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Adsorption of Water in Activated Carbons: Effects of PoreBlocking and Connectivity

John K. Brennan,*,† Kendall T. Thomson,‡ and Keith E. Gubbins†

Department of Chemical Engineering, North Carolina State University,Raleigh, North Carolina 27695-7905, and School of Chemical Engineering, Purdue University,

West Lafayette, Indiana 47907

Received December 26, 2001. In Final Form: April 1, 2002

We present a simulation study of the adsorption mechanism of water in a realistic carbon structure.Water molecules are modeled using a recently developed fixed-point charge water model optimized to thevapor-liquid coexistence properties.1 Reverse Monte Carlo techniques2 are used to generate a realisticporous carbon model composed of graphitic microcrystals consisting of rigid basal plates. Arrangementsof the carbon plates are driven by a systematic refinement of simulated carbon-carbon radial distributionfunctions to match experimentally measured radial distribution functions. The adsorption of water inactivated (having oxygenated surface groups) and nonactivated (graphitic) carbon is investigated usingthe grand canonical Monte Carlo simulation method. The adsorption behavior is found to be stronglydependent on the presence of activated sites. No appreciable adsorption occurs in the graphitic carbon untilthe pressure approaches the bulk gas saturation pressure. Effects of the surface site density, surface siteposition, and the magnitude of the surface site electrostatic charge on the adsorption behavior areinvestigated. Water adsorption is found to increase as the activated site density increases. The presenceof water vapor in porous carbons is shown to dramatically affect the connectivity of the available porespace. Finally, cluster calculations are performed to investigate the growth mechanism of water clustersin activated carbons.

I. Introduction

Activated carbons are the most widely used industrialadsorbent for removing contaminants and pollutants fromgaseous, aqueous, and nonaqueous streams. They areinexpensive to manufacture and have uniquely powerfuladsorption properties. Depending on the starting materialand the activation process, a wide range of pore structurescan be generated which makes porous carbons applicableto a wide range of technologies. However, the presence ofwater in activated carbons can severely handicap anintended process, making the use of porous carbons formany processes unappealing. For example, quite lowhumidities in gases entering industrial adsorbers cansignificantly inhibit its capacity and selectivity duringthe removal of organic and inorganic contaminants (see,e.g., ref 3). Similarly, small amounts of water often reducethe diffusion rates of alkanes in microporous media by anorder of magnitude.4 The mechanism producing theseeffects is poorly understood.

The adsorption of water in porous carbons is signifi-cantly different from that of simple, nonassociating fluids,such as nitrogen, carbon dioxide, and hydrocarbons.5 Themain underlying differences stem from the strong water-water interactions, the weak water-carbon interactions,and the formation of hydrogen bonds with oxygenatedgroups on the carbon surface. Thus, previously drawn

conclusions about the adsorption of simple fluids in porouscarbons cannot be applied to predicting the behavior ofwater in these materials. Progress in understanding thisbehavior on a molecular level has been slow. Activatedcarbons are derived from a wide variety of precursors,and their amorphous pore structure has proved to beamong the most difficult to characterize. Experimentalcharacterization of these materials faces a number ofchallenges, such as determining (a) the distribution ofcarbon microcrystal sizes, (b) the densities and species ofsurface groups, (c) the topological nature of the connectedpore structure, and (d) pore size distributions.

Although there have been numerous experimentalstudies of water adsorption in activated carbons, thescientific community lacks consensus over the adsorptionmechanism and the shape of the resulting isotherms (fora recent review see ref 6). There is general agreementthat adsorption occurs initially (at low pressures) via theformation of three-dimensional water clusters which isthen followed by micropore filling behavior at highpressures. However, significant discrepancies exist in thedetails of pore filling and the role of the surface site typeand density. Little is known of the effect of these variableson the phase transitions of water confined in porouscarbons. Additionally, for the optimization of industrialprocesses, engineers require a universal description ofthe isotherms irrespective of the surface group type ordensities and also for the adsorption process over the entirerelative pressure (P/Po) range. Several predictive modelshave been proposed; unfortunately all have failed tosatisfactorily describe the isotherms over an adequatepressure range. Furthermore, experimental measure-ments are often made on poorly characterized carbonsthat make direct comparisons with theory or simulationresults inconclusive. Molecular simulation offers the

* Corresponding author.† North Carolina State University.‡ Purdue University.(1) Errington, J. R.; Panagiotopoulos, A. Z. J. Phys. Chem. B 1998,

102, 7470.(2) Thomson, K. T.; Gubbins, K. E. Langmuir 2000, 16, 5761.(3) Gong, R.; Keener, T. C. J. Air Waste Manage. Assoc. 1993, 43,

864.(4) Bourdin, V.; Grenier, P.; Malka-Edery, A. Proceedings of the 6th

Conference on Fundamentals of Adsorption; Elsevier: Paris, 1998.(5) Gregg, S. J.; Sing, K. S. Adsorption, Surface Area and Porosity;

Academic Press: New York, 1982.(6) Brennan, J. K.; Bandosz, T. J.; Thomson, K. T.; Gubbins, K. E.

Colloids Surf., A 2001, 187, 539.

5438 Langmuir 2002, 18, 5438-5447

10.1021/la0118560 CCC: $22.00 © 2002 American Chemical SocietyPublished on Web 06/13/2002

possibility of studying the characteristic parameters ofthese systems explicitly and systematically, but to daterelatively little simulation work has been undertaken.

