Ammonia synthesis surface coverage

Fractional surface coverages of predominant adsorbed species versus dimensionless distance from the reactor inlet. (Figure adapted from Kinetic Simulation of Ammonia Synthesis Catalysis by J. A. Dumesic and A. A. Trevino, in Journal of Catalysis, Volume 116 119, copyright 1989 by Academic Press, reproduced by permission of the publisher and the authors.)... 

The passivation by oxygen of a commercial ammonia synthesis catalyst was studied with adsorption microcalorimetry by Tsarev and co-workers (240). Two types of adsorbed oxygen at 293 K were found to participate in the formation of a passivating layer. One type was characterized by differential heats of adsorption near 420 kJ mol" that were close to the heat of iron oxidation and which were independent of surface coverage for several mono-layers. The other form was obtained after a large dose, sufficient for coverage of the entire metal surface with a molecular monolayer. Subsequent adsorp-... 

In order to corroborate the main steps of the synthesis of dimethylethylamine and of the main by-products, we studied the reactivity of some intermediates and products with or without reagents (MEA, MeOH) under the same experimental conditions i) In the absence of methanol (replaced by n-heptane), monoethylamine is transformed mainly into diethylamine (DEA), the deactivation of the catalyst being very fast due to an increase of the formation of ammonia (15). Baiker and Kijenski showed, for instance, that part of the copper was transformed into copper nitride during the amination of alcohols (12). In the presence of methanol, the monoethylamine surface coverage is lower and a decrease cf the DEA formation can be observed. Methanol acts as an inhibitor in the synthesis of DEA and as a promotor of the catalyst duration. 

In addition to the skeptic s view of LH correlations, however, there must be additional reasons why this model has been used successfully for the correlation of kinetics in so many different types of catalytic reactions. Two factors seem to be most important. First is the fact that the expressions are fairly insensitive to the precise sort of kinetic scheme involved. Many widely differing types of reaction sequences can be shown to give approximate LH forms, as given in Tables 3.2 and 3.3 [see also a further good example based on the ammonia synthesis reaction by G. Buzzi Ferraris, G. Donati, F. Rejna and S. Carra, Chem. Eng. Sci., 29, 1621 (1974)]. Second is the fact that for reasonable assumptions concerning the nature of typical nonuniform surfaces it can be shown that they tend to look like uniform surfaces in overall behavior. This second point can be illustrated by an example scheme in which the heat of formation of surface complexes is a linear function of surface coverage. Consider the two-step sequence... 

This isotherm may be derived from kinetic considerations for intermediate surface coverages (0.2 < 0 < 0.8), but it does not lend itself to multicomponent adsorption and also fails to predict the limiting conditions of — 0 when Pa 0 and 9 — when 00. Even though it was used for correlating the kinetics of ammonia synthesis, the Temkin isotherm has not found much use in the kinetic analysis of solid-catalyzed gas-phase reactions. 

It can be seen that from VA to VIII groups, the initial adsorption heat of nitrogen decreases and ammonia synthesis rate reaches a maximum value, and after that the rate goes down due to the reduction of surface coverage of adsorbed nitrogen. 

It is not practical for catalyst development to look for the correlation of adsorption heats and catalytic activities from adsorption heat measurements. First, the measurement of adsorption heat is more difficult and more complicated than the evaluation of catalyst activity in general. Second the adsorption heat varies with surface coverage. Although it is known that catalytic activity does not depend on initial adsorption heat, it is still used for correlating catalytic activities of ammonia synthesis and ethylene hydrogenation because of the lack of experiment data. Therefore, there is a certain limit for the study of catalyst and catalytic reaction using adsorption heat. 

Zwietering et al. measured the amount of N— at the conditions of ammonia synthesis by weight analysis and it was 1% of equilibrium adsorption amount over iron catalyst containing 0.85% AI2O3 at 0.52-0.69 of surface N— coverage and 568 K of temperature. Aika and Ozaki further verified that H— was not a factor in the RDS, by the determination of label atom experiments. 

Recombination of adsorbed hydrogen atoms leads to the reverse process of thermal desorption whose rate is governed by Ead + E, where E is the activation energy for dissociative adsorption. If a hydrogen-covered Fe surface is heated up in vacuo, desorption will be completed at 500 Under the conditions of ammonia synthesis (>700 K) this step will hence be so rapid that the steady-state coverage of Had will be determined by the adsorption-desorption equilibrium H2+ 2Had, where the concentration of free surface sites is, of course, affected by the presence of all other surface intermediates. 

---