Method of storing a gas, in particular hydrogen

Abstract

We describe a method of storing a gas, in particular hydrogen, comprising: providing a polymer sponge, wherein said polymer sponge comprises a plurality of catalytic nanoparticles; providing a solution of reactants, catalyzed by said nanoparticles to produce said gas; absorbing said solution into said polymer sponge such that said reactants react within said polymer sponge to produce said gas; wherein said gas is held within said polymer sponge; and wherein said polymer sponge comprises a thermally responsive polymer having a volume which reduces with a change in temperature, such that said gas held within said polymer is extractable by changing a temperature of said polymer sponge.

Claims

1. A method of storing hydrogen gas, comprising: providing a polymer sponge, wherein said polymer sponge comprises a plurality of catalytic nanoparticles; providing a solution of reactants, catalysed by said nanoparticles to produce said gas; absorbing said solution into said polymer sponge such that said reactants react within said polymer sponge to produce said gas; wherein said gas is held within said polymer sponge; and wherein said polymer sponge comprises a thermally responsive polymer having a volume which reduces with a change in temperature, such that said gas held within said polymer is extractable by changing a temperature of said polymer sponge.

2. A method as claimed in claim 1 wherein said catalytic nanoparticles comprise complexes of a nanoparticle with a stabilising molecule, wherein said stabilising molecule interacts with said polymer and said nanoparticle to restrict agglomeration of said nanoparticles and stabilise a distribution of said nanoparticles during manufacture of said polymer.

3. A method as claimed in claim 2 wherein said stabilising molecule provides a steric effect to further stabilise said nanoparticles, such that the nanoparticles are restricted from agglomerating with one another.

4. A method as claimed in claim 3 wherein said stabilising molecule comprises cucurbit[n]uril.

5. A method as claimed in claim 1 wherein said nanoparticles comprise metal nanoparticles.

6. A method as claimed in claim 5 wherein said metal nanoparticles are ruthenium nanoparticles.

7. A method as claimed in claim 1 wherein said thermally responsive polymer has a volume which reduces above a threshold temperature to provide said gas.

8. A method as claimed in claim 1 comprising producing said polymer sponge by polymerising a monomer in the presence of said catalytic nanoparticles.

9. A method as claimed in claim 8 wherein said polymer comprises polyNIPAM.

10. A method as claimed in claim 1 wherein said reaction is an aqueous reaction, and wherein said solution is an aqueous solution.

11. A method as claimed in claim 1 wherein said reactants comprise a solution of metal hydride.

12. A method as claimed in claim 11 wherein said reaction comprises a reaction of a hydrogen-containing compound with water.

13. A method as claimed in claim 12 wherein said hydrogen-containing compound comprises an amine-borane compound or derivative thereof.

14. A method as claimed in claim 13 in which the amine-borane compound or derivative thereof is ammonia borane or a derivative thereof.

15. A method as claimed in claim 1 further comprising retrieving said gas from said polymer sponge, and optionally further comprising recycling said polymer sponge by washing a by-product of said reaction from said polymer sponge after retrieving said gas.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other aspects of the invention will now be further described, by way of example only, with reference to the accompanying figures in which:

(2) FIG. 1 shows the four steps in producing the metastable nanoparticles;

(3) FIG. 2 shows the XPS spectra collected on metastable ruthenium nanoparticles;

(4) FIG. 3 shows the SAXS pattern of a metastable ruthenium nanoparticles in solution;

(5) FIG. 4 shows the Host-Guest stabilization of the metal nanoparticles in solution;

(6) FIG. 5 shows the embedded metal nanoparticles in thermoresponsive sponge-like solid support;

(7) FIG. 6 shows thermoresponsive role of polymer as a function of hydrogen gas release;

(8) FIG. 7: shows DSC profile overlap with H.sub.2 release point;

(9) FIG. 8 shows recyclability of the material;

