Introduction

Hollow graphitic spheres (HGS) are interesting support materials for catalytic active metal nanoparticles. HGS can be widely used as tailored mesoporous carbon support with several advantages over other carbon materials.¹ The spheres have a high surface area and a pore system with narrow pore size distribution which makes this material interesting for the impregnation of metal nanoparticles and techniques like the confined-space alloying of bimetallic nanoparticles.² Additionally HGS have a high degree of graphitization what makes the material stable under common catalytic reaction conditions.  A typical synthesis route¹ for HGS covers 3 major steps starting with silica spheres coated with a mesoporous silica shell:

–          Impregnation of the SiO₂ template with iron nitrate and polymer-precursor

–          Carbonization/graphitization of the polymer shell

–          Leaching of the silica to obtain hollow graphitic spheres

Methods

The polymer-coated SiO₂ spheres were dry-prepared  and heated up to 1000°C with a heating rate of 5 °C min^-1. At certain points the temperature was maintained to minimize the thermal drift for the image acquisition.

Results

Starting the thermal treatment at first the activation of iron particles could be observed. Beginning at about 250°C the finely dispersed iron started to agglomerate and formed metallic nanoparticles. By further heating the iron particles became mobile and the graphitization started at about 900°C.

References

1)      C. Galeano et al., J. Am. Chem. Soc., 134, 20457-20465 (2012)

2)      C. Baldizzone, S. Mezzavilla et al, Angew. Chem. Int. Ed. 53, 14250-14254 (2014)

Figures:

Fig. 1: Cross section BF-STEM image of HGS with PtNi nanoparticles embedded in the porous shell to make the shell easier visible.

Fig. 2: HR-TEM (upper) and BF-STEM (lower) images of HGS with PtNi nanoparticles embedded in the porous shell. The graphitic parts of the shell can clearly be seen.

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EMC Abstracts - https://emc-proceedings.com/abstract/observation-of-the-graphitization-process-of-hollow-graphitic-spheres/