Kubas Binding Research Articles

“Capping” out oxygen interference: An approach to achieve efficient hydrogen storage via Kubas binding

Hydrogen bound via Kubas interaction on metal decorated light weight materials forebodes a much efficient alternative for the fast depleting fossil fuels. However, oxygen interference in hydrogen storage on metal decorated carbonaceous systems obstructs the successful realization of the theoretically promised high storage of Kubas bound H2. The interference is a consequence of preferred O2 binding at the metal site, thereby blocking it for H2 adsorption. Here, we report that arene capping of a Sc-metallacarborane (MCB) efficiently reverses the preferential adsorption of O2 over H2. The capped MCB completely repels O2 off the Sc-site allowing purely Kubas binding of H2. This is enabled by the down-shift of the d-band center of Sc below the Fermi-level from 1.29 eV in the uncapped to 5.67 eV in the capped MCB, respectively. The optimised d-band center position enables the empty Sc antibonding states to only be available for hydrogen adsorption via Kubas interaction, thereby, to completely avoid oxygen binding. This novel approach tunes a cluster electronically to enhance gas adsorption selectivity, which can be efficiently ulitized in various catalytic, sensing and gas storage systems.

High-Pressure Raman and Calorimetry Studies of Vanadium(III) Alkyl Hydrides for Kubas-Type Hydrogen Storage.

Reversible hydrogen storage under ambient conditions has been identified as a major bottleneck in enabling a future hydrogen economy. Herein, we report an amorphous vanadium(III) alkyl hydride gel that binds hydrogen through the Kubas interaction. The material possesses a gravimetric adsorption capacity of 5.42 wt % H2 at 120 bar and 298 K reversibly at saturation with no loss of capacity after ten cycles. This corresponds to a volumetric capacity of 75.4 kgH2 m(-3) . Raman experiments at 100 bar confirm that Kubas binding is involved in the adsorption mechanism. The material possesses an enthalpy of H2 adsorption of +0.52 kJ mol(-1) H2 , as measured directly by calorimetry, and this is practical for use in a vehicles without a complex heat management system.

Evidence of Nanoconfinement Effects in the Adsorption of Hydrogen on Coinage Metal Complexes Dispersed within Porous Carbon

In this article, we report the results of careful, room-temperature, high-pressure adsorption studies, in which simple complexes of copper and silver reversibly adsorb hydrogen when dispersed within nanoporous carbon. Whereas these complexes alone did not adsorb hydrogen, when they were nanoconfined within carbon, they adsorbed 1–3 mol of H2 per metal center. As a figure of merit, nanoconfined cupric formate adsorbed ∼2.6 H2/Cu at 100 bar and 20 °C, much larger than any reported metal-containing adsorption medium. On carbon alone, the heat of hydrogen adsorption decreased with an increase in adsorption extent, limiting to insufficient levels of uptake. By contrast, when these metal salts were dispersed within the carbon, the heats of adsorption increased markedly in a linear manner, meaning that the thermodynamics has moved in the right direction and is not self-limiting. Such thermodynamic behavior is associated with side-on dihydrogen binding onto a metal center, the so-called Kubas binding. Such intera...

DFT study of hydrogen adsorption on Eu‐decorated single‐ and double‐sided graphene

We carry out density functional theory calculations to study the adsorption of hydrogen molecules on Eu‐decorated single‐ and double‐sided graphene sheets. It is found that Eu prefers to adsorb on the hollow site of carbon ring above both the single‐ and double‐sided graphene. The hybridization of Eu on graphene is mainly between the Eu‐5d orbital and the C‐2p orbital. The charge of Eu is enhanced when the first hydrogen molecule approaches the Eu atom. As more hydrogen molecules are introduced, the charge population of Eu‐5d and 6p orbital is increased. Each Eu can stably bind up to six hydrogen molecules with an average adsorption energy of −0.404 and −0.392 eV for Eu decorated single‐ and double‐sided graphene, respectively. Eu atoms carry negative charge when 12 hydrogen molecules are attracted by the Eu‐decorated double‐sided graphene. The main interaction between hydrogen and Eu is similar to Kubas binding, which contains hybridization of H–σ* and Eu‐5d orbital and charge transfer between them.

Are Metal–Metal Interactions Involved in the Rising Enthalpies Observed in The Kubas Binding of H2 to Hydrazine-Linked Hydrogen Storage Materials?

