GB2114461A - Ferreous catalyst system for ammonia synthesis - Google Patents

Abstract

A ferreous catalyst system for the synthesis of ammonia from hydrogen and nitrogen comprises a typical Haber catalyst (e.g. derived from magnetite) and a promoter combination comprising lanthanum and nickel in an atomic ratio of 1:4 to 6 and preferably 1:5. The promoter combination may also comprise ruthenium, cobalt or cerium preferably in amounts of 1 to 25% by weight. The promoted catalyst has a lower activation energy permitting higher conversions to ammonia or the use of lower temperatures or pressures. Also disclosed are precursors for the catalyst system and an ammonia synthesis process using the catalyst system.

Description

SPECIFICATION Ferreous catalyst system and process for ammonia synthesis This invention relates to a ferreous catalyst system suitable for the synthesis of ammonia from nitrogen and hydrogen (ie for the Haber process), to precursors for the catalyst system, and to a process for synthesising ammonia using the catalyst.

After over sixty years of operation, the Haber process is still the only commercial process widely available for synthesising ammonia and porous iron is still the basis of the catalyst used in the process. Commercially, the Haber process is generally operated at pressures of from 150 to 1000 bar and at temperatures of from 350 to 6509C and even then only a low conversion per pass of nitrogen and hydrogen to ammonia is achieved.

Because of the high pressures and temperatures used, capital and depreciation charges can represent up to about half of the cost of the production of ammonia. Accordingly there is a long established need to find a more active catalyst which achieves higher conversions and/or permits operation at lower pressures and/or temperatures.

Current ferreous catalysts used in the Haber process are present in the form of a porous matrix having a surface area of át least 1 m2/g (preferably at least 10 m2/g and usually 15 to 20 m2/g) which is obtained by the reduction of magnetite. Almost invariably the catalyst will also contain moieties derived from the addition of alumina (mainlyAl2O3), magnesia (mainly MgO), quicklime (mainly CaO), potash (mainly K2O or KOH) and/or silica (mainly SiO2) to the magnetite prior to reduction. Alumina and magnesia inhibit the tendency for the porous matrix to coarsen during use and quicklime enhances the ability of the alumina to inhibit coarsening.Potash marginally increases the catalytic activity of the iron but only at the cost of promoting coarsening of the matrix and so improvements achievable by adding potash are limited. Silica increases the resistance of the catalyst to poisoning by water. It is still not clear to what extent the metal components of a Haber catalyst exist in the metallic, oxide, nitride, hydride or other forms and so the composition of the catalyst is conveniently described in terms of metal moieties, that is to say metal atoms whether they be combined or uncombined with other atoms. For example, it has been postulated that during the course of an ammonia synthesis reaction, the iron exists mainly in the metallic state carrying chemisorbed nitrogen whereas the quicklime, alumina and silica may form complex alumino-silicates and the potash and magnesia may survive unchanged.Typically the ferreous catalyst will comprise metal moieties in the following weight percentages which are based on the total weight of the metal moeity present in the ferreous catalyst: Al 1.4to 2.1% eg 1.77% Mg up to 0.35% eg 0.27% Ca 1 to 7% eg 3.5% K 0.7to1.3eg1% Fe Not less than 85% and preferably at least 92% Typical commercial catalysts also comprise silicon in amounts of from 0.1 to 0.5% by weight of the total metal moiety.

The porous ferreous catalyst matrix can be pyrophoric. Therefore to reduce the risk of spontaneous combustion, the matrix is often made by reducing the iron oxide in situ. Alternatively the matrix may be made by reducing the iron oxide and then treating with oxygen or ammonia to form a precursor comprising a pacifying presence of (preferably up to 10% by weight of) oxygen or ammonia which is subsequently removed in situ.

An object of this invention is to provide a catalyst system comprising a ferreous catalyst suitable for ammonia synthesis (and especially a ferreous catalyst such as those used commercially in the Haber process) and a promoter for the catalyst, which catalyst system reduces the apparent activation energy and possibly alters the mechanism of the ammonia synthesis reaction. Another object is to provide an iron oxide composition which can be reduced in situ to produce such a catalyst system or which can be reduced and temporarily pacified with oxygen or ammonia to produce a precursor for the catalyst system. Another object is to provide a process for the synthesis of ammonia in which the catalyst system is used.

