Molybdenum Nitride Catalysts Research Articles

A new molybdenum nitride catalyst with rhombohedral MoS2 structure for hydrogenation applications.

Nitrogen-rich transition-metal nitrides hold great promise to be the next-generation catalysts for clean and renewable energy applications. However, incorporation of nitrogen into the crystalline lattices of transition metals is thermodynamically unfavorable at atmospheric pressure; most of the known transition metal nitrides are nitrogen-deficient with molar ratios of N:metal less than a unity. In this work, we have formulated a high-pressure route for the synthesis of a nitrogen-rich molybdenum nitride through a solid-state ion-exchange reaction. The newly discovered nitride, 3R-MoN2, adopts a rhombohedral R3m structure, isotypic with MoS2. This new nitride exhibits catalytic activities that are three times more active than the traditional catalyst MoS2 for the hydrodesulfurization of dibenzothiophene and more than twice as high in the selectivity to hydrogenation. The nitride is also catalytically active in sour methanation of syngas with >80% CO and H2 conversion at 723 K. Our formulated route for the synthesis of 3R-MoN2 is at a moderate pressure of 3.5 GPa and, thus, is feasible for industrial-scale catalyst production.

Electrochemical Synthesis of Ammonia Based on Co3Mo3N Catalyst and LiAlO2–(Li,Na,K)2CO3 Composite Electrolyte

Cobalt molybdenum nitride (Co3Mo3N) catalyst was synthesised through ammonolysis of the corresponding precursors by flowing pure ammonia gas. The catalyst was characterised by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Ammonia was successfully synthesised from wet hydrogen and dry nitrogen at atmospheric pressure using Co3Mo3N–Ag composite as cathode, Ag–Pd alloy as anode and LiAlO2–(Li/Na/K)2CO3 composite as an electrolyte. Ammonia formation was investigated at 400, 425 and 450 °C, and the maximum observed ammonia formation rate was 3.27×10−10 mol s−1 cm−2 at 450 °C when applied at 0.8 V. The catalytic activity of Co3Mo3N for electrochemical ammonia synthesis is lower than that of Pd. The successful synthesis of ammonia demonstrates that LiAlO2–(Li/Na/K)2CO3 composite exhibits protonic or/and oxygen ion conduction.

Increasing Pt methanol oxidation reaction activity and durability with a titanium molybdenum nitride catalyst support

Titanium molybdenum nitride (Ti0.8Mo0.2N) hybrid support is prepared by a facile and efficient method, including a one-pot solvothermal process followed by a thermal treatment under ammonia at 750 °C, and this hybrid support is further decorated with Pt nanoparticles to catalyze the oxidation of methanol. The catalyst is characterized by X-ray diffraction (XRD), nitrogen adsorption/desorption, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and electrochemical measurements. XRD and TEM results show that the synthesized Ti0.8Mo0.2N is formed as a single-phase solid solution with high purity. Notable, Ti0.8Mo0.2N supported Pt catalyst exhibits a much higher mass activity and durability than that of the conventional Pt/C (E-TEK) electrocatalysts for methanol electrooxidation. The experimental data indicates that the Mo doping has the bifunctional effect that improves the performance and durability of the supported Pt NPs by inducing both co-catalytic and electronic effects.

Crystal structure and microstructural changes of molybdenum nitrides traced during catalytic reaction by in situ X-ray diffraction studies

X-ray diffraction was used to study changes in the crystal structure and microstructure of molybdenum nitrides during ammonia decomposition. In addition, electron microscopy was employed to analyse morphological changes of the nitrides caused by the catalytic reaction. Molybdenum nitride catalysts (MoxNy) were prepared via high temperature ammonolysis of molybdenum oxide (MoO3) at 650 °C and 800 °C. The materials are nanocrystalline and highly porous, the samples produced at 650 °C still contain the partly reduced precursor (MoO2). In situ X-ray diffraction studies performed on a laboratory instrument equipped with a catalysis reaction chamber reveal significant compositional and structural changes taking place during the reaction. Ball milling, known to enhance the activity of catalysts, has a deleterious effect on the molybdenum nitride specimens: it gives an initial boost due to the reduction of size and increase in dislocation content, but it also favours the formation of the less active hexagonal MoN.

