10666 DOI: 10.1021/la901107s Langmuir 2009, 25(18), 10666–10675Published on Web 08/13/2009

pubs.acs.org/Langmuir

© 2009 American Chemical Society

Binary Adsorption Equilibrium of Carbon Dioxide and Water Vapor on

Activated Alumina

Gang Li, Penny Xiao, and Paul Webley*

Cooperative Research Centre for Greenhouse Gas Technologies, Department of Chemical Engineering, MonashUniversity, Wellington Road, Clayton, Victoria 3168, Australia

Received March 30, 2009. Revised Manuscript Received June 6, 2009

Adsorption equilibria of a CO2/H2O binary mixture on activated alumina F-200 were measured at severaltemperatures and over a wide range of concentrations from 4% to around 90% of the saturated water vapor pressure.In comparison with the single-component data, the loading of CO2 was not reduced in the presence of H2O, whereas atlow relative humidity the adsorption of H2O was depressed. The binary system was described by a competitive/cooperative adsorption model where the readily adsorbed water layers acted as secondary sites for further CO2

adsorption via hydrogen bonding or hydration reaction. The combination of kinetic models, namely, a Langmuirisotherm for characterizing pure CO2 adsorption and a BET isotherm for H2O, was extended to derive a binaryadsorption equilibrium model for the CO2/H2O mixture. Models based on the ideal adsorbed solution theory of Myersand Prausnitz failed to characterize the data over the whole composition range, and a large deviation of binaryCO2/H2Oequilibrium from ideal solution behavior was observed. The extended Langmuir-BET (LBET) isotherm, analogous tothe extended Langmuir equation, drastically underestimated the CO2 loading. By incorporating the interactionsbetween CO2 andH2Omolecules on the adsorbent surface and taking into account the effect of nonideality, the realisticinteractive LBET (R-LBET) model was found to be in very good agreement with the experimental data. The derivedbinary isosteric heat of adsorption showed that the heat was reduced by competitive adsorption but promoted bycooperative adsorption.

1. Introduction

The adsorption of CO2 on solid adsorbents is receivingincreasing attention in both experimental and theoretical studiesdue to the interest in CO2 capture technologies. Water vapor iscommonly found coexisting with CO2 such as in air prepurifica-tion1 and postcombustion CO2 capture applications.

2 It is knownto substantially affect the CO2 adsorption processes. An under-standing of how CO2 and H2O interact and affect each other’sbehavior on adsorbent surfaces is therefore essential. In spite ofthe importance of this system, there are relatively few adsorptionequilibrium data sets available in the literature for this binarysystem. Rege and Yang3 measured very low concentration ofCO2/H2O vapor (<4.1% RH or relative humidity) mixtureadsorption on 13X zeolite and γ-alumina by means of Fouriertransform infrared (FTIR) spectroscopy and observed a likelyenhancement of CO2 when the concentration of CO2 < 300 ppm(parts per million). Baltrusaitis4 studied the surface reactions ofCO2 at the adsorbed water-metal oxide interface and attributedthe enhancement of CO2 loading by trace moisture to theformation of bicarbonate. However, most of the adsorptionprocesses mentioned above deal with water vapor of medium orhigh RH at ambient temperature, i.e., percentage levels. Incontrast, a typical postcombustion flue gas contains 4-10% v/vwater vapor and 8-12% v/v CO2. To the best of our knowledge,the adsorption equilibria of the CO2/H2O binarymixture over the

full range ofRHat highwater levels have not been reported in theliterature, largely because of the inconvenience of measurementand the difficulty to correlate the data.

The adsorption equilibrium of the CO2 and H2O binarymixture is known to be a highly nonideal system with complexinteractions between these two species onan adsorbent surface.3-5

A few methods have been developed to characterize the multi-component equilibrium deviation from ideality, and reviews areavailable.6,7 The most commonly used method to treat thesedeviations from ideality is the modification of ideal adsorbedsolution theory (IAST)8 by introducing nonunity activity coeffi-cients of the solution (adsorbate) to the IAST;this is called realadsorbed solution theory (RAST).9 An alternative approach is totake into account the surface energetic heterogeneity by applyingthe IAST to a local site and obtain the overall integral ofheterogeneous IAST.10 Even though the modified versions ofIAST are able to describe multicomponent systems with little tomedium nonideality,11 their applications to highly nonideal sys-tems (e.g., CO2 þ H2O on 13X)5 are not very successful yet. Thevirial mixture coefficients (VMC) method developed by AppelandLeVan12 is capable of describing highly nonideal systems suchas water and hexane binary mixture on BPL activated carbon,13

and moreover, it is robust and thermodynamically consistent.

*E-mail: paul.webley@eng.monash.edu.au.(1) Rege, S. U.; Yang, R. T.; Qian, K. Y.; Buzanowski, M. A. Chem. Eng. Sci.

2001, 56, 2745.(2) Li, G.; Xiao, P.; Webley, P.; Zhang, J.; Singh, R.Adsorption-J. Int. Adsorpt.

Soc. 2008, 14, 415.(3) Rege, S. U.; Yang, R. T. Chem. Eng. Sci. 2001, 56, 3781.(4) Baltrusaitis, J.; Schuttlefield, J. D.; Zeitler, E.; Jensen, J. H.; Grassian, V. H.

J. Phys. Chem. C 2007, 111, 14870.

(5) Bai, R. S.; Yang, R. T. Langmuir 2005, 21, 8326.(6) Do, D. D. Adsorption Analysis: Equilibria and Kinetics; Imperial College

Press: London, 1998; Chapter 5.(7) Yang, R. T. Gas Separation by Adsorption Processes; Imperial College Press:

London, 1997; Chapter 3.(8) Myers, A. L.; Prausnit, J. M. AIChE J. 1965, 11, 121.(9) Talu, O.; Zwiebel, I. AIChE J. 1986, 32, 1263.(10) Hu, X. J.; Do, D. D. AIChE J. 1995, 41, 1585.(11) Hyun, S. H.; Danner, R. P. J. Chem. Eng. Data 1982, 27, 196.(12) Appel,W. S.; LeVan,M.D.; Finn, J. E. Ind. Eng. Chem. Res. 1998, 37, 4774.(13) Rudisill, E. N.; Hacskaylo, J. J.; Levan, M. D. Ind. Eng. Chem. Res. 1992,

31, 1122.

