Muons as a Tool for Probing Supercritical Water Chemistry Paul W. Percival Simon Fraser University...

Muons as a Tool for Probing Supercritical Water Chemistry

Paul W. Percival

Simon Fraser

University

and TRIUMF

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Paul Percival, May 2009

Supercritical Water Oxidation

organic compounds are miscible in SCW

There are drastic changes in the physical properties of water close to and above the critical point (Tc = 374°C, Pc = 221

bar).

This leads to unusual chemistry:

W. Schilling and E.U. Franck, Ber. Bunsenges. Phys. Chem. 92 (1988) 631.

2000 bar, 450ºC

30% methane in water

A Flame in Water!

combustion of organic materials is possible

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Supercritical Water Oxidation

Many of these reactions involve free radical intermediates, but there is very little direct knowledge about these transient species under such extreme conditions.

SCWO facilities are being developed for the destruction of hazardous waste such as PCBs, sewage sludge, chemical weapons. It has even been proposed for water recycling on long space flights.

There is ~500 tons of blister and nerve agents stored at the Blue Grass Army Depot in Kentucky. The plan is to neutralise these materials and then destroy the hydrolysate by means of SCWO.

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Studying Reactions in High Temperature Water is

Difficult

Muons can be injected into most materials. The momentum can be adjusted for optimum penetration of the sample. Optical windows are unnecessary.

… but muon spin spectroscopy has a number of advantageous features for the study of “hostile”

environments:

the short lifetime of the muon, µSR is inherently suited to the study of transients.

The detectors are simple devices (plastic scintillators) outside the controlled sample environment.

Spin polarized muons give high sensitivity and do not need radio-frequency or other radiation.

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Muonium — a light isotope of hydrogen

Mu e

reduced mass of Mu = 0.995 mr(H)

ionization potential = 13.539 eV

Bohr radius = 0.532 Å

e

The properties of a single electron atom are determined by the reduced mass

r N e e

1 1 1 1m m m m

= + »

1p9m m =

62.2 10 s-= ´

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Centres for Muon Spin Spectroscopy

NOW

J-PARC

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The TRIUMF Joint Venture

Member Universities

University of Alberta

University of British Columbia

Carleton University

University of Manitoba

Simon Fraser University

Université de Montréal

University of Toronto

University of Victoria

Associate Members

University of Guelph

McMaster University

Queen's University

University of Regina

Saint Mary's University

York University

TRIUMF is owned and operated by a consortium of Canadian universities:

TRIUMF's operating costs are funded by a contribution from the federal government through the National Research Council of Canada (NRC).

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TRIUMF

Meson Hall

Cyclotron

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The TRIUMF Meson Hall

Cyclotron

muon beam line

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Looking down on the TRIUMF M9 Beam Area

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Apparatus for Hydrothermal Studies in HELIOS

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High pressure and temperature cell

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The Phase Diagram of Water

Pressure / bar

Temperature /°C

221

1.0

0 100 374

Triple point

Critical point

ICE

VAPOURSTEAM

LIQUID

FLUID

The Phase Diagram of Water

50—450°C250—350 bar

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µSR: Muon Spin Spectroscopy

• spin polarized muon beam

• muons are implanted in the sample

• muons decay: + e+ + + e = 2.2 s

• angular distribution of e+ has maximum in muon spin direction

• precession of muon spins in transverse magnetic field

• equivalent to free induction decay in pulsed NMR or ESR

Muon Spin Rotation

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The Muon Lifetime Experiment

/0 bace kgroundtN N -= +

ee 2.2 s + +® + + =

µ+

beamS

start stopCLOCK

M F

B

P

e+

time

N

lifetime histogram

0

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µ+

beamS

start stopCLOCK

M F

B

P

e+

time

N

µSR histogram

Muon Spin Rotation, µSR

Apply a magnetic field perpendicular to the initial spin direction.

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The µSR Histogram

0

200

400

600

800

0 1 2 3 4 5 6

time / s

cou

nt

( ) ( )[ ]/0 e 1 costN aP t t B - + + +

Subtract the constant background and divide out the exponential decay …

… to get the muon asymmetry, which represents the time dependence of the muon spin polarization.

