MoO2 Catalyst Research Articles

Efficient and Ultrastable Seawater Electrolysis at Industrial Current Density with Strong Metal-Support Interaction and Dual Cl--Repelling Layers.

Direct seawater electrolysis is emerging as a promising renewable energy technology for large-scale hydrogen generation. The development of Os-Ni4Mo/MoO2 micropillar arrays with strong metal-support interaction (MSI) as a bifunctional electrocatalyst for seawater electrolysis is reported. The micropillar structure enhances electron and mass transfer, extending catalytic reaction steps and improving seawater electrolysis efficiency. Theoretical and experimental studies demonstrate that the strong MSI between Os and Ni4Mo/MoO2 optimizes the surface electronic structure of the catalyst, reducing the reaction barrier and thereby improving catalytic activity. Importantly, for the first time, a dual Cl- repelling layer is constructed by electrostatic force to safeguard active sites against Cl- attack during seawater oxidation. This includes a strong Os─Cl adsorption and an in situ-formed MoO4 2- layer. As a result, the Os-Ni4Mo/MoO2 catalyst exhibits an ultralow overpotential of 113 and 336mV to reach 500mA cm-2 for HER and OER in natural seawater from the South China Sea (without purification, with 1m KOH added). Notably, it demonstrates superior stability, degrading only 0.37 µV h-1 after 2500 h of seawater oxidation, significantly surpassing the technical target of 1.0 µV h-1 set by the United States Department of Energy.

A versatile heterostructure junction of NiO–C-MoOx (X= 2 & 3) composite electrocatalysts for hydrogen evolution reaction

Developing high-density crystal heterostructure interfaces is highly desirable for the hydrogen evolution reaction (HER). For the first time, NiO-modified MoOx heterogeneous structures are created on carbon matrix materials using a solid-state reaction of the precursor. The solid-state reaction method uses a novel concept called “solid solvent” instead of many solvents and is simple, scalable, and less harmful to the environment. The NiO–C–MoO2 catalyst has an active surface of 40.1 m2 g−1. The NiO–C–MoO2 compound showed superior hydrogen evolution reaction (HER) performance compared to NiO–C–MoO3, with a low overpotential of only 170 mV and Tafel slope of 81.1 mV dec−1 vs. reverse hydrogen electrode (RHE). The d-band structure and modulated charge close to the Fermi level are responsible for the enhanced conductivity, increased availability of active sites, and advantageous H+ adsorption sites. This underscores the significance of optimizing the electronic structure of bifunctional electrocatalysts based on transition metals for optimal HER catalytic activity.

A difunctional interlayer of an activated carbon cloth supported with MoO2 catalyst for high-rate magnesium–sulfur batteries

The main challenges faced by magnesium-sulfur (Mg–S) batteries are the polysulfide shuttle effect and slow kinetics of S cathode, causing the severe self-discharge behavior and rapid capacity decay. Herein, we elaborately design a MoO2-loaded activated carbon cloth (denoted as MoO2@ACC) functional interlayer to realize high-performance Mg–S batteries. The ACC possesses a remarkable adsorption effect, which can effectively mitigate the polysulfide shuttle effect and enable the Mg–S batteries with a high voltage plateau. Besides, the MoO2 can observably reduce the polarization during charge/discharge process attributed by the fast reaction kinetics due to the catalytic effect. The Mg–S battery with this MoO2@ACC interlayer achieves a high energy density and good capacity retention (80.3 %) after 100 cycles (0.5 C) and effectively mitigates the Mg–S battery's self-discharge as well. This work illustrates the significant role of the interlayer with both adsorption and catalytic functions to boost the usability of the Mg–S batteries.