DOI: 10.1021/la901107s 10667Langmuir 2009, 25(18), 10666–10675

Li et al. Article

However, the expression of the VMC model is implicit, makingthe dynamic numerical computation difficult and making thehigher-order virial parameters lack sound physical meanings. TheDoong-Yangmodel14 is the extension of the potential theory15 tomulticomponent adsorption equilibrium, and it is very useful fordescribing highly nonideal systems. Rege and Yang showed thatthe Doong-Yang model provided a marginally better fit to thebinary adsorption data ofH2O andCO2 at very low concentration(as mentioned above) than the IAS Dubinin-Astakhov modelwithin the extremely low (ppm level) water partial pressure region,but itwas evenworse than thepredictedones by single-componentDubinin-Astakhov isotherm.3 Moreover, the data were mea-sured at only one temperature so the determination of tempera-ture-dependent parameters and the isosteric heat of adsorption isimpossible.

The purpose of this work is to obtain a set of binary adsorptionequilibrium data of CO2/H2O on a selected adsorbent, namely,activated alumina F-200, to study the interactions betweenadsorbed species. Initially, we will start with a simple binarymodel to characterize the ideal adsorption equilibrium for theCO2/H2Omixture. Furthermore, the nonideality of the CO2/H2Osystem will be examined with specific parameters describingadsorbate-adsorbate and adsorbate-adsorbent interactions todevelop a more accurate binary isotherm model on the basis of akinetic mechanism.

2. Theory

2.1. Extended Langmuir-BET (LBET)Model. There area few reasons to choose the Langmuir and Brunauer-Em-mett-Teller (BET) models as a basis for deriving a binarymodel. First, the Langmuir model is the most commonly usedequation for characterizing type I isotherms.16,17 Likewise,BET and its other modified versions still remain the mostimportant equations for determining multilayered adsorptionof subcritical vapors that follow type III/II isotherms,17,18

mainly due to its versatility and simplicity. Most importantly,the BET isotherm is the extension of Langmuir to multilayeradsorption, and both of them belong to kinetic models withpatchwise topology, which defines that the rate of adsorptionreaches the rate of desorption at equilibrium state. Thesefeatures make the derivation of the binary model consistentand practical. Furthermore, derivations of the binary modelfrom the kinetic procedure can be reasonably extended to thedescription of H2O and CO2 interactions, i.e., the associationrate of CO2 with water molecules on adsorbent surface is equalto the dissociation rate at equilibrium. Nevertheless, this mayrequire fundamental knowledge of the physical and chemicalreactions of water and CO2 on the adsorbent surface.

For a binary adsorption system containing gas L (literally CO2

in this work) characterized by the Langmuir isotherm and gas B(H2O vapor in this work) characterized by the BET isotherm, weassume that there are no lateral interactions between differentspecies and each gas keeps its own rate constants for adsorptionand desorption as the same as that in the single component, andthe adsorbent surface is energetically homogeneous with one siteper molecule. Then the competitive binary isotherm is proposedas follows.

The reversible adsorption of gas L on the surface of adsor-bent is

L ðgasÞ þ S ðempty siteÞT L-S ðadsorbed gas LÞ ð1aÞwhere L, S, and L-S represent equilibrium concentrations ofthe corresponding species on the surface. So at equilibrium, therate of adsorption of the specie L is equal to its rate ofdesorption,

kLaPLð1- θtÞ ¼ kLdθL ð1bÞwhere kLa and kLd are the rate constant of adsorption anddesorption, respectively. θt denotes the sum of the fractions ofsurface sites covered by all species, and θL denotes the coverageby gas L. The gas-phase pressure of component L is given byPL. Thus, the equilibrium rate constant of gasL, represented byKL, is given as

KL ¼ kLa

kLd¼ bL exp

EL

RT

� �ð1cÞ

where bL is the gas-solid affinity coefficient of gas L, EL is theheat of adsorption, R is the gas constant, and T the tempera-ture.

For gas B, representing a subcritical vapor, its first-layerreversible adsorption is

B ðgasÞ þ S ðempty siteÞT B1 - S ð1st-layer adsorbed BÞ ð2aÞAt equilibrium, it gives

kB1aPBð1- θtÞ ¼ kB1dθBl ð2bÞand

KB1 ¼ kB1a

kB1d¼ bB1 exp

EB1

RT

� �ð2cÞ

where the variables are defined in the same way as the above forgas L. Subscript 1 denotes first-layer adsorption.

Similar to the original BET model, the adsorbed molecules ofgas B in one layer can act as adsorption sites for molecules in thenext layer. So the multilayer stacking of gas B takes place asfollows:

BðgasÞ þ Bði-1Þ-S ði-1 layer adsorbed BÞT Bi - S ði layer adsorbed BÞ, ig 2 ð3aÞ

kBiaPBθBði-1Þ ¼ kBidθBi, ig 2 ð3bÞ

KBi ¼ kBia

kBid¼ bBi exp

EBi

RT

� �, ig 2 ð3cÞ

where the equilibrium rate constant and heat of adsorption forlayers i g 2 are the same at the given temperature T, i.e., KB2 =KB3=...=KBi=KB, andEB2=EB3=...=EBi=EB.Referringto eq 3a, the concept of multilayer adsorption may also beinterpreted as a polymerization reaction where the growing ofthe polymer chain represents the accumulation of layers.19

Accordingly, we can name i-layered adsorbate as i-mer.

(14) Doong, S. J.; Yang, R. T. Ind. Eng. Chem. Res. 1988, 27, 630.(15) Dubinin,M.M.; Zaverina, E.D.; Serpinsky, V. V. J. Chem. Soc. 1955, 1760.(16) Langmuir, I. J. Am. Chem. Soc. 1918, 40, 1361.(17) Brunauer, S.; Deming, L. S.; Deming, W. E.; Teller, E. J. Am. Chem. Soc.

1940, 62, 1723.(18) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. (19) Talu, O.; Meunier, F. AIChE J. 1996, 42, 809.

10668 DOI: 10.1021/la901107s Langmuir 2009, 25(18), 10666–10675

Article Li et al.

The sum of surface fractions equals unity:

θ0 þ θL þ θB1 þ 3 3 3 þ θBi ¼ 1 ð4ÞAfter some necessary algebra (Supporting Information), we

obtain the extended binary LBET isotherm for gas B and gas L,respectively,

NB ¼ MBKB1PB

ð1-KB2PBÞ½1-KB2PB þKB1PB þKLPLð1-KB2PBÞ�ð5aÞ

NL ¼ MLKLPLð1-KB2PBÞ1-KB2PB þKB1PB þKLPLð1-KB2PBÞ ð5bÞ

whereML andMB are defined as the surface adsorption sites forgas L and gas B, respectively, which are also known as themaximum monolayer adsorption capacity. We assume that thetemperature variation of ML and MB is negligible. A schematicdiagram of the LBET model is shown in Figure 1.

