Silanol-Related and Unspecific Adsorption of Molecular Ammonia on

ARTICLE pubs.acs.org/JPCC

Silanol-Related and Unspecific Adsorption of Molecular Ammonia on Highly Dehydrated Silica M. Armandi,† V. Bolis,‡ B. Bonelli,§ C. Otero Are
an,|| P. Ugliengo,*,^ and E. Garrone*,§ †

Center for Space Human Robotics @Polito, Istituto Italiano di Tecnologia, Corso Trento, 21, 10129 Torino, Italy Dipartimento DiSCAFF, Universita del Piemonte Orientale “A. Avogadro” and INSTM Unit Piemonte Orientale, Largo G. Donegani 2/3, I-28100 Novara, Italy § Department of Materials Science and Chemical Engineering, Politecnico di Torino and INSTM Unit Torino-Politecnico, Corso Duca degli Abruzzi 24, I-10129 Torino, Italy Departamento de Química, Universidad de las Islas Baleares, 07122 Palma de Mallorca, Spain ^ Dipartimento di Chimica IFM and NIS Centre of Excellence, Universita di Torino, via P. Giuria 7, 10125 Torino, Italy

)

‡

ABSTRACT: The interaction of molecular ammonia with the highly dehydrated surface of a common nonporous amorphous silica (Aerosil 300 from Degussa) comprises two phenomena: (i) the widely studied H-bonding interaction with isolated surface silanols, with typical IR features, and (ii) an unspecific interaction that escaped detection so far, being not conspicuous in the IR spectra and only clearly revealed by quantitative measurements. These two adsorption processes occur simultaneously, notwithstanding a marked difference in their corresponding interaction energy. The former process is well described by a Langmuir model, the latter by a Henry-type adsorption isotherm. Coupling of adsorption microcalorimetry with IR spectroscopy at a controlled temperature showed that the silanol-related interaction has ΔH0 =
58.4 kJ mol
1 and ΔS0 =
218 J mol
1 K
1 (reference: 303 K, 1 mbar), whereas the unspecific interaction has ΔH0 =
26.9 kJ mol
1 and ΔS0 =
135 J mol
1 K
1. The perturbation induced on the IR modes of adsorbed ammonia molecules is small in both cases, and the corresponding frequencies close to those of the gaseous species. Experimentally determined energy values and vibrational features were compared with corresponding results of ab initio calculations (comprising dispersive contributions) on the interaction of ammonia molecules with a slab model of amorphous silica recently investigated by using large-scale periodic B3LYP calculations. All features, both energetic and vibrational, of the silanol-related ammonia adsorption process are accounted for very satisfactorily. Computational results suggest that unspecific adsorption takes place on dehydrated patches of the surface and that such adsorption is dominated by dispersion interactions, while no features of H-bonding are present. The calculated ΔH0 value (
17.5 kJ mol
1) is remarkably smaller than the corresponding experimental value (
26.9 kJ mol
1), probably because of some inadequacy of the model to represent the actual structure of the dehydrated patches. The simultaneous occurrence of a weak and a relatively strong ammonia
silica interaction, as well as the absence of other possible adsorption modes, is discussed.

’ INTRODUCTION Silicon dioxide, commonly known as silica,1 constitutes the major component of the Earth's crust where it can occur in several crystalline forms, as well as in a wide variety of amorphous hydroxylated and hydrated forms. In addition to that, synthetic silica is produced in very large amounts for a broad range of technical applications that span inter alia the field of ceramics, adsorbents, catalysts, and biomaterials. Examples of technical usage of silica, mainly in the amorphous form, can be found in chromatography, selective separations and immunoassays,2
5 bioglasses and drug delivery systems,6
11 water purification,12
14 and heterogeneous catalysis (mainly as a catalyst support).15
18 Propelled by the foregoing range of applications, which heavily lean on surface properties, a countless number of articles already dealt with several aspects of the silica surface chemistry. However, many elusive details involving mainly surface acidity, r 2011 American Chemical Society

hydrogen bonding, and weak adsorbate
adsorbent interactions are still in the need of clarification. Worth adding is that, besides technical applications, improved knowledge of the silica surface would also help the understanding in several other related fields, such as the toxicity of inhaled mineral dusts19,20 and the possible role of silica in the development of prebiotic chemistry.21
23 Being the underlying amorphous solid in nature, the description of the related surface is difficult. In the past, in the absence of better models, reference has been made to selected planes of silica polymorphs.24,25 Later, computer models of disordered silica surfaces have been attempted,26 but only recently, computing power has increased enough to allow quantum-mechanical Received: April 5, 2011 Revised: October 13, 2011 Published: October 14, 2011 23344

