Palladium surface structure

C.5.2. Hydrogen Adsorption Followed by CO Adsorption. When the gas dosing sequence was reversed, the coadsorption behavior became more complex and depended on the temperature and the palladium surface structure. We start with measurements at 100 K. On hydrogen-precovered Pd(l 1 1), no CO adsorption was... 

Room temperature deposition of silver on Pd(lOO) produces a rather sharp Ag/Pd interface [62]. The interaction with a palladium surface induces a shift of Ag 3d core levels to lower binding energies (up to 0.7 eV) while the Pd 3d level BE, is virtually unchanged. In the same time silver deposition alters the palladium valence band already at small silver coverage. Annealing of the Ag/Pd system at 520 K induces inter-diffusion of Ag and Pd atoms at all silver coverage. In the case when silver multilayer was deposited on the palladium surface, the layered silver transforms into a clustered structure slightly enriched with Pd atoms. A hybridization of the localized Pd 4d level and the silver sp-band produces virtual bound state at 2eV below the Fermi level. 

In spite of the advances made by these researchers, it remains unclear how membrane surfaces undergo restructuring and how these changes influence the catalytic and transport properties of the material. Furthermore, there is a need to link surface structure and composition with long-term performance of palladium membranes under continuous reaction conditions. One... 

In the following section, CO adsorption on alumina supported palladium nanoparticles of various sizes and surface structures is examined and compared with the corresponding results for CO on Pd(l 11) and rough Pd(l 11) (119,120,152,289). The preparation and characterization of the alumina support and of deposited palladium nanoparticles have been described in detail (63,68,73,83,101,290) and only a brief summary is given here. 

Ethene adsorption, in particular on platinum and palladium surfaces, has received much attention. The atomic structure of the various adsorbed ethene... 

Mckenzie A. L., Fishei C. T. and Davis R. J., Investigation of the surface structure and basic properties of calcined hydrotalcites. J. Catal. 138 (1992) pp. 547-561. Narayann S. and Krishna K., Hydrotalcite-supported palladium catalysts. Appl. Catal. A 174 (1998) pp. 221-229. 

Surface structure and catalytic reactivity of palladium overlayers for 1,3-butadiene hydrogenation... 

Perhaps the most surprising result of all is to find the hydrogenation of benzene and its reverse among the reactions that are oblivious of surface structure in the critical region of dispersion. Fortunately, this very important result concerning the hydrogenation of benzene has been corroborated by Aben et al. (41), who studied this reaction on platinum, palladium, and nickel supported over silica, alumina, silica-alumina, and silica-magnesia. The metal crystallite sizes varied between 10 and 200 A. The authors found that for each metal the activity per exposed metal atoms was independent of the dispersion and of the support. 

Enzymes may have more complicated structures than, say, a palladium surface, but they catalyse reactions in the same way. They act as a surface or focus for the reaction, bringing the substrate or substrates together and holding them in the best position for reaction. The reaction takes place, aided by the enzyme, to give products which are then released (Fig. 4.4). Note again that it is a reversible process. Enzymes can catalyse both forward and backward reactions. The final equilibrium mixture will, however, be the same, regardless of whether we supply the enzyme with substrate or product. 

For the characteristic differences between the low and high activity states we suggest the following explanations. When the reaction starts in the low activity state, palladium is in metallic form and hence, the catalyst must be trained to develop a surface structure characteristic of high activity state in the total conversion of m-xylene. In this state the combustion occurs in a typical heterogeneous oxidation, so the conversion increases with temperature in an exponential manner. 

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