Quenching of positronium in Al2O3

supported catalysts

Department of Physics, Wuhan University, Wuhan 430072, P. R. China

Z. Q. Chen, H. J. Zhang and S. J. Wang

Positron and Positronium Chemistry

Introduction

Sample preparation

Experiments

Results and discussion

Conclusion

Outline

Introduction

• Positronium formation and annihilation is a fundamental and important problem

• Annihilation lifetime of Ps can be affected by - Spin-conversion - Chemical quenching - Magnetic quenching etc.

• Formation probability of Ps can be also affected by - Chemical centers (electron scavenger) - Electric fields etc.

Introduction

Formation and a

nnihilation of Ps

will be affected b

y the active cent

ers

Porous tructure

Ps has high for

mation probabilit

y

Catalysts

Our purpose:• Formation and annihilation mechanism of Ps in catalysts• Provide a new probe for catalysts (pore structure, active centers…)

Sample preparation

Mechanical mixing method

-Al2O3 nanopowder + MOx nanopowder (M=Ni,Fe,Cu,Cr） grinding 2h pressed into pellets (6MPa， 5min)

Impregnation method

-Al2O3 nanopowder + Ni(NO3)2·6H2O aqueous solution

dispersed in a ultrasonic bath (80oC, 3h)

dried in air (120oC, 10h) + calcination (450oC, 10h)

pressed into pellets (6MPa， 5min)

Experiment

• Positron lifetime and CDB measurement simultaneously

• Sample chamber was evacuated to a vacuum better than 110-

5 Torr

Spin conversion of Ps in NiO/Al2O3

XRD Measurement

Grain size is 19 nm for -Al2O3 and 23 nm for NiO

-Al2O3 and NiO phase can be observed in NiO/Al2O3

• NiO/Al2O3 catalyst prepared by mechanical mixing

Spin conversion of Ps in NiO/Al2O3

0 50 100 150100

101

102

103

104

105

coun

ts

time (ns)

aAl2O

3

(b): 24wt% NiO/Al2O

3

(c): NiO

(a)

(b)

(c)

Positron lifetime spectra -Al2O3: 93.65 ns (21.40%) 2.35 ns (1.38%) 410.2 ps (40.43%) 152.8 ps (36.79%)

• NiO: 43.81 ns (3.76%) 3.67 ns (4.02%) 573.1 ps (19.98%) 242.8 ps (72.23%)

0 6 12 18 24 30 36 42

0

20

40

60

80

100

lifet

ime

(ns)

NiO (wt%)

Positron lifetime as a function of NiO content in NiO/Al2O3

Spin conversion of Ps in NiO/Al2O3

Spin conversion of Ps in NiO/Al2O3

0 6 12 18 24 30 36 42

0.01

0.02

0.03

0.04

4-1 (n

s-1

)

NiO content (wt%)

λo-Ps = λo-Ps0 + k [M] → k=(7.9±0.4) ×107 s−1

Spin conversion of Ps in NiO/Al2O3

0 6 12 18 24 30 36 420

5

10

15

20

25

inte

nsity

(%)

NiO (wt%)

I4

I3

With increasing NiO content

• I4 decreases

21.40% → 11.56%

• I3 increases

1.38% → 4.18%

Spin conversion of Ps in NiO/Al2O3

Possible reason for the decrease of 4 ?

• Decrease of pore size

• Chemical reaction of Ps

• Spin conversion of Ps

Lifetime measurement alone cannot solve the problem !!!

Spin conversion of Ps in NiO/Al2O3

0 6 12 18 24 30 36 420.47

0.48

0.49

0.50

0.51

0.52

S p

aram

eter

NiO(wt%)

0 6 12 18 24 30 36 420

5

10

15

20

25

inte

nsit

y (%

)

NiO (wt%)

I4

I3

0 6 12 18 24 30 36 42

0

20

40

60

80

100

life

tim

e (n

s)

NiO (wt%)

S-parameter shows increase Spin conversion of Ps

Spin conversion of Ps in NiO/Al2O3

0 6 12 18 24 30 36 420

10

20

30

inte

nsity

(%

)

NiO (wt%)

o-Ps

p-Ps

Multi-Gaussian fitting

o-Ps converted to p-Ps (spin conversion)

100 200 300 400 500 600 700 800-4

-3

-2

-1

0

1

2

3

4

(c)

