Correlation of alkane oxidation performance with STM and tunneling spectroscopy measurements of heteropolyacid catalysts

• Published in: Catalysis today 2003-06, Vol.81 (2), p.137-148
• Main Authors: Song, In K., Lyons, James E., Barteau, Mark A.
• Format: Article
• Language: English
• Subjects: Catalysis; Catalytic reactions; Chemistry; Exact sciences and technology; General and physical chemistry; Heteropolyacid; Scanning tunneling microscopy; Selective alkane oxidation; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; Tunneling spectroscopy
• ISSN: 0920-5861; 1873-4308
• DOI: 10.1016/S0920-5861(03)00107-X

A comprehensive study of the surface electronic properties of nanostructured heteropolyacid (HPA) monolayers was carried out using scanning tunneling microscopy (STM) with the aim of developing tools for rational selection of HPAs as oxidation catalysts. A wide set of Keggin-type HPAs with different counter-cation, framework polyatom, and central heteroatom substitutions were examined. HPA samples deposited on graphite surfaces formed two-dimensional well-ordered monolayer arrays, and exhibited negative differential resistance (NDR) behavior in their tunneling spectra. Substitution of more electronegative atoms for counter-cations or for the central heteroatom shifted the NDR peaks to less negative voltages, corresponding to increased reduction potentials of the HPAs. However, 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, it was consistently observed that NDR peaks appeared at less negative potentials for higher reduction potentials of the HPAs, across all families of HPAs examined in this work. The NDR peak voltage could be utilized as a correlating parameter (as an alternative parameter) for the reduction potential; a less negative NDR peak voltage corresponds to a higher reduction potential of the HPA. A “volcano” plot constructed for the first time by relating NDR peak voltage to catalytic performance of HPAs in propane oxidation (a target reaction) showed that there was an optimum range of NDR peak voltages for the best performance in this reaction. The ability to tune NDR peak voltages of HPAs by exchange/substitution can be a strong advantage in designing HPAs in a systematic way as selective oxidation catalysts. This work has successfully demonstrated that NDR peak voltages of HPAs can serve as selection, prediction, and design tools in identifying efficient HPAs for propane oxidation.