GB1604265A - Method for synthesis of ammonia - Google Patents

Description

(54) METHOD FOR SYNTHESIS OF AMMONIA (71) I, MEHMET NAFIZ OZYAGCILAR, a citizen of Turkey, of 175 Poplar Plains Road, Toronto, Canada, do hereby declare the invention, for which I pray that a patent may be granted to me and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to the synthesis of ammonia from molecular nitrogen and hydrogen through the use of a new catalyst and novel processes based thereon. Ammonia is an important raw material in the chemical industry, particularly in the production of synthetic fertilizers. Agricultural research has shown that nitrogen is an indispensable ingredient of fertilizers for crops. The major source of that nitrogen at the present time is ammonia and one of the major goals of chemical technology in the fertilizer field is the production of ammonia at faster rates and with correspondingly lower costs.

Prior art methods for the production of ammonia from gaseous nitrogen and hydrogen have employed iron catalysts of various types. However, such catalysts require high temperatures and pressures necessitating expensive equipment of relatively low capacity, and are rapidly consumed at the temperatures and pressures required by various degradation mechanisms.

Research on the synthesis of ammonia has also progressed in the direction of room temperature synthesis in aqueous solutions where either biological "nitrogen-fixation" conditions are simulated, or a metal salt or complex is used as a catalyst together with a reducing agent. Although aqueous studies are still at the fundamental research level and have not yet been commercialized, it has been found that the metals of heterogeneous catalysts effective to synthesize ammonia at high temperatures from gaseous nitrogen and hydrogen also catalyze the ammonia synthesis when present as a salt or complex in aqueous solutions.

Although gaseous nitrogen and hydrogen theoretically can be combined at room temperature and atmospheric pressure to produce ammonia at an equilibnum yield of 95.5% on thermodynamic grounds, there is no catalyst or method for ammonia synthesis in the prior art that can be employed at room temperature to produce ammonia from gaseous nitrogen and hydrogen at commercially feasible rates.

Therefore, it has been necessary to employ much higher temperatures in the synthesis of ammonia to achieve satisfactory reaction rates. The iron catalysts previously used produce no appreciable ammonia from gaseous nitrogen and hydrogen at temperatures below approximately 3600C.

Even higher temperatures are therefore necessary for acceptable yields. However, higher temperatures in turn result in a drastic reduction in the thermodynamic equilibrium yield of the reaction due to its exothermic nature. Reductions in equilibrium yields with increasing temperatures can only be partly compensated for by increasing the operating pressure, and the pressures needed closely approach the design limits of the equipment presently available for industrial application.

Prior art processes for the synthesis of ammonia from gaseous nitrogen and hydrogen over commercial iron catalysts operate at temperatures around 5000 C. The equilibrium yield of the reaction at this temperature with only one atmosphere of pressure is well below 0.2%. Higher pressures in the range of 150 atmospheres are therefore employed to compensate for the low yield at ambient pressure. Such high pressures result in high equipment and maintenance cost. Furthermore, equilibrium yields attained at such elevated pressures are usually less than 25%. This means that a substantial portion of the effluents from reactor vessels must be recycled one or more times, adding substantially to the cost of production both in the form of added equipment and longer operating times for separation and recycle of the product stream.

By reason of the foregoing thermodynamic and kinetic considerations, the cost of producing ammonia by prior art methods is quite high, involving relatively slow production rates and costly equipment.

The cost of the iron catalyst itself is also quite substantial, mainly because special manufacturing processes must be utilized to improve the catalytic properties of the iron.

Thus, additional components called promoters must be added to the iron and, in most cases, both the iron and the promoters must be supported on a special carrier in a manner to permit sufficient contact between those constituents and the gaseous reactants.

The foregoing disadvantages encountered with reactions between nitrogen and hydrogen over prior art catalysts are avoided through the use of the present invention. The novel catalyst of this invention for the first time allows the use of substantially lower operating temperatures and pressures in achieving commercially feasible production rates. At such temperatures and pressures, equilibrium yields fall within the range of 30 to 60% and therefore actual yields are greatly improved.

According to the present invention there is provided a method of making ammonia which comprises contacting nitrogen and hydrogen at synthesis conditions with a catalyst comprising an alloy capable of forming unstable hydrides.

