Iron-ammonia catalysts activity

With regard to the sulfur bound on the catalyst surface, differences exist between the various types of ammonia catalysts, especially between those that contain alkali and alkaline earth metals and those that are free of them. Nonpromoted iron and catalysts activated only with alumina chemisorb S2N2 and thiophene. When treated with concentrations that lie below the equilibrium for the FeS bond, a maximum of 0.5 mg of sulfur per m2 of inner surface or free iron surface is found this corresponds to monomolecular coverage [382], [383], The monolayer is also preserved on reduction with hydrogen at 620 °C, whereas FeS formed by treatment above 300 °C with high H2S concentrations is reducible as far as the monolayer. For total poisoning, 0.16-0.25 mg S/m2 is sufficient. Like oxygen, sulfur promotes recrystallization of the primary iron particle. 

The presence of various active parts on the surface of the iron-ammonia catalyst was demonstrated by Emmett and Kummer (74) by means of adsorbing radioactive and nonradioactive samples of carbon... 

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. 

In the early research stages of ammonia synthesis catalyst, Mittasch et al. studied almost all metal elements and their bimetallic alloy in the Periodic Table. Several metals have no or less catal3dic functions themselves. However, the addition of some promoter could increase their activities. The addition of a secondary metal into Fe, Mo, W, Co, Ni, Pd, Pt, Os, Mn enhances their activities. The different ratio between two metals leads to a different activity. It can be concluded from these results that the addition of metals in Vlff or VI groups favors enhancing of iron-based catalyst activity. For example, Fe-Mo (1 1) catalyst has high activity. If the Mo content is less than 80%, the activity decreases after running for a long-time. These catalysts are prepared by calcinations of metal nitrates and ammonium molybdate... 

Recent commercialization efforts have focused on improved activity synthesis catalysts, which allow ammonia synthesis to be conducted at significantly lower pressures and temperatures. Catalyst manufacturers have focused on enhancing the activity of the iron-based catalyst through the use of promoters (23). 

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. 

Research in catalysts for ammonia manufacture is stiU going on, and though the use of supported metals such as mthenium may be two to three times as active as promoted iron oxide, catalysts 50—100 times more active than promoted iron oxide are required to affect process economics significantly. 

In this exercise we shall estimate the influence of transport limitations when testing an ammonia catalyst such as that described in Exercise 5.1 by estimating the effectiveness factor e. We are aware that the radius of the catalyst particles is essential so the fused and reduced catalyst is crushed into small particles. A fraction with a narrow distribution of = 0.2 mm is used for the experiment. We shall assume that the particles are ideally spherical. The effective diffusion constant is not easily accessible but we assume that it is approximately a factor of 100 lower than the free diffusion, which is in the proximity of 0.4 cm s . A test is then made with a stoichiometric mixture of N2/H2 at 4 bar under the assumption that the process is far from equilibrium and first order in nitrogen. The reaction is planned to run at 600 K, and from fundamental studies on a single crystal the TOP is roughly 0.05 per iron atom in the surface. From Exercise 5.1 we utilize that 1 g of reduced catalyst has a volume of 0.2 cm g , that the pore volume constitutes 0.1 cm g and that the total surface area, which we will assume is the pore area, is 29 m g , and that of this is the 18 m g- is the pure iron Fe(lOO) surface. Note that there is some dispute as to which are the active sites on iron (a dispute that we disregard here). 

The present paper focuses on the interactions between iron and titania for samples prepared via the thermal decomposition of iron pentacarbonyl. (The results of ammonia synthesis studies over these samples have been reported elsewhere (4).) Since it has been reported that standard impregnation techniques cannot be used to prepare highly dispersed iron on titania (4), the use of iron carbonyl decomposition provides a potentially important catalyst preparation route. Studies of the decomposition process as a function of temperature are pertinent to the genesis of such Fe/Ti02 catalysts. For example, these studies are necessary to determine the state and dispersion of iron after the various activation or pretreatment steps. Moreover, such studies are required to understand the catalytic and adsorptive properties of these materials after partial decomposition, complete decarbonylation or hydrogen reduction. In short, Mossbauer spectroscopy was used in this study to monitor the state of iron in catalysts prepared by the decomposition of iron carbonyl. Complementary information about the amount of carbon monoxide associated with iron was provided by volumetric measurements. 

