Quenching of positronium in Al 2 O 3 supported catalysts Department of Physics, Wuhan University, Wuhan , 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 annihilation of Ps will be affected by the active centers Porous tructure Ps has high formation probability 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  -Al 2 O 3 nanopowder + MO x nanopowder (M=Ni,Fe,Cu,Cr ） grinding 2h  pressed into pellets (6MPa ， 5min)  Impregnation method  -Al 2 O 3 nanopowder + Ni(NO 3 ) 2 ·6H 2 O aqueous solution dispersed in a ultrasonic bath (80 o C, 3h) dried in air (120 o C, 10h) + calcination (450 o C, 10h) pressed into pellets (6MPa ， 5min)

Experiment Positron lifetime and CDB measurement simultaneously Sample chamber was evacuated to a vacuum better than 1  Torr

Spin conversion of Ps in NiO/Al 2 O 3 XRD Measurement  Grain size is 19 nm for  -Al 2 O 3 and 23 nm for NiO   -Al 2 O 3 and NiO phase can be observed in NiO/Al 2 O 3 NiO/Al 2 O 3 catalyst prepared by mechanical mixing

Spin conversion of Ps in NiO/Al 2 O 3 Positron lifetime spectra  -Al 2 O 3 : ns (21.40%) 2.35 ns (1.38%) ps (40.43%) ps (36.79%) NiO: ns (3.76%) 3.67 ns (4.02%) ps (19.98%) ps (72.23%)

Positron lifetime as a function of NiO content in NiO/Al 2 O 3 Spin conversion of Ps in NiO/Al 2 O 3

λ o-Ps = λ o-Ps 0 + k [M] → k=(7.9±0.4) ×10 7 s −1

Spin conversion of Ps in NiO/Al 2 O 3 With increasing NiO content I 4 decreases 21.40% → 11.56% I 3 increases 1.38% → 4.18%

Spin conversion of Ps in NiO/Al 2 O 3 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/Al 2 O 3 S-parameter shows increase  Spin conversion of Ps

Spin conversion of Ps in NiO/Al 2 O 3 Multi-Gaussian fitting o-Ps converted to p-Ps (spin conversion)

Sample unpaired spins (g -1 ) I o-Ps I p-Ps  -Al 2 O %10.2% 12wt% NiO/Al 2 O 3 2.0× %12.6% 24wt% NiO/Al 2 O 3 6.7× %15.8% Unpaired electron in NiO → spin conversion ESR measurement Spin conversion of Ps in NiO/Al 2 O 3

Deconvoluted CDB spectra Spin conversion of Ps in NiO/Al 2 O 3

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 Fe 2 O 3 /Al 2 O 3 catalyst  4 decreases with increasing Fe 2 O 3 content

Chemical quenching of Ps Fe 2 O 3 /Al 2 O 3 catalyst k=(1.25  0.15)  10 7 s −1 o-Ps = o-Ps 0 + k [M]

Chemical quenching of Ps Fe 2 O 3 /Al 2 O 3 catalyst S parameter and I p-Ps decreases  Chemical quenching

Chemical quenching of Ps CuO/Al 2 O 3 catalyst  4 decreases with increasing CuO content

Chemical quenching of Ps CuO/Al 2 O 3 catalyst k=(1.83  0.05)  10 7 s −1 o-Ps = o-Ps 0 + k [M]

Chemical quenching of Ps CuO/Al 2 O 3 catalyst S parameter and I p-Ps decreases  Chemical quenching

Chemical quenching of Ps Cr 2 O 3 /Al 2 O 3 catalyst  4 decreases with increasing Cr 2 O 3 content

Chemical quenching of Ps Cr 2 O 3 /Al 2 O 3 catalyst k=(2.56  0.19)  10 7 s −1 o-Ps = o-Ps 0 + k [M]

Chemical quenching of Ps Cr 2 O 3 /Al 2 O 3 catalyst S parameter and I p-Ps decreases  Chemical quenching

Inhibition of Ps formation Fe 2 O 3 /Al 2 O 3 CuO/Al 2 O 3 Cr 2 O 3 /Al 2 O 3 Inhibition of Ps formation in all these catalysts

Monolayer dispersion of NiO on Al 2 O 3 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 Al 2 O 3 NiO/Al 2 O 3 catalyst prepared by impregnation Monolayer dispersion capacity of NiO is about 9 wt%

variation of  4, I 3 and I 4 shows two stages Monolayer dispersion of NiO on Al 2 O 3

o-Ps = o-Ps 0 + k [M] k 1 =(20.1±0.5)  10 7 s −1 (monolayer dispersion) k 2 =(4.59±0.26)  10 7 s −1 Monolayer dispersion of NiO on Al 2 O 3

NiO conten < 9 wt%: spin conversion of Ps NiO conten > 9 wt%: inhibition of Ps formation Monolayer dispersion of NiO on Al 2 O 3

Conclusion Spin conversion of Ps was observed in NiO/Al 2 O 3 catalysts by positron lifetime and CDB measurements Dispersion of NiO on  -Al 2 O 3 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 Fe 2 O 3 /Al 2 O 3, CuO/Al 2 O 3 and Cr 2 O 3 /Al 2 O 3 catalysts. The active components NiO, Fe 2 O 3, CuO, Cr 2 O 3 inhibit the formation of Ps.

Thank you ！

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 Al 2 O 3

Summary sampleMethod SI p-Ps mechanismk (10 7 s −1 ) NiO/Al 2 O 3 mixing  spin conversion 7.9  0.4 NiO/Al 2 O 3 Impregnation  spin conversion 20.1  0.9 (monolayer) 4.59  0.26 (beyond monolayer) Fe 2 O 3 /Al 2 O 3 mixing  chemical quenching 1.25  0.15 CuO/Al 2 O  0.05 Cr 2 O 3 /Al 2 O  0.19