Mid- and Near-Infrared Study of Hydroxyl Groups at a Silica Surface: H

ϵ(νOH)/ϵ(2νOH) both increase for decreasing νOH wavenumber decreases, i.e., as the strength of the H-bond increases. ... Forgot ACS ID or Pas...
2 downloads 0 Views 465KB Size
Download PDF
13300

J. Phys. Chem. C 2009, 113, 13300–13308

Mid- and Near-Infrared Study of Hydroxyl Groups at a Silica Surface: H-Bond Effect Ce´dric Carteret† Laboratoire de Chimie Physique et Microbiologie pour l’EnVironnement, Equipe Chimie et Spectrochimie des interfaces, UMR 7564, CNRS- Nancy UniVersite´, 405 rue de VandœuVre F54600 Villers-le`s-Nancy, France ReceiVed: January 29, 2009; ReVised Manuscript ReceiVed: April 22, 2009

This paper studies the IR absorption of the OH stretching modes νOH and of their first overtones 2νOH for silanol groups at silica surfaces. Heat treatment of silica induces a condensation of silanol groups. The subsequent decrease in silanol concentration results in strong modification of the spectral profiles. The spectra are interpreted in terms of distributions of OH oscillators. The mechanical anharmonicity coefficient xOH and the ratio of the fundamental/overtone extinction coefficients (νOH)/(2νOH) both increase for decreasing νOH wavenumber decreases, i.e., as the strength of the H-bond increases. The decadic integrated intensities are plotted against silanol concentrations. The intensity of the νOH mode significantly increases with hydrogen bonding, whereas that of 2νOH slightly decreases. As a consequence, compared to the fundamental mode, the first overtone provides a better picture of the distribution of OH states. Analysis of the infrared spectra also demonstrates the presence of cooperative bonding between silanol groups at the silica surface. 1. Introduction Silicas are used in many applications as adsorbates, catalytic or chromatographic supports. For this reason, many studies1-4 have been devoted to the study of their surface properties. The unsatisfied surface valences are saturated by chemisorption of water with formation of hydroxyl groups, silanols SiOH, covalently bonded to the solid. These groups play a major role in the surface reactivity of silica, which thus strongly depends on the preparation process and postsynthesis modification of the material. The infrared spectra of SiOH groups have been extensively studied,5 with special focus on the νOH stretching mode, due to its separation from the other modes and its large dependence on interactions of hydroxyl groups. In that context, most articles deal with the assignment of the different νOH subbands that arise from the heterogeneous spatial distribution of silanols at a silica surface. IR spectroscopy has long been used for studying H-bonded systems.5-12 Numerous experimental studies of hydrogen bonds have thus been reported for alcohols and water under various conditions (liquid13-24 and supercritical phases,25-27 rare gas matrices28-30 and gas phases31-34). Anharmonicity is a fundamental parameter for understanding the nature of hydrogen bonding since its value is intimately related to the shape of the potential function.7,11,12 Anharmonicity can be calculated by ab initio methods for simple H-bonded systems such as dimers.31-34 For more complicated systems, solutions, or solids, anharmonicity can be derived experimentally by recording overtone spectra in the near-infrared (NIR)10-20,28-35 or estimated by deuteration experiments.31,32 Still, the NIR spectral range is seldom examined, and, in the last two decades, only a few NIR studies have examined hydrogen-bonded systems and investigated the effect of hydrogen bonding on anharmonicity. The reasons for this relatively modest research effort are (i) overtones of hydrogen-bonded species have a weak intensity and (ii) the NIR spectra of polyatomic molecules contain numerous bands originating not only from overtones, but also from combinations and/or simultaneous transitions, which makes a precise band † E-mail: [email protected].

assignment often difficult. However, significant conclusions were drawn from such measurements: indeed they revealed (i) an increase with H-bonding of the absolute value of the mechanical anharmonicity coefficient xOH defined as xOH ) (2σ1 - σ2)/2, where σ1 and σ2 are the wavenumbers of the fundamental (νOH) and first overtone (2νOH), respectively; (ii) a concomitant wavenumber decrease and intensity increase of the νOH band; and (iii) an intensity decrease of the 2νOH band with respect to the corresponding bands of the free OH oscillator. Stronger H-bonding results in stronger effects.9,13,16,19,21-24,28-30 Furthermore, these experimental results were confirmed by ab initio calculation.31-34 A quantification of the effect of hydrogen bond on O-H overtone intensities was very recently published by Scharge et al.32 in the case of gaseous alcohols. The concomitant description of the H-bond effect on the O-H fundamental and first overtone of hydroxyl groups at oxide surfaces has, to our knowledge, not yet been performed in the literature. Studying O-H oscillators in solid phases allows avoiding the problems of simultaneous transitions and vibrational couplings, which require extreme precautions in the study of OH profiles for molecules in solution.10,11,14-16 It then appears that silanol groups at silica surface provide a unique opportunity for studying H-bonding distribution as silanol concentration can be modulated through heat treatment of the silica at various temperatures. The present work reports the first principal results obtained on the mechanical anharmonicity coefficient, the integrated intensities of νOH and 2νOH, the spectral profiles related to the chemical distributions of silanol groups, and their evolutions with H-bond strength. Finally, the existence of cooperative H-bonding at the silica surface is also discussed. 2. Experimental Section Samples. In order to obtain high-quality NIR spectra, a thick optically transparent sample was necessary. Thus, plate-shaped monoliths of porous silica were used. The porous silica glass under study was Vycor 7930 manufactured by Corning with a surface area of 200 m2 g-1 (as measured by the BrunauerEmmett-Teller (BET) method using nitrogen at 77 K as the

