Detailed Simulation and Characterization of Highly Proton Conducting

Oct 9, 2009 - Institute of Physical Chemistry and Electrochemistry & Center for Solid State Chemistry and New Materials (ZFM), Gottfried Wilhelm Leibn...
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Detailed Simulation and Characterization of Highly Proton Conducting Sulfonic Acid Functionalized Mesoporous Materials under Dry and Humidified Conditions Roland Marschall,† Pia To¨lle,‡ Welchy L. Cavalcanti,‡ Michaela Wilhelm,§ Christof Ko¨hler,‡ Thomas Frauenheim,‡ and Michael Wark*,† Institute of Physical Chemistry and Electrochemistry & Center for Solid State Chemistry and New Materials (ZFM), Gottfried Wilhelm Leibniz UniVersity HannoVer, Callinstrasse 3a, 30167 HannoVer, Germany, Bremen Center for Computational Materials Science (BCCMS), UniVersity of Bremen, Am Fallturm 1, 28359 Bremen, Germany, and Ceramic Materials and Components, UniVersity of Bremen, Am Biologischen Garten 2/IW3, 28359 Bremen, Germany ReceiVed: May 8, 2009; ReVised Manuscript ReceiVed: August 20, 2009

Two different synthesis routes to prepare highly ordered and highly proton conductive sulfonic acid (SO3H) functionalized mesoporous silica materials are presented. The highest loading of Si-MCM-41 with SO3H groups was reached via a co-condensation route, which turned out to be advantageous compared to grafting, leading to hybrid materials exhibiting proton conductivities up to 0.2 S/cm at 100% relative humidity and 413 K. Especially at low humidities and under dry conditions, we show that the high group density of cocondensed materials is crucial for the proton transport. Experimental characterization, via impedance spectroscopy and water adsorption, and theoretical investigations of a simplified model system concordantly demonstrate the most important dependencies of the proton transport properties on the water content and on the density of SO3H groups. Computational studies were performed including classical molecular dynamic (MD) simulation and free energy calculation with a quantum mechanical method, which are showing comparable trends to the performed proton conductivity measurements of the material. Furthermore, the water uptake of the functionalized material was studied by MD simulation, which revealed the water density inside the porous system to be inhomogeneous in the case of low relative humidity. 1. Introduction Mesoporous silica materials, especially Si-MCM-41, have been the focus of research of porous materials and host-guest chemistry since their discovery in 1992.1 Their highly ordered structures, large surface areas and pore volumes, and easily accessible and variable surface functionalities substantially enhance the attractiveness of these materials. By surface modification with organic moieties they become excellent starting materials for creating hybrid organic-inorganic composites with an important application potential in catalysis, separation, and nanotechnology.2 When functionalized with sulfonic acid (-SO3H) groups, mesoporous silica materials become applicable in heterogeneous acid catalysis,3 and several recent publications document the continuing actuality of these materials in catalysis research.4 However, these hybrid systems also show excellent proton conductivities, as first demonstrated by Kaliaguine et al. already in 2002 for SO3H-functionalized mesoporous silica with nonordered porosity.5 Strict pore ordering improves the proton conductivity further, whereas Si-MCM-41 is more suitable than Si-SBA-15 and Si-SBA-16, because in the narrower channels protons can hop more easily between SO3- groups located on opposite sides of a pore channel.6 As known from former theoretical studies on highly ordered functionalized structures, the density of functional groups influences the intrinsic proton conduction ability of a material.7,8 * To whom correspondence should be addressed. E-mail: michael.wark@ pci.uni-hannover.de. † Gottfried Wilhelm Leibniz University Hannover. ‡ Bremen Center for Computational Materials Science (BCCMS), University of Bremen. § Ceramic Materials and Components, University of Bremen.

The degree of functionalization with conducting groups is, however, directly associated with the pore diameters, the pore volumes, and the concentrations of OH groups on the inner surface (silanol groups) of the mesoporous SiO2, which is roughly 2.8-3.2 silanol groups/nm2 for Si-MCM-41.9 Functionalization via grafting led on average to about one anchored conducting SO3H group/nm2; the serious problem, however, is the large inhomogenity in the group distribution resulting from the anchoring starting at the pore mouths.6 Employing a “cocondensation” of sodium metasilicate with (3-mercaptopropyl)trimethoxysilane (MPMS) as a sulfur-containing silica source, higher values and more homogeneous distributions of conducting groups per square nanometer can be reached.10 These materials are promising candidates as additives in hightemperature (413-453 K) polymer electrolyte fuel cell (HTPEMFC) membranes. This temperature range is highly favorable for operating the fuel cell system, because the cooling of the whole arrangement is simplified and the tolerance of the electrodes toward CO is increased, both factors enhancing the efficiency. SO3H was chosen as functional group in the materials, because it is not only known for efficient proton conduction but also used as component in many different polymer electrolyte membranes, which leads to a good compatibility with those systems. Some first attempts to use modified Si-MCM-41 particles as inorganic additives in proton conducting membranes like Nafion or sulfonated polyether ketones (SPEEK) have been published.11,12 The mesoporous additives are used to enhance the water adsorption capacity of the membranes, as effective water storage increases the proton conductivity and facilitates the water

10.1021/jp904322y CCC: $40.75  2009 American Chemical Society Published on Web 10/09/2009

