Group VIB Metal Research Articles

Coordination Structure Modulation in Group-VIB Metal Doped Ag3PO4 Augments Active Site Density for Electrocatalytic Conversion of N2 to NH3.

Doping is considered a promising material engineering strategy in electrochemical nitrogen reduction reaction (NRR), provided the role of the active site is rightly identified. This work concerns the doping of group VIB metal in Ag3PO4 to enhance the active site density, accompanied by d-p orbital mixing at the active site/N2 interface. Doping induces compressive strain in the Ag3PO4 lattice and inherently accompanies vacancy generation, the latter is quantified with positron annihilation lifetime studies (PALS). This eventually alters the metal d-electronic states relative to Fermi level and manipulate the active sites for NRR resulting into side-on N2 adsorption at the interface. The charge density deployment reveals Mo as the most efficient dopant, attaining a minimum NRR overpotential, as confirmed by the detailed kinetic study with the rotating ring disk electrode (RRDE) technique. In fact, the Pt ring of RRDE fails to detect N2H4, which is formed as a stable intermediate on the electrode surface, as identified from in-situ attenuated total reflectance-infrared (ATR-IR) spectroscopy. This advocates the complete conversion of N2 to NH3 on Mo/Ag3PO4-10 and the so-formed oxygen vacancies formed during doping act as proton scavengers suppressing hydrogen evolution reaction resulting into a Faradaic efficiency of 54.8% for NRR.

Synthesis and Crystal Structures of Novel Group 6B Metal Carbonyl Complexes with N‐(4‐methyl‐2‐pyridinyl)‐P,P‐diphenylphosphinous Amide Ligand

AbstractThe reaction of N‐(4‐methyl‐2‐pyridinyl)‐P,P‐diphenylphosphinous amide ligand 4‐Me‐C5H3N‐2‐NH(PPh2) (1) with M(CO)6 (M=Cr, Mo, W) in refluxing toluene afforded a mononuclear group VIB metal tetracarbonyls [M(CO)4{1‐κ2P,N}] (M=Cr(2), Mo(3), W(4)). Complexes 2–4 were characterized by multinuclear NMR (1H, 13C and 31P NMR) and IR spectroscopy, and their structures were also determined by single crystal X‐ray diffraction. 2 and 4 represent the first examples of structurally characterized complexes of such ligands with group 6B metal carbonyls.

Influence of group VIB metals introduced into zeolite on the adsorption performance of VOCs in cooking oil fumes

Series samples of the MEL topology zeolite containing VIB group metals (VIB-S2) were synthesized via direct hydrothermal method for adsorption of volatile organic compounds (VOCs) from cooking oil fumes. Compared with silicalite-2 (S2, 66.24 mg/g) and ZSM-11 (Al-S2, 25.98 mg/g), VIB-S2 samples showed significantly enhanced adsorption capacity of non-methane hydrocarbons (NMHC) from cooking oil fumes (103.96 mg/g for Cr-S2, 95.88 mg/g for Mo-S2, 88.68 mg/g for W-S2). VIB group metals changed the framework structure and texture characteristics of zeolite, reduced the number of defects and hydroxyl group, and increased the proportion of non-microporous structures and increased the number of adsorption sites in VIB-S2. Consequently, VIB-S2 showed excellent organophilicity, hydrophobicity and distinct VOCs adsorption behavior to S2 and Al-S2. Systematic and quantitative correlation analysis of these factors showed that the adsorption capacity of NMHC was significantly positively correlated with hydrophobicity (correlation coefficient was 0.91, significance coefficient < 0.05), which is closely related to the non-microporous structure of samples.