The previous simulation studies of water in microporouscarbons have used the idealized slit-pore model (with theexception of the work by Segarra and Glandt7). Modelinggraphitic carbons (where all the oxygenated groups havebeen removed from the surface through heat treatmentor reduction) is relatively straightforward. Studies havebeen successfully carried out using both point-charge andoff-center square-well models of water in graphitic slitpores.8-13 The adsorption behavior is characterized bynegligible adsorption at low-to-moderate relative pres-sures followed by an onset of significant uptake that occurssuddenly at high relative pressures. Not surprisingly, suchtype V behavior can be described with simple Lennard-Jones (LJ) potentials,14 since the behavior merely requiresa fluid-fluid interaction that is much more attractive thanthe fluid-solid interaction.

Molecular simulation studies of activated carbons inwhich a significant density of oxygenated sites are presenton the carbon surfaces are considerably more difficult tomodel. Segarra and Glandt7 modeled the structure of theactivated carbon using randomly orientated platelets ofgraphite while mimicking the activation by a dipoledistributed uniformly over the edges of the platelets.However, more recent work suggests that the surface sitemodel is not sufficiently inhomogeneous to predict thecorrect trends of low-pressure adsorption data and theheats of adsorption.15 Maddox et al.,10 Muller et al.,11 andMcCallum et al.13 have used slit-pore models that containembedded surface sites to represent the activated carbon.Studies carried out by Muller et al.11 and McCallum etal.13 found the adsorption behavior to depend strongly onthe geometric arrangement of the active sites on the carbonsurface. Cooperative adsorption phenomena were foundto be critical, with water clusters forming readily fromsurfaces where two or more sites were positioned at aseparation distance suitable for both water-site andwater-water bonding. The adsorption behavior was foundto be dependent on the number of sites present on thesurface. Even a moderate density of active sites added tothe carbon surface causes the adsorption behavior tochange qualitatively. Moreover, the uptake of waterbecomes appreciable at low pressures and subsequent porefilling occurs at much lower pressures.

Reasonable agreement with experiment is obtained withthe model of McCallum et al.13 despite uncertainties inthe morphology of the carbon (which had a wide pore-sizedistribution). However, the chief deficiency of the modelis the oversimplified treatment of the carbon structure,which was represented as a distribution of unconnectedslit pores having a range of pore widths. Such an idealizedtreatment of the carbon structure has several drawbacks.First, the slit-pore model does not allow for effects due totheedgesof thecarbonmicrocrystalswhere theoxygenatedsurface sites are most likely seated. In addition, the model

cannot account for the finite size of these crystals orirregular ordering of neighboring crystals. Furthermore,since the slit-pore model represents a single pore, effectsof pore connectivity and topology cannot be studied. Inaddition, the intermolecular potentials used by McCallumand co-workers for both the water and water-siteinteractions neglected long-range interactions and po-larizability effects. The work presented in this paperaddresses some of the deficiencies of previous simulationwork by considering a long-ranged water potential modeland adsorption in a realistic carbon model.

The remaining portions of this paper are organized asfollows. Section II provides an outline of the methodologyused to generate the realistic porous carbon models.Section III describes the intermolecular potentials of thewater molecule and surface site groups, while details ofthe simulation method can be found in section IV. Resultsof Monte Carlo simulations of water adsorbed in bothgraphitic and activated carbons are presented in sectionV, followed by a discussion of our findings in section VI.

II. Molecular Models of Porous Carbons

The production of activated carbons involves twoprocesses: (1) carbonization, i.e., thermal decompositionof raw organic material (e.g., wood, peat, coal, lignite,synthetic polymers, coconut shell) in an inert atmosphere;and (2) activation to enhance porosity and surface area,usually by heating the material in the presence of anoxidizing gas stream such as air, steam, or CO2.5,16 Carbonsmanufactured through such processes are composed ofgraphite microcrystals having a range of sizes, some ofwhich are connected by covalent bonds. Inorganic atoms(typically O, N, S) are present at the edges of some of themicrocrystals in the form of chemical groups.

Since the precursors and techniques used to preparecarbons vary widely and the reactions involved areunknown, it is difficult to develop simulation protocolsthat mimic the manufacturing process. Alternatively, thereverse Monte Carlo (RMC) technique has been appliedto this problem.2 The starting configuration is a systemcontaining graphite basal plates having a range of sizes(the mean size is typically about 2 nm) that is based onexperimental evidence. The graphite microcrystals aretreated in atomic detail, and intermolecular parametersdeveloped by Steele17 are used. The microcrystals areplaced with random positions and orientations in a periodicsimulation cell, with a carbon density chosen to matchthe experimental material. Monte Carlo-type moves ofthe position and orientation of the plates are performed.It is possible to include both changes in the size of thebasal plates and cross-linking (bonding) between basalplates during this procedure. The acceptance criteria forthe moves in the RMC method are designed to adjust thestructure until the carbon-carbon structure factor or theradial distribution function matches a target experimentalstructure factor obtained from small-angle neutron scat-tering (SANS) or small-angle X-ray scattering (SAXS)techniques. The RMC method has previously been dem-onstrated to converge to the target radial distributionfunction for a variety of carbons. These model structureshave been shown to have carbon densities, porosities,surface areas, and surface energy distributions that closelymatch those of the targeted materials. The resulting poremorphologies of the carbon models generated in this work

(7) Segarra, E. I.; Glandt, E. D. Chem. Eng. Sci. 1994, 49, 2953.(8) Ulberg, D. E.; Gubbins, K. E. Mol. Simul. 1994, 13, 205.(9) Ulberg, D. E.; Gubbins, K. E. Mol. Phys. 1995, 84, 1139.(10) Maddox, M.; Ulberg, D. E.; Gubbins, K. E. Fluid Phase Equilib.

