Hubei Nuclear Solid Physics Key Laboratory, Department of Physics, Wuhan University, Wuhan 430072, People's Republic of China.
J Chem Phys. 2012 Jan 21;136(3):034701. doi: 10.1063/1.3676259.
NiO/Al(2)O(3) catalysts with different NiO loadings were prepared by impregnation method. The monolayer dispersion capacity of NiO is determined to be about 9 wt.% through XRD quantitative phase analysis. Positron lifetime spectra measured for NiO/Al(2)O(3) catalysts comprise two long and two short lifetime components, where the long lifetimes τ(3) and τ(4) correspond to ortho-positronium (o-Ps) annihilation in microvoids and large pores, respectively. With increasing loading of NiO from 0 to 9 wt.%, τ(4) drops drastically from 88 to 38 ns. However, when the NiO loading is higher than 9 wt.%, τ(4) shows a slower decrease. Variation of λ(4) (1/τ(4)) as a function of the NiO content can be well fitted by two straight lines with different slopes. The relative intensity of τ(4) also shows a fast decrease followed by a slow decrease for the NiO content lower and higher than 9 wt.%, respectively. The coincidence Doppler broadening measurements reveal a continuous increase of S parameter with increasing NiO loading up to 9 wt.% and then a decrease afterwards. This is due to the variation in intensity of the narrow component contributed by the annihilation of para-positronium (p-Ps). Our results show that the annihilation behavior of positronium is very sensitive to the dispersion state of NiO on the surface of γ-Al(2)O(3). When the NiO loading is lower than monolayer dispersion capacity, spin conversion of positronium induced by NiO is the dominant effect, which causes decrease of the longest lifetime and its intensity but increase of the narrow component intensity. After the NiO loading is higher than monolayer dispersion capacity, the spin conversion effect becomes weaker and inhibition of positronium formation by NiO is strengthened, which results in decrease of both the long lifetime intensity and the narrow component intensity. The reaction rate constant is determined to be (1.50 ± 0.04) × 10(10) g mol(-1) s(-1) and (3.43 ± 0.20) × 10(9) g mol(-1) s(-1) for NiO content below and above monolayer dispersion capacity, respectively.
采用浸渍法制备了不同 NiO 负载量的 NiO/Al(2)O(3)催化剂。通过 XRD 定量相分析，确定 NiO 的单层分散容量约为 9wt%。对 NiO/Al(2)O(3)催化剂进行的正电子寿命谱测量由两个长寿命和两个短寿命分量组成，其中长寿命 τ(3)和 τ(4)分别对应于微空隙和大孔中的正电子素（o-Ps）湮没。随着 NiO 负载量从 0 增加到 9wt%，τ(4)从 88ns 急剧下降到 38ns。然而，当 NiO 负载量高于 9wt%时，τ(4)的下降速度较慢。λ(4)(1/τ(4))随 NiO 含量的变化可以用两条斜率不同的直线很好地拟合。τ(4)的相对强度也表现出快速下降，随后在 NiO 含量低于和高于 9wt%时分别缓慢下降。符合多普勒展宽测量显示，随着 NiO 负载量增加到 9wt%，S 参数连续增加，然后随后下降。这是由于窄分量强度的变化导致的，该分量由 para-positronium（p-Ps）的湮没贡献。我们的结果表明，正电子湮没行为对 NiO 在γ-Al(2)O(3)表面的分散状态非常敏感。当 NiO 负载量低于单层分散容量时，NiO 诱导的正电子自旋转换是主要效应，这会导致最长寿命及其强度下降，但窄分量强度增加。当 NiO 负载量高于单层分散容量时，自旋转换效应变弱，NiO 对正电子素形成的抑制作用增强，导致长寿命强度和窄分量强度均下降。反应速率常数分别确定为（1.50±0.04）×10(10)gmol(-1)s(-1)和（3.43±0.20）×10(9)gmol(-1)s(-1)，分别用于 NiO 含量低于和高于单层分散容量。
