Mo Center Research Articles

Synergy of Single Atoms and Lewis Acid Sites for Efficient and Selective Lignin Disassembly into Monolignol Derivatives.

Lignin is the most abundant aromatic polymer from the natural and renewable lignocellulosic biomass resource. Developing highly efficient catalysts for lignin depolymerization to produce valuable monophenols with high yield and selectivity remains a desirable but challenging target in this field. Here, we design a synergistic catalyst combining atomically dispersed Mo centers and Al Lewis acid sites on a MgO substrate (Mo1Al/MgO) for the depolymerization of Eucalyptus lignin via the β-aryl ether bond cleavage. A near-theoretical monophenol yield of 46% with an ultrahigh selectivity of 92% for coniferyl and sinapyl methyl ether, as well as good cycling durability, was achieved simultaneously by Mo1Al/MgO in an inert N2 atmosphere. First-principles calculations and control catalytic experiments confirmed the synergistic catalysis mechanism between Mo1-O5 single-atom centers and the neighboring Al Lewis acid sites with the participation of a methanol solvent. This study validates the feasibility of designing better-performing catalysts with synergistic multiactive sites for the efficient and selective disassembly of complex renewable biopolymers into highly value-added products with lower cost and greater security.

Material of oxo-peroxo-molybdenum (VI) complex involving 4-Furoyl-3-methyl-1-phenyl-2-pyrazoline-5- one: experimental cum theoretical observations

We report here the experimental and theoretical investigation of the bis(4-furoyl-3-methyl-1-phenyl-2-pyrazolin-5-one)oxoperoxomolybdenum(VI) complex molecule. It was prepared by the reaction of (2:1) 4-furoyl-3-methyl-1-phenyl-2-pyrazolin-5-one and [MoO(O)2]2+ in an aqueous ethanol medium. Characterization was performed by elemental analysis, molar conductivity, magnetic measurements, electrochemical analysis, and infrared and electronic spectral studies. Theoretical validation was performed by density functional theory calculations using B3LYP as the LANL2DZ function. The molecular geometry results show a distorted pseudo-pentagonal bipyramidal geometry together with the O7 coordination mode around the Mo(VI) center. The FMO energies allow the determination of the atomic and molecular parameters and also represent the charge transfer across the molecule.

MoSn2Se4-decorated MXene/functionalized RGO nanohybrid for ultrastable supercapacitor and oxygen evolution catalyst

Multifunctional MXene-based electrode with good electrocatalytic performance in oxygen evolution reaction as well as superior supercapacitor performance is rare so far. In this study, Mo and/or Sn selenides were grown on a template of MXene (Ti3C2Tx)/amine functionalized reduced graphene oxide (NH2-RGO) binary composite material via facile hydrothermal method. The presence of Mo and Sn metal centers has a crucial significance to increase the reactive sites over Ti3C2Tx/NH2-RGO nanosheets. Further, the Ti3C2Tx/NH2-RGO binary structure provides a conducting matrix to the metal selenides and prevents their agglomeration tendency. Superior electrocatalytic activity was obtained from Ti3C2Tx/NH2-RGO/MoSn2Se4 electrocatalyst with an onset potential of 1.3 V [vs. Reversible Hydrogen Electrode (RHE)], lower overpotential (η10) of 50 mV, large electrochemically active surface area, and smaller Tafel slope of 55.9 mV/dec with durable electrocatalytic performance. The assembled asymmetric supercapacitor device using our synthesized material as cathode showed excellent performance with high specific capacitance (120.2 F/g), large energy density (42.7 W/h/kg), and ultrastability with 97% capacitance retention after 10,000 charging–discharging cycles. The prepared ternary composite has been synthesized for the first time and has also performed better than the previous reports of its kind. This work provides an overall insight into designing an electrode material for both efficient oxygen evolution as well as high performance supercapacitor device.