1995, 104, 145.(11) Muller, E. A., Rull, L. F.; Vega, L. F.; Gubbins, K. E. J. Phys.

Chem. 1996, 100, 1189.(12) Bandosz, T. J., Blas, F. J.; Gubbins, K. E.; McCallum, C. L.;

McGrother, S. C.; Sowers, S. L.; Vega, L. F. Proc. Mater. Res. Soc. 1998,497, 231.

(13) McCallum, C. L.; Bandosz, T. J.; McGrother, S. C.; Muller, E.A.; Gubbins, K. E. Langmuir 1999, 15, 533.

(14) Balbuena, P. B.; Gubbins, K. E. Langmuir 1993, 9, 1801.(15) Gordon, P. A.; Glandt, E. D. Langmuir 1997, 13, 4659.

(16) Rouquerol, F.; Rouquerol, J.; Sing, K. S. Adsorption by Powdersand Porous Solids; Academic Press: London, 1999.

(17) Steele, W. A., The Interaction of Gases with Solid Surfaces;Pergamon Press: Oxford, 1974.

Adsorption of Water in Activated Carbons Langmuir, Vol. 18, No. 14, 2002 5439

are quite slitlike, but highly connected. Further details ofthe RMC method can be found in the original paper.2 TheRMC models used in this work were generated from anactivated mesocarbon microbead sample (a-MCMB) ob-tained from the Osaka Gas Company (Osaka, Japan). Thecubic simulation cell of the carbon structure for most ofthe work presented here measured 10.0 nm on a side andcontained over 48 000 carbon atoms. In this work, thecarbon atoms making up the microcrystals were modeledas LJ spheres, thus ignoring any electrostatic behavior.The carbon-carbon potential parameters were taken froma standard source as εcc/kB ) 28 K and σcc ) 0.340 nm.17

Having prepared model carbons that have realistictopologies, oxygenated groups must be placed on themicrocrystals. A variety of oxygenated groups (e.g.,phenolic, lactonic, carboxylic, hydroxyl, carbonyl) areknown to be present on the surface. The relative andabsolute amounts of the groups depend on the carbonprecursors and the activating process. Typically, sitedensities range from 0 (for graphitic carbons) up to about2.65 sites/nm2. X-ray diffraction patterns indicate thatthese groups are located randomly around the edges ofthe microcrystals.18 Details of the models used in this workto represent these surface sites are given below.

III. Intermolecular Potentials for Water andSurface Sites

Water molecules are modeled using a recently developedfixed point-charge model optimized using the vapor-liquidcoexistence properties.1 The model is superior to otherfixed point-charge models in its prediction of the vapor-liquid-phase envelope. The water model consists of aBuckingham exponential-6 potential located at the oxygencenter plus point charges. The Buckingham potential isgiven by the expression

where the parameters ε, rm, and R are characteristic ofthe Buckingham exponential-6 potential. The parameterrm is the radial distance at which the potential is aminimum. The radial distance for which Uexp-6(r) ) 0(conventionally denoted by σ) can be computed from eq 1(e.g., when R ) 12, σ ) 0.876100rm). The cutoff distancermax is the smallest positive value for which dUexp-6(r)/dr ) 0 and is obtained by iterative solution of eq 1.Parameter values for the Errington and Panagiotopouloswater model are ε/kB ) 159.78 K, rm ) 0.3647 nm, R ) 12,and rmax ) 0.175 nm. The water model also includes a setof three point charges. A point charge of magnitude -2q(where q ) 0.3687e) is at the oxygen center, while twopoint charges (+q each) are placed a distance of 0.10668nm from the oxygen center. The angle formed by thecharges is fixed at 109.5°.

A variety of chemical and physical techniques are usedto identify the species and surface density of the functionalgroups on the carbon microcrystal. Frequently, however,different methods lead to quite disparate results, pre-sumably due to the complexity of the carbon structure.Recent experiments19 and simulation studies20 suggest

that, on average, the interaction appears to be roughlyindependent of the type of active surface group in whichthe oxygen is incorporated. Therefore, mixtures of surfacegroups have usually been represented as an effective singlegroup determined from a suitably weighted mean of thetotal site-water interactions.11,12,20 Although idealized,the use of an effective site can simplify the analysis of theadsorption mechanism. In this work, a single surface sitegroup was used and was modeled as a carbonyl group(CdO). The oxygen atom was modeled as a single LJ spherewith a point charge fixed at its center. The LJ energy andsize parameters were taken from the OPLS format (see,e.g., ref 21), ε/kB ) 105.76 K and σ ) 0.296 nm, respectively.The CdO bond length was taken as 0.1214 nm. Finally,the point charge at the oxygen center was taken as -0.37e,which was derived from a quantum chemical electrostaticpotential.22 To keep the overall carbon structure electri-cally neutral, a charge of the same magnitude but oppositesign was placed on the carbon atom.

Having chosen a reasonable potential model for thesurface group, sites were placed in the carbon structureusing the following procedure. A carbon atom located onthe edge of a microcrystal was randomly chosen. A surfacegroup was placed 0.1214 nm from this carbon atom, withthe CdO axis parallel to the basal plate. If the distancebetween the surface site and any carbon atom fromadjacent microcrystals was less than (σ)O + σC)/2, thenthis position was rejected and a new placement wasattempted. A range of site densities was considered inthis study: 0.25, 0.65, 1.25, 2.0, and 2.5 sites/nm2. For a125 nm3 simulation box, this range of site densitiescorresponds to 21 surface site groups for n ) 0.25 sites/nm2 and 210 surface site groups for n ) 2.5 sites/nm2.

