Room temperature liquid metal catalysts and methods of use

Abstract

A catalyst composition includes a liquid metal alloy having a melting point from about 20? C. to about 25? C., the liquid metal alloy including a primary metal and a secondary metal, the primary metal being aluminum and the secondary metal is selected from the group consisting of gallium, indium, and bismuth.

Claims

1. A catalyst composition comprising gallium and aluminum at an atomic ratio from about 2:1 to about 5:1 of gallium to aluminum, wherein aluminum includes particles having an average size from about 5 nm to about 30 nm diameter dispersed in gallium.

2. The catalyst composition of claim 1, wherein the catalyst composition has a melting point from about 20? C. to about 25? C.

3. The catalyst composition of claim 1, wherein gallium and aluminum are present at an atomic ratio of from about 2.5:1 to about 4:1 of gallium to aluminum.

4. The catalyst composition of claim 3, wherein gallium and aluminum are present at an atomic ratio of from about 2.5:1 to about 3.5:1 of gallium to aluminum.

5. The catalyst composition of claim 1 provided as a pellet of at least 0.1 g, at least 1 g, at least 10 g, at least 100 g, or at least 1000 g in mass.

6. The catalyst composition of claim 5 where the pellet is stored under an alkane.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) Various embodiments of the present disclosure are described below with reference to the following figures:

(2) FIG. 1 is a photograph of a liquid metal catalyst submerged in water generating hydrogen gas according to an embodiment of the present disclosure;

(3) FIG. 2 is a photograph of a hydrogen meter for collecting hydrogen generated by reaction of the liquid metal catalyst with water of FIG. 1;

(4) FIG. 3 is a powder X-ray diffraction (PXRD) spectrum of Al.sub.2O.sub.3 obtained as a byproduct

(5) FIG. 4 is a PXRD spectrum of recovered gallium forming the liquid metal catalysts that were used in HER according to an embodiment of the present disclosure;

(6) FIG. 5 is a bar graph of hydrogen production of gallium-aluminum (GaAl) alloy catalysts according to the present disclosure;

(7) FIG. 6 is a bar graph of hydrogen production of a GaAl alloy catalyst with different water types;

(8) FIG. 7 is a bar graph of hydrogen production of GaAl alloy catalysts and different water types;

(9) FIG. 8 is a transmission electron microscopy (TEM) image at 200 nm scale of GaAl according to the present disclosure;

(10) FIG. 9 is a scanning electron microscopy (SEM) image at 50 ?m scale of GaAl alloy catalyst according to the present disclosure; and

(11) FIG. 10 shows PXRD spectra GaAl alloy catalysts and pure aluminum and gallium for comparison.

DETAILED DESCRIPTION

(12) The present disclosure provides a liquid metal catalyst, which may be an alloy of two or more metals, one of which may be aluminum. In embodiments, the liquid metal catalyst may be a binary alloy, which includes aluminum as a primary metal and a secondary metal, which may be one of gallium, bismuth, indium, and the like. The binary alloy of the liquid metal catalyst may include aluminum and the secondary metal at an atomic ratio from about 1:1 to about 1:4 of aluminum to the secondary metal. In further embodiments, the atomic ratio of aluminum to the secondary metal may be about 4:2.

(13) In further embodiments, the liquid metal catalyst may be a ternary alloy of aluminum, a secondary metal and a tertiary metal. The secondary and tertiary metals are different and may be one of gallium, bismuth, indium, and the like. The ternary alloy of the liquid metal catalyst may include aluminum, the secondary metal, and the tertiary metal present at an approximate atomic ratio 1:1:1, 1:1:2, or 1:2:4, of aluminum, the secondary metal, and the tertiary metal.

(14) The liquid metal catalyst may be formed by combining aluminum with a secondary metal and/or tertiary metal at the atomic ratios disclosed above. Aluminum may be dissolved in the secondary and/or tertiary metals by heating the constituent metals under inert atmosphere. In further embodiments, the constituent metals may be mixed without heating under inert atmosphere by folding and pressing the constituent metals together. The liquid metal catalyst may be formed as a droplet to minimize formation of an oxide layer, which inhibits hydrogen evolution, on the surface of the liquid metal catalyst.

