Oxidation Catalysis over Solid-State Keggin-Type Phosphomolybdic Acid with Oxygen Defects

Electron-Delocalization Across High Surface Entropy Sub-1 nm Nanobelts Toward Enhanced Electrocatalytic Urea Oxidation

Integration of inherently incompatible elements into a single sublattice, resulting in the formation of monophasic metal oxide, holds great scientific promise; it unveils that the overlooked surface entropy in subnanometer materials can thermodynamically facilitate the formation of homogeneous single-phase structures. Here a facile approach is proposed for synthesizing multimetallic oxide subnanometer nanobelts (MMO-PMA SNBs) by harnessing the potential of phosphomolybdic acid (PMA) clusters to capture inorganic nuclei and inhibiting their subsequent growth in solvothermal reactions. Experimental and theoretical analyses show that PMA in MMO-PMA SNBs not only aids subnanometer structure formation but also induces in situ modifications to catalytic sites. The electron transfer from PMA, coupled with the loss of elemental identity of transition metals, leads to electron delocalization, jointly activating the reaction sites. The unique structure makes pentametallic oxide (PMO-PMA SNBs) achieve a current density of 10 mA cm-2 at a low potential of 1.34 V and remain stable for 24 h at 10 mA cm-2 on urea oxidation reaction (UOR). The exceptional UOR catalytic activity suggests a potential for utilizing multimetallic subnanometer nanostructures in energy conversion and environmental remediation.

Structure Transformation of Methylammonium Polyoxomolybdates via In-Solution Acidification and Solid-State Heating from Methylammonium Monomolybdate and Application as Negative Staining Reagents for Coronavirus Observation

We prepared polyoxomolybdates with methylammonium countercations from methylammonium monomolybdate, (CH3NH3)2[MoO4], through two dehydrative condensation methods, acidifying in the aqueous solution and solid-state heating. Discrete (CH3NH3)10[Mo36O112(OH)2(H2O)14], polymeric ((CH3NH3)8[Mo36O112(H2O)14])n, and polymeric ((CH3NH3)4[γ-Mo8O26])n were selectively isolated via pH control of the aqueous (CH3NH3)2[MoO4] solution. The H2SO4-acidified solution of pH < 1 produced "sulfonated α-MoO3", polymeric ((CH3NH3)2[(MoO3)3(SO4)])n. The solid-state heating of (CH3NH3)2[MoO4] in air released methylamine and water to produce several methylammonium polyoxomolybdates in the sequence of discrete (CH3NH3)8[Mo7O24-MoO4], discrete (CH3NH3)6[Mo7O24], discrete (CH3NH3)8[Mo10O34], and polymeric ((CH3NH3)4[γ-Mo8O26])n, before their transformation into molybdenum oxides such as hexagonal-MoO3 and α-MoO3. Notably, some of their polyoxomolybdate structures were different from polyoxomolybdates produced from ammonium molybdates, such as (NH4)2[MoO4] or (NH4)6[Mo7O24], indicating that countercation affected the polyoxomolybdate structure. Moreover, among the tested polyoxomolybdates, (CH3NH3)6[Mo7O24] was the best negative staining reagent for the observation of the SARS-CoV-2 virus using transmission electron microscopy.

Four POM-Viologen Color-Changing Materials with Fast Color Response under Various External Stimuli

Four kinds of polyoxometalate (POM)-viologen compounds were synthesized by hydrothermal method, namely (1-cby)2·[H2(SiMo12O40)]·2H2O (1), (1-cby)2·[H2(SiW12O40)]·2H2O (2), (1-cby)2·(1,1'-bcby)2·{H4[Co4(H2O)2(PW9O34)2]}·12H2O (3), (1-cby)·(1,1'-bcby)·[H(α-PW11O39)CoII(1-cby)]·8H2O (4) (1-cby·Br = 1-Cyclopropylmethyl-[4,4']bipyridinyl-1-ium bromide, 1,1'-bcby·Br = 1,1'-Bis-cyclopropylmethyl-[4,4']bipyridinyl-1-ium bromide). These four POM-viologen compounds exhibit one-dimensional supramolecular network structures. Especially, compound 3 contains a rare sandwich POM subunit {Co4(H2O)2(PW9O34)2}10-. These four compounds can be used as color-changing materials, and they all exhibit noticeable color changes upon exposure to light, heat, and electricity. The discoloration mechanism involves viologen derivatives with electron-deficient properties accepting electrons from POM with electron-rich properties under external stimulation, leading to the formation of viologen free radicals. Among them, compounds 1 and 2 also have good properties for ink-free erasable printing, double anticounterfeiting, and ultraviolet detector because of their rapid color response to ultraviolet (UV) light. In addition, compounds 1-4 also show different color changes in the detection of volatile amines.

