Palladium bulk properties

Figure 21 provides an example of the use of ESCA to define an oxidation state of a freshly reduced palladium-on-carbon hydrogenation catalyst exposed to the air. The metallic palladium peaks (Fig. 21a) are quite evident, indicating no bulk oxidation occurred. There is a strong peak for carbon, probably due to adsorbed CO2 from the air. The presence of a small amount of PdO is suggested at 337 eV in Fig. 21B. This peak is a shoulder on the palladium 3 5/2 peak and most likely represents a surface layer of oxide on the palladium. This information could not be conveniently obtained by XRD because small palladium (or PdO) crystallites cannot diffract X rays. Furthermore, XRD measures bulk properties and would not see surface oxides even if the crystallite sizes were sufficiently large to be XRD sensitive. We can therefore expect to see more frequent use of ESCA or other surface sensitive techniques to monitor the surface of catalytic materials.

Besides, PEG has been found to affect the kinetics of palladium deposition as well as both morphology and bulk properties of Pd deposits [103] bulk defectiveness increased with the size of PEG molecules. Investigations of PEG role in electrodeposition of Zn-Cr alloys have shown [104] that large PEG molecules enhance Cr(III) reduction. 

Palladium Ensembles in Zeolites. Small metal particles or clusters have attracted great interest during the last decade. The optical, electronic and catalytic characteristics of clusters are expected to change from bulk properties to molecular properties within a certain size-range. " This change is represented by the transition of the electronic band structure of a crystal to the molecular orbital levels of species few atoms in size. Since the cluster size determines the relative population of coordination sites and possibly its molecular symmetry, it is thought to be responsible for modified selectivities in a number of catalytic reactions. Controlled synthesis of stable clusters with defined size is of particular interest, because this would potentially allow to fine-tune the properties of electronic materials and catalyst systems. 

Metals and alloys, the principal industrial metalhc catalysts, are found in periodic group TII, which are transition elements with almost-completed 3d, 4d, and 5d electronic orbits. According to theory, electrons from adsorbed molecules can fill the vacancies in the incomplete shells and thus make a chemical bond. What happens subsequently depends on the operating conditions. Platinum, palladium, and nickel form both hydrides and oxides they are effective in hydrogenation (vegetable oils) and oxidation (ammonia or sulfur dioxide). Alloys do not always have catalytic properties intermediate between those of the component metals, since the surface condition may be different from the bulk and catalysis is a function of the surface condition. Addition of some rhenium to Pt/AlgO permits the use of lower temperatures and slows the deactivation rate. The mechanism of catalysis by alloys is still controversial in many instances. 

In their electrochemical surface properties, a number of metals (lead, tin, cadmium, and others) resemble mercury, whereas other metals of the platinum group resemble platinum itself. Within each of these groups, trends in the behavior observed coincide qualitatively, sometimes even semiquantitatively. Some of the differences between mercury and other. y- or p-metals are due to their solid state. Among the platinum group metals, palladium is exceptional, since strong bulk absorption of hydrogen is observed here in addition to surface adsorption, an effect that makes it difficult to study the surface itself. 

Trace impurities in noble metal nanoclusters, used for the fabrication of highly oriented arrays on crystalline bacterial surface layers on a substrate for future nanoelectronic applications, can influence the material properties.25 Reliable and sensitive analytical methods are required for fast multi-element determination of trace contaminants in small amounts of high purity platinum or palladium nanoclusters, because the physical, electrical and chemical properties of nanoelectronic arrays (thin layered systems or bulk) can be influenced by impurities due to contamination during device production25 The results of impurities in platinum or palladium nanoclusters measured directly by LA-ICP-MS are compared in Figure 9.5. As a quantification procedure, the isotope dilution technique in solution based calibration was developed as discussed in Chapter 6. 

Baetzold used extended Hiickel and complete neglect of differential overlap (CNDO) procedures for computing electronic properties of Pd clusters (102, 103). It appeared that Pd aggregates up to 10 atoms have electronic properties that are different than those of bulk palladium. d-Holes are present in small-size clusters such as Pd2 (atomic configuration 4dw) because the diffuse s atomic orbitals overlap strongly and form a low-energy symmetric orbital. In consequence, electrons occupy this molecular orbital, leaving a vacant d orbital. For a catalytic chemist the most important aspect of these theoretical studies is that the electron affinity calculated for a 10-atom Pd cluster is 8.1 eV. This value, compared to the experimental work function of bulk Pd (4.5 eV), means that small Pd clusters would be better than bulk metal as electron acceptors. 

The efficiencies of various metals for the recombination of H atoms are given in Table 8. In all cases, the recombination is first-order and the activation energy is less than 5 kJ mole 1. The fact that the state of anneal of the metal can alter 7 by a factor of ten [14] suggests that the reaction may occur on minority sites, rather than on a well-defined surface characterised by the intrinsic properties of the metal. This is certainly the explanation for the wide range of values reported for palladium. The extent of diffusion of H atoms into the bulk is also a complicating factor, making 7 dependent on the time of exposure to H atoms for palladium, this effect can be so gross as to result in distortion of the sample. 

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 catalytic activity of Pti c polymer-protected bimetallics has been found to vary strongly with composition [75]. As shown in Figure 11, such catalysts have been studied by Pt NMR. However, because the palladium NMR has not been observed, the average surface electronic properties can be determined only indirectly and tentatively. The Pt NMR in Figure 11 has shown that the interior of the alloy particles is bulklike. In the bulk alloys the f-LDOS on both Pt and Pd sites varies rapidly with composition around x = 0.8 [54]. It is supposed, but not proven, that on the surfaces of the alloy particles the f-LDOS changes strongly with composition as well and that this explains the variation in catalytic activity. 

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