Ammonia synthesis potassium, effects

The addition of potassium to Fe single crystals also enliances the activity for ammonia synthesis. Figure A3.10.19 shows the effect of surface potassium concentration on the N2 sticking coefficient. There is nearly a 300-fold increase in the sticking coefficient as the potassium concentration reaches -1.5 x 10 K atoms cm ... 

Bare S R, Strongin D R and Somoqai G A 1986 Ammonia synthesis over iron single crystal catalysts—the effects of alumina and potassium J. Phys. Chem. 90 4726... 

These effects profoundly influence the ammonia, as shown in Fig. 8.28 where the ammonia synthesis rate is plotted for two basal planes of iron and the same iron surfaces modified with 0.1 Ml of potassium. 

Potassium is a well-known promoter in the synthesis of ammonia and in Fischer-Tropsch syntheses, where it is thought to assist the dissociation of the reactants, N2 and CO, respectively [36, 37]. Empirical knowledge concerning the promoting effect of many elements has been available since the development of the iron ammonia synthesis catalyst, for which a few thousand different catalyst formulations have been tested. Recent research into surface science and theoretical chemistry has led to a rather complete understanding of how the promoter functions [38, 39]. 

The ammonia synthesis catalyst problem could be considered solved when the catalytic effectiveness of iron in conversion and its onstream life were successfully and substantially improved by adding reduction-resistant metal oxides [232] (Table 15). The iron catalysts promoted with aluminum and potassium oxides proved to be most serviceable [238]. Later, calcium was added as the third activator. Development work in the United States from 1922 can be found in [239]. 

Potassium is used as a dopant on catalysts for the methanation reaction and ammonia synthesis. Its purpose is to increase the rate of the reaction. Potassium is also used on the steam reforming catalyst, not as a promotor but as a dopant that inhibits catalyst deactivation by coke formation (ref. 1). It is reasonable that the role of potassium as a promotor of reaction rates is to lower some barrier to bond dissociation. Since molecular beam techniques afford a convenient means of measuring changes in barrier heights as well as in shapes of the barrier through measurements of the dissociation probability versus energy, the possible effect of potassium on the dissociation of CH4 is investigated. 

From a study of the mechanism of the poisoning action of water vapors mill oxygen on iron ammonia catalysts 21 and by making certain assumptions, Almquistsu has been able to calculate that in pure iron catalysts about one atom in two thousand is active toward ammonia synthesis, whereas in iron catalysts promoted by alumina about one atom in two hundred is active. This shows the remarkable added activity obtainable by the use of promoters. That the effect is complicated beyond any simple explanation is evidenced further by some of the results of Almquist and Black, These workers have shown that whereas an iron-alumina catalyst shows greater activity toward ammonia synthesis at atmospheric pressure than an iron catalyst containing both alumina and potassium oxide, the hitter catalyst is 50 per cent more active when the pressure is raised to 1(X) atmospheres. 

The addition of potassium to industrial Fe catalysts leads to an increase in activity for ammonia synthesis (N2 -I- 3H2 - 3NH3) (136). This promotion effect has been the subject of considerable attention from the surface science community, particularly with regard to the coadsorption of K or K + O and N2 (136-139). Ertl and co-workers have shown that potassium addition to single-crystal Fe surfaces can lead to a 10- to 100-fold enhancement in the rate of dissociative N2 adsorption, which is thought to be the rate-determining step in NH3 synthesis (136-139). However, Bare et al. (140) were unable to promote the activity of Fe(l 11), (100), or (110) surfaces for this reaction at 20-atm pressure with either K, K + O, or K + AlO, addition. They interpreted this result to indicate that the promotional role of K in industrial catalysis may be cooperation with other promoters, such as the support material, to cause structural rather than electronic promotion. These results were for very low conversions, however, so that the product (NH3) partial pressure was low. Strongin and... 

