Preparation, Characterization, and Surface Modification of Periodic

Article pubs.acs.org/cm

Preparation, Characterization, and Surface Modification of Periodic Mesoporous Silicon−Aluminum−Carbon−Nitrogen Frameworks O. Majoulet,† C. Salameh,† M. E. Schuster,‡ U. B. Demirci,† Y. Sugahara,§ S. Bernard,*,† and P. Miele*,† †

IEM (Institut Europeen des Membranes), UMR 5635 (CNRS-ENSCM-UM2), Universite Montpellier 2, Place E. Bataillon, F-34095, Montpellier, France ‡ Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4−6, 4195 Berlin, Germany § Department of Applied Chemistry, School of Science and Engineering, Waseda University, Ohkubo 3-4-1, Shinjuku-ku, Tokyo, 169-8555, Japan S Supporting Information *

ABSTRACT: Periodic mesoporous silicon−aluminum−carbon−nitrogen (Si/Al/C/ N) frameworks with P6mm hexagonal symmetry were synthesized by a solvent nanocasting route using mesoporous carbon (CMK-3) as hard template and preceramic polymers containing both −[R1R2Si−N(R3)]n− and −[R4Al−N(R5)]n− backbones (with R1 = R2 = R3 = R4 = H and R5 = CH2CH3) as ceramic precursors. The preceramic polymers are prepared through a simple and cost-effective procedure by blending poly(perhydropolysilazane) and poly(ethyliminoalane) as precursors of silicon nitride/silicon (Si3N4/Si) and carbon-containing aluminum nitride (Al/C/N), respectively. The blended polymers with various and controlled Al:Si ratios were infiltrated into the porous structure of CMK-3, followed by a pyrolysis-template removal cycle performed under nitrogen at 1000 °C (2 h, ceramic conversion), then in an ammonia atmosphere at 1000 °C (5 h, template removal). This procedure resulted in the formation of periodic mesoporous Si/Al/C/N frameworks with surface areas of 182−326 m2 g−1, a pore size distribution of 4.1−5.9 nm, and pore volumes in the range of 0.51−0.65 cm3 g−1. The uniformity of the mesopores and periodicity of the obtained amorphous micrometer-size powders, studied by transmission electron microscopy (TEM), small-angle X-ray diffraction (SA-XRD), and N2 sorption, are affected by the Al:Si ratio. Amorphous materials did not exhibit weight change up to 1400−1470 °C in flowing nitrogen, and their behavior in air, up to 1000 °C (with dwelling time of 5 h), is dependent on the proportion of AlN and Si3N4 phases. The as-obtained powders then were decorated with Pt (nano)particles by impregnation to form supported catalysts. The as-formed catalysts showed attractive reactivity and robustness in our probe reaction, namely, the hydrolysis of an alkaline solution of sodium borohydride at 80 °C. Our main results are reported therein. KEYWORDS: polyaluminosilazane, ordered mesoporosity, silicon−aluminum-carbon−nitrogen, platinum nanoparticles, hydrogen production

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INTRODUCTION

be viewed as more effective supports for catalysts working in harsh environments. Recently, silicon carbide (SiC), silicon nitride (Si3N4), and silicon carbonitride (Si/C/N) systems attracted increasing interest for environmental (diesel filters, water treatment) and energy (fission nuclear reactors) applications according to their properties (high thermal robustness, oxidation and corrosion resistance, low bulk density, high thermal conductivity, high mechanical strength).7,8 However, most of the actual and future industrial challenges related to silicon-containing nonoxide ceramics require the development of materials in which compositions, shapes, and textures are tuned on demand. Traditional techniques are energy-ineffective and severely limit

In modern heterogeneous catalysis, the overall performance of metal (nano)particles is dependent on their size, shape, crystal structure, and textural parameters. Performance can be optimized by suitable control and selection of the support on which (nano)particles are preferentially synthesized. First, this offers the opportunity to control their size and their shape while keeping them from aggregating. Second, this allows easy separation after the reaction, with features resulting in favorable catalytic performances.1,2 Metal-oxide type supports3−5 represent one of the most important classes of catalytic materials in hydrogenation and oxidation reactions as well as in environmental applications.6 However, under severe conditions, such as those for reducing an aqueous solution or high-temperature oxidative medium, the oxide-type ceramics do not always satisfy the requirements in terms of stabilities (chemical, thermal, and mechanical). Porous silicon-containing nonoxide ceramics can © 2013 American Chemical Society

Received: January 27, 2013 Revised: September 24, 2013 Published: September 24, 2013 3957

dx.doi.org/10.1021/cm401605a | Chem. Mater. 2013, 25, 3957−3970

Chemistry of Materials

Article

reaction, and thermochemical water splitting)6,59 could be problematic, because of possible instability. The addition of aluminum to silicon-based ceramics contributes to the improvement of their hydrothermal stability. As an illustration, the addition of Al to Si/N/O systems forming Si/Al/O/N ceramics60 results in an improved thermal and chemical stability in oxidizing atmosphere, in particular in the presence of corrosive species and water vapor, in comparison to aluminum-free Si/N/O systems. Similarly, the addition of aluminum to Si/C/N(O) results into a nonparabolic oxidation curve (at T ≥ 1000 °C), which decreases more rapidly with time, down to a negligible level. This has been well-demonstrated by An et al.61 At 1400 °C, a stationary parabolic rate is observed after 20 h, with parabolic constants ∼10 times lower than those of the aluminum-free Si/C/N(O) systems. Authors suggested that the remarkably low oxidation rates of these materials were attributed to the lower permeability of the formed oxide layer to molecular oxygen, which resulted from the incorporation of aluminum in the silica network. This passivating Si/O/Al layer is shown to hinder diffusion-controlled oxidation in the bulk. Besides, the presence of 1%−10% AlN in SiC results in materials having reduced grain size and improved microstructural uniformity, in comparison to monolithic SiC.62 It was also shown that Si/ Al/C/N ceramics possess superior creep resistance, high fracture toughness, enhanced oxidation, corrosion resistance, and improved thermal conductivity, in comparison to SiC.63−67 Such results clearly prove the potentialities of the Si/Al/C/N systems for harsh environment. Unfortunately, the produced Si/Al/C/N materials do not display substantial surface area whereas mesoporous Si/Al/C/N materials are expected to provide access to a wider range of applications by exploring their expected high thermal and mechanical stability in catalysis. A major reason for this is that the synthesis of porous Si/Al/C/ N ceramics is quite challenging, because their synthesis requires reaction conditions that exclude water and oxygen. Two general classes of Si/Al/C/N precursor synthesis have been investigated: (1) mixtures of precursors that lead to the stable phases composing the composite and (2) singlecomponent precursors in which the metals (Al) or the metalloid (Si) are precombined in a common structure. Paciorek et al. investigated the synthesis and pyrolysis of the dimers {[(Me 3 Si) 2 N] 2 AlNH} 2 and {[(Me 3 Si) 2 N] 2 Al(NH2)2}3.68 Paine et al. prepared solid solutions of AlN and SiC at temperatures of 1484 °C. Therefore, they exhibit improved microstructural stability at high temperatures in an inert atmosphere, in comparison to boron-free Si/C/N systems.37−39,47−56 However, their stability in steamed gas remains an open question. The exceptionally low oxidation rates of Si/ B/C/N fibers, as initially reported,57 have been underestimated for several reasons, e.g., the low oxide/ceramic volume ratio, the B2O3 volatilization, and/or the water-sensitive borosilicate formation and the borosilicate viscous flow. More recent studies reported diffusion rate constant values close to those for SiC and Si3N4 at 1500 °C.58 Within this context, their use as support in catalytic reactions involving water as reaction medium and/or as reactant (e.g., catalytic hydrogen generation from liquid-phase hydrogen storage materials, water-gas-shift 3958



