Pd intermetallic compounds

Fig. 20. Main 3d5,2XPS pealr from La-Pd and Ce-Pd intermetallic compounds.

The alkali metals do not form intermetallic compounds with the transition metals. Both Na and Li dissolve only ppm quantities of most transition metals, although the solubilities are greater in Li. Nickel is very soluble in Li (900 ppm at 600°C), but no intermetallic compound forms. Intermetallic compound formation is, however, observed between Li and the noble metals Rh, Ir, Pd and Pt, so this section concentrates on the preparation of these compounds. 

Beryllium forms intermetallic compounds with transition metals and phase diagrams are available Some 26 phase diagrams have also been published for Mg-transition metal systems, and intermetallic compound formation is widespread in these systems also. The extent of intermetallic compound formation decreases down group IIA, such that Ca, Sr and Ba show much less tendency for compound formation to the extent that compounds are observed only in the Ba-Pd system. 

The intermetallic compounds CePd3 and ZrPd3 exhibited higher selectivity for butene formation than Pd. On Pd the hydrogen and butadiene are adsorbed on similar sites, whereas on the intermetallic compounds different sites may be involved in these adsorption processes44. 

A large number of matrix modifiers have been developed that thermally stabilize the analyte, allowing higher ash temperatures to be used without analyte loss. In this way, more matrix may be removed leaving less to interfere with the analyte s determination. Examples of this type of matrix modifier include some transition metal ions, e g. Ni and Pd, which form thermally stable intermetallic compounds with the metalloids, e.g. As-Ni,... 

Some intermetallic compounds have the same structures as those of simple polar compounds. Quite a few AM type intermetallic compounds have the NaCl (3 2PO, Section 5.1.1) structure, but usually for those differing significantly in electronegativities. Table 5.1 includes many compounds of the types MP, MAs, MSb, and MBi. The NiAs structure (2-2PO, Section 5.2.1) is found for a few MAs and MSb compounds (Table 5.5), the MSn compounds of Fe, Ni, Cu, Pd, Pt, Rh, and Au, and MnBi, NiBi, PtBi, and RhBi. The ZnS structures (CN 4) are not usually encountered for intermetallic compounds. The compounds of Al, Ga, and In with P, As, and Sb have the zinc blende (ZnS, 3 2PT, Section 6.1.1) structure. These are semiconductors or insulators. Because the bcc structure is common for metals, it is not surprising that many 1 1 intermetallic compounds have the CsCl structure (3 2PTOT, Section 7.2.1). A few of these intermetallic compounds are included in Table 7.1 a more extensive list is given in Table 9.1. 

Metals have been considered in Chapter 4 on the elements. Some alloys are simple solid solutions, and others have the composition of intermetallic compounds. If two metals are melted together we almost always obtain a liquid solution. If the liquid is cooled it can form a solid solution, give two or more phases of the metal(s) and /or intermetallic compound(s), or a single intermetallic compound might be formed if the composition corresponds to that of the compound. Chemically similar metals of similar size have the greatest tendency to form solid solutions. The following pairs, of similar size and the same periodic group, form solid solutions for any proportions K-Rb, Ag-Au, Cu-Au, As-Sb, Mo-W, and Ni-Pd. Hume-Rothery noted that usually a metal of lower valence is likely to dissolve more of one of... 

The second mode of action of a modifier is direct reaction with the analyte to convert it into a phase with greater thermal stability, that is, to reduce analyte volatility. In this way, the charring stage can be carried out at higher temperatures, allowing a more efficient removal of the matrix but without the loss of analyte. Examples of this type of matrix modifier include transition metal ions (mainly Pd), which form thermally stable intermetallic compounds with analytes, and magnesium nitrate, which thermally decomposes to magnesium oxide, and in the process traps analyte atoms in its crystalline matrix it is thermally stable until 1100°C. In fact, the most frequently reported mixture for matrix modification consists of Pd(N03)2 and Mg(N03)2, proposed by Schlemmer and Welz as a universal chemical modifier.17... 

Finally, the use of permanent chemical modifiers must be mentioned. Such chemical modification involves coating the graphite tube with a noble metal such as Ir, Pt, W, or Zr. These modifiers behave in much the same way as aqueous Pd in that thermally stable intermetallic compounds are formed on the hot inner surfaces of the coated graphite tube. 

Our intention was to employ the reaction of neopentane for probing the Pd surface, especially when the so-called strong metal-support interactions were expected. Such interactions may be manifested both in topographical (sintering and pillbox structure) as well as in chemical changes (decoration by support species and formation of an intermetallic compound) of the surface of Pd particles, and it would be difficult to decide a priori to what extent each of these factors may contribute to the observed changes in the catalytic behavior. Our goal was to try to separate these two factors. 

These catalysts are composed of one or several metallic active components, deposited on a high surface area support, whose purpose is the dispersion of the catalytically active component or components and their stabilization [23-27], The most important metallic catalysts are transition metals, since they possess a relatively high reactivity, exhibit different oxidation states, and have different crystalline structures. In this regard, highly dispersed transition clusters of metals, such as Fe, Ru, Pt, Pd, Ni, Ag, Cu, W, Mn, and Cr and some alloys, and intermetallic compounds, such as Pt-Ir, Pt-Re, and Pt-Sn, normally dispersed on high surface area supports are applied as catalysts. 

