Patent Application: US-33434806-D

Abstract:
the invention relates to a method of storing hydrogen that employs a mixture of hydrogen and a hydrocarbon that can both be used as fuel . in one embodiment , the method involves maintaining a mixture including hydrogen and a hydrocarbon in the solid state at ambient pressure and a temperature in excess of about 10 k .

Description:
reference will now be made in detail to the preferred embodiments of the present invention , examples of which are illustrated in the accompanying drawings . at high pressure ( p ), molecular hydrogen and other closed shell systems can form a number of stoichiometric solid compounds with other simple molecules ( e . g ., h 2 o , ar , ch 4 ). as an important component of molecular cluster studies , van der waals solid compounds are of interest from a fundamental point of view . van der waals solid compounds allow the study of interactions in weakly interacting molecular and atomic systems in different crystalline environments at high density . they exhibit many interesting materials properties with relevance to many fields . for example , those containing molecular hydrogen may be significant for planetary science , because h 2 is the most abundant molecular species in the universe . these hydrogen - rich materials are also of potential technological interest . as discussed in mao and mao , “ hydrogen storage in molecular compounds ”, proc . nat . acad . sci ., vol . 101 , no . 3 , pp . 708 - 710 , jan . 20 , 2004 which is hereby incorporated in its entirety , the stability field of ( h 2 ) 4 ( ch 4 ), h4m , which was small ( 5 , 000 - 6 , 000 mpa ) at 300 ° k ., greatly expanded and became the only molecular compound between h 2 and ch 4 at 1600 k and 1 , 000 mpa . the h4m was identified by using raman spectroscopy because it was found to be the only h 2 — ch 4 compound having a hydrogen vibron peak at lower energy than the q1 ( 1 ) vibron . further , hydrogen forms at least four molecular compounds with methane . one of particular interest from a hydrogen storage standpoint is ( h 2 ) 4 ch 4 ( h4m ). h4m has the highest hydrogen content of any currently known stoichiometric compound , containing 50 . 2 wt % hydrogen including the hydrogen in methane , 33 . 4 wt % only considering the molecular hydrogen component , making it a promising hydrogen storage candidate . in addition , methane can be used as a secondary fuel . a practical hydrogen storage method should satisfy a number of requirements ( e . g ., high hydrogen content per unit mass and volume , moderate synthesis pressure p , near ambient pressure p and moderate temperature ( t ) for storage , easy hydrogen release , and environmentally friendly byproducts ). diamond anvil cell ( dac ) studies allow characterization of gas mixtures at high p over a range of t via a number of in situ techniques . melting represents a very important transition in h4m for determining its potential as a hydrogen storage material . information on the melting curve of h4m at high pressure p ( up to 6 gpa ) and low temperature t ( down to 10 ° k .) can be obtained using optical microscopy and raman spectroscopy . fig1 illustrates a schematic of an experimental set - up using a cryostat and a diamond anvil cell ( dac ) in lever arm with a close - up of opposing diamond anvils compressing a sample within a becu gasket . a dac with 0 . 7 mm diameter diamond culets , and 0 . 25 mm thick becu gaskets ( to minimize reaction with hydrogen ) and drilled 0 . 3 mm diameter holes to contain the sample . several small ruby grains were used as the pressure calibrant . the dac was inserted in a gas pressure vessel where a nominally 4 : 1 mixture of h 2 and ch 4 gases ( 80 . 6 and 19 . 4 mole %, respectively ) were compressed to 200 mpa . after clamp - sealing the sample in the gasket at 1 gpa , the dac was removed from the gas vessel and further compressed using a lever arm . the whole assembly was then introduced into a cryostat which was cooled using liquid nitrogen and liquid helium . a silicon diode was attached to the dac near the sample to measure temperature . the dac was equipped with retracting springs to ensure complete pressure reduction in the cryostat . melting and crystallization of the sample were observed in situ at high pressure p and low temperature t using optical microscopy and raman spectroscopy . the sample was initially a fluid mixture of h 2 and ch 4 . after crystallization , the sample was found to be in a single phase h4m . fig2 illustrates grown crystal grains . melting was observed by shrinking of crystal grains and rounding of their edges before they eventually completely transformed into the fluid phase . raman measurements were performed with 488 or 514 . 5 nm ar + ion laser lines . two areas of interest in the raman spectra were the hydrogen vibron region , shown in fig3 a , and the ch stretch region , shown in fig3 b . the hydrogen vibron ( q 1 ( j )) in the raman spectra for the liquid and solid were quite diagnostic for determining when the sample melted or solidified . at the p - t of interest here , the hydrogen molecules are non - rigid rotors , and essentially the only populated states are the j = 0 and j = 1 molecular rotational states , i . e ., the ground states of para ( p - h 2 ) and ortho molecules ( o - h 2 ), respectively . the hydrogen molecules at the start of the experiments are in the normal ratio of spin states ( n - h 2 ) which is a 3 : 1 mixture of the ortho - and para - species . based on the generalized binary random alloy approximation , it has been previously demonstrated that the multiplet of q lines collapses into