Proton Conducting Mesoporous Materials management in the final fuel cell. Thereby, the porous structure of the Si-MCM-41 mesoporous molecular sieve turned out to be important.12 The SO3H-functionalized Si-MCM-41 has also proven a high influence on proton conductivity at temperatures above 373 K, especially in composite membrane applications with polysiloxanes or polyoxadiazoles.13,14 Proton-conducting properties of pure polymeric materials that contain SO3H groups operating at temperatures below 373 K have already been studied in experiments as well as in numerous theoretical studies.15-17 Among the factors that facilitate the proton transfer are charge delocalization within the SO3- groups, fluctuation motions of the sulfonic head groups and side chains, and a high water content.18 Several studies focus on the morphology and the dependence of proton transport on the water content.19-23 In the hydrated environment, water plays an important role and the proton transfer mechanisms is understood from the consideration of dissociation of the proton from the acidic site, subsequent transfer of the proton to the aqueous medium, shielding of the hydrated proton by water from the conjugate base (e.g., the sulfonate anion), and finally diffusion of the proton in the confined water within the polymer matrix.24 The SO3H thus increases the total number of intrinsic protons in the membrane. Under dry conditions or a low degree of hydration, the presence of neighboring SO3H groups play an important role in the proton transfer.25 Minimal energy conformations were determined for SO3H dimers under dry conditions26 and for SO3H dimer and monomer under low hydration conditions (water cluster of up to five molecules),17,25,27 revealing an estimated deprotonation of the acid at a hydration of only two to three water molecules per SO3H group with a formation energy of the deprotonated acid of about 7 kcal/mol.17 The aim of this work is a comparison between SO3H-functionalized mesoporous materials prepared by different synthesis methods (namely postsynthetic grafting and in situ co-condensation procedure) and exploration of properties leading to enhanced proton conductivity of such material using experimental analysis methods as well as computer simulation. Structure analyses have been performed via nitrogen adsorption, infrared (IR) spectroscopy, X-ray powder diffractometry (XRD), and water adsorption measurements. Water storage capability results of the synthesized materials lead to a better insight into the dependence on water of the proton conductivity of the materials. The local water environment of the SO3H groups is analyzed by MD simulation of a functionalized slab model at different humidity and temperature. In impedance spectroscopy measurements the influences of SO3H and SO3- group density and water content on the proton conductivity were observed, while simulations neglecting the mesoporous silica materials showed the same trends as shown by free energy barrier calculations for the transfer of a proton from one SO3- group to the next depending on the presence of different amounts of water and different distances between the groups. In addition, classical MD simulations are performed to analyze the number and distance of neighboring SO3H groups in a model system of SO3H attached to immobilizing alkane chain in the dry case. 2. Experimental Methods 2.1. Synthesis of Functionalized Mesoporous SiO2. SiMCM-41 and SO3H-MCM-41 were synthesized by following the homogeneous precipitation procedure.28 The SO3H-MCM41 samples prepared using 20, 30, and 40 mol % of MPMS and subsequent oxidation10 are designated as 20, 30, and 40% SO3H-MCM-41, respectively.

J. Phys. Chem. C, Vol. 113, No. 44, 2009 19219 For comparison, postsynthetically SO3H-grafted mesoporous SiO2 was synthesized according to the procedure presented earlier, where pristine Si-MCM-41 was grafted with MPMS. Again the anchored SH groups were subsequently oxidized.6 These samples are designated as SO3H-MCM-41 x mmol, where x indicates the amount of MPMS in the postsynthetical grafting reaction. 2.2. Characterization. X-ray diffraction (XRD) was measured with a Philips X′pert diffractometer at room temperature in the range from 1° to 10°, using Cu KR radiation (λ ) 0.15418 nm). The presence of SH and SO3H groups in the functionalized powders was checked by FT-IR spectroscopy, recorded in attenuated total reflection mode on a diamond crystal using a Bruker Tensor 27 instrument. Nitrogen adsorption-desorption experiments at the boiling point of nitrogen (77 K) were carried out with a Micromeritics ASAP 2010 apparatus. To calculate the specific surface areas, the Brunauer-Emmet-Teller (BET) method29 was used; pore diameters were derived from the adsorption branches of the isotherms by using the Barrett-Joyner-Halanda (BJH) method, and pore volumes were estimated from the amount of adsorbed gas at a relative pressure of p/p0 ) 0.95. All three methods have been described in detail before.6,10 Water vapor adsorption-desorption isotherms were obtained by a volumetric BELSORP 18-3 apparatus (Bel Japan, Inc.) at 295 K with an equilibration time of 500 s. The ion exchange capacities of the SO3H-functionalized samples were determined by titration. A small amount of powder was suspended in a 0.01 M sodium hydroxide solution for 48 h, the remaining sodium hydroxide being titrated with hydrochloric acid. The proton conductivity was measured by impedance spectroscopy (IS) using a Zahner electrochemical workstation IM6e in a frequency range from 1 to 106 Hz with an oscillating voltage of 100 mV. Prior to the measurement, the functionalized powders were pressed into small pellets of 8 mm diameter and 0.5-1 mm thickness, which were inserted between two thin graphite slices (8 mm in diameter). Afterward, they were put into a polytetrafluoroethylene (PTFE) specimen holder, which was located in a gastight stainless steel body with thermocouple access to the holder. This body was connected via a stainless steel tube to a stainless steel water reservoir. Relative humidity (RH) in the cell was controlled by adjusting the temperature of the water tank. The specific conductivity was calculated according to the formula σ ) (1/R)(L/A), where R is the resistance corresponding to the phase angle closest to zero in the Bode diagram, L the thickness of the sample between the electrodes, and A the cross-sectional contact area of the electrodes. This analysis procedure is typically used to interpret proton conductivities in powders or membranes.5 2.3. Simulation Strategy. A simple pair interaction is regarded as the driving mechanism for proton transport under dry or low humidity conditions with proton percolation through water within the pores being essentially blocked. Therefore, the distribution of water in the vicinity of SO3- groups and the distance between the groups are obtained from separate simulations, relating the experimental conditions to the microscopic scale. The macroscopic parameter ranges are chosen depending on the experimental characterization and the conditions in the fuel cell. They are as follows: a pore size of the silicon dioxide material of up to 3.0 nm, the density of functional groups on the pore wall surface of up to 2 groups/nm2, spacer lengths equal

