HABER-BOSCH CATALYST COMPRISING AN ANION-VACANT LATTICE

Abstract

A composition for catalysis of a Haber-Bosch process comprises an anion vacant lattice and a Haber-Bosch catalyst (e.g. Fe Ru). Suitable anion vacant lattices include oxynitrides and oxides, which may be doped or undoped, including Ce.sub.aM.sub.bO.sub.2-XN.sub.Y (Formula III) M is one or more elements with a valence lower 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 (approximately). X is greater than 0 and less than 2. Y is greater than zero and less than or equal to X. A process employing the composition produces ammonia.

Claims

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

2. The 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. (canceled)

4. The composition according to claim 1, wherein the anion vacant lattice is doped to promote anion vacancies.

5. The composition according to claim 1, wherein the anion vacant lattice is an oxynitride.

6. The composition according to claim 5, wherein the oxynitride is a compound according to formula III:
Ce.sub.aM.sub.bO.sub.2-xN.sub.y   (Formula III) wherein M is one or more elements with a valence lower 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; 0<X<2; and 0<Y≤X.

7. The composition according to claim 6, wherein M is Sm and/or a is 0.5 to 0.9.

8. The composition according to claim 6, wherein (i) M is Pr or La; and/or (ii) a is 0.2 to 0.6.

9. The composition according to claim 5, wherein the oxynitride is a compound according to formula V or VI
Zr.sub.aM.sub.bO.sub.2-xN.sub.y   (Formula V) wherein M is titanium; and/or cerium; and or one or more elements with a valence lower 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; 0<X<2; and 0<Y≤X
Ti.sub.aM.sub.bO.sub.2-xN.sub.y   (Formula VI) wherein M is zirconium; and/or cerium; and/or one or more elements with a valence lower 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; 0<X<2; and 0<Y≤X.

10. The composition according to claim 4, wherein the anion vacant lattice is an oxygen vacant lattice and the oxygen vacant lattice comprises doped CeO.sub.2, doped ZrO.sub.2, doped TiO.sub.2, doped BaZrO.sub.3 or combinations thereof.

11. The composition according to claim 10, wherein the oxygen vacant lattice is yttrium stabilized zirconia (YSZ).

12. The composition according to claim 10, wherein the oxygen vacant lattice is a compound according to formula II;
Ce.sub.aM.sub.bO.sub.2-δ   (Formula II) wherein, M is one or more elements with 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.

13. The composition according to claim 12, wherein (i) each of “a” and “b” are independently in the range 0.1 to 0.8 and/or (ii) M is Sm, Pr, La, Gd or combinations thereof.

14. The composition according to claim 13, wherein the oxygen vacant lattice comprises Ce.sub.0.5Sm.sub.0.2O.sub.2-δ or Ce.sub.0.5Sm.sub.0.5O.sub.2-δ.

15. The composition according to claim 9, wherein the oxygen vacant lattice is a compound according to formula I;
BaZr.sub.xCe.sub.yY.sub.zO.sub.3-δ   (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.

16. The composition according to claim 15, wherein each of “x”, “y” and “z” are independently in the range 0.1 to 0.8.

17. The composition according to claim 16, wherein the oxygen vacant lattice comprises BaZr.sub.0.1Ce.sub.0.7Y.sub.0.2O.sub.3-δ.

18. (canceled)

19. A process for producing ammonia, comprising the steps of: i) providing a composition according to claim 1; and ii) exposing said composition to a mixture of nitrogen and hydrogen gas.

20. The process of claim 19, wherein the composition is exposed to a mixture of nitrogen and hydrogen at a temperature below 600° C. and a pressure below 20 MPa or the process is a batch process.

21. (canceled)

22. An anion vacant lattice according to formula III, V or VI:
Ce.sub.aM.sub.bO.sub.2-XN.sub.Y   (Formula III) wherein M is zirconium; and/or titanium and/or one or more elements with a valence lower 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 (approximately); 0<X<2; and 0<Y≤X
Zr.sub.aM.sub.bO.sub.2-XN.sub.Y   (Formula V) wherein M is titanium; and/or cerium; and/or one or more elements with a valence lower 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 (approximately); 0<X<2; and 0<Y≤X
Ti.sub.aM.sub.bO.sub.2-XN.sub.Y   (Formula VI) wherein M is zirconium; and/or cerium; and/or one or more elements with a valence lower 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 (approximately); 0<X<2; and 0<Y≤X.

23. The anion vacant lattice according to claim 22, wherein (i) M is Sm, Pr and/or La; and (ii) a is from 0.1 to 0.9.

Description

BRIEF DESCRIPTION OF FIGURES

[0078] FIG. 1 shows XRD images of the BZCY proton conducting support and the supported Ni catalyst before and after stability test.

[0079] FIG. 2 shows UV-Vis spectra of the Ni-BZCY catalyst before and after reduction.

[0080] FIG. 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.

[0081] FIG. 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).

[0082] FIG. 5 shows (a): STA analysis of dry Ni-BZCY in N.sub.2 (b): STA analysis of wet Ni-BZCY in N.sub.2.

[0083] FIG. 6 shows ammonia synthesis rate using a Ni-BZCY catalyst at different temperatures (120 mL min.sup.−1, H.sub.2:N.sub.2=3:1).

[0084] FIG. 7 shows ammonia synthesis rate using a Ni-BZCY catalyst at different flow rates (620° C., H.sub.2:N.sub.2=3:1).

[0085] FIG. 8 shows ammonia outlet concentration at different flow rates (620° C., H.sub.2:N.sub.2=3:1).

[0086] FIG. 9 shows ammonia synthesis rate using a Ni-BZCY catalyst at different feed mole ratios (200 mL min.sup.−1, 620° C.).

[0087] FIG. 10 shows ammonia synthesis rate using 60% NiO/40% MgO—CeO.sub.2 catalyst at different temperatures (120 mL min.sup.−1, H.sub.2:N.sub.2=3:1).

[0088] FIG. 11 shows ammonia synthesis rate using a Ni-BZCY catalyst over dry and wet stability tests (620° C., 200 mL min.sup.−1, H.sub.2:N.sub.2=3:1).

[0089] FIG. 12 shows the catalytic activity of pure Fe, Fe with CeO.sub.2 (5 wt %) and Fe with CeO.sub.2 (10 wt %) at a reaction pressure of 10 bar (total flow rate 80 mL min.sup.−1, H.sub.2:N.sub.2 mole ratio 3:1).

[0090] FIG. 13 shows the catalytic activity of pure Fe, Fe with CeO.sub.2 (5 wt %) and Fe with CeO.sub.2 (10 wt %) at a reaction pressure of 30 bar (total flow rate 80 mL min.sup.−1, H.sub.2:N.sub.2 mole ratio 3:1).

[0091] FIG. 14 shows the catalytic activity of pure Fe and Fe with SrFe.sub.12O.sub.19-12Sr.sub.2Br.sub.4O (5 wt %) at reaction pressure of 30 bar (total flow rate 80 mL min.sup.−1, H.sub.2:N.sub.2 mole ratio 3:1).

[0092] FIG. 15 shows the catalytic activity of Fe catalyst with BCZY (60 wt %) at various pressures (total flow rate 80 mL min.sup.−1, H.sub.2:N.sub.2 mole ratio 3:1).

[0093] FIG. 16 shows the catalytic activity of Fe.sub.2O.sub.3 with Sr.sub.1.8Fe.sub.2O.sub.5 (90 wt % and 85%) at various pressures (total flow rate 80 mL min.sup.−1, H.sub.2:N.sub.2 mole ratio 3:1).

