1. Quenching of positronium in Al 2 O 3 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

2. Introduction Sample preparation Experiments Results and discussion Conclusion Outline

3. 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.

4. 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…)

5. 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)

6. Experiment Positron lifetime and CDB measurement simultaneously Sample chamber was evacuated to a vacuum better than 1  10 -5 Torr

7. 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

8. Spin conversion of Ps in NiO/Al 2 O 3 Positron lifetime spectra  -Al 2 O 3 : 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%)

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

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

11. 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%

12. 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 !!!

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

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

15. Sample unpaired spins (g -1 ) I o-Ps I p-Ps  -Al 2 O 3 022.8%10.2% 12wt% NiO/Al 2 O 3 2.0×10 19 18.3%12.6% 24wt% NiO/Al 2 O 3 6.7×10 19 18.1%15.8% Unpaired electron in NiO → spin conversion ESR measurement Spin conversion of Ps in NiO/Al 2 O 3

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

17. 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.

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

19. 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]

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

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

22. 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]

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

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

25. 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]

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

27. 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

28. 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

29. 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%

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

31. 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

32. 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

33. 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.

34. Thank you ！

35. 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

36. 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 3 1.83  0.05 Cr 2 O 3 /Al 2 O 3 2.56  0.19