On the reduction potential of cation-exchanged heteropolyacids (HPAs)

UV-Visible spectroscopy and scanning tunneling microscopy (STM) studies were performed to explore reduction potentials of cation-exchanged Keggin-type heteropolyacid (HPA) catalysts. Absorption band edge and negative differential resistance (NDR) peak voltage of cation-exchanged HPA samples determined by UV-Visible spectroscopy and STM, respectively, were colsely related to their reduction potentials. It was observed that HPAs with higher reduction potentials showed absorption band edges at longer wave lengths and exhibited NDR peak voltages at less negative applied values. The reduction potentials of cation-exchanged HPA catalysts could also be correlated with the electronegativities of counter-cations. Substitution of more electronegative counter-cations increased reduction potentials of the HPAs. The NDR peak voltage and the absorption band edge of HPAs could be utilized as a correlating parameter for their reduction potentials.     

Scanning tunneling microscopy (STM) and tunneling spectroscopy (TS) of heteropolyacid (HPA) self-assembled monolayers (SAMS): connecting nano properties to bulk properties

Nanoscale investigation of Keggin-type heteropolyacid (HPA) self-assembled monolayers (SAMs) was performed by scanning tunneling microscopy (STM) and tunneling spectroscopy (TS) in order to relate surface properties of nanostructured HPA monolayers to bulk redox and acid properties of HPAs. Cation-exchanged, polyatomsubstituted, and heteroatom-substituted HPAs were examined to see the effect of different substitutions. HPA samples were deposited on HOPG surfaces in order to obtain images and tunneling spectra by STM before and after pyridine adsorption. All HPA samples formed well-ordered monolayer arrays, and exhibited negative difference resistance (NDR) behavior in their tunneling spectra. NDR peaks measured for fresh HPA samples appeared at less negative potentials for higher reduction potentials of the HPAs. These changes could also be correlated with the electronegativities of the substituted atoms. Introduction of pyridine into the HPA arrays increased the lattice constants of the two-dimensional HPA arrays by ca. 6 A. Exposure to pyridine also shifted NDR peak voltages of HPA samples to less negative values in the tunneling spectroscopy measurements. The NDR shifts of HPAs obtained before and after pyridine adsorption were correlated with the acid strengths of the HPAs. This work demonstrates that tunneling spectra measured by STM can fingerprint acid and redox properties of HPA monolayers on the nanometer scale. This paper is dedicated to Professor Wha Young Lee on the occasion of his retirement from Seoul National University.     

Redox properties of α2-K8P2W17(M·OH2)O61 (M = MnII,ZnII, FeII,CoII, and NiII) Wells–Dawson heteropolyacids probed by scanning tunneling microscopy and tunneling spectroscopy

Scanning tunneling microscopy (STM) and tunneling spectroscopy investigations of α₂-K₈P₂W₁₇(M·OH₂)O₆₁ (M = Mn^II, Zn^II, Fe^II, Co^II, and Ni^II) Wells–Dawson heteropolyacids (HPAs) were conducted. HPAs formed self-assembled and well-ordered arrays on graphite surface. Negative differential resistance (NDR) phenomena were observed in the tunneling spectra of the HPAs. NDR peak voltage of the HPAs appeared at less negative voltage with increasing reduction potential and with decreasing UV–visible absorption edge energy. HPAs were then applied to the electro-oxidation of methanol as a redox mediator. The oxidation activity for residual intermediates in the reaction increased as NDR peak voltage of HPA appeared at less negative voltage.     

Reduction potentials of heteropolyacid catalysts probed by scanning tunneling microscopy and UV-visible spectroscopy

Reduction potentials of heteropolyacid (HPA) catalysts were probed by scanning tunneling microscopy (STM) and UV-visible spectroscopy. Correlations between reduction potentials and NDR (negative differential resistance) peak voltages, and between reduction potentials and absorption edge energies of HPA catalysts were established. The reduction potentials of HPA catalysts have been shown to follow similar trends to the NDR peak voltages of selfassembled HPA monolayers and the absorption edge energies of bulk HPAs. In the UV-visible spectra of HPA catalysts, lower absorption edge energies corresponded to higher reduction potentials of the HPAs.     