The structure of deformation twins in BCC transition metals during nucleation and growth

Nucleation and growth of deformation twins in BCC transition metals have been investigated in many computational and experimental studies without conclusively describing the critical twin nuclei thickness and its subsequent growth. While most studies conclude that the thickness is three layers by assuming conservation of Burgers vector and nucleation from a screw dislocation, recent experimental studies have contradicted this, suggesting a minimum of six layers. Here we demonstrate, under nonzero finite pure shear stresses, that the minimum thickness during twin nucleation is two layers in Group VB and VIB metals whereas it is three layers in iron. The twinning dislocations that constitute the nuclei have been determined which are dependent on metal's electronic structure i.e, position in periodic table. Growth of twins occurs in a layer-by-fashion maintaining isosceles twin boundary structures in Group VB metals and Fe whereas the more commonly acknowledged reflection twin boundaries are favored in Group VIB.

Reactions of Group VIB metal carbonyls with two novel rigid macrocyclic Schiff base ligands incorporating acridine inter-crossing group

Two new rigid macrocyclic Schiff base ligands incorporating acridine inter-crossing group [1,1′-(acridine-3,6-diylbis(azaneylylidene))bis(methaneylylidene))bis(naphthalen-2-ol), L1, and N,N'-(acridine-3,6-diyl)bis(1-(quinolin-2-yl)methanimine), L2, were synthesized and fully characterized. The X-ray single crystal analysis of L1 explored that the compound crystallized in a monoclinic crystal system with a space group P21/c. The compound was crystalized with five water molecules. Interestingly, L1 occurred as a double zwitterion, where the hydrogen of the OH group of the hydroxynaphthalene transferred to the azomethine nitrogen to form NH+ and O− moieties with the presence of hydrogen bonding between them. Hirshfeld surface analysis of the L1 was also investigated. Reactions of the two ligands with the Group VIB metal carbonyls in air were investigated. In order to characterize the novel compounds, various analytical and spectroscopic methods were used. The analysis revealed that the complexes were formed as oxo derivatives with unique structural features. The presented compounds conducted fluorescence quenching investigations, viscosity evaluations, antimicrobial and antioxidant properties. It was concluded that the binding interaction with DNA was electrostatic binding mode rather than intercalative mode. Molecular docking of L1 and its complexes were investigated versus three macromolecular receptors (PDB: 4BJP, 1BQB and 1BNA). These targets were selected to compare the experimental biological data with the results of the docking analysis.

Unveiling the mechanism of controllable CO2 hydrogenation by group VIB metal single atom anchored on N-doped graphite: A density functional theory study

CO2 hydrogenation has raised considerable interest due to concerns about climate change. Realizing low-temperature reverse water gas shift (rWGS) reaction remains a significant challenge in the context of coupling it with the C–C growth reactions to convert CO2 to C2+ fuels. We carried out systematic DFT simulations to unveil the underlying low-temperature mechanism for the selective hydrogenation of CO2 to produce CO, over a variety of metal-based single atom catalysts (SACs) supported on the nitrogen-doped graphite. Group VIB metal-based SACs outperformed other 15 metal candidates in terms of versatile capacities in both selective activation of CO2 molecule and facilitating escaping of CO and H2O. Mo1/N3-Gt was especially outstanding by giving rise to spontaneous production of CO and O∗ through an effective electron injection into the CO2 molecule. Water formation has been identified as the potential rate-controlling step in such a catalytic reaction over Mo1/N3-Gt with an energy barrier of 1.10 eV. Herein, the H migration played a pivotal role and had tight affinity to the charge of H∗ on the active site of catalyst. The dynamic coordination environment of Moδ+ was revealed to be the dominant factor affecting the surface H∗ charge, leading to a variety of hydrogenation behaviors. The electron-deficient ligands of CO2 and O∗ on Mo1/N3-Gt, as well as additional adsorbed H2, were effective in adjusting the 4d and 5s electronic structure of central Mo and consequently resulted in nearly electric neutral surface H∗s, thus most benefiting the hydrogenation process. The optimal charge of the coordinated Mo for an outstanding selective hydrogenation performance in this scenario was found to be no less than +1.7e.

