Binding of Small Molecules to a Silica Surface - ACS Publications

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Binding of Small Molecules to a Silica Surface: Comparing Experimental and Theoretical Results DeCarlos E. Taylor,† Keith Runge,*,‡ Marshall G. Cory,§ Douglas S. Burns,§ Joseph L. Vasey,§ John D. Hearn,^ and Michael V. Henley^ †

U.S. Army Research Laboratory Weapons and Materials Research Directorate RDRL-WMB-D Aberdeen Proving Ground, Maryland 21005, United States ‡ BWD Associates, LLC 2901 NW 54th Avenue Gainesville, Florida 32653-1819, United States § ENSCO, Inc. 4849 North Wickham Road Melbourne, Florida 32940, United States ^ US AFRL RXQL 139 Barnes Drive, Suite 2 Tyndall AFB, Florida 32403-5323, United States

bS Supporting Information ABSTRACT: A multiscale method for systematically generating predictive models for probe
surface interactions and its independent experimental verification is described. The interaction of three probe molecules (H2O, NH3, and NO) with silica was studied using experiment, theoretical quantum chemistry, and molecular dynamics calculations. Quantum chemical (QC) methods were used to compute binding enthalpies and vibrational (infrared, IR) spectra of molecule
surface pairs for three unique surface silanol sites. The probe
surface IR spectral shifts induced by the interaction of the probe molecules with the surface silanol sites were also computed and compared to experiment. The computed IR results are comparable to those of experiment and (a) verified that the surface that has been created using simulation is indeed similar to the experimental surface and (b) shed insight into the underlying physical process leading to the observed shifts. The theoretically determined enthalpies of adsorption (ΔHads) compared well with experiment falling within the uncertainty of those measured using inverse gas chromatography. For water, ΔHads,350K =
13.5 kcal/mol (calculated) versus
13.6 ( 2.8 kcal/mol (experimental, 330 K < Texpt < 370 K). For ammonia, ΔHads,353K =
15.2 kcal/mol (calculated) versus
12.7 ( 2.9 kcal/mol (experimental, 323 K < Texpt < 383 K). Finally, for nitric oxide, ΔHads,253K =
4.23 kcal/mol (calculated) versus
4.03 ( 0.35 kcal/mol (experimental, 243 K < Texpt < 263 K).

’ INTRODUCTION The interaction of silica with water is important for geophysics, planetary science, and materials processing.1 An example in geophysics is the reduction of yield strength associated with the phenomenon of hydrolytic weakening which has been studied since the 1960s.2 While the interaction of silica with water has received, and continues to receive, a good deal of attention, it is not the only chemical interaction of interest for materials processing. In chemo-mechanical processes and environmental weathering, a wide variety of chemically distinct substances interact with silica surfaces and affect the short- and long-time behavior of systems that include silica. This interaction is initiated by the physisorption of small molecules, like water, ammonia, and NO, on silica surfaces. The creation of amorphous silica surfaces leads to defects that serve as binding sites for adsorbates. These binding sites consist of hydrogen-bonding and non-H-bonding silanol sites including isolated silanols, geminal silanols, and H-bonding vicinal silanols. While other configurations of defects on the surface can lead to H-bonding silanol sites, most H-bonding silanols are vicinal and, by and large, isolated and geminal silanols comprise the non-Hbonding silanol sites. The defects generally consist of hydroxylated silicon atoms near the surface where the molecular interactions form various configurations that are dependent on the nature of r 2011 American Chemical Society

the chemical interactions. To explore a range of possible chemical interaction types, we have chosen basic (ammonia, NH3), acidic (nitric oxide, NO), and amphoteric (water, H2O) adsorbates. Previous work has been conducted on water adsorption using molecular clusters with density functional theory (DFT).3,4 Similarly, ammonia interactions with silica have been modeled using molecular clusters5,6 and periodic models,7,8 and as with the water studies, binding energies and IR frequency shifts were reported. The accurate theoretical description of physisorption of small molecules on the surface of amorphous silica requires the use of quantum chemical methods. The computational demands of these methods are large and, for routine use, the higher levels of ab initio theory (e.g., coupled cluster theory) are limited to a few dozen atoms. The physical system includes many hundreds of atoms within a few angstroms of the adsorption site; hence, for computational purposes, the system must be truncated. Many truncation schemes are available in the literature;9
11 here, we choose to use a method previously developed in the context of silica fracture.12,13 Comparison must be made to experimental results to validate the level of theory and strategy involved in making these calculations. Received: June 11, 2011 Revised: November 9, 2011 Published: November 15, 2011 24734

