Redox Properties of Nano-Structured α 2 -K 6 As 2 W 18− x Mo x O 62 ( x = 0–3) Wells-Dawson Heteropolyacid Catalysts Probed by Scanning Tunneling Microscopy

Molybdenum-substituted Wells-Dawson tungstoarsenate (K6As2W18-xMoxO62 (x = 0-3)) heteropolyacids (HPAs) were investigated by scanning tunneling microscopy (STM). The effect of molybdenum substitution on the electrochemical properties of K6As2W18-xMoxO62 (x = 0-3) catalysts was elucidated. Formation of two-dimensional self-assembled HPA arrays was observed in the STM measurements, and their periodicities were well matched with the lattice constant determined by X-ray crystallography. Tunneling spectra measured on individual HPA molecule exhibited a distinctive current-voltage behavior, referred to as negative differential resistance (NDR) phenomenon. However, tunneling spectra measured on interstitial space between HPA molecules showed the same current-voltage response as graphite. NDR peak voltage shifted to less negative voltage with increasing molybdenum content in the K6As2W18-xMoxO62 (x = 0-3). A correlation between NDR peak voltages and bulk redox property of HPAs determined by electrochemical analysis and UV-visible spectroscopy measurement was then established. The trend of NDR peak voltage was well consistent with the trends of reduction potential and UV-visible absorption edge energy. Vapor-phase oxidation of benzyl alcohol was carried out as a model reaction. Yield for benzaldehyde increased with decreasing NDR peak voltage.