Use of Carbon Fiber Composite Molecular Sieves for Air Separation

A novel adsorbent material, 'carbon fiber composite molecular sieve' (CFCMS), has been developed by the Oak Ridge National Laboratory. Its features include high surface area, large pore volume, and a rigid, permeable carbon structure that exhibits significant electrical conductivity. The unique combination of high adsorptive capacity, permeability, good mechanical properties, and electrical conductivity represents an enabling technology for the development of novel gas separation and purification systems. In this context, it is proposed that a fast-cycle air separation process that exploits a kinetic separation of oxygen and nitrogen should be possible using a CFCMS material coupled with electrical swing adsorption (ESA). The adsorption of O{sub 2}, N{sub 2}, and CO{sub 2} on activated carbon fibers was investigated using static and dynamic techniques. Molecular sieving effects in the activated carbon fiber were highlighted by the adsorption of CO{sub 2}, a more sensitive probe molecule for the presence of microporosity in adsorbents. The kinetic studies revealed that O2 was more rapidly adsorbed on the carbon fiber than N{sub 2}, and with higher uptake under equilibrium conditions, providing the fiber contained a high proportion of very narrow micropores. The work indicated that CFCMS is capable of separating O{sub 2} and N{sub 2} from air on the basis of the different diffusion rates of the two molecules in the micropore network of the activated carbon fibers comprising the composite material. In response to recent enquires from several potential users of CFCMS materials, attention has been given to the development of a viable continuous process for the commercial production of CFCMS material. As part of this effort, work was implemented on characterizing the performance of lignin-based activated carbon fiber, a potentially lower cost fiber than the pitch-based fibers used for CFCMS production to date. Similarly, to address engineering issues, measurements were made to characterize the pressure drop of CFCMS as a function of carbon fiber dimensions and monolith density

Interaction of Hydrogen with Graphitic Surfaces, Clean and Doped with Metal Clusters

Producción CientíficaHydrogen is viewed as a possible alternative to the fossil fuels in transportation. The technology of fuel-cell engines is fully developed, and the outstanding remaining problem is the storage of hydrogen in the vehicle. Porous materials, in which hydrogen is adsorbed on the pore walls, and in particular nanoporous carbons, have been investigated as potential onboard containers. Furthermore, metallic nanoparticles embedded in porous carbons catalyze the dissociation of hydrogen in the anode of the fuel cells. For these reasons the interaction of hydrogen with the surfaces of carbon materials is a topic of high technological interest. Computational modeling and the density functional formalism (DFT) are helping in the task of discovering the basic mechanisms of the interaction of hydrogen with clean and doped carbon surfaces. Planar and curved graphene provide good models for the walls of porous carbons. We first review work on the interaction of molecular and atomic hydrogen with graphene and graphene nanoribbons, and next we address the effects due to the presence of metal clusters on the surface because of the evidence of their role in enhancing hydrogen storage.Ministerio de Economía, Industria y Competitividad (Grant MAT2014-54378-R

Activated Carbons Derived from High-Temperature Pyrolysis of Lignocellulosic Biomass

Biomass pyrolysis to produce biofuel and hydrogen yields large amounts of charred byproducts with low commercial value. A study was conducted to evaluate their potential for being converted into higher value activated carbons by a low-cost process. Six chars derived from various lignocellulosic precursors were activated in CO2 at 800 °C to 30–35% weight loss, and their surface area and porosity were characterized by nitrogen adsorption at 77 K. It was found that, in similar activation conditions, the surface area of the activated carbons correlates with the activation energy of the oxidation reaction by CO2, which in turn varies inversely with the carbon yield after thermolysis in nitrogen at 1000 °C. Since lignin is the most thermally-stable component of lignocellulosic biomass, these results demonstrate, indirectly, that robust, lignin-rich vegetal precursors are to be preferred to produce higher quality activated carbons. The chars derived from white pine (pinus strobus) and chestnut oak (quercus prinus) were converted to activated carbons with the highest surface area (900–1100 m2/g) and largest mesopores volume (0.85–1.06 cm3/g). These activated carbons have properties similar to those of commercially-available activated carbons used successfully for removal of pollutants from aqueous solutions

Microstructure-Dependent Gas Adsorption: Accurate Predictions of Methane Uptake in Nanoporous Carbons

We present a framework for rapidly predicting gas adsorption properties based on van der Waals density functional calculations and thermodynamic modeling. Utilizing this model and experimentally determined pore size distributions, we are able to accurately predict uptakes in five activated carbon materials without empirical potentials or lengthy simulations. Our results demonstrate that materials with smaller pores and higher heats of adsorption can still have poor adsorption characteristics due to relatively low densities of highly adsorbent pores

Nitrogen adsorption data, FIB-SEM tomography and TEM micrographs of neutron-irradiated superfine grain graphite

This manuscript provides raw nitrogen gas adsorption data, images and videos obtained from a technique that combines Focused Ion Beam (FIB) and Scanning Electron Microscopy (SEM) known as FIB-SEM tomography and Transmission Electron Microscopy (TEM) micrographs. This collection of data is useful for characterization of the effects of high fluence neutron irradiation in nuclear graphite as described in the associated manuscript, “Mesopores development in superfine grain graphite neutron-irradiated at high fluence” (Contescu et al., 2019). Nitrogen adsorption isotherms at 77 K are provided for graphite samples before and after neutron irradiation at 300, 450, and 750 °C at fluences before and after turnaround. FIB-SEM tomography reveals porosity of unirradiated and irradiated samples. Using this technique, four data sets were obtained, of which the first three are presented in video format, whereas the fourth one is a series of images provided in raw format unique to this manuscript. All microscopy data document the microstructure, surface area and porosity of superfine grain graphite G347A (Tokai Carbon, Japan) before irradiation and irradiated after turnaround at 400 °C. TEM micrographs provide unique information on irradiation damage at high neutron fluence (>27. 8 displacements per atom, dpa) in the microstructure and crystal lattice of graphite. Additional TEM micrographs are provided here, which do not duplicate the research paper published elsewhere (Contescu et al., 2019). These data sets are unique, as samples at high irradiation doses have never been measured or imaged before with the aforementioned techniques