Supported metallic oxides

Vuurman M A and Waohs I E 1992 In situ Raman speotrosoopy of alumina-supported metal oxide oatalysts J. Rhys. Chem. 96 5008-16... 

Each precious metal or base metal oxide has unique characteristics, and the correct metal or combination of metals must be selected for each exhaust control appHcation. The metal loading of the supported metal oxide catalysts is typically much greater than for nobel metals, because of the lower inherent activity pet exposed atom of catalyst. This higher overall metal loading, however, can make the system more tolerant of catalyst poisons. Some compounds can quickly poison the limited sites available on the noble metal catalysts (19). 

In this process ethylene, dissolved in a liquid hydrocarbon such as cyclohexane, is polymerised by a supported metal oxide catalyst at about 130-160°C and at about 200-500 Ibf/in (1.4-3.5 MPa) pressure. The solvent serves to dissolve polymer as it is formed and as a heat transfer medium but is otherwise inert. 

Raman spectroscopy has provided information on catalytically active transition metal oxide species (e. g. V, Nb, Cr, Mo, W, and Re) present on the surface of different oxide supports (e.g. alumina, titania, zirconia, niobia, and silica). The structures of the surface metal oxide species were reflected in the terminal M=0 and bridging M-O-M vibrations. The location of the surface metal oxide species on the oxide supports was determined by monitoring the specific surface hydroxyls of the support that were being titrated. The surface coverage of the metal oxide species on the oxide supports could be quantitatively obtained, because at monolayer coverage all the reactive surface hydroxyls were titrated and additional metal oxide resulted in the formation of crystalline metal oxide particles. The nature of surface Lewis and Bronsted acid sites in supported metal oxide catalysts has been determined by adsorbing probe mole-... 

CU/AI2O3, and AU/AI2O3 catalysts and the effects of Ii20 and CeO addition [69]. However, the additives caused a decrease in the N2 selectivity but remarkably improved the catalytic activity, in particular, a decrease in Tso over 200°C in the case of gold. Gold catalysts have a potential for NH3 oxidation at lower temperature if a proper kind of support metal oxides is selected. 

As can be seen in table 1, with different preparation methods and active metals, the average size of the copper particle for the catalysts A and D were 20.3 nm and 50.0 nm. While those of the catalysts B and C were 51.3 nm and 45.4 run, respectively. CuO, non-supported metal oxide, made by impregnation is sintered and cluster whose particle size was 30 pm. The water-alcohol method provided more dispersed catalysts than the impregnation method. 

A 5 wt.% CoOx/Ti02 catalyst was prepared via an incipient wetness technique in which an aqueous solution of Co(N03)2 6H20 (Aldrich, 99.999%) was impregnated onto a shaped Ti02 (Milleimium Chemicals, commercially designated as DT51D, 30/40 mesh), as described in detail elsewhere [6]. Other supported metal oxide catalysts, such as FeOx, CuO, and NiOx, were obtained in a fashion similar to that used for preparing the CoO, catalyst. 

Vibrational spectroscopic studies of heterogeneously catalyzed reactions refer to experiments with low area metals in ultra high vacuum (UHV) as well as experiments with high area, supported metal oxides over wide ranges of pressure, temperature and composition [1]. There is clearly a need for this experimental diversity. UHV studies lead to a better understanding of the fundamental structure and chemistry of the surface-adsorbate system. Supported metals and metal oxides are utilized in a variety of reactions. Their study leads to a better understanding of the chemistry, kinetics and mechanisms in the reaction. Unfortunately, the most widely used technique for determining adsorbate molecular structure in UHV,... 

The active sites or reaction sites proposed until now for CO oxidation over gold catalysts with different support metal oxides are as follows ... 

Wachs, I.E. (1996) Raman and IR studies of surface metal oxide species on oxide supports Supported metal oxide catalysts, Catal. Today, 27, 437. 

The catalysts which have been tested for the direct epoxidation include (i) supported metal catalysts, (ii) supported metal oxide catalysts (iii) lithium nitrate salt, and (iv) metal complexes (1-5). Rh/Al203 has been identified to be one of the most active supported metal catalysts for epoxidation (2). Although epoxidation over supported metal catalysts provides a desirable and simple approach for PO synthesis, PO selectivity generally decreases with propylene conversion and yield is generally below 50%. Further improvement of supported metal catalysts for propylene epoxidation relies not only on catalyst screening but also fundamental understanding of the epoxidation mechanism. 

