Spectroscopy in Materials ChemistryView this Special Issue
Synthesis and Characterization of an Iron Nitride Constructed by a Novel Template of Metal Organic Framework
An iron nitride with high surface area was synthesized from an iron-based metal organic framework (Fe-MOF) in this work. During the synthesis process, the Fe-MOF of MIL-53 served as a hard template, a template to impart a certain degree of morphology for iron oxide products and to form porosities for iron nitride products. Moreover, it played the roles of iron sources for the synthesis of the final iron oxides and the iron nitrides. The physicochemical properties of the materials were characterized by a series of technologies including XRD, SEM, and N2-adsorption/desorption. The results showed that the iron nitride synthesized from MIL-53 was α-N. And, the α-N showed the morphology with loosely aggregated particles which favored the formation of rich interparticle porosities. As a result, the surface area of the α-N was larger than those of samples using α-Fe2O3 as precursors and its value was 41 m2/g. In addition, the results agreed that both raw material properties (such as crystallinity and surface areas) and nitriding approaches had significant effects on the surface areas of iron nitrides. Also the results were proved by the iron oxide synthesized with different methods. This new synthetic strategy could be a general approach for the preparation of late transition metal nitrides with peculiar properties.
Transition metal nitride catalysts have attracted great attentions because they exhibit noble metal-like characteristics in many reactions, such as fuel cells, optical coatings, electrical contacts, and catalysts . The key to producing successful catalysts lies in the ability to synthesize materials of high surface areas . Synthetic strategies of transition metal nitrides with high surface areas have been intensively studied, including temperature-programmed reaction of solid-state metal compounds with gaseous nitrogen sources [3–6], pyrolysis of metal precursors , and solution reactions  as well as developed reactive hard template  and soft urea pathway . Recently, Yang and DiSalvo  reported a simple process prepared mesoporous transition metal nitrides (NbN, VN, Ta3N5, and TiN) by the ammonolysis of bulk ternary oxides that contain cadmium. Depending upon the starting composition and the ammonolysis temperature, the TMNs had pore sizes from 10 to 40 nm, domain sizes of 5–40 nm, and the surface areas 15–60 m2/g. However, most of the nitrides are early transition metal nitrides focusing on nitrides of molybdenum and tungsten, and only a few papers have reported the synthesis of late transition metal nitrides particles such as iron nitrides, due to their relatively low thermal stability [8, 12–15]. For example, Koltypin et al.  reported the sonochemical synthesis of iron nitride nanoparticles by two methods, that is, sonication of Fe(CO)5 in a decane solution under a gaseous NH3/H2 mixture and nitridation of sonochemically prepared amorphous iron under a mixed NH3/H2 stream. Later on, iron particles of various grain sizes , amorphous iron prepared through reducing ferrous sulfate by potassium borohydride , and Fe2O3  were also used as precursor to produce iron nitride by a temperature-programmed reaction in NH3. Recently, Choi and Gillan  reported the use of solvothermally moderated metal azide decomposition as a route to nanocrystalline mid to late transition metal nitrides. This method utilizes exothermic solid-state metathesis reaction precursor pairs, namely, metal halides (NiBr2, FeCl3, and MnCl2) and sodium azide, but conducts the metathesis reaction and azide decomposition in superheated toluene. The reaction temperatures are relatively low (<300°C) and yield thermally metastable nanocrystalline hexagonal Ni3N and Fe2N and tetragonal MnN. Moszyński et al.  reported that three iron samples of various mean sizes of crystallites were nitrided in gas mixtures with NH3/H2 ratio increasing stepwise up to the pure ammonia. The higher the mean size of crystallites was, the lower nitriding potential was required to initiate the phase transition observed during the nitriding process. Unfortunately, the surface areas of these iron nitrides are usually lower than 40 m2/g. In order to produce promising iron nitride catalysts, new ways to synthesize iron nitrides with higher surface areas are thus in great demand.