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Figure 1. Slab model functionalized with SO3H groups at the inner surface: blue balls, carbon; white balls, hydrogen; red balls, oxygen; light yellow balls, sulfur; yellow balls, silicon.

to those of propyl or longer hydrocarbon chains, different humidity, and a temperature between 300 and 450 K. At first, a slab model is used to study the water uptake of a functionalized SiO2 pore environment by MD depending on the pore diameter, the density of groups, and the temperature. The resulting water density inside the pores is equivalent to 100% RH at the chosen condition. In a second step, the slab model of the 3.0 nm pore was filled with lower amounts of water to simulate environments of less than 100% RH. In order to get information about the local water environment of the SO3- groups related to the experimental conditions, the radial distribution function (RDF) and the averaged number of water molecules in the vicinity of SO3groups obtained by integration of the RDF is calculated as ∫RDF (N/V)4πr2 dr, where RDF is the radial distribution function, N is the total number of water molecules, and V is the volume of the simulation box. Further, the distance between SO3- groups due to the mobility of the alkane chain is analyzed by RDF from MD simulations of the slab model with a water environment as well as of a simplified model under dry conditions only varying the alkyl chain length and neglecting also the silica substrate. Via integration of the RDF, we get the averaged number of neighboring SO3- groups in the vicinity of a SO3- group. With the environment thus specified, we can apply basic transition state theory to gain detailed insight into the energetics of the proton transport between sites under varying water conditions. The reaction rate depends on the free energy barrier of the reaction by the coefficient exp(-∆G/kT), where G is the free energy barrier, k is the Boltzmann constant, and T is the temperature. Therefore, the density functional based tight binding method (DFTB)30,31 is applied to calculate energy barriers for the transition of an excess proton between two negatively charged methyl-SO3- groups. The number of water molecules and the distances of the functional groups are chosen according to the MD simulations referring the atomic simulation to the macroscopic conditions. Nuclear quantum effects were proven to have only minor influence on the transfer of protons in small water clusters.32,33 Therefore, we treated the protons as classical particles in this study. 2.4. Simulation Details. To study the water uptake of the SiO2 pore, a 1.0 nm thick R-cristobalite slab model (shown in Figure 1) saturated with hydroxyl groups is taken. The surface was functionalized with SO3H propylsilane chains with a density of 1.3 groups/nm2. Due to the periodicity in the direction perpendicular to the surface, the variation of the length of the simulation box vector leads to a change of the surface distance, i.e., the pore diameter. Diameters in the range from 1.5 to 3

nm are modeled. The SiO2 interaction parameters are those published for the CHARMM force field.34,35 The parameters for the functional groups are taken from the OPLS force field,36 and the charges are obtained by electrostatic surface potential analysis (ESP) of Hartree-Fock calculations with a 6-31G* basis using the NWCHEM program.37,38 Water is described by the TIP3P model.39 The classical MD simulations are performed with the GROMACS program. In a first step, the slab model with a surface area of 60 nm2 was coupled to a water reservoir of similar size with more than 15 000 molecules, and a MD simulation was performed with a isobaric-isothermal ensemble (NPT) with the system coupled to the Berendsen barostat40 and to the Nose-Hoover thermostat41,42 with a coupling time of 0.5 ps at 1 bar pressure and different temperature (300, 400, 450 K). The resulting water density inside the pore is equivalent to 100% RH. To obtain the water distribution and RDF’s at lower hydration levels, a slab model (of the 3.0 nm pore) was filled with different amounts of water. Another MD simulation was performed with a isochoric-isothermal ensemble (NVT) with the system coupled the Nose-Hoover thermostat41,42 at different temperatures (300, 450 K). After an equilibration of 1.0 ns at an elevated temperature and 0.5 ns at the simulation temperature, the density profiles and RDFs of SO3H groups and water were analyzed during 1 ns of production run. For the simple model neglecting the substrate, the simulation box is formed by at least 134 SO3H and one SO3- groups immobilized via alkyl chains, which are either arranged in an equally spaced grid on a flat two-dimensional surface or on the lateral surface of a cylinder by restraining the position of the two terminal carbon atoms of the alkyl chain. The inner atoms are located on the surface of a cylinder with radius 2.8 nm. In comparison to the experiment, only the unrestrained atoms are considered as the effective chain length, which varies between three and seven carbon atoms, namely propyl, pentyl, and heptyl, respectively (Figure 2A-C). Each system has been equilibrated for at least 2 ns; afterward, during a 2 ns production run the atomic positions are collected every picosecond. All MD simulations are run in the canonical (NVT) ensemble with the system coupled to a Nose-Hoover thermostat41,42 with a coupling time of 0.5 ps at 450 K. In the cylindrical arrangement, the density of groups equals 1.0 effective propyl-SO3H/nm2, while in the flat arrangement the density of groups is in the range of 0.25-2.0 effective propyl-, pentyl- or heptyl-SO3H groups/nm2. In the following, details of the free energy barrier simulations are given. The functional groups (methyl-SO3-) are located in a defined distance from each other due to position restraints