Both heterogeneous and homogeneous CO reduction catalyst recipes often contain electrophilic components such as silica supports, metal oxides, and A1Cl3 [1,5,33,34,35,36]. 

The title Spectroscopy in Catalysis is attractively compact but not quite precise. The book also introduces microscopy, diffraction and temperature programmed reaction methods, as these are important tools in the characterization of catalysts. As to applications, I have limited myself to supported metals, oxides, sulfides and metal single crystals. Zeolites, as well as techniques such as nuclear magnetic resonance and electron spin resonance have been left out, mainly because the author has little personal experience with these subjects. Catalysis in the year 2000 would not be what it is without surface science. Hence, techniques that are applicable to study the surfaces of single crystals or metal foils used to model catalytic surfaces, have been included. 

There seems to be several mechanisms leading to the activity loss oxidation of cobalt metal, sintering of cobalt metal particles as well as sintering of the support and formation of stable cobalt-support metal oxides (silicates or aluminates). Oxidation by water is a key issue, possibly occurring on all supports and on unsupported cobalt. A thermodynamic analysis of this effect was reported by van Steen et al.,40 and they describe the FTS reaction system in terms of reactions (1) and (2) below ... 

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. 

Wachs, I.E. Molecular engineering of supported metal oxide catalysts Oxidation reactions over supported vanadia catalysts. Catalysis 1997,13, 37-54. 

The high-density polyethylene is linear and can be manufactured by (i) coordination polymerisation of monomer by triethyl aluminium and tritanium chloride, (ii) polymerisation with supported Metal Oxide Catalysts. Such as chromium or molybdenum oxides supported over alumina-silica bases. 

Polymerisation with Supported Metal Oxide Catalyst... 

In general, there are two possibilities to prepare nanocarbon-supported metal(oxide) catalysts. The in situ approach grows the catalyst nanoparticles directly on the carbon surface. The ex situ strategy utilizes pre-formed catalyst particles, which are deposited on the latter by adsorption [94]. Besides such solution-based methods, there is also the possibility of gas phase metal (oxide) loading, e.g., by sputtering [95], which is used for preparation of highly loaded systems required for electrochemical applications not considered here. 

Acid centers, structure, sulfate-supported metal oxides, 37 192-196 Acidic catalysis, 6 241 montmorillonite, 38 266-268 Acidic dissociation constant, probe molecules, 38 210... 

Acidity, 27 284, 285 catalytic performance, 30 121 crystalline titanium silicates, 41 319-320 estimating, 37 166 heteropoly compounds, 41 139-150 ion exchange and, zeolites, 31 5-6 sulfate-supported metal oxides, 37 186-187 surface, monolayer dispersion, 37 34-35 tin-antimony oxide, 30 114-115, 125-1256 Acids, see also specific compounds adsorption of, on oxide surfaces, 25 243-245... 

Metal oxides, 31 78-79, 89, 102, 123, 157-158, 191, 32 199-121 see also Amorphous metal oxides Sulfate-supported metal oxides specific oxides adsorbed oxygen on, 27 196-198 binary, surface acidity, 27 136-138 catalytic etching, 41 390-396 coordination number, 27 136 electrocatalysts, 40 127-128 Fe3(CO)i2 reaction with, 38 311-314 Lewis acid-treated, 37 169-170 multiply-valent metals, electrocatalytic oxidations, 40 154-157 superacids by, 37 201-204 surface acidity, methods for determining, 27 121... 

The snrface acid-base properties of supported oxides can be conveniently investigated by studying the adsorption of suitably chosen basic-acidic probe molecules on the solid. As shown, acidic and basic sites are often present simultaneously on solid surfaces. The knowledge of the detailed amphoteric character of supported metal oxides is of extreme interest due to the possibility of using them as catalysts in different reactions in which acidity governs the reaction mechanism. 

The major challenge in gasification is to avoid the formation of tars, which have a tendency to clog filters and condense in end-pipelines. Tars are considered as the condensable fraction of the organic gasification products, and consist mainly of different aromatic hydrocarbons with benzene as the main species. For removal of tars three types of catalysts have been widely investigated alkali metal salts, alkaline earth metal oxides and supported metallic oxides. 

Surface Oxide—Support Interactions in the Molecular Design of Supported Metal Oxide Selective Oxidation Catalysts... 

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