Gradient oxygen-injecting MoS2 nanosheets catalyst boosting reductive C-N coupling of nitroarenes: Mechanistic insight into activity reconstruction

It is very appealing to be able to improve the intrinsic activity of a low-cost MoS2 catalyst and broaden the adaptability of substrates in heterogeneous catalytic C-N bond formation from nitro compounds. A gradient oxygen-injection method was used in this study to regulate the electronic structure of the in-plane Mo site in the inert basal plane of the 2H-MoS2 nanosheet, resulting in a remarkable activity reconstruction in the reductive C-N coupling of nitroarenes with aromatic boronic acids. In the lattice of MoS2, gradient oxygen substitution results in structured changes of the in-plane Mo site, including crystal distortion brought on by asymmetric coordination and additive intergrown interfacial MoS2 and MoO3, which in turn contributes to the synergistic highly-active reactive sites of MoS2. Additionally, the interesting secondary activity reconstruction of the catalyst in the toluene solvent during the reaction canreplace partial S sites with reactive intermediate oxygen, which results in a subsequent improvement in activity of the used catalyst. The effectiveness of the modified catalyst and the crucial role of the introduced oxygen concentration and alternate position around the Mo site of MoS2 in determining the adsorption and activation of nitro compound were validated through the implementation of various reaction experimental controls and density functional theory calculations. The N-O bond of nitrobenzene was specifically made weaker and easier to break by the addition of adjacent oxygen atoms around the Mo center, which aided the subsequent coupling process. Through altering the coordination environment of the active metal in the reductive coupling reaction of nitroarenes, this work offers some novel findings and new insights into the activity reconstruction of Mo-based catalysts.

Role of Multiple Vanadium Centers on Redox Buffering and Rates of Polyvanadomolybdate-Cu(II)-Catalyzed Aerobic Oxidations.

A recent report established that the tetrabutylammonium (TBA) salt of hexavanadopolymolybdate TBA4H5[PMo6V6O40] (PV6Mo6) serves as the redox buffer with Cu(II) as a co-catalyst for aerobic deodorization of thiols in acetonitrile. Here, we document the profound impact of vanadium atom number (x = 0-4 and 6) in TBA salts of PVxMo12-xO40(3+x)- (PVMo) on this multicomponent catalytic system. The PVMo cyclic voltammetric peaks from 0 to -2000 mV vs Fc/Fc+ under catalytic conditions (acetonitrile, ambient T) are assigned and clarify that the redox buffering capability of the PVMo/Cu catalytic system derives from the number of steps, the number of electrons transferred each step, and the potential ranges of each step. All PVMo are reduced by varying numbers of electrons, from 1 to 6, in different reaction conditions. Significantly, PVMo with x ≤ 3 not only has much lower activity than when x > 3 (for example, the turnover frequencies (TOF) of PV3Mo9 and PV4Mo8 are 8.9 and 48 s-1, respectively) but also, unlike the latter, cannot maintain steady reduction states when the Mo atoms in these polyoxometalate (POMs) are also reduced. Stopped-flow kinetics measurements reveal that Mo atoms in Keggin PVMo exhibit much slower electron transfer rates than V atoms. There are two kinetic arguments: (a) In acetonitrile, the first formal potential of PMo12 is more positive than that of PVMo11 (-236 and -405 mV vs Fc/Fc+); however, the initial reduction rates are 1.06 × 10-4 s-1 and 0.036 s-1 for PMo12 and PVMo11, respectively. (b) In aqueous sulfate buffer (pH = 2), a two-step kinetics is observed for PVMo11 and PV2Mo10, where the first and second steps are assigned to reduction of the V and Mo centers, respectively. Since fast and reversible electron transfers are key for the redox buffering behavior, the slower electron transfer kinetics of Mo preclude these centers functioning in redox buffering that maintains the solution potential. We conclude that PVMo with more vanadium atoms allows the POM to undergo more and faster redox changes, which enables the POM to function as a redox buffer dictating far higher catalytic activity.

