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.