Oxygen production, rate-limiting step

First, solvent molecules, referred to as S in the catalyst precursor, are displaced by the olefinic substrate to form a chelated Rh complex in which the olefinic bond and the amide carbonyl oxygen interact with the Rh(I) center (rate constant k ). Hydrogen then oxidatively adds to the metal, forming the Rh(III) dihydride intermediate (rate constant kj). This is the rate-limiting step under normal conditions. One hydride on the metal is then transferred to the coordinated olefinic bond to form a five-membered chelated alkyl-Rh(III) intermediate (rate constant k3). Finally, reductive elimination of the product from the complex (rate constant k4) completes the catalytic cycle. 

Formic acid adsorbed with near unit sticking probability on clean Fe(lOO). The reaction product spectrum for HCOOH on Fe(lOO) is shown in Fig. 17 (95). As with Cu( 110) the reaction proceeded via two steps, one which evolved Hj at 350 K and the other which formed, CO2, and CO at 490 K. A small amount of CO was evolved at 800 K due to the reaction of residual carbon and oxygen atoms on the surface. From these results it was concluded that CO, H2, and CO2 were formed by a common rate-limiting step at 490 K. Since the H2/H2 TPD peak appears at 400 K and below, this step was determined to be a surface reaction. No water was formed. Evidently the reaction proceeded by the pathways... 

When the rate of the chemical reaction occurring at the surface is the rate-limiting step, the principles we have described to this point apply. The reaction rate can have any order, and the gas reacts with the ceramic substrate to produce products. Although our discussion to this point has focused on oxide ceramics, there are a number of nonoxide ceramics, such as carbides, nitrides, or borides, that are of importance and that undergo common decomposition reactions in the presence of oxygen. These ceramics are particularly susceptible to corrosion since they are often used at elevated temperatures in oxidizing and/or corrosive enviromnents. For example, metal nitrides can be oxidized to form oxides ... 

That rate-limiting step in oxygen production may be (1) absorption of a facile hole-acceptor species, (2) hole transfer to an absorbed or electrolyte species, or (3) desorption of oxidized products. These steps must compete with bulk and surface recombination processes. Williams and Nozik (39) have shown that... 

The initial dehydration reaction is sufficiently fast to form an equilibrium mixture of methanol, dimethyl ether, and water. These oxygenates dehydrate further to give light olefins. They in turn polymerize and cyclize to form a variety of paraffins, aromatics, and cycloparaffins. The above reaction path is illustrated further by Figure 3 in terms of product selectivity measured in an isothermal laboratory reactor over a wide range of space velocities. ( 3) The rate limiting step is the conversion of oxygenates to olefins, a reaction step that appears to be autocatalytic. In the absence of olefins, this rate is slow but it is accelerated as the concentration of olefins increases. 

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