Energy Applications of Energy Applications of Ionic LiquidsIonic Liquids

James F. WishartChemistry Department

Brookhaven National Laboratory

LEAF LEAF BNL Laser-Electron Accelerator FacilityBNL Laser-Electron Accelerator Facility

U. S. Department of EnergyU. S. Department of EnergyBasic Energy SciencesBasic Energy Sciences

Salts are held together by electrostatic forces

Ionic lattice of Na+ cationsand Cl- anions.

Na+

Separation of ions, r(Å)

0 5 10 15

0

1

2

3

-1

-2

-3

-4

-5

-6

-7

PotentialEnergy

eV

Electrostatic Energy

Z+ Z- e2Uc = -

r

Repulsive Energy

kUr = r 9

Cl-NaCl (melts at 801 C)˚

Anions:BF4

-, PF6-, NO3

-, CF3SO3-, (CF3SO2)2N-

CF3CO2-, CH3CO2

-, CH3SO3-, AlCl4-

Pick ions to pack poorly.

Disorder is our friend.Size mismatch, bulky ions

Low molecular symmetry

Rotational disorder (R1 R≠ 2)

Mix-and-match as neededCations:

Ionic Liquids - Designing “Bad” Crystals

Separation of ions

0 5 10 15

0

1

2

3

-1

-2

-3

-4

-5

-6

-7

RepulsiveEnergy

Electrostatic Energy

Z+ Z- e2Uc = -

r

PotentialEnergy NaCl

m.p. 801 C

N+ Cl-

Anions:BF4

-, PF6-, NO3

-, CF3SO3-, (CF3SO2)2N-

CF3CO2-, CH3CO2

-, CH3SO3-, AlCl4-

Pick ions to pack poorly.

Disorder is our friend.Size mismatch, bulky ions

Low molecular symmetry

Rotational disorder (R1 R≠ 2)

Mix-and-match as neededCations:

Ionic Liquids - Designing “Bad” Crystals

Cl-

Separation of ions

0 5 10 15

0

1

2

3

-1

-2

-3

-4

-5

-6

-7

Electrostatic attraction is still strong enough to make vapor pressure ~0.

If it can’t evaporate, it can’t burn.

Combine specific ions to give desired properties.

Control solubility of solids and liquids:

Phase separation (like oil and water).

Easy separation of products.

Make liquid easy to reuse/recycle.

• Inherently safer.

• More economical.

• Less environmentally burdensome.

RepulsiveEnergy

Electrostatic Energy

Z+ Z- e2Uc = -

r

N+

NaClm.p. 801 C

N+

Anions:BF4

-, PF6-, NO3

-, CF3SO3-, (CF3SO2)2N-

CF3CO2-, CH3CO2

-, CH3SO3-, AlCl4-

Pick ions to pack poorly.

Disorder is our friend.Size mismatch, bulky ions

Low molecular symmetry

Rotational disorder (R1 R≠ 2)

Mix-and-match as neededCations:

Ionic Liquids - “Extreme Solvents”

Cl-3Electrostatic attraction is still strong

enough to make vapor pressure ~0.

If it can’t evaporate, it can’t burn.

Combine specific ions to give desired properties.

Control solubility of solids and liquids:

Phase separation (like oil and water).

Easy separation of products.

Make liquid easy to reuse/recycle.

• Inherently safer.

• More economical.

• Less environmentally burdensome.

N+

Ionic liquid properties lie at the edge of or beyond those of normal liquids.

Electrochemical range ( 6 V)≤Liquidus range ( 250° C)≤Vapor pressure (very low)

Viscosity (higher than normal)Dissolve polar, non-polar, biopolymers

Intrinsically conductive

Ionic liquids provide a path to new science and technology.

Ionic Liquids are Finding Industrial Uses

Cover StoryChemical and Engineering NewsApril 24, 2006, pp. 15-21

Out Of The Ivory TowerIonic liquids are starting to leave academic labs and find their way into a wide variety of industrial applications

Photo: BASF

Ionic Liquids and EnergyDesigner Solvents for a Sustainable World

Non-VolatileNon-Combustible

Phase SeparationHighly Conductive

Solubility ControlViscosity Control

LubricantsHeat Transfer

MakingClean Fuels

Nuclear FuelRad Waste

EnzymaticCatalysis

Solar EnergyConversion

BatteriesFuel Cells

IndustrialSolvents

Ionic Liquids

Energy EfficiencyHydrogen Economy Hydrogen Economy

Solar PhotochemistryBiofuels

Solar PhotochemistryHydrogen Initiative

Nuclear EnergyWaste Management

Energy Efficiency

Biofuels

Energy Efficiency

Wishart, Energy & Env. Sci. 10.1039/b906273d

Ionic liquids and energy production

Solar PhotoconversionPhotoelectrochemical cells (electrolytes and redox

carriers)Solar Thermal Conversion

Heat transfer and storage fluidsBiofuel production

Breakdown/dissolution of recalcitrant lignocellulosic feedstocks

Fossil fuel desulfurization and extraction of aromaticsAdvanced nuclear fuel cycle

