Space confined synthesis of highly dispersed bimetallic CoCu nanoparticles as effective catalysts for ammonia borane dehydrogenation and 4-nitrophenol reduction

Highlights

• Synthesis of ultrasmall CoCu NPs using COOH functionalized SBA-16 as support.

• Bimetallic CoCu@S16LC-20 are used as multifunctional catalysts.

• CoCu@S16LC-20 can release H₂ at a rate of 6570 mL min⁻¹ g⁻¹ from NH₃BH₃ hydrolysis.

• CoCu@S16LC-20 degrades 4-nitrophenol to 4-aminophenol at a rate of 3.85 × 10⁻² s⁻¹.

• Excellent activity due to small size, 3D porous support, and synergistic effect.

Abstract

Bimetallic CoCu nanoparticles (NPs) are successfully embedded within the cage-type mesopores of COOH functionalized mesoporous silica SBA-16 by double agent chemical reduction method. The presence of the COOH groups in SBA-16 control the size and dispersion of the CoCu NPs. CoCu@S16LC-20 is used as the catalyst for the hydrolysis of ammonia borane (AB) and reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). The catalytic activity of the CoCu@S16LC-20 outperforms that of monometallic counterparts Co and Cu. In particular, Co₄₀Cu₆₀@ S16LC-20 can release H₂ at a rate of 6570 mL min⁻¹ g⁻¹ and reveals superb catalytic activity with a turn over frequency (TOF) of 16.36 mol H₂ min⁻¹ mol_catalyst⁻¹. The superior catalytic activity of the bimetallic CoCu@S16LC-20 could be ascribed to the strong synergistic effects emerging from the modified electronic structure, smaller particle size, and 3-dimensional (3D) mesoporous silica support. The smaller particle size renders more active sites, and the mesoporous silica support provides easier paths for transfer of reactants to the catalyst. The catalyst magnetic nature permits straightforward recovery from the aqueous solution after reaction completion which enhances the reusability. The bimetallic Co₄₀Cu₆₀@S16LC-20 can degrade 4-NP to 4-AP at rapid rate of 3.85 × 10⁻² s⁻¹ in presence of reducing agent NaBH₄.

