Catalyst for Ammonia Synthesis

Abstract

The invention concerns a catalyst for the low energy manufacture of ammonia; a process for manufacturing said catalyst; and a process for low energy manufacture of ammonia comprising the use of said catalyst.

Claims

1. An atomic metal catalyst, said catalyst comprising a plurality of metal atom clusters supported on the surface of a solid substrate, wherein each metal atom cluster independently comprises from about 1 to about 500 metal atoms.

2. The atomic metal catalyst according to claim 1, wherein each metal atom cluster independently comprises from about 1 to about 10 metal atoms.

3. The atomic metal catalyst according to claim 1, wherein each of said metal atom clusters comprises one or more metals selected from: lead (Pb), silver (Ag), gold (Au), platinum (Pt), molybdenum (Mo), tungsten (W), rhenium (Re) cobalt (Co), ruthenium (Ru), rhodium (Rh) and iron (Fe).

4. The atomic metal catalyst according to claim 3, wherein each of said metal atom clusters comprises one or more metals selected from Pt, Mo, Re, Co, Ru, Rh and Fe.

5. The atomic metal catalyst according to claim 4, wherein each of said metal atom clusters comprises Fe atoms.

6. The atomic metal catalyst according to claim 1, wherein said metal atom clusters cover from 0.1 to 20% of the surface of the substrate.

7. The atomic metal catalyst according to claim 1 ms, wherein said substrate is a silicon or carbon-based material, an oxide, a hydride, a nitride or a MXene.

8. The atomic metal catalyst according to claim 7, wherein said substrate is a carbon material.

9. The atomic metal catalyst according to claim 8, wherein said carbon material is doped with one or more heteroatom containing dopants, optionally wherein said dopant(s) cover from 0.1 to 20% of the surface of the substrate.

10. A method for preparing the catalyst according to claim 1, wherein said method is a cluster deposition process in which said metal atom clusters are formed and then deposited onto the surface of said substrate; or wherein said method is an atom deposition process in which individual metal atoms are deposited, and then form metal atom clusters, on said substrate surface.

11. The method according to claim 10, comprising depositing a plurality of metal atoms and/or metal atom clusters onto the surface of a solid substrate by a cluster deposition, evaporation deposition, sputter deposition or pulsed laser deposition process, wherein each metal atom cluster independently comprises from about 1 to about 500 metal atoms.

12. The method according to claim 11, wherein each metal atom cluster independently comprises from about 1 to about 10 metal atoms.

13. The method according to claim 10, wherein said method is a cluster deposition process, the method comprising the following steps: (i) providing a cluster beam deposition source comprising a plasma sputtering and gas condensation chamber, a mass filter chamber and a deposition chamber; (ii) disposing in the condensation chamber a metal catalyst target comprising metal atoms; (iii) disposing a solid substrate in the deposition chamber; (iv) performing a magnetron sputtering step in said condensation chamber that comprises sputtering said metal catalyst target with plasma so as to eject metal atoms, followed by a condensing step in which said ejected atoms form positively charged metal ion clusters by cooling in an inert gas; (v) separating and selecting on the basis of size metal ion clusters in said mass filter chamber; and (vi) depositing said metal ion clusters of chosen size on the surface of said substrate in said deposition chamber.

14. The method according to claim 13, wherein said metal atom target comprises one or more metal selected from: lead (Pb), silver (Ag), gold (Au), platinum (Pt), molybdenum (Mo), tungsten (W), rhenium (Re), cobalt (Co), ruthenium (Ru), rhodium (Rh) and iron (Fe).

15. The method according to claim 14, wherein said metal atom target comprises one or more metal selected from Pt, Mo, Re, Co, Ru, Rh and Fe.

16. The method according to claim 15, wherein said metal atom target comprises Fe atoms.

17. The method according to claim 10, wherein said substrate is a silicon or carbon-based material, an oxide, a hydride, a nitride or a MXene.

