catalysts
Article Characteristics of High Surface Area Molybdenum Nitride and Its Activity for the Catalytic Decomposition of Ammonia
Seo-Hyeon Baek 1,†, Kyunghee Yun 1,†, Dong-Chang Kang 2, Hyejin An 3, Min Bum Park 3, Chae-Ho Shin 1,* and Hyung-Ki Min 4,*
1 Department of Chemical Engineering, Chungbuk National University, Chungbuk 28644, Korea; [email protected] (S.-H.B.); [email protected] (K.Y.) 2 Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 37673, Korea; [email protected] 3 Department of Energy and Chemical Engineering, Incheon National University, Incheon 22012, Korea; [email protected] (H.A.); [email protected] (M.B.P.) 4 Lotte Chemical Research Institute, Daejeon 34110, Korea * Correspondence: [email protected] (C.-H.S.); [email protected] (H.-K.M.); Tel.: +82-43-261-2376 (C.-H.S.) † These authors contributed equally to this work.
Abstract: High surface area (>170 m2 g−1) molybdenum nitride was prepared by the temperature-
programmed nitridation of α-MoO3 with pure ammonia. The process was optimized by adjusting the experimental variables: the reaction temperature, heating rate, and molar flow rate of ammo- nia. The physicochemical properties of the as-formed molybdenum nitride were characterized by
X-ray diffraction, N2 sorption, transmission electron microscopy, temperature-programmed oxida- tion/reduction, and X-ray photoelectron spectroscopy. Of the experimental variables, the nitridation temperature was found to be the most critical parameter determining the surface area of the molyb- denum nitride. When the prepared molybdenum nitride was exposed to air, the specific surface area Citation: Baek, S.-H.; Yun, K.; Kang, rapidly decreased because of the partial oxidation of molybdenum nitride to molybdenum oxynitride. D.-C.; An, H.; Park, M.B.; Shin, C.-H.; However, the surface area recovered to 90% the initial value after H2 treatment. The catalyst with Min, H.-K. Characteristics of High the highest degree of nitridation showed the best catalytic activity, superior to that of unmodified Surface Area Molybdenum Nitride α-MoO3, for the decomposition of ammonia because of its high surface area. and Its Activity for the Catalytic Decomposition of Ammonia. Keywords: molybdenum nitride; ammonia decomposition; hydrogen production; nitridation; Catalysts 2021, 11, 192. https://doi. topotactic transition org/10.3390/catal11020192
Received: 24 December 2020 Accepted: 29 January 2021 Published: 2 February 2021 1. Introduction Transition metal oxides, carbides, nitrides, and phosphides are widely used as catalysts Publisher’s Note: MDPI stays neutral or catalyst supports [1]. Well-known oxide materials include single component oxides, with regard to jurisdictional claims in such as SiO2, Al2O3, TiO2,V2O5, MoO3, and WO3, and their combinations have been published maps and institutional affil- used as catalysts and catalyst supports for reactions such as metathesis, isomerization, iations. hydrogenation, and partial oxidation [2]. In addition, transition metal nitrides, such as Si3N4, AlN, TiN, VN, Mo2N, and WN/W2N, and carbides, such as SiC, Al4C3, TiC, VC, Mo2C, and WC/W2C, have been used in cutting tools and refractory materials owing to their high thermal and chemical stabilities, as well as mechanical strengths [3]. Thus, Copyright: © 2021 by the authors. because of their remarkable thermal and chemical stabilities, these materials are potential Licensee MDPI, Basel, Switzerland. supports or catalysts for use in heterogeneous catalysis. However, the application of This article is an open access article transition metal nitrides and carbides has been limited because of their low specific surface distributed under the terms and areas, which are a result of sintering during their high-temperature synthesis [4,5]. To conditions of the Creative Commons use these transition metal nitrides and carbides as catalysts or catalyst supports, it is Attribution (CC BY) license (https:// necessary to maintain a high surface area to increase the reactant–catalyst contact as much as creativecommons.org/licenses/by/ possible. Volpe and Boudart reported the preparation of high surface area Mo2N and W2N 4.0/).
Catalysts 2021, 11, 192. https://doi.org/10.3390/catal11020192 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, 192 2 of 15
through the topotactic nitridation of MoO3 and WO3 with NH3 at high temperatures [6]. Subsequently, the prepared transition metal nitride compounds were reacted under a CH4/H2 atmosphere to prepare carbide counterparts with a high surface area [7]. Since then, various attempts have been made to synthesize high surface area nitride/carbide materials [8–10]. For example, binary molybdenum nitrides, such as Fe3Mo3N, Co3Mo3N, and Ni3Mo3N, have been prepared by adding Fe, Co, and Ni, respectively, to high surface area Mo2N to improve its catalytic activity and stability [11–14]. As a hydrogen storage medium and carrier, ammonia has a relatively low specific energy cost, especially compared to its alternatives [15–17]. Thus, the catalytic decom- position of ammonia with energy efficient manner is a key technology to utilize it as a hydrogen carrier. Ru-based catalysts are most active for the decomposition of ammonia and are capable of completely converting ammonia to nitrogen at 450 ◦C[18,19], but Ru is expensive. Thus, various attempts have been made to replace these precious metal catalysts [20–23]. As alternative catalyst materials, transition metal nitrides have been widely applied in chemical reactions, such as ammonia synthesis [24–26], carbazole hy- drodenitrogenation [27], indole hydrodenitrogenation [28], hydrazine decomposition [29], and direct NO decomposition [30]. Stoichiometric Mo2N exists as crystalline γ-, δ-, and β-phases depending on the synthesis conditions, and, in general, molybdenum nitride synthesized from α-MoO3 is γ-Mo2N[31,32]. In this study, molybdenum nitride having a high surface area was prepared by the topotactic conversion of α-MoO3 by ammonia treatment at high temperature, and the catalytic activity of these materials for the decomposition of ammonia were investigated. The reaction parameters, such as the molar hourly space velocity (MHSV, which is de- fined as the molar rate of pure ammonia per mole of α-MoO3), reaction temperature, or ramping rate most influencing the surface area of the molybdenum nitride prepared was systematically investigated and correlated with the performance of catalytic ammonia de- composition. The prepared molybdenum nitride was characterized by various techniques, and its stability under an oxidizing, reducing, and inert atmosphere was examined.