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The Journal of Physical Chemistry C We include, among the tests that will be used in assessment, examination of the bulk silica with respect to previous experiments. Infrared signatures from theory and experiment provide a check on the structure of the surface created. Furthermore, infrared shifts upon the adsorption of water, ammonia, and nitric oxide on the silica surface explore the similarities between the theoretical and experimental systems. Finally, the heats of sorption measured by inverse gas chromatography give a direct comparison with the calculated binding enthalpies. In the next section, the theoretical (the construction of the bulk silica surface using molecular dynamics, the truncation of the theoretical clusters) and experimental (inverse gas chromatography, infrared spectroscopy) methods are described. This is followed by a section comparing the theoretically created silica surface with the surface used in experiment. Comparisons of the experimental and theoretical surfaces with adsorbates are discussed thereafter with a comparison of experimentally determined heats of sorption with theoretically derived binding enthalpies preceding our conclusions.

’ METHODS Theoretical Construction of the Hydroxylated Amorphous Silica Surface. The first step in generating a silica surface

with molecular dynamics (MD) is to create a bulk silica sample. We chose to use the silica potential of van Beest et al. (BKS)14 for this task because it is known to provide a good description of crystalline and amorphous silica. Using the CP2K software package,15 a 3072 atom cristobalite lattice was heated to 9000 K for 100 ps (1 fs time step) to generate the initial melt following the annealing schedule outlined by Huff et al.16 In the Huff work, a potential similar in form to the BKS potential used presently required a temperature of 8000 K to sufficiently displace the atoms from their original location in order to prevent recrystallization as the sample was cooled to room temperature; therefore, a similarly high temperature was used in this work. This melt was then quenched to a final temperature of 300 K using 100 K decrements with 2 ps of relaxation time at each temperature. After annealing the bulk sample, a free surface was prepared by removing a 6 Å layer of atoms taking care to maintain the proper stoichiometric ratio of silicon to oxygen. A 12 Å void space was then inserted between the central cell and its periodic image perpendicular to the cleavage plane. Since the potential cutoff was 10 Å, this cell void size was sufficient to prevent the surface atoms from interacting with any periodic images which would not be present on the experimental surface. The cleaved surface was then allowed to relax by heating to 3000 K for 100 ps and by annealing to 300 K again with 100 K decrements and 2 ps of relaxation time at each temperature. The resulting surface contained several types of defects including dangling bonds, undercoordinated silicon atoms, and highly strained two member rings. To generate a hydroxylated surface, all dangling oxygen bonds and undercoordinated silicons were passivated with hydrogen atoms and hydroxyl groups, respectively. In addition, all two-membered rings, because of their high strain energy, were opened and passivated by addition of single water molecules forming vicinal silanol units. Representative isolated, geminal, and vicinal silanol clusters were extracted from this hydroxylated surface for further refinement using quantum mechanics. The theoretical approach chosen for this work is designed to, as much as possible, create a realistic representation of the

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experimental system. In particular, the generation of an amorphous silica system which is then cleaved to create the silica surface is accomplished using an atomistic potential as described above. This provides the general structure of the three types of silanol sites which are selected to be representative of those found on the passivated system. To calculate binding enthalpies which are comparable to those experimentally determined by inverse gas chromatography, higher level and computationally more intensive calculations are required. For these calculations, the MP2/6-31++G** level of theory has been chosen as it can be implemented on clusters of a couple dozen atoms and as there is previous experience in using second-order theory within a consistent embedding hierarchy.1 The effect of the remainder of the silica system is incorporated using pseudoatoms whose positions are determined from the atomistic simulation as described below. These pseudoatoms are held fixed while the remainder of the silanol sites are optimized using MP2/6-31++G** with and without the adsorbate molecules. Experimental Determination of Silanol Density. The surface density of silanol groups on silica gel (Davisil grades 636 and 646, Sigma-Aldrich) was determined via reaction with LiAlH4 (Sigma-Aldrich). H2 produced via reaction 1 was measured with a capacitance pressure gauge.17 A small amount of silica gel (∼0.5 g) was placed in a round-bottom flask and was heated to 100 C under vacuum for more than two hours to remove physisorbed water. Five milliliters of a 2% (w/w) solution of LiAlH4 in diglymes (Sigma-Aldrich) was added to the silica gel, and the pressure change was measured. SiOH surface concentration (nSiOH/A) was calculated from the pressure change (ΔP) using the total volume of the reaction apparatus (V = 180
350 mL) and assuming the ideal gas law according to eq 1 in which A is the total surface area of the silica gel sample, R is the universal gas constant, and T is the vessel temperature (∼20 C). diglymes 4SiOH þ LiAlH4 sf SiOLi þ ðSiOÞ3 Al þ 4H2 ðgÞ ðreaction1Þ   nSiOH ΔPV 1 ¼ A RT A

