Metal oxides, catalysts reaction mechanism

In the cases of the selective oxidation reactions over metal oxide catalysts the so-called Mars-van Krevelen or redox mechanism [4], involving nucleophilic oxide ions 0 is widely accepted. A possible role of adsorbed electrophilic oxygen (molecularly adsorbed O2 and / or partially reduced oxygen species like C , or 0 ) in complete oxidation has been proposed by Haber (2]. However, Satterfield [1] queried whether surface chemisorbed oxygen plays any role in catalytic oxidation. 

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],... 

The decomposition of nitrous oxide over various metal oxides has been widely investigated by many investigators (1-3). Dell, Stone and Tiley (4) have compared the reactivity of metal oxides and shown that in general p-type oxides were the best catalysts and n-type the worst, with insulators occupying an intermediate position. It has been generally accepted (5) that this correlation indicates that the electronic structure of the catalyst is an important factor in the mechanism of the decomposition of nitrous oxide over metal oxides catalysts. The reaction is usually written (4) as... 

The hydrogenation of 1,3-butadiene over metal oxide catalysts with D2 or H2 gives rise to predominantly 2-butene, whereas conventional metal catalysts give rise to 1-butene as the main product. Explain the differences in the product distribution in light of the reaction mechanisms. 

As mentioned in the introduction, early transition metal complexes are also able to catalyze hydroboration reactions. Reported examples include mainly metallocene complexes of lanthanide, titanium and niobium metals [8, 15, 29]. Unlike the Wilkinson catalysts, these early transition metal catalysts have been reported to give exclusively anti-Markonikov products. The unique feature in giving exclusively anti-Markonikov products has been attributed to the different reaction mechanism associated with these catalysts. The hydroboration reactions catalyzed by these early transition metal complexes are believed to proceed with a o-bond metathesis mechanism (Figure 2). In contrast to the associative and dissociative mechanisms discussed for the Wilkinson catalysts in which HBR2 is oxidatively added to the metal center, the reaction mechanism associated with the early transition metal complexes involves a a-bond metathesis step between the coordinated olefin ligand and the incoming borane (Figure 2). The preference for a o-bond metathesis instead of an oxidative addition can be traced to the difficulty of further oxidation at the metal center because early transition metals have fewer d electrons. 

Furans can be converted into N- alkylpyrroles by heating with primary amines and alumina. Similar thermal conversions of furans and benzo[6]furans to their sulfur analogues in the presence of alumina or other metal oxide catalysts and hydrogen sulfide are also known. l,3-Diphenylbenzo[c]furan is converted into the thiophene by heating with phosphorus pentasulfide. The mechanism of these reactions is obscure. 

The validity of the method of course also relies on the proposed mechanism for reaction (1) being the real one, meaning that there is some danger of circularity in the reasoning. It is to be hoped that other groups are also going to use this method, in order to widen its field of application. As far as could be ascertained, the method proposed by Parkash77 to determine the number of active sites in metal oxide catalysts by selective gas chemisorption has not... 

Banares, M.A., Martfnez-Huerta, M.V., Gao, X., Fierro, J.L.G., and Wachs, I.E., in "Metal oxide catalysts active sites, intermediates and reaction mechanisms", Symposium, 220th ACS National Meeting, Washington, USA (2000d). 

The Langmuir-Hinshelwood kinetic model describes a reaction in which the rate-limiting step is reaction between two adsorbed species such as chemisorbed CO and 0 reacting to form C02 over a Pt catalyst. The Mars-van Krevelen model describes a mechanism in which the catalytic metal oxide is reduced by one of the reactants and rapidly reoxidizd by another reactant. The dehydrogenation of ethyl benzene to styrene over Fe203 is another example of this model. Ethyl benzene reduces the Fe+3 to Fe+2 whereas the steam present reoxidizes it, completing the oxidation-reduction (redox) cycle. This mechanism is prevalent for many reducible base metal oxide catalysts. There are also mechanisms where the chemisorbed species reacts... 

Most oxidation reactions over oxide catalysts are well nnderstood in terms of the redox mechanism, for example, repeated rednction and oxidation of the surface layer or bulk of the oxide catalyst. In the first step, a metal oxide catalyst oxidizes reactant molecules, such as carbon monoxide to carbon dioxide (equation 1 reduction of catalyst). In the second step, the reduced catalyst is oxidized back to its initial state by oxygen molecules supplied by the gas phase (equation 2 reoxidation of catalyst). The catalytic oxidation (equation 3) proceeds by repetition of this redox cycle. 

