GB2573125A

GB2573125A - Catalyst

Info

Publication number: GB2573125A
Application number: GB1806687.8A
Authority: GB
Inventors: Tao Shanwen; Humphreys John
Original assignee: University of Warwick
Current assignee: University of Warwick
Priority date: 2018-04-24
Filing date: 2018-04-24
Publication date: 2019-10-30
Also published as: GB201806687D0

Abstract

The invention relates to anion vacant lattices as a co-catalyst for augmenting the activity of existing Haber-Bosch process catalysts. The Haber-Bosch catalyst may be Fe, Co, Ni, Ru or combinations thereof. The anion vacant lattice may be doped to promote anion vacancies, and the anion vacancy may be an oxygen or nitrogen vacant lattice. The oxygen lattice can be CeO2, BaZrO3 or combinations thereof. The oxygen vacant lattice may be CeaMbO2-δ, where M has a valence less than 4, and “a” and “b” are independently in the range of 0.05 to 0.95, and together sum to 1, such as Ce0.8Sm0.2O2-δ. Alternatively, the oxygen lattice can have the formula BaZrxCeyYzO3-δ, such as BaZr0.1Ce0.7Y0.2O3-δ. Where the anion vacant lattice is nitrogen vacant, the lattice may be a zirconium nitride with the composition ZrN0.7. The anion vacant lattice may be present in the composition in the range of 5 to 20 wt% of the total composition. Further aspects relate to a catalyst cartridge for a Haber-process, use of the composition for production of ammonia, and a Haber-Bosch process for producing ammonia comprising exposing the composition to a mixture of nitrogen and hydrogen gas.

Description

CATALYST

FIELD OF INVENTION

The invention relates to catalysts for the Haber-Bosch process. In particular, catalytic compositions, cartridges comprising said compositions, the use of said compositions in catalysing the production of ammonia in the Haber-Bosch process, and a Haber-Bosch process wherein said composition is provided as a catalyst.

BACKGROUND

The Haber-Bosch process is one of the most important chemical reactions discovered in the 20th century. Ammonia, the foundation of nearly all chemically useful nitrogen-containing compounds, is produced from a mixture of hydrogen gas and relatively inert nitrogen gas by means of a metal catalyst. The importance of the Haber-Bosch process is underlined by the Nobel Prizes in chemistry awarded to both its pioneers after whom the process is named.

Hydrogen gas and nitrogen gas are combined in a pressurised vessel and heated. In the presence of a suitable catalyst, the hydrogen and nitrogen molecules react at the surface of the catalyst to from ammonia which is then desorbed from the catalyst.

The precise mechanism by which the reaction proceeds is not completely known but it is believed, without being bound by theory that hydrogen gas become adsorbed on the catalyst surface and forms highly reactive hydrogen species that are more capable of reacting with nitrogen gas molecules.

Numerous catalysts have been investigated and many modifications to the technique have been proposed over the last 100 years. For example, co-catalytic materials have been tested in combination with traditional Haber-Bosch catalysts in an attempt to augment the catalytic activity. Examples include K2O, CaO, CS2O, and AI2O3. Various systems have also been proposed to maximise the surface area of catalyst materials to increase reaction rates.

Attempts have been made to move away from the conventional Haber-Bosch processes because maintaining continuous, high temperature, high pressure reaction conditions is expensive. One technique that has been explored is the electrochemical production of ammonia, such as disclosed in, "Ammonia synthesis at atmospheric pressure in a BaCeo.2Zro.7Yo.iC>2.9 solid electrolyte celt'·, Vasileiou, E. et al.; Solid State Ionics 275 (2015) 110-116. These processes are advantageous in some senses as they can be conducted at lower pressures and temperatures as the electrochemical aspect of the system helps drive the reaction. However, such systems are difficult to scale up as compared to Haber-Bosch processes. Moreover, a significant proportion of the existing infrastructure for producing ammonia is adapted for Haber-Bosch processes.

Given the expensive operating costs, there is demand for improved catalyst materials to allow reactions to proceed at comparable rates under milder conditions and increase the rate of reaction under comparable conditions.

The invention is intended to address or at least ameliorate these issues.

SUMMARY OF INVENTION

There is provided in a first aspect of the invention, a composition for the catalysis of a Haber-Bosch process, the composition comprising an anion vacant lattice and a Haber-Bosch catalyst.

The term "Haber-Bosch process" is intended to refer to the production of ammonia from a mixture of both hydrogen and nitrogen gases in the presence a heterogeneous catalyst, wherein the hydrogen and nitrogen react together on the surface of the catalyst. In other words, processes akin to those based on the reaction pioneered by Fritz Haber and Carl Bosch. This process is typically conducted at high temperatures and pressure that would be familiar to a person skilled in the art. For instance, the term "Haber-Bosch process" is not considered to encompass the electrochemical synthesis of ammonia as the hydrogen and nitrogen sources are provided in separate chambers and the process is believed to occur via a completely different mechanism, requiring among other things the diffusion of active intermediate species through an electrode.

The term "Haber-Bosch catalyst" is intended to refer to any material that catalyses the production of ammonia in a Haber-Bosch process. Historically, many different materials were used as catalysts (even osmium and uranium were at one time considered as effective catalysts). Subsequent research revealed the effectiveness of other more readily available materials such as cobalt, iron, nickel and ruthenium. It is believed that these materials function well as catalysts for the Haber-Bosch process because they adsorb hydrogen gas and promote the formation of reactive hydrogen species. It is believed that these reactive hydrogen species are what allow the formation of ammonia to happen quickly. Accordingly, a "Haber-Bosch catalyst" as referred to herein is intended to encompass all materials that operate in this capacity.

