Surface oxygen species

G. D. Moggridge, J. P. S. Badyal and R. M. Lambert, X-ray photoelectron spetroscopic characterisation of oxygen surface species on a doubly promoted manganese oxide model planar catalyst significance for CH4 coupling, J. Phys. Chem., 1990, 94, 508. 

V / RHE. Figure 6.32 provides a schematic illustration of the series pathway with CO as intermediate. Methanol adsorbs on Pt forming some H-containing surface intermediate H atoms are abstracted in a sequence of steps, until absorbed CO is formed. Removal of CO requires adsorption of water to yield water-based oxygenated surface species. 

The spectra of the oxidized carbons (CWZ—Ox, CWN2—Ox, RKD3—Ox, D—Ox) are quite similar to those obtained for various carbon materials oxidized with nitric acid or dioxygen [89-91,116-126,173,174). The relative decrea.se in intensity of C—H moiety bands (near 2900 cm ) after oxidation may indicate that oxygen surface species are also formed at the expense of the aliphatic resi-... 

This condition is most damaging under open circuit conditions when no load is applied and the cell voltage is close to one volt Under high potentials, the catalyst carbon support experiences oxidizing conditions, initially resulting in an increase in oxygen surface species and associated increase in hydrophilicity. As the carbon surface further oxidizes, carbon is converted to carbon dioxide (reaction 6.4), the Pt particle cormections are severed, the catalyst layer collapses and the catalyst layer dramatically thins. 

To account for the additional nitrogen production on the oxygen pulse during an O2/NH3 pump-probe experiment, steps 8-10 are added to the reaction mechanism. In step 8, adsorbed ammonia reacts with an oxygen surface species (peroxide). The surfece lifetime of this surfece oxygen species is very short. Rather than a peroxide species, the reactive oxygen species could be a superoxide. 

It is also considered that COadsorbed is intermediate adsorbed species but there is nothing confirmed about the carbon-oxygen surface species. 

In our experiment, photocatalytic decomposition of ethylene was utilized to probe the surface defect. Photocatalytic properties of all titania samples are shown in table 2. From these results, conversions of ethylene at 5 min and 3 hr were apparently constant (not different in order) due to the equilibrium between the adsorption of gaseous (i.e. ethylene and/or O2) on the titania surface and the consumption of surface species. Moreover it can be concluded that photoactivity of titania increased with increasing of Ti site present in titania surface. It was found that surface area of titania did not control photoactivity of TiOa, but it was the surface defect in titania surface. Although, the lattice oxygen ions are active site of this photocatalytic reaction since it is the site for trapping holes [4], this work showed that the presence of oxygen vacancy site (Ti site) on surface titania can enhance activity of photocatdyst, too. It revealed that oxygen vacancy can increase the life time of separated electron-hole pairs. 

A typical result for DPV In Fig. 4a shows the presence of two redox couples with peak potentials of 0.25 V and 0.19 V ( lOmV). Similar results have also been obtained with SWV. The relative Intensities of the two peaks vary from sample to sample but are always present with activated electrodes. The similarities between the potentials found for the surface species and for the oxidation of ascorbic acid suggest that an ec catalytic mechanism may be operative. The surface coverage of the o-qulnone Is estimated to be the order of 10 mol cm . It Is currently not possible to control the surface concentration of the o-qulnone-llke species or the oxygen content of the GCE surface. 

Tripkovic AV, Popovic KD, Lovic JD. 2001. The influence of the oxygen-containing species on the electrooxidation of the C-l-C alcohols at some platinum single crystal surfaces in alkaline solution. Electrochim Acta 46 3163-3173. 

Figure 9.6 Visual representation of the platinum oxide growth mechanism, (a) Interaction of H2O molecules with the Pt electrode occurring in the 0.27 V < < 0.85 V range, (b) Discharge of 5 ML of H2O molecules and formation of 5 ML of chemisorbed oxygen (Ochem)- (c) Discharge of the second ML of H2O molecules the process is accompanied by the development of repulsive interactions between (Pt-Pt) -Ofi m surface species that stimulate an interfacial place exchange of Ochem and Pt surface atoms, (d) Quasi-3D surface PtO lattice, comprising Pt and moieties, that forms through the place-exchange process. (Reproduced with permission...

At high temperatures (> 170 K), the water desorbs and so the autocatalytic reaction cannot be sustained and is an explanation for why the H2 + 02 reaction slows, the formation of OH species now being solely dependent on the H(a) + O(a) reaction, which is the slowest step in the above scheme. That the water + oxygen reaction was fast and facile was evident from the spectroscopic studies at both nickel and zinc surfaces, when the oxygen surface coverage was low and involving isolated oxygen adatoms. 

It is frequently asserted that two weaknesses of STM are first that all atomic asperities in images need not necessarily correspond to atom surface positions and second that it is inherently difficult to establish the identity of imaged atoms when two or more surface species are involved. The latter need not, however, be a problem. In a study (for example) of the oxidation of ammonia at Cu(110) the oxygen and nitrogen adatoms form separate individual structures which run in the < 100 > and < 110 > directions, respectively, whereas under ammonia-rich conditions only imide species are formed, running in the < 110 > direction, with in situ XPS confirming their presence and the absence of surface oxygen (Chapter 5). 

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