[0094] FIGS. 17 to 19 show the ammonia synthesis rate for Fe—Ce.sub.0.8Sm.sub.0.2O.sub.2-δ with a support weight percent between 14 and 26% at 3 MPa; Fe—Ce.sub.0.8Sm.sub.0.2O.sub.2-δ with a support weight percent between 14 and 26% at 1 MPa; and Fe-20% CeO.sub.2 and Fe-SDC at 3 MPa, respectively. Catalysts were added such that the total catalyst mass was 300 mg. Reactant gases were supplied at a total volumetric flowrate of 80 mL min.sup.−1 with a H.sub.2/N.sub.2 ratio of 3. The outlet gases were passed through a 0.01M sulphuric acid trap and the produced ammonia was measured using an ISE Thermo Scientific Orion Star A214 ammonia meter.

[0095] FIG. 20 shows the activity of the 80% Fe-20% Ce.sub.0.8Sm.sub.0.2O.sub.2-δ catalyst over 200 hours on stream. Both temperature and pressure were kept constant at 450° C. and 3 MPa respectively. Feed gas was kept and a constant mole ratio of 3 to 1 H.sub.2 to N.sub.2 respectively. A gas flowrate of 80 mL min.sup.−1 was employed during the tests but was reduced to 40 mL min.sup.−1 overnight.

[0096] FIG. 21 shows the proposed reaction pathway on the catalyst in which nitrogen is dissociatively adsorbed on the Fe surface and undergoes hydrogenation. Hydrogen gas is ionised on the Ce.sub.0.8Sm.sub.0.2O.sub.2-δ surface. The reaction intermediate NH* is then reacting with OH..sub.O on the Ce.sub.0.8Sm.sub.0.2O.sub.2-δ surface at the contact points between Fe and Ce.sub.0.8Sm.sub.0.2O.sub.2-δ to undergo the final stages of hydrogenation producing adsorbed ammonia on the Ce.sub.0.8Sm.sub.0.2O.sub.2-δ surface.

[0097] FIG. 22 shows ammonia synthesis rate for the best performing Fe-20% Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y catalyst composition compared to the Fe-20% CeO.sub.2-xN.sub.y catalyst, Fe-20% CeO.sub.2-xN.sub.y calcined catalyst, Fe-20% CeO.sub.2 catalyst, and industrial magnetite Fe catalyst. All measurements were at 400° C. at either 3 MPa (left) or 1 MPa (right).

[0098] FIG. 23 shows the ammonia synthesis rate for Fe—CeO.sub.2, Fe—CeO.sub.2-xN.sub.y, Fe—Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y with a support weight percent of 20% at 3 MPa. Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y has the highest activity (uppermost line) with a peak activity at 400° C. For comparison Ce.sub.0.8Sm.sub.0.2O.sub.2-xN.sub.y has a peak at 500° C. (see FIG. 26).

[0099] FIG. 24 shows the ammonia synthesis rate for Fe—CeO.sub.2, Fe—CeO.sub.2-xN.sub.y, Fe—Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y with a support weight percent of 20% at 1 MPa. Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y has the highest activity (uppermost line) with a peak activity at 450° C.

[0100] FIG. 25 shows the activity of Fe—CeO.sub.2-xN.sub.y catalyst over 200 hours on stream. Both temperature and pressure were kept constant at 450° C. and 3 MPa respectively. Feed gas was kept and a constant mole ratio of 3 to 1 H.sub.2 to N.sub.2 respectively. A gas flowrate of 80 mL min.sup.−1 was employed.

[0101] FIG. 26 shows the activity of Fe-20% Ce.sub.aSm.sub.bO.sub.2-XN.sub.Y catalysts at various temperatures at 3 MPa.

[0102] FIG. 27 shows the activity of Fe-20% Ce.sub.aSm.sub.bO.sub.2-XN.sub.Y catalysts at various temperatures at 1 MPa.

[0103] FIG. 28 shows the activity of Fe—Ce.sub.0.5Sm.sub.0.5O.sub.2-XN.sub.y catalyst at over 200 hours on stream. Both temperature and pressure were kept constant at 400° C. and 3 MPa respectively. Feed gas was kept and a constant mole ratio of 3 to 1 H.sub.2 to N.sub.2 respectively. A gas flowrate of 80 mL min.sup.−1 was employed.

[0104] FIG. 29 shows the activity of 10% Ru—Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y at 3 MPa and 1 MPa under different temperatures.

[0105] FIG. 30 shows the Raman spectra of pure CeO.sub.2, CeO.sub.2-xN.sub.y and Ce.sub.1-zSmO.sub.2-xN.sub.y indicating the presence of oxygen vacancies in the doped cerium oxynitrides.

[0106] FIG. 31 shows the catalytic activity of 80% Fe-20% ZrO.sub.2 (99+% excluding HfO.sub.2 (2%), Alfa Aesar) and 80% Fe-20% YSZ (yttrium stabilized zirconia, PI-KEM Ltd) at 3 MPa. Reactant gases were supplied at a total volumetric flowrate of 80 mL min.sup.−1 with a H.sub.2/N.sub.2 molar ratio of 3.

[0107] FIG. 32 shows room temperature XRD patterns of Ce.sub.1-zSm.sub.zO.sub.2-xN.sub.y with z=0 to 0.5, demonstrating a single phase.

[0108] FIG. 33 shows the lattice parameter of the pure and Sm-doped cerium oxynitrides.

[0109] FIG. 34 shows room temperature XRD patterns of Ce.sub.1-zPr.sub.zO.sub.2-xN.sub.y with z=0.8 and 0.9, demonstrating a single phase.

EXAMPLES

Example 1

Synthesis of BZCY

[0110] In order to synthesise the BaZr.sub.0.1Ce.sub.0.7Y.sub.0.2O.sub.3-δ (BZCY) perovskite catalyst support a solid state reaction was employed. Firstly stoichiometric amounts of BaCO.sub.3 (99% alfa), ZrO.sub.2 (99% alfa), CeO.sub.2 (99.5% Alfa) and Y.sub.2O.sub.3 (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.sup.−1. After this NiO (99% Alfa) was added to the BaZr.sub.0.1Ce.sub.0.7Y.sub.0.2O.sub.3 powder with a weight ratio of 60% to 40% respectively. This was further wet ground in isopropyl alcohol for 12 hours. The MgO—CeO.sub.2 support for the comparison test was prepared through a combustion synthesis in which equimolar amounts of Ce(NO.sub.3).sub.3.6H.sub.2O (99.5% alfa) and Mg(NO.sub.3).sub.2.6H.sub.2O (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

[0111] 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 45 kV and 40 mA. The SEM images were obtained with ZEISS SUPRA 55-VP operating at 10 kV. 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 N.sub.2 flowing rate of 70 ml min.sup.−1. The UV-Vis measurements were carried on a Shimadzu 3600 Spectrophotometer with integrating sphere for solid samples. The samples were mixed with BaSO.sub.4 to fill in the sample holder before the measurements. The specific surface area of both the Ni-BZCY catalyst and the Ni—MgO—CeO.sub.2 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

[0112] 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 H.sub.2 and N.sub.2 with a total flow rate of 100 mL min.sup.−1 and mole ratio of 9:1 H.sub.2:N.sub.2 for 4 hours. After this the temperature, total flow rate and flow rate ratio were adjusted in order to determine the optimal conditions. H.sub.2 and N.sub.2 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 H.sub.2/N.sub.2 (3:1 m/o), then N.sub.2 passing through room temperature water was passed through the catalyst for one hour. After this process, the gas was switched to mixed H.sub.2 and N.sub.2 then slowly heated to 620° C. to continue the ammonia synthesis measurement.