Scanning tunneling microscopy and tunneling spectroscopy studies of niobium-containing H6+xP2W18?xNbxO62 (x=0, 1, 2, 3)Wells-Dawson heteropolyacid catalysts to probe their redox property and oxidation catalysis

Niobium-containing H_6+x P₂W_18−x Nb _x O₆₂ (x=0, 1, 2, 3) Wells-Dawson heteropolyacids (HPAs) were investigated by scanning tunneling microscopy (STM) and tunneling spectroscopy (TS) in order to elucidate their redox properties. The HPAs formed two-dimensional well-ordered monolayer arrays on graphite surface and exhibited a distinctive current-voltage behavior called negative differential resistance (NDR) in their tunneling spectra. NDR peak voltage measured on HPA molecule was correlated with reduction potential and absorption edge energy determined by electrochemical method and UV-visible spectroscopy, respectively. NDR peak voltage of H_6+x P₂W_18−x Nb _x O₆₂ Wells-Dawson HPAs appeared at less negative voltage with increasing reduction potential and with decreasing absorption edge energy. Oxidative dehydrogenation of isobutyraldehyde was also carried out as a model reaction to probe oxidation catalysis of the HPAs. The trend of NDR peak voltage of H_6+x P₂W_18−x Nb _x O₆₂ Wells-Dawson HPAs was well consistent with the trend of yield for methacrolein.     

UV–visible spectroscopy as a probe of heteropolyacid redox properties: application to liquid phase oxidations

UV–visible spectra of heteropolyacids (HPAs) in solution were measured as a simple method of correlating and predicting the redox properties of HPAs. The range of Keggin-type HPAs included H₃PMo₁₂O₄₀, H₃PW₁₂O₄₀, and variations of cations, heteroatoms, and/or the framework metals. The absorption edges of 0.01 molar aqueous HPA solutions were determined and compared with absorption edges of solid HPAs and with results from other methods that probe redox properties. The absorption edges of the solutions followed similar trends to the absorption edges of bulk solid HPAs, NDR (negative differential resistance) peak voltages from STM measurements on HPA monolayers, and reduction potentials measured electrochemically. HPAs that are stronger oxidizing agents are characterized by absorption edges at longer wavelength (lower energy), and NDR peaks and reduction potentials at more positive voltages. Comparison of edge energies measured in aqueous solution with the performance of HPA catalysts for liquid phase oxidations demonstrates threshold behavior for oxidation activity in each case examined. These results illustrate the potential use of UV–visible spectroscopy in solution as a simple diagnostic indicator in the selection of HPAs as oxidation catalysts.     

Scanning Tunneling Microscopy Study of H6+xP2Mo18?xVxO62 (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 H_6+x P₂Mo_18−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 H_6+x P₂Mo_18−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 H_6+x P₂Mo_18−x V _x O₆₂ appeared at less negative voltage with increasing reduction potential and oxidation catalysis.     

Investigation into the activity of heteropolyacids towards the oxygen reduction reaction on PEMFC cathodes

A total of 18 heteropolyacids (HPAs) were investigated to determine their activity as non-Pt oxygen reduction reaction (ORR) catalysts in polymer electrolyte membrane fuel cell cathodes (PEMFCs). Polarization curves, cyclic voltammetry and impedance spectroscopy determined that, of the HPAs tested, only molybdenum based HPAs are active for the ORR and that vanadium substitutions improved the activity. The reduction potentials of the HPAs in the fuel cell environment were determined by cyclic voltammetry. This showed that no activity is seen above 0.55 V, as the catalysts must first be reduced in situ by 4e⁻ before the HPA can reduce oxygen. The potential at which the HPA can be reduced has been determined to be the limiting factor when using these catalysts for ORR in PEMFCs. Power densities of 67 mW/cm² at 0.2 V were obtained using H₅PMo₁₀V₂O₄₀. Molybdenum based HPAs were covalently bonded to the carbon achieving mass loadings 3× that obtained through adsorption. Using this approach catalyst, performance was improved to 86 mW/cm² at 0.2 V. The increased loadings did not significantly increase the potentials at which the HPA becomes active for the ORR. We were able to show that MEA degradation, as measured by F⁻ emission rates, using these catalysts are reduced during accelerated testing protocols.     

