Low-temperature catalytic synthesis of graphite aerogels from polyacrylonitrile-crosslinked iron oxide and cobalt oxide xerogel powders

Abstract

We demonstrate the Fe- or Co-catalyzed preparation of graphitic carbon aerogels that bypasses the typical use of supercritical fluid drying, and takes place at much lower temperatures (800–1500 °C) than conventional graphitization (2500–3300 °C). The process starts with preparation of polyacrylonitrile (PAN)-crosslinked iron and cobalt oxide xerogel powders via surface-initiated free-radical polymerization of acrylonitrile on the porous solid networks of sol-gel-derived particle suspensions of the oxides. The resulting wet-gel oxide/polymer composite powders were dried under vacuum at 50 °C to xerogel powders, which were compressed into desirable form factors, e.g., discs or cylinders. These compacts were then aromatized (300 °C, O2), and subsequently carbonized in the range of 800 °C to 1500 °C under Ar. In situ carbothermal reduction of the oxide networks by the newly produced carbon yielded catalytic-toward-graphitization Fe(0) or Co(0) nanoparticles embedded within the carbons. Post-pyrolysis, those metallic nanoparticles were removed with aqua regia leaving behind pure graphitic carbon aerogels. Chemical characterization of materials up to aromatization was carried out with solid-state 13C NMR; characterization of the terminal graphite aerogels was based on powder-XRD, Raman, TEM, SEM, XPS and N2-sorption. Porosities in the range of 63%–78% v/v were created during aromatization, and mainly during pyrolytic carbonization. The best quality graphite aerogels were obtained with iron at 1500 °C. These materials consisted of 99.8% w/w graphitic carbon, the crystallite domain size along the (002) plane reached 170 Å, and the crystallite width along the a-axis was 70 nm.