10.1021/jp9008724 CCC: $40.75  2009 American Chemical Society Published on Web 07/01/2009

IR Study of Hydroxyl Groups at a Silica Surface

J. Phys. Chem. C, Vol. 113, No. 30, 2009 13301

adsorbate) and an average pore diameter of 7 nm. As suggested by Corning, Inc., the Vycor porous glass was washed at about 100 °C with a H2O2 solution (30% by weight, Labosi Analypur) to remove traces of adsorbed organic contaminants from the surrounding atmosphere. The cleaned glass was stored in Milli-Q water until ready for analysis. Before each analysis, the silica was dried 24 h in a desiccator with a controlled water relative humidity of 25%. Such a sample was considered pristine. The thicknesses of the studied samples were 4.75 mm in NIR and 120 µm in mid-IR (MIR). Thermogravimetric Measurements. Thermogravimetric analyses were obtained on a Setaram model TGA92 instrument under an inert water free N2 atmosphere, the gas being purified through a P2O5 trap. About 100 mg of silica was introduced in a platinum crucible at 20 °C. The weight loss was recorded as a function of temperature, which was increased at a rate of 0.2 °C min-1 from 130 to 1000 °C after a stage of five hours at 130 °C. The mass of the sample after this stage, called m0, corresponded to a fully dehydrated sample. Above 130 °C, the weight loss was due to the elimination of water following the equation

2tSi-OH f tSi-O-Sit + H2O

where N is the molar amount per unit mass of the species in the plate (mmol g-1) and F is the surface mass (g cm-2), i.e., the mass of the plate divided by its area. A relationship analogous to eq 5 then holds, if Absorbance is replaced by the Areaband:

Areaband )

∫band A(σ)d(σ) ) INF

(6)

where I is the integral decadic absorption coefficient. The integrated intensity I is usually expressed in km · mol-1. At constant F, the Areaband increases linearly as a function of N with IF as the slope. If we consider silanols at a silica surface, several types of more or less H-bonded OH oscillators coexist. Each type, labeled i, is characterized by its abundance Ni and its absorption coefficient Ii. Then, one has N ) ∑iNi and Areaband ) F∑iIiNi, leading to the definition of an apparent coefficient Iapp:

Iapp )

(1)

∑ IiNi i

)

∑ Ni

Areaband NF

(7)

i

The weight loss measured in milligrams during the experience was converted in % weight of dry silica:

∆x(%) ) 100(m0 - m)/m0

(2)

Since one water molecule forms from two silanol hydroxyl groups, the molar concentration per unit of mass of condensed hydroxyls was

∆N(mmol/g) ) 2 ·

1000 ∆x(%) 20 ) ∆x(%) 100 MH2O 18

(3)

Infrared Spectra. Infrared spectra were obtained with a Fourier transform infrared (FTIR) Perkin-Elmer 2000 spectrometer equipped with deuterated triglycine sulfate (DTGS) detector, using a globar source and a KBr beamsplitter for the MIR or a tungsten-halogen source and a quartz beamsplitter for the NIR. Transmission spectra were obtained through evacuated monoliths (pressure about 10-5 Pa) after heat treatment in the same evacuated chamber. All the spectra were recorded for samples at room temperature with a resolution of 1 cm-1. Spectral Data Treatment. When a monochromatic beam of light, characterized by its wavenumber σ, goes across a homogeneous plate of thickness l (cm), in which the molar concentration of a species is C (mol L-1), the decadic absorbance of this species is, according to Beer-Lambert law,

A(σ) ) ε(σ)Cl

(4)

where ε (mol-1 L cm-1) is the molar decadic absorption coefficient at the considered wavenumber σ. For a species on the surfaces of a silica plate, an equivalent form of eq 4 can be obtained:

A(σ) ) ε(σ)NF

(5)

We defined the R coefficient as the derivative of the Areaband with respect to the concentration N:

{

d(Areaband /F) Rj ) d(N)

}

{ } d(

)

Nj

∑ IiNi) i

d(

∑ Ni) i

(8)

Nj

If the concentration of only one species S varies around the point Nj, we can associate the derivative at the point Nj noted Rj to the integral decadic absorption coefficient of S noted IS. We postulate on the one hand that the absorption profiles of νOH and 2νOH characterize distributions of H-bonded silanol groups at the silica surface and, on the other hand, that the absorptions are only due to the transitions νOH and 2νOH in both spectral domains. For NIR and MIR silica plates in the same reference state r, i.e., after degassing at 130 °C, the absorption at wavenumbers σ1 and σ2 can be expressed using Beer-Lambert law as

A1r(σ1) ) ε1(σ1) · Nr(σ1).F1r A2r(σ2) ) ε2(σ2) · Nr(σ2).F2r

for the fundamental νOH for the overtone 2νOH

Nr(σ1) and Nr(σ2) correspond to silanol concentrations absorbing at σ1 and σ2; F1r and F2r are the surface masses of the plates. For silica plates in a chemical state k leading to a different distribution of OH oscillators, after annealing at 500 °C for instance, we obtain

A1k(σ1) ) ε1(σ1) · Nk(σ1).Fk A2k(σ2) ) ε2(σ2) · Nk(σ2).Fk

for the fundamental νOH for the overtone 2νOH

As F1k ) F1r and F2k ) F2r, we can define the following ratios:

13302

J. Phys. Chem. C, Vol. 113, No. 30, 2009

R1k(σ1) )

Carteret

A1r(σ1) Nr(σ1) ) A1k(σ1) Nk(σ1)

(9)

Nr(σ2) Nk(σ2)

(10)

and

R2k(σ2) )

A2r(σ2) A2k(σ2)

)

If silanols absorbing at σ1 and σ2 are in the same state, we verify

Nr(σ1) Nr(σ2) ≡ Nk(σ1) Nk(σ2)

so

R1(σ1) ≡ R2(σ2)

Thus, for the same value of R1 and R2, we obtain a couple of wavenumbers (σ1, σ2) for an OH group in a definite state. In order to improve the robustness of the method, we consider n states of the samples (k from 1 to n) with n being different distributions from r. From the silica plates, we obtained these various molecular state distributions of silanols by heating the sample at n different temperatures. We obtain n ratios R1(σ1) and R2(σ2), and we defined two matrices R1 of size n × f and R2 of size n × o where f and o are the numbers of data points in the fundamental (associated wavenumber σ1) and in the first overtone (associated wavenumber σ2), respectively. R1(σ1) is the column vector of size n × 1, which regroups the n values of the R1 ratio at the particular fundamental wavenumber σ1. R2(σ2) is the column vector of size n × 1 which regroups the n values of the R2 ratio at the overtone wavenumber σ2. Then the correlation matrix C of size f × o which gathers the correlation coefficients between the f × o couples (R1(σ1); R2(σ2)) can be calculated. For each fundamental wavenumber σ1, we associate a unique σ2 overtone wavenumber by selecting the couple (σ1, σ2) with the highest correlation coefficient. Small absorbances of at least one spectrum k increase uncertainty on the correspondence (σ1, σ2). For an OH vibrator (decoupled from the molecule), the second-order perturbation theory allows one to compute the mechanical anharmonicity x from the experimental wavenumbers of the fundamental and first overtone: x ) σ1 - (1/2)σ2. Usually, the wavenumbers σ1 and σ2 are identified with those of the maxima of bands νOH and 2νOH.11-15,17-20,28-30,32 However, such a procedure is justified only if the profiles of both bands are identical, in the homothetic ratio 2 on the wavenumbers, which is not the case in most experiments.11-15,17-20 Comparing the overtone and fundamental absorptions shows that it is far from being the case for H-bonded OH groups at the silica surface. To obtain the values of the anharmonicity coefficient x we thus correlated the absorption profiles in the fundamental and overtone spectral ranges. Once the wavenumber couples (σ1, σ2) have been identified, the mechanical anharmonicity coefficient xOH and the ratio of the fundamental/ overtone extinction coefficients ε(νOH)/ε(2νOH) can be estimated. 3. Results and Discussions Silica Dehydration. In order to study the hydroxyl group of silica surfaces, it is important to ensure extensive dehydration of the samples prior to analysis. Most divided silicas (nonporous or mesoporous) are readily dehydrated by evacuation at room temperature.3,4,36,37 Figure 1 displays the evolution of the absorption NIR spectrum of Vycor upon progressive evacuation

Figure 1. Evolution of the NIR spectra of Vycor silica by evacuation at various temperatures.

under vacuum (10-5 Pa). Absorption spectra allow monitoring both water and silanol contents. Under vacuum absorption signals around 5260, 6840, and 7115 cm-1 related to physisorbed water decrease, while the silanol components around 4545 and 7310 cm-1 increase, due to the lowering of hydrogen bonding to water molecules. The component at 4545 cm-1 is assigned to the combination (ν+δ)SiOH between stretching OH and bending SiOH modes of silanols, while the band at 7310 cm-1 is assigned to the overtone 2νOH. Water content can be estimated from absorption bands involving only vibrations of physisorbed water molecules. Such a criterion is fulfilled by the (ν+δ) combination of the bending (ν2) and one of the stretching modes (ν1 or ν3) of water molecules in the spectral range 5000-5350 cm-1. In the case of Vycor, the sample still contains residual water molecules after evacuation at room temperature, as evidenced by the relatively intense combination band at 5260 cm-1. We can at least consider two limiting factors for water evacuation. On one hand, the Vycor sample is monolithic silica with high and complex porosity, which limits water diffusion. On the other hand, the surface of Vycor exhibits high energetic sites for water adsorption.38,39 The absence of any absorption signal around 5200 cm-1 reveals the complete removal of water molecules above 100 °C. At 130 °C there is no modification of the silanol absorptions: the surface is fully hydroxylated and free of water molecules. This annealing temperature corresponds to the initial state of the Vycor silica surface used to study the behavior of hydroxyl groups versus an increase in heating temperature. Above 130 °C, surface dehydroxylation begins to occur. Thermal Annealing: Silanol Condensation and Evolution of the νOH and 2νOH Profiles. Figure 2A,B displays the evolution with heat treatment of the νOH and 2νOH absorption signals under 10-5 Pa. After degassing at 130 °C, molecular water is totally removed from the silica surface, and the surface is fully hydroxylated. The absorption profiles then arise from the heterogeneous spatial distributions of OH groups at the silica surface leading to a distribution of chemical states of the OH groups. The H-bonded OH with various OH · · · H distances are responsible for the intense absorption below 3700 cm-1, whereas the weak shoulder at 3735-3740 cm-l characterizes free silanol groups. In the NIR spectral range, the maximum of the overtone 2νOH at 7310 cm-1 characterizes free silanols, whereas the H-bonded silanol absorption corresponds to the asymmetry of this band toward low wavenumbers. When the annealing temperature increases, the global intensity of the bands νOH and 2νOH drops, evidencing a decrease in silanol concentration

IR Study of Hydroxyl Groups at a Silica Surface

J. Phys. Chem. C, Vol. 113, No. 30, 2009 13303

Figure 2. Transmission spectra in the νOH (Part A) and 2νOH (Part B) spectral range of Vycor 7930 silica samples after heating at 130, 220, 300, 350, 400, 450, 500, 550, 600 °C. At 130 °C, the silica is dehydrated and fully hydroxylated.