Metal organic frameworks (MOFs) as a kind of emerging nanoporous material, built from connectors and organic linkers, have attracted many attentions due to their intriguing architectures and topologies, tunable pore sizes, and wide potential applications such as gas storage and separation and drug delivery . The various pore sizes, high thermal stability, and synthetically flexible environment of MOFs make them feasible as templates for preparing nanoporous carbon materials , nanoscale NaAlH4 , transition metal nanoparticles , silver cluster , and so forth. Recently, Kim et al.  reported a versatile method to prepare metal nanoparticles supported on nanoporous carbon (M/NC3) via carbonization and carbothermal reduction (CCR) of metal-coordinated IRMOF-3 materials by postsynthetic modification (PSM) with metal precursors (i.e., Ru, W, V, and Ti). Use of IRMOF materials as templates/carbon sources led to desirable pore characteristics in the resulting materials, including high surface area (900–2000 m2/g) coupled with an increased mesoporosity (0.14–0.34 ). However, examples of metal organic framework as hard template to synthesis of iron nitride are rare. Transition metal ions are usually used as connectors; thus, MOFs provide the possibility to be a porous reactive hard template candidate. As a typical example of Fe-MOFs, the common MIL-53(Fe) is chosen, which is built from infinite chains of corner sharing octahedra interconnected by dicarboxylate groups of benzene dicarboxylate units . In this way, a 3D microporous framework with 1D diamond shaped channels with free internal diameter of about 0.85 nm is formed.
Herein, we report the synthesis of iron nitride from iron-based metal organic framework exemplified by octahedral MIL-53(Fe) not only as a reactive hard template but also as a provider of Fe source. The difference in BET surface area of iron nitride between MOFs and Fe2O3 is of interest. For this purpose, Fe2O3 is synthesized by the sol-gel method and MOFs as template ways  according to the literature, respectively. All of the Fe2O3 and MIL-53 were employed to prepare iron nitride and their BET surface areas were analyzed. The proposed preparation strategy is shown in Scheme 1.
2.1. Synthesis of MIL-53(Fe)
All the reagents are of analytic grade from Beijing Chemical Reagent Company and used as received. The MIL-53(Fe) was synthesized according to our prior report ; the mixture of FeCl3·6H2O, terephthalic acid, N,N′-dimethyl formamide (DMF), and hydrofluoric acid (HF) with molar ratios of 1 : 1 : 280 : 2 was heated in Teflon-lined autoclaves. In order to investigate the effect of the crystallinity on the BET surface area, the mixtures were heated at 150°C for 1 day and 3 days. A yellow powder MIL-53(Fe), loaded with DMF inside its pores, was obtained by filtration method. To remove the occluded DMF, the sample was heated at 150°C in air atmosphere overnight and then cooled down to room temperature. To further extract the residual traces of DMF, the sample was stirred with a large volume of deionized water (1.0 g of MIL-53 in 500 mL of water) for 4~6 hours and was filtered, and then it was dried at room temperature in air atmosphere.
2.2. Synthesis of Iron Oxide
The conventional α-Fe2O3 nanoparticle was prepared by heating the mixture of 1.0 mol/L Fe(NO3)3 solution and 2.0 mol/L citric acid solution with the molar ratio of 1 : 1 at 90°C, dropwise adding the glycol of a dispersant into the mixture until it became viscous, and then the mixture was dried at 120°C for 12 h in a crucible to form a gel. Finally, the gel was calcined at 550°C for 2 h to obtain Fe2O3 powder, with the obtained sample designated as FO-0.
The as-synthesized iron precursors (MIL-53(Fe)) were positioned at the end of the quartz tube. The quartz tube was then mounted in the middle of the tube furnace. A flow of air was introduced into the quartz tube at a rate of 30 mL/min and was heated to 400°C for 5 h at a rate of 10°C/min. Then, the system was allowed to cool naturally to room temperature. The two α-Fe2O3 samples were then collected designated as FO-1 and FO-2, corresponding to the reaction time of 1 day and 3 days, respectively.