(10) FIG. 9 shows on-off release by temperature control; and

(11) FIG. 10 shows LCST modulation with different functional groups.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(12) Hydrogen is among the leading candidates as an alternative energy source for the future. Whilst hydrogen gas is used in a variety of ways to generate energy, the most efficient process entails its conversion to electrical energy via fuel cell technologies. Fundamental technology exists for both the storage of hydrogen as well as its on-board production; however, improvement of current methods is crucial to achieve more efficient practical applications. Catalytic nanoparticles (NPs) have attracted a great deal of interest among both scientific and industrial communities owing to the unique properties derived from their characteristic large surface area-to-volume ratios. In order to achieve their small size during preparation, and to prevent subsequent particle coalescence due to the enhanced surface tension associated with small particle sizes, NPs are generally stabilized through the introduction of surfactants or surface-bound ligands and stabilizing agents. While the NPs must be covered with protective ligands for the purpose of stability and to allow them to be used in catalytic applications, the presence of the same protective ligands can also reduce substrate accessibility to the catalytic NP surface. These competing factors have presented the synthetic chemist with a dilemma, as a compromise between these competing issues must be sought. Embodiments of the present invention demonstrate a method of preparation of metastable catalytic metal nanoparticles which can be made from a variety of metals, including ruthenium, palladium, platinum, rhodium, gold, silver, copper, cobalt, nickel, and iron. This method not only offers versatility and simplicity in the preparation of organic ligand-free nanoparticles, but also allows for enhanced exposure of the catalytic surface of the NPs.

(13) For example, consider the formation of the aforementioned nanoparticles using Ruthenium. Ruthenium complexes are among the most widely studied materials due to their catalytic performance. Some ruthenium complexes have recently been shown to be effective in the hydrolysis of ammonia borane and other metal hydrides. A number of different shapes, structures and compositions of RuNPs have also been investigated for their catalytic activity in a wide range of chemical transformations. As the size of NPs has a marked effect on the overall catalytic activity, their formation is of great consequence and is controlled by an aggregation process during reduction of the metal ions. In principle the particle size can be determined by influencing the relative rates of nucleation and cluster growth. Traditionally, in order to achieve such stable aggregates on the nanometer scale, the formation process, has been carried out in the presence of stabilizing ligands. Particle collisions by Brownian motion give rise to substantial agglomeration, if repulsive interactions or stabilizing ligands are not present. According to Turkevich and co-workers, there are three reaction stages in forming metal NPs: (i) an initial nucleation step, followed by (ii) aggregation, and finally (iii) growth.

(14) For example, FIG. 1 illustrates the formation of metastable RuNPs: a) Ruthenium ions in solution; b) nucleation, forming initial seeds upon introducing NaBH.sub.4; c) growth of seed dimensions, which occur when further ruthenium ions are introduced to the formed nuclei, eventually leading to the final stable state; d) metastable cluster, when remaining ruthenium ions are no longer reduced.

(15) Host-Guest Stabilization System

(16) The preparation of metastable NPs can readily address the challenges ligands and additives introduce to catalytic nanoparticles when synthesised. The catalytically active NPs are further stabilised in a dynamic fashion by exploiting a versatile host-guest complex based on the macrocycle cucurbit[n]uril and a positively charged guest in solution, as illustrated in FIG. 2. The stability of the NPs complex is improved using this supramolecular approach, thus, allowing for a multiple use system in a catalytic reaction in solution.

(17) Additionally, FIG. 3 illustrates a process of nanoparticles stabilization by embedding the metastable catalytic nanoparticles described above in a solid matrix as well as stabilization of nanoparticles in solution used for the storage of gas, namely hydrogen. The solid support, can be utilised to promote in situ different catalytic transformations (such as c-c coupling, Suzuki, hydrogen transfer, oxidations etc.) and capture and release other gases (such as CO, CO.sub.2, N.sub.2 etc.) in a controllable manner by external temperature stimuli.

