Abstract:
A process for enhancing the kinetics of hydrogenation/dehydrogenation of complex chemical hydrides using mechanomixing and/or mechanomilling. The mechanomixing makes hydrogenation/dehydrogenation of complex chemical hydrides reversible at much reduced temperature and pressure. The mechanomilling reduces particle size or grain size of the decomposition byproducts, further increasing surface area and intimate contact of the byproducts. In the process of the present invention, complex chemical hydrides can be utilized as a reversible hydrogen storage media for various applications such as transportation, including fuel cells. The process is simple and inexpensive.

Description:
TECHNICAL FIELD 
     This invention relates to a process for enhancing the kinetics of hydrogenation/dehydrogenation of MAlH 4  and MBH 4  metal hydrides, and more particularly to such a process using mechanomixing for low temperature, low-pressure reversible hydrogen storage. 
     BACKGROUND OF THE INVENTION 
     Application of hydrogen as a fuel is attractive because it generates no polluting emissions. However, this attractive application has been hindered due to volumetric problems of storing hydrogen in gaseous or even liquid forms. Hydrogen storage alloys have been proposed and developed to the extent of commercial use in metal hydride batteries. However, the gravimetric hydrogen storage in alloys is still low and requires high pressure and temperature. 
     Complex chemical hydrides with hydrogen storage capacity have been proposed, and hydrogen generation from this class of compounds has been demonstrated. Unfortunately, hydrogenation of the decomposed complex chemical hydrides is not straightforward, and remains a scientific challenge. 
     It has been demonstrated that the decomposition of NaAlH 4  occurs with at least distinct thermal signatures. The decomposition byproducts have been proposed and identified in the past. The decomposition steps of NaAlH 4  can be summarized as follows: 
     
       
         3NaAlH 4(solid) →Na 3 AlH 6(solid) +Al+3H 2   (Eq. 1)  
       
     
     
       
         Na 3 AlH 6(solid) →3NaH (solid) +Al=3/2H 2   (Eq. 2)  
       
     
      NaH (solid) →Na+1/2H 2   (Eq. 3) 
     Several research groups have investigated the use of catalysts to enhance hydrogenation of complex chemical hydrides. Although some successes have been achieved, the kinetics and reversible hydrogen capacity of these materials remains very low. In particular, the hydrogen capacity of the materials decays very fast during hydrogenation/dehydrogenation cycles. 
     Bagdanovic (German patent No. 195-26-434, 1995) describes the catalytic effects of transition metal doping of kinetics and reversibility age of a series of alanates. This reference suggests that NaAlH 4  doped with titanium decomposes at a reduced temperature and pressure. Further, it is suggested that the reversibility of the compound is also improved by the titanium doping. Others have also observed the catalytic effects of titanium and zirconium doping. 
     Zaluski et al,  J Alloys Compd.  285, 125 (2000), have reported the kinetics enhancement of NaAlH 4  by energetic milling of alanate with carbon. Carbon was mixed with the NaAlH 4  and milled prior to any hydrogenation/dehydrogenation. No mixing or milling was conducted during the hydrogenation or dehydrogenation. However, it is also reported that the large amount of carbon may have a negative effect as it reduces the gravimetric percentage of active alanates in the composite. It is believed that the carbon actually poisons the process. 
     Thus, it would be desirable to provide a process for the reversible hydrogenation/dehydrogenation of metal hydrides that overcomes the problems associated with prior art methods. 
     SUMMARY OF THE INVENTION 
     The invention includes a process for enhancing the kinetics of hydrogenation/dehydrogenation of complex chemical hydrides using mechanomixing and/or mechanomilling. The mechanomixing makes hydrogenation/dehydrogenation of complex chemical hydrides reversible at much reduced temperatures and pressures. The mechanomilling reduces particle size or grain size of the decomposition byproducts, further increasing surface area and intimate contact of the byproducts. In the process of the present invention, complex chemical hydrides can be utilized as a reversible hydrogen storage media for various applications such as transportation, including fuel cells. The process is simple and inexpensive. 
     The process according to the present invention utilizes complex chemical hydrides of a variety of different formulas, and most preferably complex chemical hydrides generally having the formula MBH 4  where M is at least one selected from the group consisting of Na, Li and K, and where B is at least one selected from the group consisting of the elements in the third column of the periodic table. The invention can be practiced using various mixing and/or milling techniques known to those skilled in the art. The invention can be practiced with real-time mixing during decomposition. Wet milling of the decomposition products is also contemplated as producing similar results. Mixing and/or milling methods other than mechanical are also contemplated useful in the present invention. 
