| Since the first observation of positronium(Ps) by Deutsch in 1951, researchers have proposed several models on Ps formation and annihilation, and have carried out lots of works using Ps. However, the formation and annihilation of Ps is a complicated physical process influenced by many factors, up to now the Ps formation and annihilation mechanism is still not thoroughly understood. It is a basic problem of positron physics which needs more efforts to be gradually solved.Porous catalysts are multiphase system where active components are dispersed on the surfaces of porous supports. Porous catalysts are full of nanometer-sized pores, therefore Ps can be formed with a high propability and live for a long lifetime in the pores. The active components dispersed on the surfaces of porous supports will not only influence the Ps formation, but also influence the annihilation of Ps by interaction with the Ps atoms diffused into the pores. The annihilation characteristics of Ps in the pores are closely related with the pore structure, gas filled in the pores and the chemical environment of the pores. Therefore, porous catalysts are very suitable for the study of Ps formation and annihilation mechanism. However, up to now only very few research works have been reported on formation and annihilation of Ps in porous catalysts, and these limited works reported have not attained a deep understanding of the Ps annihilation mechanism.In this dissertation, we studied the formation and annihilation mechanism of Ps in five series of catalysts by measuring both positron annihilation lifetime (PAL) and coincidence Doppler broadening (CDB) spctra. To profoundly reveal the annihilation mechanism of Ps, the microstructure of the catalysts were also studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron-spin-resonance (ESR) and other experimental methods. The main results are listed as follows:1. The formation and annihilation of Ps in NiO/Al2O3 catalysts prepared by solid state reaction were studied. Two short and two long lifetime components were resolved by analyzing the PAL spectra of NiO/Al2O3 catalysts, where the two long lifetime t3 and t4 correspond to the ortho-positronium (o-Ps) annihilation in the microvoids and the large pores. With increasing NiO content, the longest lifetime t4 and its relative intensity I4 decrease significantly, while t3 and its relative intensity I3 increase slightly. The o-Ps intensity (sum of I4 and I3) obtained from PAL measurement decrease continually with NiO content. Meanwhile, the S parameter from CDB spectra increases continuously with the NiO content. Furthermore, the p-Ps intensity obtained from multi-Gaussian fitting on CDB spectra also increases gradually with NiO content. The variation of o-Ps and p-Ps intensity indicates the spin conversion of Ps in NiO/Al2O3 catalysts. The full width at half maximum (FWHM) of p-Ps become narrower with increasing NiO content, this might be caused by p-Ps converted from o-Ps. Comparing to the original p-Ps, the converted p-Ps atoms have much longer lifetime and experienced much more collisions to lose energies, thus have much lower momentum before annihialtion. The lifetime distribution of conversed p-Ps induced the increase of t3 and I3. Electron-spin-resonance (ESR) measurements reveal that the spin conversion of Ps is induced by the unpaired electrons of the paramagnetic centers of NiO.2. The monolayer dispersion of NiO in the NiO/Al2O3 catalysts prepared by impregnation method was studied by spin conversion effect of Ps. The calculation of the XRD patterns of the catalysts shows that the monolayer dispersion capacity of NiO in this series of NiO/Al2O3 catalysts is about 9wt%. When the NiO loading is less than 9wt%, the NiO dispersed in a monolayer state. While the NiO loading exceeds 9wt%, the surplus loading remains as crystalline phase in the catalysts together with the monolayer state. With increasing NiO content, t4 and I4 firstly decrease sharply with NiO loading lower than 9wt%, and then decrease slowly for NiO loading higher than 9wt%. In the two stages mentioned above, the o-Ps intensity obtained from PAL measurement decrease sharply and then decreases slowly with NiO content, while t3 and I3 increse sharply and then decreases slowly. But the p-Ps intensity, obtained from multi-Gaussian fitting of CDB spectra, firstly increase sharply (NiO loading lower than 9wt%) and then decrease gradually (NiO loading higher than 9wt%) with NiO content. It is evident that a strong spin conversion effect of Ps occurred before monolayer NiO dispersion, the inhibition effect of Ps formation by monolayer-dispersed NiO is not obvious. The spin conversion of Ps becomes weaker with NiO content exceeding monolayer dispersion capacity, the prohibition effect of Ps formation by crystalline NiO is significant. The spin conversion of Ps by NiO and the inhibition effect of Ps formation by crystalline NiO can account for the variation of t3 and I3 in the two stages.3. The formation and annihilation of Ps in Fe2O3/Al2O3, CuO/Al2O3 and Cr2O3/Al2O3 catalysts prepared by solid state reaction were studied. With increasing content of the active components, the lifetime components ti, t2 and t3 in all three catalysts keep nearly unchanged, while t4 and its intensity I4 decrease significantly, but I3 shows nearly no change. The o-Ps intensity obtained from PAL measurement decreases continually with active component, while S parameter and the p-Ps intensity decrease with active component too. This implies that the quenching of Ps is not induced by spin conversion but caused by chemical reaction of Ps. The o-Ps annihilation rate in large poresλ4 (inverse of ta) shows good linearity with increasing content of the active component for all the three series of catalysts, it indicates that the chemical quenching of o-Ps in porous catalysts is proportional to the content of the active component. In addition, the gradual decrease of both o-Ps and p-Ps intensity also indicates that the active components such as Fe2O3, CuO and Cr2O3 also inhibit Ps formation.
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