O2 dissociation

Figure 5.10. Defects consisting of oxygen vacancies constitute adsorption sites on a Ti02 (110) surface. Note how CO binds with its lone-pair electrons on a Ti ion (a Lewis acid site). O2 dissociating on a defect furnishes an O atom that locally repairs the defect. CO2 may adsorb by coordinating to an O atom, thus forming a carbonate group. [Figure adapted from W. Gopel, C. Rocher and R. Feierabend, Phys. Rev. B 28 (1983) 3427.]...

Figure 9.3 Cross-section and top views of selected states along the O2 dissociation path on Pt(lll), from the initial state (t-b-t) to the dissociated product state (fee x 2). The third image is the transition state. (Reproduced with permission from Xu et al. [2004].)...

The stoichiometric Pt3Co(lll) surface has one-quarter of the Pt atoms replaced with Co atoms. Bulk Co is very reactive toward oxygen, on which O2 dissociation is probably non-activated. The Co atoms on the alloy surface are diluted by Pt, but... 

Figure 9.15 Kinetic current density (squares) at 0.8 V for O2 reduction on supported Pt monolayers in a 0.1 M HCIO4 solution, and the calculated activation energy barriers for O2 dissociation (filled circles) and OH formation (open circles) on PtML/Au(lll), Pt(lll), PtML/ Pd(lll), and PtML/lT(lll). as a function of the calculated binding energy of atomic oxygen (BEo). The current density data for Pt(lll) were obtained fiom [Maikovic et al., 1999] and ate included for comparison. Key 1, Pt]y[L/Ru(0001) 2, Pb /bllll) 3, PtML/Rh(lH)i 4, Ptim,/ Au(lll) 5, Pt(lll) 6, PtML/Pd(lll). Surface coverage is ML O2 in O2 dissociation and ML each for O and H in OH formation. (Reproduced with permission fiom Zhang et al. [2005a].)...

Molecular oxygen was activated on the space between the two Re,5 octahedral cores of the Rejo cluster and benzene concertedly reacted on the activated O2 with the very low activation energy of 24 kJ mol, where O2 dissociates to form nearly... 

Hemoglobin binds Oj at the high PO2 (100 mm Hg) of the lung capillary beds and transports it to the peripheral tissues, where Poj is lower (-30 mm Hg) and O2 dissociates from hemoglobin. 

In conditions that lead to chronic hypoxemia, such as smoking and chronic obstructive pulmonary disease, an increased concentration of BPG In the RBCs promotes O2 dissociation from hemoglobin in tissues to support cellular function. 

Figure 8. Structure (upper panel), binding energy (middle panel), and bond distances O-Au and 0-0 (lower panel), for a few lowest energy isomers of Au 02 clusters. Except for n=2, the O2 dissociation is energetically favorable but kinetically hindered.

In this subsection we investigate the adsorption of O2 molecules on neutral Aun clusters with 5cluster substrate, which is chosen among the equilibrium structures of pure An clusters obtained in a previous work . We do not consider initial configurations with two separated O atoms. Thus, O2 dissociative adsorption is obtained only when that process occurs without any barrier. 

In order to mimic an entire catalytic cycle, a second O was removed from the oxide overlayer (equivalent to performing a second epoxidation cycle) and O2 dissociation was examined on this doubly reduced oxide overlayer. The most favorable dissociation route identified, with a barrier of 0.40 eV, proceeds through the transition state labeled (d) in Fig. 9, and involves O2 initially adsorbed parallel to the surface. 

Fig. 10. Relative energy diagram for the conversion of gas phase ethylene into ethylene epoxide (C2H4O) via an oxametallacycle (OMME) intermediate on the low coverage atomic O phase on Ag lll. States to the right-hand side of the vertical dashed line do not contain epoxide molecules and are simply related to O2 dissociative adsorption on clean Ag lll. (Same color codes as Fig. 9.) Copyright from [52].

Oxygen (O2) dissociation on single crystal platinum surfaces... 

The FeO(lll) film is, indeed, extremely stable and chemically inert under conditions typically used in UHV-based experiments. However, the situation changes dramatically in the mbar range of pressures. At low temperatures studied here (400 450 K), Pt(lll) is inactive in CO oxidation due to the well-known blocking effect of CO on O2 dissociation. The nanometer-thick FesO lll) films shows some activity, but it is negligible as compared to ultrathin FeO(lll) films, which showed an order of magnitude higher reaction rate under the same conditions (Figure 4.4.7A). Therefore, it is the thin FeO overlayer on Pt that is responsible for the enhanced reactivity of encapsulated Pt particles in CO oxidation. 

The implication of this mechanism is that the most ideal catalyst under the most optimum conditions would transform six ethylene molecules into EO and one ethylene molecule into water and carbon dioxide. Hence, according to this theory, the selectivity can never exceed 6/7. This theory also explains the favourable influence of chlorine. It was known that chlorine selectively poisons sites on Ag which can adsorb O2 dissociatively. As atomic oxygen was considered to be... 

The pK value of this reaction is about 4.8. Therefore, at physiological pH most of the Oj /HOj radical couple exists in the dissociated form as the superoxide radical anion. However, the hydroperoxyl radical is more reactive than the O2 dissociated form (Table 2). Moreover, HOj can easily diffuse through biological membranes, while membrane penetration by the anionic Oj" is much slower (G30). Therefore, even a small fraction of nondissociated HOj present in cells seems to play an important role in reactions of the Oj" /HOj couple. 

The electron affinities of clusters behave in a similar manner. This fact, undoubtedly, has a role to play in the chemistry exhibited by nanometals that has been reported in the literature recently. For example, it has been shown that Au atoms (Gold is a noble metal in the bulk state) supported on a TiOa substrate shows a marked size effect in their ability to oxidize the diatomic gas CO to CO2 via a mechanism involving O2 dissociative chemisorption and CO adsorption (Valden et al., 1998). Small Ni particles have also been found to dissociate CO (Doering et al., 1982). Smaller nanoparticles of Ag can dissociate molecular oxygen to atomic oxygen at low temperatures, whereas in the bulk state, the species adsorbed on the Ag surface is O2 (Rao et al., 1992). 

In the synthetic iron porphyrin, O2 affinity mainly depends on the strengths of the <7-donation from the lone pair of O2 to the heme-iron dz orbital and 7r-back donation from the d/r orbital on the iron to the Jt orbital of 2. To evaluate the O2 affinity and/or O2 binding dynamics in myoglobin and hemoglobin, O2 - protein interaction is a further important factor. For example, the O2 dissociation rate constant for oxymyoglobin is relatively smaller than those of O2 complexes... 

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