Catalytic hydrodeoxygenation (HDO) of phenol over supported molybdenum carbide, nitride, phosphide and oxide catalysts

The performance of titania-supported molybdenum carbide, nitride, phosphide and oxide catalysts was compared for catalytic hydrodeoxygenation (HDO) of phenol. Phenol was selected as a stable model component for lignin degradation products in fast pyrolysis bio-oil. The synthesis and formation path of the materials was evaluated by the use of complementary characterization techniques (XRD, TPR and TGA–MS). All the catalysts showed promising catalytic performance during testing in a fixed-bed reactor set-up at a temperature of 350°C and total pressure of 25bar, and the highest activity was achieved with 15wt% Mo2C/TiO2. A high selectivity towards benzene was observed with all catalysts. MoP supported on TiO2 showed the strongest tendency to aromatic ring hydrogenation and also a significant selectivity (10% level) towards methylcyclopentane, indicating contributions of acidic surface chemistry. Thus, Mo based materials are active and promising materials for establishing selectivity control in HDO of bio-oils.

Unsupported Molybdenum Carbide and Nitride Catalysts for Polychlorinated Biphenyls Hydrodechlorination

Unsupported Mo carbide and Mo nitride catalysts have been synthesized by temperature programmed synthesis, and characterized by X-ray diffraction, X-ray photoelectron spectroscopy, H2-temperature programmed reduction and desorption and N2 physisorption. These solid compounds have been tested for the hydrodechlorination (HDCl) of polychlorinated biphenyls (PCBs) at 1 bar in a fixed bed reactor. The Mo carbides and the corresponding Mo nitride phases showed a catalytic activity higher than a conventional HDS catalyst consisting of alumina-supported NiMo sulfides (~35 %). Aroclor 1260 HDCl conversion results were related to spilt-over hydrogen species that were proposed to favor the hydrogenolysis of C–Cl bond on refractory phases. The original PCBs broad distribution changed almost completely to non-chlorinated biphenyl on the carbide and nitride catalysts.

Hydrodeoxygenation of guaiacol over carbon-supported molybdenum nitride catalysts: Effects of nitriding methods and support properties

Molybdenum nitride catalysts supported on activated carbon materials with different textural and chemical properties were synthesized by nitriding supported Mo oxide precursors with gaseous NH3 or N2/H2 mixtures using a temperature-programmed reaction. The supports and catalysts were characterized by N2 physisorption, XRD, chemical analysis, TPD, FT-IR and XPS. Guaiacol (2-methoxyphenol) hydrodeoxygenation (HDO) activities at 5MPa and 300°C were evaluated in a batch autoclave reactor. Molybdenum nitrides prepared using a N2/H2 mixture resulted in more highly dispersed catalysts, and consequently more active catalysts, relative to those prepared using ammonolysis. The HDO activity was also related to pore size distribution and the concentration of oxygen-containing surface groups of the different carbon supports. Increased mesoporosity is argued to have facilitated the access to active sites while increased surface acidity enhanced their catalytic activity through modification of their electronic properties. The highest activity was thus attributed to the highest dispersion of the unsaturated catalyst species and the highest support mesoporosity. Surprisingly, addition of Co did not improve the HDO activity.

Comparison of alumina- and SBA-15-supported molybdenum nitride catalysts for hydrodeoxygenation of guaiacol

The hydrodeoxygenation of guaiacol (2-methoxyphenol) has been studied in a batch reactor over alumina- and SBA-15 silica-supported molybdenum nitride catalysts at 300°C and 5MPa of hydrogen pressure. The catalysts were prepared by nitriding supported Mo oxide precursors with ammonia gas or nitrogen–hydrogen mixtures via temperature-programmed reaction. The alumina-supported catalysts had a higher activity relative to the SBA-15 silica-supported catalysts which was essentially due to catechol production, an effect of the alumina support. The SBA-15 silica-supported catalysts transformed guaiacol directly to phenol by demethoxylation without noticeable catechol production. On both supports, nitridation by ammonolysis increased the activity by a factor of ∼1.1 relative to nitridation by nitrogen–hydrogen. On SBA-15, ammonolysis preferentially produced the γ-Mo2N phase while the N2/H2 mixture produced the β-Mo2N0.78 phase. The incorporation of Co led to a marginal improvement in exposed Mo species but generally had a diminishing effect on HDO activity. The lack of catechol production using the SBA-15 silica support is important in minimizing coking reactions and also opens up possibilities for utilizing silica supports with highly controlled pore sizes to possibly influence product distribution in HDO of more diverse feed streams derived from biomass conversion processes.

