Struktur und katalytische Eigenschaften von Molybdänoxid-Trägerkatalysatoren bei einigen Oxydationsreaktionen

Structure and Catalytic Properties of Molybdenum Oxide Supported Catalysts in Some Oxidation Reactions Molybdenum supported catalysts were prepared by using different precursor compounds such as Mo(π-C₃H₅)₄, [Mo(OC₂H₅)₅]₂, MoCl₅, (NH₄)₆Mo₇O₂₄, and their catalytic behaviour in some oxidation reactions was studied. During the preparation process, as a result of interaction between the molybdenum compound used and the support, different surface compounds with strongly differing catalytic properties have been formed. MoO₃ and supported catalysts with MoO₃ crystallites on the surface, catalyse the H₂ oxidation at temperatures above 400°C and the CO oxidation at temperatures of about 500°C. The reaction proceeds according to a redox mechanism. On surface compounds of molybdenum which exist on the surface if organic complexes are used as precursors, the catalytic H₂ oxidation occurs even at 100°C with a high reaction rate. The catalytic CO oxidation on these catalysts occurs at temperatures of about 300°C. An associative mechanism on coordinative unsaturated Mo^VI sites is discussed.     

Synthese und Struktur von Mo2NCl7

Synthesis and Structure of Mo₂NCl₇ The reaction of VN with MoCl₅ at 175 °C in a sealed glass ampoule yields the molybdenum(V) nitride chloride Mo₂NCl₇ in form of air sensitive black crystals with the triclinic space group P1¯ and a = 905.7(8); b = 975.4((6); c = 1283.4(8) pm, α = 103.13(4)°; β = 109.83(5)° und γ = 98.58(5)°. The crystal structure is built up from dinuclear units [Mo₂N₂Cl₇]^3— and [Mo₂Cl₇]³⁺, which are connected by asymmetric nitrido bridges to form endless chains. Within both dinuclear units the Mo atoms are bridged by three Cl atoms resulting in a Mo‐Mo distance of 349.2(3) pm in the unit [Mo₂N₂Cl₇]^3—. In case of [Mo₂Cl₇]³⁺, however, a shorter Mo‐Mo distance of 289.4(3) pm is observed, which can be interpreted by a single bond. Correspondingly a reduced magnetic moment of 0.95 B.M. per Mo atom is observed.     

Activation and properties of Mo=O bonds in Mo/SiO2 catalysts

The existence of the charge transfer excited triplet state [Mo⁵⁺-O^-] produced by UV-irradiation of Mo/SiO₂ catalysts, and its reactivity are evidenced by experiments of photoluminescence, photoinduced metathesis, and photoreduction of CO. Mo⁵⁺ ions can be produced separately by thermal activation and O^- ions by further adsorption of N₂O on those Mo⁵⁺ ions. The latter of which are adsorbed on Mo⁶⁺ ions are found to be more reactive than O^2- of [Mo⁶⁺ =O^2-] bond. They are able either to add a molecule such as CO or C₂H₄, or to abstract hydrogen from H₂, CH₄ or trans-dicyanoethylene, or a CN group form tetracyanoethylene (TCNE). The Mo⁵⁺ ions are able to coordinate gas phase ligands when their coordination sphere possesses vacant sites. This is the case for tetracoordinated Mo⁵⁺ _4c ions arising from reduction of tetrahedral Mo⁶⁺ ions (Eq. (7)). These Mo⁵⁺ _4c ions are similar to those produced by UV-irradiaiion (Eq. (2)). In addition, if the adsorbed molecule has a sufficiently large electron affinity, such as TCNE or O₂, an electron transfer can occur (Eq. (9) and (17)). The [Mo⁵⁺-O^-] bond obtained by thermal activation is more difficult to evidence than that obtained with UV-activation because it is not detectable by EPR. However, the EPR results obtained at low temperature show that the O^- ions adsorbed on Mo/SiO₂ catalysts as well as the [Mo⁵⁺-O^-] excited triplet state obtained by UV-irradiation of 1Mo⁶⁺=O²] interact with methanol (Eq. (16)). They are consistent with the mechanism of methanol oxidation occurring at high temperature (Eq. (4)).     