18. The method according to claim 17, wherein said substrate is a carbon material, doped with one or more heteroatom containing dopants.

19. The method according to claim 13, wherein in step (iv) said metal catalyst target is sputtered with an inert gas, preferably argon, plasma, and/or wherein said clusters are formed in step (iv) by condensation in a pressure of helium gas cooled to about 80 to about 120 K.

20. The method according to claim 13, wherein in step (vi) metal ion clusters comprising 1, 2 or 3 metal atoms are deposited on the surface of the substrate.

21. A method for producing ammonia, the method comprising: (i) disposing in a reactor a catalyst bed comprising an atomic metal catalyst according to claim 1; (ii) passing one or more sources of nitrogen (N.sub.2) and one or more sources of hydrogen (H.sub.2) over said catalyst bed; (iii) obtaining a product stream comprising ammonia (NH.sub.3).

22. The method according to claim 21, wherein step (ii) is carried out at a temperature in the range of from about 20 C. to about 250 C., and/or at a pressure of no more than about 3 MPa (30 bar).

23. The method according to claim 22, wherein step (ii) is carried out at a temperature in the range of from about 30 C. to about 75 C., and/or at a pressure of no more than about 1 MPa (10 bar).

24. The method according to claim 21, wherein the catalyst bed is reduced prior to step (ii), by exposure to H.sub.2 at a temperature up to about 400 C.

25. The method according to claim 21, wherein the one or more source of hydrogen is prepared from a green hydrogen feedstock, and/or the method is powered by renewable energy.

26. A method for producing ammonia via heterogeneous catalysis, the method comprising: (i) disposing in a reactor a catalyst bed comprising an atomic metal catalyst; (ii) passing nitrogen (N.sub.2) and hydrogen (H.sub.2) over said catalyst bed; (iii) obtaining a product stream comprising ammonia (NH.sub.3); wherein the atomic metal catalyst comprises a plurality of metal atom clusters supported on the surface of a solid substrate, wherein each metal atom cluster independently consists of from 1 to 500 metal atoms, wherein the number of metal atoms is determined by STEM; wherein the said substrate is a silicon or carbon-based material, an oxide, a hydride, a nitride or a MXene; wherein each of said metal atom clusters comprises platinum (Pt), molybdenum (Mo), rhenium (Re) cobalt (Co), ruthenium (Ru), rhodium (Rh) and iron (Fe).

Description

[0053] The Invention will now be described by way of example only with reference to the Examples below and to the following Figures wherein:

[0054] FIG. 1. Schematic illustration of the size selected cluster beam source, which combines the magnetron sputtering technique and gas condensation technique;

[0055] FIG. 2. STEM images of Fe dimer deposited on graphene oxide coated TEM grid;

[0056] FIG. 3. N1s XPS spectra of Fe dimer on N doped carbon (Fe2NC), N doped carbon (NC) alone and Fe dimer on carbon (Fe2C);

[0057] FIG. 4. N1s XPS spectra of Fe.sub.2NC system under different conditions;

[0058] FIG. 5. Fe L-edge NEXAFS of Fe2NC system and Fe2C system under different conditions;

[0059] FIG. 6. N1s peak of N doped carbon under different conditions. The temperature (T) is under room temperature (rt) and the flow of H.sub.2 and N.sub.2 is in sccm;

[0060] FIG. 7. Partial mass spectra of Fe2NC system (top spectrum) and Fe2C system (bottom spectrum). The flow of H.sub.2 and N.sub.2 is in sccm, and the partial pressure the signal of the mass is in mbar;

[0061] FIG. 8. The catalytic activity of FeTiH.sub.2 powder towards ammonia production (200 C.; 10 bar). The metal loading of Fe clusters on TiH.sub.2 powder is normalised to 0.1%. The products were analysed with gas chromatography each hour.

MATERIALS AND METHODS

[0062] The metal catalysts generated for this project were produced at the new Swansea Satellite Nanolab at Diamond Light Source (B07) with the cluster beam deposition technique (instrument built by Swansea).