Surface Science of Ammonia Synthesis Structure Sensitivity of Ammonia Synthesis Kinetics of Dissociative Nitrogen Adsorption Effects of Aluminum Oxide in Restructuring Iron Single-Crystal Surfaces for Ammonia Synthesis Characterization of the Restructured Surfaces Effect of Potassium on the Dissociative Chemisorption of Nitrogen on Iron Single-Crystal Surfaces in UHV... 

Effects of Potassium on the Adsorption of Ammonia on Iron Under Ammonia Synthesis Conditions... 

Effects of Potassium on Ammonia Synthesis Kinetics Extensive research has been completed in which the effects of potassium on ammonia synthesis over iron single-crystal surfaces of (111), (100), and (110) orientations [59] have been determined. The apparent order of ammonia and hydrogen for ammonia synthesis over iron and K/Fe surfaces has been determined in addition to the effect of potassium on the apparent activation energy Ef) for the reaction. In all the experiments, potassium was coadsorbed with oxygen because only about 0.15 ML of potassium coadsorbed with oxygen is stable under ammonia synthesis conditions (20... 

These results are consistent with earlier literature [63, 64] in which the effects of potassium on doubly promoted (aluminum oxide and potassium) catalysts were studied. It was shown that the turnover number for ammonia synthesis is roughly the same over singly (aluminum oxide) and double promoted iron when 1 atm reactant... 

Effects of Potassium on the Adsorption of Ammonia on Iron Under Ammonia Synthesis Conditions The changes in the apparent reaction order dependence in ammonia partial pressure suggest that to elucidate the effects of potassium on both iron single crystals and the industrial catalyst, it is necessary to understand the readsorption of gas-phase ammonia on the catalyst surface during ammonia synthesis The fact that the rate of ammonia synthesis is negative order in ammonia synthesis. Once adsorbed, the ammonia has a certain residence time (t) on the catalyst which is determined by its adsorption energy on iron [t cx tq exp (AH /RT)]... 

D.R. Strongin and G.A. Somorjai. The Effects of Potassium on Ammonia Synthesis over Iron Single Crystal Surfaces. J. Catal. 109 51 (1988). 

The exact role of promoters is not very well understood in many cases, but it is now generally accepted that it is related to the formation of specific electronic surface states necessary for the given catalytic reaction. It apparently does not matter how that electronic state is produced that is, whether it is formed in the preparation of the native catalyst surface or by the presence of some other component which induces the necessary state. As an example, the presence of small amounts of aluminum and potassium oxides on iron-iron oxide catalyst in the Haber ammonia synthesis greatly improves its activity. Either promoter alone has no significant effect on the process. Why Such questions remain as fodder for further industrial or graduate research. 

Research with an alkali-promoted (potassium or K2O) ruthenium catalyst has demonstrated that ammonia synthesis can be effected at lower temperatures and pressures than those required by the Haber process. As the price of energy increases, ruthenium catalysis might become increasingly important, because the energy-expensive compression process could be avoided. Another advantage of ruthenium if its diminished susceptibility to poisoning by H2O and CO. Ruthenium catalysts can carry out the direct synthesis of ammonia from N2, CO, and H2O ... 

Fig. 3.50 The effect of potassium on the rate of ammonia synthesis on an iron single crystal face ...

Fig. 3.53 The effect of potassium on the activation energy of ammonia synthesis between 623 and 723...

We conclude that anhydrous potassium hydroxide is the active species responsible for promotion by potassium and that the beneficial effect on ammonia synthesis is bifunctional, enhancing on the one hand the adsorption energy of molecular nitrogen, and reducing the adsorption energy of the product ammonia on the other. 

A SURFACE SCIENCE AND CATALYTIC STUDY OF THE EFFECTS OF ALUMINUM OXIDE AND POTASSIUM ON THE AMMONIA SYNTHESIS OVER IRON SINGLE-CRYSTAL SURFACES... 

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