Reduced Cobalt Borides Supported on Multi-Walled Carbon Nanotubes As Catalysts for Hydrogen Generation Reactions

Abstract

Nonrenewable sources of energy have caused detrimental pollution in recent decades, which has driven the search to find alternative fuel sources. Hydrogen is a powerful example of an alternative fuel, which has widespread applications and growing potential.1 Through the use of hydrogen feedstock materials (HFMs), hydrogen can be released and used in a controlled manner, which has brought attention to industrialized usage of HFMs. Specifically, sodium borohydride (NaBH4) is an HFM that contains 10.8% hydrogen by weight and can be utilized in hydrogen evolution reactions (HERs).2 The hydrolysis reaction of NaBH4 requires the use of a catalyst to speed the reaction, making a usable amount of hydrogen in a short timeframe.2 In recent studies, metal borides have gained attention as a catalyst due to their nano properties; their high surface area to volume ratio make them excellent candidates. With this study, cobalt borides were synthesized from a reduced cobalt metal organic framework (Co-MOF) precursor, which was synthesized via green chemical practices in room temperature, aqueous conditions.3 The borides were created in the presence of multiwalled carbon nanotubes (MWCNT) to prevent clustering effects and increase the stability and efficiency of the borides. To confirm the presence of borides and the complete reduction from the MOF precursor, multiple forms of characterization were used. X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) were performed to analyze the changes in crystallinity and organic functional groups, while scanning electron microscopy (SEM) and transition electron microscopy (TEM) were used to collect images of the composites. Through the imaging, the dimensions of the nanoparticle were measurable, and the size and shape of the borides clearly differ from the MOF precursors. Inductively coupled plasma (ICP) and Energy Dispersive X-ray Spectroscopy (EDS) were also used to confirm the presence of cobalt and boron, along with their respective ratios. The success of the catalyst in HERs was tested using a displacement method under various conditions in an aqueous solution of NaBH4.4 The renewability of the material along with its behavior under different temperatures gave its stability and activation energy. Five consecutive renewability trials were performed, which showed only a slight decrease from 22.50 to 17.76 mL of hydrogen produced. Additionally, activation energy was found to be 63.47 kJ mol-1 using the Arrhenius equation. The results acquired from the characterization and HER trials suggest that CoB@MWCNT can be successfully synthesized from a Co-MOF@MWCNT precursor and is an adequate catalyst for HFM hydrolysis reactions.