Chemical quenching and inhibition of positronium in Cr2O3/Al2O3 catalysts

The pore structure of Cr₂O₃/Al₂O₃ catalysts and the surface chemical properties of these pores were characterized by positron lifetime and coincidence Doppler broadening (CDB) measurements. Four lifetime components could be resolved from the positron lifetime spectrum, with two long lifetime components and two short lifetime components. The two long lifetimes τ ₄ and τ ₃ are attributed to ortho-positronium (o-Ps) annihilation in large pores and microvoids, respectively. With increasing Cr₂O₃ content, both τ ₄ and its intensity I ₄ show sharp decrease, while τ ₃ and its intensity I ₃ keep nearly unchanged. The Doppler broadening S parameters also show sharp decrease with increasing Cr₂O₃ content. Detailed analysis of the CDB spectrum reveals that the parapositronium (p-Ps) intensity also decreases with increasing Cr₂O₃ content. This indicates that the change of o-Ps lifetime τ ₄ is due to the chemical quenching by Cr₂O₃ but not spin-conversion of positronium. The decrease of o-Ps intensity I ₄ indicates that Cr₂O₃ also inhibits positronium formation.