[0113] Dilute H.sub.2SO.sub.4 (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.

[0114] In order to calculate the ammonia synthesis rate the following equation was used:

[00001]$\begin{array}{cc}{r}_{{\mathrm{NH}}_3}=\frac{\left[{\mathrm{NH}}_4^{+}\right]\times V}{t\times m}& \left(9\right)\end{array}$

where [NH.sub.4.sup.+] is ammonia concentration in mol L.sup.−1, V is volume of 0.01M H.sub.2SO.sub.4 in L, t is time in hours and m is catalyst mass in grams.

XRD Analyses

[0115] In the XRD results shown in FIG. 1 it can be seen that there are some small peaks attributed to BaCO.sub.3 and Y-doped Ce.sub.xZr.sub.1-xO.sub.2 present for BaZr.sub.0.1Ce.sub.0.7Y.sub.0.2O.sub.3 before and after being mixed with the NiO, however, after reduction at 700° C. in H.sub.2/N.sub.2 mixture (90% H.sub.2) for 4 hours, these peaks are no longer present. A possible reason is that, BaCO.sub.3 and Y-doped Ce.sub.xZr.sub.1-xO.sub.2 were converted into amorphous phase during the reduction process thus cannot be detected by XRD. The XRD peaks for the catalyst before and after the stability test are the same although the intensity of the Ni peak has increased after the stability tested showing the possible aggregation of Ni particles whilst better crystallisation is another possible reason

UV-Visible Observation

[0116] In order to identify the BaCO.sub.3 phase, the absorbance spectra of the catalyst were measured before and after reduction to investigate whether or not BaCO.sub.3 and Y-doped Ce.sub.xZr.sub.1-xO.sub.2 are converted into amorphous phases. The absorbance spectra of pure BaCO.sub.3, ZrO.sub.2, CeO.sub.2 and the catalysts before and after catalysts test were measured using a Shimadzu UV-2600 with integrating sphere. The results are shown in FIG. 2. It was observed that after reduction none of BaCO.sub.3, zirconia or ceria can be identified in the reduced catalyst. Therefore, it was shown that an amorphous phase was not formed by BaCO.sub.3, zirconia or ceria and they are not present in the reduced catalyst. One possible reason is that, the tiny amounts of BaCO.sub.3 second phase was covered by a thin layer of Ni when NiO was reduced by H.sub.2 whilst diffusion of newly formed Ni is very likely, thus BaCO.sub.3 cannot be detected by either XRD or UV-Vis spectrometer.

SEM Observation

[0117] FIG. 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 FIG. 4. The distribution of Ni (FIG. 4b) is homogeneous.

TG-DSC Analysis

[0118] 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 FIGS. 5 (a) and (b) respectively. For the dry catalyst, the initial weight loss below 100° C. (˜0.12 wt %) is due to the loss of absorbed water. Slight weight gain on cooling peaked at ˜270° C. (˜0.03 wt %) 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 (˜0.34 wt %) 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 uptake (˜0.18 wt %) was observed indicating BZCY can strongly uptake water at lower temperature.

Effect of Temperature on Catalyst Activity

[0119] When a constant flow rate was kept at 120 mL min.sup.−1 and H.sub.2:N.sub.2 were flown with a mole ratio of 3:1 the effects of changing temperature could be observed, this is shown in FIG. 6. It was observed that the activity increases up to a maximum of approximately 135 μmol g.sup.−1 h.sup.−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 FIG. 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 FIG. 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

[0120] The effect of total flow rate was then tested at a constant temperature of 620° C. with the results shown in FIG. 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.

[0121] As shown in FIG. 8, when total flow rate is plotted against ammonia outlet concentration, it rises up to a total flow rate of 120 mL min.sup.−1 before levelling off. This therefore shows that the total flow rate is independent of conversion rate over a value of 120 mL min.sup.−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.sup.−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 Gas Ratio on Catalyst Activity

[0122] To determine the optimal feed ratio the gas inlet mole ratio was adjusted between 2.6 and 3.4 (H.sub.2/N.sub.2) with the optimal being detected for a value of 3.2 with a rate of approximately 320 μmol g.sup.−1 h.sup.−1 (FIG. 9). All measurements were taken at 620° C. with a total flow rate of 200 ml/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 H.sub.2 being ionised and transferred to the support as H.sup.+ therefore adjusting the value of H.sub.2 to N.sub.2 in the reactor closer to the stoichiometric value of 3.

Effect of Temperature on Catalyst Activity of 60% NiO/40% MgO—CeO.SUB.2

[0123] 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. MgO—CeO.sub.2 composite is an excellent support for Ru catalysts for ammonia synthesis. In this study, for comparison, Ni supported in MgO—CeO.sub.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.sup.−1 (FIG. 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 (FIG. 6). However, the catalytic activity is related to the specific surface area. The specific surface area was measured to be 0.907 m.sup.2 g.sup.−1 for the Ni-BZCY catalyst and 16.940 m.sup.2 g.sup.−1 for the Ni—MgO—CeO.sub.2 catalyst. The specific surface area of Ni-BZCY is only 5.3% of that of Ni—MgO—CeO.sub.2 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

[0124] 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 H.sub.2/N.sub.2 mole ratio of 3 and a total flow rate of 200 mL min.sup.−1. The catalyst was found to be stable over this period with no loss of activity as can be seen in FIG. 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 (100 mL min.sup.−1) was bubbled through the reactor for 1 hour before being heated back to 620° C. at a rate of 1° C. min.sup.−1. This was repeated 5 times with the results shown in FIG. 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 μmol g.sup.−1 h.sup.−1 to 50 μmol g.sup.−1 h.sup.−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 (FIGS. 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. Evidence for this was observed during the XRD which showed an increase in intensity of the Ni peak after the stability test suggesting possible better crystallisation of Ni particle leading to loss of active sites on the Ni surface. 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.

[0125] As well as the BZCY promoted catalyst pure Ni was also tested with a rate of 25.12 μmol g.sup.−1 h.sup.−1 observed at 620° C. with a total flow rate of 200 mL/min and a H.sub.2/N.sub.2 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.

[0126] 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 H.sub.2 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

[0127] i) Preparation of Fe—SrFe.sub.12O.sub.19-12Sr.sub.2B.sub.2O.sub.5 Catalyst

[0128] 18.4538 g SrCO.sub.3, 7.4196 g H.sub.3BO.sub.3, 4.7907 g Fe.sub.2O.sub.3 were mixed in agate mortar and pestle, then put in an alumina crucible, pre-fired 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 fired 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—SrFe.sub.12O.sub.19-12Sr.sub.2B.sub.2O.sub.5 amorphous powder, was mixed with commercial Fe.sub.2O.sub.3 (Alfa) with a weight ratio of 9.5/0.5 for Fe.sub.2O.sub.3:Fe—SrFe.sub.12O.sub.19-12Sr.sub.2B.sub.2O.sub.5 to be used for ammonia synthesis. The loading of the composite catalysts was 300 mg after reduction to Fe: Fe—SrFe.sub.12O.sub.19-12Sr.sub.2B.sub.2O.sub.5. The H.sub.2 and N.sub.2 flow rates were 60 ml min.sup.−1 and 20 ml min.sup.−1 respectively at ambient temperature and pressure. The synthesised ammonia was collected by 100 ml (0.01M) H.sub.2SO.sub.4 solution and was measured by a Fisher Scientific Orion A214 ammonia meter.