2.3. Synthesis of Iron Nitride
The as-synthesized iron precursors (MIL-53(Fe), FO-0 and FO-2) were placed in a quartz tube and treated in a flow of nitrogen (30 mL/min) at 150°C to remove air in the system. Then, the precursors were heated in NH3 flow (60 mL/min) at a rate of 10°C min−1 to 500°C for 10 h. After that, the sample was cooled naturally to room temperature, then flowing 1% O2/N2 for passivation treatment to sample surface. The products were denoted as FN-1, FN-2, FN-3, and FN-4, corresponding to MIL-53(Fe)’s with the different reaction time of 1 day and 3 days and the different iron precursors of FO-0 and FO-2, respectively.
2.4. Characterization Techniques
X-ray diffraction (XRD) measurements were performed using a Rigaku D/MAX-2500 X-ray diffractometer with CuKα radiation. Samples were analyzed over a range of 5–80° using a step scan mode with a step rate of 4°/min. Scanning Electron Microscopy (SEM) measurements were performed using a FEI Quanta 200F microscope. Nitrogen adsorption experiments of samples were performed on an ASAP 2020 micropore analyzer. The MIL-53(Fe) samples were evacuated at 150°C for 12 h prior to exposing them to nitrogen gas, while the other samples were evacuated at 300°C for 12 h. Their surface areas were calculated using the BET equation and the volumes were determined using the BJH method.
3. Results and Discussion
3.1. XRD Patterns
To probe the effect of MOFs crystallinity on the preparation strategies, we had synthesized MIL-53(Fe) with different crystallization time. As shown in Figure 1, the XRD patterns of MIL-53(Fe), synthesized with different crystallization time, were similar with our prior report . Relative crystallinity was calculated based on the integrated areas of the MIL-53(Fe) characteristic XRD diffraction peaks at , 11.1°, 16.38°, 17.2°, 18.6°, and 22.3° using Origin software after baseline correction. By calculation, the MIL-53(Fe), with shortened crystallization time, was provided with the relative crystallinity of 82%.
The samples of MIL-53(Fe) with different crystallization time were calcined to obtain Fe2O3 powders; their XRD patterns are displayed in Figure 2. As shown in Figure 2, the XRD patterns of FO-1 and FO-2 were similar with that of FO-0 and all the diffraction peaks were able to be assigned to α-Fe2O3 with a rhombohedral phase (JCPDS card number 33-0064). Also, the results suggested that the hematite could be prepared from the iron-based metal organic framework (Fe-MOF) serving as reactive hard templates. Compared with the relative crystallinity of FO-1 and FO-2, fewer differences than those of MIL-53 used as hard templates existed. The result indicated that MIL-53 possessed little effects on the material crystallinity.
After nitriding, the FN-3 and FN-4 samples from the α-Fe2O3 precursors showed polycrystalline mixtures consisting mainly of and -Fe4N (Figures 3(c) and 3(d)). However, the FN-1 and FN-2 samples all produced ; almost no peaks related to -Fe4N were observed. Usually, iron first undergoes initial nitriding to Fe4N, and a further nitridation occurs to form (or Fe3N1+x) and Fe2N. Here, given the same nitriding conditions, MIL-53(Fe) underwent further nitridation to form . It was mainly attributed to MIL-53(Fe)’s 3D open microporous system facilitating the contact of NH3. Compared with FN-1 and FN-2, the better the crystallinity of MIL-53(Fe) was, the better the crystallinity of the resulting was. The results presented that well-crystallized MIL-53(Fe) was necessary for the formation of iron nitride with high crystallinity.
3.2. SEM Images
SEM images given in Figure 4 showed that the MIL-53(Fe) sample possessed an octahedral morphology; its content and uniformity varied depending on the material crystallinity. The low-crystallinity samples prepared by shortening the crystallization time exhibited some irregular blocks, which were unformed crystals and unreacted raw materials. Further increasing crystallization time may promote irregular particles evolving into clear-cut submicron-sized MIL-53 octahedral crystals.
