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The adsorption of H2in ZIF-76, a zeolitic imidazolate framework (ZIF) with lta topology, was investigated in a combined experimental and theoretical study. Each Zn2+ion in the structure of this ZIF is coordinated to imidazolate and 5-chlorobenzimidazolate linkers in a 3:1 ratio. The X-ray crystal structure of ZIF-76 contains a large amount of structural disorder, which makes this a challenging material for modeling. We therefore chose to parametrize and simulate H2adsorption in two distinct crystal structure configurations of ZIF-76 that differ by only the relative positions of one imidazolate and one 5-chlorobenzimidazolate linker. The simulated H2adsorption isotherms for both structures are in satisfactory agreement with the newly reported experimental data for the ZIF, especially at low pressures. The experimental initial isosteric heat of adsorption (Qst) value for H2in ZIF-76 was determined to be 7.7 kJ mol–1, which is comparable to that for other ZIFs and is fairly high for a material that does not contain open-metal sites. Simulations of H2adsorption in one of these structures resulted in Qstvalues that are in very good agreement with experiment within the loading range considered. Two notable H2binding sites were discovered from simulations in both structures of ZIF-76; however, the preferential regions of H2occupancy are reversed for the two structures. The inelastic neutron scattering (INS) spectra for H2adsorbed in ZIF-76 contain several peaks that arise from transitions of the hindered H2rotor, with the lowest energy peak occurring in the range of 6.0–7.2 meV. Two-dimensional quantum rotation calculations for H2adsorbed at the considered sites in both structures yielded rotational transitions that are in good agreement with the peaks that appear in the INS spectra. Despite the large degree of disorder in the ZIF-76 crystal structure, the overall environment in the ZIF still gives rise to interconnected INS features as discerned from our calculations. This study demonstrates how important details of the H2adsorption mechanism in a ZIF with structural disorder can be obtained from a combination of experimental measurements and theoretical calculations.

A combined experimental and theoretical study of H2sorption was carried out on two isostructural zeolitic imidazolate frameworks (ZIFs), namely ZIF-68 and ZIF-69. The former consists of Zn2+ions that are coordinated to two 2-nitroimidazolate and two benzimidazolate linkers in a tetrahedral fashion, while 5-chlorobenzimidazolate is used in place of benzimidazolate in the latter compound. H2sorption measurements showed that the two ZIFs display similar isotherms and isosteric heats of adsorption (Qst). The experimental initial H2Qstvalue for both ZIFs was determined to be 8.1 kJ mol–1, which is quite high for materials that do not contain exposed metal centers. Molecular simulations of H2sorption in ZIF-68 and ZIF-69 confirmed the similar H2sorption properties between the two ZIFs, but also suggest that H2sorption is slightly favored in ZIF-68 with regards to uptake at 77 K/1.0 atm. This work also presents inelastic neutron scattering (INS) spectra for H2sorbed in ZIFs for the first time. The spectra for ZIF-68 and ZIF-69 show a broad range of intensities starting from about 4 meV. The most favorable H2sorption site in both ZIFs corresponds to a confined region between two adjacent 2-nitroimidazolate linkers. Two-dimensional quantum rotation calculations for H2sorbed at this site in ZIF-68 and ZIF-69 produced rotational transitions that are in accord with the lowest energy peak observed in the INS spectrum for the respective ZIFs. We found that the primary binding site for H2in the two ZIFs generates high barriers to rotation for the adsorbed H2, which are greater than those in several metal–organic frameworks (MOFs) which possess open-metal sites. H2sorption was also observed for both ZIFs in the vicinity of the nitro groups of the 2-nitroimidazolate linkers. This study highlights the constructive interplay of experiment and theory to elucidate critical details of the H2sorption mechanism in these two isostructural ZIFs.

Amorphous carbon thin films are technologically important materials that range in use from the semiconductor industry to corrosion-resistant films. Their conversion to crystalline graphene layers has long been pursued; however, typically this requires excessively high temperatures. Thus, crystallization routes which require reduced temperatures are important. Moreover, the ability to crystallize amorphous carbon at reduced temperatures without a catalyst could pave the way for practical graphene synthesis for device fabrication without the need for transfer or post-transfer gate deposition. To this end we demonstrate a practical and facile method to crystallize deposited amorphous carbon films to high quality graphene layers at reduced annealing temperatures by introducing oxidizing gases during the process. The reactive gases react with regions of higher strain (energy) in the system and accelerate the graphitization process by minimizing criss-cross-linkages and accelerating C–C bond rearrangement at defects. In other words, the movement of crystallite boundaries is accelerated along the carbon hexagon planes by removing obstacles for crystallite coalescence.