One of the major requirements for multicomponent isothermsis that the equations can be reduced to valid single-componentequations. If we remove gas L or gasB from the above equations,eq 5a will become eq 6a, the single-component modified BET(also known as GAB) equation,20 which has been widely used forthe characterization ofwater adsorptionon foodmaterials,21,22 oreq 5bwill be reduced to the single-component Langmuir isothermeq 6b, respectively:

NB ¼ MBKB1PB

ð1-KB2PBÞð1-KB2PB þKB1PBÞ ð6aÞ

NL ¼ MLKLPL

1þKLPLð6bÞ

The isotherm eqs 5a and 5b reduce to the Henry’s law limit whenthe pressure is very low, a constraint demanded by statisticalthermodynamics for thermodynamic consistency. The Henry’slaw limit is not met by other isotherm equations based onDubinin’s potential theory such as the Doong-Yang model.14

Moreover, if themultilayer adsorption of gasB is small enoughto be negligible, i.e., KB2 f 0, then the extended binary LBETisotherms (eqs 5a and 5b) will reduce to the extended Langmuirisotherms for a binary system:

NB ¼ MBKB1PB

1þKB1PB þKLPLð7aÞ

NL ¼ MLKLPL

1þKB1PB þKLPLð7bÞ

So, like the extended Langmuir and its modifications,23 theadvantage of the extended Langmuir-BET isotherm is its analy-tical, explicit, and convenient expression.2.2. Realistic Interactive LBETModel. Ideally, one would

prefer to generate themulticomponent adsorption equilibria fromonly the pure-component data and single-component iso-therms.12 Obviously the above extended LBET isotherm (eqs 5aand 5b) may not be sufficient to characterize systems involving

strong interactions between adsorptive gases. Particularly in thisstudy, the binary mixture of carbon dioxide (gas L) and watervapor (gas B) on an adsorbent surface is a highly nonideal systemwhere the CO2 adsorption is in a monolayer and H2O is in amultilayer pattern.

The conventional explanation forCO2 andwater interactions isthat CO2 will have hydration reactions with H2O molecules toform unstable carbonic acid and eventually dissociate into morestable ions.24,25

CO2 þH2OTH2CO3 THCO3- þHþ ð8Þ

Intensive studies indicate that the hydration reaction inneutral solution may have different pathways and the numberof water molecules actively participating varies.26 The optimalactivation energy for the above process involving one CO2 andone water dimer has been reported as high as 65.2 kJ/mol.25

Noticeable CO2 hydrate formation requires a high criticalpressure (2-20 MPa) and a subambient temperature (273-285 K).27 However, the fast dissociation step in eq 8 was foundto generate less than one HCO3

- ion per 1000 water moleculesat 20 MPa and 318.15 K.28,29 It is also known that CO2

solubility is as low as 0.0286 mol/kg of water at 0.1 MPa and303.15 K.30 Therefore, the chemisorption capacity of CO2 onneutral water at atmospheric pressure is too small to be takeninto account.

However, special attention has to be paid to the differencebetween CO2/H2O interaction on an adsorbent surface and in thebulk phase, as the surface chemistry and the geometry of theadsorbent will play fundamental roles in changing the loading ofCO2 on adsorbed water layers at relatively low CO2 partialpressure. First, the adsorbent surface is normally not neutral andit may consist of Lewis and Broensted acid/basic sites, for instance,-OHon γ-Al2O3 surface

4 and-NH2 on amine tethered silica.31,32

On suchadsorbent surfaces,CO2 is shown to reactwith coadsorbedwater to yield adsorbed carbonate resulting in an enhancement ofCO2 uptake by a thin layer of adsorbed water. This has beencharacterized by spectroscopic shifts of IR33 and isotope studies

Figure 1. Schematic diagram of an ideal binary adsorptionmodelwithout interactions between adsorbed species, where gas L(represented in ellipsoids) adsorbs in monolayer and gas B(spheres) in multilayer. Higher layers are not graphed here forclarity purposes.

(20) Anderson, R. B. J. Am. Chem. Soc. 1946, 68, 686.(21) Siripatrawan, U.; Jantawat, P. Food Sci. Technol. Int. 2006, 12, 459.(22) Timmermann, E. O.; Chirife, J.; Iglesias, H. A. J. Food Eng. 2001, 48, 19.(23) Kapoor, A.; Ritter, J. A.; Yang, R. T. Langmuir 1990, 6, 660.

(24) Liedl, K. R.; Sekusak, S.; Mayer, E. J. Am. Chem. Soc. 1997, 119, 3782.(25) Nguyen, M. T.; Ha, T. K. J. Am. Chem. Soc. 1984, 106, 599.(26) Nguyen,M. T.; Raspoet, G.; Vanquickenborne, L.G.; VanDuijnen, P. T. J.

Phys. Chem. A 1997, 101, 7379.(27) Yang, S. O.; Yang, I. M.; Kim, Y. S.; Lee, C. S. Fluid Phase Equilib. 2000,

175, 75.(28) da Rocha, S. R. P.; Johnston, K. P.; Westacott, R. E.; Rossky, P. J. J. Phys.

Chem. B 2001, 105, 12092.(29) Tewes, F.; Boury, F. J. Phys. Chem. B 2004, 108, 2405.(30) Duan, Z. H.; Sun, R. Chem. Geol. 2003, 193, 257.(31) Harlick, P. J. E.; Sayari, A. Ind. Eng. Chem. Res. 2007, 46, 446.(32) Hicks, J. C.; Drese, J. H.; Fauth, D. J.; Gray,M. L.; Qi, G. G.; Jones, C.W.

J. Am. Chem. Soc. 2008, 130, 2902.(33) Baltrusaitis, J.; Grassian, V. H. J. Phys. Chem. B 2005, 109, 12227.

DOI: 10.1021/la901107s 10669Langmuir 2009, 25(18), 10666–10675

Li et al. Article

showing extensive exchangebetweenoxygen in adsorbedwater andoxygen atoms in both adsorbed carbonate and gas-phase carbondioxide.4 Second, there is distinct hydrogen bonding between theoxygen atoms in CO2 and the hydrogen in condensedwater.

34 CO2

molecules tend to adsorb onto gas/water interface and then formone-to-one “H-type” complex with H2O.29,35 It is a fast andreversible physisorption process via low-energy interactions, espe-cially hydrogen bonding. Tewes and Boury29 measured the ad-sorbed amount of CO2 on unit surface area of bulk water interface.Obviously, this physisorption capacity is proportional to theaccessible gas/water interface, which essentially depends on theadsorbent surface area.