A novel approach of methane dehydroaromatization using group VIB metals (Cr, Mo, W) supported on sulfated zirconia

AbstractMethane dehydroaromatization (MDHA) is a direct activation approach to covert methane to value-added chemicals in a single step. This requires no intermediate step, making it a commercially economic approach. Mo supported on HZSM-5/MCM-22 is a well-studied catalyst for this reaction, where Mo sites are responsible for activating methane to C2Hy dimers, which can oligomerize on HZSM-5 Bronsted acid sites to produce aromatics. Challenges for these bifunctional catalysts involve rapid coking and low product yield. In this study, a novel catalytic approach is introduced using group VIB metals (Cr, Mo, W) supported on sulfated zirconia (SZ) solid acid. It is believed that the Bronsted acidity of SZ should help to convert the dimers generated from metal sites to ethylene and aromatics like benzene.Here, fresh Mo, W and Cr were doped into SZ and characterized using pyridine DRIFTS, ammonia TPD, BET and SEM-EDS.. Catalytic activity for MDHA was ranked as Mo&gt;W&gt;Cr. Mo/SZ showed greater selectivity towards ethylene and benzene, followed by W/SZ, which was selective primarily towards ethylene. Cr/SZ showed the least activity under similar reaction conditions, producing only a small amount of ethylene. Higher catalytic activity for Mo/SZ was possibly due to reduced Mo oxide sites, found from XANES analysis, as well as higher acidity, observed from TPD. Deactivation was mainly due to coking, observed from subsequent TPO analysis. Further investigation is necessary to enhance the activity of this novel catalytic approach before considering for potential industrial applications.

Novel electronic properties of monoclinic MP4 (M = Cr, Mo, W) compounds with or without topological nodal line

Transition metal phosphides hold novel metallic, semimetallic, and semiconducting behaviors. Here we report by ab initio calculations a systematical study on the structural and electronic properties of hbox {MP}_4 (M = Cr, Mo, W) phosphides in monoclinic C2/c (C_{2h}^6) symmetry. Their dynamical stabilities have been confirmed by phonon modes calculations. Detailed analysis of the electronic band structures and density of states reveal that hbox {CrP}_4 is a semiconductor with an indirect band gap of 0.47 eV in association with the p orbital of P atoms, while hbox {MoP}_4 is a Dirac semimetal with an isolated nodal point at the Gamma point and hbox {WP}_4 is a topological nodal line semimetal with a closed nodal ring inside the first Brillouin zone relative to the d orbital of Mo and W atoms, respectively. Comparison of the phosphides with group VB, VIB and VIIB transition metals shows a trend of change from metallic to semiconducting behavior from hbox {VB-MP}_4 to VIIB-hbox {MP}_4 compounds. These results provide a systematical understandings on the distinct electronic properties of these compounds.

Influence of group VIB metals on activity of the Ni/MgO catalysts for methane decomposition

Mesoporous 55 wt.% Ni/MgO and group VIB metal (Cr, Mo or W)-doped catalysts were prepared using the facile "one-pot" evaporation-induced self-assembly in ethanol and wetness impregnation methods, respectively. Physicochemical properties of the as-prepared catalysts were characterized by means of the BET, XRD, SEM, TEM, HAADF, XPS, H2-TPR, TPO, and Raman techniques. The doping of Cr to 55 wt% Ni/MgO improved catalytic activity and stability for the decomposition of methane under harsh reaction conditions. The maximal initial methane conversions of 80, 87, and 75% were achieved over the 55 wt.% Ni−xCr/MgO (denoted as 55Ni−xCr/MgO; x = 5, 10, and 15 wt.%) at 675 oC and GHSV = 48,000 mL/(g h), respectively. It is concluded that the high catalytic efficiency of the Cr-doped sample in methane decomposition was associated with its high surface area, high oxygen adspecies concentration, and good low-temperature reducibility. In addition, multi-walled carbon filaments with metal-encapsulated carbon particles were deposited on the Cr-doped catalysts, and the ordered carbon filaments with different diameters and good crystallinity were produced on the surface of the 55Ni−10Cr/MgO catalyst.

