Impact of nitrogen and alkali dopants on pore architecture and surface functionalities of biochar in one-step CO2 pyrolysis

Akın S. Ş., Kır A. B., Kazanc F.

Journal of Analytical and Applied Pyrolysis, vol.186, 2025 (SCI-Expanded) identifier

  • Publication Type: Article / Article
  • Volume: 186
  • Publication Date: 2025
  • Doi Number: 10.1016/j.jaap.2025.106962
  • Journal Name: Journal of Analytical and Applied Pyrolysis
  • Journal Indexes: Science Citation Index Expanded (SCI-EXPANDED), Scopus, Academic Search Premier, Analytical Abstracts, CAB Abstracts, Chemical Abstracts Core, Chimica, Compendex, Veterinary Science Database
  • Keywords: Biochar, Heteroatom, Hierarchical porosity, Oxidative atmosphere, Pyridinic, Pyrrolic
  • Middle East Technical University Affiliated: Yes

Abstract

This study aims to elucidate the roles of common nitrogen dopants (melamine (M) and ammonium chloride (ACL)) and alkali agents (KOH (K) and NaOH (N)) on surface functional groups, pore architectures, and degrees of graphitization. Sugar beet pulp (SBP) samples were treated with 0.5 M KOH or NaOH solutions, either with or without the addition of nitrogen agents (ACL and M). Subsequently, the treated SBP samples underwent pyrolysis at 700°C under CO2 atmosphere. The resulting engineered biochars were characterized with respect to their microstructural, textural, morphological and thermal characteristics. The presence of KOH yielded higher specific surface areas for biochar samples (1188 and 1487 m²/g with and without melamine, respectively) compared to samples prepared with NaOH (706 and 557 m²/g with and without melamine, respectively). XPS results showed that KOH favored C[dbnd]O moieties, while NaOH favored C-OH moieties. Nitrogen agents introduced pyrrolic and pyridinic groups, improving structural parameters compared to treatment with only alkali agents (ID/IG: SBPC-N-ACL: 0.66 and SBPC-K-M: 0.75 SBPC-N: 0.80 and SBPC-K: 1.48). TGA analysis revealed that the agents significantly altered the pyrolysis mechanism, complementing the hydrolysis effects on the major constituents of SBP (hemicellulose, pectin, starch, and proteins) during the pre-treatment step. Specifically, alkali agents catalyzed dehydration reactions, resulting in higher char yields. Melamine acted as a nitrogen radical promoter, facilitating the cracking of long-chain volatile polysaccharides and acting as a crosslinker, leading to the formation of pyridinic groups and contributing to mesopore development under the CO₂ atmosphere.