The morphologies of the oxidized products are shown in Figure 5. The conventional FO-0 (Figure 5(a)) showed compact monodisperse nanospheres with the diameter of approximately 50 nm. However, the other samples from MIL-53(Fe) (Figure 5(b)) presented loosely aggregated particles. Among them, FO-1 prepared from MIL-53(Fe) with low crystallinity exhibited irregular particles with the diameters of about 150 nm. Interestingly, the FO-2 sample (Figure 5(c)) from well-crystallized MIL-53(Fe) exhibited octahedral crystals with the size of about 700 nm, faithfully retaining the octahedral morphology of MIL-53(Fe). It demonstrated that the high crystallinity of MIL-53(Fe) was necessary for the formation of α-Fe2O3 with a regular octahedral morphology. Although a variety of α-Fe2O3 morphologies were reported in some literatures, the octahedral morphology herein, as far as we know, is the first example. Also, the iron oxide with the octahedral morphology was creatively employed to prepare iron nitride.
In this work, the morphologies of iron nitrides prepared by different strategies are given in Figure 6. The samples from MIL-53(Fe) (Figures 6(a) and 6(b)) showed loosely aggregated particles with rich interparticle porosities. The iron nitride FN-3 corresponding α-Fe2O3 had compactly aggregated particles. Interestingly, the FN-4 samples from MIL-53 oxide produced peculiar coralloid iron nitrides (Figure 6(d)). It could be because the nitriding temperature led to the disrupting of the MIL-53(Fe) framework; the octahedral morphology disappeared in iron nitride images.
3.3. N2 Adsorption Isotherms
The textural properties and the isotherms of samples were characterized by the N2 adsorption isotherms. N2 adsorption results (Figure 7 and Table 1) showed that the shortened crystallization MIL-53 sample had a BET surface area of 611 m2·g−1. The surface area was lower than 1031 m2·g−1 for the completing crystallized sample. Similarly, Langmuir surface areas, pore volumes, and pore diameter reduced due to some raw materials blocking the pore. Also, the results (see Figure 8 and Table 1) exhibited that the FO-1 and FO-2 samples from MIL-53 had larger BET surface area and total pore volume than those of the conventional FO-0 sample using sol-gel method. Such higher surface areas for both were believed to benefit from the calcination of the MOFs template. Compared to the FO-1 sample, the FO-2 sample had a relatively higher BET surface of 170 m2·g−1. It illustrated that FO-2 sample was retaining not only the octahedral morphology of MIL-53(Fe) but also the microporous framework. The result was consistent with the SEM image result. As a result, it may be concluded that the crystallinity and BET surface of MOFs template had significant effects on the pore properties of iron oxide.
In addition, in accordance with these observed morphologies, nitrogen adsorption experiment results (see Figure 9 and Table 1) determined that the BET specific area of the sample FN-0~4 was 21, 41, 12, and 17 m2·g−1, respectively. The surface areas of the FN-1 and FN-2 samples prepared from MIL-53(Fe) were larger than those of FN-3 and FN-4 using α-Fe2O3 as the precursor. Although the nitridation reaction finally disrupted the MIL-53(Fe) framework through the decomposition of dicarboxylate units, the initial microporous crystalline framework was beneficial for the formation of interparticle porosities. Accordingly, MIL-53(Fe) was considered as a hard template to impart a certain degree of porosity to the iron nitride product. Apparently, the increasing of BET surface area for iron nitrides from MIL-53 raw material was mainly ascribed to the prior nitriding pathway.
In this work, the iron nitride was creatively prepared through thermal ammonia nitridation of the iron-based metal organic framework (Fe-MOF) serving as the reactive hard template and the iron source. The results demonstrated that the iron nitride directly prepared from the MIL-53(Fe) was provided with the optimal surface area in all the preparation strategies. The high surface area was attributed to the formation of interparticle porosity benefited from the initial microporous crystalline framework of the MIL-53(Fe). Accordingly, the MIL-53(Fe) was considered as the hard template to impart a certain degree of porosity to the iron nitride. Thus, the increasing of BET surface area for iron nitrides from MIL-53 raw material was mainly ascribed to the prior nitriding pathway. In addition, this new synthetic strategy is able to become a general approach for the preparation of late transition metal nitrides with peculiar properties.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors acknowledge the support of this work by the National Natural Science Foundation of China (Grant nos. 21106123 and 21303157).
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