Mixed metal catalysts surface composition

Since the catalytic process takes place on the surface of the mixed metal catalysts, a knowledge of the surface composition of these materials is essential to... 

It can be seen, then, that while the surface composition of a mixed metal catalyst is of critical importance to the outcome of a given reaction, there is little that may be said concerning the optimum surface concentration for a particular reaction. Even if such a prediction could be made it would be difficult to design a catalyst having the prescribed surface composition under the reaction conditions used. Much more needs to be done to optimize the use of such mixed metal catalysts, particularly in synthetically useful reactions. 

Supported mixed metal catalysts can be prepared by almost any of the procedures described above for the production of monometallic species. The discussion concerning the surface composition of these multimetallic species presented in Chapter 12 applies to supported catalysts as well but may be modified by the presence of very small crystallites in the supported catalysts. 2> 3 The factors presented above concerning the location of the metal in the support particle are also applicable here as well and can be used to prepare bimetallic catalysts having specific distribution profiles. 

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... 

Catalysts vary both in terms of compositional material and physical stmcture (18). The catalyst basically consists of the catalyst itself, which is a finely divided metal (14,17,19) a high surface area carrier and a support stmcture (see Catalysts, supported). Three types of conventional metal catalysts are used for oxidation reactions single- or mixed-metal oxides, noble (precious) metals, or a combination of the two (19). 

In addition to modification of surfaces by non-metals, the catalytic properties of metals can also be altered greatly by the addition of a second transition metaP ". Interest in bimetallic catalysts has arisen steadily over the years because of the commercial success of these systems. This success results from an enhanced ability to control the catalytic activity and selectivity by tailoring the catalyst composition . A long-standing question regarding such bimetallic systems is the nature of the properties of the mixed-metal system which give rise to its enhanced catalytic performance relative to either of its individual metal components. These enhanced properties (improved stability, selectivity and/or activity) can be accounted for by one or more of several possibilities. First, the addition of one metal to a second may lead to an electronic modification of either or both of the metal constituents. This... 

The flexibility in composition of LDHs has led to an increase in interest in these materials. As a result of their relative ease of synthesis, LDHs represent an inexpensive, versatile and potentially recyclable source of a variety of catalyst supports, catalyst precursors or actual catalysts. In particular, mixed metal oxides obtained by controlled thermal decomposition of LDHs have large speciflc surface areas (100-300 m /g), basic properties, a homogeneous and thermally stable dispersion of the metal ion components, synergetic effects between the elements, and the possibility of structure reconstruction under mild conditions. In this section, attention is focused on recently reported catalytic applications in some flelds of high industrial and scientific relevance (including organic chemistry, environmental catalysis and natural gas conversion). 

Heterogeneous catalysis is clearly a complex phenomenon to understand at the molecular level. Any catalytic transformation occurs through a sequence of elementary steps, any one of which may be rate controlling under different conditions of gas phase composition, pressure, or temperature. Furthermore, these elementary processes occur catalytically on surfaces that are usually poorly understood, particularly for mixed oxide catalysts. Even on metallic catalysts the reaction environment may produce surface compounds such as carbides, oxides, or sulfides which greatly modify... 

Li et al. developed a solid-state reaction process to synthesize perov-skite-type LaCoOs NCs with grain diameters of 15 0 run (Li et al., 2002). In the first step of the preparation, 5 run composite hydroxide NPs were s)mthesized by grinding metal nitrates liquid paste and mixing with KOH. Then the composite powders were calcined at 800 °C, yielding a single-phase oxide. Tien-Thao et al. prepared LaCo Cui J.O3 x < 0.3) by mechano-synthesis (Tien-Thao et al., 2008). The sample has various distinct Co " " ions in the perovskite lattice, which are more reducible. The reduced catalyst surface comprising cobalt and copper atoms is very selective for the hydrogenation of CO. 

