Controlled Hydrogen Release From Ammonia Borane Using Mesoporous Scaffolds.

Controlled Hydrogen Release From Ammonia Controlled Hydrogen Release From Ammonia BoraneBorane Using Using MesoporousMesoporous Scaffolds.Scaffolds.

Anna Gutowski, Benjamin Schmid, Liyu Li, R. Scott Smith, Bruce D. Kay, John Linehan, Wendy Shaw, Nancy Hess, Yongsoon Shin,

Maciej Gutowski & Tom Autrey

APS LA

March 2005

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OutlineOutlineOutline

Background NHxBHxThermal decomposition pathways

ApproachMesoporous scaffolds

ResultsThermodynamic and Kinetic Comparisons

Future Work

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NHxBHx Store significant quantity of hydrogen (>6 wt%/step)

NHxBHx Store significant quantity of hydrogen (>6 wt%/step)

Wt% H2 T (˚C)NH4BH4 NH3BH3 + H2 6.1 <25NH3BH3 NH2BH2 + H2 6.5 <120NH2BH2 NHBH + H2 6.9 >120NHBH BN + H2 7.3 >500

Two sequential steps > 12 wt% hydrogen

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Favorable Thermodynamics?

Reactants Products ∆E (kcal/mol)

NH4BH4(s) → NH3BH3(s) + H2 -2.3NH3BH3(s) → (NH2BH2)n + H2 +8.8(NH2BH2)n → (NHBH)n + nH2 -3.2(NHBH)n → BN(s) + nH2 -9.2

AB+ H2 PIB+ H2

PAB+ H2ABO

ABO = NH4BH4AB = NH3BH3

PAB = (NH2BH2)n

PIB = (NHBH)n

BH4 + 4H2O B(OH)4 + 4H2∆H = -60 kcal/mol

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2015 goal (9 & 81)

Materials for H2 StorageMaterials for HMaterials for H22 StorageStorage

NH4BH4 = NHBH + 3H2

NH4BH4 = NB+4H2

NH3BH3 = NHBH+2H2

Ref: A. Züttel, “Materials for hydrogen storage”, materials today, Septemper (2003), pp. 18-27

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Ammonia Borane vs Ethane Any similarities?

Isoelectronic IsomersIsoelectronicIsoelectronic IsomersIsomers

H3N BH3 H3C—CH3

MW 30.81 30.07Mp[˚C] 114 -172bonding dative covalentDHo[kcal/m] 31 90M[D] 5.2 0R[A] 1.66 1.53Wt% H2 19% 19%

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How is hydrogen released?How is hydrogen released?How is hydrogen released?

BH ----- HN Dihydrogen bondδ- δ+

2.02 Å

Hydride atoms act as Proton acceptor

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Thermolysis of Ammonia BoraneThermolysis of Ammonia Borane

T (˚C)NH3BH3 (NH2BH2)n + H2 + ? <120

(NH2BH2)n (NHBH)n + H2 + ? >120

Are there other ‘products’?is the hydrogen clean? (borazine)

How is the H2 released?mechanism (solid state)

What is the activation barrier?can we change it with catalysis, (other)

Can the reaction be reversible?

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Volatile Products from NH3BH3Volatile Products from NHVolatile Products from NH33BHBH33

NH3BH3 Products + H2 Products + H2

~100 ˚C ~170 ˚C

MS = 79,80, 81

MS = 2

Borazine(BHNH)3

H2

DSC: 20 – 200 ˚C (5 ˚C/min, Ar 40 ml/min)

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NH3BH3 ChallengesNHNH33BHBH33 ChallengesChallenges

Meets and exceeds DOE gravimetric & volumetric targets

Need to lower temperature (or increase rates)Minimize volatile (borazine)

Can this be reversible? Not making B-O bonds (∆H = -60 kcal/mol)Release of H2 near thermoneutral (∆H = -5 kcal/mol)

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How does nano science improve the efficiency of hydrogen storage?

How does How does nanonano science improve the science improve the efficiency of hydrogen storage?efficiency of hydrogen storage?

Hypothesis: Nano phase hydrogen storage materials can have different thermodynamic and kinetic properties compared to bulk hydrogen storage materials.

Nano particles of Hydrogen Storage materialControl Reactivity (enhanced rate of hydrogen release)Control Selectivity (prevent borazine formation)Can we prevent fusion of the nanoparticles as the reaction proceeds? (Don’t want to lose nano properties)

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Approach: Nano-scale ScaffoldsApproach: Approach: NanoNano--scale Scaffoldsscale Scaffolds

6.5 nm diapores

4.2 nm wall thickness

Array of hollow cylinders

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Nano-phase Ammonia Borane

Use mesoporous silica (SBA-15) as a scaffold 6-7 nm wide channels to hold Ammonia Borane (NH3BH3) in the nano-phase. Should also preserve nanophase. Trap borazine in pores?

Ammonia borane infiltrated

NH3BH3

Add saturated solution of NH3BH3 to

SBA-15

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Volatile Products from NH3BH3 in SBA-15 mesoporous scaffold

Volatile Products from NHVolatile Products from NH33BHBH33 in SBA-15 mesoporous scaffold

MS = 79,80, 81

MS = 2

No borazine detected from AB in SBA-15! What about hydrogen, looks different?