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Figure 2. (A) SO3H molecule with an effective propyl chain (three flexible C atoms), (B) pentyl chain (five flexible C atoms), (C) heptyl chain (seven flexible C atoms), (D) methyl-SO3H (energy barrier calculation). Restrained atoms/molecules are marked by circles, and the modified center of excess charge coordinate is marked by an arrow. Blue balls, carbon; white balls, hydrogen; red balls, oxygen; yellow balls, sulfur.

applied to the (terminal) carbon atoms, as shown in Figure 2D. The humidity of the model is varied by the numbers of additional water molecules in the environment constrained to a sphere with a radius of 1.5 nm by hard wall boundary conditions. In the low-humidity case, an umbrella sampling43,44 was performed using the DFTB method, where the reaction coordinate x is defined as the relative distance of the proton coordinate to the sulfur of the groups. Therefore, the modified center of excess charge coordinate (mCEC),45 which is implemented in the RXNCOR module of CHARMM,46 was used. The values x ) -1 and 1 are equivalent to the protonation of the right or the left neighbor. Using the additional potential k(x - x0)2 in several independent MD simulations with varied x0 between [-1, +1], the proton position is moved stepwise from one SO3H group to the other. After an equilibration of more than 6 ps per umbrella sampling step, a production run of 2 ps using a 1 fs time step for the isobar-isothermal ensemble (NPT) with a Berendsen coupling with coupling constant of 0.3 ps has been performed.40 To obtain the free energy barrier, the weighted histogram analysis method (WHAM)47,48 was used. In the dry case, the potential energy barriers are calculated using an analogous simulation scheme; for details, see ref 13. Due to the absence of water, entropic effects play consequently a minor role. Therefore, we take the potential energy as a good approximation of the free energy 3. Results and Discussion 3.1. Materials Characterization and Water Uptake. In order to get detailed information about both the porous structure and the chemical nature of the pore surface of the different assynthesized and functionalized hybrid materials, two very different adsorptives have been chosen, namely, nitrogen and water vapor. As described in detail in ref 10, pristine Si-MCM41 shows the typical type IV adsorption isotherms of nitrogen at 77 K, exhibiting a good hexagonal pore structure. The BET surface area, pore volume, and pore diameter of the Si-MCM41 achieve 1030 m2/g, 0.954 cm3/g, and 2.8 nm, respectively. Adding additional agents to the reaction mixture in a cocondensation reaction has a strong influence on the structure of the materials and their texture properties, because the micelles formed by the surfactant CTAB can expand and incorporate the additional silanes. Such pore expansion has been found, if up to 20% of the tetraethyl orthosilicate (TEOS) used as the only Si source in the Si-MCM-41 synthesis has been replaced by MPMS.10 By increasing the amount of substitution further,

however, the pores narrow and the changes in the texture become even more drastic. For the 40% SO3H-MCM-41 sample the nitrogen isotherm is of type I, and the BET equation is not applicable, which indicates the microporous nature of this sample. The loading of the materials with organic moieties is so much increased that the mesopores are narrowed and transformed to micropores of about 1.5 nm in diameter (the assessment based on the Langmuir surface area of 311 m2/g and the total pore volume of 0.171 cm3/g). The remaining hexagonal pore structure of the system can either be proven by TEM observation or XRD/adsorption measurements.10 Figure 3 depicts schematically the situation inside a pore of the 40% SO3H-MCM-41; pore diameter, size, and distances of the anchored groups are given in the correct relation. Due to the high loading, the propyl-SO3H groups directing into the pore channels are so densely packed that N2 molecules cannot enter the small voids between them. According to the ionic exchange capacity (IEC) of SO3H-functionalized materials, the co-condensation results in a loading of 1.6 mequiv/g for 20% SO3H-MCM-41 continuously increasing to 2.3 mequiv/g for the 40% SO3H-MCM-41, respectively, whereas a postsynthetic grafting route results only in loadings of about 1 mequiv/g. For a better understanding of the chemical nature of the surface of the pore walls of functionalized Si-MCM-41, adsorption isotherms of water vapor have been measured. Figure 4 exemplarily shows the adsorption isotherms of water vapor on postsynthetically grafted samples recorded at 295 K in the relative pressure range of p/p0 of 0.0-0.98, proving the strong influence of propyl-SO3H functionalization on the water management. The isotherms on the pristine Si-MCM-41 and grafted SO3HMCM-41 are of type V,49 the pore filling and emptying occurring within a narrow range of relative pressure at approximately 0.4 and 0.6, respectively. These values of relative pressure correspond to the pore diameters of about 2-3 nm, as calculated from Kelvin’s equation.50 The shift in the location of the sharp step in both adsorption and desorption isotherms toward lower relative pressure is on one hand due to a slight narrowing of the average pore width caused by the grafting but reflects also an altered interaction of the H2O molecules with the chemically modified walls.51,52 At high p/p0, pure Si-MCM-41 shows a very high water uptake up to 52% of the mass of the dry mesoporous material. This can be explained by the hydrophilic surface of the material, bearing 2.8-3.2 silanol groups/nm2. When these silanol groups

