Catalysts changes during partial oxidation

Partial oxidation reactions are usually carried out over transition metal oxides capable of changing their valent state during their interaction with reacting molecules. Naturally, zeolites with their alumina-silicate composition did not prove themselves as good oxidation catalysts. They failed also to serve as efScient catalyst supporters, since transition metals being introduced into the zeolite matrix lose their ability to activate dioxygen [3,4],... 

The model shown in Scheme 2 indicates that a change in the formal oxidation state of the metal is not necessarily required during the catalytic reaction. This raises a fundamental question. Does the metal ion have to possess specific redox properties in order to be an efficient catalyst A definite answer to this question cannot be given. Nevertheless, catalytic autoxidation reactions have been reported almost exclusively with metal ions which are susceptible to redox reactions under ambient conditions. This is a strong indication that intramolecular electron transfer occurs within the MS"+ and/or MS-O2 precursor complexes. Partial oxidation or reduction of the metal center obviously alters the electronic structure of the substrate and/or dioxygen. In a few cases, direct spectroscopic or other evidence was reported to prove such an internal charge transfer process. This electronic distortion is most likely necessary to activate the substrate and/or dioxygen before the actual electron transfer takes place. For a few systems where deviations from this pattern were found, the presence of trace amounts of catalytically active impurities are suspected to be the cause. In other words, the catalytic effect is due to the impurity and not to the bulk metal ion in these cases. 

Recently, such a temperature oscillation was also observed by Zhang et al (27,28) with nickel foils. Furthermore, Basile et al (29) used IR thermography to monitor the surface temperature of the nickel foil during the methane partial oxidation reaction by following its changes with the residence time and reactant concentration. Their results demonstrate that the surface temperature profile was strongly dependent on the catalyst composition and the tendency of nickel to be oxidized. Simulations of the kinetics (30) indicated that the effective thermal conductivity of the catalyst bed influences the hot-spot temperature. 

Initially a few particularly sensitive tubes of the bundle will run away, i.e., the reaction changes, for example, from a selective partial oxidation to a total combustion, and the temperature rises rapidly. In a multitubular reactor with thousands of tubes every tube cannot be equipped with temperature profile measurements it is therefore likely that this runaway will remain undetected, especially if it involves only a few tubes. Although temperatures above 1000 °C can often be reached in the catalyst during such runaways, there is no safety risk, provided the tube is surrounded by a liquid heat-transfer medium. Because of the good heat transfer to the fluid the tube temperature remains close to that of the heat-transfer medium, and melting of the tube does not occur. 

Grunwaldt JD, Beier M, Kimmerle B, et al. Structural changes of noble metal catalysts during ignition and extinction of the partial oxidation of methane studied by advanced QEXAFS techniques. Phys Chem Chem Phys. 2009 11 8779. 

Hannemann S, Grunwaldt JD, Kimmerle B, Baiker A, Boye P, Schroer C. Axial changes of catalyst structure and temperature in a fixed-bed microreactor during noble metal catalysed partial oxidation of methane. Top Catal. 2009 52 1360. 

Another example of catalysts that undergo structural changes during reaction are catalysts that act by a redox mechanism, such as vanadium pentoxide used for partial oxidations. As shown by Mars and Van Krevelen [12], the catalyst provides the oxygen needed for the reactions, and it becomes reduced to a lower-valence oxide in the process ... 

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