Organosilicon polymer-derived mesoporous 3D silicon carbide, carbonitride and nitride structures as platinum supports for hydrogen generation by hydrolysis of sodium borohydride

Highlights

• Mesoporous 3D SiC, Si–C–N and Si₃N₄ structures are synthesized with a control pore size and a high specific surface area.

• 3D structures serve as supports to grow platinum nanoparticles for hydrolysis of NaBH₄ in harsh conditions.

• High H₂ rates are measured for the platinum-supported Si₃N₄ component.

Abstract

Herein, mesoporous 3D silicon carbide (SiC), carbonitride (SiCN) and nitride (Si₃N₄) structures have been synthesized by nanocasting and pyrolysis using commercial organosilicon polymers as precursors of the different compositions. Detailed characterizations by BET and XRD allowed us to fix the most appropriate parameters to design mesoporous 3D structures with high specific surface areas and high pore volume. Then, the series of 3D structures has been used as supports to grow platinum nanoparticles (Pt NPs) by wet impregnation followed by reduction in hydrogen/argon flow. The Pt-supported mesoporous 3D supports kept the mesoporosity of the virgin supports to be used for catalytic hydrolysis of sodium borohydride (NaBH₄). A hydrogen generation rate of 24.2 L min⁻¹ g_Pt⁻¹ is measured for the Pt-supported mesoporous 3D Si₃N₄ structure, which is notably higher than the catalytic hydrolysis using Pt-supported mesoporous 3D SiC and SiCN structures. HRTEM investigations demonstrated the homogeneous distribution of Pt NPs over the Si₃N₄ support.

Introduction

Liquid organic and inorganic chemical hydrides like methanol, formic acid, ammonia borane and sodium borohydride are attractive for fuel cell applications, owing to some advantageous features [1]: they are in liquid state (especially in aqueous solution) and can be easily handled; they may store and produce hydrogen on-demand; they are relatively stable in ambient conditions during long periods of storage without usage; they are non-flammable and non-toxic; they are hydrogen-rich, i.e. with high gravimetric energy densities. A typical and widely-investigated example of those is the alkaline aqueous solution of sodium borohydride NaBH₄, knowing that the hydride carries 10.8 wt% of hydrogen [2], [3].

By hydrolysis, molecular hydrogen is spontaneously released by reaction of one hydridic hydrogen (H^δ−) of NaBH₄ with one protic hydrogen (H^δ+) of H₂O, which thus provides half of the H₂ generated. The by-product, sodium tetrahydroxyborate, may be recycled back into NaBH₄ via a complex process which may offer a continuous delivery of hydrogen to engine or fuel cell [4]. To achieve high effective gravimetric hydrogen storage capacities, hydrolysis has however to be accelerated by a metal-based catalyst [5], [6], [7]. For example, Kojima et al. reported an effective capacity of 9 wt% owing to the presence of a supported catalyst [7]. Furthermore, such attractive performances imply harsh operating conditions according to the fact that the reaction is exothermic and the by-product sodium tetrahydroxyborate is a strong base which significantly increases the pH of the solution. Within this context, both the catalyst and the support have to be designed with the objective to withstand these harsh operating conditions.

Catalytic activity of metal nanoparticles is dependent on their shape, size, crystal structure and textural parameters. Furthermore, it can be suitably increased by selecting a support system on which nanoparticles are preferentially synthesized with tailored shapes, sizes and crystalline structure without agglomeration. Because the transport of materials to active sites of catalyst is controlled by diffusion through the pores of the support system [8], the support is expected to be prepared with tunable pore morphologies and accessible porosity [9]. In particular, the high specific surface area (SSA) of mesopores provides selectivity and active sites to be effectively accessed for catalysis. We therefore focused our study on the preparation of mesoporous components.

Metal oxide-type supports form one of the most important classes of supports of active nanocatalysts [9], [10]. Even though these materials display excellent properties, their efficiency is limited to reactions where the conditions are not harsh. In reactions involving high temperatures, reducing media (highly alkaline) and/or fast reaction rates, these supports exhibit stability issues (thermal, chemical and mechanical). For such reactions, non-oxide ceramics represent better alternative supports.

In the category of non-oxide ceramics, silicon or transition metal carbides, carbonitrides and nitrides display attractive properties (high thermal and chemical stability, oxidation and corrosion resistance, low bulk density, high thermal conductivity, mechanical reliability) to be used in harsh environment [11], [12]. However, the preparation of non-oxide ceramics with tailored mesoporosity is a great challenge difficult to reach using conventional ceramic process. One of the ways to design such materials is to control the structure at very small length scales in an early stage of their synthesis. A precursor route is the way to reach this goal.

Preceramic polymers in which uniform chemical composition is established at molecular scale are making an increasingly important contribution to the research development and manufacture of carbide, carbonitride and nitride components as single-phase, multi-phase, solid solution and nanocomposite structures by pyrolysis in inert or reactive atmosphere [13], [14], [15], [16]. This concept called Polymer-Derived Ceramics (PDCs) route combines molecular chemistry, processing, engineering and chemistry of materials to control over elemental composition, nanostructural organization and shape of the final materials. This inherently allows proposing a large range of properties which is difficult to find in materials prepared by conventional synthesis routes. The possibility to develop materials bearing tailored mesoporosity can be envisioned by coupling the PDCs route with a templating approach [17], [18], [19], [20], [21], [22]. This process is in general applied to prepare mesoporous PDC powders through impregnation of hard template powders such as ordered mesoporous silica (SBA-15), derived carbon (CMK-3) or zeolite-derived carbon. However, powders have limited practical use. One of the solutions could be to sinter these powders at low temperature by Spark Plasma Sintering (SPS) and generate 3D structures as the preferred configuration [23], [24]. However, this strategy reduces the micro/mesoporosity and develops the macroporosity of the materials. Here, we propose to i) impregnate the porous structure of monolith-type templates with particular mesoporosity with a preceramic polymer solution, (ii) performing the subsequent pyrolysis to achieve the precursor-to-ceramic conversion, then (iii) removing the mold while generating a monolith with tailored mesoporosity. To our knowledge, this strategy has never been applied to generate fully mesoporous PDCs as monoliths.

Herein, we use three different commercial organosilicon polymers to form mesoporous 3D structures of silicon carbide (SiC), carbonitride (SiCN) and nitride (Si₃N₄). Monolithic activated carbon specimens are used as templates. As-obtained high surface area 3D Si-based carbide, carbonitride and nitride materials are used as platform to grow catalytically-active platinum nanoparticles (NPs). To assess the catalytic activity of the Pt/SiC, Pt/SiCN and Pt/Si₃N₄ materials, these systems are tested for hydrolysis of NaBH₄ to generate hydrogen in aqueous medium; the performances are finally compared. A hydrogen release of 24.2 L min⁻¹ g_Pt⁻¹ is measured for Pt/Si₃N₄. This material has been investigated by HRTEM before and after Pt NP growth. The overall flow chart of the process is shown in Fig. 1.