     The invention includes a process for the dehydrogenation/hydrogenation of a complex chemical hydride comprising: decomposing a complex chemical hydride to produce hydrogen and a plurality of byproducts, and whereby the decomposing of the complex chemical hydride produces a foamy mass; mixing the foamy mass to bring the byproducts in more intimate contact with each other and to produce a mixed byproduct mass of reduced volume; and exposing the mixed byproduct mass of reduced volume to hydrogen so that the hydrogen reacts with the byproducts to produce a complex chemical hydride having greater hydrogen content than the byproducts. 
     In another embodiment of the invention, the complex chemical hydride includes material having the formula MBH 4  where M is at least one selected from the group consisting of Na, Li and K, and where B is at least one selected from the group consisting of the elements in the third column of periodic table. 
     In another embodiment of the invention, the complex chemical hydride includes material having the formula MBH 4  where M includes Na and B includes Al. 
     In another embodiment of the invention, the mixing of the foamy mass includes moving a metal ball through the foamy mass. 
     In another embodiment of the invention, the mixing of the foamy mass includes stirring the foamy mass with a stirring rod. 
     In another embodiment of the invention, the decomposing of the complex chemical hydride comprises heating the complex chemical hydride to a temperature ranging from 50° C. to 600° C. 
     In another embodiment of the invention, the decomposing of the complex chemical hydride comprises heating the complex chemical hydride to a temperature ranging from about 100° C. to 200° C. to produce a first set of the byproducts, and thereafter heating the complex chemical hydride to a temperature ranging from greater than 200° C. to 300° C. to produce a second set of the byproducts. 
     In another embodiment of the invention, the decomposing of the complex chemical hydride comprises heating the complex chemical hydride to a temperature ranging from 100° C. to 300° C. 
     Another embodiment of the invention includes a process for the dehydrogenation/hydrogenation of a complex chemical hydride comprising: decomposing a complex chemical hydride to produce hydrogen and a plurality of byproducts, and whereby the decomposing of the complex chemical hydride produces a foamy mass, and wherein the complex chemical hydride comprises NaAlH 4 ; mixing the foamy mass to bring the byproducts in more intimate contact with each other and to produce a mixed byproduct mass of reduced volume; and exposing the mixed byproduct mass of reduced volume to hydrogen so that the hydrogen reacts with the byproducts to produce a complex chemical hydride having greater hydrogen content than the byproducts. 
     Another embodiment of the invention includes a process for the dehydrogenation/hydrogenation of a complex chemical hydride comprising: decomposing a complex chemical hydride to produce hydrogen and a plurality of byproducts, and whereby the decomposing of the complex chemical hydride produces a foamy mass, and without mixing the complex chemical hydride during the decomposing of the complex chemical hydride; mixing the foamy mass to bring the byproducts in more intimate contact with each other and to produce a mixed byproduct mass of reduced volume; and exposing the mixed byproduct mass of reduced volume to hydrogen so that the hydrogen reacts with the byproducts to produce a complex chemical hydride having greater hydrogen content than the byproducts. 
     Another embodiment of the invention includes a process for the dehydrogenation/hydrogenation of a complex chemical hydride comprising: decomposing a complex chemical hydride to produce hydrogen and a plurality of byproducts, and whereby the decomposing of the complex chemical hydride produces a foamy mass, and mixing the complex chemical hydride during the decomposing of the complex chemical hydride; mixing the foamy mass to bring the byproducts in more intimate contact with each other and to produce a mixed byproduct mass of reduced volume; and exposing the mixed byproduct mass of reduced volume to hydrogen so that the hydrogen reacts with the byproducts to produce a complex chemical hydride having greater hydrogen content than the byproducts. 
     Another embodiment of the invention includes a process for the dehydrogenation/hydrogenation of a complex chemical hydride comprising: decomposing a complex chemical hydride to produce hydrogen and a plurality of byproducts, and whereby the decomposing of the complex chemical hydride produces a foamy mass; mixing the foamy mass to bring the byproducts in more intimate contact with each other and to produce a mixed byproduct mass of reduced volume; and exposing the mixed byproduct mass of reduced volume to hydrogen at a pressure less than 400 pounds per square inch so that the hydrogen reacts with the byproducts to produce a complex chemical hydride having greater hydrogen content than the byproducts. 
