The Catalytic Mechanism of Intercalated Chlorine Anions as Active Basic Sites in Mgal-layered Double Hydroxide for COS Hydrolysis

Abstract

\n In order to make clear the role of intercalated anions in layered double hydroxides (LDHs) for catalytic hydrolysis of COS, the adsorption and reaction characteristics of COS over the simple Mg2Al-Cl-LDH model catalyst were studied by both theoretical and experimental methods. Density functional theory (DFT) calculations by CASTEP found that the chloride ions in LDH function as the key Brønsted-base sites to activate the adsorbed H2O with enlarged bond length and angle, facilitate the dissociative adsorption of intermediates including mono-thiocarbonic acid (MTA) and hydrogen thiocarbonic acid (HTA), and participate in the formation of transient states and subsequent hydrogen transfer process with decreased energy barriers during COS hydrolysis. COS hydrolysis will preferentially go through the dissociated intermediates of mono-thiocarbonates (MT) and hydrogen thiocarbonates (HT) with dramatically decreased energy barriers, and the rate-determining step of COS hydrolysis over Mg2Al-Cl-LDH will be the nucleophilic addition of C=O in COS by H2O (Ea = 1.10 eV). The experimental results further revealed that the apparent activation energy (0.89 eV) of COS hydrolysis over Mg2Al-Cl-LDH is close to theoretical value (1.10 eV), and the accumulated intermediates of MT, HT or carbonate were also observed by FT-IR around 1363 cm-1 on the used Mg2Al-Cl-LDH, which are well in accordance with the theoretical prediction. The demonstrated participation of intercalated chlorine anions in the evolution of intermediates and transient states as Brønsted-base sites during COS hydrolysis will give new insight into the basic sites in LDH materials.