Dispersed platinum supported by hydrogen molybdenum bronze-modified carbon as electrocatalyst for methanol oxidation

A new electrocatalyst, Pt/H_xMoO₃-C, for methanol oxidation, was prepared by dispersing platinum nano-particles on Vulcan XC-72 modified by hydrogen molybdenum bronze (H_xMoO₃, 0 ≤ x ≤ 2). The modification of Vulcan XC-72 with H_xMoO₃ on was accomplished by reducing the adsorbed molybdic acid and the platinum nano-particles were dispersed on the modified carbon by reducing chloroplatinic acid, with formaldehyde as the reductant. The prepared Pt/H_xMoO₃-C was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersion spectrometer, cyclic voltammetry (CV), chronoamperometry (CA), and single-cell test, with a comparison of the electrocatalyst, carbon-supported platinum (Pt/C) prepared under the same condition but without the modification. The results obtained from XRD and SEM showed that the modification of Vulcan XC-72 with H_xMoO₃ reduced the platinum particle size and improved distribution uniformity of platinum on carbon. The results, obtained from CV, CA, and the single-cell test, showed that Pt/H_xMoO₃-C exhibited better electrocatalytic activity toward methanol oxidation than Pt/C.     

Perovskite Oxide Nanosheets with Tunable Band‐Edge Potentials and High Photocatalytic Hydrogen‐Evolution Activity

Perovskite nanosheets of HCa_2?xSr_xNb₃O₁₀ and HCa₂Nb_3?yTa_yO₁₀ with controlled band‐edge potentials were prepared. They worked as highly efficient heterogeneous photocatalysts for H₂ evolution from a water/methanol mixture under band‐gap irradiation. The activity was found to depend on the composition. The highest activity was obtained with HCa₂Nb₂TaO₁₀ nanosheets, recording an apparent quantum yield of approximately 80 % at 300 nm, which is the highest value for a nanosheet‐based photocatalyst reported to date.     

A novel method of the synthesis of molybdovanadophosphoric heteropoly acid solutions

A novel eco-friendly method of the synthesis of aqueous solutions of the Keggintype Mo-V-P heteropoly acids H_3+x PV _x Mo_12−x O₄₀ (HPA-x) is proposed. At the first stage, V₂O₅ is dissolved in cooled H₂O₂ to form peroxyvanadic compounds, which spontaneously decompose to yield a H₆V₁₀O₂₈ solution. The latter is stabilized by the addition of H₃PO₄ to yield a H₉PV₁₄O₄₂ solution that is added to a boiling aqueous suspension of (H₃PO₄ + MoO₃). This suspension is gradually evaporated producing the HPA-x solution. This safe and practically wasteless method holds much promise for the preparation of HPA-x solutions with x = 2–6.     

Identification of the Active Sites on Metallic MoO2−x Nano-Sea-Urchin for Atmospheric CO2 Photoreduction Under UV,Visible, and Near-Infrared Light Illumination

We report an oxygen vacancy (V_o)-rich metallic MoO_2−x nano-sea-urchin with partially occupied band, which exhibits super CO₂ (even directly from the air) photoreduction performance under UV, visible and near-infrared (NIR) light illumination. The V_o-rich MoO_2−x nano-sea-urchin displays a CH₄ evolution rate of 12.2 and 5.8 μmol g_catalyst⁻¹ h⁻¹ under full spectrum and NIR light illumination in concentrated CO₂, which is ca. 7- and 10-fold higher than the V_o-poor MoO_2−x, respectively. More interestingly, the as-developed V_o-rich MoO_2−x nano-sea-urchin can even reduce CO₂ directly from the air with a CO evolution rate of 6.5 μmol g_catalyst⁻¹ h⁻¹ under NIR light illumination. Experiments together with theoretical calculations demonstrate that the oxygen vacancy in MoO_2−x can facilitate CO₂ adsorption/activation to generate *COOH as well as the subsequent protonation of *CO towards the formation of CH₄ because of the formation of a highly stable Mo−C−O−Mo intermediate.     