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Figure 3. Model used in simulation (see Table 1) corresponding to an idealized distribution of functional groups in a Si-MCM-41 pore with diameter of 2.8 nm. (A) Schematic view, where only the fully drawn groups (blue balls, carbon; white balls, hydrogen; red balls, oxygen; yellow balls, sulfur) are within one plane and the other pale chains are anchored behind those, representing the three-dimensionality. (B) Snapshot of the simulation; the outer two C-atoms were kept fixed during the simulation and thus represent two parts of the pore walls.

TABLE 1: Averaged Water Densities inside Pure and Functionalized SiO2 Pores of Specified Pore Diameters Modeled by the Slab Model at Specified Temperature and at 1 bar Pressure water density inside the pore (g/cm 3)

pore diameter (nm)

number of SO3H groups/nm2

300 K

400 K

450 K

3.0 3.0 2.5 2.0 1.5

0.0 1.3 1.3 1.3 1.3

0.99 0.95 0.96 0.93 0.84

0.87 0.9 0.93 0.9 0.81

0.81 0.8 0.78 0.78 0.75

are used to bind organic moieties, the alkoxysilanes need 1-2 silanol groups to attach to the Si-MCM-41 surface. Thus, the number of silanol groups and consequently the pore wall hydrophilicity are drastically decreased. In addition, functionalization with MPMS implies the addition of hydrophobic propyl chains ending in SH groups of lower hydrophilicity than OH groups. These three facts cause a decrease of water uptake for propyl-SH-functionalized Si-MCM-41, indicating the increasing hydrophobicity of the sample. As a result, the condensation in the pores is delayed, although, as found by N2 adsorption, the

Figure 4. Water adsorption isotherms at 295 K for SH-MCM-41 (∆), compared to pure Si-MCM-41 (O) and SO3H-MCM-41 (b). Loading was achieved via grafting with 20 mmol of MPMS and subsequent oxidation of the same sample for ideal comparison. IEC ) 1 mequiv/g.

pore diameter is only slightly narrowed by 10%.52 When the SH groups are subsequently oxidized, the weakly hydrophilic SH group is converted into the much more hydrophilic SO3H group. The high hydrophilicity of this end group overcompensates the hydrophobicity of the propyl chain and the decreased number of surface silanol groups, resulting in a water uptake of 56%, even slightly higher than for the pure Si-MCM-41 material. In fact, this high water storage capability and hydrophilicity makes SO3H-functionalized Si-MCM-41 suitable as efficient solid proton conductors. In parallel to the experimental sorption isotherms, the water uptake in pure and functionalized SiO2 pores of 1.5, 2.0, 2.5, and 3.0 nm diameters, respectively, was analyzed by MD simulation in a NPT ensemble. The average densities are shown in Table 1; these densities will be referred to as 100% hydration at 1 bar pressure and the indicated temperatures. As observed in the experiment, where the mass of adsorbed water is always related to the dry weight of the functionalized samples, the water uptake of the SO3H-functionalized pores is in the same range as for the empty pores, although the volume occupied by the functional groups is not considered in the calculation of the water density. There is only a minor influence of the pore diameter on the water density inside a pore. A significant drop of the average density only appears at a diameter of 1.5 nm and a temperature of 450 K. Under these conditions, the neglect of the volume of the introduced groups gets critical, as the density was estimated only by counting the water molecules and dividing this number by the volume of a unfunctionalized pore. The error introduced by neglecting the volume of the functional groups may be estimated by calculating the volume of a functional group characterized by the van der Waals radii of the atoms. The volume per group equals 0.14 nm3, which is in the range from 25% to 10% of the pore volume of the 1.5 and 3.0 nm pores, respectively. As expected, the water density is lowered by increasing the temperature; it decreases by less than 5% from 300 to 400 K but more than 10% between 400 and 450 K. In order to obtain information about the water in the vicinity of the SO3H groups, MD simulations in an NVT ensemble of the slab model were performed at different water content (between 19 and 97% of maximal water uptake) and at different temperatures (300, 400, and 450 K). In Figure 5, the average density profiles are plotted for different position on the surface.

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Figure 5. Water density profile perpendicular to the pore wall depending on the temperature (black, 450 K; gray, 300 K) and the relative humidity inside the pore. 100% equals the maximal water uptake at 1 bar pressure. The sulfur atoms of the SO3- groups are marked as spheres and the water molecules are shown in ball and stick presentation. Blue, carbon; white, hydrogen; red, oxygen; light yellow, sulfur; yellow, silicon. The point of origin to the distance perpendicular to the surface is the first layer of Si atoms, which are situated inside the pore surface.