In order to be suitable as a catalyst in the Haber-Bosch, the composition must remain sufficiently stable across the range of conditions that the process operates. Typically, the Haber-Bosch process is conducted at temperatures as high as 700°C and in excess of 20 MPa of pressure.

The term "anion vacant lattice" is intended to describe a material with a crystal structure comprising anions where some of those anions are missing so as to create anion vacancies. This is chiefly achieved using doping. Materials comprising oxygen and nitrogen anions are preferred and hence oxygen and nitrogen vacant lattices are typically employed. The material can be in either crystalline or amorphous state. The terms "oxygen vacant lattice" or "nitrogen vacant lattice" are intended to describe a crystal lattice having oxygen or nitrogen respectively as a key component of the lattice structure and which, either inherently or due to exposure to certain reaction conditions, is missing oxygen or nitrogen ions from its structure so as to leave vacancies within the lattice (having dimensions comparable to an oxygen and nitrogen ion respectively). There is no particular restriction on the type of lattice used in the present invention. The material may also be in an amorphous state. The lattice may be any of the 7 general types of lattice: triclinic, monoclinic, orthorhombic, tetragonal, trigonal, hexagonal, and cubic. Typically, the lattice may be orthorhombic, tetragonal, hexagonal or cubic. Often, the lattice will be cubic or pseudo-cubic. Typical examples of crystal structures used in the invention include perovskites and fluorites. The anion vacant lattice acts as a co-catalyst, augmenting the rate of reaction in combination with Haber-Bosch catalysts.

The inventors have surprisingly found that lattices having anion vacancies function very well as co-catalysts to conventional Haber-Bosch catalysts, leading to significant improvement in catalyst activity compared with conventional catalysts. It is believed that nitrogen gas molecules entering these vacancies will, when in close proximity to said environment, experience a weakening of the triple bond between the two nitrogen atoms resulting in an increased tendency of said nitrogen molecule to react with active hydrogen species on the surface of the same catalyst composition. The anions within the anion vacant lattices are not particularly limited, by are usually selected from fluorine, chlorine, bromine, iodine, sulphur, selenium, oxygen, nitrogen or combinations thereof. Most typically, the anions in the anion vacant lattices are oxygen and/or nitrogen.

It is typically the case that the composition is configured for catalysis of a Haber-Bosch process. The Haber-Bosch process is a heterogeneous reaction where gases adsorb onto a solid catalyst surface, react and then desorb. Accordingly, the composition is typically formulated for this purpose. This may include providing a minimum surface area of the solid composition so as to ensure efficient reaction rate. For instance, the composition may be provided as: a powder, a coating on a high surface area support; a coating on the supporting particles; impregnated within a porous medium; or a combination thereof.

Whilst there is no particular restriction on the choice of Haber-Bosch catalyst, it is typically the case that the Haber-Bosch catalyst comprises a metal compound selected from the group consisting of: Co, Ni, Fe, Ru, or combinations thereof. More typically, the metal compound is Fe, Ru, or combinations thereof and even more typically, the metal compound is Fe. More typically still, the Haber-Bosch catalyst is an iron oxide (e.g. Fe2C>3). Reference to "Co", "Ni", "Fe", "Ru" or other Haber-Bosch catalyst materials is intended to encompass compounds comprising those elements, such as oxides or alloys, as well as their elemental forms. As will be appreciated, the high temperatures and hydrogen concentrations in Haber-Bosch processes means certain catalysts are liable to be reduced and so the material introduced into the system may change in situ.

Typically, the anion vacancies in the anion vacant lattice are created by doping a parent anion lattice (e.g. an oxide or a nitride). Some crystal lattices, when heated or pressurised during a Haber-Bosch process, naturally lose anions (such as oxygen or nitrogen) from their structure, thereby forming vacancies in situ. However, in order to assist this process and/or to create or maximise the number of anion vacancies, dopant ions can be used to create a charge mismatch thereby introducing vacancies into predominantly regular lattices. This is also advantageous not only because it increases the number of vacancies but because (depending on the size of the charge mismatch) it can increase the magnitude of the effect felt by a nitrogen triple bond within the vacancy. The choice of dopant (either relatively electron rich or relatively electron poor) can change the character of the environment surrounding the anion vacancy, in particular the magnitude of the influence upon the nitrogen triple bond. Accordingly, doping allows tailored environments to be created for different scenarios.

Whilst there is no particular restriction on the choice of the oxygen vacant lattice to be doped, the oxygen vacant lattice is typically an oxide. Most typically, the oxygen vacant lattice is a fluorite or perovskite structure such as ceria, zirconia, aluminium oxide, magnesium oxide, iron oxide or combination thereof (all of which may be doped). Of these, ceria and iron oxides are typically the category of materials used most often. Typical examples of suitable oxygen vacant lattice materials include, but are not limited to: BaZr03, CaZr03, CaAI03, Ce02, MgO, Zr02, BaCe03, SrZr03, LnCe03, LnZr03, SrCe03, Sri.sFe205, B12O3, Sn02, LnFe03, LnCo03, SrCe03, SrFei20i9-12Sr2B20s or combinations thereof (wherein "Ln" represents lanthanides). SrFei20i9-12Sr2B20s is often used and may be in an amorphous form such as an amorphous glass. Typical examples of nitrogen vacant lattices include nitrogen deficient zirconium nitride (such as ZrNo.7). As will be appreciated by the person skilled in the art, the choice of dopant used depends upon the lattice to which it is applied and the character of the environment that is desired. Accordingly, each of the above mentioned materials can be doped to replace one or more of the elements contained therein.

