Silver catalyst, structure

Claus, P. and Hofmeister, H. 1999. Electron microscopy and catalytic study of silver catalysts Structure sensitivity of the hydrogenation of crotonaldehyde. J. Phv.s. Chem. B. 103, 2766-2775. 

We have studied the steady-state kinetics and selectivity of this reaction on clean, well-characterized sinxle-crystal surfaces of silver by usinx a special apparatus which allows rapid ( 20 s) transfer between a hixh-pressure catalytic microreactor and an ultra-hixh vacuum surface analysis (AES, XPS, LEED, TDS) chamber. The results of some of our recent studies of this reaction will be reviewed. These sinxle-crystal studies have provided considerable new insixht into the reaction pathway throuxh molecularly adsorbed O2 and C2H4, the structural sensitivity of real silver catalysts, and the role of chlorine adatoms in pro-motinx catalyst selectivity via an ensemble effect. 

A significant cost advantage of alkaline fuel cells is that both anode and cathode reactions can be effectively catalyzed with nonprecious, relatively inexpensive metals. To date, most low cost catalyst development work has been directed towards Raney nickel powders for anodes and silver-based powders for cathodes. The essential characteristics of the catalyst structure are high electronic conductivity and stability (mechanical, chemical, and electrochemical). 

As in the case of normal supported catalysts, we tried with this inverse supported catalyst system to switch over from the thin-layer catalyst structure to the more conventional powder mixture with a grain size smaller than the boundary layer thickness. The reactant in these studies (27) was methanol and the reaction its decomposition or oxidation the catalyst was zinc oxide and the support silver. The particle size of the catalyst was 3 x 10-3 cm hence, not the entire particle in contact with silver can be considered as part of the boundary layer. However, a part of the catalyst particle surface will be close to the zone of contact with the metal. Table VI gives the activation energies and the start temperatures for both methanol reactions, irrespective of the exact composition of the products. 

Similar to that of the Bu jtpy-silver catalyst, the crystal structure of the bath-ophenanthroline-silver complex has a disilver(I) core and two BP ligands stacked over each other, similar to the corresponding palladium(II)-BP complex. 

A detailed infrared study of adsorbed species has also been carried out by Force and Bell. ° They used a supported silver catalyst under full reaction conditions at a temperature of 493 K. By measuring the infrared spectra of adsorbed reactants and products in the presence and absence of oxygen, the authors were able to assign most of the bands present during the reaction. The remaining bands were then compared with spectra obtained from known systems and hence the authors were able to propose likely structures for the reaction intermediates and a reaction mechanism. ... 

In view of evidence such as that in Fig. 8-5, it is unlikely that detailed quantitative descriptions of the void structure of solid catalysts will become available. Therefore, to account quantitatively for the variations in rate of reaction with location within a porous catalyst particle, a simplified model of the pore structure is necessary. The model must be such that diffusion rates of reactants through the void spaces into the interior surface can be evaluated. More is said about these models in Chap. 11. It is sufficient here to note that in all the widely used models the void spaces are simulated as cylindrical pores. Hence the size of the void space is interpreted as a radius 2 of a cylindrical pore, and the distribution of void volume is defined in terms of this variable. However, as the example of the silver, catalyst indicates, this does not mean that the void spaces are well-defined cylindrical pores. 

The aim of this work is to explore the applicability of the sol-gel method for the preparation of Ag/Si02 and Cu/Si02 catalysts and to see whether such a method can yield silver and copper species stabilized by the carrier. Characterization of the catalyst structure by several physical and chemical techniques, including N2 adsorption-desorption isotherms, mercury porosimetry measurements, X-ray diffraction and transmission electron microscopy, has been used to correlate the microstructure of Ag/Si02 and Cu/Si02 catalysts with their catalytic performance. 

Silver catalysts have been used for the partial oxidation of methanol to formaldehyde this is a very important process in the chemical industry. The role of the silver catalyst and, in particular, the influence of its atomic structure on the catalytic process have been extensively studied with various surface science tools [44—50]. In these investigations, Raman spectroscopy was employed to identify and confirm the role of the oxygen species for the catalytic process. These studies were performed under reaction conditions close to those in industrial processes using Ag(lll) and Ag(llO) samples. Upon extended exposure to oxygen at high temperatures, both samples restructure to (111) planes with a well-defined microstructure and with mesoscopic roughness (on a scale of 1 pm). Therefore, in the course of the oxygen pretreatment, the local nature of the surface of the two samples becomes nearly identical and, hence, their Raman spectra are quite similar [44]. 

Silver is the only efficient catalyst for ethylene epoxidation providing the Tninimimi of by-products. Why is it so unique The question still remains open. Supported silver catalysts have one peculiar property the influence of the size of the silver particles on the reaction rate is observed within particle sizes of 100-1000 A [1-4]. Tins effect is explained by the change of surface structure, silver particle morphology, influence of support, etc. 

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