High resolution electron microscopy catalysts

High resolution electron microscopy has recently demonstrated the capability to directly resolve the atomic structure of surfaces on small particles and thin films. In this paper we briefly review experimental observations for gold (110) and (111) surfacest and analyse how these results when combined with theoretical and experimental morphological studies, influence the interpretation of geometrical catalytic effects and the transfer of bulk surface experimental data to heterogeneous catalysts. 

Catalyst surface areas were measured using the multi-point BET method on a Carlo-Erba Ins. Sorpty 1750. Before the measurements, the samples were heated under dynamic vacuum at 573 K for 1 h in order to remove adsorbed water and impurities. Measurements were made at liquid nitrogen temperature with nitrogen as the adsorbate gas. Powder X-ray diffraction measurements were performed on a Siemens Model D-500 diffractometer with Co Kc monochromatic radiation (X = 1.78901 A) and the high resolution electron microscopy was carried out on a Topcon EM-002B microscope. To prevent artefacts no solvents were used in the preparation and mounting of samples for HRTEM. 

We discuss the combined use of high-resolution electron microscopy, electron diffraction, optical diffractometry and computer graphics for investigating zeolitic structure. Particular attention is given to twinned faujasitic materials and to intergrowth structures in ZSM-5 and ZSM-11 catalysts. 

For many heterogeneous catalysts the presence of 2 nm crystallites may be deleterious to performance. Unfortunately the positive identification of 1 nm Pt crystallites on a high surface area 7-Al203 support is extremely difficult. Although EXAFS has been used, it is not a routine technique. High resolution electron microscopy has not been successful with low metals loading. 

Improved annular dark field detectors for the scanning transmission electron microscopes may assist in detecting lnm crystallites. For tailoring a catalyst to a specific reaction, one must be able to relate the structure of the site to catalytic activity and selectivity. Possibly future developments in high resolution electron microscopy will address this problem. 

Hardacre el al. (7 75, 174) investigated the properties, structure, and composition of cerium oxide films prepared by cerium deposition on Pt(lll), finding that the activity for CO oxidation is enhanced on Pt(lll) that is partially covered by ceria. It was suggested that new sites at the Pt-oxide interface become available for reaction. A remarkable observation is the high activity for CO oxidation when the Pt(lll) sample is fully encapsulated by ceria (Pt was undetectable by XPS and AES). It was proposed that an ultrathin, disordered ceria film becomes the active catalyst. It was also demonstrated by XPS and AES that Pt dramatically increases the reducibility of cerium oxide that is in intimate contact with Pt. This result suggests that intimate contact between the noble metal and oxide phases is indeed crucial to facile oxygen release from ceria. High-resolution electron microscopy demonstrated the presence of direct contact between ceria and noble metal for supported Pt-Rh catalysts (775). Hardacre et al. (173,174) related the catalytic activity of the ceria phase to partially reduced cerium oxide. 

High Resolution Electron Microscopy (HREM) has proven as a very useful technique in the structural characterisation of supported metal catalysts (383-386) in general and, in particular, of noble metal catalysts supported on ceria-based oxides (52,70,72,97,105,109,117,124,135,137,139,144,147,155,171,182-184.194.203,209, 210,218,226,234,235,387) ... 

Using the accumulated data obtained from in-situ and high resolution electron microscopy studies, Baker and co-workers (ref. 22) developed a model to account for the growth of filamentous carbon resulting from metal catalyzed decomposition of selected hydrocarbons. The main steps in the mechanism are adsorption and decomposition of hydrocarbons at the leading face of catalyst particle, followed by dissolution of carbon in the metal and diffusion to the trailing faces where carbon is precipitated from solution to form the filament. Deactivation of the catalyst occurs... 

Extents of deactivation of an industrial catalyst CR 4/6 (Rhone-Poulenc, France) were determined by X-ray photoelectron (XPS) and IR-spectroscopy, high resolution electron microscopy (EM) and by activity in the Claus reaction. 

Datye AK, Smith DJ (1992) Characterization of heterogeneous catalysts by high resolution electron microscopy. Catal Rev Sci Fng 34 129... 

The catalyst employed in this work was a commercial Ru/C catalyst (Aldrich, ref 20,618-0). Inductive coupled plasma-atomic emission spectroscopy (ICP-AES) was used to measure the ruthenium content in the catalyst after dissolution of the solid in an acidic solution, and for the determination of the concentration of various metal ions in the solution after the oxidation treatment. The sizes of ruthenium particles were measured by high resolution electron microscopy (JEOL JEM 2010). 

Structure Elucidation from Crystal Powders. For many practical materials, such as polymers and zeolite catalysts, it is impossible to synthesize large crystals. Therefore the structure has to be found from powders. Powder XRD (preferably using synchrotron radiation) and neutron diffraction are the most important techniques, but experiments using other analysis methods like High Resolution Electron Microscopy (HREM) and Electron Diffraction (ED), MAS-NMR and EXAFS can add valuable information (8). 

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