Figure 3. Evolution of the fundamental and first overtone silanol absorptions (band area) versus the silanol molar loss by heat treatment. A: νOH and B: 2νOH.

(eq 1). The condensation first involves the more strongly H-bonded population of silanol groups, up to about 450 °C and then, sequentially, the silanols with the lowest vOH wavenumber. This is illustrated by the decrease of both the broad absorption below 3700 cm-1 and the low wavenumber shoulder of 2νOH. Concomitantly, less perturbed states appear as evidenced by the increase of both high frequency sides at 3747 and 7330 cm-1. The difference between the profiles νOH and 2νOH diminishes with increasing annealing temperature. At 600 °C, both profiles are very similar. In that case, both maxima can be correlated to yield an anharmonicity coefficient of 84 cm-1, close to the value reported for isolated silanol at the silica surface.35 The decrease of silanol IR absorptions can be correlated to the decrease of silanol concentration upon increasing annealing temperature. As spectral profiles are modified with heat treatment, it is more appropriate to use band areas rather than absorbance at a given wavenumber as the infrared variable. The curve Area ) f(∆N) is not linear at all for the fundamental νOH transition (Figure 3A). In contrast, the graph for the overtone 2νOH displays a good linearity (Figure 3B). As thermal analysis cannot provide the total silanol concentration N0 in fully hydroxylated silica because of the constant presence of silanols in the material beyond 1000 °C, measurements at annealing temperatures above 400 °C were fitted linearly, the extrapolation toward Area ) 0 thus yielding an estimate of N0. According to such a procedure, the total concentration of silanol is 3.07 ((0.06) mmolOH/g. The amount N(T) of remaining silanols at each annealing temperature can then be determined as N(T) ) N0 - ∆N(T). Vycor 7930 is characterized by a specific surface of 200 m2 · g-1. Thus, a concentration of 3.07 mmolOH · g-1 corresponds

to a silanol density of 3.07 × 6.10-20/(2.0 × 102 × 1018) ≈ 9.2 OH · nm-2. It is important to point out that the concentrations in surface hydroxyl groups thus obtained are higher than those generally reported. The density of surface silanol groups on amorphous silicas can vary considerably according to the preparation method. In attempting to quantify the silanol groups on a fully hydroxylated silica surface, a number of techniques and approaches have been employed such as deuteriumexchange processes coupled with mass spectrometric analysis,40 MAS NMR,41-45 Raman,41 thermogravimetry,42,46-49 thermal analysis,3,36 chemical grafting, and chemical reaction methods.2,3,36,50 The amount of silanols established by these techniques varied from 4 to 16 OH per nm-2.36,37,40-50 On most fully hydroxylated silicas there are about 4-7 OH/nm2, this quantity being more or less independent of surface area. However, this number can in some cases exceed 10 OH/nm2.36,44,46,49 Furthermore, theoretical values calculated on the basis of geometric considerations range from 4.5 up to 13.5 OH/nm2.39 Mechanical Anharmonicity Coefficient: Correlation between Fundamental and Overtone Wavenumbers. Figure 4 presents the ratio curves R1 and R2 for the transitions νOH and 2νOH. Both curves display a similar shape. It is then possible to correlate σ1 and σ2 (Figure 5) as developed in the data treatment section. The values of xOH are reported in Figure 6 for the spectral range 3550-3750 cm-1. The uncertainty on the calculated values increases from a few inverse centimeters at 3700 cm-1 to 30 cm-1 at 3550 cm-1. The values of xOH obtained in the high wavenumber absorption ranges, i.e., for the molecular states where OH groups are weakly perturbed, are in agreement with those deduced from absorption maxima in cases where the fundamental and overtone bands are rather narrow. Toward low frequencies, i.e., for the states in which OH groups are more

13304

J. Phys. Chem. C, Vol. 113, No. 30, 2009

Carteret

Figure 6. Calculated mechanical anharmonicity xOH versus wavenumber of the fundamental.

Figure 4. Plots of the ratios of the distributions: NT/N130°C with T ) 220, 300, 350, 400, 450, 500, 550, and 600 °C. (A) R1(T) for the fundamental νOH and (B) R2(T) for the first overtone 2νOH. See Spectral Data Treatment section for detailed explanations.