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The Journal of Physical Chemistry C models of the amorphous surface to be developed.27
29 The complex nature of the (even partially) hydrated surface is witnessed by the correspondingly rich IR spectrum in the O
H stretching region.4,30,31 Thermal treatment in vacuo at a relatively high temperature, for example, above 700 K, leads to a much simpler situation. IR spectra of highly dehydrated silica typically shows an absorption band at 3747 cm
1, coming from noninteracting silanol species,4,30,31 as the only feature in the O
H stretching mode region. Its very small half-width further suggests a rather homogeneous nature of the O
H species involved.4,30
32 NMR spectroscopy basically confirms this finding and further indicates that only a minor fraction of such hydroxyl groups consists of geminal O
H species.4,33 The surface concentration34
37 of silanol species is about 1 OH/100 Å2: this corresponds to an average distance between adjacent silanols of the order of 10 Å, which suggests that no mutual interaction takes place. For these reasons, the species is termed isolated silanol: in the following, it will be represented by the symbol tSiO
H. Detailed IR studies of the vibrational properties of tSiO
H are available, and all modes have been singled out and characterized, including the elusive low-lying torsional mode.36 The IR spectroscopic study of the interaction of tSiO
H with simple molecules constituted a popular research field in the 70s, and in the 80s, these types of interactions were modeled quantum-mechanically via a cluster approach.38,39 Evidence being that they are basically all equal and noninteracting, the tSiO
H species can be regarded as constituting an ideal ensemble in the thermodynamic sense. Accordingly, the interaction between the isolated silanols of dehydrated silica and a number of adsorbed molecules, among which unsaturated hydrocarbons,40,41 carbonyl compounds,42 and furan derivatives43 can be described by Langmuir-type isotherms, as far as both volumetric data and optical isotherms are concerned. The interaction of ammonia with the dehydrated surface of amorphous silica seems to constitute an exception. IR studies30,31,34
36 show that H-bonding between the H-acceptor ammonia molecule and the H-donor tSiO
H species takes place and shows its typical spectroscopic features, no other process having been singled out. Nevertheless, measured volumetric isotherms have a non-Langmuir-type behavior, and correspondingly, the differential heats of adsorption measured via microcalorimetry show a steadily declining trend with increasing coverage, instead of keeping a constant (coverageindependent) value.44 The foregoing, seemly inconsistent observation, prompted us to re-examine ammonia adsorption on dehydrated silica: the solution of a scientific puzzle was the driving force, not indeed the study of the surface acidity, which, on the one hand, has been already examined in detail, and for which, on the other hand, the use of an ammonia molecule as a probe molecule is not entirely advisable.45 The method employed was the joint use of microcalorimetry and IR spectroscopy, under conditions as close as possible. Besides the sample of severely dehydrated, amorphous silica employed, a sample of all-silica MFI zeolite (commonly termed silicalite) was used. A word of warning is required: current silicalite has Silicon defects, originating so-called hydroxyl nests, which show an unexpected acidic behavior, so to be used in reactions, such as the Beckmann rearrangement. The one employed here is flawless, so it shows no polar terminations and acidity and can be regarded as an example of locally “flat”

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rigid silica patches only composed of siloxane moieties, smoothly joined together. The experimental results obtained were analyzed in the light of ab initio quantum-mechanical computations (inclusive of dispersion interactions) of ammonia adsorption on a recently developed model of the dehydrated surface of amorphous silica.29 A satisfactory overall understanding of ammonia adsorption on dehydrated amorphous silica was thus obtained, which showed that the adsorption process is more complex than expected, as illustrated below.

’ MATERIALS AND METHODS The chosen material was an amorphous nonporous silica (Aerosil 300 from Degussa) obtained by SiCl4 flame pyrolysis; it had a specific surface area of 296 m2 g
1 (see ref 44). The sample was outgassed (activated) for 2 h at 1073 K (residual pressure p < 10
5 mbar) to get rid of adsorbed water and mutually interacting silanols. A defect-free inert all-silica MFI zeolite was also employed as an example of an apolar silica surface characterized by unspecific interaction with NH3. Details on the preparation and structural characterization of the sample are found in refs 44 and 46. The sample in this latter case was preoutgassed overnight at only 303 K (residual pressure p < 10
5 mbar), in that this temperature was sufficient to get rid of all of the physisorbed species. Adsorption Volumetry and Microcalorimetry. Heats of adsorption were measured at 303 K by means of a heat-flow microcalorimeter (Calvet MS Standard by Setaram, France) connected to a gas-volumetric glass apparatus, allowing the simultaneous determination of both integral heats evolved (ΔQinti) and amounts adsorbed (Δna,i) in the adsorption of successive small doses of the adsorptive. The ammonia pressure was monitored up to ∼100 mbar by means of a transducer gauge (Barocell 0
133 mbar, Edwards).44 Total adsorbed amounts (Na = ΣiΔna,i) and integral heats (Qint = ΣiΔQinti), when reported as a function of the equilibrium pressure, yield the volumetric and calorimetric isotherms, respectively. Differential heats of adsorption, which are generally defined as the derivative of the Qint = f(Na) function, qdiff = dQint/ dNa (see ref 44), will be determined in the present case from the equilibrium data (interpolated volumetric and calorimetric isotherms), as explained in the Results and Discussion section. After a first adsorption run on the activated sample, ammonia was evacuated by pumping off overnight (p e 10
5 mbar), and a second run was performed. The first and second run isotherms (both volumetric and calorimetric) coincided, indicating that the interaction between NH3 and silica is reversible. IR Spectroscopy. The IR cell used was a commercial one (AABSPEC), equipped with a capacitance pressure gauge (CTR100, Oerlikon-Leybold) and an electronically controlled heating element, so that a constant temperature inside the cell could be imposed, which was monitored by a K-thermocouple (in contact with the sample wafer) connected to a digital thermometer (CHY 502 A, Tersid). The accuracy of the pressure and temperature measurements was (0.20% and (0.05% (of the read value), respectively.47 The Aerosil 300 sample was pretreated following the same procedure as above. A series of IR spectra were recorded while keeping the sample at the constant temperature of 303 K 23345