Time evolution of ion fluxes incident at the substrate plane during reactive high-power impulse magnetron sputtering of groups IVb and VIb transition metals in Ar/N2

Reactive transition-metal (TM) nitride film growth employing bias-synchronized high power impulse magnetron sputtering (HiPIMS) requires a detailed knowledge of the time evolution of metal- and gas-ion fluxes incident at the substrate plane in order to precisely tune momentum transfer and, hence, provide the recoil density and energy necessary to eliminate film porosity at low deposition temperatures without introducing significant film stress. Here, the authors use energy- and time-dependent mass spectrometry to analyze the evolution of metal- and gas-ion fluxes at the substrate plane during reactive HiPIMS sputtering of groups IVb and VIb TM targets in Ar/N2 atmospheres. The time- and energy-integrated metal/gas ion ratio NMe+/Ng+ incident at the substrate is significantly lower for group IVb TMs (ranging from 0.2 for Ti to 0.9 for Hf), due to high N2 reactivity which results in severely reduced target sputtering rates and, hence, decreased rarefaction. In contrast, for less reactive group VIb metals, sputtering rates are similar to those in pure Ar as a result of significant gas heating and high NMe+/Ng+ ratios, ranging from 2.3 for Cr to 98.1 for W. In both sets of experiments, the peak target current density is maintained constant at 1 A/cm2. Within each TM group, NMe+/Ng+ scales with increasing metal-ion mass. For the group-VIb elements, sputtered-atom Sigmund-Thompson energy distributions are preserved long after the HiPIMS pulse, in contradistinction to group-IVb TMs for which the energy distributions collapse into narrow thermalized peaks. For all TMs, the N+ flux dominates that of N2+ ions, as the molecular ions are collisionally dissociated at the target, and N+ exhibits ion energy distribution functions resembling those of metal ions. The latter result implies that both N+ and Me+ species originate from the target. High-energy Ar+ tails, assigned to ionized reflected-Ar neutrals, are observed with heavier TM targets.

Synthesis and inner-sphere hydrophosphorylation of (η1-N)-N-benzyl-N-(1,3-dimethylbut-2-enylidene)amine-(dicarbonyl)(1,3,5-triorganyl-1,3,5-triazacyclohexane) molybdenum(0) and -tungsten(0)

Complexes of group VIb metals (Mo and W) has been prepared with N-benzyl-(3-methylbut-2-enylidene)amine, where the latter is coordinated with the dicarbonyl(1,3,5-triorganyl-1,3,5-triazacyclohexane)-metal(0) fragment exclusively through the lone electron pair of the imine nitrogen atom. The reactions of the synthesized metal complexes with diethyl phosphite in various conditions always involved the addition of the phosphite across the С=N bond to form, depending on the conditions, aminophosphonates or their inner-sphere rearrangement products amidophosphates. Both organophosphorus compounds remain in the coordination sphere of the transition metal complex.

Synthesis and characterization of group VIB metal carbonyl heteroacene complexes

Group VIB metal carbonyl heteroacene complexes were synthesized and characterized. Single crystal X-ray diffraction confirms that all metal carbonyl complexes bond to the ligand through the phenanthroline unit. Their absorption spectra are red-shifted compared to that of the ligand and their cyclic voltammetry revealed that their reduction occurred at less reducing potentials than the corresponding reduction in ligand.

He–He and He–metal interactions in transition metals from first-principles

We investigated the atomistic mechanism of He–He and He–metal interactions in bcc transition metals (V, Nb, Ta, Cr, Mo, W, and Fe) using first-principles methods. We calculated formation energy and binding energy of He–He pair as function of distance within the host lattices. The strengths of He–He attraction in Cr, Mo, W, and Fe (0.37–1.11 eV) are significantly stronger than those in V, Nb, and Ta (0.06–0.17 eV). Such strong attractions mean that He atoms would spontaneously aggregate inside perfect Cr, Mo, W, and Fe host lattices in absence of defects like vacancies. The most stable configuration of He–He pair is <100> dumbbell in groups VB metals, whereas it adopts close <110> configuration in Cr, Mo, and Fe, and close <111> configuration in W. Overall speaking, the He–He equilibrium distances of 1.51–1.55 Å in the group VIB metals are shorter than 1.65–1.70 Å in the group VB metals. Moreover, the presence of interstitial He significantly facilitates vacancy formation and this effect is more pronounced in the group VIB metals. The present calculations help understand the He-metal/He–He interaction mechanism and make a prediction that He is easier to form He cluster and bubbles in the groups VIB metals and Fe.