Figure 6. The average number of water molecules in the vicinity of the SO3- groups obtained from sulfur-water RDFs, as explained in section 2.3; the thick lines show the number of water molecules in the vicinity of selected SO3- groups (A and B to show the deviation from the averaged result) in the case of 53% of maximal water uptake.

In all simulations, the density decreases close to the surface, while the maximal density is observed at distances of about 0.5-1.0 nm from each wall. This distance reflects roughly the length of the linkers in alkylsulfonate groups, indicating that the water density is highest in the region of the very hydrophilic SO3- groups. In the high hydration cases of 97% (at 450 K) and 82% (at 300 K), a constant density of 1 g/cm3 similar to the density of (liquid) bulk water is reached inside the pore. In the case of 53% (at 450 K) and 45% (at 300 K), a very inhomogeneous distribution of water molecules is observed. In some regions a connection between the two surfaces is formed by high density paths of water, while the water density in other parts of the pore equals zero. In the case of 20% hydration, the middle of the pore is empty; the connecting water bridges completely disappeared. To analyze the vicinity of the SO3- groups depending on the humidity (Figure 6), the RDF is integrated by volume. The average number of water molecules in a distance of 0.8 nm around a SO3- group differs among 15, 35, and 48 molecules for 450 K and water uptake of 20%, 53%, and 97%, respectively.

Figure 7. Proton conductivities for the pressed powders of SO3HMCM-41 5 mmol (white box), 10 mmol (gray box), and 20 mmol (black box) (postsynthetically grafted) and for (co-condensation) 20% (white dot), 30% (gray dot), and 40% (black dot) SO3H-MCM-41 at 100% RH, compared to an about 500 µm thick Nafion foil (black triangle) measured under the same conditions. The connecting lines are only for guiding the eyes. (Results are taken from refs 6 and10.).

In the simulations leading to the water bridges (53% of maximal water uptake at 450 K and 45% at 300 K), the nonaveraged local density differs from the average by about 25%, due to the inhomogeneity of the distribution. 3.2. Dependence of Proton Conductivity on Water Content and SO3H Group Density. A comparison of the proton conductivities of the different SO3H-functionalized Si-MCM41 materials measured under full hydration (100% RH) is shown in Figure 7. Typically, proton conductivities of these materials increase continuously with temperature, even above 373 K. No conductivity drop above 373 K is visible, which is often the case when sulfonated fluorocarbon polymers like Nafion were measured at elevated temperatures.52,53 As expected, the proton conductivities are increasing with increasing the number of SO3H groups in the pores of the SiMCM-41 materials. In the case of grafted samples (squares in Figure 7), proton conductivities increase continuously with the

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Figure 8. Dependence of proton conductivity for (postsynthetically grafted) SO3H-MCM-41 20 mmol on the relative humidity [0% (white dot), 50% (gray dot), 100% (black dot)] in the measurement cell; connecting lines are only for guiding the eyes.

amount of MPMS offered in the grafting procedure. In these samples, the ion exchange capacities increase from 0.8 to 0.9-1 mequiv/g.54 Theoretically, from the offered amounts of MPMS, IECs up to 1.67 mequiv/g might be possible, but blocking effects and hindrance of MPMS diffusion into the pores restrict the loading.6 In a co-condensation process, the limitations of a grafting process are no longer important because the functionalization agent MPMS is already present in the synthesis mixture and is incorporated in the silica framework together with the hydrolyzing and condensing silica precursor TEOS. Thus, the loading of SO3H groups in the mesoporous silica materials could be significantly enhanced to roughly 1.6 SO3H groups/nm2, and also their distribution was found to be more homogeneous.10 Taking the surface area of the material into account, an IEC of 2.3 mequiv/g results for the 40% SO3H-MCM-41 sample. At a RH of 100% with the density of water inside the pore found to be similar to bulk water, a proton conductivity of 0.2 S/cm is reached. The majority of the SO3H is deprotonated and most of the proton transport will take place only via the water molecules in the pores, slightly supported by the remaining SO3groups. Vehicular and structure diffusion of protons both take place, similar as in bulk water neglecting the influence of the pore in a first approximation. Under real conditions in a fuel cell operating at elevated temperature, however, the hydration is lower. Therefore, we analyzed the influence of the RH on the proton conductivity of maximum sulfonated Si-MCM-41 materials via IS measurements and computational methods. In Figure 8 (postsynthetically grafted) and in Figure 9 (cocondensation), the performances of the two classes of functionalized materials are shown under different RH. At dry conditions, the co-condensation sample exhibits a 3-4 orders of magnitude higher conductivity than the grafted sulfonated sample and reaches 10-4 S/cm at 413 K. This demonstrates that via co-condensation, even at dry conditions, good proton conductivity values can be achieved, due to the very high and homogeneous loading of the system. At high degrees of loading neighboring SO3H groups can more easily get in contact, enabling proton transfer, as was shown by MD simulation (see below). For both samples, the increase with temperature from 333 to 373 K is low (only 1 order of magnitude) in the dry condition case compared to the hydrated case, where the change results

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Figure 9. Dependence of proton conductivity for 40% SO3H-MCM41 (co-condensation) on the relative humidity [0% (white box), 50% (gray box), 100% (black box)] in the measurement cell; connecting lines are for guiding the eyes.