ð1Þ

The pressure rise was rapid and was almost complete after approximately 15 min. The pressure was corrected by the observed leak rate of the reaction vessel, which was typically less than 0.3 Torr/min. Vicinal and H-bonded geminal surface silanols were quantified by first reacting isolated and non-H-bonded geminal silanols with hexamethyl disilazane (HMDS, Sigma-Aldrich) via reaction 2. HMDS does not react with H-bonded surface silanols,18 and so it provides a facile means for chemically covering nonH-bonded silanols for subsequent LiAlH 4 reaction. A small amount of silica gel (0.5
1.5 g) was placed in a round-bottom flask, was heated to 100 C, and was exposed to a constant flow of HMDS-saturated argon. The reacted surface was then exposed to LiAlH4 to quantify the remaining H-bonded surface silanols. 2SiOHisolated þ HMDSðgÞ f 2SiOSiðCH3 Þ3 þ NH3 ðgÞ ðreaction2Þ Inverse Gas Chromatography. Experimental measurements of the enthalpies of adsorption, ΔHads, for the various probes on 24735

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silica gel were made with inverse gas chromatography (IGC), a well-established method for investigating physicochemical properties of heterogeneous interactions.19 Detailed descriptions of IGC experimental methods have been documented,20 and so only a brief description is provided here. A dilute gas-phase probe molecule (e.g., H2O, NH3, NO) was passed through a conditioned stationary phase (silica gel was conditioned at 100 C in pure carrier gas), and the elution profile was measured. In the linear part of the isotherm, the net retention volume (Vn, volume of gas needed to elute the probe molecule) is directly proportional to the partition coefficient (K): Vn = KA in which A is the surface area (or mass) of the stationary phase.21 The van’t Hoff relation yields ΔHads by relating Vg (Vn normalized to the stationary phase mass) to the column temperature (Tc). Equation 2 shows the relationship between Vg and ΔHads in which R is the universal gas constant (8.31415 J/K-mol or 1.98722  10
3 kcal/K-mol) and C is a constant. ln

ΔHads Vg ¼
þ C RTc Tc

ð2Þ

Column flow rates were measured and maintained with a 100 sccm (standard cubic centimeters per minute) mass flow controller (MKS Instruments). Vg was determined by using eq 3 in which F is the flow rate at standard temperature and pressure, tr and t0 are the retention times of the retained and unretained probes, respectively, m is the mass of stationary phase, and j is the James-Martin correction factor.12 For the conditions in these experiments, for which the inlet and outlet pressures are almost equivalent, j is a negligible correction (i.e., j ≈ 1). Vg ¼ jF

