Effects of Temperature on Adsorption of Methanol on Graphitized Thermal Carbon Black: A Computer Simulation and Experimental Study

Abstract

We report a computer simulation and experimental study of adsorption of methanol on a highly graphitized thermal carbon black over a range of temperatures to investigate the adsorptive capacity, the isosteric heat, and the configuration of molecules in the adsorbed layer. We include coverage beyond the monolayer region, not previously studied in detail. The adsorption is shown to be strongly affected by hydrogen bonding which results in strong fluid–fluid interaction between the adsorbed molecules. The adsorption isotherms are initially sigmoidal, however, and in the multilayer region the isotherms follow typical type II behavior. The isosteric heat of adsorption of methanol is distinctly different from that of simple gases because of the dominance of the strong interactions between adsorbed molecules over the solid–fluid interactions. The configuration of adsorbed molecules in the submonolayer region favors the formation of 2D cyclic clusters of four or five methanol molecules, which maximizes the hydrogen bonding, while near the end of the completion of the first layer methanol molecules form molecular chains with some degree of 2D-zigzag pattern which again maximizes the hydrogen bonding between all molecules. This is in qualitative agreement with the X-ray diffraction measurements obtained by Morishige (Morishige, K.; Kawamura, K.; Kose, A. J. Chem. Phys. 1990, 93, 5267). When the second and higher layers are formed above the surface, the adsorbate forms clusters in three-dimensional configurations; however, the molecular chains with some degree of 2D-zigzag pattern in the first layer still remain.

Sorption of methanol, dimethyl carbonate, methyl acetate, and acetone vapors in CTA and PTMSP: General findings from the GAB Analysis

“…The isosteric heats of soption of the tested compounds in CTA showed maxima in the vicinity the of the unit relative surface coverage. Similarly, the isosteric heats of adsorption of a variety of sorbates on carbon black show maxima near the unit surface coverage, which was explained as a consequence of the formation of a dense adsorption monolayer on the adsorption centers of high affinity . This conclusion, though somewhat simplistic, is in a reasonable agreement with the later molecular simulations .…”

“…Similarly, the isosteric heats of adsorption of a variety of sorbates on carbon black show maxima near the unit surface coverage, which was explained as a consequence of the formation of a dense adsorption monolayer on the adsorption centers of high affinity . This conclusion, though somewhat simplistic, is in a reasonable agreement with the later molecular simulations . Interestingly, the isosteric heats of sorption in CTA increased from acetone through MeOAc to DMC, thus indicating that the energy of interaction among CTA and the sorbate increases with the increasing number of oxygen atoms in the sorbate molecules.…”

On the isosteric heat of adsorption of non-polar and polar fluids on highly graphitized carbon black

“…The maximum heat of 49 kJ/mol deserves elaboration: at this point (c) methanol molecules interact with (i) clusters on the edges of graphene layers, mainly due to the hydrogen bonding of the OH groups, and (ii) graphite surface mainly due to methyl group, resulting in an enhancement in the isosteric heat. For loadings in the range between points (d) and (e), the heat reaches a plateau of 43 kJ/mol, which is the sum of the FS interaction and fluid-fluid interaction at the boundary of the 2D-cluster on the basal plane [29]. The isosteric heat approaches the heat of liquefaction for second and higher layers (loadings greater than point f).…”

On the resolution of constant isosteric heat of propylene adsorption on graphite in the sub-monolayer coverage region

“…Our initial premise is that the argument of Kiselev and co-workers is plausible because we have tested their experimental data for many sorbates against computer simulation and found perfect agreement (argon [4], nitrogen [5], methane [6], ethane [7], ethylene [7], carbon tetrachloride [8], carbon dioxide [9], benzene [3], n-butane, n-pentane and n-hexane [10] , methanol [11,12], ethanol [12] and ammonia [13]). Therefore, the hypothesis is that the increase in isosteric heat from the increasing FF contribution, compensates exactly for the decrease in isosteric heat from the SF contribution, in the case of propylene adsorption.…”