Accordingly this invention provides a porous ferreous catalyst system suitable for use in the synthesis of ammonia from nitrogen and hydrogen wherein the system comprises (a) a ferreous catalyst comprising iron moiety and (b) a promoter combination comprising lathanum and nickel moieties in a ratio of 1 atom lathanum to 4 to 6 atoms nickel and wherein the total weight of lanthanum and nickel moieties is from 1 to 30 (preferably 5 to 20)% by weight of the weight of iron moiety in the ferreous catalyst. The presence of the promoter combination results in a lowering of the apparent activation energy of the synthesis reaction by at least 15 kcals/mole which enables higher conversions to be achieved for a particular combination of temperature and pressure or permits the same conversions to be achieved at lower temperatures and/or pressures.It is possible that under the conditions of the synthesis reaction, any lanthum and nickel combination breaks down enabling lathanum and nickel moieties to disperse separately. The lanthanum may form a nitride.

Preferably the atomic ratio of lanthanum to nickel is 1:5 for at this ratio the hydrogen absorption has been found to be especially high and the rates of hydrogen absorption and desorption are least affected by impurities. The effectiveness of the promoter may be further increased by including in the combination one or more auxiliary metal moieties such as cobalt or cerium and preferably ruthenium in amounts of form 1 to 25% by weight of the total weight of metal moeity in the promoter combination.

A possible well understood method for associating the ferreous catalyst with the promoter combination to produce the catalyst system comprises melting together particles of partially reduced iron oxide or partially reduced ferreous catalyst with ground particles of the intermetallic compound Lands (optionally mixed with auxiliary metal moeity) which have been combined in a conventional furnace and under appropriate conditions. The mixture is next compressed into pellets and then reduction is completed in situ to produce a catalyst system having a surface area of about 1 to 5 m2/.

The invention also provides an iron oxide (preferably magnetite) composition suitable for reduction to produce a catalyst system according to this invention wherein the composition comprises (a) an iron oxide and (b) a promoter combination comprising lanthanum and nickel in an atomic ratio of 1 :4to 6 and in an amount such that the weight of lanthanum and nickel moieties is from 1 to 30% by weight of iron moiety in the composition.

The invention also provides a precursor for a catalyst system according to this invention the precursor comprising: (a) a ferreous catalyst; (b) a promoter combination comprising lanthanum and nickel moieties in an atomic ratio of from 1:4 to 6 and in an amount such that the weight of the lanthanum and nickel moieties is from 1 to 30% by weight of the iron moiety of the ferreous catalyst, and (c) a pacifying presence of oxygen or ammonia.

The invention further provides a process for the synthesis of ammonia from nitrogen and hydrogen using a ferreous catalyst wherein the ferreous catalyst is a catalyst according to this invention. The invention especially provides such a process when performed at pressures below 150 bar or temperatures below 400"C.

The invention is further illustrated by the following examples of which Example A is comparative.

Examples 1 to 4 andA Various catalyst systems as specified in Table 1 were prepared by first reducing a ground magnetic composition which consisted of the following components: Alumina 2 to 3.5% Magnesia 0.1 to 0.4% Quicklime 2 to 3.5% Potash 0.7 to 2% Silica 0.2 to 0.4% Fe304 to 100% All percentages are by weight based on the weight of the composition.

Reduction was performed for 22 hours at 620"C at 1 bar absolute using a hydrogen flow rate of 18 to 20 litres/minute. A precursor was obtained in which about 90% by weight of the iron oxide has been reduced to metallic iron and which comprised a porous matrix. The matrix was ground to a fine powder.

Various amounts of a promoter combination as specified in Table 1 were ground to a fine powder and shaken overnight with the powdered precursor to produce an intimate mixture. The mixture (orforthe purposes of Example A, the ground precursor alone) was then compresed using a pressure of 464 bar into cylindrical pellets 3 mm in diameter and 3 mm high and approximately 0.09 g in weight. The bulk density of the tablets was 3.8 g/cm3 and they comprised approximately 50% by volume of pores.