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Figure 1. Overall synthetic path employed to generate periodic mesoporous Si/Al/C/N ceramics labeled ompSi/Al/C/Nxy (with x = Al = 1 and y = Si = 1−3). (PHPS, AQUAMICA NN-310, average molecular weight of 974) was provided by Mitsuya Boeki, Ltd., Japan. Elemental contents, 1H and 29 Si NMR, and FTIR data are reported below. Anal. Found (wt %): Si, 65.1; N, 26.2; H, 8.3; O, 0.4. FTIR (KBr/ cm−1): ν (N−H) = 3374 (m), ν (Si−H) = 2125 (s), ν (N−H) = 1180 (s), ν (Si−N) = 840−1020 (s). 1H NMR (500 MHz, C6D6, δ/ppm): 1.0 (NH), 4.96 (SiH). 29Si NMR (79.43 MHz, C6D6, δ/ppm): −39.0 ppm (HSiN3/H2SiN2). The synthesis of poly[N-(ethylimino)alane] (PEIA, [HAlNEt]n) with an empirical formula of [Al1.0N0.9C2.1H6.8]n (n ≈ 8) has been already reported by our group.74 FTIR and 1H and 27Al NMR data are reported below. FTIR (KBr/cm−1). ν(C−H) = 2954 (s), 2898 (s), 2803 (m), ν(Al− H) = 1850 (m), δsym(CH3) = 1378 (w), δasym(CH3) = 1463 (w), ν(C− N) = 1095 (w), ν(Al−N) = 694 (m). 1H NMR (500 MHz, C6D6, δ/ ppm): 1.1−1.6 (CH3), 3.1−3.6 (CH2), 4.6 (AlH). 27Al NMR (130.20 MHz, δ/ppm): 136 ppm (HAlN3). Polymer Synthesis. Homogeneous solutions consisting of various ratios of PHPS and PEIA, thereby various Al (x):Si (y) ratios (x:y = 1:1, 1:2, 1:3) based on the monomeric unit of each polymer led to blended polymers called polyaluminosilazanes labeled PASZxy (PASZ11, PASZ12, and PASZ13). The quantities of PHPS and PEIA used for synthesis of each blended polymers are detailed in Table 1.

process of ordered mesoporous carbon CMK-3. We used preceramic polymers containing both −[R1R2Si−N(R3)]n− and −[R4Al−N(R5)]n− backbones (with R1 = R2 = R3 = R4 = H and R5 = CH2CH3) (see Figure 1), according to a synthetic procedure recently reported.73 Morphological and (nano)structural characterizations by scanning electron microscopy (SEM), transmission electron microscopy (TEM), nitrogen physisorption (77 K), elemental analysis, and X-ray diffraction (XRD) were performed. Thermal decomposition of ceramics was studied by TGA. Second, the as-obtained micrometer-sized powders served as a platform to synthesize Pt (nano)particles via an impregnation method. The as-formed supported catalysts Pt@Si/Al/C/N were tested for the generation of hydrogen by hydrolysis of alkaline aqueous sodium borohydride at 80 °C as a proof of concept. This probe reaction is advantageous to assess the robustness of the support, because it offers harsh experimental conditions, in terms of solution alkalinity, very high rates of H2 production over the catalytic sites, presence of water, and temperature. Our main results are reported herein.

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EXPERIMENTAL SECTION

Materials. The molecular and polymeric precursors are unstable in air. Therefore, all manipulations were carried out under inert conditions. All ceramic products were handled in a glovebox, because of their high surface area and affinity for moisture before characterization. Argon (>99.995%) was purified by passing through successive columns of phosphorus pentoxide, siccapent, and BTS catalysts. Schlenks were dried at 120 °C overnight before pumping under vacuum and filling them with argon for synthesis. Manipulation of the chemical products was made inside an argon-filled glovebox (MBraun MB200B) where the O2 and H2O concentrations were kept at 18 MΩ cm) were used. Poly(perhydropolysilazane)

Table 1. Experimental Parameters Fixed during the Synthesis of Blended Polymers sample

PEIA (g)

PHPS (g)

chemical formula

PASZ11 PASZ12 PASZ13

0.180 0.111 0.173

0.113 0.140 0.327

[Si1.0Al1.0N1.7H9.3C2.1]n [Si1.0Al0.5N1.3H6.4C1.1]n [Si1.0Al0.3N1.1H4.8C0.7]n

The reaction is conducted in a three-necked round-bottom flask equipped with a gas inlet tube and a glass stopper. The flask was charged with a precise quantity of PEIA (Table 1) and toluene (100 mL). Depending on the Al:Si ratio targeted, a precise quantity of PHPS (Table 1) was added dropwise to this solution. After stirring at 30 °C until complete dissolution and getting a clear solution, the solvent was removed via an ether bridge at a reduced pressure to yield a white solid. Chemical analysis, FTIR, and 1H NMR data are reported below. 3959



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ammonia and nitrogen (75:25 in flow ratio) using silica crucibles (sample weight of ∼20 mg) at ambient atmospheric pressure. FTIR spectra of PASZxy and the heat-treated samples isolated at different intermediate temperatures in the range of 100−1000 °C (labeled PASZxy-T, with T being the intermediate temperature), were analyzed at RT. Fourier transform infrared (FTIR) spectra were obtained from a Nicolet Magna 550 Fourier transform-infrared spectrometer in a KBr matrix (dried at 120 °C in air). High-temperature thermogravimetric analysis (HT-TGA, Setaram Setsys 2400 CS evolution equipment) of ordered mesoporous Si/Al/C/N materials was performed in flowing nitrogen from 25 °C to 1700 °C at a heating rate of 5 °C min−1, using tungsten carbide crucibles. In addition, TGA (Setaram, Model TGA 92 16.18) of the same sample was performed in static air with a moisture level of 35% (measured every hour, then averaged) from 25 °C to 1000 °C (dwelling time of 5 h) at a heating rate of 5 °C min−1, using silica crucibles. Small-angle powder X-ray diffraction patterns (SAXRD) were recorded using a Philips Model PW 3040/60 X’Pert PRO XRD system operating at 30 mA and 40 kV, and between 0.7° and 5.0° with a step size of 0.0167°. In addition, powder XRD was applied from 20° to 80° to identify the global structure of the Si/Al/C/N phase. The mesoporous materials were observed by SEM (Hitachi, Model S800) equipped with energy-dispersive spectroscopy analysis (EDX) to obtain information on the carbon, silicon, and aluminum contents of the material. Nitrogen and oxygen contents have been measured by a LECO TC-600 oxygen/nitrogen analyzer (Institut des Sciences Chimiques de Rennes, UMR CNRS 6226, Université de Rennes 1, France). TEM images were taken with a transmission electron microscope (Philips, Model CM 200FEG) operated at 200 kV. All powders were dispersed on carbon-film-covered copper grids for analysis. Nitrogen adsorption/desorption isotherms were measured on a Model Sorptomatic 1900 analyzer (Fisons). Before adsorption measurements, all samples were outgassed for 4 h at 150 °C in the degas port of the adsorption analyzer. The Brunauer−Emmett−Teller (BET) method was used to calculate the specific surface area. The pore-size distribution was derived from the adsorption branches of the isotherms using the Barrett−Joyner−Halenda (BJH) method. The total pore volume (Vp) was estimated from the amount of N2 adsorbed at a relative pressure of P/P0 = 0.99. Supported Catalysts. The preparation of the platinum (Pt) supported ompSi/Al/C/Nxy (denoted Pt@ompSi/Al/C/Nxy) (with x = Al = 1 and y = Si = 1−3) was performed by conventional impregnation method. Typically, an aqueous solution of Pt2+ was prepared. In parallel, 50 of mg support was dispersed in 3 mL of deionized water and stirred for 1 h. Then, the metal solution was added under stirring with the objective to introduce 0.5 mg of Pt. The total volume was adjusted to 5 mL. Our target was 1 wt % of Pt in the supported catalyst. The mixture was stirred for 18 h. The Pt2+ cations were reduced by adding sodium borohydride (NaBH4, 0.2 mg) and the solution was stirred for 2 h. Finally, water was extracted in an oven at 80 °C for 24 h, and the as-obtained material was milled and sieved to be calcined at 350 °C for 2 h. The presence of platinum was verified by EDX spectroscopy, powder XRD, and TEM. The specific surface area and the mesoporosity of the nanocomposites was investigated by N2 sorption. The catalytic ability of the supported catalysts was assessed in hydrolysis of sodium borohydride. The hydrogen generation measurement was done according to our general procedure as reported elsewhere.77 Typically, the catalyst (16 mg) is milled and sieved, and then transferred in a reactor (glass tube) sealed with a septum. It is connected to a water-filled inverted buret, via a cold trap (0 °C) for steam. To start the hydrogen generation, 1 mL of a stabilized (2 wt % NaOH) aqueous solution of NaBH4 (120 mg) is injected over the catalyst. H2 evolution is recorded and analyzed post-hydrolysis.

