Mixed metals surface composition

The effect of precursor-support interactions on the surface composition of supported bimetallic clusters has been studied. In contrast to Pt-Ru bimetallic clusters, silica-supported Ru-Rh and Ru-Ir bimetallic clusters showed no surface enrichment in either metal. Metal particle nucleation in the case of the Pt-Ru bimetallic clusters is suggested to occtir by a mechanism in which the relatively mobile Pt phase is deposited atop a Ru core during reduction. On the other hand, Ru and Rh, which exhibit rather similar precursor support interactions, have similar surface mobilities and do not, therefore, nucleate preferentially in a cherry model configuration. The existence of true bimetallic clusters having mixed metal surface sites is verified using the formation of methane as a catalytic probe. An ensemble requirement of four adjacent Ru surface sites is suggested. 

As it can be observed in Table 13.1, Ir supported over pure oxides exhibited low acidity, but Ir supported on mixed Nb20s-Si02 displayed an important enhancement in the surface acidity with surface coverage by niobia increases. Binding energies (BE) of core-level electrons and metal surface composition were obtained from XP spectra. The BE values of Si 2p, Ti 2p3/2, Nb 3ds/2 were 103.4, 458.5 and 123 eV respectively, which are exactly the expected values considering the presence of oxides of Si (IV), Ti (IV) and Nb (V). With regard to Ir 4f7/2 core level, a... 

Fig. 2.26 Influence In203-Sn02 mixed metal oxide composition on (1, 2) sensor response and (3) specific surface area of gas-sensing matrix calcined at 600 °C (1) response to 1,000 ppm of CO at Tpg =250 °C and (2) response to 450 ppm of NO2 at Tp =150 °C (Adapted with permission from Aifan et al. (2006). Copyright 2006 Elsevier)...

The composition and chemical state of the surface atoms or molecules are very important, especially in the field of heterogeneous catalysis, where mixed-surface compositions are common. This aspect is discussed in more detail in Chapter XVIII (but again see Refs. 55, 56). Since transition metals are widely used in catalysis, the determination of the valence state of surface atoms is important, such as by ESCA, EXAFS, or XPS (see Chapter VIII and note Refs. 59, 60). 

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). 

There is a wide variety of solid electrolytes and, depending on their composition, these anionic, cationic or mixed conducting materials exhibit substantial ionic conductivity at temperatures between 25 and 1000°C. Within this very broad temperature range, which covers practically all heterogeneous catalytic reactions, solid electrolytes can be used to induce the NEMCA effect and thus activate heterogeneous catalytic reactions. As will become apparent throughout this book they behave, under the influence of the applied potential, as active catalyst supports by becoming reversible in situ promoter donors or poison acceptors for the catalytically active metal surface. 

Physical properties of binary or ternary Ru/Ir based mixed oxides with valve metal additions is still a field which deserves further research. The complexity of this matter has been demonstrated by Triggs [49] on (Ru,Ti)Ox who has shown, using XPS and other techniques (UPS, Mossbauer, Absorption, Conductivity), that Ru in TiOz (Ti rich phase) adopts different valence states depending on the environment. Possible donors or acceptors are compensated by Ru in the respective valence state. Trivalent donors are compensated by Ru5+, pentavalent acceptors will be compensated by Ru3+ or even Ru2+. In pure TiOz ruthenium adopts the tetravalent state. The surface composition of the titanium rich phase (2% Ru) was found to be identical to the nominal composition. 

It is a matter of speculation as to whether or not the activity would pass through a significant maximum at a surface composition between 0 and 30% Rh. It is interesting to note in this connection that the magnetic susceptibility (156, 157) and the electronic specific heat coefficient (156) increase from low values at 60% Ag-Pd through pure palladium and reach a maximum at - 5% Rh-Pd, thereafter decreasing smoothly to pure rhodium. Activity maxima have also been reported for reduced mixed oxides and supported alloys of group VIII metal pairs. For example, in the... 

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). 

In an ABS/metal composite, 10% iron powder has been admixed. The main reasons for choosing iron powder as short fiber fillers were its reasonably good mechanical and thermal properties as well as its capabilities of mixing and surface bonding with polymers (79). The shape of the iron particles was spherical. 

The effects of changes in the surface composition of mixed oxides SA and ST, and the initial concentration of dissolved metal ions, are revealed in changes of (i) the pH values corresponding to adsorption of 50% of the ions (pH5o%) and (ii) the pH range of adsorption from 10 to 90% of the ions (ApHi0. 90%) (Tables 2-4). 

The study of catalysis by mixed-metal electrodes is confronted with intricacies not encountered in work with single-metal surfaces. An important issue that is seldom addressed pertains to the character of the topmost layer of the electrocatalyst. Almost all of the stractural and compositional analysis undertaken have employed methods that provide information not only of the outermost layer but also of the sub-surface environment hence, ambiguities may reside in the results that lead to umeliable correlations between interfacial structure, composition and reactivity. 

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... 

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