H2 Adsorption Isotherms Research Articles

Microwave Synthesis of a Nanoporous Hybrid Material, Chromium Trimesate

Recently, the domain of nanoporous materials has been enlarged very much to the development of porous hybrid materials designated as metal-organic frameworks (MOF), porous coordination polymers or organic-inorganic hybrids which are the most recently highlighted class of materials consisting of metal ions linked together by organic bridging ligands in the framework. The attraction of combining properties from both inorganic and organic components has led to a quest of research toward new hybrid materials with potential applications including gas storage, catalysis, separation, and molecular recognition. Very recently, Ferey and co-workers have reported a novel hybrid material, chromium trimesate (designated as MIL-100), which has a hierarchical pore system (micro: 5-9 A; mesoporous: 25-30 A) with a very high Langmuir surface area. Syntheses of the porous hybrid materials have been carried out mainly by hydrothermal or solvothermal synthesis generally in a period of several days. Microwave techniques have attracted growing attention for the rapid synthesis of nanoporous materials requiring several days to prepare under hydrothermal conditions. Potential advantages of this technique in the synthesis of porous materials include phase selectivity, narrow particle size distribution and facile morphology control besides fast crystallization. However, microwave technique has not been applied to the synthesis of nanoporous hybrid materials yet even though the microwave syntheses of not only organic molecules but also inorganic materials have been often studied. In this communication, we report the first successful result on the microwave synthesis of porous chromium trimesate as an organic-inorganic hybrid material. The porous chromium trimesate (MIL-100) was synthesized in aqueous media similar to the previous reported method except using microwave irradiation as a heating source. The molar composition of reactant mixture was 1.0 Cr: 0.67 H3BTC (benzene tricarboxylic acid): 2.0 HF: 290 H2O. The reactant mixture was loaded in a Teflon autoclave, which was sealed and placed in a microwave oven (Mars-5, CEM). The autoclave was heated to the reaction temperature of 220 C and kept for a predetermined time. The structure and crystallinity of the synthesized samples were determined by X-ray powder diffraction. The TGA pattern was obtained with a thermal analyzer in the air flow. The sorption experiments were carried out volumetrically. Figure 1 shows the XRD patterns of as-synthesized chromium trimesate obtained by varying crystallization time at 220 C under microwave irradiation. The XRD patterns of the samples obtained from microwave irradiation are well consistent with the pattern of MIL-100 synthesized for 4 days at 220 C using conventional hydrothermal heating. However, the chromium trimesate materials synthesized using microwave method contained unreacted metallic chromium species until 2 h of crystallization. The crystal yield of the chromium trimesate based on chromium from microwave synthesis for 4 h is 44%, which is comparable with the result of 45% in the conventional synthesis for 4 days. The TGA profile of Figure 2 reveals a stability of the title compound up to 270 C. The thermal stability determined with TGA are very similar to the previous results. The chromium trimesate synthesized by microwave heating shows nitrogen adsorption-desorption isotherm very similar to the isotherm of MIL-100 (Data not shown), representing the permanent porosity and similarity of the pore structure of the hybrid materials synthesized by both methods. For multiplayer coverage of N2, we estimate the apparent BET surface area and pore volume to be 1700 m/g and 0.97 mL/g, respectively. From H2 adsorption isotherm (Figure 3), the adsorption capacity of hydrogen at –196 C and 1 atm is estimated to be about 150 mL/g (STP), which is comparable with the results adsorbed on various MOFs and porous hybrid materials. The re-adsorption isotherm after the desorption at –196 C of the pre-adsorbed hydrogen is same as the adsorption isotherm on the fresh sample, representing that there is no chemisorption site in the chromium trimesate