Precisely optimizing polysulfides adsorption and conversion by local coordination engineering for high-performance Li-S batteries

Incorporating electrocatalysts into lithium-sulfur (Li-S) batteries is a promising strategy to relieve the deleterious shuttle effect and sluggish conversion kinetics of lithium polysulfides (LiPSs). However, atomic modulation of the electrocatalysts to boost the catalytic activity is still challenging because of their intrinsic structural complexity. Herein, we report the theoretical prediction and experimental realization of Mo single atoms with different ligand coordinations (Mo-NxC3−x), wherein we find that the LiPSs adsorption and conversion are well modulated by the atomic coordination species of single Mo centers. The resultant Mo-N2C1 displays a moderate bonding strength with LiPSs in comparison with the Mo-N1C2 and Mo-N3 counterparts, which facilitates the charge transfer kinetics and reduces the Li2S precipitation/decomposition energy barrier. Consequently, the constructed Li-S batteries with Mo-N2C1 present a durable cyclability with a low capacity decay rate of 0.055% each cycle over 1000 cycles at a high current rate of 10 C and a decent areal capacity of 4.27 mAh cm−2 after 100 cycles with a low electrolyte/sulfur ratio of 8 µL mg−1. This work demonstrates that optimizing LiPSs adsorption and conversion through local composition and coordination modulation is an effective strategy for developing efficient and durable electrocatalysts for advanced Li-S batteries.

Heterostructured Mo and Co-based phosphides as high-performance bifunctional electrocatalysts for overall water splitting.

Transitional metal phosphides are efficient and durable electrocatalysts for water splitting. In this work, Mo-CoP/Co2P/NF heterostructures are reported to exhibit bifunctional electrocatalyst properties in various electrolytes. The Co phosphides were found to be possessing a hydrogenase-like structure in these heterostructures with P as the proton-acceptor site and Co as the hydride-acceptor site, making them highly active during the HER process. Moreover, the electronic structure of Co ions could be modified, or the transfer of electrons could be accelerated due to the different valence states of Mo. Additionally, the Mo centers possessed superior adsorption properties toward hydrogen. Consequently, excellent performance for the electrocatalytic HER was exhibited by the Mo-CoP/Co2P/NF-300 heterostructure with small overpotentials of 86.6 mV and 48 mV at a current density of 10 mA cm-2 in 1.0 M KOH and 0.5 M H2SO4 solutions, respectively. Furthermore, it also exhibited efficient OER activity in an alkaline solution with a low overpotential of 245 mV at 30 mA cm-2. Post-analysis revealed the changes in the surface and the formation of Co oxyhydroxide during the OER process, and the formation of Co-P-O during the HER process. The high HER and OER performances are attributed to these transformations of morphologies and compositions. Consequently, a two-electrode electrolyzer based on Mo-CoP/Co2P/NF-300 required voltages of 1.59 V and 1.703 V at 20 mA cm-2 and 100 mA cm-2, respectively, and maintained long-term stability.

Nitrogen Reduction to Ammonia by a Phosphorus-Nitrogen PN 3 P-Mo(V) Nitride Complex: Significant Enhancement via Ligand Postmodification

Nitrogen Reduction to Ammonia by a Phosphorus-Nitrogen PN ³ P-Mo(V) Nitride Complex: Significant Enhancement via Ligand Postmodification

Enhanced Heterogeneous Fenton-like Process for Sulfamethazine Removal via Dual-Reaction-Center Fe-Mo/rGO Catalyst

A heterogeneous Fenton-like catalyst with single redox site has a rate-limiting step in oxidant activation, which limited its application in wastewater purification. To overcome this, a bimetallic doping strategy was designed to prepare a heterogeneous Fenton-like catalyst (Fe-Mo/rGO) with a double-reaction center. Combined with electrochemical impedance spectroscopy and density functional theory calculation, it was confirmed that the formation of an electron-rich Mo center and an electron-deficient Fe center through the constructed Fe-O-Mo and Mo-S-C bonding bridges induced a higher electron transfer capability in the Fe-Mo/rGO catalyst. The designed Fe-Mo/rGO catalyst exhibited excellent sulfamethazine (SMT) degradation efficiency in a broad pH range (4.8-8.4). The catalytic performance was hardly affected by inorganic anions (Cl-, SO42- and HCO3-) in the complicated and variable water environment. Compared to Fe/rGO and Mo/rGO catalysts, the SMT degradation efficiency increased by about 14.6 and 1.6 times in heterogeneous Fenton-like reaction over Fe-Mo/rGO catalyst. The electron spin resonance and radical scavenger experiments proved that ·O2-/HO2· and 1O2 dominate the SMT removal in the Fe-Mo/rGO/H2O2 system. Fe and Mo, as active centers co-supported on rGO, significantly enhanced the electron transfer between catalyst, oxidant, and pollutants, which accelerated the reactive oxygen species generation and effectively improved the SMT degradation. Our findings offer a novel perspective to enhance the performance of heterogeneous Fenton-like catalysts by accelerating the electron transfer rate in the degradation of organic pollutants.