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The Journal of Physical Chemistry C (the temperature adopted for calorimetric measurements) and varying the ammonia pressure inside the cell, which thus acted as an open system.48,49 Computational Methods. All calculations were carried out using the CRYSTAL09 code.50,51 Recent work29 has shown that a slab model apt to simulate the surface of amorphous silica can be derived from bulk cristobalite, which was “melt” at 6000 K using classical molecular dynamics and then quenched to room temperature, slowly enough to avoid the presence of strained two-membered (Si
O)2 rings in the final optimized glasslike structure. A slab about 15 Å thick was cut out from the resulting bulk glass, and the unsaturated Si/Si
O valences were satisfied by OH/H groups, respectively. The surface was then dehydroxylated in subsequent steps, in which couples of nearby hydroxyl species were eliminated after visual inspection, simulating the thermal dehydration of a real silica sample. Resulting slabs exhibited a OH density ranging from 7.2 to 1.2 OH/100 Å2. Each structure was fully optimized at the B3LYP/6-31G (d,p)52,53 level using a massive parallel version of CRYSTAL09 running on 128 CPUs at the Barcelona Supercomputing center. The reader is referred to ref 29 for details. In the present case, the layer with 1.2 OH/100 Å2 calculated at the B3LYP/6-31G(d,p) level was chosen as the starting structure. The resulting unit cell (12.4 Å  13.1 Å  ≈15 Å (thickness)) contains more than 180 atoms, without any symmetry constraints. Because the target of the calculation was the interaction of ammonia with the surface, the adopted strategy was as follows. Instead of the more satisfactory, widely used, B3LYP52,53 functional, we turned to the functional proposed by Perdew
Burke
Ernzerhof54 (PBE), which is less effective but allows faster calculations. Besides PBE (not inclusive of dispersion), we also considered the functional (hereafter, PBE-D) inclusive of the term accounting for dispersive interactions suggested by Grimme.55,56 To reduce further the computational burden, we (i) decreased the number of atoms to 120, by reducing the thickness of the slab to ≈10 Å; (ii) fully relaxed the free slab structure only at the PBE level of computation; and (iii) optimized at the PBE-D level only the positions of atoms in OH and NH3, which are those expected to change position significantly during the NH3 adsorption process. The Gaussian basis set for geometry optimization and frequency calculation was taken from the CRYSTAL Web site (http://www.crystal.unito.it). As the 6-31G(d,p) is affected by a rather large basis set superposition error (BSSE), single-point energy calculations on the PBE-D/6-31G(d,p) structures have been performed with the more flexible TZP basis set from Ahlrichs and co-workers,57 in which the most diffuse Gaussian function for Si has been removed from the original TZP basis set to avoid numerical instability during the energy calculation. As the unit cell is rather large, the Hamiltonian matrix is diagonalized at the Γ point only without serious error in the total energy. Coulomb series have been truncated adopting as values of the tolerances (7, 7) (see ref 51 for details). Integration of the exchange/correlation functional over radial and angular coordinates is performed setting the XLGRID option,51 which reduces the error in the total integrated electron density to 6  10
5 electrons upon the 1080 electrons in the unit cell. The SCF process has been stopped when the energy difference between cycles was smaller than 10
8 and 10
10 hartree in geometry optimizations and frequency calculations, respectively. Internal coordinates and cell parameters optimization has been

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Table 1. Interaction Energies (ΔE, ΔEC) and Heat of Adsorption (ΔHC(303)) of NH3 at the PBE-D Level for the Two Sites Considered Computed with 6-31G(d,p) (Bare Numbers) and with TZP (in Parentheses) Basis Setsa adsorption complex


ΔE


ΔEC


ΔHC(303)
D
ΔHexp(303)

OH1N

84.1 (70.2) 68.6 (61.5)

66.4 (59.3) 14.4

OH1NN

60.1 (47.3) 44.6 (40.5)

43.4 (39.3) 16.5

SN

42.5 (26.0) 18.5 (18.4)

17.7 (17.5) 21.8

58.4 26.9

a

D is the pure Grimme dispersion contribution to the interaction energies. The superscript C indicates values corrected for BSSE. For the OH1NN structure, the interaction energy has been computed with respect to NH3 and the OH1N structure. For comparison, the experimental values of ΔH (ΔHexp) are given in the last column. All values are in kJ mol
1.

performed by using a quasi-Newton algorithm.58,59 Convergence is tested on the root-mean-square (rms) and on the absolute value of the largest components of both the gradient and the estimated nuclear displacement using the default parameters.51 Phonon frequencies within the harmonic approximation are computed at the Γ point (k = 0) by diagonalizing the massweighted Hessian matrix, obtained from the second derivative of the total energy with respect to the atom displacements (0.003 Å)60 for the atoms belonging to the fragments considered for the PBE-D geometry optimization. The interaction energy, ΔE, for the NH3 molecule adsorbed on the silica surface, S, is defined as ΔE = E(S-NH3//S-NH3)
E(S//S)
EM(NH3//NH3), in which E(S-NH3//S-NH3) is the unit cell energy of the silica/NH3 complex, E(S//S) is the unit cell energy of the free silica slab, and EM(NH3//NH3) is the energy of the free NH3. The interaction energy was corrected for the basis set superposition error (BSSE) following the Boys
Bernardi recipe:61 the BSSE-corrected value is referred to as ΔEC (Table 1). The ΔEC value was then transformed into the standard enthalpy of adsorption ΔH0(303) by using the computed frequencies calculated at the PBE-D/6-31G(d,p) level in the harmonic approximation (evaluated for free NH3, free silica, and the adsorption complexes considering only the atoms belonging to the optimized fragments, vide supra) and standard formulas of statistical mechanics. The same approach was followed to compute the variation in entropy, ΔS, due to loss of translational and rotational degrees of freedom of the NH3 molecule upon adsorption.

