Mechanistic Investigation of the Catalytic Decomposition of Ammonia (NH₃) on an Fe(100) Surface: A DFT Study

Catalytic decomposition of ammonia (NH₃) is a promising chemical reaction in energy and environmental applications. Density functional theory (DFT) calculations were performed to clarify the detailed catalytic mechanism of NH₃ decomposition on an Fe(100) surface. Specifically, the elementary steps of the mechanism were calculated for the general dehydrogenation pathway of NH₃. The adsorption of two types of ammonia dimers (2NH₃), locally adsorbed NH₃ and hydrogen-bonded NH₃, were then compared, revealing that locally adsorbed NH₃ is more stable than hydrogen-bonded NH₃. By contrast, the dehydrogenation of dimeric NH₃ results in a high energy barrier. Moreover, the catalytic characteristics of NH₃ decomposition on a nitrogen (N)-covered Fe surface must be considered because the recombination of nitrogen (N₂) and desorption have an extremely high energy barrier. Our results indicate that the catalytic characteristics of the NH₃ decomposition reaction are altered by N coverage of the Fe surface. This study primarily focused on energetic and electronic analysis. Finally, we conclude that Fe is an alternative catalyst for the decomposition of NH₃ in CO_<i>x</i>-free hydrogen production

Interpenetration of Metal Organic Frameworks for Carbon Dioxide Capture and Hydrogen Purification: Good or Bad?

Using grand canonical Monte Carlo (GCMC) simulations with our recently developed first-principles-based force fields, we report the effects of porosity and interpenetration on the CO₂ uptake in 14 prototypical MOFs (metal organic frameworks). The maximum CO₂ capacity for both total and excess uptakes at high pressures (e.g., 50 bar) correlates well with the pore volume of MOFs and zeolitic imidazolate frameworks, rather than the surface area, which agrees well with the experimental results. The interpenetration between MOFs leads to smaller pore volume (higher density) lowering the maximum CO₂ uptake at high pressures. However, the interpenetrating MOFs produce new CO₂ adsorption sites with high binding affinity (approximately twice that of noninterpenetrating MOFs), such as shared spaces created by two organic linkers of adjacent MOFs, enhancing CO₂ uptake at low pressures (e.g., 2 bar). For H₂ uptake at 298 K, on the other hand, the interpenetration does not provide positive effects. For these reasons, the interpenetration of MOFs remarkably enhances the selectivity of CO₂ over H₂, by more than 3 times that of noninterpenetrating MOFs. These results also show that smaller pores in MOFs are, indeed, advantageous for the CO₂/H₂ separation

ReaxFF Molecular Dynamics Simulations of Water Stability of Interpenetrated Metal–Organic Frameworks

Molecular dynamics (MD) simulations using the reactive force field (ReaxFF) have been performed to elucidate the underlying water-induced disruption mechanism of several prototypical interpenetrated MOFs (IRMOF-9, IRMOF-13, and SUMOF-4). Through the comparison to the corresponding noninterpenetrated MOFs (IRMOF-10 and IRMOF-14), for both the interpenetrated and noninterpenetrated MOFs, structural collapse was always accompanied by the dissociation of the water molecules, with the produced OH^– and H⁺ forming chemical bonds with the Zn²⁺ ion and O atom of the ligand, respectively. However, the water stability of the interpenetrated MOFs is less than that of the corresponding noninterpenetrated structures. The reasons for the differences between the MOFs in the resistance to water attack are clarified. The water resistance of the noninterpenetrated MOFs is mainly attributed to the strength of the Zn–O_ligand, but, the hydrogen bond has little effect. However, a trade-off between the strength of the Zn–O_ligand bond and the hydrogen bond determines the water stability of the interpenetrated MOFs. We expect that our understanding of the water-disruption mechanisms of MOFs will provide helpful guidance for the design of MOFs with a high water-resistance. Additionally, this work shows that ReaxFF simulations could be a useful technique for predicting the hydrothermal stability of MOFs

High H₂ Uptake in Li-, Na-, K-Metalated Covalent Organic Frameworks and Metal Organic Frameworks at 298 K