Converting formaldehyde in methanol with MoO2 under irradiation: A pollution-free strategy for cleaning air

Direct photocatalytic reduction of toxic formaldehyde (HCHO) in value-added chemicals and fuels is promising because that not only abates the environmental pollution, but also solves the energy shortage. Herein, self-supported MoO2 and MoO3 nanoparticles growing on Mo meshes were comparatively applied to the photocatalytic conversion of HCHO. Under UV–visble lights, MoO2 reduces HCHO in methanol (CH3OH) while MoO3 oxidizes HCHO in carbon oxide and water. Their contrary photocatalytic capacities were revealed. Compared with MoO3, the lower work function of MoO2 enables an electron-rich interface, realizing a complete reduction of 30 ppm HCHO to CH3OH in 30 min. Theoretical calculations clarify that a large number of delocalized electrons on MoO2 attracts HCHO molecule and activates its CO bond, facilitating subsequent hydrogenation and reduction of HCHO to CH3OH. As for MoO3, the wider bandgap and higher potential of valence band govern the photocatalytic oxidation of HCHO.

Aldehyde and Ketone Hydroboration Mediated by a Heterogeneous Single‐Site Molybdenum‐Dioxo Catalyst: Scope and Mechanistic Implications

AbstractEfficient hydroboration of aldehydes and ketones is demonstrated using a single‐site MoO2 catalyst supported on activated carbon. Under mild conditions with low catalytic loadings, the reaction exhibits chemoselectivity towards carbonyl reduction over halides, alkene, alkyne, nitrile, and ester groups, affording high yields of desired products. Furthermore, competition studies between aldehyde and ketone reduction using HBpin and HBcat indicate that HBpin preferably functionalizes aldehydes, while HBcat can functionalize either substrate under specific conditions. Mechanistic studies suggest that the reaction proceeds via the initial formation of a molybdenum‐hydride species upon B−H activation and subsequent molybdenum‐alkoxide species upon carbonyl activation by the Mo center. The experimental activation energy of 14.3 kcal/mol alignssatisfactorily with the DFT computed ΔH≠ of 18.7 kcal/mol, further supporting the proposed mechanism. Combined experimental/computational analyses reveal that excess HBpin can possibly deactivate the catalyst. The presence of excess benzaldehyde efficiently mitigates deactivation by HBpin, providing significantly higher catalyst recyclability. Overall, this non‐toxic, air‐ and moisture‐stable, active, and selective catalyst demonstrates significant potential for green processes.

AcesExperimental and theoretical study on photocatalytic nitrogen reduction catalyzed by Fe doped MoO2

Transition metal oxides are suitable for photocatalytic nitrogen reduction (pNRR) due to their properties of narrow band gap and high energy density. Still, it is also limited by the defects of high photogenerated carrier recombination and low adsorption capacity of nitrogen molecules, which hinder the further improvement of the activity. For high carrier recombination and low activation rate of the nitrogen molecule, herein, Fe-doped MoO2 catalyst (Fe−MoO2) was synthesized for pNRR. Combined with the experimental characterization and theoretical calculation, it shows that the doping of Fe atoms, one hand, improves the conduction band position of MoO2 and enhances the reduction ability, which is favorable to nitrogen reduction. On the other hand, it not only enhances the adsorption of Mo sites and N2 molecules, but also changes the surface electronic structure and promotes the electron transfer from MoO2 to Fe sites, and the two act synergistically to adsorb and activate nitrogen more effectively. The activity of Fe−MoO2 for pNRR is as high as 96.62 μmol g−1h−1, which is about 2.5 times that of pristine MoO2, and the activity decreases by only 10% after three cycles.

Metallic MoO2 as a Highly Selective Catalyst for Electrochemical Nitrogen Fixation to Ammonia under Ambient Conditions

AbstractThe development of earth‐abundant and non‐noble based metal catalysts for electrochemical nitrogen reduction towards sustainable ammonia production from N2 and H2O in the ambient atmosphere holds immense potential in realizing an alternate route to energy‐intensive Haber‐Bosch process. Herein, we have employed highly conductive MoO2 synthesized by hydrothermal route as an electrocatalyst for ammonia generation. The MoO2 catalyst was employed for electrochemical nitrogen reduction in N2 saturated 0.1 m HCl electrolyte in a two‐compartment cell. Ammonia yield rate of 2.30 μg h−1 mg−1cat and 2.40 % Faradaic efficiency has been obtained at a potential of −0.3 V vs. RHE. The MoO2 catalyst is selective for ammonia generation without any hydrazine formation. The metallic nature of MoO2 catalyst assists in achieving comparable activity for the electrochemical nitrogen reduction. The above catalyst has the potential for providing better ammonia yield rate by combining or doping with other active elements.