PASZ11. Anal. Found (wt %): Si, 24.7; Al, 23.8; N, 21.0; H, 8.3; C, 22.2 [Si1.0Al1.0N1.7H9.3C2.1]n ([87.47]n). FTIR (KBr/cm−1): ν(N−H) = 3426 (w), ν(C−H) = 2954 (s), 2898 (s), 2805 (m), ν(Si−H) = 2125 (s), ν(Al−H) = 1860 (m), δasym(CH3) = 1463 (w), δsym(CH3) = 1378 (w), δ(N−H): 1173 (m), δ(N−Si−N) = 1020−840 (vs), ν(C−N) = 1095 (s), ν(Al−N) = 720 (m). 1H NMR (500 MHz, C6D6, δ/ppm): 0.8−0.9 (NH), 1.2−1.6 (CH3), 3.25−3.4 (CH2), 4.45 (AlH), 4.9 (SiH). 29Si NMR (79.43 MHz, δ/ppm): −37.2 ppm (HSiN3/H2SiN2). 27 Al NMR (130.20 MHz, δ/ppm): 135 ppm (HAlN3). TGA (N2, 1000 °C, 80.3% ceramic yield): 25−200 °C: Δm = 4.6%; 200−1000 °C: Δm = 15.1%. PASZ12. Anal. Found (wt %): Si, 35.9; Al, 17.3; N, 22.4; H, 8.2; C, 16.1 [Si1.0Al0.5N1.3H6.4C1.1]n ([79.45]n). FTIR (KBr/cm−1): ν(N−H) = 3425 (w), ν(C−H) = 2954 (s), 2898 (s), 2803 (m), ν(Si−H) = 2125 (s), ν(Al−H) = 1850 (m), δasym(CH3) = 1466 (w), δsym(CH3) = 1378 (w), δ(N−H): 1173 (m),), ν(C−N): 1095 (m), δ(N−Si−N) = 1020− 840 (vs), ν(C−N) = 1080 (s), ν(Al−N) = 720 (m). 1H NMR (500 MHz, C6D6, δ/ppm): 0.8−0.9 (NH), 1.3−1.6 (CH3), 3.25−3.3 (CH2), 4.45 (AlH), 4.86 (SiH). 29Si NMR (79.43 MHz, δ/ppm): −36.9 ppm (HSiN3/H2SiN2). 27Al NMR (130.20 MHz, δ/ppm): 135 ppm (HAlN3). TGA (N2, 1000 °C, 81.2% ceramic yield): 25−200 °C: Δm = 6%; 200−1000 °C: Δm = 12.8%. PASZ13. Anal. Found (wt %): Si, 43.3; Al, 12.5; N, 23.7; H, 7.5; C, 13.0 [Si1.0Al0.3N1.1H4.8C0.7]n ([64.82]n). FTIR (KBr/cm−1): ν(N−H) = 3425 (w), ν(C−H) = 2956 (s), 2898 (s), 2803 (m), ν(Si−H) = 2125 (s), ν(Al−H) = 1845 (m), δasym(CH3) = 1458 (w), δsym(CH3) = 1378 (w), δ(N−H): 1171 (m), δ(N−Si−N) = 1020−840 (vs), ν(C− N) = 1110 (s), ν(Al−N) = 717 (m). 1H NMR (500 MHz, C6D6, δ/ ppm): 0.9 (NH), 1.3 − 1.6 (CH3), 3.25−3.35 (CH2), 4.45 (AlH), 4.9 (SiH). 29Si NMR (79.43 MHz, δ/ppm): −37.3 ppm (HSiN3/H2SiN2). 27 Al NMR (130.20 MHz, δ/ppm): 135 ppm (HAlN3). TGA (N2, 1000 °C, 87.5% ceramic yield): 25−200 °C: Δm = 6.5%; 200−1000 °C: Δm = 6.0%. Preparation of Hard Templates. SBA-15 was prepared by hydrothermal synthesis and then was used as a hard template to prepare CMK-3 via impregnation with sucrose, according to established procedures.75,76 Textural and structural parameters are reported in Table 1. Preparation of Si/Al/C/N Frameworks. Nanocasting of the samples PASZ11, PASZ12, and PASZ13 were carried out through an impregnation process in a Schlenk-type flask. Only the preparation of Si/Al/C/N framework from PASZ12 is described below, since all mesoporous ceramics are prepared in a similar manner. PASZ12 (0.226 g) was dissolved in 1.5 mL of toluene under stirring and added to 0.160 g (polymer:template weight ratio of 1.4) of the CMK-3 template dehydrated at 100 °C for 2 h at reduced pressure prior to use. The mixture was then stirred at room temperature (RT) for 48 h under static vacuum. After absorption of the polymer into the pores of the template, a filtration step was performed and then the solvent was evaporated at low pressure at 40 °C to generate a black powder. The composites are transferred into a silica tube inserted in a horizontal tube furnace (Nabertherm type RS 80/500/11, Germany). Subsequently, the samples were subjected to a N2 cycle of ramping of 1 °C min−1 to 1000 °C, dwelling at that temperature for 2 h, and then cooling to RT at a rate of 2 °C min−1. A constant nitrogen volumetric flow of 0.039 mL s−1 was passed through the tube. After the polymerto-ceramic conversion process, the composite underwent a final thermal treatment under a mixture of nitrogen and ammonia atmospheres in the same horizontal tube furnace from RT to 1000 °C and kept at this temperature for 5 h to remove CMK-3, while generating samples ompSi/Al/C/Nxy (with x = Al = 1 and y = Si = 1− 3). A constant volumetric flow of 0.039 m s−1 for each gas was passed through the tube. Characterizations. Thermogravimetric analyses (TGAs) of the polymer-to-ceramic conversion were recorded on a Setaram system (Model TGA 92 16.18). Experiments were performed at 5 °C min−1 up to 1000 °C in flowing nitrogen for the polymer-to-ceramic conversion using silica crucibles (sample weight of ∼40 mg) at ambient atmospheric pressure. The CMK-3 removal step was monitored by TGA at 5 °C min−1 up to 1000 °C in a mixture of

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RESULTS AND DISCUSSION Preparation of the Periodic Mesoporous Si/Al/C/N materials. The present study has a double objective. The first objective consists in the preparation and characterization of periodic mesoporous Si/Al/C/N ceramics. The second objective is focused on the modification of their surface with 3960



