MALDI-TOF Mass Spectrometry of Nanosized MoO<Subscript>2</Subscript>. Structure and Relative Stability of Isomers of Lower Molybdenum Oxide Cations

MALDI-TOF was used to study molybdenum dioxide (MoO₂) containing a nanosized fraction. The composition of cationic clusters of nonstoichiometric lower molybdenum oxides in the gas phase was determined, and the thermodynamic stabilities and configurations of isomers were calculated for selected symmetric molecular structures and for cations MoSO ₈ ⁺ and Mo₅O ₉ ⁺ . Molecular orbital analysis was performed for two trigonal-bipyramidal clusters Mo₅O₈ and Mo₅O₉. Changes in molybdenum–molybdenum interatomic distances in going from MoO ₈ ⁺ and Mo₅O ₉ ⁺ cations to neutral clusters are discussed.     

Studies on molybdenum carbide supported HZSM-5 (Si/Al = 23, 30, 50 and 80) catalysts for aromatization of methane

Here in, for the first time we are reporting molybdenum carbide reduction into metallic molybdenum during methane aromatization on HZSM-5 (Si/Al ratio = 23, 30, 50 and 80) at methane space velocity of 1800 mL.g_cat⁻.h⁻. Benzene yield was influenced by the surface metallic molybdenum through the non-aromatic carbon deposits formation via linear hydrocarbons degradation on HZSM-5 with fewer acidity (Si/Al ratio = 30, 50 and 80). Our XPS analysis results demonstrated improved surface metallic molybdenum in spent Mo₂C/HZSM-5 = 80 (0.71 atom. %) and 50 (0.54 atom. %) samples over Mo₂C/HZSM-5 = 30 (0.33 atom. %) and 23 (0.20 atom. %) samples. Furthermore, HR-TEM and FFT analysis images clearly established fine distribution of distorted spherical shaped Mo₂C particles with 6–14 nm size in spent Mo₂C/HZSM-5 = 23. On the other hand, Mo₂C particle size was increased upto 22 nm in Mo₂C/HZSM-5 = 80. The ease reduction of Mo₂C into metallic molybdenum and aggregation of Mo₂C particles in spent higher Si to Al ratio (50 and 80) samples was associated with weak interactions between Mo₂C and the HZSM-5 with fewer acidity. At 700 °C, the order of benzene yield as follows: Mo₂C/HZSM-5 = 80 (2.2%) < Mo₂C/HZSM-5 = 50 (3.25%) < Mo₂C/HZSM-5 = 30 (5.2%) < Mo₂C/HZSM-5 = 23 (8.0%).     

Effect of the formation of secondary pores in zeolite ZSM-5 on the properties of molybdenum-zeolite catalysts for methane aromatization

A study is performed of 4% Mo/ZSM-5 (30) catalysts for methane aromatization prepared by solid-phase synthesis with mechanical mixing of a zeolite with MoO₃ followed by calcination at 550°C. Zeolite etched with sodium hydroxide solutions and dealuminated with aluminum nitrate solutions is used as a support. Catalytic studies of the catalysts are conducted. The effect of treating the initial zeolite on the properties of catalysts in methane aromatization is determined. The effect subsequently treating a zeolite support has on the acid sites of a catalyst is confirmed by means of temperature-programmed reduction and the temperature-programmed desorption of NH3. The formation of molybdenum ions in the +5 oxidation state during catalysis and the presence of graphitized carbon deposits on a spent catalyst’s surface are confirmed by EPR and temperature-programmed oxidation.     

A facile preparation of nanocrystalline Mo2C from graphite or carbon nanotubes

Nanocrystalline Mo₂C powders were successfully synthesized at 500 °C by reacting molybdenum chloride (MoCl₅) with C (graphite or carbon nanotube) in metallic sodium medium. X-ray powder diffractometer (XRD), transmission electron microscope (TEM), X-ray photoelectron spectroscope (XPS) and surface area analyzer (BET method) were used to characterize the samples. Experiments reveal that the carbon source used for the carbide synthesis has a great effect on the particle size and the surface area of the samples. When micro-sized graphite was used as C source the obtained nanocrystalline Mo₂C powder consists of particles of 30∼100 nm, with a surface area of 2.311 m²/g. When carbon nanotubes were used as C source, the as-synthesized Mo₂C sample is composed of particles of 20∼50 nm, with a surface area of 23.458 m²/g, which is an order of magnitude larger than that of the carbide prepared from the graphite.     