Water-substrate interactions are of two kinds, thosewith the carbon atoms making up the microcrystals andthose with the oxygenated surface groups. However, anunusual difficulty arises when determining unlike-pairinteraction parameters for the Errington and Panagioto-poulos water model. The Buckingham exp-6 water modelis a three-parameter potential (ε, rm, and R), while theLennard-Jones potential is a two-parameter potential (εand σ). It is apparent then that estimates of the unlikepair interactions cannot be made directly. However, ithas been shown recently that for simple fluids, theparameters of a Buckingham exp-6 potential are trans-ferable to a Lennard-Jones potential by a judicious choiceof the repulsion parameter, R (without changing theoriginal ε and rm values).23 The Lennard-Jones potentialis, with this choice

where ε/kB ) 159.78 K, σ ) 0.3226 nm, and R ) 10.8. Thewater-substrate interactions were then calculated byusing the Lorentz-Berthelot combining rules to determineεws and σws, while Rws ) (RwRs)1/2, where w ) water ands ) a carbon atom or a surface site.

IV. Simulation Details

The adsorption behavior of water in porous carbons wasdetermined by using the grand canonical ensemble MonteCarlo (GCMC) simulation method. Biased insertion and

(18) Bansal, R. C.; Donnet, J. Carbon Black; Donnet, J., Bansal, R.C., Wang, M., Eds.; Marcel Dekker: New York, 1993.

(19) Barton, S. S.; Evans, M. J. B.; MacDonald, J. Langmuir 1994,10, 4250.

(20) Muller, E. A.; Hung, F. R.; Gubbins, K. E. Langmuir 2000, 16,5418.

(21) Jorgensen, W. L.; Swenson, J. J. Am. Chem. Soc. 1985, 107, 569.(22) Sigfridsson, E.; Ryde, U. J. Comput. Chem. 1998, 19, 377.(23) Panagiotopoulos, A. Z. J. Chem. Phys. 2000, 112, 7132.

Uexp-6(r) )

[∞ r < rmax

ε

1 - 6/R[6R

exp(R[1 - rrm]) - (rm

r )6] r > rmax(1)

ULJ(r) ) ε[ 6R - 6(rm

r )R

- RR - 6(rm

r )6] (2)

5440 Langmuir, Vol. 18, No. 14, 2002 Brennan et al.

deletion sampling techniques were used to reach equili-brated states, as well as for statistically reliable finalaverages.24 For most of the work reported here, the cubicsimulation cell length was 10.0 nm (porosity ) 0.485) witha small portion of the work carried out in a simulation cellof length 5.0 nm (porosity ) 0.579). The number of watermolecules adsorbed in the porous carbon ranged from 0to 6000 molecules; the number of carbon atoms in the10.0 and 5.0 nm cells was 48 012 and 7150, respectively.A spherical cutoff of 3.9σw was used without applying long-range corrections. Ewald summations were employed toaccount for the long-range electrostatic interactions.24 Avirial equation of state was used to convert activity valuesimposed in the GCMC simulations into pressure values.11

Simulations were equilibrated for (5-10) × 106 GCMCmoves followed by a production run of (2-5) × 107 moves.A GCMC move consisted of one of the following: thetranslation and rotation of a water molecule; the insertionof a water molecule from the bulk phase into the porouscarbon; or the deletion of a water molecule from the porouscarbon. Block averages of these moves were used todetermine uncertainties of the overall averages.

V. Simulations of Water Adsorbed in PorousCarbons

Graphitic Carbon. The adsorption of water in non-activated (graphitic) carbons was simulated at a tem-perature of 300 K. The adsorption isotherms for twographitic carbon models at different porosities (0.579 and0.485) are shown in Figure 1. Negligible adsorption occursin the carbon at low pressures, and not until the bulkpressure is near the bulk saturation point does significantuptake occur. This behavior is a consequence of the weakcarbon-water interaction relative to the strong water-water interaction. The adsorption isotherms agree quali-tatively with experimental data for many graphiticcarbons,5,16 although experimentally the onset of signifi-cant water uptake and capillary condensation may occurat lower pressures than found in the simulations. Thedifference is attributed to structural heterogeneity (e.g.,

nonaromatic rings of carbon atoms or heteroatoms thatcause cross-linking of carbon plates) in actual carbonsthat is not present in the RMC-generated models. Suchheterogeneity provides adsorption sites for water mol-ecules. As expected, the amount of water adsorbed nearP/Po ) 1 increases with increasing porosity because thereis more void space available for water molecules to occupy.A snapshot of water adsorbed in the graphitic carbon modelat P/Po ) 0.85 is shown in Figure 2. Note how the watermolecules have uniformly filled the available pore space.Although the water-carbon interactions are weak, thehigh bulk phase pressure induces the pore filling behaviorillustrated in Figure 2.

