Influence mechanism of metal modification on CO₂ adsorption of nitrogen-rich biochar

To enhance the adsorption capacity of biochar and prepare efficient adsorbents, elemental analysis was conducted on corn stalks, rice husks, and Chlorella. Chlorella, with the highest nitrogen content (8.85%), was selected as the raw material. Nitrogen-rich biochar was prepared using a one-step activation method, and metal-doped nitrogen-rich biochar adsorbents were fabricated by impregnating metal salts onto it. The effects of different metals coupled with nitrogen-doped biochar on CO₂ adsorption were investigated. SEM-EDS characterization tests indicated that metals were successfully incorporated into the surface of nitrogen-rich biochar and were evenly distributed. XPS test results showed that the nitrogen-containing groups underwent migration and transformation before and after metal doping, but pyrrolic nitrogen (N-5) remained the dominant form. Experimental results revealed that the specific surface areas of nitrogen-rich biochar, Zn-doped nitrogen-rich biochar, and Na-doped nitrogen-rich biochar were 1394.13, 1334.70, and 1185.98 m²/g, respectively, and their micropore rates were 37.11%, 36.70%, and 35.52%, respectively. Compared with nitrogen-rich biochar, the specific surface areas of Zn-doped and Na-doped nitrogen-rich biochar decreased by 59.43 and 208.15 m²/g, respectively, and their micropore rates decreased by 0.41% and 1.59%, respectively. Metal doping facilitated the chemical adsorption of CO₂ by nitrogen-rich biochar, and Na-doped nitrogen-rich biochar exhibited the highest CO₂ adsorption capacity, which was 42.9% and 16.7% higher than that of nitrogen-rich biochar and Zn-doped nitrogen-rich biochar, respectively. Molecular mechanism studies on the adsorption of CO₂ by Na and Zn coupled with N-5 were conducted from the perspective of quantum chemical theory. The theoretical calculation results indicated that the adsorption energy of CO₂ on the surface of CN5@Na₂O (pyrrolic biochar coupled with Na₂O) was 24.98 kJ/mol higher than that of CN5@ZnO (pyrrolic biochar coupled with ZnO), suggesting that metal doping promoted CO₂ adsorption, and the promoting effect of Na was superior to that of Zn. This study not only provides theoretical support for the preparation of nitrogen-containing groups in biochar and metal-doped nitrogen-rich biochar but also offers an important reference for achieving low-cost and high-efficiency CO₂ capture technology.