Adsorption of Acetone on Nonporous and Mesoporous Silica - The

J. Phys. Chem. C 2009, 113, 16517–16529

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Adsorption of Acetone on Nonporous and Mesoporous Silica Valentina Crocella`, Giuseppina Cerrato, Giuliana Magnacca, and Claudio Morterra* Department of Chemistry IFM and NIS Centre of Excellence, UniVersity of Turin, Consortium INSTM (Florence), Research Unit of Turin UniVersity, Via P. Giuria 7, 10125 Torino, Italy ReceiVed: April 28, 2009; ReVised Manuscript ReceiVed: July 24, 2009

The ambient temperature solid/gas interaction of acetone with a nonporous silica (Aerosil 200) and two mesoporous silicas (a spherical MCM-41 preparation, with highly uniform and very small meso-pores, and a reference conventional MCM-41 preparation), vacuum activated at different temperatures, has been investigated by the combined use of gas-volumetry/adsorption-microcalorimetry and in situ FTIR spectroscopy. In the early stages of acetone uptake, when no physical adsorption occurs yet, two specifically adsorbed (chemisorbed) acetone species form, characterized by the formation of two H-bonds with suitably spaced silanol couples, and of one (stronger) H-bond with single silanols, respectively. Both types of chemisorbed acetone are fully reversible, upon evacuation, in the case of nonporous silica, whereas they are partly irreversible, though to a rather different extent, in the case of mesoporous silicas. The energy involved in the acetone adsorption process, expressed in the form of (differential) molar adsorption heats evolved for admission of small doses of adsorptive, depends on activation (dehydration) conditions and is definitely larger in the case of the mesoporous silica characterized by very small-bore meso-porosity. In particular, on small-pores spherical MCM-41, somewhat stronger H-bondings are formed, as indicated by lower stretching frequencies of the acetone carbonyl group, but also other forces are likely to be active in the adsorption process occurring inside the narrow meso-pores network. The comparison between overall (i.e., nonresolved) gas-volumetric quantitative uptake data and band-resolved IR spectroscopic data allows, on one side, an approximate though realistic evaluation of the number of hydroxyl groups involved in the two types of specific acetone adsorption and, on the other side, yields clear indication that also the hydroxyl groups, that were initially involved in mutual H-bonding interactions, become involved in H-bonding adsorption of acetone molecules. 1. Introduction Extremely vast is the scientific literature aimed at evaluating the physical/chemical characteristics of sites active at the surface of adsorbing systems by following, with the use of mainly spectroscopic tools, the behavior of suitable adsorbed molecular probes. During a recent spectroscopic investigation of the acid/basic features of some silica-supported cationic species,1 a study carried out using acetone as a probe and the aldolic condensation of acetone as a test reaction, it turned out that the necessary preliminary understanding of the plain silica/acetone system was quite far from being satisfactory. The study of acetone adsorption on silica surfaces is not new, as it was first carried out from CCl4 solutions2 and successively from both solution and the gaseous phase.3 Both studies2,3 evidenced the formation of two H-bonded adsorbed species, characterized by slightly different stability and slightly different spectral position (∆ν ≈ 10 cm-1) of the analytical carbonyl stretching vibration (hereafter referred to as the νCO vibrational mode). The acetone complex characterized by slightly higher stability and slightly lower νCO frequency was tentatively assigned by the first authors2 to acetone molecules interacting with two suitably positioned surface OH groups (silanols), whereas the second authors3 favored for that adspecies the interaction with a surface silanol whose oxygen atom was H-bonded with a neighboring OH group. Both studies2,3 assigned the species characterized by a higher νCO frequency to the * To whom correspondence should be addressed. Phone: +39 011 6707589. Fax: +39 011 6707855. E-mail: [email protected].

