Equilibrium catalytic oxidation

Cox MP, Ertl G, Imbihl R, Riistig J. 1983. Non-equilibrium surface phase transitions during the catalytic oxidation of CO on Pt(lOO). Surf Sci 134 L517. 

It is often necessary to employ more than one adiabatic reactor to achieve a desired conversion. The catalytic oxidation of SOj to SO3 is a case in point. In the first place, chemical equilibrium may have been established in the first reactor and it would be necessary to cool and/or remove the product before entering the second reactor. This, of course, is one good reason for choosing a catalyst which will function at the lowest possible temperature. Secondly, for an exothermic reaction, the temperature may rise to a point at which it is deleterious to the catalyst activity. At this point, the products from the first reactor are cooled prior to entering a second adiabatic reactor. To design such a system, it is only necessary to superimpose on the rate contours the adiabatic temperature paths for each of the reactors. The volume requirements for each reactor can then be computed from the rate contours in the same way as for a... 

More generally, co is independent of the external gas pressure k is the Boltzmann constant (1.38 x 10 erg deg ) and T is the temperature in Kelvin. Furthermore, the equilibrium between co and a collapsed CS plane fault is maintained by exchange at dislocations bounding the CS planes. Clearly, this equilibrium cannot be maintained except by the nucleation of a dislocation loop and such a process requires a supersaturation of vacancies and CS planes eliminate supersaturation of anion vacancies (Gai 1981, Gai et al 1982). Thus we introduce the concept of supersaturation of oxygen point defects in the reacting catalytic oxides, which contributes to the driving force for the nucleation of CS planes. From thermodynamics. 

It is clear that equilibrium measurements of surface thermodynamics cannot predict surface composition under the dynamic conditions of catalytic oxidation. Nevertheless, such measurements will provide a sounder base than bulk thermodynamics for understanding the surface chemistry and permit working backward, from direct measurements of surface chemistry during reaction, to predictions concerning the microenvironment at the surface under reaction conditions. 

Fig. 10.1. Sketch of S02, 02, N2 feed gas descending a reactive catalyst bed. It assumes that equilibrium is attained before the gas leaves the bed and that composition and temperature are uniform horizontally at all levels. Rapid catalytic oxidation requires an input gas temperature -690 K, Table 7.2.

Maximum equilibrium S03 production is favored by a cool equilibrium temperature (but warm enough for rapid catalytic oxidation). Temperature exerts a much greater influence on this maximum than pressure or catalyst bed feed gas composition. 

A small amount of Ar is also present in catalytic S02 oxidation feed gases. Like C02, it has no effect on equilibrium % S02 oxidized equations. 

An excellent example of an optimum operation design is the determination of operating conditions for the catalytic oxidation of sulfur dioxide to sulfur trioxide. Suppose that all the variables, such as converter size, gas rate, catalyst activity, and entering-gas concentration, are fixed and the only possible variable is the temperature at which the oxidation occurs. If the temperature is too high, the yield of SO, will be low because the equilibrium between SO, SO, and 0, is shifted in the direction of SO, and 0,. On the other hand, if the temperature is too low, the yield will be poor because the reaction rate between SO, and 0, will be low. Thus, there must be one temperature where he amount of sulfur trioxide formed will be a maximum. This particular temperature would give the... 

Adsorption isotherms play a key role in either the design of the adsorption-based process for the disposal of wastes containing VOCs or modeling the catalytic oxidation process. The equilibrium data for mesoporous sorbents are fitted to combined model of Langmuir and Sips equations. This hybrid isotherm model with four isotherm parameters... 

The oxidation of [Cu(bpy)2] by O2 is reported to be second order in Cud) whereas the oxidation by H2O2 is reported to be first order with respect to Cu(I), in general accord with the above mechanism (306, 309, 719, 988). The dioxygen is most likely present as a superoxide 02 ion in the complex [Cu(bpy)2(02)] (306). The [Cu(bpy)2] /02 system is an active catalytic oxidation systems (presumably by the intermediacy of [Cu(bpy)2(02)] ) for the oxidation of alcohols to aldehydes and ketones (655). The importance of predissociation of bpy ligands is not yet established though evidence has been presented for the equilibrium... 

In a Claus plant plant H2S is converted into sulfur however, the conversion is not complete (94-98%). About 1% H2S and 0.5% COS remain in the off-gas due to the thermodynamics of the Claus equilibrium reaction. Van Nisselrooy and Lagas [162] developed a catalytic process, called Superclaus, which is based on bulk sulfur removal in a conventional Claus section, followed by selective catalytic oxidation of the remaining H2S to elemental sulfur. Iron oxides and chromium oxides supported... 

