Studying Metal Oxide Catalysts

Owing to the importance of metal oxides in catalysis their structure is widely studied. The aim of such work is to establish structure-activity relationships, whereby the structural parameters of the materials responsible for their activity, and their mode of action, are identified. Industrial catalysts, however, are often highly complex materials and identifying such relationships is not a simple task. 

Metal Oxide Catalysis. Edited by S. David Jackson and Justin S. J. Hargreaves Copyright 2009 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-31815-5 

Furthermore, both bulk and surface properties of the materials can play important roles. For instance the oxidation of alkanes over vanadium-based oxides is beUeved to proceed via a Mars-van Krevelen mechanism [8, 9] with lattice (bulk) oxygen being the active oxygen species. In metal oxide-based cracking catalysts, however, activity is directed by the surface acidity of the material. 

There are, therefore, a number of distinct structural characteristics which must be identified in order to fuUy understand the action of oxide catalysts. Bulk properties of interest include the identification of distinct crystallographic phases present in the catalyst the local environment of the nuclei in either crystalline phases or amorphous materials and the redox properties of the catalyst. Surface properties impacting on catalytic activity include the local environment of nuclei at the surface acid-base behavior the number and concentration of acid sites, including hydroxyl groups and the nature of these acid sites. 

Solid-state NM R is perhaps unique in its ability to probe each of these characteristics. In particular, its sensitivity to local geometry and coordination environments provides advantages over many more traditional characterization methods. However, no single NMR experiment can furnish the investigator with data in each and aU of these areas. A number of distinct, innovative, approaches have been developed to probe different structural features. A number of these are outlined below. 

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Alkalis are the most important electropositive promoters of metal and metal oxide catalysts. They are used in many important industrial catalysts but are also quite suitable for fundamental studies since they can be easily introduced under vacuum conditions on well-characterized model metal surfaces. 

The activity of the Au/metal oxide catalysts is extremely sensitive to the method of preparation. The Au/metal oxide catalysts were prepared by the co-precipitating method [1]. During the course of this study, we have determined that the activity and the stability of the catalyst for room temperature CO oxidation were a function of Ph of the solution, temperature of precipitation, aging temperature and time, catalyst wash procedure, and calcination. 

We have summarized below recent results concerning spectroscopic / flow reactor investigations of hydrocarbons partial and total oxidation on different transition metal oxide catalysts. The aim of this study is to have more information on the mechanisms of the catalytic activity of transition metal oxides, to better establish selective and total oxidation ways at the catalyst surface, and to search for partial oxidation products from light alkane conversion. 

At an industrial scale, the esterification catalyst must fulfill several conditions that may not seem so important at lab-scale. This must be very active and selective as by-products are likely to render the process uneconomical, water-tolerant and stable at relatively high temperatures. In addition, it should be an inexpensive material that is readily available on an industrial scale. In a previous study we investigated metal oxides with strong Bronsted acid sites and high thermal stability. Based on the literature reviews and our previous experimental screening, we focus here on application of metal oxide catalysts based on Zr, Ti, and Sn. 

Other metal oxide catalysts studied for the SCR-NH3 reaction include iron, copper, chromium and manganese oxides supported on various oxides, introduced into zeolite cavities or added to pillared-type clays. Copper catalysts and copper-nickel catalysts, in particular, show some advantages when NO—N02 mixtures are present in the feed and S02 is absent [31b], such as in the case of nitric acid plant tail emissions. The mechanism of NO reduction over copper- and manganese-based catalysts is different from that over vanadia—titania based catalysts. Scheme 1.1 reports the proposed mechanism of SCR-NH3 over Cu-alumina catalysts [31b],... 

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

To investigate the effect of the synthesis method on the structure-reactivity relationship of the supported metal oxide catalysts, a series of V205/Ti02 catalysts were synthesized by equilibrium adsorption, vanadium oxalate, vanadium alkoxides and vanadium oxychloride grafting [14]. The dehydrated Raman spectra of all these catalysts exhibit a sharp band at 1030 cm characteristic of the isolated surface vanadium oxide species described previously. Reactivity studies with... 

