Characterization of narrow micropores in almond shell biochars by nitrogen, carbon dioxide, and hydrogen adsorption

• CO2 isotherms may be needed to fully characterize the pore structure of biochars.
• Biochars may contain significant number of narrow micropores.
• Narrow micropores appear sensitive to washing in the case of high-ash biochar.
• Results indicate that biochar may have hydrogen storage potential.

Characterization of biochars usually includes surface area and pore volume determinations by nitrogen adsorption. In this study, we confirmed that there is a substantial pore volume in biochars created via slow pyrolysis from low- and high-ash almond shells that cannot be characterized in this fashion due to the narrow pore sizes. Hydrogen and carbon dioxide were used, in addition to nitrogen, to characterize these narrow micropores. All isotherms, when appropriately normalized, fell on the same characteristic curve which aided in the characterization and understanding of the pore structure. Based on the characteristic adsorption isotherm, the average pore size diameter was estimated to be 0.4–0.8 nm. When washed, the biochars’ pore volume increased but the narrow micropore structure remained. While the biochars were alkaline, the pH of the wash solution had no or little impact on the adsorption of CO2. Overall, the results suggest that N2 isotherms should be complemented with CO2 isotherms for proper characterization of biochars. Alignment of such normalized isotherms to characteristic curves can assist in generating a more complete understanding of the pore structure over the entire region of pore diameters.