Separations by extraction and electrodeposition

Wishart, Energy & Env. Sci. 10.1039/b906273d

Ionic liquids and energy storage

BatteriesLi Intercalation types.Li metal batteries – inertness is still a quest.

SupercapacitorsEssential technology for buffering electrical loads in

regenerative systems (hybrid vehicles, machinery).High stability and capacity are possible with ILs.

Wishart, Energy & Env. Sci. 10.1039/b906273d

Ionic liquids and efficient energy utilization

Fuel cellsProtic ILs as proton carriers

Gas separation (purification, H2 production, carbon captureSupported ionic liquid membranes

Electroactive polymers and ILsElectrochromic windows or displaysElectromechanical actuators/energy harvestersSensors

Catalytic processesPhase transfer catalysis (gas/liquid, liquid/liquid)

Wishart, Energy & Env. Sci. 10.1039/b906273d

DOE Workshop on Basic Research for ANES

235 invited experts from:31 universities11 national laboratories6 industries3 government agencies11 foreign countries

Panels:Materials under Extreme Conditions Chemistry under Extreme Conditions Separations Science Advanced Actinide Fuels Advanced Waste Forms Predictive Modeling and SimulationCross-cutting themes

http://www.sc.doe.gov/bes/reports/list.html

Advanced Nuclear Energy Systems

Multi-tier reactor systems extract more energy and reduce the high-level waste burden.

U, Np, PuAm, CmCs, SrTc, I, etc.

Np, PuAm, Cm

Cs, SrTc, I, etc.

Cs, SrTc, I, etc.

U, Pu

Np, Am, Cm

Ionic Liquids and Nuclear Processing

Ionic liquids could be used to process nuclear fuel, waste, and radiological contamination.

Beneficial features:Solvent properties and substrate ligation can be controlled by designLow volatilityCombustion resistanceHigh conductivityWide electrochemical windows.

“Electrodeposition of cesium at mercury electrodes in the tri-1-butyl-methyl-ammonium bis((trifluoromethyl)sulfonyl)imide RTIL”

P.-Y. Chen, C. L. Hussey, Electrochimica Acta 49, 5125 (2004)

28 mM Cs+ in Bu3MeN+NTf2– at a HMDEas a function of scan rate.

N

+

F

F

F

S

O

O

N

F

F

F

S

O

O

Solvation and Separation Properties of ILs

C4C2

C6

C12

Mixing oil (non-polar) and water (polar)

Changing anion changes selectivity

Sr2+/K+ Ratio

Ionic Liquids for Reprocessing

Inorg. Chem. 2003, 42, 2197

(EXAFS)

Electrochimica Acta 2004, 49, 5125

Inorg. Chem. 2003, 42, 5348

Green Chemistry, 2003, 5, 682

Green Chemistry, 2005, 7, 151

Inorg. Chem. 2002, 41, 1692

Green Chemistry, 2002, 4, 152–158

Non-flammable, highly conductive and wide electrochemical windows

Actinides and Fission Products in ILs

Chemical speciation - structures and formulasOxidation and reduction chemistry - oxidation states

Solubilities of materials in ionic liquidsExtraction - separating one element from the others

Plating out by electrodeposition

Effects of radiation on ionic liquids?

Initial Events in Radiolysis

solvent

e-

e-e-

hole“hole” + energetic

secondary electron

thermal electron

thermalization distance

presolvated “dry” states

solvated electron

High-energy electrons transfer energy to the medium, resulting in excitation and ionization of the constituents.

High-energy electron

solvent*

Ionization

ExcitationProductssolvent

Early Reactions in Radiolysis

Early reactions define radiation damage pathways

Recombination of hole and electronhole + e- → IL or IL*

Dissociation (bond breakage)hole → fragments (X + Y•)

Scavenging by solutes (or IL cations)e- + S or C+ → S- or C•

hole + S → hole- + S+

hole e-thermal electron

e-

solvated electronIL*

S"

S S-

S"+

We want to understand and control the reaction pathways to reduce net radiation damage and to control TRU and FP metal ion oxidation states for optimal separations.