Carbon nanofibers confined polyoxometalate derivatives as flexible self-supporting electrodes for robust sodium storage

Flexible self-supporting film electrodes, which eliminate the need for additional adhesive, conductive agents, or current collectors, offer significant advantages in terms of mechanical properties, specific capacity, and energy density for energy storage applications. In this study, we successfully developed a flexible film electrode by incorporating derivatives of Mo and Fe-based polyoxometalates (POMs-D) into carbon nanofibers (CNFs). The integration of CNFs significantly enhanced the structural stability of POMs-D, while the internally formed electrical field facilitated efficient electron transfer, resulting in good performance in sodium storage. The film electrode demonstrated a high capacitive contribution of 90.0 % for sodium uptake/release at a scan rate of 1.0 mV s-1. It maintained a capacity of approximately 170 mA h g-1 even after 8000 cycles at a current density of 3.0 A g-1. Moreover, the film electrode exhibited a decent capacity with a 40.0-fold increase in current density, along with high power capability and energy density in sodium-ion hybrid supercapacitors, showcasing the versatility. These findings unveil the structure-functionality relationship and offer an advanced approach for developing high-performance film electrode materials, opening new possibilities in the fields of material science and energy storage.

Syngas Production by Chemical Looping Dry Reforming of Methane over Ni-modified MoO3 /ZrO2

We investigated supported-MoO3 materials effective for the chemical looping dry reforming of methane (CL-DRM) to decrease the reaction temperature. Ni-modified molybdenum zirconia (Ni/MoO3 /ZrO2 ) showed CL-DRM activity under isothermal reaction conditions of 650 °C, which was 100-200 °C lower than the previously reported oxide-based materials. Ni/MoO3 /ZrO2 activity strongly depends on the MoO3 loading amount. The optimal loading amount was 9.0 wt.% (Ni/MoO3 (9.0)/ZrO2 ), wherein two-dimensional polymolybdate species were dominantly formed. Increasing the loading amount to more than 12.0 wt.% resulted in a loss of activity owing to the formation of bulk Zr(MoO4 )2 and/or MoO3 . In situ Mo K-edge XANES studies revealed that the surface polymolybdate species serve as oxygen storage sites. The Mo6+ species were reduced to Mo4+ species by CH4 to produce CO and H2 . The reduced Mo species reoxidized by CO2 with the concomitant formation of CO. The developed Ni/MoO3 (9.0)/ZrO2 was applied to the long-term CL-DRM under high concentration conditions (20 % CH4 and 20 % CO2 ) at 650 °C, with two pathways possible for converting CH4 and CO2 to CO and H2 via the redox reaction of the Mo species and coke formation.

Preparation and isolation of mono-Nb substituted Keggin-type phosphomolybdic acid and its application as an oxidation catalyst for isobutylaldehyde and Wacker-type oxidation

The potassium and proton mixed salt of mono-Nb substituted Keggin-type phosphomolybdate, KH3[PMo11NbO40], was isolated in a pure form by reacting Keggin-type phosphomolybdic acid (H3[PMo12O40]) and potassium hexaniobate (K8Nb6O19) in water, followed by freeze-drying. The all protonic form, H4[PMo11NbO40], was isolated via proton exchange with H-resin and subsequent freeze-drying. The most crucial factor to isolate KH3[PMo11NbO40] and H4[PMo11NbO40] in pure forms is the evaporation of water using the freeze-drying method. Using a similar procedure, the potassium salt of the di-Nb substituted compound K5[PMo10Nb2O40] was isolated. H4[PMo11NbO40] exhibited high catalytic activity for oxidizing isobutylaldehyde to methacrolein and moderate catalytic activity for the Wacker-type oxidation of allyl phenyl ether when combined with Pd(OAc)2.

Enhanced catalysis of a vanadium-substituted Keggin-type polyoxomolybdate supported on the M3O4/C (M = Fe or Co) surface enables efficient and recyclable oxidation of HMF to DFF

In the reaction of oxidizing 5-hydroxymethylfurfural (HMF), attaining high efficiency and selectivity in the conversion of HMF into DFF presents a challenge due to the possibility of forming multiple products. Polyoxometalates are considered highly active catalysts for HMF oxidation. However, the over-oxidation of products poses a challenge, leading to decreased purity and yield. In this work, metal-organic framework-derived Fe3O4/C and Co3O4/C were designed as carriers for the vanadium-substituted Keggin-type polyoxomolybdate H5PMo10V2O40·35H2O (PMo10V2). In this complex system, spinel oxides can effectively adsorb HMF molecules and cooperate with PMo10V2 to catalyze the aerobic oxidation of HMF. As a result, the as-prepared PMo10V2@Fe3O4/C and PMo10V2@Co3O4/C catalysts can achieve efficient conversion of HMF into DFF with almost 100% selectivity. Among them, PMo10V2@Fe3O4/C exhibits a higher conversion rate (99.1%) under milder reaction conditions (oxygen pressure of 0.8 MPa). Both catalysts exhibited exceptional stability and retained their activity and selectivity even after undergoing multiple cycles. Studies on mechanisms by in situ diffuse reflectance infrared Fourier transform spectroscopy and X-ray photoelectron spectroscopy revealed that the V5+ and Mo6+ in PMo10V2, together with the metal ions in the spinel oxides, act as active centers for the catalytic conversion of HMF. Therefore, it is proposed that PMo10V2 and M3O4/C (M = Fe, Co) cooperatively catalyze the transformation of HMF into DFF via a proton-coupled electron transfer mechanism. This study offers an innovative approach for designing highly selective and recyclable biomass oxidation catalysts.