Despite the difficulty in distinguishing the effect of chemisorp-tion and physiorption, the total amount of coadsorbed CO2 is afunction of the surface area of adsorbed water layers and thedensity of the Lewis and Broensted sites occupied by the waterlayers. So based on eqs 1-3, we will have the following reactionsas illustrated in Figure 2:

LðgasÞ þ Bði-1Þ - S ði - 1 layer adsorbed BÞTL-Bði-1Þ - S, ig2 ð9aÞ

kLBði-1ÞaPLθBði-1Þ ¼ kLBði-1ÞdθLBði-1Þ, ig2 ð9bÞ

KLBði-1Þ ¼ kLBði-1ÞakLBði-1Þd

¼ bLBði-1Þ expELBði-1ÞRT

� �, ig2 ð9cÞ

where gas L denotes CO2 and B denotes H2O, and L-B(i-1)-Sdenotes coadsorption of L and i-1 layered B on the adsorbentsurface S. In addition to the previous assumptions of a flat andopen adsorbent surface, we assume that the reactivity of thesurface -OH groups of different water layers is constant. There-fore, bLB and ELB are constants regardless of layer numbers.

Noting that the reactions of eqs 9 and 3 are competing witheach other, at equilibrium, we will have

X¥j¼1

θLBj ¼ KLBPL

X¥i¼1

θBi ð10Þ

By adding eq 9 into the summation equation of total coveragein eq 4 and doing some necessary manipulation, the final expres-sion of the R-LBET model (realistic interactive Langmuir-BETbinary model) is

For H2O : NB ¼MBKB1PBð1þKLBPLÞ

ð1-KB2PBÞ½1-KB2PB þKB1PB þKB1PBKLBPL þKLPLð1-KB2PBÞ�ð11aÞ

For CO2 : NL ¼MLKLPLð1-KB2PBÞ þMLBKB1PBKLBPL

1-KB2PB þKB1PB þKB1PBKLBPL þKLPLð1-KB2PBÞð11bÞ

where MLB is the maximum adsorption sites for CO2 on waterlayers when the adsorbent surface is fully covered by water.

TheR-LBETmodel can be reduced to the extended LBETbinarymodel (eqs 5a and 5b) if we remove all the interactive terms (withLB subscriptions) from eqs 11a and 11b.2.3. Ideal Adsorbed Solution (IAS) Model. The well-

known IAS theory8,36 assumes that the adsorbed phase is anideal solution of the adsorbed components and the surfacepotential of the mixture is the same as the surface potentials ofall pure components. The basic working equations are as follows,

Pyi ¼ Pi ¼ Pi0ðπÞxi for i ¼ 1, 2, 3 3 3 , n ð12Þ

which is the analogue of Raoult’s law in vapor-liquid equi-librium,

andXni¼1

yi ¼ 1,Xni¼1

xi ¼ 1 ð13Þ

where Pi0 is the hypothetical equilibrium pressure of the pure

component that is at the same spreading pressure π and tempera-ture T of the adsorbed components on the surface. That is,

πA

RT¼

Z P01

0

N1

p1dP1 ¼

Z P02

0

N2

p2dP2 ¼ 3 3 3

¼Z P0

n

0

Nn

pndPn ð14Þ

in whichNi(Pi) is the adsorption isotherm. In the case of CO2 andH2O binary system of this study, Ni(Pi) is represented by theLangmuir and the modified BET isotherms, respectively.

The total adsorbed amountNt and the amount contributed byeach component Ni can be calculated from the equation

1

Nt¼

Xni¼1

xi

N0i

ð15Þ

and Ni ¼ Ntxi ð16Þwhere Ni

0 is the amount adsorbed of the pure component in thestandard state at the pressure Pi

0, which is fixed by eq 14. In thisstudy, a simple analytic solution forPi

0(π) canonly be obtained forthe Langmuir isotherm, whereas for themodified BET isotherm, atedious numerical or graphical method has to be employed. Thealgorithm to calculate the ultimate adsorbed amount for eachcomponent has been elaborated in the literature.6,37 Therefore, thisIAS theory in combination with the Langmuir and modified BETequation is referred to as the IAS-LBET model.

Figure 2. Schematic diagram of the binary adsorption model ofCO2 (L) and H2O (B) and their interactions (LB) in the form ofsurface coverage θ.

(34) Sato, H.; Matubayasi, N.; Nakahara, M.; Hirata, F. Chem. Phys. Lett.2000, 323, 257.(35) Sadlej, J.; Makarewicz, J.; Chalasinski, G. J. Chem. Phys. 1998, 109, 3919.

(36) Sircar, S.; Myers, A. L. Chem. Eng. Sci. 1973, 28, 489.(37) Valenzuela, D. P.; Myers, A. L. Adsorption Equilibrium Data Handbook;

Prentice Hall: Englewood Cliffs, NJ, 1989; Chapter 3.

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Article Li et al.

3. Experimental Section

3.1. Materials.Carbon dioxide gas was supplied by evapora-tion of refrigerated liquid carbon dioxide with a purity of 99.99%(BOCGases, Australia). Ultrapurewater was supplied by aMilli-Q apparatus (Millipore, U.S.A.). The adsorbent investigated inthis study is activated alumina F-200manufactured by Engelhard(U.S.A.), 2 mmmedian pellet size. The samples were regeneratedat 330 �C under vacuum condition for 24 h with MicromeriticsVacPrep (Micromeritics, U.S.A.).

3.2. Adsorption Measurement. The pure-componentadsorption/desorption isotherm data for CO2 andN2 on activatedalumina F-200 were obtained by an ASAP 2010 gas adsorptionanalyzer (Micromeritics, U.S.A.) at different temperatures overthe pressure range 0-118 kPa. Surface area and pore size distribu-tion of activated alumina F200 were measured with the sameapparatus by conducting liquid nitrogen adsorption tests at 77 K.