X + Y•

S'

S'-x x

(im•, py•)

Ionic Liquid Radiolysis

Ionic liquids will undergo radiolysis when used for nuclear processing.

What happens when ILs are irradiated?

We must understand the primary radiation chemistry of ionic liquids.

Effects of ionic liquid composition on:The primary species produced by radiolysis (identity and yields).

Electron species, hole(s), reduced cations (Im•, Py•)

Spectroscopic and thermodynamic properties

Reactivity of primary species in neat solvent and with solutes.

Energetic transients: pre-solvated (“dry”) electron reactivity and relaxation.

How can we use it?Can the reactivity of primary species be exploited to achieve:

Maximum radiation stability to prevent degradation and lowered efficiencyor

A high yield of specific products to assist the separations process?

Integrated research program on ionic liquids-based separations systems

Mark DietzU. Wisconsin-Milwaukee

Sheng DaiORNL

Huimin LuoORNL

Chuck HusseyU. Mississippi

Jim WishartBrookhaven

Ilya Shkrob (and son)Argonne

Radiation chemistry

Separations chemistry

Supported by U.S. DOE

In cooperation withCEA Saclay

(Baldacchino, Renault, Le Caër)and CEA Marcoule (Moisy)

Identify ionic liquid radiolysis products and understand how they interfere with separations. With that comprehension, create more stable ionic liquid sepatations systems. Control and direct the damage.

A picosecond-synchronized UV laser pulse generates photoelectrons,which are accelerated to 9 MeV by high fields (80 MV/m)

in the one-foot long resonant-cavity structure.

2856 MHz microwave power: 15 MW

5 picosecond UV laser pulse

Photocathode Electron Gun Accelerators:Laser-Microwave Synchronization at LEAF

Wishart, Cook, Miller Rev. Sci. Inst. 75, 4359 (2004)

Pulse-ProbeTransient Absorption:

Excess electrons in C4mpyrr NTf2

60

50

40

30

20

10

0

Abso

rban

ce x

103

1600140012001000800600400

Wavelength, nm

40

30

20

10

0

Absorbance x10 -3

20 ps 40 ps 80 ps 160 ps 320 ps 1 ns 2 ns 4 ns 8 ns Digitizer 10 ns

Ionic Liquid:C4mpyrr+ NTf2

-

–

+

Slow electron solvation in ionic liquidsobserved by pulse-probe radiolysis at LEAF

The average electron solvation time in C4mpyrr+ NTf2- is 260 ps.

Solvation in ionic liquids operates by translation, not rotation.

50

40

30

20

10

0

Abso

rban

ce x

103

6004002000

Time, ps

600 nm 700 nm 800 nm 900 nm 1000 nm 1300 nm 1400 nm 1500 nm 1600 nm

Solvationresponse

0.4

0.2

0.0

-0.2

Fact

or A

mpl

itude

15001200900600

Wavelength, nm

0.12

0.10

0.08

0.06

0.04

0.02

0.00

Fact

or A

mpl

itude

43210

Tim e, ns

Solvated electron

Electron Gun Accelerator

Sample

Delay

Detectors

Probe Beam

266 nmUV Beam

Electron Beam

Faraday Cup240-1700 nm

LEAF’s Pulse-ProbeTransient Absorption Detection System

Optical Fiber Single-Shot Detection (OFSS)

100 fibers = 100 delay timesFiber/fiber ∆t ~ 15 psTime window = 1.5 ns

Risetime = 1.6 ps (to 0.5 ps) Probes: 800 nm, OPA

Signal

Reference

200

150

100

50

0

Abs

orba

nce

(mO

D)

12008004000Time in ps

Solvated ElectronAverage of 25 shots

Andrew CookRev. Sci Inst., 2009.

EPR studies of irradiated ionic liquid glasses

Done at ANL by Ilya Shkrob and Sergey Chemerisov.

EPR of matrix-isolated species : Ionic liquid glasses and crystalline salts were treated

at 77 K by:Repetitive electron pulses:(3 MeV VdG, average dose ~1.5 kGy)Or repetitive KrF excimer laser pulses at 248 nm

Scanned with 9.44 GHz Bruker ESP300E with cryostatSpectra taken at several temperatures upon warming.