Use of the catalyst defined above enables the production of ammonia from gaseous hydrogen and nitrogen by a commercially economic process at lower temperatures and pressures than heretofore possible, thereby reducing the cost of process equipment and operating and maintenance costs associated therewith.

The catalysts used in the method of the invention are relatively resistant to poisoning by impurities that usually exist in commercially available hydrogen and nitrogen reactant gases and are capable to yielding higher rates of ammonia production at ambient temperatures than ere ore possible and can be used to promote ammonia synthesis in aqueous solutions containing molecular hydrogen and nitrogen.

The catalysts used in the method of the invention the cost of which can be substantially less than the cost of prior art catalysts for such processes, are beiieved to operate by weakening or breaking the chemical bonds of nitrogen and hydrogen molecules.

he reaction rates attainable with the catalysts referred to at any given temperature and pressure are at least an order of magnitude (factor of 10) and sometimes several orders of magnitude greater than reaction rates experienced with prior art catalysts and processes.

Consequently, the cost of ammonia production is substantially lowered.

Furthermore, equipment requirements are much less stringent and less costly by reason of the lower operating pressures that can be employed at the process temperature selected. In addition, the catalyst itself is free from many problems commonly encounted with prior art catalysts such as sintering and physical attrition. To the contrary, it has been found that the catalytic activity of the new catalyst employed in the present invention increases with aging in the presence of the hydrogen component of the reactant atmosphere. This enhancement of activity is the result of cracks and fissures formed in the catalyst particles, both microscopically (surface cracks and macroscopically breakage into smaller particles), with the attendant increase in the active surface area of the catalyst. There is also much less poisoning or deactivation of the catalyst through smothering of adsorbing sites by the absorption of impurities that normally exist in commercially available nitrogen and hydrogen reactants. With proper control of the process conditions, activation of the catalyst bed can continue simultaneously with the production reaction.

The new catalysts of the present invention comprise an alloy capable of forming unstable hydrides. Such an alloy includes at least one metal selected from IB, IIB, IIIB, IVB, VB, VIB, VIIB and VIII of the periodic table. (Handbook of Chemistry and Physics, 2nd Edt. 1970. Pub. Van Norstrand Reinhold & Co.). Preferably, the alloy composition will contain a phase which can form unstable metal hydrides, and a second phase which is a metal which can be at least partially hydrided. It is theorized that if a catalyst alloy contains two phases, one capable of initiating hydrogen spillover and another which only has hydrogenation activity when supplied with hydrogen atoms, then occurrence of hydrogen spillover between these phases will increase the hydrogenation activity of the catalyst.

The preferred catalysts of the present invention are alloys which contain at least two phases. The first phase may consist of an intermetallic compound containing Ti, Zr or V which compound is an unstable hydride former. The second phase is free Ti, Zr or V. Particularly preferred compounds include TiCo and TixCr in which x > 1 as well as other titanium alloys that form unstable hydrides and have excess Ti; Zr Fey, ZrCo ZrCr where y < 2, as well as other Zr alloys that form unstable hydrides and have excess Zr; and TiFe ,M1, where M is a transition metal such as Cr, Mn, Co, Ni, Mo and Cu. Further examples include: MoTi" MoTi, TiV, TiV2, TiV9, Ti3V, Ti,V, Ti V Ti9Zr, Tier, TiZr9, MoTiFe, Ti ze2Mo, TiFe2V, TiFezZr, NiTi1s, NiTi20, NiTi25, TiCo, Ti1 sCo, Ti25Co, TiCr, Ti, sCr2, TiCr2, TiMo, Ti2Mo, TiMn, Ti2Mn, MoTiBa, VTiFe, VTiFeBa, VTiBa, ZrFeV, ZrNi, ZrNi2, Zr2Ni, Zr3Ni, ZrMol ,,, ZrMo2, ZrMo2.29 ZrFe,,, ZrFe2, ZrFe2,2 ZrCo,,, ZrCo2, ZrCo22, ZrMnl.", ZrMn2, ZrMn2.2 MoV, MoV2, MoV2Fe, and VBa. Any of the above alloys with additional excess Zr, Ti, and/or V are also preferred.