Pure water isotherms and water/CO2 binary isotherms weremeasured on a custom-built apparatus (BIAU = binary isother-mal adsorption unit). This is a batch breakthrough adsorptionapparatus that relies on both gravimetric and volumetric techni-ques for its data collection. Similar methods for measurement ofmulticomponent adsorption equilibria can also be found in litera-ture.38 The schematic diagram of the BIAU is shown in Figure 3.To control the temperature, the unit is immersed in a constant-temperature water bath with (0.1 �C control stability and acirculating water pump to keep the temperature uniform in thebath. The major parts of the unit include an adsorption column, avapor generator, and a humidity transmitter that are all fullyimmersed in the water bath. The adsorption column consists of astainless steel cylinder, approximately 2 cm in diameter and 12mLin volume, which is designed to hold enough samples (8-10 g foractivated alumina F-200) in this work. P150 (0.1 mm in diameter)meshes and 1/8 in. fittings are set at the inlet and the outlet.

The vapor generator consists of a glass bubbler sealed bySwagelok stainless steel fitting for holding pressure up to200 kPa. Ultrapure water was used as the vapor source. Dry car-bon dioxide (from liquidCO2, LindeGas, Australia) gas was splitinto two stream lines with needle valves and flowmeters indepen-dently. Onewas fed into the bubbler to carry the water vapor, andthe other one was passed through a coil heat exchanger immersedin awater bath.When they reached the experimental temperature,both streams were merged into one line again. For such athermostatic system, the RH of the CO2/H2O binary mixturecan be easily adjusted by finely tuning the flow rates while keepingthe total pressure constant. It should be noticed that all gases havefinite solubility in water, so it is important to saturate the bubblerwith experimental gas in advance under experimental pressureand temperature before loading the sample. Therefore, humidCO2 (or other customer gas) with variable RH was prepared andready to be fed into the adsorption column. At equilibrium, thetotal loading of the CO2/H2Owas obtained directly bymeasuringthe weight change of the adsorption column. Subsequently, theadsorbed amount of individual H2O was obtained bymultiplyingthe weight change of the water bubbler by the fraction of theadsorbed water derived from the volumetric integration of thewater breakthrough curve. Finally, the CO2 loading was calcu-lated by mass balance. The weight change of the adsorption unitwasmeasuredbyabalancewith readability of 0.1mgand capacityof 210 g (Sartorius BP210S, Germany).

The partial pressure and temperature of the water vapor wasmeasured by a humidity and temperature transmitter (VaisalaHUMICAPHMT330,Finland)with anaccuracyof 0.1Pa (0.1%RHresolution) and a span of 0-100%RH.The calibration of thehumidity transmitter was conducted by the static volumemethod.A certain amount of water was injected with a gas chromatogra-phy syringe into a rigid, sealed, and dry glass container filled with

the environmental gas (air or CO2) and preinstalled with thehumidity transmitter. The container was kept in a thermostaticwater bath. The reading of the humidity transmitter was recordedat equilibrium. The experiments were repeated with air and CO2,respectively.

4. Results and Discussions

4.1. Single-Component Adsorption on F-200. The adsorp-tion of single-component CO2 and H2O were measured respec-tively at a number of different temperatures. The experimentaldata points were fit bymodels based on a least-squares minimiza-tion method by adjusting the parameters to reach the minimumsum of squares error. As the success of determination of theproposed binarymodels depends to a large extent on the ability ofthe single-component isotherms to fit the pure-gas adsorptiondata accurately, the average relative error,Rave, has been used forevaluating the goodness of the models. That is,

Rave ¼ 100

n

Xni¼1

jðNcal, i -Nexp , iÞ=Nexp , ij ð17Þ

where Nexp is the experimental adsorption data and Ncal is thecorresponding value calculated by the model.

The activated alumina F-200 has a BET surface area of366.8 m2 and a broad pore size distribution with large fractionof mesopores as measured in this work. The single-site Langmuirmodel was fit to the experimental data of CO2 adsorption onactivated alumina F-200 at the two temperatures in order todetermine the parameters (Table 1). As shown in Figure 4, themodel matches the data well at medium and higher part of theexperimental pressure range, while there is relatively a noticeabledeviation at lower partial pressure range. This deviation is mostlydue to the limitation of the Langmuir model with the assumptionof a homogeneous surface, whereas the real adsorbent surfacealways shows some degree of heterogeneity. It is well-known thatthe alumina surface presents ionic oxide sites (AlþO2-A1þ) forCO2 adsorption by ion-quadrupole interaction39 and also a largenumber of hydroxyl groups (if activated below 500 �C) accoun-table for CO2 chemisorption by forming bicarbonate.33 Never-theless, an important observation is that CO2 adsorption/desorption is fully reversible (within experimental error), which

Figure 3. Schematic diagram of the binary isothermal adsorptionunit.

(38) Keller, J.; Staudt, R. Gas Adsorption Equilibria: Experimental Methods andAdsorptive Isotherms; Springer Science: Boston, 2005; Chapter 4. (39) Peri, J. B. J. Phys. Chem. 1966, 70, 3168.

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is a critical prerequisite for our binary experiments. The averagerelative error for CO2 is as small as only 5.66%. In addition, aspresented in Figure 4, N2 adsorption on F-200 is nearly negligibleat room temperature, so it follows that air is eligible to be used asthe carrier gas for H2O adsorption.

For water adsorption on activated alumina F-200, the experi-mental results obtained byBIAUat three temperatures of 293.15,298.15, and 303.15 K are shown in Figure 5. Although thetemperature range is not large, the water saturation pressuresare 2.337, 3.166, and 4.242 kPa, respectively. First of all, as ameasure of validation, a set of literature data40 at 298.15 K werealso plotted in Figure 5 for comparison. It shows that theexperimental data are in good agreement with the literature data.Themechanism of water adsorption on activated alumina is quiteclear: the adsorption is mainly due to hydrogen bonding with thesurface hydroxyl groups and some fraction of chemi- or quasi-chemi-sorption by compensating surface ionic defects.41,42

Despite the imperfection of the basic assumption of the model,the modified BET equation (eq 6a), has still been used for fittingthe H2O adsorption data in this work, which allows the extensionof its applicability range toRH=0.8 or even higher, instead of theusual range of 0.05-0.35 of the conventional BET equation.6,20,43

The higher flexibility of the modified BET equation is because theequation has an extra parameterKLB for characterizing the heat ofmultilayer adsorption, whereas for the conventional BET, thisvalue is fixed as the heat of liquefaction of water vapor. As a result,the average relative error for fitting the experimental data is 9.28%,which is still reasonably small, given the wide range of relativehumidity from 0.94% (at 303.15 K) to 92.3% (at 293.15 K). Theparameters of themodifiedBET isotherm are presented in Table 2.