Reduction of the iron oxide in the tabletted precursor was then completed by reducing at 550"C under 1 bar of hydrogen for 7.5 hours to produce the catalyst system. The surface area of the ground, tabletted catalyst prior to final reduction in situ was 3.2 m2/g. The system was purged with argon before use in an ammonia synthesis. Each catalyst system was specified in Table 1 was in turn introduced into an ammonia synthesis reactor. A stoichiometric 3:1 ratio of hydrogen to nitrogen was passed through the reactor at a pressure of 100 bar and at different temperatures and space velocities and the conversion to ammonia was measured.

From a graph of reciprocal temperature ("K-') versus a function K (see later) representing the rate constant, the value of the apparent activation energy for the reaction was deduced for each catalyst system. The various apparent activation energies are shown in Table 1.

TABLE 1 Example Promoter * Amount Apparent Combination Promoter Combination Activation Energy kcal/mole A None - 40.4 1 LaNi5+ 20% Ru 10% 16.1 2 LaNi5 20% 15.3 3 LaNi5+ 5% Ru 20% 14.6 4 ***(MM)Ni5 10% 19.2 * % by weight of the weight of the ferreous catalyst used ** % by weight of the total weight of La, Ni and Ru moieties MM MM is mischmetal.

The apparent activation energies quoted in Table 1 are less than half of that obtained with the standard commercially available catalyst of Example A and this suggests that a new reaction mechanism may have been induced. It is probable that with techniques able to produce higher surface areas, the activation energies achievable will approach the theoretical minimum value. Even with the present relatively coarse catalyst systems of Examples 1,2 and 3 it is possible to achieve a conversion at 337"C which can only be achieved using the unpromoted catalyst of Example A if a temperature of 500"C is employed.

It may be possible to achieve results if the lanthanum moiety of the promoter combination is exchanged for mischmetal. Thus, if desired, some or all of the lanthanum moiety of the promoter combination may be replaced by mischmetal.

The function K1 (which represents the rate constant) is derived using the equation of Temkin et al., W=KPhAPnS where W = rate of conversion of H2 adn N2 to NH3 A and B are constants such that A + B = 1 and for present purposes A and B are assumed to be 0.5 Pn is the partial pressure of nitrogen Ph is the partial pressure of hydrogen For a given temperature W, Pn and Ph can be determined experimentally and hence a value for K at that temperature can be derived.

Claims (9)

1. A porous ferreous catalyst system suitable for use in the synthesis of ammonia from nitrogen and hydrogen wherein the system comprises (a) a ferreous catalyst comprising iron moiety and (b) a promoter combination comprising lanthanum and nickel moieties in a ratio of 1 atom lanthanum to 4 to 6 atoms nickel and wherein the total weight of lanthanum and nickel moieties is from 1 to 30% by weight of the weight of iron moiety in the ferreous catalyst.

2. A catalyst system according to claim 1 wherein the atomic ratio of lanthanum to nickel is 1 to 5.

3. A catalyst system according to claim 1 wherein the promoter combination comprises the intermetallic compound Lands or species derived therefrom.

4. A catalyst system according to any one of claims 1 to 3 wherein the promoter combination comprises 1 to 25% by weight (based on the total weight of metal moieties in the promoter combination) of auxiliary metal moiety.

5. A catalyst system according to claim 4 wherein the auxiliary metal moiety comprises ruthenium, cobalt or cerium.

6. An iron oxide composition suitable for reduction to produce a catalyst system as defined in claim 1 wherein the composition comprises (a) an iron oxide (b) a promoter combination comprising lathanum and nickel in an atomic ratio of 1:4 to 6 and in an amount such that the weight of lanthanum and nickel is from 1 to 30% by weight of iron moiety in the composition.

7. A precursor for a catalyst system as claimed in any one of claims 1 to 5 comprising (a) a ferreous component and (b) a promoter combination comprising lanthanum and nickel moieties in an atomic ratio of from 1:4 to 6 and in an amount such that the weight of the lanthanum and nickel moieties is from 1 to 25% by weight of the iron moiety of the ferreous component and (c) a pacifying presence of oxygen or ammonia.

8. A process for the synthesis of ammonia from nitrogen and hydrogen using a ferreous catalyst as claimed in any one of claims 1 to 5, or as produced from the iron oxide composition as claimed in claim 6 or from the precursor as claimed in claim 7.

9. A process as claimed in claim 8 when performed at a pressure below 150 bar.