ii) Preparation of Fe—BaZr.sub.0.1Ce.sub.0.7Y.sub.0.2O.sub.3-δ

[0129] Stoichiometric amounts of BaCO.sub.3 (99% alfa), ZrO.sub.2 (99% alfa), CeO.sub.2 (99.5% Alfa) and Y.sub.2O.sub.3 (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.sup.−1. The obtained BaZr.sub.0.1Ce.sub.0.7Y.sub.0.2O.sub.3-δ powder was mixed with commercial Fe.sub.2O.sub.3 (Alfa) with weight ratio of 4/6 for Fe.sub.2O.sub.3:BaZr.sub.0.4Ce.sub.0.7Y.sub.0.2O.sub.3-δ to be used for ammonia synthesis. The loading of the composite catalysts was 300 mg after reduction to Fe: BaZr.sub.0.1Ce.sub.0.7Y.sub.0.2O.sub.3-δ, The H.sub.2 and N.sub.2 flow rates were 60 ml min.sup.−1 and 20 ml min.sup.−1 respectively at ambient temperature and pressure. The synthesised ammonia was collected by 100 ml (0.01M) H.sub.2SO.sub.4 solution and was measured by a Fisher Scientific Orion A214 ammonia meter.

iii) Preparation of Fe—Ce.sub.0.8Sm.sub.0.2O.sub.2-δ (SDC)

[0130] 0.001 mol 0.3487 g Sm.sub.2O.sub.3 was dissolved in dilute nitric acid at a temperature around 60° C. until Sm.sub.2O.sub.3 powder was completely dissolved to form an aqueous samarium nitrate solution. 0.008 mol, 3.4738 g Ce(NO.sub.3).sub.3.6H.sub.2O 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 the pH value reached 10. The reaction was allowed to continue at room temperature for 1 hour. The obtained precipitate was filtered and washed with deionised water several times. 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 a heating/cooling rate of 5° C. min.sup.−1. The obtained Ce.sub.0.8Sm.sub.0.2O.sub.2-δ powder was mixed with commercial Fe.sub.2O.sub.3 (Alfa) with weight ratio of 9.5/0.5 for Fe.sub.2O.sub.3:Ce.sub.0.8Sm.sub.0.2O.sub.2-δ to be used for ammonia synthesis. The loading of the composite catalysts was 300 mg after reduction to Fe: Ce.sub.0.8Sm.sub.0.2O.sub.2-δ. The H.sub.2 and N.sub.2 flow rates were 60 ml min.sup.−1 and 20 ml min.sup.−1 respectively at ambient temperature and pressure. The synthesised ammonia was collected by 100 ml (0.01M) H.sub.2SO.sub.4 solution and was measured by a Fisher Scientific Orion A214 ammonia meter.

iv) Preparation of Fe—CeO.SUB.2

[0131] Dissolve 0.01 mol, 4.3423 g Ce(NO.sub.3).sub.3.6H.sub.2O in deionised water to obtain 0.05M aqueous solution. Dilute ammonia solution was slowly added into the cerium nitrate solution with stirring until the pH value reaches 10. Allow the reaction to continue at room temperature for 1 hour. The obtained precipitate was filtered and washed by water to remove the remaining 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 a heating/cooling rate of 5° C. min.sup.−1. The obtained CeO.sub.2 powder was mixed with commercial Fe.sub.2O.sub.3 (Alfa) with weight ratio of 9:1 and 9.5/0.5 for Fe.sub.2O.sub.3:CeO.sub.2 to be used for ammonia synthesis. The loading of the composite catalysts was 300 mg after reduction to Fe:CeO.sub.2. The H.sub.2 and N.sub.2 flow rates were 60 ml min.sup.−1 and 20 ml min.sup.−1 respectively at ambient temperature and pressure. The synthesised ammonia was collected by 100 ml (0.01M) H.sub.2SO.sub.4 solution and was measured by a Fisher Scientific Orion A214 ammonia meter.

v) Preparation of Fe—Sr.sub.1.8Fe.sub.2O.sub.5

[0132] Sr(NO.sub.3).sub.2 and Fe(NO.sub.3).sub.3.9H.sub.2O 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.sup.−1. The resulting Sr.sub.1.8Fe.sub.2O.sub.5 powder was then reduced in H.sub.2/N.sub.2 (total flowrate 50 ml min.sup.−1, mol ratio 3:1) at 800° C. for 12 hours with a heating and cooling rate of 5° C. min.sup.−1 to exsolve the excess Fe on to the surface as nanoparticles. The obtained Sr.sub.1.8Fe.sub.2O.sub.5 powder was mixed with commercial Fe.sub.2O.sub.3 (Alfa) with weight ratio of 9/1 and 8.5/1.5 for Fe.sub.2O.sub.3:Sr.sub.1.8Fe.sub.2O.sub.5 to be used for ammonia synthesis. The loading of the composite catalysts was 300 mg after reduction to Fe: Sr.sub.1.8Fe.sub.2O.sub.5. The H.sub.2 and N.sub.2 flow rates were 60 ml min.sup.−1 and 20 ml min.sup.−1 respectively at ambient temperature and pressure. The synthesised ammonia was collected by 100 ml (0.01M) H.sub.2SO.sub.4 solution and was measured by a Fisher Scientific Orion A214 ammonia meter.

Fe—Ce.sub.0.8Sm.sub.0.2O.sub.2-δ (SDC) Investigation

[0133] The activities of Fe-SDC on ammonia synthesis at high temperature at 3 MPa and 1 MPa are shown in FIGS. 17 and 18 respectively. Generally the catalyst with 20 wt % SDC exhibits the highest activity of the different SDC promoter ratios tested. At this ratio, the rates for N.sub.2 cleavage on Fe and H.sub.2 dissociation on SDC match well to maximize the production of ammonia. For both measured pressures, the highest ammonia production rate was observed at 450° C.

[0134] Table 1 provides a comparison of selected highly active ammonia synthesis catalysts. Activity was measured at optimal pressure and temperature. The purity of the gas supply used was also compared.

TABLE-US-00001 Activity H.sub.2/N.sub.2 gas Synthesis (mmol g.sup.−1 refer- Catalyst Purity conditions h.sup.−1) ence Co.sub.3Mo.sub.3N 99.9999% T = 400° C. 5.36 (1) with further P = 10 MPa purification 10% Ru/Ba-Ca(NH.sub.2).sub.2 99.99995% T = 360° C. 60.4 (2) P = 0.9 MPa 1.2% Ru/C12A7:e.sup.− 99.99995% T = 400° C. 8.245 (3) P = 1.0 MPa 8.3% Ru/LaScSi 99.99995% T = 400° C. 19 (4) P = 1.0 MPa 5% Ru/Pr.sub.2O.sub.3 Unreported T = 400° C. 19 (5) P = 1.0 MPa Fe—LiH Unreported T = 350° C. 12 (6) P = 1 MPa Wüstite based industrial 99.99995% T = 450° C. 16 (2) Fe catalyst P = 0.9 MPa Magnetite based  99.995% T = 450° C. 1.7 This industrial Fe catalyst P = 1 MPa work 80% Fe-  99.995% T = 450° C. 8.7 This 20% Ce.sub.0.8Sm.sub.0.2O.sub.2-δ P = 1 MPa work (1) C. J. H. Jacobsen, Chemical Communications, 1057-1058 (2000). (2) M. Kitano, Y. Inoue, M. Sasase, K. Kishida, Y. Kobayashi, K. Nishiyama, T. Tada, S. Kawamura, T. Yokoyama, M. Hara, H. Hosono, Angewandte Chemie International Edition 57, 2648-2652 (2018). (3) M. Kitano, Y. Inoue, Y. Yamazaki, F. Hayashi, S. Kanbara, S. Matsuishi, T. Yokoyama, S.-W. Kim, M. Hara, H. Hosono, Nature Chemistry 4, 934-940 (2012). (4) J. Z. Wu, Y. T. Gong, T. Inoshita, D. C. Fredrickson, J. J. Wang, Y. F. Lu, M. Kitano, H. Hosono, Adv. Mater. 29, (2017). (5) K. Sato, K. Imamura, Y. Kawano, S. Miyahara, T. Yamamoto, S. Matsumura, K. Nagaoka, Chemical Science 8, 674-679 (2017). (6) P. Wang, F. Chang, W. Gao, J. Guo, G. Wu, T. He, P. Chen, Nature Chemistry 9, 64-70 (2016).