Heterogeneous liquid-phase lactonization of 1,4-butanediol using H3+xPMo12−xVxO40 (x=0–3) catalysts immobilized on polyaniline

In order for successful application of heteropolyacids (HPAs) as heterogeneous catalysts to liquid-phase lactonization of 1,4-butanediol, H_3+xPMo_12−xV_xO₄₀ (x=0–3) HPAs were immobilized on polyaniline (PANI) by one-step and two-step methods. Aniline was polymerized in the presence of HPA in the one-step preparation method, while HPA was immobilized on the ready-made PANI support in the two-step method. It was found that HPAs were molecularly dispersed and strongly immobilized in/on the PANI support as charge-compensating components. Most HPAs in the two-step HPA---PANI catalysts were immobilized only on the surface of PANI support. The surface areas of the two-step HPA---PANI catalysts were much higher than those of the one-step catalysts, which is important from the practical point of applications. Thermal stability of PANI support was much enhanced by the binding with HPA, and thermal stability of the two-step HPA---PANI catalysts was superior to the one-step catalysts. In the lactonization of 1,4-butanediol, catalytic activities were in the following order: two-step HPA---PANI>one-step HPA---PANI>unsupported HPA; H₆PMo₉V₃O₄₀---PANI>H₅PMo₁₀V₂O₄₀---PANI>H₄PMo₁₁V₁O₄₀---PANI>H₃PMo₁₂O₄₀---PANI. High activity of V-containing two-step catalysts and easiness of catalyst recovery in the liquid-phase reaction make them good candidates for an energy-saving lactonization process of 1,4-butanediol.     

Negative Differential Resistance in ZnO Nanowires Bridging Two Metallic Electrodes

The electrical transport through nanoscale contacts of ZnO nanowires bridging the interdigitated Au electrodes shows the negative differential resistance (NDR) effect. The NDR peaks strongly depend on the starting sweep voltage. The origin of NDR through nanoscale contacts between ZnO nanowires and metal electrodes is the electron charging and discharging of the parasitic capacitor due to the weak contact, rather than the conventional resonant tunneling mechanism.     

Redox Properties and Catalytic Oxidation Activities of Polyatom-Substituted H n PW11M1O40 (M = V, Nb, Ta, and W) Keggin Heteropolyacid Catalysts

Redox properties and catalytic oxidation activities of polyatom-substituted H _n PW₁₁M₁O₄₀ (M = V, Nb, Ta, and W) Keggin heteropolyacids (HPAs) were examined. Reduction potentials and UV–visible absorption edge energies of H _n PW₁₁M₁O₄₀ (M = V, Nb, Ta, and W) HPA catalysts in solution were determined by an electrochemical method and UV–visible spectroscopy measurements, respectively. It was observed that reduction potentials of H _n PW₁₁M₁O₄₀ (M = V, Nb, Ta, and W) HPA catalysts increased and UV–visible absorption edge energies of the HPA catalysts decreased with decreasing electronegativity of substituted polyatom. It was also found that the lower absorption edge energy corresponded to the higher reduction potential of the HPA catalyst. Vapor-phase oxidation of benzyl alcohol was carried out as a model reaction to probe the redox properties of H _n PW₁₁M₁O₄₀ (M = V, Nb, Ta, and W) HPA catalysts. Yield for benzaldehyde increased with increasing reduction potential and with decreasing absorption edge energy of the HPA catalyst, and in turn, with decreasing electronegativity of substituted polyatom. Reduction potential of H _n PW₁₁M₁O₄₀ (M = V, Nb, Ta, and W) HPA catalysts measured by an electrochemical method and absorption edge energy of the HPA catalysts measured by UV–visible spectroscopy could be utilized as a probe of oxidation catalysis of the HPA catalysts.     