a single q 1 ( 1 ) line with a diminishing q 1 ( 0 ) over a small range of pressure . as shown in the fluid raman vibron spectra , the hydrogen q 1 ( 1 ) peak , ν f1 in fig3 a , dominate with a weak q 1 ( 0 ) side peak ( ν f2 ) at slightly higher frequency . the hydrogen in h4m solid , however , shows two equally strong peaks ( ν s1 and ν s2 ) and a weak side peak ( ν s3 ). previous publications indicated that the observed doublet was due to a two phase region ( i . e ., ν s2 corresponding to fluid hydrogen and ν s1 due to h4m ), but we found that both peaks occurred when the sample completely solidified . the ν s1 and ν s2 doublet can only be attributed to h4m and must be due to either factor group splitting or two distinct h 2 sites in the unit cell . the weak ν s3 peak is thus the q 1 ( 0 ) branch of ν s2 . this information will provide constraints on the crystal structure and symmetry of h4m , which have not yet been determined . the slight negative shift of ν s1 relative to ν f1 may indicate weak bonding and stabilization of hydrogen in h4m . the vibrational properties of pure crystalline ch 4 are well understood . the totally symmetric c — h stretching mode ν 1 ( a 1 ) of ch 4 gives the strongest raman band between 2900 and 2950 cm − 1 , and the triply degenerate v 3 ( f 2 ) mode gives a weak band at 3100 and 3150 cm − 1 . in the present hydrogen and methane mixture , the ν 1 ( a 1 ) remains a single peak , whereas the ν 3 ( f 2 ) splits into doublet and triplet at low temperatures , as shown in fig3 b . there is little difference between the raman spectra of ch 4 in the fluid and in h4m solid . the sample was repeatedly cycled down in temperature to crystallize the fluid and then down in pressure to melt the solid in order to follow the melting line . the p - t conditions at which crystallization or melting of the sample was observed are listed in table 1 below and plotted in fig4 . there appeared to be considerable metastability at moderate p - t during the temperature t cycle ; the fluid required significant undercooling to solidify ; e . g ., at 3 and 4 gpa the undercooling is as much as 40 ° k . the hysteresis appears much less during the pressure p cycle ; e . g ., at 1720 k the p hysteresis is only 0 . 3 gpa . without wishing to be bound by theory , it is thought that this may have been due to the fact that pressure p was changed with significant strain and disturbance of the sample that promoted the phase transformation . hysteresis in the phase diagram at low temperature t is not unexpected and is an encouraging sign since the melting curve intersects ambient pressure at 64 ° k . at lower t ( i . e ., about 100 k ), the degree of undercooling and under pressuring seemed significantly reduced . the melting point was determined as the midpoint between the onset of crystallization and the onset of melting . the data was fit to an empirical melting simon - glatzel law , which is shown in fig4 . the data are within experimental error of the fit . extrapolating to low pressure , the melting curve intersects ambient pressure at 64k . after reaching liquid nitrogen temperature , the solid was cooled to 10 ° k . using liquid helium and then pressure p was slowly released . the solid was quenched at 10 ° k . and the nominal vacuum of the cryostat . upon increasing temperature t , the sample decomposed at 23 ° k ., which is higher than the liquid boiling point of hydrogen . in other words , the sample lost all of its hydrogen and left a solid methane at 23 ° k . this is less than the 64 ° k . predicted from the melting curve , but one factor to consider is the negligible hydrogen partial pressure in the helium cryostat . h4m will be able to be retained at much higher temperature t with a modest hydrogen partial pressure ( e . g ., 10 bar ), thus enabling storage of high hydrogen content in a solid at a practical p - t . although , the melting curves of h4m and pure hydrogen are similar above 1 gpa , they differ significantly at lower pressure p where the application to hydrogen storage is relevant . the h4m solid is stabilized to 40 ° k . higher temperature t than pure hydrogen at the same pressure p , as shown in fig4 . the melting curve of fig4 for h4m from ambient to high pressure p ( 6 gpa ) and down to low temperature t ( 10 ° k .) shows that h4m has potential to store hydrogen at ambient p and low t . further experiments on larger sample volumes in gas reaction chambers with an appreciable hydrogen partial pressure will help clarify the stability at low t and would provide more accurate p measurements . also , the use of promoter molecules may stabilize h4m to higher temperature tat ambient pressure p . previous x - ray diffraction studies indicate that the phase may have a tetragonal methane substructure , but these experiments were limited due to the small pressure p interval over which h4m is stable at ambient temperature t . additional x - ray and neutron diffraction experiments at lower temperature t are required to address this question . the wide stability range of this hydrogen - rich van der waals compound provides important information on the origin and density dependence of the intermolecular interactions in this interesting class of materials . it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention . thus , it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents . all publications and patent applications herein are incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference . w . l . vos , l . w . finger , r . j . hemley , h . k . mao , phys . rev . lett . 71 ( 1993 ) 3150 . 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