Surface Valence State Effect of MoO2+x on Electrochemical Nitrogen Reduction.

The valance of Mo is critical for FeMo cofactor in ambient ammonia synthesis. However, the valence effect of Mo has not been well studied in heterogeneous nanoparticle catalysts for electrochemical nitrogen reduction reaction (NRR) due to the dissolution of Mo as MoO4 2− in alkaline electrolytes. Here, a MoO2+ x catalyst enriched with surface Mo6+ is reported. The Mo6+ is stabilized by a native oxide layer to prevent corrosion and its speciation is identified as (MoO3) n clusters. This native layer with Mo6+ suppresses the hydrogen evolution significantly and promotes the activation of nitrogen as supported by both experimental characterization and theoretical calculation. The as‐prepared MoO2+ x catalyst shows a high ammonia yield of 3.95 µg mgcat −1h−1 with a high Faradaic efficiency of 22.1% at −0.2 V versus reversible hydrogen electrode, which is much better than the MoO2 catalyst with Mo6+ etched away. The accuracy of experimental results for NRR is confirmed by various control experiments and quantitative isotope labeling.

Improving oxygen vacancies by cobalt doping in MoO2 nanorods for efficient electrocatalytic hydrogen evolution reaction

AbstractRecently, molybdenum dioxide (MoO2) has gained intensive attention as an eco‐friendly and earth abundant catalyst for electrocatalytic hydrogen evolution from water splitting. However, the catalytic activity of MoO2 catalyst for hydrogen evolution reaction (HER) is severely limited by the less exposed active sites. Herein, we present Co‐doped MoO2 for efficient HER through a facile wet chemistry synthesis followed by calcination treatment process. The optimized Co‐MoO2‐0.01 nanorods (NRs) delivers a very low overpotential of 26 mV at 10 mA cm−2 and a small Tafel slope of 30.9 mV dec−1, which is much better than that of pure MoO2 NRs and the commercial Pt/C catalyst in HER. Experimental and theoretical results reveal that Co doping not only produce more oxygen vacancies in MoO2, which can activate adjacent oxygen atoms as active sites and thus increase the exposed active sites on the surface of catalyst, but also enhance the electrical conductivity of catalyst during HER process. In a word, this work provides a new promising synthetic strategy for developing earth‐abundant and cost‐effective HER electrocatalysts through rational defect‐engineering design.

Furfural hydrogenation, hydrodeoxygenation and etherification over MoO2 and MoO3: A combined experimental and theoretical study

Valorization of lignocellulosic biomass, particularly catalytic hydrotreatment of hemicellulose-based furfural (FUR), has been studied for the production of value-added chemicals. A three-phase batch reactor has been used for hydrotreatment in isopropanol over various commercially available unsupported MoOx catalysts, at various temperatures (170–230 °C), pressures (0–80 bar H2), catalyst loadings (0–2 wt%), and reactant concentrations (5–20 wt%). No significant difference in catalytic activity or selectivity has been observed among the three different MoO3 and one MoO2 catalysts, while NiMo/Al2O3, Mo2C and WO3 were much less active. Data-points collected have been used to propose a detailed reaction pathway network for a micro-kinetic model, which also took into consideration the thermodynamics, and adsorption, desorption, and surface reaction kinetics. The alcoholysis of FUR yielded valuable isopropyl levulinate (IPL) as the major product under all tested reaction conditions, while other value-added compounds (furfuryl alcohol, isopropyl furfuryl ether, furfuryl acetone, angelica lactone) were observed in smaller quantities. It was found that neither the presence nor the absence of the gaseous H2 pressure contributes to the global reaction rate, or selectivity, since the solvent acts as a sufficient hydrogen donor. Additionally, density functional theory (DFT) calculations provided further insight into the active planes present by the implementation of the Wulff construction. Furthermore, the reaction mechanism was explained based on reaction energies, which were in silico determined and compared for several surfaces. The results were consistent with the characterization and activity-testing results. The furfural ring-opening reaction, yielding valuable IPL in the absence of gaseous H2, over a cheap bulk MoOx is reported for the first time.