Quantum nature of adsorbed hydrogen on single-wall carbon nanohorns

We have measured N2 adsorption isotherms on single-wall carbon nanohorns (SWNHs) over the temperature range of 77–92 K, and isosteric heat of adsorption, q st, were determined. Adsorption measurements for SWNHs have been also done for for H2 at 20 K, and for H2 and D2 at 77 K, respectively. We have performed grand canonical Monte Carlo (GCMC) simulations of N2, H2, and D2 for single-wall carbon nanotube (SWNT) models to compare with the experimental data. Simulated N2 adsorption isotherm on a SWNT bundle model is in reasonably good agreement with the experimental isotherm on the SWNH assembly over a wide range of pressures at 77 K; however, simulated q st-values for the SWNT bundle suggest that the SWNH particles are incompletely arranged in the SWNH assembly. Simulated endohedral isotherm of classical H2 inside an isolated SWNT model at 20 K showed that a density of adsorbed H2 in the internal space of the SWNH particle is quite smaller than that of classical H2 because of large quantum effects. In simulating H2 and D2 adsorption isotherms on the model SWNT bundle at 77 K, GCMC simulations based on the Feynman-Hibbs (FH) effective potential were applied to introduce quantum effects to the statistical properties generated by the classical Lennard-Jones (LJ) potential. We found that an adsorption ratio of H2 to D2 on the SWNT bundle from the FH-GCMC simulations is less than 0.91 depending on adsorption pressure; this is because the potential field of the SWNT bundle for H2 is relatively weaker than that for D2 even at 77 K, due to the wide quantum spreading of a H2 molecule.

Adsorption Equilibrium of the Mixture CH4 + N2 + H2 on Activated Carbon

Adsorption equilibrium data of the mixture CH4 + N2 + H2 on activated carbon JX101 were collected with a dynamic method for the temperature range 283 K to 313 K. Since the critical temperature of the components is much lower than the temperature tested, this set of data was used to test the applicability of the available prediction models for the condition of above-critical temperatures. Some models rely on the adsorption data of pure components; therefore, the adsorption isotherms of pure CH4, N2, and H2 on the same carbon were also collected. Six prediction models were tested. They are the extended Langmuir (EL) model, the loading ratio correlation (LRC) model, the ideal adsorption solution theory (IAST), the Flory−Huggins vacancy solution model (FHVSM), the model based on micropore size distribution and the extended Langmuir equation (MPSD−EL), and the model based on micropore size distribution and the ideal adsorption solution theory (MPSD−IAST). The performance of a model was valued with the average ...

Comparison of DFT characterization methods based on N2, Ar, CO2, and H2 adsorption applied to carbons with various pore size distributions

Adsorption isotherms of N2, Ar, CO2, and H2 were measured on selected activated carbons representing various pore size distributions. Isotherm data were analyzed using the DFT method. A good agreement was obtained between N2 and Ar results in a wide range of pore sizes. Both N2 and Ar results agree with CO2 analysis at 273 K in the range of small micropores where CO2 analysis is most applicable. Also, in this range of pores the analysis of H2 adsorption data at 77 K gives results which are consistent with the results obtained from the more conventional vapor-phase adsorptives. However, the range of PSDs calculated from H2 data extends to pore sizes smaller than those that can be characterized by other adsorbates.

Multi-component Adsorption Characteristics of Hydrogen Isotopes on Synthetic Zeolite 5A-type at 77.4K

The pressure swing adsorption (PSA) method, operating rapid desorption, has become applied to medium-massive air-component separation. In this study, with the aim of applying the PSA method to a hydrogen isotope separation process, the multi-component adsorption characteristics of H2-D2 or H2-HD-D2 on a synthetic zeolite at the liquid nitrogen temperature 77.4 K are investigated by using a volumetric apparatus, where the system effects of thermal transpiration and diffusion are taken into account. The adsorption isotherms for pure H2 and D2 are shown in a wide pressure range between 10-2 and 105Pa. The isotherms are closely approximated by an error functional expression proposed here. The separation factors resulting in adsorption are measured in a wide range of amount adsorbed. The separation factors of D2/H2 are estimated in accordance with the ideal adsorbed solution theory using the adsorption isotherms for H2 and D2. The behavior of the estimated factors agrees with that of the experimental ones. The result suggests that values around 3 are expectable for the factors of D2/H2 at amounts adsorbed roughly between 0.01 and 1 mol/kg.

