Density functional theory and experimental study on the chemisorption and catalytic decomposition of benzene over exposed bio-char surface: The influence of unsaturated carbon atoms and potassium

Abstract

• Interaction between benzene and char was investigated using DFT method. • Three possible chemisorption and evolution paths were proposed and calculated. • This work firstly proves that active surface area with unsaturated carbon atoms, is crucial for the activation and decomposition of aromatic tar compounds. Catalytic decomposition over char surface is an effective approach for tar removal. In this work, the density functional theory (DFT) and experiments were used to study the complicated mechanism of the interaction between specific sites on char surface and tar compound at molecular level. Two kinds of bio-char models were used and compared. Three molecular reaction paths were built and corresponding species were optimized. The characteristics of electron density of key components and reaction potential energy were calculated and analyzed. Results show that heterogeneous adsorption of benzene on the active sites of char surface could effectively enhance the decomposition of benzene due to the electron shift caused by chemisorption on char surface. The decomposition temperature is largely reduced from 1100℃ to 900℃ or lower. The band gaps indicate that benzene tends to be more likely excited by model char without potassium, which has more dangling bond on char surface. Potential energy results of three reaction paths show that activation energies of about 208.64 kJ/mole and 163.32 kJ/mole are needed for ring breakage of benzene in Path-1 and Path-2, which are much lower than that of thermal decomposition. The energy barrier for benzene decomposition over char-K is 270.04 kJ/mole. Potassium plays a negative effect on the cracking of tar due to the pore polycondensation and occupation of active sites. Kinetic analysis reveals that the char-tar interactions are largely affected by temperature and higher temperature benefits the breakage of aromatic ring. Experimental results indicate that improving the porosity and surface area of char, especially the active surface area with unsaturated carbon atoms, is crucial for the activation and decomposition of aromatic tar compounds.