Gas separation: processes – Solid sorption – Inorganic gas or liquid particle sorbed
Reexamination Certificate
2001-11-19
2003-07-22
Smith, Duane (Department: 1724)
Gas separation: processes
Solid sorption
Inorganic gas or liquid particle sorbed
C095S106000, C502S417000, C420S900000, C423S448000
Reexamination Certificate
active
06596055
ABSTRACT:
BACKGROUND OF THE INVENTION
Hydrogen is a widely used commodity in the chemical and petroleum processing industries. Typically it is manufactured, usually by a reforming of natural gas, and is delivered to the users' sites by pipeline, as liquid H
2
or as the highly compressed gas in cylinders. The transport of hydrogen as a cryogenic liquid or as compressed gas are capital and energy-intensive processes which result in a significantly higher cost for the delivered gas. Therefore, there has been a large research effort directed at finding lower cost alternatives, principally on developing materials that could effectively “capture” hydrogen at or near ambient conditions and release the gas as desired, at the point of use. Recently such efforts have been greatly stimulated by the emerging technology of H
2
-driven fuel cells which, for mobile systems, ideally require a safe and cost-effective means for an on-board storage of hydrogen.
Most of the research towards ways to “contain” hydrogen has focused on the reversible chemical reaction and absorption of H
2
by various metals and metal alloys to form metal hydrides. Representative systems are LaNi
5
, FeTi and various magnesium-rich alloys, such as Mg
2
Ni and Mg
2
Fe.
In general, the hydride-forming metals/alloys that demonstrate favorable thermodynamic properties display poor H
2
capacity, whereas hydride-forming metals/alloys with a relatively high H
2
capacity generally have unfavorable thermodynamic properties. While substantial research efforts have been focused upon the synthesis and study of new generations of bi-, tri-, and multi-metallic alloys that demonstrate incremental improvements in hydrogen capacity and adsorption/desorption kinetics, it can be argued that the art has reached a point of diminishing returns with respect to advancing the functional characteristics of these systems, casting doubt on their commercial viability and application at a large scale for H
2
storage.
The sorption of hydrogen by various new structural forms of carbon has recently gained widespread attention. It has been known for some time that high-surface-area activated carbon and also certain alkali-metal graphite intercalation compounds will reversibly sorb considerable quantities of hydrogen, but only at cryogenic temperatures. Such systems therefore do not offer practical or economic advantages over the use of compressed or liquified hydrogen. Rodriquez et al in U.S. Pat. No. 5,653,951 (1997) claim the storage of hydrogen in various layered carbon nanostructures including carbon nanofibers and carbon nanotubes. Hydrogen storage data is only given for carbon nanofiber materials which take up ~1.22 cc of H
2
/gram at 295 K, 3.5 psia H
2
pressure. This corresponds to a uptake of only ca. 0.01 wt % H
2
, a capacity which is far too small for any practical application. Bulk graphite, which has a surface area less than that of carbon nanofibers, would be expected to show an even smaller H
2
capacity.
Recently, Chambers, Rodriquez et al reported in
J. Phys. Chem B
1998, 102, 4253 that carbon nanofiber materials of unspecified specific origin reversibly sorb very large, 50 wt % or greater, quantities of hydrogen under high H
2
pressure. These results have not been confirmed by others, and have been directly disputed in a number of publications. See Rzepka, M. et al,
J. Phys Chem B
, (1998) 102, 10894.
A. C. Dillon et al in
Nature Vol.
386, p. 379 (1997) reported on an unusual sorption of hydrogen at near-ambient temperatures by single-walled carbon nanotube (SWNT) materials. The SWNT materials are relatively recently discovered new structural forms of carbon which essentially consist of rolled-up single sheets of graphite, with an external diameter of ca 12 Å and a very large length-to-diameter aspect ratio. The SWNT's are usually bundled together and appear by electron microscopy as long fibers which can be shortened and their properties otherwise modified by selective oxidation processes. SWNT materials are expected to adsorb H
2
at low (cryogenic) temperatures, analogous to activated carbons, due to their high surface area. However, the ambient-temperature H
2
sorption results of Dillon et al have been directly disputed in a recent publication by Ye, Y., et al,
Appl. Phys Lett
. (1999) 74, 2307.