     These and other objects, features and advantages of the present invention will become apparent from the following brief description of the drawings, detailed description of the preferred embodiments, and appended claims and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic illustration of a system useful in practicing the process according to the present intention; 
     FIG. 2 is a graphical illustration of the reversible hydrogenation/dehydrogenation of a complex metal hydride and the influence of mechanomixing at the end of each dehydrogenation to process according the present invention; 
     FIG. 3A is a graphic representation of the differential scanning calorimetry of a Ti doped complex chemical hydride; and 
     FIG. 3B is a graphic representation of the differential scanning calorimetry of a complex chemical hydride processed according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a schematic illustration of a system  10  useful in practicing a process according to the present invention. The system  10  includes a hydrogen source  12  which may be a pressure vessel having hydrogen under pressure contained therein or may be another source type for hydrogen such as a fuel reformation system for reforming a fuel such as methanol or gasoline to produce hydrogen. Plumbing  14  may be provided from the hydrogen source  12  to a hydrogenation/dehydrogenation reaction vessel  16 . Pressure gauges  18 , filters  20 , valves  22 , and quick connects  24  may be provided in the plumbing  14  between the hydrogen source  12  and the reaction vessel  16  as desired. The reaction vessel  16  receives a complex chemical hydride such as a hydride having the formula MBH 4  where M is at least one selected from the group consisting of Na, Li and K, and where B is at least one selected from the group consisting of the elements in the third column of the periodic table (B, Al, Ga, In, Tl, Sc, Y, La, Ac). The complex chemical hydride is preferably in a solid phase, but may be wet or in a liquid phase. The reaction vessel  16  is constructed and arrange to receive one or more mixing and/or milling devices  28 ,  29 . A mixing device  28  may include a stainless-steel ball(s) which may be caused to move around in the vessel by mechanical means such as by rotating a stirring rod  29  with a paddle  27  on the end thereof. A motor (not shown) may be provided to rotate the stirring rod  29 . Alternatively, the reaction vessel  16  may be removed by from the setup and vibrated with the steel ball  28  therein to mix the complex chemical hydrides during and/or after the decomposition process. The system includes a heating element  30  such as a tube furnace to supply heat to the complex chemical hydride to decomposes the same. A controller  32  is provided to monitor and control the heat applied to the complex chemical hydride by the tube furnace  30 . Plumbing  34  may be provided between the reaction vessel  16  and a hydrogen measuring device  36  such as a syringe, or a hydrogen application device, such as a fuel cell. Vacuum pumps  40 , needle valves  42 , mass flow meters  44 , and sensors  46  may be provided in the plumbing  34  between the reaction vessel  16  and the measuring device  36  or hydrogen application device as desired. The above-described system was used to conduct and verify the reversibility of a hydrogenation/dehydrogenation process according to the present invention. 
     Sodium aluminum hydride was purified by re-crystallizing commercial NaAlH 4 . A 1.0M solution of NaAlH 4  in THF (tetrahydrofuran) was prepared using the Shlenk method. In a typical experiment, 10 g of NaAlH 4  in 100 ml of THF was stirred for one hour and filtered through a glass filter. The filtrate was concentrated to about 30 ml in vacuum, whereby NaAlH 4  started to separate from the solution. 100 ml Pentane was added to the THF solution and the mixture was stirred for three hours under an argon atmosphere causing NaAlH 4  to fully separate from the solution as a fine precipitate. The suspension was stirred for two more hours, filtered, and the remaining THF was removed by washing thrice with pentane/toluene. Then, the solid sample was filtered and dried under vacuum overnight at room temperature. After drying, NaAlH 4  was obtained as a fine white powder. Lithium aluminum hydride was prepared by using the same process from a commercial 1.0 M solution of LiAlH 4  in tetrahydrofuran. After drying, LiAlH 4  was obtained as a fine white powder. 
     The mechanomixing process has been applied at the end of each dehydrogenation process. The mechanomixing process has improved the kinetics of hydrogenation process due to: (1) intimate contact of the decomposition byproducts, which is essential for hydrogenation to take place, and (2) activation of the byproducts due to generation of defects and non-stoichiometric surface composition. In addition to the intimate mixing, the destabilization of the bulk by surface activation is also required for enhanced hydrogenation. The following examples provide detailed information about hydrogenation/dehydrogenation of NaAlH 4  according to the present invention. 