( ) ( )cosaP t t +-0.2

-0.1

0

0.1

0.2

0 1 2 3 4 5 6

time / s

mu

on

asy

mm

etr

y

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Muonium in Supercritical Water

Percival, Brodovitch, Ghandi et al., Phys. Chem. Chem. Phys. 1 (1999) 4999

400°C, 245 bar

0.0 0.5 1.0 1.5 2.0 2.5 3.0

time / s

-0.2

-0.1

0.0

0.1

0.2

Asy

mm

etr

y D signal

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Muonium in supercritical water at 196 G

400°C

245 bar

0.00 0.05 0.10 0.15 0.20

time / µs

Mu

on

Asy

mm

etr

y

200 220 240 260 280 300 320 340

Frequency / MHz

Fou

rier

Pow

er

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Muonium Kinetics

Mu A products+ ¾¾®

M

[Mu][A][Mu] [Mu]

dk

dt- = =

Muonium reactions are pseudo-first order because…

only a few million Mu atoms are needed for an experiment.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Muo

n A

sym

met

ry The Mu signal decays

exponentially

time / µs

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Muonium Kinetics

M

[Mu][A][Mu] [Mu]

dk

dt- = =

0 M[A]k = +

0 is the first-order rate constant for pure solvent

In addition to chemical decay, the muon decays radioactively, with a lifetime which is independent of chemical environment.

0M [A]k

-=second-order rate constant

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.00 0.05 0.10 0.15 0.20

[MeOH]

Mu

de

cay

rate

/

s-1

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Mu + Benzene

A fall-off of rate is common for reactions in high T

water

0 100 200 300 400Temperature / °C

0.0

0.2

0.4

0.6

0.8

1.0

1.2

k Mu

/ 10

10 M

-1s-1

250 bar> 310 bar

0 100 200 300 400Temperature / oC

107

108

109

1010

1011

k Mu

/ M-1

s-1

350 bar

245 bar

< 200 bar

1 bar (lit.)

aqMu OH MuOH e

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Diffusion and collision-controlled kinetics

Mu A MuA products

Reaction rates in liquids depend on diffusion of the reactants

to form the encounter pair, as well as the activated chemical

reaction.

Mu Aff Ai Mud 4k R R D D difo fbs act

1 1 1

k k k

Ract exp /ak f A E RT collR -1

enc coll

PZf

PZ where

The key factor in the fall-off with temperature seems to be the drop in the number of collisions between a pair of reactants over the duration of their encounter.

many collisions per encounter at low temperature

collision encounter for gas-like behaviour at high T

Ghandi, Percival et al Phys. Chem. Chem. Phys. (2002)

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H + OH H2O

0 100 200 300 400 500Temperature /°C

0.0

2.0

4.0

6.0

8.0

k / 1

010 M

-1s-1

M3

M1

AECL

M2

current PWR reactorsnext generation

reactors

Data limited to 200°CBuxton and Elliot, JCS Far. Trans. 89 (1993) 485

Ghandi and Percival, J. Phys. Chem. A 107 (2003) 3006

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0.1

1.0

10.0

100.0

0 100 200 300 400 500

Temperature / °C

k /

10

6 M-1

s-1

H abstraction from methanol by Mu (H)

H + CH3OH

Mezyk and Bartels, 1994

Percival et al., 2007

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Supercritical-Water-Cooled Reactor

Canada is one of ten countries (the Generation IV International Forum) working together to lay the groundwork for fourth generation nuclear reactor systems.

The priority R & D areas for Canada include “improved understanding of radiolysis under supercritical water conditions and the effect of radiolysis products on corrosion and stress corrosion cracking”.

The Supercritical-Water-Cooled Reactor (SCWR) system is a high-temperature, high-pressure water cooled reactor that operates above the thermodynamic critical point of water (374°C, 22 Mpa)

The SCWR system is primarily designed for efficient electricity production.

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Muoniated Free Radicals

e.g. cyclohexadienyl

µSR:precession frequencies muon hyperfine coupling

µLCR:resonance fields other nuclear hyperfine couplings

hyperfine couplings distribution of unpaired electron spin

Temperature dependence of hyperfine couplings intramolecular motion

R4R3

R2 C

Mu

R1

C

H Mu

0.33

-0.10

0.48

0.33

-0.10•

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Muon Avoided Level Crossing Resonance

6.8 7.0 7.2 7.4 7.6 7.8 8.0 8.2

Magnetic Field / kG

A+–

A–

Magnetic Field

Ene

rgy

e X, ,

Mixing of spin levels related by flip-flop transitions of the muon and another nucleus results in loss of muon spin polarization at a characteristic field (Bres) determined by the difference in muon and nuclear hyperfine constants.