Spin regulation for efficient electrocatalytic N2 reduction over diatomic Fe-Mo catalyst

Electrocatalytic nitrogen reduction reaction (eNRR) is a promising method for the sustainable production of ammonia as an alternative to the traditional energy-intensive Haber-Bosch process. In this work, an efficient strategy by atomic spin regulation to promote NRR through Fe-transition metal (TM) hybrid heteronuclear dual-atom catalysts has been studied. By means of DFT computations, the stability, activity, and selectivity of 30 kinds of Fe-based dual-atoms anchored on N-doped porous graphene are systematically investigated to evaluate their catalytic performance. Fe/MoNC is screened as an excellent NRR catalyst with the limiting potentials of 0.63 V, and also suppresses HER. In the Fe/MoNC, the neighboring Fe atom regulates the spin state of the Mo center in MoN4 from high-spin state (2.63 μB) to medium-spin state (0.74 μB), which can effectively relieve the strong overlapping between Mo 4d orbital with the NxHy intermediates, promote the desorption of reaction product, and eventually achieve a lower limiting potential. Interestingly, the archetype of the active center of nitrogenase is also a FeMo-cofactor, which is consistent with our screening results. The work may provide new insight into the mechanism of nitrogenase, and promote the rational design of efficient NRR catalysts by atomic spin regulation.

Total Synthesis of the Tetracyclic Pyridinium Alkaloid epi-Tetradehydrohalicyclamine B.

The first total synthesis of a tetracyclic marine pyridinium alkaloid hinged on recent advances in chemoselectivity management: While many classical methods failed to afford the perceptively simple pyridine-containing core of the target, nickel/iridium photoredox dual catalysis allowed the critical C-C bond to be formed in good yield. Likewise, ring closing alkyne metathesis (RCAM) worked well in the presence of the unhindered pyridine despite the innately Lewis acidic Mo(+6) center of the alkylidyne catalyst. Finally, an iridium catalyzed hydrosilylation was uniquely effective in reducing a tertiary amide without compromising an adjacent pyridine and the lateral double bonds; this transformation is largely without precedent. The second strained macrocycle enveloping the core was closed by intramolecular N-alkylation with formation of the pyridinium unit; the reaction proceeded site- and chemoselectively in the presence of an a priori more basic tertiary amine.

Total Synthesis of the Tetracyclic Pyridinium Alkaloid epi‐Tetradehydrohalicyclamine B

AbstractThe first total synthesis of a tetracyclic marine pyridinium alkaloid hinged on recent advances in chemoselectivity management: While many classical methods failed to afford the perceptively simple pyridine‐containing core of the target, nickel/iridium photoredox dual catalysis allowed the critical C−C bond to be formed in good yield. Likewise, ring closing alkyne metathesis (RCAM) worked well in the presence of the unhindered pyridine despite the innately Lewis acidic Mo(+6) center of the alkylidyne catalyst. Finally, an iridium catalyzed hydrosilylation was uniquely effective in reducing a tertiary amide without compromising an adjacent pyridine and the lateral double bonds; this transformation is largely without precedent. The second strained macrocycle enveloping the core was closed by intramolecular N‐alkylation with formation of the pyridinium unit; the reaction proceeded site‐ and chemoselectively in the presence of an a priori more basic tertiary amine.

Structural Exploration and Protein Binding Efficiencies of Binuclear Dioxidomolybdenum(VI) Complexes Constructed from ONO Chelator and Linear N−N Ditopic Spacer

Binuclear dioxido Mo(VI) complexes(1 and 2) of general formula [(MoO2L1−2)2(bpe)] have been synthesized by varying a auxiliary bridging ligand on the primary ligand framework. The impact of spacer variation on metal-organic supramolecular architectures, DFT calculations and BSA binding efficiencies of the complexes has been studied. The intermolecular interactions in the crystal structures are also studied by Hirshfeld surface analysis. FRET, Time-resolved fluorescence measurement and molecular docking of the complexes were also carried out.

