(345i) Describing High Surface Coverage Effects Under Reaction Conditions Using Van Der Waals Functionals: CO Adsorption and Aqueous Environments

Many catalytic reactions happen at high surface coverages where interactions between adsorbates play a key role.  An accurate description of these interactions is therefore important to model the catalytic activity and selectivity. In this presentation we demonstrate that recent vdW-DF functional [1] provide an accurate description for two challenging situations: the stability of CO on Co and Pt surfaces, and the conversion of oxygenates in aqueous environments. Fischer-Tropsch (FT) synthesis, the conversion of synthesis gas, a mixture of CO and H₂, to long-chain hydrocarbons, (CH₂)_n and H₂O, involves a sequence of C-O scission, C-C forming and C and O hydrogenation steps and is performed at high CO partial pressures of about 10 bar and relatively low temperatures of 220 °C. Under these conditions, the Co catalyst is nearly saturated with CO, as also indicated by the zero-order kinetics in CO. Yet, the actual CO saturation coverage is less clear. Two stable CO-surface structures with 1/3 and 7/12 ML coverage have observed in single crystal studies [2,3]. First principle phase diagram calculations with the vdW-DF functional confirm the stability of these two phases and show a true first-order phase transition between the two phases. For comparison, CO adsorption on Pt was also studied, where a similar phase transition is not observed. As we illustrate, the presence of these two phases significantly affects the stability of the CH_x species on the surface and hence the activity and selectivity of FT synthesis. Also under aqueous conditions, adsorbate-adsorbate interactions have an important effect on the activity and selectivity. Van der Waals functionals [4] again give an accurate description of the hydrogen-bonding interactions, as well as the adsorption of methoxy and aldehyde species on Pt. Using vdW-DF calculations, we demonstrate that the presence of water switches the gas-phase selectivity for methanol dehydrogenation because hydrogen-bonding interactions are very different for the C-H and the O-H dehydrogenation transition state. 

References

[1] Dion, M.; Rydberg, H.; Schroder, E.; Langreth, D. C.; Lundqvist, B. I. Phys. Rev. Lett. 2004, 92, 246401

[2] Beitel, G. A.; Laskov, A.; Oosterbeek, H.; Kuipers, E. W. J. Phys. Chem. 1996, 100, 12494

[3] Zhuo, M.; Borgna, A.; Saeys, M.; J. Catal., 2013, 290, 217

[4] Klimes, J.; Bowler, D.R.; Michelides, A. Phys. Rev. B 2011, 83, 195131