It is often the case that the oxygen vacant lattice, to which a dopant may be added, is selected from: CeCh, BaZrCh, B12O3, SnCh and Sri.8Fe20s; and more typically Ce02, BaZrCh, or combinations thereof. Typically, the oxygen vacant lattice is CeCh. These materials have been found to be particularly effective starting materials for creating oxygen vacant lattices. This is particularly surprising as they have very different lattice parameters.

The amount of dopant included within the anion vacant lattice will naturally vary depending upon the number of vacancies required and the ability of the material to retain it general structure. Typically, the dopant present within an anion vacant lattice is a minority component i.e. there is more of the material being replaced than there is dopant replacing it. Usually, the dopant is present in an amount in the range 1 mol% to 30 mol% of the total anion vacant lattice, sometimes in an amount in the range 5 mol% to 20 mol% of the total anion vacant lattice and often in the range 10 mol% to 15 mol% of the total anion vacant lattice. A typical example of an oxygen vacant lattice used in the invention is shown in formula I;

(formula I) wherein; "a" represents a value between 0 and 0.2 and each of "x", "y" and "z" are independently in the range 0.01 to 0.99, typically 0.05 to 0.95, with the proviso that "x", "y" and "z" together sum to 1. The inventors have found that cerium and yttrium doped barium zirconium oxides (BZCYO) are not only stable at standard Haber-Bosch process operating conditions but also perform very well compared to existing catalysts on the market. Typically, each of "x", "y" and "z" are independently in the range 0.1 to 0.8 and most typically the oxygen vacant lattice comprises BaZro.iCeo.7Yo.203-5, where δ effectively symbolises the number of moles of oxygen vacancy.

In another embodiment of the invention, the oxygen vacant lattice may be a compound according to formula II;

(formula II) wherein, M is an element with a valence lower than 4, typically a lanthanide or rare earth element other than cerium, such as Sm, Eu, Gd or combinations thereof, "a" and "b" are independently in the range 0.05 to 0.95, with the proviso that "a" and "b" together sum to 1. Typically, M is Sm. The inventors have found that samarium doped cerium oxide shows good results in promoting the Haber-Bosch process in conjunction with a suitable Haber-Bosch catalyst. Often, each of "a" and "b" are independently in the range 0.1 to 0.8 and it may be the case that the oxygen vacant lattice comprises Ceo.8Smo.202-5, where δ effectively symbolises the number of moles of oxygen vacancy.

Both these materials have been found to be stable under standard Haber-Bosch process conditions which is particularly advantageous because, in industry, such process are typically run on a continuous basis. Accordingly, catalyst longevity is important to prevent regular starting and stopping of the process.

As explained above, it is believed that the anion vacant lattice activates nitrogen molecules so that they are more prone to react with active hydrogen species. However, the catalyst is still required to drive the hydrogen portion of the reaction. Accordingly, it is desirable to have a balance of both the Haber-Bosch catalyst and the anion vacant lattice cocatalyst. It is typically the case that the amount of anion vacant lattice present is in the range 1 wt% to 70 wt% of the total composition. More usually, the amount of anion vacant lattice present in the composition is in the range 2 wt% to 60 wt% of the total composition, and often in the range 3 wt% to 40 wt% of the total composition. More typically, the amount of anion vacant lattice present in the composition is in the range 3 wt% to 20 wt% of the total composition, and usually in the range 3 wt% to 10 wt% of the total composition. Often the amount of anion vacant lattice present is in the range 4 wt% to 6 wt% of the total composition, most typically about 5% of the total composition.

There is also provided in a second aspect of the invention, a catalyst cartridge for a Haber-Bosch process, the cartridge comprising the composition according to the first aspect of the invention. In industrial applications of the Haber-Bosch process, the reaction is performed (typically under high pressure) within a reaction vessel. The catalyst is typically suspended within the reaction vessel in a cradle or support structure so as to ensure sufficient exposure of the mixed hydrogen and nitrogen gases to the catalyst. This also permits easy introduction and removal of the catalyst, as compared to simply pouring powder into a reactor. Accordingly, catalyst compositions are often provided in a cartridge format which can simply be inserted into a reactor prior to operation and disposed of once

the catalyst has degraded or fallen below a threshold activity. Accordingly, the term "cartridge" as used herein is intended to encompass containers configured to house and permit gaseous interaction with portions of heterogeneous catalyst held therein. The cartridges are typically adapted for easy insertion and removal from a reactor.

The composition is typically provided in the form of a powder due to the large surface area it provides. However, any large surface area arrangement or formulation for heterogeneous catalysis would be suitable (such as those described above), provided the support is stable under typical Haber-Bosch process conditions. Alternatively, the catalyst may be mixed with binders or other materials so as to form particles of a particular size and distribution. The catalyst may also be provided on a support, such as a porous support, typically having a high surface area.

There is also provided, in a third aspect of the invention, a Haber-Bosch process for producing ammonia, comprising the steps of i) providing a composition according to the first aspect of the invention and ii) exposing said composition to a mixture of nitrogen and hydrogen gas.

The conditions of the process can be varied based on the speed of reaction desired and operational requirements of the system. The skilled person would be familiar with the equilibrium process that occurs in a Haber-Bosch reaction and the importance of controlling temperature and pressure to most efficiently favour the formation of ammonia. With the present catalyst, it has been found that less energy intensive conditions are required to provide results comparable to the prior art. Accordingly, the reaction conditions of the process are typically milder than industry standard often below 600°C and below 20MPa. A typical Haber-Bosch process involves a reactor adapted to contain pressurised gas, an area within the reactor to hold the catalyst so as to ensure maximum exposure of the reagent gases thereto, and means for providing and extracting the atmosphere within the reactor. Such reactors are often equipped with external separation means to collect ammonia and return unreacted hydrogen and nitrogen to the reagent source streams. Various systems can be employed to ensure maximum heat retention through this process.

