Fe-Ca-Catalyzed Conversion of Cellulose Into Highly Ordered Graphitic Carbon Revealed by Solid-State 1 3C NMR and Mössbauer Spectroscopy.

Abstract

An earth-abundant Fe-Ca bimetallic catalyst was developed to drive the structural transformation of renewable cellulose into highly ordered graphitic carbon. Solid-state ¹³C NMR T₁ relaxation measurements monitored the gradual reorganization of carbon as the temperature changed. The results demonstrate a clear correlation between elevated graphitization temperatures and longer relaxation time, with the Fe-Ca-catalyzed biocarbon at 1800°C achieving a T₁ value of 102.5 s, which closely approaches the 117.5 s threshold for commercial graphite. ⁵⁷Fe Mössbauer spectroscopy revealed a highly coordinated sequential reduction mechanism, shifting from nanoscale superparamagnetic oxides at low temperatures (400°C-600°C) to a reactive iron carbide intermediate at 800°C. At high temperatures (1400°C-1800°C), calcium serves as a vital structural modifier that prevents Fe nanoparticle sintering, thereby maintaining high metal dispersion and enabling complete deoxygenation and advanced graphitic crystallization. Although the resulting highly reduced ferromagnetic α-Fe matrix induces a strong bulk magnetic susceptibility (BMS) effect, which complicates conventional carbon chemical-shift identification, advanced ¹³C T₁ and Raman analyses confirm the formation of a highly ordered graphitic carbon network. This work provides profound atomic-scale insights into the valorization of bio-feedstocks into highly crystalline carbon materials suitable for energy storage applications, offering a sustainable alternative to fossil-derived graphite.