(15) The liquid metal catalyst may be a binary alloy of Ga and Al. Various atomic ratios of the GaAl alloys may be used to optimize the production of hydrogen. GaAl alloys having a higher Ga ratio produce larger amounts of hydrogen, compared to Al-rich alloys as shown in hydrogen production bar graphs of FIG. 7. In embodiments, gallium and aluminum may be present in the alloy at an atomic ratio from about 2:1 to about 5:1. In further embodiments, the ratio may be from about 2.5:1 to about 4:1 of gallium to aluminum. In additional embodiments, the ratio may be from 2.5:1 to about 3.5:1 of gallium to aluminum.

(16) Ga:Al alloys have a melting point from about 20? C. to about 25? C. Ga:Al alloys may be formed by mechanically combining Ga and Al, such as by folding two metals at a temperature from about 20? C. to about 30? C. Folding the two metals results in production of aluminum nanoparticles, which may have an average particle size from about 5 nm to about 30 nm. Folding may include any application of pressure on the two metals that results in formation of the alloy.

(17) Suitable aluminum may be obtained from any source such as foil, recyclable aluminum (e.g., cans, packaging, trays, etc.). Suitable aluminum foil may have a thickness of less than 0.5 mm. In embodiments, aluminum foil may have a thickness of less than 0.4 mm. Aluminum may also be provided as particles having an average diameter from about 0.01 mm to about 1 mm, and in embodiments may be about 0.5 mm and in further embodiments may be about 0.04 mm. The disclosed range has been found to be more effective mixing of the two metals.

(18) Hydrogen generation occurs at the interface between aluminum and water, thus requiring a pristine aluminum surface. As noted above, aluminum does not generate hydrogen gas, as a passivating oxide layer prevents any reaction from occurring with water. According to the present disclosure, gallium may be used to dissolve aluminum, destroying any passivating aluminum oxide film and forming aluminum nanoparticles, enabling aluminum to split water and form aluminum hydroxide and hydrogen gas. The elemental distribution maps (FIGS. 8 and 9) show that aluminum is present in several areas on the gallium surface, indicating that aluminum nanoislands form within a sea of gallium by simple mechanical mixing.

(19) Each element maintains their individual crystal structures as seen in PXRD data (FIGS. 3, 4, and 10). Aluminum is present in the form of nanoparticles, as indicated by TEM data (FIGS. 8 and 9), which may have an average particle size from about 5 nm to about 30 nm, which in embodiments may be from about 10 nm to about 20 nm. At these nano-aluminum sites, a series of hydrogen bond exchanges occur to liberate hydrogen, a pathway similar to that of the theoretical Grotthuss mechanism. The byproduct Al(OH).sub.3/Al.sub.2O.sub.3 formed is porous and gets swept away to expose a new surface of Al on the nanoparticles again for further reaction. Density function theory calculations performed on 13-atoms clusters, such as Al.sub.13, GaAl.sub.12, and in-depth analysis of complementary Lewis acid/base pairs of these metal clusters reveal that doping of Ga in an Al.sub.13 cluster reduces the transition state barrier for water splitting via simultaneous breaking of an OH bond and AlH bond.

(20) The liquid metal catalysts according to the present disclosure have a melting point from about 20? C. to about 25? C., thereby acting as a room temperature liquid metal alloy. The use of liquid metal catalysts having a relatively low melting point enables continuous generation of hydrogen and oxidation of aluminum, which otherwise would be hindered by the formation of an oxide layer in solid phase aluminum.

(21) In particular, the liquid metal catalysts according to the present disclosure may be used in HER. The rate of hydrogen generation from the HER according to present disclosure may be affected by the pH and temperature at which HER is carried out. Accordingly, the HER may be carried out at a pH from about 9 to about 13, in embodiments from about 10 to about 12. The HER may also be carried at a temperature from about 22? C. and 100? C., in embodiments from about 30? C. to about 80? C., and in further embodiments, from about 40? C. to about 60? C. HER may be carried with any suitable water, however, certain impurities present in the water may affect the rate of hydrogen generation. However, as shown, they have minimal effect on the process.

(22) The method for hydrogen generation according to the present disclosure includes providing a liquid metal catalyst and exposing the catalyst to a hydrogen containing compound such as water or an aqueous solution. Exposure to the compound may be carried by placing the catalyst composition in a liquid container.

