Taner Yildirim

Titanium-Decorated Carbon Nanotubes as a Potential High-Capacity Hydrogen Storage Medium

We report a first-principles study, which demonstrates that a single Ti atom coated on a single-walled nanotube (SWNT) binds up to four hydrogen molecules. The first H2 adsorption is dissociative with no energy barrier while the other three adsorptions are molecular with significantly elongated H-H bonds. At high Ti coverage we show that a SWNT can strongly adsorb up to 8 wt % hydrogen. These results advance our fundamental understanding of dissociative adsorption of hydrogen in nanostructures and suggest new routes to better storage and catalyst materials.

Unusual and Highly Tunable Missing-Linker Defects in Zirconium Metal-Organic Framework UiO-66 and Their Important Effects on Gas Adsorption

UiO-66 is a highly important prototypical zirconium metal-organic framework (MOF) compound because of its excellent stabilities not typically found in common porous MOFs. In its perfect crystal structure, each Zr metal center is fully coordinated by 12 organic linkers to form a highly connected framework. Using high-resolution neutron power diffraction technique, we found the first direct structural evidence showing that real UiO-66 material contains significant amount of missing-linker defects, an unusual phenomenon for MOFs. The concentration of the missing-linker defects is surprisingly high, ∼10% in our sample, effectively reducing the framework connection from 12 to ∼11. We show that by varying the concentration of the acetic acid modulator and the synthesis time, the linker vacancies can be tuned systematically, leading to dramatically enhanced porosity. We obtained samples with pore volumes ranging from 0.44 to 1.0 cm(3)/g and Brunauer-Emmett-Teller surface areas ranging from 1000 to 1600 m(2)/g, the largest values of which are ∼150% and ∼60% higher than the theoretical values of defect-free UiO-66 crystal, respectively. The linker vacancies also have profound effects on the gas adsorption behaviors of UiO-66, in particular CO2. Finally, comparing the gas adsorption of hydroxylated and dehydroxylated UiO-66, we found that the former performs systematically better than the latter (particularly for CO2) suggesting the beneficial effect of the -OH groups. This finding is of great importance because hydroxylated UiO-66 is the practically more relevant, non-air-sensitive form of this MOF. The preferred gas adsorption on the metal center was confirmed by neutron diffraction measurements, and the gas binding strength enhancement by the -OH group was further supported by our first-principles calculations.

Methane Storage in Metal–Organic Frameworks: Current Records, Surprise Findings, and Challenges

We have examined the methane uptake properties of six of the most promising metal organic framework (MOF) materials: PCN-14, UTSA-20, HKUST-1, Ni-MOF-74 (Ni-CPO-27), NU-111, and NU-125. We discovered that HKUST-1, a material that is commercially available in gram scale, exhibits a room-temperature volumetric methane uptake that exceeds any value reported to date. The total uptake is about 230 cc(STP)/cc at 35 bar and 270 cc(STP)/cc at 65 bar, which meets the new volumetric target recently set by the Department of Energy (DOE) if the packing efficiency loss is ignored. We emphasize that MOFs with high surface areas and pore volumes perform better overall. NU-111, for example, reaches ~75% of both the gravimetric and the volumetric targets. We find that values for gravimetric uptake, pore volume, and inverse density of the MOFs we studied scale essentially linearly with surface area. From this linear dependence, we estimate that a MOF with surface area 7500 m(2)/g and pore volume 3.2 cc/g could reach the current DOE gravimetric target of 0.5 g/g while simultaneously exhibiting around ~200 cc/cc volumetric uptake. We note that while values for volumetric uptake are based on ideal single crystal densities, in reality the packing densities of MOFs are much lower. Finally, we show that compacting HKUST-1 into wafer shapes partially collapses the framework, decreasing both volumetric and gravimetric uptake significantly. Hence, one of the important challenges going forward is to find ways to pack MOFs efficiently without serious damage or to synthesize MOFs that can withstand substantial mechanical pressure.

High-Capacity Methane Storage in Metal−Organic Frameworks M2(dhtp): The Important Role of Open Metal Sites

We found that metal-organic framework (MOF) compounds M(2)(dhtp) (open metal M = Mg, Mn, Co, Ni, Zn; dhtp = 2,5-dihydroxyterephthalate) possess exceptionally large densities of open metal sites. By adsorbing one CH(4) molecule per open metal, these sites alone can generate very large methane storage capacities, 160-174 cm(3)(STP)/cm(3), approaching the DOE target of 180 cm(3)(STP)/cm(3) for material-based methane storage at room temperature. Our adsorption isotherm measurements at 298 K and 35 bar for the five M(2)(dhtp) compounds yield excess methane adsorption capacities ranging from 149 to 190 cm(3)(STP)/cm(3) (derived using their crystal densities), indeed roughly equal to the predicted, maximal adsorption capacities of the open metals (within +/-10%) in these MOFs. Among the five isostructural MOFs studied, Ni(2)(dhtp) exhibits the highest methane storage capacity, approximately 200 cm(3)(STP)/cm(3) in terms of absolute adsorption, potentially surpassing the DOE target by approximately 10%. Our neutron diffraction experiments clearly reveal that the primary CH(4) adsorption occurs directly on the open metal sites. Initial first-principles calculations show that the binding energies of CH(4) on the open metal sites are significantly higher than those on typical adsorption sites in classical MOFs, consistent with the measured large heats of methane adsorption for these materials. We attribute the enhancement of the binding strength to the unscreened electrostatic interaction between CH(4) and the coordinatively unsaturated metal ions.