[0135] At 1 MPa and 450° C., the activity of Fe catalyst promoted with 20 wt % SDC is 8.7 mmolg.sup.−1 h.sup.−1, which is lower than the Wustite based industrial Fe catalyst, which is reported 16 mmol g.sup.−1 h.sup.−1 at 0.9 MPa and 450° C. but much higher than the magnetite Fe-based industrial catalyst (1.7 mmol g.sup.−1 h.sup.−1) (Table 1). Therefore, high activity for the SDC promoted Fe catalyst has been demonstrated, which is significantly higher than that of the industrial Fe-based catalyst.

[0136] The inventors propose that this improvement is related to the extrinsic oxygen vacancies deliberately introduced through the doping of Sm.sub.2O.sub.3 in CeO.sub.2. For comparison, the activity of Fe catalyst promoted with 20 wt % CeO.sub.2 was also measured at 3 MPa over the same range of temperatures (FIG. 20). It was observed that the activity of SDC promoted Fe is much higher than that of CeO.sub.2 promoted Fe at temperatures above 350° C. This experiment clearly demonstrates that the introduction of extrinsic oxygen vacancies can significantly improve the catalytic activity of Fe catalysts at moderate temperature. At higher temperatures, the oxygen vacancies are activated providing active sites for the reaction thus, the activity of Fe-SDC catalyst is much higher due to the oxygen vacancy concentration of SDC being significantly greater than that for pure CeO.sub.2.

[0137] In order to further confirm the critical role extrinsic oxygen vacancies have on promoting the Fe-based catalyst, pure ZrO.sub.2 and 8 mol % Y.sub.2O.sub.3-stabilized ZrO.sub.2 (YSZ) were also used to promote Fe-based catalyst with the same 20 wt % weight ratio. Extrinsic oxygen vacancies are present in YSZ whist there are no extrinsic oxygen vacancies in ZrO.sub.2. It was clearly observed that the activity of Fe-YSZ catalyst is much higher than that for Fe—ZrO.sub.2 at temperatures above 300° C. (FIG. 31). This experiment further confirms that extrinsic oxygen vacancies have obvious promotion effects on the catalytic activity. It was observed that pure ZrO.sub.2 also exhibits promotion effects at temperatures above 400° C. Similar to pure CeO.sub.2, this is probably related to the formation of intrinsic oxygen vacancies when zirconium is exposed to hydrogen at high temperatures. It is well known that the reduction of CeO.sub.2 is much easier than ZrO.sub.2. Therefore under the same reducing condition, the oxygen vacancy concentration is much higher for CeO.sub.2 and a higher catalytic activity is achieved. On the other hand, the reduction of CeO.sub.2 will release electrons which can be donated to iron and then to the anti-bond of N≡N of adsorbed N.sub.2, facilitating the cleavage of N.sub.2, increasing the NH.sub.3 production rate.

[0138] However, the reduction of SDC is much more difficult than that for CeO.sub.2, in other words, under the ammonia synthesis conditions, the provided electrons from SDC will be a lot lower than that from CeO.sub.2. Therefore the higher promotion effect observed from SDC is not due to the donation of electrons for cleavage of N.sub.2, it is therefore mainly from the dissociation of H.sub.2 through oxygen vacancies. It has been reported that H.sub.2 adsorption is high between 400 and 500° C. but sharply decreases above 500° C. This is consistent with the peak activity for Fe—CeO.sub.2 and Fe-SDC catalysts where the highest activity was at 450° C. and the 2.sup.nd highest at 500° C. (FIGS. 17 and 19). Therefore, when investigating oxide promoters, the effect from H.sub.2 dissociation is more important than that of electron donation. Both pure CeO.sub.2 and SDC exhibit no catalytic activity towards ammonia synthesis when tested on their own without iron, indicating they cannot cleave N.sub.2 by themselves. The cleavage must rely on iron. This provides further evidence that the promotion effect from SDC and YSZ is mainly from the extrinsic oxygen vacancies. To further confirm the promotion effect of extrinsic oxygen vacancies, commercial ZrO.sub.2 and YSZ were also used to promote Fe-based catalyst. It was also observed from the obtained XRD patterns that complete reduction of Fe.sub.2O.sub.3 was achieved under reaction conditions. The formula of 8 mol % Y.sub.2O.sub.3 doped ZrO.sub.2 is approximately Zr.sub.0.85Y.sub.0.15O.sub.2-δ. The doping level in YSZ is lower than that for SDC, thus the concentration of extrinsic oxygen vacancies in YSZ is lower than that for SDC, leading to lower activity. On the other hand, a lot fewer electrons are provided from YSZ compared to SDC as reduction of the later is relatively easier. This has been demonstrated when they have been used as electrolytes for solid oxide fuel cells. Therefore, the promotion effect from SDC is more significant than that for YSZ. This increase in activity for both the ceria based and zirconia based promotors when the amount of extrinsic oxygen vacancies are increased provides clear evidence to the promotional effect of oxygen vacancies to the ammonia synthesis reaction.

[0139] The activities of each of the optimal compositions of catalyst at 450° C. and 3 MPa are highlighted in FIG. 22 along with the activity of an industrial iron catalyst under the same reaction conditions. From this it was observed that the activity of 80% Fe-20% SDC was nearly 3 times higher than that of a magnetite Fe-based industrial catalyst. The reduction in activity as the pressure was decreased was also less pronounced for the catalysts promoted by SDC. It is notable that the calcined catalyst (middle bars) has lower activity than the equivalent catalyst that has not been calcined. This supports the role of anionic vacancies in promoting the reaction.

