Monometallic stability

Li and Armor reported that Co-exchanged zeolites present a very high catalytic performance for the CH4-SCR, even in oxygen excess conditions [1, 3], Bimetallic Pt-and Pd-Co zeolites have revealed an increase of activity, selectivity towards N2 and stability, when compared with monometallic Co catalysts [4-8] even in the presence of water in the feed. Recent works show that these catalytic improvements are due to the presence of specific metal species as isolated metal ions, clusters and oxides and their location inside the cavities or in the external surface of zeolite crystallites [9, 10],... 

Hitherto no monometalated molecular pnictide exists without solvation of the main group metal atom. Therefore, the monomeric species L (Fig. 2) can only be stabilized if the Li ion has its coordination sphere enlarged through donor solvation. More importantly, the lithium phosphanides of the type K undergo oligomerization processes to form dimer, tetramer, hexamer, or polymeric assemblies M—Q (Fig. 2), which dissociate in solution more easily than related amides (2, 11, 12). 

Bimetallic Catalysts and Promoters. Shah and co-workers compared the methane decomposition activities and stabilities for monometallic (Pd, Mo or Ni) and bimetallic M-Fe (M = Pd, Mo or Ni) catalyst above 673 Their studies showed that the bimetallic M-Fe catalysts produced hydrogen at significantly higher rates than the monometallic (M) catalysts. The Pd-Fe catalyst was found to be the most active methane decomposition catalyst at 973 K. 

Coreduction of Mixed Ions. Coreduction of mixed ions is the simplest method to synthesize bimetallic nanoparticles. However, this method cannot be always successful. Au/Pt bimetallic nanoparticles were prepared by citrate reduction by Miner et al. from the corresponding two metal salts, such as tetrachloroauric(III) acid and hexachloroplatinic(IV) acid (24). Reduction of the metal ions is completed within 4 h after the addition of citrate. Miner et al. studied the formation of colloidal dispersion by ultraviolet-visible (UV-Vis) spectrum, which is not a simple sum of those of the two monometallic nanoparticles, indicating that the bimetallic nanoparticles have an alloy structure. The average diameter of the bimetallic nanoparticles depends on the metal composition. By a similar method, citrate-stabilized Pd/Pt bimetallic nanoparticles can also be prepared. 

An attempt to prepare PVP-stabilized Au/Pd bimetallic nanoparticles was made by the successive reduction procedure (40). When preparation of Au nanoparticles precedes the reduction of Pd ion, only mixtures of Pd and Au monometallic nanoparticles were produced. On the other hand, when the preparation of Pd nanoparticles precedes the reduction of Au ions, some bimetallic nanoparticles were found. However, the Au/Pd bimetallic nanoparticles thus obtained did not have a core/shell structure, although the bimetallic nanoparticles obtained by the coreduction have such a structure (Fig. 9.1.4). 

The examples are shown in Figure 9.1.10, which gives x-ray diffractograms of three types of physical mixtures of PVP-stabilized Pd, Pt, and Au monometallic nanoparticles, and the corresponding PVP-stabilized bimetallic nanoparticles (53). The diffraction patterns of the physical mixtures are consistent with the sum of two individual patterns, and are clearly different from those of the bimetallic nanoparticles, which have two broader peaks, indicating that several interatomic lengths exist in a single particle. By XRD one can easily understand if the obtained multi-metallic nanoparticles have an alloy structure or are simple physical mixtures of monometallic particles. 

If two different external electrophiles are to be added to the dimetallic species, diastere-oselectivity can only be achieved if the two carbon-metal bonds are properly discriminated against the reaction with a first electrophile. Moreover, the resulting monometallic species has to exhibit significant configurational stability. Coordination by a heteroatom, which turned out to be essential for achieving substrate-induced diastereoselection, also nicely served these purposes. 

In another approach, transition metals have been used to stabilize the chemically reactive sesquifulvalene [116, 122]. In an early report, the bimetallic (Fe-Cr) complex was reported to exhibit hyperpolarizabilities up to 570 x 10 ° esu. However, for some other ferrocene derivatives of the sesquifulvalene complexes multi-photon luminescence has been observed which prohibits the determination of the hyperpolarizability by HRS. However, replacement of Fe(II) by Ru(II) results in luminescence-free derivatives [122, 123[. Monometallic manganese(I) sesquifulvalene complexes, on the other hand, are reported to exhibit only modest hyperpolarizabilities [124]. 

Studies with palladium(II) revealed that in [Pd(en)(OH)2]/ribitol solutions with a molar ratio of 3 1 or 2 1 the dimetallated complex [Pd2(en)2(Ribtl,2 3,4H 4)] is formed as main species. The remaining terminal hydroxyl function is employed in an intramolecular S hydrogen bond to 02 causing the enhanced stability compared to the minor, bis(terminal) species [Pd2(en)2(Ribtl,2 4,5H 4)]. The monometallated terminal complex [Pd(en)(Ribtl,2H 2)] is the main species in equimolar solutions. The detection of the isomeric complex [Pd(en) (Ribt2,3H 2)] as a minor species gives reason to state that terminal chelation in erythro polyols is more favorable than a chelation with two adjacent secondary hydroxyl functions [19]. This is confirmed by the observation that (dppp)Pt " forms the terminal 1,2-diolato complex with ribitol with marked excess compared to the 2,3-erythro bound isomer (molar ratio 83 17, respectively) [22]. 

The bimetallic Pt-Re/Al203-Cl catalyst is the most widely used in naptha reforming. The addition of Re strongly improves the stability of the traditional monometallic Pt catalyst. Such improvement is explained by a double effect of rhenium stabilization of the metallic phase on the support and higher resistance to deactivation by coke deposition [1-8]. Nevertheless the role and the nature of the interaction between Pt and Re are the subject of many controversies [9-14]. 

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