Adsorption and separation of hydrogen isotopes in carbon nanotubes: Multicomponent grand canonical Monte Carlo simulations

Adsorption isotherms of hydrogen isotopes from molecular simulations in carbon nanotubes and interstices are presented. The adsorption of pure isotopes follows Henry’s law up to moderate coverages. A modified path integral grand canonical Monte Carlo (PI-GCMC) technique for mixture adsorption is presented and applied to adsorption of isotope mixtures in carbon nanotubes. Adsorption isotherms of H2–T2 mixtures in nanotubes and interstices are determined at 20 and 77 K. Selectivities for T2 over H2 are calculated over a range of pressures. Selectivity in the nanotubes and interstices increases with pressure until the nanotube is saturated. Comparison of simulation results with predictions based on ideal adsorbed solution theory (IAST) shows good agreement up to moderate loadings. At higher loadings, selectivities determined from multicomponent simulations remain roughly constant, whereas IAST predicts continued increase in selectivities. Isotherms for H2–D2 and the selectivities of D2 over H2 are determined for adsorption in (10,10) nanotubes and in the interstitial channels of closed (10,10) nanotubes.

Adsorption of H2 and D2 on Carbon Nanotube Bundles

Adsorption isotherms of H2 and D2 deposited on single wall, closed end carbon nanotube bundles have been measured. Two temperature ranges were covered, a) T>77 K to study adsorption on high energy binding sites, and b) T 77 K. We observe two well defined steps in the isotherms that correspond to adsorption on at least two distinct locations, grooves/interstitials and the graphene surface. The calculated isosteric heats of adsorption, Qst, in the grooves/interstitials are approximately twice the value on the graphene.

Hydrogen adsorption on graphite and in carbon slit pores from path integral simulations

A method for computing adsorption isotherms for quantum fluids by combining grand canonical Monte Carlo with the path integral technique is presented. The method is used to compute the isosteric heat of adsorption for H2, HD, and D2 as a function of coverage for several different graphite-hydrogen potentials, and these results are compared with available experimental data. Adsorption isotherms for H2, HD, and D2 on graphite are computed and compared with experiment. The agreement between simulations and experiment is very good in most cases. Adsorption of para-hydrogen in graphite slit pores is studied by grand canonical path integral Monte Carlo. Comparison with classical hydrogen at the same absolute and reduced temperatures indicates that the quantum effects suppress capillary condensation for some values of the slit width.

Adsorption isotherms of He and H2 at liquid He temperatures

The adsorption isotherms of He and H2 at 4.2 K on stainless steel and Pyrex glass and of He on stainless steel at eight different temperatures between 1.9 and 4.2 K for pressures between 10−12 and 10−5 Torr have been measured. The measured data have been compared to different theoretical models describing adsorption isotherms. The Dubinin–Radushkevich–Kaganer (DRK) equation describes the measured He isotherms well for pressures down to about 10−9 Torr. The adsorption isotherms of He on stainless steel appear to follow the DRK equation as long as the energy to compress the gas from the measured equilibrium pressure to the saturated vapor pressure at the temperature of the measurement is smaller than the isosteric heat of adsorption. At pressures lower than about 10−9 Torr, the isotherms deviate from the DRK equation. The measurements show that the empirical constants in the DRK equation for He on stainless steel have similar values to He on Cu plated stainless steel while for He on the Pyrex glass surface the values are different. The isosteric heat of adsorption for He on stainless steel has been calculated from the measured isotherms and it decreases almost linearly from 8.5 meV/atom at a surface coverage of 1.8×1014 He atoms/cm2 down to 2.5 meV/atom at a surface coverage of 1.1×1015 He atoms/cm2. In the region where the DRK equation is not valid, the He isotherms can be described by a simple power law P=βSα, where P is the equilibrium pressure, S the surface coverage, α=1.63, and β varies with temperature.

