Hydrogen Uptake Values Research Articles

Investigation of hydrogen adsorption sites in ferrocene-based hypercrosslinked polymers by DFT calculations

Herein, two types of Fc-based hypercrosslinked polymers (HHCP and DHCP) with high BET surface area (953.9 and 1062.7 m2 g−1) were prepared by direct one-step phenolic polycondensation aiming at high uptake capacity towards H2. As expected, the hydrogen uptake values of HHCP and DHCP were notably higher than those of porous polymers with a higher surface area under the same conditions. The maximum H2 uptake of HCPs at 77 K/1.0 bar are determined to be 2.51 wt % for HHCP and 2.74 wt % for DHCP, respectively. Despite the pore structure playing a determinant role in hydrogen adsorption, the adsorption sites in the polymer framework can be another essential factor to influence the hydrogen adsorption capacity. The interactions between the Fc building block and H2 are revealed by DFT calculation to fully illustrate the role of Fc in H2 adsorption. The results indicate that both iron atoms and benzene rings in Fc-based HCPs contribute to the adsorption of hydrogen. Moreover, it is interesting to find that the most stable adsorption site around the Fc building unit is the position directly above the Cp ring. The results in this work may more reasonably explain the phenomenon that the introduction of Fc units could enhance the adsorption of H2 in many studies, which also provides possible insights into the application of metallocene compounds in H2 adsorption.

Boronic Acid Moiety as Functional Defect in UiO-66 and Its Effect on Hydrogen Uptake Capacity and Selective CO2 Adsorption: A Comparative Study.

Herein, we use linker fragmentation approach to introduce boronic acid moieties as functional defects into Zr-based metal-organic frameworks (MOFs, UiO-66). Our findings show that the amount of permanently incorporated boronic acid containing ligand is directly dependent on the synthesis method. The accessible boronic acid moieties in the pore surfaces significantly improve the hydrogen uptake values, which are 3.10 and 3.44 wt % at 21 bar, 77 K for dimethylformamide (DMF)/H2O and DMF/HCI synthesis methods, respectively. Also, CO2 selectivity of the resulting MOFs over N2 and CH4 significantly increases due to the quadrupolar interaction between active surfaces and CO2 molecules. To the best of our knowledge, both hydrogen storage and selectivity of CO2 for UiO-66 are the highest reported values in the literature to date. Furthermore, another striking result that emerged from the high-pressure hydrogen uptake isotherms is the direct correlation between the defects and hysteric adsorption behavior, which may result in the shift from rigidity to flexibility of the framework due to the uncoordinated sites.

Morphology-controllable synthesis and characterization of carbon nanotube/polypyrrole composites and their hydrogen storage capacities

Sphere-like and layer-by-layer growth mechanisms of polypyrrole are controlled by changing pyrrole monomer concentration and using carbon nanotubes (CNT) as template. Pristine polypyrrole has sphere-like structures but remarkable change in types of polypyrrole growth is observed from spherical-like to layer-by-layer structures in the presence of CNT. Acid treatment enhances polypyrrole coverage on CNT surface by preventing agglomeration of polypyrrole due to an increase in surface oxygen groups and sp2 bonds in CNT structure. The crystallinity of powders comparably decreases after polypyrrole coating due to the amorphous structure of polypyrrole and a sharp decrease in the intensity of 002 peak. The influence of surface functionalization and polymer coating on the structural parameters of multi-walled CNT and their composites is investigated by tailoring the feeding ratio of polypyrrole. The hydrogen sorption measurements at ambient conditions by Intelligent Gravimetric Analyzer demonstrate that hydrogen uptake of CNT/polypyrrole composite is 1.66 wt.% which is almost 3 times higher than that of pristine CNT. Higher hydrogen uptake values are obtained by keeping the mass ratio of pyrrole monomer and CNT equal by using non-functionalized CNT in composite production. Hydrogen adsorption/desorption kinetics of polypyrrole/CNT composites is improved by increasing adsorption sites after polymer coating and acid treatment. The desorption curves of these modified surfaces are higher than their adsorption curves at lower pressures and hysteresis loop is observed in their isotherms since hydrogen is chemically bonded to the modified surfaces by the conversion of carbon atoms from sp2 to sp3 hybridization.

