Redox mechanism kinetics

Campbell and coworkers269 also published a kinetics study of the reverse water-gas shift over Cu(110) in 1992, and the results were cast in terms of the redox mechanism (reverse of Scheme 60, left side). A hydrogen-induced surface phase transition was suggested to impact the rate at high H2/C02 ratios, as the rate was found to exhibit a saturation-like behavior with increasing P(H2) when 5 Torr of C02 was used, but continued on a log-linear trend when 150 Torr of C02 was... 

Hinrichsen, Muhler, and co workers—micro kinetic analysis parameterized by redox model. Hinrichsen et al.317 investigated the elementary steps by micro kinetic analysis by applying temperature and concentration-programmed experiments over Cu/Zn0/Al203, and modeling the data with the simplified redox mechanism in the spirit of Ovesen, Topsoe, and coworkers.303 This included 3 steps (1) dissociative adsorption of H2 on Cu metallic surface (2) dissociative adsorption of H20 leading to an adsorbed H2 molecule and an O adatom and a reduction step by CO to form gas phase C02 and a free active site (see Scheme 71). 

Jakdetchai and Nakajima/Wang and coworkers—theoretical models favor redox mechanism. Beginning in 2002, a number of theoretical models were published in Theochem studying the water-gas shift reaction over Cu(110), Cu(lll), and Cu(100) surfaces. Perhaps the first was by Jakdetchai and Nakajima,325 relying on the AMI method. The main goal of the study was (1) to determine whether or not theoretical calculations are consistent with a redox or associative (e.g., formate) mechanism and (2) whether the kinetics are described best by a Langmuir-Hinshel-wood expression or an Eley-Rideal expression. That is, in the case of a redox model, does the adsorbed O adatom react with adsorbed CO or directly with gas phase CO Their approximate A//a[Pg.205]

The reaction kinetics for the system containing only CO, H2, and Oz in the gas feed could be best represented by the redox mechanism [49], Such a redox reaction can be described by the following two-step reaction ... 

The kinetics of selective CO oxidation over the Cu Cej r02, nanostructured catalysts can be well described by employing Mars and van Krevelen type of kinetic equation derived on the basis of a redox mechanism ... 

In our studies we have demonstrated that the redox mechanism that was used to model dynamic behavior of CO oxidation is consistent with a kinetic model of the selective CO oxidation obtained under steady-state mode of operation [62], We propose the following tentative scheme (Figure 7.15) for the selective CO oxidation over the CuolCe(J902 v catalyst CO and H2 adsorb on the... 

To summarize, one can say that the electrochemical performance of CNT electrodes is correlated to the DOS of the CNT electrode with energies close to the redox formal potential of the solution species. The electron transfer and adsorption reactivity of CNT electrodes is remarkably dependent on the density of edge sites/defects that are the more reactive sites for that process, increasing considerably the electron-transfer rate. Additionally, surface oxygen functionalities can exert a big influence on the electrode kinetics. However, not all redox systems respond in the same way to the surface characteristics or can have electrocatalytical activity. This is very dependent on their own redox mechanism. Moreover, the high surface area and the nanometer size are the key factors in the electrochemical performance of the carbon nanotubes. 

Several authors have proposed that CH4 combustion over PdO occurs via a redox mechanism [82-85]. Methane activation through assisted hydrogen extraction is generally regarded as the rate-determining step, although there is not a general consensus on the nature of the adsorption sites. Further, desorption of H2O by decomposition of surface hydroxyls has been reported to play a key role in reaction kinetics at temperatures below 450 °C [67, 86]. 

In spite of the redox mechanism discussed above, the following Eley-Rideal (ER) kinetic expression has been proposed in the literature ... 

Group VIB, oxyanions, redox reactions, kinetics and mechanism, 40 269-274 Group VIB carbonyls infrared spectra, 19 85 matrix photochemistry, 19 84-88 substituted, matrix photochemistry, 19 89,... 

