Bimetallic bonding

Hi ly dispersed supported bimetallic catalysts with bimetallic contributions have been prepared from molecular cluster precursors containing preformed bimetallic bond [1-2]. For examples, extremely high dispersion Pt-Ru/y-AUOa could be prepared successfully by adsorption of Pt2Ru4(CO)ison alumina [2]. By similar method, Pt-Ru cluster with carbonyl and hydride ligands, Pt3Ru6(CO)2i(p3-H)(p-H)3 (A) was used in this work to adsorb on MgO support. The ligands were expectedly removable from the metal framework at mild conditions without breaking the cluster metal core. 

In this chapter, an overview is presented of studies that deal with the electronic and chemical properties of Pd in bimetallic systems. We will focus on palladium for three main reasons. First, bimetallic catalysts that contain Pd or other Group-10 metals have many uses isomerization of hydrocarbons, olefin hydrogenation, CO oxidation, alcohol synthesis, acetylene trimerization, etc. [8,10,19-21]. Second, palladium is very sensitive to the formation of bimetallic bonds [22-24]. And third, there is a vast number of experimental and theoretical articles in the literature that examine the properties of Pd in bimetallic systems [14,15,19-23,25-44]. From this large volume of work, one can get a general idea of how deep is our knowledge about the basic nature of bimetallic bonding and how it affects the properties of a metal. 

The core levels of Pd are also very sensitive to the formation of bimetallic bonds. Figure 4 shows Pd 3d XPS spectra for different coverages of palladium on... 

Fig. 3 Effects of bimetallic bonding on the properties of Pd Shift in the first peak of the Pd 4d band, the one closer to the Fermi level, as a function of metal substrate. Reprinted from ref [15].

In bimetallic systems like Pd/Al, Pd/Ti and Pd/Zn the bonding interactions between the metals are so strong that intermixing and formation of bulk alloys take place [35,46-50]. Alloy formation does not occur in many other bimetallic systems (PdfTa [25], Pd/W [26,53], Pd/Re [27], Pd/Ru [27], Pd/Rh [27]) and one can examine the strength of the corresponding bimetallic bonds using thermal... 

Mo(llO) [15,65], Re(0001) [27], Ru(OOOl) [27] and Rh(lll) [32], When going from a rhodium to a tantalum substrate, there is an increase of 150 K in the palladium desorption temperature, which indicates an enhancement of 10-12 kcal/mol in the strength of the Pd-substrate bond. At the same time, the Pd 3ds/2 core-level shift increases by more than a factor of fom. The stronger the bimetallic bond, the larger the electronic perturbations in the Pd atoms. The strongest metal-metal bonds are seen in systems that combine a metal with an electron-rich d band (Pd) and a substrate with an electron-poor d band (Ta,W,Mo). 

The type of correlation seen in Figure 9 has also been observed for Ni [22], Cu [22], Au [68] and Zn [69] overlayers. This suggests that in general the core level shifts do reflect changes in initial state induced by bimetallic bonding. And in most cases the formation of a strong metal-metal bond is associated with substantial perturbations in the electronic properties of the bonded metals [22,23,68,69]. 

From the experiments described above it is obvious that bimetallic bonding can have a tremendous impact on the chemical properties of a metal. It is important to establish in what kind of bimetallic systems one can expect the largest changes in chemical behaviour. Figxire 14 displays the desorption temperature observed for Pd-bonded CO on a series of bimetallic surfaces and Pd(lOO) [15,27,35,64,67,73-77],... 

Fig. 14 Effects of bimetallic bonding on the properties of Pd. The graph displays the CO desorption temperature and the shifts in the Pd 3ds/2 binding energy with respect to the surface atoms of Pd(lOO). Solid bars shift in Pd 3ds/2 binding energy hatched bars Pd CO desorption temperature. Reprinted from ref [35].

Bimetallic bonding can also produce interesting changes in the reactivity of a metal towards hydrogen (i.e. H2,gas -2Hads) [15,43,81-83]. Experimental evidence indicates that hydrogen dissociatively chemisorbs on Pd/Nb(110) at room... 

Photoemission studies have shown that in many cases the formation of a bimetallic bond induces positive core-level shifts for both metals [17,86,87,88,]. This, obviously, is not consistent with a simple metal->metal charge transfer [60,90], The phenomenon could be a consequence of combining inter- and intra-atomic charge redistributions (for example, d-sp rehybridization) induced by bimetallic... 

The FPLAPW method has also been used to study bimetallic bonding in PdAV(llO), Pd/Re(0001) and Pd/Ru(0001) [34], In general, electron density plots show an important shift of electrons from the Pd layer toward the metal-metal... 

Theoretical studies show that bimetallic bonding increases the stability of the Pd 4d valence band [14,23,34,36,40,90,98], The variation in surface core level shifts for metal overlayers is accompanied by a similar shift in the center of gravity of the admetal d band [34,40,90]. In the top panel of Figure 18 is shown the calculated density-of-states (DOS) at the Fermi level for a palladium monolayer on four different metal substrates. As one moves from Pd/Ru to Pd/Ta, there is a substantial... 

DF slab calculations have been used to study in a systematic way the effects of bimetallic bonding on the valence band of Pd and many other metals [14,36,101,102]. For metal overlayers, the strain induce by the metal substrate on... 

A large number of studies described in this book indicate that the formation of a heteronuclear metal-metal bond can lead to important changes in the chemical properties of the bonded elements. In many cases, bimetallic bonding induces a large redistribution of charge around the metals [48-50]. In principle, this redistribution of charge could affect the reactivity of a metal toward sulphur. A very... 

In the previous section we have discussed several cases in which bimetallic bonding increases the overall reactivity of a system towards sulphur. If the opposite occurs, such a phenomenon could be useful for the prevention of sulphur poisoning. In practical terms, the idea is to find bimetallic systems that have a good catalytic activity and are less sensitive to the presence of sulphur-containing molecules in the feedstream than pure metals. Sn/Pt and Pd/Rh satisfy these requirements [26-29]. 

Stable CT-bonded Pt—Hg complexes may be prepared when perfluorinated groups or sterically large groups near to the bimetallic bond are present [equation (25) X = halogen or Hg is eliminated on treatment with CF3CO2H yielding... 

Dong et al. [35] analyzed the multiple bimetallic bond of Cr-Cr, V-Mn, V-Tc, Nb-Mn, Cr-Mo, Mo-Mo, and Nb-Tc using CASPT2. They determined the bimetallic bond length, NBO, EBO, formal shortness ratio (FSR) values, and Mulliken charge populations for these systems. The V-Mn molecule presents the shortest bond length value and the largest FSR value in the series. 

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