(23) The hydrogen containing compound may be an aqueous alkaline medium, which may be prepared by dissolving an alkaline compound including alkali metal hydroxides such as lithium hydroxide, sodium hydroxide and potassium hydroxide, and tetraalkylammonium hydroxides such as tetramethylammonium hydroxide and tetraethylammonium hydroxide. Suitable solvents include pure water or water that is mixed with various water-miscible solvents including alcohols such as methyl and ethyl alcohols, dimethylformamide, dimethylacetamide, ethyleneglycol, diethyleneglycol and the like. The aqueous alkaline medium may include from about 1% by to about 30% by weight of the alkaline compound dissolved therein. The generated hydrogen may be collected or syphoned for later use. In further embodiments, the generated hydrogen may be used directly with any system and or apparatus that utilizes hydrogen as a source of fuel, such as a fuel cell.

(24) The liquid metal catalyst alloys according to the present disclosure may be shaped into pellets of predetermined size and stored under cyclohexane and used when needed to prevent moisture from reducing effectiveness of the alloy. The pellets may be sized such that when contacting water, the pellets generate a predetermined amount of hydrogen. In embodiments, the pellets may be sized to produce 1 kg of hydrogen gas. Since the pellet size depends on the atom ratio of Ga:Al, which affects the hydrogen production, the pellet may be at least 0.1 g, at least 1 g, at least 10 g, at least 100 g, or at least 1000 g in mass

(25) The liquid metal catalyst is efficient in producing hydrogen during HER by producing from about 80% to about 90% of theoretical amount of hydrogen from water at about 25? C. During contact of the liquid metal catalyst with water, the resultant products include hydrogen gas (H.sub.2), aluminum oxide (Al.sub.2O.sub.3), and regenerated secondary metal, which may be recovered at a yield of from about 80% to about 95% using any suitable solid separation technique such as filtration. The collected secondary and tertiary metals may then be recycled to form additional liquid metal catalyst compositions for subsequent use during HER.

(26) In the liquid metal catalyst, aluminum activation is achieved by forming an alloy with one or more low melting point metals. Room temperature liquid metals are useful for their ability to create low melting point eutectic alloy. Since the liquid metal catalyst is a eutectic alloy having a melting point from about 20? C. to about 25? C., the liquid metal catalyst maintains its liquid phase under room temperature. This in turn allows HER to occur at temperatures near or at room temperature.

(27) After using the GaAl liquid metal catalyst, Ga may be recovered and recycled to make additional GaAl liquid metal catalyst. Ga may be recovered from the alloy after the reaction by filtration and an aqueous rinse. Recovery of the gallium may be about 98% or above using this method. The recovered gallium was not distinguishably different from fresh gallium in its ability to produce hydrogen in the presence of aluminum as shown in FIG. 7.

(28) Low eutectic melting point obviates the need for adding energy to the alloy to keep it in its liquid phase. Although it is envisioned that additional energy may be added to the liquid metal catalyst to maintain its liquid phase. In embodiments, the liquid metal catalyst may have a melting point from about 25? C. to about 35? C., in which case the catalyst may be heated above room temperature to maintain its liquid phase.

(29) The liquid metal catalysts according to the present disclosure allow for hydrogen production on demand to avoid storage and transportation of hydrogen gas, which requires liquefaction of hydrogen gas. The method of forming the liquid metal catalyst also avoids using heat and other energy intensive procedures, such as grinding and/or ball milling. Instead, the liquid metal catalyst may be formed by folding and pressing the constituent metals to mix the metals.

(30) The present disclosure also provides for hydrogenation of alkenes using GaAl alloy disclosed herein. Hydrogenation of this alkene previously required pressurized systems and/or extreme temperature. This reaction uses a metal catalyst under pressurized hydrogen and heat to induce the transformation. The GaAl alloy may be used to generate hydrogen under atmospheric pressure and room temperature. Any alkene, such as alkene 4-phenyl-1-buten-4-ol, may be used with a hydrogenation catalyst, which may be any suitable catalyst, such as a nickel-based catalyst, a platinum-based catalyst, a ruthenium-based catalyst, an iridium-based catalyst, and the like. Hydrogen may be generated ex situ by contacting GaAl alloy with a hydrogen generating compound, such as water, alcohol (e.g., methanol). Hydrogen may be transferred, e.g., via a cannula, into the reaction mixture. The alkene may be hydrogenated from about 30 minutes to about 2 hours, and in embodiments, may be about an hour.

