Structure effects catalyst characterization

This chapter deals with the study of structural properties of catalysts and catalytic model surfaces by means of interference effects in scattered radiation. X-ray diffraction is one of the oldest and most frequently applied techniques in catalyst characterization. It is used to identify crystalline phases inside catalysts by means of lattice structural parameters, and to obtain an indication of particle size. Low energy electron diffraction is the surface sensitive analog of XRD, which, however, is only applicable to single crystal surfaces. LEED reveals the structure of surfaces and of ordered adsorbate layers. Both XRD and LEED depend on the constructive interference of radiation that is scattered by relatively large parts of the sample. As a consequence, these techniques require long-range order. 

Mossbauer spectroscopy has matured into one of the classical techniques for catalyst characterization, although its application is limited to a relatively small number of elements which exhibit the Mossbauer effect. The technique is used to identify phases, determine oxidation states, and to follow the kinetics of bulk reactions. Mossbauer spectra of super-paramagnetic iron particles in applied magnetic fields can be used to determine particle sizes. In favorable cases, the technique also provides information on the structure of catalysts. The great advantage of Mossbauer spectroscopy is that its high-energy photons can visualize the insides of reactors in order to reveal information on catalysts under in-situ conditions. 

A brief review of the diffraction phenomenon and the effect of crystallite size is presented. Applications of XRD to catalyst characterization are illustrated, including correlation of XRD powder patterns to molecular structural features, determination of Pt crystallite size and others. Factors that affect the appearance of XRD powder patterns, such as framework structure perturbations, extra-framework material, crystal morphology, impurities, sample preparation, instrument configurations, and x-ray sources, are discussed. 

The method of preparation of the catalyst has been found to alter the effect of the promoter (196). With standard VPO prepared with an organic solvent, the effects of cobalt and of iron were found to be the same as those previously described 182,193-195,202,208). The improvement in catalytic performance is proposed to be a consequence of the stabilization of dimers, which are the proposed active sites. However, catalysts prepared from V0P04 2H2O in organic solvents are not characterized by a promotional effect of iron. This lack of promotion is attributed to the loss of crystallinity and surface area of the rosette crystals formed by in the preparation. Similarly, the increase in activity attributed to cobalt is thought to be a structural effect, influencing the development of the (100) plane of (VO)2P207. 

Almost all group 4 metal complexes require a cocatalyst to generate an active metal-alkyl cationic species. Ordinary alkylaluminums - used in conventional Ziegler Natta catalysts - are insufiicient to activate these compounds on their own. The principal activator nsed is methylalumoxane (MAO), a structurally enigmatic material with a mixture of nuclearities. Its purpose is to alkylate the metal dichloride and to abstract one of the reactive hgands to form the ion pair active catalyst. The interaction is dynamic and a large excess of MAO is needed for effective catalyst performance, thus inhibiting a comprehensive characterization of these catalysts. 

Sinfelt has greatly contributed to the catalyses of bimetallic nanoparticles [18]. His group has thoroughly studied inorganic oxide-supported bimetallic nanoparticles for catalyses and analyzed their microstructures by an EXAFS technique [19-22]. Nuzzo and co-workers have also studied the structural characterization of carbon-supported Pt/Ru bimetallic nanoparticles by using physical techniques, such as EXAFS, XANES, STEM, and EDX [23-25]. These supported bimetallic nanoparticles have already been used as effective catalysts for the hydrogenation of olefins and carbon-skeleton rearrangement of hydrocarbons. The alloy structure can be carefully examined to understand their catalytic properties. Catalysis of supported nanoparticles has been studied for many years and is practically important but is not considered further here. 

In a relatively short period of time, Raman spectroscopy has emerged as one of the leading catalyst characterization techniques, especially for molecular-level structural information under reaction conditions. It is now possible to obtain real-time in situ Raman analysis of working catalysts with the modem spectrometer systems that have been developed in recent years. The continual development of new and improved Raman spectrometer systems and cell designs will continue to resolve some of the remaining problems in Raman characterization studies of catalysts (fluorescence and photochemical effects). These new capabilities are expected to accelerate the number and types of catalyst issues that are resolvable with Raman spectroscopy. Thus, it is anticipated the exponential growth of Raman spectroscopy of catalysts, shown in Fig. 1, will continue for many more years. 

Homo- and co-polymerization of ethylene were performed by using a catalyst system composed of TiCl4/THF/NgCla complex with AlEtj at TO C. In order to investigate the role of MgCla in the catalyst 6 catalysts with different composition (Mg/Ti - 0.42 -16.5) were characterized by means of elemental analysis, IR spectroscopy, x-ray powder diffraction, and SEM technique. The catalytic activity of polymerization increased linearly with the Ng/Ti ratio of catalyst within the experimental range. The activity of copolymerization with 1-hexene also increased with Hg/Ti ratio. The enhancement of polymerization rate by the addition of 1-hexene in the reaction medium was observed only for the catalyst of Mg/Ti ratio smaller than 2.5. The effect of crystallization conditions during the catalyst preparation on the chemical composition and physical structure of catalysts was discussed. The variation caused by different crystallization conditions had considerable influences on the rate profiles of homo-and copolymerization of ethylene. 

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