Bimetallic oxide clusters

Fig. 14.11 Mass distribution of Au Fe O demonstrating the production of small bimetallic oxide clusters...

The exceptional renewed interest in supported gold catalysts leads us to mention here a few recent examples of the formation of in situ gold carbonyl species from supported gold that can be related to the preparation of supported Au nanoclusters [53-55]. On the other hand, bimetallic carbonyl cluster salts of [Au4Fe4(CO)i,5] and [AuFe4(CO)i,5] have been used recently in the preparation of Au/Fe0,t/Ti02 catalysts for the total oxidation of dichlorobenzene and toluene [56]. 

When the three-component system described in Section 1.39.5 is subjected to hydrothermal conditions of higher pH (5 to 8) and more extreme temperatures and reaction times, the resultant oxide materials often exhibit architectures in which the secondary metal is directly incorporated into a bimetallic oxide network and/or in which the molybdate cluster identity is lost upon fusing into one- or two-dimensional molybdenum oxide substructures. 

Mixed metal catalysts can also be made using PVD [41,42] and by activation of rapidly quenched metal alloys such as Au5FeZni4 and Au5Ag Zn,4 under CO oxidation conditions at 553 K [52]. Other methods for preparing mixed metal catalysts include the use of dendrimer-stabilized bimetallic particles, and adsorption of bimetallic molecular clusters [2],... 

Our first attempt of a successive reduction method was utilized to PVP-protected Au/Pd bimetallic nanoparticles [125]. An alcohol reduction of Pd ions in the presence of Au nanoparticles did not provide the bimetallic nanoparticles but the mixtures of distinct Au and Pd monometallic nanoparticles, while an alcohol reduction of Au ions in the presence of Pd nanoparticles can provide AuPd bimetallic nanoparticles. Unexpectedly, these bimetallic nanoparticles did not have a core/shell structure, which was obtained from a simultaneous reduction of the corresponding two metal ions. This difference in the structure may be derived from the redox potentials of Pd and Au ions. When Au ions are added in the solution of enough small Pd nanoparticles, some Pd atoms on the particles reduce the Au ions to Au atoms. The oxidized Pd ions are then reduced again by an alcohol to deposit on the particles. This process may form with the particles a cluster-in-cluster structure, and does not produce Pd-core/ Au-shell bimetallic nanoparticles. On the other hand, the formation of PVP-protected Pd-core/Ni-shell bimetallic nanoparticles proceeded by a successive alcohol reduction [126]. 

Gold-platinum bimetallic clusters for aerobic oxidation of alcohols under ambient conditions Miyamura H, Matsubara R, Kobayashi S (2008) Chem Commun 2031-2033. 

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],... 

Cluster or bimetallic reactions have also been proposed in addition to monometallic oxidative addition reactions. The reactions do not basically change. Reactions involving breaking of C-H bonds have been proposed. For palladium catalysed decomposition of triarylphosphines this is not the case [32], Likewise, Rh, Co, and Ru hydroformylation catalysts give aryl derivatives not involving C-H activation [33], Several rhodium complexes catalyse the exchange of aryl substituents at triarylphosphines [34] ... 

Nanoparticles of various metals and metal oxides have been prepared on nanocarbons including Pt [139], Pd [206,207], Ag [208,209], Au [210], MnOx [211], TiOx [212], NiOx [213], CoOx [213], Ru02 [214] and various bimetallic clusters [215]. For example,... 

In D2O, HD was found instead of 0-H2. It is presently assumed that binding of hydrogen to a metal ion in the bimetallic active site weakens the H-H bond sufficiently to enable this reaction. Oxidation of the hydride is expected to be a two-electron process, and hydrogenases should, therefore, contain a redox unit capable of accepting these two electrons simultaneously. I assume here that the bimetallic center plus the conserved proximal Fe-S cluster perform this task. 

Figure 3.5. Continued. The H2-NAD reaction is inhibited neither in air nor in the presence of CO. C,The possible reactions of hydrogen with the Fe-Fe site of active [Fe]-hydrogenases. In the oxidized state, the bimetallic center shows a S = 1/2 EPR signal, presumably due to an Fe -Fe pair (an Fe -Fe pair cannot be excluded). Whether the unpaired spin is localized on iron (Pierik et al. 1998a) or elsewhere (Popescu and Mtlnck 1999) is not known. Hydrogen is presumably reacting at the vacant coordination site on Fe2 (Fig. 3.1C). After the heterolytic splitting, the two reducing equivalents from the hydride are rapidly taken up by the Fe-Fe site (one electron) and the attached proximal cluster (one electron). Subsequently, the electron is transferred from the proximal cluster to the other Fe-S clusters in the enzyme. Under equilibrium conditions, the proximal cluster in the active enzyme appears to be always in the oxidized [4Fe-4S] state (Popescu and Mtlnck 1999). Protons are not shown.

Nickel-iron hydrogenases [NiFe] (Figure 8.2) are present in several bacteria. Their structure is known [22, 23] to be a heterodimeric protein formed by four subunits, three of which are small [Fe] and one contains the bimetallic active center consisting of a dimeric cluster formed by a six coordinated Fe linked to a pentacoordinated Ni (III) through two cysteine-S and a third ligand whose nature changes with the oxidation state of the metals in the reduced state it is a hydride, H, whereas in the oxidized state it may be either an oxo, 0, or a sulfide,... 

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