Hydrogen storage on silicon, carbon, and silicon carbide nanotubes: A combined quantum mechanics and grand canonical Monte Carlo simulation study

Grand canonical Monte Carlo (GCMC) simulation combined with ab initio quantum mechanics calculations were employed to study hydrogen storage in homogeneous armchair open-ended single walled silicon nanotubes (SWSiNTs), single walled carbon nanotubes (SWCNTs), and single walled silicon carbide nanotubes (SWSiCNTs) in triangular arrays. Two different groups of nanotubes were studied: the first were (12,12) SiNTs, (19,19) CNTs, and (15,15) SiCNTs and the second were (7,7) SiNTs, (11,11) CNTs, and (9,9) SiCNTs with the diameters of ∼26 and ∼15 Å for the first and second groups, respectively. The simulations were carried out for different thermodynamic states. The potential energy functions (PEFs) were calculated using ab initio quantum mechanics and then fitted with (12,6) Lennard-Jones (LJ) potential model as a bridge between first principles calculations and GCMC simulations. The absolute, excess, and delivery adsorption isotherms of hydrogen were calculated for two groups of nanotubes. The isosteric heat of adsorption and the radial distribution functions (RDFs) for the adsorbed molecules on different nanotubes were also computed. Different isotherms were fitted with the simulation adsorption data and the model parameters were correlated. According to the results, the hydrogen uptake values in (19,19) CNT array exceeded the US DOE (Department of Energy) target of 6.0 wt% (FY 2010) at 77 K and 1.0 and 2.0 MPa for absolute and excess uptakes, respectively. The results also show that SiNTs and SiCNTs are not more useful materials compared with corresponding CNTs for hydrogen storage.

Poly phenylenediamine and its TiO 2 composite as hydrogen storage material

Poly phenylenediamine was synthesized from 1,4-phenylenediamine in presence of potassium persulphate and salicylic acid. The structure of the formed poly phenylenediamine was traced using FTIR and its morphology was examined by transmittance electron microscope (TEM). Gel permeation chromatography (GPC) was used to evaluate the polymer molecular weight which showed that the value of its molar mass is 20,000 g mol −1 and it has polydispersity index of 1.01. Different concentrations of TiO 2 were incorporated in the poly phenylenediamine structure via coordination bond between the oxygen atom of TiO 2 and the hydrogen atom of N–H group of polymer. The prepared composites were characterized using FTIR, TA, TEM and SEM/EDX. The TEM micrographs revealed that the composites have 2-D network structure and its morphology changed from a dendritic structure for the pure polymer to layered structure of the composite. The polymer and its composite are completely insulators. Their hydrogen storage capacity has been estimated at −193 °C and the composite reported higher hydrogen uptake values than the pure polymer. The reason is suggested to be due to the layered structure of composite which gives it the privilege to store more hydrogen in its interlayer vicinity.

Influence of MIL‐101 Doping by Ionic Clusters on Hydrogen Storage Performance up to 1900 Bar

Ultra-high-pressure hydrogen-storage performance (up to 1900 bar) was investigated for mesoporous chromium terephthalate MIL-101 and its inclusion compounds containing ionic clusters [Re(4)S(4)F(12)](4-) and [SiW(11)O(39)](7-) within the porous framework. The maximum specific hydrogen uptake values (total) for MIL-101 are 12.3 (at 81) and 7.2 wt. % (at 293 K). Such unique measurement conditions allowed us to identify the density of the absorbed hydrogen directly from the excess sorption isotherm curves. The corresponding density values were found to be almost comparable at low temperature, but significantly different at ambient temperature, which indicated an increase of more than double in the number of hydrogen binding sites in the case of the inclusion compounds with rhenium clusters.

Controlling Hydrogenation of Graphene on Transition Metals

A monatomic layer of graphite (MG or graphene) adsorbed on the (111) faces of transition metals Pt, Ir, and Ni, has been employed for controlling the atomic hydrogen adsorption site selectivity and the amount of hydrogen adsorbed upon saturation. The variations in the graphene-metal chemical bonding caused by hydrogenation and the values of saturated hydrogen coverage have been studied by X-ray photoemission and X-ray absorption spectroscopy. The hydrogenation of the graphene/metal systems has also been compared to the hydrogen adsorption on highly oriented pyrolytic graphite under the same experimental conditions. It has been found that graphene adsorption on the transition metal substrates can drastically enhance the hydrogen uptake values. The highest values have been observed for MG/Ir(111), less for MG/Pt(111), even less for MG/Ni and the least for the adsorption on bulk graphite. The high level of H coverage on MG/Ir and MG/Pt has been assigned to the preferential H adsorption on the more bonding patches (pores) of the MG/metal coincidence lattice. This adsorption creates unpaired electrons which contribute to a strengthening of the graphene-metal bonds. In this way, the densest possible graphane-like patches can be formed on MG/Pt and MG/Ir. On the MG/Ni interface the formation of graphane is obstructed by the strong interfacial bonding.

Hydrogen storage: a comparison of hydrogen uptake values in carbon nanotubes and modified charcoals

We compared the hydrogen uptake weight percentages (wt.%) of different carbonized materials, before and after modification, for their application in hydrogen storage at room temperature. The Sievert's method [T.P. Blach, E. Mac, A. Gray, J. Alloys Compd. 446-447 , 692 (2007)] was used to measure hydrogen uptake values on: (1) Taiwan bamboo charcoal (TBC), (2) white charcoal (WC), (3) single-walled carbon nanotubes (SWCNTs) bought from CBT Inc. and (4) homemade multi-walled carbon nanotubes (MWCNTs) grown on TBC. Modified samples were coated with a metal catalyst by dipping in KOH solutions of different concentrations and then activated in a high temperature oven (800 °C) under the atmospheric pressure of inert gas. The results showed that unmodified SWCNTs had superior uptake but that Taiwan bamboo charcoal, after modification, showed enhanced uptake comparable to the SWCNTs. Due to TBC's low cost and high mass production rate, they will be the key candidate for future hydrogen storage applications.