Investigations of molybdenum redox phenomenon in Li2O-MoO3-P2O5 phosphate glasses

Two series of glasses have been prepared and characterized. One with varying Li₂O/P₂O₅ ratio and the other with varying Mo/P ratio. The relationship between the formation of the reduced state of molybdenum in phosphate glasses and the type of gases released in heating batch materials has been investigated. Effect of temperature on the valence state of molybdenum is also studied. Oxidation-reduction (redox) equilibrium of Mo⁵⁺/Mo⁶⁺ and environment of molybdenum (V) in these series of lithium-molybdenum-phosphate glasses are related to the glass composition and the possible structural units formation in the glasses.     

High Intensity Trip-Multiplet Transitions,a Reality! Synthesis,Properties, and Crystal Structure of l-Bis(triphenylphosphine)iminium trans-Dibromophthalocyaninato(2–)molybdate(III)

Oxophthalocyaninato(2–)molybdenum(IV), activated by bromine oxidation prior to use, reacts with fused triphenylphosphine in the presence of bis(triphenylphosphine)iminium bromide to yield linear-bis(triphenylphosphine)iminium trans-dibromophthalocyaninato(2–)molybdate(III), ^l(PNP)^trans[Mo(Br)₂pc^2?]. It crystallizes triclinic with crystal data: a = 10.506(1) Å, b = 12.436(2) Å, c = 12.918(2) Å, α = 76.186(1)°, β = 67.890(1)°, γ = 68.689(1)°; space group P1 (No. 2); Z = 1. Mo^III is in a pseudo-octahedral coordination geometry with the bromo ligands in trans-arrangement. The Mo? N_p and Mo? Br distance is 2.043(10) and 2.588(1) Å, respectively. The PNP cation adopts a linear conformation. In the IR spectrum v_as(Mo? Br) is observed at 218 cm^?1 and v_as(P? N) of the linear (P? N? P) core at 1406 cm^?1. Cyclic and differential-pulse voltammetry show two quasi-reversible cathodic processes at ?1.15 and ?0.53 V vs. Ag/AgCl. The first is assigned to a phthalocyaninate directed reduction (pc^2?/pc^3?), while the latter arises from a Mo directed reduction (Mo^III/Mo^II). Spectral monitoring confirms the reversible Mo^III/Mo^II reduction. Two quasi-reversible anodic processes at 0.60 and 1.27 V are assigned to the successive Mo directed oxidation with redox couples Mo^III/Mo^IV and Mo^IV/Mo^V. For the first time, three very intense spin-allowed trip-quartet transitions are observed in the electronic absorption spectra at 7140 (TQI), 16890 (TQ2) and 18700 cm^?1 (TQ3) together with a sing-quartet transition at 15850 cm^?1 and characteristic ?Q”? region with maximum at 28500 cm^?1 and ?N”? region at 37400 cm^?1. All electronic excitations are of comparable intensity. A prominent low temperature emission at 6690 cm^?1 is assigned to a spin-forbidden trip-sextet.     

Synthesis and properties of (VO)<Subscript>0.09</Subscript>V<Subscript>0.18</Subscript>Mo<Subscript>0.82</Subscript>O<Subscript>3</Subscript> · 0.54H<Subscript>2</Subscript>O microrods

The (VO)_0.09V_0.18Mo_0.82O₃ · 0.54H₂O microrods of hexagonal symmetry system with the unit cell parameters a = 10.586 Å and c = 3.698 Å were obtained for the first time under hydrothermal conditions (T = 160°C, τ = 30?50 h). Particles were 1–2 μm in diameter and up to 45 μm in length. The compound is thermally stable up to 469°C. The core-electron Mo3d, V2p, and O1s and valence-band X-ray photoelectron spectra and IR spectra of the samples were studied. The molybdenum atoms in the complex oxide have the oxidation state Mo⁶⁺. The vanadium atoms introduced into the h-MoO₃ lattice in molybdenum positions have the oxidation state V⁵⁺. Approximately one-third of vanadium atoms as vanadyl ions (VO)²⁺ are located in the channels of h-MoO₃ lattice, thus stabilizing the latter.     

