Role of Adsorbed Oxygen

Above room temperature, oxygen adsorbs on a-Fe203 in at least three different forms that are characterized by different temperatures of desorption 

Experiments were then performed to preadsorb the various forms of oxygen on an iron oxide surface. These were then followed by the butene adsorption-desorption experiments to determine the amounts of the two types of oxidation sites. Typical results are shown in Table V (6). The amounts of oxidation products are independent of the presence of preadsorbed oxygen. In fact, even the thermally desorbed isomers are not affected by the preadsorbed oxygen. This absence of effect of the preadsorbed oxygen was observed also in pulse experiments. It was found that the amounts of butadiene, C02, and butene isomers formed from a butene pulse passing over the catalyst at 100,200, or 300°C are independent of preadsorbed oxygen (6). 

While preadsorbed oxygen has no effect, the presence of gaseous oxygen drastically changes the product distribution in the thermal desorption and pulse reaction of butene on a-Fe203, as can be seen from results shown in Table VI (6). On this oxide, thermal desorption in an 02 instead of an He carrier results in a much lower yield of hydrocarbons and a much higher yield of C02, The same is observed in pulse reactions. Thus, on a-Fe203, adsorbed butene, adsorbed butadiene, and/or butadiene precursors must be very 

Effect of Preadsorbed Oxygen on the Oxidation of cis-2-Butene in Adsorption-Desorption Experiments -6 

Adsorbed oxygen species Adsorption temperature (°C) Thermally desorbed products (1017 molecules/m 2) B.D. I 

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These reactions demonstrate the Brflnsted base role of adsorbed oxygen perviously found on Ag(llO) and show further that more active transition metals which themselves activate C-H bonds catalytically oxidize via a two-step mechanism in which the surface intermediates are scavenged by adsorbed oxygen. 

The role of adsorbed oxygen species in the mechanism of alkane transformation, on the contrary, is more questionable. The effect induced by the substitution of O2 with N2O and IR indications are in agreement with this interpretation, but, on the other hand, activated electrophilic oxygen species form on reduced sites, preferably in tetrahedral coordination (79). The partial reduction of tetrahedral V =0 with formation of tetrahedral v after propane oxidative dehydrogenation can be observed using UV-Visible diffuse reflectance, ESR and V-NMR spectroscopies. It is thus not possible to assign unequivocally the active species in propane selective activation to a tetrahedral V =0 species or to or V -0-0 species formed in the... 

Sancier et al. (43) used oxygen-18 to examine the relative role of adsorbed versus lattice oxygen in propylene oxidation over a silica-supported bismuth molybdate catalyst as a function of temperature. At 400°C they observed the formation of predominantly acrolein[I60] rather than acrolein[I80], indicating significant participation of lattice oxygen. However, as the reaction temperature was decreased, the authors concluded that the role of adsorbed oxygen became more important. 

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. 

These effects are most striking on silver since it is, itself, a very unreactive surface. There is every reason to expect, however, that oxygen will behave similarly on other metals. More complex reaction behavior will, of course, be observed as the intrinsic reactivity of the metal increases. Oxygen adsorbed on platinum should show similar properties. In fact the formation of surface OH groups from HjO and 0(a) was recently reported 145). The ability of platinum itself to break C-H and C-C bonds complicates oxidation mechanisms, but future work should provide a greater understanding of the relative role of surface oxygen in oxidation catalysis. 

Electron paramagnetic resonance has played a major role in the characterization of adsorbed oxygen species and the use of 17Oz has enabled a major advance to be made in the understanding of the nature of the various oxygen species and how they can be bonded to the surface. The use of IR spectroscopy as a technique has tended to be neglected because of the... 

Muhler M, et al. On the role of adsorbed atomic oxygen and C02 in copper-based methanol synthesis catalysts. Catal Lett. 1994 25(1—2) 1—10. 

In this chapter, we have concentrated on MgO, one of the most studied and better understood oxide materials. We have shown that even on such a simple nontransition metal oxide about a dozen of different surface defect centers have been identified and described in the literature. Each of these centers has a somewhat different behavior toward adsorbed metal atoms. It becomes immediately clear that the precise assignment of the defect sites involved in the interaction, nucleation, and growth of the cluster is a formidable task. Nevertheless, thanks to the combined use of theory and experiment, the progress in this direction has been particularly significant and promising. For instance, a lot of evidence has been accumulated that points toward the role of the oxygen vacancies, the F centers. At the moment, these sites seem the most likely sites for nucleation and growth of small metal clusters. 

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