Synthesis and formation mechanism of iron nanoparticles in graphitized carbon matrices using biochar from biomass model compounds as a support

Abstract Biochar, a carbon-rich valuable by-product obtained from the hydrothermal carbonization of biomass model compounds (cellulose, hemicellulose and lignin), was utilized as a support for synthesis of carbon encapsulated iron nanoparticles (CEINP) to promote green chemistry and engineering. Core-shell structures consisting of dark metal cores ranging from 40 to 80 nm and a light matrix with graphitic structure appeared in the CEINP. Rietveld refinement analysis applied to the X-ray diffraction (XRD) patterns of cellulose and hemicellulose biochar revealed similar phase compositions (graphite, α-Fe, Fe3C, CFe15.1 and CFe21.2). Interestingly, poorly ordered graphite phase was detected in the XRD diffractogram of CEINP from lignin biochar and HRTEM images have also shown the presence of disordered graphitic layers encapsulating the iron nanoparticles. CEINP from cellulose and hemicellulose on the other hand showed distinct graphite (crystalline) phases. Nitrogen physisorption measurements showed that the graphitization induced by iron nanoparticles introduced mesopores into the carbon matrix of CEINP from cellulose and hemicellulose biochar while CEINP from lignin biochar retained microporous structure due to low graphitization. Based on the characterization results, a detailed dissolution-precipitation mechanism is discussed in the context of bulk iron-carbon equilibrium phase diagram to explain the formation of different phases observed in the CEINP.