Oxygen, adsorption modeling

Figure 9. Scheme of oxygen adsorption models (a) die Griffiths model, (b) die Pauling model, and (c) the Yeager model. 

Duarte H A and Salahub D R 1998 Embedded cluster model for chemisorption using density functional calculations oxygen adsorption on the AI(IOO) surface J. Chem. Phys. 108 743... 

R. L.C., Kreuzer, H.J. and Kruse, N. (2006) Oxygen adsorption on gold nanofacets and model clusters. Journal of Chemical Physics, 125, 054703-054707. 

Nevertheless, there is a whole series of experimental results enabling one to propose and substantiate a sufficiently general model consistently describing the effect of oxygen adsorption on electric conductivity of partially reduced oxides observed in experiments. Let us con sider these data. 

Thus, the model proposed explains the effect of CO on electric conductivity of several oxides only in case when oxygen is present in ambient volume which was observed in numerous experiments. Accordingly, the fact of existence of relatively narrow temperature interval in which an adsorbent is sensitive to CO becomes clear. This can be linked with the fact that if the operational temperature To is small the reaction products (in case of CO this is CO2) cannot get desorbed (see expression (2.80)), i.e. regeneration of the centers of oxygen adsorption is not feasible. If Tq is very high both adsorption of oxygen and reducing gas should be ruled out. 

Figure 4. Bridge model of oxygen adsorption on PANI.

In order to study the possible reasons and mechanisms of the catalytic activity of conducting polymers (CP), the electronic structure of some molecular CPs clusters and its adsorption complexes with oxygen were modeled [6], In the CP-O2 complex, the CP surface is an electron density donor. For example, in the case of PANI, the bond orders in adsorbed O2 molecules decrease by about 30%, and the bond lengths L increase by about 24%. Thus, the adsorbed O2 molecules have a fairly high degree of activation and can readily interact with the protons. 

Two models of oxygen adsorption are considered, vertical form and parallel form, which are illustrated in Fig. 9.1 and Fig. 9.2. For all the cases, the adsorbate/substrate system is optimized by GGA. In optimization, all atoms on the pyrite substrate are fixed only the O atoms are allowed to move. The initial 0—0 double bond length and the distance between Fe atom and O atom are 0.121 nm and 0.196 run, respectively. To simplify the calculation, the adsorption coverage will not be considered. 

The storage of NO on BaO has also been analyzed in the literature by adopting a detailed mechanistic kinetic model [76]. The model consists of 10 elementary reversible steps adsorption of oxygen, adsorption of NO, adsorption of NO2 and oxidation of NO to NO2 at the Pt sites, adsorption of NO2 at the BaO sites, release of NO into the gas phase and oxidation of BaO to BaO2, formation of BaONOs and of... 

Che and Naccache (199) have studied the kinetics of 02 formed on slightly reduced anatase using EPR. They found that the adsorption could be explained on the basis of different formation rates for 02 adsorbed at different sites, with zero- and first-order kinetics for the oxygen and Ti3+ concentrations, respectively. Using the same approach, Hauser (200) has extended this work and proposed different models to explain the kinetics based on the formation of 02, O2-, and 02 ions for which activation energies around 1 kcal/mol were obtained. Nikisha et al. (201) have studied the oxygen adsorption kinetics using EPR, conductivity, and volumetric measurements. 

Results were modelled on the basis of CO adsorption/desorption equilibrium with oxygen adsorption as the rate determining step and an irreversible Langmuir-Hinshelwood surface reaction. It was assumed that adsorption and... 

An Eley—Rideal model with dissociative adsorption of oxygen is proposed by Bhattacharyya et al. [48]. Because the oxygen adsorption is assumed to be irreversible, the model is identical with a redox model and... 

Co-adsorption and mutual interactions between the reactants on the surface form the basis for understanding the microscopic steps of the reaction. Since product formation takes place rather rapidly above room temperature, this information mainly became available from low-temperature studies. As a result, these processes are much more complicated than can be described by a Langmuir-type adsorption model (i.e., simple competition for free adsorption sites) and, moreover, an asymmetric behavior is found which means that pre-adsorbed CO inhibits the adsorption of oxygen, whereas the reverse is not the case. At very low surface concentrations of CO and Oad these will be randomly distributed over the surface as illustrated schematically by Fig. 32a (88). 

Calorimetric data indicate that in the case of oxygen adsorption on oxygenated silver (the surface sites of which we denote as Z) [i.e., in the case of the process corresponding to stage 4 of scheme (220)], the surface behaves in such a way as if it were uniform, in accordance with Assumption 3 formulated in Section IX. Thus the model of an ideal surface layer may be used to obtain the kinetic equations. Applying the stage steady-state conditions... 

There are three adsorption models for molecular oxygen adsorption (Fig. 9) ... 

The rate constants kx and k for CO and 02 adsorption, respectively, are determined by the respective initial sticking coefficients (i.e., at zero coverage). The coverage dependence of CO adsorption follows a precursor kinetics that can be modeled by the exponent r [being between 3 and 4 for Pt(110) (22)]. The kinetics of oxygen adsorption is modeled by the requirement for two neighboring empty adsorption sites, which can be occupied by either O or CO. We will return to this model later at various points. 

As a hemoglobin model, using imidazole attached polystyrene) with the picket fence porphyrin-Co(H) (XIV) the reversible oxygen adsorption-desorption reaction is observed (148,149). 

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