Interestingly, the inventors have found that, when the composition of the invention is used to catalyse the process, purification of the incoming hydrogen and nitrogen gas streams is not required. Accordingly, one of the advantages that the present composition offers is the ability to perform Haber-Bosch processes without the need for extensive purification of reagents. As such, it is typically the case that the hydrogen and nitrogen used in the process have a purity of greater than 90%, more typically greater than 95%, more typically greater than 97% and often greater than 98% or 99%. Substantially pure gases may also be used as impurities, whilst tolerable, may ultimately increase damage to the catalyst or system in general. The impurities are typically selected from traditional components found in air (e.g. water vapour, oxygen, carbon monoxide, carbon dioxide, noble gases, helium and the like) and particulate matter such as small metal particles or dust particles.

It is typically the case that the catalyst is prepared using a solid state reaction, precipitation, co-precipitation, ball-milling, infiltration, sol-gel processes, combustion synthesis or solvent thermal synthesis.

In a fourth aspect of the invention, there is provided a use of a composition according to the first aspect of the invention for the production of ammonia in a Haber-Bosch process.

The invention will now be described with reference to the accompanying figures and specific examples.

BRIEF DESCRIPTION OF FIGURES

Figure 1 shows XRD images of the BZCY proton conducting support and the supported Ni catalyst before and after stability test.

Figure 2 shows UV-Vis spectra of the Ni-BZCY catalyst before and after reduction.

Figure 3 shows SEM images of the unreduced catalyst (a), the reduced catalyst before stability test (b) and the reduced catalyst after stability test (c). The magnification factor was 10000.

Figure 4 shows a SEM image of the reduced catalyst before stability test with highlighted area of element mapping (a), EDS mapping for Ni (b), EDS mapping for Ba (c), EDS mapping for Zr (d), EDS mapping for Ce (e), EDS mapping for Y (f), EDS mapping for O (g)·

Figure 5 shows (a): STA analysis of dry Ni-BZCY in N2(b): STA analysis of wet Ni-BZCY in N2.

Figure 6 shows ammonia synthesis rate using a Ni-BZCY catalyst at different temperatures (120 ml. min-1, H2:N2 = 3:1).

Figure 7 shows ammonia synthesis rate using a Ni-BZCY catalyst at different flow rates (620°C, H2:N2 = 3:1).

Figure 8 shows ammonia outlet concentration at different flow rates (620°C, H2:N2 = 3:1).

Figure 9 shows ammonia synthesis rate using a Ni-BZCY catalyst at different feed mole ratios (200 ml. min 1, 620°C).

Figure 10 shows ammonia synthesis rate using 60%NiO / 40%MgO-CeC>2 catalyst at different temperatures (120 mL min'1, H2:N2 = 3:1).

Figure 11 shows ammonia synthesis rate using a Ni-BZCY catalyst over dry and wet stability tests (620°C, 200 mL min'1, H2:N2 = 3:1).

Figure 12 shows the catalytic activity of pure Fe, Fe with Ce02 (5wt%) and Fe with CeC>2 (10wt%) at a reaction pressure of 10 bar (total flow rate 80mL min'1, H2 : N2 mole ratio 3:1).

Figure 13 shows the catalytic activity of pure Fe, Fe with CeCh (5wt%) and Fe with Ce02 (10wt%) at a reaction pressure of 30 bar (total flow rate 80mL min'1, H2 : N2 mole ratio 3:1).

Figure 14 shows the catalytic activity of pure Fe and Fe with SrFei20i9-12Sr2Br40 (5wt%) at reaction pressure of 30 bar (total flow rate 80mL min'1, H2 : N2 mole ratio 3:1).

Figure 15 shows the catalytic activity of Fe catalyst with BCZY (60wt%) at various pressures (total flow rate 80mL min'1, H2 : N2 mole ratio 3:1).

Figure 16 shows the catalytic activity of Fe203 with Sri.8Fe20s (90wt% and 85%) at various pressures (total flow rate 80mL min'1, H2 : N2 mole ratio 3:1).

Figure 17 shows the catalytic activity of Fe2C>3 with ZrNo.7 (15wt%) at various pressures (total flow rate 80mL min'1, H2 : N2 mole ratio 3:1).

EXAMPLES

Example 1

Synthesis of BZCY

In order to synthesise the BaZro.iCeo.7Yo.203-5 (BZCY) perovskite catalyst support a solid state reaction was employed. Firstly stoichiometric amounts of BaCCb (99% alfa), ZrC>2 (99% alfa), Ce02 (99.5% Alfa) and Y2O3 (99.9% Alfa) were weighed and mixed using a pestle and mortar. The resulting mixture was then wet ground in isopropyl alcohol for 12 hours. After drying at 80°C the mixture was then fired at 1000°C for 3 hours with a heating and cooling rate of 5°C min-1. After this NiO (99% Alfa) was added to the BaZro.1Ceo.7Yo.2O3 powder with a weight ratio of 60% to 40% respectively. This was further wet ground in isopropyl alcohol for 12 hours. The Mg0-Ce02 support for the comparison test was prepared through a combustion synthesis in which equimolar amounts of Ce(N03)3 6H2O (99.5% alfa) and Mg(N03)2 6H2O (98% alfa) were dissolved in deionised water, citric acid (99% alfa) was then added with the mole ratio of 1:1 against total moles of metal ions. This solution was then heated on a hot plate at 200°C until the combustion was complete with the resulting powder fired at 500°C for 2 hours.

