Molybdenum Species Research Articles

Developing Catalysts for Proton and Anion Exchange Membrane Water Electrolyzers

To achieve carbon neutrality, extensive research is ongoing to produce green hydrogen via water electrolysis combined with renewable energy sources. Understanding and developing catalysts for membrane electrode assembly (MEA) type water electrolysis systems, such as proton exchange membrane water electrolysis (PEMWE) and anion exchange membrane water electrolysis (AEMWE), are critical steps toward this goal. Therefore, this presentation will focus on catalyst development for PEMWE and AEMWE, with emphasis on cost-effective catalysts.For oxygen evolution reaction (OER) catalysis in PEMWE, the primary challenge is to minimize iridium (Ir) usage for sustainable green hydrogen production. Layered monoclinic iridium nickel oxide (IrNiOx) platelets were synthesized using a molten salt method and employed for the OER. These platelets exhibited high OER activity with reduced Ni dissolution in acidic conditions. When applied in MEA, they demonstrated enhanced interconnectivity within the catalyst layer, promoting electron transfer. Even at low Ir loading (0.2 mgIr cm-2), the platelets showed good performance with an initial cell voltage of 1.70 V at 1 A cm-2 and minimal degradation over 100 hours. This highlights the effectiveness of incorporating transition metals into Ir oxide to reduce Ir usage in PEMWE.For oxygen evolution reaction (OER) catalysis in AEMWE, attention is focused on nickel iron (NiFe) hydroxide catalysts for their high activity and stability in alkaline conditions. However, the lack of initial electrical conductivity of as-prepared NiFe LDH limits its potential as an electrocatalyst. To address this issue, a monolayer structuring approach was proposed. This method improved mass transport to allow a high energy conversion efficiency of 72.6% with exceptional stability over 50 hours at 1 A cm-2. Additionally, lack of initial electrical conductivity hinders determination of electrochemically active surface area (ECSA) of NiFe LDH using conventional double layer capacitance method. Use of electrochemical impedance spectroscopy (EIS) at reactive OER potentials to extract the capacitance that is hypothesized to arise due to reactive OER intermediates (O*, OH*, OOH*) adsorbed on the catalyst surface was thus investigated. This allowed the estimation of ECSA and intrinsic activity of NiFe LDH, validating the methodology through rigorous catalyst loading and support studies.For hydrogen evolution reaction (HER) catalysis in AEMWE, replacing platinum (Pt) with Ni-based alloys containing molybdenum (Mo) species has shown promise. Dynamic active sites of Ni catalysts with Mo species require innovative approaches to maintain initial performance, warranting material innovation or careful manipulation of operating protocol. The approaches in which this is made possible is shortly presented, together with the innovations that allows probing of such phenomenon.

Phosphorous-doped nickel-cobalt bimetallic molybdate/three-dimensional hierarchical porous carbon for high-efficiency solid-state supercapacitors

The limited electrical conductivity and insufficient quantity of electrochemically active sites in transition-metal oxides impede their extensive utilization in high-performance supercapacitors. Herein, we present a successful synthesis of phosphorus-doped NixCo1-xMoO4 (P-NCMO) on three-dimensional hierarchical porous carbon (HPC) derived from enzymatically hydrolyzed lignin through a co-precipitation pathway and subsequent phosphorylation process. The electrochemical performance was improved by optimizing the content of P doping. Additionally, the incorporation of phosphorus into P-NCMO led to a decrease in Co-O bonding energy and facilitated the formation of molybdenum species with lower oxidation states, thereby enhancing surface redox chemistry and improving electrochemical performance. As a result, the optimized HPC/P-NCMO-2 electrode exhibits a high specific capacity of 1003 C g−1 at a current density of 1 A g−1, and it maintains 89.87 % of its initial capacity after 10,000 cycles at a current density of 1 A g−1. The optimized HPC/P-NCMO-2 assembled as the positive electrode and activated carbon as the negative electrode in an asymmetric supercapacitor delivers a high energy density of 94.7 Wh kg−1 at a power density of 800 W kg−1, while maintaining excellent cycle life.

