Transmission electron microscopy catalyst characterization

MgO-supported model Mo—Pd catalysts have been prepared from the bimetallic cluster [Mo2Pd2 /z3-CO)2(/r-CO)4(PPh3)2() -C2H )2 (Fig. 70) and monometallic precursors. Each supported sample was treated in H2 at various temperatures to form metallic palladium, and characterized by chemisorption of H2, CO, and O2, transmission electron microscopy, TPD of adsorbed CO, and EXAFS. The data showed that the presence of molybdenum in the bimetallic precursor helped to maintain the palladium in a highly dispersed form. In contrast, the sample prepared from the monometallie precursors was characterized by larger palladium particles and by weaker Mo—Pd interactions. ... 

Transmission electron microscopy is one of the techniques most often used for the characterization of catalysts. In general, detection of supported particles is possible, provided that there is sufficient contrast between particles and support - a limitation that may impede applications of TEM on well-dispersed supported oxides. The determination of particle sizes or of distributions therein is now a routine matter, although it rests on the assumption that the size of the imaged particle is truly proportional to the size of the actual particle and that the detection probability is the same for all particles, independent of their dimensions. 

Transition metal oxides, rare earth oxides and various metal complexes deposited on their surface are typical phases of DeNO catalysts that lead to redox properties. For each of these phases, complementary tools exist for a proper characterization of the metal coordination number, oxidation state or nuclearity. Among all the techniques such as EPR [80], UV-vis [81] and IR, Raman, transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS) and NMR, recently reviewed [82] for their application in the study of supported molecular metal complexes, Raman and IR spectroscopies are the only ones we will focus on. The major advantages offered by these spectroscopic techniques are that (1) they can detect XRD inactive amorphous surface metal oxide phases as well as crystalline nanophases and (2) they are able to collect information under various environmental conditions [83], We will describe their contributions to the study of both the support (oxide) and the deposited phase (metal complex). 

The structure of the catalysts was characterized by X-ray diffraction, IR-spectroscopy and transmission electron microscopy, their thermal stability was followed by thermal analytical method. The specific surface area and pore size distribution of the samples were determined by nitrogen adsorption isotherms. 

Ffirai and Toshima have published several reports on the synthesis of transition-metal nanoparticles by alcoholic reduction of metal salts in the presence of a polymer such as polyvinylalcohol (PVA) or polyvinylpyrrolidone (PVP). This simple and reproducible process can be applied for the preparation of monometallic [32, 33] or bimetallic [34—39] nanoparticles. In this series of articles, the nanoparticles are characterized by different techniques such as transmission electronic microscopy (TEM), UV-visible spectroscopy, electron diffraction (EDX), powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) or extended X-ray absorption fine structure (EXAFS, bimetallic systems). The great majority of the particles have a uniform size between 1 and 3 nm. These nanomaterials are efficient catalysts for olefin or diene hydrogenation under mild conditions (30°C, Ph2 = 1 bar)- In the case of bimetallic catalysts, the catalytic activity was seen to depend on their metal composition, and this may also have an influence on the selectivity of the partial hydrogenation of dienes. 

The forty-eighth volume of Advances in Catalysis includes a description of a new and increasingly well understood class of catalysts (titanosilicates), a review of transmission electron microscopy and related methods applied to catalyst characterization, and summaries of the chemistry and processes of isobutane-alkene alkylation and partial oxidation and C02 reforming of methane to synthesis gas. 

Roth, C., Martz, N., and Fuess, H., Characterization of different Pt-Ru catalysts by x-ray diffraction and transmission electron microscopy, Phys. Chem. Chem. Phys., 3, 315,2001. 

Roster, A.J., Ziese, U., Verkleij, A.J., Janssen, A.H., de Graaf J., Geus, J.W., and de Jong, K.P. (2000) Development and application of 3-dimensional transmission electron microscopy (3D-TEM) for the characterization of metal-zeolite catalyst systems. Stud. 

Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) are recognized as powerful and versatile tools for the characterization of supported metal catalysts, because real-space images of catalysts with spatial resolution down to 0.1 nm can be recorded and combined with high-spatially resolved spectroscopic information. However, TEM has been used mainly for ex situ characterization, for example, of catalysts after gas treatments. 

II. Transmission Electron Microscopy (Limitations of Conventional TEM in the Characterization of Catalysts)... 

High-resolution transmission electron microscopy (HRTEM) has matured markedly in the preceding decade and has emerged as a powerful technique for investigation of nanostructured metal catalysts at the atomic level, even under working conditions. The ability to image the dynamic structure and morphology of supported metal nanocluster catalysts in such detail makes HRTEM an essential complement to the arsenal of spectroscopic techniques used for characterization of... 

With improvements in the preparation of more active HDS catalysts, MoS2 crystallites became smaller, and traditional physical techniques for characterization such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) became limited. In fact, today s best catalysts do not exhibit XRD patterns, and the active catalyst particles can no longer be observed directly by TEM. Thus, new techniques were required to provide structural information about Co(Ni)-Mo-S catalysts. As modern surface science characterization procedures evolved, they were immediately applied to the study of CoMoSx-based... 

Characterization of the Ru/KY and Ru/NaY catalysts used in this study by transmission electron microscopy showed that the metal was well-dispersed within the zeolite as particles small enough to fit inside the supercages, less than 1 nm in diameter. 

The promoted WZ catalysts containing lwt% Pt and/or lwt% Fe203 were characterized by CO chemisorption and by transmission electron microscopy (TEM) in combination with EDX.33 The state of iron and its redox behavior was also analyzed by x-ray absorption spectroscopy (XANES and EXAFS at the Fe K-edge), by in situ EPR spectroscopy and by Mossbauer spectroscopy. 

Characterization of Catalysts by Scanning Transmission Electron Microscopy... 

The electron microscope offers a unique approach for measuring individual nano-sized volumes which may be catalytically active as opposed to the averaging method employed by spectroscopic techniques. It is just this ability of being able to observe and measure directly small crystallites or nano-volumes of a catalyst support that sets the microscope apart from other analyses. There have been many studies reported in the literature over the past fifteen years which emphasize the use of analytical and transmission electron microscopy in the characterization of catalysts. Reviews (1-5) of these studies emphasize the relationship between the structure of the site and catalytic activity and selectivity. Most commercial catalysts do not readily permit such clear distinction of physical properties with performance. The importance of establishing the proximity of elements, elemental distribution and component particle size is often overlooked as vital information in the design and evaluation of catalysts. For example, this interactive approach was successfully used in the development of a Fischer-Tropsch catalyst (6). Although some measurements on commercial catalysts can be made routinely with a STEM, there are complex catalysts which require... 

Electron microscopy, with its high spatial resolution, plays an important role in the physical characterization of these catalysts. Scanning electron microscopy (SEM) is used to characterize the molecular sieve particle sizes and morphologies as a function of preparation conditions. Transmission electron microscopy (TEM) is used to follow the changes in the microstructure of the iron silicates caused by different growth conditions and subsequent thermal and hydrothermal treatments. 

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