Oxygen detailed model

A detailed model for the oxygen reduction reaction at semiconductor oxide electrodes has been developed by Presnov and Trunov [341, 345, 346] based on concepts of coordination chemistry and local interaction of surface cation d-electrons at the oxide surface with HO, H20, and 02 acceptor species in solution. The oxygen reduction reaction is assumed to take place at active sites associated with cations at the oxide surface in a higher oxidation state. These cations would act as donor-acceptor reduction (DAR) sites, with acceptor character with respect to the solid by capture of electrons and donor electronic properties with respect to species in solution. At the surface, the long-range oxide structure is lost and short-range coordination by hydroxide ions and water molecules in three octahedral positions may occur [Fig. 16(b)], One hydroxide ion can compensate coulombically for the excess charge on surface M2+ cations with two coordinated water mole-... 

The most important features of both the reflectance and the photolumi-nescence spectra have been explained by the preceding model since it is based on ideal surface structures essentially determined by (001) planes. Thus, several likely possibilities, such as the presence of surface defects, impurities, and remaining adsorbates, the relaxation of the planes exposed at the surface, the impurity-induced reconstruction of the surfaces, and changes in the force constants, have been excluded (80). A more detailed model is needed in which the ion pair of the metal cation and oxygen anion can be taken into account on the basis of such experimental evidence as the hydrogen adsorption on MgO obtained by Coluccia and Tench (65) and Ito et al. (90). 

NO form a number of higher oxides, such as N02 and N03 that compete with adsorbed NO and atomic oxygen for platinum surface sites. Several atomistically detailed models of the NO oxidation reaction based on DFT-derived parameters for the reaction kinetics have been reported [58,59]. These models are successful in describing the sensitivity of the reaction kinetics to surface coverage. They are somewhat limited in terms of the surface species and the reaction steps considered. 

Recent experimental and theoretical work has produced a rather clear understanding, albeit with some speculation, of the electronic processes involved in the chemisorption of H atoms on the low index surfaces of silicon. Even these very simple systems exhibit complicating features, however, and it is probably unrealistic to expect such detailed models to be available for any other gas-isemiconductor combination. Nevertheless, we will discuss in the following three sections the results which have been obtained at the next level of complexity, viz. oxygen on Si and GaAs, and chlorine on Si. 

The reactivity trends observed for both the nucleophilic addition and cycloaddition of alkene radical cations parallel trends observed for the reactions of carbocations with nucleophiles and alkenes. However, the observed variations in reactivity towards oxygen - and substituent effects on the competition between addition of methanol and the neutral monomer for diphenylethene radical cations indicate that variations in both spin and charge density are important in determining the overall reactivity patterns. It is clear that further experimental and theoretical studies are required to provide a detailed model for understanding and ultimately predicting the reactivity of radical cations. 

Ammonia oxidation was included in the detailed model [12] (results not included). In this model, two reaction steps were added, where ammonia on the surface was oxidized either with oxygen alone or in a second step in a combination with hydroxyl groups. The hydroxyls are produced according to ... 

Different SCR reaction occurs depending on the NO2 to NOx ratio, where standard SCR occurs with NO only, rapid SCR with equimolar amount of NO and NO2, and slow NO2 SCR with NO2 only. In several global kinetic models, these three reactions are added. In more detailed models, more surface species are considered, for example, nitrites, nitrates, HNO3, oxygen, and hydroxyls. N2O is an unwanted by-product during the SCR process over copper zeolites that are increasing with the NO2 content. The mechanism for the N2O production is suggested to be from decomposition of ammonium nitrate. In addition, there are models available that incorporate the urea decomposition and hydrolysis, in addition to the SCR reactions. 

In this section we shall present a brief review of the processes which contribute to the chemistry in CSEs before describing some detailed models developed recently for the external envelopes of carbon- and oxygen-rich stars. We shall begin by discussing the chemistry at the stellar surface followed by a description of the different chemical processes which occur as one moves outward from the star through the CSE. 

Scalo and Slavsky (1980) were the first to present a detailed model of the chemical processes which occur throughout the CSE. Their model calculated molecular abundances from the LTB region out to the external envelope where photoprocesses dominate and included a detailed calculation of the thermal balance within the CSE. This model showed the importance of neutral chemistry in an oxygen-rich CSE, in contrast to the case in carbon-rich CSEs. Neutral chemistry is much more important because OH becomes the most abundant reactive species in the envelope. Since neutral radical chemistry is important, the detailed results of chemical abundance calculations are sensitive to the presence of activation energy barriers in several reactions and hence to the temperature profile assumed for the CSE. Since current experimental studies involving radical reactions are almost all carried out at room temperature, and in any case above 200 K, small activation barriers, for example less than 100 K, are not ruled out and can inhibit important reactions in the outer CSE where the temperature is lower than 100 K (see the article by I.W.M. Smith, this volume). 

A detailed model of the chemistry in oxygen-rich envelopes has been constructed by Nejad and Millar (1988) and considers, in a fashion similar to that described above for IRC+10216, the chemistry of H,0,C, and N species when several parents are injected into the outer envelope. Table IV gives calculated column densities for a... 

The oxidation reactions of alkanes, alkenes and aromatic hydrocarbons treated in this section are described in detail in monographs by Calvert et al. (2000, 2002, 2008), and reaction mechanisms for air quality models are summarized by StockweU et al. (2012). Also, Calvert et al. (2011) and Mellouki et al. (2003) reviewed the oxidation reactions of oxygenated volatile organic compounds (OVOCs) which are not treated in this book. Detailed models for the photolysis... 

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