ðtr
t0 Þ Tc m 273:15

ð3Þ

Sample-loop injections were used to introduce the probe molecules to the silica gel columns. A certified mixture of NO in N2 (4870 ppm, Matheson Tri Gas, Inc.) was used without further dilution to fill the sample loop (0.10 mL). N2 elution provided a measure of the dead time for each injection on the silica gel column (∼2.5 g) housed in a 1/4 in. o.d. glass tube and held in place with small plugs of deactivated glass wool. The column temperature was maintained at subambient temperatures using liquid CO2 to cool the GC oven (HP model 5890), and NO and N2 elution profiles were measured with electron impact ionization (EI) mass spectrometry (HP model 5972). NH3 (chemical purity grade, Union Carbide Co.) was injected into an evacuated lecture bottle (0.5 L) and was backfilled with 1000 psi N2. This NH3/N2 mixture was used to fill the sample loop (91 μL) with an optional N 2 dilution flow to further decrease the NH3 concentration. NH3 was injected onto a silica gel column (5
12 mg) housed in 1/16 in. o.d. Teflon tubing and held in place with a small amount of Teflon filter paper. The column temperature was controlled with the GC oven (HP model 6890N), and NH3 was detected by an electrolytic conductivity detector (model 5200, OI Analytical). H2O measurements were made with a commercial IGC system (SMS, Inc.) and were made at constant partial pressure. H2O injections on silica gel (0.14 g) were accomplished by filling a sample loop (0.225
1.125 mL) with a variable amount of H2O vapor by mixing dry and H2Osaturated He flows (saturated flow was generated by bubbling He through a temperature-controlled H2O bath). H2O was detected by a change in thermal conductivity of the gas stream.

Figure 1. Radial distribution function for BKS generated silica.

Infrared Spectroscopy. Transmission Fourier transform infrared (FTIR) spectra of probe molecules adsorbed onto silica were collected using a Nicolet Magna FTIR Spectrometer 750. Pellets of CAB-O-SIL fumed silica (Eager Plastics, Inc.) were pressed and were held in place with Teflon spacers in a glass gas cell sealed with zinc selenide windows pressed against Buna rubber o-rings. Fumed silica was chosen because it provided a much better material than silica gel for pressing pellets without the use of KBr which could contribute to adsorption of the probe molecules and which was, therefore, avoided. Silica pellets were dried by heating the entire glass cell (∼140 C) and by purging with a constant flow of dry N2. Probes were introduced in a N2 stream and were passed over the pellet at room temperature (H2O and NH3) or
80 C (NO). Thirty-two scans were recorded at better than 1 cm
1 resolution and were averaged for each spectrum. New pellets were prepared for each probe molecule. Chemicals and Gases. All chemicals were purchased from the indicated vendor and were used without further purification unless indicated otherwise. Reverse osmosis water was generated in-house. Gases (He, N2, H2, Ar, UHP grade) were purchased from Airgas. Compressed air was made and purified in-house with an Aadco pure air generator model 737. A certified mixture of NO (4870 ( 2% ppm in N2, used for IGC measurements) and CP grade NO (used for FTIR measurements) were purchased from Matheson Tri Gas. Ammonia was purchased from Union Carbide Corporation.

’ RESULTS Comparison of Theoretical and Experimental Surfaces. We verify the quality of the prepared silica sample by examination of the radial distribution function (RDF) as well as the coordination numbers of silicon and oxygen. The RDF for the amorphous sample generated with the BKS potential is presented in Figure 1. The first peak in the RDF occurs at ∼1.69 Å, which is in good agreement with the experimental peak value of 1.6 Å.22 The data in Table 1 compares the computed pair distribution function peak positions to experiment. Again, the theoretical results for each atom pair are in excellent agreement with the experimental values. Table 2 shows the percentage of silicon (oxygen) atoms as a function of coordination number. For a quality bulk silica sample, the vast majority of silicon atoms should have a coordinate number of 4, and the oxygen coordination number should be 2. 24736

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is consistent with the previous results of Zhuralev.23 Results are shown in Table 3, and within the uncertainty of the measurement, there is no difference between the total silanol surface densities for the two grades of silica gel. H-bonded surface silanols were quantified as described above. The average abundances of H-bonded surface silanols were 34 and 48% of the total surface silanols for Davisil grades 636 and 646, respectively, but this apparent difference is within the uncertainty of the measurement. Moreover, adsorption results from IGC experiments were the same for both grades of silica gel. As a negative control, silica gel was exposed to trimethylsilyl chloride, which reacts with all surface silanols,24 and then was exposed to LiAlH4. The results for this negative control exhibited no pressure change within the uncertainty of the measurement indicating that there were no available surface silanols for reaction with LiAlH4. Truncation of Theoretical Clusters. The clusters removed from the surface were all extracted by cutting across Si
O bonds, and the terminal oxygen atoms of the clusters were replaced by pseudoatoms. The pseudoatom used for cluster termination was parametrized to mimic the chemical effect of the neighboring SiO4 unit that was removed after extraction from the surface. To develop a pseudoatom representative of SiO4 groups, pyrosilicic acid was used as our reference system (Figure 2) where the equilibrium geometry was obtained at the MP2/6-31++G** level of theory using the ACES II/III software package.25 Using pyrosilicic acid, an O
Si(OH)3 linkage was replaced by a valence electron only fluorine atom (Figure 2) to which an effective core potential (ECP) of the usual form