Many IL families have been studied (cations and anions).Recently we have accumulated a large number of spectra of Ils with different

anions and are now analyzing them.. Aromatic anions interest us..New strategy : site specific deuteration of thecations to localize the damage

sites. Underway – we are doing the synthesesJ. Phys. Chem. B, 2007, 111, 11786 & 2009, 113, 5582

Radiolysis of aliphatic cations

J. Phys. Chem. B,

2007, 111, 11786

with Eli Shkrob

The radiolysis of imidazolium cations

J. Phys. Chem. B, 2007, 111, 11786 & 2009, 113, 5582 with Eli Shkrob

Inferred from calculations and EPR spectraEvidence from bulk radiolysis products.

Colored intermediates (charge-resonance bands)Identify by Resonance Raman?

Br– -> Br0 -> Br3–

SCN– -> SCN0 -> (SCN)2–

DCA– -> DCA0 -> (DCA)2–

PF6–

Charge transfer

Decomposition of NTf2- anion

• •CF3 and N• -centered (anion) radicals are observed by EPR

• CF3S•O2 radical is not observed (while CF3S•O3 was observed)

• DFT calculations indicate low barrier for •CF3 + SO2 dissociation (<0.3 V)

• There might be no barrier due to the hydration of the released SO2 by clusters of water in the IL

• A product of •CF3 and TfN-● nitrene radical anion addition are observed by MS

• SO32- is observed by product analysis (forms insoluble salts of Sr2+)

-

NTf2- NTf2

-

TfN-

C2 C2v Cs

N: a=9.5 Gb=(24.1,-12.7,-11.3) G

N: ai=11.1 Gb=(23.9,-12,-11.9) G

S N SO

OO

O

S N SO

OO

OS

N

OOCF3 CF3 CF3 CF3

CF3

-

NTf2- NTf2

-

TfN-

C2 C2v Cs

N: a=9.5 Gb=(24.1,-12.7,-11.3) G

N: ai=11.1 Gb=(23.9,-12,-11.9) G

S N SO

OO

O

S N SO

OO

OS

N

OOCF3 CF3 CF3 CF3

CF3

Eli Shkrob

Are Radiation-Durable IL-Based Extraction Systems Possible?

Ionic liquids are subject to radiation damage.Bulk radiolysis studies: damage “accumulates slowly with dose.”BNFL; CEA (G ~ 2-3/100eV for ion loss), Bartels GH2

= 0.26 - 2.5

Separations bottom line: durable performance of the system.PUREX: 30 wt% Tributylphosphate in hydrocarbon - dealkylation leads

to dibutylphosphoric acid that interferes with extraction.The hydrocarbon diluent directs reactivity to the extractant (DEA).

EPR measurements on radiolysis of (MeO)3PO and (EtO)3PO in ILs:Radicals from phosphates ≤ 4% of radicals from ILs

KEY to durability: Protect extractant from damage and avoid degradation products that interfere with extraction (or electrodeposition).

Direct the radiolysis into benign pathways.J. Phys. Chem. B, 2007, 111, 11786

The Special Case for Borated Ionic Liquids

Criticality calculations from LANL:water-reflected sphere of Pu solution.

Processing fissile materials is very dangerous because of the risk of runaway chain reactions (“criticality accidents”).

“Criticality Safe” systems: It would be safer if criticality prevention was an inherent feature of the processing system instead of relying on administrative controls that are subject to human error.

10B (20% abundance) has a very large thermal neutron cross section.

Use of borated ILs could dramatically lower the risk of criticality accidents.

1 kg Pu in 1 L butyl-methyl-imidazolium+ BF4

- (4 M)

Some Boron-Containing Ionic Liquid Families

BF4- salts

(CnF2n+1)BF3- salts

B(CN)4-

Boronium cations

Chris Reed, U. C. Riverside, with John HolbreyChemically inert. X = H, Cl, BrLoaded with thermal neutron scavengers.Melting points 45 C and up.˚Larsen, Holbrey, Tham, Reed JACS 122 (2000) 7264

Reasonable fluid properties.Commercially available.Xu, Wang, Nieman, AngellJPCB 107 (2003) 11749

(Car)boranes: RC(0-2)B(12-10)H(11-5)X(0-6)-

BOB: bis(oxalato)borate-

Fox, et al. (Davis, Rogers)Chem. Comm. (2005) 3679

D. Gabel, U. Bremen

Ionic Liquid Radiation Chemistry - Summary

Radiolysis products (= damage) accumulate more slowly in some ILs than in molecular solvents (or PUREX mixture), but other ILs are sensitive.

Some borated ions are reactive under radiation, but carborane and borane clusters are relatively inert and promising as additives to reduce criticality risk.

We are applying our knowledge of radiation chemistry to design radiation-resistant ILs that will be the basis for stable separation systems.

