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1.
J Phys Chem Lett ; 11(23): 10029-10036, 2020 Dec 03.
Artigo em Inglês | MEDLINE | ID: mdl-33179928

RESUMO

It has been well-established that unfavorable scaling relationships between *OOH, *OH, and *O are responsible for the high overpotentials associated with oxygen electrochemistry. A number of strategies have been proposed for breaking these linear constraints for traditional electrocatalysts (e.g., metals, alloys, metal-doped carbons); such approaches have not yet been validated experimentally for heterogeneous catalysts. Development of a new class of catalysts capable of circumventing such scaling relations remains an ongoing challenge in the field. In this work, we use density functional theory (DFT) calculations to demonstrate that bimetallic porphyrin-based MOFs (PMOFs) are an ideal materials platform for rationally designing the 3-D active site environments for oxygen reduction reaction (ORR). Specifically, we show that the *OOH binding energy and the theoretical limiting potential can be optimized by appropriately tuning the transition metal active site, the oxophilic spectator, and the MOF topology. Our calculations predict theoretical limiting potentials as high as 1.07 V for Fe/Cr-PMOF-Al, which exceeds the Pt/C benchmark for 4e ORR. More broadly, by highlighting their unique characteristics, this work aims to establish bimetallic porphyrin-based MOFs as a viable materials platform for future experimental and theoretical ORR studies.

2.
ACS Appl Mater Interfaces ; 12(35): 39074-39081, 2020 Sep 02.
Artigo em Inglês | MEDLINE | ID: mdl-32805928

RESUMO

Catalytic systems whose properties can be systematically tuned via changes in synthesis conditions are highly desirable for the next-generation catalyst design and optimization. Herein, we present a two-dimensional (2D) conductive metal-organic framework consisting of M-N4 units (M = Ni, Cu) and a hexaaminobenzene (HAB) linker as a catalyst for the oxygen reduction reaction. By varying synthetic conditions, we prepared two Ni-HAB catalysts with different crystallinities, resulting in catalytic systems with different electric conductivities, electrochemical activity, and stability. We show that crystallinity has a positive impact on conductivity and demonstrate that this improved crystallinity/conductivity improves the catalytic performance of our model system. Additionally, density functional theory simulations were performed to probe the origin of M-HAB's catalytic activity, and they suggest that M-HAB's organic linker acts as the active site with the role of the metal being to modulate the linker sites' binding strength.

3.
Inorg Chem ; 57(12): 7222-7238, 2018 Jun 18.
Artigo em Inglês | MEDLINE | ID: mdl-29863849

RESUMO

We investigate the (surface) bonding of a class of industrially and biologically important molecules in which the chemically active orbital is a 2 p electron lone pair located on an N or O atom bound via single bonds to H or alkyl groups. This class includes water, ammonia, alcohols, ethers, and amines. Using extensive density functional theory (DFT) calculations, we discover scaling relations (correlations) among molecular binding energies of different members of this class: the bonding energetics of a single member can be used as a descriptor for other members. We investigate the bonding mechanism for a representative (H2O) and find the most important physical surface properties that dictate the strength and nature of the bonding through a combination of covalent and noncovalent electrostatic effects. We describe the importance of surface intrinsic electrostatic, geometric, and mechanical properties in determining the extent of the lone-pair-surface interactions. We study systems including ionic materials in which the surface positive and negative centers create strong local surface electric fields, which polarize the dangling lone pair and lead to a strong "electrostatically driven bond". We emphasize the importance of noncovalent electrostatic effects and discuss why a fully covalent picture, common in the current first-principles literature on surface bonding of these molecules, is not adequate to correctly describe the bonding mechanism and energy trends. By pointing out a completely different mechanism (charge transfer) as the major factor for binding N- and O-containing unsaturated (radical) adsorbates, we explain why their binding energies can be tuned independently from those of the aforementioned species, having potential implications in scaling-driven catalyst discovery.

4.
Chem Rev ; 118(5): 2302-2312, 2018 03 14.
Artigo em Inglês | MEDLINE | ID: mdl-29405702

RESUMO

Despite the dedicated search for novel catalysts for fuel cell applications, the intrinsic oxygen reduction reaction (ORR) activity of materials has not improved significantly over the past decade. Here, we review the role of theory in understanding the ORR mechanism and highlight the descriptor-based approaches that have been used to identify catalysts with increased activity. Specifically, by showing that the performance of the commonly studied materials (e.g., metals, alloys, carbons, etc.) is limited by unfavorable scaling relationships (for binding energies of reaction intermediates), we present a number of alternative strategies that may lead to the design and discovery of more promising materials for ORR.

