Space-confined preparation of high surface area tungsten oxide and tungsten nitride inside the pores of mesoporous silica SBA-15

Microporous and Mesoporous Materials 211 (2015) 147e151

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Space-confined preparation of high surface area tungsten oxide and tungsten nitride inside the pores of mesoporous silica SBA-15 € hler a, *, Annemette Hindhede Jensen c, 1, Simon Meyer a, Hans Beyer b, Klaus Ko Erik Christensen c, Niels J. Bjerrum c €t München, Ernst-Otto-Fischer-Straße 1, 85748 Garching, Germany Department of Chemistry, Catalysis Research Center, Technische Universita €t München, Lichtenbergstraße 4, 85748 Garching, Germany Department of Chemistry, Institute of Technical Electrochemisty, Technische Universita c Department of Energy Conversion and Storage, Technical University of Denmark, Kemitorvet, Building 207, 2800 Kgs. Lyngby, Denmark a

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a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 October 2014 Received in revised form 16 February 2015 Accepted 3 March 2015 Available online 10 March 2015

For the direct preparation of high surface area nitride materials, a lack of suitable precursors exists. Indirect preparation by gas phase nitridation (e.g. by ammonia) requires high temperatures and often results in sintering. The present work demonstrates that the space-confined preparation of W2N inside the pores of ordered mesoporous silica SBA-15 offers a possibility to reduce sintering phenomena and thus to obtain smaller particles, porous structures and a higher surface area material. The preparation was pursued in a two-step approach. First, WO3 was introduced into the channels of SBA-15 and second, ammonolysis was conducted for its conversion to W2N. When performed in the presence of the exotemplate, SBA-15 acts as a stabilizer and small W2N particles (6e7 nm) with a high specific surface area (40 m2 g
1) are obtained after template removal. When the template is, however, removed before nitridation, it cannot stabilize the W2N particles and enhanced sintering occurs. © 2015 Elsevier Inc. All rights reserved.

Keywords: High surface area Mesoporous tungsten oxide Tungsten nitride Space-confined preparation

1. Introduction Interstitial alloys of transition metals have often been proposed as heterogeneous catalysts for a large variety of reactions since the discovery of “Pt-like” properties of tungsten carbide by Levy and Boudart in 1973 [1]. By dissolution of atoms like carbon, nitrogen or oxygen in the lattice of early transition metals, the chemical and physical properties of the host metal are significantly changed [2]. Besides the most widely investigated and applied carbides, increasing interest has been drawn to transition metal nitrides as recently summarized in a review article by Hargreaves [3]. Nitrides have mainly been reported to be active in hydrogenation and dehydrogenation reactions, including ammonia synthesis, but have also been used as base catalysts [4e6]. With this background as motivation, many synthesis procedures to transition metal nitrides with high surface area and defined morphology have been developed. Classically, a metal oxide precursor is converted to the respective nitride by temperature-programmed ammonolysis as

* Corresponding author. Tel.: þ49 89 289 13233; fax: þ49 89 289 13183. €hler). E-mail address: [email protected] (K. Ko 1 Present address: SiOx, Bybjergvej 7, 3060 Espergaerde, Denmark. http://dx.doi.org/10.1016/j.micromeso.2015.03.003 1387-1811/© 2015 Elsevier Inc. All rights reserved.

was first shown for molybdenum and tungsten nitride by Volpe and Boudart [7]. Essentially, the preparation details have not changed since these early works [8]. Synthesis modifications have mainly been carried out on the tungsten precursor side. The temperatureprogrammed ammonolysis was proven to be suitable for conversion of tungstic acid [9], solegel derived WO3 [10], W(NH2)6
xClx [11] and scheelites [12] to W2N. Shi et al. employed the ammonolysis technique to prepare ordered mesoporous chromium and cobalt nitrides [13]. By impregnation of mesoporous silica SBA-15 with oxide precursors, they obtained Cr2O3- or Co3O4- SBA-15 composites with the transition metal oxides trapped in the channels of the silica. They could show that the mesoporous structure could be retained after nitridation to CrN and CoN, respectively. We present a similar approach for the preparation of mesoporous tungsten nitride here. As displayed in Scheme 1, the silica is impregnated with a solution of a WO3 precursor which yields a WO3/SBA-15 composite after calcination. The conversion of WO3 to W2N was investigated both before and after removal of the silica template by etching with hydrofluoric acid. It is shown that the space confinement by SBA-15 avoids sintering during the preparation and leads to W2N with smaller particles and higher surface area. This two-step process offers the possibility of preparing non-

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Scheme 1. Representation of the exotemplated W2N preparation.

oxide materials with a defined morphology. While a very large variety of mesoporous oxides was synthesized by means of templating [14], the preparation of non-oxides often lacks suitable direct precursors [15].