One of the more common routes to fabricate graphene is by chemical vapor deposition (CVD). This is primarily because of its potential to scale up the process and produce large area graphene. For the synthesis of large area monolayer Cu is probably the most popular substrate since it has a low carbon solubility enabling homogeneous single-layer sheets of graphene to form. This process requires a very clean substrate. In this work we look at the efficiency of common pretreatments such as etching or wiping with solvents and compare them to an oxidation treatment at 1025 °C followed by a reducing process by annealing in H2. The oxidation/reduction process is shown to be far more efficient allowing large area homogeneous single layer graphene formation without the presence of additional graphene flakes which form from organic contamination on the Cu surface.

Electronic structure calculations and simulations of H2sorption were performed in four members of the M-MOF-74 series: Mg-MOF-74, Ni-MOF-74, Co-MOF-74, and Zn-MOF-74. Notable differences were observed in the partial charge and polarizability of the metal ions derived from the electronic structure calculations. The modeling parameters obtained from the electronic structure calculations were found to influence certain features in the experimentally observed H2sorption trends in the M-MOF-74 series. The simulations were performed with the inclusion of explicit many-body polarization, which was required to reproduce the experimental H2sorption observables (i.e., sorption isotherms and isosteric heats of adsorption (Qst)) and the H2–metal interaction in all four MOFs using classical molecular simulation. Consistent with experimental measurements, the simulations captured the following trend for the H2–metal interaction strength: Ni-MOF-74 > Co-MOF-74 > Mg-MOF-74 > Zn-MOF-74. The calculations revealed that stronger H2–metal interactions within the M-MOF-74 series corresponded to shorter H2–metal distances and higher induced dipoles on the metal-sorbed H2molecules. In addition, it was observed that there was a strong correlation between the H2–metal interaction and the polarization contribution. Although Mg-MOF-74 has the highest calculated partial charge for the metal ion within the series, the Mg2+ion has a very low polarizability compared to the other M2+ions; this explains why the H2–metal interaction in this MOF is weaker compared to those for Ni-MOF-74 and Co-MOF-74. The sterics interactions, reflected in the crystal structure for all four MOFs, also played a role for the observed H2sorption trends. Zn-MOF-74 has the lowest H2uptakes and Qstwithin the series due to an unfavorable geometric environment for the Zn2+ions within the ZnO5clusters. Lastly, the two-dimensional quantum rotational levels were calculated for the H2–metal interaction in all four MOFs using the potential energy function employed herein and the calculated transitions were in good agreement with the corresponding peaks that were observed in the experimental inelastic neutron scattering (INS) spectra for the respective MOFs. This observation serves both to provide atomistic resolution to the spectroscopic experiments and to validate the molecular force field.

Grand canonical Monte Carlo (GCMC) simulations of H2sorption were performed in Mg-MOF-74, a metal–organic framework (MOF) that displays very high H2sorption affinity. Experimental H2sorption isotherms and isosteric heats of adsorption (Qst) values were reproduced using a general purpose materials sorption potential that includes many-body polarization interactions. In contrast, using two models that include only charge–quadrupole interactions failed to reproduce such experimental measurements even though they are the type normally employed in such classical force field calculations. Utilizing the present explicit polarizable model in GCMC simulation resulted in a Mg2+–H2distance of 2.60 Å, which is close to a previously reported value that was obtained using electronic structure methods and comparable to similar experimental measurements. The induced dipole distribution obtained from simulation assisted in the characterization of two previously identified sorption sites in the MOF: the Mg2+ions and the oxido group of the linkers. The calculated two-dimensional quantum rotational levels for a H2molecule sorbed onto the Mg2+ion were in good agreement with experimental inelastic neutron scattering (INS) data. Although the H2–metal interaction in MOFs may be thought of as a quantum mechanical effect, this study demonstrates how the interaction between the sorbate molecules and the open-metal sites in a particular highly sorbing MOF can be captured using classical simulation techniques that involve a polarizable potential.

Grand canonical Monte Carlo (GCMC) simulations were performed to investigate CO2and H2sorption in an rht-metal–organic framework (MOF) that was synthesized with a ligand having a nitrogen-rich trigonal core through trisubstituted triazine groups and amine functional groups. This MOF was synthesized by two different groups, each reporting their own distinct gas sorption measurements and crystal structure. Electronic structure calculations demonstrated that the small differences in the atomic positions between each group’s crystal structure resulted in different electrostatic parameters about the Cu2+ions for the respective unit cells. Simulations of CO2sorption were performed with and without many-body polarization effects and using our recently developed CO2potentials, in addition to a well-known bulk CO2model, in both crystallographic unit cells. Simulated CO2sorption isotherms and calculated isosteric heats of adsorption, Qst, values were in excellent agreement with the results reported previously by Eddaoudi et al. for both structures using the polarizable CO2potential. For both crystal structures, the initial site for CO2sorption were the Cu2+ions that had the higher positive charge in the unit cell, although the identity of this electropositive Cu2+ion was different in each case. Simulations of H2sorption were performed with three different hydrogen potentials of increasing anisotropy in both crystal structures and the results, especially with the highest fidelity model, agreed well with Eddaoudi et al.’s experimental data. The preferred site of H2sorption at low loading was between two Cu2+ions of neighboring paddlewheels. The calculation of the normalized hydrogen dipole distribution for the polarizable model in both crystal structures aided in the identification of four distinct sorption sites in the MOF, which is consistent to what was observed in the experimental inelastic neutron scattering (INS) spectra. Lastly, while the experimental results for the two groups are quantitatively different, the sorption mechanisms (for both crystal structures and sorbates) are broadly similar and not inconsistent with either set of experimental data; the theoretical sorption isotherms themselves resemble those by Eddaoudi et al.

