chemical hydrides: amine boranes - ucsb · 5 approachesapproaches scanning calorimetry combined...
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Chemical Hydrides: Amine BoranesChemical Hydrides: Amine BoranesChemical Hydrides: Amine Boranes
International Symposium on Materials Issues in Hydrogen Production and
Storage August 2006
Acknowledgements Department of Energy, Office of Basic Energy Sciences and Energy Efficiency and Renewable Energy.
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NH BH (3)
Hydrogen-rich ammonia borane (AB)HydrogenHydrogen--rich ammonia borane (AB)rich ammonia borane (AB)
George Thomas
Hydrogen Densities of Materials
0
50
100
150
200
0 5 10 15 20 25 30Hydrogen mass density (mass %)
Hyd
roge
n vo
lum
e de
nsity
(kgH
2m
-3)
100
liquidhydrogen
700 bar
350 bar
CH4 (liq)
C2H5OH
C8H18
C3H8
C2H6NH3
CH3OH
Mg2NH4
LaNi5H6
FeTiH1.7
MgH2
KBH4
NaAlH4
NaBH4
LiAlH4
LiBH4
AlH3TiH2
CaH2
NaH
2015 system targets
2010 system targets
3 3
NH3BH3(2)
NH3BH3(1)
Mg(OMe)2.H2O
11M aq NaBH4
hexahydrotriazine
decaborane
LiNH2(2)
LiNH2(1)
NH4BH4(4)
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NHxBHx Store hydrogen (>6 wt%/step)NHxBHx Store hydrogen (>6 wt%/step)
Wt% H2 T (K)NH4BH4 NH3BH3 + H2 6.1 <300NH3BH3 (NH2BH2)n + H2 6.5 <375(NH2BH2)n (NHBH)n + H2 6.9 >375(NHBH)n BN + H2 7.3 >>800
Two sequential steps > 12 wt% hydrogen!
Solid state chemistry: thermodynamics and kinetics of hydrogen release?
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Reactants Products ∆E (kJ/mol)
NH4BH4(s) → NH3BH3(s) + H2 -10NH3BH3(s) → (NH2BH2)n + H2 +37(NH2BH2)n → (NHBH)n + nH2 -13(NHBH)n → BN(s) + nH2 -38
AB+ H2
PIB+ H2
PAB+ H2ABH2
ABH2 = NH4BH4AB = NH3BH3
PAB = (NH2BH2)n
PIB = (NHBH)n
Thermodynamics
BH4 + 4H2O B(OH)4 + 4H2∆H = -250 kJ/mol
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ApproachesApproachesApproaches
Scanning calorimetry combined with mass detection and gravimetric analysis
Thermodynamics of hydrogen lossKinetics and insight into mechanism of H-release
NMR and Raman spectroscopy Solid state to study phase transitions and molecular dynamicsVariable temperature for in-situ kinetic investigations
Neutron spectroscopy QENS (for dynamics of H motion)INS (structural properties)
Combination of experiment & theory to gain fundamental understanding of chemical & physical properties of AB
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Ammonia borane isoelectronicisoelectronic with ethane
NH3BH3 CH3CH3
MW 30.81 30.07Mp[˚C] 124 -183M[D] 5.2 0bonding N->B dative C-C covalentR[A] 1.58 (1.62) 1.53kg H2/kg 191 2.1kg H2/liter 143 1.3
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Background crystalline NH3BH3Background crystalline NHBackground crystalline NH33BHBH33
• Synthesis (Shore and Parry in 1955)• AB in equilibrium with diammoniate of diborane
2NH3BH3 ↔ [NH3BH2NH3][ BH4]
• NH3BH3 H2 + polyaminoborane at < 370 K.∆Hrxn = -20 kJ/mol (Wolf et al. 2000)
•First order-disorder phase transition at 225 K, the low T structure is orthorhombic, the high T tetragonal (Reynhardt & Hoon)•Electrostatic bonding between (N)H+ and (B)H-
(dihydrogen bond) (Crabtree et al. 1999)
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0 500 1000 1500 2000 2500 3000
Rel
ativ
e H
eat R
elea
sed
(a. u
.)
