Theoretical investigation of cadmium vapor adsorption on kaolinite surfaces with DFT calculations

Abstract As in-furnace sorbent, kaolinite has the poorer performance of cadmium (Cd) capture than that of lead (Pb) capture during combustion, whether for oxide capture or chloride capture. Pb vapor capture by kaolinite has been investigated theoretically before, and in this paper, the density functional theory (DFT) was used again to investigate Cd vapor capture. The emphasis was on the comparison of Pb capture and Cd capture. The calculations show that Cd adsorption and Pb adsorption have the same active sites including the exposed O atoms and unsaturated Al atoms on Al surface, however, the inertia of Cd atom causes the poorer performance of CdCl 2 capture. During the transition from unstable CdCl 2 adsorption to stable CdO adsorption through de-HCl process, the middle products are easy to desorb from Al surface for the weak attraction between Cd atom and O atoms. This situation does not happen during PbCl 2 adsorption for the stronger activity of Pb atom. The bonding between O atom (from oxide) and Al atoms is the main cause of oxide adsorption, so the poorer performance of CdO capture than that of PbO capture has nothing to do with chemical adsorption but diffusion process.