Defect Pyrochlore-Type Mott–Schottky Photocatalysts for Enhanced Ammonia Synthesis at Low Pressure

The ambient ammonia synthesis coupled with distributed green hydrogen production technology can provide promising solutions for low-carbon NH₃ production and H₂ storage. Herein, we reported Ru-loaded defective pyrochlore K₂Ta₂O_6−x with remarkable visible-light absorption and a very low work function, enabling effective visible-light-driven ammonia synthesis from N₂ and H₂ at low pressure down to 0.2 atm. The photocatalytic rate was 2.8 times higher than that of the best previously reported photocatalyst and the photo-thermal rate at 425 K was similar to that of Ru-loaded black TiO₂ at 633 K. Compared to perovskite-type KTaO_3−x with the same composition, the pyrochlore exhibited a 3.7-fold increase in intrinsic activity due to a higher photoexcited charge separation efficiency and a higher conduction band position. The interfacial Schottky barrier and spontaneous electron transfer between K₂Ta₂O_6−x and Ru further improve photoexcited charge separation and accumulate energetic electrons to facilitate N₂ activation.     

Selective skeletal isomerization of alkanes over partially reduced MoO<Subscript>3</Subscript>

The surface area and the pentane isomerization activity of Pt/MoO₃ were enlarged by H₂ reduction. The enlargements was observed only when the reduction proceeded through the formation of hydrogen molybdenum bronze, H_xMoO₃. The catalytic activities of H₂-reduced MoO₃ with different noble metals for pentane isomerization and 2-propanol dehydration depended on the ability of noble metal to produce the H_xMoO₃ phases. H₂-reduced Pt/MoO₃ was more active for pentane isomerization than Pt/H, and its activity was comparable to that of Pt/HZSM-5. In heptane isomerization, H₂-reduced Pt/MoO₃ exhibited a lower activity than Pt/H, although heptane was isomerized very selectively. Strong adsorption of heptane onto H₂-reduced Pt/MoO₃ is likely to be a reason for its lower heptane isomerization activity.     

Activation of Water-Splitting Photocatalysts by Loading with Ultrafine Rh–Cr Mixed-Oxide Cocatalyst Nanoparticles

The activity of many water-splitting photocatalysts could be improved by the use of Rh^III–Cr^III mixed oxide (Rh_2−xCr_xO₃) particles as cocatalysts. Although further improvement of water-splitting activity could be achieved if the size of the Rh_2−xCr_xO₃ particles was decreased further, it is difficult to load ultrafine (<2 nm) Rh_2−xCr_xO₃ particles onto a photocatalyst by using conventional loading methods. In this study, a new loading method was successfully established and was used to load Rh_2−xCr_xO₃ particles with a size of approximately 1.3 nm and a narrow size distribution onto a BaLa₄Ti₄O₁₅ photocatalyst. The obtained photocatalyst exhibited an apparent quantum yield of 16 %, which is the highest achieved for BaLa₄Ti₄O₁₅ to date. Thus, the developed loading technique of Rh_2−xCr_xO₃ particles is extremely effective at improving the activity of the water-splitting photocatalyst BaLa₄Ti₄O₁₅. This method is expected to be extended to other advanced water-splitting photocatalysts to achieve higher quantum yields.     

Insight into the Mechanism of Hydrogenation of Amino Acids to Amino Alcohols Catalyzed by a Heterogeneous MoOx‐Modified Rh Catalyst

Hydrogenation of amino acids to amino alcohols is a promising utilization of natural amino acids. We found that MoO_x‐modified Rh/SiO₂ (Rh–MoO_x/SiO₂) is an efficient heterogeneous catalyst for the reaction at low temperature (323 K) and the addition of a small amount of MoO_x drastically increases the activity and selectivity. Here, we report the catalytic potential of Rh–MoO_x/SiO₂ and the results of kinetic and spectroscopic studies to elucidate the reaction mechanism of Rh–MoO_x/SiO₂ catalyzed hydrogenation of amino acids to amino alcohols. Rh–MoO_x/SiO₂ is superior to previously reported catalysts in terms of activity and substrate scope. This reaction proceeds by direct formation of an aldehyde intermediate from the carboxylic acid moiety, which is different from the reported reaction mechanism. This mechanism can be attributed to the reactive hydride species and substrate adsorption caused by MoO_x modification of Rh metal, which results in high activity, selectivity, and enantioselectivity.     

