Structural Evolution in Iron-Catalyzed Graphitization of Hard Carbons

Abstract

Despite the recent interest in catalytic graphitization to obtain graphite-like materials from hard-carbon sources, many aspects of its mechanism are still poorly unknown. We performed a series of in situ experiments to study phase transformations during graphitization of a hard-carbon precursor using an iron catalyst at temperatures up to 1100 °C and ex situ total scattering experiments up to 2000 °C to study the structural evolution of the resulting graphitized carbon. Our results show that upon heating and cooling, iron undergoes a series of reductions to form hematite, magnetite, and wustite before forming a carbide that later decomposes into metallic iron and additional graphite and that the graphitization fraction increases with increasing peak temperature. Structural development with temperature results in decreasing sheet curvature and increased stacking, along with a decrease in turbostratic disorder up to 1600 °C. Higher graphitization temperatures result in larger graphitic domains without further ordering of the graphene sheets. Our results have implications for the synthesis of novel biomass-derived carbon materials with enhanced crystallinity.

Iron-catalyzed graphitization for the synthesis of nanostructured graphitic carbons

“…For example, in situ synchrotron XRD data from Gomez-Martin et al showed that the onset of graphitization of a wood precursor corresponded to the appearance of an Fe 3 C phase. 222 During cooling, the Fe 3 C phase transforms to γ-Fe then α-Fe, triggering a second carbon precipitation step. This change in the crystalline composition on cooling highlights the importance of studying graphitization in situ .…”

“…216 Fe and Fe 3 C are both phases that are commonly iden-tied in in situ X-ray diffraction studies of iron-catalyzed graphitization, oen within the same sample. 146,149,222,223 This raises the possibility that both dissolution-precipitation and carbide decomposition are potential mechanisms in catalytic graphitization. However, other pathways may also be possible.…”

“…Some studies used nitrogen gas flow through 223 or around 244 the capillary while another partially carbonized the sample before sealing it within a quartz capillary. 222 In all these studies, a hot air blower was used to heat the sample to the required temperature (Fig. 17).…”

Graphitization by Metal Particles

“…However, by taking into account the higher chemical potential of amorphous carbon compared with graphite, Parmon showed such a high carbon solubility into the metal particles is thermodynamically favorable and could result in a meta-stable carbide melt forming at the surface . A finding supported by other studies on Au and mixed Co/Fe nanoparticles; , and recently by in situ XRD of growth around iron particles …”

“…Despite the success of these models in explaining many experimental observations, more recent in-situ experiments, have detected graphitic carbon formation during heating rather than upon cooling as expected from the dissolution/precipitation mechanism. High-resolution transmission electron microscopy performed in situ has shown graphenic layers forming around metal particles at 500 °C; and X-ray diffraction (XRD) shows graphitic carbon formation around 700 °C. ,− A surface-mediated rearrangement of carbon into graphitic carbon is clearly more significant than precipitation; especially on smaller metal particles that do not behave as bulk materials, and operating at lower temperatures is more attractive for scalable production.…”