Supported bimetallic particles, preparation

Figure 8.2 Open routes for the preparation of supported bimetallic particles from carbonyl species generated on the surface of a support from an initial M L metal precursor.

The SEA approach can be applied to a novel system in three steps (1) measure the PZC of the oxide (or carbon) and choose a metal cation for low-PZC materials and an anion for high-PZC materials, (2) perform an uptake-pH survey to determine the pH of the strongest interaction in the appropriate pH regime (high pH for low PZC and vice versa), and (3) tune the calcination/reduction steps to maintain high dispersion. Highly dispersed Pt materials have been prepared in this way over silica, alumina, and carbon. Other oxides can be employed similarly. For bimetallics, the idea is to first adsorb a well-dispersed metal that forms an oxide intermediate with a PZC very different to the support. In this way the second metal can be directed onto the first metal oxide by SEA. Reduction may then result in relatively homogeneous bimetallic particles. 

B. D. Chandler, A. B. Schabel, C. F. Blanford, and L. H. Pignolet, The preparation and characterization of supported bimetallic Pt-Au particle catalysts from molecular cluster and chloride salt precursors, J. Catal. 367-383 (1999). 

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. ... 

The PtRu bimetallic system has been the catalyst of choice for MeOH oxidation in acid elecfrolyfes since its discovery by workers at Shell in the early 1960s2 In practice, PtRu lowers the overpotential for MeOH oxidation by >200 mV compared to pure Pt. The MeOH oxidation reaction on Pt and PtRu is probably the most studied reaction in fuel cell electrocatalysis due to its ease of sfudy in liquid electrolytes and the many possible mechanistic pathways. In recent years, the deposition of PtRu particles onto novel carbon supports and the novel PtRu particle preparation routes have proved popular as a means to demonstrate superiority over conventional PtRu catalysts. 

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. 

The use of hetero-metallic (MM )carbonyl complexes as precursors can lead to the preparation of supported catalysts having weU-defined bimetallic entities in which the intimate contact between M and M remains in the final catalyst and the atomic ratio M/M of the aggregates is that of the bimetallic carbonyl precursor used. This is illustrated in Figure 8.1, in which the definite interaction of the MjM (CO) complex with the functional group (F) of a surface (S) produces a new anchored surface species. This new surface species could evolve with an appropriate treatment producing tailored bimetallic particles. 

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. 

The now classical methods used for the preparation of supported gold catalysts are hardly capable of giving particles that are both small and bimetallic, when the precursors in solution do not interact strongly with each other. During the subsequent thermal treatment performed to get metal particles, the metals must have enough mobility to migrate on the support, interact with each other, and form bimetallic particles. However, phase separation can be a common problem, especially when the metal ratio falls in the miscibility gap (Section 2.6), or if the intended composition is not thermodynamically stable. 

This method has not been used very much to prepare bimetallic particles. It is always performed at fixed pH as in Section 4.2.3, i.e. the same recipe was applied without attempt to understand the underlying chemistry. Platinum-gold and palladium-gold (Pt or Pd Au = 5 95) have been deposited this way on titania-silica supports.66 After calcination at 673 K, the particles are small (<5nm), and no separate platinum or palladium metal particles are found. With Pd-Au/CeC>2 (Pd Au = 2 to 100) calcined at 673 K, however, only large gold particles (>8nm) and small palladium particles (<3nm) were found at gold loadings above lwt.%.184... 

Supported mixed metal catalysts are also prepared by other means such as the deposition of bimetallic colloids onto a support O and the decomposition of supported bimetallic cluster compounds.208 The photocatalytic codeposition of metals onto titania was also attempted with mixed results.209 with a mixture of chloroplatinic acid and rhodium chloride, very little rhodium was deposited on the titania. With aqueous solutions of silver nitrate and rhodium chloride, more rhodium was deposited but deposition was not complete. In aqueous ammonia, though, deposition of both silver and rhodium was complete but the titania surface was covered with small rhodium crystallites and larger silver particles containing some rhodium. With a mixture of chloroplatinic acid and palladium nitrate both metals were deposited but, while most of the resulting crystallites were bimetallic, the composition varied from particle to particle.209... 