(For this analysis, two atoms are considered coordinated if they are within 1.1 times the ab initio Si
O bond length of 1.66 Å.) As shown, over 98% of the silicon (oxygen) atoms in the sample are 4 (2) coordinated. These results indicate that the long- and shortrange order of the amorphous silica sample is consistent with prior experiments. Surface silanol concentrations for two grades of silica gel (Davisil grades 636 and 646) were measured. According to the product literature (and verified in-house), Davisil grades 636 and 646 have surface areas of 480 and 300 m2/g, respectively, and they are both expected to have the same surface silanol concentration. We find an average of 2.43 surface silanols per nm2 which Table 1. Summary of Theoretical and Experimental Characterized Bulk Silica property

experimenta

BKS

density (g/cm3)

2.32

2.1
2.2

RDF Pair/Peak Positions (Å) Si
O/1st

1.7

1.62

Si
O/2nd

4.1

4.15

O
O/1st

2.7

2.65

O
O/2nd

5.1

4.95

Si
Si/1st Si
Si/2nd

3.2 5.2

3.12 5.18

Vibrational Frequencies (cm
1) isolated vicinal

3742

3743

3730/3658

3530

VL ðrÞ ¼

a

Experimental density (this work), RDF peak positions (ref 15), and vibrational frequencies (this work).

2

3

4

silicon

0.0

1.7

98.3

oxygen

98.7

1.3

0.0

Table 3. Measured Surface Silanol Concentrations
2


2

[SiOH]H‑bonded [nm ] % SiOHH‑bonded b

Davisil 646

average

2.18 ( 0.49

2.68 ( 0.93

2.43 ( 0.53b

0.75 ( 0.16 34% ( 11b

1.28 ( 0.20 48% ( 18b

1.01 ( 0.13b 41% ( 11b

a

[SiOH]surf [nm ]

a

Davisil 636 a

a a

αiL c e
ςiL r2 iL

ð4Þ

was assigned for which L ranges from 0 to the maximum angular momentum of the basis set. In practice, to reduce the size of the computational basis, the polarization functions are removed from the pseudoatoms; therefore, only projections through L = 1 are required. The choice of fluorine for the pseudoatom, following the work of Zhang et al.,10 was motivated by the necessity of having a monovalent atom to properly cap the cluster. The exponents and coefficients for the fluorine ECP were determined using a combination of quasi-Newton and genetic algorithms such that the geometry of the fragment in Figure 2 matched that from the full pyrosilicic acid reference molecule. The converged ECP parameters for the pseudoatom are contained in Table 4, and a comparison of optimized internal coordinates from the fragment and the reference molecule is shown in Table 5. Surface IR Signatures. A transmission FTIR spectrum of fumed silica is shown in Figure 3 where the sharp transition at

Table 2. Percent Coordination in BKS Generated Silica coordination

∑i r

Uncertainties are the standard deviations of the measurements. Propagation of uncertainties.

Figure 2. Pyrosilicic acid (reference system) and pseudoatom substitution. 24737

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Table 4. Converged Effective Core Potential Parameters for Pseudoatom in Figure 2 α

ζ

C

1

1

14.94278


0.99997

0 0

0 2

9.17289 26.88892

1.14530 19.28774

L

Table 5. Comparison of Full Molecule and Truncated Cluster Geometry coordinate

full molecule

truncated cluster

R(Si
O) [Å]

1.615

1.615

R(Si
OH) [Å]

1.659

1.652

A(O
Si
O) [degrees]

106.9

105.9

Figure 4. MP2/6-31++G** optimized structures of H2O, NH3, and NO interacting with the isolated, vicinal (H-bonding), and geminal silanol sites.

Figure 3. Experimental silica FTIR transmission spectrum of CAB-OSIL fumed silica at 45 and 135 C and IR spectra calculated for three silica models.