Activated Carbon. The adsorption behavior of wateradsorbed in an activated microporous carbon model(porosity ) 0.485) is shown in Figure 3 for surface sitedensities, n ) 0.25, 0.65, 1.25, 2.0, and 2.5 sites/nm2. Alsoshown is the adsorption of water in the microporous carbonmodel without activated sites. In contrast to the adsorptionof water in the graphitic carbon, significant uptake occurseven at low pressures when activated sites are present onthe edges of the carbon plates. As the surface site densityincreases, the amount of water adsorbed increases. Theadsorption mechanism for this trend can be described asthe following. At low pressures, uptake of water occursvia the bonding of isolated water molecules to theoxygenated surface sites only, i.e., adsorption of waternear a carbon surface without a surface site does not occur.As the bulk phase pressure is increased, further adsorptionoccurs either to a surface site directly or to other watermolecules already adsorbed at a surface site. For the n )0.25, 0.65, and 1.25 sites/nm2 isotherms, when all surfacesites are enclosed by a cluster of water molecules littlefurther uptake occurs as the pressure increases until nearthe bulk saturation point. This is attributed to the surfacesites (with their resident water clusters) being located farfrom each other so that the bridging of these water clusterscannot occur at low to moderate pressures. However, at

(24) Frenkel, D.; Smit, B. Understanding Molecular Simulation;Academic Press: New York, 1996.

Figure 1. Adsorption isotherms (T ) 300 K) for water ingraphitic carbon for two carbon models at different porosities,0.579 (9) and 0.485 (b). P/Po is the relative pressure, where Pois the bulk gas saturation pressure at 300 K.

Figure 2. Snapshot of water adsorbed in graphitic carbon(porosity ) 0.485) at P/Po ) 0.85 and T ) 300 K. Blue spheresrepresent oxygen atoms while yellow spheres represent hy-drogen atoms. The size of the carbon atoms (represented asbrown spheres) has been reduced for visual clarity.


higher site densities (2.0 and 2.5 sites/nm2), continuousuptake occurs as the pressure increases (as evidenced bythe less dramatic rise in the isotherms). This behavior isattributed to water molecules seeking locations where theycan bond either to several preadsorbed water moleculesor to a water molecule and a surface site simultaneously.Note the significant uptake of water at low pressures forsite densities 2.0 and 2.5 sites/nm2. Porous carbonmaterials used in a separation or purification processcontaining site densities of this magnitude would suffersignificant pore blocking even in low-humidity gas streams.

Snapshots of molecular configurations of water adsorbedin an activated carbon model (n ) 0.65 sites/nm2) at

relative pressures P/Po ) 0.05 and 0.2 are shown in Figure4. At P/Po ) 0.05, water molecules have begun to adsorbbut only at the surface sites. At P/Po ) 0.2, furtheradsorption occurs around water molecules already ad-sorbed at the surface sites. Several of these clusters haveagglomerated, resulting in a three-dimensional networkof water molecules that spans the entire pore structure.

Desorption isotherms were determined for site densities0.25, 0.65, and 2.5 sites/nm2 and are given in Figure 5,together with the adsorption curves. The adsorption-desorption hysteresis loop for all site densities is quitenarrow and appears to vanish at high site densities.Notably, the hysteresis loop is negligible for n ) 2.5 sites/nm2 but is most pronounced for adsorption in the graphiticcarbon models (not shown). Lack of significant hysteresissuggests a pore filling mechanism rather than capillarycondensation. Snapshots given in Figure 4 suggest thatpore filling occurs by a cluster growth mechanism. Furtheranalysis of the pore filling mechanism is given below.

The effect of the placement of the surface sites on theadsorption of water was investigated. Several differentrandom arrangements for surface site densities n ) 0.25and 0.65 sites/nm2 were considered. The resulting iso-therms were the same within statistical uncertainty forthe different surface site arrangements, and the overalladsorption behavior was found to be indistinguishablefor the different placements. However, this study wascarried out using a single realization of the porous carbonmodel. Performing a similar study using multiple realiza-tions of the carbon model may provide further insightinto this effect. Moreover, the relative number of surfacesites to carbon atoms in the structure is small; thus thiseffect may not be discernible at such low surface sitedensities. For example, when n ) 0.65 sites/nm2 in a 125nm3 simulation box, there are 55 surface sites per 7150carbon atoms.

At this point, it is useful to compare the adsorptionbehavior of a nonassociating fluid, such as methane, withthe behavior of water in our carbon models. For this study,

Figure 3. Adsorption isotherms for water adsorbed in porouscarbon at site densities: 0.25 (4), 0.65 (9), 1.25 (0), 2.0 ([), and2.5 (O) sites/nm2 at 300 K. Also shown is the adsorption isothermfor the carbon model with no activated sites (b). The porosityof the carbon is 0.485. P/Po is the relative pressure.

Figure 4. Molecular configuration snapshots of water adsorbed in activated carbon at T ) 300 K for a site density of 0.65 sites/nm2.Snapshots are shown at two different pressures: (a) P/Po ) 0.05 and (b) P/Po ) 0.2. Green spheres represent oxygen surface groups.For visual clarity, the front right quarter of the simulation box has been removed. The orientation of the carbon sample is the samefor both snapshots.


methane was modeled as a single-site LJ particle withε/kB ) 148.1 K and σ ) 0.381 nm, where electrostaticbehavior was neglected. A comparison of the adsorptionbehavior of methane and water in an activated carbonwith a site density of 0.65 sites/nm2 is given in Figure 6.The adsorption of the two fluids was simulated atapproximately the same reduced temperature, one-halfof their respective critical temperature. Dramatic differ-ences in the adsorption behavior of the two fluids are seenin Figure 6. Due to the much stronger carbon-methaneinteractions, methane adsorbs in the porous carbon evenat very low pressures. Furthermore, the shape of themethane isotherm corresponds to monolayer formationfollowed by pore filling as the pressure is increased. Notethat the final uptake of water is much larger than thefinal uptake of methane, due to the smaller-sized watermolecule as well as to hydrogen bonding that causesoverlap of the water molecules. Also note that a capillarycondensation transition does not occur for methane evenat conditions well above the bulk saturation point

