Unveiling the promoting mechanism of N-doping in porous biochar for formaldehyde adsorption: experimental and DFT studies

Abstract

Nitrogen (N) doping in porous carbon adsorbents has been proven to be one of the effective strategies to enhance formaldehyde (HCHO) adsorption performance. However, the intrinsic promoting mechanism of specific nitrogen species (pyridinic-N, pyrrolic-N, and graphitic-N) remains unclear, hindering the rational design of porous carbon adsorbents. Herein, we prepared a series of nitrogen-doped porous biochars with alterable N species via one-step pyrolysis of urea and waste jujube pits in different proportions (BCU-x, x = 0–3), and the correlation between specific N species and HCHO adsorption performance was established for the first time. Experimental results show that the amount of surface pyrrolic-N (N-5) groups was the dominant factor in determining HCHO adsorption performance. Combined with DFT calculation results, it was revealed that the introduction of pyrrolic-N species significantly increased the inhomogeneity of electrostatic potential (ESP) distribution and the maximum absolute ESP value of carbonaceous models (increased from 15.94 kcal mol⁻¹ to 50.15 kcal mol⁻¹) and consequently enhanced the affinity between polar HCHO and carbonaceous models (varied from −4.98 kcal mol⁻¹ to −7.85 kcal mol⁻¹). Specifically, the O atom of HCHO tended to interact with the H atom attached to the pyrrolic-N moiety, and a hydrogen bond interaction (N–H⋯OCH₂) existed. Therefore, the BCU-2 sample with the richest pyrrolic-N species exhibited the optimal HCHO adsorption capacity, of up to 21.25 mg g⁻¹, which was nearly 3.5 times higher than that of pristine biochar. This study clarifies the intrinsic promotion mechanism of specific nitrogen species in HCHO adsorption and provides general guidelines for the further design of high-performance carbonaceous adsorbents for HCHO removal.