Figure 5. Correlation between wavenumbers σ1 (νOH) and σ2 (2νOH). This correlation is obtained from the spectral data treatment developed in section 2.

perturbed, xOH reaches values roughly twice that obtained for free OH. These results agree fairly well with the conclusions obtained on alcohols both in solution11-20 and in the gas phase:31,32 i.e., the anharmonicity coefficient increases with increasing H-bond energy. Integrated Intensities of νOH and 2νOH. Spectral Distributions of Silanol Groups. Taking into account the correlation between σ1 and σ2, the ratio of the molar extinction coefficients ενOH/ε2νOH can be estimated (Figure 7). The curve ενOH/ε2νOH indicates a very important modification of this ratio according to the state of perturbation of the silanol group. Burneau and Limouzi16 had validated the hypothesis of a unique relation between ενOH and σ1 or ε2νOH and σ2 for the OH group of methanol. However, supplementary

Figure 7. Calculated ratio of the molar absorption coefficients of the first overtone (2νOH) and fundamental (νOH) OH absorptions versus σ1(νOH).

measurements are necessary to estimate the contribution of each of the coefficients ενOH and ε2νOH to the variation of the ratio. The coefficients ενOH(σ1) and ε2νOH(σ2) must not be confused with ενOH, apparent(σ1) and ε2νOH, apparent(σ2). Indeed, these latter coefficients are estimated through the relation εapparent(σ) ) A(σ)/ C0l, where C0 represents the total concentration of OH groups and not the concentration C(σ) of the OH groups contributing to the absorption at wavenumber σ. Thus, ε(σ) is always higher than εapparent(σ), the difference increasing with increasing broadening of the distribution. Figure 8 shows the evolutions of the band areas of νOH and (2νOH) band with silanol concentration. The νOH area increases strongly in a nonlinear way with silanol concentration (Figure 8A). In contrast, the area of the first overtone increases linearly up to silanol concentrations of 1-1.2 mmol/g (Figure 8B). Both curves can be well fitted by polynomial functions. Using these fits, the derivatives (dA/dN) can be calculated accurately (Figure 9). Figure 9 displays the Iapp and R values versus silanol concentration. For the free OH groups, at low concentration, the estimates of the integral decadic absorption coefficients I0(νOH) and I0(2νOH) by using either Iapp or R are very close with values of 26 and 1.12 km · mol-1 for the fundamental and overtone, respectively. When NOH increases, the concentration of H-bonded silanols increases. Iapp corresponding to the fundamental mode (Figure 9Aa) then increases strongly (×5), whereas that corresponding to the first overtone (Figure 9Ba) decreases slightly (×0.8). The evolutions of R (Figure 9Ab,Bb) are more pronounced in both spectral ranges. For strongly perturbed silanols, the integral decadic absorption coefficients can be estimated at NOH ) 3 mmol/g; values of 270 and 0.55 km · mol-1 are obtained for IHB,νOH and IHB,2νOH, respectively.

IR Study of Hydroxyl Groups at a Silica Surface

Figure 8. Evolution of the band areas of the fundamental and first overtone silanol absorptions with the total silanol concentration. Experimental data points and polynomial fits: (A) νOH and (B) 2νOH.

For silanols engaged in H-bonds, the value obtained for the fundamental mode (270 km.mol-1) is in rather good agreement with that reported by Davydov et al.51 (190 km · mol-1). For the free silanols, I0(νOH) was found close to the mean value of the literature46,52-54 (30 km · mol-1). No data for the overtone mode were reported in literature. In summary, the integrated absorption coefficient of free SiOH groups drops by a factor around 20 between the fundamental and the first overtone. This drop is more dramatic for H-bonded silanol, as it reaches 500. The effect of H-bonding on the infrared intensities of fundamental and first overtone has been widely debated in the literature.9,13,16,19,21-24,38,30-34,55,56 Luck’s group reported that the infrared integrated intensity of fundamental OH stretching of methanol in different solvents strongly increases with increasing hydrogen bonding, whereas the integrated intensity of the first overtone does not exhibit any significant change.21-24 Similar results have been reported for other alcohols in solutions.13,16,19 Measurements in low temperature matrices (N2 or Ar) carried out on water or methanol diluted samples revealed a strong intensity increase of the fundamental OH-stretching transition of the hydrogen-bonded hydroxyl group and a decrease of the first overtone.28-30 Very recently, Scharge et al.32 reported the first experimental overtone intensity data set for H-bonded alcohol in the gas phase. The integrated coefficient of the first overtone 2νOH of free trifluoroethanol is about 13 times smaller than that of the fundamental νOH, whereas, in the dimer, it is about 400 times smaller for the donor OH stretch. Burneau and Limouzi16 and Luck et al.22,23 mentioned that the distribution of OH vibrations in OH · · · B complexes is better described by the 2νOH absorption than by the fundamental νOH one, as the

J. Phys. Chem. C, Vol. 113, No. 30, 2009 13305

Figure 9. Apparent integrated decadic molar absorption coefficient (Iapp) and derivative R versus the total silanol concentration: (A) νOH and (B) 2νOH; (a) Iapp and (b) R.

fundamental band overemphasizes H-bonded silanols. Simple theoretical models based on expressions of the transition moment were developed to understand the effect of hydrogen bonding on IR intensities of fundamental and first overtone. For the first overtone, changes in the intensity of 2νOH were interpreted as resulting from the compensation of terms from mechanical and electrical anharmonicities in the expression of the transition moment.55,56 As far as the fundamental is concerned, the integrated absorption coefficient is known to be linearly correlated to the absorption wavenumber.6,9,22,57-60 For instance, in materials such as quartz or silicate glass, Paterson57 derived the frequency dependence of the hydroxyls absorption intensity in the 3 µm region as

I(σ) ) 0.5(3780 - σ)

using km · mol-1 units

(11) with I being the integral decadic absorption coefficient, equivalent to Iapp (spectral data treatment) and σ being the median wavenumber, i.e., the center of gravity of the area of the absorption band. By comparing IR and Raman spectra of liquid methanol, Burneau et al.59 showed that the OH dipole moment derivative (dp/dr)e can be related to the wavenumber σ. From this relation, Burneau et al.60 further proposed the following expression relating I with the νOH stretching wavenumber:

I(σ) ) [6462 - 1.687σ]2 /(1000 · ln 10) using km · mol-1 units (12)

13306

J. Phys. Chem. C, Vol. 113, No. 30, 2009

Carteret

Figure 10. Correlation of the integrated intensity of OH fundamental stretching absorption versus wavenumber.