2. Results and Discussion
Figure1a shows the plots of NH 3 decomposition and H2 and H2O production during temperature-programmed nitridation, as well as the reduction of α-MoO3 and MoO2, as a function of reaction temperature. H2O, which is generated by the reduction of oxides ◦ by NH3, was detected from ca. 320 C, and the evolution of H2 via the decomposition ◦ of NH3 was observed at ca. 400 C. The evolution of H2 over both oxides started from the peak temperature in the H2O evolution curves, indicating that the decomposition of NH3 proceeds over nitrided α-MoO3 (γ-Mo2N). Dewangan et al. reported that the formation of γ-Mo2N by the nitridation of α-MoO3 with NH3 proceeds via an intermediate γ-Mo2OxN1-x or MoO2 phase [32]. Thus, the hydrogen produced by the initial dissociation of NH3 was not detected and was mostly consumed during the reduction of both oxides. In addition, the decomposition of NH3 and generation of H2 over α-MoO3 require higher temperatures than MoO2 because of the formation of an intermediate γ-Mo2OxN1-x phase from α-MoO3. The reaction parameters affecting the Brunauer–Emmett–Teller (BET) surface area, total pore volume, and average pore diameter during the nitridation of α-MoO3 were in- vestigated by changing the nitridation temperature, heating rate, and MHSV (Figure1b–d , respectively). As shown in Figure1b, the BET surface area of α-MoO3 increased proportion- ally with increase in nitridation temperature, reaching its maximum value of 150 m2 g−1 at ◦ 2 −1 630 C; this is approximately 50 times higher than that (3 m g ) of pristine α-MoO3. In line with the increase in the BET surface area, the total pore volume of α-MoO3 increased pro- portionally to the nitridation temperature up to 770 ◦C, subsequently decreasing rapidly because of the phase transformation of γ-Mo2N to the other phase, such as β-Mo2N. ◦ Wei et al. reported the transformation of γ-Mo2N to β-Mo2N at 600 C and β-Mo2N to ◦ metallic Mo at around 880 C based on the observation of N2 release in their temperature- Catalysts 2021, 11, 192 3 of 15
programmed reduction (TPR) experiments under flowing H2 [33]. Thus, the reduction of the specific surface area at nitridation temperatures higher than 630 ◦C can be attributed to the formation of the β-Mo2N phase. However, a continuous increase in the total pore volume up to 770 ◦C was observed, probably because of the formation of mesopores during the partial transformation of γ-Mo2N to β-Mo2N[34]. The differentiation of γ-Mo2N and β-Mo2N by X-ray diffraction is quite difficult because of the heavy overlap of the character- istic peaks; however, the BET surface areas of γ-Mo2N and β-Mo2N fall into the range of 40–145 and 2–17 m2 g−1, respectively [25,35,36]. Thus, it can be reasonably speculated that ◦ the phase change of γ-Mo2N to β-Mo2N occurred at the temperatures higher than 630 C. The rapid increase in the average pore diameter with associated reduction in the surface area and pore volume over the samples nitrided at temperatures higher than 800 ◦C is a result of the formation of metallic Mo particles. This can be further supported by the observation of X-ray diffraction peaks corresponding to metallic Mo at the temperatures higher than 800 ◦C, as shown in Table S1. In fact, the heating rate during high-temperature treatment of oxide materials is an important factor determining the surface area and particle size [2]. Figure1c shows the textural properties (i.e., BET surface area, total pore volume, ◦ and average pore diameter) of α-MoO3 nitrided in flowing NH3 at 630 C and a MHSV −1 ◦ −1 of 6.2 h at different heating rates (30–240 C h ). For the nitridation of α-MoO3 in ◦ flowing NH3, a low heating rate at temperatures below 450 C is advantageous for the generation of γ-Mo2OxN1-x and HxMoO3 rather than MoO2 [32]. Under these conditions, the H2 formed by the dissociation of NH3 diffuses into the α-MoO3 lattice and produces the HxMoO3 phase by a topotactic reaction in which the loosely held double-thick layers of [MoO6] octahedra in α-MoO3 are broken by H2 insertion [37]. On the other hand, the fast ◦ heating of α-MoO3 below 450 C disturbs the diffusion of H2 into the α-MoO3 lattice, and, as a result, the decomposition of α-MoO3 to MoO2 dominates. As shown in Figure1c, the 2 −1 surface area of nitrided α-MoO3 was maintained higher than 150 m g when a heating rate of less than 120 ◦C h−1 was used but steadily reduced at higher heating rates. This ◦ −1 implies that the diffusion of H2 is limited at heating rates higher than 120 C h , resulting in the decomposition of α-MoO3 to MoO2. This result is also well correlated with the rapid increase in the average pore diameter of this material at heating rates higher than 120 ◦C h−1. Unlike the changes in surface area and average pore diameter, the total pore volume of nitrided α-MoO3 hardly changed over the entire range of heating rates. Figure1d shows the changes in the BET surface area, total pore volume, and average pore diameter of ◦ α-MoO3 nitrided with NH3 at 630 C as a function of MHSV. The α-MoO3 nitrided with −1 NH3 at a low MHSV of 10 h has a relatively low surface area and total pore volume, probably because of the thermal reduction of α-MoO3 to MoO2 at this reaction temperature. ◦ Spevack and McIntyre also observed the thermal reduction of α-MoO3 to MoO2 at 600 C using Raman spectroscopy [38]. The surface area of nitrided α-MoO3 gradually increased with increasing MHSV and leveled off at MHSV values higher than 40 h−1. In addition, the total pore volume of the nitrided sample rapidly increased until the MHSV reached 20 h−1 and was slightly reduced at higher MHSV. The theoretical mass reduction in the transformation of α-MoO3 to γ-Mo2N is 28.5 wt.%, and the nitrogen content in γ-Mo2N is 6.8 wt.%. As shown in Table S1, the mass reduction occurred below the theoretical value for all nitride samples obtained after nitridation, and, in most cases, the MoO2 phase was detected. Thus, to prevent the rapid surface oxidation of the nitrided sample on exposure to air, the sample was passivated with 1% O2/Ar at room temperature, and the weight difference between the theoretical and practical values of the sample can be explained by oxygen adsorption and diffusion into the nitride samples. ◦ Above a nitridation temperature of 700 C, the MoO2 phase was no longer detected, and only the γ-Mo2N phase was observed by X-ray diffraction (XRD) analysis (Figure S1 and Table S1). The lattice parameter of the sample nitrided at 700 ◦C was similar (0.4166 nm) to that of stoichiometric γ-Mo2N(a = 0.4165 nm), which has a face-centered cubic (fcc) structure [31,32]. Above a nitridation temperature of 800 ◦C, metallic Mo was detected by XRD analysis, and this was likely formed by the partial decomposition of γ-Mo2N Catalysts 2021, 11, 192 4 of 15
Catalysts 2021, 11, x FOR PEER REVIEW 3 of 15 (Table S1). This formation of metallic Mo caused a rapid decrease in the surface area and total pore volume, and the porous nature of the sample was lost.
Figure 1. ((aa)) NH NH33 decompositiondecomposition and and production production of of H H2O2O and and H H2 during2 during the the temperature-programmed temperature-programmed nitridation nitridation of α of- ◦ αMoO-MoO3 (open3 (open symbols) symbols) and and MoO MoO2 (closed2 (closed symbols) symbols) from from 200 200 to to630 630 °C forCfor 2 h. 2 h.Standard Standard nitridation nitridation conditions conditions were were an −1 −1 −1 −1 −1 ◦ −1 ◦ −1 ◦ −1 ◦ anMHSV MHSV of 64.2 of 64.2 h (10 h L(10 h Lfor h 1 gfor α-MoO 1 g α3-MoO) at heating3) at heating rates of rates200 °C of h 200 to 200C h °C toand 200 thenC 30 and °Cthen h to 30 630C °C h followedto 630 byC followedmaintenance by maintenance at this temperature at this temperature for 2 h. (b–d for) Changes 2 h. (b–d in) Changes textural inproperties textural propertiesof nitride samples of nitride obtained samples after obtained the nitri- after dation of α-MoO3 as a function of (b) reaction temperature at a heating rate of 30 °C h−1 and◦ MHSV−1 of 64.2 h−1, (c) heating−1 the nitridation of α-MoO3 as a function of (b) reaction temperature at a heating rate of 30 C h and MHSV of 64.2 h , −1 −1 (ratec) heating (heating rate to (heating630 °C at to an 630 MHSV◦C at of an 64.2 MHSV h ), ofand 64.2 (d h) −MHSV1), and (heating (d) MHSV at a (heating rate of 90 at °C a rate h ). of 90 ◦C h−1).