Metals frequently used as catalysts are Fe, Ru, Pt, Pd, Ni, Ag, Cu, W, Mn, and Cr and some of their alloys and intermetallic compounds, such as Pt-Ir, Pt-Re, and Pt-Sn [5], These metals are applied as catalysts because of their ability to chemisorb atoms, given an important function of these metals is to atomize molecules, such as H2, 02, N2, and CO, and supply the produced atoms to other reactants and reaction intermediates [3], The heat of chemisorption in transition metals increases from right to left in the periodic table. Consequently, since the catalytic activity of metallic catalysts is connected with their ability to chemisorb atoms, the catalytic activity should increase from right to left [4], A Balandin volcano plot (see Figure 2.7) [3] indicates apeak of maximum catalytic activity for metals located in the middle of the periodic table. This effect occurs because of the action of two competing effects. On the one hand, the increase of the catalytic activity with the heat of chemisorption, and on the other the increase of the time of residence of a molecule on the surface because of the increase of the adsorption energy, decrease the catalytic activity since the desorption of these molecules is necessary to liberate the active sites and continue the catalytic process. As a result of the action of both effects, the catalytic activity has a peak (see Figure 2.7). 

Here, AH(A-B) is the partial molar net adsorption enthalpy associated with the transformation of 1 mol of the pure metal A in its standard state into the state of zero coverage on the surface of the electrode material B, ASVjbr is the difference in the vibrational entropies in the above states, n is the number of electrons involved in the electrode process, F the Faraday constant, and Am the surface of 1 mol of A as a mono layer on the electrode metal B [70]. For the calculation of the thermodynamic functions in (12), a number of models were used in [70] and calculations were performed for Ni-, Cu-, Pd-, Ag-, Pt-, and Au-electrodes and the micro components Hg, Tl, Pb, Bi, and Po, confirming the decisive influence of the choice of the electrode material on the deposition potential. For Pd and Pt, particularly large, positive values of E5o% were calculated, larger than the standard electrode potentials tabulated for these elements. This makes these electrode materials the prime choice for practical applications. An application of the same model to the superheavy elements still needs to be done, but one can anticipate that the preference for Pd and Pt will persist. The latter are metals in which, due to the formation of the metallic bond, almost or completely filled d orbitals are broken up, such that these metals tend in an extreme way towards the formation of intermetallic compounds with sp-metals. The perspective is to make use of the Pd or Pt in form of a tape on which the tracer activities are electrodeposited and the deposition zone is subsequently stepped between pairs of Si detectors for a-spectroscopy and SF measurements. 

In the realm of theory also, greater demands will be made. As such studies (37—39) as those of Cu—Ni (Fig. 13) and Ag—Pd (Fig. 14) have shown, the d levels of the two species in transition metal alloys tend to maintain their atomic identities, at least when the levels in the pure components are sufficiently well separated in energy. However, neither calculation nor experiment has been done with refinement sufficient for quantitative testing of a theory, such as the coherent potential approximation, designed to describe the d band behavior. In pure metals and intermetallic compounds, band calculations can be compared directly with experiment if transition probabilities and relaxation effects are understood. With care they can be used also in evaluation of the effective interelectronic terms which enter equations such as (18a). Unfortunately, one cannot, by definition, produce a set of selfconsistent band calculation results for a matrix of specific valence electron snpmdl.. . configurations thus, direct estimates for I of Eq. (18a) or F of Eq. (18b) cannot be made. However, band calculations for a set of systems can indicate whether or not it is reasonable to factor level shifts into volume and electron count terms, in the manner of Eqs. (18a) and (23). When this cannot be done, one must revert to a more general expression for a level shift, such as Eq. (1). 

Other hydrides of intermetallic compounds include the A M (A = rare earth, M = Ni, Co) or the MoSi structure intermetallics (where Mo = Cu, Pd Si = Ti, Zr, Hf) , etc. . The formation of these hydrides does not differ from that of those already covered however, because of their technological value, the formation of Mg Ni hydrides is discussed. 

From studies of x-ray absorption spectroscopy [51,52], it is known that Pd has a tendency to lose d electrons when forming bulk intermetallic compounds. Figure 17... 

As further matrix modifiers Mg(N03)2 and often Pd(N03)2 are used. The can be used separately but are often also used as a mixture. The mechanisms of their stabilizing action, although having been investigated intensively are not completely known, but seem to relate to the formation of intermetallic compounds with the analytes. In the case of Pd such as is used for the determination of Sn, the selectivity for the stabilization of tin, for example in determinations in organic media, is based on the formation of Pd3Sn2, which can be shown by x-ray diffraction. 

In previous works [8,9], bimetallic Bi-Pd catalysts supported on activated carbon and characterized by various Bi/Pd molar ratios ((Pd+Bi)=10 wt.%) were prepared from the thermal degradation of Bi and Pd acetate-type precursors under nitrogen at 773 K. Because several binary Bi-Pd alloys were heavily suspected in the supported catalysts, three intermetallic compounds, Bi2Pd, BiPd and BiPda were also prepared from the same precursors, according to the same... 

XRD Most bimetallic catalysts are characterized by poorly resolved XRD spectra, suggesting an amorphous or microcristalline structure. Metallic bismuth and an intermetallic compound (Bi2Pd) were however observed in the catalysts in which the Bi/Pd molar ratio is equal to 2. Figure 1 shows the X-ray diffraction pattern of the bimetallic supported catalyst Ac.lPd2Bi/Co. 

XPS The XPS data collected on the pure intermetallics before use in the catalytic tests are are listed in table 2. As indicated by this table, palladium is again partially covered by bismuth (Pd/Bi exp- > Pd/Bi theor) and is mainly in the metallic form, while bismuth is present on the surface of these intermetallic compounds in the oxidized form. 

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