TPD study and carbazole hydrodenitrogenation activity of nitrided molybdena–alumina

The relationship between the activity and surface molybdenum species of nitrided 12.5% MoO₃/Al₂O₃ was studied in the hydrodenitrogenation (HDN) of carbazole at 573 K and 10.1 MPa total pressure. The surface molybdenum species were determined by the desorption of nitrogen gas during TPD. The surface area of NH₃-cooled Mo/Al₂O₃ nitrided at 773 and 1173 K was decreased by 8% and 61% from 245 m² g⁻¹ of the fresh MoO₃/Al₂O₃, respectively. The NH₃-cooled Mo/Al₂O₃ catalysts had slightly higher surface area than the He-cooled catalysts. The HDN rate increased with increasing nitriding temperature in the HDN of carbazole on the nitride catalysts. The NH₃-cooled Mo/Al₂O₃ catalysts nitrided at 1173 K were the most active in carbazole HDN and the He-cooled catalyst nitrided at 773 K was the least.     

Simultaneous hydrodesulfurization of thiophene and hydrogenation of cyclohexene over dimolybdenum nitride catalysts

A series of unsupported dimolybdenum nitride (γ-Mo₂N) catalysts differing in surface area were prepared by temperature programmed reduction of MoO₃ with a mixture of NH₃:N₂ (90:10). Characterization of catalysts by BET, XRD, TPR and XPS techniques was carried out. The samples were used as catalysts in hydrotreating reactions (simultaneous hydrodesulfurization of thiophene and hydrogenation of cyclohexene). Low surface area γ-Mo₂N materials show much higher specific conversions than those with higher surface area. These results indicate that HDS and HYD reactions over γ-Mo₂N seem to be structure-sensitive. The relative exposure extent of crystalline planes (1 1 1) and (2 0 0) over the different catalysts can be associated with their hydrogen adsorption capacities and with their catalytic performances. The catalytic activities are significantly affected by the catalyst pretreatment conditions.     

Synthesis of bulk and alumina-supported γ-Mo2N catalysts by a single-step complex decomposition method

A single-step complex decomposition method for the synthesis of bulk and alumina-supported γ-Mo₂N catalysts is described. The complex precursor (HMT)₂(NH₄)₄Mo₇O₂₄·2H₂O (HMT: hexamethylenetetramine) is converted to γ-Mo₂N under a flow of Ar in a temperature range of 823–1023 K. Furthermore, decomposition of the precursor in a NH₃ flow forms γ-Mo₂N in a temperature range of 723–923 K. Compared with direct decomposition of the precursor in Ar, the reaction in NH₃ shows obvious advantages that the nitride forms at a lower temperature. In addition, alumina-supported γ-Mo₂N catalysts with different nitride loadings can be prepared from the alumina-supported complex precursor in the Ar or NH₃ flow. The resultant catalysts exhibit good dibenzothiophene HDS activities, which are similar to the γ-Mo₂N/γ-Al₂O₃ 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.     

Hydrodesulfurization over supported monometallic, bimetallic and promoted carbide and nitride catalysts

The preparation of alumina-supported β-Mo₂C, MoC_1−x (x≈0.5), γ-Mo₂N, Co–Mo₂C, Ni₂Mo₃N, Co₃Mo₃N and Co₃Mo₃C catalysts is described and their hydrodesulfurization (HDS) catalytic properties are compared to conventional sulfide catalysts having similar metal loadings. Alumina-supported β-Mo₂C and γ-Mo₂N catalysts (Mo₂C/Al₂O₃ and Mo₂N/Al₂O₃, respectively) are significantly more active than sulfided MoO₃/Al₂O₃ catalysts, and X-ray diffraction, pulsed chemisorption and flow reactor studies of the Mo₂C/Al₂O₃ catalysts indicate that they exhibit strong resistance to deep sulfidation. A model is presented for the active surface of Mo₂C/Al₂O₃ and Mo₂N/Al₂O₃ catalysts in which a thin layer of sulfided Mo exposing a high density of sites forms at the surface of the alumina-supported β-Mo₂C and γ-Mo₂N particles under HDS conditions. Cobalt promoted catalysts, Co–Mo₂C/Al₂O₃, have been found to be substantially more active than conventional sulfided Co–MoO₃/Al₂O₃ catalysts, while requiring less Co to achieve optimal HDS activity than is observed for the sulfide catalysts. Alumina-supported bimetallic nitride and carbide catalysts (Ni₂Mo₃N/Al₂O₃, Co₃Mo₃N/Al₂O₃, Co₃Mo₃C/Al₂O₃), while significantly more active for thiophene HDS than unpromoted Mo nitride and carbide catalysts, are less active than conventional sulfided Ni–Mo and Co–Mo catalysts prepared from the same oxidic precursors.     