Figure 11. νOH absorption A(σ) of Vycor (a) and spectral distribution D(σ) ) A(σ)/I(σ) using different functions I(σ): (b) Burneau,61 (c) Paterson,58 and (d) this work. The different functions I(σ) are presented in the text.

with I being the integral decadic absorption coefficient, equivalent to R (spectral data treatment), for a species absorbing at wavenumber σ. The I(σ) values obtained for OH groups at silica surfaces are in excellent agreement with this general trend (Figure 10). Indeed, a linear relation is observed:

I(σ) ) 1776 - 0.468σ

using km · mol-1 units

(13) with I being the integral decadic absorption coefficient, equivalent to Iapp (spectral data treatment) and σ beeing the median wavenumber, i.e., the center of gravity of area of the absorption band. Note that relations 11 and 13 are obtained in the same manner (I equivalent to Iapp) in contrast with relation 12, where I is equivalent to R. Relation 12 gives higher values at low wavenumber. This is related to the previous discussion about the fact that, for perturbed states, values of R are higher than values of Iapp. The strong increase of the νOH intensity with increasing H-bond interactions can be overcome by correcting the integral molar absorption by its variation with wavenumber σ.22,60 The absorption spectra A(σ) are then transformed into the corrected ones DOH(σ) following the relationship61

DOH(σ) ) A(σ)/I(σ)

(14)

Figure 11 displays the profiles normalized to their maximum for the initial spectrum corresponding to A(σ) and for DOH(σ) obtained with the different corrective functions (eqs 11, 12, 13). The use of the functions significantly reduces the importance of the strongly perturbed silanols, and the maximum of the distribution is shifted to 3740 cm-1 instead of 3510 cm-1 for the initial absorption spectrum. The calculated distributions get closer to the profile of the first overtone 2νOH. The distributions in Figure 11c and Figure 11d still display a marked shoulder toward 3680-3690 cm-1 and a bump at 3520 cm-1 while distribution 11b reduces more significantly the importance of the perturbed states. The initial spectrum A(σ) highlights H-bonded hydroxyls, whereas the spectrum transformed by I(σ) allows reaching the distribution of hydroxyls at the silica surface. To conclude, the spectral difference between the fundamental and the first overtone absorptions can be understood and taken

Figure 12. Integrated intensities I of fundamental νOH and first overtone 2νOH versus molar fraction of H-bonded silanol XB.

into account by correcting the intensity of the fundamental mode by I(σ) (eq 14). Cooperative Effect. Spectra obtained at various annealing temperatures were considered as the combination of two spectral contributions: H-bonded (B) and non-H-bonded (NB) species. The spectrum obtained for an annealing temperature of 700 °C was taken as the reference spectrum for NB silanols. The area assignable to free silanols ANB(T) can then be determined at each annealing temperature T. The concentration in NB silanol groups is given by ANB/I0. The mole fraction of NB OH groups, XNB is then calculated as the ratio NNB/N, where N is the concentration in NB and B silanol groups. The mole fraction of H-bonded OH groups is thus XB ) (1 - XNB). Figure 12 displays the correlation between the apparent integrated decadic absorption coefficient Iapp and XB over the whole range of studied annealing temperature. As XB increases, a monotonic variation could be expected. In fact, the observed behavior is quite different. In the first part of the curves (XB e 0.7), the linear variations are due to progressive dimerization. Above this limit, larger aggregates characterized by higher absorption coefficients start forming. Such a cooperative effect increases the average energy of bonding with every new silanol added to a chain through O-H · · · O bonds. This effect is of primary significance for understanding the behavior of hydrogen-bonded systems.25-27,61-67 For instance, Barlow et al.26 and Lalanne et al.27 have shown that their spectroscopic data on H-bonding of pentanol and hexanol could not be properly interpreted without taking into account the cooperative effect of hydrogen bonding. This effect was also shown to be very significant to understand the features of the broad OH stretching band of liquid metha-