(b)

rela

tive

inte

nsity

field(mT)

(a) (a): 24wt%NiO/Al2O

3

(b): 12wt%NiO/Al2O

3

(c): -Al2O

3 Sampleunpaired spins

(g-1)Io-Ps Ip-Ps

-Al2O3 0 22.8% 10.2%

12wt%

NiO/Al2O3

2.0×1019 18.3% 12.6%

24wt%

NiO/Al2O3

6.7×1019 18.1% 15.8%

Unpaired electron in NiO → spin conversion

ESR measurement

Spin conversion of Ps in NiO/Al2O3

-6 -4 -2 0 2 4 60

50000

100000

150000

200000

250000

-6 -4 -2 0 2 4 6

-6 -4 -2 0 2 4 60

50000

100000

150000

200000

250000

-6 -4 -2 0 2 4 6

-Al2O

3

Counts

12wt% NiO/Al2O

3

6wt% NiO/Al2O

3

Counts

E (keV)

24wt% NiO/Al2O

3

E (keV)

p-Ps

Deconvoluted CDB spectra

Spin conversion of Ps in NiO/Al2O3

Spin conversion of Ps in NiO/Al2O3

0 6 12 18 24 30 36 420,90

0,95

1,00

1,05

1,10

1,15

1,20

1,25

FWH

M o

f p-P

s (k

eV)

NiO content (wt%)

width of p-Ps peak decreases with increasing NiO content

The p-Ps converted from o-Ps may survive for long time to have a more complete thermalization.

Chemical quenching of Ps

• Fe2O3/Al2O3 catalyst

0 6 12 18 24 30 36 420

200

400

600

lifet

ime

(ps)

-Fe2O

3 (wt%)

0 6 12 18 24 30 36 420

10

60

80

100

lifet

ime

(ns)

Fe

2O

3 content (wt%)

3

4 decreases with increasing Fe2O3 content

Chemical quenching of Ps

0 6 12 18 24 30 36 42

0

10

20

inte

nsity

(%

)

Fe2O

3 (wt%)

I4

I3

0 6 12 18 24 30 36 420,008

0,010

0,012

0,014

0,016

0,018

4 (ns

-1)

a-Fe2O

3 content (wt%)

• Fe2O3/Al2O3 catalyst

k=(1.25 0.15) 107 s−1

o-Ps = o-Ps0 + k [M]

Chemical quenching of Ps

0 6 12 18 24 30 36 420,45

0,46

0,47

0,48

S p

ara

mete

r

Fe2O

3 (wt%)

0 6 12 18 24 30 36 420

10

20

30

inte

nsi

ty (

%)

Fe2O

3(wt%)

p-Ps

o-Ps

• Fe2O3/Al2O3 catalyst

S parameter and Ip-Ps decreases Chemical quenching

Chemical quenching of Ps

• CuO/Al2O3 catalyst

0 6 12 18 24 30 36 420

100

200

300

400

500

600

700

lifet

imes

(ps

)

CuO content (wt%)

0 6 12 18 24 30 36 420

10

50

60

70

80

90

100

lifetim

es

(ns)

CuO content (wt%)

4 decreases with increasing CuO content

Chemical quenching of Ps

0 6 12 18 24 30 36 420,010

0,012

0,014

0,016

0,018

0,020

(

ns-1)

CuO content (wt%)

• CuO/Al2O3 catalyst

k=(1.83 0.05) 107 s−1

o-Ps = o-Ps0 + k [M]

Chemical quenching of Ps

• CuO/Al2O3 catalyst

0 6 12 18 24 30 36 420,480

0,485

0,490

0,495

0,500

S p

aram

eter

CuO content (wt%)0 6 12 18 24 30 36 42

4

6

8

10

12

p-P

s in

ten

sitie

s (%

) CuO content (wt%)

S parameter and Ip-Ps decreases Chemical quenching

Chemical quenching of Ps

• Cr2O3/Al2O3 catalyst

0 6 12 18 24 30 36 420

20

40

60

80

100

3

lifet

ime

(ns)

Cr

2O

3 content (wt%)

4

0 6 12 18 24 30 36 420

100

200

300

400

500

600

lifet

ime

(ps)

Cr2O

3 content (wt%)

4 decreases with increasing Cr2O3 content

Chemical quenching of Ps

• Cr2O3/Al2O3 catalyst

0 6 12 18 24 30 36 42

0

10

20

inte

nsi

ty (

%)

