As an initial step towards understanding the mechanism behind photocatalysis in graphitic carbon nitride (g-C3N4), we present a first-principles density functional theory (DFT) study of H2O molecular adsorption on the tri-s-triazine-based structure. The optimization of the system determined that the most stable configuration would be on top of the two-coordinated nitrogen atom in an orientation where one O–H bond is parallel to the surface and the other one is pointing to the surface. The adsorption energy at the most stable configuration was found to be 0.82 eV with a barrier energy of ∼0.02 eV. Partial density of states (PDOS) and charge density distribution analysis show that, primarily, the bonding occurs between the hydrogen atom of the water molecule and the two-coordinated nitrogen atom of g-C3N4. Results of this study would be useful not only to better understand the mechanism behind H2O molecule adsorption but also to give insight into the role of the catalyst in the photocatalytic process.

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The absorption of lithium in montmorillonite [LiSi8(Al3Mg)O20(OH)4] was investigated using Density Functional Theory (DFT). The final position of lithium after absorption was found to be in good agreement with an experimental observation where lithium atom migrated from the interlayer into the vacant octahedral site of montmorillonite. The lithium absorbed on montmorillonite was held together by a very strong attraction between ions and exhibited an insulating behavior as depicted from the density of states curve. Due to the presence of lithium in the octahedral site of montmorillonite, the OH group reoriented itself perpendicular to the ab plane and an electron of lithium was transferred in order to compensate the existing net charge of montmorillonite caused by isomorphous substitutions. Relative small charge transfer was observed between lithium and montmorillonite.

In order to clarify the adsorption mechanism of the O 2 molecule on Co-polypyrrole composite metallo-organic catalyst, we have investigated the interaction between the molecule and Co-(n)pyrrole model clusters (n ¼ 4; 6) using the density functional theory. The stable adsorption site of the O 2 molecule on Co-(4)pyrrole is found to be at the O-O center of mass located on top of the Co atom in side-on configuration, while for the case of Co-(6)pyrrole cluster, the O 2 molecule is slightly deviated from the side-on configuration. The O-O bonds of the O 2 /Co-(4)pyrrole and the O 2 /Co-(6)pyrrole systems have elongated by 10.84 and 9.86%, respectively. The elongation mechanism of O 2 on Co-(n)pyrrole is induced by the interaction between the cobalt d-orbitals and the O 2 anti-bonding à orbital, which results in a charge transfer from the cobalt atom toward the O 2 molecule. This effect seems important in the adsorption of the O 2 molecule on Co-(n)pyrrole. It is likely that the extra charge in the O 2 molecule would fill its anti-bonding orbital and consequently weaken the O-O bond. In Co-(4)pyrrole, the elongation of the O 2 bond is larger than that of Co-(6)pyrrole since a complete side-on configuration has more symmetric overlapping between the cobalt d-orbitals and the O 2 anti-bonding orbital.

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