Oxygen adsorbed, with

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 ... 

Here, the desirable reaction is four-electron reduction, which requires that oxygen adsorbs with the rupture of the 0=0 bond. Since the traditional electrode materials such as pyrolytic graphite or activated carbon do not facilitate 0=0 bond rupture, a catalyst is needed to accelerate oxygen adsorption with 0=0 bond rupture, the four-electron reduction, or the hydrogen peroxide decomposition. A variety of catalysts have been examined for their activity, stability, and cost (Table 23.3). These catalysts can be divided into three categories ... 

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. 

An alternative surface reaction which has been suggested is a reaction between an adsorbed oxygen atom with an adsorbed carbon monoxide molecule to form carbon dioxide which is immediately desorbed. The reaction rate is again given by the equation above. 

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.

Figure 4. Side and top views of the energetically most favorable complexes formed between protonated cinchonidine and methyl pyruvate which would yield (R)-methyl lactate (left) and (S)-methyl lactate (right), respectively, upon hydrogenation. The complexes have been accomodated on a space filling model of platinum (111) surface in order to illustrate the space requirements of the adsorbed complexes. For the sake of clarity, in the side views the carbon atoms of the reactant are marked with a white square and the oxygen atoms with an o. Data taken from ref. [41].

The reaction of the adsorbed oxygen species with methane... 

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. 

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. 

Figure 3.12 Series of STM images, recorded during reaction of adsorbed oxygen atoms with coadsorbed CO molecules at 247 K, all from the same area of a Pt(l 1 1) crystal. Before the experiment, a submonolayer of oxygen atoms was prepared and CO was continuously supplied from the gas phase...

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... 

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