TheFigure reaction2 shows parameters the N 2 adsorption–desorption affecting the Brunauer–Emmett–Teller isotherms of α-MoO (BET)3 andsurface the area, anal- totalogous pore samples volume, nitrided and average at different pore diameter temperatures during for the 7 h.nitridation The α-MoO of α3-MoOsample3 were shows in- vestigateda typical type by changing II isotherm, the whichnitridation is characteristic temperature, of heating non-porous rate, and materials MHSV [ 39(Figure]. This 1b– is − d,further respectively). supported As byshown its very in Figure low BET 1b, the surface BET surface area (2.8 area m2 ofg α1-MoO) and3 totalincreased pore volumepropor- 3 −1 2 tionally(0.007 cm withg increase). However, in nitridation the shape temperature, of the N2 adsorption–desorption reaching its maximum isotherm value of of nitrided 150 m −1 2 −1 gα-MoO at 6303 gradually°C; this is approximately changed to type 50 IV,times characteristic higher thanof that a mesoporous(3 m g ) of pristine material, α-MoO as the3. ◦ Innitridation line with temperature the increase increased in the BET from surface 450 to area, 700 theC, andtotal the pore hysteresis volume loopof α-MoO related3 in- to creasedthe narrow proportionally slit-type pores to the was nitridation observed temp overerature all nitrided up to 770α-MoO °C, 3subsequently. The maximum decreas- BET 2 −1 3 −1 ingsurface rapidly area because (145 m ofg the) and phase total transformation pore volume (0.106 of γ-Mo cm 2Ng to) the were other observed phase, for suchα-MoO as β3- ◦ Monitrided2N. Wei at 650et al.C. reported The maximum the transformation Barrett–Joyner–Halenda of γ-Mo2N to (BJH)β-Mo2 poreN at diameter600 °C and calculated β-Mo2N tofrom metallic the N 2Moadsorption at around branch 880 °C also based gradually on the increased observation from of 1.2 N to2 release 2.2 nm in with their the tempera- increase ture-programmedin nitridation temperature. reduction (TPR) experiments under flowing H2 [33]. Thus, the reduc- tion ofFigure the specific3 shows surface the results area of at simultaneous nitridation temperatures thermogravimetric higher (TG)/differentialthan 630 °C can be ther- at- tributedmal analysis to the (DTA)–mass formation of spectrometry the β-Mo2N (MS) phase. analyses However, of the a continuous nitrided α-MoO increase3 (i.e., inγ the-Mo total2N) poreunder volume inert, oxidizing, up to 770 °C and was reducing observed, atmospheres probably (He,because O2, andof the 20% formation H2/He, of respectively). mesopores duringThe TG-DTA the partial traces transformation shown in Figure of3a,b γ-Mo contain2N to three β-Mo steps2N [34]. of weight The differentiation loss and changes of inγ- ◦ Mothe2 heatN and flow β-Mo in flowing2N by X-ray O2: 0–250,diffraction 250–700, is quite and 700–900difficult becauseC, corresponding of the heavy to the overlap exother- of themic characteristic oxidation of γ peaks;-Mo2N however, to γ-Mo2 OthexN BET1-x or surface MoO2 areas(14% massof γ-Mo increase),2N and totalβ-Mo oxidation2N fall into to theα-MoO range3 (5% of 40–145 mass increase), and 2–17 and m2 meltingg−1, respectively and evaporation [25,35,36]. of Thus,α-MoO it3 ,can respectively. be reasonably The ◦ speculatedsharp endothermic that the peakphase observed change inof theγ-Mo DTA2N curveto β-Mo around2N occurred 800 C representsat the temperatures the phase ◦ highertransformation than 630 of°C. solid The αrapid-MoO increase3 (melting in pointthe average = 795 poreC) to diameter the liquid with phase. associated As shown re- ductionin Figure in3 c,the massive surface consumptionarea and pore of volume O 2 (m/z over= 32)the andsamples evolution nitrided of Nat2 temperatures(m/z = 28) at ◦ higherca. 450 thanC in 800 an °C oxidizing is a result atmosphere of the formation were observed of metallic by Mo quadrupole particles. massThis can spectrometry be further supported by the observation of X-ray diffraction peaks corresponding to metallic Mo at the temperatures higher than 800 °C, as shown in Table S1. In fact, the heating rate during
Catalysts 2021, 11, 192 5 of 15
(QMS), indicating that the total oxidation of γ-Mo2OxN1-x to α-MoO3 occurred at this tem- perature. This is further confirmed by the fact that only peaks corresponding to α-MoO3 ◦ were observed in the XRD pattern (Figure S2c) of γ-Mo2N treated at 700 C in flowing 3 −1 5% O2/He (100 cm min ). The theoretical mass increases for the oxidation of γ-Mo2N to MoO2 and α-MoO3 are 24.3% and 39.8%, respectively. The actual mass increase of less than 20% under an O2 atmosphere is probably due to the partial oxidation of the prepared γ-Mo2N to γ-Mo2OxN1-x or MoO2, especially in the surface region during the passivation ◦ process. The mass losses observed up to 400 C under H2 and He atmospheres mainly orig- inate from the reaction adsorbed ammonia on the partially oxidized α-MoO3 (MoO3·NH3 ◦ → γ-MoOxN1-x), and the mass reduction at temperatures higher than 600 C is probably due to the phase transformation of γ-Mo2N to β-Mo2N, which is well correlated with the decrease in the surface area of this material, as shown in Figure1b. However, as shown in Figure S2b, a significant amount of the MoO2 phase was observed after treatment with 3 −1 γ-Mo2N in a flow of pure He (100 cm min ), probably caused by the decomposition of surface γ-Mo2OxN1-x formed during passivation in 1% O2/He to MoO2. The generation of ◦ nitrogen-containing compounds (i.e., NO, NO2, and N2O) and H2 below 300 C in the He atmosphere supports the presence of reaction with adsorbed ammonia and lattice oxygen ◦ (Figure3d). In addition, the increase in N 2 release at temperatures higher than 600 C confirms the phase transformation to the nitrogen-deficient β-Mo2N phase, and this change Catalysts 2021, 11, x FOR PEER REVIEWwas more significant under the reducing conditions (20% H /He), resulting in the5 higher of 15 2 weight loss shown in Figure3a.
Figure 2. N2 adsorption–desorption isotherms of (a) α-MoO3 and nitrided samples prepared by nitridation of α-MoO3 in Figure 2. N2 adsorption–desorption isotherms of (a) α-MoO3 and nitrided samples prepared by nitridation of α-MoO3 in purepure ammonia ammonia flow flow at at (b (–bf–)f 450,) 450, 550, 550, 600, 600, 650, 650, and and 700 700◦C, °C, respectively. respectively. Nitridation Nitridation conditions: conditions: pure pure ammonia ammonia flow flow rate rate of of 3.4 L h−1 for 0.3 g α-MoO3 at a heating rate of 200 °C h−1 to 200 °C and, then, 60 °C h−1 to the final temperature, followed 3.4 L h−1 for 0.3 g α-MoO at a heating rate of 200 ◦C h−1 to 200 ◦C and, then, 60 ◦C h−1 to the final temperature, followed by maintenance at that temperature3 for 7 h. Insets shows Barrett–Joyner–Halenda pore size distribution calculated from byadsorption maintenance branch. at that temperature for 7 h. Insets shows Barrett–Joyner–Halenda pore size distribution calculated from adsorption branch. Figure 3 shows the results of simultaneous thermogravimetric (TG)/differential ther- mal analysis (DTA)–mass spectrometry (MS) analyses of the nitrided α-MoO3 (i.e., γ- Mo2N) under inert, oxidizing, and reducing atmospheres (He, O2, and 20% H2/He, respec- tively). The TG-DTA traces shown in Figure 3a,b contain three steps of weight loss and changes in the heat flow in flowing O2: 0–250, 250–700, and 700–900 °C, corresponding to the exothermic oxidation of γ-Mo2N to γ-Mo2OxN1-x or MoO2 (14% mass increase), total oxidation to α-MoO3 (5% mass increase), and melting and evaporation of α-MoO3, respec- tively. The sharp endothermic peak observed in the DTA curve around 800 °C represents the phase transformation of solid α-MoO3 (melting point = 795 °C) to the liquid phase. As shown in Figure 3c, massive consumption of O2 (m/z = 32) and evolution of N2 (m/z = 28) at ca. 450 °C in an oxidizing atmosphere were observed by quadrupole mass spectrometry (QMS), indicating that the total oxidation of γ-Mo2OxN1-x to α-MoO3 occurred at this tem- perature. This is further confirmed by the fact that only peaks corresponding to α-MoO3 were observed in the XRD pattern (Figure S2c) of γ-Mo2N treated at 700 °C in flowing 5% O2/He (100 cm3 min−1). The theoretical mass increases for the oxidation of γ-Mo2N to MoO2 and α-MoO3 are 24.3% and 39.8%, respectively. The actual mass increase of less than 20% under an O2 atmosphere is probably due to the partial oxidation of the prepared γ-Mo2N to γ-Mo2OxN1-x or MoO2, especially in the surface region during the passivation process. The mass losses observed up to 400 °C under H2 and He atmospheres mainly originate from thereaction adsorbed ammonia on the partially oxidized α-MoO3 (MoO3∙NH3 → γ- MoOxN1-x), and the mass reduction at temperatures higher than 600 °C is probably due to the phase transformation of γ-Mo2N to β-Mo2N, which is well correlated with the decrease in the surface area of this material, as shown in Figure 1b. However, as shown in Figure S2b, a significant amount of the MoO2 phase was observed after treatment with γ-Mo2N in a flow of pure He (100 cm3 min−1), probably caused by the decomposition of surface γ- Mo2OxN1-x formed during passivation in 1% O2/He to MoO2. The generation of nitrogen-
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containing compounds (i.e., NO, NO2, and N2O) and H2 below 300 °C in the He atmos- phere supports the presence of reaction with adsorbed ammonia and lattice oxygen (Fig- ure 3d). In addition, the increase in N2 release at temperatures higher than 600 °C confirms the phase transformation to the nitrogen-deficient β-Mo2N phase, and this change was Catalysts 2021, 11, 192 6 of 15 more significant under the reducing conditions (20% H2/He), resulting in the higher weight loss shown in Figure 3a.