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stretching and deformation bands of PEIA and PHPS (see Figure SI1 in the Supporting Information) without indication of possible reactions between both polymers: νN−H (∼3425 cm−1), νC−H (2956−2803 cm−1), νSi−H (2125 cm−1), νAl−H (1860−1840 cm−1),δasCH3 (1463 cm−1), δsCH3(1378 cm−1), δNH (1173−1171 cm−1), νC−N (1110−1080 cm−1), a broad N−Si− N asymmetric stretching (δN−Si−N) at 840−1020 cm−1 and νAl−N (720 cm−1). The chemical formula (Table 1) changed in accordance with the fixed Al:Si ratio, from [Si1.0Al1.0N1.7H9.3C2.1]n for PASZ11 to [Si1.0Al0.3N1.1H4.8C0.7]n for PASZ13 and tended to confirm the absence of reactions between both precursors. As expected, the proportion of carbon and hydrogen gradually decreased from PASZ11 to PASZ13, in relation with the decrease in the Al:Si ratio (and, thereby, the PEIA:PHPS ratio). In addition, the Al:Si ratio measured by elemental analysis is also in good agreement with the one fixed during the polymer preparation. The direct solution infiltration of CMK-3 with the blended polymers at RT for 48 h under static vacuum proved to be an effective method to fully infiltrate the pores of the template. The polymer:template weight ratio was optimized at 1.4. An important step for the successful synthesis is the subsequent filtration step that consists of selectively removing the precursor molecules that are deposited outside while the capillary forces keep the precursor molecules effectively inside the pores. A further crucial part is the post-treatment of the infiltrated template at low pressure (5 × 10−2 mbar at 40 °C) in order to slowly remove the solvent while generating a black powder. Lastly, the subsequent heat treatments also are challenging, because the ceramic yield must be the highest possible to keep the maximum of products confined in the porosity of the template after the ceramic transformation and obtain after the template removal step a porous component with the expected ordered mesoporosity. Ceramic materials were obtained by pyrolysis of the polymers up to 1000 °C (1 °C min−1) in flowing nitrogen. Several factors, such as the polymer structure, the molecular weight, and the degree of cross-linking (i.e., the structure of the polymer backbone and the nature of the functional groups that are attached to silicon, aluminum and/or nitrogen), strongly influence the ceramic yield. An additional important issue in this regard is the decomposition chemistry of the precursors, including the ability to cross-link further during the heat treatment. As is suggested by the substantially enhanced ceramic yield for the blended polymers from 80.3 (PASZ11) to 87.5% (PASZ13), compared to that of the two individual precursors (75.3% for PHPS and 61.7% for PEIA) under the same conditions (see Figure SI2 in the Supporting Information), the blended polymers reacted during the pyrolysis. The increased ceramic yield of PASZ13 is attributed to the presence of more reactive sites (PASZ13 contains the highest ratio of PHPS, which display three reactive sites (2 Si− H bonds and 1 N−H bond) in the monomeric unit), which allowed further cross-linking during the polymer-to-ceramic conversion. However, the precise nature of the reaction occurring in the porous structure of the template between the two precursors is presently unknown. Within this context, we investigated FTIR studies on pyrolyzed intermediates. Based on the infrared spectra of different pyrolyzed intermediates from PASZ12 (see Figure SI3 in the Supporting Information) and the literature,73,81,82 dehydrocoupling reactions are evident: reactions between of Si−H and N−H units forming Si−N bonds (see reaction 1 in Table 2) mainly occurred in the temperature range of 500−1000 °C. Alternatively, the loss of

Pt (nano)particles. The supported catalysts have been then tested in a probe reaction for hydrogen generation in aqueous medium. On the basis of the general “molecular building block” approach,73 we used precursors of the separated ceramic components that can be mixed homogeneously in solution in relative simplicity. Requested characteristics for the preparation of the polymer-derived mesoporous Si/Al/C/N ceramics are as follows: (1) The synthesis of the precursors remains simple. (2) The precursors are soluble in a common solvent for processability. (3) The polymer-to-ceramic conversion occurs at the lowest possible temperature and the decomposition temperature of each precursor should match as closely as possible. (4) Precursors are expected to react together during pyrolysis, avoiding phase separation at temperatures that are too low. (5) The segregation of one of the individual component ceramic phases should occur at the highest temperature for stability. We have used two polymeric precursors; both satisfy the aforementioned criteria. Poly(perhydropolysilazane) (PHPS, [SiH2−NH]n) is a soluble thermoset polymer containing only silicon, nitrogen, and hydrogen. Its pyrolysis under inert atmosphere results in a silicon nitride/silicon composite, i.e., Si3N4 + Si78−80 in a relatively high ceramic yield (75.3% at 900 °C under argon73). Poly[N-(ethylimino)alane] (PEIA, [HAlNEt]n) yields AlN by pyrolysis under ammonia at 1000 °C, then under nitrogen at 1800 °C.74 Using nitrogen as the atmosphere from RT to 1800 °C results in a carbon-containing AlN with ∼20 wt % carbon. The PHPS solution was mixed at 30 °C with a solution of PEIA in toluene, according to a controlled Al:Si ratio, to form a homogeneous colorless liquid mixture. The mixture was then dried to remove all volatile compounds. The RT mixture of PHPS and PEIA generated white powders (= polyaluminosilazane, labeled PASZxy (with Al = x = 1 and Si = y = 1−3)). We focused on this restricted ratio range because, outside this range, the effect of the polymer in the lowest proportion is negligible. Chemical composition and structural characterization of the three compounds investigated by microanalysis, NMR in solution, and FTIR spectroscopy indicated that no obvious reaction of the precursor mixtures is observed prior to their use for the nanocasting and pyrolysis experiments. Most characteristic are the results of 1H NMR investigations (recorded in C6D6). The spectra of the three samples displayed broad signals that are characteristic of PEIA and PHPS. The presence of Al− H bonds was confirmed by the appearance of a broad singulet at δ = 4.45 ppm, whereas the presence of Si−H bonds appeared through the broad signal emerging at δ = 4.86−4.9 ppm. The protons in CH2 groups linked to nitrogen atoms were represented by a triplet signal at δ = 3.24−3.4 ppm. Finally, the presence of two sets of signals in the range δ = 1.2−1.6 ppm was assigned to protons in CH3 groups, whereas the signal relative to protons in NH units was identified at δ = 0.8−0.9 ppm. 27Al and 29Si NMR confirmed peak positions observed in PEIA and PHPS, respectively. (See the Experimental Section.) Solid-state 13C and 15N NMR is under investigation to have a precise idea of the structure of PASZ and its evolution during pyrolysis. The FTIR spectra also showed the characteristic 3961