Materials characterisation

The catalyst was characterised using both X-ray Diffraction (XRD) and Scanning electron microscopy (SEM). The crystal structures were determined using a Panalytical X'Pert Pro Multi-Purpose Diffractometer (MPD) with Cu K alpha 1 radiation working at 45kV and 40mA. The SEM images were obtained with ZEISS SUPRA 55-VP operating at lOkV. Thermal gravimetry-differential scanning calorimetry (TG-DSC) analyses of pre-reduced Ni-BZCY catalysts were carried out on a NETSCH F3 thermal analyser in flowing N2 to 800°C with an N2 flowing rate of 70 ml min-1. The UV-Vis measurements were carried on a Shimadzu 3600 Spectrophotometer with integrating sphere for solid samples. The samples were mixed with BaS04 to fill in the sample holder before the measurements. The specific surface area of both the Ni-BZCY catalyst and the Ni-MgO-Ce02 catalyst was measured using a QUADRASORB SI surface area analyser. Both of the reduced samples were degassed at 350°C before carrying out surface area analysis at liquid nitrogen temperature.

Catalyst activity measurement

To measure the catalytic activity 0.48 g of catalyst was loaded into an alumina reactor and was supported in the centre on glass fibre. The catalyst was then reduced at 700°C in H2 and N2 with a total flow rate of 100 ml. min-1 and mole ratio of 9:1 H2:N2 for 4 hours. After this the temperature, total flow rate and flow rate ratio were adjusted in order to determine the optimal conditions. H2 and N2 from gas cylinders were directly used without any purification process. For the stability test, the catalyst was cooling down to room temperature under the protection of mixed H2/N2 (3:1 m/o), then N2 passing through room temperature water was passed through the catalyst for one hour. After this process, the gas was switched to mixed H2 and N2 then slowly heated to 620°C to continue the ammonia synthesis measurement.

Dilute H2SO4 (0.01 M) was used to collect any produced ammonia which was then measured using ISE Thermo Scientific Orion Star A214 ammonia meter. Both hydrogen and nitrogen were used from the cylinder with no further purification.

In order to calculate the ammonia synthesis rate the following equation was used:

(9) where [NH4+] is ammonia concentration in mol L'1, V is volume of 0.01M H2SO4 in L, t is time in hours and m is catalyst mass in grams. XRD analyses

In the XRD results shown in Figure 1 it can be seen that there are some small peaks attributed to BaC03 and Y-doped CexZn-x02 present for BaZro.iCeo.7Yo.203-5 before and after being mixed with the NiO, however, after reduction at 700°C in H2/N2 mixture (90%H2) for 4 hours, these peaks are no longer present. A possible reason is that, BaC03 and Y-doped CexZn-x02 were converted into amorphous phase during the reduction process thus cannot be detected by XRD. The XRD graphs from the catalyst before and after the stability test are identical showing that the catalyst undergoes no changes during the measurements. UV-visible observation

In order to identify the BaCCb phase, the absorbance spectra of the catalyst were measured before and after reduction to investigate whether or not BaC03 and Y-doped CexZn-x02 are converted into amorphous phases. The absorbance spectra of pure BaCCb, Zr02, Ce02 and the catalysts before and after catalysts test were measured using a Shimadzu UV-2600 with integrating sphere. The results are shown in Figure 2. It was observed that after reduction none of BaC03, zirconia or ceria can be identified in the reduced catalyst. Therefore, it was shown that an amorphous phase was not formed by

BaC03, zirconia or ceria and they are not present in the reduced catalyst. One possible reason is that, the tiny amounts of BaC03 second phase was covered by a thin layer of Ni when NiO was reduced by H2 whilst diffusion of newly formed Ni is very likely, thus BaC03 cannot be detected by either XRD or UV-Vis spectrometer. SEM observation

Figure 3a&b show the SEM pictures of unreduced NiO-BZCY catalyst. The big particles are BCZY oxide with small NiO particles homogeneously distributed in the oxide matrix. After the reduction (Fig. 3c&d), the particle size slightly became larger. Element mapping of reduced Ni-BZCY is shown in Figure 4. The distribution of Ni (Figure 4b) is homogeneous. TG-DSC analysis

In order to figure out the effects of moisture on the properties of the reduced Ni-BZCY catalyst, TG-DSC analyses were carried out for both dry and wet reduced Ni-BZCY catalysts. For the wet catalyst, reduced Ni-BZCY catalyst was exposed to flowing air through room temperature for 1 hour before the TG-DSC measurement. The TG-DSC data for both samples are shown below in Figure 5 (a) and (b) respectively. For the dry catalyst, the initial weight loss below 100°C (~ 0.12wt%) is due to the loss of absorbed water. Slight weight gain on cooling peaked at ~ 270°C (~ 0.03wt%) was observed, possibly due to the adsorption of steam by BZCY. When the wet reduced Ni-BZCY was used, the initial weight loss continued at a much higher temperature, until ~ 250°C with larger weight loss (~ 0.34wt%) indicating BZCY can hold water to a higher temperature. A shoulder weight gain peaked around 450°C was observed which is due to water uptake, which was also observed in protonic conducting oxides. On cooling, more water update (~ 0.18wt%) was observed indicating BZCY can strongly update water at lower temperature.

Effect of temperature on catalyst activity

When a constant flow rate was kept at 120ml_ min-1 and H2:N2 were flown with a mole ratio of 3:1 the effects of changing temperature could be observed, this is shown in Figure 6. It was observed that the activity increases up to a maximum of approximately 135 pmol g-1 h 1 at 620°C before dropping again. At lower temperature, the catalytic activity of the Ni-BZCY catalyst is not high enough. At a higher temperature, the produced ammonia may decompose, leading to lower production rate. In Figure 5b, a weight loss at ~ 650°C was observed due to the loss of updated water. This temperature is very close to the highest catalytic activity as shown in Figure 6. Therefore promotion effect of the BZCY could be related to the updated water at high temperature.