Enhanced H2 adsorption in isostructural metal-organic frameworks with open metal sites: strong dependence of the binding strength on metal ions.

Metal-organic frameworks (MOFs) with open metal sites exhibit a much stronger H2 binding strength than classical MOFs, due to the direct interaction between H2 and the coordinately unsaturated metal ions. Here we report a systematic study of the H2 adsorption on a series of isostructural MOFs, M2(dhtp) (M = Mg, Mn, Co, Ni, Zn). The experimental, initial isosteric heats of adsorption for H2 (Qst) of these MOFs range from 8.5 to 12.9 kJ/mol, with increasing Qst in the following order: Zn, Mn, Mg, Co, and Ni. The H2 binding energies derived from first-principles calculation follow the same trend as the experimental observation on Qst, confirming the electrostatic Coulomb attraction between the H2 and the open metals being the major interaction. We also found a strong correlation between the metal ion radius, the M-H2 distance, and the H2 binding strength, which provides a viable, empirical method to predict the relative H2 binding strength of different open metals.

Graphene Oxide Framework Materials: Theoretical Predictions and Experimental Results†

A series of idealized model systemswith various diboronic acid linker concentrations (and con-sequently different pore size, pore volume, and surface areas)were examined (see Supporting Information for details).Structural optimization yielded a circa 1.1 nm interlayerseparation for these ideal structures. The simulated absolute

Carbon capture in metal–organic frameworks—a comparative study

Metal–organic frameworks (MOFs) have been shown to be excellent materials for storage of carbon dioxide, implying that they could be useful for removal of carbon dioxide from flue gas stacks, however their performance in industrially relevant swing adsorption processes for carbon capture has not been studied. Here we show that the efficacy of MOFs for carbon capture depends dramatically on the process and that some MOFs can provide significant carbon capture under typical pressure and vacuum swing processes. In particular, MOFs that possess coordinatively unsaturated metal centers offer as much as 9 mmol g−1 swing capacity under certain conditions. The results herein clearly show that there is no single ideal compound for carbon capture applications and that different materials can perform better or worse depending on the specific process conditions. In addition to their capture performances, we have also investigated their selectivity to carbon dioxide over that of nitrogen and methane. The analysis provided clearly demonstrates that the performance of a given MOF cannot be determined without also considering the detailed industrial process in which the MOF is to be applied.

Alkali and alkaline-earth metal amidoboranes: structure, crystal chemistry, and hydrogen storage properties.

Alkali- and alkaline-earth metal amidoboranes are a new class of compounds with rarely observed [NH2BH3](-) units. LiNH2BH3 and solvent-containing Ca(NH2BH3)2 x THF have been recently reported to significantly improve the dehydrogenation properties of ammonia borane. Therefore, metal amidoboranes, with accelerated desorption kinetics and suppressed toxic borazine, are of great interest for their potential applications for hydrogen storage. In this work, we successfully determined the structures of LiNH2BH3 and Ca(NH2BH3)2 using a combined X-ray diffraction and first-principles molecular dynamics simulated annealing method. Through detailed structural analysis and first-principles electronic structure calculations the improved dehydrogenation properties are attributed to the different bonding nature and reactivity of the metal amidoboranes compared to NH3BH3.

Giant anharmonicity and nonlinear electron-phonon coupling in MgB2: a combined first-principles calculation and neutron scattering study.

First-principles calculations of the electronic band structure and lattice dynamics for the new superconductor MgB (2) are carried out and found to be in excellent agreement with our inelastic neutron scattering measurements. The numerical results reveal that the E(2g) in-plane boron phonons near the zone center are very anharmonic and strongly coupled to the planar B sigma bands near the Fermi level. This giant anharmonicity and nonlinear electron-phonon coupling is key to quantitatively explaining the observed high T(c) and boron isotope effect in MgB (2).

ADSORPTION AND DISSOCIATION OF HYDROGEN MOLECULES ON BARE AND FUNCTIONALIZED CARBON NANOTUBES

Interaction between hydrogen molecules and bare as well as functionalized single-wall carbon nanotubes SWNT is investigated using first-principles plane wave method. It is found that the binding energy of the H2 physisorbed on the outer surface of the bare SWNT is very weak, and cannot be enhanced significantly either by increasing the curvature of the surface through radial deformation, or by the coadsorption of a Li atom that makes the semiconducting tube metallic. Although the bonding is strengthened upon adsorption directly to the Li atom, its nature continues to be physisorption. However, the character of the bonding changes dramatically when SWNT is functionalized by the adsorption of a Pt atom. A single H2 is chemisorbed to the Pt atom on the SWNT either dissociatively or molecularly. The dissociative adsorption is favorable energetically and is followed by the weakening of the Pt-SWNT bond. Out of two adsorbed H2, the first one can be adsorbed dissociatively and the second one is chemisorbed molecularly. The nature of bonding is a very weak physisorption for the third adsorbed H2. Palladium also promotes the chemisorption of H2 with relatively smaller binding energy. Present results reveal the important effect of transition metal atom adsorbed on SWNT and these results advance our understanding of the molecular and dissociative adsorption of hydrogen for efficient hydrogen storage.