Throughout the remainder of the present specification, the catalysts comprising of titanium hydrides, in association with a binary alloy or iron and titanium, will be discussed, but it is to be understood that any of the catalyst compositions of the present invention, as set forth hereinabove, may be used.

For example, where titanium hydrides are specified hereinbelow, it is to be understood that zirconium or vanadium hydrides may be used in their place and similarly, the other binary alloys set forth hereinabove may be used in place of FeTi.

The catalyst described below contain, besides the alloy, titanium hydrides, primarily titanium dihydride which is a stable compound. More definitively, it has been found that titanium metal combines with hydrogen to form an extremely active catalyst for the production of ammonia from gaseous nitrogen and hydrogen.

Particularly, as described and claimed in out copending U.K. Patent Application Nos. 20830/78 (Serial No. 1604263) and 20831/78 (Serial No. 1604264), the reaction appears to be enhanced when the hydrided titanium is associated with a binary alloy of iron and titanium, particularly as a hydrided titanium phase in the iron-titanium alloy (which itself may be converted to iron titanium hydride). The alloy itself may also be used as a catalyst under some circumstances.

The specific alloy associated with the hydrided titanium in the preferred embodiment described in the aforementioned applications has a titanium to iron mole ratio of 1.0 and is available from the International Nickel Company.

This alloy is further described in a book entitled Constitution of Binary Alloys, First Supplement, as authored by R.P. Elliot and published by McGraw-Hill, New York, New York, 1965, and in a paper of Reilly, et al.

referenced more fully below. This alloy is formed from relatively pure iron and titanium by a melting process requiring temperatures in the range of 1500 to 1900 C. Although this alloy can be made from commercial grade iron, the purity of electrolytic iron is preferred.

The combined catalyst may be made by the same process as the binary alloy since an alloyed mixture containing more than 1 mole of titanium for each mole of iron will produce a composition consisting of two phases, namely, the intermetallic compound having a titanium to iron mole ratio of one and pure titanium, the latter being converted to titanium dihydride in an initial hydriding step. Titanium dihyide is a stable compound which remains as such throughout the production process and is continually activated (decontaminated) as long as it is in contact with significant amounts of hydrogen.

The following range of compositions for iron-titaniumaloys, only some of which contain associated free hydrided titanium, have been determined to be catalytically active for combining molecular hydrogen and nitrogen to produce ammonia and are listed in the order of increasing catalytic activity for this reaction: 1/2 mole Ti: 1 mole Fe, 1 mole Ti:l mole Fe, 1.1 mole Tri: 1 mole Fe, 2 moles Ti: I mole Fe, and 3 moles Tri: 1 mole Fe. Thus, the greater the amount of titanium associated with the alloy, the greater the reaction rate and yield for ammonia synthesis. Combined catalyst compositions richer in titanium dihydride than a mole ratio of 2 to 1 do not appear to be commercially available at the present time due to the difficulties experienced in the manufacture of corresponding iron-titanium alloys. However, it is believed that even higher catalytic reaction rates may be attainable if greater quantities of free titanium are associated with the irontitanium compound or other alloys.

The catalysts of the invention are active for ammonia formation at all temperatures around and above l800C. and at all pressures at and above atmospheric, the higher the temperature or the pressure or both, the greater the reaction rates attainable. The upper limits of temperature and pressure for production reactions are set only by the physical design parameter of the equipment employed.

Although hydrided titanium appears to exhibit increased catalytic activity when alloyed with or otherwlse associated with the iron-titanium bi-metallic compound, and although the bi-metallic compound shows some catalytic activity itself, the catalytically more active component is considered to be titanium dihydride. For further information on the independent properties of each of these compounds, particular reference is made to the article entitled "Formation and Properties of Iron Titanium Hydride" by J. J. Reilly and R. H.

Wiswall, Jr., of Brookhaven National Laboratory, published in Inorganic Chemistry, viol. 13, No. 1, 1974, at pages 21W222, and to the book by W. M. Mueller et al., entitled Metal Hydrides as published by Academic Press, New York, New York., 1968.