Table 6. Silanol Vibrational Transitions for Fumed Silica measured [cm
1]

publisheda [cm
1]

MP2 [cm
1]

isolated

3743

3747

3742

terminal internal

3715 3647

3720 3650

3730

H-bonded

3530

3550

3658

assignment

a

Reference 26.

3743 cm
1 is attributed to isolated silanols on the surface of the silica and where the broad transitions at 3647 and 3530 cm
1 are attributed to bulk silanols and H-bonded surface silanols, respectively. A small shoulder at 3715 cm
1 corresponds to terminal H-bonded silanols (i.e., the oxygen is interacting with a neighboring silanol, but the hydrogen is not). Upon heating, the intensity of the peak at 3530 cm
1 is reduced as the surface silanols condense and evaporate from the surface. A small amount of hydrocarbon contamination is observed (presumably from the O-ring seals), but the agreement with published work, as shown in Table 6, suggests it does not significantly affect probe adsorption. Vibrational Analysis. As the system investigated by FTIR is amorphous, the features of the spectrum are broad with the exception of the sharp transition at 3743 cm
1. The intent of the vibrational analysis presented here is to confirm that the

theoretical clusters chosen are representative of those created in experiment. The vibrational analysis that has been computed gives single frequencies from normal-mode analysis, and if these frequencies are within the range of the experimental broad peaks, we conclude that the theoretical clusters are representative. There are several concerns that are commonly addressed in matching IR spectra to calculated frequencies including (a) scaling factors, which for the current level of theory is about 0.95,27 and (b) the role of anharmonicity. These concerns have not been included in the frequencies and frequency shifts presented here (i.e., the frequencies are unscaled) as quantitative comparison of IR spectra is not our focus. Figure 3 compares the experimental IR spectra at 45 and 135 C with the calculated IR spectrum from three different MP2/6-31++G** optimized surface clusters (isolated, vicinal, and geminal). The calculated νSiO
H = 3742 cm
1 for the isolated silanol (green line) compares extremely well with the experimentally measured νSiO
H = 3743 cm
1. In addition, consistent with experiment, the νSiO
H of the vicinal silanol (red line) is redshifted from the isolated silanol (νSiO
H,expt = 3530 cm
1 vs νSiO
H,MP2 = 3730/3658 cm
1). These results suggest that the simulated surface is indeed similar to the experimental surface. Many of the vibrational degrees of freedom of the internal Si
O
Si structure (∼1200 cm
1, ∼3500 cm
1) are not captured in the theoretical spectrum because of the limited size of the cluster models that were used. Adsorption IR Shifts. The adsorption of water, ammonia, or NO causes shifts in the IR spectra that are indicative of the type of silanol binding site occupied. A comparison of the magnitude and direction of these shifts is an indicator of the fidelity of the 24738

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Table 7. Comparison of Vibrational Transitions (νSiO
H and ΔνSiO
H) for Silanol
Adsorbate Systems silica system ν [cm
1]

experimenta theory (MP2/6-31++G**)

Δν [cm
1]

a

isolated

SiOH

SiOH
H2O

SiOH
NH3

SiOH
NO

isolated

3743

3440

3040

3530

vicinal

3530

isolated

3742

3479

3078

3666

vicinal

3658

3437

3042

3655

geminal

3732

3578

3151

3678

experiment


303


703


224

theory


263


664


76

vicinal

experiment theory


90
221


490
616


31
15

geminal

theory


154


581


54

Transitions for silanol
adsorbate complexes are not distinguishable.