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After the polymer-to-ceramic conversion, our standard protocol to remove carbon template uses a nitrogen/ammonia atmosphere37−39 through heat treatment to 1000 °C, while keeping the sample at this temperature for 5 h. We investigated TGA of a “test sample” (obtained by pyrolysis under nitrogen at 1000 °C of PASZ12 without infiltration) under nitrogen/ ammonia up to 1000 °C with a dwelling time of 5 h to follow the behavior of the Si/Al/C/N phase under ammonia: The TGA curve (see Figure SI4 in the Supporting Information) did not indicate a fundamental weight change after this treatment. Chemical analyses performed before (Si1.0Al0.5O0.2C1.0N1.4; Si, 36.7 wt %; Al, 19.3 wt %; C, 15.5 wt %; N, 24.7 wt %; O, 3.8 wt %) and after (Si1.0Al0.5C0.9N1.3; Si, 39.9 wt %; Al, 20.9 wt %; C, 14.0 wt %; N, 24.8 wt %; O, 0.4 wt %) ammonia treatment were similar. This points to the fact that the Si/Al/C/N phase is stable under these conditions. After cooling, gray samples going from dark (ompSi/Al/C/ N11) to light (ompSi/Al/C/N13) are obtained. The chemical formula of samples coupling EDX (Si, Al, and C contents) and carrier gas heat extraction (LECO TC-600, N and O contents) changed with the Al:Si ratio from Si1.0Al0.9O0.6C1.1N1.6 (ompSi/ Al/C/N11, Si, 29.0 wt %; Al, 24.1 wt %; C, 14.0 wt %; N, 23,1 wt %; O, 9.8 wt %) to Si1.0Al0.3O0.2C0.4N1.0 (ompSi/Al/C/N13, Si, 46.9 wt %; Al, 15.6 wt %; C, 9.0 wt %; N, 23.0 wt %; O, 5.5 wt %). For the ompSi/Al/C/N12 sample, a chemical formula of Si1.0Al0.4O0.1C1.1N1.2 was calculated based on the weight percentage of the elements that compose the materials (Si, 39.4 wt %; Al, 17.5 wt %; C, 14.6 wt %; N, 24,7 wt %; O, 3.8 wt %). First, we suggest that the oxygen content level of the samples is mainly due to physisorbed/chemisorbed water confined to the porous structure of the material, despite the extreme precautions taken to protect the samples against moisture. The reason why the oxygen content is high for ompSi/Al/C/N11 can be explained by its high sensitivity to oxidation, as shown later. Second, a comparison of the composition of the polymers and the ceramic materials showed that the Al:Si ratio remains constant during pyrolysis. This indicated the absence of evolution of Si- and Al-based gaseous species as byproducts during the pyrolysis, confirming the chemical interaction of both polymers during heat treatment. Moreover, the Si:N ratio remains almost unchanged. The most apparent change (excepted for hydrogen) in the chemical compositions is the different carbon content of the precursors

Table 2. Reactions Representing the Mechanisms That Occur during the PASZxy-to-Si/Al/C/N Conversion reaction

reaction number

Si 2N−H + H−Si → Si 2−N−Si + H 2

(1)

Si 2N−H + H−Al → Si 2−N−Al + H 2

(2)

•

•

Al−NCH 2CH3 → Al + NCH 2CH3 •

(3)

•

(4)

Si• + •NCH 2CH3 → Si−NCH 2CH3

(5)

Si−H → Si + H •

•

H + NCH 2CH3 → HNCH 2CH3 •

•

Al 2N−CH 2CH3 → Al 2N + CH 2CH3 •

•

Si + Al 2N → Al 2N−Si

(6) (7) (8)

•

•

(9)

•

•

(10)

Si + CH 2CH3 → Si−CH3CH3

H + CH 2CH3 → CH3CH3

the NH groups (as shown on the intermediate pyrolyzed at 300 °C in Figure SI3 in the Supporting Information) with the concomitant disappearance of Al−H groups indicated that Al− H and N−H units reacted to form Al−N bonds (see reaction 2 in Table 2) below 300 °C. The heterolytic cleavage of Al−N bonds (reaction 3 in Table 2) leading to the formation of nitrogen terminals (N•)73 may involve the attack of silicon and hydrogen radicals (reaction 4 in Table 2) to form Si−N bonds (reaction 5 in Table 2) and ethylamine (reaction 6 in Table 2). Furthermore, the homolytic cleavage of C−N bonds (reaction 7 in Table 2) is expected to involve the formation of Si−N bonds (reaction 8 in Table 2), Si−C bonds (reaction 9 in Table 2) and alkanes (reaction 10 in Table 2). The disappearance of the bands that are characteristic of alkyl groups confirmed the occurrence of these reactions. After heat treatment to 1000 °C, only a broad band in the wavenumber range 1200−700 cm−1 is present. It indicated the presence of Al−N, Si−C, and Si−N bonds in the material. The chemical interaction between both precursors from the polymer state to the ceramic state under nitrogen to 1000 °C (dwelling time of 2 h) is not only a way of preventing the appearance of an unexpected porosity but it also finds great interest in limiting the degree of phase separation in the ceramics at high temperature.

Figure 2. HRTEM images of the ompSi/Al/C/N12 sample: (a) longitudinal projection (along the mesopores) and (b) cross-sectional projection (across the mesopores). 3962



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Figure 3. (a) Small-angle X-ray (SA-XRD) patterns (Cu Kα radiation) of ompSi/Al/C/Nxy (with x = Al = 1 and y = Si = 1−3) samples, (b) XRD patterns for the as-obtained ompSi/Al/C/N12 sample at different temperatures, (c) N2 adsorption−desorption isotherms recorded at 77 K of ompSi/Al/C/Nxy (with x = Al = 1 and y = Si = 1−3) samples, and (d) pore size distribution of ompSi/Al/C/Nxy (with x = Al = 1 and y = Si = 1− 3) samples.

Table 3. Textural Properties of SBA-15 and CMK-3 Templates, and ompSi/Al/C/Nxy (with x = Al = 1 and y = Si = 1−3) SA-XRD Data SBA-15 CMK-3 ompSi/Al/C/N11 ompSi/Al/C/N12 ompSi/Al/C/N13

BET Data

d100 spacing (nm)

cell parameter, a (nm)

BET surface area (m g )

pore diameter (nm)

wall thickness (nm)

pore volume (cm3 g−1)

9.4 8.5 6.5 6.7 6.8

10.8 9.8 7.5 7.7 7.8

545 1355 182 214 326

6.9 3.5 5.1 5.9 4.1

3.9

0.75 1.00 0.51 0.65 0.61

2

and the ceramics, which proved the evolution of carbon-based groups during pyrolysis. TEM Observations. The samples were generated as micrometer-sized platelike powders, as shown in the SEM image of the representative ompSi/Al/C/N12 sample (see Figure SI5 in the Supporting Information). Figure 2 depicts the corresponding HRTEM images of the sample which clearly displays an excellent order after the template removal. When viewed down the [100] direction of the sample (Figure 2a), the HRTEM image shows one-dimensional (1D) channels that represent hexagonally ordered pores arranged in a linear array. The distance between the centers of the adjacent channels is ∼7.5 nm. This value is similar to the value measured for the CMK-3 template (8.2 nm), indicating that the mixed polymer framework is sufficiently rigid and cross-linked to withstand pore collapse and deterioration of the nanoscale order of the CMK-3 hard template. The cross-sectional HRTEM image (Figure 2b) clearly displays a hexagonal (honeycomb-like) arrangement of the mesopores with regular diameters of ∼7 nm. Based on these observations, it can be assumed that the sample exhibits a structure with an ordered

−1

2.4 1.8 3.7

hexagonal arrangement of cylindrical channels similar to that observed in silica template SBA-15. The ordered structure was investigated by powder XRD and nitrogen gas adsorption measurements (see Figure 3). X-ray Diffraction (XRD). Figure 3a shows the small-angle X-ray (SA-XRD) patterns of the ompSi/Al/C/Nxy (with x = Al = 1 and y = Si = 1−3) samples, demonstrating the well-ordered structure of the specimens with an ordering that is improved from ompSi/Al/C/N11 to ompSi/Al/C/N13, which is consistent with the increase of the ceramic yield of the corresponding polymers from PASZ11 to PASZ13. This clearly demonstrated the strong correlation between the ceramic yield of preceramic polymers and the retention of the ordered porosity of the derived ceramics. A clear diffraction peak with d = 6.8 nm (2θ = 1.30°) and two weak peaks at 3.96 (2θ = 2.23°) and 3.45 (2θ = 2.56°) nm were observed for ompSi/Al/C/N13, whereas the SA-XRD patterns of the ompSi/Al/C/N11 and ompSi/Al/C/N12 samples exhibited a slight loss of order, as illustrated by the gradual decrease in the intensity of both the (100) and higher-order reflections. These peaks can be assigned to the (100), (110), and (200) reflections of the two3963