(P/Po > 1). This behavior can be interpreted by consideringthe molecular configuration snapshot of methane in theactivated carbon shown in Figure 7a. The pore widththroughout the carbon is only 2 or 3 methane moleculardiameters (average pore width = 0.9 nm (see Figure 9),where σmethane ) 0.381 nm). Therefore after monolayers ofmethane molecules are formed on the surface of the basalplates, there is little pore space remaining for a capillarycondensation transition to occur. The differences in theadsorption behavior of an associating fluid and a nonas-sociating fluid are clearly evident in Figure 7. Methanemolecules prefer to form layers on the carbon surface,while water molecules preferentially form a three-dimensional network of clusters.

The dependence of water adsorption on the magnitudeof the surface site electrostatic potential was also deter-mined. The original surface site charge (q ) -0.37e) wasreduced to q ) -0.185e and adsorption isotherms forseveral different site densities were determined. Asexpected, the qualitative behavior and shape of theresulting isotherm remained the same. However, thequantitative behavior was considerably changed as seenin Figure 8, for the site density n ) 0.65 sites/nm2. Whenthe surface site charge is -0.185e, the water moleculesare less attracted to the surface sites, and thus higherpressures are required to adsorb the same uptake as whenq ) -0.37e. As the site density was increased, thequantitative dependency of the adsorption behavior onthe surface site attraction was found to increase. However,the overall adsorption mechanism and qualitative be-havior did not change.

Next, we quantified the effect of the presence of wateron the size of the available pore space. Pore size distribu-tion calculations were performed using a standard geo-metric-based technique2 for the empty carbon structureas well as for the carbon with different amounts of wateradsorption. The resulting pore size distribution calcula-tions for site densities of 0.25 and 0.65 sites/nm2 are shownin Figure 9. For both cases, when small amounts of waterare present (P/Po ) 0.07) the available pore space isrelatively unchanged. However, when moderate amountsof water are adsorbed in the carbon (P/Po ) 0.26), theavailable pore space is considerably reduced. This isevidenced by the shift to smaller pore sizes as well as thenarrower distribution, which indicates that only small-sized pores are accessible when moderate amounts of waterare adsorbed. As seen from Figure 9, the average poresize is reduced by a factor of 2 when P/Po ) 0.26. Thisoverall behavior is intensified as the site density isincreased.

Next, we examine the effect of the presence of water onthe connectivity of the available pore space. It has beenpostulated that water clusters formed at or across theopening of a pore will effectively block the entire porespace. Such pore blocking could significantly reduce theadsorption capacity and diffusion rates of organic mol-ecules in the carbon. We have investigated this proposedpore-blocking mechanism by the following scheme. Latticecoordinates were superimposed onto the carbon sample,and a test particle (chosen to represent a methanemolecule) was placed at each of the lattice sites. If thedistance between a test particle and a carbon atom wasgreater than or equal to (σmethane + σcarbon)/2, then the latticesite was considered to be in the available pore space. Next,a probing molecule was systematically moved throughoutthis available pore space, and the volume of the spacethat was physically continuous and connected was de-termined. The results of these calculations are shown inFigure 10 for different amounts of water present in the

Figure 5. Desorption isotherms for site densities of 0.25 (2),0.65 (0), and 2.5 (b) sites/nm2. The corresponding adsorptionisotherms are the same as shown in Figure 3.

Figure 6. Comparison of the adsorption behavior of methane([) and water (9) in an activated carbon with site density of0.65 sites/nm2. The two fluids are each simulated at atemperature of approximately one-half of their respectivecritical temperature. The amount adsorbed is plotted as afunction of ê/êo, where ê is the activity of the adsorbed fluid andêo is the activity of the bulk gas at saturation.


activated carbon. Carbon atoms, surface sites, and watermolecules have been removed for visual clarity, while theavailable pore space has been filled. In Figure 10a, nowater is present and the entire available pore space isphysically continuous and connected as indicated by thelone pink color. However when a trace amount of wateris present (Figure 10b), the total available pore volumeis reduced as expected. More importantly though, the porespace is no longer physically continuous. Figure 10b showsthat the available pore space is now separated into threeregions (colored pink, green, and blue). Therefore, it isimpossible for a diffusing gas molecule to access the pinkregion if it initially resides in either the green region orthe blue region. Furthermore, although the pore spacescolored green and blue are empty, these spaces areinaccessible from the bulk fluid, as well as from the pinkregion. As the amount of water in the carbon increases

further (Figure 10c), the overall available pore space isdramatically reduced and is further separated into foursmall regions (colored pink, green, blue, and yellow). Nowonly the pink region is accessible to molecules from thebulk gas. This behavior would render the carbon virtuallyuseless as an adsorbent in an industrial process.