The highly selective catalytic hydrogenation of CO2 to CO over transition metal nitrides

Three transition metal-like facet centered cubic structured transition metal nitrides, γ-Mo₂N, β-W₂N and δ-NbN, are synthesized and applied in the reaction of CO₂ hydrogenation to CO. Among the three nitride catalysts, the γ-Mo₂N exhibits superior activity to target product CO, which is 4.6 and 76 times higher than the other two counterparts of β-W₂N and δ-NbN at 600 ℃, respectively. Additionally, γ-Mo₂N exhibits excellent stability on both cyclic heating–cooling and high space velocity steady state operation. The deactivation degree of cyclic heating–cooling evaluation after 5 cycles and long-term stability performance at 773 and 873 K in 50 h are all less than 10%. In-situ XRD and kinetic studies suggest that the γ-Mo₂N itself is able to activate both of the reactants CO₂ and H₂. Below 400 ℃, the reaction mainly occurs at the surface of γ-Mo₂N catalyst. CO₂ and H₂ competitively adsorbe on the surface of catalyst and CO₂ is the relatively stronger surface adsorbate. At a higher temperature, the interstitial vacancies of the γ-Mo₂N can be reversibly filled with the oxygen from CO₂ dissociation. Both of the surface and bulk phase sites of γ-Mo₂N participate in the high temperature CO₂ hydrogenation pathway.     

还原温度对Mo基催化剂物相及其加氢脱氧性能的影响

以碳纳米管(CNTs)为载体,通过控制催化剂合成的还原温度制备了一系列负载型Mo基催化剂。采用XRD、TEM、N₂物理吸附、XPS以及NH₃/H₂-TPD等技术对催化剂进行了表征,并研究了Mo基催化剂对硬脂酸催化加氢脱氧性能的影响。结果表明:随着还原温度的升高,催化剂表面的Mo物种逐渐被还原,还原过程为:MoO₃→MoO₂→Mo→Mo₂C。还原温度为450℃和550℃时,催化剂的活性相为MoO₂;还原温度为600℃时,催化剂的活性相为MoO₂/Mo/β-Mo₂C的混合相;还原温度为650℃和700℃时,催化剂的活性相全部转化为β-Mo₂C。与活性相MoO₂催化剂相比,β-Mo₂C催化剂具有更高的加氢脱氧活性。此外,还原温度为600℃的MoO₂/Mo/β-Mo₂C混合相催化剂因具有较大的比表面积、较多的酸中心数量和较强的H₂吸附能力,使得该催化剂在硬脂酸加氢脱氧反应中表现出最优越的催化活性。     

Use of unsupported and silica supported molybdenum carbide to treat chloroarene gas streams

The gas phase catalytic hydrodechlorination (HDC) of mono- and di-chlorobenzenes (423 K ≤ T ≤ 593 K) over unsupported and silica supported Mo carbide (Mo₂C) is presented as a viable means of detoxifying Cl-containing gas streams for the recovery/reuse of valuable chemical feedstock. The action of Mo₂C/SiO₂ is compared with MoO₃/SiO₂ and Ni/SiO₂ (an established HDC catalyst). The pre- and post-HDC catalyst samples have been characterized in terms of BET area, TG-MS, TPR, TEM, SEM, H₂ chemisorption/TPD and XRD analysis. Molybdenum carbide was prepared via a two step temperature programmed synthesis where MoO₃ was first subjected to a nitridation in NH₃ followed by carbidization in a CH₄/H₂ mixture to yield a face-centred cubic (-Mo₂C) structure characterized by a platelet morphology. Pseudo-first order kinetic analysis was used to obtain chlorobenzene HDC rate constants and the associated temperature dependences yielded apparent activation energies that decreased in the order MoO₃/SiO₂ (80 ± 5 kJ mol⁻¹) ≈ MoO₃ (78 ± 8 kJ mol⁻¹) > Ni/SiO₂ (62 ± 3 kJ mol⁻¹) ≈ -Mo₂C (56 ± 6 kJ mol⁻¹) ≈ -Mo₂C/SiO₂ (53 ± 3 kJ mol⁻¹). HDC activity was lower for the dechlorination of the dichlorobenzene reactants where steric hindrance influenced chloro-isomer reactivity. Supporting -Mo₂C on silica served to elevate HDC performance, but under identical reaction conditions, Ni/SiO₂ consistently delivered a higher initial HDC activity. Nevertheless, the decline in HDC performance with time-on-stream for Ni/SiO₂ was such that activity converged with that of -Mo₂C/SiO₂ after three reaction cycles. A temporal loss of HDC activity (less extreme for the carbides) was observed for each catalyst that was studied and is linked to a disruption to supply of surface active hydrogen as a result of prolonged Cl/catalyst interaction.     