Figure 10. Average number of sulfur atoms (in SO3- groups) in the vicinity of a studied SO3H groups obtained from sulfur-sulfur RDF, as explained in section 2.3. Different model systems are compared by varying the density of SO3H-containing linkers, the effective alkyl chain length, and the model type (slab model, simple model neglecting the substrate, and simple model with cylindrical ordering of the groups).

in at least 2 orders of magnitude. The effect of water on the proton conductivity is stronger in the case of the grafted sample, where an increase by approximately 4 orders of magnitude is measured going from dry conditions to 100% RH, while in the case of the co-condensation samples the change is only by 1-3 orders of magnitude. The most obvious difference in the two samples is the density of SO3H groups inside the pore system, as was pointed out already. Under completely dry conditions, a MD simulation of model systems with different group density was performed, in addition to the MD simulation of the slab model at different humidity conditions and 1.3 nm distances of the functional groups. A homogeneous arrangement of the functional groups on the inner porous surface is considered and the SiO2 material is neglected. The differences of the co-condensation and grafting samples are reduced in this simulation to differences in the density of functional groups. Figure 10 shows the distribution of sulfur of SO3- groups in the vicinity of a studied SO3- group, resulting from the integral of the sulfur-sulfur RDF (see section 2.3). For a density of 1 group/nm2, there are on average three other sulfur atoms in a distance of 0.5 nm around the sulfur atom, while for lower density, only one or no other sulfur atom is that close. This means that only a smaller number of neighboring groups is getting in contact, so that the direct proton transport

Proton Conducting Mesoporous Materials is lowered, as seen in experiment (see Figures 8 and 9). The stepwise increase of the curves indicates strong inhomogeneity. Three next SO3- neighbors are at an equal distance of 0.5 nm, while the second next SO3- neighbors are situated at about 1 or 2 nm distances in the cases of densities of 2 and 1 group/ nm2, respectively. Further, the influence of the chain length and the geometric arrangement was analyzed. A longer chain length has a similar effect as a higher density of linkers; the plot in Figure 10 shows a local maximum at a sulfur-sulfur distance of 1 nm in the case of the effective pentyl chain, while in the case of heptyl chain the increase is continuously steeper than in the case of propyl. A slight lowering of the number of next SO3- neighbors in both cases indicates sterical hindrance. The cylindrical model with a density of 0.5/nm2 shows a qualitatively similar behavior as the 1/nm2 and 0.5/nm2 flat models, but at similar density, a significantly higher number of next SO3neighbors and a stronger increase in the range of second neighbor SO3- groups is observed as an effect of the cylindrical arrangement. As the different models reflect a qualitatively similar behavior, the choice of the model does not influence the general behavior in the dry case, but for a quantitative estimation, the physical parameters of the model, such as density and pore diameter, would have to be interpreted with caution in comparison to the experimental results. Experimentally in comparison to the results from the slab model simulation, the stepwise increase is suppressed due to the interaction with water, and the number of neighbors in the vicinity of the SO3- group decreases in the short distance range. Therefore, the proton transport over elongated distances becomes more important. In general, for a long-range transport in the dry or low hydrated case, we also considered the proton transport over the elongated distances between groups to be the limiting step, which has to be overcome through vehicular proton transport by a moving functional group or through transport with intermolecular water molecules between groups if the distance exceeds 1 nm. Further, in the case of inhomogeneous distribution of functional groups, as in the case of the postsynthetically grafted materials, regions of low densities are limiting the direct proton transport between functional groups. Therefore, we analyzed the effect of water on the free energy barrier for the proton transfer, taking into account different distances between the SO3H groups. A low free energy barrier leads to a higher reaction rate for the proton transport, which corresponds to a higher conductivity of the material. As shown in Figure 11, the barrier strongly depends on the number of water molecules that are included in the system. For a distance of 2 nm between the methyl SO3H groups, the barrier is lowered from about 26 to 15 kcal/mol by increasing the number of water molecules from 10 to 18. At 450 K, these differences in barrier heights lead to a change in the reaction rate [∼exp(-∆G/kT)] of 8 orders of magnitude, going from a factor of 2.5 × 10-13 to 5.1 × 10-5. For a shorter distance between the groups (1.2 nm), the free energy barrier at 10 molecules is even lower with 8 kcal/mol, corresponding to a reaction rate coefficient of 1.2 × 10-4. Upon increasing to 18 molecules, the barrier stays constant. Here the effect of the additional water molecules is much smaller than for the larger distance (2 nm). This indicates that the local water environment is the dominant factor for the barrier height. For small distances the additional molecules are not directly involved in the reaction and, thus, contrary to the case of larger distances, do not contribute. A number of on average 10-20 molecules in the vicinity of the SO3H group describes the low humidity case of less than 20% hydration, but due to the inhomogeneities, low density regions also exist

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Figure 11. Free energy barrier plot for the proton transfer between two methyl SO3H molecules in the presence of 10 (gray line) or 18 (dark line) water molecules. The methyl SO3H molecules are restrained at the carbon atom with distances between the carbon atoms of about 2 nm (dashed) and 1.2 nm (continuous), which leads to reachable minimum distances of 1.5 and 0.7 nm, respectively, between the oxygen atoms of different SO3- groups.

