Heterogeneous catalysis mixed catalysts

In theory, almost all metal oxides can be synthesized by sol-gel process. Among others, an abundant literature can be found on SiOj, AljOj, TiOj, ZrOj as well as on the corresponding mixed oxides [19, 67,71-73]. Thus all porous oxide materials used in heterogeneous catalysis as catalyst support or precursors of catalyst supports can be prepared by sol-gel process. In the field of inorganic gels, the present review will focus mainly on silica gels. 

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... 

Equation (2) (as an ordinary differential equation) and Eq. (3) apply now with Eq. (4). As already implied, a laboratory well-mixed reactor for heterogeneous catalysis is more difficult to realize than a PFR. Many versions have been used 12), and Froment and Bischoff 13) illustrate reactors with external recycle, with internal recycle 1,14), and with an internal spinning basket 15). When using these reactors for experiments in the transient regime, it is important to keep to a minimum the volume outside the bed of catalyst. Internal recycle reactors involve bearings exposed to hot reactive gases and require a magnetic drive system for leak-proof operation. Exter-... 

Decades of heterogeneous catalysis research have found empirically that combinations of transition metals are often better catalysts than the pure metal components by themselves. It remains challenging in many cases, however, to understand why these multi-metallic materials are better catalysts than the pure components. We focus in this review on combinations of transition metals where mixing of the components has occurred on atomic length-scales (as opposed to the conceptually simpler situation where the two components co-exist in pure but separated forms). We will refer to these combinations genetically as alloys, although below we define a more precise nomenclature for the range of possible materials that exist. 

In addition to practical applications, metal cluster-derived catalysts, particularly intrazeolite metal cluster compounds, may aid in the identification of catalytically important bonding and structural patterns and thereby further our molecular understanding of surface science and heterogeneous catalysis. The ship-in-bottle technique for the synthesis of bulky metal-mixed metal cluster compounds inside zeolites and/or interlayered minerals has gained growing attention for the purpose of obtaining catalytic precursors surrounded by the interior constraint, imposing molecular shape selectivity. Such approaches may pave the way to offer the molecular architecture of hybrid (multifunctional) tailored catalysts to achieve the desired selectivity and stability for industrial processes. 

Strength (FLS) empirical approach are discussed in Section 3 as methods for determining the molecular structures of metal-oxide species from their Raman spectra. The state-of-the-art in Raman instrumentation as well as new instrumental developments are discussed in Section 4. Sampling techniques typically employed in Raman spectroscopy experiments, ambient as well as in situ, are reviewed in Section S. The application of Raman spectroscopy to problems in heterogeneous catalysis (bulk mixed-oxide catalysts, supported metal-oxide catalysts, zeolites, and chemisorption studies) is discussed in depth in Section 6 by selecting a few recent examples from the literature. The future potential of Raman spectroscopy in heterogeneous catalysis is discussed in the fmal section. 

Sudah, O.S. Chester, A.W. Kowalski, J.A. Beeckman, J.W Muzzio, F.J. Quantitative characterization of mixing processes in rotary cal-ciners. Powder Technol. 2002, 126 (2), 166-173. Delmon, B. Formation of final catalyst. In Handbook of Heterogeneous Catalysis Ertl, G., Knozinger, H., Weitkamp, J., Eds. Wiley-VCH Weinheim, Germany, 1997 Vol. 1, 264-286. Patterson, H.B.W. Hydrogenation of Fats and Oils Theory and Practice AOCS Press Champaign, IL, 1994. 

Supported metals are used extensively in heterogeneous catalysis. In the present investigation platinum is loaded onto titania and titania-alumina supports to study the SMSI effects in detail. The catalysts were characterized by X-ray Diffraction(XRD), Stepwise Temperature Programmed Reduction (STPR) and chemisorption measurements. All the samples exhibit eharacteristic behaviour showing SMSI effect after HTR, though there is only moderate interaction in the mixed oxide sample. From STPR studies, the reducibility of platinum and the support in supported platinum systems is shown to depend on the extent of the interaction at the interface. 

Hydrogen gas is adsorbed onto the surface of these metal catalysts, and the catalyst weakens the H—H bond. In fact, if H2 and D2 are mixed in the presence of a platinum catalyst, the two isotopes quickly scramble to produce a random mixture of HD, H2, and D2. (No scrambling occurs in the absence of the catalyst) Hydrogenation is an example of heterogeneous catalysis, because the (solid) catalyst is in a diffo-ent phase from the reactant solution. In contrast, homogeneous catalysis involves reactants and catalyst in the same phase, as in the acid-catalyzed dehydration of an alcohol. 

In heterogeneous catalysis, there are many examples where addition of a second component can change the overall catalytic reactivity in the system by changing its solid-state chemistry. An example of this includes the addition of Co + to the M0S2 and NiS2 systems discussed in Chapter 5. The mixed metal sulfides offer significantly increased activity due to changes in the chemical reactivity of the sulfide surface. We introduce here the solid-state chemistry of oxide catalysts (see also reference 3). A more detailed discussion on mixed metal oxides is presented in Chapter 5. 

Misono, M. (ed) (2013) Heterogeneous Catalysis of Mixed Oxides Perovskite and Heteropoly Catalysts, 1st edn, Elsevier, Amsterdam, Netherlands. 

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