Energetics and configurations of He–He pair in vacancy of transition metals

We report an atomistic study of energetics and configurations of He–He pair in vacancy of bcc transition metals (V, Nb, Ta, Cr, Mo, W, and Fe) using first-principles methods. The results show He-vacancy attractions in the group VB metals are 2–3 times weaker than those in Cr, Mo, W and Fe. The 〈111〉 dumbbell configuration for He–He pair in vacancy is the most stable except for V. Calculated formation energies of He–He pair in vacancy of group VB metals (3.2–3.95eV) are systematically lower than those of group VIB and VIIIB metals (4.25–5.67eV), while He–He distance for V metal is greater than other metals. He–metal repulsion is stronger than He–He due to longer He–metal distance in metal vacancy. Calculated densities of states provide a reasonable explanation for most stable configuration; thus the stability of He–He pairs depends strongly on the electronic structure of the host and insignificantly on its atomic size. Moreover, the group-specific trends of He–He and self-interstitial configurations are discussed.

3+2] Cycloadditions of Molybdenum(II) Azide Complexes with Nitriles and an Alkyne

A series of products of [3+2] cycloaddition reactions of molybdenum azide complexes with nitriles and an alkyne were prepared and characterized. These include the tetrazolate complexes Mo(η3-C3H5)(CO)2(en)(N4CR) (R = CH3 (1), C6H5 (2), C(CH3)3 (3), CH═CHCN (4)) and the triazolate complexes Mo(η3-C3H5)(CO)2(L2){N3C2(CO2CH3)2} (L2 = en (6), dppe (7)). They are the first examples of a complex with a heterocyclic ligand prepared via a reaction of a group VIB metal azide with an unsaturated dipolarophile. All these complexes are fluxional in solution, with those containing en ligands displaying broad unresolved proton NMR signals at room temperature. With the exception of 1, their solution behavior was studied by low-temperature NMR spectroscopy. In contrast, the dppe complexes display average NMR signals at room temperature due to a fast exchange. The solid-state structures of these cycloaddition products were also determined by single-crystal X-ray diffraction analysis. The tetrazolate complexes 1, 2, and 4 ...

ChemInform Abstract: Diphosphazanes as Ligands. Symbiosis of Phosphorus Chemistry and Organometallic Chemistry

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Photochemical reactions of group VIB metal carbonyls with heterocyclic Schiff bases derived from 4-amino -3-hydrazino-5-mercapto-1,2,4-triazole

The new complexes, M(CO)5(Schiff base) [M = Cr; 1, Mo; 2, W; 3, Schiff base = 4-salicylidenamino-3-hydrazino-5-mercapto-1,2,4-triazole, SAHMT, a; 4-(2-hydroxynaphthylidenamino)-3-hydrazino-5-mercapto-1,2,4-triazole, 2HNAHMT, b; 4-(3-hydroxybenzylidenamino)-3-hydrazino-5-mercapto-1,2,4- triazole, 3HBAHMT, c; 4-(4-hydroxybenzylidenamino)-3-hydrazino-5-mercapto-1,2,4- triazole, 4HBAHMT, d; 4-(5-bromosalicylidenamino)-3-hydrazino-5-mercapto-1,2,4-triazole, 5BrSAHMT, e; were synthesized by photochemical reaction of metal carbonyls M(CO)6 (M = Cr, Mo, W) with new heterocyclic Schiff bases derived from 4-amino-3-hydrazino-5-mercapto-1,2,4-triazole, a–e. The ligands and complexes have been characterized by elemental analysis, EI-mass spectrometry, FT-IR, 1H and 13C-{1H}-NMR spectroscopy. The spectroscopic studies show that Schiff bases, a–e, are monodentate and coordinate via azomethine N donor to the central metal atom in M(CO)5(Schiff base) (M = Cr, Mo, W).