Enhancement of the Catalytic Activity of a Dawson-type Heteropoly Acid Induced by the Loading on a Silica Support

The Friedel–Crafts alkylation, benzylation of anisole with benzyl alcohol, was carried out over heteropoly acids (HPAs) supported on SiO₂. A Dawson-type HPA, in which Nb atoms were partially substituted with W atoms, exhibited the highest activity among various kinds of HPAs. The catalyst was reusable in the reaction after the filtration and it kept high activity even after the washing with hot water when the catalyst was calcined at 743 K. In contrast, the unsupported catalyst was almost inactive after the thermal treatment. The change in the activity was in good agreement with the degree in the acid amount measured by NH₃-TPD. Unlike the Dawson-type heteropoly acid, the activity of the Keggin-H₃PW₁₂O₄₀/SiO₂ remarkably dropped through the repeated use for the reaction. Based on the structural analysis of Dawson-type HPAs, the partially decomposed HPA on the support of SiO₂ was ascribed to the active species.     

Atomic-Level Characterization of Cubic Silicon Carbide Surfaces—A Review

Scanning tunneling microscopy (STM) images of the cubic β-SiC (100) and β-SiC (111) surfaces are taken after annealing to 1200°C to eliminate the surface oxide. Low-energy electron diffraction (LEED) patterns of the β-SiC (111) surface show a 6 √ 3 × 6 √ 3 geometry, while STM images show a 6 × 6 geometry. Contrast reversal is observed as tunneling voltage bias is reversed. Spectroscopic I/V measurements indicate the presence of a graphite layer on the top surface. A model of the surface is proposed where an incommensurate graphite monolayer is grown over a (1 × 1) Si-terminated β-SiC (111) surface. This model helps to explain the discrepancy between the 6 √ 3 × 6 √ 3 LEED pattern and the 6 × 6 geometry observed in STM images. Charge transfer between certain carbon atoms and silicon atoms gives rise to the 6 × 6 geometry and the contrast reversal.     

Spin negative differential resistance in edge doped zigzag graphene nanoribbons

The nonlinear spin-dependent transport properties in zigzag graphene nanoribbons (ZGNRs) edge doped by an atom of group III and V elements are studied systematically using density functional theory combined with non-equilibrium Green’s functions. The dopant type, acceptor or donor, and the geometrical symmetry, odd or even, are found critical in determining the spin polarization of the current and the current–voltage characteristics. For ZGNRs substitutionally doped on the lower-side edge, the down (up) spin current dominates in odd-(even-) width ZGNRs under a bias voltage around 1 V. Remarkably, in even-width ZGNRs, doped by group III elements (B and Al), negative differential resistance (NDR) occurs only for down spins. The bias range of the spin NDR increases with the width of ZGNRs. The clear spin NDR is not observed in any odd-width ZGNRs nor in even-width ZGNRs doped by group V elements (N and P). This peculiar spin NDR of edge doped ZGNRs suggests potential applications in spintronics.     

Scanning tunneling microscopy characterization of ammonia synthesis catalysts

The possibility of studying the texture of catalysts at the atomic level by Scanning Tunneling Microscopy (STM) is demonstrated. It is shown that high resolution pictures can be obtained even for a passivated ammonia synthesis catalyst using a scanning tunneling microscope operating in air. The STM results reveal that the catalyst has a spongy pore structure with both hole and slit shaped pores. The catalyst surfaces are observed to contain fine quite regular ridge structures with a direction that tentatively is related to the (110) direction of iron. The presence of the ridge structures is apparently the result of the topotactic nature of the reduction process and a preference to expose (111) Fe facets.     

Composite electrolyte of heteropolyacid (HPA) and polyethylene oxide (PEO) for solid-state dye-sensitized solar cell

A novel heteropolyacid (HPA) and polyethylene oxide (PEO) composite electrolyte was prepared and applied as the solid electrolyte for solid-state dye-sensitized solar cell. Advanced HPA-PEO composite electrolytes could be prepared by the optimized mixture solvent of chloroform and methanol (ChMe), which possessed the enhanced morphological properties, ionic conductivity and amorphicity. It is due to the presence of well-dispersed HPAs on the texture of PEO with the strong interaction between them. DSSCs fabricated with HPA-PEO/ChMe electrolyte showed significantly high photovoltaic (PV) performance with the overall conversion efficiency of 3.1%, open circuit voltage of 0.524 V and a short circuit current of 9.7 mA/cm². HPA in the composite electrolytes may act as an electron acceptor to prohibit the photo-reduction of I⁻, resulting in the advanced photocurrent density and stability without significant decline of the PV performance for 7 days.     

