Oxygen, adsorbed

Perhaps the most fascinating detail is the surface reconstruction that occurs with CO adsorption (see Refs. 311 and 312 for more general discussions of chemisorption-induced reconstructions of metal surfaces). As shown in Fig. XVI-8, for example, the Pt(lOO) bare surface reconstructs itself to a hexagonal pattern, but on CO adsorption this reconstruction is lifted [306] CO adsorption on Pd( 110) reconstructs the surface to a missing-row pattern [309]. These reconstructions are reversible and as a result, oscillatory behavior can be observed. Returning to the Pt(lOO) case, as CO is adsorbed patches of the simple 1 x 1 structure (the structure of an undistorted (100) face) form. Oxygen adsorbs on any bare 1 x 1 spots, reacts with adjacent CO to remove it as CO2, and at a certain point, the surface reverts to toe hexagonal stmcture. The presumed sequence of events is shown in Fig. XVIII-28. 

Chemisorption occurs when the attractive potential well is large so that upon adsorption a strong chemical bond to a surface is fonued. Chemisorption involves changes to both the molecule and surface electronic states. For example, when oxygen adsorbs onto a metal surface, a partially ionic bond is created as charge transfers from the substrate to the oxygen atom. Other chemisorbed species interact in a more covalent maimer by sharing electrons, but this still involves perturbations to the electronic system. 

Brongersma H H and Theeten J B 1976 The structure of oxygen adsorbed on Ni(OOOI) as determined by ion scattering spectroscopy Surf. Sc/. 54 519-24... 

For storage of a very small amount of highly oxygen-sensitive substance, it is recommended that an empty tube (D) is continuously evacuated at 100-120°C for several hours before use, to remove the oxygen adsorbed in the glass. 

Based on the gas adsorption behavior, Ko-zawa and Yamashita proposed a hypothesis [20, 21] that the cross—section of the fine pores of EMD is a cavity shape as shown in Fig. 16. The cross-section of the pore by computer calculation is a circle (Fig. 16A). Nobody knows the real shape of the cross-section as yet. Kozawa s belief in the cavity shape (Fig. 16B) is based on the results of experiments involving the oxygen adsorbed and desorbed from the pore walls [20]. 

Electrocatalytic reactions, such as the transformation of O2 from the zirconia lattice to oxygen adsorbed on the film at or near the three-phase-boundaries, which we denote by 0(a), have been found to take place primarily at these three phase boundaries.5 8 This electrocatalytic reaction will be denoted by ... 

Figure 5.44. In situ SERS spectra69 of oxygen adsorbed on Ag/YSZ at 300°C under (a) open circuit conditions and with die cell operating in the potentiostatic mode with (b) UWR = -2 V and (c) Uwr = +2 V. Spectra (b) and (c) were obtained after the system had reached steady state, w = 200 mW, photon counter time constant, x = 2 s, ssw = 2 cm l. Reprinted with permission from WILEY-VCH.

D.I. Kondarides, G.N. Papatheodorou, C.G. Vayenas, and X.E. Verykios, In situ High Temperature SERS study of Oxygen adsorbed on Ag Support and Electrochemical Promotion Effects, Ber. Buns. Phys. Chem. 97, 709-720 (1993). 

On smooth Au surfaces, the adsorbed oxygen, which is only possible in molecular form [5], is not selective to form PO [6]. Therefore partial oxidation is contributed by oxygen adsorbed on Ti02. However, there are no direct experimental evidence whether O2 is adsorbed dissociatively or non-dissociatively. It is generally accepted that O2 adsorbs on Ti02 in a molecular form [7] and is activated at the Au/Ti02 interface [1]. 

The exact epoxidation mechanism is still not quite clear. However, in all possible mechanisms, the interaction between Au and Ti02 is essential. According to a mechanism suggested by Hayashi et al [1], molecular oxygen adsorbed on Ti02 is activated, probably to a... 

The most essential question is why the CO-free sites are secured for H2 adsorption and oxidation. Watanabe and Motoo proposed a so-called bifunctional mechanism originally found at Pt electrodes with various oxygen-adsorbing adatoms (e.g., Ru, Sn, and As), which facilitate the oxidation of adsorbed COad at Pt sites [Watanabe and Motoo, 1975a Watanabe et al., 1985]. This mechanism has been adopted for the explanation of CO-tolerant HOR on Pt-Ru, Pt-Sn, and Pt-Mo alloys [Gasteiger et al., 1994, 1995], and recently confirmed by in sim FTIR spectroscopy [Yajima et al., 2004]. To investigate the role of such surface sites, we examined the details of the alloy surface states by various methods. 

Watanabe M, Shibata M, Motoo S. 1985. Electrocatalysis hy ad-atoms. PartXn. Enhancement of carbon monoxide oxidation on platinum electrodes by oxygen adsorbing ad-atoms (Ge, Sn, Pb, As, Sb and Bi). J Electroanal Chem 187 161-174. 

Shibata M, Furuya N, Watanabe M. 1987. Electrocatalysis by ad-atoms. Part XXL Catal3ftic effects on the elementary steps in methanol oxidation by non-oxygen-adsorbing ad-atoms. J Electroanal Chem 229 385-394. 

When reducing gases interact with the oxygen adsorbed in a charged form there is a product created that easily delivers electron into the conductivity zone with fast desorption [85-88] ... 

Figure 3.22 shows the result of verification of equations (3.28) and (3.29) under conditions of interaction of oxygen adsorbed on ZnO film with molecules of polar and chemically analogous solvents for the same concentration of dissolved oxygen. The values s of these solvents vary from 17 to 81. Experimental points are fairly satisfactorily plotted against the straight line IgB - i/s which is in consistency with the requirements of equations (3.28) and (3.29). 

Note that in weakly polar (.e < 5) and nonpolar solvents, it is impossible to blow off oxygen adsorbed on ZnO film with an inert gas hydrogen, nitrogen, etc.), similar to the case of gas or saturated vapour phase (polar liquid at any e) at room temperature, i.e., under the conditions, where f = 1, and no liquid layer is condensed on the film. 

Physical adsorption is a readily reversible process, and alternate adsorption and desorption stages can be carried out repeatedly without changing the character of the surface or the adsorbate. Chemisorption may or may not be reversible. Often one species may be adsorbed and a second desorbed. Oxygen adsorbed on charcoal at room temperature is held very strongly, and high temperatures are necessary to accomplish the desorption. CO and/or C02 are the species that are removed from the surface. Chemical changes like these are prima facie evidence that chemisorption has occurred. 

Calvet microcalorimeters are particularly convenient for such studies. Figure 19 show s, for instance, the evolution of the differential heat of adsorption of oxygen, measured at 30°C with a Calvet calorimeter, as a function of the total amount of oxygen adsorbed on the surface of a sample (100 mg) of nickel oxide, NiO(200) (19, 73). The volume of the first... 

Hydrogen peroxide then can be reduced or decomposed in water and oxygen. Thus, oxygen reduction can proceed by two parallel ways (a) the four-electron reaction of conversion into water and (b) the two-electron reaction of transformation into hydrogen peroxide. The four-electron reaction can proceed, if oxygen adsorbs with a rupture of chemical bonds in 02 ... 

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