J. Phys. Chem. B 2006, 110, 7967-7974
7967
Improved Hydrogen Release from LiB0.33N0.67H2.67 with Noble Metal Additions Frederick E. Pinkerton,*,† Martin S. Meyer,† Gregory P. Meisner,† and Michael P. Balogh§ Materials and Processes Laboratory and Chemical and EnVironmental Sciences Laboratory, General Motors Research and DeVelopment Center, MC 480-106-224, 30500 Mound Road, Warren, Michigan 48090-9055 ReceiVed: October 10, 2005; In Final Form: February 9, 2006
The hydrogen release behavior of the quaternary hydride LiB0.33N0.67H2.67 has been successfully improved through the incorporation of small quantities of noble metal. Adding 5 wt % Pd either as Pd metal particles or as PdCl2 reduced the temperature T1/2 corresponding to the midpoint of the hydrogen release reaction by ∆T1/2 ) -43 °C and -76 °C, respectively. PtCl2 and Pt nanoparticles supported on a Vulcan carbon substrate proved to be even more effective, with ∆T1/2 ) -90 °C. The amount of NH3 released during dehydrogenation is reduced compared to that from additive-free material, and, more importantly, at temperatures below 210 °C hydrogen is released with no detectable NH3. In contrast to additive-free LiB0.33N0.67H2.67, which melts completely above 190 °C and releases hydrogen from the liquid state only above ∼250 °C, hydrogen release from LiB0.33N0.67H2.67 + 5 wt % Pt/Vulcan carbon is accompanied by partial melting plus a cascade through a series of solid intermediate phases. Calorimetric measurements indicate that both additive-free and Ptadded LiB0.33N0.67H2.67 release hydrogen exothermically, and hence the reverse reaction is thermodynamically unfavorable. By exposing partially dehydrogenated samples to high H2 pressures at modest temperatures, fractional hydrogen uptake (roughly 15% of the released hydrogen) has been achieved. The mechanism by which noble metals promote hydrogen release is not known, but the behavior is consistent with that expected for a catalyst, including a large effect with small additions and saturation of the effect at low concentration.
Introduction The promise of hydrogen fuel cells for clean, efficient automotive propulsion has stimulated intense interest in highperformance on-board hydrogen storage systems. Storing hydrogen as a solid hydride is conceptually attractive because it offers a volumetric hydrogen density greater than that of either compressed gas or liquid hydrogen storage1 without highpressure containment or cryogenic tanks. To date, however, no solid material offers the ideal combination of high gravimetric hydrogen density and fast sorption kinetics at practical temperatures and pressures. Conventional metal hydrides either have low gravimetric density (e.g., LaNi5H6, 1.4 wt % hydrogen2) or unacceptably high operating temperatures (e.g., Mg alloys and Mg2Ni, ∼300 °C3). In the past few years, a number of promising new materials have emerged, including substantial advances in reversible storage using Ti-doped sodium alanate,4-8 reversible systems based on alkali metal and alkaline earth nitrides and amides,9-15 and reduced-temperature dehydrogenation in lithium borohydride.16 Vajo et al.17 recently reported reversible storage of ∼9 wt % in LiBH4 destabilized with MgH2, illustrating the potential of similar coupled reactions for tuning the thermodynamic properties of hydrogen storage systems. Although these systems remain limited by modest hydrogen capacity, slow kinetics, or high operating temperatures, they provide considerable encouragement for the development of new hydrogen storage materials. We recently reported the discovery of a new quaternary hydride in the Li-B-N-H system that released >10 wt % * Author to whom correspondence should be addressed (e-mail
[email protected]). † Materials and Processes Laboratory. § Chemical and Environmental Sciences Laboratory.
hydrogen when heated.18 Nearly single-phase material was formed from a mixture of 2LiNH2 + LiBH4 either by ballmilling or by heating mixed powders above ∼95 °C. Material at this composition melted at ∼190 °C and released all of its hydrogen above 250 °C, forming a mixture of solid Li3BN2 polymorphs. H2 release from 2LiNH2 + LiBH4 mixtures has subsequently been confirmed by Aoki et al.19 Powder X-ray diffraction (XRD) of ball-milled samples found the new phase to have a body-centered cubic crystal structure with lattice constant a ) 10.76 Å. We tentatively identified this phase as “Li3BN2H8” and hereafter denote it the R-phase. A recent XRD study20 performed on single crystals obtained by recrystallization from the melt determined the equilibrium composition of the R-phase to be Li4BN3H10 with lattice constant a ) 10.68 Å. Investigation of (LiNH2)x(LiBH4)1-x materials21 found the LiNH2:LiBH4 ) 2:1 composition (x ) 0.67, or LiB0.33N0.67H2.67) to be optimal for hydrogen release, providing maximum hydrogen production with minimum concurrent ammonia production. Samples made at the 3:1 composition (x ) 0.75) corresponding to Li4BN3H10 released less hydrogen and >3 times as much ammonia during dehydrogenation compared to the 2:1 composition. For this reason, the composition LiB0.33N0.67H2.67 was selected for the current study. The new compound has several disadvantages as a source of hydrogen: (1) the >250 °C hydrogen release temperature is still rather high, (2) a small quantity of undesirable NH3 is produced simultaneously with H2 release, and (3) initial calorimetric measurements indicate that hydrogen release is exothermic and rehydrogenation is thus not thermodynamically favored. Furthermore, dehydrogenation occurs from the liquid phase; although this is not necessarily a disadvantage for an irreversible hydrogen source, the Li3BN2 product tends to be a monolithic solid that may kinetically inhibit further reaction and
10.1021/jp0557767 CCC: $30.25 © 2006 American Chemical Society Published on Web 03/25/2006
7968 J. Phys. Chem. B, Vol. 110, No. 15, 2006
Pinkerton et al.
TABLE 1: Additives to LiB0.33N0.67H2.67, Listing the Weight Fraction of Additive Expressed as a Percent, the Corresponding Mole Fraction of the Metal per Mole of LiB0.33N0.67H2.67 Expressed as a Percent, an Estimate of the Particle Size after Ball-Milling, the Corresponding Specific Surface Area, and the Temperature Shift ∆T1/2 of the Midpoint of the Dehydrogenation Weight Loss Measured by TGA
additive TiCl3 Pd
mole specific ∆T1/2 weight fractiona particle surface (%) (%) size (nm) area (m2/g) (°C) -6
5
0.7
5 10
1.1 2.1
e100
∼8
-43 -64
∼40
-76
PdCl2
8.3
1.1
e20
Pt
5
0.6