[0140] Alongside the clear promotion effect of ceria and doped ceria towards the synthesis of ammonia with an iron catalyst, the resistance to catalyst poisoning should also be noted. The purity of both reaction gases used in our experiments was 99.995% with no further purification, both oxygen and water were present in the 50 ppm impurities. It can be seen that our measured activity for an industrially used commercial promoted Fe catalyst is lower than that reported elsewhere, this lower activity is an indicator of the negative effects of oxygenates in the feed gas in our experiment. However, the high activities obtained for the ceria and doped ceria supported catalyst indicate excellent tolerance towards impurities in the feed gas. As shown in FIG. 20, during the measured 200 hour test at 450° C. and 3 MPa, the 80% Fe-20% SDC is fairly stable although less pure H.sub.2 and N.sub.2 (99.995%) was used as the feeding gas. The catalyst is observed to keep its high activity over this period showing its resistance to gas feed impurities. The activity of our catalysts is comparable to the leading Fe-based industrial catalysts tested under extreme gas purity (99.99995%). It was observed that the activity measured at the start of each group during the 200 hours test was slightly lower than each of the others. This is due to the reactor needing time to achieve a stable through put after the flow rate was increased from 40 mL min.sup.−1 to 80 mL min.sup.−1 at the start of each group. This experiment indicates that the SDC promoted Fe catalyst has high oxygenate tolerance and ammonia can be continuously produced from less pure H.sub.2 and N.sub.2. This will reduce the requirement on H.sub.2 and N.sub.2 purification, saving the cost on equipment, purification catalysts and maintenance for the gas purification process. The energy input on the gas purification process will also be reduced thus increasing the overall efficiency. This feature is particularly useful when renewable electricity is used as the energy source for ammonia synthesis. Intensive purification on H.sub.2 produced through the splitting of water and N.sub.2 through the separation from air may not be required, making the localised ammonia synthesis process less complicated and more feasible.

[0141] The mechanism in which oxygen poisons an ammonia synthesis catalyst occurs through the growth of large iron crystals formed through the continuous oxidation and reduction cycles that take place. CeO.sub.2 based materials are excellent combustion catalysts. The presence of a CeO.sub.2 based promoter in the composite catalysts will catalyse the reaction between H.sub.2 and trace amounts of O.sub.2, forming H.sub.2O. The other oxygenates such as CO, H.sub.2O can effectively adsorb on the surface of CeO.sub.2-based materials. Without wishing to be bound by theory, under these conditions SDC is used as a reservoir to reversibly store oxygenates, thus decreasing the chance for oxidation of Fe causing the sintering of Fe to become less significant. H.sub.2O from the reaction between the H.sub.2 syngas and O.sub.2 impurity, or the H.sub.2O impurity itself will interact with oxygen vacancies to form protonic defects further reacting with NH* to form NH.sub.3 (FIG. 21). This protection from large iron crystal growth through repeated oxygenation cycles is evident when examining the higher surface area exhibited by the catalysts promoted with SDC and YSZ after reduction. Therefore the negative effect of the oxygenates on Fe particles will be minimised, leading to good stability.

[0142] In brief, the addition of an oxide promoter with extrinsic oxygen vacancies, such as SDC or YSZ, to an iron catalyst has shown an improvement in both activity and gas impurity tolerance over the conventional fused iron catalysts, opening up an exciting new class of ammonia synthesis catalysts. This provides a new strategy to develop novel ammonia synthesis catalyst with both high activity and high tolerance to oxygenates for practical applications, particularly for low carbon ammonia synthesis using renewable electricity as the energy source.

Example 3

[0143] i) Preparation of Fe—CeO.sub.2-xN.sub.y

[0144] 0.02 mol, 8.6844 g Ce(NO.sub.3).sub.3.6H.sub.2O was mixed with 0.2 mol, 12.012 g urea in a ceramic evaporating dish. Then 50 ml water was added into the mixture to dissolve the mixture. The ceramic evaporating dish was then put on a hotplate. This mixture was continuously stirred at 120° C. for 24 hours before increasing the temperature to 400° C. for combustion. The resulting gel like product was then combusted at 400° C. to obtain the powder product of CeO.sub.2-xN.sub.y.

[0145] The obtained CeO.sub.2-xN.sub.y powder was mixed with commercial Fe.sub.2O.sub.3 (Alfa) with weight ratio of 85/15 for Fe.sub.2O.sub.3:CeO.sub.2-xN.sub.y to be used for ammonia synthesis. The loading of the composite catalysts was 300 mg after reduction to Fe: CeO.sub.2-xN.sub.y. The H.sub.2 and N.sub.2 flow rates were 60 ml min.sup.−1 and 20 ml min.sup.−1 respectively at ambient temperature and pressure. The synthesised ammonia was collected by 100 ml (0.01 M) H.sub.2SO.sub.4 solution and was measured by a Fisher Scientific Orion A214 ammonia meter.

[0146] Nitrogen and the value of x and y were confirmed through XRF analysis. The results are given in the table below. This gives a composition of CeO.sub.1.42N.sub.0.39 and CeO.sub.1.37N.sub.0.42 before and after the stability test respectively.

[0147] In order to calculate the given composition it was assumed that Ce had a valance of +4, O a valance of −2, and N a valance of −3 with an overall charge of zero (neutrality). From this the following charge balance can be constructed.

(+4)(a)+(−2)(2−x)+(−3)(y)=0

[0148] Where a is the number of moles of Ce in the composition. For CeO.sub.2-xN.sub.y a=1.

[0149] This gives:

x=3/2y

[0150] From the XRF data the value of y can be obtained through a direct mole ratio of N to Ce, allowing for x to be calculated from the above equation.

[0151] In order to confirm that all nitrogen observed in the XRF results was in the CeO.sub.2-xN.sub.y composition, pure commercial CeO.sub.2 was tested. It can be observed in table 2(c) that no nitrogen was present in the pure commercial CeO.sub.2.

[0152] A significant amount of carbon as well as excess oxygen were detected in all XRF results for cerium samples including pure commercial CeO.sub.2. This is due to the strong adsorption of CO.sub.2 and CO on the CeO.sub.2 surface at room temperature as has been reported elsewhere (C. Slostowski, S. Marrea, P. Dagaulta, O. Babotb, T. Toupanceb, C. Aymonier, Journal of CO.sub.2 Utilization 20, 52-58 (2017), (8) I. Yanase, K. Suzuki, T. Ueda, H. Kobayashi, Materials Letters 228, 470-474 (2018)). No nitrogen signal was picked up from commercial CeO.sub.2 indicating the adsorption of N.sub.2 on the CeO.sub.2 surface is negligible.

[0153] Due to the adsorption of CO.sub.2 on the CeO.sub.2 surface, extra signals of carbon and oxygen are shown in XRF measurements. Therefore the observed oxygen content is the sum of the oxygen from absorbed CO.sub.2 and in those in the oxynitrides. Under the circumstance a direct measurement of oxygen content and y value cannot be directly obtained from the XRF results. However, the absorption of nitrogen on commercial CeO.sub.2 is negligible, it is assumed that the adsorption of nitrogen on cerium oxynitride is also negligible. Therefore, the measured molar ratio between Ce and N will be accurate. From the Ce:N molar ratio, based on the charge neutrality principle for a molecule, the content of oxygen in cerium oxynitrides can be then deduced.

[0154] This gives a composition of CeO.sub.1.42N.sub.0.39 and CeO.sub.1.37N.sub.0.42 before and after the stability test respectively, i.e. Y=0.39 before and 0.42 after. The content of nitrogen in the cerium oxynitride increased a little, which means the oxynitride is stable under the ammonia synthesis condition.

ii) Preparation of Fe—CeO.sub.0.8Sm.sub.0.2O.sub.2-xN.sub.y

[0155] 0.016 mol, 6.9475 g Ce(NO.sub.3).sub.3.6H.sub.2O, 0.004 mol, 1.7778 g Sm(NO.sub.3).sub.3.6H.sub.2O and 0.2 mol, 12.012 g urea were mixed in a ceramic evaporating dish. Then 50 ml water was added to dissolve the mixture. The ceramic evaporating dish was then put on a hotplate. This mixture was continuously stirred at 120° C. for 24 hours before increasing the temperature to 400° C. The resulting gel like product was then combusted at 400° C. to obtain the powder product of Ce.sub.0.8Sm.sub.0.2O.sub.2-xN.sub.y.