These metal and metal oxide catalysts must work as a kind of electron pool which brings about multi-electron process for H2 and 02 generation. Silver colloids were studied as electron pool for H2 formation under y-irradiation in the aqueous system composed of Ag° colloids, acetone, 2-propanol and SDS S9). The colloids (average diameter 140 A) of 2.5 x 10 4 M can store 1 coulomb/1, corresponding to the storage of 450 electrons/particle 60 ... 

A systematic study to identify solid oxide catalysts for the oxidation of methane to methanol resulted in the development of a Ga203—M0O3 mixed metal oxide catalyst showing an increased methanol yield compared with the homogeneous gas-phase reaction.1080,1081 Fe-ZSM-5 after proper activation (pretreatment under vacuum at 800-900°C and activation with N20 at 250°C) shows high activity in the formation of methanol at 20°C.1082 Density functional theory studies were conducted for the reaction pathway of the methane to methanol conversion by first-row transition-metal monoxide cations (MO+).1083 These are key to the mechanistic aspects in methane hydroxylation, and CuO+ was found to be a likely excellent mediator for the reaction. A mixture of vanadate ions and pyrazine-2-carboxylic acid efficiently catalyzes the oxidation of methane with 02 and H202 to give methyl hydroperoxide and, as consecutive products, methanol and formaldehyde.1084 1085... 

In contrast to lead, the possible poisoning by metallic elements, derived from the vehicle system, is not easily documented. Many formulations of automotive catalysts contain both base and noble metals, but the detailed effect of such combinations on the particular reactions is rarely known with precision. One study was concerned with the effect of Cu on noble metal oxidation catalysts, since these, placed downstream from Monel NOx catalysts, could accumulate up to 0.15% Cu (100). The introduction of this amount of Cu into a practical catalyst containing 0.35% Pt and Pd in an equiatomic ratio has, after calcination in air, depressed the CO oxidation activity, but enhanced the ethylene oxidation. Formation of a mixed Pt-Cu-oxide phase is thought to underlie this behavior. This particular instance shows an example, when an element introduced into a given catalyst serves as a poison for one reaction, and as a promoter for... 

The next work [63] discusses the use of hydrogen peroxide and alkyl peroxides as effective oxidants in applied studies. Effective oxidation catalysts consisting of promoter transition metals are described. 

A controlled modification of the rate and selectivity of surface reactions on heterogeneous metal or metal oxide catalysts is a well-studied topic. Dopants and metal-support interactions have frequently been applied to improve catalytic performance. Studies on the electric control of catalytic activity, in which reactants were fed over a catalyst interfaced with O2--, Na+-, or H+-conducting solid electrolytes like yttrium-stabilized zirconia (or electronic-ionic conducting supports like Ti02 and Ce02), have led to the discovery of non-Faradaic electrochemical modification of catalytic activity (NEMCA, Stoukides and Vayenas, 1981), in which catalytic activity and selectivity were both found to depend strongly on the electric potential of the catalyst potential, with an increase in catalytic rate exceeding the rate expected on the basis of Faradaic ion flux by up to five orders of... 

EPR studies of transition metal-oxide catalysts have shown that oxygen molecules and atoms on their surface form radicals of several types whose parameters are mainly listed in Table 8.3. Here and in further Tables, for better comparison and representation, we include the appropriate data obtained for some diamagnetic oxides and relative compounds. 

Multiwall carbon nanotubes (MWNT) were obtained according to the method described in [4, 5]. The structure of MWNT and PTFE-MWNT composites was studied with use of transmssion electron microscope JEM-100CXII. Average diameter of nanotubes was 10-20 nm, surface area (determined by argon desorption method) - 250-400 m2/g, bulk density of MWNT powder 20-40 g/dm3. As-obtained MWNT were used which contained 6-20% of minerals (rests of metal oxide catalyst). 

Furfural 69 has been used as a chemical feedstock for the production of furan via two production methods involving the decarbonylation of furfural <2005MI7>. Processes in both the liquid and gas phases were described for the preparation of furan through the decarbonylation of furfural using noble metal and metal oxide catalysts. The results of the study led the authors to state that the research trends for preparing furan based on the decarbonylation of furfural should mainly be concentrated on more effective catalysts and environmentally friendly processes. 

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