Effect of total flow rate on catalyst activity

The effect of total flow rate was then tested at a constant temperature of 620°C with the results shown in Figure 7. It can be seen that the activity increases with increasing flow rate. This increase in activity expected to be due solely to the increase in reactant gas, in order to confirm this ammonia outlet concentration was plotted against total gas flow rate.

As shown in Figure 8, when total flow rate is plotted against ammonia outlet concentration, it rises up to a total flow rate of 120 ml. min'1 before levelling off. This therefore shows that the total flow rate is independent of conversion rate over a value of 120 mL min-1 in our experiments and that the activity measured at this these flow rates is solely due to catalytic activity. However, at total gas flow rates less than 120 ml min'1, lower outlet ammonia concentration was observed. The possible reason is that, majority of the mixed gas passed through the edge of the glass fibre where the loading of catalyst was relatively lower thus the contact time with the catalyst was short leading to reduced ammonia formation.

Effect of feed aas ratio on catalyst activity

To determine the optimal feed ratio the gas inlet mole ratio was adjusted between 2.6 and 3.4 (H2/N2) with the optimal being detected for a value of 3.2 with a rate of approximately 320 pmol g 1 h 1 (Figure 9). All measurements were taken at 620°C with a total flow rate of 200ml/min. The reason for this deviation from the normal may be due to the proton conducting nature of the BZCY support with some of the fed H2 being ionised and transferred to the support as H+ therefore adjusting the value of H2 to N2 in the reactor closer to the stoichiometric value of 3.

Effect of temperature on catalyst activity of 60%NiQ / 40%MaO-CeQ2 In order to examine the promotion effects of the proton conducting nature of the catalyst support, a Ni catalyst supported on a non-proton conductor was tested under the same conditions. Mg0-Ce02 composite is an excellent support for Ru catalysts for ammonia synthesis. In this study, for comparison, Ni supported in MgO-CeC>2 composite was also synthesised and the catalytic activity was also investigated. This was tested over the temperature range of 600°C to 640°C with a hydrogen to nitrogen mole ratio of 3 and a total flow rate of 120 mL min'1 (Figure 10). From this it can be seen that the maximum flow rate achieved was at 620°C mirroring that results obtained for the BZCY support. However, the activity of this catalyst is around 4 times lower than the activity of the Ni catalyst when used with the BZCY proton conducting support (Figure 6). However, the catalytic activity is related to the specific surface area. The specific surface area was measured to be 0.907 m2 g'1 for the Ni-BZCY catalyst and 16.940 m2 g'1 for the Ni-MgO-CeC>2 catalyst. The specific surface area of Ni-BZCY is only 5.3% of that of Ni-MgO-CeCh but the catalytic activity to ammonia synthesis is much higher. This experiment further demonstrates that proton-conducting oxide BZCY has obvious promotion effects on ammonia synthesis.

Stability of catalytic activity in the presence of moisture

The stability of ammonia synthesis catalysts in the presence of an oxidant is a big challenge. The catalyst stability was investigated over 144 hours at 620°C with a H2/N2 mole ratio of 3 and a total flow rate of 200 mL min'1. The catalyst was found to be stable over this period with no loss of activity as can be seen in Figure 11. After this the effect of wetting the catalyst was also investigated. To perform these experiments the reactor was cooled to room temperature and wet nitrogen (lOOmL min'1) was bubbled through the reactor for 1 hour before being heated back to 620°C at a rate of 1°C min'1. This was repeated 5 times with the results shown in Figure 11. It can be seen from the results that there is a drop in activity after each cycle with an overall linear drop over the 5 cycles. The activity drops to approximately a fifth of its original value after 5 cycles going from approximately 250 pmol g1 h1 to 50 pmol g'1 h'1 with a further drop expected on further wetting cycles. This loss of activity was suspected to be caused either due to the poisoning effect of the water on the Ni catalyst after being wetted at room temperature because slight oxidation of Ni on the surface may happen as the case for Fe-based catalysts. However, upon examining the XRD patterns and SEM images of the reduced catalyst after the stability test no major changes were observed from the freshly reduced catalyst (Figures 1&3). However, a trace amount of NiO may still have been formed after treating the catalyst but is beyond the measurement limit for XRD. The oxidation and reduction cycles that the Ni catalyst undergoes in the wetted catalyst may also damage the active sites on the catalyst greatly speeding up the degradation of the catalyst that would be noticed over the catalysts life time. This effect of enhanced catalyst degradation may also be attributed to the heating and cooling cycles in-between each data point on the wetted catalyst stability test.

As well as the BZCY promoted catalyst pure Ni was also tested with a rate of 25.12 pmol g1 h1 observed at 620°C with a total flow rate of 200 mL/min and a H2/N2 ratio of 3. This is roughly ten times lower than that for the BZCY promoted catalyst when the same weight of nickel oxide was used. This therefore shows the excellent promotion effects that can be achieved using the BZCY proton conducting support.

When investigating materials as potential supports for ammonia synthesis catalysts the electro negativity of the support is a strong consideration. In this work, we have shown that another desirable effect of a support material may be its ability to conduct protons.

This promoting ability of proton conducting supports can be explained by the ionisation of the H2 gas fed to the reactor. By using a proton conducting support it is proposed that the dissociated hydrogen on the active sites is then transferred in to the support freeing the site for the adsorption of nitrogen.