simulated model to the experimental system. In the case of each adsorbate, the above-described truncated binding sites are filled with adsorbate molecules. Optimized structures describing the interaction of water, ammonia, and NO with each of the three silanol sites were completed at the MP2/6-31++G** level of theory using Gaussian0319 and are presented in Figure 4. In the case of the geminal silanol, there is an interacting and a noninteracting OH with respect to H2O. For ammonia, the binding is found to occur through the H3N 3 3 3 H
OSi linkage. We did not explicitly investigate, at the quantum chemical (QC) level, the interactions of NH3 with the surface Si
O
Si functional units. As shown in Figure 4, interactions of NO with the silanol occur with both the N (NO) and the O (ON). Vibrational Analysis. Changes in the IR spectrum associated with the SiO
H stretch that result from the interaction of water with the surface were investigated. Table 7 shows that the predicted shifts for the interaction of H2O with vicinal silanols are larger than the experimentally measured shift (
90 cm
1). On the other hand, the predicted shift for H2O interaction with isolated silanols agrees well with experiment (
303 cm
1). The main change in the IR spectrum that results from the interaction of water with the surface is a shift in the H-bonded SiO
H stretch to 3440 cm
1. This broad transition is due to adsorption of H2O to both isolated and vicinal silanols. Upon adsorption of H2O to isolated silanols, the narrow line width of the
OH stretch broadens and is red-shifted, and so it is no longer distinct from the vicinal silanol
OH transition. Geminal silanols are not distinguishable from isolated (if non-H-bonded) or vicinal (if H-bonded) silanols in IR spectroscopy.10 As shown in Table 7, the apparent experimental Δν(O
H) for adsorption of water to isolated and vicinal silanols are
303 and
90 cm
1, respectively. Calculations for the water interaction with isolated silanols (
263 cm
1) are somewhat less than what is measured experimentally. The theoretical (
221 cm
1) and experimental (
90 cm
1) shifts for Δν(O
H)vicinal have the same sign and yet are significantly different. In the case of vicinal/geminal, these silanols have two
OH groups. One group interacts with the probe, and the other does not. MP2 calculations indicate shifts of
30 and
15 cm
1 for the other vicinal/geminal Si
OH shift corresponding to the noninteracting vicinal/geminal Si
OH. The intensity of the predicted band increases with shifts (ΔνSiO
H) relative to that of the bare surface of (a)
263 cm
1 for interaction with SiO
Hisolated, (b)
221 cm
1 for interaction with SiO
Hvicinal, and (c)
154 cm
1 for interaction with SiO
Hgeminal.

When the adsorbate is ammonia (see Table 7), theoretical predictions for interaction with vicinal silanols are larger than the experimentally measured shift (
490 cm
1), and predictions for interaction with isolated silanols are smaller than the experimentally measured shift (
703 cm
1). The intensity of the predicted band increases with shifts (ΔνSi
OH) relative to that of the bare surface of (a)
664 cm
1 for interaction with SiO
Hisolated, (b)
616 cm
1 for interaction with SiO
Hvicinal , and (c)
581 cm
1 for interaction with SiO
Hgeminal. Both experiment and MP2 indicate that the NH3
silica shifts are significantly larger than shifts for H2O and NO. The main change in the experimental IR spectrum that results from the interaction of NH3 with the surface is a large shift in the H-bonded SiO
H stretch to 3040 cm
1. This broad transition has contributions from NH3 adsorbed to both isolated and vicinal silanols yielding shifts of
703 and
490 cm
1, respectively. These shifts agree with calculations for the NH3 adsorption on isolated (
664 cm
1) and vicinal (
616 cm
1) silanol sites. Finally, for the NO adsorbate, Table 7 shows that the predicted shift for the interaction of NO with each silanol type for all three of the truncated systems is generally larger than the experimentally measured shift for vicinal silanols (
31 cm
1). The intensity of the predicted band increases with small shifts (ΔνSiO
H) relative to that of the bare surface of (a)
76 cm
1 for interaction with Si
OHisolated, (b)
15 cm
1 for interaction with Si
OHvicinal, and (c)
54 cm
1 for interaction with Si
OHgeminal. These shifts of the silanol stretches, νSiO
H, are much smaller than those predicted for H2O and NH3. The main change in the experimental IR spectrum that results from the interaction of NO with the surface is a relatively small shift in the H-bonded SiO
H stretch to 3499 cm
1. This shift can be due to NO adsorption to both isolated and vicinal silanols; however, adsorption to the isolated silanols would require a
244 cm
1 shift which is much larger than the predicted shift (
76 cm
1). The predicted shift for NO adsorption to vicinal silanols is small (
15 cm
1), which is in agreement with the measured shift (
31 cm
1). Thus, we can conclude that the surface
adsorbate clusters are similar, though not identical, to the surface adsorption configurations encountered in the experimental system. Comparison of Heats of Sorption and Binding Enthalpies. Computation of Binding Enthalpies. The ab initio quantum chemistry software packages used for this work consisted of Gaussian0328and GAMESS.29 These well-known programs provide high-level solutions to the time-independent Schr€odinger equation employing both correlated-wave function- and density-functional-based methodologies. In addition to energies, analytic gradients (a necessity for 24739