’ RESULTS AND DISCUSSION Computational Results for the Surface Silanols. The model structure resulting from dehydration is shown in Figure 1. The most conspicuous features of the surface are two types of isolated OH groups, OH1 and OH2. Another possible site, made of adjacent siloxane bridges, will be apparent further on. The two surface silanols (OH1 and OH2) are different regarding their environments. The species labeled OH1 is a genuinely isolated site, the calculated spectroscopic features of which are in excellent agreement with the known experimental characteristics of the tSiO
H species. These are, in detail, the following: (i) the harmonic O
H stretching frequency (exptl 3820 cm
1, calcd 3809 cm
1), (ii) the Si
OH stretching mode (exptl 975 cm
1, calcd 846 cm
1), (iii) the Si
O
H bending mode (exptl 834 cm
1, calcd 735 cm
1), and (iv) the torsional mode (exptl 105 cm
1, calcd 104 cm
1). Considering that 23346

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Figure 2. Volumetric (section a) and calorimetric (section b) isotherms measured at 303 K for the adsorption of ammonia on (i) Aerosil 300 outgassed at 1073 K (triangles) and (ii) a nondefective silicalite outgassed at 303 K (dashed line).

Figure 1. Ab initio optimized model of the highly dehydroxylated amorphous silica surface. Oxygen in surface OH groups appears as red balls, corresponding H atoms in gray. Upper section: side view. Lower section: view from above (lower side of the slab not shown for clarity).

anharmonicity was not taken into account, OH1 can be regarded as being a good model for the isolated silanol. The OH2 species is, instead, in a very weak H-bonding interaction with a nearby distorted Si
O
Si bridge (the OH2 3 3 3 (OSi2) distance is 2.34 Å): the harmonic stretching mode comes, consequently, at 3730 cm
1 (79 cm
1 lower than that of OH1), and the torsional mode at 376 cm
1. Although the energy to excite this mode is less than 2RT at ambient temperature (so probably causing the silanol species to behave as an almost free OH rotor at ambient temperature), the stretching mode is bound to be perturbed by the surroundings, as it happens with hydroxyl species sitting in narrow pores of zeolites.62 We conclude that species OH2, although possible on a thermodynamic ground, is not actually formed in the severe dehydration process, leading to a surface population of around 1 (OH)/100 Å2. Such a process probably follows other paths, among which the formation of strained 2-Si rings not envisaged in the present model. Volumetric and Calorimetric Data. Figure 2 reports volumetric isotherms (section a) and calorimetric isotherms (section b) concerning the adsorption of ammonia at 303 K on Aerosil 300 outgassed at 1073 K. For comparison, data of ammonia adsorption on the defect-free silicalite are also reported (dashed line isotherms). Further details on samples and data collection

procedures were given elsewhere.44 As anticipated above, the overall shape of the Aerosil 300 isotherm is non-Langmuirian. It is worthy of note that the adsorbed amount at relatively high pressures is larger than 0.5 mmol g
1, the value corresponding in the present case to a ca. 1:1 ratio between ammonia molecules and isolated silanols. The curve in Figure 3 reports the corresponding differential heat of adsorption as a function of coverage. Triangles are the experimentally determined points from the ΔQint/ΔNa versus coverage plot (not reported for the sake of brevity), as described in ref 63. The interpolated curve is described below. Heterogeneity is indeed observed, the differential heats of adsorption being found to range between ca. 60 and 30 kJ mol
1. IR Spectra at 303 K and Their Elaboration. Figure 4 reports a set of IR spectra in the O
H stretching region, measured at 303 K at different equilibrium pressures of ammonia. The absorption band at 3747 cm
1 typical of the tSiO
H groups is observed to decrease with increasing pressure, while a broad band envelope centered at about 3070 cm
1 grows; this broad band is due to the O
H stretching mode of tSiO
H species engaged in H-bonding with ammonia. The observed shift of the O
H stretching mode of silanols is ca. 677 cm
1, in agreement with the literature values.30,31,34
36,44 It is worthy of note that the shape of the broad 3070 cm
1 band remains strictly constant regardless of coverage. In addition, weak bands are seen in the region of 3500
3300 cm
1, where the N
H stretching modes of ammonia molecules not exceedingly perturbed are expected. The asymmetric and symmetric stretching modes of gaseous ammonia fall at 3443 and 3336 cm
1, respectively: the latter mode is more intense than the former, and indeed, a narrow band at 23347

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Figure 3. Differential heats of adsorption of ammonia qdiff = dQint/dna on (i) Aerosil 300 outgassed at 1073 K (triangles) and (ii) nondefective silicalite outgassed at 303 K (dashed line). The dotted vertical line corresponds to a 1:1 molar ratio between NH3 and isolated silanols at the surface of Aerosil 300 (ca. 1 OH/100 Å2). The solid upper curve has been obtained by numerical differentiation of eqs 3 and 4.

3336 cm
1 is visible at relatively high pressures in the spectra of Figure 4 without any sizable companion at 3443 cm
1—the latter mode being less intense, as stated before. In the band envelopes, three components can be identified at 3416, 3400 (arrows), and 3325 cm
1. We ascribe the two bands close to 3400 cm
1 to the asymmetrical mode of two different species; the corresponding symmetrical counterparts are not singled out and merge together at 3325 cm
1. The N
H stretching region of the spectra in Figure 4 suggests that (i) two forms of molecularly adsorbed ammonia exist, (ii) in both cases, adsorbed ammonia is little perturbed from the gaseous state, and (iii) the species absorbing at 3400 cm
1 grows more slowly with increasing pressure than that at 3416 cm
1, though eventually catching up at high pressure values. All of these findings will find correspondence with further results reported below, both computational and volumetric. Spectra in Figure 4 can be processed as follows. Let the intensity of the tSiO
H species be A under a given circumstance, and AM in the absence of ammonia
silica interactions. The decrease in intensity of the 3747 cm
1 band, AM
A, when divided by AM, yields the fraction θ of tSiO
H species involved in that interaction, independent from adsorption processes possibly occurring on other parts of the silica surface. Under the assumptions that (i) silanols constitute an ideal ensemble of sites and (ii) a simple 1:1 interaction with ammonia takes place, the equilibrium constant, b, of the interaction process is b ¼ θ=½ð1
θÞp