The Yaghi laboratory has developed porous covalent organic frameworks (COFs), COF102, COF103, and COF202, and metal–organic frameworks (MOFs), MOF177, MOF180, MOF200, MOF205, and MOF210, with ultrahigh porosity and outstanding H₂ storage properties at 77 K. Using grand canonical Monte Carlo (GCMC) simulations with our recently developed first principles based force field (FF) from accurate quantum mechanics (QM), we calculated the molecular hydrogen (H₂) uptake at 298 K for these systems, including the uptake for Li-, Na-, and K-metalated systems. We report the total, delivery and excess amount in gravimetric and volumetric units for all these compounds. For the gravimetric delivery amount from 1 to 100 bar, we find that eleven of these compounds reach the 2010 DOE target of 4.5 wt % at 298 K. The best of these compounds are MOF200-Li (6.34) and MOF200-Na (5.94), both reaching the 2015 DOE target of 5.5 wt % at 298 K. Among the undoped systems, we find that MOF200 gives a delivery amount as high as 3.24 wt % while MOF210 gives 2.90 wt % both from 1 to 100 bar and 298 K. However, none of these compounds reach the volumetric 2010 DOE target of 28 g H₂/L. The best volumetric performance is for COF102-Na (24.9), COF102-Li (23.8), COF103-Na (22.8), and COF103-Li (21.7), all using delivery g H₂/L units for 1–100 bar. These are the highest volumetric molecular hydrogen uptakes for a porous material under these thermodynamic conditions. Thus, one can obtain outstanding H₂ uptakes with Li, Na, and K doping of simple frameworks constructed from simple, cheap organic linkers. We present suggestions for strategies for synthesis of alkali metal-doped MOFs or COFs

Tuning Metal–Organic Frameworks with Open-Metal Sites and Its Origin for Enhancing CO₂ Affinity by Metal Substitution

Reducing anthropogenic carbon emission is a problem that requires immediate attention. Metal–organic frameworks (MOFs) have emerged as a promising new materials platform for carbon capture, of which Mg-MOF-74 offers chemospecific affinity toward CO₂ because of the open Mg sites. Here we tune the binding affinity of CO₂ for M-MOF-74 by metal substitution (M = Mg, Ca, and the first transition metal elements) and show that Ti- and V-MOF-74 can have an enhanced affinity compared to Mg-MOF-74 by 6–9 kJ/mol. Electronic structure calculations suggest that the origin of the major affinity trend is the local electric field effect of the open metal site that stabilizes CO₂, but forward donation from the lone-pair electrons of CO₂ to the empty d-levels of transition metals as in a weak coordination bond makes Ti and V have an even higher binding strength than Mg, Ca, and Sc

Correction and Addition to “Tuning Metal–Organic Frameworks with Open-Metal Sites and Its Origin for Enhancing CO₂ Affinity by Metal Substitution”

Correction and Addition to “Tuning Metal–Organic Frameworks with Open-Metal Sites and Its Origin for Enhancing CO₂ Affinity by Metal Substitution

High-Throughput Screening to Investigate the Relationship between the Selectivity and Working Capacity of Porous Materials for Propylene/Propane Adsorptive Separation

An efficient propylene/propane separation is a very critical process for saving the cost of energy in the petrochemical industry. For separation based on the pressure-swing adsorption process, we have screened ∼1 million crystal structures in the Cambridge Structural Database and Inorganic Crystal Structural Database with descriptors such as the surface area of N₂, accessible surface area of propane, and pore-limiting diameter. Next, grand canonical Monte Carlo simulations have been performed to investigate the selectivities and working capacities of propylene/propane under experimental process conditions. Our simulations reveal that the selectivity and the working capacity have a trade-off relationship. To increase the working capacity of propylene, porous materials with high largest cavity diameters (LCDs) and low propylene binding energies (<i>Q</i>_st) should be considered; conversely, for a high selectivity, porous materials with low LCDs and high propylene <i>Q</i>_st should be considered, which leads to a trade-off between the selectivity and the working capacity. In addition, for the design of novel porous materials with a high selectivity, we propose a porous material that includes elements with a high crossover distance in their Lennard-Jones potentials for propylene/propane such as In, Te, Al, and I, along with the low LCD stipulation