Time (min)
85 °C
80 °C
75 °C70 °C
Neat AB
Thermal decomposition of solid AB
•Rates increase with temperature
•Activated process
•Sigmoidal kinetics
•Induction period
•Thermodynamics
•∆Hrxn = -21±3 kJ/mol
NHNH33BHBH33(s) (s) (NH(NH22BHBH22)(s) + H)(s) + H22
Solid state kinetics analysis
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Stabilizing polymerizationStabilizing polymerizationStabilizing polymerization
+
Large dipole moment → instability
Relieving instability•Coiling of oligomers → cyclization•Branching of oligomers
Simulated annealing reveals “coiled” or “cyclic” structure of oligomers, stabilized by dihydrogen bonds – precursors to
observed cyclic products
Ammonia borane decompositionAmmonia borane decompositionAmmonia borane decomposition
-(NH2BH2NH2)-
NH3BH3
(NH3)2BH2BH4
-(NH2BH(NH2)2- -(NH2BH3
11B NMR 800 MHz11B NMR 800 MHz
Complex mixture of cyclic, branched polymeric products
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Nano-phase ammonia borane
Use mesoporous silica (SBA-15) as a scaffold. The 6-7 nm wide channels will hold Ammonia Borane (NH3BH3) in the nano-phase. Should also preserve nanophase.
Ammonia borane infiltrated
NH3BH3
Add saturated solution of NH3BH3 to
SBA-15
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0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0.0
0.2
0.4
0.6
0.8
1.0
0 100 200 300 400 500 600
Rel
ativ
e H
eat R
elea
se Extent of Reaction
Time /min
Neat Amonnia Borane80 °C
Amonnia Borane on SBA Scaffold 50 °C
50 100 150 200 250
Rel
ativ
e Yi
eld
(arb
. uni
ts)
Temperature (°C)
Hydrogenm/e = 2
Borazinem/e = 80
Neat ABAB:SBA-15
Borazine (BHNH)3
Ammonia borane in SBA-15
Change in Reactivity and Selectivity
Angew. Chem. Int. Ed. 2005, 44, 3578.
Scaffolds: enhance rates of H2 release enhance purity of H2(little borazine) change in thermodynamics
x20
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Fundamental Science QuestionsFundamental Science Questions
NH3BH3 (NH2BH2)n + H2 + ? <100 ˚C
Is the mechanism intra or intermolecular?What is the activation barrier?Can we change it with catalysis, (other)Can we control the decomposition pathways?
How is H2 formed from solid AB?How is H2 formed from solid AB?
Need approaches to study solid state kinetics
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Phase transformation kinetics
r (t)
Avrami Equation XCrystal = 1 - exp (-(kt)n)
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Avrami rate as function of T (70-85 C)AvramiAvrami rate as function of T (70rate as function of T (70--85 C)85 C)
Xreaction = 1 - exp (-(kt)3)
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Arrhenius analysis of k(Avrami) vs. temperature Arrhenius analysis of Arrhenius analysis of k(Avramik(Avrami) vs. temperature ) vs. temperature
Activation barrier for H2 formation from solid state ammonia borane is ca. 150 kJ/mol.
Ea ~ 150 kJ/mol
1
Kinetic model for hydrogen formationKinetic model for hydrogen formationKinetic model for hydrogen formation
Sigmoidal kinetic behaviorInduction, Nucleation & Growth
Induction: Rate limiting. How to decrease (scaffold)?
Nucleation:Physical or Chemical Change
Growth: hydrogen formationHow do you modify pathways, change thermodynamics?
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Optical microscope studiesOptical microscope studiesOptical microscope studies
Time 0 75C 100C
7 min 10 min 13 min 14 min
Crystal of AB (~0.1 mm)
Induction, nucleation, growthInduction, nucleation, growthInduction, nucleation, growth
3
5
43
21
1
2
3 45
Raman microscopyRaman microscopyRaman microscopy
“top” of crystal (1) is crystalline AB, bottom (5) is amorphous AB + ??. Beginning of nucleation?
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Growth?Growth?Growth?
2.02 Å
Literature: Reaction Pathways to H2Intermolecular
2 NH3BH3 NH3BH2-NH2BH3 + H2
or
IntramolecularNH3BH3 NH2=BH2 + H2
n NH2=BH2 (NH2BH2-NH2BH2)n
BH ----- HN Dihydrogen bondδ- δ+
Hydride atoms act as Proton acceptors
Intermolecular vs. IntramolecularIntermolecular vs. IntramolecularIntermolecular vs. Intramolecular
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Ionic pathway2 NH3BH3 [NH3BH2NH3]+[BH4]- (isomerization, no H2 loss)
AB DADB
NH3BH3 + [NH3BH2NH3 ]+[BH4]-[NH3BH2NH2-BH2NH3][BH4]- + H2
Neutral pathway2 NH3BH3 NH3BH2-NH2BH3 + H2
If [NH3BH2NH3]+[BH4]- is an intermediate, should see it by in-situ solid state 11B NMR
Nucleation? Nucleation? Do we have dehydrocoupling of ionic or neutral intermediates?
