Catalysts, bimetallic clusters

For highly dispersed ruthenium-copper catalysts (bimetallic clusters), to... 

When a second metallic element is added to the common single metal-supported catalyst, bimetallic clusters of particles in the size range 1-50 nm are formed. About 50-95% of the metal atoms in the range of 1-3 nm are exposed to the surface. A particularly important effect of such microparticle clusters supported on silica or alumina is on selectivity, which can often be enhanced to significantly higher levels. 

Sulfided bimetallic clusters which mimic the metal composition of commercial hydrodesulfurization (HDS) catalysts have been prepared and their homogeneous catalytic behavior studied. Reaction of thiophenol with [Mo2Co2(/z4-S)... 

A MgO-supported W—Pt catalyst has been prepared from IWsPttCOIotNCPh) (i -C5H5)2l (Fig. 70), reduced under a Hs stream at 400 C, and characterized by IR, EXAFS, TEM and chemisorption of Hs, CO, and O2. Activity in toluene hydrogenation at 1 atm and 60 C was more than an order of magnitude less for the bimetallic cluster-derived catalyst, than for a catalyst prepared from the two monometallic precursors. 

MgO-supported model Mo—Pd catalysts have been prepared from the bimetallic cluster [Mo2Pd2 /z3-CO)2(/r-CO)4(PPh3)2() -C2H )2 (Fig. 70) and monometallic precursors. Each supported sample was treated in H2 at various temperatures to form metallic palladium, and characterized by chemisorption of H2, CO, and O2, transmission electron microscopy, TPD of adsorbed CO, and EXAFS. The data showed that the presence of molybdenum in the bimetallic precursor helped to maintain the palladium in a highly dispersed form. In contrast, the sample prepared from the monometallie precursors was characterized by larger palladium particles and by weaker Mo—Pd interactions. ... 

We have found EXAFS to be a very effective method for obtaining structural information on bimetallic cluster catalysts (8,12-15,17) These types of catalysts, and bimetallic catalysts in general, have been the subject of extensive research in the EXXON laboratories since the 1960 s (18-25). In this paper we present a brief review of the results of some ofour EXAFS investigations on bimetallic cluster catalysts. 

We plan to make studies on palladium-copper, iridium-copper, and platinum-copper catalysts to extend our investigation of the effect of varying miscibility of the components on the structural features of the bimetallic clusters present. With these additional systems, the whole range from complete immiscibility to total miscibility of copper with the Group VIII metal will be encompassed. 

Extended X-ray absorption fine structure (EXAFS) studies have been very useful for obtaining structural information on bimetallic cluster catalysts. The application to bimetallic systems is a particularly good one for illustrating the various factors which have an influence on EXAFS. Moreover, the applicability of EXAFS to this area has been very timely, in view of the enormous interest in bimetallic systems in both catalytic science and technology. 

The results of the EXAFS studies on supported bimetallic catalysts have provided excellent confirmation of earlier conclusions (21-24) regarding the existence of bimetallic clusters in these catalysts. Moreover, major structural features of bimetallic clusters deduced from chemisorption and catalytic data (21-24), or anticipated from considerations of the miscibility or surface energies of the components (13-15), received additional support from the EXAFS data. From another point of view, it can also be said that the bimetallic catalyst systems provided a critical test of the EXAFS method for investigations of catalyst structure (17). The application of EXAFS in conjunction with studies employing ( mical probes and other types of physical probes was an important feature of the work (25). 

In order to verify the presence of bimetallic particles having mixed metal surface sites (i.e., true bimetallic clusters), the methanation reaction was used as a surface probe. Because Ru is an excellent methanation catalyst in comparison to Pt, Ir or Rh, the incorporation of mixed metal surface sites into the structure of a supported Ru catalyst should have the effect of drastically reducing the methanation activity. This observation has been attributed to an ensemble effect and has been previously reported for a series of silica-supported Pt-Ru bimetallic clusters ( ). 

Surface Composition Measurements. The surface composition and metal dispersion for a series of silica (Cab-O-Sll) supported Ru-Rh bimetallic clusters are summarized In Table I. Surface enrichment In Rh, the element with the lower heat of sublimation, was not observed over the entire bimetallic composition range. In fact, to within the experimental limit of error of the measurements, surface compositions and catalyst compositions were nearly equal. A small local maximum In the dispersion was observed for the catalyst having a surface composition of 50% Rh. 

Metal dispersions were observed to decrease as the concentration of Ru was Increased. This same trend was observed for the Ru-Rh catalysts and was in marked contrast to observations on silica-supported Ft-Ru catalysts W. In this case a large Increase in dispersion was obtained as a result of bimetallic clustering in the cherry model configuration. 

The surface-catalyst composition data for the silica-supported Ru-Rh cuid Ru-Ir catalyst are shown in Figure 1. A similcir plot for the series of silica-supported Pt-Ru bimetallic catalysts taken from ref. P) is included for comparison purposes. Enthalpies of sublimation for Pt, Ru, Rh and Ir are 552, 627, 543, and 648 KJ/mole. Differences in enthalpies of sublimation (a<75 KJ/mole) between Pt and Ru cind between Rh and Ru are virtually identical, with Pt euid Rh having the lower enthalpies of sublimation. For this reason surface enrichment in Pt for the case of the Pt-Ru/Si02 bimetallic clusters cannot be attributed solely to the lower heat of sublimation of Pt. Other possibilities must also be considered. 

Methanatlon Studies. Because the most effective way to determine the existence of true bimetallic clusters having mixed metal surface sites Is to use a demanding catalytic reaction as a surface probe, the rate of the CO methanatlon reaction was studied over each series of supported bimetallic clusters. Turnover frequencies for methane formation are shown In Fig. 2. Pt, Ir and Rh are all poor CO methanatlon catalysts In comparison with Ru which Is, of course, an excellent methanatlon catalyst. Pt and Ir are completely inactive for methanatlon In the 493-498K temperature range, while Rh shows only moderate activity. 

A comment regarding the dispersion of the Ru-Rh/Si02 and the Ru-Ir/Si02 is in order. For the case of the supported Pt-Ru catalysts. Increases in dispersion as a result of clustering were very large ( ). This effect was particularly noticeable for bimetallic particles which conform to the cherry model. Evidently, the formation of an inner core enriched in one of the two metals, followed by an outer layer enriched in the other metal, inhibits further crystal growth. For the alumina-supported Pt-Ru bimetallic clusters, the effect, although present, is considerably smaller. 

Recently, metal cluster catalysts composed of two different metallic elements are of interest from both scientific and technological points of view because of their interesting physiochemical properties [46, 47]. Bimetallic clusters are known to exhibit specific reactivity. Their catalytic efficiency is also controlled by their size. 

As an aside, we should mention that the same principles apply to the formation of bimetallic clusters on a support. In the case of Pt-Re on AI2O3 it has been shown that hydroxylation of the surface favors the ability of Re ions to migrate toward the Pt nuclei and thus the formation of alloy particles, whereas fixing the Re ions onto a dehydroxylated alumina surface creates mainly separated Re particles. As catalytic activity and selectivity of the bimetallic particles differ vastly from those of a physical mixture of monometallic particles, the catalytic performance of the reduced catalyst depends significantly on the protocol used during its formation. The bimetallic Pt-Re catalysts have been identified by comparison with preparations in which gaseous Re carbonyl was decomposed on conventionally prepared Pt/Al203 catalysts. ... 

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]. 

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