Catalytic conversion of spent coffee grounds into graphitic and nanotube biocarbon

Graphitic biocarbon comprising of graphene-like shells, nanotube-like structures, spherical clusters of graphitic biocarbon encapsulated Fe, and amorphous biocarbon, were simultaneously synthesized from spent coffee grounds. The study investigated the influence of two distinct types of non-toxic, easily accessible metal catalysts on graphitization, and the evolution of the catalyst-impregnated biocarbon nanostructures with temperature. The findings show that using Ca as a catalyst produces a highly porous graphitic structure with a larger crystallite size (L c = 14.38 nm) and nanotube-like structures that become denser with temperature, increasing in diameter from 58.48 nm at 1200 °C to 93.64 nm at 1800 °C. Conversely, Fe as catalyst produces non-porous graphitic structures with a smaller crystallite size (L c = 4.06 nm). Notably, unlike previous literature reports, nanotube-like structures were observed only in Ca-catalyzed biocarbon, not in the Fe-catalyzed ones. • The choice of metal catalyst influences the resulting graphitic structures obtained • Notable distinction in the crystallite sizes obtained for Ca and Fe catalyzed biocarbon • Highly porous graphitic structure obtained with Ca as catalyst, whereas using Fe results in non-porous graphitic structures • Nanotube-like graphitic structures synthesized from coffee grounds using Ca as catalyst