Dopant depends on morphological and electrochemical characteristics of LiMn 2– X Mo X O4 cathode nanoparticles

For the first time, solid solutions of LiMn_2–X Mo _X O₄ nanoparticles were synthesized by combustion method at 700 °C in air. The synthesized LiMn_2–X Mo _X O₄ (X?=?0.0–0.2) nanoparticles were characterized by X-ray powder diffraction, Fourier transform infrared spectroscopy (FT-IR), Field emission-scanning electron microscopy, and Particle size analysis. The unit-cell constant is increasing from 8.237 to 8.293 Å with the increase of Mo, the presence of Mo at X?≤?0.05 in LiMn_2–X Mo _X O₄ nanoparticles retained the spinel structure (Fd-3m), whereas on increasing the Mo (X?≥?0.05 %), the ordering of Li⁺ ions in both octahedral and tetrahedral cationic position leads to the lowering of symmetry (P4₁32). On increasing the Mo content, prominent peak splitting and broadening are observed at 600–500 and 830 cm^?1 for Li–Mn–O and Mo–O respectively in the FT-IR spectra. The TG/DTA spectrum reveals that the convenient formation of Li mangano-molybdate is at 700 °C. The voltammograms of all the samples show two redox peaks centered around 4 V except for the sample with higher Mo doping (X?=?0.2). The sample with X?=?0.03 shows higher redox peak current values. A marginal increase of 146 Ω R _ct value was found for the LiMn_1.97Mo_0.03O₄ nanomaterial after 10th cycle which is rather high for the rest of the materials. A discharge capacity retention of 88 % at 50th cycle is observed for X?=?0.03 sample, while the other samples exhibit drastically reduced capacity. The LiMn_1.97Mo_0.03O₄ nanoparticle can able to deliver higher and constant discharge capacity, and it may be a good alternative for the existing cathode materials.     

Electron transfer. 109. The reaction of dimeric molybdenum(V) with carboxylato-bound chromium(V). A third mechanistic variation

The bridged dimer of molybdenum(V), Mo₂O₄²⁺ (aq) is oxidized to Mo(VI) by carboxylato-bound chromium(V). Reaction of bis(chelated) Cr(V) with excess (Mo^V)₂ yields a chelated Cr(III) complex, but this conversion proceeds through a pink Cr(IV) intermediate, indicating that the oxidation of (Mo^V)₂ entails a series of le^? steps, passing through a reactive transient, the mixed valence complex, Mo^VMo^VI. When experiments are carried out in buffers of the ligating acid, 2-ethyl-2-hydroxybutanoic acid, two stages of ligation of (Mo^V)₂ by the ligand anion, characterized by rate constants near 10⁴ and 0.14 M^?1 s^?1 (19°C; pH 3.0; μ = 0.6 M) must be considered. In quick mixing experiments, the first, but not the second, of these proceeds before the redox reaction gets under way, and autocatalytic redox profiles are observed. If the slower ligation is allowed to reach completion before Cr(V) is added, reduction to Cr(IV) is greatly accelerated and conforms to the superposition of two processes, whereas the reduction of Cr(IV) to Cr(III) is slow and exhibits a rate independent of [Cr^IV]. A proposed sequence applicable to the latter conditions includes reductions of Cr(V) at two ligation levels, slow unimolecular conversion of (Mo^V)₂ to an activated form, and rapid reduction of the latter with Cr(IV). Here Cr(IV) has assumed the role of a scavenger for the reactive form of (Mo^V)₂.     

CO and NO adsorption on VO x /SBA-15 catalysts: an FT-IR spectroscopic study

The adsorption of CO and NO over VO _x -SBA-15 mesoporous materials with different vanadium content was investigated by FT-IR spectroscopy. Vanadium complexes were reduced in situ by hydrogen atmosphere at 450 °C for 3 h. Spectra of reduced samples show increasing in intensity of silanol groups, caused by dissociation of V–O(Si) bonds and formation of new H–O(Si) bonds. Reduction occurs with formation of water. The band corresponds to overtone of V=O stretching modes decreases in intensity because of oxygen withdrawing from V=O species. Presence of V⁴⁺ and V³⁺ species was observed. Inspection of CO adsorbed IR spectra evidenced existence at least two different type of V³⁺–CO complexes on the silica surface differing in both stretching frequencies and complex stabilities. We did not found principal difference between spectra of absorbed CO at ?196 °C on the samples with different concentration of vanadium, probably because of relative low degree of reduction. As well as heterogeneity of surface V³⁺ and V⁴⁺ species was evidenced by adsorption of NO. Both V³⁺ and V⁴⁺ ions possess two effective coordinative vacancies and as a result can adsorb two NO molecules forming dinitrosyls. A part of V³⁺ cations forms only mononitrosyls characterize by band at 1724 cm^?1. Results obtained after NO adsorption reveal existence of three different kinds of vanadium species. Probably two of them are isolated and associated vanadium sites. The third type of vanadium has different surrounding than other two types. It was demonstrated that NO is a better probe than CO for testing the oxidation and coordination state of reduced vanadium species.     