The growth mechanism of the three-dimensional net-work of water clusters that are formed in the porouscarbons was also investigated. The size of water clustersand the frequency of cluster sizes were calculated usingthe cluster multiple labeling scheme of Hoshen andKopelman.25 At low temperatures a hydrogen bond isformed when two water molecules are approximately 0.94molecular diameters apart.26 Therefore, a water moleculewas designated as part of a particular cluster if themolecule was within a distance of (0.94σwater) ) 0.3003nm from another water molecule in the cluster. The growthmechanism of water clusters in porous carbons is il-lustrated in Figure 11 for increasing amounts of water.The amount of water present is quantified by the reducedwater density, Fw* ) Nwσw*/V. For the smallest amountof water present in the carbon (Fw* ) 0.034), small waterclusters having a range of sizes reside in the pore space.These clusters are distributed throughout the pore spaceand will have a minimal effect on a given process, e.g., theseparation or purification of a gas mixture. When largeramounts of water are present (Fw* ) 0.067), again a rangeof small-sized water clusters can be found, but now a single,large cluster (comprised of ca. 100 water molecules) isalso present. As the amount of water is increased further(Fw* ) 0.2), two very large clusters exist. The presence ofthese large clusters will have a large impact on theadsorption capacities and diffusion rates for a givenprocess. The growth mechanism of these water clustersoccurs by several small (but proximal) clusters growinguntil they are large enough to bridge togethersformingsingle, large clusters within the porous carbon.

(25) Hoshen, J.; Kopelman, R. Phys. Rev. B 1976, 14, 3438.(26) Postorino, P.; Tromp, R. H.; Ricci, M.-A.; Soper, A. K.; Neilson,

G. W. J. Chem. Phys. 1994, 101, 4123.

Figure 7. Molecular configuration snapshots of (a) methane and (b) water in an activated carbon (n ) 0.65 sites/nm2). The fluidsare at similar conditions of one-half their critical temperature and about 0.3 P/Po. The orientation of the carbon sample is the samefor both snapshots.

Figure 8. Effects of the magnitude of the surface siteelectrostatic charge on the adsorption behavior of water in anactivated carbon. Site density is 0.65 sites/nm2 and T ) 300 K.


VI. DiscussionWe have studied the adsorption behavior of water in

porous activated carbons using molecular simulation. Therealistic morphologies and topologies of the carbons thatwere generated allowed phenomena such as the con-nectivity of the pore space and pore blocking to be studied.Qualitative agreement with the observed adsorptionbehavior in experimental porous carbons was found. Theadsorption was found to be strongly dependent on thedensity of the oxygenated surface sites in the carbons.For low site densities, negligible adsorption occurs untilthe pressure approaches the bulk saturation point,whereas significant uptake occurs at even low pressuresfor moderate and high site densities. We have determinedthe growth mechanism of the water clusters that readilyform in porous carbons. Further investigation of thebehavior of these clusters revealed that small clusters

form at the surface sites at low P/Po, but at higherpressures several proximal clusters can bridge togetherto form a single large clustersbehavior detrimental toseparation and purification processes. We demonstratedthat if these clusters occur at the openings of pores, theycan effectively block the entire pore. These phenomenaare believed to be responsible for the observed reductionsin adsorption capacity and diffusion rates for adsorbateswhen water is present in the bulk gas stream. Previousstudies based on idealized slit pore models were not ableto examine these effects, due to the omission of poreconnectivity in the model.

Further progress in this area, and quantitative com-parison with experiment, will require a coordinatedexperimental and molecular simulation study for a seriesof well-characterized carbons that are representative ofthose used in research studies and industrial applica-

Figure 9. Effect of the presence of water in an activated carbon on the pore size distribution. Different amounts of water presentcorrespond to the P/Po values taken from the isotherms of Figure 3 for site densities: (a) 0.25 sites/nm2; (b) 0.65 sites/nm2. Ordinateunits are in cubic centimeters of micropore volume per gram of carbon per angstrom.


tions.27 On the experimental side, well-characterizedcarbons with a narrow distribution of pore size would bemost appropriate for initial studies. In addition to ad-sorption of water for a range of temperatures, heats ofadsorption should be measured, including the isostericheat at zero coverage. The latter provides an importantsource of information on the water-carbon and water-site interactions. Characterization methods should includeboth wide and small-angle X-ray or neutron scattering(preferably both), to be used with the reverse Monte Carlomethod to determine carbon model structures; elementalanalysis; and surface chemistry analysis to determine thespecies as surface density of the various surface groups.Methods of surface chemistry characterization for carbonshave been reviewed recently.27 On the molecular simula-

tion side, further improvements to the structural modelsof the carbons, and also to the intermolecular potentialmodels, are likely to be needed. The porous carbon modelsgenerated for this study are reasonable representationsof carbons such as activated carbon fibers (ACF) or carbonsthat have been treated at high temperature, where ringdefects (rings not made up of six carbon atoms) have beenmostly removed. However, many other types of carbonscontain defects in the carbon rings, such as five- or seven-member rings. These defective carbon rings result inmorphologies that are highly disordered, with basal platesthat are bent and curved. Recently the reverse MonteCarlo method has been extended to account for suchdefects,28 so that it can be applied to highly disorderedcarbons.

The simple water and surface-site intermolecularpotential models can capture the predominant adsorptionbehavior, but they may require some refinement to achievequantitative agreement with the behavior of experimentalsystems. While a detailed model that accurately representsthe system of interest is desirable, it must be simpleenough that the calculations remain tractable. The waterand surface site models used here omit some interactionsthat may be significant, and this could limit theirpredictive capabilities. Among the most important inter-actions that are neglected are (1) direct electrostaticinteractions between water molecules and the carbonsurface, (2) induction interactions between water mol-ecules and the carbon surface, (3) induction interactionsbetween water molecules and the oxygenated surfacegroups,and (4)higherorder two-bodydispersionandthree-body interactions. For the direct electrostatic interactionsbetween the fluid molecules and the carbon surface electricfield, recent studies suggest the use of surface quadrupoleslocated in the carbon basal plane.29-33 The influence of

(27) Bandosz, T. J.; Biggs, M. J.; Gubbins, K. E.; Hattori, Y.; Iiyama,T.; Kaneko, K.; Pikunic, J.; Thomson, K. Chem. Phys. Carbon, in press.

