Preparation and Characterization of Carbon-Encapsulated Iron Nanoparticles and Its Application for Core-Shell Type of Catalyst

Introduction

Spherical iron oxide and carbon-encapsulated iron nanoparticles have been prepared by ultrasonic irradiation followed by annealing at various temperatures. As the annealing temperature of the as-prepared α-Fe₂O₃ nanoparticles was increased, the sample transformed into γ-Fe₂O₃, Fe₃O₄, and Fe nanoparticles via the reduction process without requiring any additional reducing agents such as H₂ gas, thus creating a carbon shell surrounding the nanoparticles. By controlling the experimental conditions, Fe nanoparticles of various sizes can be formed with diameters in the range 100 - 800 nm; these nanoparticles are tightly encapsulated by 20 nm thick carbon shells. Because of their high saturation magnetization 212 emu·g^-1, the carbon-encapsulated Fe nanoparticles can be used for magnetic resonance imaging (MRI) with a dramatically enhanced efficiency compared to commercially available T₂contrast agents. Also, it can be used to catalysts system in liquid phase hydrogenation of biomass derived levulinic acid (LA) to γ-valerolactone (GVL) because of the excellent catalytic activity of Fe/C core-shell type as precious/noble metals in liquid phase using water as a solvent.

Experimental

To perform the facile synthesis of 300 nm α-Fe₂O₃ nanoparticles, the sono-chemistry method was used, inducing ultrasonic decomposition of raw materials under ambient conditions. Iron (III) acetylacetonate, Fe(acac)₃, in octyl ether was sonicated to generate localized hot spots within the acoustic cavitation of collapsing bubbles during ultrasonic irradiation (reaction time is 10 min and the temperature reaches approximately 257 °C). As the annealing temperature of the as-prepared α-Fe₂O₃ nanoparticles was increased, the sample transformed into various forms: α-Fe₂O₃ → γ-Fe₂O₃ → Fe₃O₄ → Fe. It should be noted that the reduction phenomenon occurred without any additional reducing agents such as H₂gas.                    

During the ultrasonic irradiation process, the carbon species generated from the octyl ether would then be embedded in the as-prepared α-Fe₂O₃ nanoparticles acting as a reducing agent when the annealing temperature increased. The LA hydrogenation was carried out in a 100 ml batch type reactor. Typically, 5.0 g of LA in 45 ml of DI water were added into the reactor, in which 0.2 g of Fe/C was charged. After tighten of reactor, the initial hydrogen pressure was maintained to 5 bar. The reaction was carried out at 170 ^oC for 3h using 1000 of RPM speed.    

After cooling of reactor, the product mixture and catalyst were separated by simple filtration. Qualitative and quantitative analysis of products were done by Gas chromatography equipped with Cyclo-Sil B column and FID detector. For recycle study, the recovered catalyst was washed with ethanol and dried it at 120 ^oC for overnight and used for the next run.

Result and Discussion

We confirmed the chemical composition of the as-prepared α-Fe₂O₃ using energy-dispersive X-ray analysis (EDS). Results from EDS spectra of as-prepared α-Fe₂O₃ nanoparticles showed that the sample contained Fe, O, and 52.6 wt % C. Our experimental evidence leds us to believe that the embedded carbon species play an important role as reducing agent in the phase transformation of iron oxide. Embedded carbon species have been studied and developed for their use as reducing agents for rare-earth materials, such as heat treatment of Eu³⁺-containing materials under a reducing atmosphere (carbon or CO gas). When the annealing temperature increased, the as-prepared α-Fe₂O₃ nanoparticles were rapidly transformed into γ-Fe₂O₃ and Fe₃O₄ via the reduction process; their carbon contents were decreased to 36.5 wt % and 21.6 wt %, respectively. We found that these species could be directly converted into a carbon shell on the surface of the Fe nanoparticles when the annealing temperature was 700 °C. We confirmed the phase transformation from Fe₃O₄to Fe by XRD as the annealing temperatures increased (Figure 1a). Examination of the sample by FE-SEM and TEM indicates that the carbon shell growth produces Fe nanoparticles encapsulated in a carbon shell (Figure 1b and c). HR-TEM (Figure 1d) image shows that most of the Fe nanoparticles were encapsulated by carbon shell with a diameter of 20 nm.

After successful characterization of core-shell Fe/C material, we investigated its hydrogenation activity for LA hydrogenation to GVL in a liquid phase using water as a green solvent. The hydrogenation was carried out in a batch type reactor at 170 ^oC of reaction temperature and using 5 bar of H₂ pressure. The core-shell type of Fe/C catalysts work excellently for this reaction and gave 99.6% of GVL selectivity at 99.5% conversion of LA within 3 h of reaction time, and it could also allow its reusability for 5 times without any significant deactivation in catalytic activity.