Nonoxidative conversion of methane into aromatic hydrocarbons on Ni-Mo/ZSM-5 catalysts

The nonoxidative conversion of methane into aromatic hydrocarbons on high-silica zeolites ZSM-5 containing nanosized powders of molybdenum (4.0 wt %) and nickel (0.1–2.0 wt %) was studied. Data on the acid characteristics of the catalysts and the nature and amount of coke deposits formed on the surface of the catalysts were obtained using the thermal desorption of ammonia and thermal analysis. The microstructure and composition of Ni-Mo/ZSM-5 catalysts were studied by high-resolution transmission electron microscopy and energy-dispersive X-ray analysis. The formation of various chemical species in the samples was detected: oxide-like clusters of Mo within zeolite channels (∼1 nm), molybdenum carbide particles (5–30 nm) on the outer surface of the zeolite, and Ni-Mo alloy particles with different compositions (under reaction conditions, carbon filaments grew on these particles). It was found that, as the Ni content was increased from 0.1 to 2.0 wt %, the rate of deactivation of the catalytic system increased because of blocking pores in the zeolite structure by filamentous carbon up to the formation of condensed coke deposits.     

XPS study of potassium-promoted molybdenum carbides for mixed alcohols synthesis via CO hydrogenation

The X-ray photoelectron spectroscopy (XPS) was used to investigate the surface characteristic of potassium-promoted or un-promoted both β-Mo_2C and α-MoC_(1-x) pretreated by syngas at different temperatures, and the promotional effect of potassium on the catalytic performance was also studied. XPS results revealed that the content of surface Mo and its valence distribution between β-Mo_2C and α-MoC_(1-x) were quite different. Promoted by potassium, the remarkable changes were observed for surface composition and valence of Mo distribution over β-Mo_2C. Potassium had strong electronic effect on β-Mo_2C, which led to a higher Mo~(4+) content. On the contrary, potassium had little electronic effect on α-MoC_(1-x), and K-Mo interaction was weak. Therefore, Mo~0 and Mo~(2+) became the dominant species on the catalyst surface, and the Mo~(4+) content showed almost no increase as the pretreatment temperature enhanced. In terms of catalytic performance of molybdenum carbides, the increase in Mo~0 most likely explained the increase in hydrocarbon selectivity, yet Mo~(4+) might be responsible for the alcohols synthesis.     

Preparation and characterization of bimetallic PdMo/Y-zeolite: catalytic properties in methane combustion

This work deals with the preparation and the characterization of palladium and palladium–molybdenum supported on HY and NaY zeolites, with the aim to study the effect of molybdenum on the properties of palladium. Catalytic performances were tested in the reaction of methane combustion. The introduction of molybdenum in palladium exchanged zeolites NaY and HY was realized in dynamic or static regime (under vacuum) using Mo(CO)₆ vapor at ambient temperature. Pd was found to migrate in supercages under the influence of Mo(CO)₆, which produces by decomposition, Mo⁵⁺ species revealed by EPR spectroscopy and consequently palladium was reduced. Catalytic results show that the activity of PdHY increases with time during a relatively long period compared to the other samples. This activation in stream can be attributed to a slow migration of palladium to supercages. Nevertheless, PdHY and PdMoNaY were less active than PdNaY at 500 °C. The catalytic activity of monometallic samples increases with time, whereas it decreases for bimetallic ones. The comparison of the catalytic activities of Pd and PdMo supported on NaY and HY suggests that the basicity of the support enhances the oxidation ability of palladium by an increase of the electronic density of the metal particles at the surface. The pretreatment conditions exerted also a great effect on the behavior of mono and bimetallic catalysts. The reduction in hydrogen at 500 °C led to a decrease of the combustion activity depending on the nature of the catalyst.     

Mo2B4O9—Connecting Borate and Metal-Cluster Chemistry

We report on the first thoroughly characterized molybdenum borate, which was synthesized in a high-pressure/high-temperature experiment at 12.3 GPa/1300 °C using a Walker-type multianvil apparatus. Mo₂B₄O₉ incorporates tetrahedral molybdenum clusters into an anionic borate crystal structure—a structural motif that has never been observed before in the wide field of borate crystal chemistry. The six bonding molecular orbitals of the [Mo₄] tetrahedron are completely filled with 12 electrons, which are fully delocalized over the four molybdenum atoms. This finding is in agreement with the results of the magnetic measurements, which confirmed the diamagnetic character of Mo₂B₄O₉. The two four-coordinated boron sites can be differentiated in the ¹¹B MAS-NMR spectrum because of the strongly different degrees of local distortions. Experimentally obtained IR and Raman bands were assigned to vibrational modes based on DFT calculations.     