β-Mo2C/MoO2 heterostructures for hydrogen evolution reaction: Morphology and catalytic activity regulated by heteroatom doping

Improving the activity of non-noble metallic electrocatalyst for hydrogen evolution reaction (HER) is an important issue for hydrogen energy utilization. In this work, we reported a new strategy for morphology regulation of β-Mo2C/MoO2 heterostructure via heteroatom doping to enhance the electrocatalytic HER activity. Electron microscopy observations found that N and S doping resulted in nanosphere and nanorod morphology, respectively. Amongst the S doping catalyst (denoted as S@β-Mo2C/MoO2) exhibited remarkably improved electrocatalytic HER activity and stability as compared to the pristine β-Mo2C/MoO2 catalyst, whereas the N doping induced significant degradation of catalytic activity and stability. The mechanism investigations reveal that the nanorod morphology of S@β-Mo2C/MoO2 endows it lower charge transfer resistance, higher electrochemical active surface area and lower valence state of Mo species, which contributes positively and importantly to its better electrocatalytic HER activity.

Strategic Design of MoO2 Nanoparticles Supported by Carbon Nanowires for Enhanced Electrocatalytic Nitrogen Reduction

Ammonia is an industrially relevant chemical that can be directly synthesized from water and air using renewable energy through the electrochemical nitrogen reduction reaction (NRR). However, because of the inert nature of nitrogen, current attempts at synthesizing ammonia under aqueous conditions result in low selectivity and yield rates. The poor electrocatalytic performance is mainly attributed to competing hydrogen evolution, underexposed active sites, inadequate electrode contact, and poor stabilization/destabilization of key reaction intermediates. Herein, we present a catalyst composed of MoO2 with surface vacancies dispersed over conductive carbon nanowires that mitigates these obstacles for NRR by providing a high surface area with stable catalytic sites and an underlying conductive support, where a variety of X-ray spectroscopy techniques are used to characterize the MoO2 catalyst. This uniquely engineered catalyst exhibits exceptional Faradaic efficiencies of over 30% and yields of 21.2 μg h–1 mg–1 at a low potential of −0.1 V vs RHE under ambient aqueous conditions.

Solvent-Free Hydrodeoxygenation of Triglycerides to Diesel-like Hydrocarbons over Pt-Decorated MoO2 Catalysts.

In the present work, the solvent-free hydrodeoxygenation of palm oil as a representative triglyceride model compound to diesel-like hydrocarbons was evaluated in a batch reactor using Pt-decorated MoO2 catalysts. The catalysts with various Pt loadings (0.5–3%) were synthesized by an incipient wetness impregnation method. The metallic Pt and MoO2 phases were detected in the XRD patterns of as-prepared catalysts after the reaction and acted as active components for the deoxygenation reactions. The XPS experiments confirmed the existence of metallic Pt and PtOx species. The XANES investigation of Mo L3-edge spectra elucidated a change in the valence state by the transformation of MoO3 into MoO2 species after the deoxygenation reaction. The TEM observation revealed the formation of Pt nanoparticles in the range of 1–3 nm decorated on MoO2 species. The number of acid sites increased with stronger metal–support interactions on increasing the Pt loading. The catalytic performance of the MoO2 catalyst significantly improved with a small amount of Pt decoration. However, the further increase in Pt loading did not relatively increase the deoxygenation activity due to the formation of the agglomerated Pt particles. The high performance of the decorated catalysts could be attributed to the moderate acidity from the Pt dispersed on MoO2 toward decarbonylation and decarboxylation reactions.