H-bonding interaction of acetone with one H-bonding free (i.e., isolated) surface silanol. More recently, a theoretical work by Kachurovskaya et al.,4 based on an ab initio cluster approach, indicated that an acetone complex formed by H-bonding with two separated terminal OH groups, simulating a bifurcated acetone interaction, should present somewhat higher adsorption energy (∼17 kcal mol-1), whereas little energy difference would exist (∼8-10 kcal mol-1) between acetone H-bonded to either the free OH group of an isolated silanol or the free OH group of an H-bonded vicinal OH pair. No interaction between acetone molecules and OH groups already involved in mutual H-bonding interaction has been considered in the mentioned work,4 whereas the involvement of H-bonded OH groups in acetone adsorption was explicitly excluded by other authors (e.g., see refs 5 and 6). The present contribution deals with the gas/solid interaction of acetone with silicas (either pyrogenic nonporous or ordered mesoporous). Its aim is to obtain, by the combined use of in situ FTIR spectroscopy and adsorption microcalorimetry/gasvolumetry, a better spectroscopic description as well as a quantitative/energetic evaluation of acetone uptake on different silica preparations and on silicas activated in different thermal conditions (i.e., possessing different degrees of surface hydration). 2. Experimental Section Materials. Ambient temperature adsorption/desorption of acetone has been studied on the following silicas: (1) Aerosil 200, hereafter referred to as AT, where T represents the temperature (K) at which the sample was vacuum

10.1021/jp903910n CCC: $40.75  2009 American Chemical Society Published on Web 08/24/2009

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activated before adsorption experiments. Aerosil 200 is a pyrogenic amorphous nonporous silica, obtained by SiCl4 flame hydrolysis (Degussa, Frankfurt A.M., Germany; lot.: 1490), and kindly supplied by Eigenmann & Veronelli SpA (Milano, Italy). Activation temperatures were: 303 K (A303 is a nominally fully hydrated Aerosil, from which only physically adsorbed water and other volatile atmospheric contaminants have been removed); 673 K (A673, employed only for IR experiments, corresponds to a medium dehydration stage); and 1100 K (A1100 corresponds to a high dehydration stage). Literature data report the estimated residual surface hydroxylation of Aerosil specimens treated in vacuo at various temperatures,7 but it is difficult to reconcile some aspects of these “specific” Aerosil-OH data with data contained in other more general silica-OH overviews.8,9 In the present study, literature OH population figures will be compared with surface OH contents suggested by acetone adsorption. Specific surface area (SSA) of Aerosil 200 is estimated by the manufacturer to be ∼200 m2 g-1. SSA of all our AT samples, preliminarily outgassed at 423 K (2 h), was determined by N2 adsorption at 77 K using a semiautomatic apparatus (ASAP 2010, Micromeritics), and analyzed with the standard BET model for SSA determination.10 It turned out to keep constant at 196 ((5%) m2 g-1. The selection of commercial Aerosil 200 as representative of nonporous silicas has been suggested by its high purity, large use, and widely known physical/chemical surface features. (2) A mesoporous silica of the MCM-41 family, synthesized in the form of regular spheres of ∼490 nm average diameter. This material, usually termed MCM-41/Sph490,11 is here referred to as MCMT, where T represents the temperature (K) at which the sample of spherical mesoporous silica was vacuum activated prior to acetone adsorption/desorption experiments. All preparative details and physicochemical features of MCM-41 Sph490 have been described elsewhere.11 It is only recalled that BET SSA (N2 adsorption at 77 K) is 820 ((5%) m2 g-1, and stable for thermal treatments at T as high as ∼830 K; the overall pore volume VP is 0.25 cm3 g-1; the average pore diameter DP (BJH method) is slightly above ∼20 Å, but the sharp peak of the narrow unimodal pore-size distribution curve is centered at ∼18 Å (adsorption isotherm branch), i.e., just outside the range of microporosity. Still, the t-plot graphical method10 indicates that no real micropores are appreciably present. Activation temperatures were 303 K (MCM303 corresponds to a nominally fully hydrated system, from which only physically adsorbed water and other volatile atmospheric contaminants have been removed) and 673 K (MCM673 corresponds to a medium dehydration stage, in which virtually all H-bonding interactions among surface silanols should be eliminated). No higher activation temperatures were adopted for MCM-41/Sph490, to remain sufficiently far from the conditions at which the regular mesoporous network starts collapsing. The selection of this mesoporous silica preparation has been suggested by the high homogeneity of its geometrical and physical features and by its possible adoption as a controlled drug-releasing system.11 (3) For comparison purposes relative to the gas-volumetric/ microcalorimetric analysis, we used also a “conventional” MCM-41 specimen (i.e., prepared according to the standard route12), that is most often adopted as a catalyst/catalytic support (e.g., see ref 13, and references therein). This reference MCM41 preparation, vacuum activated only at 673 K (medium dehydration) and referred to in the text as MCM-R673, is characterized by a SSA of 1012 ((5%) m2 g-1, a definitely larger mesopore volume (0.5 cm3 g-1), a (typical) broader

