Reaction regenerative mechanism

The mechanism and kinetics of the WGS reaction over Fe-Cr catalysts have been the subject of numerous publications. Despite intense investigations, still there is no full agreement as to the reaction mechanism. The two competing approaches are a redox (regenerative) mechanism first proposed by Kulkova and Temkin as early as 1949 which presumes reduction of an oxide center (O) by a CO molecule yielding CO2 and a vacant surface center ( ), followed by reoxidation of the vacant center by water that produces hydrogen and regenerates the oxide center for the catalytic cycle. 

The literature pertaining to the catalytic properties of magnetite focuses primarily on the water-gas shift reaction. A number of reaction kinetics studies have been reported in which WGS reaction pathways have been proposed (1,2,7-18), In short, two types of mechanisms have been put forward, these being the adsorptive and regenerative mechanisms. In the adsorptive pathway, reactants adsorb on the surface where they react to form surface intermediates, followed by decomposition to products and desorption from the surface (12-18), Support for this adsorptive mechanism has been provided by tracer studies and apparent stoichiometric number analyses. Two such adsorptive mechanisms consistent with experimental observations are shown below. 

For a uniform surface, the regenerative mechanism predicts the following kinetic expression for the forward rate of the WGS reaction ... 

This review has focused on recent research directed toward characterization of the active sites for water-gas shift over magnetite-based catalysts. The reaction can be described by a regenerative mechanism wherein gas phase or weakly adsorbed CO reduces anion sites and steam oxidizes the resultant surface oxygen vacancies. Kinetic relaxation techniques indicate this to be a primary pathway. The sites which participate in this reaction comprise only about 10% of the BET monolayer, and these sites can be titrated using CO/CO2 adsorption at 663 K. In contrast, the total cation site density is effectively titrated with NO at 273 K. In fact, the ratio of the extent of CO/CO2 adsorption to the extent of NO adsorption provides a measure of the fraction of the magnetite surface which is active for water-gas shift. 

Two mechanisms are thought to be important for the water-gas shift reaction a regenerative mechanism in which the catalyst surface is successively oxidized by H2O and reduced by CO, and an associative mechanism in which adsorbed reactant species interact to form an adsorbed intermediate, generally thought to be a formate, which decomposes to water-gas shift products (50). 

The presence of both Fe and Fe " on MgO suggests that the ability to alter oxidation state is not sufficient to ensure the dominance of the regenerative mechanism The dominance of this reaction over magnetite suggests that the electron hopping which takes place in the octahedral sites facilitates the oxidation/reduction cycles necessary for the regenerative mechanism ... 

The mechanism of the catalyzed shift reaction for both copper- and iron-based catalysts remains controversial. Two types of mechanism have been proposed adsorptive and regenerative. In the former, the reactants adsorb on the catalyst surface, where they react to form surface intermediates such as formates, followed by decomposition to products and desorption from the surface. In the regenerative mechanism, on the other hand, the surface undergoes successive oxidation and reduction cycles by water and carbon monoxide, respectively to form the corresponding hydrogen and carbon dioxide products of the WGS reaction. 

The redox mechanism is also known as oxidation—reduction or regenerative mechanism of Rideal-Elay type. In this mechanism, a redox reaction takes place at the catalyst surface. More in detail, water oxidizes the catalyst surface and CO re-reduces the oxidized surface. Alternatively, a bifimctional way can be considered in which the CO adsorbed on the metal is oxidized by the support and the water fills the support oxygen vacancy (Ladebeck Kochloefl, 1995). If the symbol reported in the following expressions represents the active site, then this mechanism can be summarized as in the following reactions (1.5) and (1.6) ... 

The mechanism of the WGS reaction is a matter of considerable controversy, which is centered upon whether the reaction proceeds via an associative mechanism or a regenerative mechanism. Both mechanisms were first proposed by Armstrong and Hilditch (232) in 1920. In the associative mechanism, the adsorption of CO and H2O onto a catalyst surface results in the formation of an intermediate of unspecified structure that subsequently decomposes into the reaction products. 