[0063] The cluster beam deposition source (FIG. 1) is a vacuum-based, magnetron-sputtering, gas condensation source, equipped with a lateral time-of-flight mass filter [7]. A metal target is sputtered by a DC Argon plasma; the hot metal atoms are condensed into clusters in a pressure of helium gas cooled to 100K by liquid nitrogen; positively charged clusters are extracted and focused by an ion optics into the mass filter for size selection. The resolution of the mass filter was about 1 atom in 20, and the transmission efficiency for the selected mass was >50%. In this project, a series of atomic metal catalysts (Fe and Pt) including metal single atoms, dimers and trimers were produced. The atomic catalysts were deposited onto transmission electron microscopy (TEM) grids (graphene oxide coated grid, 3 mm in diameter) for TEM characterisation [8] (1% surface coverage), and XPS and near edge X-ray absorption fine structure (NEXAFS) study [9] (3%-4% surface coverage). In order to prevent the surface diffusion of atomic catalysts, N-doping (5% surface coverage) of the carbon (graphene oxide) was also conducted prior to the Fe deposition. These materials were imaged by scanning transmission electron microscopy (STEM) at Swansea University and Diamond Light Source.

[0064] XPS and NEXAFS experiments under near-ambient pressures were conducted at the B07 beamline at Diamond Light Source. Ambient pressure XPS and NEXAFS, together with multi mass spectrometry systems, were conducted to validate the catalytic activity of different atomic catalysts towards thermochemical ammonia synthesis, by exposing the samples to pure N.sub.2 and N.sub.2+H.sub.2 at temperatures between room temperature and 400 C., and a pressure range of 10.sup.4 to 10 mbar. The adsorbed reactive intermediates (e.g. *NHx) and molecules were monitored in real time via chemical shifts in the N 1 s core level. The chemical/oxidation state, etc. were studied by monitoring the Fe 2 p core levels.

[0065] In a further example, a thin layer of iron clusters (approx. maximum cluster size 1 nm) was deposited onto TiH.sub.2 particles (average particle size 20 m), enclosed within a metal cup having a stirring function, using a magnetron sputtering technique. Then, the catalytic activity of the iron cluster coated TiH.sub.2 particles towards ammonia production was measured using a high pressure reactor, and the products were analysed using gas chromatography or liquid chromatography. Specifically, the reaction was tested at a temperature of 200 C. and a pressure of 1 MPa (10 bar) in a mixture of N.sub.2 (10 ml/min) and H.sub.2 (30 ml/min) following dilution of the catalyst with SiC powder to improve the heat transfer.

RESULTS

[0066] Images after deposition of the Fe dimers fabricated by cluster beam deposition are shown in FIG. 2. As the landing energy of those dimer is 9 eV per atom, they may break into single atoms on surface impact. The bright dots shown in the STEM images are Fe dimers and single atoms.

[0067] Three samples were tested for ammonia synthesis, namely, Fe dimers on an N-doped carbon (graphene oxide) support (Fe.sub.2NC), the N-doped carbon support itself (NC) and Fe dimers on the carbon support without doping (Fe.sub.2C). FIG. 2 shows the N1s XPS spectra of those samples under vacuum conditions. The deposition chamber of the cluster source was vented with N.sub.2 gas after cluster deposition and then the samples were transferred (in air) to the beamline for measurement. The N in the bare NC system is from the N doping process. Both pyridinic N and pyrrolic N are found on the surface of this support. The addition of Fe dimers on the N doped carbon introduces a new peak corresponding to FeNx species. This implies that the Fe dimer is very active for N.sub.2 fixation. Once the NC system is decorated by Fe dimers, the N.sub.2 molecules are adsorbed on the surface of those dimers. This can also be confirmed by the experiments with the Fe.sub.2C system without dosing N.sub.2 into the chamber. Here there is no N doping and the surface N is from the chamber venting process and air transfer.