Crocella` et al. distribution of mesoporosity with average pore-size of ∼26 Å and peak of the pore-size distribution curve centered at ∼23 Å. Samples of this “conventional” MCM-41 preparation were kindly supplied by Dr. Maria Botavina (Department of Chemistry I.F.M., University of Turin). Reagents. High-purity liquid acetone (Chromasolv for HPLC, Sigma-Aldrich) was used without any further purification and rendered gas-free by several “freeze-pump-thaw” cycles. Techniques. IR Spectroscopy. In situ IR spectra were recorded (4 cm-1 resolution; 4000-1200 cm-1 spectral interval) on a FTIR spectrometer (Bruker IFS 113v, equipped with MCT cryodetector) at “beam temperature” (BT), i.e., the temperature reached by samples in the IR beam. (For insulating white samples, either in vacuo or under a low adsorptive pressure, BT is estimated to be (at least) some 30 K higher than RT). The homemade quartz infrared cell, characterized by a very small optical path,14 was connected to a conventional high vacuum line (residual p ≈ 10-5 Torr). This setting allowed us to perform, in strictly in situ conditions, both sample thermal treatments (2 h, at the selected activation temperature), and adsorption-desorption cycles of the molecular probe. The powdery silica samples were compressed in the form of selfsupporting pellets, usually of ∼10 mg cm-2 thickness, and mechanically protected with a pure gold frame. Activated samples were first contacted at BT with increasing doses of acetone vapor, usually up to an equilibrium pressure of ∼3-5 Torr (the adsorption run), and then evacuated at BT for increasing times (up to 1.5 h; the desorption run), in order to test the desorption spectral response of adsorbed species. At any step of the adsorption/desorption cycles, in situ IR spectra were recorded (128 scans). In all IR experiments, the approximate equilibrium adsorptive pressure was monitored by a Pirani gauge and a conventional Hg manometer. Spectral intensities were usually normalized to the sample weight and, when comparing different silicas, also to the BET surface area. Computer spectral resolution and integrated absorbance of the resolved spectral components play an important role in this work, as the comparison of resolved spectral data with nonresolved quantitative (gas-volumetric) data leads to their tentative resolution. Bands resolution was carried out using the FIT routine by Bruker, and imposing, for each mass-normalized spectrum, only the number of spectral components to be resolved, whereas all major spectral parameters (spectral position, half-bandwidth, percent of Gaussian profile) were allowed to float freely. Adsorption Microcalorimetry/Gas-Volumetry. Heats of adsorption of acetone vapor were measured, at 298 K, by means of a heatflow microcalorimeter (Calvet C80, Setaram, France) connected to a grease-free high-vacuum gas-volumetric glass apparatus (residual p ≈ 10-5 Torr) equipped with a Ceramicell 0-100 Torr gauge (by Varian). A well established stepwise procedure was followed (see, for instance, ref 15, and references therein) that allows, during the same experiment, both integral heats evolved (-Qint) and adsorbed amounts (na) to be determined for very small increments of the adsorptive pressure. Adsorbed amounts and integral heats evolved, normalized to the unit surface area, have been plotted vs pressure in the form of volumetric (quantitative) and calorimetric isotherms, respectively. Evolved heats (values obtained by the best-fit curve passing through the experimental points in the calorimetric isotherm) have been also reported vs adsorbed amounts: the slope of this curve represents the average adsorption heat released during the adsorption process. The adsorption heats observed for each small dose of gas admitted over the sample

Adsorption of Acetone on Silica

Figure 1. Quantitative isotherms [section (a); surface area normalized adsorbed amounts na (µmol m-2) vs adsorptive equilibrium pressure pe (Torr)] and surface area normalized calorimetric isotherms [section (b); released integral heats -Qint (J m-2) vs adsorptive equilibrium pressure pe (Torr)] of the adsorption at 298 K of acetone vapor on A303 (squares) and A1100 (circles). Empty symbols represent primary adsorption runs, solid symbols the secondary adsorption ones. Inset to section (a) Langmuir plot presentation [pe/na (Torr µmol-1 m2) vs adsorptive equilibrium pressure pe (Torr)] of the two quantitative isotherms. Crossed-square symbols: A303; crossed-circles: A1100.

