Density Functional Theory calculations of metal-water interactions and the role of water and surface defects in chemical reactions

Role of water in chemical reactions and interactions of water with metal surface is of fundamental interest in catalysis and related fields. Water interactions are not only important in aqueous solution but for any surface exposed to atmospheric pressure. Low coordination sites, such as step and kinks, have been shown to significantly affect molecular-surface interaction and can affect both the overall reaction rate and the selectively of a chemical reaction. This structure sensitivity of catalytic reactions is of a particular importance in heterogeneous catalysis. Recent progress in nanoparticle synthesis further emphasizes the need for increased fundamental understanding of the potential energy landscape around low coordination sites. The high density of low coordination sites along with the high surface to volume ratios are some of the most important characteristics of nanoparticles that often makes them significantly better catalyst than the single crystalline form of the same metal.

We use DFT do describe water adsorption, diffusion, dissociation and early cluster formation on terrace, steps and kinks on Pt(111). The adsorption energy of a single water molecule increases as it moves from the flat terrace up to the step edge and then atop a kink atom. The highest activation barrier for diffusion from the flat terrace to a defects site is only 0.22 eV making the water molecule mobile at about 100K. The stronger binding on the step and kinks makes the reverse barriers much larger and, therefore, the diffusion from the step or kink back to the flat terrace is only accessible at higher temperature. At lower temperatures wetting of the surface will therefore begin at a defect sites.

Calculations of water-surface interaction is computationally challenging but including water in studies of chemical reactions is very important and can effect both activation barriers and reveal low energy reaction path not accessible in calculations assuming ultra-high vacuum.

Surface defects also play an important role in the dissociation of water molecules. The calculated reaction energy for water dissociation, H2Oads → OHads + Hads, on flat terrace, step and kink shows an interesting trend where the reaction energy on the flat terrace is almost twice the energy at a kink and three times the energy on a step. Water molecule dissociation on flat Pt(111) terrace is therefore less likely than at a defect site. The reaction energy on a step or a kink is about half the reaction energy on the flat terrace making the defect sites significantly more promising for water dissociation.