Introduction

Hydrogen has been considered as one of the best alternative energy carriers to satisfy the increasing demand for a sustainable and clean energy supply due to its advantages of high energy density, abundance, and environmental friendly nature [1], [2], [3]. The development of an effective hydrogen generation/storage system as a long-term solution for a secure energy is one of the most difficult challenges encountered by the mankind [4], [5], [6], [7]. Ammonia borane (NH₃BH₃, AB) has become an appealing contender for chemical hydrogen storage applications due to its high hydrogen content (19.6 wt%), low molecular weight (30.86 g mol⁻¹), easy storage, and convenient transportation owing to its high stability in neutral aqueous solution at room temperature [4], [8], [9]. More importantly, the hydrolysis of ammonia borane can generate an equivalent amount of hydrogen at room temperature in the presence of a suitable catalyst by following the equation: NH₃BH₃ + 2H₂O → NH₄BO₂ + 3H₂. A wide range of catalysts including noble metals such as Pt [10], Pd [11], Ag [12], and Ru [13] etc. have been tested for the hydrolytic dehydrogenation of ammonia borane with rapid hydrogen generation. However, the extensive applications of these metals are circumscribed because of their high cost and limited availability in nature. Therefore, low cost non-noble metals such as Co [14], Fe [15], Ni [16], and Cu [17] have been employed and designed as catalysts for the hydrolytic dehydrogenation of ammonia borane. Unfortunately, most of these low cost materials are fairly unstable in the ambient environment due to the magnetism induced agglomeration and easy oxidation. Various research groups have suggested the synthesis of bimetallic alloy nanoparticles by combining both noble and non-noble metals aiming to reduce the expenditure of the catalysts [18], [19], [20], [21], [22], [23]. By adjusting the ratio of the two metal precursors, the composition of bimetallic nanoparticles can be controlled. Surprisingly, the catalytic activity of the bimetallic nanoparticles in the hydrolysis of ammonia borane was found to be composition dependent, and in most of the time the catalytic activity of one specific combination was better than their monometallic counterpart. The enhancement of the catalytic activities of the bimetallic nanoparticles has been supposed to be caused by the synergistic structural effects and fine tuning of the electronic structure of the bimetallic nanoparticles [24], [25], [26], [27], [28]. Development of cheap metal catalysts with improved catalytic activity is touted as very important for practical applications by considering the low cost issue. The size distribution of the metal catalyst significantly determines its activity in the catalytic reaction. In general, uniformly dispersed metal nanoparticles can supply abundant active sites for the catalytic reaction to take place. However, these ultrafine metal nanoparticles aggregate easily during the catalytic reaction, and results in reduction of their activity and reusability tremendously. The use of an appropriate support is the most worthy idea to avoid the aggregation of the nanoparticles. In addition, it is evidenced that proper metal-support interaction can further boost the catalytic performance. Various materials such as active carbon [29], [30], [31], [32], magnetic carbon [33], graphene [34], [35], carbon nanotube [36], mesoporous, and fibrous silica [37], [38], have been widely utilized as the supports to fabricate metal nanoparticles with required sizes and shapes. Mesoporous silicas such as SBA-15, MCM-41, FDU-12 and SBA-16 etc. have been employed as effective supports to confine metal nanoparticles because of the uniform pore size and large surface area [39], [40], [41], [42]. The highly ordered mesoporous channels in these kinds of supports not only control the growth of metal nanoparticles, but also preclude the aggregation of metal nanoparticles. Mesoporous silicas with 3-dimensional (3D) pore arrangements, such as FDU-12, SBA-16, SBA-1 etc. are especially suitable for entrapping metal nanoparticles as their multidirectional mesopores cut down the possibility of pore blocking tremendously. The interpenetrating mesoporous architecture of the support also allows easy transportation of reactant and product molecules throughout the catalytic reaction. However, agglomeration of metal nanoparticles could not be controlled very efficiently due to the weak electrostatic attraction between the metal ions and the silanol on the surface of the mesoporous silica support. One viable option to enhance the intensity of the electrostatic attraction is to modify the surface of the mesoporous silica support with desired organic functional groups such as thiol (SO₃) [43], carboxylic acid (COOH) [44], and amino (NH₂) [45] etc. The confinement of noble metal free bimetallic nanoparticles within the mesopores of organic group functionalized mesoporous silica is therefore highly advantageous considering its economic prospect and durability in catalytic reactions.

Herein, a series of CoCu bimetallic nanoparticles with different Co/Cu ratios were prepared using COOH functionalized mesoporous silica SBA-16 with 3D pore arrangement as the support via the double agent chemical reduction approach. NH₃BH₃ was used as the co-reducing agent along with NaBH₄ to restrict the growth of the metal nanoparticles precisely via slowing down the reducing rates of metal ions. The different Co/Cu ratios were acquired by introduction of various amounts of cobalt and copper precursors. The homogenously dispersed COOH functional groups within the mesoporous silica support can govern the distribution and growth of the nanoparticles extensively. The prepared mesoporous silica supported bimetallic CoCu nanoparticles were used as the catalysts for the release of hydrogen from the hydrolytic dehydrogenation of ammonia borane. Because of the homogenous dispersion and smaller size of bimetallic CuCo nanoparticles, and 3D porous structure of the support, the catalyst thus obtained is anticipated to exhibit excellent catalytic activity for the hydrolysis of ammonia borane. In addition, the magnetic nature of the prepared catalyst permits its easy separation from the aqueous solution only with the use of an external magnetic field rather than widely used filtration and centrifugation techniques. The magnetic behavior of the bimetallic CoCu nanoparticles thus can play an important role in the convenient recycling process. The catalytic performance of the bimetallic CoCu nanoparticles were further tested for the catalytic conversion of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of NaBH₄. Due to the altered electronic structure emerging from the CoCu alloy formation and the strong synergistic effects, bimetallic catalyst exhibited excellent catalytic activity compared to the monometallic counterparts.