Compound catalyst of ReMoSx@HSSZ-39 and SAPO-34 zeolites for high performance conversion of CO2 to C2-4 hydrocarbons

We report here a high-performance catalyst ReMoSx@HSSZ-39 compounded with SAPO-34 for conversion of CO2 to C2-4 hydrocarbons, in which the ReMoSx clusters are responsible for highly efficient hydrogenation of CO2 to methanol as the intermediate and then the surrounding zeolitic acid sites for further conversion of methanol to hydrocarbons. The investigation finds that uniform ReMoSx clusters can be reliably fabricated in zeolitic cages by co-introduction of rhenium and molybdenum species into the HSSZ-39 zeolite and followed by sulfidation, which constitutes an intimate bifunction catalyst through better cooperation of ReMoSx clusters and the zeolitic acid sites. Compared with MoSx@HSSZ-39, the introduction of rhenium significantly suppresses CO generation. By mixing some SAPO-34 zeolite, with increased density of protonic acid, more than 86 % C2-4 hydrocarbons in organic products and only 21 % CO at 32.5 % of CO2 conversion are obtained and the composite catalyst shows excellent stability with more than 95 % C2-4 hydrocarbons in organic products, less than 30 % CO and above 26.6 % of CO2 conversion after 1000 h on stream.

Boron doped Mo/HMCM-22 catalyst for improving coke resistance in methane dehydroaromatization

Mo/HMCM-22 is one of the candidate catalysts for methane dehydroaromatization (MDA) reaction. However, serious coke deposition would deactivate the catalyst, resulting in a rapid decrease of aromatics yield. Here, we adjusted the acidity of MCM-22 zeolite by doping boron into the framework. Boron species replace some of framework aluminum and prompt aluminum to locate in sinusoidal channel. It optimizes the ratio of strong/weak Brønsted acid, as well as enhances the interaction between molybdenum and HMCM-22 to facilitate the dispersion of molybdenum species. After 10 h of MDA reaction, the yield of aromatics over Mo/H[B]MCM-22 (SBR 30) is 3.5 times higher than that of Mo/H[B]MCM-22 (SBR 5). It decreases by only 4.8 % in comparison to the initial yield. Meanwhile, Mo/H[B]MCM-22 (SBR 30) shows significantly better resistance to coke deposition. Therefore, the established structure–activity relationships provide valuable insights for future research on the modification of zeolite acidity in methane dehydroaromatization reaction.

Efficiently activate peracetic acid by FeS2-anchored molybdenum species for the rapid degradation of sulfamethazine

Peracetic acid (PAA) based advanced oxidation processes are increasingly used as alternative strategies for sulfonamide antibiotics (SAs) degradation. In this study, molybdenum (Mo) species were successfully anchored on the FeS2 surface. This catalyst (Mo-FeS2) could be used as an excellent PAA activator for the removal of sulfamethazine (SMT). Electrochemical measurements revealed that anchored Mo species lowered the electron transfer resistance on the FeS2 surface, and density functional theory (DFT) calculation suggested more electrons can be shifted from FeS2 to Mo, which is prone to transfer electrons for PAA activation. After 30 min reaction, 87.6 % of SMT could be removed by Mo-FeS2/PAA at pH 4, and reactive species (RSs) including HO•, CH3COO• and Fe(IV) were responsible for the SMT degradation. Increasing the dosages of PAA (0.02 – 0.5 mM) or Mo-FeS2 (0.1 – 0.4 g/L) facilitated the SMT degradation. DFT calculation results suggest that the aniline ring of SMT is vulnerable to attack induced by RSs. The degradation pathways of SMT, including Smiles rearrangements, hydroxylation of aniline ring, oxidation of the –NH2 group and coupling reaction, were proposed according to the identified oxidation products. Sulfamethoxazole, sulfadiazine, sulfanilamide and sulfathiazole could also be degraded by Mo-FeS2/PAA (>50 %), suggesting that the Mo-FeS2/PAA would be a competitive strategy for the remediation of SAs-contaminated water.