By comparing the twopure-component adsorptions,we can seethe maximum adsorbed amounts of water are significantly higherthan CO2. MB, the maximum monolayer adsorption of H2O, is6.0807mol/kg, nearly 4 times that ofML (1.6898mol/kg) forCO2,indicating a higher density ofH2Oon the alumina surface becauseof the smaller molecular size of H2O (van der Waals diameter0.275 nm) versus CO2 (diameter 0.47 nm). Furthermore, theparameter EB1 (69.580 kJ/mol) representing the association en-ergy between the first-layer water and the surface sites is sub-stantially higher than EL (27.265 kJ/mol) of CO2, indicating thatH2O adsorption energy is much stronger than CO2 on activatedalumina F-200.4.2. Binary H2O/CO2 Adsorption on F-200. In order to

characterize the coadsorption behavior of H2O/CO2 and test theproposed binary models, binary adsorption of H2O/CO2 onactivated alumina F-200 was measured at three different tem-peratures by keeping the CO2 partial pressure nearly constantwhile increasing H2O partial pressure stepwise. The experimentalresults were first recorded in terms of total adsorption weightdirectly measured by the gravimetric method (Figure 6 parts a, c,and e). Then the adsorbed amount of individual H2O was deter-mined by the volume integral, and subsequently the CO2 loadingwas determined by subtraction (Figure 6 parts b, d, and f). Thepredictions of the binary adsorption by the extendedLBETmodeland IAS-LBETmodel are quite straightforward and completelybased on the obtained parameters of the single-componentmodels as shown by Tables 1 and 2. However, for the R-LBETmodel, simultaneous fitting of the experimental binary data ofboth H2O and CO2 at multitemperatures has to be made toextract the interactive parameters MLB, bLB, and ELB, which arepresented in Table 3.

As shown by Figure 6, the adsorbent surface is initiallyprecovered by CO2, and with the introduction of water into thesystem, there is a distinct depression of water loading at lowrelative humidity. This is because the surface sites, i.e., ionicoxides and structural hydroxyls, for H2O adsorption are basicallythe same as that for CO2. Therefore, H2O has to compete withCO2 to be adsorbed on the alumina surface. With the increase ofwater humidity, the inhibitory effect of CO2 on H2O is reducedand the adsorption of H2O quickly recovers to the level of single-component adsorption. Competitive adsorption is a commonphenomenon in a multicomponent system; for example, Rudisillet al.13 noted that, in the presence of water, hexane partial

Table 1. Parameters of the Langmuir Isotherm (eq 6b) for Single-Component CO2 Adsorption and the Average Relative Error of the

Fitting

adsorptive ML (mol/kg) bL (1/kPa) EL (kJ/mol) Rave (%)

CO2 1.6898 2.732 � 10-7 27.265 5.66

Figure 4. Single-component isotherms for CO2 and N2 on acti-vated alumina F-200 at different temperatures. Symbols denoteexperimental data. Lines denote model fitting by single-site Lang-muir equations.

Figure 5. Water adsorption isotherms on activated aluminaF-200at different temperatures. Symbols denote experimental data.Lines denote model fitting by modified BET equation.

(40) Serbezov, A. J. Chem. Eng. Data 2003, 48, 421.(41) Desai, R.; Hussain, M.; Ruthven, D. M. Can. J. Chem. Eng. 1992, 70, 699.(42) Kotoh, K.; Enoeda, M.; Matsui, T.; Nishikawa, M. J. Chem. Eng. Jpn.

1993, 26, 355.(43) Everett, D. H. Langmuir 1990, 6, 1729.

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pressure has to be increased dramatically to maintain the samehexane loading on activated carbon; Bowen and Vane44 studiedthe competitive coadsorption of ethanol, acetic acid, andwater on

CBV ZSM-5 zeolite, and the data were fitted with the dual-siteextended Langmuir model with reasonable accuracy. As it hasbeen discussed earlier that H2O is more strongly adsorbed thanCO2, therefore CO2 should have gradually been displaced by theincreasing partial pressure of H2O. However, Figure 6 shows that

Table 2. Parameters of the Modified BET Isotherm (eq 6a) for Single-Component H2O Adsorption and the Average Relative Error of the Fitting

adsorptive MB (mol/kg) bB1 (1/kPa) EB1 (kJ/mol) bB2 (1/kPa) EB2 (kJ/mol) Rave (%)

H2O 6.0807 5.78 � 10-12 69.580 6.96 � 10-9 43.149 9.28

Figure 6. Binary adsorptionofH2OandCO2onactivated aluminaF-200with constantCO2partial pressure of 102.125kPa, (a) total loadingin weight as directly measured by gravimetric method and (b) individual loadings in mol at 293.15 K; (c) total and (d) individual loadings at298.15K; (e) total and (f) individual loadings at 303.15K. Solid symbols denote binary experimental data. Dotted lines are the prediction byIAS-LBET model; dashed lines are predicted results by LBET model, whereas solid lines are fitting results by R-LBET model.

(44) Bowen, T. C.; Vane, L. M. Langmuir 2006, 22, 3721.

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the adsorbed amount of CO2 in the binary system is almostunchanged except a slight decrease at higher water humidity.Neither the extended LBET model nor the IAS-LBET model isable to correctly predict the binary CO2 loading, although each ofthem shows correct prediction of the water loading.Meanwhile, ithas to be noted that the IAS-LBET model is only capable ofpredicting binary adsorption up to 42% of RH in this study,because above the maximum limit the spreading pressure of CO2

cannot match the H2O one (eq 14), no matter how high the CO2

pressure is set. So at three temperatures, the average relative erroris 36.2% for the extended LBET model and 47.43% (within thevalid fitting range) for the IAS-LBET model. This indicates theH2O/CO2 system displays a strong deviation from ideality asassumed by IAS theory. In contrast, the R-LBET model exhibitsgood agreement with the individual adsorption data of water andCO2 simultaneously with a practically small error of 8.43%(Table 3). Furthermore, the review of the total loading(Figure 6 parts a, c, and e), indicates that there is an obviousincrement of total loading of the binary mixture compared withthe one of the single-component H2O over the entire H2O partialpressure range. This result has been accurately simulated byR-LBET model with an average relative error of 3.86% only.Not surprisingly, the extended LBET model and IAS-LBETmodel all underestimate the total loading with large errors of18.30% and 12.86%, respectively.