Engineering PdIr Nanostructures Synergistically Induced by Self-assembled Surfactants and Halide Ions for Alcohol Electrooxidation

The design and synthesis of metallic nanocatalysts with distinct nanostructures and composition is still a noteworthy topic in the electrochemistry field. In this work, we have realized the morphological evolution of PdIr nanostructures in aqueous solution through the synergistic effect of self-assembled functional surfactants and different halide ions, and achieved precise control of the kinetic and thermodynamic crystalline growth due to the different reduction potential between PdCl₄²⁻, PdBr₄²⁻, and PdI₄²⁻. The actual precursors of PdCl₄²⁻ resulted in ultrathin nanodendrites, PdCl_xBr_(4−x)²⁻ for nanosheets and fewer branched nanodendrites, PdCl_xI_(4−x)²⁻ for nanorings, nanoflowers and multiply concave nanocubes. Owing to the synergistic advantages of structure and composition (alloyed Ir), PdIr nanodendrites exhibited enhanced electrocatalytic activity, anti-poisoning ability, and stability toward alcohols (including ethanol, methanol, and glycerol) electrooxidation reactions. The results would be helpful for thoroughly understanding how structure-directing surfactants and halide ions synergistically determine the production of advanced metallic nanocrystals.     

Preparation of Ultrathin Two-Dimensional TixTa1−xSyOz Nanosheets as Highly Efficient Photothermal Agents

Although two-dimensional (2D) metal oxide/sulfide hybrid nanostructures have been synthesized, the facile preparation of ultrathin 2D nanosheets in high yield still remains a challenge. Herein, we report the first high-yield preparation of solution-processed ultrathin 2D metal oxide/sulfide hybrid nanosheets, that is, Ti_xTa_1−xS_yO_z (x=0.71, 0.49, and 0.30), from Ti_xTa_1−xS₂ precursors. The nanosheet exhibits strong absorbance in the near-infrared region, giving a large extinction coefficient of 54.1 L g⁻¹ cm⁻¹ at 808 nm, and a high photothermal conversion efficiency of 39.2 %. After modification with lipoic acid-conjugated polyethylene glycol, the nanosheet is a suitable photothermal agent for treatment of cancer cells under 808 nm laser irradiation. This work provides a facile and general method for the preparation of 2D metal oxide/sulfide hybrid nanosheets.     

Raman spectroscopy study of species responsible for the fast proton conductivity in H2Sb4O11.3H2O

The Raman spectroscopy study of crystallized antimonic acid trihydrate powders in the 20–4000 cm⁻¹ confirme the explicit formula : (H₃O⁺)_2−x[Sb₄O_11−xOH_x]^−(2−x).(3+x−2)H₂O. Furthermore it emphasizes the presence of hydrogen bonds between either the SbOSb bridge and H₃O⁺ or the protonated Sb(OH)Sb bridge and H₂O.     

Room‐Temperature and Aqueous‐Phase Synthesis of Plasmonic Molybdenum Oxide Nanoparticles for Visible‐Light‐Enhanced Hydrogen Generation

A straightforward aqueous synthesis of MoO_3?x nanoparticles at room temperature was developed by using (NH₄)₆Mo₇O₂₄?4 H₂O and MoCl₅ as precursors in the absence of reductants, inert gas, and organic solvents. SEM and TEM images indicate the as‐prepared products are nanoparticles with diameters of 90–180 nm. The diffuse reflectance UV‐visible‐near‐IR spectra of the samples indicate localized surface plasmon resonance (LSPR) properties generated by the introduction of oxygen vacancies. Owing to its strong plasmonic absorption in the visible‐light and near‐infrared region, such nanostructures exhibit an enhancement of activity toward visible‐light catalytic hydrogen generation. MoO_3?x nanoparticles synthesized with a molar ratio of Mo^VI/Mo^V 1:1 show the highest yield of H₂ evolution. The cycling catalytic performance has been investigated to indicate the structural and chemical stability of the as‐prepared plasmonic MoO_3?x nanoparticles, which reveals its potential application in visible‐light catalytic hydrogen production.     