Radiation effects in ILs are manageable and not a roadblock to reprocessing use.

Acknowledgments

Alison Funston, Tomasz Szreder, Marie ThomasPedi Neta (NIST), Jan Grodkowski, (NIST, Inst. Nucl. Chem. & Tech., Warsaw)Prof. Ed Castner, Dr. Hideaki Shirota, Tania Fadeeva, Heather Lee (Rutgers Univ.)Profs. Sharon Lall-Ramnarine (QCC, CUNY) and Robert Engel (Queens Coll.,

CUNY)Prof. Mark Kobrak (Brooklyn College, CUNY), Prof. Shawn Abernathy (Howard U.)Ilya Shkrob, Sergey Chemerisov (ANL)Andy Cook, John Miller, J.P. Kirby, Paul Poliakov, Sean McIlroy, Paiboon

Sriarunothai

Summer Students (2003-2007, 2008):Andre Grange, Kimberly Odynocki, Rabindra Ramkirath, Elina Trofimovsky, Neel Khanna, Alex Reben, Heidi Martinez, Vanessa Hernandez, Annu Itty Ipe, Heather Lee, Hughton Walker, Kathryn Sims, Kandis Stubblefield , Katherine Ureña, Steve Bagienski, Alejandra Castaño, Jinhee Gwon, Jasmine Hatcher, Jockquin Jones, Kijana Kerr, Charlene Lawson, Xing Li, Gina Zhou (PST)

U.S. Department of Energy, Basic Energy Sciences, Chemical Sciences Division BNL Laboratory-Directed Research and Development (LDRD) ProgramBNL Office of Educational Programs, BNL Diversity OfficeCUNY Louis Stokes Alliance for Minority Participation

New York Regional Alliancefor Ionic Liquid Studies

Ionizing Radiations:X-raysGamma raysAlpha particles (He2+)Beta particles (e-)Protons (H+)Nucleons (C6+, O8+)Neutrons (n0)

Radiation Chemistry is the study of chemistry resulting from the interaction of ionizing radiation with matter.

Secondary electrons are expelled.Radicals and ions are produced.Bonds are broken.

Features of Pulse Radiolysis:No back reaction (A* + B <=> A+ + B-)Not limited by excited state lifetimes.Oxidation and reduction schemes are simple.Convenient dose measurement, shot-to-shot.No chromophore required - wider range of systems.Broad spectral windows.

Radiation Chemistry

Pulse Radiolysis Kinetics Measurements

Pulse radiolysis is used to measure the speed of chemical reactions.

electron accelerator

e-

light source

filter or monochromatorselects wavelength

detector

sample

A short pulse of electrons initiates chemical reactions.

Detection using time-resolved (transient) absorption:

Profile vs. time shows how the concentration of reactive species changes.Wavelength dependence (spectrum) identifies the species.

The electrons pass completely through the solution, depositing energy into the sample along the way.

N

+

F

F

F

S

O

O

N

F

F

F

S

O

O

2 0

1 5

1 0

5

0

8 0 06 0 04 0 02 0 00

T i m e , n s

N e a t M B 3 N N T f 2 4 0 0 n m 6 5 0 n m 1 0 5 0 n m

Solvated Electron in MB3N+ NTf2-

N

F

F

F

S

O

O

N

F

F

F

S

O

O

+ Š

Dosimetry referenced to (SCN)2

- in water.

Z-Density corr: 1.16Wishart and Neta, JPC B, 107, 7261 (2003). Other spectra: Dorfman and Galvas (1975).

e–solv decay: ≤ 400 ns

Hole(?) decay: 50 ns

ILs: 9 ≤ ε ≤ 15 byDielectric spect’ry

(Wakai, et al.)

IL polarities have also been ranked with solvatochromic dyes (e.g., betaine-30):Alkylammoniums are similar to acetonitrile.Imidazoliums appear more polar (due to H-bonding C-2 proton).

Electron yield: G = 0.7 per 100 eV absorbed.

