Supported mixed metals

Support Effect.—In the process of making mixed metal supported catalysts it is natural to wonder what interaction takes place between the metal components and between metals and support. Bouwman and Biloen have answered these questions for Pt/y-AUOa, Ge/y-AUOa, and Pt,Ge/y-Al203. They used XPS and from observed line positions determined valence states. From intensities they inferred either aggregation of metal or its disappearance into the support. As they point out, some care is required in interpretation of line positions. Thus Pt°, for example, may lie as single atoms in the electric fields of the ions of the support. This problem they dealt with by making comparisons with evaporated... 

Today the most efficient catalysts are complex mixed metal oxides that consist of Bi, Mo, Fe, Ni, and/or Co, K, and either P, B, W, or Sb. Many additional combinations of metals have been patented, along with specific catalyst preparation methods. Most catalysts used commercially today are extmded neat metal oxides as opposed to supported impregnated metal oxides. Propylene conversions are generally better than 93%. Acrolein selectivities of 80 to 90% are typical. 

In addition to these principal commercial uses of molybdenum catalysts, there is great research interest in molybdenum oxides, often supported on siHca, ie, MoO —Si02, as partial oxidation catalysts for such processes as methane-to-methanol or methane-to-formaldehyde (80). Both O2 and N2O have been used as oxidants, and photochemical activation of the MoO catalyst has been reported (81). The research is driven by the increased use of natural gas as a feedstock for Hquid fuels and chemicals (82). Various heteropolymolybdates (83), MoO.-containing ultrastable Y-zeoHtes (84), and certain mixed metal molybdates, eg, MnMoO Ee2(MoO)2, photoactivated CuMoO, and ZnMoO, have also been studied as partial oxidation catalysts for methane conversion to methanol or formaldehyde (80) and for the oxidation of C-4-hydrocarbons to maleic anhydride (85). Heteropolymolybdates have also been shown to effect ethylene (qv) conversion to acetaldehyde (qv) in a possible replacement for the Wacker process. 

Catalysis by Metal Oxides and Zeolites. Metal oxides are common catalyst supports and catalysts. Some metal oxides alone are industrial catalysts an example is the y-Al202 used for ethanol dehydration to give ethylene. But these simple oxides are the exception mixed metal oxides are more... 

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). 

A variety of mixed metal catalysts, either as fused oxides (42 7 8) or coprecipitated on supports (25 0) or as physical mixtures of separate catalysts (5P), have been tested in aniline reductions. In the hydrogenation of ethyl p-aminobenzoate, a coprccipitated 3% Pd, 2% Rh-on-C proved superior to 5% Rh-on-C, inasmuch as hydrogenolysis to ethyl cyclohexanecarboxylate was less (61) (Table 1). 

These data show hydrogenolysis to increase with temperature, a general observation supported by many experiments. Here the influence of temperature is less with the mixed-metal catalysts. 

Chapter 11 analyzes the recently discovered mechanistic equivalence of electrochemical promotion and metal-support interactions on ionic and mixed conducting supports containing Zr02, Ce02 or Ti02. The analysis focuses on the functional identity and operational differences of promotion, electrochemical promotion and metal support interactions. 

It will also be shown that the absolute electrode potential is not a property of the electrode but is a property of the electrolyte, aqueous or solid, and of the gaseous composition. It expresses the energy of solvation of an electron at the Fermi level of the electrolyte. As such it is a very important property of the electrolyte or mixed conductor. Since several solid electrolytes or mixed conductors based on ZrC>2, CeC>2 or TiC>2 are used as conventional catalyst supports in commercial dispersed catalysts, it follows that the concept of absolute potential is a very important one not only for further enhancing and quantifying our understanding of electrochemical promotion (NEMCA) but also for understanding the effect of metal-support interaction on commercial supported catalysts. 

Promotion, electrochemical promotion and metal-support interactions are three, at a first glance, independent phenomena which can affect catalyst activity and selectivity in a dramatic manner. In Chapter 5 we established the (functional) similarities and (operational) differences of promotion and electrochemical promotion. In this chapter we established again the functional similarities and only operational differences of electrochemical promotion and metal-support interactions on ionic and mixed conducting supports. It is therefore clear that promotion, electrochemical promotion and metal-support interactions on ion-conducting and mixed-conducting supports are three different facets of the same phenomenon. They are all three linked via the phenomenon of spillover-backspillover. And they are all three due to the same underlying cause The interaction of adsorbed reactants and intermediates with an effective double layer formed by promoting species at the metal/gas interface (Fig. 11.2). 

Having discussed the functional equivalence of classical promotion, electrochemical promotion and metal-support interactions on 02 -conducting and mixed electronic-ionic conducting supports, it is useful to also address and systematize their operational differences. This is attempted in Figure 11.15 The main operational difference is the promoter lifetime, Tpr, on the catalyst surface (Fig. 11.15). 