(qdiff) have been finally reported as a function of coverage, in order to obtain the (differential) enthalpy changes associated with the proceeding adsorption process. The differential-heat plots presented here were obtained by taking the middle point of the partial molar heats (∆Qint/∆na, kJ/mol) vs na histogram relative to the individual adsorptive doses, prepared as small as possible. As for IR experiments, also the quantitative (gasvolumetric) and energetic (microcalorimetric) reversibility of acetone adsorption was investigated through adsorption-desorption-adsorption cycles. After the adsorption run I (primary isotherm), samples were outgassed overnight (at 298 K), and then an adsorption run II (secondary isotherm) was performed, to check if secondary and primary adsorption runs coincided. 3. Results and Discussion Part I. Acetone on Nonporous Silica (Aerosil 200). Gas-Volumetric and Microcalorimetric Measurements. Figure 1 reports two quantitative isotherms (section a) and the corresponding calorimetric isotherms (section b) relative to acetone adsorption on nonporous silica activated in the two extreme conditions, i.e., 303 and 1100 K, respectively. The following can be noted: (i) As a maximum equilibrium pressure of ∼8.5 Torr was reached, the isotherms can be reasonably ascribed to the sole specific (i.e., chemisorptive) interaction of acetone with the silica surface. At 303 K, acetone vapor pressure is 283.2 Torr,16 and a maximum relative pressure was reached (p/p° ≈ 0.03) at which no liquid-like physical adsorption is expected to be present yet. (ii) The inset to Figure 1a, reporting a Langmuir-plot presentation of the quantitative isotherms, indicates that a definitely non linear plot is obtained for the highly hydrated A303 system (lower trace, crossed square symbols), whereas for the highly dehydrated A1100 system (upper trace, crossed circles), a plot more similar to, though not coincident with, a straight line is obtained. (A central portion of the plot, between

J. Phys. Chem. C, Vol. 113, No. 37, 2009 16519 ∼1.5 and ∼6 Torr, can be approximated by a straight line, but for lower and higher pressures downward deviations from linearity are evident). The first observation suggests, for highly hydrated A303, a largely heterogeneous adsorption process, consistent with Okunev’s hypothesis3 that, at the surface of highly hydrated (porous) silica gel, two forms of specific H-bonding adsorption should be present. Only one such species would still be formed on highly dehydrated silica gel,3 and IR spectra in the next section will confirm that Okunev’s hypothesis is basically correct also for nonporous silica. The overall nonLangmuir behavior of both AT specimens suggests that, also in the presence of basically only one type of acetone/surface interaction, the acetone/silica system remains rather heterogeneous, at least in terms of sites energy distribution. This interpretation will be confirmed by differential adsorption heats (next figure). (iii) Both chemisorption isotherm sets of Figure 1a, none of which presents the two-plateaus shape sometimes reported for adsorption from solution,3 indicate that the specific acetone adsorptive capacity of nonporous silica is a continuous growing function (isotherm profile I, BDDT classification10), whose initial slope and maximum uptake decline severely with increasing activation temperature (i.e., dehydration). This indicates that specific acetone interactions with nonporous silica involve only H-bonding complexes with surface hydroxyls, as previously suggested for porous silica gel.3 The non-Langmuir nature of type I isotherms of Figure 1a prevents a direct calculation of the maximum chemisorption coverage (i.e., the monolayer capacity nm), while the need of avoiding the onset of physical adsorption prevents the direct observation of nm in the form of a chemisorption plateau. Empirical best-fit curves through the experimental points were then resorted to, and yielded (approximate) nm plateau values corresponding to 2.67 and 1.51 µmol m-2 for A303 and A1100, respectively. [The best-fitting function was a combination of exponentials of the form y ) y0 + a1(1 - e-x/b1) + a2(1 - e-x/b2). The choice was done on the basis of what equation gave an nm value close enough to that obtained by applying Langmuir’s model to the whole isotherm of A1100 (∼1.8 µmol m-2)]. On account of acetone cross-sectional area (σ ) 0.34 nm2 per molecule),17 maximum coverages of 0.54 and 0.31 are obtained for acetone chemisorption on A303 and A1100, respectively (∼0.37, adopting Langmuir’s algorithm for A1100). In both cases the estimated monolayer capacity is very far from unity even if, for A303, maximum hydration conditions have been deliberately adopted. (iv) Comparison between primary and secondary adsorption isotherms indicates that acetone uptake on nonporous silica is reversible in terms of both adsorbed amounts and released heats, as mere data fluctuations within narrow margins of experimental error are observed. The reversibility of the adsorption process will be confirmed by IR spectra: a datum to be regarded with some caution, as the sample temperature is bound to be somewhat different in the two experimental situations. The inset of Figure 2 reports, as a function of adsorbed amounts, the area-normalized integral adsorption heats, as obtained from the curves fitting the experimental data of the two AT systems. Neither of the plots can be regarded as linear, as indicated by the deviation of the solid-line curves from the dotted lines tangent to the initial heat-release trend. This behavior confirms that chemisorptive acetone uptake on nonporous silica is an heterogeneous process, in which more energetic sites react first. Still, the heterogeneity turns out to