Phase Engineering and Alkali Cation Stabilization for 1T Molybdenum Dichalcogenides Monolayers

AbstractThe technological barriers of the dimensional engineering and interfacial instability seriously hinder the scalable production of metallic (1T) transition metal dichalcogenides (TMD) monolayers. In this article, a facile and fast electron injection strategy is developed to modulate the d orbits of Mo center in trigonal prismatic 2H phases (MoS2 and MoSe2); meanwhile various cations (Li+, Na+, and K+) reinforce the in‐plane 1T‐atomic arrangement and expand the out‐of‐plane spacing for easy exfoliation. Theoretical and experimental evaluations further elucidate the decisive electron‐donating capability and suitable ionic radius in stabilizing 1T coordination. The as‐tailored 1T‐MoS2/MoSe2 anodes can achieve the robust Na+ storage in the half cells (5000 cycles at 5 A g–1) and extreme power output of 3134.9 W kg–1 in the full cell. This phase‐engineering approach enables the precise dimensional manipulation of the 1T TMDs, which further extends their application horizons as the cation host for the power‐oriented battery systems.

Surface Anchoring and Active Sites of [Mo3S13]2- Clusters as Co-Catalysts for Photocatalytic Hydrogen Evolution.

Achieving light-driven splitting of water with high efficiency remains a challenging task on the way to solar fuel exploration. In this work, to combine the advantages of heterogeneous and homogeneous photosystems, we covalently anchor noble-metal- and carbon-free thiomolybdate [Mo3S13]2– clusters onto photoactive metal oxide supports to act as molecular co-catalysts for photocatalytic water splitting. We demonstrate that strong and surface-limited binding of the [Mo3S13]2– to the oxide surfaces takes place. The attachment involves the loss of the majority of the terminal S22– groups, upon which Mo–O–Ti bonds with the hydroxylated TiO2 surface are established. The heterogenized [Mo3S13]2– clusters are active and stable co-catalysts for the light-driven hydrogen evolution reaction (HER) with performance close to the level of the benchmark Pt. Optimal HER rates are achieved for 2 wt % cluster loadings, which we relate to the accessibility of the TiO2 surface required for efficient hole scavenging. We further elucidate the active HER sites by applying thermal post-treatments in air and N2. Our data demonstrate the importance of the trinuclear core of the [Mo3S13]2– cluster and suggest bridging S22– and vacant coordination sites at the Mo centers as likely HER active sites. This work provides a prime example for the successful heterogenization of an inorganic molecular cluster as a co-catalyst for light-driven HER and gives the incentive to explore other thio(oxo)metalates.

Insight into the methylene C–H bond cleavage of ethylbenzene during ethylbenzene hydroxylation using EBDH as a catalyst, a DFT study

The hydroxylation of ethylbenzene to ( S)-1-phenylethanol with the help of ethylbenzene dehydrogenase (EBDH) is a stereospecific catalytic reaction. This hydroxylation process involves the C–H bond cleavage of methylene part of ethylbenzene and transfer of its hydrogen to the oxygen atom attached with the metal at the active site of EBDH as a first step, which leads to the formation of an intermediate. The second step involves the transfer of OH from the active-site metal back to the carbon of intermediate, resulting in the formation of ( S)-1-phenylethanol. This C–H bond cleavage could be homolytic or heterolytic and directly affect the reaction mechanism of ethylbenzene hydroxylation. In this article, density functional theory studies were performed on the ethylbenzene-bound EBDH active-site model complexes to investigate the impact of the C–H bond cleavage of methylene part of ethylbenzene on the reaction mechanism of ethylbenzene hydroxylation. For this, different protonation states and participation of amino acid residues near the Mo center of EBDH were considered. Models with protonation of His192, Lys450, and Asp223 and model without protonation were investigated for comparison. Computed relative energies indicate that the overall lowest energy barrier pathway results when ionic (heterolytic) and radical (homolytic) pathways are combined.