Mechanistic insight into the reactivity of oxotransferases by novel asymmetric dioxomolybdenum(VI) model complexes

The asymmetric molybdenum(VI) dioxo complexes of the bis(phenolate) ligands 1,4‐bis(2‐hydroxybenzyl)‐1,4‐diazepane, 1,4‐bis(2‐hydroxy‐4‐methylbenzyl)‐1,4‐diazepane, 1,4‐bis(2‐hydroxy‐3,5‐dimethylbenzyl)‐1,4‐diazepane, 1,4‐bis(2‐hydroxy‐3,5‐di‐tert‐butylbenzyl)‐1,4‐diazepane, 1,4‐bis(2‐hydroxy‐4‐flurobenzyl)‐1,4‐diazepane, and 1,4‐bis(2‐hydroxy‐4‐chlorobenzyl)‐1,4‐diazepane (H₂(L1)–H₂(L6), respectively) have been isolated and studied as functional models for molybdenum oxotransferase enzymes. These complexes have been characterized as asymmetric complexes of type [MoO₂(L)] 1–6 by using NMR spectroscopy, mass spectrometry, elemental analysis, and electrochemical methods. The molecular structures of [MoO₂(L)] 1–4 have been successfully determined by single‐crystal X‐ray diffraction analyses, which show them to exhibit a distorted octahedral coordination geometry around molybdenum(VI) in an asymmetrical cis‐β configuration. The Mo? O_oxo bond lengths differ only by ≈0.01 Å. Complexes 1 , 2 , 5 , and 6 exhibit two successive Mo^VI/Mo^V (E_1/2, ?1.141 to ?1.848 V) and Mo^V/Mo^IV (E_1/2, ?1.531 to ?2.114 V) redox processes. However, only the Mo^VI/Mo^V redox couple was observed for 3 and 4 , suggesting that the subsequent reduction of the molybdenum(V) species is difficult. Complexes 1 , 2 , 5 , and 6 elicit efficient catalytic oxygen‐atom transfer (OAT) from dimethylsulfoxide (DMSO) to PMe₃ at 65 °C at a significantly faster rate than the symmetric molybdenum(VI) complexes of the analogous linear bis(phenolate) ligands known so far to exhibit OAT reactions at a higher temperature (130 °C). However, complexes 3 and 4 fail to perform the OAT reaction from DMSO to PMe₃ at 65 °C. DFT/B3LYP calculations on the OAT mechanism reveal a strong trans effect.     

Polymerization of acrylonitrile by V5+–lactic acid system in aqueous sulfuric acid

Kinetics of polymerization of acrylonitrile by the redox system V⁵⁺–lactic acid in sulfuric acid at 20–35°C was studied. Oxidation of lactic acid by V⁵⁺ in the absence of monomer was also carried out under identical conditions. The rates of polymerization, V⁵⁺ disappearance and the chainlengths of polyacrylonitrile were measured. From the results it is concluded that the polymerization reaction is initiated by an organic free radical arising from the V⁵⁺–lactic acid reaction with termination by V⁵⁺ ions. Mutual termination of active polymer radicals does not appear to operate under the conditions studied. The various rate parameters were evaluated.     

An infrared study of molybdenum /V/ hydroxide

The IR spectra of paramagnetic Mo/V/ oxygen compounds that are claimed in the literature to be unstable, were measurement in the 4000–3000 and 1200–250 cm^?1 regions. The spectra are poor but show a few bands in the valence and in the OH groups vibrations regions. Mo/V/ hydroxide can be reversibly or irreversibly dehydroxylated thermally in vacuum. The irreversible dehydroxylation of amorphous species containing Mo/V/ hydroxide takes place at >350°C, and is connected with a rearrengement of the atomic OMoO linkage and with crystallization of MoO₃, MoO₂ and Mo₄O₁₁ phases.     