Low Temperature WGS Catalysts for Hydrogen Station and Fuel Processor Applications

Generally, water gas shift (WGS) reaction is a very important step in the industrial production of hydrogen, ammonia and other bulk chemicals utilizing synthesis gases. In this paper, we are reporting WGS reaction carried out in our research group for the application of hydrogen station and fuel processor. We prepared various Mo₂C, Pt–Ni-based and Cu-based catalysts for low temperature WGS reaction. The characteristics of the prepared catalyst were analyzed by N₂ physisorption, CO chemisorptions, XRD, SEM and TEM technologies, and compared with that of commercial Cu-Zn/Al₂O₃ catalyst. It was found that prepared catalysts displayed reasonably good activity and thermal cycling stability than commercial LTS (Cu–Zn/Al₂O₃) catalyst. It was found that the deactivation of commercial LTS catalyst during the thermal cycling run at 250 °C was caused by the sintering of active metal even though it shows high activity at less than 250 °C. The deactivation of Mo₂C catalyst during the thermal cycling run was caused by the transition of Mo^δ+, Mo^IV and Mo₂C on the surface of Mo₂C catalyst to Mo^VI(MoO₃) with the reaction of H₂O in reactants. However, they showed higher stability than the commercial LTS catalyst during thermal cycling test. The Pt–Ni/CeO₂ catalyst after the thermal cycling shows slightly deactivation due to the sintering of Ni metal. Among Cu-based catalysts, it was found that Cu–Mo/Ce_0.5Zr_0.5O₂ catalyst has higher WGS activity and stability over commercial LTS catalyst. The results suggested that Pt–Ni/CeO₂ and Cu–Mo/Ce_0.5Zr_0.5O₂ catalysts are desirable candidates for application in hydrogen station and fuel processor system even though all other catalysts deactivated slowly during the thermal cycling run.     

Electron Paramagnetic Resonance Study on Supported Molybdenum Catalysts. Part 1: Interaction of Molybdenum Oxide with Silica and Signal Anlsotropy

The effects of reduction, evacuation and adsorbate adsorption on Mo/SiO₂ catalysts prepared by impregnation were studied by means of the electron paramagnetic resonance (EPR) technique. Pentavalent molybdenum species in the overlayer (Mo₀⁵⁺, g? = 1.941, g| = 1.895) and at the interface (Mo_i⁵⁺, g? = 1.952, g| = 1.866) between MoO₃ and SiO₂ were distinguished in the EPR spectra; they were observed from samples reduced at T < 573 K and T > 573 K respectively. The difference in their nature reflected the extent of their interaction with the support. Mo₀⁵⁺ existed in the MoO3 overlayer of the catalyst and interacted only weakly with the support. Mo_i⁵⁺ from the Mo ions connected directly to SiO₂ was affected strongly by the support. The anisotropic factor of the measured Mo_i⁵⁺ signals increased with the extent of support interaction. Superoxide ions (O₂, g₁ = 2.016, g₂ = 2.011, g₃ = 2.006) were found upon oxygen adsorption on samples evacuated at temperatures above 573 K. The observed O₂ may exist at the interface between MoO₃ and the support.     

Porous Molybdenum Carbide Nanostructured Catalyst toward Highly Sensitive Biomimetic Sensing of H2O2

There are great challenges to fabricate a highly selective and sensitive enzyme‐free biomimetic sensor. Herein for the first time a unique nanostructure of porous molybdenum carbide impregnated in N‐doped carbon (p‐Mo₂C/NC) is synthesized by using SiO₂ nanocrystals‐templating method and is further used as an enzyme‐free electrochemical biosensor toward highly selective, sensitive detection of H₂O₂, of which the limit of detection, dynamic detection range and sensitivity accomplish as 0.22 μM, 0.05–4.5 mM and 577.14 μA mM^?1 cm^?2, respectively, and are much superior to the non‐porous molybdenum carbide impregnated in N‐doped carbon (Mo₂C/NC). The sensor is also used to monitor H₂O₂ released from A549 living cells. This work holds a great promise to be used to monitor the presence of H₂O₂ in biological research while offering an important knowledge to design a highly selective and sensitive biomimetic sensor by synthesizing a porous catalyst to greatly improve the reaction surface area rather than conventionally only relying on dispersing the catalyst material into porous carbon substrate.     

