Bimetallic ammonia cracking catalyst containing ruthenium and rhodium

Abstract

A catalyst composition for converting ammonia (NH3) into nitrogen (N2) and hydrogen (H2) comprises ruthenium (Ru) and rhodium (Rh) as catalytically active materials. The Ru and Rh are supported on a support material. The composition comprises Rh in a mass ratio of 0.2 to 0.5 of the total mass of Rh and Ru combined, and less than 3 wt% Ru and less than 1.5 wt% Rh based on the mass of the composition.

Description

[0001] P102052W001

[0002] BIMETALLIC AMMONIA CRACKING CATALYST

[0003] Field of the invention

[0004] This invention relates to catalytic ammonia (NH3) cracking, or decomposition, into nitrogen (N2) and hydrogen (H2). The invention provides a catalyst composition for converting ammonia into nitrogen and hydrogen. The invention also provides a process for preparing a catalyst composition for converting ammonia into nitrogen and hydrogen. The invention further provides a process for converting ammonia into nitrogen and hydrogen, a process and system for power generation and a fuel combustion and exhaust system.

[0005] Background to the invention

[0006] Ammonia is a carbon-free fuel that provides an attractive alternative to fossil fuels. More relevant for present purposes is that, due to the established ammonia infrastructure, mild storage conditions, and high energy density, ammonia has also been identified as a promising hydrogen (H2) storage medium and a key part in hydrogen economy.

[0007] Ammonia decomposition, or cracking, involves the catalytic decomposition of ammonia into hydrogen and nitrogen. Exploiting such decomposition in the production of hydrogen as a fuel for use, for example, in a fuel cell system or in a hydrogen fuelled internal combustion engine is attractive, considering that ammonia has a high energy density and a high hydrogen storage capacity. Moreover, no carbon dioxide (CO2) is produced in the decomposition.

[0008] The on-site / on-board usage of ammonia as a source of hydrogen remains a challenge, however, since it requires a facile conversion of ammonia to hydrogen at low temperatures, typically temperatures below 450°C. Therefore, the development of effective ammonia cracking catalysts is becoming increasingly important to provide a viable way to convert ammonia into hydrogen.

[0009] Catalytic ammonia decomposition may also be useful in the treatment of ammonia emissions, in applications where oxygen is absent or not present in sufficient quantities to employ ammonia oxidation. For example, exhaust gas from an ammonia-fuelled internal combustion engine may comprise slipped ammonia which should be treated before emission to the atmosphere. P102052W001

[0010] Supported ruthenium (Ru) catalysts are widely regarded as the most active ammonia cracking catalysts but have a significant cost implication. The possibility to utilise lower cost alternatives such as iron (Fe), cobalt (Co), chromium (Cr), nickel (Ni) and copper (Cu) has been demonstrated, but these alternatives fail to reach appreciable activity at low temperatures.

[0011] McCullough et al. (Material Discovery and High Throughput Exploration of Ru Based Catalysts for Low Temperature Ammonia Decomposition, Materials 2020, 13, 1869, MDPI, 16 April 2020) analyses the ammonia cracking activity of a variety of y-alumina (Y-AI2O3) supported bimetallic potassium (K) promoted ruthenium catalysts, concluding that partial substitution of ruthenium with magnesium (Mg), calcium (Ca), strontium (Sr), scandium (Sc), yttrium (Y), tantalum (Ta), hafnium (Hf), and zirconium (Zr) results in catalyst formulations that are more active than a baseline potassium promoted ruthenium catalyst, and that such metals may therefore be employed to reduce ruthenium loading on ammonia cracking catalysts without sacrificing catalytic activity.

[0012] A need remains, nevertheless, to provide improved ruthenium-based ammonia cracking catalysts that achieve higher catalytic activities at lower ruthenium loadings.

[0013] Summary of the invention

[0014] In the present invention, it was surprisingly found that catalytic ammonia cracking activity of ruthenium-based catalyst compositions may be enhanced by the inclusion of small amounts of rhodium (Rh) in the composition, as compared to catalyst compositions comprising ruthenium only.

[0015] Thus, broadly, the invention provides a bimetallic ruthenium-rhodium ammonia cracking catalyst composition.