Y-type zeolites for the catalytic oxidative degradation of organic azo dyes in wastewater

Cobalt- and iron-containing catalysts active in the oxidation of organic dyes with hydrogen peroxide have been prepared from granular synthetic NaY and HY zeolites without a binder by ion exchange followed by heat treatment at 350–500°C. It has been demonstrated by X-ray photoelectron spectroscopy that cobalt and iron in these catalysts are in the form of Co²⁺ and Fe³⁺ ions on the support surface. The FeHY and CoNaY catalysts are most effective and stable in the oxidation of the anionic dye carmoisine in weakly acidic and alkaline media.     

Properties and deactivation of the active sites of an MoZSM-5 catalyst for methane dehydroaromatization: Electron microscopic and EPR studies

The MoZSM-5 (4.0 wt % Mo) catalyst has been characterized by high-resolution transmission electron microscopy, EDXA, and EPR. Two types of molybdenum-containing particles are stabilized in the catalyst in the course of nonoxidative methane conversion at 750°C. These are 2-to 10-nm molybdenum carbide particles on the zeolite surface and clusters smaller than 1 nm in zeolite channels. According to EPR data, these clusters contain the oxidized molybdenum form Mo⁵⁺. The surface Mo₂C particles are deactivated at the early stages of the reaction because of graphite condensation on their surface. Methane is mainly activated on oxidized molybdenum clusters located in the open molecular pores of the zeolite. The catalyst is deactivated after the 420-min-long operation because of coke buildup on the zeolite surface and in the zeolite pores.     

Study of reduced Mo/Al2O3 metathesis catalysts prepared via metal complexes

The Mo/Al₂O₃ catalysts for propene metathesis were prepared both via anchoring Mo complexes of various nuclearities and by conventional method of impregnation. The catalysts from metal complexes were found to be active in metathesis at ambient temperature after reduction with H₂ or CO at 400–500°C. The average oxidation state of Mo in the activated catalysts was determined with regard to oxygen consumption needed for oxidation of the reduced Mo species to Mo⁶⁺.     

FT-IR study of the nature and stability of NOx surface species on ZrO2, VOx/ZrO2 and MoOx/ZrO2 catalysts

The adsorption of NO, NO/O₂ mixtures and NO₂ on pure ZrO₂ and on two series of catalysts supported on ZrO₂, one containing vanadia and the other molybdena (ZV and ZMo, respectively), has been investigated. The V and Mo surface contents of the latter were ≤3 atoms nm⁻² and ≤5 atoms nm⁻², respectively. All samples had been previously submitted to a standard oxidation treatment. On all samples, only extremely minor amounts of NO_x surface species are formed by NO interaction at room temperature (RT). NO_x surface species are formed in greater amounts on pure ZrO₂ when NO and O₂ are coadsorbed at RT; they are mainly nitrites, small amounts of nitrates, and small amounts of (O₂NO−H)^δ− species; when ZrO₂ is warmed to 623 K in the NO/O₂ mixture, nitrites decrease, nitrates and (O₂NO−H)^δ− species increase. The same NO_x species as on the ZrO₂ surface free from V (or Mo) are formed on ZV (or ZMo) samples with surface V (or Mo) density <1.5 atoms nm⁻²; however, they occur in decreased amount with increasing V (or Mo) coverage. On ZV samples with a surface V density of 1.5–3 atoms nm⁻² (or ZMo samples with a surface Mo density of 1.5–5 atoms nm⁻²) when NO and O₂ are coadsorbed at RT, there is formation of small amounts of nitrites, nitrates (both on ZrO₂ surface free from V (or Mo) and at the edges of V- or Mo-polyoxoanions) and NO₂ ^δ+ species, associated with V⁵⁺ (or Mo⁶⁺) of very strong Lewis acidity; when samples are warmed up 623 K in the NO/O₂ mixture, nitrites disappear, nitrates increase, NO₂ ^δ+ species remain constant or slightly decrease. When NO₂ is allowed into contact at RT with oxidized samples, surface situations almost identical to those obtained for each sample warmed to 623 K in NO/O₂ mixture is reached. The NO_x surface species stable at 623 K, the temperature at which catalysts show the best performance in the selective catalytic reduction (SCR) of NO by NH₃, are nitrates, both on ZrO₂ and on polyvanadates or polymolybdates at high nuclearity. On the contrary, nitrites and NO₂ ^δ+ species are unstable at 623 K.     