Hydrogen‐Evolution Catalysts Based on Non‐Noble Metal Nickel–Molybdenum Nitride Nanosheets

Not noble but effective: A new class of heterogeneous hydrogen-evolving electrocatalysts based on inexpensive components was developed. The carbon-supported nickel–molybdenum nitride (NiMoNx, see picture) catalyst had a nanoscale sheet structure comprising a few layers and an abundance of highly accessible reactive sites.

Hydrodeoxygenation of oleic acid and canola oil over alumina-supported metal nitrides

Nitrides of molybdenum, tungsten and vanadium supported on γ-Al 2O 3 were prepared by temperature-programmed reaction with NH 3 and tested as catalysts for hydrodeoxygenation of oleic acid and canola oil at 380–410 °C and 7.15 MPa H 2. The molybdenum nitride catalyst was found superior to the vanadium and tungsten nitrides for catalytic hydrotreating of oleic acid in terms of fatty acid conversion, oxygen removal and production of normal alkanes (diesel fuel cetane enhancers). The supported molybdenum nitride favoured the hydrodeoxygenation of oleic acid to n-C 18H 38 three times out of four compared to decarbonylation and decarboxylation. A 450-h long hydrotreating test performed at 400 °C and 8.35 MPa H 2 with Mo 2N/Al 2O 3 and canola oil, indicated that oxygen removal exceeded 90% over the duration of the experiment and that the yield of middle distillate hydrocarbons (diesel fuel) ranged between 38 and 48 wt% (based on liquid feed).

Synthesis of bulk and alumina-supported γ-Mo 2N catalysts by a single-step complex decomposition method

A single-step complex decomposition method for the synthesis of bulk and alumina-supported γ-Mo 2N catalysts is described. The complex precursor (HMT) 2(NH 4) 4Mo 7O 24·2H 2O (HMT: hexamethylenetetramine) is converted to γ-Mo 2N under a flow of Ar in a temperature range of 823–1023 K. Furthermore, decomposition of the precursor in a NH 3 flow forms γ-Mo 2N in a temperature range of 723–923 K. Compared with direct decomposition of the precursor in Ar, the reaction in NH 3 shows obvious advantages that the nitride forms at a lower temperature. In addition, alumina-supported γ-Mo 2N catalysts with different nitride loadings can be prepared from the alumina-supported complex precursor in the Ar or NH 3 flow. The resultant catalysts exhibit good dibenzothiophene HDS activities, which are similar to the γ-Mo 2N/γ-Al 2O 3 prepared by traditional TPR method. The catalyst prepared by decomposition in an Ar flow exhibits highest activity. It proves that such a single-step complex decomposition method possesses the potential to be a general route for the preparation of molybdenum nitride catalysts.

Carbon-covered Alumina: A Superior Support of Noble Metal-like Catalysts for Hydrazine Decomposition

Carbon-covered alumina (CCA) were synthesized from mesoporous alumina and a series of carbon sources (including sucrose, furfuryl alcohol, and benzene). They had structural properties of alumina and surface characteristics of carbon. When they were used as supports for molybdenum carbide, nitride, and phosphide catalysts, significantly higher activities were obtained in hydrazine decomposition as compared to those supported on the conventional alumina. The difference in the interactions of catalytic active sites with the CCA and with the alumina supports was preliminarily deemed to be the main cause of the better performance of CCA supported catalysts. Carbon contents on alumina and carbon sources were found to be important for CCA to be a good support. Carbon deposited on alumina in a near monolayer form showed the best activities. In contrast with sucrose and furfuryl alcohol, benzene as the carbon source readily yielded CCA supports with a hydrophobic surface, which resulted in relatively low dispersions of metal and, in turn, decreased activity of the supported catalysts.