The main emphasis was laid, in this initial work, on Haber s catalysts, e.g., osmium and uranium compounds, as well as on a series of iron catalysts. Some other metals and their compounds which we tested are, as we know today, less accessibble to an activation by added substances than iron. Therefore, they showed no improvement or only small positive effects if used in the form of multicomponent catalysts. Finally, the substances which we added to the metal catalysts in this early stage of our work were mostly of the same type as those which had proved to favor the nitride formation, e.g., the flux promoting chlorides, sulfates, and fluorides of the alkali and alkaline earth metals. Again, we know today that just these compounds do not promote, but rather impair the activity of ammonia catalysts. 

A brief survey of the positive or negative effects of various elements (viz., of their oxides) on the activity of iron as an ammonia catalyst follows ... 

A favorable combination of valence forces of both components seems to be the basic principle of the nickel-molybdenum ammonia catalyst. It has been found (50) that an effective catalyst of this type requires the presence of two solid phases consisting of molybdenum and nickel on the one hand and an excess of metallic molybdenum on the other. Similar conditions prevail for molybdenum-cobalt and for molybdenum-iron catalysts their effectiveness depends on an excess of free metal, molybdenum for the molybdenum-cobalt combination and iron for the molybdenum-iron combination, beyond the amounts of the two components which combine with each other. A simple explanation for the working mechanism of such catalysts is that at the boundary lines between the two phases, an activation takes place. In the case of the nickel-molybdenum catalyst, the nickel-molybdenum phase will probably act preferentially on the hydrogen and the molybdenum phase on the nitrogen. 

Recently, independent publications by Kojima and Aika and workers from Topsoe have reported on the high activity and stability of Cs/ C03M03N based catalysts for ammonia synthesis. The activity has been found to be significantly higher than for the commercial iron based catalyst. Table 3 gives a comparison of the ammonia synthesis rates of Cs and K promoted catalysts at various promoter loadings (2, 5, 10 and 30 mol%) for low conversion at 400°C and atmospheric pressure. ... 

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. 

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. 

A number of substances show considerable activity as ammonia catalysts. Fe, Os, and Re and nitrides of Mo, W, and U are the best known. Iron in the form of promoted iron catalysts is by far the most important, maybe the only type in industrial use, and except for a few comparisons, iron catalysts will be the only type dealt with in this paper. Furthermore, the discussion will be limited to the type of catalysts made by fusing iron oxides together with the promoter components and subsequently reducing the catalysts. This limitation is not too important, since this type of catalyst is the one most widely used and also the type on which most fundamental work has been done. 

Table 16 gives a composition survey of commercial ammonia catalysts in the years 1964-1966. The principal component of oxidic catalysts is more or less stoichiometric magnetite, Fe304, which transforms after reduction into the catalytically active form of a-iron. 

Insofar as small crystals of nonreducible oxides dispersed on the internal interfaces of the basic structural units (platelets) will stabilize the active catalyst surface Fe(lll), the paracrystallinity hypothesis will probably hold true. But the assumption that this will happen on a molecular level on each basic structural unit is not true. The unique texture and anisotropy of the ammonia catalyst is a thermodynamically metastable state. Impurity stabilization (structural promotion) kinetically prevents the transformation of platelet iron into isotropic crystals by Ostwald ripening [154]. Thus the primary function of alumina is to prevent sintering by acting as a spacer, and in part it may also contribute to stabilizing the Fe(lll) faces [155], [156], [298],... 

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