_

Collaboration with Engel group:Wishart et al., Radiat. Phys. Chem. 72, 99 (2005)

Electron solvation can be slow in ionic liquids

F

F

F

S

O

O

N

F

F

F

S

O

O

N

+

N

+

O

H

O

H

4

Solvent Viscosity, cP τsolvSH8 (NTf2)2 4570 25 ns1,2,6-(OH)3hexane 2490 1 nsMeOH, EtOH, EG <1-20 <10 ps

Viscosity 1120 cP at 20˚C

O

N

+

N

+

2

O

“SH8” “SE6”

Wishart and Neta, JPC B, 107, 7261 (2003)

Reaction of the electron with pyrene in MB3N+ NTf2-

490 nm - Pyrene anion

Pyr+

Pyr-

Pyr-PyrH•

8 0

6 0

4 0

2 0

0

Abs

orba

nce

x103

8 0 06 0 04 0 02 0 00

T i m e , n s

A r g o n - p u r g e d N o p y r e n e 8 . 3 m M p y r e n e 2 1 . 3 m M p y r e n e 3 2 . 8 m M p y r e n e

3 0

2 0

1 0

0

Abs

orba

nce

x103

8 0 06 0 04 0 02 0 00

T i m e , n s

N o p y r e n e 8 . 3 m M 2 1 . 3 m M 3 2 . 8 m M

k(e-solv) = 1.7 x 108 M-1 s-1

e-solv decay at 1030 nm

N

+

Missing e-solv

at “zero” time

IncreasingPyr- anionat “zero” time

Solvated electron capture is slow:

Pyrene

Solid circles: ~1 µsOpen circles: ~70 µs18 mM pyrene.

e- + pyrene → pyrene-

Pre-solvated electron capture is significant.

Electron reactivity vs. solvation

Wishart and Neta, JPC B, 107, 7261 (2003) and recent work

e-

“dry” electron

e-

solvated electron

C37k(e-

solv)

S-

G[scav]/G0 = exp(-[scav]/C37)

Solvated electron capture is slower than in normal liquids (~1010).Dry electron scavenging is remarkably efficient (high Q37).

MeBu3N+ HxBu3N+ C4mpyrr+

Pre-solvated electron reactivityis important in ionic liquids

Pre-solvated electrons are mobile and reactive.In most normal solvents, they only last picoseconds.In some ionic liquids, they last 1000x longer.

+

+-

+

+-

+

+-

+

+-

+-

-+

-

-+

-

-+

-

-e-

In ionic liquids, solvation can be so slow that even low scavenger concentrations compete effectively.

Implications:• Concentrations of solutes that are too low to react with e-

solv may still react with e-

pre. Complication for reprocessing use?

• Energetic e-pre may react to form unique products unavailable from e-

solv.

• Implies new strategies for stabilization.

• Easier to generate intermediates for chemical reactivity studies.

• Solvation studies over a range of ionic liquids are necessary.

OFSS observes pre-solvated electron scavenging

Preferential loss of the solvation process with increasing benzophenone quencher concentration.Implies greater reactivity for the pre-solvated state.

150

100

50

0

Abs

orba

nce

(mO

D)

1400120010008006004002000Time in ps

Benzophenone in P14 NTf2900 nm UFSS

Neat IL

25 mM

50 mM

76 mMBenzophenone in C4mpyrr+ NTf2

-

900 nm OFSS

Radiolysis yields of hydrogen gas

RTILs G(H2) (µmol/J)

hmim Tf2N 0.026

hDMAP Tf2N0.026

bmpyrrTf2N0.065

Et3NH Tf2N0.072

P88814 Tf2N 0.25

Tarábek, Bartels, et al., Radiat. Phys. Chem. 78 (2009) 168.

Organics G(H2) (µmol/J)

Benzene 0.004

Imidazole 0.003

Pyridine 0.003

n-Bu-benzene0.026

Pyrrolidine 0.66

Me3N 0.98

New scientific opportunities for pulse radiolysis

Radiation-induced chemistry at interfaces:• Suspensions of colloids and solids in nuclear reprocessing• Integrity/reactivity of fuel cladding

Charge-transfer processes in nanoscale materials, surfaces:Radiolysis is an excellent method for inducing charge transfer.• Photo/electrochemistry (e.g., Grätzel cells)• Batteries• Catalysis• Polymer composites (including conducting polymers)• Micelles

Radiolysis operates on the bulk material.Interface-specific spectroscopic techniques are needed.• Anisotropy-dependent methods (SHG, SFG, etc.)• Surface-enhanced effects• Others?

Radiolysis of Carborane Salts (neat and in ILs)

Carborane: CB11H6Br6- BOB: bis(oxalato)borate-

When carborane salts are dissolved in NTf2- liquids

sharing the same saturated cation, the solvated electron is observed to react with the carborane anion (k = 3–7 x 108 dm3 mol-1 s-1), possibly producing bromide ion (H-atoms were not detected).

Popularized by C. A. Angell, ASUAttractive fluid properties.Potentially radiation sensitive.Radiolysis studies needed.