The titanium metal, either alone or as associated with the bi-metallic alloy, is first reacted with hydrogen to clean the material and form titanium dihydride, preferably as a separate series of steps prior to use of this compound as the catalyst in the ammonia production reaction. The hydridin steps first remove reaction inhibiting oxides and other impurities on the catalyst particles and then produce titanium dihydride as well as iron titanium hydrides when associated with the bi-metallic alloy.

The catalyst composition should be ordered from the manufacturer in particulate form of relatively small size (less than 16 mesh, U.S. size). As received, the particles are coated with an oxide layer. In this state, the metal will not form its hydride, which is the compound active as a catalyst. These activation steps also remove other surface and internal impurities such as carbon and nitrogen compounds and absorbed gases other than nitrogen and hydrogen. Initial cleaning of the particles and formation of the catalyst is preferably accomplished by subjecting a particulate bed of titanium to hydrogen gas at tem,peratures in the range of 200"C. to 400 C. and pressures in the range of 150 to 200 psia for an extended period. The reactant gases are then passed through the bed for the production reaction.

Where the catalyst is associated with the iron-titanium I to I binary alloy, the initial immersion is followed by out-gassing and then alternately pressurizing the catalyst bed with hydrogen at much higher pressure and outgassing the hydrogen so that the alloy is successively hydrided and dehydrided.

This process breaks up the supported catalyst particles into smaller particles and also produces multiple cracks in the surface of each individual particle, thereby greatly increasing the reactive surface area of the catalyst bed. The dehydriding cycle is carried out at approximately 200 C. or greater, preferably about 4000 C. with outgassing by pure helium purging at approximately atmospheric pressure, and the hydriding cycle is generally carried out at ambient temperature (20 to 25"C.) and 1,000 psia. Alternatively, and preferably due to the possibility of impurites in the helium, outgassing may be accomplished by drawing a slight vacuum of 1 or 2 inches of water in the reactor vessel containing the catalyst bed. This size reduction process is preferably continued until the average particle size is 200 mesh or less. (U.S. Sieve) Following the steps for preparation of the catalyst, a gaseous feed stream comprised of nitrogen and hydrogen is continually passed over the catalyst bed in a production reaction carried out at a temperature and pressure selected for the highest or other desired level of yield of ammonia in the product. Although significant yields of the product are attainable at temperatures as low as 1800C. and pressures as low as atmospheric, commercial yields generally require higher temperatures in the range of 275 to 3250C. and higher pressures in the range of 500 to 1500 psia (30 to 100 atm.).

Even greater temperatures and pressures would further increase reaction rates, but such operations are limited at the present time by restrictions imposed by design parameters of the process equipment available. In addition, higher temperatures could also have adverse effects on the ammonia synthesis reaction by increasing the rate of ammonia dissociation into nitrogen and hydrogen to unacceptable levels. In other words, excessively high temperatures could reverse the synthesis reaction because of thermodynamic limitations arising from the exothermic nature of the reaction. However, at temperatures and pressures within the range specified above, yields approaching 100% of theoretical are attainable.

When the partial pressure of the hydrogen in the reactor vessel is equal to or greater than the equilibrium dissociation pressure of iron titanium hydride and the catalyst is associated with the 1 to 1 binary alloy, that alloy phase will be converted to iron titanium hydride which is a stable compound only under such conditions.

When the catalyst bed is outgassed, this hydride compound reverts to the binary alloy, aiding in the break up of the particles.

However, it is believed that the catalytic reaction itself would not be inhibited by the hydrided form of the inter-metallic compound if such pressures were employed in the production process. The partial pressure of hydrogen to be used at a given temperature to achieve the hydrided state of the alloy can be determined from the equilibrium dissociation pressure of iron titanium hydride at that temperature, the latter relationship being set forth in the literature. For the specific temperature and pressure relationship utilized in this invention, see particularly the article entitled "Formation and Properties of Iron Titanium Hydride" referenced above.

The preferred processes for both making the combined form of the catalyst and subsequently producing ammonia will now be described. A catalyst bed consisting of as purchased particles of titanium and iron alloyed at a ratio of 3 to 1 is charged into a conventional reactor vessel, such as that presently used in the production of ammonia with prior art catalysts. This composition contains a titanium metal phase associated with an iron-titanium 1 to I compound phase with two moles of free titanium for every mole of carrier compound.

