Potassium ammonia synthesis

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... 

The industrial catalysts for ammonia synthesis consist of far more than the catalyticaHy active iron (74). There are textural promoters, alumina and calcium oxide, that minimise sintering of the iron and a chemical promoter, potassium (about 1 wt % of the catalyst), and possibly present as K2O the potassium is beheved to be present on the iron surface and to donate electrons to the iron, increasing its activity for the dissociative adsorption of N2. The primary iron particles are about 30 nm in size, and the surface area is about 15 m /g. These catalysts last for years. 

The performance of many metal-ion catalysts can be enhanced by doping with cesium compounds. This is a result both of the low ionization potential of cesium and its abiUty to stabilize high oxidation states of transition-metal oxo anions (50). Catalyst doping is one of the principal commercial uses of cesium. Cesium is a more powerflil oxidant than potassium, which it can replace. The amount of replacement is often a matter of economic benefit. Cesium-doped catalysts are used for the production of styrene monomer from ethyl benzene at metal oxide contacts or from toluene and methanol as Cs-exchanged zeofltes ethylene oxide ammonoxidation, acrolein (methacrolein) acryflc acid (methacrylic acid) methyl methacrylate monomer methanol phthahc anhydride anthraquinone various olefins chlorinations in low pressure ammonia synthesis and in the conversion of SO2 to SO in sulfuric acid production. 

A classical example of promotion is the use of alkalis (K) on Fe for the ammonia synthesis reaction. Coadsorbed potassium (in the form of K20) significantly enhances the dissociative adsorption of N2 on the Fe surface, which is the crucial and rate limiting step for the ammonia synthesis5 (Fig. 2.1). 

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 ammonia synthesis and the Fischer-Tropsch synthesis, where it is thought to assist the dissociation of the reactants,... 

Catalysts consisting of more than one component are often superior to monometallic samples. Model studies with potassium on Fe surfaces revealed, for example, the role of the electronic promoter in ammonia synthesis. A particularly remarkable case was recently reported for a surface alloy formed by Au on a Ni(lll) surface where the combination of STM... 

The above are equilibrium reactions, and their successful exploitation requires that they be carried out under conditions in which the equilibrium favors the product. Specifically, this requires that the adsorbed species in Reactions (D)-(I) not be held so tightly on the catalyst surfaces as to inhibit the reaction. On the other hand, strong interaction between adsorbate and catalyst is important to break the bonds in the reactant species. Optimization involves finding a compromise between scission and residence time on the surface. Although we are especially interested in metal surfaces, those constituents known as promoters in catalyst mixtures are also important. It is known, for example, that the potassium in the catalyst used for the ammonia synthesis shifts Equilibrium (F) to the right and also increases the rate of Reaction (D) by lowering its activation energy from 12.5 kJ mole to about zero. 

It should be noted that the results for the formic acid decomposition donor reaction have no bearing for ammonia synthesis. On the contrary, if that synthesis is indeed governed by nitrogen chemisorption forming a nitride anion, it should behave like an acceptor reaction. Consistent with this view, the apparent activation energy is increased from 10 kcal/mole for the simply promoted catalyst (iron on alumina) to 13-15 kcal/mole by addition of K20. Despite the fact that it retards the reaction, potassium is added to stabilize industrial synthesis catalysts. It has been shown that potassium addition stabilizes the disorder equilibrium of alumina and thus retards its self-diffusion. This, in turn, increases the resistance of the iron/alumina catalyst system to sintering and loss of active surface during use. 

One strong point of SIMS is its ability to detect elements that are present in trace amounts, and as such the technique is highly suited to the detection of poisons on a catalyst caused by contaminants in the reactor feed. Chlorine, for example, poisons the iron catalyst used in ammonia synthesis because it suppresses the dissociation of nitrogen molecules. Plog et al. [18] used SIMS to show that chlorine impurities may coordinate to potassium promoters, as evidenced by a KCI2- signal, or to iron, visible by an FeCh- peak. The SIMS intensity ratio... 

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]. 

Ammonia synthesis catalysts have traditionally been based on iron and have been made by the reduction of magnetite (Fe304). The difference between different commercially available products lies in optimized levels of metal oxide promoters that are included within the magnetite structure. These metal oxides promote activity and improve the thermal stability of the catalyst. Typical promoters are alumina (AI2O3X potassium oxide (K2O), and calcium oxide (CaO). The interactions between the many components in the catalyst can radically affect 1) the initial reducibility, 2) the level of catalyst activity that is achieved, 3) the long-term catalyst performance and 4) the long-term catalyst stability204. 

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]. 

Table 20. Ammonia synthesis activity of metals supported on carbon with potassium metal promotion (ml Nib,/ mL catalyst, 573 K, 1283.13 mbar. H N = 3 1...

The steam reforming of hydrocarbon feedstocks is a common industrial process which produces hydrogen for use in methanol or ammonia synthesis. A variety of hydrocarbons, e.g. natural gas or naphthas, can be used as the reactant in the steam reforming process, This use of a variety of reactant feed types places considerable demands upon the catalyst manufacturer since all hydrocarbons have different reactivities and, most importantly, disparate tendencies to generate carbonaceous deposits, ICI produce a range of catalysts for use with a number of hydrocarbon reactants. For the reforming of heavy naphtha feedstocks, which show a considerable propensity for carbon deposition, ICI provides a potassium promoted nickel based catalyst (ref 1). The object of this paper is to describe the mechanism by which alkali provides resistance to carbon formation in nickel catalysts. 

Formally, ammonia synthesis is closely related to Fischer-Tropsch synthesis. Industrial operation involves the use of an iron catalyst promoted with calcium and potassium oxides. However, the reason we consider this process here is not directly in connection with alkali promotion of the catalyst. We are concerned with a remarkable achievement reported by Yiokari et al. [15], who use a ton-conducting electrolyte to achieve electrochemical promotion of a fully promoted ammonia synthesis catalyst operated at elevated pressure. Specifically, they make use of a fully promoted industrial catalyst that was interfaced with the proton conductor CaIno.iZro.903-a operated at 700K and 50 bar in a multipellet configuration. It was shown that under EP the catalytic rate could be increased by a factor of 13 when... 

The base isopilocereine, obtained in small yield by the potassium-ammonia cleavage of pilocereine 111) and 0-methylpilocereine (113), was shown by degradation and synthesis to have the dimeric formula CLVII (114-119) assigned previously to pilocereine. 

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. 

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