dx.doi.org/10.1021/jp205479v |J. Phys. Chem. C 2011, 115, 24734–24742

The Journal of Physical Chemistry C

ARTICLE

efficient exploration of potential energy surfaces) and analytic/ numerical Hessians (vibrational frequency analyses) are also available. These codes run very efficiently in parallel and were run using from 1 to 256 processors on a variety of computer architectures including an IBM P6 and several Linux clusters. Minimum energy configurations and vibrational frequencies were computed at the MP2/6-31++G** level of theory. Optimal geometries of each adsorbate were determined for all three silanol clusters extracted from the surface. During the cluster geometry optimizations, the pseudoatoms were constrained to remain at their relative positions in the bulk sample. This was done to mimic the effect of the bulk on the defect site which anchors the terminal SiO4 units surrounding the reactive center. The binding enthalpy of each adsorbate was computed as H ¼ Eelec þ Evib þ Etrans þ Erot þ PV

ð5Þ

in which H is the enthalpy; Eelec is the MP2 electronic energy including the zero point energy but excluding the basis set superposition error;30 and Evib, Etrans, and Erot are the vibrational, translational, and rotational contributions to the enthalpy. Because the pseudoatoms represent nuclear and electronic constraints from the remainder of the system, their positions are fixed in the optimization, and vibrational analysis is carried out neglecting the contributions from the pseudoatom degrees of freedom. Operationally, the normal modes involving motion of the fixed pseudoatoms are removed by assigning an infinite mass to the pseudoatoms effectively removing their contribution to the mass-weighted Hessian before diagonalization. An appropriate comparison of the theoretical values to experiment comes from statistical thermodynamic concerns. To appropriately average the calculated enthalpies for the isolated, geminal, and vicinal sites with each adsorbate, we assume equal populations of each and that each type of silanol site is available for adsorption. Both of these assumptions are supported by the experimental characterization described above (see Table 3). Furthermore, assuming that the specific heat at constant pressure is the same for each type of silanol site, we obtain eq 6

∑i Hi e
H =kT ÆÆHææ ¼ ∑i e
H =kT i

i

ð6Þ

in which H is the enthalpy computed using eq 6 above. The ensemble-averaged enthalpies were determined using the midpoint for the experimental temperature range. ΔHads for H2O, NH3, and NO adsorbing on silica were measured using IGC, and all plots according to eq 2 were linear (R2 > 0.95, see Figures S1
S3 of the Supporting Information for elution profiles and linear fits). Measurements were made as a function of probe injection amount for H2O and NH3 as shown in Figure 5 to investigate the potential for adsorbate
adsorbate interactions to affect the results. Two pieces of evidence suggest NH3 measurements were made in the linear part of the isotherm (i.e., adsorbate
adsorbate interactions were negligible): (1) there was no dependence of ΔHads on the injection amount (see Figure 5) and (2) the ratio of NH3 molecules to surface sites was less than 0.0014 (i.e., nNHS/nSiOH , 1). The average ΔHads for NH3
silica was
12.7 ( 2.9 kcal/mol for which the uncertainty is the standard deviation of the measurements. It is unclear what caused the large scatter in the measurements of NH3 adsorption enthalpy (note the 12 kcal/mol range at 11 nmol

Figure 5. Enthalpy measurements for H2O and NH3 on silica as f(probe injection amount). NH3 was injected at constant absolute pressure, and H2O was injected at constant partial pressure (note the top x scale). Error bars represent the uncertainty of the linear fit (eq 2).

injection in Figure 5), but these measurements seem to be outliers (see Figure S4 of the Supporting Information for a histogram of the results). H2O appears to have a slight trend toward larger ΔHads at higher injection amounts; however, upon closer inspection, this dependence is within the scatter of the experimental data for P/P0*Vloop < 0.25 mL. Averaging all the measurements together, ΔH for water on silica gel was determined to be
13.6 ( 2.8 kcal/mol for which the uncertainty is the standard deviation of the averaged measurements. This compares well with published results of ΔHads =
15 kcal/mol for an approximate adsorbed water coverage of 0.5 μmol/m2, whereas the coverage in these experiments is