5.
Phys Chem Chem Phys ; 19(5): 3575-3581, 2017 Feb 01.
Artigo em Inglês | MEDLINE | ID: mdl-28094377

RESUMO

While natural gas is an abundant chemical fuel, its low volumetric energy density has prompted a search for catalysts able to transform methane into more useful chemicals. This search has often been aided through the use of transition state (TS) scaling relationships, which estimate methane activation TS energies as a linear function of a more easily calculated descriptor, such as final state energy, thus avoiding tedious TS energy calculations. It has been shown that methane can be activated via a radical or surface-stabilized pathway, both of which possess a unique TS scaling relationship. Herein, we present a simple model to aid in the prediction of methane activation barriers on heterogeneous catalysts. Analogous to the universal radical TS scaling relationship introduced in a previous publication, we show that a universal TS scaling relationship that transcends catalysts classes also seems to exist for surface-stabilized methane activation if the relevant final state energy is used. We demonstrate that this scaling relationship holds for several reducible and irreducible oxides, promoted metals, and sulfides. By combining the universal scaling relationships for both radical and surface-stabilized methane activation pathways, we show that catalyst reactivity must be considered in addition to catalyst geometry to obtain an accurate estimation for the TS energy. This model can yield fast and accurate predictions of methane activation barriers on a wide range of catalysts, thus accelerating the discovery of more active catalysts for methane conversion.

6.
Nat Mater ; 16(2): 225-229, 2017 02.
Artigo em Inglês | MEDLINE | ID: mdl-27723737

RESUMO

While the search for catalysts capable of directly converting methane to higher value commodity chemicals and liquid fuels has been active for over a century, a viable industrial process for selective methane activation has yet to be developed. Electronic structure calculations are playing an increasingly relevant role in this search, but large-scale materials screening efforts are hindered by computationally expensive transition state barrier calculations. The purpose of the present letter is twofold. First, we show that, for the wide range of catalysts that proceed via a radical intermediate, a unifying framework for predicting C-H activation barriers using a single universal descriptor can be established. Second, we combine this scaling approach with a thermodynamic analysis of active site formation to provide a map of methane activation rates. Our model successfully rationalizes the available empirical data and lays the foundation for future catalyst design strategies that transcend different catalyst classes.

7.
J Phys Chem Lett ; 6(9): 1586-91, 2015 May 07.
Artigo em Inglês | MEDLINE | ID: mdl-26263318

RESUMO

The development of metal organic frameworks (MOFs) with high porosity, large surface area, and good electrical properties would offer opportunities for producing functionalized porous materials suitable for energy storage, conversion, and utilization. Realizing these applications remains challenging because of the limited numbers of electrically conductive porous MOFs that are known. We apply density functional theory (DFT) to assess a large number of potentially electrically conductive MOFs generated by infiltrating known materials with conjugated and redox-active 7,7,8,8-tetracyanquinododimethane (TCNQ) molecules. DFT results demonstrate that TCNQ coordinating with dimeric Cu paddlewheels can create molecular chains in a variety of MOFs. Several of these materials feature the formation of multiple dimensional conducting chains, making the materials promising for electrical conductivity.

8.
Langmuir ; 31(30): 8453-68, 2015 Aug 04.
Artigo em Inglês | MEDLINE | ID: mdl-26158777

RESUMO

Generic force fields such as UFF and DREIDING are widely used for predicting molecular adsorption and diffusion in metal-organic frameworks (MOFs), but the accuracy of these force fields is unclear. We describe a general framework for developing transferable force fields for modeling the adsorption of alkanes in a nonflexible MIL-47(V) MOF using periodic density functional theory (DFT) calculations. By calculating the interaction energies for a large number of energetically favorable adsorbate configurations using DFT, we obtain a force field that gives good predictions of adsorption isotherms, heats of adsorption, and diffusion properties for a wide range of alkanes and alkenes in MIL-47(V). The force field is shown to be transferable to related materials such as MIL-53(Cr) and is used to calculate the free-energy differences for the experimentally observed phases of MIL-53(Fe).

9.
ChemSusChem ; 5(10): 2058-64, 2012 Oct.
Artigo em Inglês | MEDLINE | ID: mdl-22764080

RESUMO

A fundamental study on the adsorption properties of primary, secondary, and tertiary amine materials is used to evaluate what amine type(s) are best suited for ultradilute CO(2) capture applications. A series of comparable materials comprised of primary, secondary, or tertiary amines ligated to a mesoporous silica support via a propyl linker are used to systematically assess the role of amine type. Both CO(2) and water adsorption isotherms are presented for these materials in the range relevant to CO(2) capture from ambient air and it is demonstrated that primary amines are the best candidates for CO(2) capture from air. Primary amines possess both the highest amine efficiency for CO(2) adsorption as well as enhanced water affinity compared to other amine types or the bare silica support. The results suggest that the rational design of amine adsorbents for the extraction of CO(2) from ambient air should focus on adsorbents rich in primary amines.


Assuntos
Ar , Aminas/química , Dióxido de Carbono/química , Adsorção , Dióxido de Carbono/isolamento & purificação , Silanos/química , Água/química
10.
Phys Rev Lett ; 97(10): 105502, 2006 Sep 08.
Artigo em Inglês | MEDLINE | ID: mdl-17025826

RESUMO

We predict a previously unknown phase transformation from wurtzite to a graphitelike (P6(3)/mmc) hexagonal structure in [0110]-oriented ZnO nanowires under uniaxial tensile loading. Molecular dynamics simulations and first principles calculations show that this structure corresponds to a distinct minimum on the enthalpy surfaces of ZnO for such loading conditions. This transformation is reversible with a low level of hysteretic dissipation of 0.16 J/m3 and, along with elastic stretching, endows the nanowires with the ability to recover pseudoelastic strains up to 15%.

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