Scanning Tunneling Microscopy Study of H6+x P2Mo18−x V x O62 (x = 0–3) Wells–Dawson Heteropolyacid Catalysts: Correlation of NDR Peak Voltage with Reduction Potential and Oxidation Catalysis

Abstract

Scanning tunneling microscopy (STM) and tunneling spectroscopy studies were carried out to examine the redox properties of vanadium-containing H6+x P2Mo18−x V x O62 (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 H6+x P2Mo18−x V x O62 HPAs were correlated with reduction potentials determined by temperature-programmed reduction and with catalytic activity for oxidative dehydrogenation of isobutyraldehyde to methacrolein. The NDR peak voltage of H6+x P2Mo18−x V x O62 appeared at less negative voltage with increasing reduction potential and oxidation catalysis. Negative differential resistance (NDR) peak voltage of H6+x P2Mo18−x V x O62 determined by scanning tunneling microscopy appeared at less negative voltage with increasing reduction potential and with increasing oxidation catalysis.