Models of two linked M(III) and M(II) (M = Ti, V, Cr) binding sites in hydrazine-linked hydrogen storage materials have been studied quantum chemically using density functional theory. The results compare favorably with previous experimental and computational results. Strong evidence is observed that the H2 molecules bind to the metal in a Kubas manner. As seen previously in monometallic analogues,(1, 2) altering the transition metal across the first row of the periodic table reduces the number of H2 molecules that can be bound, and replacing a hydrazide ligand with a hydride increases the M-H2 interaction energy. Evidence is presented for metal–metal interactions, which can influence the H2 binding enthalpy and may help to explain the observed metallic properties and rising H2 binding enthalpies with coverage of the experimental materials. An alternate explanation for the rising enthalpies is also proposed, involving a pressure-induced deformation of the structure with concomitant twisting of the bonds i...

Single Pd atoms in activated carbon fibers and their contribution to hydrogen storage

Palladium-modified activated carbon fibers (Pd-ACF) were synthesized by melt-spinning, carbonization and activation of an isotropic pitch carbon precursor premixed with an organometallic Pd compound. The hydrogen uptake at 25 °C and 20 bar on Pd-ACF exceeded the expected capacity based solely on Pd hydride formation and hydrogen physisorption on the microporous carbon support. Aberration-corrected scanning transmission electron microscopy (STEM) with sub-Ångstrom spatial resolution provided unambiguous identification of isolated Pd atoms occurring in the carbon matrix that coexist with larger Pd particles. First principles calculations revealed that each single Pd atom can form Kubas-type complexes by binding up to three H 2 molecules in the pressure range of adsorption measurements. Based on Pd atom concentration determined from STEM images, the contribution of various mechanisms to the excess hydrogen uptake measured experimentally was evaluated. With consideration of Kubas binding as a viable mechanism (along with hydride formation and physisorption to carbon support) the role of hydrogen spillover in this system may be smaller than previously thought.

Kubas-Type Hydrogen Storage in V(III) Polymers Using Tri- and Tetradentate Bridging Ligands

Oxalic acid, oxamide, glycolic acid, and glycolamide were employed as 2-carbon linkers to synthesize a series of one-dimensional V(III) polymers from trismesityl vanadium(III)·THF containing a high concentration of low-valent metal sites that can be exploited for Kubas binding in hydrogen storage. Synthesized materials were characterized by powder X-ray diffraction (PXRD), nitrogen adsorption (BET), X-ray photoelectron spectroscopy (XPS), infrared spectroscopy (IR), Raman spectroscopy, thermogravimetric analysis, and elemental analysis. Because each of these organic linkers possesses a different number of protons and coordinating atoms, the products in each case were expected to have different stoichiometries with respect to the number of mesityl groups eliminated and also a different geometry about the V(III) centers. For example, the oxalate and glycolate polymers contained residual mesityl groups; however, these could be exchanged with hydride via hydrogenolysis. The highest adsorption capacity was recorded on the product of trismesityl vanadium(III)·THF with oxamide (3.49 wt % at 77 K and 85 bar). As suggested by the high enthalpy of adsorption (17.9 kJ/mol H(2)), a substantial degree of performance of the vanadium metal centers was retained at room temperature (25%), corresponding to a gravimetric adsorption of 0.87 wt % at 85 bar, close to the performance of MOF-177 at this temperature and pressure. This is remarkable given the BET surface area of this material is only 9 m(2)/g. A calculation on the basis of thermogravimetric results provides 0.88 hydrogen molecule per vanadium center under these conditions. Raman studies with H(2) and D(2) showed the first unequivocal evidence for Kubas binding on a framework metal in an extended solid, and IR studies demonstrated H(D) exchange of the vanadium hydride with coordinated D(2). These spectroscopic observations are sufficient to assign the rising trends in isosteric heats of hydrogen adsorption observed previously by our group in several classes of materials containing low-valent transition metals to the Kubas interaction.

Bis(benzene) and Bis(cyclopentadienyl) V and Cr Doped Mesoporous Silica with High Enthalpies of Hydrogen Adsorption

Hexagonally packed mesoporous silica synthesized using a dodecylamine template was treated with bis(cyclopentadienyl) chromium, bis(cyclopentadienyl) vanadium, and bis(benzene) vanadium. The obtained composite samples were characterized by nitrogen adsorption, X-ray powder diffraction, and X-ray photoelectron spectroscopy. These materials were then studied for hydrogen adsorption and storage capacities to determine the effect of metal doping and the degree of Kubas binding to the metals in these systems. By studying the hydrogen adsorption at two temperatures of 77 and 87 K, the heats of adsorption were also obtained by fitting the data in Clapeyron−Clausius equation. The composite samples all possess higher adsorption and storage capacities than pristine silica, and all show the same rising trend of adsorption enthalpies as a function of surface coverage observed in previous studies from our group on mesoporous oxides with reduced transition metal centers on the surface. The highest enthalpy recorded was...

