Structure composition, catalyst

Due to these virtues, solid state NMR is finding increasing use in the structural analysis of polymers, ceramics and glasses, composites, catalysts, and surfaces. 

The properties of the zeolite play a significant role in the overall performance of the catalyst. Understanding these properties increases our ability to predict catalyst response to changes in unit operation. From its inception in the catalyst plant, the zeolite must retain its catalytic properties under the hostile conditions of the FCC operation. The reaclor/regenerator environment can cause significant changes in chemical and structural composition of the zeolite. In the regenerator, for instance, the zeolite is subjected to thermal and hydrothermal treatments. In the reactor, it is exposed to feedstock contaminants such as vanadium and sodium. 

Analytical electron microscopy (AEM) permits elemental and structural data to be obtained from volumes of catalyst material vastly smaller in size than the pellet or fluidized particle typically used in industrial processes. Figure 1 shows three levels of analysis for catalyst materials. Composite catalyst vehicles in the 0.1 to lOim size range can be chemically analyzed in bulk by techniques such as electron microprobe, XRD, AA, NMR,... 

In HRTEM, very thin samples can be treated as weak-phase objects (WPOs) whereby the image intensity can be correlated with the projected electrostatic potential of crystals, leading to atomic structural information. Furthermore, the detection of electron-stimulated XRE in the electron microscope (energy dispersive X-ray spectroscopy, or EDX, discussed in the following sections) permits simultaneous determination of chemical compositions of catalysts to the sub-nanometer level. Both the surface and bulk structures of catalysts can be investigated. 

Alumina is one of the most commonly used supports for nickel catalysts 111, 178,194-204). Ni/Al203 exhibits carbon deposition (180) that depends on the catalyst structure, composition, and preparation conditions. 

Characterization is a central aspect of catalyst development [1,2], The elucidation of the structures, compositions, and chemical properties of both the solids used in heterogeneous catalysis and the adsorbates and intermediates present on the surfaces of the catalysts during reaction is vital for a better understanding of the relationship between catalyst properties and catalytic performance. This knowledge is essential to develop more active, selective, and durable catalysts, and also to optimize reaction conditions. 

Zhang and Shi [36] found that the dual-bound composite catalyst layer exhibited higher performance than either a PTFE-bound CL or a thin-film CL, as shown in Figure 2.9. Optimization of the dual-bound CL showed that impregnation of Nation between the two layers could lead to decreased cell performance [37]. Thus, the optimal structure for a dual-bound CL was a separate hydrophilic layer on top of a hydrophobic layer. 

HREM methods are powerful in the study of nanometre-sized metal particles dispersed on ceramic oxides or any other suitable substrate. In many catalytic processes employing supported metallic catalysts, it has been established that the catalytic properties of some structure-sensitive catalysts are enhanced with a decrease in particle size. For example, the rate of CO decomposition on Pd/mica is shown to increase five-fold when the Pd particle sizes are reduced from 5 to 2 nm. A similar size dependence has been observed for Ni/mica. It is, therefore, necessary to observe the particles at very high resolution, coupled with a small-probe high-precision micro- or nanocomposition analysis and micro- or nanodiffraction where possible. Advanced FE-(S)TEM instruments are particularly effective for composition analysis and diffraction on the nanoscale. ED patterns from particles of diameter of 1 nm or less are now possible. 

For more than five decades, the methods of surface physics and chemistry have provided some of the most incisive results advancing our understanding of the catalytic action of solids at the molecular scale. Characterizations by physical methods have demonstrated the dynamic nature of catalyst surfaces, showing that their structures, compositions, and reactivities may all be sensitive to temperature and the composition of the reactive environment. Thus, the most insightful catalyst characterizations are those of catalysts as they function. This volume of Advances in Catalysis is dedicated to the topic of physical characterization of solid catalysts in the functioning state. Because the literature of this topic has become so extensive, the representation will extend beyond the present volume to the subsequent two volumes of the Advances. 

The properties of these new materials as catalyst support were tested on Fischer-Tropsch process (CO-H2 reaction) in a fixed bed differential reactor. Three materials were tested a) CON, a conventional activated carbon b) SC-155 (G40.60) and c) C-155 (G20.20). All of them were previously iron doped until 5% metallic iron wt/wt was reached. The test conditions were Reaction temperature =270°C H2/CO ratio=3, pressure = latm. The main properties of the tested catalyst supports and their performance in the first hour test are shown in Table 2. SC-155 (G40.60) and C-155 (G20.20) were selected for this test in order to compare materials with near the same specific surface area but with different structural composition, and CON was selected because it is of common use and has very different texture characteristics respect to the other two materials. 

Catalysts vary both in terms of compositional material and physical structure (18). The catalyst basically consists of the catalyst itself, which is a finely divided metal (14,17,19) a high surface area carrier and a support structure (see Catalysts, supported). Three types of conventional metal catalysts are used for oxidation reactions single- or mixed-metal oxides, noble (precious) metals, or a combination of the two (19). 

When I speak of product formulation and design, I refer to the systematic identification of the molecular structure or material formulation that would meet a specifically defined need. In other words, you know what you want, but you don t know what structure or formulation will take you there. This fairly broad definition is applicable to a wide variety of situations. For example, engineering materials, polymer composites, catalysts and fuel additives, agrochemicals, and pharmaceutical problems all fit into this framework. 

Modem two-stage catalysts are so-called composite ones, containing both a zeolite-Y and an ASA component, where they should be balanced so as to achieve both a high activity and a stable yield structure in recycle. The FIMS analysis of the unconverted material obtained over such a composite catalyst demonstrates that a more balanced conversion of the different feed components can be achieved (Fig. 6.12 (c)). And indeed, such catalysts perform quite satisfactorily in practice (59). 

Table I indicates the microstructure of polybutadienes prepared by means of the indicated catalyst systems. It shows that at some given ratio of components, not necessarily at all ratios, a particular catalyst system is capable of yielding polymer with the indicated structural composition. The patent literature, in some cases, contains conflicting data, indicating the influence of unspecified, and pos-sibly unknown, factors.

The catalysts were prepared by co-precipitation method from aqueous solution of metal nitrates of Cu, Zn, Fe, and Cr and NaOH aqueous solution. Potassium was impregnated to the precipitate with KjCOj aqueous solution. The composition of catalysts were as follows CAT A K/Cu-Zn-Fe=0.077/l-l-3, CAT B K/Cu-Zn-Fe-Cr=0.077/l-l-3-0.1. The hydrogenation of CO2 was performed with a conventional flow reactor for about 150 hours at 300 °C and 7.0MPa. The structures of catalysts were identified by means of Rigaku RINT 2000 X-ray diffractometer. The observation of catalyst particles and the microanalysis of their compositions were carried out by means of Hitachi HF-2000 field emission transmission electron microscope and Kevex DELTA plus 1 energy-dispersive X-ray spectrometer. 

In this paper we present results related to the atomic structure and catalytic properties of Pd overlayers on various substrates. A reaction has been chosen to test the catalytic properties of these systems, it is the 1,3-butadiene hydrogenation, a reaction for which Pd is known to be the best catalyst. In the following, after a short description of the experimental approach, the 1,3-butadiene hydrogenation reaction and the specific properties of Pd for this reaction will be presented. Then the reactivity of several Pd overlayers obtained either by surface segregation in Pd-based alloys or by atomic beam deposition on a metal will be investigated and discussed in terms of structure, composition related to surface segregation and surface stress. The influence of the surface orientation of the substrate will be discussed. 

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