Correlation of negative differential resistance (NDR) peak voltages of nanostructured heteropolyacid (HPA) monolayers with one electron reduction Potentials of HPA catalysts
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abstract
An extensive and quantitative study on the surface electronic properties of nanostructured Keggin-type heteropolyacid (HPA) monolayers was carried out using scanning tunneling microscopy (STM) to relate surface electronic properties to bulk redox properties of HPAs. Cation-exchanged RPMo12O40 (R = H3, Zn3/2, Co3/2, Cu3/2, Bi1); heteroatom-substituted HnXW12O40 (X = P, Si, B, Co) and HnXMo12O40 (X = P, As, Si); and polyatom-substituted HnPW11M1O40 (M = W, Mo, V), H3PMoxW12-xO40 (x = 0, 3, 6, 9, 12), H3+xPMo12-xVxO40 (x=0-3), and H3+xPW12-xVxO40 (x=0-3) HPAs were examined to elucidate the effect of different substitutions. All HPA samples formed two-dimensional well-ordered monolayer arrays on graphite surfaces and exhibited negative differential resistance (NDR) behavior in their tunneling spectra. Substitution of more electronegative atoms for countercations or for the central heteroatom shifted the NDR peaks to less negative voltages, corresponding to increased reduction potentials of the HPAs. On the other hand, substitution of more electronegative framework polyatoms shifted the NDR peaks to more negative voltages, corresponding to decreased reduction potentials. Irrespective of the exchanged/substituted positions, however, a comprehensive correlation between NDR peak voltage and reduction potential of HPAs established for all families of HPAs examined in this work revealed that NDR peak voltage could be utilized as a correlating parameter for the reduction potential of HPAs; a less negative NDR peak voltage corresponds to a higher reduction potential of the HPA. It is concluded that NDR peak voltages of HPAs can provide a selection and design basis for HPA catalysts efficient for selective oxidation reactions.
