Molecular oxygen dissociation

In the first step, molecular oxygen is adsorbed onto a catalyst surface site, (), forming 02(ads). The molecular oxygen dissociates to produce two adsorbed oxygen species, which may themselves adsorb gas-phase hydrocarbon molecules to form CnHn-O on the catalyst surface, as in step 3. The adsorbed hydrocarbon-oxygen species may further react with adsorbed oxygen to produce, via partially oxidised intermediates, carbon dioxide and water. These are able to dissociate from the catalyst surface into the gas phase. 

Clearly, once the propene molecule has generated a reduced patch of R atoms, there will be a competition between NO adsorption/ dissociation and the re-oxidation of the sites by molecular oxygen dissociation. The experimental evidence strongly suggests that as the temperature is increased we move from a region in which NO adsorption/ dissociation is favoured (low temperature), to one in vstoch re-oxidation by O2 is preferred (high temperature). Such a hypothesis explains why tiie peak NO activity decreases as the temperature of peak NO conversion increases. 

Parker D, Eppink A. (1997) Photoelectron and photofragment velocity map imaging of state-selected molecular oxygen dissociation/ionization dynamics. J. Chem. Phys. 107 2357-2362. 

Ozone can be generated by a variety of methods, the most common of which involves the dissociation of molecular oxygen electrically (silent discharge) or photochemicaHy (uv). The short-Hved oxygen atoms (lifetime s) react rapidly with oxygen molecules to form ozone. The widely employed technique... 

The reaction rate of molecular oxygen with alkyl radicals to form peroxy radicals (eq. 5) is much higher than the reaction rate of peroxy radicals with a hydrogen atom of the substrate (eq. 6). The rate of the latter depends on the dissociation energies (Table 1) and the steric accessibiUty of the various carbon—hydrogen bonds it is an important factor in determining oxidative stabiUty. 

The absorption wavelengtlrs quoted here are for the complete dissociation of these molecules to the atoms in their ground state. The thermochemical data also show that a temperature of nearly 4000 K is requhed before the atomic oxygen concentration is equal to that of molecular oxygen, and almost 7000 K for the nitrogen atom population to be equal to the molecular nitrogen concentration, at one atmosphere pressure. 

It is concluded [634] that, so far, rate measurements have not been particularly successful in the elucidation of mechanisms of oxide dissociations and that the resolution of apparent outstanding difficulties requires further work. There is evidence that reactions yielding molecular oxygen only involve initial interaction of ions within the lattice of the reactant and kinetic indications are that such reactions are not readily reversed. For those reactions in which the products contain at least some atomic oxygen, magnitudes of E, estimated from the somewhat limited quantity of data available, are generally smaller than the dissociation enthalpies. Decompositions of these oxides are not, therefore, single-step processes and the mechanisms are probably more complicated than has sometimes been supposed. 

The adsorption of C02 on metal surfaces is rather weak, with the exception of Fe, and no molecular or dissociative adsorption takes place at room temperature on clean metal surfaces. At low temperatures, lower than 180 to 300 K, a chemisorbed COf" species has been observed by UPS6 on Fe(lll) and Ni(110) surfaces, which acts as a precursor for further dissociation to CO and adsorbed atomic oxygen. A further step of CO dissociation takes place on Fe(l 11) above 300 to 390 K. 

H. 10 An important role of stratospheric ozone, Os, is to remove damaging ultraviolet radiation from sunlight. One result is the eventual dissociation of gaseous ozone into molecular oxygen gas. Write a balanced equation for the dissociation reaction. 

H2 adsorption is weak on the anatase surfaces [8], No dissociative adsorption of H2 takes place over the smooth surfaces of Au at temperatures below 473 K [9,10]. On small Au particles, adsorption is possible at low temperature. Dissociative adsorption of H2 can be accelerated by the negatively charged molecular oxygen species at steps, edges, comers of Au particles [5]. 

Pt(lll) at 105 K (70x70 A), (b) Model illustrating the growth of O chains by the collision of precursor molecular oxygen with the ends of O chains where they dissociate. (Reproduced from Ref. 28). 

In the presence of oxygen, the situation is more complicated, as more active species can be formed, which can contribute to NO decomposition or formation in the plasma. It was found [46] that the excited states of N2 do not contribute significantly to NO evolution, but mainly influence the dissociation or quenching of 02. As the concentration of 02 increases, their contribution to 02 dissociation also increases. Only nitrogen atoms in the ground state (N(2D)) or excited state (N(4S)) may contribute directly to NO formation by reactions with molecular oxygen ... 

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