Monolayer dispersion of CoO on Al2O3 probed by positronium atom
Monolayer dispersion of CoO on Al2O3 probed by positronium atom
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DOI：
10.1016/j.apsusc.2013.12.162
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发表时间：
2014-02
影响因子：
6.7
通讯作者：
Z. Liu;Heng Zhang;Zongyun Chen
中科院分区：
文献类型：
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作者：
Z. Liu;Heng Zhang;Zongyun Chen
采用浸渍法制备了CoO/Al 2 O3催化剂，CoO含量为0 ~ 24wt%。X-射线衍射和X-射线光电子能谱测量表明在N2中煅烧后形成CoO。X射线衍射定量分析表明CoO/Al 2 O3催化剂中CoO的单层分散量约为3wt%。用正电子湮没寿命和符合多普勒展宽测量研究了CoO在Al 2 O3上的分散状态。正电子湮没寿命测量结果显示了两个长寿命分量τ 3和τ4，分别对应于微孔和大孔中正电子偶素的湮没寿命。结果表明，正电子偶素对CoO在Al_2O_3上的分散状态非常敏感。CoO的存在显著降低了τ4的寿命和强度。对符合多普勒展宽测量结果的详细分析表明，当CoO含量低于单层色散时，CoO诱导了正电子素的自旋转换反应。当钴含量高于单层分散容量时，正电子素形成的抑制成为主导效应。
CoO/Al2O3catalysts were prepared by wet impregnation method with CoO contents ranging from 0 wt% to 24 wt%. X-ray diffraction and X-ray photoelectron spectroscopy measurements suggest formation of CoO after calcined in N2. Quantitative X-ray diffraction analysis indicates monolayer dispersion capacity of CoO in CoO/Al2O3catalysts to be about 3 wt%. Positron annihilation lifetime and coincidence Doppler broadening measurements were performed to study the dispersion state of CoO on Al2O3. The positron lifetime measurements reveal two long lifetime componentsτ3andτ4, which correspond toortho-positronium annihilation lifetime in microvoids and large pores, respectively. It was found that the positronium atom is very sensitive to the dispersion state of CoO on Al2O3. The presence of CoO significantly decreases both the lifetime and the intensity ofτ4. Detailed analysis of the coincidence Doppler broadening measurements suggests that with the CoO content lower than the monolayer dispersion, spin conversion reaction of positronium is induced by CoO. When the cobalt content is higher than the monolayer dispersion capacity, inhibition of positronium formation becomes the dominate effect.