Bonding Langmuir-Hinshelwood mechanism

There are no hydrogen-hydrogen bonds in CH3OH, so it seems reasonable that H2 must dissociate in the gas phase prior to adsorbing on the catalyst. The carbon-oxygen triple bond in CO reverts to a single bond in the final product. These characteristics of the three components are captured in the four-step modified Langmuir-Hinshelwood mechanism, where chemical reaction is considered to... 

Other DFT calculations have shown that co-adsorption of H2O and O2 on Aug clusters, free or supported on MgO(lOO), leads to the formation of an 02- -H20 complex involving partial proton sharing or proton transfer, and leading to a hydroperoxy-like complex (HO2) [178]. This favors the activation of the 0—0 bond, i.e. the bond extension to values characteristic of a peroxo- or superoxo-Uke state. Consequently, the reaction with CO can occur with a small activation barrier of -0.5 eV, either through an Eley-Rideal mechanism if O2 is adsorbed on the top face of Aug clusters or through a Langmuir-Hinshelwood mechanism if O2 is adsorbed on the periphery of the cluster. 

The oxidation of carbon monoxide takes place in the car exhaust cleaning by means of catalysts based on the platinum metals and represents the simplest heterogeneously catalyzed reaction [54,55]. It involves chemisorption of CO and dissociative chemisorption of oxygen, and COad -f Oad react with each other to CO2 via the Langmuir-Hinshelwood mechanism. As an example. Fig. 6.15 shows the ordered structures formed by these adsorbates on a Rh(l 11) surface [56]. The CO molecules are in this case bonded to the surface "on top" and always exhibit the tendency to form densely packed adlayers if the coverage becomes high enough (Fig 6.15a). The O atoms, on the other hand, occupy... 

Between processes of adsorption and chemisorption there is a close relationship, since in the process of evolution of adsorbed species one can observe formation of transient complexes that are products of reactions proceeding by the Langmuir-Hinshelwood mechanism. According to the accepted classification of reactions in a surface layer of SiOi they are subdivided into two main classes, namely reactions with substitution of structural hydroxyl protons (SeI processes) and reactions with substitution of OH groups at silicon atoms (SnI processes). Reactions of heterolytic decomposition of siloxane bonds proceeding by the AdN3 mechanism form a separate class. 

The PD method provides a simple and effective route to produce supported Au and Pt catalysts efficient in CO oxidatiom With P25 supported catalysts, the best results were obtained for An at pH 10, while for ZnO supported catalysts toe most active were those prepared with no pH adjustment. On P25, Au is more active than Pt, while Pt provides better resnlts for ZnO. This might be related to toe smaller particle size of Au when supported on P25 and a stronger SMSl effect, which are important factors in the Bond-Thompson mechanism preferred by Au, which requires a significant participation of toe support lattice oxygen. With Pt catalysts, toe dependence on toe support is not so high, reflecting toe preference for a Langmuir-Hinshelwood mechanism, which involves primarily the bare active metal centre. 

AU experiments to be described below are interpreted on the basis of the Langmuir-Hinshelwood (LH) mechanism for CO electro-oxidation suggested by GUman more than 40 years ago [Gihnan, 1964]. According to GUman s model, water needs to be activated on a free site on the surface, leading to surface-bonded OH ... 

Many catalyzed surface reactions can be treated as a two-step process with an adsorption equilibrium followed by one rate-determining step (diffusion, surface reaction, or desorption). The surface reaction kinetics are usually discussed in terms of two limiting mechanisms, the Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms. In the LH mechanism, reaction takes place directly between species which are chemically bonded (chemisorbed) on the surface. For a bimolecular LH surface reaction. Aawith competitive chemisorption of the reactants, the rate of reaction is given by the following expression ... 

The mechanism for this reaction is not of the Langmuir-Hinshelwood form, but rather involves the reaction between a surface-bonded NO molecule with an NO molecule in the solution phase along with a simultaneous electron transfer. The Tafel slope in acidic solution is (120 mV), which implies that the first electron-transfer step is rate-determining (see Addendum to this chapter). The reaction is first order in H+ and shows no apparent isotope effect. The following mechanism has been proposed ... 

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