ð1Þ

which coincides with the Langmuir-type equation: θ ¼ ðAM
AÞ=AM ¼ bp=½1 þ bp

ð2Þ

The corresponding optical isotherm (concerning only the interaction of the ammonia molecule with tSiO
H species) is reported in the inset of Figure 4. The curve is observed to obey strictly the Langmuir equation for ideal adsorption, eq 2, with b = 0.117 ( 0.005 mbar
1. The intensity of the growing band envelope as a function of pressure can also be used to trace optical isotherms. We have done so for a few frequency values, including 3070 cm
1 (maximum), leaving aside the 3500
3300 cm
1 region where ammonia molecules absorb. Also, these optical isotherms have a

Figure 4. IR spectra of Aerosil 300 outgassed at 1073 K in the 3800
2600 cm
1 range, taken at 303 K by increasing the NH3 equilibrium pressure in the 0.00
40.0 mbar range. Inset: optical isotherm obtained by plotting the fraction of silanols engaged as a function of the NH3 equilibrium pressure; the dotted line is the curve-fit obtained according to the Langmuir model.

remarkable Langmuirian nature, with equilibrium constants coinciding with those computed from the decreasing intensity of the silanol band. It is so concluded that (i) tSiO
H species behave also as an ideal ensemble in their interaction with ammonia, (ii) the main IR spectroscopic features in Figure 4 are ascribable to this interaction, as the same Langmuir isotherm is always involved, and (iii) a second adsorption process takes place. The experimental evidence concerning the presence of this second, unspecific adsorption process is weak in the IR spectra, and it is more convincingly provided (see below) by the non-Langmuir shape of the isotherms in Figure 2, and by the fact that the adsorbed amount is larger than ca. 0.5 mmol g
1. For these reasons, the occurrence of this second adsorption process has so far escaped detection. The Unspecific Adsorption Process. Two possibilities arise. Either a second (independent) site is available at the surface or the tSiO
H species already interacting with one ammonia molecule can coordinate a second one. A possible structure corresponding to the latter hypothesis has been calculated ab initio and is described below. There is evidence, however, that this is not the case. If adsorption consisted of a two-step process, intensities of IR absorption bands would not follow the observed Langmuir behavior so neatly. Further IR evidence denying the occurrence of a two-step adsorption process is given below. The occurrence of another type of adsorption site for ammonia at the surface of dehydrated amorphous silica is, therefore, suggested. Indeed, adsorption of ammonia occurs at the surface of defect-free silicalite, (dashed line curves in the lower part of Figure 2a,b), where only adsorption on siloxane bridges is likely to take place. The related differential heat of adsorption is in this case around 10 kJ mol
1, as reported in ref 44 and illustrated in Figure 3, for comparison (dashed line in the lower part of the figure). We have, therefore, closely inspected the structure model in Figure 1 looking for sites constituted by Si
O
Si bridges. Indeed, the PBE/6-31G(d,p) electrostatic potential colormapped on the electron density (Figure 6, top left) shows, besides the OH1 site, a patch of negative potential just above a three Si
O membered ring, labeled site S hereafter. 23348

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Figure 5. Volumetric (section a) and calorimetric (section b) isotherms measured at 303 K concerning the adsorption of ammonia on Aerosil 300 outgassed at 1073 K (triangles). Solid curves represent the fit obtained by applying a Langmuir
Henry model (eqs 3 and 4). Both total amounts and individual components are reported.

Model of Adsorption with Two Independent Sites. Under the assumption of two independent sites, the isotherms in Figure 2 (both volumetric and calorimetric) concerning adsorption of ammonia on Aerosil 300 can be simulated by the superposition of two local isotherms, one of which (at least) is of a Langmuir type. The first attempt was to simulate the volumetric isotherm through two Langmuir functions, the former of which has b = 0.117 mbar
1 as the equilibrium constant (the value coming from IR spectroscopic measurements). Such a simulation is inconclusive, because too many parameters are involved (even if only three). The weak adsorption process taking place at siloxane sites has to be described through a Henry-type equation, which involves one parameter less than the Langmuir formula. Related equations are

N a ¼ N 1 b1 p=ð1 þ b1 pÞ þ N 2 b2 p

ð3Þ

Q int ¼ N 1 b1 q1 p=ð1 þ b1 pÞ þ N 2 b2 q2 p

ð4Þ

Na is the adsorbed amount per unit surface area, and Qint the corresponding integral heat evolved. Ni (i = 1, 2) is the number of sites per unit surface area involved in the ith process, bi the related equilibrium constant, and qi the corresponding molar heat of adsorption. Actually, the term N2b2 acts as a single unknown parameter and b1 is known so that only two parameters are involved. Volumetric data in Figure 2 are represented with great accuracy by eq 3, with N1 = 0.462 ( 0.023 mmol g
1 and N2b2 = 3.34 ( 0.32  10
3 mmol g
1 mbar
1. The N1 value is in excellent agreement with the actual density of silanols on

