy is a powerful technique for characterization of solids and surfaces, and is well suited for examining oxides and supported oxide catalysts. Over the past few years, our group successfully examined a number of catalytic systems using UV Raman spectroscopy. The use of UV excitation prevents fluorescence from the Raman spectra by exciting the sample at a frequency where fluorescence does not occur. In this study, Raman spectra were obtained from the 1% chromium on alumina catalyst during exposure to propane or propene under reaction conditions. Calcined catalysts were compared to catalysts activated in hydrogen. 

Suitable NMR experiments are able to characterize equilibrium structures and the frequencies and amplitudes of molecular motion. Such information is essential for the experimental verification of theoretical calculations on the structure and molecular dynamics of catalytic systems. 

In the last decade, there has been an increase in the research effort in this field. Coke characterization has been included in many papers where deactivation is a major issue. Several characterization techniques have been used to study coke deposits and to obtain information regarding reaction mechanism, deactivation mechanism, and regeneration conditions. One of the most widely used techniques is temperature-programmed oxidation. Because of its simplicity and utility, this technique has been widely accepted and used in the characterization of coke in a large variety of catalytic systems. 

The statistical copol5miers of polyarylesterketones, involving naphdia-lene cycle in the main ehain, ean be produeed [388] by low-temperature polycondensation of bisphenyloxide, 4,4 - -bis(P-naphtoxy)benzophe-none with chloranhydrides of aromatic bicarbonic acids - terephthaloyl-chloride and isophthaloylchloride (I) in the presence of catalytical system AlCl3/N-methylpirrolydone/ClCH2CH2Cl (copolymers are characterized by improved thermo- and chemical stabiUty), and also by the reaction of hydroquinone with l,4-bis(4,4 -flourobenzoyl)naphthalene (II) in the presence of sodium and potassium carbonates in bisphenylsulfone [389],... 

Cyclic voltammetry allows a precise kinetic characterization of such systems starting from the observation of catalytic... 

In situ SXAS for characterizing heterogeneous catalytic systems under reaction conditions is a relatively young techniqne and its potential needs to be fully recognized in the future. It provides an alternative method to look into chemical and structural details of a Uve catalytic system, especially those systems using the 3d-transition-metal-based catalysts. The examples in this chapter present current status and... 

Preparation and characterization of catalytic supports with variable composition in the system Si02-Al203-AlP0. ... 

Detecting and measuring the size of nanoparticles in supported metal catalysts is of utmost importance for the overall characterization of the system, a factor that is related to the catalytic behavior. Some properties of nanomaterials can be inferred by modeling its cluster structure and size, and a model is used to simulate the scattering of rays by the sample. In general, the parameters involved in the model are optimized until the theoretical pattern fit the experimental results. 

---