Once the binary parameters were determined (Table 3), theR-LBETmodel was further used to predict and compare with theexperimental data forbinaryadsorptionof 15.53%CO2 (balancedwith air) and water vapor on activated alumina F-200 withincreasing humidity at 298.15 K. As shown in Figure 7, theR-LBET model shows excellent predictions of the total adsorbedamount. This suggests that the R-LBET model is capable ofcharacterizing the binary adsorption isotherm of CO2 and H2Ovapor with a set of physically meaningful parameters, where thesingle-component isotherm for CO2 is Langmuir and for H2O isBET. However, in the absence of binary experimental data, theextended LBET model would still be a possible option for theprediction of binary CO2/H2O adsorption equilibria.4.3. CO2 /H2O Binary Interactions. The superior perfor-

mance of the R-LBET over the other tested models is chieflybecause of the successful incorporation of the binary parametersto describe the interactions between water and CO2 in a highlynonideal system. As elaborated previously, the interaction isrepresented by CO2 reactions with water i-mer (i = 1, 2, ..., n)on the adsorbent surface, where the adsorbed water layers willcreate new sites for CO2 adsorption by hydrogen bonding,electrostatic interaction, and other low-energy interactions. Onactivated alumina F-200 surface, the maximum monolayer ad-sorption capacity of CO2 onto water layer MBL (2.8 mol/kg) ishigher than ML (1.6898 mol/kg), the maximum monolayeradsorption capacity of pure CO2 on activated alumina, but stilllower than the water one (MB = 6.0807 mol/kg), indicating thatCO2 molecules are more compact on the adsorbed water surfacethan directly on alumina. This is most probably because of thehigher density of -OH groups on the adsorbed water surfacebeing the accessible sites for CO2, given that MB is much largerthan ML. It has been reported that the adsorbed water layer has

twice as many as the original structural hydroxyls of surfaces.42

Moreover, the molecular area of adsorbed CO2 on the watersurface was found to be smaller than that on solid adsorbentsurface.29 The association energy ELB of CO2 with n-layeradsorbedwater (water n-mer) on activated alumina F-200 surfaceis found to be 16.709 kJ/mol, which is the lowest among all theenergy parameters determined in this study: ELB < EL < EB2 <EB1. First, it indicates that the adsorption of CO2 on water layersis weaker than directly on activated alumina surface; second, onthe surface of the adsorbedwater layer, CO2 adsorption is inferiorto the competition of water (multilayer) adsorption. However,ELB is more than twice the enthalpy of CO2 adsorption on bulkliquid water interface (6.938 kJ/mol) where CO2 is in the form of“H-type” 1/1 complex with H2O.29 It has to be pointed out thatcarbon dioxide tends to form carbonate with the first-layeradsorbed water because of the strong electrostatic charge of the-OHgroups caused by the ionic oxide sites, and it should have anadsorption energy of a similar magnitude to the CO2 hydrationenergy in solution, although no data have been reported; how-ever, as the layers of adsorbed water build up, the strength of theelectrostatic charge will decrease and CO2 is more likely to form acomplex with multilayered water with a lower association energyrather than carbonate. As much as 4 molecules of water can beinvolved in the bicarbonate reaction.Hence, the 16.709kJ/mol forELB obtained by data correlation is probably the “average” valueof CO2 adsorption onto all the different water layers on theactivated alumina surface, due to the assumption of constantELB

in our R-LBET model, and it should be treated with caution.The sorption enhancement of CO2 depends on the summation

effect of both competition and cooperation by the coadsorbedwater. This does not necessarily mean the loading of CO2 inbinary system will be higher than that in single-componentadsorption. In this study, the nearly unchanged CO2 loading inthe presence ofH2Ounder the experimental conditions (at around1 bar CO2) indicates that the CO2 desorbed by water competitionis compensated by the assisted adsorption of CO2 with thecooperation of water. Otherwise, the CO2 loading would havebeen drastically reduced as the predicted result by IAS theory,without considering the contribution of the enhancement effect.We can also anticipate the loading of CO2 at high pressures anddifferent humidity, even though there is a lack of high-pressureexperimental data due to the limitation of the apparatus.

Table 3. Binary Parameters of the R-LBET Isotherm for H2O and

CO2 Adsorption at Three Temperatures and the Average Relative

Error of the Fitting

adsorptive MLB (mol/kg) bLB (1/kPa) ELB (kJ/mol) Rave (%)

H2O þ CO2 2.8000 5.339 � 10-6 16.709 8.43

Figure 7. Total adsorption of H2O and CO2 binary mixture onactivated aluminaF-200measuredwith 15.53%v/vCO2 (balancedwith air) at 104.5 kPa and 298.15 K.

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As shown by Figure 8a, CO2 loading is slightly reduced in thepresence of high-humidity water vapor within the CO2 partialpressure range of 0.1-300 kPa, coinciding with the range of datain the current work. This is also in accordance with the experi-mental result byRege andYang3 that there is little or no incrementof CO2 adsorption on γ-Al2O3 in the CO2 concentration range of0.1-4% v/v. Rege also claimed a significant CO2 enhancement atextremely low concentrations of CO2 (<300 ppm), but it wasbased on the comparison with the loading of pure componentpredicted by single-component Dubinin-Astakhov isothermwithout addressing its accuracy rather than real experimentaldata. Nevertheless, at high CO2 partial pressure above 300 kPa,CO2 loading is considerably promoted by the coadsorbed waterwith increasing humidity. A similar result has been reported bySun et al.45 that CO2 sorption amount is nearly doubled bypreadsorbed H2O in activated carbon at high CO2 pressure(>2.0MPa) by the hydration reaction and is directly proportionalto the adsorbed water amount. The absolute value of the CO2

enhancement predicted by R-LBET model is, however, not assignificant as that reported by Sun et al.45 in carbon showing apronounced “S” shape inflection, because ourmodel assumesCO2

adsorption is only related to the surface area, whereas in reality

CO2 carbonate may also fill up the pores at high pressure.Figure 8b shows that, at low relative humidity, water adsorptionwas depressed by higher partial pressured CO2 while the influenceof CO2 on H2O loading is insignificant at other conditions.4.4. Isosteric Heat of Adsorption of Binary System. The

heat of adsorption is of great importance in adsorption-based unitoperations and requires precise knowledge of the adsorptionequilibria for its estimation. However, there are limited data forheat of adsorption in the literature for pure gas adsorption andvirtually none for binary gas mixtures CO2/H2O.46 The isostericheat of adsorption of component i in a multicomponent gassystem47 is defined as

ΔHi

RT2¼ D ln Pi

DT

� �Ni

ð18Þ

which is the analogue of the conventional isosteric heat ofadsorption of a pure gas, namely, the van’t Hoff equation. Itcan be directly measured calorimetrically48 or calculated fromadsorption equilibrium data of multicomponent mixtures. Parti-cularly for binary gas mixture, ΔHi can be obtained from binaryequilibrium data by applying eq 18 as follows:47