(28) Pikunic, J.; Pellenq, R. J.-M.; Thomson, K. T.; Rouzaud, J.-N.;Levitz, P.; Gubbins, K. E. Stud. Surf. Sci. 2001, 132, 647.

(29) Vernov, A. V.; Steele, W. A. Langmuir 1992, 8, 155.(30) Hansen, F. Y.; Bruch, L. W.; Roosevelt, S. E. Phys. Rev. B 1992,

45, 11238.

Figure 10. Available pore space when water is present in an activated carbon having a site density of 0.65 sites/nm2. Availablepore space is represented schematically as overlapping spheres. The depiction of the spheres was chosen arbitrarily and is forillustration purposes only. Each color represents distinct regions of physically continuous pore space. The sequence of figuresrepresent different amounts of water present: (a) Fw* ) 0 (P/Po ) 0); (b) Fw* ) 0.034 (P/Po ) 0.10); (c) Fw* ) 0.2 (P/Po ) 0.14), whereFw* ) Nwσw

3/V, Nw ) number of water molecules, σw ) Lennard-Jones size parameter for water model, and V ) total volume ofporous carbon sample. Carbon atoms, surface sites, and water molecules have been removed to enhance visualization of the porespace. Note that periodic boundary conditions24 have been used, and thus continuous pore space can extend across the simulationcell boundaries.

Figure 11. Frequency of water cluster size as a function of thecluster size at differing amounts of water present in the porouscarbon: Fw* ) 0.034 (b), 0.067 (0), and 0.2 (2), where Fw* isdefined in the caption of Figure 10. The site density is 0.65sites/nm2 and T ) 300 K.


induction effects, due to the interaction between chargeson the water molecules with the induced dipole in thegraphite and oxygenated groups, can be estimated via theusual many-body induction terms.34 In these calculations,it is important to include the effects of anisotropy in thepolarizability of graphite.35,36 Higher order two-bodydispersion and three-body fluid-substrate interactionshave been considered by several authors.20,36-39 For simplenonpolar fluids interacting with graphite, these forces arebelieved to contribute 5-15% of the total fluid-substrateenergy.36-39 Although the inclusion of many-body polar-ization to date has not offered a significant improvementin the description of the thermodynamics and structurefor bulk water,40-43 it may prove important for water neara carbon surface. Such effects are likely to be of particularimportance for confined aqueous mixtures since selective

adsorption will be affected by dissimilar polarizibilities ofthe mixture components induced by the carbon surface.Of the available water models that include many-bodypolarization, those proposed by Errington and Panagioto-poulos44 and by Chialvo and Cummings45,46 are of par-ticular interest, since, as in the model used here, thepotentials are optimized using wide-ranging thermody-namic and structural data.

Acknowledgment. This work was funded by theDivision of Basic Sciences, Department of Energy (Grant# DE-FA02-99 ER14847). Computing resources wereprovided by the National Partnership for AdvancedComputational Infrastructure (MCA93S011) through theSan Diego Supercomputing Center. J.K.B. is grateful toNSF for support through a NSF-CISE Postdoctoralfellowship (ACI-9896106).

LA0118560(31) Hansen, F. Y.; Bruch, L. W.; Taub, H. Phys. Rev. B 1995, 52,8515.

(32) Whitehouse, D. B.; Buckingham, A. D. J. Chem. Soc., FaradayTrans. 2 1993, 89, 1909.

(33) Bruch, L. W.; Cole, M. W.; Zaremba, E. Physical Adsorption:Forces and Phenomena; Oxford University Press: Oxford, 1997.

(34) Gray, C. G.; Gubbins, K. E.Theory of Molecular Fluids; ClarendonPress: Oxford, 1984.

(35) Nicholson, D. Surf. Sci. 1987, 181 L189.(36) Nicholson, D. Fundamentals of Adsorption; Mersmann A. B.,

Scholl, S. E., Eds.; Engineering Foundation: New York, 1991.(37) Lachet, V.; Boutin, A.; Tavitian, B.; Fuchs, A. H. J. Phys. Chem.

B 1998, 102, 9224.(38) Nicholson, D.; Pellenq, R. J. M. Adv. Colloid Interface Sci. 1998,

76-77, 179.(39) Pellenq, R. J. M.; Nicholson, D. Mol. Phys. 1998, 95, 549.

(40) Gubbins, K. E.; Quirke, N. Molecular Simulations and IndustrialApplications; Gubbins, K. E., Quirke, N., Eds.; Gordon and Breach:Amsterdam, 1996.

(41) Kiyohara, K.; Gubbins, K. E.; Panagiotopoulos, A. Z. Mol. Phys.1998, 94, 803.

(42) Caldwell, J.; Dang, L. X.; Kollman, P. A. J. Am. Chem. Soc.1990, 112, 9144.

(43) Mountain, R. D. J. Chem. Phys. 1995, 103, 3084.(44) Errington, J. R.; Panagiotopoulos, A. Z., in press.(45) Chialvo, A. A.; Cummings, P. T. Fluid Phase Equilib. 1998, 151,

73.(46) Chialvo, A. A.; Cummings, P. T.; Simonson, J. M.; Mesmer, R.

E.; Cochran, H. D. Ind. Eng. Chem. Res. 1998, 37, 3021.


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