3 −1 Figure 3. (a) TGA and (b) DTA curves of γ-Mo2N in flowing helium, pure O2, and 20% H2/He (50 cm min ). Evolution of Figure 3. (a) TGA and (b) DTA curves of γ-Mo2N in flowing helium, pure O2, and 20% H2/He (50 cm3 min−1). Evolution of fragment ions observed by QMS during the temperature-programmed decomposition of γ-Mo2N in flowing (c) 5% O2/He fragment ions observed by QMS during the temperature-programmed decomposition of γ-Mo2N in flowing (c) 5% O2/He (100 cm3 min−1) and (d) pure He. This nitride sample was obtained by the nitridation of α-MoO at a reaction temperature 3 −1 3 (100 cm min ) ◦and (d) pure He. This nitride◦ sample−1 was obtained by the−1 nitridation of α-MoO3 at a reaction−1 temperature of 700 C for 7 h at a heating rate of 60 C h in flowing ammonia (10 L h for 1 g of α-MoO3, i.e., MHSV = 64.2 h ). of 700 °C for 7 h at a heating rate of 60 °C h−1 in flowing ammonia (10 L h−1 for 1 g of α-MoO3, i.e., MHSV = 64.2 h−1). Figure4 shows the transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns of α-MoO3 and its nitrided analogs prepared at differ- Figure 4 shows the transmission◦ electron microscopy (TEM) images and selected area ent temperatures (550–700 C). α-MoO3 has plate-like morphology with an orthorhombic electron diffraction (SAED) patterns of α-MoO3 and its nitrided analogs prepared at dif- crystal lattice (a = 3.96, b = 13.86, and c = 3.70 Å) [40]. Because the nitridation proceeds ferentat temperatures high temperatures, (550–700 the generation °C). α-MoO of new3 has pores plate-like observed morphology in the samples with is probably an orthorhom- bic crystaldue to lattice the extraction (a = 3.96, of lattice b = 13.86, oxygen and from c = the 3.70 crystalline Å) [40]. oxynitride Because phase.the nitridation Volpe and proceeds at highBoudart temperatures, [6] and Jaggers the [generation5] reported that of newα-MoO pores3 is transformed observed toinγ -Mothe 2samplesN under anis probably due toNH the3 atmosphere extraction via of a lattice reaction oxygen pathway from in which the γcrystalline-Mo2OxN1- xoxynitrideor MoO2 are phase. reaction Volpe and intermediates. The reaction pathway for α-MoO3 nitridation is mainly dependent on the Boudart [6] and Jaggers [5] reported that α-MoO3 is transformed to γ-Mo2N under an NH3 heating conditions (i.e., temperature and heating rate) [32,41]. In addition, γ-Mo2OxN1-x atmosphereis known via to bea reaction transformed pathway to γ-Mo in2N which at a relatively γ-Mo2O lowxN nitridation1-x or MoO temperature2 are reaction (ca. interme- ◦ ◦ diates.500 TheC), reaction whereas MoOpathway2 starts for to react α-MoO with3 NHnitridation3 only at temperatures is mainly dependent higher than 657 on Cthe heating ◦ and is completely converted to γ-Mo2N at 785 C[6,31]. This is further supported2 byx the1-x conditions (i.e., temperature and heating rate) [32,41]. In addition,◦ γ-Mo O N is known observation of characteristic peaks corresponding to pure γ-Mo2N at 700 C, as shown in to be transformed to γ-Mo2N at a relatively low nitridation temperature (ca. 500 °C), Figure S1. 2 3 whereas AsMoO shown starts in Figure to react4a, the with SAED NH image only of atα -MoOtemperatures3 clearly shows higher reflections than 657 cor- °C and is completelyresponding converted to the (100), to γ-Mo (101),2N and at (001)785 °C facets [6,31]. along This the [010]is further direction supported [5]. After by the the obser- vationtransformation of characteristic of α-MoO peaks3 to corresponding the fcc-structured toγ-Mo pure2O γxN-Mo1-x by2N nitridation at 700 °C, at as high shown tem- in Figure S1. peratures, reflections corresponding to the (002), (020), and (022) facets were also observed along the [100] direction [11,32]. Energy dispersive X-ray spectroscopy (EDS) mapping (Figure4f) shows the uniform distribution of nitrogen on the γ-Mo2N sample prepared at 700 ◦C by nitridation and also indicates the presence of molybdenum oxynitrides, which are generated during the passivation process. However, the large amount of oxygen observed
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◦ Catalysts 2021, 11, x FOR PEER REVIEWin the EDS maps of γ-Mo2N prepared at 700 C suggests the presence of XRD-invisible7 of 15
MoO2 clusters in the sample.
Figure 4. TEM images and SAED patterns of (a) commercial α-MoO3 and nitrided samples treated at (b–d) 550, 600, and Figure 4. TEM images and SAED patterns of (a) commercial α-MoO3 and nitrided samples treated at (b–d) 550, 600, and 700◦ °C, respectively; (e) α-MoO3 and nitrided samples treated at 700◦ °C; and (f) EDS mapping of the nitrided sample shown 700 C, respectively; (e) α-MoO3 and nitrided samples treated at 700 C; and (f) EDS mapping of the nitrided sample shown in (e). Nitridation conditions: pure ammonia flow rate of 3.4 L h−1 for 0.3 g α-MoO3 (MHSV = 72 h−1) at heating rates of 200 in (e). Nitridation conditions: pure ammonia flow rate of 3.4 L h−1 for 0.3 g α-MoO (MHSV = 72 h−1) at heating rates of °C h−1 to 200 °C and 60 °C h−1 to the final temperature, followed by maintenance at 3that temperature for 7 h. 200 ◦C h−1 to 200 ◦C and 60 ◦C h−1 to the final temperature, followed by maintenance at that temperature for 7 h.