CVD preparation of alumina-supported niobium nitride and its activity for thiophene hydrodesulfurization

Niobium nitride was synthesized on a Si(400) substrate and a γ-alumina pellet using a CVD method with a stream of NbCl₅/Ar, NH₃, and H₂ gases at 723–973 K under reduced pressure. The composition and surface properties of the deposited niobium nitride were analyzed using XRD and XPS measurements. The activity of alumina-supported niobium nitrides for the hydrodesulfurization (HDS) of thiophene at 673 K and atmospheric pressure was determined. The alumina had a surface area of 177 m² g⁻¹ and the alumina-supported niobium nitride catalyst had surface areas of 179–190 m² g⁻¹. Although the catalysts had low activity in the initial stages, the activity increased after 200–300 min started to about three times the initial activities. XPS analysis indicated that the activity of the niobium nitride catalysts was decreased by sulfur accumulation on the surface and nitrogen released from niobium nitride. The relationship between the surface properties of the niobium nitride catalysts and the activities for thiophene HDS is discussed.     

Activity, selectivity and durability of VO–ZSM-5 catalysts for the selective catalytic reduction of NO by ammonia

ZSM-5 zeolite was loaded with vanadyl ions (VO²⁺) by treatment of Na–ZSM-5 with aqueous VOSO₄ solution at pH 1.5–2. The catalytic material was tested for the selective catalytic reduction of NO with ammonia at temperatures between 473 and 823 K and normal pressure using a feed of 1000 ppm NO, 1000 (or 1100) ppm NH₃ and 2% O₂ in He. The catalyst proved to be highly active, providing, e.g. initial NO conversions of >90% at 620 l g⁻¹ h⁻¹ (≈400 000 h⁻¹) and 723 K, and selective, providing nitrogen yields equal to NO conversion at equimolar feed in a wide temperature range and only minor N₂O formation at NH₃ excess. Admixture of SO₂ (200 ppm) resulted in an upward shift of the useful temperature range, but did not affect the catalytic behaviour at temperatures ≥623 K. No SO₂ conversion was noted at T ≤ 723 K and 450 l g⁻¹ h⁻¹. The poisoning effect of water (up to 4.5 vol%) was weak at temperatures between 623 and 773 K. VO-ZSM-5 catalysts are gradually deactivated already under dry conditions, probably by oxidation of the vanadyl ions into pentavalent V species. This deactivation is considerably accelerated in the presence of water.     

A comparative study of supported MoO3 catalysts prepared by the new “slurry” impregnation method and by the conventional method: their activity in transesterification of dimethyl oxalate and phenol

