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