Recent sol-gel methods have been recognized as promising procedures to prepare catalysts [12-14]. The sol-gel methods allows a unique way of catalyst design, because they represent an ab initio synthesis of the final solid from well defined molecular compounds [13]. By suitable choice of reagents, reaction and drying conditions, such technique allows to predefine pore structure, porosity, composition, surface polarity and crystallinity or amorphicity of metal oxides [12]. In principle, any metal that forms stable oxides can be forced to copolymerise with other metals in sol-gel procedures to provide mixed metal oxides [13]. 

Interactions between the precious metal and support influence the performance of the catalyst. Beil (1987) has defined metal-support interaction as depending on contact between the metal particle and the support which can be a dissolution of the dispersed metal in the lattice. The interaction could also depend on the formation of a mixed metal oxide, or the decoration of the metal particle surface with oxidic moieties derived from the support. It is possible that in this study, the differences in catalytic performance of the same active material supported on different washcoats can be attributed to any of these phenomena. Another explanation could be that the support materials exhibit different acid-base properties. According to the Bronsted and Lewis definitions, a solid acid shows a tendency to donate a proton or to accept an electron pair, whereas a solid base tends to accept a proton or to donate an electron pair. The tendency of an oxide to become positively or negatively charged is thus a function of its composition, which is affected by the preparation method and the precursors used. Refer to the section Catalyst characterization for further discussion on the influence of support material on catalyst performance. To thoroughly examine the influence of the support... 

Transition metal carbides (mainly of W and Mo) have been shown to be effective catalysts in some chemical reactions that are usually catalyzed by noble metals such as Pt and Pd (ref.1). Their remarkable physical properties added to lower cost and better availability could make them good candidates for substitute materials to noble metals in automobile exhaust catalysis. Hence, for this purpose, we have prepared several catalysts of tungsten carbide and W,Mo mixed carbides supported on y alumina with different Mo/W atom ratios. The surface composition has been studied by XPS while the quantitative determination of catalytic sites has been obtained by selective chemisorption of hydrogen and of carbon monoxide. The catalytic performances of these catalysts have been evaluated in the simultaneous conversion of carbon monoxide, nitric oxide and propane from a synthetic exhaust gas. 

There are some excellent review articles on different aspects of mesostructured materials, such as synthesis, properties, and applications. " Extensive research effort has been devoted to the exploitation of new phases (lamellar, cubic, hexagonal structures), expansion of the pore sizes (about 2-50 nm are accessible), and variable framework compositions (from pure silica, through mixed metal oxides to purely metal oxide-based frameworks, and inorganic-organic hybrid mesostructures). Another research focus is on the formation of mesostructured materials in other morphologies than powders, e.g. monolithic materials and films, which are required for a variety of applications including, but not limited to, sensors (based on piezoelectric mass balances or surface acoustic wave devices), catalyst supports, (size- and shape-selective) filtration membranes or (opto)electronic devices. The current article is focused... 

Mg-Al mixed oxides obtained by thermal decomposition of anionic clays of hydrotalcite structure, present acidic or basic surface properties depending on their chemical composition [1]. These materials contain the metal components in close interaction thereby promoting bifunctional reactions that are catalyzed by Bronsted base-Lewis acid pairs. Among others, hydrotalcite-derived mixed oxides promote aldol condensations [2], alkylations [3] and alcohol eliminations reactions [1]. In particular, we have reported that Mg-Al mixed oxides efficiently catalyze the gas-phase self-condensation of acetone to a,P-unsaturated ketones such as mesityl oxides and isophorone [4]. Unfortunately, in coupling reactions like aldol condensations, basic catalysts are often deactivated either by the presence of byproducts such as water in the gas phase or by coke build up through secondary side reactions. Deactivation has traditionally limited the potential of solid basic catalysts to replace environmentally problematic and corrosive liquid bases. However, few works in the literature deal with the deactivation of solid bases under reaction conditions. Studies relating the concerted and sequential pathways required in the deactivation mechanism with the acid-base properties of the catalyst surface are specially lacking. 

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