A new preparation method for supported MoO₃ catalyst, slurry impregnation, has been described and compared with the conventional impregnation method. Slurry MoO₃/water is used instead of the solution ammonium heptamolybdate, AHM [(NH₄)₆Mo₇O₂₄]. The MoO₃/γ-alumina, MoO₃/active carbon, and MoO₃/silica catalysts with different Mo loadings were prepared by slurry and by conventional method. The low solubility of MoO₃ was sufficient to transport molybdenum species from solid MoO₃ to the adsorbed phase. The equilibrium was achieved after several hours at 95 °C based on the loading amount of molybdenum. Only the process of drying was needed; calcination was not necessary and was left out. This is an important advantage for active carbon support because oxidative degradation of active carbon impregnated by molybdena starts at a relatively low temperature of about 250 °C during calcination on air. The activity was tested in the transesterification of dimethyl oxalate (DMO) and phenol at 180 °C. The dependences of catalytic activity on Mo loadings for the slurry prepared catalysts were similar to the dependences for the samples prepared by the conventional impregnation method with AHM. The activities of the slurry impregnation MoO₃/γ-Al₂O₃ catalysts were almost the same as those of catalysts prepared conventionally. Although the performances of slurry impregnation MoO₃/SiO₂ catalysts for transesterification of DMO were slightly better than those of the corresponding catalysts prepared by conventional impregnation, no waste solution and no calcining nitrogenous gases were produced. Therefore, we conclude that the new slurry impregnation method for preparation of supported molybdenum catalysts is an environmentally friendly process and a simple, clean alternative to the conventional preparation using solutions of (NH₄)₆Mo₇O₂₄. The present work will lead to a remarkable improvement in the catalyst preparation for the transesterification reaction.     

Hydrodenitrogenation of pyridine over alumina-supported iridium catalysts

The catalytic properties of alumina-supported Ir catalysts (≈1 wt% Ir) were studied in the hydrodenitrogenation (HDN) of pyridine at 320°C and 20 bar of pressure in the absence as well as presence of parallel hydrodesulfurization (HDS) of thiophene. The effects of Ir precursor (Ir(AcAc)₃, Ir₄(CO)₁₂, H₂IrCl₆, (NH₄)₂IrCl₆), metal dispersion and sulfur addition were investigated. Ir₄(CO)₁₂ gave the most active catalyst which was ascribed to a lower amount of contaminants originated from the starting Ir compounds rather than to a better Ir dispersion. The decrease of Ir dispersion by sintering in air led to much higher decrease of the rate of C–N bond hydrogenolysis than that of pyridine hydrogenation. The Ir dispersion determined partly the HDN selectivity; a better dispersed Ir phase gave a lower amount of intermediate piperidine. Presulfidation of the reduced catalyst led to 20% decline of the rates of both consecutive HDN steps. An additional and much larger activity decline was caused by the simultaneous execution of HDS. The competitive adsorption of thiophene (or H₂S) was selectively affecting C–N bond hydrogenolysis more than pyridine hydrogenation. The alumina-supported Ir catalysts possessed much higher HDN activity and HDN/HDS selectivity than a conventional NiMo system.     

Selective oxidative dehydrogenation of propane over surface molybdenum-enriched MgMoO4 catalyst

Drastic activity increases were observed by the treatments of the magnesium-rich MgMo_0.99O_y catalysts, which are poorly active for the oxidative dehydrogenation of propane, with inorganic or organic acid to remove excess magnesium on the surface. MoO₃ loading on magnesium-rich MgMo_0.99O_y catalysts also resulted in drastic activity increases. The activity increases followed non-effective loadings of MoO₃ in the range 0–2 wt%, because it is necessary to neutralize the surface magnesium with MoO₃ before the formation of molybdenum-rich surface. The pH of the aqueous (NH₄)₆Mo₇O₂₄ solution for the MoO₃ loading apparently influenced the activity. Under the acidic conditions the MoO₃ loading resulted in the drastic activity increase but under the basic conditions the effect of the MoO₃ loading was poor, suggesting that a cluster-type MoO₃ on MgMoO₄ surface is responsible for the activity of propane oxidative dehydrogenation.     

Simultaneous characterization of acidic and basic properties of solid catalysts by a new TPD method and their correlation to reaction rates