2. Experimental 2.1. Materials All chemicals were purchased from SigmaeAldrich, Germany and used without further purification. Gases were obtained from Westfalen, Germany in a purity of 99.95% (NH3) and 99.996 (Ar), respectively. The preparation of hexagonally ordered SBA-15 was pursued following standard procedures [16,17]. Tribloc copolymer Pluronic® P123 (4 g) was dissolved in water (105 mL) overnight. After heating the solution to 35  C, aqueous HCl (20 mL, 37%) was added and the mixture was stirred for 10 min. Subsequently, tetraethyl orthosilicate (8.8 mL) was added dropwise within 30 min and the solution was stirred at 35  C for 4 h until a white precipitate was formed. After hydrothermal treatment at 100  C for 24 h, it was recovered by filtration, washed with water and calcined under air at 550  C for 5 h (1 K min
1 heating rate). For the exotemplated preparation of WO3, SBA-15 (0.5 g) was impregnated twice with a solution of ammonium metatungstate (1st: 1.54 g, 2nd: 1.03 g) in water equal to a final molar ratio of WO3/ SiO2 ¼ 5/4. After each incipient wetness impregnation step, the mixture was dried at 60  C for 5 h and calcined at 620  C (1 K min
1 heating rate) for 4 h under air. The silica template was removed by treatment with 40% hydrofluoric acid for 1 h at RT and the remaining powder was washed with water. The product was dried and is designated as mWO3 in the following. The notation mWO3/ SBA-15 describes the composite of mWO3 and SBA-15 before removal of the silica template. Nitridation of WO3 was conducted by treatment of the powders in a horizontal tube furnace at 700  C (5 K min
1 heating rate) for 3 h with pure ammonia (100 mL min
1). After the treatment, the gas was changed to Ar and the samples were cooled to RT. Depending on the removal of the SBA-15 template before or after the nitridation, the samples are donated as W2N(1) and W2N(2), respectively, according to Scheme 1.

2.2. Characterization Powder IR spectra were measured on a BioRad FTS 575C FT-IR spectrometer equipped with a MIRacle ATR unit from Pike Technologies.

X-ray powder diffraction analysis was performed on a Philips X'Pert diffractometer using Cu Ka1 irradiation in the range of 5e70  2q. For qualitative phase analysis of the measured XRD patterns, the database of the International Centre for Diffraction Data (ICDD) was used. Low-angle XRD patterns were obtained with a STOE Stadi P diffractometer with Cu Ka1 irradiation in the range between 0.35 and 5  2q. Single-point BET surface area analysis was performed on a Micromeritics Autochem 2910 after outgassing the samples under He for 1 h at 300  C. The porosity of SBA-15, WO3 and W2N powders was determined by nitrogen physisorption at 77 K on a Quantachrome Autosorb-iQ instrument. The samples were pretreated under vacuum at 350  C for 12 h prior to physisorption measurements. Adsorption and desorption isotherms of all samples were recorded in the relative pressure range of 10
5  (p/p0)  0.995. The diameter of pores accessible to nitrogen and the pore volume was determined from the desorption branches of the isotherms using the BJH method developed by Barrett, Joyner and Halenda [18]. Transmission electron microscopy (TEM) was measured on a JEOL JEM 2010 microscope with a LaB6 cathode at 120 kV. 3. Results and discussion 3.1. Exotemplated preparation of WO3 For the preparation of mWO3 as precursor for W2N, an exotemplated route was pursued. In contrast to existing publications by Kang et al. [19] and Rossinyol et al. [20,21], ammonium metatungstate (NH4)6H2W12O40 was used as WO3 precursor instead of phosphotungstic acid. This way, a clean decomposition to only gaseous side products according to equation (1) was obtained during calcination without phosphorous impurities. (NH4)6H2W12O40(s) / 12WO3(s) þ 6NH3(g) þ 4H2O(g)

(1)