Adsorption isotherms of H2 and mixtures of H2, CH4, CO, and CO2 on copper plated stainless steel at 4.2 K

Adsorption isotherms in the pressure range 10−11–10−6 Torr have been measured for H2 and mixtures of H2 and CH4, CO, and CO2 on copper plated stainless steel at 4.2 K. The measurements have been focused on the behavior of the isotherms at low surface coverage, up to a few monolayers of adsorbed gas on the cold surface. The isotherms were measured in a static situation with a small amount of gas injected for each point on the isotherm. Coadsorption measurements of H2 with CH4, CO, and CO2 as well as adsorption of H2 on condensates of CH4, CO, and CO2 have been made. A cryotrapping effect of H2 is seen when coadsorbing the gas mixtures, which is especially strong for the mixture of H2 and CO2. The measurements show that CO2 condensed at 4.2 K may have a porous structure that H2 can penetrate, while CO and CH4 have rather dense structures when adsorbed at 4.2 K. The measurements have been done within the framework of the Large Hadron Collider project [The Large Hadron Collider Accelerator Project, CERN/AC/95-05 (LHC), 20 October 1995] at CERN, Geneva, Switzerland, where superconducting magnets will be used to produce a magnetic field of about 9 T around the arcs of the 27 km long quasicircular accelerator. Synchrotron radiation produced by the circulating proton beam in the superconducting magnets will induce desorption of neutral gas molecules, mainly H2, CH4, CO, and CO2, from the inner surface of the vacuum chamber. The desorbed gas molecules will be physisorbed on the cold surfaces in the vacuum chamber and they may induce vacuum instabilities in the accelerator. Hence, it is of importance to have knowledge of the mixed adsorption isotherms of these gases at low temperatures.

Hydrogen adsorption on alkali metal surfaces: Wetting, prewetting and triple-point wetting

We report the results of a quartz microbalance study of the multilayer adsorption of H2 and D2 on sodium and on rubidium. On sodium H2 and D2 display the well known phenomenon of triple-point wetting. Interestingly low temperature adsorption isotherms of H2 on the evaporated Na surface show sharp layering transitions. On rubidium a weaker substrate H2 and D2 undergo a wetting transition in the liquid phase followed by sharp presetting transitions away from liquid-vapor coexistence. The prewetting phase diagram of H2 on Rb has been quantitatively mapped out.

Pretreatment effects and NO x decomposition on alumina supported Rh/MoO3 catalysts

Rh-MoO3/Al2O3, Rh/Al2O3 and MoO3/Al2O3 catalysts have been prepared and subjected to various pretreatments including high temperature reduction, high temperature oxidation and oxidation/ reduction cycles. After each series of treatments, surfaces were analysed by XPS and FTIR of adsorbed NO. The effectiveness of these surfaces in dissociating NO was studied by TPRS in the temperature range 300–773 K. Reduction rates of Mo oxides in H2 were determined gravimetrically while rhodium dispersion was determined from H2 adsorption isotherms at 298 K. Results indicate that molybdenum and rhodium exist in close contact in both oxidised and reduced forms. H2 chemisorption was suppressed following HTR of the catalyst due to coverage of rhodium by molybdenum oxide species but this could be reversed by HTO (773 K) followed by low temperature reduction. Although a small proportion of Mo could be reduced following HTR, cycles of HTR/HTO produced Rh/Mo oxide phases in which a proportion of Mo could be reduced to Mo° at 473 K. The presence of reduced Mo would appear to play an important role in the improved performance of Rh-MoO3/Al2O3 catalysts.