[0016] More specifically, one aspect of the invention provides a catalyst composition for converting ammonia (NH3) into nitrogen (N2) and hydrogen (H2), the catalyst composition comprising catalytically active material supported on a support material, which catalytically active material comprises ruthenium (Ru) and rhodium (Rh), wherein the composition comprises Rh in a mass ratio of 0.2 to 0.5 of the total mass of Rh and Ru combined, and wherein the composition comprises less than 3 wt% Ru and less than 1.5 wt% Rh based on the mass of the composition. P102052W001

[0017] The composition may comprise Rh in a mass ratio of 0.2 to 0.4 of the total mass of Rh and Ru combined, preferably in a mass ratio of 0.25 to 0.36, most preferably 0.27 to 0.36.

[0018] The support material may be a refractory metal oxide. For example, the support material may be selected from the group consisting of alumina, silica, titania, zirconia, ceria, zinc oxide, tin oxide, magnesium oxide, yttrium oxide, tantalum oxide, hafnium oxide, lanthanum oxide and a mixed or composite oxide thereof, e.g. a mixed oxide or a composite oxide of two or more thereof. Preferably, the refractory oxide support material is alumina. The alumina may be gamma phase alumina, theta phase alumina, alpha phase alumina, delta (5), eta (q) and kappa (K) or a mixture thereof.

[0019] In an alternative embodiment of the invention, the support material is a molecular sieve. For example, the support material may be a zeolite, such as a small, medium or large pore zeolite. The zeolite may preferably be a small pore zeolite, for example having a framework type selected from AEI, AFX, CHA, LTA or ERI, a medium pore zeolite, for example having a framework type selected from FER, MEL, MFI, STI and STT, or a large pore zeolite, for example having a framework type selected from AFI, BEA, MAZ, MOR, Y, FAU and OFF.

[0020] Alternatively, the support material may be a carbon-based material. For example, the support material may be activated carbon, charcoal, lampblack, carbon nanofiber carbon nanotubes. [ALSO SUBSTRATE CAN BE CARBON]

[0021] The composition may comprise, by mass, from 0.5 wt% to 2.5 wt% Ru based on the mass of the composition.

[0022] Preferably, the composition comprises, by mass, from 0.75 wt% to 2 wt% Ru based on the mass of the composition.

[0023] More preferably, the composition comprises, by mass, from 1 wt% to 2 wt% Ru based on the mass of the composition.

[0024] Still more preferably, the composition comprises, by mass, from 1 wt% to 1.5 wt% Ru based on the mass of the composition.

[0025] The composition may comprise, by mass, from 0.1 wt% to 1 .5 wt% Rh based on the mass of the composition. P102052W001

[0026] Preferably, the composition comprises, by mass, from 0.1 wt% to 1 wt% Rh based on the mass of the composition.

[0027] More preferably, in such an embodiment of the invention, the composition comprises, by mass, from 0.2 wt% to 0.5 wt%, for example 0.3 wt% to 0.5 wt% or about 0.4 to about 0.5 wt%, Rh based on the mass of the composition.

[0028] In the composition, the ruthenium and the rhodium would typically, independently, be present in metallic form or oxide form.

[0029] The catalytically active material may further comprise a promoter metal, such as an alkali or alkaline earth metal. Preferably, the promoter metal is selected from potassium (K), caesium (Cs), sodium (Na), lithium (Li) or mixtures of one or more thereof. The catalyst composition may comprise, by mass, from 1 % to 10%, more preferably from 2% to 6%, e.g. 2%, of the promoter metal-based on the mass of the composition. The promoter would typically be present in the catalyst composition as an oxide, but may alternatively or additionally be present in the form of a hydroxide, nitrate, acetate or carbonate.

[0030] The catalytically active material is supported by the support material, that is, the catalytically active material may be disposed on or within the support material. The catalytically active material may be disposed directly onto or is directly supported on the support material (e.g. there is no intervening material between the catalytically active material and the support material). The catalytically active material may be dispersed on the support material (e.g. particles of the catalytically active material are dispersed over the surface of a particulate refractory oxide). Alternatively, the catalytically active material may be present within the pores of the support material, for example, the support material is impregnated with the catalytically active material.

[0031] The catalyst support material may be loaded with the catalytically active material by any known means, for example by means of precipitation or ion exchange. The Ru, Rh and optional promoter may be loaded separately or at the same time. For example, the catalyst support material in powder form may be combined with an aqueous solution comprising a Ru salt, a Rh salt and, where applicable, a salt of the promoter metal. Suitable salts may include nitrate, acetate, hydroxide, chloride or oxalate salts thereof.