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dimensional (2D) hexagonal lattice (space group P6mm) with a lattice parameter a100 = 7.5−7.8 nm, which is in good agreement with the local measurement made by the HRTEM observations. The values of 7.5−7.8 nm are slightly smaller than that measured for CMK-3 (9.8 nm), suggesting that a limited framework shrinkage is occurring during the polymer-toceramic conversion. Table 3 reports the structural XRD properties of all specimens, as well as those of SBA-15 and CMK-3. The absence of diffraction peaks in the wider-angle XRD patterns (Figure 3b) of the as-obtained ompSi/Al/C/N12 (ompSi/Al/C/N12−1000) demonstrates that the channel walls of the samples are amorphous in nature. The amorphous state is globally retained after heat treatment to 1400 °C (ompSi/Al/C/N12−1400) with only the appearance of a diffuse α-Si3N4 peak emerging at ∼26°. Crystallization of these frameworks only occurs at prolonged reaction times (2 h) at 1700 °C (ompSi/Al/C/N12−1700) into the 2H solid solution (AlN, SiC). N2 Adsorption−Desorption Isotherms. The nitrogenadsorption isotherms of ompSi/Al/C/Nxy (with x = Al = 1 and y = Si = 1−3) samples are shown in Figure 3c. Table 2 reports the corresponding BET results. The isotherms of SBA-15 and CMK-3 are similar to those described in the literature and, therefore, are not discussed here.75,76 Based on the IUPAC classification of the sorption isotherms,83 the isotherms of ompSi/Al/C/Nxy (with x = Al = 1 and y = Si = 1−3) samples show a IV-type curve typical for mesoporous adsorbents with a distinct capillary condensation, which suggests that the materials have uniform mesoporous channels. They exhibit a hysteresis loop with a capillary condensation at a relative pressure (P/P0) that is related to the Al:Si ratio, typically from 0.45 to 1.00. A H2 hysteresis loop is observed for the ompSi/Al/C/N13 sample. A superposition of H1 and H2 hysteresis loops is rather observed for ompSi/Al/ C/N11 and ompSi/Al/C/N12. The H1 hysteresis loop suggests the presence of cylindrical mesopores. The H2 hysteresis loop is characteristic of capillary condensation into ink-bottle-shaped mesopores. The samples exhibit specific BET surface areas ranging from 182 m2 g−1 to 326 m2 g−1 associated with uniform mesopores dimensions (Figure 3d), according to the nitrogen desorption determined by the Barrett−Joyner− Halenda (BJH) analysis. The pore-size distributions are in the range of 4.1−5.9 nm and pore volumes vary from 0.51 cm3 g−1 to 0.65 cm3 g−1. By comparison, close mesopore volumes are observed for the ompSi/Al/C/N11 and ompSi/Al/C/N13 samples. This is in agreement with SA-XRD observations and emphasizes the necessity of an optimal filling of the hard template with a high ceramic yield precursor. According to the cell parameters and the mesopores diameter, wall thicknesses in the range of 1.8−3.7 nm are calculated. High-Temperature Thermogravimetric Analysis. The PDCs route is not only a means to produce conventional ceramics such as AlN74 or BN;84 it also offers access to siliconcontaining PDCs that are amorphous and exhibit atomically homogeneous elemental distributions.37−39,47−56 Their main interest is the stability of their amorphous network at very high temperature. The high-temperature behavior of these materials is a function of their molecular origin (the structural arrangement and the chemistry of the preceramic polymer54,85), ), the form of the material (fibers, coatings, powders, ...), the texture (porous → dense) and the surrounding atmosphere. Another important issue that affects the high-temperature

behavior of ceramics is the specific surface area. Janakiraman et al. showed that the specific surface area of powders strongly and detrimentally influences the thermal stability of Si/B/C/N bulk materials.86 We investigated thermogravimetric (TGA) experiments up to 1700 °C in a nitrogen atmosphere (Figure 4) to

Figure 4. High-temperature TGA experiments recorded under a nitrogen atmosphere from RT to 1700 °C (5 °C min−1) for ompSi/ Al/C/Nxy (with x = Al = 1 and y = Si = 1−3) samples.

follow the weight change of samples at high temperature. Data have been discussed based on the combination of XRD (Figure 3b) and empirical formulas as given in the previous section. Then, TGA has been used to follow the weight change of the samples in air up to 1000 °C (dwelling time of 5 h; see Figure 5).

Figure 5. High-temperature TGA experiments recorded in air from RT to 1000 °C (5 °C min−1; dwelling time at 1000 °C = 5 h) for ompSi/Al/C/Nxy (with x = Al = 1 and y = Si = 1−3) samples.

Samples show almost no mass change up to 1400−1470 °C in flowing nitrogen, demonstrating the excellent stability of the amorphous channel walls. Heat treatment at temperatures higher than 1400−1470 °C resulted in a relatively rapid decomposition step associated with a continuous weight loss up to 1700 °C and a complete loss of specific surface area. The sample ompSi/Al/C/N13 is the most stable with a decomposition shifted to higher temperature by ∼70 °C and associated with the lowest final weight loss measured at 1700 3964



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°C (18.7 wt %). The weight loss measured after heat treatment at 1700 °C gradually increased from ompSi/Al/C/N13 to ompSi/Al/C/N11 (27.5 wt %). Based on the fact that (i) AlN does not thermodynamically decompose below 1700 °C, (ii) Si3N4 and C are in contact during sintering, and (iii) a 2H solid solution (AlN, SiC) is identified after heat treatment to 1700 °C, the reaction depicted in eq 11, which is known to proceed under standard conditions at 1484 °C, mainly occurred in the materials. Si3N4 + 3C → 3SiC + 2N2

Both SiC and Si3N4 react with water vapor to form SiO2.91 This is also the case for silicon. As an illustration, the oxidation reactions of Si3N4 and Si progress as indicated in eqs 16 and 17.92

(13)

In air, Si3N4 and SiC are known to form a dense and continuous SiO2 layer, according to eqs 14 and 15 (without bubbles/cracks) with a sharp oxide/ceramic interface at T ≥ 1000 °C.90 (14)

2SiC(s) + 3O2 (g ) → 2SiO2 (s) + 2CO(g )

(15)

(17)

(18)