Optimization of hydrogen storage capacity in silica-supported low valent Ti systems exploiting Kubas binding of hydrogen

Silica-based materials grafted with low valent Ti fragments for Kubas-type binding of hydrogen were optimized for hydrogen adsorption capacity by varying the surface area, pore size, loading levels, and type of organometallic precursor. All materials were characterized by XRD, nitrogen adsorption, and XPS where appropriate. The surface area of HMS silica was optimized by varying silica-to-surfactant molar ratio, and also by tuning the pore size by varying the surfactant’s carbon chain length (C 6, C 8, C 10, C 12). Then Ti fragments originating from either benzyl, allyl, or methyl Ti precursors were grafted onto the optimal HMS surface at different loading levels to arrive at Ti grafted HMS materials with H 2 storage capacities and binding properties superior to those previously reported by our group for benzyl Ti (III) species on silica. HMS prepared with dodecylamine using a silica:surfactant ratio of 3:1 and subsequently grafted with 0.2 M equiv. of TiBz 4 had the highest H 2 adsorption at 2.45 wt% at 77 k and 60 atm, which equates to an average of 3.98 H 2 molecule per Ti metal center, just one H 2 molecule short of the theoretical saturation limit of 5 H 2/Ti predicted by the 18-electron rule. The H 2 adsorption capacities of Me 3Ti-HMS and Allyl 3Ti-HMS prepared using the same optimized sample of C 12-HMS silica at a 3:1 Si:surfactant ratio possessed H 2 adsorption values corresponding to 2.4 and 2.27 H 2 per Ti center, respectively, at 60 atm and 77 K. This performance level is significantly lower than that of the benzyl Ti (III) system. The binding enthalpies of the benzyl Ti (III) material increase with H 2 coverage to 23 kJ/mol, while the enthalpies for the newly synthesized Me 3Ti-HMS and Allyl 3Ti-HMS materials increase with H 2 coverage to a maximum of 2.66 and 4.17 kJ/mol, respectively. XPS studies on these materials suggested a trend in π-back donating ability on the Ti (III) centers of methyl > allyl > benzyl, opposite that observed experimentally. The reason for the diminished performance of the allyl and methyl Ti (III) systems may thus be related to the presence of THF ligands blocking coordination sites in the allyl and methyl systems. THF is not present in the benzyl system because this solvent is not required for synthesis.

H2 Storage Materials (22KJ/mol) Using Organometallic Ti Fragments as σ-H2 Binding Sites

Low-coordinate Ti (III) fragments with controlled geometries designed specifically for sigma-H2 binding were grafted onto mesoporous silica using tri- and tetrabenzyl Ti precursors. The hydrogen storage capacity was tested as a function of precursor and precursor loading level. At an optimal loading level of 0.2 mol equiv tetrabenzyl Ti the total storage capacity at -196 degrees C was 21.45 wt % and 34.10 kg/m(3) at 100 atm, and 3.15 wt % and 54.49 kg/m(3) for a compressed pellet under the same conditions. The adsorption value of this material was 1.66 wt %, which equates to an average of 2.7 H2 per Ti center. The adsorption isotherms did not reach saturation at 60 atm, suggesting that the theoretical maximum of 5 H2 per Ti in this system may be reached at higher pressures. The binding enthalpies rose with surface coverage to a maximum of 22.15 kJ/mol, which is more than double that of the highest recorded previously and within the range predicted for room temperature performance. The adsorption values of 0.99 at -78 degrees C and 0.69 at 25 degrees C demonstrate retention of 2.4 H2 and 1.1 H2 per Ti at these temperatures, respectively. These findings suggest that Kubas binding of H2 may be exploited at ambient temperature to enhance the storage capacities of high-pressure cylinders currently used in hydrogen test vehicles.

Calculations of Dihydrogen Binding to Doped Carbon Nanostructures

We used second-order Moller-Plesset perturbation theory to calculate the binding energies of hydrogen molecules to various carbon nanostructures, in an effort to find improved hydrogen storage materials. For the central concave site of corannulene, a new binding geometry was found, with the H2 molecule in a parallel configuration, and an improved corrected binding energy of 9.8 kJ/mol was discovered. We studied the binding of hydrogen to B and Be doped small carbon nanostructures. For B doped corannulene, Kubas binding was observed in a metastable state. For C11H8Be, a binding energy of 17.5 kJ/mol and, for C16Be2H8, a binding energy of 12.3 kJ/mol due to Kubas-type non-dissociative adsorption were found. Benzene-B,N-2COOH and pyrazine dicarboxylate were proposed as higher binding energy building blocks for the metal−organic framework materials. For both benzene-B,N-2COOH and pyrazine dicarboxylate, we found low physisorption binding energies.