Supported bimetallic catalysts preparation

J. Margitfalvi, S. Szabo, andF. Nagy, Supported bimetallic catalysts prepared by controlled surface reactions. Stud. Surf. Sci. Catal. 27, 373-409 (1983). 

Hi ly dispersed supported bimetallic catalysts with bimetallic contributions have been prepared from molecular cluster precursors containing preformed bimetallic bond [1-2]. For examples, extremely high dispersion Pt-Ru/y-AUOa could be prepared successfully by adsorption of Pt2Ru4(CO)ison alumina [2]. By similar method, Pt-Ru cluster with carbonyl and hydride ligands, Pt3Ru6(CO)2i(p3-H)(p-H)3 (A) was used in this work to adsorb on MgO support. The ligands were expectedly removable from the metal framework at mild conditions without breaking the cluster metal core. 

There has been great interest in the preparation of bimetallic transition metal cluster complexes containing palladium.899-902 Bimetallic palladium-ruthenium clusters have been shown to be good precursors to supported bimetallic catalysts.903,904... 

Supported bimetallic catalysts have gained unquestionable importance in subjects such as refining, petrochemistry and fine chemistry since their earliest use in the 1950s [1, 2]. The catalytic behavior of such a system is influenced by the size of the metal particles and by the interactions among them and with the support and other catalyst components. The second metal may influence the first metal through electronic interactions or by modifying the architecture of the active site. Very often, the interactions between the two metals are complex and largely unknown, and consequently the preparation procedure critically influences the nature of the catalytic system obtained. 

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... 

On the other hand, hi- or multi-metallic supported systems have been attracting considerable interest in research into heterogeneous catalysis as a possible way to modulate the catalytic properties of the individual monometalUc counterparts [12, 13]. These catalysts usually show new catalytic properties that are ascribed to geometric and/or electronic effects between the metalUc components. Of special interest is the preparation of supported bimetallic catalysts using metal carbonyls as precursors, since the milder conditions used, when compared with conventional methods, can render catalysts with homogeneous bimetallic entities of a size and composition not usually achieved when conventional salts are employed as precursors. The use of these catalysts as models can lead to elucidation of the relationships between the structure and catalytic behavior of bimetalUc catalysts. 

Preparation of Supported Bimetallic Catalysts Containing Gold... 

The preparation of a successful supported bimetallic catalyst is quite a difficult proposition. The main problem is to ensure that the two components reside in the same particle in the finished catalyst, and to know that it is so. The main physical techniques to characterise bimetallic particles are hydrogen chemisorption, XRD, TEM, EDX, XPS, XAFS,197Au Mossbauer (Section 3.3) and CO chemisorption coupled by IR spectroscopy (Section 5.3). The characterisation of bimetallic catalysts is not always thoroughly done, and there is the further complication of structural changes (particularly of the surface) during use. In situ or post-operative characterisation would reveal them, but it is rarely done. 

Surface redox reactions represent a relatively new approach for preparing supported bimetallic catalysts. Three main techniques have already been identified ... 

The characterization of supported metal catalysts is a matter of some complexity and supported bimetallic catalysts even more so. Nevertheless the development and application of methods for determining catalyst structure is essential for an understanding of why the performance of a selected combination of metal(s) and support varies as a function of preparative variable, activation procedure, reaction conditions, or time. Although some aspects of catalyst structure can be routinely determined, the basic measurement of absolute metal dispersion by selective chemisorption/gas titration is still the subject of many publications and the necessity of cross-checking by instrumental methods is generally appreciated. The characterization of supported metal catalysts also involves some less accessible properties, e.g., the sites available on crystallites as a function of size, high-temperature... 

Utilisation of sol-gel technique with respect to the preparation of supported bimetallic catalysts allows to produce catalysts with homogeneous distribution of finely dispersed metals. Another advantage includes improved thermal stability of the metals, higher surface areas, well-defined pore size distribution and ability to control the microstructure of the carrier. 

As part of a major research topic concerning characterization and mode of operation of novel (bi-)metallic catalysts for liquid phase catalysis, we have now developed a fast and simple preparation method for supported bimetallic catalysts, even of less noble metals. This method involves reduction of bimetallic... 

Analytical techniques have advanced to give high lateral spatial resolution. It is now possible to measure the chemical composition of Inm individual crystallites of a supported bimetallic catalyst. This will allow the development of catalyst preparation methods to make bimetallic catalysts of more homogeneous composition. This could be particularly important where a monometallic crystallite generates an unwanted by-product. 

Methods of Controlled Surface Reactions (CSRs) and Surface Organometallic Chemistry (SOMC) were developed with the aim to obtain surface species with Sn-Pt interaction. In CSRs two approaches have been used (i) electrochemical, and (ii) organometallic. Characteristic feature of the organometallic approach is that both CSR and SOMC results in almost exclusively supported alloy type bimetallic nanoclusters. Studies on the reactivity of tin organic compounds towards hydrogen adsorbed on different transition and noble metals have revealed new aspects for the preparation of supported bimetallic catalysts. 

Supported bimetallic catalysts. Supported bimetallics with extremely high dispersions can be made from molecular bimetallic precursors in which the two metals are bonded to each other. Pt-Ru clusters dispersed on y-AljOj were prepared by decarbonylation of molecularly adsorbed Pt2Ru4(CO)ig by treatment in He or Hj at temperatures in the range of 573-673 K.- EXAFS data show that, after decarbonylation, the Pt-Ru interactions were largely maintained, but the Pt-Ru cluster frame was changed. The average Pt-Pt bond distance apparently increased slightly (from 2.66 to 2.69 A), and the Ru-Ru distance decreased from 2.83 to 2.64 A. 

One of the major challenges to understanding bimetallic catalysts is to obtain supported clusters with relatively well understood and uniform compositions. Supported bimetallic catalysts are usually prepared by deposition of metal salts onto a support followed by caldnation and reduction by H2 at elevated temperature. lliis approach leads to nonuniformities in cluster composition and structure and even physical separation of the two metals. These difficulties have prompted research on catalysts prepared from molecular metal clusters since they provide precursors of well defined stoichiometry and structure. 

The previous examples are all supported gold catalysts. Supported bimetallic catalysts have been explored as well. Scurrell and coworkers developed a series of Au-M/ Fe Oj (M=Ag, Bi, Co, Cu, Mn, Ni, Pb, Ru, Sn, Tl) catalysts for WGS [70,71], These catalysts were prepared by deposition-coprecipitation using HAuCl, FeCNOjjj, and metal salts as precursors, followed by calcination in air at 400°C. Au-Ru/Fe Oj showed the highest activity. However, there is no evidence showing that the so-called bimetallic catalysts are bimetallic. The thermal decomposition of metal salts in air usually leads to the formation of metal oxides instead [61]. 

Generally supported bimetallic catalysts are being prepared using the same procedures as for the production of monometallic supported catdysts, viz., impregnation, deposition-precipitation, and ion exchange. These procedures, however, usually result in supported catalyst precursors of a non-uniform chemical composition of tiie individual active particles. The variation of the cherrucal composition is mainly due to a lack of interaction between the two metals to be alloyed during the various steps of the preparation procedure. 

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. 

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