Inclusion complexes of cucurbit[n]urils (n = 7, 8) with η5‐cyclopentadienyl methyl tricarbonyl molybdenum(II) and their use in epoxidation catalysis

There are very few known examples of supramolecular compounds comprising molybdenum species hosted inside the portals/cavities of cucurbit[n]urils (CBn). In this work, CB7 and CB8 macrocycles have been studied as hosts for the carbonyl complex [CpMo(CO)3Me] (1) (Cp = η5‐C5H5). Compounds were isolated in the solid state and characterized as genuine 1:1 inclusion complexes (1@CBn) by elemental and thermogravimetric analyses, powder X‐ray diffraction, scanning electron microscopy, 13C{1H} cross‐polarization magic‐angle spinning NMR, FT‐IR, Raman, and diffuse reflectance UV–Vis spectroscopies. The host–guest structures can act as supramolecular precatalysts for olefin epoxidation. Based on the model reaction of cis‐cyclooctene with hydroperoxide oxidants (tert‐butylhydroperoxide or hydrogen peroxide), the structural features of 1@CBn as well as the operating conditions influence the catalytic process. The metal species in 1@CBn undergo oxidative decarbonylation in situ, giving oxidized metal species that are catalytically active for olefin epoxidation. The type of oxidant and solvent influences the catalytic activity and stability. 1@CB8 was more stable than 1@CB7 with regard to catalyst recycling and reuse. Based on the substrate scope investigation, for relatively large olefins, such as the fatty acid methyl ester methyl oleate, the size of the macrocyclic host may be a determining factor for catalytic activity.

Effect of molybdenum valence in low Mo/Sn ratio catalysts for the oxidation of methanol to dimethoxymethane

A series of Mo/Sn (1:20, molar ratio) catalysts were prepared by two-step hydrothermal synthesis method, and the effect of calcination temperature of tin precursors on the reaction performance of methanol oxidation to dimethoxymethane (DMM) was investigated. The crystal structure, surface properties, redox property and valence change of molybdenum species of the catalyst were characterized by XRD, Raman, FT-IR, XPS, NH3-TPD and H2-TPR. The results showed that Mo1Sn20-600°CSn catalyst exhibited better performance than other catalysts, achieving DMM selectivity of 90% with methanol conversion of 30% at 140 °C. From the characterization results, the surface properties of the tin precursors affected the structure of catalyst, the degree of molybdenum oxide dispersion and valence of molybdenum species, and further influenced the performance of the catalysts. The high temperature calcination of tin precursors is more favorable for the generation of Mo6+ in the Mo1Sn20 catalyst.

Facet-dependent catalysis of γ-Bi2MoO6 for selective oxidation of isobutene to methacrolein

γ-Bi2MoO6 nanosheets catalyzed the selective oxidation of isobutene to methacrolein with a conversion of 88% and a selectivity of 67% due to the preferential termination of the reactive molybdenum species on the {010} facets.

Immobilisation of Molybdenum in a Sulphate-Reducing Bioreactor

This work presents a biological remediation process for molybdenum-bearing wastewater which may lead to the fabrication of biogenic Mo chalcogenide particles with (photo)catalytic properties. The process is based on dissimilatory sulphate reduction, utilising sulphate-reducing bacteria (SRB), and reductive precipitation of molybdate which is the predominant species of molybdenum in oxygenated water/wastewater. The SRB culture was established in a biofilm reactor which was fed with synthetic solutions containing sulphate (17.7 mM), molybdate molybdenum (2 mM), divalent iron (1.7 mM) and ethanol as the carbon/electron donor. The performance of the bioreactor was monitored in terms of pH, sulphate and molybdenum (Mo(VI) and total) content. The presence of thiomolybdate species was studied by scanning UV-Vis absorbance of samples from the reactor outflow while the reactor precipitates were studied via electron microscopy coupled with energy dispersive spectrometry, X-ray diffractometry and laser light scattering. A molar molybdate/sulphate ratio of 1:12.5 proved effective for molybdate reduction and recovery by 76% in 96 h, whereas sulphate was reduced by 57%. Molybdenum was immobilised in the sulphidic precipitates of the bioreactor, presumably via two principal mechanisms: (i) microbially mediated reduction and precipitation, and (ii) thiomolybdate formation and sorption/incorporation into iron sulphides.