(18) Synthesis Methods

(19) System 1: Synthesis of Metastable Metal Nanoparticles (MNPs) Up to 5 nm

(20) The synthesis of the metal nanoparticle will now be described, using Ruthenium as an example metal, but noting that the aforementioned metals could also be used. A solution of 1:1 (v/v) aqueous ethanol is added of a (30 mM) stock solution of RuCl.sub.3*H.sub.2O or any other metal complex respectively (Na.sub.2PdCl.sub.4/PtCl.sub.6/RhCl.sub.3*H.sub.2O/HAuCl.sub.4/AgClO.sub.3/CuCl.sub.2/CoCl.sub.2/NiCl.sub.2 or FeCl.sub.2) is then added to form a solution of 0.5 mM. The solution is sonicated for 1 min and 1 molar ratio of (0.1M) NaBH.sub.4 of 1:1 (v/v) aqueous ethanol is immediately added under swirling for 15 min using bench-top incubator at 300 rpm at room temperature. The color of the solution should immediately change. The solution is then left to age for 24 h. The method could be applied for lower concentrations down to 0.1 mM for getting larger clusters and up to 2.5 mM for smaller clusters). Water only may be used a solvent system for metastable nanoparticles formation, however, a 1:1 (v/v) aqueous ethanol solvent system is found to be the most stable solvent mixture.

(21) System 2: Supramolecular Dynamic Stabilization of Metastable MNPs in Solution by CB Molecules

(22) The nanoparticles can also be stabilized in solution. 1-adamantylamine (1 eq.) as a guest can be added to a CB[n] solution and heated to 50 C. under sonication for 15 min then is added to the MNPs (any of the metastable MNP solutions mentioned in system 1 described above) under vigorous swirling for 1 hour at room temperature. The color of the black-dark solution should become brighter, indicating a rapid CB[n] distribution in solution. The solution is then swirled using a bench-top incubator. This type of stabilization can be achieved using many other positively charged guests which can form a strongly bound host-guest complex, using the same molar ratio and under the same conditions.

(23) System 3: Immobilization of CB[n]-MNPs Assemblies on a Thermoresponsive Polymeric Support

(24) Finally, the nanoparticles can be stabilized in a solid polymer matrix, forming a sponge-like material. The solid supported MNPs are prepared by dissolving N-isopropylacrylamide (NIPAm), N,N-methylenebisacrylamide (MBA; 0.086 molar ratio to NIPAm, (3-Acrylamido-propyl) trimethylammonium chloride (AMPTMA; 75 wt % in water; 0.013 molar ratio to NIPAm) and 4,4-Azobis 4-cyanovaleric acid (ACPA; 0.020 molar ratio to NIPAm) in a Ruthenium nanoparticle solution (RuNP:CB[n]; 1.610.sup.4 molar ratio to NIPAm, 0.5 mM) prepared in a 1:1 (v/v) mixture of ethanol and water. This solution is then degassed with bubbling nitrogen for 20 min. This mixture is then added dropwise into a degassed solution of toluene (0.090 molar ratio to NIPAm), Span 80 (0.116 molar ratio to NIPAm) and dodecane (8.610.sup.6 molar ratio to NIPAm) in a three-neck flask fitted with a mechanical stirrer. The nitrogen inlet and outlet is then removed and the flask heated to 70 C. while stirring at 360 rpm for 4 h. After this time, the particles are collected by decanting of the supernatant, washed with acetone and water and dried in a vacuum oven (60 C., 0 mbar, 4 h). The non-temperature responsive materials were prepared in an equivalent fashion using Acrylamide in place of NIPAm. Synthesis could be made for other MNPs than RuNPs as mentioned in system 1 procedure for concentrations from 0.1 mM.

(25) The stabilized nanoparticles can then be reacted with a metal hydride, such as ammonia borane, to produce hydrogen gas, which is thought to be stored in the pores area of the polymeric matrix. On slight heating, catalytically generated and stored hydrogen gas from metal hydride compounds will be released.