Molecular and Electronic Structures, Brönsted Basicities, and Lewis Acidities of Group VIB Transition Metal Oxide Clusters

The structures and properties of transition metal oxide (TMO) clusters of the group VIB metals, (MO(3))(n) (M = Cr, Mo, W; n = 1-6), have been studied with density functional theory (DFT) methods. Geometry optimizations and frequency calculations were carried out at the local and nonlocal DFT levels with polarized valence double-zeta quality basis sets, and final energies were calculated at nonlocal DFT levels with polarized valence triple-zeta quality basis sets at the local and nonlocal DFT geometries. Effective core potentials were used to treat the transition metal atoms. Two types of clusters were investigated, the ring and the chain, with the ring being lower in energy. Large ring structures (n > 3) were shown to be fluxional in their out of plane deformations. Long chain structures (n > 3) of (CrO(3))(n) were predicted to be weakly bound complexes of the smaller clusters at the nonlocal DFT levels. For M(6)O(18), two additional isomers were also studied, the cage and the inverted cage. The relative stability of the different conformations of M(6)O(18) depends on the transition metal as well as the level of theory. Normalized and differential clustering energies of the ring structures were calculated and were shown to vary with respect to the cluster size. Brönsted basicities and Lewis acidities based on a fluoride affinity scale were also calculated. The Brönsted basicities as well as the Lewis acidities depend on the size of the cluster and the site to which the proton or the fluoride anion binds. These clusters are fairly weak Brönsted bases with gas phase basicities comparable to those of H(2)O and NH(3). The clusters are, however, very strong Lewis acids and many of them are stronger than strong Lewis acids such as SbF(5). Brönsted acidities of M(6)O(19)H(2) and M(6)O(18)FH were calculated for M = Mo and W and these compounds were shown to be very strong acids in the gas phase. The acid/base properties of these TMO clusters are expected to play important roles in their catalytic activities.

Synthesis, Structure, and Hydrophosphorylation of π‐Complexes Derived from tert‐Butyl(1,3‐dimethyl‐2‐butenylidene)amine and Homoligand Iron(0) and Group VIb Metal Carbonyls.

New iron(0), chromium(0), molybdenum(0), and tungsten(0) π-complexes containing η2-, η 3-, and η4-coordinated tert-butyl(1,3-dimethyl-2-butenylidene)amine ligands were synthesized under conditions of photochemical and thermal activation. The geometric, electronic, and energetic parameters of the coordinated 1,3-azadiene were calculated by nonempirical methods, and factors determining the mode of coordination were established. The obtained π-complexes reacted with dialkyl phosphonates in the presence of triethylamine to afford organometallic α-aminophosphonates; hydrophosphorylation in the absence of a catalyst gave the corresponding amidophosphate which then underwent rearrangement into α-aminophosphonate.

Phase stability and alloying behavior in the Mo-Si-B system

The effect of transition metal (TM) substitution for Mo has been examined in terms of the phase stability and multiphase microstructures in the Mo-Si-B ternary system. The metal-rich portion of the ternary Mo-Si-B system at equilibrium is comprised of thermally stable bcc Mo(ss) phase, a ternary-based Mo5SiB2 (T2) phase, and a binary-based metal-rich silicide (Mo3Si [A15]). The structures that are developed by following systematic alloying with a wide range of TMs, which are substitutional in both Mo(ss) and T2 phases (group IVB, VB, and VIB metals), have been analyzed to elucidate the roles of the substitution on the stability of the three phase fields of Mo(ss) + T2 + Mo3Si. In particular, the borosilicide ternary-based T2 phase shows an extended solid solution with a wide range of TMs. The extended solubility in the T2 phase essentially mimics the alloying behavior of the TM-based bcc phase. The critical factor for the phase stability appears to be the existence of a unique feature of bcc-like TM clusters within the T2 lattice structure. The combined criteria of atomic size factor and the valence electron concentration per atom (e/a) have been used to elucidate the observed alloying behavior.