The reactor is then heated and outgassed by drawing a vacuum at 4000 C. for approximately six to eight hours to desorb and expel contaminant gases.

Following the outgassing and while maintaining the vessel at 4000C., the reactor is pressurized with hydrogen to 200 psia and maintained at temperature and pressure until the formation of titanium dihydride has been completed, which requires approximately four to six hours. The catalyst forming process also removes the oxide films and other absorbed impurities from the catalyst so as to enhance diffusion of hydrogen into the alloy, as well as to permit adsorption of the reactant gases during the production reaction. The initial hydrogen treatment is best carried out with the hydrogen gas confined to the reactor vessel in a static condition, instead of utilizing any type of flow regime with pure hydrogen which could result in excessive temperatures from the hydride formation reaction. Upon removal of the oxide films, titanium dihydride begins forming in the presence of the hydrogen with the evolution of sufficient heat to raise the reactor temperature significantly. When the reactor temperature levels off and starts to fall back to that maintained externally (400 C.), formation of the titanium dihydride catalyst is complete.

A process to enhance the activity of the supported catalyst thus formed is then commenced by allowing the reactor to cool to near ambient temperature (20 to 250C.) while continuously drawing a vacuum over the catalyst bed to outgas the hydrogen.

Upon reaching ambient temperature, the reactor is then pressurized again with hydrogen to a pressure above the equilibrium dissociation pressure of the hydride form of the iron-titanium alloy at the prevailing temperature. A hydrogen pressure of 1,000 psia is sufficient to accomplish this hydriding step at the usual ambient temperatures encountered. After such pressurization has been maintained for approximately one-half hour, the reactor is again heated to approximately 4000C. and immediately allowed to cool upon reaching the temperature while maintaining a slight vacuum throughout the heating and cooling cycle to outgas the hydrogen in a dehydriding step. These hydriding and dehydriding cycles break the catalyst particles without destroying the integral bond between the titanium dihydride phase and the intermetallic compound phase, and are preferably repeated until the desired particle size is attained. This usually requires three to four cycles, depending on the original particle size and the dimensions of the catalyst bed. The catalyst is then ready for the production reaction.

Following the last dehydriding step of the size reduction process, the reactor is heated to 3000 C. and pressurized with hydrogen to 80 atm. A reactant composition comprised of 1 mole of nitrogen to 3 moles of hydrogen is then introduced into the reactor and the product stream drawn off on a continuous basis at a flow rate determined by space velocity (ratio of feed rate to total weight of catalyst) which should not exceed 500 cubic meters per hour per ton of catalyst as determined with reference to standard conditions of temperature and pressure. A variety of other feed compositions, such as hydrogen to nitrogen mole ratios of 2 to 1 or 1 to 1 may also be employed within the scope of this invention. However, it is desirable to always maintain sufficient hydrogen in the feed stream to achieve continuous activation of the catalyst.

Higher space velocities and corresponding feed rates are also possible, but may give tower yields and would require higher pumping energy inputs.

Nevertheless, faster throughput and lower yields may be more economical overall depending on the optimum parameters of the separation and recycle equipment employed to handle the product downstream of the reactor vessel.

The product stream leaving the catalyst bed will contain the nitrogen and hydrogen reactants and the ammonia product. If desired, the ammonia can be separated from the product stream in a conventional fashion and the reactants recycled to the reactor vessel. One such separation scheme involves cooling the product stream to a temperature in the range of 25"C. to 100 C., which is usually low enough to totally condense the ammonia product and then passing the stream through a condensate separator and recycling the gas effluent consisting of the uncombined nitrogen and hydrogen back to the reactor.

Actual condensate temperature in this case would be determined by the process economics, taking into account the cooling, heating and pumping operations required, as well as the partial pressure of the ammonia in the product stream. Some of the ammonia product might also be recycled, depending of course on the parameters of the separation equipment.