(31) The following Examples illustrate embodiments of the present disclosure. These Examples are intended to be illustrative only and are not intended to limit the scope of the present disclosure. Also, parts and percentages are by weight unless otherwise indicated. As used herein, room temperature or ambient temperature refers to a temperature from about 20? C. to about 25? C.

EXAMPLES

Example 1

(32) This example describes preparation of binary and ternary liquid metal alloys. Aluminum was dissolved in gallium to form a liquid metal binary alloy to prepare samples 1-9. In samples 1-3, the liquid metal binary alloy was formed by heating aluminum and gallium at the stated ratios as noted in Table 1, below. Samples 4-9 were prepared by mixing aluminum and gallium. A droplet of each sample was submerged in water, which generated hydrogen gas, as shown in FIG. 1. Samples 1-4 and 6-9 were submerged in deionized water and sample 5 was submerged in ocean water. The reaction scheme for preparation of the aluminum/gallium liquid metal alloy and HER is shown below in Formulas (I) and (II), respectively:

(33) $\begin{array}{cc}\mathrm{Al}+\mathrm{Ga}.fwdarw.\mathrm{Al}/\mathrm{Ga}& \left(I\right)\\ 2\mathrm{Al}/\mathrm{Ga}+3H_{2}O.fwdarw.{\mathrm{Al}}_2{O}_3+3H_2+2\mathrm{Ga}& \left(\mathrm{II}\right)\end{array}$

(34) The amount of hydrogen generated by each of the samples 1-9 was measured and collected using a hydrogen meter shown in FIG. 2. Volumes of the collected hydrogen gas were recorded, and amount and yield were also calculated, which are also noted in Table 1. Sample 8 was prepared using recycled gallium, which was collected from prior use of the liquid metal catalysts. Sample 9 was prepared using 99.9999% pure gallium. It was observed that samples 8 and 9 had the highest yields of hydrogen gas out of all the samples 1-9.

(35) TABLE-US-00001 TABLE 1 Volume of Amount Sample Ratio Method Hydrogen (mmol) Yield 1 (1:1, Ga:Al) Heat gun 17.6 mL 0.70 23% 2 (2:1, Ga:Al) Heat gun 24.1 mL 0.96 32% 3 (2:4, Ga:Al) Heat Gun 48.5 mL 1.94 16.6%.sup. 4 (2:4, Ga:Al) Mechanical 71.9 mL 2.87 24% mixing 5 (3:1, Ga:Al) Mechanical 54 mL 2.16 36% ocean water mixing 6 (3:1, Ga:Al) Mechanical 106 mL 4.22 70% mixing 7 (4:1, Ga:Al) Mechanical 101 mL 4.01 66% mixing 8 (3:1, Ga:Al) Mechanical 123.8 mL 4.95 83% recycled Ga mixing 9 (3:1, Ga:Al) Mechanical 130.6 mL 5.22 87% ultrapure Ga mixing

(36) The aluminum oxide and gallium metal, which were byproducts of HER, were collected and characterized using PXRD. The plots of PXRD spectra of aluminum and gallium are shown in FIGS. 3 and 4, respectively.

Example 2

(37) This example describes preparation of binary liquid GaAl metal alloys (in a 1:2, 1:1, 2:1, 3:1, 4:1, 6:1 atomic ratio [Ga:Al]) using aluminum foil.