We propose a new TPD method for simultaneously characterizing the acidic and basic properties of solid catalysts by utilizing the co-adsorption of NH₃ and CO₂ on catalysts. First CO₂ was adsorbed on the catalyst sample; then NH₃ was adsorbed on it. Another adsorption sequence of NH₃ and CO₂, and CO₂ and NH₃ single adsorptions were also conducted. The TPD measurements were carried out by heating the catalyst sample from 373 to 773 K at a heating rate of 2.5 K min⁻¹ in a helium stream under a total pressure of 1.3 kPa. In solid acid catalysts, there is little difference in the NH₃-TPD spectra between single and co-adsorption systems. This results from the absence of any induction effect between the acid and base sites, because the number of base sites in the solid acid catalyst is very small. In contrast, in a solid acid–base catalyst of alumina, a remarkable difference in the NH₃-TPD spectra was observed between single adsorption and co-adsorption systems. The difference in the TPD spectra between single and co-adsorption systems was ascribed to a strong induction effect appearing on the acid and base sites, which was proved by an in situ IR measurement. The validity of the TPD method was examined by correlating the number of the strong acid sites to catalytic activities of dehydrolysis of ethanol over solid acid and solid acid–base catalysts. In solid acid–base catalysts, the number of strong acid sites was calculated from the activation energy distribution for the desorption of NH₃ in a co-adsorption system because of the strong induction effect. A proportional relationship between the intrinsic reaction rate constant, which is based on the concentration of ethanol within the catalyst, and the number of strong acid sites could be obtained, regardless of the catalysts or their types or pore structure.     

Lithium-doped sulfated-zirconia catalysts for oxidative coupling of methane to give ethylene and ethane

Li-doped sulfated-zirconia catalysts were found to be effective for oxidative coupling of methane (OCM). The catalyst performances depend on the sulfate content and calcination temperature. A maximum C₂ yield is attained over the catalysts, which contain 6 wt.% sulfate and calcined at 923–973 K, being closely related to the preparation conditions of sulfated-ZrO₂ as solid super-acids. When the performances of the Li-doped sulfated-ZrO₂ (Li/SZ) catalysts were tested at 1023 K as a function of reaction time, both the C₂ and CO_x selectivities remained constant over the range of 8 h, but the CH₄ conversion decreased from 17.5% to 11.9%. The nature of Li/SZ catalysts for the OCM was investigated by X-ray diffraction, XPS, and NH₃ and CO₂ TPD measurements. It could be postulated that the sulfated-ZrO₂ surface could play an important role in the formation of a catalytically active structure by Li-doping.     

AFeO3 (A=La, Nd, Sm) and LaFe1−xMgxO3 perovskites as methane combustion and CO oxidation catalysts: structural, redox and catalytic properties

Catalytic methane combustion and CO oxidation were investigated over AFeO₃ (A=La, Nd, Sm) and LaFe_1−xMg_xO₃ (x=0.1, 0.2, 0.3, 0.4, 0.5) perovskites prepared by citrate method and calcined at 1073 K. The catalysts were characterized by X-ray diffraction (XRD). Redox properties and the content of Fe⁴⁺ were derived from temperature programmed reduction (TPR). Specific surface areas (SA) of perovskites were in 2.3–9.7 m² g⁻¹ range. XRD analysis showed that LaFeO₃, NdFeO₃, SmFeO₃ and LaFe_1−xMg_xO₃ (x·0.3) are single phase perovskite-type oxides. Traces of La₂O₃, in addition to the perovskite phase, were detected in the LaFe_1−xMg_xO₃ catalysts with x=0.4 and 0.5. TPR gave evidence of the presence in AFeO₃ of a very small fraction of Fe⁴⁺ which reduces to Fe³⁺. The fraction of Fe⁴⁺ in the LaFe_1−xMg_xO₃ samples increased with increasing magnesium content up to x=0.2, then it remained nearly constant. Catalytic activity tests showed that all samples gave methane and CO complete conversion with 100% selectivity to CO₂ below 973 and 773 K, respectively. For the AFeO₃ materials the order of activity towards methane combustion is La>Nd>Sm, whereas the activity, per unit SA, of the LaFe_1−xMg_xO₃ catalysts decreases with the amount of Mg at least for the catalysts showing a single perovskite phase (x=0.3). Concerning the CO oxidation, the order of activity for the AFeO₃ materials is Nd>La>Sm, while the activity (per unit SA) of the LaFe_1−xMg_xO₃ catalysts decreases at high magnesium content.     