ΔH1

RT2¼

1P

DN1

DT

� �P, y1

DN2

Dy1

� �P,T

- DN1

Dy1

� �P,T

DN2

DT

� �P, y1

� �þ 1

y1

DN1

DP

� �T, y1

DN2

DT

� �P, y1

- DN1

DT

� �P, y1

DN2

DP

� �T, y1

� �

DN1

Dy1

� �T,P

DN2

DP

� �T, y1

- DN1

DP

� �T, y1

DN2

Dy1

� �T,P

ð19Þ

and with an analogous equation for component 2.Herein, eq 19 was employed to determine the isosteric heat of

adsorption of H2O and CO2, respectively, from the equilibriumdata simulated by R-LBET model, effectively using the R-LBETmodel as a data-smoothing function. As shown from Figure 9a,the heat of adsorption of H2O at 0 kPa constant pressure of CO2

(i.e., pure H2O) is 69.58 kJ/mol at zero loading, which is justslightly higher than the literature data of 69 kJ/mol on carbon

molecular sieve with surface functional groups.49 This high valueis consistent with the strong water binding with primary sites on theactivated alumina surface. The isosteric value then decreases asmultilayer adsorption of water takes place. At higher loading(>10 mol/kg), corresponding to RH above 55%, the isosteric heatlevels off at 43.3 kJ/mol asmultilayer adsorption ofwater in the formof clustering and polymerization become dominant. This value iscompatible with the heat of water liquefaction of 44.2 kJ/mol,50,51

Figure 8. Binary CO2/H2O adsorption equilibria over full range of partial pressure predicted byR-LBETmodel in terms of (a) CO2 loadingand (b) H2O loading.

(45) Sun, Y.; Wang, Y. X.; Zhang, Y.; Zhou, Y. P.; Zhou, L. Chem. Phys. Lett.2007, 437, 14.(46) Sircar, S.; Cao, D. V. Chem. Eng. Technol. 2002, 25, 945.(47) Sircar, S. J. Chem. Soc., Faraday Trans. 1 1985, 81, 1527.

(48) Sircar, S.; Mohr, R.; Ristic, C.; Rao, M. B. J. Phys. Chem. B 1999, 103,6539.

(49) Rutherford, S. W. Langmuir 2006, 22, 9967.(50) Bolis, V.; Busco, C.; Ugliengo, P. J. Phys. Chem. B 2006, 110, 14849.(51) Iwaki, T.; Jellinek, H. H. G. J. Colloid Interface Sci. 1979, 69, 17.

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Li et al. Article

indicating that the assumption of the multilayer mechanism is stilladequate to characterize the adsorption of H2O, even though multi-layer adsorption may possibly merge into capillary condensation athigher relative humidity region. In the presence of CO2, the H2Oisosteric heat significantly decreases within the region of lower waterloading (<10 mol/kg), probably because the heat of adsorption ofH2O is partially counteractedby the endothermic desorptionofCO2;at higherwater loading, the influenceofCO2 is notnoticeable.On theother hand, in Figure 9b, the heat of adsorption of CO2 is drasticallyreduced by the competitive coadsorption of H2O at CO2 loading of<1.68 mol/kg. At lower partial pressure of H2O (<0.05 kPa at298.15 K) where the adsorbent surface is partially covered by H2O,theCO2 isosteric heat decreaseswith increasingCO2 loading becausethe higher-energy adsorption sites (on activated alumina surface) arefilled preferentially at low CO2 pressure and the low-energy sites (onadsorbed water layers) are filled at higher CO2 pressure. At higherH2O pressure (e.g., 1.25 kPa or 40% RH), the adsorbent surface isfully covered by H2O molecules, and hence CO2 only adsorbs ontothe water layers with a low isosteric heat of 16.6 kJ/mol. Further-more, the isosteric value of CO2 increases exponentially with CO2

loading probably due to the enhancement of CO2 adsorption byhydration reaction with H2O of high humidity. However, many ofthese effects are complicatedandspeculative, and they require furthercalorimetric analysis for better definition.

5. Conclusions

The adsorption isotherms of the binary mixture of CO2 andH2O on activated alumina F-200 and the adsorption of theircorresponding single components were measured experimentallyby the combination of gravimetric and volumetricmethods. It wasfound that the adsorption equilibrium of single-component CO2

could be characterized by the Langmuir isotherm and the single-component H2O by the modified BET isotherm. In the binarymixture, CO2 and H2O were adsorbed onto the same type ofadsorption sites and compete with each other. The adsorption ofH2O was mildly depressed by the competition of CO2 at lowerrelative humidity, whereas the loading of CO2 in binary system is

almost unchanged except a slight decrease at higher water humid-ity due to the enhancement effect by cooperative adsorption ofH2O, in which CO2 was believed to have a secondary adsorptionon adsorbed water layers by forming complexes with watermolecules. Binary adsorption models, namely, the IAS-LBET,extended LBET, and the realistic interactive LBET (R-LBET),based on the Langmuir and modified BET isotherms for single-component adsorption, were derived to further study this highlynonideal binary system. Among them, only the R-LBET modelwas acceptable for correlating the binary data over the entirewaterhumidity region and also showed a good prediction of binaryadsorption at different CO2 partial pressures. This is because thebinary parameters with appropriate physical meanings wereincorporated into the R-LBET model for describing the interac-tions between CO2 and H2O molecules on activated aluminasurface, which could be determined by data fitting, even thoughthe real intermolecular interaction may be far more complicated.The competing models investigated here (IAS-LBET, extendedLBET) substantially underestimated the loading of CO2 of thebinary mixture, and particularly, the IAS theory failed to predictthe binary adsorption at higher H2O relative humidity because ofthe large difference between the maximum spreading pressures ofCO2 and H2O. The isosteric heat of adsorption of the binarysystem showed a noticeable decrease in comparison with those ofsingle components, except that the isosteric heat of CO2 increasedexponentially at a very high pressure of CO2 and high waterhumidity due to the speculative CO2 hydration reaction.

Acknowledgment. The project is financially supported byCooperative Research Centre for Greenhouse Gas Technologies(CO2CRC). We thank Mr. Barry Hooper for his review andtechnical support.

Supporting InformationAvailable: Somenecessary algebrafor the complete derivation of the binary LBET model. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

Figure 9. Estimated isosteric heat of adsorption of component (a) H2O and (b) CO2 of the binary gas mixture at constant partial pressure ofthe other component on activated alumina F-200. Symbols denote data points calculated from binary R-LBET model.