Selective and Tunable Near‐Infrared and Visible Light Transmittance of MoO3−x Nanocomposites with Different Crystallinity

In this Communication, we report MoO_3−x nanocomposites in which the near‐infrared and visible light transmittance can be selectively modulated through the crystallinity. The MoO_3−x nanocomposites were fabricated by a hydrothermal method, and their optical properties were characterized by UV‐Vis spectrometer. The obtained results proved the possibility to tune the nanocomposite's optical properties in the UV/Visible spectral region: crystalline MoO₃ mainly regulates the near‐infrared range (800–2600 nm), and amorphous MoO_3−x mainly changes the visible range from 350 nm to 800 nm and MoO_3−x , with semi‐crystalline structures mainly modulating around 800–1000 nm. These kinds of optical modulations could be attributed to small polar absorption, free electron absorption and plasmon absorption according to different crystallinity. Our work may create new possibilities for future applications such as photochromism, photocatalysis, and electrochromism.     

Oxo-Hydroxoferrate K2−xFe4O7−x(OH)x: Hydroflux Synthesis,Chemical and Thermal Instability,Crystal and Magnetic Structures

The reaction of Fe(NO₃)₃⋅9 H₂O with KOH under hydroflux conditions at about 200 °C produces red crystals of K_2−xFe₄O_7−x(OH)_x in a quantitative yield. In the crystal structure, edge-sharing [FeO₆] octahedra form Fe₂O₆] honeycomb nets. Pillars consisting of pairs of vertex-sharing [FeO₄] tetrahedra link the honeycomb layers and form columnar halls in which the potassium ions are located. The trigonal (P 1m) and the hexagonal (P6₃/mcm) polytypes of K_2−xFe₄O_7−x(OH)_x show oriented intergrowth. The sub-stoichiometric potassium content (x≈0.3) is compensated by hydroxide ions. K_2−xFe₄O_7−x(OH)_x is an antiferromagnet above 2 K and its magnetic structure was determined by neutron powder diffraction. Under ambient conditions, K_2−xFe₄O_7−x(OH)_x hydrolyzes and K₂CO₃ ⋅ H₂O forms gradually on the surface of the K_2−xFe₄O_7−x(OH)_x crystals. Upon annealing at air at about 500 °C, the potassium atoms in the columnar halls start to order into a superstructure. The thermal decomposition of K_2−xFe₄O_7−x(OH)_x proceeds via a topotactic transformation into K_1+x′Fe₁₁O₁₇, adopting the rhombohedral β’’ or the hexagonal β-aluminate-type structure, before γ-Fe₂O₃ is formed above 950 °C, which then converts into thermodynamically stable α-Fe₂O₃.     

Deposition of NiO onto MoO3/γ-Al2O3 extrudates by slurry impregnation method

NiO-MoO₃/γ-Al₂O₃ catalysts were prepared by the reaction of γ-Al₂O₃ extrudates with an aqueous slurry of MoO₃, followed by the reaction of the MoO₃/γ-Al₂O₃ catalyst with an aqueous slurry of NiO, Ni(OH)₂, NiCO₃·2Ni(OH)₂·xH₂O, or 2NiCO₃·3Ni(OH)₂·4H₂O and by subsequent drying. The NiO deposition was examined with electron probe microanalysis. The deposited Ni efficiently increased the activity in benzothiophene hydrodesulfurization.     

Effect of MoO<Subscript>3</Subscript> additions on the thermal stability and crystallization kinetics of PbO–Sb<Subscript>2</Subscript>O<Subscript>3</Subscript>–As<Subscript>2</Subscript>O<Subscript>3</Subscript> glasses

The present paper reports on the effect of MoO₃ on the glass transition, thermal stability and crystallization kinetics for (40PbO–20Sb₂O₃–40As₂O₃)_100−x –(MoO₃) _x (x = 0, 0.25, 0.5, 0.75 and 1 mol%) glasses. Differential scanning calorimetry (DSC) results under non-isothermal conditions for the studied glasses were reported and discussed. The values of the glass transition temperature (T _g) and the peak temperature of crystallization (T _p) are found to be dependent on heating rate and MoO₃ content. From the compositional dependence and the heating rate dependence of T _g and T _p, the values of the activation energy for glass transition (E _g) and the activation energy for crystallization (E _c) were evaluated and discussed. Thermal stability for (40PbO–20Sb₂O₃–40As₂O₃)_100−x –(MoO₃) _x glasses has been evaluated using various thermal stability criteria such as ΔT, H _r , H _g and S. Moreover, in the present work, the K _r(T) criterion has been considered for the evaluation of glass stability from DSC data. The stability criteria increases with increasing MoO₃ content up to x = 0.5 mol%, and decreases beyond this limit.     