As shown in Figure 4a, the SAED image of α-MoO3 clearly shows reflections corre- To examine the stability of γ-Mo2N under oxidizing conditions, a sample was exposed tosponding air for up to tothe 60 (100), days, (101), and theand textural (001) facets properties along the are compared[010] direction in Figure [5]. After5. Compared the trans- formation of α-MoO3 to the fcc-structured2 −1 γ-Mo2OxN1-x by nitridation◦ at high tempera- to the initial surface area (120 m g ) of γ-Mo2N prepared at 700 C for 7 h, those of the samplestures, reflections exposed corresponding to air gradually to decreased the (002), over(020), time, and as(022) did facets their totalwere porealso volumes.observed along the [100] direction [11,32]. Energy dispersive X-ray spectroscopy (EDS) mapping After 60 days, we measured the N2 adsorption–desorption isotherm of the γ-Mo2N sample, which(Figure revealed 4f) shows a the change uniform to a distribution type II nonporous of nitrogen material, on the as γ-Mo well2N as sample reductions prepared in the at surface700 °C by area nitridation and total and pore also volume indicates of 88% the and presence 81%, respectively, of molybdenum compared oxynitrides, to the initialwhich values.are generated This is during a result the of passivation the diffusion process. of oxygen However, into the the nitride large amount crystal lattice,of oxygen which ob- resultsserved inin thethe formationEDS maps of of molybdenum γ-Mo2N prepared oxynitride at 700 and °C thesuggests observed the reductionpresence of in XRD- pore volumeinvisible and MoO specific2 clusters surface in the area. sample. After exposure to air for seven days, the initial γ-Mo2N phaseTo did examine not change the stability significantly, of γ-Mo as shown2N under by oxidizing the XRD analysis,conditions, which a sample contained was noex- peaksposed corresponding to air for up to to 60 molybdenum days, and the oxide textur phasesal properties (Figure S3).are compared However, afterin Figure air exposure 5. Com- ◦ ◦ forpared seven to the days, initial the surface reflection area corresponding (120 m2 g−1) of to γ-Mo the (111)2N prepared facet shifted at 700 from °C for 37.4 7 h, tothose 37.2 of inthe 2 θsamples. This small exposed shift to (∆ air= 0.2gradually◦) indicates decreased the expansion over time, of as the did fcc their unit total cell frompore 0.4163volumes. to 0.4180After 60 nm. days, This we volume measured expansion the N induced2 adsorption–desorption by oxygen diffusion isotherm into bulk of theγ-Mo γ-Mo2N2N causes sam- poreple, which blocking revealed and, finally, a change the transformationto a type II nonporous to γ-Mo material,2OxN1-x, whichas well results as reductions in a decrease in the insurface the surface area and area total and pore total volume pore volume. of 88% However, and 81%, the respectively, oxynitride compared sample exposed to the toinitial air recoveredvalues. This the is textural a result properties of the diffusion of the initial of oxygen nitride into (by morethe nitride than 90% crystal the initiallattice, values) which ◦ afterresults H2 intreatment the formation at 500 ofC molybdenum for 2 h (Figure oxynitride S4). and the observed reduction in pore volumeAs shownand specific in Figure surface6, the area. distribution After exposure of Mo, N, to and air Ofor in seven the γ days,-Mo2 Nthe sample initial exposedγ-Mo2N tophase air for did three not dayschange was significantly, characterized as byshown X-ray by photoelectron the XRD analysis, spectroscopy which contained (XPS) depth no profilingpeaks corresponding analysis. As theto molybd sputteringenum time oxide of the phases Ar ion (Figure beam S3). increased, However, the concentrationafter air expo- ofsure Mo for increased seven days, and thosethe reflection of N and correspond O decreased.ing Theto the initial (111) surface facet shifted ratios of from N/Mo 37.4° and to O/Mo37.2° in in 2θ the. This samples small are shift 0.7 (Δ and = 0.2°) 0.4, indicates respectively, the andexpansion these ratios of the continuously fcc unit cell from decreased 0.4163 to 0.4180 nm. This volume expansion induced by oxygen diffusion into bulk γ-Mo2N causes pore blocking and, finally, the transformation to γ-Mo2OxN1-x, which results in a
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decrease in the surface area and total pore volume. However, the oxynitride sample ex- posed to air recovered the textural properties of the initial nitride (by more than 90% the initial values) after H2 treatment at 500 °C for 2 h (Figure S4).
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decrease in the surfacewith area the and increase total in sputteringpore volume. time. The However, high O/Mo ratio,the oxynitride especially in the sample surface region,ex- posed to air recovered indicatesthe textural the oxidation properties of the γof-Mo the2N surfaceinitial duringnitride the (by passivation more than with air.90% As the Ar ion sputtering proceeded, the ratio of N/Mo and O/Mo gradually decreased and approached initial values) after H2 treatment at 500 °C for 2 h (Figure S4). the values of Mo2N0.7O0.3.
Figure 5. Changes in (a) textural properties and (b) N2 adsorption–desorption isotherms of nitride sample as a function of exposure time in air. The inset image shows the BJH pore size distribution calculated from the adsorption branch. The nitride samples were prepared by heating 1 g α-MoO3 at 700 °C for 7 h at heating rates of 200 °C h−1 to 200 °C and 60 °C h−1 to the final temperature in pure ammonia at a flow rate of 3.4 L h−1 (MHSV = 21.6 h−1).
As shown in Figure 6, the distribution of Mo, N, and O in the γ-Mo2N sample exposed to air for three days was characterized by X-ray photoelectron spectroscopy (XPS) depth profiling analysis. As the sputtering time of the Ar ion beam increased, the concentration of Mo increased and those of N and O decreased. The initial surface ratios of N/Mo and O/Mo in the samples are 0.7 and 0.4, respectively, and these ratios continuously decreased
with the increase in sputtering time. The high O/Mo ratio, especially in the surface region, Figure 5. Changes in (a) textural properties and (b)N2 adsorption–desorption isotherms of nitride sample as a function Figure 5. Changes in (a) textural properties and (b) N2 adsorption–desorption isotherms of nitride indicatesof exposure the time oxidation in air. The inset of imagethe γ shows-Mo2 theN BJHsurface pore size during distribution the calculatedpassivation from the with adsorption air. As branch. Ar ion sample as a function of exposure time in air. The inset image◦ shows the BJH pore◦ size−1 distribution◦ sputteringThe nitride samples proceeded, were prepared the byratio heating of 1N/Mo g α-MoO an3 atd 700 O/MoC for gradually 7 h at heating ratesdecreased of 200 C handto approached 200 C and calculated60 ◦C h− 1fromto the the final adsorption temperature in branch. pure ammonia The nitride at a flow ratesamples of 3.4 L we h−1re(MHSV prepared = 21.6 hby−1 ).heating 1 g α-MoO3 at the700 °Cvalues for 7 ofh at Mo heating2N0.7O rates0.3. of 200 °C h−1 to 200 °C and 60 °C h−1 to the final temperature in pure ammonia at a flow rate of 3.4 L h−1 (MHSV = 21.6 h−1).
As shown in Figure 6, the distribution of Mo, N, and O in the γ-Mo2N sample exposed to air for three days was characterized by X-ray photoelectron spectroscopy (XPS) depth profiling analysis. As the sputtering time of the Ar ion beam increased, the concentration of Mo increased and those of N and O decreased. The initial surface ratios of N/Mo and O/Mo in the samples are 0.7 and 0.4, respectively, and these ratios continuously decreased with the increase in sputtering time. The high O/Mo ratio, especially in the surface region, indicates the oxidation of the γ-Mo2N surface during the passivation with air. As Ar ion sputtering proceeded, the ratio of N/Mo and O/Mo gradually decreased and approached the values of Mo2N0.7O0.3.
Figure 6. (a) X-ray photoelectron spectroscopy depth profiles of the major elements in γ-Mo N, and (b) O/Mo and N/Mo Figure 6. (a) X-ray photoelectron spectroscopy depth profiles of the major2 elements in γ-Mo2N, −1 andatomic (b) ratiosO/Mo as aand function N/Mo ofAr atomic ion sputtering ratios time.as a function Depth profiles of Ar were ion obtained sputtering by Ar ion time. sputtering Depth at 0.03profiles nm s were. γ-Mo N obtained by temperature-programmed nitridation of 1.0 g α-MoO at 700 ◦C for 7 h in pure NH at a flow rate of 2 −1 3 3 obtained3.4 L h−1. by Ar ion sputtering at 0.03 nm s . γ-Mo2N obtained by temperature-programmed nitri- dation of 1.0 g α-MoO3 at 700 °C for 7 h in pure NH3 at a flow rate of 3.4 L h−1. The XPS results are shown in Figure7 and Table1. XPS measurements were per- The XPS resultsformed are shown to investigate in Figure the degree 7 and of nitridation,Table 1. reductionXPS measurements of MoO3, and thewere oxidation per- state of Mo with respect to the increase in nitridation temperature. The N 1s peak in the formed to investigate the degree of nitridation, reduction of MoO3, and the+ oxidation state spectra was deconvoluted into three peaks: O-N, NH3, and NH4 species centered at of Mo with respect to397.4–397.7 the increase, 399.0–399.4 in nitridation, and 401.2–401.3 temperature. eV, respectively, The N and 1s an peak overlapped in the Mospectra 3p3/2 peak (395.2–395.6 eV) was also observed [42,43]. The binding energy of Mo 3p (398.7 eV) was deconvoluted into three peaks: O-N, NH3, and NH4+ species centered at 397.4–397.7,3/2 in the unmodified α-MoO3 shifted to lower energy after high-temperature nitridation be- 399.0–399.4, and 401.2–401.3cause of the eV, changes respectively, in the oxidation and state an (Mo overlapped6+ → Mo6+, Mo 5+Mo, and 3p Mo3/24+ peak) and electroneg- (395.2– 395.6 eV) was also observed [42,43]. The binding energy of Mo 3p3/2 (398.7 eV) in the un- modified α-MoO3 shifted to lower energy after high-temperature nitridation because of Figure 6. (a) X-ray photoelectron spectroscopy depth profiles of the major elements in γ-Mo2N, and (b) O/Mo and N/Mo atomic ratios as a function of Ar ion sputtering time. Depth profiles were obtained by Ar ion sputtering at 0.03 nm s−1. γ-Mo2N obtained by temperature-programmed nitri- dation of 1.0 g α-MoO3 at 700 °C for 7 h in pure NH3 at a flow rate of 3.4 L h−1.
