Density functional theory analysis of the adsorption behaviors of H2O and CO2 on the CaCl2(110) surface

The co-adsorption models of H2O and CO2 molecules on the CaCl2(110) surface are established by the first-principles method based on the density functional theory. The adsorption structures, electron distribution, density of states, and reaction pathways are calculated and analyzed to reveal the reaction mechanism of H2O and CO2 molecules on the CaCl2(110) surface. The results indicate that the adsorption site of the Ca-5c atom is more active than that of Ca-6c when H2O is adsorbed on the CaCl2(110) surface. The chemical co-adsorption processes of H2O and CO2 on the CaCl2(110) surface are described via the Langmuir-Hinshelwood mechanism. Their adsorption energies are -0.903 eV and -1.284 eV, which were lower than that of a single H2O molecule. The H2O molecule loses electrons during the adsorption process and the Ca-5c atom bonded acts as the acceptor and transmitter of electrons. The energy required for the dissociation of H2O is significantly reduced in the presence of co-adsorbed CO2. Moreover, the dissociation of H2O is promoted, and then the HCl molecule is formed by the OH- bonded with the H atom on the CaCl2(110) surface, and the reaction pathway requires only 1.03 eV when H2O is bonded with chemisorbed CO2 to form an HCO3- complex. In particular, this adsorption process follows a Langmuir-Rideal mechanism. The findings can lay a theoretical foundation for the chlorine liberation mechanism from CaCl2 and also provides technical support for the pyro-hydrolysis of CaCl2-containing solid wastes.