Supported bimetallic clusters (with well-defined metal frames) are commonly prepared from organometallic precnrsors, where the bimetallic clnster frame is already present in the strnctnre with reactive ligands that can be removed under specific treatments [57, 58]. Snpported bimetallic particles and alloys, which are used in naphtha reforming (Re-Pt, Sn-Pt, Ir-Pt) and antomobile exhaust conversion (Rh-Pt) will not be reviewed here. Recent reviews on this topic can be found elsewhere [59]. 

Elements for the deposition of heterobinuclear complexes for preparing bimetallic catalysts have been reported by Ichikawa [48]. The use of heterobinuclear complexes as precursors for bimetallic catalysts was aimed at the fine-tuned control of a homogeneous composition in the nuclearity of bimetallic particles. As compared to catalysts prepared by NSD methods, mixed-metal cluster-derived catalysts should retain more bimetallic ensembles. However, the use of heterobinuclear complexes is limited by the availability of high-nuclearity clusters and by the narrow choice of metallic couples. The different types of interaction of these complexes with the support are the same as previously described for the NSD methods. 

This technique was used to determine the morphological cha teristics of both the pure metal and bimetallic particles supported on single crystal graphite surfaces. The samples used in these experiments were prepared by two methods (a) single metal/graphite specimens were made by evaporation of the metal in the form of a wire from a tungsten... 

Obviously a controlled preparation of bimetallic catalysts is needed in order to imderstand its role upon activity, selectivity and deactivation resistance. On this way, the controlled formation of surface bimetallic particles has been reported in catalysts prepared by the redox method [1]. In the present work, in order to define the role of the redox method in the surface properties of the R-Au alumina supported catalysts, we report the preparation, characterization and catalytic properties of a set of bimetallic catalysts with different gold content. The catalysts were evaluated using methylcyclopentane (MCP) hydrogenolysis as the test reaction. 

Bimetallic catalysts can be obtained by surface organometallic chemistry on metals. These catalysts are prepared by the controlled reaction under hydrogen between tetra n-butyl tin and silica supported rhodium particles. For a given amount of tin fixed, these solids exhibit increasing activities and selectivities for the conversion of acetophenone to phenylethanol. 

The method described allows fast and consistent production of silica supported bimetallic palladium-copper catalysts in the liquid phase at room temperature, without the need for high temperature reduction. The catalysts show homogeneous dispersion of the mixed metal particles over the support surface and are ready to use immediately after preparation. 

As mentioned above, the addition of promoters, and even the formation of bimetallic particles, can provide carbon-supported iron catalysts with better performances in CO hydrogenation. The method of preparation of these systems is going to determine the final effect, always taking advantage of the relative inertness of the carbon surface. The interaction between the different components of the active phase can be maximized by using mixed-metal carbonyl complexes. Furthermore, use of these precursors allows for the preparation of catalysts with... 

In solvated metal atom dispersion (SMAD) method solvated atoms prepared at very low-temperature are used as transient, highly reactive organometallic reagent for the deposition of Sn-Pt bimetallic particles onto different supports. In another approach chemical vapor deposition (CVD) using tin organometallic compounds was applied. For example, the selective reaction of Sn(CH3)4 vapour with Pt nanoparticles supported on Si02 appears to be very promising preparation method. 

Metal loading low due to the limited capacity of the microemulsion Precise control of the metal microstructure size, composition, elemental distribution of each bimetallic particle The preparation is carried out in absence of support The reduction of the metal precursor take place in absence of the support The size of the particles is determined by the size of the micelles... 

A MgO-supported [ReOs3] catalyst for CO hydrogenation was prepared from [H3ReOs3(CO)i3], The bimetallic particles were stable under catalytic conditions and Re was found to prevent formation of the otherwise observed [OsioC(CO)24] J l The same precursor on y-Al203 leads to an active MMCD catalyst for n-butane hydrogenolysisJ " ... 

Bimetallic bifunctional catalysts containing different proportions of Ni and Pt supported in HUSY zeolite were prepared and characterized by TEM, punctual EDX analysis and -hexane isomerization. The EDX analysis of the Ni and Pt bimetallic catalysts shows that the metal particles contain both metals and from HRTEM it was observed that the bimetallic particles have crystallographic parameters of metallic nickel. The presence of small platinum amounts in the nickel catalysts produces more active catalysts for the -hexane isomerization, and presents also higher selectivity for the formation of dibranched hexane than the ones containing only platinum. 

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