L. Aymard, et al,
J. Electrochem. Soc
. (1999) 146, 2015 reported that carbon additives have been shown to have a favorable effect on the electrochemical performance of multi-metal alloys for nickel-metal hydride batteries. Here, carbon is incorporated interstitially in the alloy electrode where inter alia, it aids in the diffusion of hydrogen into the bulk of the alloy. K. Funaki, et al.,
J. Alloys Comp
. (1998) 270, 160 have shown that the introduction of graphite by mechanical alloying into MgNi, a well-known metal hydride forming composition, yields alloy compositions of formula MgNiC
x
where x≦1.31. Upon hydrogenation of MgNiC
x
the atomic ratio of hydrogen plus carbon to metal, (H+C)/M, remains constant indicating that the hydride sites in the metal alloy are simply replaced by carbon atoms. Thus, there is no evident increase in hydrogen storage capacity. G. Sandrock,
J. Alloys Comp
. (1999) 293-95, 877 reports that the addition of sulfur, selenium, and carbon, non-metal elements to, specifically, Ti—Mn Laves phase alloys is reported to “pave the way” for increasing the H
2
storage capacity of these alloys.
In U.S. Pat. No. 4,716,736 (1988) J. A. Schwartz teaches “metal assisted cold storage of hydrogen”. Here the well-known capability of activated high surface area microporous (not substantially graphitic) carbons to physically adsorb H
2
at cryogenic temperatures is said to be somewhat enhanced by the presence of an added highly dispersed transition metal, eg Pd, Pt, component. The utility of this system is however, restricted to cryogenic temperatures, ie, at less than 273 K; Examples provided are at 77 K and 85 K. It is theorized here that the H
2
molecule is adsorbed onto the metal as H atoms, (monatomic hydrogen), which “spills” onto the carbon surface, this activated hydrogen thus “filling the available sites on the activated carbon”—as physisorbed H
2
.
The concept of hydrogen spillover, see “Hydrogen Spillover” by P. A. Sermon and G. C. Bond,
Cat Rev.
8(2) 211 (1973) has its genesis in fundamental studies with supported metal catalysts, particularly with such systems as are used for chemical hydrogenation reactions. The metal has the role of “activating” hydrogen by reversibly dissociating H
2
into metal-H atom species on its surface. But it's has been observed that, for instance, by heating Pt dispersed on carbon catalysts (designated as Pt/C) at 623 K, Pt/Al
2
O
3
at 473-573 K, Pd/C at 473 K, and also by Pt/WO
3
, the amount of H
2
taken up is in excess of the known H
2
-sorption capacity of the metal alone. Numerous studies have provided support for the theory that some of the H
2
“spills over” from the metal to the support but the nature of this “transferred” hydrogen is not presently known. The quantity of this hydrogen on the support is usually very small, amounting to only several atoms of H for every H that's bound to the metal.
N. Rodriguez and T. Baker, U.S. Pat. No. 6,159,538, provides further data on their prior literature report in
J. Phys. Chem
B 1998, 108, 4253 of H
2
-uptake by layered nanostructures which include graphite, carbon nanofibers, multi-walled carbon nanotubes etc., that have been treated with an inert gas at elevated temperatures. The H
2
absorption is claimed to take place when the materials are subjected to flowing hydrogen at a pressure from 1000 psia to 3000 psia. The patent describes a use of a nanostructure that is intercalated with a minor amount of a suitable metal, which serves to increase the gap between the nanomaterial's layers.
BRIEF SUMMARY OF THE INVENTION
The present invention is a process for the transport and storage of hydrogen by reversible sorption and containment within carbon-metal hybrid materials. The process
Cooper Alan Charles
Pez Guido Peter
Air Products and Chemicals Inc.
Lawrence Frank M.
Rodgers Mark L.
Smith Duane
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