(38) On weigh paper, about 0.108 g/4 mmol of aluminum foil was shaped into a cup and into the center of the aluminum, about 0.837 g/12 mmol of gallium at room temperature was added to the aluminum to make a 3:1 Ga:Al alloy. The weigh paper was folded in fourths, thereby pressing the aluminum and gallium together through the weigh paper. Gallium was pressed and rubbed into the aluminum until the resulting alloy appeared homogenous and shiny in lustre. Once all the aluminum was dissolved into the gallium, the paper was folded in half and cooled, by pressing a dry ice pellet over the paper, solidifying the alloy. The GaAl alloy appeared dull grey after the alloy hardened. The alloy was peeled off the paper and re-weighed to calculate the weight. The alloy pieces were then stored under cyclohexane for further use. To store the alloy, the alloy pieces were re-melted and using a plastic syringe were withdrawn and then expelled into a bottle of cyclohexane. GaAl pellets stored under cyclohexane were tested periodically over several months of storage for their hydrogen production, with results comparable to freshly made alloys. The alloys can therefore be stored and used when needed without fear of reduction and premature hydrogen generation. Similar process was used to make 1:2, 1:1, 2:1, 3:1, 6:1 (Ga:Al) compositions by varying ratio of Ga to Al.

Example 3

(39) This example describes preparation of a binary liquid GaAl metal alloy (6:1 Ga:Al) using aluminum from a soda can.

(40) A top of a soda can was cut into pieces and was ground into a powder. About 0.108 g/4 mmol of the aluminum powder was placed on a piece of weigh paper and into the center of the aluminum about 1.68 g/24 mmol of gallium at room temperature was added to make 6:1 atomic ratio of Ga:Al alloy. The same process for combining the metals was followed as in Example 2 until a GaAl alloy was obtained, which was stored in cyclohexane.

Example 4

(41) This example describes preparation of a binary liquid GaAl (3:1 atomic ratio Ga:Al) metal alloy using aluminum from a baking tray. The same process was followed to make the GaAl metal allows as in Example 2.

Example 5

(42) This example describes preparation of a binary liquid GaAl (3:1 atomic ratio Ga:Al) metal alloy using aluminum from contaminated aluminum foil (previously used to as food wrap). The same process was followed to make the GaAl metal allows as in Example 2.

Example 6

(43) This example describes preparation of a binary liquid GaAl (3:1 atomic ratio Ga:Al) metal alloy using recycled Ga. Ga was collected using a vacuum filtration apparatus and separated visually from the collected residue. The process for forming the GaAl was the same as the process of Example 2.

(44) New commercial grade aluminum foil was initially used to optimize the alloy ratio, but expansion to waste aluminum in the form of baking trays and food wrappers demonstrated that hydrogen is efficiently generated regardless of the aluminum source. The top of a soda can was shown to produce about 80% of the theoretical amount of H.sub.2 produced with fresh aluminum. The top of the can was used as opposed to the sides because the top contained the highest concentration of aluminum without any paint or polymer coating. When testing the soda can it was discovered that a higher ratio of gallium was used to dissolve the aluminum completely due to its greater thickness.

Example 7

(45) This example describes measuring HER using a gas burette of the hydrogen meter shown in FIG. 2. Hydrogen production of each of alloys of Examples 3-6 was tested using deionized water. A nugget of each of the alloys was placed in a round-bottom flask and melted using a magnetic stirrer warmed to about 60? C. About 10 mL of deionized water was added to the flask, causing H.sub.2 gas to evolve immediately. By the controlled release of pressure in the closed system into a graduated buret, the volume of gas generated and was measured by water displacement. After approximately 15 minutes, the temperature of the displaced water and barometric pressure were also measured. The HER of each of the alloys is reproduced in the bar graph of FIG. 5.

(46) In addition to deionized water, H.sub.2 evolution was also measured using tap water, ocean water, and de-gassed COCA-COLA? (carbonated sweetened beverage) and 3:1 atomic ratio GaAl alloy of Example 2. The results of H.sub.2 evolution of 3:1 atomic ratio GaAl with deionized water, tap water, ocean water, and de-gassed soda are shown in FIG. 6.

(47) Besides deionized water, other water sources were shown to successfully generate hydrogen using 3:1 atomic ratio GaAl alloy. Tap water produced substantial amounts of hydrogen, similar to deionized water. Ocean water or simulated sea water (e.g., saline) produced approximately 30% of the theoretical amount of H.sub.2 and about 40% of the amount produced using deionized water.

