Xinye Wang

Theoretical investigation of lead vapor adsorption on kaolinite surfaces with DFT calculations.

Kaolinite can be used as the in-furnace sorbent/additive to adsorb lead (Pb) vapor at high temperature. In this paper, the adsorptions of Pb atom, PbO molecule and PbCl2 molecule on kaolinie surfaces were investigated by density functional theory (DFT) calculation. Si surface is inert to Pb vapor adsorption while Al surfaces with dehydroxylation are active for the unsaturated Al atoms and the O atoms losing H atoms. The adsorption energy of PbO is much higher than that of Pb atom and PbCl2. Considering the energy barriers, it is easy for PbO and PbCl2 to adsorb on Al surfaces but difficult to escape. The high energy barriers of de-HCl process cause the difficulties of PbCl2 to form PbO·Al2O3·2SiO2 with kaolinite. Considering the inertia of Si atoms and the activity of Al atoms after dehydroxylation, calcination, acid/alkali treatment and some other treatment aiming at amorphous silica producing and Al activity enhancement can be used as the modification measures to improve the performance of kaolinite as the in-furnace metal capture sorbent.

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

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 CdCl2 capture. During the transition from unstable CdCl2 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 PbCl2 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.