[0032] The catalyst composition may be disposed on a substrate. The substrate may be a flow- through substrate or a filter substrate, such as a wall flow filter substrate. P102052W001

[0033] The term “disposed on” in this context may encompass both having the composition directly disposed on the substrate, i.e. with no intervening material, and / or indirectly disposed on the substrate, i.e. with intervening material. If the substrate is porous, then the term “disposed on” may also encompass having the composition disposed therein, for example within the pores of the substrate, i.e. wherein the composition is disposed thereon and / or therein. The catalytic composition is typically disposed on the substrate in the form of a washcoat. The term “washcoat” as used herein is well-known in the field and refers to an adherent coating that is applied to a substrate usually during the production of a catalyst.

[0034] When the catalyst composition is coated on a substrate, the substrate is preferably a ceramic substrate or a metallic substrate.

[0035] When the substrate is a ceramic substrate, the ceramic substrate may be made of any suitable refractory material, e.g., alumina, silica, titania, ceria, zirconia, magnesia, zeolites, silicon nitride, silicon carbide, zirconium silicates, magnesium silicates, aluminosilicates, metallo aluminosilicates (such as cordierite and spudomene), or a mixture or mixed oxide of any two or more thereof. Cordierite, a magnesium aluminosilicate, and silicon carbide are particularly preferred.

[0036] When the substrate is a metallic substrate, the metallic substrate may be made of any suitable metal, and in particular heat-resistant metals and metal alloys such as titanium and stainless steel as well as ferritic alloys containing iron, nickel, chromium, and / or aluminum in addition to other trace metals.

[0037] When the substrate is a flow-through substrate, the flow-through substrate is preferably a flow-through monolith having a honeycomb structure with many small, parallel thin-walled channels running axially through the substrate and extending throughout from an inlet or an outlet of the substrate. The channel cross-section of the substrate may be any shape, but is preferably square, sinusoidal, triangular, rectangular, hexagonal, trapezoidal, circular, or oval. The flow-through substrate may also be of high porosity which allows the catalyst to penetrate into the substrate walls.

[0038] When the substrate is a wall flow filter substrate, the filter substrate is preferably a wall-flow monolith filter. The channels of a wall-flow filter are alternately blocked, which allow a gas stream to enter a channel from the inlet, then flow through the channel walls, and exit the P102052W001 filter from a different channel leading to the outlet. Particulates in the gas stream are thus trapped in the filter.

[0039] Alternatively, the substrate may be a plate-type substrate. The plate-type substrate can be made of a metal such as stainless steel The plate substrate may be a metal mesh or a corrugated metal sheet onto which the catalyst composition may be deposited.

[0040] The catalyst composition may be added to the substrate by any known means, such as a washcoating procedure, wherein a solution, slurry or suspension of the catalyst composition in a solvent is coated onto the substrate. The coated substrate then typically undergoes a calcination step, to remove solvent and to fix the catalytically active material to the substrate.

[0041] In an alternative embodiment of the invention, instead of being coated on a substrate, the catalyst composition may be pelletised. In other words, the catalyst composition, typically including a suitable binder in such a case, may be formed into pellets using a suitable pelletising process.

[0042] In another alternative embodiment of the invention, instead of being coated on a substrate or pelletised, the catalyst composition may be in the form of an extruded catalyst composition.

[0043] Another aspect of the invention provides a process for preparing a catalyst composition according to the invention, the process including contacting a catalyst support material with a solution comprising dissolved catalytically active material precursors of ruthenium and rhodium such that the support material is loaded with ruthenium and rhodium as catalytically active materials.

[0044] A further aspect of the invention provides a process for converting ammonia into nitrogen and hydrogen, the process including contacting a gaseous stream that comprises ammonia with a catalyst composition according to the invention at a temperature above 200°C.

[0045] Preferably, in such an embodiment of the invention, the temperature at which the gaseous stream comprising ammonia is contacted with the catalyst composition is in a range of from > 200°C to 500°C, preferably in the range 300 to 500°C, more preferably >350 to 500°C.

[0046] A further aspect of the invention provides a process for generating power comprising: (a) converting ammonia into nitrogen and hydrogen by contacting a gaseous stream comprising ammonia with a catalyst composition according to the invention at a temperature above P102052W001

[0047] 200°C; and (b) feeding hydrogen produced by step (a) to a hydrogen-fuelled internal combustion engine or fuel cell.

[0048] A further aspect of the invention provides a power generation system comprising: an ammonia source; an ammonia cracking unit; and a hydrogen-fuelled internal combustion engine or fuel cell; wherein the ammonia cracking unit comprises the catalyst composition according to the invention.