The MR rate law relies on the assumption that the SCR reaction is governed by a redox mechanism and therefore predicts a kinetic dependence on oxygen. It has been derived assuming that (i) two types of sites for NH3 adsorption (acidic non-reducible sites) and for NO + NH3 activation/reaction (redox sites, associated with vanadium), respectively, prevail on the catalyst surface (ii) NH3 blocks the redox sites (iii) reoxidation of the redox sites is rate controlling. 

Kinetic redox models, as formulated by Mars and van Krevelen [204], have not been considered in any recent work. Although the combined dependence on both propene and oxygen pressures does arise in certain investigations, the authors seem to ignore redox mechanisms completely and correlate their data with Langmuir—Hinshelwood type models. 

Although this model cannot correctly reflect a redox mechanism, it indicates that the reduction and reoxidation rates have the same order of magnitude, and hence both influence the kinetics. 

There are only a few recent publications. Anshits et al. [29,30] have carried out adsorption studies with various Cu—O phases and determined kinetics at low pressure in a static system. One of their conclusions is that the kinetics of partial and complete oxidation are very different. The mechanism of the latter is supposed to be of the associative type, contrary to the redox mechanism of the partial oxidation. A kinetic study with a continuously stirred vessel (375—400°C, 1 atm) was carried out by Laksh-manan and Rouleau [185]. In contrast to the redox mechanism, a singlesite Langmuir—Hinshelwood model is proposed, for which the k values and activation energies are determined. 

The kinetics are unanimously reported to depend on both the butene and oxygen partial pressures. Reaction orders close to 0.5 for both reactants are found by several authors, and for various catalysts (Sn/Sb = 1/4, 1/3, 2/1) at about 450°C [36,278,329]. Sazonova et al. [278] proved that the reaction proceeds on a partially reduced surface through a redox mechanism. They used a Sn/Sb = 2/1 catalyst at about 450°C with flow... 

Ammosov and Sazonov [21,22,24] demonstrated that for iron antimonates the initial selectivities are lower than for bismuth molybdates due to a higher rate of the parallel combustion reaction. It is proved that both selective oxidation and combustion occur by a redox mechanism. In another publication [23], the same authors report the kinetics of the butene and butadiene combustion reactions. 

Kinetic investigations have appeared in the literature since 1965. A redox mechanism is generally accepted [254], and has been confirmed by pulse experiments which demonstrated the equal activity of the catalyst in the presence and absence of oxygen. The results of Pernicone [254] and Liberti et al. [187] seem to indicate that the rate-determining step is either hydrogen abstraction from methanol or desorption of formaldehyde. 

The synthesis of paratolunitrile (PTN) and terephtalonitrile (TPN) by reaction of paraxylene with nitrogen monoxide was studied over a series of aerogel chromium oxide alumina catalysts. The stabilization of the active phase was interpreted on the basis of Cr O support interactions. Kinetic studies show that the reaction follows a "redox" mechanism for the formation of PTN and a Langmuir Hinshelwood mechanism for the production of TPN. 

Before discussing particular carbon electrode materials, we should define the qualities on which a choice of material will be based. These are the criteria that matter the most to the user, and the importance of each will vary with the application. For example, a carbon electrode to be used for detecting eluents from a liquid chromatograph should have a low background current and long stability, whereas an electrode used for studying redox mechanisms should usually exhibit fast electron transfer kinetics. The criteria relevant to carbon electrodes are conveniently classified into four types. 

The kinetics of the hydrogen oxidation reaction under excess 02 satisfactorily obeyed the following kinetic equation, typical of a two-step redox mechanism ... 

The kinetic data obtained were interpreted on the basis of a redox mechanism, with two main steps (1) dissociative adsorption of 02 (surface oxidation) and (2) interaction of NH3 with oxygen adsorbed in atomic form (surface reduction). Both steps are complex, comprising several elementary reactions. This mechanism is again consistent with the rate equation (16.1). 

The kinetics of the reaction CO + 02 on Cr3C2 obeys equation (16.1) under both excess oxygen and excess CO. Moreover, pulse experiments demonstrated that the rate of the catalytic reaction was comparable to that of CO oxidation by chemisorbed oxygen. This suggests that the catalytic reaction proceeds by a redox mechanism. 

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