Nitrogen-Doped Carbon Encapsulation of Fe Composites: Synthesis Tuning Their Physical Properties for Syngas Conversion to Valuable Compounds
João Pedro S. Nascimento, Alcineia C. Oliveira, Fabiano A. N. Fernandes, Gardênia S. Pinheiro, Gilberto D. Saraiva, Ramon R. Peña-Garcia, Marco A. Morales Torres, José Jiménez-Jiménez, Enrique Rodríguez-CastellónAbstract
Nitrogen-doped carbon encapsulation of Fe composites was achieved through polymer-assisted coprecipitation synthesis with urea. SEM-EDS, XRD, HRTEM, Raman, and FTIR spectroscopy revealed nitrogen-doped graphitic carbon layers encapsulating the Fe spinel phase (γ-Fe2O3, Fe3O4) or other spinel-related phases. The key innovation lies in the design of nitrogen-doped carbon-supported Fe composites with the addition of heteroatoms, e.g., FexNC/MFe (M = Li, Mg, Al, Ti, Co, or Ni), considerably affecting their catalytic performances in syngas conversion via Fischer–Tropsch reaction. A novel aspect of these solids was revealed by XPS and textural properties results that showed the modulation of the porous structure of the composites by the existence of surface iron carbides and Fe3N iron nitrides covering a carbon matrix strongly bonded to the Fe2+/Fe3+ entities from the FexNC/NiFe. Mössbauer and Raman spectroscopy and XRD confirmed the presence of a bulk structure consisting of spinel ferrites, besides the introduction of defects into the structure. These facts enhanced the syngas conversion to 44% and improved the selectivity toward light and heavy hydrocarbons in the FTS reaction at a pressure of 20 atm, 270 °C, and CO/H2 molar ratio of 1.