After calcination, the SBA-15 exotemplate was removed by etching with an aqueous HF solution. Quantitative removal was proven by the disappearance of the SieO stretching vibrational mode in the IR spectrum of the sample (result not shown). Fig. 1 (right) shows the wide-angle XRD pattern of the formed mWO3 in comparison to the composite of WO3 and SBA-15 and a reference pattern for monoclinic WO3. According to the good agreement of all reflections, mWO3 is phase-pure. No crystalline impurities are detected. The complex diffraction pattern resulting from the low symmetry of the monoclinic crystal system does unfortunately not allow the application of the Scherrer equation to individual

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Fig. 1. Low-angle (left) and wide-angle (right) diffraction patterns of a) SBA-15, b) WO3/SBA-15, c) mWO3, and a WO3 reference pattern (r) from the ICDD database for comparison.

reflections. The analysis of particle sizes is hence only based on transmission electron microscopy for mWO3. Low-angle XRD measurement gives two broad reflections for mWO3, corresponding to d-spacings of 8.4 and 4.2 nm, respectively. A part of the material hence consists of an ordered structure; the low peak intensities however indicate that the mesoporous regularity of SBA-15 could not be completely retained during the preparation. Multipoint BET analysis gives a type IV isotherm, typical for mesoporous materials (Fig. 2b). The hysteresis loop can be identified as type H2 containing sloping adsorption and desorption branches [22]. The specific surface area of the material was found to be 39 m2 g
1. Calculation of the BJH pore size distribution from the desorption branch of the isotherm results in the graph displayed in the inset in Fig. 2b. The small distribution of pore sizes with a maximum at 9.4 nm shows that the material has a uniform mesoporosity with only few smaller pores of 3.8e4.8 nm in diameter. TEM measurement supports this finding as shown in a representative micrograph in Fig. 3. The sample consists of small particles with diameters between 6 and 9 nm with the majority of

Fig. 2. BET adsorptionedesorption isotherm and BJH pore size distribution of a) SBA15 and b) mWO3.

particles having diameters of 7e8 nm. This size equals the diameter of the pores of the SBA-15 template as evidenced by the pore size distribution in Fig. 2a. The space-confined preparation inside the SBA-15 pores hence was successful. Domains of interconnected particles are observed which exist in ordered arrays. Aside these domains, non-agglomerated particles are present. Despite being partially ordered, the material is accordingly not ordered mesoporous throughout its entire volume. This observation correlates with the weak intensifies of the reflections in the low-angle diffraction pattern. 3.2. Conversion of mesoporous WO3 to W2N After preparing mWO3 as described above, temperatureprogrammed ammonolysis was performed to form W2N. On the one hand, the SBA-15 template was removed prior to the WO3eW2N conversion (denoted as W2N(1)) and on the other hand, the composite material of mWO3 and SBA-15 was subjected to ammonolysis, followed by etching of the template with a HF solution after nitridation (denoted as W2N(2)). As shown by the wide-angle XRD patterns in Fig. 4 (right), phase-pure W2N is obtained independent of the preparation route.

Fig. 3. TEM image of mWO3; measured at a magnification of 100 k.

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Fig. 4. Low-angle (left) and wide-angle (right) diffraction patterns of a) SBA-15, b) W2N/SBA-15, c) W2N(1), d) W2N(2) and a W2N reference pattern (r) from the ICDD database for comparison.

Beside the reflections of the W2N (1 1 1), (2 0 0) and (2 2 0) planes, no crystalline phases are detected. The sharp peaks of the WO3 precursor lattice vanished completely, indicating a quantitative conversion of WO3 to W2N. W2N formation by WO3 ammonolysis is accordingly possible inside the SBA-15 pores. This means that no diffusion limitations of NH3 or its decomposition products N2 and H2 exist and may indicate non-complete filling of the SBA-15 pores with WO3. Application of the Scherrer equation to the three distinct reflections results in average particle sizes of 9 ± 2 nm for W2N/ SBA-15, 12 ± 2 nm for W2N(1) and 11 ± 1 nm for W2N(2). In the lowangle range of the diffraction pattern, a weak reflection can be detected in the case of W2N(2) but not of W2N(1). As for mWO3, the intensity is low, indicating partial ordering but no ordered mesoporosity. The BET isotherms for W2N(1) and W2N(2) are of type IV as depicted in Fig. 5, similar to mWO3. Both materials accordingly are mesoporous. The measured specific surface areas are 29 m2 g
1 for

Fig. 5. BET adsorptionedesorption isotherm and BJH pore size distribution of a) W2N(1) and b) W2N(2).

