DFT study on the chemisorption and reforming of naphthalene over bio-char: The detailed mechanism of carbon deposition and hydrogen production

Abstract

Tar removal over char bed is a potential method for syngas cleaning. The heterogeneous interaction mechanism between char surface and tar molecule is crucial and still not fully revealed. In this work, the detailed interaction between bio-char surface and naphthalene was studied by experiment and density functional theory (DFT) at molecular level. Three bio-char models and corresponding reaction paths were proposed. The geometric optimization and frequency calculation of each species were conducted. Mulliken charges and electrostatic potential results of char models indicate that oxygenated functional groups could weaken the electron cloud density of adjacent carbon atoms and make the carbon atoms positively charged, which appears as a “Lewis acid center” and benefit the adsorption of tar molecule. The orbital electron results show that the band gaps of three char models are 1.2965 eV, 2.7903 eV and 3.4366 eV, respectively, indicating the unpaired electrons on char surface will enhance the reactivity of char compared with oxygenated functional groups. The potential energy evolution shows that the initial adsorption of naphthalene on char surface is an exothermic reaction. The activation energies for naphthalene decomposition over char surface are 207.83 kJ/mole, 164.01 kJ/mole, and 159.17 kJ/mole, which are much lower than that of thermal decomposition (446 kJ/mole). The chemisorption of naphthalene on char surface changes the electron density distribution, which greatly enhance tar decomposition. Experimental results proved that the oxidative pyrolysis char is rich in micropores and oxygenated functional groups, which largely enhanced the naphthalene reduction efficiency over bio-char surface.