[0068] All the samples were tested for ammonia synthesis under mixed N.sub.2 and H.sub.2 gas atmosphere, after sample reduction in H.sub.2 at temperature up to 400 C. FIG. 4 shows the evolution of the N1s peak under different conditions for the Fe.sub.2NC system. The intensity of FeNx/NHx increases once N.sub.2 is introduced into the chamber, which means the NN triple bond splits and FeNx or NHx species are formed. The intensity of this peak decreases when N.sub.2 is depleted in the chamber.

[0069] The reduction process of the Fe dimer catalysts on N-doped carbon and bare carbon were monitored with Fe L-edge NEXAFS spectra as shown in FIG. 5. Both Fe.sup.2+ and Fe.sup.3+ states are found in the samples after deposition of metal atom clusters. The Fe.sup.3+ can be quickly reduced to Fe.sup.2+ in an H.sub.2 atmosphere (e.g. 500 Pa (5 mbar), at a temperature of 300 C. or above). Reduction to Fe.sup.2+ was similarly achieved for the Fe.sub.2NC system. In the case of the Fe.sub.2C system, the Fe.sup.3+ became much more difficult to reduce to Fe.sub.2+ in subsequent reduction cycles, implying that there is change in the surface morphology, e.g. sintering, or atomic configuration during the heating process.

[0070] The presence of the N1s peak (NC sample) shown in FIG. 6 before conducting reduction indicates the successful doping of N into carbon (graphene oxide). This peak disappears in the reduction process. The formation of ammonia is detected in this process, which means the bonds between the doped N atoms and the carbon support are not strong enough to prevent the N forming NH.sub.3 in H.sub.2 atmosphere. The N peak cannot be recovered by reintroducing N.sub.2 gas as there is no catalyst for NN bond splitting.

[0071] The catalytic activity of Fe dimers was monitored by mass spectrometry as shown in FIG. 7. The mass of ammonia is 17 amu. However, water (18 amu) also creates a secondary signal of 17 amu, so the 17 amu is not a good indicator of formation of ammonia. As ammonia forms a strong signal at 16 amu in mass spectrometry, here we use 16 amu as the indication of ammonia. The NH.sub.3 signal shown in pink was obtained from the subtraction of the contribution of water at 17 amu. The mass spectrometry was done in a pressure of 5 mbar in the main chamber. In the Fe.sub.2NC system, it can be seen that the ammonia synthesis reaction can be triggered by introducing a minimal flow of N.sub.2 (1 sccm) to the chamber with a temperature <50 C. The successful synthesis of ammonia can be confirmed by the signal increase of both 16 amu and NH.sub.3 signal (described above). Under high temperature testing, the Fe.sub.2C system shows similar behaviour as the Fe.sub.2NC system for the formation of ammonia.

[0072] FIG. 8 shows the catalytic activity of the FeTiH.sub.2 particle system towards ammonia production. Notably, the catalyst was seen to stabilise during the first two hours of the reaction (as shown in the drop of catalytic activity) before stabilising for the remaining duration of measurement (7 hours).

SUMMARY

[0073] Few atom Fe catalysts (1, 2 and 3 atoms) have been successfully synthesised and deposited with the cluster beam deposition technique. So far the Fe dimers have been tested for ammonia synthesis on two different supports, including carbon and N-doped carbon. Both the Fe.sub.2NC system and the Fe.sub.2C system show catalytic activity for N.sub.2 reduction to ammonia. Compared with the Fe2C system, the Fe2NC system is much more stable as shown when subjected to high temperature reduction cycles. The Fe dimers can catalyse this reaction with pressure as low as 5 mbar, with higher pressures expected to achieve higher yields, and temperature <50 C.

[0074] Fe clusters (approx. maximum cluster size 1 nm) were also deposited by magnetron sputtering onto TiH.sub.2 particles, and the resultant Fe catalyst was shown to be catalytically active towards ammonia production under milder conditions than those conventionally used in the Haber-Bosch process.

REFERENCES

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