This work focuses on investigating the conversion of olive stone waste into porous, graphitic carbon materials using a sequence of thermal and catalytic treatments. Three distinct processing strategies were investigated: (i) chemical activation with potassium hydroxide (AC-OS-KOH), (ii) thermal pyrolysis in an inert atmosphere yielding biochar (C-OS), and (iii) catalytic graphitization using transition metals (Ni or Fe) either in combination with KOH activation (AC-OS-KOH-Ni, AC-OS-KOH-Fe) or applied directly to biochar (C-OS-Ni). The structural, morphological, and textural properties of the resulting carbon materials were characterized using X-ray diffraction, scanning electron microscopy, and carbon dioxide (CO2) physisorption at 0°C.
Among all synthesized materials, AC-OS-KOH and AC-OS-KOH-Fe displayed superior microporosity and well-developed pore architectures, leading to enhanced CO2 adsorption capacities compared with nonactivated and nickel-catalyzed samples. Notably, the dual strategy of chemical activation and nickel catalysis facilitated the transformation of olive stone precursors into graphitic-like porous carbon with a crystallinity index reaching 61%, indicating successful partial graphitization. CO2 adsorption–desorption experiments were conducted at 25°C and 50°C under two CO2 concentrations (90% and 10%, balanced with N2). The KOH-activated carbons, with or without metal doping, exhibited fast adsorption–desorption kinetics, in contrast to the sluggish performance of the C-OS-Ni sample. This behavior underscores the critical role of micropore size and volume in governing CO2 molecular diffusion and access to active sites. At elevated CO2 concentration (90%), AC-OS-KOH demonstrated the greatest adsorption capacity, achieving 13.64 wt.% at 25°C and 8.98 wt.% at 50°C. In contrast, under diluted CO2 conditions (10%), the AC-OS-KOH-Fe sample showed superior performance, indicating a strong link between pore size distribution and selective gas adsorption. Furthermore, the KOH-activated carbons maintained consistent adsorption performance across six consecutive adsorption–desorption cycles, confirming their stability and regeneration potential for practical CO2 capture applications.
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