Figure 6. Top-left section: electrostatic potential map calculated for the silica surface model, as mapped on the surface of constant electron density = 10
6 au. Positive/negative values of the electrostatic potential (0.02 au) appear as blue/red colors. Other sections: optimized structures of the silica slab interacting with (i) a single NH3 molecule adsorbed at surface site OH1 (species OH1N, top-right), (ii) two NH3 molecules mutually H-bonded (species OH1NN, bottom-left), and (iii) a silanol-free region (species SN, bottom-right).

the surface of Aerosil 300 outgassed at 1073 K, and this can be considered to be a piece of evidence in support of the interpretation. A similar procedure was applied to the data concerning integral heats of adsorption, by using eq 4, this time both bi and Ni values being known, and the unknowns being the two qi values. A safer procedure to determine q1 and q2, however, is as follows. By eliminating the common term N1b1/(1 + b1p) between eqs 3 and 4, one gets Q int ¼ q1 N a þ cp

ð5Þ

with c ¼
ðq1
q2 ÞN 2 b2

ð6Þ

Equation 5 allows one to apply the linear least-squares method to the triplets of data, Qint(i), Na(i), and p(i), to obtain an unbiased value of q1, which results to be q1 = 58.4 ( 1.1 kJ mol
1, whereas for c, the value of
0.1403 ( 0.01 kJ mbar
1 was obtained. From the values of c, q1, and N2b2, it is inferred that q2 = 26.9 ( 0.7 kJ mol
1. Availability of the above parameters allows decomposing both volumetric and calorimetric isotherms into the corresponding Langmuir and Henry contributions (Figure 5). It is observed that the number of molecules adsorbed in the unspecific mode is always smaller than that of molecules H-bonded to silanols but that it grows steadily with pressure, whereas the number of H-bonded species is necessarily limited by the Langmuir nature of the related local isotherm. At relatively high pressures, the number of molecules adsorbed in each mode becomes similar. This means that N2, the population of sites S, is at least of the order of N1, the population of tSiO
H sites. Moreover, the behavior of the Langmuir and Henry curves in Figure 5 can be usefully compared to the trends in intensity shown by the bands at 3416 and 3400 cm
1, respectively, in 23349

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The Journal of Physical Chemistry C Figure 4. The two bands probably have a close extinction coefficient, but the component at 3400 cm
1 starts to be visible at higher pressures than that at 3416 cm
1 (as shown by the arrows in Figure 4), so that it is possible to conclude that the species at 3416 cm
1 is due to the H-bonded species (corresponding to the Langmuir curve in Figure 5) and that at 3400 cm
1 is actually due to the unspecific mode of adsorption (corresponding to the Henry curve in Figure 5), which starts concomitantly but becomes relevant at higher coverage. It is worthy of note that the species more weakly interacting actually shows a displacement from the corresponding mode of the unperturbed gaseous molecule larger than that of the more strongly held species. This again suggests that the two modes of interaction are different in nature. From the above data, one may calculate the curve representing the differential heat of adsorption, dQ/dN. The initial value (p = 0) and the final value (p = ∞), calculated analytically by utilizing the relationship qdiff = (dQint/dp)/(dNa/dp), are ðqdiff Þ0 ¼ ðq1 þ Kq2 Þ=ð1 þ KÞ with K ¼ ðN 2 =N 1 Þðb2 =b1 Þ and ðqdiff Þ∞ ¼ q2 The differential heat at zero coverage is 59.5 kJ mol
1, a value marginally larger than q1; it is, however, worthy of note that (qdiff)0 is influenced by the weak adsorption process. All other points have been calculated by numerical differentiation and are reported as a continuous curve in Figure 3. The agreement between experimental points and the continuous curve is very satisfactory. Comparison between Calculated and Experimental Features. Figure 6 shows the three types of NH3 adsorption species considered: (i) NH3 adsorbed on OH-free patches of the silica surface (species SN), (ii) a single NH3 molecule at a silanol site OH1 (species OH1N), and (iii) two NH3 molecules at the same hydroxyl site (species OH1NN). The corresponding interaction energies are reported in Table 1. A further (in principle) possible species would be an NH3 molecule acting as a H-donor to the O atom of a silanol group; however, this species was not considered because IR spectroscopic evidence disproves its occurrence. Note that such a species should give rise to a bathochromic shift of the O
H stretching mode of the H-acceptor silanol, in close similarity to what is observed in the free interaction of a pair of silanols. Yet, at no stage of the adsorption process was ever a lowfrequency partner of the 3747 cm
1 band observed, in agreement with the very limited tendency of the isolated ammonia molecule to act as a H-donor.30,31 The value of ΔHC(303) computed with the larger TZP basis set (see the Computational Methods for details) for OH1N results to be
59.3 kJ mol
1 (Table 1), in excellent agreement with the experimental value of
59.5 kJ mol
1. The dispersive contribution, D, to the interaction energy is
14.4 kJ mol
1, that is, about one-fourth of the total. Comparison between Figures 1 and 6 (top right) reveals a rotation of the OH1 group during the geometry optimization to allow the adsorbed NH3 molecule to H-bind to a nearby siloxane Si
O
Si bridge (distance H2NH 3 3 3 O = 2.25 Å) while making a rather short H-bond with the silanol group (OH 3 3 3 N 1.68 Å). The most remarkable calculated feature is the bathochromic frequency shift of the tSiO
H stretching mode caused by

