Yaping Zhou

A comparative study of hydrogen adsorption on superactivated carbon versus carbon nanotubes

Abstract Adsorption isotherms of hydrogen on activated carbon and multiwalled carbon nanotubes (MWNT) were collected using volumetric method for the range of 233– 298 K and pressures up to 11 Mpa . The same shape of isotherms revealed a common mechanism of adsorption. However, the amount of H 2 adsorbed on MWNT is 3–5 times less than on activated carbon, but the surface concentration of H 2 on MWNT is 4–6 times higher than on activated carbon. Temperature exerts much less effect on the adsorption of H 2 on MWNT than on the adsorption of H 2 on activated carbon. Isosteric heat of adsorption was evaluated from a set of isotherms basing on the Clausius–Clapeyron equation, and −1.7 kJ/mol was obtained for MWNT, −6.4 kJ/mol was obtained for activated carbon. Considering the much less quantity adsorbed, the much less heat of adsorption and the much smaller surface area, carbon nanotubes seem not to be a promising carrier of hydrogen for practical applications.

Fundamentals of high pressure adsorption

High-pressure adsorption attracts research interests following the worlds attention to alternative fuels, and it exerts essential effect on the study of hydrogen/methane storage and the development of novel materials addressing to the storage. However, theoretical puzzles in high-pressure adsorption hindered the progress of application studies. Therefore, the present paper addresses the major theoretical problems that challenged researchers: i.e., how to model the isotherms with maximum observed in high-pressure adsorption; what is the adsorption mechanism at high pressures; how do we determine the quantity of absolute adsorption based on experimental data. Ideology and methods to tackle these problems are elucidated, which lead to new insights into the nature of high-pressure adsorption and progress in application studies, for example, in modeling multicomponent adsorption, hydrogen storage, natural gas storage, and coalbed methane enrichment, was achieved.

Sorption equilibria of CO2/CH4 mixture on activated carbon in presence of water.

The sorption isotherms of CO2 + CH4 mixtures on an activated carbon were collected in the presence of water at a temperature suitable for hydrate formation. The equilibrium composition of both phases was determined. The initial concentration of CO2 in mixtures was set at 33, 38 and 42%, and the total pressure was up to 10 MPa. CO2 hydrates were firstly formed following the increase of total pressure, and CO2 dominates the sorbed phase composition. CO2 concentration in the sorbed phase begins to decrease when the partial pressure of methane allows for the formation of methane hydrates. Competition for hydrate cavities was observed between CO2 and CH4 as reflected in the isotherm shape and phase composition at equilibrium. The formation pressure of hydrates is lower for mixtures than for pure gases, and the highest sorption capacity of each gas decreased in the mixture sorption either.

Effect of Microstructure and Surface Modification on the Hydrogen Adsorption Capacity of Active Carbons

The effect of the microstructure of coconut-shell-based activated carbons, and their treatment with chemicals, on hydrogen adsorption capacity was studied. Active carbons from coconut shells using physical activation could be improved for hydrogen adsorption by treatment with HF or NH3•H2O. Treatment with HNO3 had no obvious effect and the treatment with H3PO4 lowered the hydrogen adsorption capacity considerably. All of the effects could be accounted for by changes in the specific surface area, pore size distribution, and surface chemistry of the active carbons. The specific surface area of carbons was the dominant factor for hydrogen adsorption no matter what kind of treatment was performed.