Catalytic performance of a novel ceramic-supported vanadium oxide catalyst for NO reduction with NH3

A novel TiO₂/Al₂O₃/cordierite honeycomb-supported V₂O₅–MoO₃–WO₃ monolithic catalyst was studied for the selective reduction of NO with NH₃. The effects of reaction temperature, space velocity, NH₃/NO ratio and oxygen content on SCR activity were evaluated. Two other V₂O₅–MoO₃–WO₃ monolithic catalysts supported on Al₂O₃/cordierite honeycomb or TiO₂/cordierite honeycomb support, two types of pellet catalysts supported on TiO₂/Al₂O₃ or Al₂O₃, as well as three types of pellet catalysts V₂O₅–MoO₃–WO₃–Al₂O₃ and V₂O₅–MoO₃–WO₃–TiO₂ were tested for comparison. The experiment results show that this catalyst has a higher catalytic activity for SCR with comparison to others. The results of characterization show, the preparation method of this catalyst can give rise to a higher BET surface area and pore volume, which is strongly related with the highly active performance of this catalyst. At the same time, the function of the combined carrier of TiO₂/Al₂O₃ cannot be excluded.     

A study of the catalytic activity of cobalt–zinc manganites for the reduction of NO by hydrocarbons

In this work the catalytic behaviour of pure zinc manganite, ZnMn₂O₄, and cobalt–zinc manganites for the reduction of NO by propane and propene is reported. The NO and N₂O decomposition as well as the reduction of N₂O by propane and propene were also investigated. The catalysts are prepared starting from carbonate monophasic precursors that are decomposed in air at 973 K for 24 h. In all cases a spinel-like phase is obtained. Pure zinc manganite is an efficient catalyst for the NO reduction with both propane and propene and the selectivity to N₂ and CO₂ was almost one. However the presence of cobalt in the catalyst enhances the catalytic activity, in particular when propene is used as reducing agent of NO. All catalysts are stable up to 873 K upon contacting with the propane containing reactant stream whereas in the case of propene they preserve the original spinel structure up to about 773 K. In fact with propene the catalysts start to lose their stability as the reaction temperature increases above 773 K and disaggregate, by reduction of the spinel framework Mn³⁺ cations to Mn²⁺, forming a complex mixture of ZnO and MnO oxides. Despite the collapsing of the spinel phase, the disaggregated polyphasic catalysts still show a good activity and selectivity. An hypothesis for explaining this unusual behaviour is formulated. Finally, the reaction mechanisms presented in literature are consequently revisited on the basis of the results found in this work.     

MgO-supported Mo, CoMo and NiMo sulfide hydrotreating catalysts

The most common preparation of high surface area MgO (100–500 m² g⁻¹) is calcination of Mg(OH)₂ obtained either by precipitation or MgO hydration or sol–gel method. Preparation of MoO₃/MgO catalyst is complicated by the high reactivity of MgO to H₂O and MoO₃. During conventional aqueous impregnation, MgO is transformed to Mg(OH)₂, and well soluble MgMoO₄ is easily formed. Alternative methods, that do not impair the starting MgO so strongly, are non-aqueous slurry impregnation and thermal spreading of MoO₃. Mo species of MoO₃/MgO catalyst are dissolved as MgMoO₄ during deposition of Co(Ni) by conventional aqueous impregnation. This can be avoided by using non-aqueous impregnation. Co(Ni)Mo/MgO catalysts must be calcined only at low temperature because Co(Ni)O and MgO easily form a solid solution. Literature data on hydrodesulfurization (HDS) activity of MgO-supported catalysts are often contradictory and do not reproduced well. However, some results suggest that very highly active HDS sites can be obtained using this support. Co(Ni)Mo/MgO catalysts prepared by non-aqueous impregnation and calcined at low temperature exhibited strong synergism in HDS activity. Co(Ni)Mo/MgO catalysts are much less deactivated by coking than their Al₂O₃-supported counterparts. Hydrodenitrogenation (HDN) activity of Mo/MgO catalyst is similar to the activity of Mo/Al₂O₃. However, the promotion effect of Co(Ni) in HDN on Co(Ni)Mo/MgO is lower than that on Co(Ni)Mo/Al₂O₃.     

Microstructure of a Molybdenum Nitride Layer Formed by Nitriding Molybdenum Metal

For pure molybdenum nitrided at 1373 K in NH₃ gas, microstructural observations of a molybdenum nitride layer were conducted through a transmission electron microscope. The molybdenum nitride layer consisted of two molybdenum nitrides: an outer one of γ-Mo₂N and an inner one of β-Mo₂N. A great number of (011)[0     1] type twins were observed in the β-Mo₂N phase, whereas the microstructure of the γ-Mo₂N phase was almost free from lattice defects except for slightly observed {111}〈112〉 type twin.