(190h) Water Molecule Behaviors In TiO2 Nano-Slits with Heterogeneous Modification by Carbon

It is well known that Titanium dioxide (TiO₂) is biocompatible and environmentally friendly. Consequently, TiO₂ is widely applied in many fields, such as implant materials, photocatalysis, pigment, even cosmetic additive, etc. Recently, TiO₂ is used as catalyst support in industrial catalysis, because of its activation on catalyst deposit and chemical reaction., For the concentration of reaction and product near catalysts will break the reaction equilibria, the transport of reactants (absorption of reactant and desorption of product) is also critical to the circle of whole catalysis, besides the reaction part. A kind of mesoporous TiO₂, synthesized by our group [1], is used to be catalyst support for desulfurization from gasoline. The conversion of organic sulfur into hydrogen sulfide was always unsatisfactory until the surface of TiO₂ modified by carbon heterogeneously. Moreover, our theoretical study [2] on mass transport at solid-liquid interface shows that the transport coefficient is the key point in many processes like dissolution and sorption. Since our previous work of simulation approves that water molecule behaviors on carbon and TiO₂ surfaces are widely divergent [3], we preferred to simulating the behaviors of water molecules on heterogeneous carbon- TiO₂ surface.

In this work, we study the static structure properties of water confined to nano-slits, made by slab geometry of rutile (110) covered by graphite (0001). The model contains two slabs of rutile(110), covered by a piece of graphite(0001), opposite to each other with water molecules between them, and their distance is ranging from 0.4 to 2.0 nm by NVT-ensemble (T=300K) molecular dynamics(MD) simulations. In the models with coverage of carbon being 7%, the 1×1 nm² piece of graphite(0001) is in the center of the rutile (110) surface(about 3.6×3.9 nm²), while the one being 93%, 1×1 nm² carbon atoms is dug from the whole one. The 0% and 100% model is no graphite covered and wholly graphite covered, respectively.

We are interested in diffusion of the water molecules in the nano-slits. Results from MD simulations showed that the covering of carbon indeed increased the diffusion of water molecules, especially at the center of the slits, and the one at slits center is increased while the coverage enlarged. However, the diffusion near surfaces did not follow the rules above. Due to our previous work, the molecules of the first layer near the rutile (110) surface are immobile. When carbon covers the TiO₂ surface, the water molecules near surfaces show an interesting result. At the coverage of 7%, the water of first layer near the surface is a little mobile, particularly near the edge of carbon piece. On the other side, at the coverage of 93%, the one near the surface is still immobile, and the diffusion of second layer the water is reduced. The results conclude that the edge of the carbon covering could enhance desorption of water from TiO₂ surfaces. Hence, the small amount coverage and well dispersed of carbon would be the better choice. Such conclusion could also be grafted into many oxides models.

References

[1] M. He, X. H. Lu, X. Feng, L. Yu and Z. H. Yang, A simple approach to mesoporous fibrous titania from potassium dititanate, Chem. Commun., 2004, 19, 2202-2203.

[2] (a)Y. H. Ji, X. H. Lu, Z. H. Yang, and X. Feng, Thermodynamic analysis of the theoretical energy consumption in the removal of organic contaminants by physical methods, Sci. China Chem., 2010, 53 (3), 671-676.(b) Y. H. Ji, X. Y. Ji, C. Liu, X. Feng and X. H. Lu, Modelling of Mass Transfer Coupling with Crystallization Kinetics in Microscale, Chem. Eng. Sci., 2010, 65(9), 2649-2655.

[3] Wei M, Zhou J, Lu X, Zhu Y, Liu W, Lu L, Zhang L. Diffusion of Water Molecules Confined in Slits of Rutile TiO₂(110) and Graphite(0001). Fluid Phase Equilib., 2011, 302, 316–32