[0049] A further aspect of the invention provides a fuel combustion and exhaust system comprising a fuel combustor and an exhaust gas treatment system, wherein the fuel combustor is in fluid communication with the exhaust gas treatment system and is an ammonia-containing-fuel- combustor, and wherein the exhaust gas treatment system comprises the catalyst composition according to the invention. The term “ammonia-containing-fuel-combustor” as used herein may encompass a fuel combustor that is designed to burn a fuel that comprises ammonia. The ammonia-containing-fuel-combustor may be an ammonia-fuelled internal combustion engine.

[0050] Brief description of the drawings

[0051] The features of the invention are exemplified in the following examples, with reference to the appended drawings, in which:

[0052] Figure 1 shows the effect of Rh presence and Ru / Rh mass ratio on catalytic ammonia cracking performance;

[0053] Figure 2 shows the effect of Ru / Rh ratio on catalytic conversion of ammonia and ammonia decomposition rate at 300°C;

[0054] Figure 3 shows the apparent activation energy barrier (Ea) of catalyst compositions according to the invention compared to a Ru-only composition;

[0055] Figure 4 shows the light-off curve of Ru coated and Ru-Rh coated monoliths respectively in 5% NH3 / N2; and

[0056] Figure 5 shows steady state ammonia conversion over Ru coated and Ru-Rh coated monoliths respectively in 5% NH3 / N2.

[0057] Examples P102052W001

[0058] Example 1

[0059] Catalyst compositions according to the invention were made using a conventional washcoat synthesis method.

[0060] The catalyst compositions were prepared by dispersing the support material and precursor salts of Ru and, where applicable, Rh in water to form a slurry with a solids content of less than 50%. A particulate gamma alumina was used as the support material. The ruthenium and rhodium precursor were ruthenium acetate and rhodium nitrate respectively

[0061] The pH of the slurry comprising the dissolved precursors was adjusted using nitric acid (HNO3), tetraethylammonium hydroxide (TEACH) and ammonia hydroxide (NH4OH) and was maintained for at least 1 hour. The slurry was coated on a substrate and then dried at 80°C and then calcined at approximately 250°C for 2 hours in air.

[0062] The following catalyst compositions were made:

[0063] Table 1

[0064] The performance of the respective catalyst compositions for ammonia cracking was investigated by contacting a packed bed reactor comprising 0.2g of each catalyst composition, in powder format, with a 1% NH3 / Ar gas stream at a rate of 400 ml / min over a temperature range of from 150°C to 400°C.

[0065] The results are shown in Figure 1 , from which it can be seen that each of the Ru-Rh bimetallic catalyst compositions exhibited superior ammonia cracking performance compared to the Ru-only catalyst composition (composition 1), wherein the best improvement is achieved by P102052W001 the 1 wt% Ru and 0.4-0.5 wt% Rh compositions (compositions 3 and 4), while the 0.5 wt% Rh-only composition (composition 7) showed no low temperature activity.

[0066] An assessment was also made of the effect of Ru / Rh ratio, which is represented in Figure 2 and shows that as the Rh / (Ru+Rh) ratio increases to 0.35, catalytic conversion of ammonia and ammonia decomposition rate per gram Ru+Rh increases compared to both the Ru-only and Rh-only catalyst compositions, indicating a synergistic effect between Ru and Rh.

[0067] As the Rh / (Ru+Rh) ratio increases further, to >0.4, a decline in cracking performance is observed, suggesting that increasing the amount of Rh blocks surface Ru sites on the catalyst composition, suggesting the existence of an optimal range for the Rh / (Ru+Rh) ratio.

[0068] The kinetic parameter of compositions 1 , 2 and 3 was evaluated by the apparent activation energy barrier (Ea) as shown in Figure 3. The Ru-only formulation (composition 1) shows an Ea of 93 kJ / mol. As the Rh / (Ru+Rh) ratio increases to 0.2 and ~ 0.3 (compositions 2 and 3), the Ea decreases to 77 and 66 kJ / mol respectively. The decreased Ea suggests that the combination of Ru and Rh creates unique active sites which facilitate the ammonia decomposition reaction.