IR Study of Hydroxyl Groups at a Silica Surface nol.67 Here, we present direct evidence of cooperative effect for OH groups at silica surfaces. Conclusion MIR and NIR spectra of hydroxyl groups on silica have been recorded as a function of annealing temperature. Temperature increase induces the progressive dehydroxylation of the surface through silanol condensation. The decrease in the amount of silanol groups (NOH) results in strong spectral modifications. The self-association of OH groups decreases when NOH decreases as the amount of H-bonded species and the strength of the H-bonds decrease. Measurements in both spectral ranges allow investigating the effects of hydrogen bonding on the behavior of transitions involving the OH oscillators, especially in the fundamental νOH and first overtone 2νOH spectral regions. Our results confirm and extend conclusions previously reported for self-associated species and binary systems in solution. Upon increase of the H-bond strength, the mechanical anharmonicity coefficient of the OH stretching gradually increases, and the intensity of the fundamental mode increases with concomitant wavenumber lowering, whereas the intensity of the first overtone depends on the strength of hydrogen bonds to a much smaller degree. Accordingly, the absorption profile of the first overtone provides a better picture of the distribution of states of the hydroxyl groups than that of the fundamental, which overemphasizes the most perturbed hydroxyls engaged in H-bonds. A calibration for silanol groups supports the view that, in the νOH range, the hydroxyls absorption intensity is wavenumber dependent: it increases linearly with wavenumber lowering. The contradictory behavior of the fundamental band and first overtone bands can be reconciled if the fundamental spectra are corrected by a function IνOH ) f(σ). Finally, it appears that cooperative effects are of primary significance for understanding the behavior of systems with hydrogen bonds. Acknowledgment. I wish to dedicate this paper to Pr. Andre´ Burneau. Without his impulse I would certainly never have started this work. I am very grateful to Professors Bernard Humbert and Jean-Pierre Perchard and to Dr. Laurent J. Michot for fruitful discussions. I thank Lionel Aranda for the thermogravimetric analyses. References and Notes (1) Iler, R. K. The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties and Biochemistry of Silica; J. Wiley & Sons: New York, 1979. (2) Vansant, E. F.; Van Der Voort, P.; Vrancken, K. C. Characterization and Chemical Modification of the Silica Surface, Elsevier: Amsterdam, 1995. (3) Legrand, A. P. The Surface Properties of Silicas; J. Wiley & Sons: New York, 1998. (4) Bergna H. E. Roberts W. O. Colloidal Silica: Fundamentals and Applications; Surfactant Science Series; CRC Press Taylor and Francis: Boca Raton, 2006; Vol. 131. (5) Burneau, A.; Gallas, J. P. In Surface Properties of Silicas; Legrand, A. P., Ed.; J. Wiley & Sons: New York, 1998; Chapter 3A, p 147. (6) Joesten, M. D.; Schaad, L. Hydrogen Bonding; Marcel Dekker, Inc: New York, 1974. (7) Schuster, P.; Zundel, G.; Sandorfy, C. The Hydrogen Bond. II. Structure and Spectroscopy; North Holland; Amsterdam, 1976. (8) Marechal, Y. The Hydrogen Bond & the Water Molecule: The Physics & Chemistry of Water, Aqueous & Bio-media; Elsevier Science & Technology: New York, 2007. (9) Iogansen, A. V. Spectrochimica A 1999, 55A (7-8), 1585–1612. (10) Burneau, A. J. Mol. Liq. 1990, 46, 99, and references therein. (11) Sandorfy, C. J. Mol. Struct. 2002, 614 (1-3), 365–366, and references therein.

