Improved Hydrogen Release from LiB0.33N0.67H2.67 with Noble Metal Additions

Abstract

The hydrogen release behavior of the quaternary hydride LiB(0.33)N(0.67)H(2.67) has been successfully improved through the incorporation of small quantities of noble metal. Adding 5 wt % Pd either as Pd metal particles or as PdCl(2) reduced the temperature T(1/2) corresponding to the midpoint of the hydrogen release reaction by DeltaT(1/2) = -43 degrees C and -76 degrees C, respectively. PtCl(2) and Pt nanoparticles supported on a Vulcan carbon substrate proved to be even more effective, with DeltaT(1/2) = -90 degrees C. The amount of NH(3) released during dehydrogenation is reduced compared to that from additive-free material, and, more importantly, at temperatures below 210 degrees C hydrogen is released with no detectable NH(3). In contrast to additive-free LiB(0.33)N(0.67)H(2.67), which melts completely above 190 degrees C and releases hydrogen from the liquid state only above approximately 250 degrees C, hydrogen release from LiB(0.33)N(0.67)H(2.67) + 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 Pt-added LiB(0.33)N(0.67)H(2.67) release hydrogen exothermically, and hence the reverse reaction is thermodynamically unfavorable. By exposing partially dehydrogenated samples to high H(2) 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.