The differential line shape is from field modulation.

e X, ,

p p

LCRep

1

2

A A A AB

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17.2 17.4 17.6 17.8 18.0

Field / kG

A+-A

-

(b)

10.4 10.6 10.8 11.0 11.2

Field / kG

A+-A

-(a)

Isopropyl from dehydration of 1-propanol

HH

HC

Mu

CC

H

HH

68.2

65.4

-57.7

Ap /MHz

LCR distinguishes the signs of the hfcs

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0.8 1.0 1.2 1.4 1.6 1.8 2.0

Field / kG

A+-A

-

(a)

9.4 9.6 9.8 10.0 10.2 10.4 10.6 10.8

Field / kG

A+-A

-

(b)

The Mu adducts of acetone are isomeric radicals

CH2Mu

O

H

CH3

O

Mu

CH3CH3

92°C136 bar

350°C250 bar

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How to Study a keto-enol Equilibrium by Radical

Trapping

At 25°C the keto-enol equilibrium constant of acetone in

water is 5 10-9

O

OH

O

Mu

CH2Mu

O

H

Mu

Mu

High temperature:

Low temperature:

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Thanks…

…to my collaborators in the

Simon Fraser University Muonium Chemistry Group

in particular, Dr. Jean-Claude Brodovitch

and my recent graduate students:

Brenda Addison-Jones (Ph.D. 2000)

Khashayar Ghandi (Ph.D. 2002)

Iain McKenzie (Ph.D. 2004)

Brett McCollum (Ph.D. 2008)

TRIUMF Centre for Molecular and Materials Science

Drs. Syd Kreitzman, Donald Arseneau, Bassam Hitti and staff

For funding: NSERC of Canada

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Pions Decay to Give Muons

At TRIUMF, pions are produced by bombardment of a Be or C production target with 500 MeV protons. We take the positive pions, which decay to positive muons:

+ +® + In the pion rest frame:

Momentum is

conserved:

Spin is conserved:

¬¾¾ ¾¾®e

p p

=-

=-

uur uur

uur uur

Spin and momentum are related through

helicity:

ˆ ˆ 1.

ph h

p

×

= =±r rr r

For the neutrino, h = -1, i.e. the spin and momentum are anti-parallel.

Therefore, muons are produced with and p anti-parallel. p p

¬¾¾¾® ¬¾¾

¾ ¾¾®e

= 26 ns

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Muon Beams are Spin Polarized

There are two commonly used types of muon beam:

Surface Muon Beams collect and transport muons which are created from the decay of pions at or near the surface of the pion production target.

select these muons to get these spins:

beam line

Decay Muon Beams collect muons from pions which decay in flight.

In the rest frame only backward { } and forward muons are transported.

In the laboratory frame, both momentum bites travel in the forward direction.

The forward (momentum) muons (p~180 MeV/c) have backward spin; the backward muons (p~80 MeV/c) have forward spin.

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Muon Decay

ee 2.2 s + +® + + =

The 3 particle decay results in a spectrum of positron energies.

The spatial distribution of positrons is asymmetric and depends on the muon spin polarization.

Consider the decay pattern which results in the maximum positron energy:

The e+ is emitted in the direction of the muon spin at the moment of decay. More generally,

Energy

N

1max 2

52.8 MeV

E m=

=

e+µ

e

e

[ ]( , )1 ( )cos

dN EC D E

dE d

= +W

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Alcohols are dehydrated in superheated water

e.g. dehydration of tert-butanol forms isobutene

– H2OCH3

CH3

CH3

OH

CH3

CH3

CH2

Mu CH3

CH3

CH2Mu

which can be spin-labelled by adding Mu

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tert-butyl is also formed by dehydration of butanols

9.6 9.8 10.0 10.2 10.4 10.6

Field / kGA

+-A

-

0 200 400

Frequency / MHz

Pow

er

O H

H3C C H2M u

Mu•C H3

O H - H2O

- H2OO H

H3C C H2M u

Mu•C H3

O H - H2O

- H2O

sec-butanol

isobutene

tert-butanol

350°C, 250 bar

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Muon ALCR spectrum of tert-butyl radical in water

200°C

250 bar

10.0 10.5 11.0 11.5

Field / kG

A+-A

-CH3

CH3

CH2Mu

μ pres

μ p

1

2 γ γ

A AB

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1-propanol and 2-propanol both give isopropyl

0 200 400

Frequency / MHz

Pow

er

10.4 10.6 10.8 11.0 11.2

Field / kGA

+-A

-

O H

H3C C H2M u

Mu•H

O H- H2O

- H2OO H

H3C C H2M u

Mu•H

O H- H2O

- H2O

1-propanol

2-propanol

propene

350°C, 250 bar

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High pressure system

Muons as a Tool for Probing Supercritical Water Chemistry Paul W. Percival Simon Fraser University...

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