Radio-frequency thermal plasma-induced novel chainmail-like core-shell MoO2 as highly stable catalyst for converting syngas to higher alcohols

A novel core-shell catalyst with crystalline MoO2 core and chainmail-like amorphous MoOX shell (noted as MoO2-Pla) was prepared by radio-frequency induction thermal plasma method, and its catalytic performance for higher alcohols synthesis from syngas (CO and H2) was compared with that of the MoO2 catalysts synthesized by hydrothermal method and temperature programed reduction method (noted as MoO2-Hyd and MoO2-TPR). It was found that the chainmail-like shell formed under such extreme condition was both active to the reaction and inert to the thermal sintering, which was very fascinating and significantly different from those of the previous reported core-shell catalysts. The unusual amorphous species in the shell increased the CO conversion of MoO2-Pla catalyst by 109 and 213.5%, respectively, compared with MoO2-Hyd and MoO2-TPR. More importantly, the shell formed under such ultra-high temperature (over 3000 °C) and ultra-fast quenching process exhibited ultra-stable chainmail-like particle surface, which endowed the catalyst with high catalytic stability within 300 h of experimental determination. This work provided a new alternative strategy and method for the design of high activity and high stability catalysts.

Solvent-controlled selective hydrodeoxygenation of bio-derived guaiacol to arenes or phenols over a biochar supported Co-doped MoO2 catalyst

A novel magnetic Co-doped MoO2@C was prepared by in-situ co-pyrolysis and used for catalyzing hydrodeoxygenation (HDO) of bio-derived guaiacol to arenes or substituted phenols controlled by solvent type. Considering the variation of product distributions from the HDO of guaiacol using different solvents, the reaction intermediates of each stage were traced to explore its HDO pathways over Co-MoO2@C. The results show that guaiacol conversion to catechol via specific demethylation instead of direct demethoxygenation induces the entire reactions. Subsequently, catechol was converted to arenes or phenols by selective dehydroxylation or alkylation over Co-MoO2@C using different solvents. The synergy among Co, MoO2, and biochar plays a crucial role in activating H2 and ethanol, which is the key to achieving high reactant conversion and product selectivities under relatively lower initial H2 pressure (0.8 MPa). This solvent-controlled trend is also applicable to the catalytic HDO of the ethanol-soluble portion from bagasse. Furthermore, possible mechanisms for bimetallic synergic catalysis of the formation and transfer of active hydrogen atoms were proposed.

Application of Ti-doped MoO2 microspheres prepared by spray pyrolysis to partial oxidation of n-dodecane

The present investigation is focused on improving the performance of molybdenum dioxide (MoO2) by doping with Ti for the partial oxidation (POX) of n-dodecane. Ti-doped MoO2 nanoparticles were synthesized via solvothermal cracking of polycrystalline MoO3 microparticles prepared by ultrasonic spray pyrolysis in the presence of a Ti precursor. Partial oxidation of n-dodecane was conducted at 850 °C with an O2/C ratio of 0.5. The 6 at% Ti-doped MoO2 was fully converted into orthorhombic carbide phase (β-Mo2C) during the reaction. This carbide sample showed high catalytic activity and stability with conversion and H2 yield of 94.4% and 86.3% after 24 h on stream, respectively. On the other hand, un-doped MoO2 was partially converted into the carbide phase during the reaction, which led to mixed oxide and carbide phases. This mixed phase showed poor catalytic activity and rapid deactivation after only 6 h of operation. Our ammonia temperature programmed desorption (TPD) and pyridine diffuse reflectance infrared Fourier transform (DRIFT) tests suggest that the addition of Ti to MoO2 improves both the density and strength of Lewis acid sites, and hence improves hydrocarbon activation. This increased surface carbon activation would enhance the carburization process of the Ti-doped MoO2 catalyst and retain the carbide phase under the POX condition.

