Scanning Tunneling Microscopy Study of H_(6+x)P2Mo_(18-x)V_xO_(62) (x = 0-3) Wells-Dawson Heteropolyacid Catalysts: Correlation of NDR Peak Voltage with Reduction Potential and Oxidation Catalysis

摘要

Scanning tunneling microscopy (STM) and tunneling spectroscopy studies were carried out to examine the redox properties of vanadium-containing H_(6+x)P2Mo_(18-x)V_xO_(62)(x = 0, 1, 2, 3) Wells-Dawson heteropolyacid (HPA) catalysts. The HPAs formed two-dimensional well-ordered monolayer arrays on a graphite surface and exhibited a distinctive current-voltage behavior called negative differential resistance (NDR). The NDR peak voltages of H_(6+x)P2Mo_(18-x)V_xO_(62) HPAs were correlated with reduction potentials determined by temperature-programmed reduction and with catalytic activity for oxidative dehydrogena-tion of isobutyraldehyde to methacrolein. The NDR peak voltage of H_(6+x)P2Mo_(18-x)V_xO_(62) appeared at less negative voltage with increasing reduction potential and oxidation catalysis.