Catalytic activities and surface properties of zeolite-supported molybdenum nitrides for NO reduction with H 2

A series of zeolite (H-ZSM-5)-supported molybdenum nitride catalysts with Mo loading ranging from 2 to 30 wt.% were synthesized by temperature-programmed nitridation in a flow of NH 3. The surface properties of the nitride samples were characterized by XPS, H 2-TPR, and XRD techniques and their catalytic activities were evaluated for NO reduction with H 2. For the fresh samples, molybdenum nitrides coexisted with oxides on the zeolite. With the increase of Mo loading from 2 to 30 wt.%, the degree of nitridation increased linearly with the increase of Mo loading. We observed that a catalyst with higher Mo loading exhibited higher initial activities. The nitrided 2 wt.% Mo/H-ZSM-5 catalyst was the most stable and NO conversion to N 2 remained unchanged within a test period of 15 h. For the catalysts with Mo loading above 2 wt.%, catalytic activities decreased with time on stream. After 15 h, the nitrided 2 wt.% Mo/H-ZSM-5 catalyst was the most active among the tested catalysts. The results of H 2-TPR measurements for the used and oxygen-saturated catalysts revealed that catalyst deactivation was a result of oxygen incorporation into the nitride lattices. The strong interaction between the molybdenum species and H-ZSM-5 zeolite as well as the lowering of H 2-reduction temperature of surface oxygen might be the reasons for the good performance of the nitrided 2 wt.% Mo/H-ZSM-5 catalyst for NO reduction with H 2.

The reaction route and active site of catalytic decomposition of hydrazine over molybdenum nitride catalyst

The catalytic properties of the passivated, reduced passivated, and fresh bulk molybdenum nitride for hydrazine decomposition were evaluated in a microreactor. The reaction route of hydrazine decomposition over molybdenum nitride catalysts seems to be the same as that of Ir/ γ-Al 2O 3 catalysts. Below 673 K, the hydrazine decomposes into N 2 and NH 3. Above 673 K, the hydrazine decomposes into N 2 and NH 3 first, and then the produced NH 3 further dissociates into N 2 and H 2. From the in situ FT-IR spectroscopy, hydrazine is adsorbed and decomposes mainly on the Mo site of the Mo 2N/ γ-Al 2O 3 catalyst.

Acetone condensation over molybdenum nitride and carbide catalysts

Acetone condensation was carried out over high surface area molybdenum nitride and carbide catalysts. Rates and selectivities for these materials were compared to those for MgO (a solid base catalyst), USY (a solid acid catalyst) and 1% Pt/SiO 2 (a supported noble metal catalyst). The Mo 2N catalyst as well as MgO and USY catalyzed the condensation of acetone to products including mesityl oxide (MO), mesitylene and isophorone. Areal rates for the Mo 2C and Mo 2N catalysts were higher than those for the MgO and USY catalysts. The Mo 2N catalyst was most active for mesityl oxide production. The formation of mesitylene and isophorone over this catalyst indicated that Mo 2N possessed strong acid and base sites, respectively. Results from the thermal desorption of CO 2 and NH 3 confirmed the presence of these sites. The hydrogenation of acetone to isopropanol (IPA) and its subsequent dehydration to propylene were predominant reactions for the Mo 2C and 1% Pt/SiO 2 catalysts. Differences between the Mo 2N and Mo 2C catalysts are explained based on their base, acid and metallic properties.

In situ IR Spectroscopic Studies on Molybdenum Nitride Catalysts: Active Sites and Surface Reactions

AbstractFor Abstract see ChemInform Abstract in Full Text.