(48) Visual comparison between the reaction the alloy with deionized water compared with ocean water showed a much less vigorous reaction for the latter. However, ocean water was still shown to be a viable hydrogen source. This yield demonstrated that it is possible to use a variety of water sources with this alloy to achieve excellent hydrogen production. Collected hydrogen gas was subjected to gas chromatography-mass spectrometry analysis, which confirmed that no detectable chlorine gas was formed, a pervasive problem that prevents the use of electrode-based water splitting, particularly in saltwater.

Example 8

(49) This example describes analysis of the 3:1 atomic ratio Ga:Al alloy of Example 2.

(50) The 3:1 atomic ratio Ga:Al alloy of Example 2 was analyzing using transmission electron microscopy (TEM). TEM of the 3:1 atom ratio Ga:Al alloy revealed nanoparticles of aluminum having a diameter from about 5 nm to 30 nm (FIG. 8). Both electron energy loss spectroscopy and selected area electron diffraction confirmed the dark areas are Al and the lighter areas in between Al nanoparticles are Ga. Furthermore, scanning electron microscopy with energy dispersive spectroscopy analysis showed a homogeneous distribution of Al particles in the Ga (FIG. 9).

(51) Powder X-ray diffraction (PXRD) was performed on Ga:Al alloys of Example 2 (FIG. 10). Ga and Al references were calculated from single crystal X-ray structure from the American Mineralogist Crystal Structure Database (AMCSD). Orthorhombic gallium (Cmca) has characteristic diffraction peaks at 30.3, 30.6, 45.4, 46.4, 57.6, 63.6, 76.3 and 77.0? (2?). Cubic aluminum (Fm3m) has characteristic diffraction peaks at 38.5, 44.7, 65.1, and 75.3? (2?). Utilizing in-plane PXRD, characteristic peaks of orthorhombic Ga and cubic Al are seen in all 6:1 to 1:4 Ga:Al mixtures. This suggests both aluminum and gallium retain their crystal structure after mixing and do not form a homogenous alloy.

Example 9

(52) This example describes hydrogenation of alkene, i.e., 4-Phenyl-1-butene-4-ol, using hydrogen generated by reaction of 3:1 atomic ratio GaAl alloy of Example 2.

(53) In a 100-mL round-bottom flask, about 1 g, 26.43 mmol of NaBH4 was suspended in about 24 mL of EtOH and 1.25 mL, and 2.5 mmol of 2M NaOH. The reaction mixture was filtered and set aside at about 25? C. In a separate 100-mL round-bottom flask, about 0.314 g, 1.25 mmol of nickel(II) acetate tetrahydrate was dissolved in about 12.5 mL EtOH. To the nickel solution, about 1.25 mL, 1.38 mmol of the NaBH.sub.4 solution was added slowly via syringe to commence H.sub.2, which results in H.sub.2 evolution. Once the hydrogen evolution abates, the stirring was stopped and about 0.187 mL, 1.25 mmol of the alkene, 4-phenyl-1-butene-4-ol, was added to the reaction flask and a balloon was then connected to the reaction flask.

(54) About 0.63 g (4 mmol of theoretical H.sub.2) of 3:1 GaAl alloy of Example 2 was placed into a separate round-bottom flask and melted with a warm stir bar. The alloy containing flask was connected via cannula to the reaction flask containing the nickel catalyst and the alkene. With both flasks stirring, about 10 mL of deionized water was added to the alloy flask to generate hydrogen that is then led into the reaction flask. After approximately 15 min, the two flasks were disconnected, and a second flask containing about 0.787 g (5 mmol of theoretical H.sub.2) of 3:1 GaAl alloy of Example 2 was attached and the same procedure as above was conducted to generate hydrogen gas. After the hydrogen had evolved, the reaction mixture was allowed to stir for about 1 h. The reaction mixture was then centrifuged, and the supernatant decanted. The solid catalyst was rinsed with about 2?10 mL of diethyl ether, centrifuged, and the supernatant decanted. The combined supernatants were then concentrated by rotary evaporation and the product, 1-phenyl-1-butanol, analyzed via .sup.1H and .sup.13C NMR. The same process was repeated using about 10 mL of methanol instead of deionized water and achieved the same result.

(55) It will be appreciated that of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims. Unless specifically recited in a claim, steps or components of claims should not be implied or imported from the specification or any other claims as to any particular order, number, position, size, shape, angle, or material.