Neon and CO 2 Adsorption on Open Carbon Nanohorns Vaiva Krungleviciute, ‡ Carl A. Ziegler, ‡ Shree R. Banjara, ‡ Masako Yudasaka, † S. Iijima, † and Aldo D. Migone* ,‡ † Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, 305-8565, Japan ‡ Department of Physics, Southern Illinois University Carbondale, 1245 Lincoln Dr., Neckers 483A, Carbondale, Illinois 62901-4401, United States ABSTRACT: We present the results of a thermodynamics and kinetics study of the adsorption of neon and carbon dioxide on aggregates of chemically opened carbon nanohorns. Both the equilibrium adsorption characteristics, as well as the dependence of the kinetic behavior on sorbent loading, are different for these two adsorbates. For neon the adsorption isotherms display two steps before reaching the saturated vapor pressure, corresponding to adsorption on strong and on weak binding sites; the isosteric heat of adsorption is a decreasing function of sorbent loading (this quantity varies by about a factor of 2 on the range of loadings studied), and the speed of the adsorption kinetics increases with increasing loading. By contrast, for carbon dioxide there are no substeps in the adsorption isotherms; the isosteric heat is a nonmonotonic function of loading, the value of the isosteric heat never differs from the bulk heat of sublimation by more than 15%, and the kinetic behavior is opposite to that of neon, with equilibration times increasing for higher sorbent loadings. We explain the difference in the equilibrium properties observed for neon and carbon dioxide in terms of differences in the relative strengths of adsorbate−adsorbate to adsorbate− sorbent interaction for these species. ■ INTRODUCTION Studies of gas adsorption on new nanoporous sorbents are attracting considerable interest, 1−4 much of which is derived from the potential applications that adsorption on these materials have for solving current technological problems. 5 Adsorption provides a practical approach for achieving the separation of gaseous mixtures. 6 (This process has been suggested as a feasible alternative for addressing the pressing need for new and more efficient methods for selectively capturing CO 2 from a mixture of combustion byproduct gases. 7,8 ) Adsorption has also been suggested as a solution to the on-board storage of novel fuels: gas storage is a significant problem in the implementation of the use of alternative fuels (such as natural gas, methane, or hydrogen) in transportation applications. 9 While it is possible that an adsorption related solution (a combination of adsorption and other approaches in a single storage device, for example) may be found to satisfy the storage needs for transportation, it is unlikely that, given the enormous magnitude of the CO 2 sequestration needs, 10 any practical long-term storage solution for CO 2 can be found using industrially produced sorbents. An effective sorbent for use either in gas mixture separation applications or in the storage of nontraditional fuels for transportation has to possess a number of physical character- istics: adequate kinetics of adsorption and desorption for the gases of interest, appropriate binding energies, structural stability, etc. A condition that often remains implicit is that the binding energy of the targeted species to the sorbent has to be sufficiently large when compared to the energy of interaction between molecules of the target species (i.e., the target species has to be more attracted to the sorbent than it is to itself). In this study we report the results of an investigation of the adsorption characteristics of two simple molecular gases, Ne and CO 2 , on aggregates of opened dahlia-like carbon nanohorns (i.e., nanohorns that have been chemically treated make access to their interior spaces possible). We have explored the kinetics of adsorption on the nanohorn aggregates for these two gases, and we have determined the equilibrium thermodynamic properties of neon and of CO 2 on this sorbent. In previous studies we have reported on the equilibrium properties of Ne, 11 CF 4 , 11 and CO 2 12 adsorbed on the outside surfaces of aggregates of closed nanohorns (i.e., as-produced, untreated nanohorns). For neon and CF 4 on closed nanohorns we found that the strength of the gas−sorbent interaction on the strongest binding sites present on the aggregates was considerably higher than the gas−gas interaction (we compared the binding energy on these sites to the latent heat of condensation of the adsorbate species). 11 The binding energy on the strongest sites of the closed nanohorn aggregates is about a factor of 2 larger than the heat of condensation for neon and nearly 1.4 times larger than the heat of condensation for CF 4 . 11 The situation is quite different for CO 2 , where the Received: March 25, 2013 Revised: June 18, 2013 Published: June 26, 2013 Article pubs.acs.org/Langmuir © 2013 American Chemical Society 9388 dx.doi.org/10.1021/la401033u | Langmuir 2013, 29, 9388−9397