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H-bonding, which results to be 939 cm
1. The experimental value is much smaller (677 cm
1). It has to be taken into account, though, that the frequency of the O
H stretching mode of H-bonded complexes heavily depends on temperature, which was 303 K for experiment, whereas calculated values refer to absolute zero. IR measurements run at 4 K64 show a bathochromic shift around 950 cm
1, in excellent agreement with the calculated value of 939 cm
1, probably fortuitous because anharmonicity was not taken into account when computing the frequency shift, and the PBE functional is known to overestimate frequency shifts in strongly H-bonded systems. As far as the unspecific adsorption is concerned, the comparison between computational and experimental results is less cogent. For the geometry of the OH1NN case, the OH 3 3 3 N distance decreases from 1.68 to 1.60 Å as a consequence of the H-bond with the second NH3 molecule (N
H 3 3 3 N distance of 1.97 Å) while the second NH3 establishes a very weak N
H 3 3 3 O bond (2.45 Å) with the oxygen of the OH2 group and with one siloxane bridge (2.40 Å). This structure, already ruled out because of the evidence that the overall process does not involve two steps, can also be excluded on the basis of the calculated IR features, as follows. A definite increase in the shift of the tSiO
H stretching frequency Δν(OH), from 939 to 1257 cm
1, is computed, that is, by more than 300 cm
1, because the H-bonding is strengthened (as a consequence of the decrease of the H 3 3 3 O distance, vide supra). The effect of an actual temperature of 303 K would probably decrease the value of Δν(OH) calculated at 0 K down to ca. 200 cm
1 only. No such shift was, however, observed: instead, the shape of the Δν(OH) band is remarkably constant. We take this as evidence that this double adduct is not formed. Interestingly, the OH1NN complex shows an interaction enthalpy TZP ΔHC(303) value of
39.3 kJ mol
1, which is even larger than the experimental value obtained for the unspecific interaction (26.9 kJ mol
1). Coming to the SN case, calculations suggest a weak, but not negligible, interaction of NH3 with siloxane patches of the silica surface. The NH 3 3 3 O distances are 2.49, 2.63, and 2.74 Å, and the ΔHC(303) value computed with the larger TZP basis set is
17.5 kJ mol
1. This is significantly smaller than the corresponding experimental value of
26.9 kJ mol
1. This suggests that the SN structure is only partially valid as a model for the unspecific adsorption. Similarly, note that the purely dispersive contribution, D, to the interaction energy is
21.8 kJ mol
1, which shows that the SN complex is exclusively held by dispersion forces, while the contribution to the interaction energy from the H-bonding PBE part is slightly repulsive. Accordingly, calculated N
H stretching modes undergo only a small hypsochromic shift, about 16 cm
1. Such a result is at variance with the experiment, which indicates a bathochromic shift of ca. 50 cm
1. The disagreement is, however, not so large: probably, the indication here is that the actual site for the unspecific adsorption has more basic siloxane oxygen atoms than the model SN, capable of a slight H-bonding contribution to the interaction, but that dispersion is the main reason for stability. In conclusion, the unspecific adsorption can be described as taking place at dehydrated patches of the silica surface, basically described by the proposed model: the actual structure of the Aerosil 300 surface probably exhibits more strained patches, able to accommodate ammonia with larger dispersive interactions and some H-bonding contributions. By the same token, with a 23350

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The Journal of Physical Chemistry C crystalline substrate, such as nondefective silicalite, the opposite would likely be the case; that is, only slight deformations of the surface are possible, allowing more limited dispersive interactions. In any case, the conclusion is that siloxane oxygen atoms at the surface of both silicalite and dehydrated amorphous silica do not show great basicity toward adsorbed ammonia. Indeed, it is known that siloxane bridges are not engaged in H-bonding even when water is adsorbed on silica.65 Thermodynamic Considerations. From the standard relationships ΔG10 =
RT ln b1 and ΔG0 = ΔH0
TΔS0, and the numerical values of b1 and q1 (the latter assumed to coincide with ΔH0), the ΔS0 value of the interaction between isolated silanols and ammonia (standard pressure of 1 mbar) results to be
218 J mol
1 K
1. Concerning the unspecific adsorption, an estimate of the related equilibrium constant b2 can be obtained by fixing a value for the population of sites, N2, because only the product N2b2 is actually measurable. Visual inspection of Figure 5 shows that a reasonable value for N2 is about 2N1. Note that small uncertainties in the population of sites will not affect the result much, because of the logarithmic relationship between the equilibrium constant and the standard change in free energy. This assumption implies a b2 value of ≈3.62  10
3 mbar
1, and a ΔS0 value of ≈
135 J mol
1 K
1. Clearly, a compensation effect is operating when passing from one adsorption process to the other, because a halved interaction enthalpy value corresponds to a nearly halved standard entropy change. Similarly, as discussed below, an unfavorable standard change in entropy is probably the reason why the species OH1NN does not form, notwithstanding a favorable interaction enthalpy. Regarding calculated standard changes in entropy values, it has to be noted that ab initio calculations (in the present case, those run at the PBE-D level) do provide such quantities by means of classical formulas of statistical thermodynamics. The values obtained are
181,
164.0, and
193 J mol
1 K
1 for OH1N, SN, and OH1NN, respectively. The experimental values of the first two are instead
218 and
135 J mol
1 K
1, respectively (no value is experimentally available for OH1NN). Note, however, that the calculated values of ΔS0 refer to a standard state at 1 bar, whereas the corresponding experimental values refer to a standard state at 1 mbar. This change of reference state has no effect on the standard adsorption enthalpy, but, within the perfect gas approximation, ΔS0 changes by the amount of
55 J mol
1 K
1. This amount should be added to the calculated ΔS0 values before comparison to the corresponding experimentally derived values of ΔS0. Hence, the ΔS0 experimental values of
218 and
135 J mol
1 K
1 for the OH1N and SN complexes would correspond to calculated values of
236 and
219 J mol
1 K
1, respectively. As noted for the enthalpy of interaction and spectroscopic features, the agreement between calculated and experimental results is very satisfactory for the species OH1N. In the case of a relatively strong adsorption, the approximation of a rigid solid seems to be appropriate. For the weaker SN case, instead, the computation of ΔS0 yields a result diverging from experiment. The reason is probably as follows. The vibrations of the molecule against the solid (hindered translation/rotation of the adsorbed NH3) are likely to scale with the overall interaction energy and are thus expected to fall at low frequency, so contributing much to the entropy of the adsorbed phase. However, the harmonic approximation adopted for the relevant statistical

