Rhenium-platinum clusters

Augustine and Sachtler, on reduction, rhenium oxides migrate over the support surface and become reduced in contact with a prereduced platinum particle. In this way, each platinum particle can generate a mixed platinum-rhenium cluster that has the beneficial effects of bimetallic catalysts on selectivity and stability. 

Perrhenate and related building blocks are constituents of several cluster compounds where they act as terminal groups in organometallic rhenium oxides such as in [(cp Re)3(//2-0)3(/U3-0)3Re03]+ (49)21 Qj. jjj heterometallic clusters such as the structurally related [(Re)3(//f dppm)3(/u -0)3Re03]+ (dppm = bis(diphenylphosphino)methane) and Pt4 P(C6H 11)3)4 (//-C0)2(Re04)2]. A series of platinum-rhenium and platinum-rhenium-mercury clusters with Pt-Re multiple bonds has been isolated from reactions of Pt3 precursors with Rc207 or perrhenate. " ... 

Platinum nanoparticles, preparation, 12, 78 Platinum-nitrogen bonds, in platinacycles, 8, 508 Platinum-osmium carbonyl clusters, characteristics, 8, 420 Platinum-oxygen bonds, in platinacycles, 8, 505 Platinum particles, surface reactivity, 12, 542 Platinum-phosphorus bonds, in platinacycles, 8, 508 Platinum-rhenium carbonyl clusters, characteristics, 8, 420 Platinum(II)-ruthenium(II) binary complexes, preparation,... 

Another catalyst system that could be considered in the bimetallic cluster category is supported platinum-rhenium (5), which represents still another type of system in the sense that a Group VIIA metallic element (rhenium) is incorporated with the Group VIII metal component. Platinum and rhenium have different crystal structures (fee vs. hep) (8) and do not exhibit complete miscibility in the bulk (Ref. 45, p. 820). However, these factors may have limited... 

The data on the catalyst containing rhenium alone indicate signficant chemisorption of carbon monoxide, but no chemisorption of hydrogen. As expected, the platinum catalyst chemisorbs both carbon monoxide and hydrogen, and the values of CO/M and H/M are nearly equal. The platinum-rhenium catalyst exhibits a value of CO/M about twice as high as the value of H/M. This result approximates what one would expect if hydrogen chemisorbed on only the platinum component of the catalyst. While this chemisorption behavior is consistent with the possibility that the platinum and rhenium are present as two separate entities in the catalyst, they do not rule out the possibility that bimetallic clusters of platinum and rhenium are present. 

Thus the Shell workers conclude that Pt-Re bonds are present at the surface in platinum-rhenium catalysts, whether or not chemisorbed sulfur is present. A platinum-rhenium entity of the type envisioned by these investigators, if it indeed exists, could also be called a bimetallic cluster. 

Non-ionic thiourea derivatives have been used as ligands for metal complexes [63,64] as well as anionic thioureas and, in both cases, coordination in metal clusters has also been described [65,66]. Examples of mononuclear complexes of simple alkyl- or aryl-substituted thiourea monoanions, containing N,S-chelating ligands (Scheme 11), have been reported for rhodium(III) [67,68], iridium and many other transition metals, such as chromium(III), technetium(III), rhenium(V), aluminium, ruthenium, osmium, platinum [69] and palladium [70]. Many complexes with N,S-chelating monothioureas were prepared with two triphenylphosphines as substituents. 

The rhenium interacts strongly with the oxygen atoms of the support and also with platinum platinum interacts less strongly with the support than rhenium. One is tempted to generalize that when one of the metals in a supported bimetallic cluster is noble and the other oxophihc, the oxophUic metal interacts more strongly with the support than the noble metal if the bimetalhc frame of the precursor is maintained nearly intact, then this metal-support interaction helps keep the noble metal highly dispersed. 

In the preparation of faujasite zeolite-supported Pt-Re catalysts, bimetallic PtRe clusters have been reported to be predominantly formed when a carbonyl rhenium precursor (Re2(CO)io) is contacted with zeolite in which platinum has been previously introduced and reduced. The preexisting Pt clusters may act as nucleation sites. After reduction, these Pt-Re systems show a high selectivity to CH4 in the hydrogenolysis of n-heptane [58]. 

These heterogeneous catalysts consist of muitimetallic clusters, containing metals, such as platinum, iridium, or rhenium, supported on porous acidic oxide supports, such as alumina. The catalysts are said to be bifunctional because both the metal and the oxide play a part in the reactions. The metal is believed to carry out reversible dehydrogenation of paraffins to olefins, while the oxide is believed to carry out isomerization. 

F, 1 NMR studies on the dynamic behaviour in solution of rhenium-platinum mixed metal clusters containing P-donor ligands... 

C, Carbide, iron complex, 26 246 ruthenium cluster complexes, 26381-284 CHF3O3S, Methanesulfonic add, trifluoro-, iridium, manganese, and rhenium com-iriexes, 26 114,115,120 platinum complex, 26 126 CHOS2, Dithioaubonic add, 27 287 CH2> Methylene, osmium complex, 27 206 CH2O2, Formic add, rhenium complex, 26 112... 

The synthesis and characterisation of the platininn-containii species (PPh3)HPt(/i-PPh2XM"CO)M(CO)4 (M = Cr, Mo, W) has been reported by Shao et al The range of rhenium-platinum clusters has been extended by the preparationof the complex [Re3Pt(/L(-H)3(CO)i4]. Osmium-rhodium complexes are also mentioned ... 

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