The XPS results are shown in Figure 7 and Table 1. XPS measurements were per- formed to investigate the degree of nitridation, reduction of MoO3, and the oxidation state of Mo with respect to the increase in nitridation temperature. The N 1s peak in the spectra was deconvoluted into three peaks: O-N, NH3, and NH4+ species centered at 397.4–397.7, 399.0–399.4, and 401.2–401.3 eV, respectively, and an overlapped Mo 3p3/2 peak (395.2– 395.6 eV) was also observed [42,43]. The binding energy of Mo 3p3/2 (398.7 eV) in the un- modified α-MoO3 shifted to lower energy after high-temperature nitridation because of
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the changes in the oxidation state (Mo6+ → Mo6+, Mo5+, and Mo4+) and electronegativity ativity(3.44 → ( 3.443.04,→ Pauling3.04, Pauling scale) of scale) the ofbonding the bonding elements. elements. In addition, In addition, the intensities the intensities of the of thepeaks peaks corresponding corresponding to Mo to 3p Mo3/2 3pand3/2 O-Nand increased O-N increased as high-t as high-temperatureemperature nitridation nitridation pro- proceededceeded because because of the of the increased increased proportion proportion of ofMo Mo in inMo Mo2N2 Ncompared compared to tothat that in in α-MoOα-MoO3 3 and the the presence presence of of γγ-Mo-Mo2O2OxNx1-Nx.1- Onx. Onthe theother other hand, hand, the peak the peakintensities intensities of adsorbed of adsorbed spe- + + species,cies, such such as NH as NH3 and3 andNH4 NH, were4 , werereduced reduced at higher at higher nitridation nitridation temperatures temperatures because because de- desorptionsorption was was easier. easier. The The O O1s 1speaks peaks in inthe the spectra spectra were were deconvoluted deconvoluted into into three three peaks peaks −− - - corresponding to to O 2 (530.6–530.7(530.6–530.7 eV), eV), OH OH (531.8–531.9(531.8–531.9 eV), eV), and and O-N O-N (532.9–533.1 (532.9–533.1 eV) eV) species originating from α-MoO-MoO3,, adsorbed H2O,O, and γ-Mo-Mo22OOxxNN1-1-x, xrespectively, respectively [44]. [44 ].As As −− - - the nitridation temperature increase increased,d, the atomic concentration of O O22 andand OH OH decreased,decreased, whereas thatthat ofof O-NO-N increased.increased. However,However, ofof thethe threethree components,components, thethe peakpeak correspondingcorrespond- − − toing O to2 Owas2 was dominant, dominant, suggesting suggesting the the formation formation of oxide of oxide layers layers after after passivation passivation with with dilute Odilute2 and O the2 and presence the presence of XRD-invisible of XRD-invisible MoO2 MoOclusters.2 clusters. The Mo The 3d Mo peak 3d waspeak deconvoluted was decon- intovoluted five into peaks, five representing peaks, representing Mo6+, Mo 5+Mo, Mo6+, Mo4+, Mo-N,5+, Mo4+ and, Mo-N, Mo-OH and centered Mo-OH at centered 232.5–232.8, at 230.7–230.8,232.5–232.8, 229.7,230.7–230.8, 228.9–229.0, 229.7, and228.9–229.0, 233.6–233.8 and eV, 233.6–233.8 respectively eV, [45respectively,46]. As the [45,46]. nitridation As temperaturethe nitridation was temperature increased, was the atomicincreased, concentration the atomic of concentration Mo6+ decreased, of Mo whereas6+ decreased, that of Mo-Nwhereas increased. that of Mo-N Moreover, increased. the peak Moreover, of Mo-OH the waspeak drastically of Mo-OH reduced was drastically after nitridation reduced at highafter temperatures.nitridation at high temperatures.
Figure 7. A B C Figure 7. (A) (N) 1s N and 1s and Mo Mo 3p 3p3/2,3/2 (B,() O) O1s, 1s, and and (C () Mo) Mo 3d 3d XPS XPS spectra spectra of (a)(a) αα-MoO-MoO3 3and and molybdenum molybdenum nitride nitride samples samples prepared by nitridation of α-MoO at (b–e) 550, 600, 650, and 700 ◦C, respectively, in pure ammonia flow. prepared by nitridation of α-MoO3 3at (b–e) 550, 600, 650, and 700 °C, respectively, in pure ammonia flow.
TableTable 1. 1.Binding Binding energiesenergies andand atomic percentages obtained from the the XPS XPS analyses analyses of of (a) (a) αα-MoO-MoO3 3andand molybdenum molybdenum nitrides nitrides preparedprepared atat (b–e)(b–e) 550,550, 600,600, 650,650, andand 700700 ◦°C,C, respectively.
N 1sN 1s O 1s O 1s Mo Mo 3d 3d Binding Energy (eV), Binding Energy (eV), Binding Energy (eV), SampleSample Binding Energy (eV), (at.%) Binding Energy (eV), (at.%) Binding Energy (eV), (at.%) (at.%) (at.%) (at.%) Mo 4+ 2− 4+ 5+ 6+ Mo- MoO-N 3p3/2 NHO-N3 NHNH3 NHO 4+ OOH-2− OH- O-N O-N Mo-NMo-N MoMo4+ Mo5+ MoMo6+ Mo-OH 3p3/2 OH (a) 398.7 398.7 530.7 530.7531.9 531.9 232.8232.8 233.7233.7 (a) MoO3 ------MoO3 (100) (100) (88.1) (88.1)(11.9) (11.9) (77.8)(77.8) (22.2)(22.2) (b) 395.6 397.4 399.0 401.2 530.6 531.8 533.1 229.0 229.7 230.8 232.5 233.6 (b) MN (550)395.6 1 397.4 399.0 401.2 530.6 531.8 229.0 229.7 230.8 232.5 233.6 MN (41.9) (29.5) (24.1) (4.5) (69.4) (21.7)533.1 (8.9)(8.9) (5.1) (36.5) (20.5) (32.7) (5.2) 1 (41.9) (29.5) (24.1) (4.5) (69.4) (21.7) (5.1) (36.5) (20.5) (32.7) (5.2) (550) 395.2 397.5 399.2 401.2 530.6 531.8 532.9 229.0 229.7 230.8 232.7 233.8 (c)(c) MN MN (600)395.2 397.5 399.2 401.2 530.6 531.8 229.0 229.7 230.8 232.7 233.8 532.9 (9.7) (600) (44.0) (33.6)(44.0) (17.9)(33.6) (4.5)(17.9) (4.5)(69.2) (69.2)(21.1) (21.1) (9.7) (24.2)(24.2) (20.5)(20.5) (18.7)(18.7) (31.5)(31.5) (5.1)(5.1) (d) 395.2 397.6 399.2 401.3 530.7 531.9 532.9 228.9 229.7 230.8 232.7 233.8 (d) MN (650)395.2 397.6 399.2 401.3 530.7 531.9 532.9 228.9 229.7 230.8 232.7 233.8 MN (46.9) (33.8) (15.1) (4.2) (68.8) (20.8) (10.4) (32.7) (19.7) (17.2) (25.5) (4.9) (46.9) (33.8) (15.1) (4.2) (68.8) (20.8) (10.4) (32.7) (19.7) (17.2) (25.5) (4.9) (650) 395.2 397.7 399.4 401.3 530.7 531.8 532.9 228.9 229.7 230.7 232.7 233.8 (e) MN (700) (e) MN 395.2 397.7(49.4) 399.4(34.1) 401.3(13.1) (3.4)530.7 (68.1)531.8 (20.5)532.9 (11.4) 228.9(34.2) 229.7(19.3) (17.1)230.7 (25.0)232.7 (4.4)233.8 (700) (49.4) (34.1) (13.1) (3.4) (68.1) (20.5) (11.4) (34.2) (19.3) (17.1) (25.0) (4.4) 1 Values in parentheses denote the nitridation temperature in degrees Celsius. 1 Values in parentheses denote the nitridation temperature in degrees Celsius.
