Study on co/competitive adsorption mechanism of CO₂/C₃H₆O on the surface of metal oxide-coupled pyrrole nitrogen biochar

Biomass has a wide range of sources and is porous, and it is a raw material for the preparation of adsorbents with high application value. The adsorption effect of metal oxide-modified biochar on CO₂ and acetone can be significantly improved, but the together/competitive relationship and adsorption mechanism of metal oxide-modified biomass-based adsorbent for simultaneous adsorption of multiple components are not clear. Based on this, the co-adsorption relationship between CO₂ and C₃H₆O on the surface of metal oxide-doped nitrogen-rich biochar was carried out, which is of great significance for the multi-component synergistic adsorption and removal of biomass-based adsorbents. In this study, the adsorption mechanism of CO₂ and C₃H₆O (CO₂&C₃H₆O) on the surface of different metal oxide-coupled pyrrole biochar (CN5@MO_x, MO_x=ZnO, CaO, Na₂O) was explored by comparing the adsorption capacity, adsorption energy, state density and charge differential density analysis. Firstly, the adsorption capacity and adsorption energy of CO₂/C₃H₆O single component were calculated from the CN5@MO_x surface, and the calculation results show that at 333 K and 100 kPa, the adsorption capacity of CO₂/C₃H₆O on the surface of the CN5@Na₂O is 3.65 and 15.34 mmol/g, and the adsorption energy is −145.86 and −132.47 kJ/mol, respectively, which are higher than that of CO₂/C₃H₆O on the surface of CN5@CaO and CN5@ZnO. It was concluded that Na₂O-doped pyrrole-nitrogen biochar had the best adsorption effect on CO₂/C₃H₆O one-component adsorption. The common/competitive adsorption of CO₂&C₃H₆O on the CN5@MO_x surface was further studied. The calculation results show that there are critical temperatures for the adsorption of CO₂&C₃H₆O on the surface of CN5@Na₂O, CN5@CaO and CN5@ZnO (333, 353 and 393 K, respectively), and the adsorption capacity of CO₂&C₃H₆O coexistence system on the CN5@MO_x surface is higher than that of CO₂/C₃H₆O single component after the critical temperature. The adsorption energy of CO₂&C₃H₆O on the surface of CN5@Na₂O, CN5@CaO and CN5@ZnO was at least 141.59, 112.77 and 31.75 kJ/mol higher than that of CO₂ or C₃H₆O single-component adsorption, respectively, and the adsorption of CO₂ and C₃H₆O on the CN5@MO_x surface showed a synergistic promotion effect, and the CN5@Na₂O had the best co-adsorption effect on CO₂&C₃H₆O. Finally, the electron density difference and density of state were used to analyze the mechanism of synergistic adsorption of CO₂&C₃H₆O on the CN5@MO_x surface, and it was concluded that the adsorption force of CO₂ was generated by the indirect interaction between C₃H₆O and CO₂, and the electron cloud of Na and C₃H₆O in Na₂O overlapped, and charge transfer occurred, which enhanced the interaction force between the two. The binding energy of the main formant of C₃H₆O and CN5 in the p orbital on the CN5@Na₂O surface is 3.43 eV lower than that of CN5@ZnO, making the most stable adsorption of C₃H₆O on the CN5@Na₂O surface.