1:1 wt:wt AB:SBA-15

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Hydrogen at lower temperature Hydrogen at lower temperature Hydrogen at lower temperature

Hydrogen released from NH3BH3 at lower temperature when it is embedded in scaffold!

Little borazine!

Can we quantify a difference in the barrier for hydrogen release?

What is ∆Hrxn?

Use isothermal DSC50 100 150 200 250

Rel

ativ

e Yi

eld

(arb

. uni

ts)

Temperature (°C)

Hydrogenm/e = 2

Borazinem/e = 80

Neat ABAB:SBA-15

DSC temperature ramp 1 ºC/min

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Thermochemistry Comparison

∆Hrxn for loss of H2 from AB (neat vs. scaffold)

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0.00

0.02

0.04

0.06

0.08

0.10

0 100 200 300 400 500 600

Hea

t Rel

ease

d (m

W/m

g)

Time (min) -tramp

Neat AB80 °C

Thermochemistry H2 releaseThermochemistryThermochemistry HH22 releaserelease

-5 kcal/mol

Isothermal DSC 80 ºC

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Thermochemical ComparisonThermochemicalThermochemical ComparisonComparison

0.00

0.02

0.04

0.06

0.08

0.10

0 100 200 300 400 500 600

Hea

t Rel

ease

d (m

W/m

g)

Time (min) -tramp

Neat AB80 °C

AB:SBA-1550 °C

-5 kcal/mol

-0.3 kcal/mol

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Change in ThermochemistryChange in Change in ThermochemistryThermochemistry

NH3BH3

AB SBA-15 85 ºC

Neat AB 85 ºC

800 MHz

(NH2BH2)

(NH2BH2)

cyc(NH2BH2) [(NH3)2BH2]BH4

Requires change in products

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Kinetic Comparison

Ea for loss of H2 from AB (neat vs. scaffold)

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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

Temperature dependence of H2 loss from ABTemperature dependence of HTemperature dependence of H22 loss from ABloss from AB

Isothermal DSC (kinetics)

τ1/2 75 ºC?

NHNH33BHBH33(s) (s) (NH(NH22BHBH22)(s) + H)(s) + H22

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Temperature dependence of H2 lose from AB/SBA15 NH3BH3(s) (NH2BH2)(s) + H2

Temperature dependence of HTemperature dependence of H22 lose from AB/SBA15 lose from AB/SBA15 NHNH33BHBH33(s) (s) (NH(NH22BHBH22)(s) + H)(s) + H22

0 50 100 150

Rel

ativ

e H

eat R

elea

sed

(a. u

.)

Time (min)

60 °C55 °C

50 °C

Ammonia borane in SBA-15

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Kinetic ComparisonKinetic ComparisonKinetic Comparison

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

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Rates as function of temperatureRates as function of temperatureRates as function of temperature

0.0

0.2

0.4

0.6

0.8

1.0

0 500 1000 1500 2000 2500 3000

Exte

nt o

f rea

ctio

n

Time/min

70 °C75 °C80 °C85 °C

50% conversion

half-life

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Arrhenius treatment of H2 formation ArrheniusArrhenius treatment of Htreatment of H22 formation formation

Rate of hydrogen release is 1 to 2 orders of magnitude faster with mesoporous scaffold

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∆Hrxn = -5 kcal/mol

Ea = 28 kcal/mol

*∆Hrxn = -0.3 kcal/mol

*Ea = 12 kcal/mol

Ene

rgy

Ene

rgy

(B) AB:SBA-15 *PAB + H2(A) AB PAB + H2

Summary Summary Summary

Selectivity of H2 release from ABNo borazine seen in volatile products or left behind in scaffold.No cyclized products observed in NMR and DSC data show process is less exothermic

Reactivity for H2 release from AB1-2 orders of magnitude faster!

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65 Å

•Mechanism of H2 formation from AB inter or intramolecular?In neat AB are the dihydrogen bonded hydrogen atoms ‘preset’ for molecular hydrogen formation?

A barrier for nucleation and a barrier for growth. What is nucleation site/event?

•Why is the rate of H2 release from AB faster in a mesoporous scaffold?Chemical (catalytic surface interaction) Physical (nano-phase crystal defects)

Change in products yields change in thermodynamics, what is mechanism for linear polymer growth, (B ---- OSi)?

Research Needs

•How general is the ‘mechanism’? Will this work help our understanding of H2 formation from materials containing both hydridic and protic hydrogen?

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J Fulton, Y Chen, M Balasubramanian XAFS N Hess, L Daemon, C Brown Neutron ScatteringW Jiang Ion Beam Synthesis nano BN D Matson RESS nano BNH Synthesis C Yonker in-situ Hi Pressure NMR

S Addleman G FryxellMesosV Viswanathan Fuel CellsG Whyatt Systems EngK Peterson Hi PressureJ Coleman SEMD McCready, XRDY Shin, S Li SBA-15C Wang TEM

FY04 NanoScience & Technology Initiative this work will be part of the Center of Excellence in Chemical Hydrogen

Storage

AcknowledgementsAcknowledgementsAcknowledgements