[0069] Example 2

[0070] A Ru-Rh bimetallic catalyst composition of the invention, comprising 0.88 wt% Ru and 0.5 wt% Rh supported on alumina such that the composition comprised, by weight, 98 wt% alumina, 0.88 wt% Ru and 0.5 wt% Rh (composition 8), was scaled up and coated on a ceramic monolith of 3-inch diameter to provide a coating density of 38g Ru / ft3and 22g Rh / ft3.

[0071] The ammonia cracking performance of the coated monolith was compared to an identical monolith coated with a Ru only catalyst composition (composition 9) with a coating density of 38g Ru / ft3in a 5%NHs / N2 gas stream. The results are represented in Figure 4 and Figure 5.

[0072] As shown in Figure 4, it was found that the light off curve shifts to lower temperature as Rh is added to Ru.

[0073] The steady state conversion of ammonia represented in Figure 5 also confirms that the Ru- Rh bimetallic monolith is twice as active as Ru monolith with the same Ru loading.

Claims

P102052W001Claims1. A catalyst composition for converting ammonia (NH3) into nitrogen (N2) and hydrogen (H2), the catalyst composition comprising catalytically active material supported on a support material, which catalytically active material comprises ruthenium (Ru) and rhodium (Rh), wherein the composition comprises Rh in a mass ratio of 0.2 to 0.5 of the total mass of Rh and Ru combined, and wherein the composition comprises less than 3 wt% Ru and less than 1 .5 wt% Rh based on the mass of the composition.

2. The catalyst composition according to claim 1 , wherein the composition comprises Rh in a mass ratio of 0.2 to 0.4 of the total mass of Rh and Ru combined, more preferably 0.25 to 0.35.

3. The catalyst composition according to claim 1 or claim 2, wherein the support material is selected from the group consisting of alumina, silica, titania, zirconia, ceria, zinc oxide, magnesium oxide, lanthanum oxide and a mixed or composite oxide thereof, e.g. a mixed oxide or a composite oxide of two or more thereof.

4. The catalyst composition according to any one of claims 1 to 3, which comprises, by mass, from 0.5 wt% to 2.5 wt% Ru based on the mass of the composition, preferably 0.75 wt% to 2 wt%, more preferably 1 wt% to 2 wt%, such as 1 wt% to 1.5 wt% based on the mass of the composition.

5. The catalyst composition according to any one of claims 1 to 4, which comprises, by mass, from 0.1 wt% to 1.5 wt% Rh based on the mass of the composition, preferably 0.1 wt% to 1 wt%, more preferably 0.2 wt% to 0.5 wt%,most preferably 0.3 wt% to 0.5 wt%.

6. The catalyst composition according to claim 5, which comprises from about 0.4 wt% to about 0.5 wt% Rh based on the mass of the composition.

7. The catalyst composition according to any one of claims 1 to 6, wherein the catalyst composition is coated on a substrate.

8. The catalyst composition according to claim 7, wherein the substrate is a flow- through substrate or a wall flow filter substrate.P102052W0019. The catalyst composition according to any one of claims 1 to 6, wherein the catalyst composition is pelletised.

10. The catalyst composition according to any one of claims 1 to 6, wherein the catalyst composition is in the form of an extruded catalyst composition.11 . A process for preparing a catalyst composition according to any one of claims 1 to 10, the process including contacting a catalyst support material with a solution comprising dissolved catalytically active material precursors of Rh and Ru such that the support material is loaded with Rh and Ru as catalytically active materials by impregnation or ion exchange.

12. A process for converting ammonia into nitrogen and hydrogen, the process including contacting a gaseous stream that comprises ammonia with a catalyst composition according to any one of claims 1 to 10 at a temperature above 200°C, preferably in the range 300 to 500°C, more preferably >350 to 500°C.

13. A process for generating power comprising: (a) converting ammonia into nitrogen and hydrogen by contacting a gaseous stream that comprises ammonia with a catalyst composition according to any one of claims 1 to 10 at a temperature above 200°C;(b) feeding hydrogen produced by step (a) to a hydrogen-fuelled internal combustion engine or fuel cell.

14. A power generation system comprising: an ammonia source; an ammonia cracking unit; and a hydrogen-fuelled internal combustion engine or fuel cell; wherein the ammonia cracking unit comprises the catalyst composition as claimed in claim 1-10.

15. A fuel combustion and exhaust system comprising a fuel combustor and an exhaust gas treatment system, wherein the fuel combustor is an ammonia-containing-fuel- combustor and is in fluid communication with the exhaust gas treatment system, and wherein the exhaust gas treatment system comprises the catalyst composition as claims in any of claims 1 to 10.

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