Wishart and Szreder, BNL, with C. Reed, U.C. Riverside

6 0

5 0

4 0

3 0

2 0

1 0

01 6 0 01 4 0 01 2 0 01 0 0 08 0 06 0 04 0 0

W a v e l e n g t h , n m

P u l s e R a d i o l y s i s o f A r g o n - S a t u r a t e dM e t h y l t r i o c t y l a m m o n i u m C B 1 1 H 6 B r 6 T r a n s i e n t a b s o r b a n c e t i m e s l i c e s

2 . 4 n s 2 5 n s 2 0 0 n s

τ 1 = 6 n s

τ 2 = 4 . 3 µ s ( 2 n d o r d e r )

O 2 , N 2 O s l o w t h e s e c o n d s t e p

Gε = 20,000

Spectrum ofoxidized carborane

Radiolysis of BOB Ionic Liquids

Radiolysis of neat C4mpyrr BOB shows no e-solv absorption transient.

In water:e-

aq + oxalate2- -> k = 2.0 x 107 M-1 s-1

e-aq + BOB- -> k = 2.8 x 107 M-1 s-1

In C4mpyrr+ NTf2- :e-

solv + BOB- -> k = 3.1 x 108 M-1 s-1

e-solv + pyrene -> k = 5.9 x 108 M-1 s-1

BOB reactivity with electrons is a problem for radiation stability.EPR results also indicate BOB- is radiolytically oxidized.

Radiation Physics and Chemistry, in press.

Laser-Electron Accelerator Facility

40

30

20

10

0

Abso

rban

ce x

103

43210Time, ns

Benzophenone anion solvation in two ionic liquids

185 mM bzph in MB 3 N NTf 2Time constants: 175 ps and 2.8 ns

380 mM bzph in mbpyrr NTf 2Time constants: 73 ps and 1.9 ns

800 nm

680 nm

OFSS: Benzophenone anion solvation in ILs

Benzophenone anion solvation appears biphasic in these ILs:

MBpyrr NTf2 (94 cP): 73 and 1900 ps.

MB3N NTf2 (750 cP): 175 and 2800 ps.

O

—

30

25

20

15

10

5

0

Abso

rban

ce x

103

900800700600500Wavelength, nm

Benzophenone anion40 ns after the electron pulse.

N

+

N

+

75mM bzphin P14 NTf2

Electrochemical Processing in Ionic Liquids

“Electrodeposition of cesium at mercury electrodes in the tri-1-butyl-methyl-ammonium bis((trifluoromethyl)sulfonyl)imide RTIL”

P.-Y. Chen, C. L. Hussey, Electrochimica Acta 49, 5125 (2004)

28 mM Cs+ in Bu3MeN+NTf2– at a HMDEas a function of scan rate.

N

+

F

F

F

S

O

O

N

F

F

F

S

O

O

Inorganic Chemistry, 44, 9497 (2005)

[UIVCl6]2-

[UVCl6]-

[UIIICl6]3-

Cs+

Cs(Hg)

F

F

F

S

O

O

N

F

F

F

S

O

O

3 5

3 0

2 5

2 0

1 5

1 0

5

01 6 0 01 4 0 01 2 0 01 0 0 08 0 06 0 04 0 0

W a v e l e n g t h , n m

“NaCl”

layeredCCCCAAAACCCC

CACAACACCACA

Spectra: Funston and Wishart, ACS Symp. Ser. 901Lattice structures inferred from crystal structures of

Pr4N NTf2 Forsyth et al., Chem Mater. 2002and C4mpyrr NTf2 Choudhury et al., JACS 2005

Solvated Electrons in Ionic Liquids

N

+

First observation of the solvated electron in an ionic liquid.Wishart and Neta, JPC B, 107, 7261 (2003).

Others: Dorfman and Galvas, 1975

N

+

Exist for microseconds in some ionic liquids.Long enough to do chemistry.

Absorption spectrum tends to peak in near-infrared: 1000 - 1500 nm.Indicates that electrons are moderately trapped - less than in water.

Spectra are sensitive to cation structure, and possibly lattice environment.Hydroxyl-bearing cations show typical alcohol spectra.

2.16 M

3.31 M

2.64 M

N

+

LEAF Facility Layout

L a s e r

R o o m

C o n t r o l R o o m

A c c e l e r a t o r V a u l t

E x p e r i m e n t a l S t a t i o n s

K ly s t r o n

R F S y s te m

E le c t r o n G u n

C o n t r o l C o n s o le

T a r g e t A

T a r g e tB

( A r e a fo r f u tu r e ta r g e ts )

D e te c to r sP ro b e

U V

P o w e r S u p p l ie s

Solvation Dynamics:Control of Electron Transport vs. Trapping

Watercross: open-water snowmobile racing

Dynamical Control of Charge Transport vs. Trapping

Watercross: as long as the snowmobiles move fast enough, they don’t sink.