First, we postulated that oxidation in ompSi/Al/C/Nxy (with x = Al = 1 and y = Si = 1−3) samples occurred by inward diffusion of oxygen due to the presence of nanochannels resulting in a higher sensitivity of the material against oxygen, in comparison to the bulk analogue. Second, the weight increase measured on the three samples is caused by the formation of Al2O3 and/or SiO2 which, depending on the ratio between both, are known to form mullite (solid solution with the composition Al2[Al2+2xSi2−2x]O10−x with 0.17 ≤ x ≤ 0.5)94 at high temperature, based on the equilibrium diagram for the Al2O3−SiO2 system.95 Mullite is extremely stable in air up to 1725 °C. The large increase of the weight of the sample ompSi/Al/C/ N11 is clearly attributed to the high proportion of AlN forming alumina in air. A chemical composition of Si1.0Al0.7O1.7C0.4 has been found after heat treatment in air with no detectable nitrogen. In addition, a SSA value of 21.04 m2 g−1 has been measured, which means that sintering occurred in sample ompSi/Al/C/N11, indicating that this sample is not attractive in a harsh environment. The increased proportion of silicon in samples ompSi/Al/C/N12 and ompSi/Al/C/N13 considerably decreased the weight gain after heat treatment in air. The resistance of the sample ompSi/Al/C/N12 against oxidation is even excellent, which is confirmed by the absence of changes in the morphological (SEM, see Figure SI6 in the Supporting Information), structural (the XRD pattern does not change after exposure to air at 1000 °C; see Figure SI7 in the Supporting Information), and textural properties (SSA = 171 m2 g−1) for sample ompSi/Al/C/N12. After oxidation, the ompSi/Al/C/N12 sample displayed a chemical formula of Si1.0Al0.5O0.6C0.5N0.8., Although sample ompSi/Al/C/N13 displays a similar behavior with a TG profile closer to ordered mesoporous ceramics derived from pure PHPS, a SSA value of 29.6 m2 g−1 is measured after heat treatment in air at 1000 °C. Reproducibility of the experiments confirmed this tendency. These studies point to the fact that the role of aluminum is one of the key parameters that affects the stability of these materials. Consequently, the high-temperature decomposition of these metastable ceramics into thermodynamically stable phases must be deeply explored. We are studying the high-temperature decomposition under nitrogen, argon (up to 1800 °C), and air (up to 1500 °C) by coupling TGA, DTA, solid-state NMR, GC-MS, TEM, XRD, BET, XPS, and microanalysis. To simulate and predict the phase equilibrium and phase reactions, thermodynamic calculation using the calculation of phase diagrams (CALPHAD) method is also carried out. Our objective is to understand the high-temperature stability of these polymer-derived Si/Al/C/N ceramics, relative to the

(12)

Si3N4(s) + 3O2 (g ) → 3SiO2 (s) + 2N2(g )

Si(s) + 2H 2O(g ) → SiO2 (s) + H 2(g )

SiO2 (s) + 2H 2O(g ) → SiO(OH)3− + H+

Bowen et al. reported that initial reaction product of AlN in water was found to be a porous amorphous AlOOH and then changed to a crystalline Al(OH)3 at RT (eq 13), which melts at ∼300 °C.89 AlN(s) + 3H 2O(g ) → Al(OH)3 (s) + NH3(g )

(16)

Normally, the SiO2 layer acts as a protective layer and decreases the reaction rate in high-temperature and high-pressure water; however, the SiO2 layer is believed to be unstable and dissolved into water through eq 18.93

(11)

Accordingly, the total mass of the ceramic should decrease due to the loss of gaseous nitrogen. Consequently, we suggested that the difference in the final weight loss was directly linked to the proportion of free carbon and silicon nitride in the final ceramics. The better stability associated with a lower final weight loss of the sample ompSi/Al/C/N13 is clearly a consequence of the low carbon content in the ceramic material. We studied the behavior of samples in static air (35% of moisture) by means of HT-TGA from 25 °C to 1000 °C (5 °C min−1 with a dwelling time of 5 h at 1000 °C) (see Figure 5). Depending on the Al:Si ratio, the mesoporous materials can show an excellent stability with only a small detectable weight change when heating to 1000 °C keeping at this temperature for 5 h. This is particularly true for the sample ompSi/Al/C/ N12: the TGA curve can be shared into four temperature ranges. The first weight loss from RT to 200 °C (4 wt %) caused by the evolution of water confined in the porous structure of the sample. It is common to all samples. Then, a second step from 200 °C to 870 °C in which almost no weight changes are recorded. A third range, from 870 °C to 1000 °C corresponds to a small weight increase (2.50 wt %). In a fourth range during the plateau at 1000 °C, oxidation in air proceeds with a continuous weight gain (1.40 wt % measured after a dwelling time of 5 h). The behavior is similar for the sample ompSi/Al/C/N13 in the temperature range from RT to 700 °C. Above 700 °C, the weight increased faster (5%) up to 1000 °C (including the plateau). For the sample ompSi/Al/C/N11, a continuous increase of the weight is measured in the temperature range of 150−1000 °C. A final weight increase of 17.5% is measured after a dwelling time of 5 h at 1000 °C. Based on the phases that are formed (AlN, Si3N4) and those identified by XRD (Figure 3b, SiC), we can discuss the different oxidation mechanisms. In air, surface oxidation of AlN occurs above 700 °C, and even at RT. The following reaction is known to occur:87,88 4AlN(s) + 3O2 (g ) → 2Al 2O3(s) + 2N2(g )

Si3N4(s) + 6H 2O(g ) → 3SiO2 (s) + 4NH3(g )

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Figure 6. Pt@ompSi/Al/C/N12 data: (a) N2 adsorption−desorption isotherms recorded at 77 K, (b) Ssmall-angle X-ray (SA-XRD) patterns (Cu Kα radiation), and (c) XRD patterns.

Figure 7. Pt@ompSi/Al/C/N12 data: (a) EDS elemental map overlapping Pt (yellow), (b) TEM image, (c) corresponding size histogram for the dispersion of the Pt (nano)particles, and (d) hydrogen evolution by hydrolysis of NaBH4 in the presence of the Pt/omp-Si/Al/C/N−1n supported catalysts at 80 °C. 3966



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Al:Si ratio, and highlight the interest of sample ompSi/Al/C/ N12. This will be published separately. Supported Mesoporous Pt@Silicon−Aluminum−Carbon−Nitrogen Catalytic Frameworks. In this work, we focused on supported Pt as the catalytic site and considered the use of our periodic mesoporous Si/Al/C/N framework as a basic host to construct Pt@ompSi/Al/C/Nxy (with x = Al = 1 and y = Si = 1−3) catalytic metal@ceramic nanocomposites for the hydrolysis of alkaline solution of sodium borohydride at 80 °C. The attractiveness of a supported catalyst lies in the low amount of platinum loaded. We supported only 1 wt % platinum. The successful loading of platinum on the Si/Al/C/ N frameworks was verified by EDX. Elemental analysis revealed that the presence of platinum is in good agreement with the target 1 wt %. Values of 1.0, 0.8, and 0.8 wt % were measured for Pt@ompSi/Al/C/N11, Pt@ompSi/Al/C/N12, and Pt@ ompSi/Al/C/N13, respectively. We report in Figures 6 and 7 the results recorded for Pt@ ompSi/Al/C/N12. It was selected because it exhibited the highest pore diameter and pore volume, and it was shown to be the most stable. It was characterized by SA-XRD, BET, and XRD (see Figure 6). After surface modification, the shape of the isotherm and the hysteresis loop of Pt@ompSi/Al/C/N12 (Figure 6a) are almost identical to the isotherm of the parent ompSi/Al/C/ N12. The hysteresis loop extends over a lower relative pressure range (0.60−0.90) revealing that the highly ordered structure is maintained even after surface modification or encapsulation of the Pt (nano)particles. The specific BET surface was found to decrease from 214 m2 g−1 for ompSi/Al/C/N12 to 102 m2 g−1 for Pt@ompSi/Al/C/N12 and the specific pore volume decreased from 0.65 cm3 g−1 to 0.51 cm3 g−1. These results are due to the formation of the (nano)particles inside the pore channels. The absence of an abrupt change in the pore volume and surface area of the support after the platinum deposition further proves that the pores of the support are not blocked by platinum (nano)particles larger than the pore size of the support, revealing that this support hinders the agglomeration or migration of the (nano)particles. The SA-XRD of Pt@ ompSi/Al/C/N12 (Figure 6b) shows a small decrease in the intensity of the lowest angle, most probably due to the filling of a part of the pores with platinum (nano)particles whose presence was proven by powder XRD (Figure 6c). The pattern showed the characteristic peaks of Pt (ICDD No. 004-0802) at 2θ = 40° (111), 46° (200), and 67° (220). Peaks are broad and weak, suggesting the formation of small crystallites. Using the Scherrer’s formula, an average crystallite size of 16 nm was calculated based on the (111) peak. The distribution of Pt for Pt@ompSi/Al/C/N12 was also verified by EDX. Elemental mapping (Figure 7a) revealed that the Pt atoms are uniformly distributed in the samples. The morphology of Pt@ompSi/Al/C/N12 was investigated by TEM (Figure 7). The presence of Pt in the form of (nano)particles is confirmed. The size of the (nano)particles was measured and the size dispersion plotted. With Pt@ ompSi/Al/C/N12, the size is centered to 7 nm (Figure 7c). These results reveal the pore-size-controlled growth of the (nano)particles using mesoporous Si/Al/C/N samples. Similar results were obtained with Pt@ompSi/Al/C/N11 (see Figure SI8 in the Supporting Information), even though very few large particles (up to ca. 40 nm) are observed. However, with Pt@ ompSi/Al/C/N13 (see Figure SI9 in the Supporting