Five new metal–organic frameworks (MOFs, termed MOF-324, 325, 326 and IRMOF-61 and 62) of either short linkers (pyrazolecarboxylate and pyrazaboledicarboxylate) or long and thin alkyne functionalities (ethynyldibenzoate and butadiynedibenzoate) were prepared to examine their impact on hydrogen storage in MOFs. These compounds were characterized by single-crystal X-ray diffraction, and their low-pressure and high-pressure hydrogen uptake properties were investigated. In particular, volumetric excess H2uptake by MOF-324 and IRMOF-62 outperforms MOF-177 up to 30 bar. Inelastic neutron-scattering studies for MOF-324 also revealed strong interactions between the organic links and hydrogen, in contrast to MOF-5 where the interactions between the Zn4O unit and hydrogen are the strongest. These data also show that smaller pores and polarized linkers in MOFs are indeed advantageous for hydrogen storage.

The interactions of hydrogen molecules with Cu+and FeO+extraframework cations in chabazite were studied using periodic DFT (B3LYP) calculations. Dispersion forces were accounted for by adding Grimme’s correction to the DFT energy at the B3LYP-optimized geometries. Two potential binding sites are found for FeO+cations inside chabazite as for Cu+. The preferred site I is located in the middle of the six-membered ring, where FeO+coordinates to three oxygen atoms of the zeolite framework, while in the less stable site IV FeO+coordinates to two oxygen atoms of the eight-membered ring (ΔE= 17.3 kJ mol−1). The interaction of H2with FeO+(Eads= −17.5 kJ mol−1) is weaker than that with Cu+(Eads= −85 kJ mol−1), so that binding is only favorable at the more open site IV, where no framework reorganization is needed. For both CuCHA and FeOCHA, the geometry of the adsorbed H2−M+complex is governed by M d → H2σ* back-donation, which also contributes to a large extent to the final adsorption energy and controls the barrier to H2rotation. The M−H distance, the polarization of the metal d orbital involved in the back-donation toward the H2molecule, as well as the H2charge density show that the M d → H2σ* back-donation is larger in H2−Cu+than in H2−FeO+. Both the binding energy and the barrier to rotation are therefore much greater for H2−CuCHA than for H2−FeOCHA (15.1 vs 7.4 kJ mol−1), in agreement with the available experimental data. A comparison between site I and site IV along with results on the possible adsorption of a second H2molecule per cation site suggests that the different adsorption sites observed by experiment are more likely due to the presence of different metal cation environments rather than binding of more than one H2per cation site.

While metal-doped mesoporous silicate SBA-15 has been studied extensively for its catalytic properties, the adsorption of molecular hydrogen in this system has not been considered. We report herein that the titanium-doped material (10% Ti) adsorbs nearly twice the amount of hydrogen at 77 K and a pressure of 25 bar than pure SBA-15, Hydrogen adsorption isotherms also show that binding energies in the Ti-doped material are more than 50% greater than in SBA-15. Neutron vibrational spectroscopy shows clear evidence for interaction of hydrogen with Ti, which is responsible for part of the increase in hydrogen sorption in the material.

The deposition of volatile cis-(CH3)2Au(O,O′-acac) onto silica partially dehydroxylated at 400 °C leads to molecular dispersion of the organogold complex. X-ray absorption near-edge structure demonstrates that the gold retains its oxidation state upon binding. IR and 1H magic-angle spinning NMR spectra suggest that the molecular framework also remains intact. Computational models involving hydroxyl-terminated octasilsesquioxane cube clusters to represent silica predict the most energetically favorable interaction to be hydrogen-bonding between two surface hydroxyls and the oxygen donor atoms of the chelated acetylacetonate ligand. This mode of adsorption was confirmed by analysis of the IR and the Au LIIIedge extended X-ray absorption fine structure. The surface hydroxyls most likely to participate in the attachment of the gold complex to silica are vicinal silanol pairs that are not involved in hydrogen bonding to other silanols.