Pre-steady-state kinetic studies established that the appearance of the NADH chromophore on addition of substrate was a two-step process, and these steps can now be identified as closure of the active site and hydride transfer. This study indicated that the on-enzyme equilibrium for addition of water or homocysteine to the enone was close to unity (and the value in free solution), whereas the equilibrium for oxidation of NAD by bound adenosine was 10 times more favourable than in free solution. The focusing of the catalytic power of the enzyme on the oxidation step avoids the formation of abortive complexes by hydride transfer between enone and NADH, yielding 4,5-dehydroadenosine and NAD ". This happens about 10 " times faster than productive hydride transfer at the beginning and end of the catalytic cycle, with the slow rate (close to that of model reactions) apparently arising from a conformationally modulated increase in the distance the hydride has to be transferred. 

Online mass spectrometry data presented and discussed in the previous sections suggest that catalytic hypophosphite oxidation on nickel in D2O solutions proceeds via the coupling of anodic (19.11) and cathodic (19.12) half-reactions at the catalyst surface. The classical mixed-potential theory for simultaneously occurring electrochemical partial reactions [14] presupposes the catalyst surface to be equally accessible for both anodic (19.11) and cathodic (19.12) half-reactions. Equilibrium mixtures of H2, HD, and D2 should be formed in this case due to the statistical recombination of Hahalf-reactions (19.11) and (19.12) for example, the catalytic oxidation of hypophosphite on nickel in D20 solution under open-circuit conditions should result in the formation of gas containing equal amounts of hydrogen and deuterium (H/D=l) with the distribution H2 HD D2= 1 2 1 (the probability of HD molecule formation is twice as high as for either H2 or D2 formation [75]). Therefore, to get further mechanistic insight, the distribution of H2, HD, and D2 species in the evolved gas was compared to the equilibrium values at the respective deuterium content [54]. 

On catalytic oxidation, conduritol C (66) gives (in just 10 min.) the ketone (66a), the quasi-equatorial hydroxyl group at C-1 (but neither the axial hydroxyl group at C-2 nor the quasi-equatorial hydroxyl group at C-4) being oxidized. The same ketone (66a) can also be prepared by oxidation of natural conduritol A (67). Both possible half-chair conformations of conduritol A (67) are equally stable, and hence there presumably exists an equilibrium in which the group at C-1, as well as that at C-4, is attacked and a racemate (66a) is produced. 

In the case of the alkyl a-D-lyxopyranosides (85) an equilibrium mixture of both chair-conformations is presumably present. Both the Cl form and the IC form possess two axial substituents, so that, according to ReeveSj neither of the two forms appears to have preferential stability. " The catalytic oxidation of benzyl a-D-lyxopyranoside (85 R = CHaCeHs) apparently gives, however, only benzyl a-D-t/ireo-pentopyranosid-3-ulose... 

Commercially it is obtained by catalytic oxidation of ammonia as already noted. Direct combination of the elements occurs only at very high temperatures, and to isolate the small amounts so formed (a few volume per cent at 3000°) the equilibrium mixture must be rapidly chilled. Though much studied, this reaction has not been developed into a practical commercial synthesis. Nitric oxide reacts instantly with 02 ... 

In the model, we assume that the rate of NO catalytic oxidation is so high that equilibrium concentration of NO, NO2 and O2 are retained throughout the reactor. This hypothesis is not directly suggested by the experimental findings, because they just prove that equilibrium conditions are established at the reactor exit. However, it is supported by NO oxidation measurements carried out with a contact time about five times lower than the minimum value investigated in the experimental campaign here presented, in which the equilibrium conversion of NO to NO2 was reached. 

The details of the process of catalytic oxidation, represented by kz, above, remain rather obscure. If the foregoing analysis is correct, the surface at the time catalytic oxidation begins must be partially silver nitrate and partially silver acetylide, the proportions of the two compounds depending upon the temperature. After the silver nitrate-silver acetylide equilibrium has been attained there are at least three different ways in which incoming acetylene molecules might react. 

Molecules could be adsorbed on the silver nitrate surface to form an activated complex which then reacts with gaseous oxygen, and this would result in a reaction rate proportional to 1 — reaction rate. There are some indications, however, that neither process is the correct one. It was observed that the evolution of carbon dioxide is very abrupt within the experimental error no carbon dioxide whatever was produced until the silver nitrate-silver acetylide equilibrium had been attained. It is difficult to understand why some catalytic oxidation should not have occurred on either of the two surfaces before this condition was reached. 

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