Selective liquid hydrogenation of long chain linear alkadienes on molybdenum nitride and carbide modified by oxygen

The as-synthesized molybdenum nitrides and carbide has higher conversion of alkadienes and selectivity of alkenes for selective hydrogenation of longer chain alkadienes (C 10–C 13) than the supported palladium catalyst used in industry at higher temperature and pressure. The introduction of oxygen element to the surface of molybdenum nitride or carbide is favorable to improvement of selectivity of alkenes. The passivated molybdenum nitride or carbide catalyst pretreated in H 2 at 450 °C has higher yield of alkenes than corresponding unpassivated and passivated molybdenum nitride or carbide, respectively. It was proved by XPS that the modification of molybdenum nitride or carbide by oxygen results in the change of oxidation states of molybdenum and their content. Decreasing of Mo 0 content and improvement of Mo 2+ and Mo 3+ are favorable to the elevation of selectivity of alkenes.

Hydrodesulfurization over intrazeolite molybdenum nitride clusters prepared by using hexacarbonyl molybdenum as a precursor

Metal nitride catalysts have received extensive attention because of their potential high performance for hydrodesulfurization (HDS). In the present study, highly dispersed Mo nitride clusters having a composition of Mo 2N are synthesized in zeolite pores by means of a CVD method using Mo(CO) 6 as a precursor. The catalytic properties of the molybdenum nitride catalysts for the HDS of thiophene are compared with that of an intrazeolite molybdenum sulfide catalyst. The molybdenum nitride catalyst shows a more stable thiophene HDS activity than the molybdenum sulfide catalyst. Molybdenum nitride clusters are only partially sulfided even after a prolonged HDS reaction.

Microcalorimetric and IR spectroscopic studies of CO adsorption on molybdenum nitride catalysts

The adsorption of CO on both nitrided and reduced passivated Mo2N catalysts in either alumina supported or unsupported forms was studied by adsorption microcalorimetry and infrared (IR) spectroscopy. The CO is adsorbed on nitrided Mo2N catalysts on three different surface sites: 4-fold vacancies, Moδ+ (0 < δ < 2) and N sites, with differential heats of CO adsorption decreasing in the same order. The presence of the alumina-support affects the energetic distribution of the adsorption sites on the nitrided Mo2N, i.e. weakens the CO adsorption strength on the different sites and changes the fraction of sites adsorbing CO in a specific form, revealing that the alumina supported Mo2N phase shows lower electron density than pure Mo2N. On reduced passivated Mo2N catalysts the CO was found to adsorb mainly on Mo4+ sites, although some slightly different surface Moδ+ (0 < δ < 2) sites are also detected. The nature, density and distribution of surface sites of reduced passivated Mo2N/γ-Al2O3 were similar to those on reduced MoO3/γ-Al2O3.

In Situ IR Spectroscopic Studies on Molybdenum Nitride Catalysts: Active Sites and Surface Reactions

Recent IR spectroscopic studies on the surface properties of fresh Mo2N/γ-Al2O3 catalyst are presented in this paper. The surface sites of fresh Mo2N/γ-Al2O3, both Moδ+ (0<δ<2) and N sites, are probed by CO adsorption. Two characteristic IR bands were observed at 2045 and 2200 cm-1, due to linearly adsorbed CO on Mo and N sites, respectively. The surface N sites are highly reactive and can react with adsorbed CO to form NCO species. Unlike adsorbed CO on reduced passivated one, the adsorbed CO on fresh Mo2N/γ-Al2O3 behaves similarly to that of group VIII metals, suggesting that fresh nitride resembles noble metals. It is found that the surface of Mo nitrides slowly transformed into sulfide under hydrotreating conditions, which could be the main reason for the activity drop of molybdenum nitride catalysts in the presence of sulfur-containing species. Some surface reactions, such as selective hydrogenation of 1,3-butadiene, isomerization of 1-butene, and hydrodesulfurization of thiophene, were studied on both fresh and reduced passivated Mo2N/γ-Al2O3 catalysts using IR spectroscopy. The mechanisms of these reactions are proposed. The adsorption and reaction behaviors of these molecules on fresh molybdenum nitride also resemble those on noble metals, manifesting the unique properties of fresh molybdenum nitride catalysts. Mo and N sites are found to play different roles in the adsorption and catalytic reactions on the fresh Mo2N/γ-Al2O3 catalyst. Generally, Mo sites are the main active sites for the adsorption and reactions of adsorbates; N sites are not directly involved in catalytic reactions but they modify the electronic properties of Mo sites.