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Catalysts 2021The, 11, 192 conversions of NH3 during the nitridation of α-MoO3 at various temperatures10 of 15 were shown in Figure 8. It is obvious that the nitridation of α-MoO3 and catalytic decom- position of NH3 over nitrided α-MoO3 proceed with the concurrent manner, and their per-
formances are increases Thewith conversions the elevation of NH of3 during temperature. the nitridation However, of α-MoO a3 sharpat various increase temperatures of NH3 conversion waswere observed shown in between Figure8. It 550 is obvious and 600 that the°C, nitridation probably of dueα-MoO to3 theand catalyticfacilitated decom- position of NH3 over nitrided α-MoO3 proceed with the concurrent manner, and their 3 3 nitridation of α-MoOperformances. As shown are in increases Figure with 1b, thethe elevation BET surface of temperature. areas of However, nitrided a sharp α-MoO increase ◦ was almost proportionalof NH to3 conversion the temperatures was observed up between to 600 550 °C, and while 600 C, no probably specific due correlation to the facilitated between the surface nitridationarea and oftheα-MoO conversion3. As shown of inNH Figure3 was1b, observed. the BET surface However, areas of nitrided the atomicα-MoO 3 ◦ content of Mo-N specieswas almost in Mo proportional 3d XPS analysis to the temperatures (Table 1) upwas to 6004.7 C,times while increased no specific at correlation this between the surface area and the conversion of NH3 was observed. However, the atomic temperature range (550–600content of °C) Mo-N indicating species in Mothe 3dfacilitated XPS analysis structural (Table1) was change 4.7 times from increased α-MoO at this3 ◦ or Mo2OxN1-x to Mo2temperatureN. In addition, range (550–600the inductionC) indicating period the facilitatedin the conversion structural change of NH from3 wasα-MoO 3 observed during theor decomposition Mo2OxN1-x to Mo 2ofN. NH In addition,3 at all the temperatures induction period except in theconversion 700 °C indicating of NH3 was ob- ◦ served during the decomposition of NH3 at all temperatures except 700 C indicating that that the transition of α-MoO3 or Mo2OxN1-x to Mo2N still proceeds at the reaction condition. the transition of α-MoO3 or Mo2OxN1-x to Mo2N still proceeds at the reaction condition.
Figure 8. Evolution of NH3 conversion during the temperature programmed nitridation of α-MoO3 at different temperatures Figure 8. Evolution of NH3 −conversion1 during the temperature programmed nitridation◦ of α-MoO3 in flowing pure ammonia, 3.4 L h for 0.3 g of α-MoO3;(a) 450, (b) 500, (c) 550, (d) 600, (e) 650, and (f) 700 C. at different temperatures in flowing pure ammonia, 3.4 L h−1 for 0.3 g of α-MoO3; (a) 450, (b) 500, (c) 550, (d) 600, (e) 650, and (f) 700 °C.
Figure 9a shows the XRD patterns of molybdenum nitride samples prepared at 650 °C for 7 and 24 h, respectively. It is obvious that the remaining portion of MoO2 on the molybdenum nitride sample reduced at the prolonged reactions (nitridation and NH3 de- composition). In addition, this further nitridation of sample during the reaction conditions results in the increase of both BET surface area and total pore volume (Figure 9b), which
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Figure9a shows the XRD patterns of molybdenum nitride samples prepared at 650 ◦C for 7 and 24 h, respectively. It is obvious that the remaining portion of MoO on the are favorable for the catalytic performance of molybdenum nitride (Figure 9c). The appar-2 molybdenum nitride sample reduced at the prolonged reactions (nitridation and NH3 ent activation energiesdecomposition). (Ea) of ammonia In addition, decomposition this further nitridation between of sample400 and during 525 the °C reaction over two condi- molybdenum nitridetions samples results in prepared the increase at of 650 both °C BET for surface nitridation area and times total pore from volume 7 and (Figure 24 9hb), which are favorable for the catalytic performance of molybdenum nitrideα (Figure9c). The were calculated using the following relationship: −rNH3 = ko exp(Ea/RT)PNH3 . Here, rNH3◦ is apparent activation energies (Ea) of ammonia decomposition between 400 and 525 C over the number of molestwo of molybdenum NH3 consumed nitride samplesper second prepared per at gram 650 ◦C of for base nitridation MoO times3. As from shown 7 and in 24 h α Figure 8a, the NH3 wereconversion calculated was using higher the following over the relationship: sample prepared−rNH3 = k oforexp( a Elongera/RT)P NH3nitrida-. Here, tion time, indicatingrNH3 thatis the the numbernitridation of moles period of NH is3 anotherconsumed decisive per second parameter per gram ofdetermining base MoO3. As the catalytic activityshown of molybdenum in Figure8a, the NHnitrides.3 conversion Figure was 8b higher shows over thethe sample Arrhenius prepared plots for a longerfor nitridation time, indicating that the nitridation period is another decisive parameter deter- NH3 consumption overmining the the two catalytic molybdenum activity of molybdenum nitride catalysts. nitrides. Figure The8b apparent shows the Arrhenius activation plots energies of ammoniafor decomposition NH3 consumption were over the calculat two molybdenumed to be 28.4 nitride and catalysts. 29.4 kcal/mol The apparent over activation the samples nitrided forenergies 7 and of24 ammonia h, respectively decomposition. The were activation calculated energies to be 28.4 of and the 29.4 molybdenum kcal/mol over the nitride catalysts weresamples slightly nitrided higher for 7 andthan 24 those h, respectively. of molybdenum The activation carbide energies (21.3 of the kcal/mol) molybdenum nitride catalysts were slightly higher than those of molybdenum carbide (21.3 kcal/mol) and supported Ru andcatalysts supported (17.9–20.3 Ru catalysts kcal/mol) (17.9–20.3 reported kcal/mol) in reported the literature in the literature [47]. [Overall,47]. Overall, the results of this studythe results reveal of thisthe study potential reveal theof high potential surface of high area surface molybdenum area molybdenum nitrides nitrides as as catalysts for H production via ammonia decomposition. catalysts for H2 production via 2ammonia decomposition.
FigureFigure 9. 9.(a )( XRDa) XRD patterns, patterns, (b)N 2(badsorption-desorption) N2 adsorption-desorption isotherms, (isotherms,c) catalytic activity (c) catalytic of molybdenum activity nitride of molyb- as a functiondenum of nitride reaction as temperature a function in of the reaction decomposition temperature of ammonia, in the and decomposition (d) Arrhenius’ plot of of ammonia, NH3 consumption and (d over) ◦ molybdenumArrhenius’ nitride plot of catalysts NH3 fromconsumption 400 to 525 C.over The molybdenum molybdenum nitride nitrid samplese catalysts were prepared from 400 in pure to 525 ammonia °C. The flow −1 ◦ −1 ◦ ◦ −1 ◦ (3.4molybdenum L h ) using 0.3 nitride g α-MoO samples3 at heating were rates prepared of 5 C min in pureto 200 ammoniaC and 1 C minflow (3.4to 650 L hC−1 followed) using by0.3 maintenance g α-MoO3 atat that heating temperature rates for of 75 or °C 24 min h. −1 to 200 °C and 1 °C min−1 to 650 °C followed by maintenance at that temperature for 7 or 24 h.
