Song In K, Shnitser Russell B, Cowan Jennifer J, Hill Craig L, Barteau Mark A
Center for Catalytic Science and Technology, Department of Chemical Engineering, University of Delaware, Newark, DE 19716, USA.
Inorg Chem. 2002 Mar 11;41(5):1292-8. doi: 10.1021/ic010832d.
Nanoscale characterization of acid and redox properties of Keggin-type heteropolyacids (HPAs) with different heteroatoms, H(n)MW(12)O(40) (M = P, Si, B, Co), was carried out by scanning tunneling microscopy (STM) and tunneling spectroscopy (TS) in this study. HPA samples were deposited on highly oriented pyrolytic graphite surfaces to obtain images and tunneling spectra by STM before and after pyridine adsorption. All HPA samples formed well-ordered 2-dimensional arrays on graphite before and after pyridine exposure. NDR (negative differential resistance) peaks were observed in the tunneling spectra. Those measured for fresh HPA samples appeared at less negative voltages with increasing reduction potential of the HPAs and with increases in the electronegativity of the heteroatom, but with decreases in the overall negative charge of the heteropolyanions. These results support the conclusion that more reducible HPA samples show NDR behavior at less negative applied voltages in their tunneling spectra. Introduction of pyridine into the HPA arrays increased the lattice constants of the 2-dimensional HPA arrays by ca. 6 A. Exposure to pyridine also shifted NDR peak voltages of H(n)MW(12)O(40) (M = P, Si, B, Co) samples to less negative values in the TS measurements. The NDR shifts of HPAs obtained before and after pyridine adsorption were correlated with the acid strengths of the HPAs, suggesting that tunneling spectra measured by STM could serve to probe acid properties of HPAs. These results show how one can relate the bulk acid and redox properties of HPAs to surface properties of nanostructured HPA monolayers determined by STM.
本研究通过扫描隧道显微镜（STM）和隧道光谱（TS）对具有不同杂原子的Keggin型杂多酸（HPA），即H(n)MW(12)O(40)（M = P、Si、B、Co）的酸和氧化还原性质进行了纳米级表征。将HPA样品沉积在高度取向的热解石墨表面上，以便在吡啶吸附前后通过STM获得图像和隧道光谱。所有HPA样品在吡啶暴露前后在石墨上均形成了有序的二维阵列。在隧道光谱中观察到了负微分电阻（NDR）峰。对于新鲜的HPA样品测量得到的那些峰，随着HPA还原电位的增加以及杂原子电负性的增加，出现在负电压较小的位置，但随着杂多阴离子总负电荷的减少而出现。这些结果支持了以下结论：更多可还原的HPA样品在其隧道光谱中在负电压较小的情况下表现出NDR行为。将吡啶引入HPA阵列使二维HPA阵列的晶格常数增加了约6 Å。在TS测量中，暴露于吡啶还使H(n)MW(12)O(40)（M = P、Si、B、Co）样品的NDR峰电压向负电压较小的值移动。吡啶吸附前后获得的HPA的NDR位移与HPA的酸强度相关，这表明通过STM测量的隧道光谱可用于探测HPA的酸性性质。这些结果表明了如何将HPA的体相酸和氧化还原性质与由STM确定的纳米结构HPA单层的表面性质联系起来。
