Adsorbed oxygen species

It is generally concluded that 0 species are the most reactive in CO oxidation and in homomolecular O2 exchange under illumination. The latter reaction proceeds via 

A similar mechanism, involving only 1 surface O per exchanged O2 molecule, was proposed by Courbon et a/. It was shown by studying 02/ 02 exchange and CO oxidation simultaneously, that CO has a much greater reactivity with 0 than does O2. 

Homomolecular O2 exchange is very slow on ZnO at 298 K in the dark. However, cooling to 77 K produces a sudden increase in rate. There is debate over the active intermediate for this strange, non-photocatalytic reaction. Tanaka et al. and Hirota et al. favour O4 and 4 species, respectively, whereas Russian workers conclude that these cannot be involved. Tanaka et al. published e.s.r. studies using 02 in support of neutral O4 species, but Gundrizer et al. state that such species in liquid O2 are inactive in exchange and, therefore, that it is more likely for the intermediate on ZnO to be a non-radical , molecularly adsorbed species. It is not clear in their paper what exactly is meant by this nor how such species can function as intermediates. 

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Martel R, Avouris Ph and Lyo l-W 1996 Molecularly adsorbed oxygen species on Si(111)-(7 7) STM-induced dissociative attachment studies Science 272 385... 

The reaction of the adsorbed oxygen species with methane... 

To distinguish the different actions of different oxygen species over the LiNiLa0x/Al203 on the CO selectivity, the sanq)le was pretreated by 5% H2/Ar flow at 1123K for 0.5 hr. After the pretreatment, the NiO was reduced to the reduced nickel and the surface adsorbed oxygen species was conqrletely consumed, and then the CH4 pulse reaction was performed. The results are different from the results of CH4 pulse reaction on the fresh sample. The... 

Ag(l 11) that was emersed from 1 mM NaOH + 0.5 mM NaF at specified potentials. The data were deconvoluted into a number of peaks. Above —0.8 V vs. Hg/HgO, the data indicate the presence of a nnmber of adsorbed oxygen species. They coupled the XPS with thermal desorption in an effort to identify the species. The peaks at 529.6eV and 531.6eV conld be assigned to adsorbed oxygen and OH species, respectively. 

For potentials higher than 0.5 V vs. RHE, the formation of adsorbed oxygen species at Ru as well as at Pt will block the catalytic surface, leading to a decrease in the methanol adsorption kinetics. Therefore, in a potential range higher than 0.5 V vs. RHE, the kinetics of methanol oxidation is optimized at a Ru-poor catalyst, because methanol adsorption is not blocked and because the presence of Ru provides the extra oxygen atom needed to complete the oxidation of adsorbed CO to CO2. 

The release of N2 occurs within function 3. It involves the dissociation of NO (via a dinitrosyl-adsorbed intermediate), followed by subsequent formation of N2 and scavenging of the adsorbed oxygen species left from NO dissociation. The removal of adsorbed oxygen is due to the total oxidation of an activated reductant (CxHyOz). This reaction corresponds to a supported homogeneous catalytic process involving a surface transition metal complex. The corresponding catalytic sequence of elementary steps occurs in the coordinative sphere of the metal cation. 

A dinitrosyl (NO)2 species forms, whose dissociation leads to N2 the remaining adsorbed oxygen species have to be scavenged by the activated reductant, C, HvOz to recover the free active site, permitting the catalytic cycle to turn over (Figure 5.1, function 3) [10]. 

Let us note that there is no need for any organic nitroso intermediate for N2 formation. Therefore, two adsorbed oxygen species Oads (ex-NO) are remaining, strongly adsorbed... 

If the two Oads species are not scavenged, then the reaction will stop. This is the case, for instance, of NO decomposition on Cu/ZSM-5 [25], Adsorbed oxygen species have to be scavenged either by an activated form of the initial HC reductant, such as QH O , (alcohol, aldehyde, etc.) or by the initial HC if their total oxidation is simultaneous with NO decomposition-reduction to N2. These oxygenates and/or HC suffer a total oxidation to C0/C02 and H20, regenerating the active site this is the principle of catalysis. Once the active site is recovered, the reaction continues to turn over. This is the catalytic cycle . 

Figure 5.1 shows that function 1 is therefore the oxidation of NO to N02, this last one being subsequently delivered to function 2 to oxidize HC to CxHyOz which will be delivered to function 3 to scavenge the adsorbed oxygen species left by the (NO)2 -adsorbed dinitrosyl species - decomposition. 

Similar results have been obtained over alumina alone, in the presence of propene [27], The initial HC of the feed (propene) has to first transform to oxygenates (alcohol, aldehyde, etc.) - simultaneously to the NO decomposition (function 3) - to scavenge adsorbed oxygen species left by NO decomposition and regenerate the active sites of function 3. The mild oxidation of HC to oxygenates is the role of function 2 of the present model. 

NO is then reduced to N2 and mild oxygenated species are completely oxidised to C02, by reacting with the adsorbed oxygen species left during the NO reducing process, so regenerating the active sites responsible for the deNOx process ... 

Based on these experimental findings it is drawn that adsorbed oxygen species should play an important role to form adsorbed intermediates and adsorption sites for reaction gas components. Therefore, to know the detailed reaction mechanism of ethylene oxidation, it is necessary to clarify a situation of the adsorbed layer formed during the reaction, especially on the adsorbed oxygen species available for the progress of reaction. 

In order to analyze the adsorption behavior of carbon dioxide on silver it was necessary to understand the adsorption behavior of and its reactivity, because the adsorption of CO strongly relatedto the adsorbed oxygen species as will be described later. For this reason, the following transient experiments were purformed. 

From the above results, one can conclude the existence of the two types of adsorbed oxygen species which may be speculated as monoatomic and diatomic oxygen species. The monoatomic oxygen species can produce (In) and the diatomic oxygen species can produce C2Hi,0 and decompose (In). 

Especially for the low temperature water gas shift reaction the mechanistic scheme, proposed here, seems to correspond to the three different adsorbed oxygen species, proposed by Kobaya-shi (13) for the ethylene oxidation on silver, whereas the importance of some surface complexes of CO - 1 0 type has been revealed (14) by analysing steady state data. 

It is known that CO oxidation involves a surface reaction between adsorbed CO molecules and an adsorbed oxygen species, which we shall refer to as a surface oxide, but which may be an adsorbed oxygen... 

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