1 research outputs found
Base Metal Catalyzed Graphitization of Cellulose: A Combined Raman Spectroscopy, Temperature-Dependent X‑ray Diffraction and High-Resolution Transmission Electron Microscopy Study
Microcrystalline
cellulose (MCC) spheres homogeneously loaded with
the nitrate salts of copper, nickel, cobalt, or iron are excellent
model systems to establish the temperature at which highly dispersed
base metal nanoparticles are formed as well as to establish the temperature
at which catalytic graphitization occurs during pyrolysis in the temperature
regime <i>T</i> = 500–800 °C. Temperature-dependent
X-ray diffraction (TD-XRD) and high-resolution transmission electron
microscopy (HRTEM) showed that the base metal nanoparticles are smoothly
formed from related base metal oxides via carbothermal reduction (fcc
copper, <i>T</i> < 500 °C; fcc nickel, <i>T</i> < 500 °C; fcc cobalt, <i>T</i> = 570 °C;
bcc iron, <i>T</i> = 700 °C). Moreover, it is shown
that at distinct temperatures nickel (<i>T</i> ≥
800 °C), cobalt (<i>T</i> ≥ 800 °C), and
iron (<i>T</i> ≥ 715 °C) nanoparticles catalyze
the conversion of the amorphous carbon support into ribbons of turbostratic
graphitic carbon according to Raman spectroscopy and TD-XRD. Copper,
however, was found to be inactive. Furthermore, HRTEM revealed that
nickel (500 °C ≤ <i>T</i> < 800 °C)
and cobalt nanoparticles (700 °C ≤ <i>T</i> <
800 °C) after their initial formation become encapsulated by
graphite-like shells prior to the onset of catalytic graphitization.
This does not occur in the presence of iron nanoparticles. This distinction
is attributed to the temperature required to access iron nanoparticles
by carbothermal reduction and their concomitant mobility. Evidence
(HRTEM) is provided that for the onset of catalytic graphitization
nickel and cobalt nanoparticles first have to escape from their graphite-like
shells. Therefore, iron nanoparticles are the most active catalyst.
Our results further show that (metastable) metal carbides play a pivotal
role in catalytic graphitization. This is demonstrated by the inactivity
of copper nanoparticles, the distinct onset temperatures of catalytic
graphitization, and the identification of cementite in the case of
iron nanoparticles