Electrons work the same way - if they move faster than the liquid can respond, they are very mobile.

We can now design ionic liquid systems to have solvation processes on selected timescales.

Control of energetics → product distributionsControl of charge separation and recombination

Blue and green points: M. Maroncelli

(C6)3C14P+

R4N+

pyrrolidinium+

imidazolium+

Tetrafluoroborate ILs: Radiation Chemistry

Imidazolium and pyridinium BF4 salts make low melting, low viscosity ILs.Their radiation chemistry is focused on the formation and reactivity of

im• and py• radicals. (Neta and coworkers)No radiolysis product studies have been done.

Saturated cations form higher-melting salts (C3mpyrr BF4: 64˚C, others higher).Reaction of e-

solv with BF4- in ILs has not been studied, however:

e-solv does not react with BF4

- in water (k > 2 x 105 M-1 s-1)Unfortunately, BF4

- is known to hydrolyze, just like PF6-, producing HF.

Solvated electrons in ILs react with acid, leading to degradation via H atom. (Grodkowski, Neta, Wishart JPCA 107, 9794 (2003)

CW radiolysis studies of PF6- salts show that they darken more than

other ILs.Therefore, BF4

- salts may be susceptable to degradation in the same way.

(Perfluoroalkyl)BF3- salts may be better because they don’t hydrolyze.

Ionic Liquids in Photoelectrochemical Devices

1-propyl-3-methylimidazolium iodide1-ethyl-3-methylimidazolium thiocyanate

Ionic liquid crystal as a hole transport layer of dye-sensitized solar cellsYamanaka et al.Chem Comm, 2005, 740

R. Sastrawan et al.Solar Energy Materials & Solar Cells (2006)

Ionic liquids are finding roles in energy applications where charge transport processes predominate.

Photoelectrochemical cells convert solar energy to electricity using chemical reactions.

Chem. Mater. (2004)

B. Li et al.Solar Energy Materials & Solar Cells (2006)

Ionic Liquids and Nuclear Processing

Ionic liquids could be used to process nuclear fuel, waste, and radiological contamination.

Beneficial features:Solvent properties and substrate ligation can be controlled by designLow volatilityCombustion resistanceHigh conductivityWide electrochemical windows.

Interested organizations:U. S. Dept. of Energy (Separations and Heavy Element Chemistry Programs)British Nuclear Fuels, Ltd. and associated companiesFrench Atomic Energy CommissionChina, Japan, Russia, India

Solvated Electron in MB3N+ NTf2-

N

F

F

F

S

O

O

N

F

F

F

S

O

O

+ Š

Dosimetry referenced to (SCN)2

- in water.

Z-Density corr: 1.16Wishart and Neta, JPC B, 107, 7261 (2003). Other spectra: Dorfman and Galvas (1975).

e–solv decay: ≤ 400 ns

Hole(?) decay: 50 ns

ILs: 9 ≤ ε ≤ 15 byDielectric spect’ry

(Wakai, et al.)

IL polarities have also been ranked with solvatochromic dyes (e.g., betaine-30):Alkylammoniums are similar to acetonitrile.Imidazoliums appear more polar (due to H-bonding C-2 proton).

Electron yield: G = 0.7 per 100 eV absorbed.

_

Laser photolysis measurements: 4 ns relaxation in MB3N NTf2, C4mpyrr NTf2: 220 ps

Slow solvation in ionic liquids

+

+-

+

+-

+

+-

+

+-

+-

-+

-

-+

-

e+

-

-+

+-

+

+-

+

+-

+

+-

+-

-+

-

-+

-

-+

-

-e-

+

+-

+

+-

+-

+

+-

+-

-+

-+

-

-+

-

-+- +

+-

+

+-

+-

+

+-

+-

-+

-+

-

-+

-

-+

-+

+-

+

+-

+-

+

+-

+-

-+

-+

-

-+

-

-+

-

+

+-

+

+-

+

+-

+

+-

+-

-+

-

-+

-

-+

-

-Radiation

Light

Solvation

Relaxation

Pulse radiolysis: probe by absorption

Laser photolysis: probe by fluorescence

Solvation of the electron in MB3N NTf2 is slow (~ 4 ns) but hard to observe.

Ordinary liquids take about one picosecond.