Enhancement of Diarylamine Antioxidant Activity by Molybdenum Dithiocarbamates.

Molybdenum dithiocarbamates (MDTCs) are indispensable lubricant additives. Although their role as antiwear agents is well established, they have also been attributed antioxidant properties that are not understood. MDTCs do not inhibit autoxidation, but they markedly enhance the capacity of diphenylamines (DPAs)─ubiquitous radical-trapping antioxidants (RTAs)─to do so. We find this synergy to be evident not only at elevated temperatures (160 °C in n-hexadecane) but also at moderate temperatures, where autoxidations can be continuously monitored and kinetics more easily interpreted (100 °C in squalane). Interestingly, the synergy disappeared in an unsaturated hydrocarbon (n-hexadec-1-ene), where the RTA activity of the DPA is known to result from the diarylnitroxide derived therefrom. Autoxidations of squalane carried out in the presence of the diarylnitroxide─wherein it is a poor inhibitor─were much better inhibited in the presence of MDTC, suggesting that it converts the nitroxide to (a) more competent RTA(s). Indeed, preparative experiments revealed two species: DPA and a DPA dimer into which a single oxygen atom had been incorporated. This conversion is accelerated by the oxidation of MDTC to a dioxo molybdenum species. A mechanism is proposed to account for these observations, and the implications of our findings and their interpretation are discussed.

Central composite design approach for concurrent desulfurization and denitrogenation of model liquid fuel over Mo‐AAC

AbstractThe primary objective of this work was to investigate the simultaneous removal of refractory sulfur (dibenzothiophene, DBT) and nitrogen compounds (indole, IND) from model gasoline. This research aimed to develop an efficient adsorption process to mitigate the emission of sulfur oxide and nitrogen oxides, which pose direct threats to human health and the environment. Additionally, the study focused on addressing technical challenges encountered in the refinery process, particularly the poisoning of catalysts due to the presence of these compounds. To achieve the study's goals, granular activated carbon (GAC) was subjected to acid treatment for modification. Molybdenum species were then incorporated into the acid‐treated GAC using the wet impregnation technique. The composition and distribution of elements on the surface of the modified GAC (Mo‐AAC) were analyzed via X‐ray photoelectron spectroscopy (XPS). Various characterization techniques were employed to confirm the uniform dispersion of molybdenum species on the GAC surface. The specific surface area of the adsorbent was increased from 257 m2/g to 316 m2/g following the acid treatment and molybdenum modification. The study explored the impact of several key parameters on the removal of DBT and IND using central composite design (CCD). A regression model was derived using CCD through hybrid central composite design, and its adequacy was verified with the means of tests such as analysis of variance (ANOVA), a lack of fit test, and residuals distribution consideration. Experimental results match well with the results obtained by CCD model, justifying the accuracy of models. The results showed that all four parameters have a significant impact on removal of DBT and IND. Increasing temperature and the adsorbent dose positively affected the removal of both DBT and IND, indicating that higher temperatures and greater adsorbent dosages improved removal efficiency. On the other hand, the concentration of DBT and IND had a negative impact, signifying that higher initial contaminant concentrations hindered removal efficiency. The study determined the optimum initial contaminant concentrations for DBT and IND, temperature, and adsorbent dose to be 186 mg/L, 50 mg/L, 25.5°C, and 32 g/L, respectively. These conditions yielded the highest removal efficiency. The research demonstrated that Mo‐AAC has the capability to effectively reduce the sulfur and nitrogen content of model gasoline, making it a promising solution for addressing environmental and refinery challenges associated with these compounds.