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Figure 2. Differential molar adsorption heats -qdiff (kJ mol-1), obtained by graphical differentiation based on experimental points, plotted as a function of surface area normalized adsorbed amounts na (µmol m-2). Squares: A303; circles: A1100. Empty symbols represent primary adsorption runs, solid symbols, the secondary adsorption ones. The broken-trace horizontal line represents the standard molar enthalpy of liquefaction of acetone at 300 K [(-qL)300 ) (∆vaph°)300 ) 31.1 kJ mol-1]. Inset: Solid-line curves: surface area normalized integral adsorption heats -Qint (J m-2), obtained from the best-fit curves of the experimental points reported in Figure 1b, are plotted as a function of the corresponding surface area normalized adsorbed amounts na (µmol m-2), as obtained from the best-fit quantitative isotherms of Figure 1a. Dotted straight lines: the straight lines, tangent to the initial heat-release trends, represent the heat-release vs coverage plots that would be observed in the case of an homogeneous adsorption process.

be different in the two cases examined, as for A303 a relatively long quasi-linear initial trend is observed. Main plots in Figure 2 report, as a function of acetone uptake (na), the differential molar adsorption heats (-qdiff ) qst; kJ mol-1)15,18 evolved for each dose of acetone admitted on the two AT systems. Useful indications derive from these plots: (i) Data scattering is larger for A303 than for A1100, mostly as a consequence of the different plot trends: rather flat in the former case, and quite steep in the latter case. qdiff plots confirm the presence of mere data fluctuations, as no different trends are present in corresponding primary and secondary adsorption runs. Also q° figures (i.e., qdiff values extrapolated to zero coverage, and representing the energetic contribution of the strongest sites fraction) are virtually coincident for corresponding primary and secondary runs, although they are quite different for the two AT systems. (ii) qdiff plots of A303 and A1100 present very different extensions, as different are the uptake ranges covered. But, within the different coverage ranges, they also exhibit rather different shape and trend. The qdiff plot of A303 (empty and full squares) starts at q° ≈ 80-90 kJ mol-1, a medium-low molar adsorption enthalpy, reasonable for a reversible specific adsorption, remains almost constant (∼70-75 kJ mol-1) for acetone concentrations up to ∼1.5 µmol m-2, then declines slowly, and starts tending to the asymptotic value of acetone molar liquefaction enthalpy (-qL ≈ 31 kJ mol-1), which will be characteristic of the uptake phase dominated by liquid-like physical adsorption. The observed trend indicates that there is an initial adsorption phase, dominated by a stronger interaction of higher and virtually constant molar adsorption enthalpy, followed without interruption by a second phase, dominated by an heterogeneous interaction that, starting from adsorption enthalpies not so different from those characteristic of the stronger and rather homogeneous first phase, declines toward qdiff values as low as q L. The qdiff plot of A1100 (empty and full circles) is simpler to interpret: it starts at a relatively high q° value (∼125 kJ mol-1), a medium-high figure for a reversible H-bonding interaction,