Example 2

Catalyst Preparation Method i) Preparation of Fe-SrFei20i9-12Sr2B20s Catalyst 18.4538g SrCCh, 7.4196g H3BO3, 4.7907g Fe203 were mixed in agate mortar and pestle, then put in an alumina crucible, pre-fire at 700°C for 24 hours. The pre-fired powder was ground and mixed in an agate mortar then put back in the same alumina crucible and fire at 1250°C for 2 hours. The melt in the alumina crucible was quenched to a steel plate at room temperature to obtain a glass material. The obtained Fe-SrFei20i9-12Sr2B20s powder, in an amorphous, was mixed with commercial Fe203 (Alfa) with weight ratio of 9.5/0.5 for Fe203: Fe-SrFei20i9-12Sr2B20s to be used for ammonia synthesis. The loading of the composite catalysts was 300mg after reduction to Fe: Fe-SrFei20i9-12Sr2B20s. The H2 and N2 flowing rate was 60 ml min'1 and 20 ml min'1 respectively at ambient temperature and pressure. The synthesised ammonia was collected by 100ml (0.01M) H2SO4 solution and was measured by a Fisher Scientific Orion A214 ammonia meter. ii) Preparation of Fe-BaZro.iCeo.7Yo.203-5

Stoichiometric amounts of BaC03 (99% alfa), Zr02 (99% alfa), Ce02 (99.5% Alfa) and Y2O3 (99.9% Alfa) were weighed and mixed using a pestle and mortar. The resulting mixture was then wet ground in isopropyl alcohol for 12 hours. After drying at 80°C the mixture was then fired at 1000°C for 3 hours with a heating and cooling rate of 5°C min- 1. The obtained BaZro.iCeo.7Yo.203-5 powder was mixed with commercial Fe203 (Alfa) with weight ratio of 4/6 for Fe203: BaZro.iCeo.7Yo.203-5 to be used for ammonia synthesis. The loading of the composite catalysts was 300mg after reduction to Fe: BaZro.iCeo.7Yo.203-5. The H2 and N2 flowing rate was 60 ml min'1 and 20 ml min'1 respectively at ambient temperature and pressure. The synthesised ammonia was collected by 100ml (0.01M) H2SO4 solution and was measured by a Fisher Scientific Orion A214 ammonia meter. iii) Preparation of Fe-Ceo.7Smo.202-5

Dissolve 0.001 mol, 0.3487g S1TI2O3 in dilute nitric acid at a temperature around 60°C until S1TI2O3 powder was completely dissolved to form an aqueous samarium nitrate solution. 0.008 mol, 3.4738g Ce(NC>3)3.6H20 was added into the as-prepared samarium nitrate solution to form a mixed nitrate solution. The concentration in terms of total metal ions is around 0.05M. Dilute ammonia solution was slowly added into the cerium nitrate solution with stirring until a pH value reaches 10. Let the reaction to continue at room temperature for 1 hour. The obtained precipitate was filtered and washed by water to remove the remained ions. After drying at room temperature inside a fume cupboard, the dried precipate was transferred into an alumina crucible and fired at 600°C for 2 hours with heating/cooling rate of 5°C min'1. The obtained Ceo.8Smo.202-a powder was mixed with commercial Fe203 (Alfa) with weight ratio of 9.5/0.5 for Fe203: Ceo.sSmo.202-6 to be used for ammonia synthesis. The loading of the composite catalysts was 300mg after reduction to Fe: Ceo.8Smo.202-5. The H2 and N2 flowing rate was 60 ml min-1 and 20 ml min'1 respectively at ambient temperature and pressure. The synthesised ammonia was collected by 100ml (0.01M) H2SO4 solution and was measured by a Fisher Scientific Orion A214 ammonia meter. iv) Preparation of Fe-Ce02

Dissolve 0.01 mol, 4.3423g Ce(N03)3.6H20 in deionised water to obtain 0.05M aqueous solution. Dilute ammonia solution was slowly added into the cerium nitrate solution with stirring until a pH value reaches 10. Let the reaction to continue at room temperature for 1 hour. The obtained precipitate was filtered and washed by water to remove the remained ions. After drying at room temperature inside a fume cupboard, the dried precipitate was transferred into an alumina crucible and fired at 600°C for 2 hours with heating/cooing rate of 5°C min'1. The obtained Ce02 powder was mixed with commercial Fe203 (Alfa) with weight ratio of 9:1 and 9.5/0.5 for Fe203:Ce02 to be used for ammonia synthesis. The loading of the composite catalysts was 300mg after reduction to Fe:Ce02. The H2 and N2 flowing rate was 60 ml min'1 and 20 ml min'1 respectively at ambient temperature and pressure. The synthesised ammonia was collected by 100ml (0.01M) H2SO4 solution and was measured by a Fisher Scientific Orion A214 ammonia meter. v) Preparation of Fe-Sri.8Fe20s

Sr(N03)2 and Fe(N03)3-9H20 were dissolved in deionised water with a mol ratio of 1.8 to 2 respectively. Citric acid and EDTA were then added with mol ratio of 1:1:1 to metal ions. This mixture was continuously stirred for 1 hour at 30°C before increasing to 200°C. The resulting gel like product was then combusted at 200°C to obtain the powder product. This was calcined at 700°C for 12 hours with a heating and cooling rate of 5°C min'1. The resulting Sri.8Fe20s powder was then reduced in H2/N2 (total flowrate 50ml min'1, mol ratio 3:1) at 800°C for 12 hours with a heating and cooling rate of 5°C min-1 to exsolve the excess Fe on to the surface as nanoparticles. The obtained Sri.8Fe20s powder was mixed with commercial Fe2C>3 (Alfa) with weight ratio of 9/1 and 8.5/1.5 for Fe203: Sri.8Fe20s to be used for ammonia synthesis. The loading of the composite catalysts was 300mg after reduction to Fe: Sri.8Fe20s. The H2 and N2 flowing rate was 60 ml min-1 and 20 ml min'1 respectively at ambient temperature and pressure. The synthesised ammonia was collected by 100ml (0.01M) FI2SO4 solution and was measured by a Fisher Scientific Orion A214 ammonia meter. vi) Preparation of Fe-ZrNo.7 composite catalyst

