Quenching of positronium in Al 2 O 3 supported catalysts

1. Positron and Positronium Chemistry Quenching of positronium in Al2O3 supported catalysts Z. Q. Chen, H. J. Zhang and S. J. Wang Department of Physics, Wuhan University, Wuhan 430072, P. R. China

2. Outline • Introduction • Sample preparation • Experiments • Results and discussion • Conclusion

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 Catalysts Porous tructure Ps has high formation probability Formation and annihilation of Ps will be affected by the active centers • 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 -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)

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/Al2O3 • NiO/Al2O3 catalyst prepared by mechanical mixing • Grain size is 19 nm for -Al2O3 and 23 nm for NiO • -Al2O3 andNiO phase can be observed in NiO/Al2O3 XRD Measurement

8. Spin conversion of Ps in NiO/Al2O3 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%)

9. Spin conversion of Ps in NiO/Al2O3 Positron lifetime as a function of NiO content in NiO/Al2O3

10. λo-Ps = λo-Ps0 + k [M] →k=(7.9±0.4) ×107 s−1 Spin conversion of Ps in NiO/Al2O3

11. Spin conversion of Ps in NiO/Al2O3 With increasing NiO content • I4 decreases 21.40% → 11.56% • I3 increases 1.38% → 4.18%

12. 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 !!!

13. Spin conversion of Ps in NiO/Al2O3 S-parameter shows increase  Spin conversion of Ps

14. Spin conversion of Ps in NiO/Al2O3 Multi-Gaussian fitting o-Ps converted to p-Ps (spin conversion)

15. Spin conversion of Ps in NiO/Al2O3 ESR measurement Unpaired electron in NiO → spin conversion

16. Spin conversion of Ps in NiO/Al2O3 Deconvoluted CDB spectra

17. Spin conversion of Ps in NiO/Al2O3 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 • Fe2O3/Al2O3 catalyst 4 decreases with increasing Fe2O3 content

19. Chemical quenching of Ps • Fe2O3/Al2O3 catalyst o-Ps = o-Ps0 + k [M] k=(1.25 0.15) 107 s−1

20. Chemical quenching of Ps • Fe2O3/Al2O3 catalyst S parameter and Ip-Ps decreases  Chemical quenching

21. Chemical quenching of Ps • CuO/Al2O3 catalyst 4 decreases with increasing CuO content

22. Chemical quenching of Ps • CuO/Al2O3 catalyst o-Ps = o-Ps0 + k [M] k=(1.83 0.05) 107 s−1

23. Chemical quenching of Ps • CuO/Al2O3 catalyst S parameter and Ip-Ps decreases  Chemical quenching

24. Chemical quenching of Ps • Cr2O3/Al2O3 catalyst 4 decreases with increasing Cr2O3 content

25. Chemical quenching of Ps • Cr2O3/Al2O3 catalyst o-Ps = o-Ps0 + k [M] k=(2.56 0.19) 107 s−1

26. Chemical quenching of Ps • Cr2O3/Al2O3 catalyst S parameter and Ip-Ps decreases  Chemical quenching

27. Inhibition of Ps formation Fe2O3/Al2O3 Cr2O3/Al2O3 CuO/Al2O3 Inhibition of Ps formation in all these catalysts

28. Pore structure of catalysts • Dispersion state of active components on the surface of the supports Monolayer dispersion of NiO on Al2O3 What can we do by using Ps as a probe in catalysts?

29. Monolayer dispersion of NiO on Al2O3 • NiO/Al2O3 catalyst prepared by impregnation Monolayer dispersion capacity of NiO is about 9 wt%

30. Monolayer dispersion of NiO on Al2O3 variation of 4, I3 and I4 shows two stages

31. Monolayer dispersion of NiO on Al2O3 o-Ps = o-Ps0 + k [M] k1=(20.1±0.5)  107 s−1 (monolayer dispersion) k2=(4.59±0.26)  107 s−1

32. Monolayer dispersion of NiO on Al2O3 • NiO conten < 9 wt%: spin conversion of Ps • NiO conten > 9 wt%: inhibition of Ps formation

33. 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.

34. Thank you！

35. Monolayer dispersion of NiO on Al2O3 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.

36. Summary