Browsing by Author "Felderhoff, Michael"

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A Few Layers Graphene Encapsulated Fe-Based Nanoparticles Synthesized from Ferrocene Containing Precursors: CVD Optimization and Evaluation for Possible Nanocatalyst Performance towards H2 Production
(Pergamon-Elsevier Science Ltd, 2026) Demirbas, Derya; Kutluay, Sinan; Agaogullari, Duygu; Suzer-Cicek, Layda; Mertdinc-Ulkuseven, Siddika; Padberg, Gero; Felderhoff, Michael; Süzer-Çiçek, İlayda
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 degrees 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 Mo & uml;ssbauer 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 (Ms = 22.4-33.5 emu/g, Hc = 82.3-278.3 Oe). Fe/Fe3C@C nanoparticles obtained under optimum conditions achieved very high H2 production rate (HPR = 54200 mLH2 gcat h- 1) values, with low activation energy (Ea = 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.
Citation - WoS: 3
Citation - Scopus: 3
Graphene Encapsulated Fe-Based Nanoparticles Synthesized From Iron(ii) Sulfate Heptahydrate Containing Precursors: Influence of Chemical Vapor Deposition Parameters
(Elsevier, 2024) Öveçoğlu, Mustafa Lutfi; Ağaoğulları, Duygu; Felderhoff, Michael; Winkelmann, Frederik; Demirbaş, Derya; Mertdinç Ülküseven, Sıddıka; Mertdinc-Ulkuseven, Siddika
Importance of process parameters on thermal, microstructural, and magnetic properties of synthesized core/shell nanoparticles was investigated during their production via chemical vapor deposition (CVD). Herein, iron(II) sulfate heptahydrate and fumed silica powders were mixed in ethanol, and the solution was used for precursor preparation by utilizing spray dryer. These prepared precursors were treated in the CVD process under methane/ hydrogen (CH4/H2) 4 /H 2 ) gas flow to synthesize graphene-encapsulated core/shell nanoparticles. CVD studies were performed at various temperatures (900-1000 degrees C), holding times (60, 90 min), and gas flow rates (100, 200 mL/ min). After CVD studies, purification was applied to remove uncoated nanoparticles, and remaining fumed silica phases originated from the precursor via selective acid leaching using hydrofloric acid (HF) and hydrochloric acid (HCl) solutions. X-ray diffractometry, Raman and Mossbauer spectroscopy, Zeta potential measurement, thermogravimetry combined with differential scanning calorimetry, scanning and transmission electron microscopy/energy-dispersive spectroscopy, and vibrating sample magnetometry (VSM) results yielded the optimized CVD parameters as 950 degrees C, 60 min, CH4/H2: 4 /H 2 : 1/1 and 50 mbar. The characterization results proved that multilayer graphene (d-spacing: 0.34 nm) encapsulated Fe/Fe3C 3 C nanoparticles (average core size: 46.9 nm, shell thickness: 16.6 nm) can be successfully synthesized by using CVD process followed by a leaching treatment. VSM results revealed that synthesized nanoparticles had soft ferromagnetic properties (Ms: s : 90.6-185 emu/ g; Hc: c : 255.4-301.6 Oe). Characterization results deepen the understanding of process parameters of CVD system on characteristics of core/shell nanoparticles.