3. Experimental 3.1. Catalyst Preparation Molybdenum nitride having a high specific surface area was synthesized by the tem- perature-programmed nitridation of α-MoO3 (Sigma–Aldrich, St. Louis, MO, USA) in a flow of pure ammonia. The optimal synthetic parameters were investigated by changing the pure ammonia flow rate (2–11.5 L h−1), heating rate (30–240 °C h−1), and nitridation temperature (425–846 °C). The molar hourly space velocity (MHSV, h−1) is defined as the molar flow rate of pure ammonia per mole of α-MoO3 as the molybdenum nitride precur- sor. Typically, pure molybdenum nitride phase was obtained by the nitridation of α-MoO3 at a reaction temperature of 700 °C for 7 h at a heating rate of 200 °C h−1 to 200 °C and 60
Catalysts 2021, 11, 192 12 of 15
3. Experimental 3.1. Catalyst Preparation Molybdenum nitride having a high specific surface area was synthesized by the temperature-programmed nitridation of α-MoO3 (Sigma–Aldrich, St. Louis, MO, USA) in a flow of pure ammonia. The optimal synthetic parameters were investigated by changing the pure ammonia flow rate (2–11.5 L h−1), heating rate (30–240 ◦C h−1), and nitridation temperature (425–846 ◦C). The molar hourly space velocity (MHSV, h−1) is defined as the molar flow rate of pure ammonia per mole of α-MoO3 as the molybdenum nitride precursor. Typically, pure molybdenum nitride phase was obtained by the nitridation of α-MoO3 at a reaction temperature of 700 ◦C for 7 h at a heating rate of 200 ◦C h−1 to 200 ◦C and 60 ◦C h−1 ◦ −1 −1 to 700 C in flowing ammonia (10 L h for 1 g of α-MoO3, i.e., MHSV = 64.2 h ). After 3 −1 cooling, the samples to room temperature in flowing N2 (100 cm g ), passivation was 3 −1 performed using 1% O2/N2 (100 cm min ) for a minimum of 1 h to avoid the rapid oxidation of the external nitride surface.
3.2. Catalyst Characterization XRD analyses were performed to examine the crystal phases of the prepared catalysts using a D8 Discover with a GADDS detector (Bruker AXS, Billericay, MA, USA). The generator current and voltage were 30 mA and 50 kV, respectively, and Cu Kα X-rays (0.15418 nm) were used. The analyses were carried out between 2θ values of 10◦ and 90◦ at a scanning rate of 0.4◦ min−1. To measure the specific surface area, pore size distribution, and total pore volume of the catalyst used, nitrogen adsorption–desorption measurements were performed at liquid nitrogen temperature (−196 ◦C) using an ASAP 2020 (Micromeritics, Norcross, GA, USA). The specific surface area was measured in the relative pressure (P/P0) range of 0.05–0.2 using the BET formula. The total pore volume was calculated from the amount of nitrogen adsorbed at P/P0 = 0.995. The pore size distribution was calculated from the adsorption isotherm using the BJH equation. XPS measurements were performed on a PHI Quantera II analyzer (Al Kα =1486.6 Ev, Ulvac-PHI Inc., Chigasaki, Kanagawa, Japan), and the results were used to investigate the electronic states of the surface elements in the molybdenum nitride samples. The binding energy was corrected based on the C 1s peak at 284.6 eV. Changes in the morphology of the catalysts were observed through TEM, and elemental analyses of the catalyst were performed via EDS analysis. SAED measurements were performed to confirm the crystalline phases. An FEI/Talos F200X (Thermo Fisher Scientific, Waltham, MA, USA) was used for TEM measurements, and an Oxford X-Max50 (Oxford Instruments, Abingdon-on-Thames, UK) was used for EDS analyses; Mn Kα radiation (hυ ≤ 129 eV) produced at an acceleration voltage of 200 kV was used. Samples used for TEM and EDS analyses were dispersed in toluene (>99.8%, SAMCHUN, Pyungtaek, Korea) for 5 min using a small fraction of the prepared catalyst. Temperature-programmed decomposition (TPD) experiments were performed to investigate the decomposition of molybdenum nitride in pure He, O2, and 3 −1 20% H2/He at flow rates of 100 cm min . The TPD experiments were carried out at room temperature to 700 ◦C at a heating rate of 10 ◦C min−1, and the decomposed gases were analyzed by QMS 200 (Balzers, Nashua, NH, USA). Simultaneous TG-DTA (TA Instruments, New Castle, DE, USA) analyses of molybdenum nitride was performed in 3 −1 flowing helium, pure oxygen, and 20% H2/He at flow rates of 50 cm min .
3.3. Catalytic Activity Tests The catalytic activity of molybdenum nitride prepared by the temperature-programmed nitridation of α-MoO3 for ammonia decomposition was tested at atmospheric pressure. The conversion of ammonia is defined using Equation (1).
2 FH produced Conversion of ammonia (%) = 3 2 × 100 (1) FNH3 fed Catalysts 2021, 11, 192 13 of 15
Here, FH2 and FNH3 are the molar flow rates of produced hydrogen and reactant ammonia, respectively. The reactants and products (ammonia, hydrogen, and water) were analyzed using nitrogen as carrier gas and a gas chromatograph (Varian 450GC, Varian Inc., Palo Alto, CA, USA) equipped with a Carbosphere-packed column (1/8” × 1.8 m) and a thermal conductivity detector (TCD).
4. Conclusions
Temperature-programmed nitridation of α-MoO3 was performed to prepare molybde- 2 −1 num nitride with a high surface area (>170 m g ). MoO2 was observed as the intermediate phase. A mixture of molybdenum metal and molybdenum nitride was observed by XRD ◦ analyses after the nitridation of α-MoO3 at temperatures over 800 C, and this phase transformation caused a rapid decrease in the specific surface area. When the prepared molybdenum nitride was exposed to air, the diffusion of oxygen into the bulk nitride resulted in a volume expansion, and a corresponding gradual reduction in the surface area and total pore volume arising from pore blocking was also observed. The easy diffusion of oxygen into the molybdenum nitride resulted in the formation of oxynitrides, but up to 90% of the initial specific surface area was recovered after H2 treatment. The catalytic activity of molybdenum nitride for ammonia decomposition was dependent on the degree of nitridation, and, of the molybdenum nitride samples, that with the highest degree of nitridation showed the highest catalytic activity. The molybdenum nitride obtained by the nitridation at 650 ◦C for 24 h exhibits the conversion of ammonia higher than 94% at the same temperature without significant catalyst deactivation.
Supplementary Materials: The following are available online at https://www.mdpi.com/2073-434 4/11/2/192/s1: Figure S1: XRD patterns of nitride samples obtained after nitridation of MoO3 as a function of (a) reaction temperature and (b) MHSV (h−1) at 650 ◦C. Standard nitridation conditions: −1 −1 ◦ −1 pure ammonia flow rate of 3.36 L h for 0.3 g MoO3 (MHSV = 72 h ) at heating rates of 200 C h to 200 ◦C and 60 ◦C h−1 to the final temperature, followed by maintenance at this temperature for 7 h. Figure S2: (a) XRD pattern of nitride sample and those after temperature-programmed decomposition of the nitride sample at (b) 700 ◦C at a heating rate of 10 ◦C min−1 in flow of pure He (100 cm3 min−1) ◦ ◦ −1 3 −1 and at (c) 700 C at a heating rate of 10 C min in flow of 5% O2/He (100 cm min ). The nitride sample in (a) was obtained by the nitridation of MoO3 under the following reaction conditions: reaction temperature of 700 ◦C for 7 h at a heating rate of 60 ◦C h−1 in pure ammonia at a flow rate −1 −1 of 10 L h for 1 g of MoO3 (MHSV = 64.2 h ). Figure S3: XRD patterns of molybdenum oxynitride samples exposed to air as a function of exposure time: (a) passivated samples after nitridation at ◦ 700 C for (b) 3, (c) 5, and (d) 7 days; Figure S4: N2 adsorption–desorption isotherms and pore size distribution of molybdenum nitride exposed to air for 60 days and (b) treated at 500 ◦C for 2 h in 3 −1 pure H2 (30 cm min ); Table S1: Mass variation, chemical compositions, and XRD phases of nitride samples obtained after the nitridation of MoO3 as a function of reaction temperature, heating rate, and MHSV. Nitridation was carried out at the final temperature for 7 h. Author Contributions: Conceptualization: H.-K.M. and C.-H.S.; methodology: S.-H.B., K.Y., D.-C.K., H.-K.M., and C.-H.S.; investigation: S.-H.B., K.Y., D.-C.K., H.A., M.B.P., H.-K.M., and C.-H.S.; resources: S.-H.B., D.-C.K., and C.-H.S.; data curation: S.-H.B., K.Y., and D.-C.K.; writing—original draft preparation: S.-H.B., K.Y., and D.-C.K.; writing—review and editing: S.-H.B., K.Y., and D.-C.K.; supervision: H.-K.M. and C.-H.S.; project administration: H.-K.M. and C.-H.S.; funding acquisition: C.-H.S. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Research Year of Chungbuk National University in 2020 and the C1 Gas Refinery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2015M3D3A1A01064899). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest. Catalysts 2021, 11, 192 14 of 15
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