The disordered cluster compound CaMo5(Mo0.38Ti0.62)O10

The title compound, calcium pentamolybdenum titanium decaoxide, is isomorphous with the AMo₅(Ti_0.7Mo_0.3)O₁₀ (A = Sr and Eu) compounds. The smaller size of calcium induces a higher molybdenum content on the capping sites of the bioctahedral Mo₁₀ clusters, leading to more Mo₁₁ and Mo₁₂ clusters in the crystal structure. The oxygen framework derives from the stacking of close‐packed layers along the a direction in the …ABAC… sequence. The Ca²⁺ ions occupy large cavities which result from the fusion of two cubooctahedra and are surrounded by ten O atoms. The Ti⁴⁺ ion is octahedrally coordinated by the O atoms.     

Graphene Oxide Supported Molybdenum Cluster: First Heterogenized Homogeneous Catalyst for the Synthesis of Dimethylcarbonate from CO2 and Methanol

The octahedral molybdenum cluster‐based compound, Cs₂Mo₆Brⁱ₈Br^a₆ was immobilized on graphene oxide (GO) by using a facile approach. High resolution transmission electron microscopy results revealed that molybdenum clusters were uniformly distributed on the GO nanosheets. Cs₂Mo₆Brⁱ₈Br^a₆ was attached to the GO support via chemical interaction between apical ligands of Mo₆Brⁱ₈Br^a₆ cluster units and oxygen functionalities of GO, as revealed by XPS studies. The developed material was used for the synthesis of dimethyl carbonate by reduction of carbon dioxide. The synthesized catalyst, that is, GO–Cs₂Mo₆Brⁱ₈Br^a_x, exhibited higher catalytic efficiency than its homogeneous analogue without using dehydrating agent. The catalyst was found to be efficiently recyclable without significant loss of catalytic activity.     

Catalytic Mo<Subscript>2</Subscript>C coatings for the water gas shift reaction: Electrosynthesis in molten salts

Electrosynthesis methods using molten salts are suggested for obtaining a new catalytic system based on the Mo₂C/Mo composition for the water gas shift reaction. The coatings obtained by the discharge of the carbonate ion on a molybdenum substrate and by the simultaneous reduction of the electroactive species MoO ₄ ^2? and CO ₃ ^2? are catalytically more active than bulk Mo₂C or the commercial catalyst Cu-ZnO-Al₂O₃ by one and three orders of magnitude, respectively.     

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)).     

Influence of Molybdenum on Ceria Activity and CO2 Selectivity in Propene Total Oxidation

A total oxidation of propene into CO₂ is obtained on pure ceria at 673 K. However, in the presence of molybdenum, propene can be partially oxidized at room temperature. The Electron Paramagnetic Resonance (EPR) indicates changes in the oxidation state of molybdenum occurring upon interaction with propene. It has been found that the concentration of Mo(V) influences the propene conversion. The interaction between propene and molybdenum leads to the formation of surface species that, depending on the strength of their bonding to the surface, can be decomposed to ethene or coke. These results have been confirmed by infrared (FTIR) study. The oxidation reaction of propene is in competition with that of coke or ethene deposit on the catalyst surface, which can explain the decrease of the catalyst activity and selectivity in the presence of high molybdenum loadings.     

Study of MO2N catalyst activity and stability in CO oxidation

Unsupported molybdenum nitride powder with S_g of 115 m²g⁻¹ (passivated) has been prepared by the temperature-programmed reaction of MoO₃ in H₂/N₂ mixture. It exhibited high catalytic activity in CO oxidation. DTA experiments in the air flow and O₂ temperature-programmed pulse reaction (TPPR) showed that the optimal oxidation temperature for the Mo₂N catalyst was under 450°C because of its instability at high temperature in the presence of O₂.     

The effect of active component distribution in a porous structure of supported catalyst

The catalytic process in supported catalyst with nonuniform distribution of active component among the pores of different size is considered. Some previous as well as new estimates are presented. The limit value of diffusivity in small pores (D _L < 10^?8 cm²/s) is established, at which the distribution can takes an effect. That is possible in the case of capillary condensation, when micro-pores are filled with liquid while macro-pores are filled with gas. Such diffusivity is also observed at configuration or surface diffusion in zeolite channels. Then the distribution may influence the pellet effectivity when the activity of active component inside zeolite crystals is higher than that on its external surface.     

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.     

Oscillatory motion of coke in deactivated HZSM-5 zeolite

We have observed motion of coke in deactivated HZSM-5 with the displacement vector alternately directed toward the outer surface of the zeolite crystals and into the interior of the zeolite structure. The driving force for the oscillations is the higher chemical potential of massive carbon μ_m at 500 °C than for carbon clusters. The switching mechanism involves an alternating jumpwise change in the value of Δμ.