(26) Thermoresponsive catalytic production of hydrogen gas is achieved as shown in FIG. 4a. RuNP-polymer catalytic composites produce and store H.sub.2 gas below the lower critical solution temperature (LCST) of the NIPAM-based thermo-responsive polymer matrix (20 C.), whereas heating above the LCST (43 C.) allows for matrix collapse and H.sub.2 gas release. When the material does not display an LCST, as in acrylamide-based (AM) materials, no H.sub.2 gas is released with temperature. As shown in FIG. 4b, LCST of the NIPAM-based material is clearly evident using differential scanning calorimetry and provides a trigger for the release of stored H.sub.2 gas. In addition, this material is reusable; a recyclability plot is shown in FIG. 4c; the catalytic activity of these composite materials is retained after several cycles as the catalytically active RuNPs embedded in the thermoresponsive matrix are highly stabilized (inset). Inductively coupled plasma mass spectrometry results demonstrate that little to no Ru is lost after each catalytic cycle. Moreover, the release of H.sub.2 gas is controllable with a temperature trigger whereby on and off correspond to environmental temperatures above and below the LCST of the composite material, respectively, as shown in FIG. 4d.

(27) A facile variability of the LCST of the material is demonstrated in FIG. 4e by alteration of the relative hydrophobicity of the monomer loading through addition of either a hydrophobic comonomer t-butylacrylamide (tBAm) or a hydrophilic comonomer acrylamide (Am). The temperature at which the H.sub.2 gas can be released is, therefore, tunable across a broad range. All samples contain an equivalent molar loading of acrylamide monomers and AMPTMA (0.013 molar ratio to Am monomer), MBA (0.086 molar ratio to Am monomer) and ACPA (0.02 molar ratio to Am monomer).

(28) Materials:

(29) N-Isopropylacrylamide (NIPAm) was purchased from Aldrich and recrystallized twice from hexane. 4,4-Azobis(4-cyanopentanoic acid) (ACPA) was purchased from Aldrich and was recrystallized from methanol. All materials were purchased from Aldrich and used as received.

CONCLUSION

(30) RuNPs are stabilized without protective organic ligands or additional supports while simultaneously exhibiting a high catalytic activity. Embodiments of the present invention demonstrate a simple yet efficient method to prepare metastable RuNPs in a 1:1 (v/v) water:ethanol mixture. The preparation process was concentration dependant on the RuCl.sub.3*H.sub.2O precursor with respect to control over the NP size. Despite the lack of a conventional organic or inorganic stabilizing ligands present on the NP surface, metastable RuNPs were shown to be stable for many months likely on account of surrounding Ru.sup.3+ ions, the presence of which was confirmed by XPS. Moreover, a consistent interparticle distance between the NPs was readily observed by SAXS measurements; this is attributed to the repulsion forces arising from these surrounding ions. Thus, the highly charged NP surfaces appear to gain long-term stability and control over size through charge-charge repulsion as opposed to direct ligand attachment. Additionally, the metastable RuNPs exhibit an fcc structure and were shown to be a promising catalytically active material for production of H.sub.2 through the hydrolysis of ammonia-borane in water at room temperature. The activation energy of 27.5 KJ*mol.sup.1 for the catalytic hydrolysis was found to be remarkably low and resulted in a turnover number of 218 per minute, rendering the metastable RuNPs as an extremely promising candidate for the production of hydrogen gas under mild conditions for practical applications.

(31) The metastable NPs embedded in thermoresponsive polymer forms a sponge-like Ru-polymer composites which can produce, store and release H.sub.2 gas in a controlled manner. The RuNPs-polymer composites can produce and store 3 equiv of H.sub.2 gas per NH.sub.3BH.sub.3 below a phase transition temperature of (42 C.) and upon heating to above that temperature, release the gas. Despite the slow kinetics achieved in the course of this study compared to traditional non-thermoresponsive supported NPs for catalytic H.sub.2 production, the low regeneration temperature in this work (42 C.) can be further lowered or raised upon tuning the LSCT.

(32) Thus embodiments of the invention demonstrate a simple approach for the preparation of thermoresponsive and porous polymer-ruthenium nanoparticle composite materials that catalytically produce and store hydrogen gas, and the inherent thermoresponsiveness allows for on-demand release of the stored gas. The catalytically active ruthenium nanoparticles are embedded into the polymer in a dynamic fashion by exploiting a versatile host-guest system based on the macrocycle cucurbit[n]uril. The materials demonstrate consistent behavior over many cycles and the catalytic activity and release temperature are easily modulated by the formulation.

(33) No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.