A few layers graphene encapsulated Fe-based nanoparticles synthesized from ferrocene containing precursors: CVD optimization and evaluation for possible nanocatalyst performance towards H₂ production

This study focuses on optimizing the synthesis of a few-layer graphene-encapsulated iron-based nanoparticles (Fe/Fe₃C@C), prepared through spray drying, chemical vapor deposition (CVD), and leaching processes using ferrocene-based precursors, and their application as nanocatalysts for hydrogen (H₂) production via sodium borohydride (NaBH₄) methanolysis. Ferrocene-impregnated silica powders were prepared by spray drying them from a solution containing ferrocene, fumed silica, and ethanol. Then, these prepared powders, known as precursor powders, were subsequently introduced into the CVD system. Both the reduction of ferrocene and the encapsulation of Fe-based nanoparticles by graphene layers occurred in-situ during the CVD process. CVD temperature and the flow rates of CH₄ and H₂ gases are critical parameters that effects of the microstructural, thermal, and magnetic properties of synthesized nanoparticles. The CVD system was performed at temperatures ranging from 850 to 1000 °C, with variable gas flow rates of 50 or 100 mL/min. Additionally, acid leaching with hydrofluoric (HF) and hydrochloric (HCl) acids ensured the synthesis of pure powders free from silica and uncoated Fe, confirming the chemical stability of the nanoparticles. The presence of graphene in all synthesized samples within these parameter ranges were confirmed by Raman spectroscopy. Phase identifications were carried out using X-ray diffraction (XRD) and Mössbauer spectroscopy, revealing the Fe and trace amount Fe₃C as core phases. Transmission electron microscopy (TEM) revealed the core-shell structure of the nanoparticles with a few layers of graphene coatings. Based on the coercivity and magnetic saturation values obtained from vibrating sample magnetometry (VSM), synthesized core-shell nanoparticles exhibited soft magnetic properties (M_s = 22.4-33.5 emu/g, H_c = 82.3-278.3 Oe). Fe/Fe₃C@C nanoparticles obtained under optimum conditions achieved very high H₂ production rate (HPR = 54200 mL_H₂ gcat⁻¹ min⁻¹) and turnover frequency (TOF = 3101 h⁻¹) values, with low activation energy (E_a = 20.08 kJ mol⁻¹) value, highlighting their potential as an efficient and promising candidate catalyst for industrial-scale H₂ production via the NaBH₄ methanolysis reaction. In addition, it was found that the Fe/Fe₃C@C nanoparticles retained 48% and 71% of their initial activity after 5 consecutive cycles, as measured by the HPR and TOF values, respectively.