Direct Synthesis of Hydrogen Peroxide from Hydrogen and Oxygen over Mesoporous Silica-Shell-Coated, Palladium-Nanocrystal-Grafted SiO₂ Nanobeads

Many studies have been conducted on core–shell structured nanocatalysts thanks to their high thermal and physical stability. However, for a typical core–shell structure, shell thickness and pore size that affect mass transfer through the shell are difficult to control. Herein, we synthesized a different type of core–shell catalyst, in which a mesoporous silica shell encapsulates the Pd-nanocrystals-grafted-SiO₂ nanobeads. With the preparation method introduced, we successfully controlled the thickness of the shell layer and generated a mesoporous texture over the shell layer. In activity tests, the production rate of hydrogen peroxide significantly increased when using the mesoporous shell catalyst over the microporous shell catalyst of similar shell thickness. The thickening of the mesoporous shell layer reduced the production rate of hydrogen peroxide. Thus, the thinner the thickness of a mesoporous shell, the more favorable in terms of pore-diffusion rate. However, the shell thickness should be adequately adjusted, because an extremely thin shell layer cannot protect the core Pd crystals from thermal agglomeration in a calcination and reduction process

Activity, Selectivity, and Durability of Ruthenium Nanoparticle Catalysts for Ammonia Synthesis by Reactive Molecular Dynamics Simulation: The Size Effect

We report a molecular dynamics (MD) simulation employing the reactive force field (ReaxFF), developed from various first-principles calculations in this study, on ammonia (NH₃) synthesis from nitrogen (N₂) and hydrogen (H₂) gases over Ru nanoparticle (NP) catalysts. Using ReaxFF-MD simulations, we predict not only the activities and selectivities but also the durabilities of the nanocatalysts and discuss the size effect and process conditions (temperature and pressure). Among the NPs (diameter = 3, 4, 5, and 10 nm) considered in this study, the 4 nm NPs show the highest activity, in contrast to our intuition that the smallest NP should provide the highest activity, as it has the highest surface area. In addition, the best selectivity is observed with the 10 nm NPs. The activity and selectivity are mainly determined by the hcp, fcc, and top sites on the Ru NP surface, which depend on the NP size. Moreover, the selectivity can be improved more significantly by increasing the H₂ pressure than by increasing the N₂ pressure. The durability of the NPs can be determined by the mean stress and the stress concentration, and these two factors have a trade-off relationship with the NP size. In other words, as the NP size increases, its mean stress decreases, whereas the stress concentration simultaneously increases. Because of these two effects, the best durability is found with the 5 nm NPs, which is also in contrast to our intuition that larger NPs should show better durability. We expect that ReaxFF-MD simulations, along with first-principles calculations, could be a useful tool in developing novel catalysts and understanding catalytic reactions

Band Gap Engineering of Paradigm MOF‑5

Recently, metal–organic frameworks (MOFs) have demonstrated great potential in photocatalysis and luminosity applications. However, most MOFs are dielectrics with substantial band gaps which limits applications of MOFs in the visible-light region. In this paper, we systematically tune the band gap of paradigm MOF-5 by substituting new atoms for the corner elements (X₄Y), in computer simulations using density functional theory. The new proposed materials are labeled X₄Y–MOF-5 (X = Zn, Cd, Be, Mg, Ca, Sr, Ba; Y = O, S, Se, Te). These new materials have band gaps ranging from 1.7 to 3.6 eV. The underlying mechanism of tunability of band gap can be ascribed to the electronic states of chalcogen atoms (O, S, Se, Te) in the X₄Y nodes and carbon atoms in the BDC linkers. The substantial tunability of band gap leads to a large absorption range covering the visible spectrum. These proposed new materials may be useful for future applications in visible-light promoted photocatalysis or luminosity. The tunability of other properties such as bulk modulus, chemical bonding, and optical properties were also investigated. These novel materials may also be useful for devices in nanoelectronics or optoelectronics