Monometallic metal-support interaction

In heterogeneous catalysis by metal, the activity and product-selectivity depend on the nature of metal particles (e.g., their size and morphology). Besides monometallic catalysts, the nanoscale preparation of bimetallic materials with controlled composition is attractive and crucial in industrial applications, since such materials show advanced performance in catalytic processes. Many reports suggest that the variation in the catalyst preparation method can yield highly dispersed metal/ alloy clusters and particles by the surface-mediated reactions [7-11]. The problem associated with conventional catalyst preparation is of reproducibility in the preparative process and activity of the catalyst materials. Moreover, the catalytic performances also depend on the chemical and spatial nature of the support due to the metal-support interaction and geometrical constraint at the interface of support and metal particles [7-9]. 

The most active catalysts for methanol oxidation are presently based on bifunctional systems such as Pt-Ru. Yet the evaluation of a metal-support interaction would require the analysis of a simpler catalytic system (i.e., a monometallic catalyst) this would avoid the interference of all those aspects such as degree of alloying, changes in crystallographic parameters, chemical state of the promoting element, which significantly affect the activity, and thus a comprehensive interpretation of the data actually available. 

Mono- and Bimetallic Supported Catalysts. - The key factor in designing supported metal catalysts is the knowledge about the reaction mechanisms and information about the role of different types of active sites in a given step of the catalytic reaction. The performance of supported mono-functional monometallic catalysts is governed by the metal particle size, metal dispersion, overall morphology of the metal nanocluster, the character of metal-support interaction, and the electronic properties of the metal. In bifunctional supported metal catalysts in addition to the above listed factors the metal/acid balance, and the type and strength of the acid function play a key role in the overall performance. 

Coq, B., Kumbhar, P. S., Moreau, C., Moreau, R, Figueras, F. (1994) Zireonia-supported monometallic Ru and bimetallic Ru-Sn, Ru-Fe catalysts role of metal support interaction in the hydrogenation of cinnamaldehyde. y. Phys. Chem. 98(40), 10180-10188... 

These results were interpreted in terms of a substantial surface enrichment in Cu, driven by Cu s lower heat of sublimation [23]. The reactivity of these catalysts for CO oxidation, and the clear spectroscopic evidence for surface Pt - CO species indicate that, at least for the heterogeneous systems, particle surface stoichiometries are very sensitive to metal-adsorbate interactions. Similar arguments were presented for the PtAu/silica system, in which monometallic Au particles severely sinter under dendrimer removal conditions. In this case, the retention of small bimetallic particles after activation was attributed to the strength of Pt-silica interactions, which effectively anchored the bimetallic nanoparticles to the support [24],... 

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. ... 

The NSD of precursors mainly applies when Ml and M2 are transition metals. The basic principle of NSD of precursors, together or successively, is such that the interaction of both precursors with the solid surface was stronger than between the two precursors. On that account the two deposited precursors are separated on the support, and surface diffusion will be necessary to yield the bimetallic aggregates during the activation process. NSD will use the same deposition methods as for monometallic catalysts (vide supra). 

The bimetallic Pt-Re/Al203-Cl catalyst is the most widely used in naptha reforming. The addition of Re strongly improves the stability of the traditional monometallic Pt catalyst. Such improvement is explained by a double effect of rhenium stabilization of the metallic phase on the support and higher resistance to deactivation by coke deposition [1-8]. Nevertheless the role and the nature of the interaction between Pt and Re are the subject of many controversies [9-14]. 

In contrast to the metal clusters in the Pt/Si02 and Ir/Si02 reference catalysts (19), those in the Pt/Al203 and lr/Al203 reference catalysts exhibit interatomic distances lower than the distances in the corresponding pure metals, which are 2.775 A and 2.714 A (33), respectively, for platinum and iridium. The contraction observed when the clusters are dispersed on alumina indicates an interaction with the carrier that is not apparent in the silica-supported clusters. The finding that the distance contractions are more pronounced for the bimetallic platinum-iridium catalyst than for the monometallic reference catalysts provides additional evidence that the bimetallic catalyst is. not simply a mixture of platinum clusters and iridium clusters. 

In summary, several investigators have attempted to use TPR to elucidate the structure and the interactions of Re and Pt in supported catalysts. The interaction between the two metals has been found to be influenced by the Re loading, the ratio of the two metals, the chlorine content, the water partial pressure in the H2 used for reduction, and especially, the temperature of drying or calcination preceding reduction. When an alumina supported Re-Pt sample prepared from salt precursors was dried at 200 °C, TPR showed that the co-reduction of the two metals occurred at a temperature intermediate between that characteristic for the two monometallic counterparts. In contrast, when the catalyst was dried or pre-oxidized at 500 °C, two separate reduction peaks were observed (see Figure 40, taken from ref. 

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. 

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