Molybdenum-Catalyzed Directed Activation of Aryl Chlorides and Fluorides

AbstractA low-valent molybdenum species generated by the reduction of a molybdenum precursor with phenylmagnesium bromide catalytically cleaves a C–Cl or C–F bond in an aromatic ketone under mild conditions, followed by cyclization to produce a hydroxyphthalan (1,3-dihydro-2-benzofuran-1-ol) derivative.

Oxidized Copper and Molybdenum Species Exclusively Boosting Electrocatalytic Hydrogen Evolution in Non-Extreme pH Carbonate Buffer Electrolyte

Oxidized Copper and Molybdenum Species Exclusively Boosting Electrocatalytic Hydrogen Evolution in Non-Extreme pH Carbonate Buffer Electrolyte

MoOx monolayers over TiO2 for the selective oxidation of isobutene

The morphology effect of TiO2 on the dispersion of molybdena species and the performance of the resulting MoOx/TiO2 catalysts in selective oxidation of isobutene to methacrolein were examined. Two-dimensional MoOx monolayers in Mo–O–Mo bonds were well dispersed on TiO2 nanosheets that were preferentially enclosed by the {001} facets. However, aggregated MoOx clusters enriched in terminal MoO bonds were formed over TiO2 nanospindles dominated by the {101} facets. The superior performance of the MoOx monolayers, regarding activity, selectivity and stability, was ascribed to the synergetic effect between MoOx monolayers and TiO2 (001) facets.

Identification of reversible and irreversible deactivation of MDA catalysts: A key to design a viable process

We report a detailed kinetic study of methane dehydroaromatization on three bifunctional Mo/HZSM-5 catalysts (0.9 wt%, 2.7 wt%, and 17.2 wt% Mo). Two main deactivation modes are present: (i) irreversible damage by zeolite amorphization during the isothermal pre-treatment in an inert atmosphere and (ii) reversible deactivation by coke deposition (“soft”, “hard” and carbide species). The former occurs at high Mo loading (>4.0 w%) and provides low activity catalysts. Catalysts with a lower and well-balanced molybdenum loading (<4 w%) suffer mainly from the second deactivation mode. The catalyst decay is modelled by semi-empirical laws including time on stream and coke level. The latter initiates textural and structural catalyst modifications leading to an activity loss. Three deactivation descriptors, 1: coke level on the spent samples, 2: loss of micropore volume, and 3: monoclinic/orthorhombic phase transition of the zeolite are identified. Further characterization indicates that both molybdenum and carbon species are responsible for the catalytic deactivation of well-balanced catalysts. At least 50% of the initially dispersed molybdenum migrates to the external surface of the zeolite to form large clusters of molybdenum carbide. The coke extracted from the zeolite micropores are confirmed to be mainly unsubstituted polyaromatics.

Catalytic hydropyrolysis of biomass over NiMo bimetallic carbon-based catalysts

Catalytic hydropyrolysis (CHP) is an emerging and promising technology to convert lignocellulosic biomass resources into alternative fuels and valuable chemicals. In this work, NiMo bimetallic carbon-based catalysts were designed and applied in the thermal conversion of alkaline lignin at 450 °C, 500 °C, and 550 °C under 1 atm H2. The CHP process exhibited a higher deoxygenation extent of around 60% compared with pyrolysis (FP) and hydropyrolysis (HP) processes. The bio-oil with higher H/C ratio and lower O/C ratio corresponding to a chemical formula of CH1.167O0.245 was obtained at 500 °C over the IW catalyst prepared via incipient wetness impregnation. CO2 and CO were generated via the decarboxylation and decarbonylation reactions, respectively. As for the CHP of real biomass, the deoxygenation extent reached about 55% over different modified catalysts. The ball milling-melting catalyst showed a considerable liquid product yield of 35.0 wt% and the best bio-oil quality with the chemical formula of CH1.456O0.339. The remarkable hydrodeoxygenation activity of carbon-derived catalysts was attributed to the synergistic effect of metallic Ni0 and intermediate oxidation state of Mo2+/Moδ+. The active hydrogen was adsorbed on the nickel sites, and the molybdenum species were responsible for eliminating the oxygen in aromatic oxygenates.

