Catalysts microanalysis

The powders of zeolites of various trademarks are used to produce petroleum-refining catalysts. In this connection, it is very important to have complete information concerning not only chemical composition and distribution of impurity elements, but also shape, surface, stmcture and sizes of particles. It allows a more detailed analysis of the physical-chemical characteristics of catalysts, affecting their activity at different stages of technological process. One prospective for solving these tasks is X-ray microanalysis with an electron probe (EPMA). 

Figure 7 Microanalysis of a CuO/ZnO methanol synthesis catalyst with a field-emission STEM (a) EOS data showing Cu and Zn K-lines and (b) EELS data showing Cu and Zn L-edges with dotted lines indicating background levels. Spectra were taken simultaneously from a 2-nm diameter area. Signal intensities above background show that approximately the same relative amounts of Cu and Zn were measured by each method.

The application of surface analytical techniques, most notably X-ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES), or its spatially resolved counterpart. Scanning Auger Microanalysis (SAM), is of great value in understanding the performance of a catalyst. However, the results obtained from any of these techniques are often difficult to interpret, especially when only one technique is used by itself. 

Microanalysis of a Copper-Zinc Oxide Methanol Synthesis Catalyst Precursor... 

Although conventional electron-probe microanalysis appears to be unsuitable for analysis of the exposed surface layer of atoms in an alloy catalyst, recent developments have shown that X-ray emission analysis can still be used for this purpose (89, 90). By bombarding the surface with high energy electrons at grazing incidence, characteristic Ka radiation from monolayer quantities of both carbon and oxygen on an iron surface was observed. Simultaneously, information about the structure of the surface layer was obtained from the electron diffraction pattern. 

Although x-ray microanalysis in the STEM is the most developed form of analytical electron microscopy, many other types of information can be obtained when an electron beam interacts with a thin specimen. Figure 2 shows the various signals generated as electrons traverse a thin specimen. The following information about heterogeneous catalysts can be obtained from these signals ... 

To a limited extent, the type of results reported here could be obtained with a TEM instrument fitted with a STEM adapter, especially if a field-emission gun is used (10). We will refer here, however, only to the use of dedicated STEM instruments such as the HB5 or HB501 made by VG Microscopes Ltd. having a cold field emission gun. Most of the instruments of this type are specialized for microanalysis using EDS or ELS and some have been applied very effectively for compositional analysis and associated studies on catalyst particles, as reported elsewhere in this volume. 

TABLE 15.2. Microanalysis of PtSn catalysts by EDX (atomic ratio). 

These results suggest that the (101) superstructure observed on the (001) -phase at the catalyst s operating temperature is closely related to Bi2M02O9. A quantification of the microanalysis of the jS-preparation shows a Bi-deficiency. Similar results are observed in the reaction of the a-phase in propylene. In a C3 H6-O2 mixture under working conditions both phases show the presence of this superstructure similar to the jS-structure. The ETEM results are consistent with XPS and Raman data which show that the surface structure of the active bismuth molybdate is close to the jS-phase and that the jS-phase is more active (Matsurra et al 1980, Burrington et al 1983). In these studies dramatic increases in the activity... 

In this book, we have highlighted the unique contributions of electron microscopy, microanalysis and ED to our understanding of catalysis and the rational design of advanced catalysts at the nano-scale and processes. EM methods, including in... 

EDX microanalysis performed on Cr-doped catalyst showed a large number of agglomerates formed from the primary NigAlg phase. (Al/Ni /" 0.22, Cr/Ni ... 

Scanning electron x-ray microanalysis techniques reveal that metal deposits are frequently nonuniform through the catalyst pellet cross section after demetallation reactions. Metal profiles measured after HDM studies with Ni and VO-etioporphyrin (Agrawal and Wei, 1984), Ni-tetra(3-methylphenyl)porphyrin (Ware and Wei 1985a), and VO-tetraphenyl porphyrin (West, 1984) demonstrate the inhomogeneity. Examples in... 

Hydrodemetallation reactions are revealed to be diffusion limited by examination of metal deposition profiles in catalysts obtained from commercial hydroprocessing reactors. Intrapellet radial metal profiles measured by scanning electron x-ray microanalysis show that vanadium tends to be deposited in sharp, U-shaped profiles (Inoguchi et al, 1971 Oxenrei-ter etal., 1972 Sato et al., 1971 Todo et al., 1971) whereas nickel has been observed in both U-shaped (Inoguchi et al., 1971 Todo et al., 1971) and... 

Flego [1] recommends the use of micro devices for automated measurement and microanalysis of high-throughput in situ characterization of catalyst properties. Murphy et al. [5] stress the importance of the development of new reactor designs. Micro reactors at Dow were described for rapid serial screening of polyolefin catalysts. De Bellefon ete al. used a similar approach in combination with a micro mixer [6], Bergh et al. [7] presented a micro fluidic 256-fold flow reactor manufactured from a silicon wafer for the ethane partial oxidation and propane ammoxidation. 

Some catalyst activation processes are extremely important this is the case for oxides used as catalysts and supports (AI2O3, SiC>2, TiC>2, ZrC>2, silica-aluminas), and zeolites. Extremely elaborate procedures are used. This concerns bulk, not supported systems, and is dealt with in Section A.2.1. The case ofSiC>2 mixed with active phases (e.g. in oxidation) has little relevance to the subject of the present section, as it seems that SiOj does not play the role of a real support, but rather that of a diluent or spacer. An electron microscopy study coupled with microanalysis on a typical oxidation catalyst (propene to acrolein) shows that only a small fraction of the active phases is attached to silica or is situated in its immediate proximity [69]. There are not many cases... 

Figure 22 Si02 supported, MAO-activated zirconocene catalyst grains, Scanning Electron Microscopy (SEM) micrograph and element mapping by Energy-Dispersive X-ray Microanalysis.

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