Redox Properties of Heteropolyacid Catalysts Probed by Electrochemical Analysis, UV-visible Spectroscopy, and Scanning Tunneling Microscopy

Abstract

Heteropolyacids (HPAs) are polymeric metal-oxide clusters that exhibit the diverse range of structures and compositions. Because of their robust structures and unique redox properties, HPAs have been widely employed as catalysts for several redox reactions. Catalytic oxidations over HPAs have been extensively studied over the past few decades and mixed-addenda HPAs have attracted recent attention due to the variety in stoichiometric combinations and outstanding redox natures. Physicochemical properties and catalytic activity of mixed-addenda HPAs can be easily tuned at molecular level by changing the constituent elements including counter-cation, central heteroatom, or framework addenda atom. Because a number of elements including metals, semimetals, and even non-metals could be incorporated into the HPA frameworks, a number of mixed-addenda HPAs with different structures can be designed as a candidate for promising oxidation catalyst. In this work, several series of transition metal-substituted HPAs were designed and synthesized. They were investigated by several experimental techniques including electrochemical analysis, UV-visible spectroscopy, and scanning tunneling microscopy (STM) in order to elucidate the effect of transition metal-substitution on redox properties and catalytic activities in oxidation catalysis. Furthermore, reliabilities of absorption edge energy and negative differential resistance (NDR) peak voltage as alternative parameters for the reducibility were also examined. First, molybdenum-substituted H6P2W18-xMoxO62 (x=0, 3, 9, 15, 18) Wells-Dawson HPAs were prepared by etherate method to elucidate the effect of molybdenum-substitution on the redox properties and catalytic activity of Wells-Dawson-type tungstophosphates. Electrochemical measurements were conducted to elucidate the redox properties of HPAs. Several tungsten-based redox transitions were observed in the cyclic voltammogram of H6P2W18O62. However, H6P2Mo18O62 exhibited molybdenum-based redox transitions. Interestingly, molybdenum-substituted Wells-Dawson HPAs showed an additional molybdenum-centered redox transition at more positive potential. First electron reduction potentials increased with increasing molybdenum-substitution. UV-visible spectroscopy measurements were conducted to probe the electronic structure of bulk H6P2W18-xMoxO62 (x=0, 3, 9, 15, 18) Wells-Dawson HPAs. Absorption edge energy determined from the linear fit of [F(R∞)•hν]1/2 (Tauc plot) decreased with increasing molybdenum content. Scanning tunneling microscopy (STM) measurements were performed for the further investigation about the local surface electronic structure of H6P2W18-xMoxO62 (x=0, 3, 9, 15, 18) Wells-Dawson HPAs. In STM measurements, two-dimensional self-assembled HPA arrays were observed. Tunneling spectra taken at bright corrugations showed a distinctive current-voltage responses, referred to as negative differential resistance (NDR) phenomenon. NDR peak voltage appeared at less negative voltage with increasing the molybdenum content. Gas-phase oxidative dehydrogenation of ethanol to acetaldehyde was carried out as a model reaction to probe the oxidation catalysis of H6P2W18-xMoxO62 (x=0, 3, 9, 15, 18) Wells-Dawson HPAs. Yield for acetaldehyde (oxidation product) increased with increasing molybdenum content. Among the tested catalysts, H6P2Mo18O62 with the highest reduction potential showed the best catalytic performance. In order to explore the effect of group 5 metal (V and Nb)-substitution on redox properties and catalytic activity of Wells-Dawson-type tungstophosphates. α2-K7P2W17V1O62 and α2-K7P2W17Nb1O62 Wells-Dawson HPAs were synthesized via direct incorporation of transition metal into the mono-lacunary species to yield the selectively-substituted structures. α-K6P2W18O62 and α2-K6P2W17Mo1O62 were also prepared for the comparison. In the electrochemical analysis, molybdenum- and vanadium-substituted Wells-Dawson tungstophosphates showed additional molybdenum- and vanadium-based redox transitions, respectively, at more positive potential. However, niobium-substituted Wells-Dawson tungstophosphates showed significantly shifted redox transitions. First electron reduction potential increased in the order of α2-K7P2W17Nb1O62 < α-K6P2W18O62 < α2-K6P2W17Mo1O62 < α2-K7P2W17V1O62. Absorption edge energy determined by UV-visible spectroscopy decreased in the order of α2-K7P2W17Nb1O62 > α-K6P2W18O62 > α2-K6P2W17Mo1O62 > α2-K7P2W17V1O62. STM images clearly showed the formation of self-assembled and well-ordered HPA arrays on HOPG surface. The trend of NDR peak voltage was also well consistent with that of absorption edge energy. Gas-phase oxidative dehydrogenation of benzylamine was carried out as a model reaction to probe oxidation catalysis. Yield for dibenzylimine (oxidation product) increased in the order of α2-K7P2W17Nb1O62 < α-K6P2W18O62 < α2-K6P2W17Mo1O62 < α2-K7P2W17V1O62. Heteropolytungstates with different central atom, α-HnXW12O40 (X=Co2+, B3+, Si4+, and P5+) Keggin HPAs were prepared to elucidate the effect of central atom on the redox properties and catalytic activity. All α-HnXW12O40 HPAs exhibited well-defined reversible and stepwise tungsten-centered redox transitions during the electrochemical measurements. First electron reduction potential increased in the order of α-H6CoW12O40 < α-H5BW12O40 < α-H4SiW12O40 < α-H3PW12O40. Absorption edge energy determined by UV-visible spectroscopy decreased in the order of α-H6CoW12O40 > α-H5BW12O40 > α-H4SiW12O40 > α-H3PW12O40. The trend of NDR peak voltage was also well consistent with that of absorption edge energy. Among the tested, PO43- anion with smaller negative charge and lager size was the most effective to enhance the reducibility. Another series of heteropolytungstates containing AsO43- as a central unit were also examined to elucidate the effect of transition metal-substitution on the redox properties and catalytic activities of Wells-Dawson-type tungstoarsenates. A series of α-K6As2W18-xMoxO62 (x=0-3) Wells-Dawson HPAs were prepared via direct incorporation of transition metal into the mono-, di-, and tri-lacunary species. In electrochemical analysis, α-K6As2W18O62 exhibited four tungsten-centered redox transitions. However, molybdenum-substituted α-K6As2W18-xMoxO62 (x=1-3) exhibited an additional molybdenum-centered redox transition at more positive potential. First electron reduction potential increased with increasing molybdenum content. Absorption edge energy determined by UV-visible spectroscopy decreased with increasing molybdenum content. It is interesting to note that NDR peak voltage appeared at less negative voltage with increasing molybdenum content. These results are nearly same with the results in the series of H6P2W18-xMoxO62 (x=0, 3, 9, 15, 18). Gas-phase oxidative dehydrogenation of benzyl alcohol was carried out as a model reaction to track the oxidation catalysis. Yield for benzaldehyde (oxidation product) increased with increasing molybdenum content. Furthermore, group 5 metal-substituted α2-K7As2W17V1O62 and α2-K7As2W17Nb1O62 Wells-Dawson HPAs were synthesized via direct incorporation of transition metal into the mono-lacunary species to yield the selectively-substituted structures. α-K6As2W18O62 and α2-K6As2W17Mo1O62 were also prepared for the comparison. In the electrochemical analysis, molybdenum- and vanadium-substituted Wells-Dawson tungstoarsenates showed additional molybdenum- and vanadium-based redox transitions, respectively, at more positive potential. However, niobium-substituted Wells-Dawson tungstoarsenates showed significantly shifted redox transitions. First electron reduction potential increased in the order of α2-K7As2W17Nb1O62 < α-K6As2W18O62 < α2-K6As2W17Mo1O62 < α2-K7As2W17V1O62. Absorption edge energy determined by UV-visible spectroscopy decreased in the order of α2-K7As2W17Nb1O62 > α-K6As2W18O62 > α2-K6As2W17Mo1O62 > α2-K7As2W17V1O62. Gas-phase oxidative dehydrogenation of benzylamine was carried out as a model reaction to track the oxidation catalysis. Yield for dibenzylimine (oxidation product) increased in the order of α2-K7As2W17Nb1O62 < α-K6As2W18O62 < α2-K6As2W17Mo1O62 < α2-K7As2W17V1O62. In summary, several series of transition metal-substituted HPA catalysts with different addenda atoms, central atoms, contents, and structures were prepared, characterized, and applied to the model reactions in order to elucidate the redox properties and catalytic activities in oxidation catalysis. Reliabilities of absorption edge energy and NDR peak voltage as alternative parameters for the redox properties were also examined. It was found that redox properties were easily tunable by changing constituent elements. In addition, it could be concluded that redox properties of HPAs, which is closely related to the electronic structure, play an important role to determine the catalytic activities in the oxidation catalysis and absorption edge energy and NDR peak voltage can be utilized as alternative parameters to estimate the reducibility of HPAs.