Information), the particles size is heterogeneous, with many particles 20−40 nm in size. In fact, these results are consistent with the pore distribution found for the support (Figure 3). With the sample ompSi/Al/ C/N13, the pore distribution is 4.1 nm. Such small pores do not probably enable the incorporation of platinum (nano)particles, as well as their stabilization. Most of the (nano)particles are formed on the external surface of the Pt@ompSi/ Al/C/N13. These results indicate the great interest of ompSi/ Al/C/N12 in preventing the aggregation of (nano)particles and stabilizing the formed (nano)particles, some of them being probably inside the mesoporous channels. The activity of Pt@ompSi/Al/C/Nxy (with x = Al = 1 and y = Si = 1−3) for the hydrolysis of sodium borohydride at 80 °C was investigated. Hydrolysis is an interesting probe reaction because the experimental conditions can be considered as being severe. In our laboratory, the mechanical stability is generally assessed in hydrolysis at 80 °C, because the process offers the following features: very high rates of H2 production on the catalytic sites, very alkaline solution (>10), and presence of water.96 In other words, a catalyst that passes this test can then be considered for reactions requiring harsher conditions (e.g., water-gas-shift reaction, Fischer−Tropsch synthesis, or thermochemical water splitting).6 Note that the catalytic ability of the Pt-free supports ompSi/ Al/C/Nxy (with x = Al = 1 and y = Si = 1−3) was assessed and, as expected, they were found to be inactive in hydrolysis of NaBH4. The hydrogen generation results for Pt@ompSi/Al/C/ Nxy (with x = Al = 1 and y = Si = 1−3) are presented in Figure 7d. The hydrolysis starts immediately; no induction period is observed. This is in agreement with the metallic state of platinum, since the occurrence of an induction period is generally observed with oxidized metals.77 Hydrogen generation rates (slope of the curves) of 1, 1, and 0.7 mL min−1 were measured for Pt@ompSi/Al/C/N13, Pt@ompSi/Al/C/N12, and Pt@ompSi/Al/C/N11, respectively. Such rates are consistent with the very low amount of platinum (ca. 1 wt %). Expressed per gram of platinum, the rates are 6.7, 6.7, and 4.7 L min−1 gPt−1, respectively, which is therefore an attractive performance.97 Accordingly, the Pt@ompSi/Al/C/Nxy (with x = Al = 1 and y = Si = 1−3) catalysts have passed our first catalytic test and, thus, the Si/Al/C/N ceramics are potential supports for catalytic applications taking place in severe experimental conditions. It should be mentioned that the deposition of Pt (nano)particles has been demonstrated on CMK-3 and SBA-15;98,99 however, these templates have not been used in our study, since the former affects the hydrolysis kinetics, because of surface protons and the latter is composed of Si−O−Si bonds that are unstable at high pH (>10 in our hydrolysis conditions). Works are planned to prepare new supported catalysts to optimize their catalytic reactivity in high-temperature reactions such as soot combustion (e.g., ceria- and/or platinum-doped Si/Al/C/N) and Fischer−Tropsch conversion (e.g., Co-doped Si/Al/C/N).

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CONCLUSION First examples of periodic mesoporous silicon−aluminum− carbon−nitrogen frameworks have been prepared and characterized to be used as catalytic support of metal (nano)particles. Samples displaying P6mm hexagonal symmetry were synthesized by a solvent nanocasting route using mesoporous carbon (CMK-3) as hard template and blended polymers with various 3967



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ACKNOWLEDGMENTS Authors gratefully acknowledge the JSPS Core-to-Core Program and the “Cluster de Recherche MACODEV” of the Region Rhône-Alpes, France for their financial support. Authors would like to acknowledge Dr. Franck Tessier (Institut des Sciences Chimiques de Rennes, UMR CNRS 6226, Université de Rennes 1, France) for providing us access to a LECO TC-600 oxygen/nitrogen analyzer.

and controlled Al:Si ratios prepared through a simple and costeffective procedure from poly(perhydridosilazane) and poly(ethyliminoalane) as silicon nitride (Si3N4) and aluminum carbonitride (Al/C/N) precursors, respectively. The solution infiltration-ceramic conversion-template removal cycle performed under nitrogen at 1000 °C (2 h, ceramic conversion) then in an ammonia atmosphere at 1000 °C (5 h, template removal) resulted in the formation of periodic mesoporous Si/Al/C/N frameworks with surface areas of 182−326 m2 g−1, a pore size distribution of 4.1−5.9 nm, and pore volumes varying from 0.51 cm3 g−1 to 0.65 cm3 g−1. By characterization using electron microscopy, X-ray diffraction, N2 sorption, chemical analyses, and high-temperature thermogravimetric analysis, it was demonstrated that the amorphous micrometer-size powders display excellent mesoporous uniformity and periodicity, depending on the ceramic yield and, therefore, on the Al:Si ratio fixed during the polymer preparation. The materials show almost no mass change up to 1400−1470 °C in flowing nitrogen and the behavior in air up to 1000 °C is closely dependent on the Al:Si ratio. The as-obtained micrometer-sized amorphous Si/Al/C/N ceramic powders were then considered as potential catalyst supports to synthesize and disperse platinum (nano)particles. Such highly stable mesoporous materials allowed them to act as a pore-size-controlling support. The Pt (nano)particles were thus supported by powder impregnation and successfully assessed in our probe reaction, i.e., hydrolysis of alkaline aqueous solution of sodium borohydride at 80 °C. This opens prospects of use in catalytic reactions offering harsh oxidative and thermal environments. The mesoporous Si/Al/C/N supports could have the robustness to effectively increase the service life of supported catalysts.

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ASSOCIATED CONTENT

S Supporting Information *

FTIR spectra of PHPS and PEIA and blended polymers (Figure SI1), TG experiments of PHPS and PEIA and blended polymers (Figure SI2), FTIR spectra of PASZ12 and its heattreated samples (Figure SI3), TG experiments of a tested sample under ammonia (Figure SI4), SEM observations of the periodic mesoporous Si/Al/C/N frameworks (Figure SI5), SEM images of the ompSi/Al/C/N12 sample after heattreatment in air at 1000 °C (dwelling time of 5 h, Figure SI6), XRD pattern of the ompSi/Al/C/N12 sample after heat treatment in air at 1000 °C (dwelling time of 5 h, Figure SI7), mapping performed by EDX to analyze the Pt dispersion, TEM image, related plot for the dispersion of the Pt (nano)particles, and XRD pattern of the Pt@ompSi/Al/C/N11 sample (Figure SI8) and of the Pt@ompSi/Al/C/N13 sample (Figure SI9). This information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +33 467 149 159 (S.B.), +33 467 149 104 (P.M.). Fax: +33 467 149 119 (S.B.), +33 467 149 119 (P.M.). E-mail address: [email protected] (S.B.), Philippe. [email protected] (P.M.). Notes

The authors declare no competing financial interest. 3968



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