Adsorption oxygen species formation

From the pH dependence of the peak potential (close to 60 mV/decade), it was proposed that the oxidation involves adsorption of oxygenated species, leading to the formation of either the oxide or the hydroxide ... 

The specific adsorption of bisulfate anions is observed in H2SO4 in both EXAFS and XANES data and, in agreement with voltammetry, is seen to impede oxygen adsorption. Significant specific anion adsorption was found in 6 M TFMSA, but not in 1 M TFMSA [Teliska et al., 2007]. As mentioned above, this specific anion adsorption suppresses OH adsorption (particularly the formation of subsurface O), causes the Pt nanoparticle to become more round, and weakens the Pt-Pt bonding at the smface. The specific anion adsorption becomes site-specific only after lateral interactions from other chemisorbed species such as OH force the anions to adsorb into specific sites. 

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 formation of oxygen species on ZnO has been of interest for some time (77). Thermal activation of ZnO at about 500°C in vacuo gives an EPR signal at g = 1.96 which is thought to arise from a donor species such as Zn+ ions (755, 167-170). Adsorption of oxygen decreases the signal at 1.96,... 

It is clear from the kinetics that both ethylene and oxygen adsorption are important since both compounds appear in the rate equations with non-zero orders. Moreover, it is well known that ethylene is not adsorbed on pure silver, but that it does adsorb on a surface that is partially covered with oxygen. This implies that ethylene is either adsorbed on top of pre-adsorbed oxygen or on silver sites that are activated by the presence of oxygen (i.e. by formation of surface oxides, or another form of electron transfer or polarization). Consequently, two different mechanisms arise for the formation of ethylene oxide. The (direct) combustion of ethylene is another point of discussion. Although many favour the idea that different oxygen species are involved, others assume the same oxygen species, but different forms of ethylene adsorption. 

Although these reactions have been researched extensively and are the subjects of numerous patents, the precise reaction mechanism is not fully understood. The controversy has mostly centered on the nature of the oxygen species responsible for ethylene oxide formation (103). The results of various surface characterization studies indicate that there are at least three types of adsorbed oxygen species on silver monoatomic chemisorbed oxygen, diatomic (molecular) oxygen, and subsurface oxygen. The first results from a dissociative adsorption of oxygen on a silver surface ... 

In addition, from the slope of these variations as a function of O2 pressure in a log-log plot, the nature of the oxygen species controlling the adsorption equilibrium between the semiconductor free electrons and the gas (for given illumination, temperature and pressure range) can be deduced, provided the ways of formation of these oxygen species from gaseous O2 are assumed. For example, the predominance of... 

From these results it appears that the addition of tin to platinum greatly favors the formation of AA eomparatively to AAL. This can be explained by the bifunctionnal mechanism where ethanol is adsorbed dissociatively at platinnm sites, either via an 0-adsorption or a C-adsorption process followed by the oxidation of these adsorbed residues by oxygenated species formed on Sn at lower potentials giving AA. 

The first stage of the process is the strong adsorption and activation of the N2O molecule accompanied by the formation of a chemisorbed monoatomic singlet oxygen species. The latter is involved in further stages in the reactions of electrophilic substitution in the aromatic riing (or insertion of the oxygen atom into the C-H bond. 

From these experiments, it was concluded that a fraction of the oxygen ions adsorbed at 250° presents, at room temperatme, the same reactivity toward carbon monoxide as oxygen species chemisorbed at room temperature, since in both cases carbon dioxide is formed. From the differential heats of interaction of carbon monoxide at room temperature (hrst adsorption) with this fraction of the oxygen species adsorbed at 250°, it has been possible to show that the heat of adsorption of these adsorbed ions (62 kcal/mole) is very similar to initial heat of adsorption of oxygen at 30° (60 kcal/mole) (Fig. 4). Since the formation of O2 ions is not probable at 250° on a divided reactive oxide, we assume that at both temperatures (30 and 250°) oxygen is adsorbed as O ions. 

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