Defect Engineering of High-Entropy Oxides for Superior Catalytic Oxidation Performance

High-entropy oxides (HEOs) are crucial in various fields (power storage/conversion, electronic devices, and catalysis) owing to their adjustable structural characteristics, fabulous stability, and massive components. However, the current strategies for synthesizing HEOs suffer from low surface area and limited active sites. Herein, we present a salt-assisted strategy with remarkable universality for the preparation of HEOs with high surface area [e.g., HP-(FeCrCoNiCu)xOy: 59 m2/g, HP-(ZnMgNiCuCo)xOy: 49 m2/g, and HP-(CrMnFeNiZn)xOy: 11 m2/g], where HP means high porosity. Especially, HP-(FeCrCoNiCu)xOy with rich-oxygen vacancies promotes catalytic efficiency for hydrocarbon and alcohol oxidation owing to its hierarchical texture and massive oxygen vacancies. Furthermore, density functional theory is utilized to well illustrate the relationship of the structure and catalytic efficiency within the catalysts. This work offers realistic pathway for the large-scale application of HEOs in catalytic areas.

Electron-Delocalization-Stabilized Photoelectrocatalytic Coupling of Methane by NiO-Polyoxometalate Sub-1 nm Heterostructures

The oxidative coupling of methane to C2 oxygenates merits great scientific and technological potential yet remains a challenge due to its inferior selectivity. Subnanomaterials (SNMs) with "p-n-p-n"-type heteroconstructions feature enhanced external field coupling properties and tunable electronic structures, serving as promising catalysts for the selective partial oxidation of methane. Here we develop NiO-polyoxometalate (POM) subnanocoils with a thickness of 1.8 nm, showing excellent catalytic activity toward photoelectrochemical coupling of methane into a C2 product under mild conditions (1 bar, 25 °C) with a notable productivity (up to 4.48 mmol gcat-1 h-1) and a high selectivity (>99%). Under photoelectrochemical coupling, C-H bonds can be activated by NiO, and the resulted *COOH intermediates are stabilized by the delocalized electrons in POM clusters. The contiguous active sites of NiO and POM at the molecular level allow the in situ coupling of *COOH into oxalate. This work points out an economic way for the oxidation of methane under mild conditions and may enlighten the design of functional SNMs from fundamental standpoints.

Tuning Oxygen Vacancies in Oxides by Configurational Entropy

Tuning surface oxygen vacancies is important for oxide catalysts. Doping elements with different chemical valence states or different atomic radii into host oxides is a common method to create oxygen vacancies. However, the concentration of oxygen vacancies in oxide catalysts is still limited to the amount of foreign dopants that can be tolerated (generally less than 10% atoms). Herein, a principle of engineering the configurational entropy to tune oxygen vacancies was proposed. First, the positive relationship between the configuration entropy and the formation energy of oxygen vacancies (Eov) in 16 model oxides was estimated by a DFT calculation. To verify this, single binary oxides and high-entropy quinary oxides (HEOs) were prepared. Indeed, the concentration of oxygen vacancies in HEOs (Oβ/α = 3.66) was higher compared to those of single or binary oxides (Oβ/α = 0.22-0.75) by O1s XPS, O2-TPD, and EPR. Interestingly, the reduction temperatures of transition metal ions in HEOs were generally lower than that in single-metal oxides by H2-TPR. The lower Eov of HEOs may contribute to this feature, which was confirmed by in situ XPS and in situ XRD. Moreover, with catalytic CO/C3H6 oxidation as a model, the high-entropy (MnCuCo3NiFe)xOy catalyst showed higher catalytic activity than single and binary oxides, which experimentally verified the hypothesis of the DFT calculation. This work may inspire more oxide catalysts with preferred oxygen vacancies.

Facile synthesis of polyoxometalate supported on magnetic graphene oxide as a hybrid catalyst for efficient oxidation of aldehydes

In the present study, Anderson-type polyoxometalate [N(C₄H₉)₄] [FeMo₆O₁₈(OH)₆] (FeMo₆) was immobilized on amino-modified magnetic graphene oxide and employed as a new hybrid catalyst in oxidation of aldehydes to carboxylic acids. The synthesized hybrid catalyst Fe₃O₄/GO/[FeMo₆] was characterized using thermogravimetric analysis (TGA), scanning electron microscopies (SEM), Fourier transform infrared (FT-IR), vibrating sample magnetometry (VSM), energy-dispersive X-ray analysis (EDX), Raman spectroscopy and inductively coupled plasma atomic emission spectroscopy (ICP-OES). The results indicated that our catalyst was quite active in oxidizing the aldehydes to their corresponding carboxylic acids in the presence of hydrogen peroxide. The synthesized catalyst can be easily separated from the reaction medium and reused for six consecutive runs without a significant reduction in reaction efficiency.