Ethylene adsorption reaction order

Despite the poisoning action of Cl for oxygen dissociative adsorption on Ag, it is used as moderator in the ethylene epoxidation reaction in order to attain high selectivity to ethylene oxide. The presence of Cl adatoms in this... 

One way to explain the observed reaction orders is to also allow for a noncompetitive dihydrogen adsorption step in the sequence. This added complexity makes sense because more surface sites are available to dihydrogen than ethylene because of the very small size of a H2 molecule. The catalytic cycle for ethylene... 

Further experiments need be done to clarify this point. One may surmise perhaps that the Langmuir adsorption isotherm applies for electrosorption of formic acid (as is the case of, e.g., ethylene " ) and the discrepancy between reported values of the reaction order arises due to the gradual change from the linear 6-c relationship observed at low values of the coverage to an approximate Freundlich-like region at intermediate-coverage... 

Taking n = 4, as discussed previously for ethylene adsorption, the reaction order for ethylene is predicted to be —0.25 as compared with the experimental value of —0.20. From Table 17, variation of the reaction order of the organic species for the anodic oxidation can be observed. With reference to last equation, it is evident that different reaction orders are expected if the organic species require different numbers of adjacent, free, metal sites for adsorption. For example, butadiene, which has two conjugated double bonds, should require approximately twice as many sites as ethylene for adsorption. This is in fact observed since... 

It is worth noting the change in the reaction order with respect to ethylene, from positive to negative, upon positive current application. This strongly suggests competitive adsorption of ethylene and the backspil-lover oxide ions O. ... 

In our experiment, photocatalytic decomposition of ethylene was utilized to probe the surface defect. Photocatalytic properties of all titania samples are shown in table 2. From these results, conversions of ethylene at 5 min and 3 hr were apparently constant (not different in order) due to the equilibrium between the adsorption of gaseous (i.e. ethylene and/or O2) on the titania surface and the consumption of surface species. Moreover it can be concluded that photoactivity of titania increased with increasing of Ti site present in titania surface. It was found that surface area of titania did not control photoactivity of TiOa, but it was the surface defect in titania surface. Although, the lattice oxygen ions are active site of this photocatalytic reaction since it is the site for trapping holes [4], this work showed that the presence of oxygen vacancy site (Ti site) on surface titania can enhance activity of photocatdyst, too. It revealed that oxygen vacancy can increase the life time of separated electron-hole pairs. 

Four different experiments were realized by labeling either the methanol or the ethylene molecules (Figure 7). The reactions were studied in static conditions adsorbing either methanol (A and B) or ethylene (C and D) prior to the second reactants. The l C-NMR spectra of Figure 7 reveals that the order of adsorption of the reactants is very important for the reactivities at first, surface alkylation occurs and is followed by separated reaction pathways for CH3OH and C2H/. 

We see in these formulae that the kinetically measured velocity constant has only in one special case, namely, in the case of zero order, the meaning of a real velocity constant. In all the other cases, it contains implicit adsorption coefficients, e.g., in the form kb = const., or (in retarded reactions) kb/b = const. Only in the case of broken order is it possible to determine k and b separately from kinetic measurements, as e.g., Schwab and Zorn (5) did for the ethylene hydrogenation. 

It should be noted that the correlations being discussed here are far from perfect and exceptions can be found in nearly each of the reaction series. (For the ethylene-hydrogen and deuterium-ammonia reactions, the correlation between catalytic activity and per cent d-character is nearly quantitative.) This is to be expected in view of the experimental difficulties involved in preparing clean and reproducible metal surfaces, particularly where different metals are being compared. In any attempt to correlate catalytic properties with work functions, it should also lie recognized that the work function is affected by adsorption, and therefore that the work functions of metals under catalytic conditions, or even their relative order, may be somewhat different than those of the clean metals. 

Numerous papers and several review articles889,899-907 deal with adsorption studies and discuss the kinetics and mechanism of the silver-catalyzed epoxidation of ethylene. A simple triangular kinetic scheme of first-order reactions satisfies the experimental observations (Scheme 9.23). On the best industrial catalysts fci/ 2 is 6, and k2/ 3 is 2.5. 

The log—log plot of the adsorption isotherm, which can possibly be correlated to the pressure-dependency of the catalytic reaction rate, is very flat. The adsorption of ethylene on nickel increases only by 10% for an increase of the equilibrium pressure by a factor of 10, although the surface is still far from being covered by a monolayer. The work of Laidler et al. (3), who studied the ammonia-deuterium exchange reaction on a promoted iron catalyst by means of the microwave method, also throws doubt on the zero-order kinetics with respect to observations made by Farkas (4). 

Pre-adsorbed chloride reduced the amount of adsorbed ethylene and enhanced the adsorption of products.Assuming that chloride also reduces the surface concentration of atomic oxygen, then catalyst selectivity will rise since the partial oxidation reaction is first order on surface species while the complete oxidation is second order. 

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