Commercial Zr power in water was first put in a fridge at a temperature below zero degree to convert the liquid into solid (Zr mixed with ice). The Zr-ice mixture was transferred into a freeze dryer to remove the water in order to get pure Zr powder. The Zr power was mixed with urea with a molar ratio of Zr to urea of 1 : 0.35. The Zr-urea mixture was put in a zirconia container, adding zirconia ball. The weight ratio of zirconia balls to chemicals is roughly 20:1. After putting on the lid, the container was sealed by sello-tape and ball-milled at 400 rpm for 72 hours. The obtained powder was washed by water for several times with the use of centrifuge. The powder was dried at 30°C for overnight to obtain ZrNx powers with x close to 0.7. The obtained ZrNo.7 powder was mixed with commercial Fe203 (Alfa) with weight ratio of 8.5/1.5 for Fe203: Sri.8Fe20s to be used for ammonia synthesis. The loading of the composite catalysts was 300mg after reduction to Fe: ZrNo.7. The H2 and N2 flowing rate was 60 ml min'1 and 20 ml min'1 respectively at ambient temperature and pressure. The synthesised ammonia was collected by 100ml (0.01M) H2SO4 solution and was measured by a Fisher Scientific Orion A214 ammonia meter.

Claims

1. A composition for catalysis of a Haber-Bosch process, the composition comprising an anion vacant lattice and a Haber-Bosch catalyst.

2. A composition according to claim 1, wherein the Haber-Bosch catalyst comprises a metal compound selected from the group consisting of: Fe, Co, Ni, Ru, or combinations thereof.

3. A composition according to claim 2, wherein the metal compound is selected from: Fe, Ru, or combinations thereof.

4. A composition according to claim 3, wherein the metal compound is Fe.

5. A composition according to any preceding claim, wherein the composition is configured for catalysis of a Haber-Bosch process.

6. A composition according to any preceding claim, wherein the anion vacant lattice is doped to promote anion vacancies.

7. A composition according to any preceding claim, wherein the anion vacant lattice is an oxygen vacant lattice.

8. A composition according to claim 7, wherein the oxygen vacant lattice is selected from: CeCh, BaCeCh, LnCeCh, SrCeCb, ZrCh, BaZrCh, LnZrCb, SrZrCb, CaZrCb, Sri.eFezOs, LnFe03, B12O3, Sn02, LnCoCb, CaAICh, SrFei20i9-12Sr2B20s, MgO or combinations thereof or combinations thereof, wherein "Ln" represent lanthanides.

9. A composition according to claim 8, wherein the oxygen vacant lattice is selected from: Ce02, BaZrCh or combinations thereof.

10. A composition according to claim 7, wherein the oxygen vacant lattice is a compound according to formula II;

(Formula II)

wherein, M has a valance of less than 4, "a" and "b" are independently in the range 0.05 to 0.95, with the proviso that "a" and "b" together sum to 1.

11. A composition according to claim 10, wherein each of "a" and "b" are independently in the range 0.1 to 0.8.

12. A composition according to claim 11, wherein the oxygen vacant lattice comprises Ceo.8Smo.202-5.

13. A composition according to claim 7, wherein the oxygen vacant lattice is a compound according to formula I;

(Formula I) wherein, each of x, y and z are independently in the range 0.05 to 0.95, with the proviso that x, y and z together sum to 1.

14. A composition according to claim 13, wherein each of "x", "y" and "z" are independently in the range 0.1 to 0.8.

15. A composition according to claim 14, wherein the oxygen vacant lattice comprises BaZro.iCeo.7Yo.203-a.

16. A composition according to any of claims 1 to 6, wherein the anion vacant lattice is a nitrogen vacant lattice.

17. A composition according to claim 16, wherein the nitrogen vacant lattice is zirconium nitride.

18. A composition according to claim 17, wherein the nitrogen vacant lattice is ZrNo.7.

19. A composition according to any of claims 6 to 18, wherein the dopant is present in an amount in the range 1 mol% to 30 mol% of the total anion vacant lattice.

20. A composition according to claim 19, wherein the dopant is present in an amount in the range 5 mol% to 20 mol% of the total anion vacant lattice.

21. A composition according to claim 20, wherein the dopant is present in an amount in the range 10 mol% to 15 mol% of the total anion vacant lattice.

22. A composition according to any preceding claim, wherein the amount of anion vacant lattice present in the range 1 wt% to 60 wt% of the total composition.

23. A composition according to claim 22, wherein the amount of anion vacant lattice present in the composition is in the range 5 wt% to 20 wt% of the total composition.

24. A composition according to claim 23, wherein the amount of anion vacant lattice present in the composition is in the range 10 wt% to 15 wt% of the total composition.

25. A catalyst cartridge for a Haber-process, the cartridge comprising the composition according to any preceding claim.

26. A catalyst cartridge according to claim 25, wherein the composition is provided as a powder.

27. A catalyst cartridge according to claim 25, further comprising a support for heterogeneous catalysis; wherein the composition is provided as a coating for said support.

28. A Haber-Bosch process for producing ammonia, comprising the steps of: i) providing a composition according to any of claims 1 to 24; and ii) exposing said composition to a mixture of nitrogen and hydrogen gas.

29. Use of a composition according to any of claims 1 to 24 for the production of ammonia in a Haber-Bosch process.