STORAGE OF HYDROGEN ON CARBON MATERIALS: EXPERIMENTS AND ANALYSES

ABSTRACT Superactivated carbon and carbon nanotubes are both considered potential hydrogen carriers. Adsorption isotherms of H2 on activated carbon AX-21 and multi-wall carbon nanotubes were collected with a volumetric method for the temperature range of 77, 233–298 K and pressures up to 7 or 10 MPa. Based on the experimental data for 233–298 K, the limiting heats of adsorption of 7.6 and 1.8 kJ/mol were obtained for activated carbon and carbon nanotubes, respectively. The absolute adsorption was determined with a recently presented method, and the adsorption behavior of H2 on carbon nanotubes was thus reasonably explained. A comparison was given for the storage capacities of compression alone and of filling powder or pellets of the two materials. It was concluded that adsorption of H2 on carbon nanotubes is too weak to enhance storage, but activated carbon enhances storage capacity considerably. The weight percentage of hydrogen stored in carbon powder reaches 10.8% at 77 K and 6 MPa, including the quantity compressed in the void space, and 4.1 kg H2 was stored in a 100-liter container filled with carbon pellets for the same condition.

A Research Note on the Adsorption of CO2 and N2

Abstract Experiments were made for the adsorption of CO 2 and N 2 on typical adsorbents to investigate the effects of porous structure and surface affinity of adsorbents as well as those of adsorption temperature and pressure that might cause the variation of adsorption mechanism. It is shown that polar surface tends to enlarge the adsorption difference between CO 2 and N 2 , and the difference is more sensitive to temperature than the adsorbents with non-polar surface. The adsorbents with non-polar surface are not much sensitive to the effect of water vapor, though the water vapor interferes the separation remarkably. The separation coefficient linearly increases with the micropore volume per unit surface area of activated carbons, but no rule is shown on mesoporous silicon materials. The function of adsorption mechanism on the separation is not as much as expected.

Intensified Reaction of Dilute Thiophenes in Nanoreactor

A discovery that catalytic reactivity is intensified on reducing the size of reactor toward several nanometers scale was applied for the separation of dilute species, for example, the thiophenic compounds in transportation fuels, in the present study. A characteristic reaction of the dilute species was selected as the separation mechanism, and a nanoreactor was formed with preloading reactant/catalyst in volumes of mesopore dimension. Probability for the dilute species to contact the nanoreactor radically enlarged due to the integration of such volumes in a porous material. Because all reagents and the reaction product stayed inside the nanoreactor, separation of fuel from sulfuric compounds and the surplus chemicals used in reaction becomes simple. It was experimentally shown that the nanoreaction exhibited first-order kinetics, and all thiophenes and benzothiophenes contained in different types of model fuels were completely removed at moderate conditions.

A Study of the Removal of Minor Amounts of H2S from Natural Gas by Activated Carbon

Studies have been made of the removal of minor amounts of H2S from natural gas by activated carbon. The surface alkalinity of the carbon had a considerable effect on the H2S capacity, although this was not permanent. Thus, when the carbon surface became neutral, the H2S content remained constant after the initial cycles of the purification/regeneration operation. The presence of water in natural gas was found to be critical for the use of activated carbon as a means of sweetening natural gas. However, although it enhanced the H2S capacity of the carbon, it made regeneration of the latter very difficult.

Storage of hydrogen–methane mixture in the complex sorbent of activated carbon–TiMn1.5 alloy

Abstract The possibility of storing the mixture of H2 and CH4 in a complex sorbent composed of superactivated carbon and TiMn1.5 alloy was experimentally studied. The storage capacity of the complex sorbent was shown nearly equal to the sum of the storage capacity of the component sorbents. Therefore, sorptive storage is technically feasible for a clean fuel mixed with natural gas and hydrogen. As a consequence, the storage pressure is considerably decreased comparing to that of compression technology.

Synthesis of Zeolite SSZ-13 for N2 and CO2 Separation

Zeolite SSZ-13 was synthesized by a traditional hydrothermal method, and characterized by powder X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The porous structure of the samples was determined from nitrogen uptake measurements at 77 K, and the surface properties were analyzed by Fourier transform infrared spectroscopy. The zeolite SSZ-13 had a type I isotherm with a uniform pore-size distribution and most pores were less than 1 nm. The Langmuir surface area was 703 m2/g. The adsorption of various gases on SSZ-13 was studied experimentally and a high selectivity for CO2 over N2 was achieved.