     EXAMPLE 1 
     One gram of purified NaAlH 4  was placed in a stainless steel reaction vessel and heat treated up to 300° C. The volume of the generated hydrogen was measured using a simple inverted graduated cylinder  36  as shown in FIG.  1 . The decomposed byproducts were milled using steel balls (in the vessel) that were moved by vibrating the vessel for 10 minutes. The pressure of hydrogen was adjusted to 100 bar hydrogen at 160° C. isotherm for a period of 3 to 9 hours. The hydrogen pressure was released to ambient pressure, and the sample was heated to 300° C. and released hydrogen was measured again. This cycle was repeated at least 20 times. The amount of hydrogen generated in each cycle is shown in FIG.  2 . 
     EXAMPLE 2 
     In order to compare the effectiveness of the mechanomixing as compared to that of catalytic effects, the NaAlH 4  was catalyzed by Ti according to the following procedure. The NaAlH 4  was doped with Ti catalyst at 2 mole % level. Hydrogenation/dehydrogenation cycles similar to experiment 1 was performed. In order to see the effect of one variable on the properties, the time was kept constant (6 hours). The same powder was used while hydrogen pressure was changed. FIGS. 3A-3B shows the effect of Ti doping on hydrogenation/dehydrogenation cycles. As shown in FIG. 2, mechanical mixing resulted in a uniform dispersion of particles and provides a more enhanced kinetics for hydrogenation/dehydrogenation cycles. We have noticed by microscopy observation that a more compact mixture (reduced volume) was observed after mechanomixing, resulting in better hydrogenation kinetics. It is noteworthy to mention that NaAlH 4  is formed not only from Na 3 AlH 6  particles (Na 3 AlH 6 +2Al+3H 2 →3NaAlH 4 ) but also from NaH and Al particles (3NaH+3Al+H 2 →3NaAlH 4 ). 
     This invention has shown that mechanical mixing is an effective process for producing a uniform mixture of byproduct particles. The mixed byproduct shows the best cycle life with the first hydrogen capacity (&gt;4 wt. %). It maintains 82% of the first cycle discharge capacity even after 20 cycles (FIG.  2 ). From x-ray analysis, it is found that the NaAlH 4  is the main product after each hydrogenation process. 
     The advantage of mechanical mixing during and/or after decomposition over other techniques becomes more apparent as the decomposed byproducts remain in intimate contact and also at much smaller particle size. The smaller particle size provides higher surface area for hydrogenation reaction, and the molecular level mixing of the components is essential to achieve fast kinetics. 
     As a sample of the complex chemical hydride decomposes upon the application of heat, the sample becomes foamy or frothy as a result of the byproducts produced (which includes hydrogen gas) in the decomposition process. Mixing and/or milling the sample brings the byproducts into intimate contact with each other. Furthermore, mixing and/or milling inhibits grain growth of the byproducts or reduces the particle size or grain growth of the byproducts so that a more reactive byproduct will be present for the hydrogenation step. The mixing and/or milling reduces the foamy or frothy nature of the sample. The process can be accomplished at a low pressure ranging from atmosphere to 400 pounds per square inch or less, and at a temperature ranging from about 100° C. to 600° C., and preferably from about 160° C. to 300° C. 
     The terms “mixing” or “mechanomixing” as used herein mean blending so that the constituent parts are intermingled to provide a more homogeneous mixture. The terms “milling” or “mechanomilling” as used herein mean grinding to reduce the particle or grain size of the constituent parts. 
     As will be appreciated from FIG. 2, the repeatability of the hydrogenation of the non-catalyzed complex chemical hydride without mixing (line A), or with a catalyzed complex chemical hydride without mixing (line B′ after three hours of hydrogenation or line B″ after nine hours of hydrogenation) dramatically drops off only after a few hydrogenation/dehydrogenation cycles. In contrast, using the complex chemical hydride in a process including mixing and/or milling according to the present invention produces repeatable, reliable, and consistent hydrogenation after numerous hydrogenation/dehydrogenation cycles (line C′ after hydrogenation for three hours and line C″ after hydrogenation for nine hours). 
     As will be appreciated from a comparison of FIGS. 3A and 3B, even the addition of the catalyst such as Ti to a complex chemical hydride such as NaAlH 4  does not produce as good as results as does mixing the complex chemical hydride sample after decomposition (FIG.  3 B). FIG. 3B shows that a substantially greater amount of NaAlH 4  is been formed at much lower pressures in a process according to the present invention that mixes complex chemical hydride after the decomposition step compared to the catalyzed complex chemical hydride without mixing.