Fe2O

3 (wt%)

I4

I3

0 6 12 18 24 30 36 420,008

0,010

0,012

0,014

0,016

0,018

4 (ns

-1)

a-Fe2O

3 content (wt%)

k=(2.56 0.19) 107 s−1

o-Ps = o-Ps0 + k [M]

Chemical quenching of Ps

• Cr2O3/Al2O3 catalyst

0 6 12 18 24 30 36 420.47

0.48

0.49

0.50

0.51

0.52

S p

aram

eter

Cr2O

3 content (wt%)

0 6 12 18 24 30 36 420

5

10

15

20

25

30

p-Ps

inte

nsity

(%

) Cr

2O

3 content (wt%)

o-Ps

S parameter and Ip-Ps decreases Chemical quenching

Inhibition of Ps formation

0 6 12 18 24 30 36 42

0

10

20

inte

nsity

(%

)

Fe2O

3 (wt%)

I4

I3

0 6 12 18 24 30 36 42

0

10

20

inte

nsi

ty (

%)

Fe2O

3 (wt%)

I4

I3

Fe2O3/Al2O3

CuO/Al2O3

Cr2O3/Al2O3

Inhibition of Ps formation in all these catalysts

Monolayer dispersion of NiO on Al2O3

What can we do by using Ps as a probe in catalysts?

• Pore structure of catalysts

• Dispersion state of active components on the surface of the supports

Monolayer dispersion of NiO on Al2O3

• NiO/Al2O3 catalyst prepared by impregnation

35 40 45 50 55 60 65 70

24wt%

inte

nsity

(ar

b. u

nits

)

2 (degree)

(200)

(440)l

2

6wt%

12wt%

0 6 12 18 24 30 36 420

1

2

3

4

5

6

I NiO

/I

Al2O

3

NiO content (wt%)

Monolayer dispersion capacity of NiO is about 9 wt%

0 6 12 18 24 30 36 420

5

10

15

20

25

I3

inte

nsity

(%

) NiO content (wt%)

I4

variation of 4, I3 and I4 shows two stages

Monolayer dispersion of NiO on Al2O3

0 1 2 3 4 5 60.010

0.015

0.020

0.025

0 6 12 18 24 30 36 420.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

4ns

-1

NiO content (wt%)

(n

s-1

)

NiO content (wt%)

o-Ps = o-Ps0 + k [M]

k1=(20.1±0.5) 107 s−1

(monolayer dispersion)

k2=(4.59±0.26) 107 s−1

Monolayer dispersion of NiO on Al2O3

0 6 12 18 24 30 36 420.50

0.52

0.54

0.56

0.58

S p

aram

eter

NiO content (wt%)

0 6 12 18 24 30 36 4210

15

20

25

Ip-Ps

inte

nsity

(%

) NiO content (wt%)

Io-Ps

• NiO conten < 9 wt%: spin conversion of Ps

• NiO conten > 9 wt%: inhibition of Ps formation

Monolayer dispersion of NiO on Al2O3

Conclusion

Spin conversion of Ps was observed in NiO/Al2O3 catalysts by positron lifetime and CDB measurements

Dispersion of NiO on -Al2O3 was characterized by Ps atom. The monolayer dispersion capacity of 9 wt% was obtained.

Chemical reaction of Ps with the active components was observed in Fe2O3/Al2O3, CuO/Al2O3 and Cr2O3/Al2O3 catalysts.

The active components NiO, Fe2O3, CuO, Cr2O3 inhibit the formation of Ps.

Thank you！

0 1 2 3 4 5 60.90

0.95

1.00

1.05

1.10

1.15

FW

HM

of p

-Ps

(keV

)

NiO content (wt%)

width of p-Ps peak decrease with increasing NiO content

The p-Ps converted from o-Ps may survive for long time to have a more complete thermalization.

Monolayer dispersion of NiO on Al2O3

Summary

sample Method S Ip-Ps mechanism k (107 s−1)

NiO/Al2O3 mixing spin

conversion7.90.4

NiO/Al2O3 Impregnation spin conversion

20.10.9

(monolayer)4.590.26

(beyond monolayer)

Fe2O3/Al2O3

mixing chemical quenching

1.25 0.15

CuO/Al2O3 1.83 0.05

Cr2O3/Al2O3 2.56 0.19