Base Metal Catalyzed Graphitization of Cellulose: A Combined Raman Spectroscopy, Temperature-Dependent X-ray Diffraction and High-Resolution Transmission Electron Microscopy Study

Abstract

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 T = 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, T < 500°C; fcc nickel, T < 500°C; fcc cobalt, T = 570°C; bcc iron, T = 700°C). Moreover, it is shown that at distinct temperatures nickel (T ≥ 800°C), cobalt (T ≥ 800°C), and iron (T ≥ 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 ≤ T < 800°C) and cobalt nanoparticles (700°C ≤ T < 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.

Well‐Defined Cobalt Catalyst with N‐Doped Carbon Layers Enwrapping: The Correlation between Surface Atomic Structure and Electrocatalytic Property

“…It was noteworthy that the uniform deposition of PDA layer on the surface of Co–MOFs was able to offer sufficient confinement to suppress the Co metal particles from further irreversible fusion and aggregation at high temperature, resulting in the well dispersion of Co NPs after pyrolysis, while the PDA‐free Co–MOFs resulted in the structure collapsed and formed self‐aggregated uncontrollable large Co NPs under the same pyrolysis process due to the lack of the protection of the surface PDA coating layer. High‐resolution TEM (HRTEM) image of Co‐MOFs@PDA‐800 clearly shows that the Co NPs are encapsulated by graphitic carbon layers (≈8 nm) with crystal lattice spacing of 3.4 Å corresponding to the (002) plane (Figure d), which should be attributed to the carbon graphitization under the catalytic behavior of Co NPs at high temperature . This result can also be confirmed by X‐ray diffraction (XRD) results ( Figure a).…”

Implications of the Molybdenum Coordination Environment in MFI Zeolites on Methane Dehydroaromatisation Performance

“…This shift has been reported to be due to a contribution from a second D2 band (1620 cm −1 ) attributed to edges of graphitic crystallites . Thus, the observed shift suggests the presence of very small carbon crystallites with large surface/bulk ratio and therefore a high number of edges . A weak shoulder can be also observed at 1200 cm −1 , this Raman shift has been named as a D4 band and it is ascribed to the stretching vibrations of sp 2 ‐sp 3 C−C bonds in conjugated aliphatic species .…”

The Reductive Dehydration of Cellulose by Solid/Gas Reaction with TiCl4 at Low Temperature: A Cheap, Simple, and Green Process for Preparing Anatase Nanoplates and TiO2/C Composites

“…The Raman spectrum of TiO 2 –600 also shows a broad D‐band centred at 1342 cm −1 (6‐membered aromatic rings in graphitic materials related, attributed to defects in graphite) and a broad G band at 1586 cm −1 (ascribed to the sp 2 hybridized carbon network) . The I (D)/ I (G) ratio, shown to be inversely related to the crystalline thickness of graphitic materials, can be tentatively used here to determine for TiO 2 –C600 a correlation length of 1–1.25 nm that indicates nanosized crystalline sp 2 ‐hybridized carbon clusters embedded in a disordered matrix of both amorphous sp 3 ‐ and sp 2 ‐hybridized carbon, a situation similar to other cellulose pyrolyzed at this temperature …”