Molybdenum dioxide as an alternative catalyst for direct utilization of methane in tubular solid oxide fuel cells

Direct utilization of methane in conventional tubular solid oxide fuel cells (SOFCs) with a Ni-YSZ anode support was proposed using MoO2 as a partial oxidation catalyst for methane reforming at the cell inlet. A promising stability was achieved for a continuous operation in a mixture of methane and air (1, 1.5) for >150h under a load voltage of 0.7V at 750°C. The addition of the MoO2 catalyst at the anode inlet made tubular SOFCs with the conventional Ni-YSZ anode an efficient and practically promising candidate for direct hydrocarbon utilization.

Direct hydrodeoxygenation of algal lipids extracted from Chlorella alga

AbstractBACKGROUNDHydrodeoxygenation (HDO) of Chlorella algal oil extracted with supercritical hexane and stearic acid as a model compound was investigated over several supported Ni and Mo2N–MoO2 catalysts in a semibatch reactor at 300 °C under 30 bar in the presence of hydrogen.RESULTSThe results showed that all catalysts were very active in HDO of stearic acid. With crude algal oil, a maximum conversion of lipids of 35% was found with an acidic Ni‐HY‐80 zeolite catalyst after 360 min. The main products in algal oil HDO were fatty acids; whereas maximally only 7% hydrocarbons were formed with Ni‐HY‐80. The main hydrocarbon products were formed via decarbonylation, as the hydrogenation/dehydration route was suppressed.CONCLUSIONSHydrodeoxygenation of crude Chlorella algal oil was demonstrated over nickel supported on beta zeolite and with a molybdenum oxide–nitride catalyst. Strong catalyst deactivation occurred for the former catalyst, whereas the latter retained its activity. © 2016 Society of Chemical Industry

Hydrogen-transfer dehydration between alcohols over V2O3 and MoO2 catalysts for the formation of corresponding alkanes and aldehydes

Conversion of alcohols in a gas phase under N2 flow at 573K was carried out using V2O3 and MoO2 oxides with low valence oxidation states. It was found in the reaction of ethanol that equimolar amounts of ethane and acetaldehyde were catalytically formed as the main products over the oxides. Bi-products were small amounts of ethene and C4 compounds. Reactions of other alcohols (methanol, 1-propanol and 2-propanol) over the V2O3 and MoO2 catalysts also led to the equimolar formation of corresponding alkanes and aldehydes or ketone. It was confirmed by XRD and XPS that the low valence states of V2O3 and MoO2 were unchanged during the reactions and the oxides stably worked as the catalyst. Based on catalytic reaction results obtained under various reaction conditions (reaction temperature, contact time, introduction of H2 and C2H4 into reaction stream) and on experiments of kinetic isotope effects on the ethanol reaction, a reaction scheme is proposed, in which hydrogen transfer reaction between two alcohol molecules adsorbed on metal–O2−–metal sites on the surface of V2O3 and MoO2 catalysts takes place via 6-membered transition state, followed by dehydration.

Activity and stability studies of MoO2 catalyst for the partial oxidation of gasoline

Abstract The present investigation is focused on the performance of molybdenum dioxide (MoO 2 ) as a catalyst for the partial oxidation of isooctane. Metallic character and high oxygen mobility exhibited by this oxide appear to enhance its catalytic activity, which can be explained in terms of the Mars–van Krevelen mechanism. An oxygen-to-carbon ratio (O/C) of 0.72 seems to stabilize the catalytic performance, which could reach H 2 yields of 78% and carbon conversions of 100%, at 700 °C and 1 atm, after 20 h on stream. In addition, the catalyst was tested for sulfur tolerance using thiophene as model sulfur compound. Our findings indicate that the catalytic activity is barely affected even at sulfur concentrations as large as 500 ppm, after 7 h on stream. Finally, the catalyst performance was compared to that of a nickel catalyst using premium gasoline as fuel. MoO 2 displayed a stable performance whereas the Ni catalyst deactivated due to coke formation.