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thermodynamic formulas can be in substantial error for such low frequency, large amplitude atomic motions. Also, the rigid model for the surface does not take into account perturbation of low-lying vibrational modes of the solid brought about by the adsorption process, and possible mixing of adsorbent and adsorbate modes having a similar frequency. The standard entropy change for the unspecific adsorption is rather close to the value for the standard entropy of vaporization (152.4 J mol
1 K
1 at the boiling point): this is obvious evidence of a much larger freedom of motion enjoyed by the ammonia molecule when weakly interacting with a dehydrated patch than with an isolated silanol. Niwa and co-workers have carried out a detailed study of the enthalpy and entropy of adsorption on zeolitic systems.66 Their findings, however, are not readily compared to the present values, because the basic chemical process is different (proton transfer instead of coordination). The simultaneous occurrence of a weak and a relatively strong interaction is due to a large difference in adsorption entropies that compensates for the rather large difference in adsorption enthalpy, yielding comparable values for the free energy of adsorption at 303 K. According to the experimental results, these amount to 8.0 and 14.0 kJ mol
1, respectively. Note that positive values are due to the assumption that the standard state is 1 mbar: expected negative values are obtained with a standard pressure of 1 bar, which brings about a difference of 16.6 kJ mol
1. Similarly, an unfavorable role of the entropic term is likely to be the reason why the interaction of a second ammonia molecule with the OH1N species, yielding the species OH1NN, is not observed: indeed, if the computed value is assumed for both enthalpy and entropy of adsorption, a standard free energy change of 35.1 kJ mol
1 is calculated at 303 K, far larger than those observed for the two spontaneous processes.

’ CONCLUSIONS Adsorption of molecular ammonia on the highly dehydrated surface of Aerosil 300 from Degussa comprises both the widely studied H-bonding interaction of ammonia with surface silanols, showing typical IR absorption features, and another interaction that is inconspicuous in the IR spectra and clearly only revealed by quantitative measurements. This latter interaction mode, herein referred to as unspecific adsorption, had escaped detection so far. The two adsorption processes occur simultaneously, notwithstanding a marked difference in the involved interaction energy, ΔH0 =
58.4 kJ mol
1 and ΔH0 =
26.9 kJ mol
1, respectively. To a high accuracy, the former process is described by a Langmuir-type model, the latter by a Henry-type isotherm. Comparison of the experimentally determined energetic values and vibrational features with results of ab initio calculations (comprising dispersive contributions) shows that all features are accounted for very satisfactorily as far as the silanol-related adsorption process is concerned. Computational results suggest that the unspecific adsorption takes place at dehydrated, partially strained patches at the silica surface, and it is dispersion-dominated: the contribution of H-bonding is limited, but it accounts for the presence in the experimental spectra of a new IR absorption band, weak and inconspicuous, at 3400 cm
1, that is, only a little red shifted from the corresponding value of unperturbed ammonia (3443 cm
1). Adsorption on the surface of defect-free silicalite is probably a 23351

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The Journal of Physical Chemistry C similar process, although involving an adsorption heat of only ca. 10 kJ mol
1. We interpret the difference in the interaction energies as due to a different compliance of the crystalline and the amorphous matrix, allowing the formation of strained surface patches only in the latter case. Similarly, the difference between the calculated ΔH0 value (
17.5 kJ mol
1) for the unspecific adsorption process and that measured experimentally (
26.9 kJ mol
1) probably reflects some inadequacy of the model to represent really strained silica surfaces. The simultaneous occurrence of a weak and a relatively strong interaction is ascribable to the large difference in adsorption entropies, which compensates for the rather large difference in adsorption enthalpy. Similarly, an unfavorable role of the entropic term is probably the reason why the interaction of a second ammonia molecule with the OH1N species (yielding the OH1NN species) is not observed.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (E.G.), piero.ugliengo@ unito.it (P.U.).

’ ACKNOWLEDGMENT P.U. acknowledges the CINECA supercomputing centre for allowance of computer time through the SILDRUG-HP10A7WAF8 “Large Scale B3LYP-D simulation of silica-based carriers for drug delivery” project, and the Italian Ministry MUR (Project COFIN-2004: “Study of physical and chemical surface features of silica powder in relationships with the response elicited in cellular systems”. V.B. and P.U. acknowledge the Italian Ministry MUR (Project COFIN-2006: “Interface phenomena in silica-based nanostructured biocompatible materials contacted with biological systems”) and the Regione Piemonte-Italy (Project CIPE2004: “Nanotechnologies and Nanosciences. Nanostructured materials biocompatible for biomedical applications”) for financially supporting this work. ’ REFERENCES (1) Iler, R. K. The Chemistry of Silica; Wiley-Interscience: New York, 1979. (2) Chedid, A.; Haux, P.; Natelson, S. Clin. Chem. 1972, 18, 384. (3) Hayrapetyan, S. S.; Khachayryan, H. G. Acta Chromatogr. 2005, 15, 162. (4) Morrow, B. A.; Gay, I. D. In Adsorption on Silica Surfaces; Papirer, E., Ed.; Marcel Dekker: New York, 2000; Vol. 90, pp 9
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