More commonly, uranium has been used as a catalyst component for mixed-metal oxide catalysts for selective oxidation. Probably the most well known of these mixed oxide catalysts are those based on uranium and antimony. The uranium-antimony catalysts are exceptionally active and selective and they have been applied industrially. An interpretation of the catalyst structure and reaction mechanism has been reported by GrasselU and coworkers [42, 43] who discovered the catalyst The USb30io mixed oxide has been extensively used for the oxidation/ammoxida-tion reaction of propylene to acrolein and acrylonitrile. The selective ammoxida-tion of propylene was investigated by GrasseUi and coworkers [44], and it has been demonstrated that at 460 °G a 62.0% selectivity to acrolein with a conversion of 65.2% can be achieved. Furthermore, Delobel and coworkers [45] studied the selective oxidation of propylene over USb30io, which at 340 °C gave a selectivity to acrolein of 96.7%. 

A great number of catalysts have been tried in the oxidation of methane at atmospheric pressure with the hope of obtaining intermediate products of oxidation. It appears, however, that catalysts tend to carry the reaction to equilibrium, at which state methanol, formaldehyde and formic acid are present in only extremely minute traces. This is well illustrated by the work of Wheeler and Blair," who studied the influence of catalysts in connection with their work on the mechanism of combustion. When methane was oxidized in the presence of metallic and metallic oxide catalysts, no formaldehyde could be detected even at very short times of contact. The formaldehyde produced in the circulation experiments was in a concentration much greater than that required for equilibrium in the reaction ... 

The kinetics of methanol oxidation over metal oxide catalysts were elegantly derived by Holstein and Machiels [16], The kinetic analysis demonstrated that the dissociative adsorption of water must be included to obtain an accurate kinetic model. The reaction mechanism can be represented by three kinetic steps equilibrated dissociative adsorption of methanol to a surface methoxy and surface hydroxyl (represented by K,), equilibrated dissociative adsorption of water to two surface hydroxyls (represented by K ), and the irreversible hydrogen abstraction of the surface methoxy intermediate to the formaldehyde product and a surface hydroxyl (the rate determining step, represented by kj). For the case of a fully oxidized surface, the following kinetic expression was derived ... 

Chapter III. Heterogeneous Hydrocarbon Reactions with Participation of Solid Metals and Metal Oxides III. 1. Mechanisms of the Interaction between Alkanes and Catalyst Surfaces... 

Nonetheless, rate expressions more complex than a simple power law are sometimes useful. For example, a power law expression does not provide any insight into the reasons for changing reactant order (i.e., a changing value of a ) with temperature or organic reactant concentration. However, such effects are frequently observed in oxidation reactions and are often consistent with more fundamentally based rate expressions. Consider, for example, what one would suppose to be the simple oxidation of methane. Golodets (p. 445) states that methane oxidation over metal oxide catalysts may be interpreted by the following mechanism ... 

The predominant application of metal oxide catalysts is due to their oxidation and acid-base behavior. In the following, these areas are discussed separately, although it is clear that in many materials, for example, heteropolyacids, which combine both strong acidity and oxidation efficacy (37,38), and the sulfated metal oxides, where controversy exists as to whether the observed low temperature isomerization pathways are catalyzed by superacid or redox mechanisms (39-41), the distinction between acid-base and oxidation properties is somewhat arbitrary. To illustrate their principles, a number of different reaction types are discussed. Dehydrogenation reactions, ammoxidation, and the WGS reaction have been included imder oxidation catalysts since they constitute major industrial applications of metal oxide-based catalysts. In the case of acid-base catalysis, some of the recent activity in the area of biodiesel is described as an illustration of the complementarity of both acid catalysis and base catalysis. There are a number of additional applications of oxides as catalysts, such as in photocatalysis (42), which have not been reviewed here because of limitations of space. Oxidation Activity. 

Although many metal oxide catalysts and supported noble metal catalysts have been developed for WGS and PROX [5-7], catalysts with better performance are still desired. The development of new catalysts is not only important for achieving better activity, selectivity, and stability of catalysts, but also important for gaining fundamental insights (e.g., nature of active sites, reaction mechanisms, and deactivation mechanisms). Gaining fundamental insights can not only satisfy scientists curiosity, but also help in developing better catalysts. 

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