Crocella` et al. though not surprisingly high, as it is known that high activation temperatures may induce, in pure silicas, the formation of few highly energetic sites (e.g., see ref 19); then qdiff falls sharply to values (∼75-80 kJ mol-1) roughly corresponding to the long and virtually flat initial portion of A303 qdiff plot; also this qdiff level is just crossed and quickly abandoned, as qdiff keeps declining with a regular trend similar to that exhibited by A303 during the second adsorption phase. This indicates that, besides few initial strong interactions (some of which may correspond to the long first-phase interactions of A303), acetone specific interaction with highly dehydrated A1100 is mainly due to one type of heterogeneous sites, most likely corresponding to the second (weaker) sites family present on highly hydrated A303. qdiff figures of Figure 2 have been compared with the adsorption energies reported by Kachurovskaya et al.4 for molecular models of acetone H-bonded to H-bonding free silanols (on silica gel, it is claimed, but nothing in the models accounts for the fact that OH groups may be located in pores and/or on an open surface). It is noted that: (i) The stronger adsorbed species, abundant on A303 and characterized by an almost constant adsorption enthalpy (-70/ 75 kJ mol-1), compares well with the model of bifurcated adsorbed acetone, for which the formation of two H-bondings on the same carbonyl group leads to a calculated adsorption energy of -16.9 kcal mol-1 (-71 kJ mol-1). Bifurcated acetone complexes should be abundant on a highly hydrated surface (A303), whereas they may even not form at all on a highly dehydroxylated surface (A1100). (ii) The weaker chemisorbed species, characterized by molar adsorption enthalpies spread in the ∼-70 to ∼-40 kJ mol-1 range, compares reasonably well with two Kachurovskaya’s models of 1:1 free-OH/acetone complexes, for which the calculated adsorption energy is -8.2 and -10.5 kcal mol-1, respectively (-34.4 and -44.1 kJ mol-1). 1:1 terminal-OH/ acetone complexes may possibly represent the only species formed on a highly dehydroxylated surface, whereas they should be expected to represent only one of the adspecies formed on a highly hydroxylated surface. This assignment, suggested by the comparison between acetone adsorption energies of experimental and theoretical origin, ought to be confirmed by some spectroscopic evidence. In Situ IR Measurements. The main analytical vibrational feature for adsorbed acetone is the νCO mode (spectral range: 1750-1670 cm-1). Upon either coordination or H-bonding interaction, νCO undergoes downward spectral shifts with respect to the unperturbed molecule.20,21 In the present case, no coordinative adsorption can occur (no coordinatively unsaturated cationic species are available at the surface of pure silicas) and, on the basis of what reported in the literature2,3 and in the discussion above, it can be predicted that (at least) two νCO components should be expected in the spectra of acetone adsorbed on nonporous silica. For this reason, it was decided to anticipate the presentation/discussion of the spectral features of surface hydroxyls, that are the surface sites for specific acetone interactions. νOH Spectral Range. Figure 3 reports the starting (background) νOH spectra relative to the three AT samples: their preliminary presentation will allow us to show the spectral modifications brought about by acetone adsorption in a differential form (i.e., after absorbance subtraction of the normalized background νOH spectrum). Although the νOH spectrum of pyrogenic silicas has been known for a long time,21-23 the curves in Figure 3 deserve a few comments.


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Figure 3. Absorbance IR spectra, mass-normalized to the weight of the heaviest sample, relative to the O-H stretching spectral region (3900-3000 cm-1) of activated A303 (1), A673 (2), and A1100 (3) specimens.