[0156] The obtained Ce.sub.0.8Sm.sub.0.2O.sub.2-xN.sub.y powder was mixed with commercial Fe.sub.2O.sub.3 (Alfa) with weight ratio of 85/15 for Fe.sub.2O.sub.3:Ce.sub.0.8Sm.sub.0.2O.sub.2-xN.sub.y to be used for ammonia synthesis. The loading of the composite catalysts was 300 mg after reduction to Fe: Ce.sub.0.8Sm.sub.0.2O.sub.2-xN.sub.y. The H.sub.2 and N.sub.2 flow rates were 60 ml min.sup.−1 and 20 ml min.sup.−1 respectively at ambient temperature and pressure. The synthesised ammonia was collected by 100 ml (0.01 M) H.sub.2SO.sub.4 solution and was measured by a Fisher Scientific Orion A214 ammonia meter.

[0157] Nitrogen and the value of x and y were confirmed through XRF analysis. The results are given in the table below. This gives a composition of Ce.sub.0.799Sm.sub.0.201O.sub.1.541N.sub.0.239 i.e. a=0.80, b=0.20, X=1.54 and Y=0.24. In order to calculate the given composition it was assumed that Ce had a valance of +4, Sm a valance of +3, O a valance of −2, and N a valance of −3 with a total composition charge of 0.

[0158] From this the following charge balance can be constructed.

(+4)(a)+(+3)(b)+(−2)(2−x)+(−3)(y)=0

[0159] Where a is the number of moles of Ce and b is the number of moles of Sm in the composition.

[0160] This gives:

[00002]$x=\frac{3.Math.y-4.Math.a-3.Math.b+4}{2}$

[0161] From the XRF data the value of y can be obtained through a direct mole ratio of N to Ce and Sm, allowing for x to be calculated from the above equation.

iii) Preparation of Fe—Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y

[0162] 0.01 mol, 4.3422 g Ce(NO.sub.3).sub.3.6H.sub.2O, 0.01 mol, 4.4445 g Sm(NO.sub.3).sub.3.6H.sub.2O and 0.2 mol, 12.012 g urea were mixed in a ceramic evaporating dish. Then 50 ml water was added to dissolve the mixture. The ceramic evaporating dish was then put on a hotplate. This mixture was continuously stirred at 120° C. for 24 hours before increasing the temperature to 400° C. The resulting gel like product was then combusted at 400° C. to obtain the powder product of Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y.

[0163] The obtained Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y powder was mixed with commercial Fe.sub.2O.sub.3 (Alfa) with weight ratio of 85/15 for Fe.sub.2O.sub.3:Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y to be used for ammonia synthesis. The loading of the composite catalysts was 300 mg after reduction to Fe: Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y. The H.sub.2 and N.sub.2 flow rates were 60 ml min.sup.−1 and 20 ml min.sup.−1 respectively at ambient temperature and pressure. The synthesised ammonia was collected by 100 ml (0.01 M) H.sub.2SO.sub.4 solution and was measured by a Fisher Scientific Orion A214 ammonia meter.

[0164] Nitrogen and the value of x and y were confirmed through XRF analysis. This gives a composition of Ce.sub.0.49Sm.sub.0.51O.sub.0.51N.sub.0.82 and Ce.sub.0.52Sm.sub.0.40O.sub.0.48N.sub.0.86 before and after the stability test respectively, i.e. Y=0.82 before and 0.86 after. The content of nitrogen in the samarium doped cerium oxynitride increased a little, which means the oxynitride is stable under the ammonia synthesis conditions.

TABLE-US-00002 Sample Specific surface area (m.sup.2 g.sup.−1) Fe.sub.2O.sub.3 36.541 CeO.sub.2-xN.sub.y 21.574 Fe.sub.2O.sub.3—-CeO.sub.2-xN.sub.y 26.451 Fe—CeO.sub.2-xN.sub.y 6.056 Fe—CeO.sub.2-xN.sub.y after 200 hour 7.386 stability test CeO.sub.2 46.710 Fe.sub.2O.sub.3—CeO.sub.2 18.220 Fe—CeO.sub.2 5.499 CeO.sub.2-xN.sub.y calcined 24.351 Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y 36.436 Fe.sub.2O.sub.3—Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y 31.913 Fe—Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y 7.022 Fe—Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y after 5.796 200 hour stability test
iv) Ce.sub.aSm.sub.bO.sub.2-XN.sub.y

[0165] CeO.sub.2-xNy and Ce.sub.aSm.sub.bO.sub.2-xN.sub.y were synthesised from cerium nitrate, samarium nitrate and urea as described above.

TABLE-US-00003 Denoted Lattice Composition from composition parameter a (Å) XRF results CeO.sub.2-xN.sub.y 5.4107(2) CeO.sub.1.42N.sub.0.39 C.sub.e0.9Sm.sub.0.1O.sub.2-xN.sub.y 5.4230(2) Ce.sub.0.903SM.sub.0.097O.sub.1.469N.sub.0.322 Ce.sub.0.8Sm.sub.0.2O.sub.2-xN.sub.y 5.4348(1) Ce.sub.0.799SM.sub.0.201O.sub.1.541N.sub.0.239 Ce.sub.0.7Sm.sub.0.3O.sub.2-xN.sub.y 5.4442(1) Ce.sub.0.688SM.sub.0.322O.sub.0.782N.sub.0.718 Ce.sub.0.6Sm.sub.0.4O.sub.2-xN.sub.y 5.4533(5) Ce.sub.0.605SM.sub.0.395O.sub.0.496N.sub.0.871 Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y 5.4695(5) Ce.sub.0.494SM.sub.0.506O.sub.0.514N.sub.0.822 CeO.sub.2-xN.sub.y calcined CeO.sub.1.67N.sub.0.22 CeO.sub.2-xN.sub.y promotor CeO.sub.1.37N.sub.0.42 after stability

[0166] It can be observed that the calcination (heating in air) increases the proportion of oxygen and decreases the proportion of nitrogen whereas this is not observed during stability testing under Haber-Bosch conditions (hydrogen and nitrogen atmosphere).

[0167] This is believed to be due to the presence of a large amount of N.sub.2 in the precursors whilst the oxygen partial pressure is much lower than that in air.

[0168] XRD patterns of pure and Sm-doped CeO.sub.2-xNy are similar to CeO.sub.2 (FIG. 32). Without being bound by theory, the inventors propose that these oxynitrides have the same or similar structure to CeO.sub.2. The indexed lattice parameters of these oxynitrides were obtained (FIG. 33): the lattice parameter increases with the increased Sm doping level and the trend follows Vegard's law. This indicates that all of the investigated oxynitrides are likely to be single phases, rather than a mixture. Pure CeO.sub.2 exhibits cubic fluorite structure. In the fluorite structure, the coordination numbers (CNs) for the cation and anion are 8 and 4 respectively. The ionic size of Ce.sup.4+ and Sm.sup.3+ ions at CN=8 is 0.97 Å and 1.079 Å respectively. Therefore, the doping of larger Sm.sup.3+ ions is believed to explain the increased lattice parameters of Ce.sub.aSm.sub.bO.sub.2-xN.sub.y. The existence of nitrogen defects and oxygen vacancies in the oxynitrides may also affect the lattice parameters but the effect of Sm-doping is more significant.

[0169] Raman spectra for oxide and oxynitride samples were recorded on a Renishaw inVia Reflex Raman Microscope (Gonzo) equipped with DPSS laser at an excitation wavelength λ=532 nm, using ×5 objective and Renishaw CCD detector.