11B NMR 800 MHz Ammonia Borane1111B NMR 800 MHz Ammonia BoraneB NMR 800 MHz Ammonia Borane
Time from top to bottom: mins.Red: 0Blue: 10Green: 15Yellow: 20Aqua: 25Black: 30Pink: 40Bright Green: 120
T = ~90 °C
-(NH2BH2NH2)-
NH3BH3
(NH3)2BH2BH4
-(NH2BH(NH2)2--(NH2BH3
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Nucleation seedingNucleation seedingNucleation seeding
BH3NH3 doped with seeded material
-0.2
0
0.2
0.4
0.6
0.8
1
0 1000 2000 3000 4000
Time(min)
q(m
W)
AB Isothemal 70C
AB doped with AB seeded tostart of exotherm
Nucleation seeding of solid AB enhances rate of H2 release.
b) Mix seeds with fresh AB
a) Heat AB for 1000 minutes to synthesize ‘seeds’
ac) Heat AB mixture decreased induction time
c
What is the nucleation step? We must know this to better control rates of H2 evolution!
∆Hrxn ~ -21 kJ/mol
Ea = 155 kJ/mol
Ener
gy
AB (NH2BH2)n + H2
Intermolecular reaction of AB leads to H2 in the solid stateDiammoniate is important intermediateRates of H2 release can be controlled in the solid state
Enhance kinetics with nucleation seeding or scaffold
Solid state AB energeticsSolid state AB Solid state AB energeticsenergetics
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Role of dihydrogen bonds in H2 release:Role of Role of dihydrogendihydrogen bonds in Hbonds in H22 release:release:
BH ----- HN Dihydrogen bondδ- δ+
Hydride atoms act as Proton acceptor*
~2Å
*Klooster, et. al. JACS, 1999, 121, 6337
Use neutron scattering (INS and QENS) methods to study properties of H-rich materials.
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Inelastic neutron scatteringInelastic neutron scatteringInelastic neutron scattering
0
50
100
0 500 1000 1500 2000cm-1
inte
nsity
10 K75 K230 K
11BH3NH3 Vibrational spectra
FDS provides approach to measure low frequency vibrational modes due to intermolecular interactions
Experimental data to bench mark theory
To facilitate identification of modes use selective D/H labeling to ‘hide’ transition.
B-N stretch
B-N torsion
Intermolecular modes
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Vibrational studies of dihydrogen bondingVibrationalVibrational studies of studies of dihydrogendihydrogen bondingbonding
BH ----- HN δ- δ+
linear
cyclic
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Computational analysis of dihydrogen bondingComputational analysis of Computational analysis of dihydrogendihydrogen bondingbonding
BH ----- HN δ- δ+
linear cyclic
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NH3BH3 SummaryNH3BH3 SummarySummary
•12 wt% hydrogen at relatively low temperatures (<400 K)
•exothermic ~ 20 kJ/mol – need to regenerate by chemical pathway
•Mechanism of H2 formation – Nucleation and Growth
•Induction period can be enhanced with scaffold or seeding
•INS provides low frequency vibrational modes to yield insight into di-hydrogen bonds – benchmark theory
•Computational approaches to understand dynamics
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Future DirectionsFuture DirectionsFuture Directions
MH ----- HY Dihydrogen bondδ- δ+
M is electropositive (Li, Mg, Ca, Na, Al, B) Y is electronegative (N, O, P, etc.)
AB will react with itself or with other hydridic and/or protic hydrogen. E.g., MgH2 ---H3NBH3 ---H2NLi
Are thermodynamics of alternative reactions more favorable?
Understanding Understanding dihydrogendihydrogen bonding interactions. How bonding interactions. How universal are these interactions in H storage materials?universal are these interactions in H storage materials?
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ContributorsContributorsNancy Hess
Anna GutowskaYongsoon Shin
Liyu Li Chun Lu
Shari Li Scott Smith Bruce Kay
Ashley Stowe Abhi KarkamkarDave Heldebrant
FSUNaresh Dalal
Ozge Gunaydin-SenR Achey
NCNRCraig Brown
Terry UdovicTeresa Jacques
Eugene Mamontov
VisitorsBenjamin Schmid (UO)
Mark Bowden (IRL) Rene Schellenberg (TUBF)
Rebecca Lowton (Oxford)
LANSCELuke DaemenThomas Proffen
Monika Hartl
Maciej GutowskiDon CamaioniGreg Schenter
Jun Li Venci Parvanov
Shawn KathmannJohn LinehanWendy Shaw
Andrew Lipton Paul Ellis
Jesse Sears
PNNL