Chain-breaking acceptor (CB-A) antioxidants, on the other hand, act by oxidizing alkyl radicals in a stoichiometric reaction and hence are only effective under oxygen-deficient conditions (reaction lOd). Antioxidants with structures based on benzofuranone derivatives (lactones) and hydroxylamines, as well as on quinones and stable free radicals, are good examples of CB-A antioxidants (91-96). Hindered amine derivatives [often referred to as hindered amine stabilizers (HAS) eg, AOs 25-27, Table 3 also function by a chain-breaking mechanism and, through their transformation products, are able to trap both R. and ROO in a cyclical regenerative mechanism (50,55,62,94,97-100) for simplified reaction mechanism, see Scheme 11. 

Among the possible reaction mechanism proposed in the literature, the regenerative mechanism is generally the most accepted to describe the WGS reaction, in which water molecules are adsorbed and dissociated on reduced sites to generate hydrogen while oxidizing the site (Eq. (20.5)). Subsequently, CO is oxidized to CO2 and reduces the oxidized site to complete the catalytic cycle (Eq. (20.6)),... 

A chain reaction is one in which the key intermediate generated in one step then re-enters the reaction sequence in another. A chain mechanism includes at least two such regenerative reactions and a pair of intermediates. This allows the intermediates to cycle repetitively. These intermediates may be atoms or radicals, or indeed any high-energy or reactive species that can undergo suitable reactions. 

If the follow-up chemical reactions regenerate the initial electroactive reactant, the mechanism (2.178) is transformed into a regenerative catalytic mechanism as represented by the following scheme [130] ... 

In the presence of an oxidant, e.g., chlorate or bromate ions, the electrode reaction is transposed into an adsorption coupled regenerative catalytic mechanism. Figure 2.85 depicts the dependence of the azobenzene net peak current with the concentration of the chlorate ions used as an oxidant. Different curves in Fig. 2.85 correspond to different adsorption strength of the redox couple that is controlled by the content of acetonitrile in the aqueous electrolyte. In most of the cases, parabolic curves have been obtained, in agreement with the theoretically predicted effect for the surface catalytic reaction shown in Fig. 2.81. In a medium containing 50% (v/v) acetonitrile (curve 5 in Fig. 2.85) the current dramatically increases, confirming that moderate adsorption provides the best conditions for analytical application. 

The applicability of the foregoing procednre has been tested by modeling simple reaction under semi-infinite diffusion conditions (reaction 1.1) and EC mechanism coupled to adsorption of the redox couple (reaction (2.177)) [2]. The solutions derived by the original and modified step-function method have been compared in order to evaluate the error involved by the proposed modification. As expected, the precision of the modified step-function method depends solely on the value of p, i.e., the number of time subintervals. For instance, for the complex EC mechanism, the error was less than 2% for p>20. This slight modification of the mathematical procedure has opened the gate toward modeling of very complex electrode mechanisms such as those coupled to adsorption equilibria and regenerative catalytic reactions [2] and various mechanisms in thin-film voltammetry [5-7]. 

A cyclical depiction of an enzyme-mediated reaction written to account for the regenerative nature of catalytic processes. The reaction cycle for a typical one-substrate one-product enzyme mechanism may be written as follows ... 

The stability of silicon electrodes contacting an aqueous electrolyte is a severe problem in regenerative solar systems. As mentioned previously, the standard electrode potential of a silicon element is negative enough to induce an electrochemical reaction mechanism, giving rise to an insulating surface silicon oxide in the absence of complexing reactants. On the... 

Catalytic current — Figure. Reaction scheme of a reductive regenerative catalytic electrode mechanism (the charge of the species is omitted)... 

Further studies showed that using a combination of sonication and phase-transfer catalyst (PTC) the rate and yield of the reaction of alkene with dichlorocarbene which resulted from chloroform and sodium hydroxide pellets in situ could be improved efficiently [40], Compared with the results reported by Regen [41 ] where sonication alone was used, they found that mechanical stirring was not necessary under high-power sonication. Other findings included the fact that the ratio of NaOHialkene could be decreased from 10 1 to 3 1 and the reaction period could be shortened from 5 h to 10—15 min in the presence ofO. 1-0.05% PTC. 

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