A Mixed‐Addenda Mo/W Organofunctionalised Hybrid Polyoxometalate

AbstractA mixed‐addenda W/Mo hybrid polyoxometalate cluster – K6[P2W15Mo2O61(POC6H5)2] (1) was synthesised from the condensation of K10[P2W15Mo2O61] and PO3C6H7 under acidic conditions. Single‐crystal X‐ray diffraction confirmed the structure of the hybrid cluster and the presence of two Mo centres in the cap of the lacunary cluster. The electronic effects of metal substitution were studied by cyclic voltammetry, spectroelectrochemistry and electron paramagnetic resonance spectroscopy and supported by density functional theory calculations. Comparing 1 to its tungsten‐only analogue K6[P2W17O61(POC6H5)2] (2), a more positive potential for the first reduction process was induced by the substitution of W for Mo, consistent with a significant lowering of the cluster LUMO energy.

Molybdenum bound nitrogen‐doped graphene catalyst for reduction of N2 to NH3 and NH2NH2, using FLP as a co‐catalyst: A DFT study

Chemists have long been pursuing the aim of synthesising practically viable low temperature catalysts with high stability, activity and selectivity towards the generation of ammonia under less expensive reaction conditions by using H2 and N2. In this regard, we have used molybdenum bound nitrogen‐doped graphene as a catalyst in the present computational investigation. We developed a new concept of the Frustrated Lewis Pair [AH]+[BH]− (FLP‐H2), which can be used as a source of hydrogen to functionalise the coordinated dinitrogen into ammonia. Furthermore, the utilisation of FLP‐H2 can prevent the competition in binding of N2 and H2 with Mo centre and enable the product formation. Our computed thermodynamic and kinetic barriers clearly reveal that the molybdenum bound nitrogen‐doped graphene can be used as a catalyst to activate and functionalise the dinitrogen. In the proposed catalytic cycle, H+ and H− can be added in an alternate manner using the FLP‐H2 source to yield NH3 and NH2NH2. Some thermodynamically viable penta‐coordinated [M]H intermediates have also been identified in the current study.

S K-edge XAS of CuII, CuI, and ZnII oxidized Dithiolene complexes: Covalent contributions to structure and the Jahn-Teller effect

Reduced dithiolene ligands are bound to high valent Mo centers in the active site of the oxotransferase family of enzymes. Related model complexes have been studied with great insight by Prof. Holm and his colleagues. This study focuses on the other limit of dithiolene chemistry: an investigation of the 2-electron oxidized dithiolene bound to low-valent late transition metal (TM) ions (ZnII, CuI, and CuII). The bonding descriptions of the oxidized dithiolene [N,N-dimethyl piperazine 2,3-dithione (Me2Dt0)] complexes are probed using S K-edge X-ray absorption spectroscopy (XAS) and the results are correlated to density functional theory (DFT) calculations. These experimentally supported calculations are then extended to explain the different geometric structures of the three complexes. The ZnII(Me2Dt0)2 complex has only ligand-ligand repulsion so it is stabilized at the D2d symmetry limit. The CuI(Me2Dt0)2 complex has additional weak backbonding thus distorts somewhat from D2d toward D2h symmetry. The CuII(Me2Dt0)2 complex has a strong σ donor bond that leads to both a large Jahn-Teller stabilization to D2h and an additional covalent contribution to the geometry. The combined strong stabilization results in the square planar, D2h structure. This study quantifies the competition between the ligand-ligand repulsion and the change in electronic structures in determining the final geometric structures of the oxidized dithiolene complexes, and provides quantitative insights into the Jahn-Teller stabilization energy and its origin.

Iron and zinc porphyrin linked MoO(dithiolene) complexes in relevance to electron transfer between Mo-cofactor and cytochrome b5 in sulfite oxidase.

Oxo-molybdenum (dithiolene) complexes covalently linked individually to iron and zinc porphyrin have been synthesized to show an electron transfer between the two metal centres in relevance to electron transfer from Mo-cofactor to cytochrome b5 domains in the oxidative half of the catalytic cycle of native sulfite oxidase. This association has been investigated by electrochemical, EPR measurement and X-ray absorbance spectroscopy techniques.