Monolayer dispersion of NiO in NiO/Al2O3 catalysts probed by positronium atom.
Monolayer dispersion of NiO in NiO/Al2O3 catalysts probed by positronium atom.
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DOI：
10.1063/1.3676259
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发表时间：
2012-01
期刊：
影响因子：
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通讯作者：
H. Zhang;Zhiqiang Chen;S. J. Wang
中科院分区：
文献类型：
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作者：
H. Zhang;Zhiqiang Chen;S. J. Wang
采用浸渍法制备了不同NiO负载量的NiO/Al（2）O（3）催化剂。NiO的单层分散容量被确定为约9wt. %通过XRD定量相分析。NiO/Al（2）O（3）催化剂的正电子寿命谱包括两个长寿命和两个短寿命分量，其中长寿命τ（3）和τ（4）分别对应于微孔和大孔中的正电子偶素（o-Ps）湮灭。随着NiO负载量从0增加到9重量%，τ（4）从88 ns急剧下降到38 ns。然而，当NiO负载量高于9重量%时，τ（4）显示出较慢的下降。λ（4）（1/τ（4））随NiO含量的变化可用两条斜率不同的直线拟合。当NiO含量低于和高于9wt. %时，τ（4）的相对强度也显示出快速下降，然后缓慢下降，分别符合多普勒展宽测量表明，随着NiO负载量的增加，S参数持续增加，直到9wt. %然后再下降。这是由于由对正电子素（p-Ps）的湮灭贡献的窄分量的强度的变化。结果表明，NiO在γ-Al（2）O（3）表面的分散状态对正电子偶素的湮没行为非常敏感。当NiO负载量低于单层色散容量时，NiO引起的正电子素自旋转换是主要效应，导致最长寿命及其强度降低，而窄组分强度增加。当NiO负载量大于单层分散容量后，自旋转换效应减弱，NiO对正电子素形成的抑制作用增强，导致长寿命强度和窄组分强度均下降。当NiO含量低于单层分散容量时，反应速率常数为（1.50 ± 0.04）× 10（10）g mol（-1）s（-1），当NiO含量高于单层分散容量时，反应速率常数为（3.43 ± 0.20）× 10（9）g mol（-1）s（-1）。
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.