(345i) Describing High Surface Coverage Effects Under Reaction Conditions Using Van Der Waals Functionals: CO Adsorption and Aqueous Environments
AIChE Annual Meeting
Tuesday, November 5, 2013 - 5:30pm to 5:45pm
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  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 CHx 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  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.
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