Silica-supported Ru catalysts

Silica-supported Ru catalysts present specific problems in that ruthenium oxides are mobile on silica. The approach used here avoids this problem by crating insitu-formed supported Ru organo complexes. The genesis of the Ru complex formation can be followed fiom the thermal... 

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 ( ). 

Table II. Catalyst Composition and Surface Composition for Silica-Supported Ru-Ir Bimetallic Catalysts ...

As was the case for the silica-supported Ru-Rh bimetallic catalysts, there was no significant surface enrichment in either metal over the entire range of bimetallic catalyst compositions. 

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. 

Another work on the Hquid phase hydrogenation of acetophenone is that of Casagrande et al. The reaction was studied over a series of silica-supported bimetallic catalysts with various Ru/Cr atomic ratios, which were prepared by reduction at room temperature with aqueous sodium tetrahydroborate. The nanostructured catalysts are very active in the low-pressure hydrogenation of acetophenone, although the selectivity towards 1-phenylethanol did not surpass 22% at 90% conversion. The addition of chromium salts to the starting solution gave rise to... 

These were prepared by tethering Rh and Pt complexes to silica-supported metal catalysts (metal = Pd, Ni, Ru, Au). The catalysts are very active in the hydrogenation of benzene derivatives to the corresponding substituted cyclohexanes under mild conditions. The activities are higher than those of the separate homogeneous complexes, complexes just tethered to silica, or the silica-supported heterogeneous catalysts. When the sol-gel-entrapped [Rh2Co2(CO)12] complex was heat-treated at 100°C, immobilized metallic nanoparticles were formed.425 The catalyst thus prepared efficiently catalyzed substituted benzene derivatives. 

The feasibility of determining approximately the surface composition of silica-supported Ru-Pt catalysts by using infrared spectroscopy and two adsorbing gases has been explored, as a possible alternative to more complex instrumental methods and with the advantage of greater sensitivity to the outermost surface layer. The infrared absorption bands of CO selectively adsorbed on Pt sites and NO selectively adsorbed on Ru sites were stable in vacuo at room temperature. If both extinction coefficients and stoicheiometric coefficients are taken to be independent of surface composition, a simple correlation between the absorbance ratio and the bulk metal composition was obtained, suggesting that the surface and bulk compositions were similar. 

A fascinating example of a support effect is seen in the case of silica-supported Ru-Cu and Os-Cu catalysts prepared by Sinfelt and co-workers. From examinations of electron micrographs, these authors claim that a proportion, sometimes a high proportion, of the metal particles is present as... 

Dispersion is defined as the ratio of surface atoms to total atoms in the metal crystallites, and it is determined from chemisorption measurements (26,29). A typical hydrogen chemisorption isotherm is shown in Figure 2.4 for a silica-supported ruthenium catalyst containing 5 wt% ruthenium. The quantity H/Ru in the right-hand ordinate of the figure is the ratio of the number of hydrogen atoms adsorbed to the number of ruthenium atoms in the catalyst. The catalyst was treated with a stream of hydrogen in an adsorption cell at 500°C, after which the cell was evacuated and cooled to room temperature for the determination of the isotherm. The adsorption is... 

A series of supported Ru catalysts were prepared on different silica carriers using triethanolamine as the impregnation aid with different thermal treatments applied. The resulting catalysts were characterized by TG/DTA, TEM, IR, Raman, EXAFS, NMR, and hydrogenation reactions. 

Joongjai Panpranot, James G. Goodwin, Jr., and Abdelhamid Sayari, Effect of Hydrogen Partial Pressure on the Surface Reaction Parameters during CO Hydrogenation on Ru-Promoted Silica-Supported Co Catalysts, J. of Catalysis, 213, 78-85 (2003). 

Supported Rli- and Ru-Sn catalysts have shown high activity for reduction of NO by CO [1]. Especially, the effective removal of NO was observed in 02-rich atmosphere. The high removal of NO in the oxidative atmosphere was attributable to a cleaning effect of Sn in the bimetallic system by spillover of oxygen atoms adsorbed on Rli or Ru sites to Sn sites [2]. Recently, we have also reported that oxidation of CO over the silica supported Rli catalysts was eidianced by incorporation of Sn [3]. The temperature which CO oxidation... 

The behavior of the CO/H2 synthesis reaction has been studied over silica-supported Ru-Fe catalysts, and an optimum range in the Ru Fe ratio was found to exist in which olefin production was maximized and methane formation was minimized. The catalyst samples were characterized by hydrogen and CO chemisorption, x-ray diffraction measurements, and Mossbauer spectroscopy. Alloy formation was verified at different Ru Fe ratios, and changes in specific activity and selectivity were observed as this ratio varied. Between Ru Fe ratios of 1/2 to 2, 45 mol% of the total hydrocarbon product was C2-C5 olefins while less than 40 moWo was comprised of methane. 

Robertson and Webb ° have synthesized silica-supported Ru carbonyl catalysts which are active for the hydrogenation of but-l-ene. On heating, these catalysts undergo certain irreversible changes, accompanied by the loss of CO. Using [ CO]-labelled Ru3(CO)i2 on silica, the authors were able to show that decomposition occurred according to the following reaction sequence ... 

Schemes Activation of the silica-supported Ru(ll) complex catalyst and hydrogenation of BT. 2.4.2.2.S...

Narayan, R.L. and King, T.S. 1998. Hydrogen adsorption states on silica-supported Ru-Ag and Ru-Cu bimetal-hc catalysts investigated via microcalorimetry. Thermochim. Acta 312 105-114. 

TABLE 22-1. SURFACE AREAS OF SILICA SUPPORTED Ru-Cu CATALYSTS PREPARED BY SOL-GEL TECHNIQUES (NIWA ET AL, 1987)... 

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. 

Supported (alumina, silica) Ru catalysts The Mossbauer data show that RuCl3 (l-3)H20 reacts chemically when supported onto alumina, but does not when impregnated on a silica support. The study further shows that a supported ruthenium catalyst converts quantitatively into RUO2 upon calcination, and that the reduction of a supported ruthenium catalyst converts all of the ruthenium into the metallic state... 

A significant volume of literature relates to our work. Concerning choice of support, Montassier et al. have examined silica-supported catalysts with Pt, Co, Rh Ru and Ir catalysts.However, these systems are not stable to hydrothermal conditions. Carbon offers a stable support option. However, the prior art with respect to carbon-supported catalysts has generally focused on Ru and Pt as metals.Additionally, unsupported catalysts have also been reported effective including Raney metals (metal sponges).Although the bulk of the literature is based on mono-metallic systems, Maris et al. recently reported on bimetallic carbon-supported catalysts with Pt/Ru and Au/Ru. In contrast, our work focuses primarily on the development of a class of rhenium-based carbon supported catalysts that have demonstrated performance equal to or better than much of the prior art. A proposed reaction mechartism is shown in Figure 34.2 °l... 

---