Theoretical investigation of sodium capture mechanism on kaolinite surfaces

Kaolinite is a highly effective in-furnace sorbent/additive to inhibit the damage caused by alkali released from fuels. The adsorption and capture mechanisms of different sodium species vapor (Na, NaCl and NaOH) on kaolinite surface were investigated by density functional theory (DFT) calculation. The effects of sodium speciation and kaolinite phase change on the adsorption behavior of sodium on Al- and Si-terminated surfaces were examined. The results show that Na atom is mainly adsorbed on hollow site of two kaolinite surfaces in the chemical adsorption manner. The adsorption energies of NaCl and NaOH on Al-terminated surface are -149.58 and -211.37 kJ/mol, respectively, which are much larger than that of the atomic sodium on hollow site. Compared to Al-terminated surface, the interaction between two sodium species and Si-terminated surface is slightly weak. Kaolinite has a greater ability for sodium adsorption and immobilization during the in-depth dehydroxylation of kaolinite at high temperatures. The mechanism of sodium species captured by kaolinite sorbent was summarized and presented on the basis of XRD analysis and DFT results. The whole sodium capture process can be divided into three stages: physical adsorption stage, internal diffusion stage and chemical adsorption stage. There is no clear boundary among the three divided stages, and the sodium capture process can occur simultaneously and continually until the kaolinite structure is saturated and deactivated.