High‐Performance Chemically Regenerative Redox Fuel Cells Using a NO3−/NO Regeneration Reaction

In this study, we proposed high‐performance chemically regenerative redox fuel cells (CRRFCs) using NO₃⁻/NO with a nitrogen‐doped carbon‐felt electrode and a chemical regeneration reaction of NO to NO₃⁻ via O₂. The electrochemical cell using the nitrate reduction to NO at the cathode on the carbon felt and oxidation of H₂ as a fuel at the anode showed a maximal power density of 730 mW cm⁻² at 80 °C and twofold higher power density of 512 mW cm⁻² at 0.8 V, than the target power density of 250 mW cm⁻² at 0.8 V in the H₂/O₂ proton exchange membrane fuel cells (PEMFCs). During the operation of the CRRFCs with the chemical regeneration reactor for 30 days, the CRRFCs maintained 60 % of the initial performance with a regeneration efficiency of about 92.9 % and immediately returned to the initial value when supplied with fresh HNO₃.     

Unexpectedly complex structure and dynamic behavior of bis [(η5-cyclopentadienyl)dicarbonyl molybdenum ]-μ-pent-3-yne (MoMo)

The title compound, (η⁵-C₅H₅)₂Mo₂(CO)₄ (μ-EtCCEt) in its crystalline form, has a molecular structure that lacks symmetry. Two (ηs-C₅H₅)Mo groups are connected by an MoMo bond 2.977(1) Å in length with the usual sort of crosswise acetylene bridge. There are two terminal CO groups on Mo(1) and one on Mo(2). The fourth CO group is in a semi-bridging posture with Mo(2)—C = 1.936(6) and Mo(1)—C = 2.826(6). The ¹³CO NMR spectrum at ?126°C has six lines, indicative of the presence of two isomers, in each of which two CO's are either statically or dynamically equivalent. Complex changes occur in the spectrum as the temperature is increased so that at ?40°C there is only a single line. A detailed interpretation of these spectra is not yet available.     

Relationship between V4+ and Mo6+ Contents in V2O5 doped with MoO3

The V⁴⁺ content in V₂O₅ doped with MoO₃ is measured by a spectroscopic method. The influence of the oxygen pressure is also considered. Up to roughly 3.5 at.-% Mo/(Mo+V) the V⁴⁺ fraction is equal to the Mo⁶⁺ fraction for samples sintered in air. Increase of PO₂ gives a decrease in the measured values of the V⁴⁺ fraction for the 5, 10 and 33 at.-% Mo-doped samples.     

Synthese und Kristallstruktur von (PPh4)2[Mo2NCl9]2, einem μ-Nitridokomplex mit Molybdän(V), und -(VI)

Synthesis and Crystal Structure of (PPh₄)₂[Mo₂NCl₉]₂, a μ-Nitrido Complex with Molybdenum (V) and (VI) The title compound is formed as a by-product in the partial oxidation of Mo₂NCl₇ with chlorine in POCl₃ solution, when the reaction mixture is treated with PPh₄Cl. The crystals, which are sensitive to moisture, are black in reflectance and red in transmittance. A more effective synthesis is the direct reaction of PPh₄[MoNCl₄] with MoCl₅ in dichloro methane. (PPh₄)₂[Mo₂NCl₉]₂ was characterized by the i.r. spectrum and by a structural analysis with X-ray data. The compound crystallizes triclinic in the space group P1 with two formula units per unit cell (9225 independent observed reflexions, R = 0.058). The cell parameters are (20°C): a = 1144 pm, b = 1517 pm, c = 2000 pm, α = 79.8°, β = 80.1°, γ = 72.1°. (PPh₄)₂[Mo₂NCl₉]₂ consists of PPh₄⊕ cations and the anions [Mo₂NCl₉]₂^22?, which dimerize via chloro bridges with Mo? Cl bons lengths of 243 pm and 287 pm. In the [Mo₂NCl₉]₂^2? units the molybdenum atoms are linked by Mo^VI?N? Mo^V bridges (bond angles 179° and 174°, resp.) with Mo? N bond lengths of 167 pm and 212 pm.