Y沸石中的正电子湮没

POSITRON ANNIHILATION IN ZEOLITE Y

本文测量了NaY、HY和NH_4Y在不同抽空温度下的正电子寿命谱,得到了四个寿命组份,并对它们的归属进行了讨论。其中两个长寿命组份τ_s和τ_4归结为沸石内表面和笼内o-Ps的湮没。o-Ps容易被沸石内的质子酸氧化,而使其湮没寿命τ_3和τ_4缩短。利用正电子寿命谱可以监测NH_4Y沸石的脱氨过程。质子酸的存在对o-Ps的形成并无影响,但随着质子酸浓度增加,o-Ps在沸石内表面湮没的几率大于笼内,故I_3大于I_4。o-Ps的在沸石内表面和笼内的湮没速率与质子酸浓度成线性关系,以一级反应动力学处理得到的o-Ps在沸石内表面和笼内的氧化反应表观速率常数分别为9.67×10~7(mmol/g)_^(-1)s^(-1)和2.36×10~7(mmol/g)^(-1)s^(-1)。

Positron annihilation lifetime measurements were carried out under different evacuation temperatures for zeolite NaY, HY and NH_4Y. Four lifetime components have been observed in these systems. The two shorter ones, τ_1 and τ_2, were assigned to positron annihilation in the bulk, self-annihilation of ρ-Ps, pick-off annihilation of ο-Ps in the bulk and annihilation of positron trapped in the cages. The two longer ones, τ_3 and τ_4, were assigned to annihilation of ο-Ps on the surface and in the cages of the zeolites respectively. The Bronsted acid in the zeolites oxidized ο-Ps readily, so that the ο-Ps lifetime τ_3 and τ_4 were reduced significantly in zeolite HY. The deammoniation of NH_4Y at higher evacuation temperatures can be monitored through measurements of ο-Ps lifetimes. Bronsted acid in the zeolites does not inhibit the formation of ο-Ps. However, ο-Ps annihilates more readily on the surface as the concentration of Bronsted acid is increased, and hence the value of I_3 is much larger than I_4 in HY. The ο-Ps annihilation rates on the surface and in the cages, λ_3 and λ_4, are linearly proportional to the concentration of Bronsted acid in the zeolites. First-order kinetics has been applied to the oxidation reactions of ο-Ps and apparent rate constants k_3 and k_4, are 9.67×10~7 (mmol/g)^(-1)s^(-1) and 2.36×10~7 (mmol/g)^(-1) s^(-1) respectively. The experimental results obtained demonstrate that positron annihilation spectroscopy is a sensitive. technique to probe the surface properties and pore structure of porous materials.