Poly[propane‐1,3‐diammonium [tetra‐μ3‐oxo‐hexaoxotrimolybdenum(VI)] dihydrate]

The crystal structure of the title compound, {(C₃H₁₂N₂)[Mo₃O₁₀]·2H₂O}_n, is composed of [Mo₃O₁₀]²⁻ anionic chains, propane‐1,3‐diammonium cations and solvent water molecules. The [Mo₃O₁₀]²⁻ chain is constructed from edge‐sharing MoO₆ octahedra. The protonated propane‐1,3‐diamine cations and solvent water molecules are located between the chains and are linked to the O atoms of the inorganic chains by hydrogen bonds.     

Adsorption properties of crystalline silicas: (II) Adsorption of anionic surfactants and delamination

Lamellar crystalline silicas (crystalline silicic acids, chemical composition SiO₂·xH₂O; examples: H₄Si₁₄O₃₀·xH₂O, H₄Si₂₀O₄₂·xH₂O) are distinguished from the amorphous forms by their layered structure and exceptional adsorption properties. One outstanding example is the reaction with anionic surfactants. Several types of crystalline silicas (typical H₄Si₂₀O₄₂·xH₂O) can intercalate ionic pairs consisting of surfactant anion and gegen ion into the interlayer space. The saturation value of SDS adsorption is 0.475 mmol SDS/g H₄Si₂₀O₄₂·3H₂O. The acid H₄Si₁₄O₃₀·xH₂O adsorbs anionic surfactants at the external surfaces only (saturation value 0.04 mmol/g H₄Si₁₄O₃₀·0.8 H₂O). When anionic surfactants are adsorbed in the interlayer space, the layer separation increases to such an extent that the crystals disarticulate in a fan-like manner or delaminate into thinner packets of layers or smaller aggregates. Washing-out the SDS ionic pairs or drying reconstitutes the parallel layer orientation and leads to re-aggregation of the packets and fragments.     

Potassium Ion Conductivity in the Cubic Labyrinth of a Piezoelectric,Antiferromagnetic Oxoferrate(III) Tellurate(VI)

Orange-colored crystals of the oxoferrate tellurate K_12+6xFe₆Te_4−xO₂₇ [x=0.222(4)] were synthesized in a potassium hydroxide hydroflux with a molar water–base ratio n(H₂O)/n(KOH) of 1.5 starting from Fe(NO₃)₃ ⋅ 9H₂O, TeO₂ and H₂O₂ at about 200 °C. By using (NH₄)₂TeO₄ instead of TeO₂, a fine powder consisting of microcrystalline spheres of K_12+6xFe₆Te_4−xO₂₇ was obtained. K_12+6xFe₆Te_4−xO₂₇ crystallizes in the acentric cubic space group I 3d. [Fe^IIIO₅] pyramids share their apical atoms in [Fe₂O₉] groups and two of their edges with [Te^VIO₆] octahedra to form an open framework that consists of two loosely connected, but not interpenetrating, chiral networks. The flexibility of the hinged oxometalate network manifests in a piezoelectric response similar to that of LiNbO₃.The potassium cations are mobile in channels that run along the <111> directions and cross in cavities acting as nodes. The ion conductivity of cold-pressed pellets of ball-milled K_12+6xFe₆Te_4−xO₂₇ is 2.3×10⁻⁴ S ⋅ cm⁻¹ at room temperature. Magnetization measurements and neutron diffraction indicate antiferromagnetic coupling in the [Fe₂O₉] groups.     

Influence of Pt and Pd on the catalytic activity of tungsten and molybdenum oxides in the deep oxidation of methane

It has been shown that the phases H_xMO₃ and MO_3−x (M = Mo, W), obtained by reduction of the oxides WO₃ and MoO₃ with hydrogen with supported Pt(Pd) (0.5 mass %), have higher catalytic activity in the deep oxidation of methane than the catalysts Pt/Al₂O₃ and Pd/Al₂O₃ with the same amount of supported metal. At temperatures above 700 K the activity of these catalysts decreases in consequence of the thermal decomposition of the phases H_xMO₃ and MO_3−x and they become similar in activity with Pt(Pd)/Al₂O₃. __________ Translated from Teoreticheskaya i éksperimental’naya Khimiya, Vol. 44, No. 2, pp. 126–129, March–April, 2008.