Although catalytic wet oxidation of acetic acid, phenol, and p-coumaric acid has been reported for Co-Bi composites and CoOx-based mixed metal oxides [3-5], we could find no studies of the wet oxidation of CHCs over supported CoO catalysts. Therefore, this study was conducted to see if such catalysts are available for wet oxidation of trichloroethylene (TCE) as a model CHC in a continuous flow fixal-bed reactor that requires no subsequent separation process. The supported CoOx catalysts were characterized to explain unsteady-state behavior in activity for a certain hour on stream. 

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. 

In order to verify the presence of bimetallic particles having mixed metal surface sites (i.e., true bimetallic clusters), the methanation reaction was used as a surface probe. Because Ru is an excellent methanation catalyst in comparison to Pt, Ir or Rh, the incorporation of mixed metal surface sites into the structure of a supported Ru catalyst should have the effect of drastically reducing the methanation activity. This observation has been attributed to an ensemble effect and has been previously reported for a series of silica-supported Pt-Ru bimetallic clusters ( ). 

Methanatlon Studies. Because the most effective way to determine the existence of true bimetallic clusters having mixed metal surface sites Is to use a demanding catalytic reaction as a surface probe, the rate of the CO methanatlon reaction was studied over each series of supported bimetallic clusters. Turnover frequencies for methane formation are shown In Fig. 2. Pt, Ir and Rh are all poor CO methanatlon catalysts In comparison with Ru which Is, of course, an excellent methanatlon catalyst. Pt and Ir are completely inactive for methanatlon In the 493-498K temperature range, while Rh shows only moderate activity. 

Figure 9.16 ORR activity of two mixed-metal monolayer electrocatalysts supported on Pd(l 11), expressed as the kinetic current density at 0.85 V as a function of the M Pt ratio in the Pd-supported Pt-M monolayer. (Reproduced with permission from Zhang et al. [2005b].)...

Mixed-metal dendrimers containing up to 6 Pt(IV)-based organometallic species in the branches and 12 peripheral ferrocene units (8) have recently been synthesized and their electrochemical behavior investigated [13]. As in the previously discussed examples, multi-electron reversible oxidation processes, assigned to the equivalent, non-interacting ferrocene units, have been observed. The authors point out that cyclic voltammetry is a powerful tool to support the structure of the dendrimers containing ferrocene units. 

As catalysis proceeds at the surface, a catalyst should preferably consist of small particles with a high fraction of surface atoms. This is often achieved by dispersing particles on porous supports such as silica, alumina, titania or carbon (see Fig. 1.2). Unsupported catalysts are also in use. The iron catalysts for ammonia synthesis and CO hydrogenation (the Fischer-Tropsch synthesis) or the mixed metal oxide catalysts for production of acrylonitrile from propylene and ammonia form examples. 

A central question with respect to supported metal catalysts is that of the structure of the metal-support interface. Various possibilities have been proposed, varying from interfaces consisting of a mixed metal aluminate or silicate layer [17] or the presence of metal ions which serve as anchors between particle and support [18] to the attractive interaction between ions of the support and the dipoles that these ions induce in the metal particle [19]. EXAFS highlights the atomic surroundings of an atom in the catalyst, and if the supported metal particles are sufficiently small, oxygen atoms in the metal-support interface give a measurable contribution to the EXAFS spectrum. 

Support for these proposals comes from several sources first, the disproportionation reactions of mixed metal clusters such as Rh2Co2(CO)j2 or Fe2Ru(CO)12 ... 

The examples introduced above refer to the characterization of the most common types of catalysts, usually supported metals or single, mixed, or supported metal oxides. Many other materials such as alloys [199,200], carbides [201-203], nitrides [204,205], and sulfides [206] are also frequently used in catalysis. Moreover, although modem surface science studies with model catalysts were only mentioned briefly toward the end of the review, this in no way suggests that these are of less significance. In fact, as the ultimate goal of catalyst characterization is to understand catalytic processes at a molecular level, surface studies on well-defined model catalysts is poised to be central in the future of the field [155,174], The reader is referred to the Chapter 10 in this book for more details on this topic. 

A systematic study of differently supported Ru catalysts showed that carbon catalysts provide very high selectivities to higher hydrocarbons (C10-C20) and the CNT-supported catalyst is among the most active systems of all [138]. In parts this is related to the inertness of carbon preventing the formation of hardly reducible mixed metal oxides with the support, such as CoAl204 [139,140], which is, besides coking, the main reason for catalyst deactivation. The carbon surface functionalized with oxygen... 

These routes rely on the direct transformation of soluble molecular species into supported metal (or mixed metal) particles. One method that has recently become popular is the "polyol" method. This takes a solution of metal salts, the carbon support, and a polyalcohol such as ethylene glycol. On heating, the polyol acts as both stabilizer and reductant, forming reduced metal particles on the carbon. It has been used successfully to prepare Ft and PtRu catalysts. ... 

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). 

Recent Advances on NO Reduction Related to Mixed Oxides, Supported Metals and Ion-Exchange Mesoporous... 

Since the strategy was initially based on catalytic purposes, the surfaces considered initially were mostly (i) highly divided oxides (here are included simple oxides, mixed oxides, zeoUtic materials, mesoporous systems, hybrid organic inorganic materials, metal organic frameworks, etc.) and (ii) highly divided metals (supported or unsupported small metal particles). 

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