[0170] Raman spectra of these samples were also collected and plotted (FIG. 30). Pure CeO.sub.2, raw and calcined CeO.sub.2-xN.sub.y show a sharp F2g peak at 465 cm.sup.−1, which corresponds to the typical fluorite structure of CeO.sub.2. A peak at 570 cm.sup.−1 is attributed to oxygen vacancies.

[0171] At 570 cm.sup.−1, no peak was observed for pure CeO.sub.2 and calcined CeO.sub.2-xN.sub.y indicating low oxygen vacancies in these samples. The peak for raw CeO.sub.2-xN.sub.y and Ce.sub.0.9Sm.sub.0.1O.sub.2-xN.sub.y at 570 cm.sup.−1 is very weak, which is attributed to a low concentration of oxygen vacancies.

[0172] With increased Sm doping level, this peak becomes stronger indicating higher concentration of oxygen vacancies. Sample Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y has the highest doping level and the peak at 570 cm.sup.−1 is also the strongest. This experiment demonstrates there are a large number of oxygen vacancies in the Sm-doped cerium oxynitrides, particularly at high doping level.

[0173] FIGS. 23 and 24 demonstrate that Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y has greater activity than Fe—CeO.sub.2 and Fe—CeO.sub.2-xN.sub.y. The activities of the following Ce.sub.aSm.sub.bO.sub.2-xN.sub.y catalysts are shown in FIGS. 26 and 27 at 3 MPa and 1 MPa respectively:

Ce.sub.0.9Sm.sub.0.1O.sub.2-xN.sub.y
Ce.sub.0.8Sm.sub.0.2O.sub.2-xN.sub.y
Ce.sub.0.7Sm.sub.0.3O.sub.2-xN.sub.y
Ce.sub.0.6Sm.sub.0.4O.sub.2-xN.sub.y
Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y

[0174] In general, sample Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y, with the measured composition of Ce.sub.0.49Sm.sub.0.51O.sub.0.51N.sub.0.82 exhibits the highest activity. The possible reason is that, it has the highest concentration of anion vacancies. About ⅓ of the anion sites are vacant. This reactant nitrogen may have strong interaction with these anion vacancies to facilitate the reaction between N.sub.2 and H.sub.2, forming ammonia. As expected, the activity at 3 MPa (FIG. 26) is much high than the activity at 1 MPa (FIG. 27).

[0175] The results for the stability test of Fe—Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y at 3 MPa and 400° C. are shown in FIG. 28. After the initial slow decrease of the activity at the first 120 hours, the catalyst became stable until the measured 220 hours.

v) Preparation of Praseodymium Doped Cerium Oxynitrides Ce.sub.aPr.sub.bO.sub.2-xN.sub.y and 85% Fe.sub.2O.sub.3: 15% Ce.sub.0.2Pr.sub.0.8O.sub.2-xN.sub.y

[0176] Ce.sub.0.1Pr.sub.0.9O.sub.2-xN.sub.y, Ce.sub.0.2Pr.sub.0.8O.sub.2-xN.sub.y, Ce.sub.0.5Pr.sub.0.5O.sub.2-xN.sub.y and Ce.sub.0.8Pr.sub.0.2O.sub.2-xN.sub.y were synthesised from cerium nitrate, praseodymium nitrate and urea analogous to the method described below for Ce.sub.0.2Pr.sub.0.8O.sub.2-xN.sub.y with adjusted molar ratio for cerium to praseodymium for each sample. The oxynitrides were found to exist as a single phase, rather than a mixture (see FIG. 33).

[0177] 0.0046 mol Ce(NO.sub.3).sub.3.6H.sub.2O, 0.0184 mol Pr(NO.sub.3).sub.3.6H.sub.2O and 0.23 mol urea were mixed in a ceramic evaporating dish. Then 50 ml water was added to dissolve the mixture. The ceramic evaporating dish was then put on a hotplate. This mixture was continuously stirred at 120° C. for 24 hours before increasing the temperature to 400° C. The resulting gel like product was then combusted at 400° C. to obtain the powder product of Ce.sub.0.2Pr.sub.0.8O.sub.2-xN.sub.y.

[0178] The obtained Ce.sub.0.2Pr.sub.0.8O.sub.2-xN.sub.y powder was mixed with commercial Fe.sub.2O.sub.3 (Alfa) with weight ratio of 85/15 for Fe.sub.2O.sub.3:Ce.sub.0.2Pr.sub.0.8O.sub.2-xN.sub.y to be used for ammonia synthesis. The loading of the composite catalysts was 300 mg after reduction to Fe: Ce.sub.0.2Pr.sub.0.8O.sub.2-xN.sub.y. The H.sub.2 and N.sub.2 flow rates were 60 ml min.sup.−1 and 20 ml min.sup.−1 respectively at ambient temperature and pressure. The synthesised ammonia was collected by 100 ml (0.01 M) H.sub.2SO.sub.4 solution and was measured by a Fisher Scientific Orion A214 ammonia meter.

vi) Preparation of Ru—Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y

[0179] 0.01 mol, 4.3422 g Ce(NO.sub.3).sub.3.6H.sub.2O, 0.01 mol, 4.4445 g Sm(NO.sub.3).sub.3.6H.sub.2O and 0.2 mol, 12.012 g urea were mixed in a ceramic evaporating dish. Then 50 ml water was added to dissolve the mixture. The ceramic evaporating dish was then put on a hotplate. This mixture was continuously stirred at 120° C. for 24 hours before increasing the temperature to 400° C. The resulting gel like product was then combusted at 400° C. to obtain the powder product of Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y.

[0180] 0.1054 g Ru.sub.3C.sub.12O.sub.12 (Alfa 99%) was dissolved in 50 mL of tetrahydrofuran (Fisher 99.5%) and continuously stirred for 4 hours. 0.450 g Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y powder was added to this solution and continuously stirred for 24 hours. Tetrahydrofuran was evaporated at room temperature and the obtained composite catalyst powder was loaded in to the reactor to give a loading of 0.300 g after reduction to Ru—Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y with a weight ratio of 10/90 respectively. The H.sub.2 and N.sub.2 flow rates were 60 ml min.sup.−1 and 20 ml min.sup.−1 respectively at ambient temperature and pressure. The synthesised ammonia was collected by 100 ml (0.01 M) H.sub.2SO.sub.4 solution and was measured by a Fisher Scientific Orion A214 ammonia meter. The catalytic activity was carried out after reducing the Ru—Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y catalyst in the mixture of N2/H2 (molar ratio 1:3) at 450° C. for overnight.

[0181] The activities of Ru—Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y at 30 bar and 10 bar under different reaction temperatures are shown in FIG. 29. At the 3 MPa, the activity of Ru—Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y is much higher than that for Fe—Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y although the weight ratio is different. The Ru catalyst promoted by oxynitride Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y is still active at a temperature as low as 200° C. whilst under the same pressure, the Fe-based catalysts do not show good activity at a temperature of 300° C. This indicates that materials with anion vacancies such as oxynitrides are excellent promoters for Ru catalyst as well. As the cost of Ru catalyst is high, one of the strategies is to introduce a small amount of Ru (less than 20 wt %, ideally less than 5 wt % of the total weight) Ru into the low-cost Fe-based catalyst promoted by materials with anion vacancies, typically oxynitrides, such as Ce.sub.0.5Sm.sub.0.5O.sub.2-xN.sub.y in order to achieve both low-cost and high activity.