Although but a single embodiment of the present invention has been described, other embodiments and variations will occur to those skilled in the art. For example, the hydrogen and nitrogen reactant may be contacted with the catalyst while in physical states other than a gas. Thus, aqueous solutions and other carriers containing free nitrogen and hydrogen molecules may be passed over or in contact with the catalyst and the hydrogen and nitrogen thereby reacted to produce ammonia. All such processes are within the contemplation of the present invention.

It is possible, of course, to use various combinations of temperature and pressure, and such uses are intended to be covered by the present invention. It is also possible to combine the unstable hydride-forming intermetallic compounds of the present invention with known catalytically active metals for this reaction such as ruthenium and osmium, either in the form of mixtures or multi-component (e.g. ternary, quaternary or higher) alloys, or to support the catalytic compositions on an inert carrier material or other substrate.

Furthermore, many other changes in the process steps are possible and such changes are within the scope of the disclosure. By way of further example, activation of the catalyst can be achieved, although at a slower rate, by exposure to the hydrogen in the feed stream itself, particularly if the product stream was to be recycled until the desired level of product was achieved.

Our copending UK Patent Applications No. 20830/78 (Serial No. 1604263) and 20831/78 (Serial No. 1604264) described and claim a method of making ammonia which comprises contacting nitrogen and hydrogen at synthesis conditions with a catalyst comprising an alloy of titanium and iron. In Application No. 20830/78 (Serial No. 1604263) the mole ratio of titanium to iron is greater than 1.0 and in Application No. 20831/78 (Serial No. 1604264) the mole ratio of titanium to iron is greater than 0.5 but not greater than 1.0.

In the present application the use of catalyst comprising an alloy of iron and titanium as the sole metals of the alloy, the mole ratio of titanium to iron being greater than 0.5 is hereby disclaimed.

Subject to the Foregoing Disclaimer WHAT I CLAIM IS: 1. A method of making ammonia which comprises contacting nitrogen and hydrogen in synthesis proportions at synthesis conditions with a catalyst comprising an alloy capable of forming unstable hydrides.

2. A method according to Claim 1 wherein said alloy includes at least one metal selected from groups IB, IIB, IIIB, IVB, VB, VIB, VIIB and VIII of the periodic table.

3. A method according to Claim 1 or Claim 2 in which the catalyst contains a phase which can form unstable metal ydrides and a second phase which is a metal which can at least be partially hydrided.

4. A method according to Claim 3 in which the first phase consists of an intermetallic compound containing Ti, Zr or V which compound is capable of forming unstable hydride and the second phase is free Ti, Zr or V.

5. A method according to Claim 1 in which the catalyst comprises a titanium alloy containing excess Ti and capable of forming unstable hydride.

6. A method according to Claim 5 in which the catalyst comprises a compound of the for

Claims (18)

**WARNING** start of CLMS field may overlap end of DESC **. present invention has been described, other embodiments and variations will occur to those skilled in the art. For example, the hydrogen and nitrogen reactant may be contacted with the catalyst while in physical states other than a gas. Thus, aqueous solutions and other carriers containing free nitrogen and hydrogen molecules may be passed over or in contact with the catalyst and the hydrogen and nitrogen thereby reacted to produce ammonia. All such processes are within the contemplation of the present invention. It is possible, of course, to use various combinations of temperature and pressure, and such uses are intended to be covered by the present invention. It is also possible to combine the unstable hydride-forming intermetallic compounds of the present invention with known catalytically active metals for this reaction such as ruthenium and osmium, either in the form of mixtures or multi-component (e.g. ternary, quaternary or higher) alloys, or to support the catalytic compositions on an inert carrier material or other substrate. Furthermore, many other changes in the process steps are possible and such changes are within the scope of the disclosure. By way of further example, activation of the catalyst can be achieved, although at a slower rate, by exposure to the hydrogen in the feed stream itself, particularly if the product stream was to be recycled until the desired level of product was achieved. Our copending UK Patent Applications No. 20830/78 (Serial No. 1604263) and 20831/78 (Serial No. 1604264) described and claim a method of making ammonia which comprises contacting nitrogen and hydrogen at synthesis conditions with a catalyst comprising an alloy of titanium and iron. In Application No. 20830/78 (Serial No. 1604263) the mole ratio of titanium to iron is greater than 1.0 and in Application No. 20831/78 (Serial No. 1604264) the mole ratio of titanium to iron is greater than 0.5 but not greater than 1.0. In the present application the use of catalyst comprising an alloy of iron and titanium as the sole metals of the alloy, the mole ratio of titanium to iron being greater than 0.5 is hereby disclaimed. Subject to the Foregoing Disclaimer WHAT I CLAIM IS:

1. A method of making ammonia which comprises contacting nitrogen and hydrogen in synthesis proportions at synthesis conditions with a catalyst comprising an alloy capable of forming unstable hydrides.