J. Phys. Chem. C, Vol. 113, No. 30, 2009 13307 (12) Sandorfy, C. J. Mol. Struct. 2006, 90 (1-3), 50–54, and references therein. (13) Couzi, M.; Huong, P. V. Spectrochimica A 1970, 26 (1), 49–58. (14) Bourderon, C.; Sandorfy, C. J. Phys. Chem. 1973, 59, 25272536. (15) Peron, J. J.; Sandorfy, C. J. Chem. Phys. 1976, 65, 3153–3157. (16) Burneau, A.; Limouzi, J. Mol. Phys. 1977, 33, 399. (17) Rospenk, M.; Zeegers-Huyskens, Th. J. Phys. Chem. A 1997, 101, 8428–8434. (18) Pawelka, Z.; Zeegers-Huyskens, Th. Vibrational Spectroscopy 1998, 18 (1), 41–49. (19) Leroux, N.; Samyn, C.; Zeegers-Huyskens, Th. J. Mol. Struct. 1998, 448 (2-3), 209–220. (20) Czarnik-Matusewicz, B.; Zeegers-Huyskens, Th. J. Phys. Org. Chem. 2000, 13 (5), 237–243. (21) Luck, W. A. P.; Ditter, W. Ber. Bunsenges. Phys. Chem. 1968, 72, 365–370. (22) England-Kretzer, L.; Fritzsche, M.; Luck, W. A. P. J. Mol. Struct. 1988, 175, 277–82. (23) Singh, S.; Schioeberg, D.; Luck, W. A. P. Spectrosc. Lett. 1981, 14 (2), 141–55. (24) Singh, S.; Fritzsche, M.; Kuemmerle, I.; Luck, W. A. P.; Zheng, H. Y. Spectrosc. Lett. 1985, 18 (4), 283–99. (25) Palombo, F.; Tassaing, T.; Danten, Y.; Besnard, M. J. Chem. Phys. 2006, 125 (9), 094503/1–094503/8. (26) Barlow, S. J.; Bondarenko, G. V.; Gorbaty, Y. E.; Yamaguchi, T.; Poliakoff, M. J. Phys. Chem. A 2002, 106, 10452. (27) Lalanne, P.; Andanson, J. M.; Soetens, J.-C.; Taasing, T.; Danten, Y.; Besnard, M. J. Phys Chem. A 2004, 108, 3902. (28) Perchard, J. P.; Mielke, Z. Chem. Phys. 2001, 264 (2), 221–234. (29) Perchard, J. P. Chem. Phys. 2001, 266 (1), 109–124. (30) Perchard, J. P. Chem. Phys. 2001, 273 (2-3), 217–233. (31) Larsen, R. W.; Zielke, P.; Suhm, M. A. J. Chem. Phys. 2007, 126, 194307. (32) Scharge, T.; Luckhaus, D.; Suhm, M. A. Chem. Phys. 2008, 346, 167–175. (33) Howard, D. L.; Kjaergaard, H. G. J. Phys. Chem. A 2006, 110 (34), 10245–10250. (34) Howard, D. L.; Kjaergaard, H. G. J. Phys. Chem. A 2006, 110 (31), 9597–9601. (35) Burneau, A.; Carteret, C. Phys. Chem. Chem. Phys. 2000, 2, 1757– 1762. (36) Legrand, A. P.; Hommel, H.; Tuel, A.; Vidal, A.; Balard, H.; Papirer, E.; Levitz, P.; Czernichowski, M.; Erre, R.; Van Damme, H.; Gallas, J. P.; Hemidy, J. F.; Lavalley, J. C.; Barres, O.; Burneau, A.; Grillet, Y. AdV. Colloid Interface Sci. 1990, 33, 91. (37) Blin, J. L.; Carteret, C. J. Phys. Chem. C 2007, 111, 14380–14388. (38) Carteret, C. in preparation. (39) Puibasset, J.; Pellenq, R. J.-M. J. Chem. Phys. 2003, 118, 5613. (40) Zhuravlev, L. T. Langmuir 1987, 3, 316–318. (41) Humbert, B. J. Non-Cryst. Solids 1995, 191, 29–37. (42) Ek, S.; Root, A.; Peussa, M.; Niisnisto, L. Thermochim. Acta 2001, 379, 201–212. (43) Sindorf, D. W.; Maciel, G. E. J. Phys. Chem. 1983, 87, 5516– 5521. (44) Le´onardelli, S.; Facchini, L.; Fretigny, C.; Tougne, P.; Legrand, A. P. J. Am. Chem. Soc. 1992, 114, 6412. (45) Chuang, I. S.; Maciel, G. E. J. Phys. Chem. B 1997, 101, 3052– 3064. (46) Gallas, J. P.; Lavalley, J. C.; Burneau, A.; Barre`s, O. Langmuir 1991, 7, 1235. (47) De Farias, R. F.; Airoldi, C. J. Therm. Anal. 1997, 53, 751–756. (48) Peng, L.; Qisui, W.; Xi, L.; Chaocan, Z. Colloids Surf., A 2009, 334 (2009), 112–115. (49) Humbert, B.; Carteret, C.; Burneau, A.; Gallas, J. P. In Colloidal Silica: Fundamentals and Applications (Surfactant Science Series; Bergna, H. E., Roberts, W. O., Eds.; CRC Press Taylor and Francis: Boca Raton, 2006; Vol. 131, Chapter 26, p 295. (50) Morrow, B. A.; McFarlan, A. J. Langmuir 1991, 7, 1695–1701. (51) Davydov, V. Ya.; Kiselev, A. V.; Kiselev, S. A. Colloı¨d J. Engl. Trans. 1979, 41, 178. (52) Curthoys, G.; Davydov, V. Ya.; Kiselev, A. V.; Kiselev, S. A; Kuznetsov, B. V. J. Colloid Interface Sci. 1974, 48, 58. (53) Baumgarten, E.; Wagner, R.; Lentes-Wagner, C. Fresenius Z. Anal. Chem. 1989, 335, 375. (54) Galkin, G. A.; Kiselev, A. V.; Lygin, V. I. Russ. J. Phys. Chem. (Engl. Trans.) 1969, 43, 1117. Galkin, G. A.; Kiselev, A. V.; Lygin, V. I. Russ. J. Phys. Chem. (Engl. Trans.) 1969, 43, 1292. (55) Singh, S.; Luck, W. A. P. Chem. Phys. Lett. 1981, 78 (1), 117– 120. (56) Di Paolo, T.; Bourderon, C.; Sandorfy, C. Can. J. Chem. 1972, 50, 3161.

13308

J. Phys. Chem. C, Vol. 113, No. 30, 2009

(57) Paterson, M. S. Bull. Mineral. 1982, 105, 20. (58) Libowitzky, E.; Rossman, G. R. Am. Mineral. 1997, 82 (11-12), 1111–1115. (59) Burneau, A.; Perchard, J. P.; Zuppiroli, G.; Limouzi, J.; Mare´chal, E. Mol. Phys. 1980, 41, 1373. (60) Burneau, A.; Barre`s, O.; Gallas, J. P.; Lavalley, J. C. Proceedings of the International Workshop on FTIR Spectroscopy; Vansant, E. F., Ed.; University of Antwerp: Antwerp, Belgium, 1990; 108-117. (61) Sear, R. P.; Jackson, G. J. Chem. Phys. 1996, 105, 1113. (62) Veytsmant, B. A. J. Phys. Chem. 1993, 97, 7144–7146.

Carteret (63) Kleeberg, H.; Klein, D.; Luck, W. A. P. J. Phys. Chem. 1987, 91, 3200–3203. (64) Luck, W. A. P. J. Mol. Struct. 1998, 448 (2-3), 131–142. (65) Gorbaty, Y. E.; Bondarenko, G. V.; Kalinichev, A. G.; Okhulkov, A. V. Mol. Phys. 1999, 96, 1659. (66) Yukhnevich, G. V. Spectrosc. Lett. 1997, 30, 901. (67) Ohno, K.; Shimoaka, T.; Akai, N.; Katsumoto, Y. J. Phys. Chem. A 2008, 12 (32), 7342–7348.

JP9008724