LTA分子筛负载六原子银团簇的结构和磁性
Structure and magnetic properties of six-atom silver clusters supported on LTA zeolite
摘要：
12%（重量）通过在NaA沸石中Ag $^+$对Na $^+$的水溶液离子交换制备含银催化剂。氢还原导致银簇的形成。在Ag $K$ -边缘处使用直接{ \it原位}还原的X射线吸收光谱实验的结果产生4.00的Ag-Ag配位数，这与ca的相当单分散的结构相容。六个原子团簇。其中$<$ 0.1%具有EPR活性，显示出具有6个等效银核的各向同性参数。在1.8 K至300 K的温度范围内的磁化率表现出居里-外斯行为，与$\approx$ 0.81 $μ_{\rm B}$每个Ag原子的磁化强度，依赖于还原历史。因此，整体磁化强度显示出比EPR观察到的更多的未成对电子，但平均每个原子仍少于一个未成对电子。这表明存在抗磁性、EPR活性自旋-1/2和EPR沉默高自旋物种的混合物。有趣的是，中性自由Ag $_6$团簇通过理论计算被预测为抗磁性的和平面三角形几何形状（{ \it Phys. Rev. A.} { \bf 75}（2007）063204）。
英文摘要：
12\% (wt.) silver containing catalysts were prepared by aqueous ion-exchange of Ag$^+$ against Na$^+$ in NaA zeolite. Hydrogen reduction leads to the formation of silver clusters. The results of X-ray absorption spectroscopy experiments at the Ag $K$-edge using direct {\it in-situ} reduction yield a Ag-Ag coordination number of 4.00, which is compatible with a quite mono-disperse structure of ca. six-atomic clusters. $<$ 0.1\% of them is EPR active, revealing isotropic parameters with six equivalent silver nuclei. The magnetic susceptibility in a temperature range of 1.8 K to 300 K exhibits Curie-Weiss behavior, with a magnetization up to $\approx$ 0.81 $μ_{\rm B}$ per Ag atom, dependent on reduction history. Thus, the overall magnetization reveals many more unpaired electrons than are observed by EPR, but is still less than a single unpaired electron per atom on average. This suggests that a mixture of diamagnetic, EPR active spin-1/2 and EPR silent high-spin species is present. Interestingly, neutral free Ag$_6$ clusters were predicted by theoretical calculations to be diamagnetic and of planar triangular geometry ({\it Phys. Rev. A.} {\bf 75} (2007) 063204).
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