2. A method according to Claim 1 wherein said alloy includes at least one metal selected from groups IB, IIB, IIIB, IVB, VB, VIB, VIIB and VIII of the periodic table.

3. A method according to Claim 1 or Claim 2 in which the catalyst contains a phase which can form unstable metal ydrides and a second phase which is a metal which can at least be partially hydrided.

4. A method according to Claim 3 in which the first phase consists of an intermetallic compound containing Ti, Zr or V which compound is capable of forming unstable hydride and the second phase is free Ti, Zr or V.

5. A method according to Claim 1 in which the catalyst comprises a titanium alloy containing excess Ti and capable of forming unstable hydride.

6. A method according to Claim 5 in which the catalyst comprises a compound of the formula TiXCo or Ti,Cr in which x > l.

7. A method according to Claim 1 in which the catalyst comprises a zirconium alloy containing excess zirconium and capable of forming unstable hydride.

8. A method according to Claim 7 in which the catalyst comprises a compound of the formula ZrFe,, ZrCo, or ZrCr, where y < 2.

9. A method according to Claim 1 wherein the catalyst comprises a compound of the formula TiFe0,MI7 where M is a transitional metal.

10. A method according to Claim 1 wherein the catalyst comprises a compound of the formula MoTi4, MoTi, TiV, TiV2, Tits, Ti3V, Ti7V, TigV, Tiger, TiZr, TiZr9 MoTiFe TiFe Mo, TiFe2V, TiFe2Zr, NiEIt5, Nii,, Nli25, TiCo, Ti Co Ti Co, TiCr, Ti,ffiCr2, TiCr2, TiMo, Ti2Mo, TiMn, Ti2Mn, MoTiBa, VTiFe, VTiFeBa, VTiBa, ZrFeV, ZrNi, ZrNi2, Zr2 Ni, Zr3Ni, ZrMo,,, ZrMo2, ZrMo2,2, ZrFel,s, ZrFe2, ZrFe2 2, ZrCol " ZiCo2, ZrCo2.2, ZrMn,,, ZrMn2, ZrMn22, MoV, MoV2, MoV2Fe, and VBa.

11. A method according to any preceding claim wherein the catalyst is activated by exposing the alloy to hydrogen at hydriding conditions to hydride at least a portion of the alloy.

12. A method according to Claim 11, wherein said hydriding conditions include exposing said alloy to hydrogen at an elevated temperature and pressure effective to remove oxides from exposed surfaces thereof.

13. A method according to any preceding claim wherein said synthesis conditions include a catalyst temperature of at least 275"C and said reactants at a total pressure of at least 30 atm.

14. A method according to any preceding claim wherein the proportions of hydrogen to the proportion of nitrogen is equal to a mole ratio of 3 to 1 and wherein said

synthesis conditions include contacting the catalyst with a gaseous mixture of said reactants at a rate not exceeding 500 cubic meters of gas per hour per ton of catalyst as determined with reference to standard temperature and pressure conditions.

15. A method according to any preceding claim, wherein preparation of the catalyst includes the steps of exposing granules of said alloy to hydrogen at a pressure at least equal to the equilibrium dissociation pressure of iron titanium hydride at the prevailing temperature of the granules to ydride the alloy, and outgassing said granules at dehydriding conditions effective to break said granules into smaller particles.

16. A method according to Claim 15, wherein exposure of said granules to hydrogen includes a first step of exposing said granules to gaseous hydrogen under conditions effective to remove oxides from exposed surfaces of said alloy.

17. A method of making ammonia according to Claim I and substantially as hereinbefore described.

18. Ammonia whenever produced by a method according to any preceding claim.