Thermodynamics of CO₂ adsorption on cellulose‐derived biochar prepared in ionic liquid

Abstract

This work focused on thermodynamic analysis of carbon dioxide (CO2) adsorption on a promising biochar as a CO2 adsorbent. The biochar was prepared by catalytic pyrolysis of cellulose material in ionic liquid at moderate temperature. The adsorption characteristics, such as adsorption capacity, interfacial potential, Gibbs free energy change, enthalpy change, entropy change, and internal energy change, influenced by adsorption temperature and gas pressure, were systematically investigated. The results indicated that CO2 adsorption on cellulose‐derived biochar was a spontaneous, physical, exothermic, and entropic decrement process, accompanied by adsorption capacity of 5.2 mmol/g and interfacial potential of −18.2 J/g at 273 K and 100 kPa. The process could be well described by adsorption potential theory. Then a quasi‐Gaussian distribution of site energy was verified for CO2 adsorption. The interfacial potential was found to be a monotropic function of the amount of CO2 adsorbed, and the latter was actually a differential of the former via adsorption potential. The positive temperature effect and negative pressure effect on negative Gibbs free energy change indicated that reducing adsorption temperature and increasing gas pressure were beneficial to CO2 uptake, accompanied by the increase of adsorption capacity and the reduction of interfacial energy, entropy, enthalpy, and internal energy. The strongest temperature effects on entropy change, enthalpy change, and internal energy change existed at given pressure or temperature. The pressure effect was stronger and more sensitive to pressure at lower adsorption pressure. More interestingly, the peak pressure or peak temperature with the strongest pressure effect possibly existed during CO2 adsorption.

Adsorption thermodynamics of CO2 on nitrogen‐doped biochar synthesized with moderate temperature ionic liquid

“…The interface potential ϕ was defined as a kind of chemical potential difference μ s − μ s0 based on the fundamental differential equations, [ 49,50 ] just like that described in the previous literature. [ 47,51 ] The absolute value of interface potential reflected the interaction strength between CO 2 molecules and the biochar, which was beneficial to understand the site energy distribution. Combined with Equation () and the slip model, Equation () was obtained to calculate the differentials of interface potential via adsorption temperature and pressure, which represented the related temperature effect and pressure effect, respectively.…”

“…The adsorption thermodynamics of CO 2 on the nitrogen‐doped biochar was investigated based on the isotherms at 273, 298, 323, and 348 K. The related analysis methods have been introduced in our previous paper. [ 47 ] Differently, instead of the D‐R equation (see Table 2), the slip model was used to extend the isotherms at more temperatures except at 273, 298, 323, and 348 K. Then, different thermodynamic systems were divided to analyze the mass and energy transfer processes based on the fundamental thermodynamic equations. Those thermodynamic parameters such as Gibbs free energy change were derived under the conditions of constant temperature and isopiestic pressure and obtained by the means of the Gibbs–Helmholtz equation.…”

“…The interface potential ϕ was defined as a kind of chemical potential difference μ s À μ s0 based on the fundamental differential equations, [49,50] just like that described in the previous literature. [47,51]…”