CONTENTS 1. Introduction 703 2. Simulation Methods 704 2.1. The Model 704 2.2. Force Field Potentials and Parameters 704 2.3. Simulating Gas Adsorption with Grand Canonical Monte Carlo 705 2.4. Quantum Chemical Calculations 706 3. Gas Storage in MOFs without Strongly Binding Sites 706 3.1. Optimal Design Characteristics Revealed by Simulation 707 3.1.1. CH 4 Storage 707 3.1.2. H 2 Storage 708 3.2. Comparison of Simulated Adsorption Isotherms with Experiment for MOFs and COFs without Strongly Binding Sites 709 3.2.1. CH 4 Storage 709 3.2.2. H 2 Storage 710 3.3. Acetylene Storage 711 4. Adsorption in MOFs with Open Metal Sites in the Nodes 712 4.1. H 2 Interactions with Open Metal Sites 712 4.2. Methane Adsorption in M-MOF-74 712 4.3.
The reactivity of heterogeneous metal catalysts can be a strong function of the coverage of adsorbates. For example, Pt-catalyzed NO oxidation to NO 2 requires high concentrations of chemisorbed (surface-bound) O, but the development of surface oxides is detrimental to reaction kinetics. Quantifying the structures, properties, and especially the conditions that produce various adsorbate coverages is essential to developing qualitatively and quantitatively correct models of surface reactivity. In this work, we examine these ideas in the context of oxidation reactions on Pt(111), the lowest energy face of bulk Pt. We use extensive supercell density functional theory (DFT) calculations to catalog and characterize the stable binding sites and arrangements of chemisorbed O on Pt(111), as a function of O coverage, θ. O atoms are found to uniformly prefer FCC binding sites and to arrange to minimize various destabilizing interactions with neighbor O. These destabilizing interactions are shown to have electronic and strain components that can either reinforce or oppose one another depending upon O-O separation. Because of the nature and magnitudes of these lateral interactions, the thermodynamically stable O orderings partition into four coverage regimes of decreasing adsorption energy: 0 < θ e 1/4 monolayer (ML), 1/4 < θ e 1/2 ML, 1/2 < θ e 2/3 ML, and 2/3 < θ e 1 ML. We use equilibrium models to quantify the oxygen chemical potentials µ O necessary to access each of these regimes. These equilibrium models can be used to relate surface coverage to various external environmental conditions and assumptions about relevant reaction equilibria: dissociative equilibrium of the surface with O 2 (g) can produce coverages up to 1/2 ML; either NO 2 decomposition or "NO-assisted" O 2 dissociation can access coverages approaching 2/3 ML, as observed during NO oxidation catalysis, and equilibrium with a solidoxygen storage material, like ceria-zirconia, can buffer equilibrium coverages at a constant 1/4 ML O. These various oxidation reaction energies can be summarized in a single "Ellingham" free energy diagram, providing a convenient representation of the relationship between surface coverage and reaction thermodynamics, and a useful guide toward relevant coverage regimes for more detailed study of reaction kinetics. † This year marks the Centennial of the American Chemical Society's Division of Physical Chemistry. To celebrate and to highlight the field of physical chemistry from both historical and future perspectives, The Journal of Physical Chemistry is publishing a special series of Centennial Feature Articles. These articles are invited contributions from current and former officers and members of the Physical Chemistry Division Executive Committee and from J. Phys. Chem.
Metal-organic frameworks (MOFs) are permanently porous solids, which are promising hydrogen storage materials. However, the maximum H 2 adsorption energies in MOFs are only around 10 kJ 3 mol -1 , leading to small adsorption capacities at ambient temperature. In this work we use ab initio calculations and grand canonical Monte Carlo (GCMC) simulations to explore metal alkoxide functionalization for improving H 2 storage in IRMOF-1, IRMOF-10, IRMOF-16, UiO-68, and UMCM-150. We examine functionalization with lithium, magnesium, manganese, nickel, and copper alkoxides. We show that lithium and magnesium alkoxides physically bind H 2 and manganese, nickel, and copper alkoxides chemically bind H 2 . H 2 binding energies calculated with quantum mechanics are -10, -22, -20, -78, and -84 kJ 3 mol -1 , respectively, for the first hydrogen molecule. Of these, lithium and manganese alkoxides bind H 2 too weakly to enhance adsorption at ambient temperature, even at 100 bar. Owing to the strong binding energies, Ni and Cu exhibit high uptake at low pressure, but metal alkoxide sites saturate at pressures as low as 1 bar. They thus exhibit poor deliverable capacities [wt % (100 bar)wt % (2 bar)]. Magnesium alkoxide exhibits low uptake at low pressure and high uptake at high pressure and is a promising functional group for enhanced ambient-temperature hydrogen storage in all MOFs studied.
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