Figure 12. Energy barrier plot for the proton transfer between oxygen atoms belonging to two methyl SO3H molecules: + (DFTB), × (DFT, B3LYP/6-311G*); for details, see ref 13.

in cases of more than 20% humidity; see section 3.1. These results also confirm the data presented in Figures 8 and 9, where it was shown that water has a much stronger effect on the proton conductivity of SO3H-MCM-41 in the case of low group density (grafted sample, Figure 8) with larger distances between the functional groups, compared to the co-condensed samples (Figure 9). Representing the extreme situation of a completely dry pore, Figure 12 shows the potential energy barriers for a proton jump as a function of the O-O distance between neighboring SO3H groups. The curves may be interpreted as an upper limit for the free energy in the low humidity case. The potential energy barrier strongly increases for increasing distance between the oxygen atoms that are involved in the proton transport; an O-O distance of only 0.33 nm (corresponding to a C-C distance of far less than 1.2 nm) results already in a barrier of about 30 kcal/mol. Therefore, in the limit of completely dry conditions, a short next neighbor distance of the SO3H molecules is crucial for the proton transport between the groups. This is again in agreement with the results presented in the Figures 8 and 9, where a high SO3H group density established via co-condensation resulted in 4 orders of magnitude higher proton conductivity under dry measurement conditions compared to grafted samples.

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4. Conclusion Two methods for the preparation of highly proton conductive, highly ordered SO3H-functionalized Si-MCM-41 sampless postsynthetical grafting and in situ co-condensation synthesiss are presented. Pellets pressed from the synthesized powders showed very high proton conductivities of up to 0.2 S/cm at 100% RH. Whereas for other kinds of proton conducting membranes, e.g., Nafion, the proton conductivity decreases drastically with temperature,13,54 the proton conductivity of these materials increases continuously with temperature. In the experiments, high loading with organic moieties achieved by the co-condensation synthesis turned out to be a crucial advantage compared to the postsynthetic grafting, especially in the dry case, leading to an efficient proton transport even in presence of little water. Molecular dynamics simulations show that this and the stronger dependence of the conductivity on the water transport for the grafted sample are a direct result of the interplay between water content and group density. At lower humidity, the water is inhomogeneously distributed within the pore, even at elevated temperatures. For a low density of SO3H groups, as obtained via grafting, it follows from the simulations that the proton transport barrier varies strongly with the number of surrounding water molecules, while for a larger density, as in the cocondensated samples with a smaller distance between groups, the effect on the barrier is comparatively smaller. The conductivity also depends on the number of excess protons, which also is expected to change with the water content of the pore. As a further effect, the simulation results suggest that also the change of the barrier of proton hopping due to the change in density and thus distance between the functional groups, which contributes via Boltzmann’s law exponentially to the transport, plays an important role. Further, the high IECs enhance the guided proton transport through the pore channels, resulting in proton conductivities in the range of those of the most promising solid proton conductors, like heteropolyacids or solid boron phosphates.55,56 Acknowledgment. The work was supported by the Deutsche Forschungsgemeinschaft (DFG) (WA 1116/15, SPP1181) and the DAAD (D/07/01324). The authors thank Dr. Michael Hoffmann (IFAM, Bremen), Dr. Volker Weiss (BCCMS, University of Bremen), Dr. Jiri Rathousky (J. Heyrovsky Institute Prague), and Prof. Ju¨rgen Caro (Institute of Physical Chemistry, Leibniz University Hannover) for their fruitful contributions. Roland Marschall gratefully acknowledges a Georg-Christoph-Lichtenberg scholarship by the Ministry of Science and Culture of the German State of Lower Saxony. The WHAM program was implemented by Alan Grossfield. References and Notes (1) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) (a) Stein, A. AdV. Mater. 2003, 763, 15. (b) de A.A. Soler-Illia, G. J.; Sanchez, C.; Lebeau, B.; Patarin, J. Chem. ReV. 2002, 102, 4093. (c) Vinu, A.; Hossain, K. Z.; Ariga, K. J. Nanosci. Nanotech 2005, 5, 347. (d) Hoffmann, F.; Cornelius, M.; Morell, J.; Fro¨ba, M. Angew. Chem., Int. Ed. 2006, 45, 3216. (3) Melero, J. A.; van Grieken, R.; Morales, G. Chem. ReV. 2006, 106, 3790. (4) (a) Mbaraka, I. K.; Shanks, B. H. J. Catal. 2005, 229, 365. (b) Shimizu, K. I.; Hayashi, E.; Hatamachi, T.; Kodama, T.; Higuchi, T.; Satsuma, A.; Kitayama, Y. J. Catal. 2005, 231, 131. (c) Sow, B.; Hamoudi, S.; Zahedi-Niaki, M. H.; Kaliaguine, S. Microporous Mesoporous Mater. 2005, 79, 129. (d) Mbaraka, I. K.; McGuire, K. J.; Shanks, B. H. Ind. Eng. Chem. Res. 2006, 45, 3022. (e) Reddy, S. S.; Raju, B:D.; Kumar, V. S.; Padmasri, A:H.; Narayanan, S.; Rama Rao, K:S. Catal. Commun. 2007, 8, 261.

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