NMR investigation on the occurrence of Na species in porous carbons prepared by NaOH activation

A detailed investigation was conducted about the process of alkali activation of charred rice hulls using NaOH. A carbon-rich precursor was initially prepared from the pyrolysis of rice hulls under N₂ atmosphere, part of it being leached with HF to remove silica. The precursor was then mixed with NaOH, heat-treated at activation temperatures from 600 to 800 °C, and part of the product was finally washed with distilled water. Thermogravimetric curves under O₂ flux showed a strong reduction in the ash content of the activated samples, indicating the consumption of silica during the activation process. From X-ray diffractometry, ²⁹Si, and ²³Na NMR spectroscopy, it was possible to identify the formation of sodium carbonate and silicates in the non-washed samples. After washing, all these compounds were removed and specific surface area measurements indicated a substantial porosity development, with larger surface area values obtained for the samples prepared from the HF-leached precursor. The use of ²³Na NMR spectroscopy indicated the retention of sodium in the washed samples, in a chemical environment distinct from carbonates and silicates. The shapes and positions of the observed resonance lines pointed to a disordered environment, associated with oxygenated surface groups within the porous structure of the activated carbons.