Scanning tunneling microscopy and tunneling spectroscopy studies of niobium-containing H_(6+x)P_2W_(18-x)Nb_xO_(62) (x=0, 1, 2, 3) Wells-Dawson heteropolyacid catalysts to probe their redox property and oxidation catalysis

摘要

Niobium-containing H_(6+x)P_2W_(18-x)Nb_xO_(62) (x=0, 1, 2, 3) Wells-Dawson heteropolyacids (HPAs) were investigated by scanning tunneling microscopy (STM) and tunneling spectroscopy (TS) in order to elucidate their redox properties. The HPAs formed two-dimensional well-ordered monolayer arrays on graphite surface and exhibited a distinctive current-voltage behavior called negative differential resistance (NDR) in their tunneling spectra. NDR peak voltage measured on HPA molecule was correlated with reduction potential and absorption edge energy determined by electrochemical method and UV-visible spectroscopy, respectively. NDR peak voltage of H_(6+x)P_2W_(18-x)Nb_xO_(62)Wells-Dawson HPAs appeared at less negative voltage with increasing reduction potential and with decreasing absorption edge energy. Oxidative dehydrogenation of isobutyraldehyde was also carried out as a model reaction to probe oxidation catalysis of the HPAs. The trend of NDR peak voltage of H_(6+x)P_2W_(18-x)Nb_xO_(62) Wells-Dawson HPAs was well consistent with the trend of yield for methacrolein.