Palladium atomic properties

In all of this work there was little suggestion that the surface states of the palladium might behave differently from bulk states. Selwood (17) indicated that, from some sorption-magnetic susceptibility data for hydrogen sorbed on palladium which was finely dispersed on alumina gel, the ultimate sorption capacity was approximately at the ratio 2H/Pd. Trzebiatowsky and coworkers (25) deposited palladium on alumina gel in amounts ranging from 0.46 to 9.1% of gel weight. They found the palladium to be present in a normal crystal lattice structure, but its susceptibility was less than for the bulk metal. This suggested to the present authors that the first layer of palladium atoms laid down on the alumina gel underwent an interaction with the alumina, which has some of the properties of a semiconductor. Such behavior was definitely shown in this laboratory (22) in the studies on the sorption of NO by alumina gel. Much of this... 

The electronic structure of palladium atoms, 4d 5s , is unique and may be responsible for this specific catalytic property for the acetylene cyclotrimerization. 

Palladium-silica catalysts prepared from tetra-ammine palladous nitrate (to avoid chlorine introduction) showed a marked reduction effect , viz, the specific activity for benzene hydrogenation decreased with increased reduction temperature, i.e., 573 or 723Various explanations were considered, including a metal-support interaction. After reduction at 873 K, X-ray diffraction provided clear evidence of chemical reaction and at lower temperatures silicon insertion into palladium might still occur, which could either disrupt the palladium ensembles required for benzene adsorption or modify the properties of single palladium atoms, if these are the active sites. 

The electronic structure of palladium atoms, 4d °5s°, is unique, and may be responsible for this specific catalytic property for acetylene cyclotrimerization. This raises the question of whether other transition metal atoms are also reactive for this reaction. Results are shown for deposited Rh (4c/ 5i ) and Ag (4c/ atoms. Ag atoms are almost unreactive (Fig. 3a) on supported Rh atoms, however, benzene is formed, and desorbs at around 430 K (Fig. 3a). 

M—C a bonds are very few. In this chapter more precisely, larger metallomacrocycles with at least one M—C a bond with or without additional heteroatoms are reviewed. Unlike in other chapters, here the compounds are reviewed on the basis of the metal atom that is present in the ring, since the generalization of the synthesis, structure elucidation, and physical and chemical properties of all titled metallomacrocycles is rather intricate. Hence the chapter is framed to focus on two classes of metallomacrocycles (i) with mercury atoms, and (ii) with platinum and/or palladium atoms. 

A very different model is called the minimum polarity model [131]. According to this model each component of an alloy has an electronic structure similar to that in its pure phase. Consequently the addition of silver to palladium would dilute palladium without changing very much its individual properties. This model has been improved [132] by assuming short range interactions between palladium atoms and silver atoms in the vicinity of palladium. 

An alternative approach consists of the use of a O2-DMSO system for the direct reoxidation of palladium without any need of benzoquinone. These systems have been proposed to involve colloidal palladium(0) and the role of DMSO would then be to dissolve such cluster-like particles. The coordination properties of DMSO are expected to keep the clusters in solution by coordination to individual palladium atoms. 

Mononuclear cyclopalladated liquid crystalline materials whose molecular structure consists of two different thermotropic ligands, connected by a palladium atom, 55, have been also prepared. The mesomorphic properties depend on the length of the chain. Monotropic or enantiotropic nematic and/or smectic phases can be found [131]. 

In the past, d-band theory has been successfully used to explain the reactivity of a wide range of catalysts. It was taken for granted that the participation of the structureless and wide sp band was not relevant. However, we want to emphasize that this approach is an oversimplification that could lead to wrong results. We have included two different pictures of the same system in order to show the behavior of the electron bands during the adsorption process. The electronic properties of a hydrogen atom approaching to the surface of a three palladium atoms cluster on Au(lll) are shown in Figure 1.16. 

As is known [25], on palladium, a molecular adsorption of CO is predominant, which determines its high selectivity with respect to methanol. The decrease in palladium activity towards methanol synthesis can hardly be explained by the effect of ruthenium dilution, since molecular adsorption of CO and its hydrogenation to CH3OH can, most probably, proceed on smaller centers than are required for dissociative adsorption of CO. This phenomenon may be accounted for by the ligand effect, that is, by the change of electronic properties of a palladium atom due to the presence of ruthenium atoms in its first coordination sphere. 

The platinum-group metals (PGMs), which consist of six elements in Groups 8— 10 (VIII) of the Periodic Table, are often found collectively in nature. They are mthenium, Ru rhodium, Rh and palladium, Pd, atomic numbers 44 to 46, and osmium. Os indium, Ir and platinum, Pt, atomic numbers 76 to 78. Corresponding members of each triad have similar properties, eg, palladium and platinum are both ductile metals and form active catalysts. Rhodium and iridium are both characterized by resistance to oxidation and chemical attack (see Platinum-GROUP metals, compounds). 

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