Niobium-based catalysts prepared by reactive radio-frequency magnetron sputtering and arc plasma methods as non-noble metal cathode catalysts for polymer electrolyte fuel cells

Two vacuum methods, reactive radio-frequency (RF) magnetron sputtering and arc plasma deposition, were used to prepare niobium-based catalysts for an oxygen reduction reaction (ORR) as non-noble metal cathodes for polymer electrode fuel cells (PEFCs). Thin films with various N and O contents, denoted as NbO_x and Nb-O-N, were prepared on glassy carbon plates by RF magnetron sputtering with controlled partial pressures of oxygen and nitrogen. Electrochemical measurements indicated that the introduction of the nitrogen species into the thin film resulted in improved ORR activity compared to the oxide-only film. Using an arc plasma method, niobium was deposited on highly oriented pyrolytic graphite (HOPG) substrates, and the sub-nanoscale surface morphology of the deposited particles was investigated using scanning tunneling microscopy (STM). To prepare practical cathode catalysts, niobium was deposited on carbon black (CB) powders by arc plasma method. STM and transmission electron microscopy observations of samples on HOPG and CB indicated that the prepared catalysts were highly dispersed at the atomic level. The onset potential of oxygen reduction on Nb-O-N/CB was 0.86 V vs. a reversible hydrogen electrode, and the apparent current density was drastically improved by the introduction of nitrogen.     

STM tip-induced local electrochemical dissolution of silver

Local dissolution/deposition processes under in situ scanning tunneling microscopy (STM) imaging conditions are studied in the systems Ag(111)/Ag⁺, ClO₄⁻ and Ag(111)/Ag⁺, SO₄²⁻. The results show that in both systems the local kinetics of these processes strongly depend on the polarization conditions. At STM-tip potentials more positive than the Ag/Ag⁺ equilibrium potential, a local dissolution of the Ag(111) substrate is observed even at cathodic substrate overpotentials at which the overall substrate current density is cathodic. This tip-induced Ag dissolution is in agreement with results obtained recently in the system Cu(111)/Cu²⁺. The enhanced local Ag dissolution is explained by a reduced Ag⁺ concentration underneath the STM tip promoted by both an electrostatic repulsion of Ag⁺ and a reduction of the mass transport due to the shielding effect of the tip. The possibility for a preparation of negative Ag nanostructures by STM tip-induced electrochemical dissolution is demonstrated.     

STM investigation of nano-structures fabricated on passivated Si surfaces

We have successfully fabricated nano-structures on passivated Si surfaces and investigated those structures by using scanning tunneling microscope (STM) and atomic force microscope (AFM). Ag nano-dots were formed on Sb-passivated Si(100) surface via self-organization mechanism and the single-electron charging effect was observed by STM at room temperature. Thermal nitridation and subsequent oxygen-induced etching of Si surfaces resulted in the formation of silicon nano-dots using silicon nitride islands as masks. Au/Ti nano-wire was also fabricated via a selective ion etching of Au/Ti thin film using carbon nanotube (CNT) mask. These results suggest new fabrication method of nano-structures using surface chemical reactions without artificial lithography techniques.     

Potential dependence of surface crystal structure of iron passive films in borate buffer solution

The effect of passivation potential on surface crystal structure, apparent thickness and passivity of oxide films formed on pure iron prepared by plasma sputter deposition was investigated. The crystallinity was improved with passivation potential and the width of atomically flat terraces was expanded to 6 nm when passivating at 750 mV for 15 min, as observed by ex situ scanning tunneling microscopy (STM) after aging in air (<30% RH). Apparent thickness and passivity are linearly dependent on passivation potential. The former weakly depends on passivation duration, the latter strongly depends on passivation duration. This is well explained by the correlation between crystal structure and passivity.