Mo promoted Ni-ZrO2 co-precipitated catalysts for green diesel production

The promoting action of Mo-species was studied for Ni-ZrO2 co-precipitated catalysts with high nickel loading used in the transformation of sunflower oil into green diesel. The catalysts were thoroughly characterized and evaluated using a high-pressure semi-batch reactor. The significant promoting action of molybdenum species was clearly demonstrated concerning the Ni-ZrO2 catalysts as Mo addition resulted in almost duplication of green diesel yield (from 35.2 to 68.4 %). This is manifested through the acceleration of the hydrodeoxygenation pathway of the reaction network. The increase of the activation temperature increases further the catalytic efficiency of the promoted catalysts resulting in 70.1 % green diesel yield. The promoting action of molybdenum was partly attributed to the increase of the specific surface area and the metallic nickel surface brought about by molybdenum but, principally, to a synergy between the oxygen vacancies developed on the very well dispersed Mo-species - and the nickel surface sites.

Enhanced aromatic yield from catalytic pyrolysis of pine wood via ultrasonic assisted Mo modified HZSM-5

Catalytic pyrolysis of biomass over HZSM-5 can produce aromatic hydrocarbons, but it also produces a lot of by-products and causes rapid coking of the catalyst. Mo/HZSM-5 was modified through ultrasonic treatment to enhance the catalytic efficiency and mitigate the coking in this study. It is found that ultrasonic treatment is conducive to the diffusion of more Mo species into the channel of HZSM-5 and enhance the interaction between molybdenum species and HZSM-5 acid sites, which promoted the synergistic catalysis of Mo species and HZSM-5 and increased the yield of aromatic hydrocarbons. The activity of the ultrasonic Mo modified catalyst increased 54%, which led to aromatic carbon production increasing from 5.09 mmol∙gcat−1 to 7.82 mmol∙gcat−1. By adjusting the WHSV, the carbon yield of aromatic can reach 21.7%. The regeneration performance of Ul-Mo/HZSM-5 is excellent, and the activity does not decrease after regenerating seven times.

Formation and Stability of Molybdenum and Tungsten Species in Peroxy Solution

Formation and Stability of Molybdenum and Tungsten Species in Peroxy Solution

Post-Synthesis Strategies to Prepare Mesostructured and Hierarchical Silicates for Liquid Phase Catalytic Epoxidation

Olefin epoxidation is an important transformation for the chemical valorization of olefins, which may derive from renewable sources or domestic/industrial waste. Different post-synthesis strategies were employed to introduce molybdenum species into mesostructured and hierarchical micro-mesoporous catalysts of the type TUD-1 and BEA, respectively, to confer epoxidation activity for the conversion of relatively bulky olefins (e.g., biobased methyl oleate, DL-limonene) to epoxide products, using tert-butyl hydroperoxide as an oxidant. The influences of (i) the type of metal precursor, (ii) type of post-synthesis impregnation method, (iii) type of support and (iv) top-down versus bottom-up synthesis methodologies were studied to achieve superior catalytic performances. Higher epoxidation activity was achieved for a material prepared via (post-synthesis) incipient wetness impregnation of MoO2(acac)2 (acac = acetylacetonate) on (pre-treated) siliceous TUD-1 and calcination; for example, methyl oleate was converted to the corresponding epoxide with 100% selectivity at 89% conversion (70 °C). Catalytic and solid-state characterization studies were conducted to shed light on material stability phenomena.