W2N(1) and 40 m2 g
1 for W2N(2). These values appear small when normalized by mass but the BET surface area of W2N(2) equals 33% of the SBA-15 template surface area (787 m2 g
1) when normalizing it to molar amounts and thus taking into account the high molar mass of W2N. In comparison to W2N(1), the porosity is more uniform in case of W2N(2). As shown by the BJH pore size distribution, the maximum lies at 6.7 nm and the pores accordingly have smaller average diameters than the mWO3 pores (9.4 nm). W2N(1) shows a less uniform pore size distribution with a maximum at 17.1 nm. TEM images shown in Fig. 6 confirm this observation. While W2N(1) consists of agglomerated particles with diameters between 8 and 17 nm, the particle size is smaller and more uniform in case of W2N(2). The majority of particles of the latter material has similar diameters of 6e7 nm and exists in partially ordered domains. It can be seen that a part of the particles is non-agglomerated while others adopted the ordered structure of the SBA-15 template. In comparison to the average particle diameters, calculated from the XRD line broadening, TEM analysis results in slightly lower values, especially for W2N(2). The discrepancy of 4e5 nm hints on few larger crystallites outside the displayed region which may be due to extraporous agglomeration. As summarized in Table 1, by converting mWO3 to W2N inside the channels of the exotemplate, a better structural retention is obtained. Apparently, the space confinement by SBA-15 successfully avoids particle growth during the conversion. Etching of the template by HF treatment after ammononlysis does not influence the morphology, similar to the preparation of mWO3. When the template is on the contrary removed before the conversion of mWO3 to W2N, no stabilizing agent is present during the ammonolysis, leading to sintering and formation of larger particles. The specific surface area is reduced this way. The narrow pore size distribution of W2N(2) is unique compared to literature results. When converting commercial WO3 to W2N by ammonolysis, less defined structures with surface areas of 17 m2 g
1 [23] or 21 m2 g
1 [24], respectively were obtained. Ko et al. prepared W2N nanoplates with mesoporosity and surface areas up to 50 m2 g
1 by nitridation of layered WO3$H2O [25] and Giordano et al. obtained small W2N particles (2e10 nm) with a specific surface area of 64 m2 g
1 [26,27]. In their so-called soft urea pathway, the latter authors gelled WCl4 with urea in an ethanol solution which was transformed to W2N by thermal treatment in N2.

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Fig. 6. TEM images of a) W2N(1) and b) W2N(2); both measured at a magnification of 100 k.

Table 1 Textural properties of the prepared WO3 and W2N powders. Sample

Specific surface area (m2 g
1)

TEM particle size (nm)

XRD particle size (nm)

Average pore diameter (nm)

Pore volume (cm3 g
1)

mWO3 W2N(1) W2N(2)

39 29 40

7e8 8e17 6e7

e 12 ± 2 11 ± 1

9.4 17.1 6.7

0.104 0.115 0.082

4. Conclusions

(grant no. 10-093906). Dr. Marianne Hanzlik and Dr. Konstantinos Chatziapostolou are acknowledged for the recording of TEM images.

References [1] [2] [3] [4] [5] [6]

A two-step approach for the preparation of mesoporous W2N was established. Due to a lack of direct W2N precursors, exotemplated preparation of WO3 was first performed and was then converted to W2N by temperature-programmed ammonolysis. The space-confined preparation of WO3 inside the SBA-15 pores leads to high surface area WO3 with small particles of 8e9 nm in diameter. The material can be converted to phase-pure W2N both before and after removal of the SBA-15 template by HF etching. In absence of the template as a stabilizer, significant sintering occurs during the ammonolysis at 700  C. When performed before the removal of the SBA-15, a good stabilization of the particles is observed and a high surface area (40 m2 g
1) material is obtained after HF etching. It consists of small W2N particles with diameters of 6e7 nm and exhibits a narrow pore size distribution. The preparation approach can be transferred to different materials, prepared by gasesolid reactions and represents a promising method for the synthesis of high surface area non-oxides. Acknowledgements The authors gratefully acknowledge the Danish Council for Strategic Research for financial support within the MEDLYS project

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