Preparation of Mesoporous Silica and Carbon Materials with Multilength‐Scale Pores and Hydrogen Sorption Application

AbstractA facile method was developed for the preparation of bimodal mesoporous siliceous materials with nanostructured pores in which a cationic organic template, n‐hexadecyltrimethylammonium hydroxide, was employed as a base catalyst as well as a mesopore structure‐directing agent during the slow condensation of tetraethoxysilane. It was possible to alter the pore orientations of smaller pores by adding neutral auxiliary templates such as Brij76 and P123. A series of biomodal mesoporous carbons was also obtained from the bimodal mesoporous silica materials through a nanocasting process. The resulting bimodal mesoporous carbons exhibited decent hydrogen uptake values at 77 K. The maximum hydrogen uptake was 1.33 wt.‐% for the sample having the narrowest pores, while keeping a relatively large pore volume. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Synthesis and high performance of ceria supported nickel catalysts for hydrodechlorination reaction

Catalysts containing 10 wt% Ni supported on CeO 2 were prepared by two ways, namely, co-precipitation method using nickel nitrate precursor and impregnation method using nickel nitrate and nickel acetylacetonate as two separate precursors. The catalysts were characterized by pulse chemisorption of H 2, X-ray diffraction, and temperature programmed reduction (TPR) techniques and evaluated for the gas phase hydrodechlorination (HDC) of chlorobenzene to benzene in a fixed-bed down-flow glass reactor at 573 K under normal atmospheric pressure. The hydrogen uptake values were used to determine the catalyst properties of Ni/CeO 2 like dispersion, metal area, and particle size. Among the two preparatory routes, co-precipitation method gave better catalytic performance in terms of hydrogenation activity, benzene selectivity, and coking resistivity than impregnated Ni/CeO 2 catalysts. This may be attributed to high dispersion of smaller NiO crystallites and the appearance of the second reduction peak at a higher temperature (578 K) in TPR profile with co-precipitated Ni/CeO 2 catalyst. This indicates that a strong interaction may take place between the NiO crystallites and CeO 2 on the surface of co-precipitated Ni/CeO 2 catalyst. Contrary to general expectation that the large Ni particles are preferable for HDC reaction, it is observed that smaller metal particles with high dispersion, as in the case of co-precipitated Ni/CeO 2 catalyst, promotes better catalyst with longer life.

High value carbon materials from PET recycling

Poly(ethylene) terephthalate (PET), has become one of the major post-consumer plastic waste. In this work special attention was paid to minimising PET residues and to obtain a high value carbon material. Pyrolysis and subsequent activation of PET from post-consumer soft-drink bottles was performed. Activation was carried out at 925°C under CO2 atmosphere to different burn-off degrees. Textural characterisation of the samples was carried out by performing N2 adsorption isotherms at −196°C. The obtained carbons materials were mainly microporous, presenting low meso and macroporosity, and apparent BET surface areas of upto 2500m2g−1. The capacity of these materials for phenol adsorption and PAHs removal from aqueous solutions was measured and compared with that attained with commercial active carbons. Preliminary tests also showed high hydrogen uptake values, as good as the results obtained with high-tech carbon materials.

Biotreatment of coals and coal-related compounds by hydrogen-utilizing microorganisms

Desulfovibrio desulfuricans and Acidianus brierleyi were used to study hydrogenation of coal, pretreated coal, asphaltenes, and model compounds [diphenyl methane (DPM) and fumarate] under anaerobic conditions. This study involved three primary aspects: (1) determination of net hydrogen-uptake, (2) identification of the biohydrogenated product of fumarate, and (3) testing the influence of hydrogen uptake/biohydrogenation of coal in terms of direct liquefaction yield. The net hydrogen uptake values (from Warburg and GC assays) by the coals or the model compounds (controls) were less than that of the biotreated samples. The greated hydrogen uptake (net, 1878 μmol g−1; coal control, 245; cell control, 127) occurred in untreated coal KCER No. 4677 in the presence of D. desulfuricans. The net hydrogen uptake by coals varied depending upon the coal type and the microorganism. Model compound DPM showed its highest hydrogen uptake rate when catalysed by D. desulfuricans (150 μmol g−1 h−1). D. desulfuricans also hydrogenated 36% of the fumarate to succinate in the presence of hydrogen. Biotrated coal KCER No. 4677 (with 1878 μmol H2 g−1) was subsequently subjected to direct liquefaction. It showed a net increase in liquefaction yield of 5·4% as a result of the biotreament.