(174d) Computer Simulations of Chemical Reactions and Proton Transport in Porous Materials and Functionalized Surfaces

Atomically-detailed computer simulations can be used to predict properties of materials that have not yet been synthesized, providing a way to screen hypothetical materials, identifying the most promising candidates for subsequent experimental exploration. We present electronic structure density functional theory (DFT) calculations for two classes of materials: the first a catalytic porous material for CO₂ hydrogenation and the second a functionalized graphene material having the ability to conduct protons anhydrously.

We show that catalytic moieties can be integrated into robust Zr-based metal organic frameworks (MOFs) that can be used to reduce CO₂ with H₂. We examine functionalized UiO-66 and UiO-67 and demonstrate that Lewis acid and base sites fixed within the MOFs can be very effective at heterolytically splitting H₂ into protic and hydridic species that can then be added simultaneously to CO₂ to produce formic acid. We investigate the effects of confinement on the reaction energies and barriers. We show that the reaction energies and barriers can be tuned by changing the functional groups and have identified Brønstedâ??Evansâ??Polanyi (BEP) relationships that can be used to construct a Sabatier analysis. The optimum properties of the catalytic moieties can then be identified.

Intermediate temperature proton exchange membrane (PEM) fuel cells have many potential advantages over conventional (low temperature) PEM fuel cells, including faster kinetics, higher quality waste heat, and reduced poisoning of the electrodes. However, existing PEM materials are limited to operating at low temperatures (below about 80°C) because of the requirement that the membranes need to be well-hydrated to function. Thus, there is a need to develop new materials that can conduct protons at high rates under low-water conditions. We have performed DFT calculations to show that hydroxylated graphene is capable of conducting protons under anhydrous conditions. We identify diffusion pathways and barrier heights for proton conduction along 1-dimenstional and 2-dimensional chains of hydroxyl groups on the surface of functionalized graphene. The barriers computed from DFT indicate that proton conduction should be faster than through hydrated Nafion membranes currently used in PEM fuel cells.