(i) A303 corresponds to the maximum possible surface hydroxylation, as at BT only physically adsorbed water has been removed in vacuo, and on pure silicas there is no molecularly coordinated water. Still, a fair number of terminal OH species free from H-bonding interactions are present, as indicated by the strong sharp peak at ∼3745 cm-1. The largely incomplete surface hydroxylation of pyrogenic silica is responsible for a maximum coverage of specifically adsorbed acetone definitely lower than one (and roughly estimated above as ∼ 0.65). (ii) At ν < 3700 cm-1, the νOH spectrum of A303 contains a broad and complex absorption due to H-bonded OH species.21 An apparent maximum at ∼3530 cm-1 can be assigned to the contributions deriving from long-chain H-bonded OH species, whereas an evident shoulder at ∼3645 cm-1 can be ascribed to the H-bonding interaction present in surface silanol pairs,24 the free OH of which has been reported to absorb weakly at ∼3720 cm-1.24 A tiny OH band at ∼3716 cm-1 is indeed present in spectrum (1) of Figure 3, and will be rendered even more evident in the acetone uptake spectral pattern of the next figure. The well resolved spectral evidence for silanol pairs on A303 is part of the scenario of a largely incomplete surface hydroxylation of nominally “fully hydrated” silica. (iii) The νOH spectrum of A673 (curve 2) presents a dominant peak at ∼3747 cm-1, much sharper than that in spectrum (1), due to still very abundant free terminal OH species. Of marginal intensity is the residual low-ν tail due to H-bonded OH species, most probably ascribable to few residual silanol pairs. (iv) The νOH spectrum of highly activated A1100 (curve 3) contains only a sharp peak at ∼3747 cm-1 (free silanols), the decreased intensity of which is responsible for the much decreased acetone adsorptivity evidenced by the isotherms of Figure 1a. The integrated absorbance (I; cm-1) of the ∼3747 cm-1 OH band will be used in the following to approximate acetone coverage, since in each adsorption step the difference (I0 - I) between the starting band intensity and the residual one, ratioed against the starting band intensity [(I0 - I)/I0], is proportional to the covered fraction of adsorbing sites. Figures 4-6 report the differential spectral modifications produced in the νOH spectral region of A303, A673, and A1100, respectively, upon acetone adsorption up to an equilibrium pressure of 5 Torr (p/p° ≈ 0.017). Figures 5 and 6 report also, as section (b), the differential spectral pattern relative to the modifications produced in the νOH spectra of A673 and A1100 during the last steps of acetone desorption at BT. The following can be noted: (i) On highly hydrated A303, the first acetone doses seem to perturb selectively free surface OH species, and this early interaction concerns both terminal free OH groups (∼3742

Figure 4. Absorbance differential spectra in the νOH spectral range of A303, relative to the adsorption of the first acetone doses. All OH spectra were normalized against the background νOH spectrum of the bare A303 sample [i.e., spectrum (1) of Figure 3]. In adsorption differential spectral patterns, bands relative to species that (upon adsorption) form or increase are pointing up, bands relative to species that decrease are pointing down. Acetone equilibrium pressures (Torr) are (1) 0.1, (2) 0.2, (3) 0.3, (4) 0.5, (5) 0.7, (6) 1.0, (7) 2.0, (8) 3.0, (9) 4.0, and (10) 5.0. Inset: Blown-up absorbance spectral pattern of free surface OH groups, decreasing in intensity upon acetone adsorption.

Figure 5. Absorbance differential spectra in the νOH spectral range of A673, relative to the adsorption of the first acetone doses [section (a)], and to the last steps of acetone desorption [section (b)]. All OH spectra were normalized against the background νOH spectrum of the bare A673 sample [i.e., spectrum (2) of Figure 3]. In adsorption differential spectral patterns, bands relative to species that (upon adsorption) form or increase are pointing up, bands relative to species that decrease are pointing down; in desorption differential spectral patterns, bands relative to species that (upon desorption) form or increase will decrease their pointing down intensity, and bands relative to species that decrease will reduce their pointing up intensity. In section (a), acetone adsorption equilibrium pressures (Torr) are (1) 0.1, (2) 0.2, (3) 0.3, (4) 0.5, (5) 0.7, (6) 1.0, (7) 2.0, (8) 3.0, (9) 4.0, and (10) 5.0. In section (b), adsorbed acetone evacuation times (sec) are: (I) 20, (II) 40, (III) 60, (IV) 120, (V) 240, and (VI) 480. Inset to section (a): Blown-up absorbance spectral pattern of free surface OH groups, decreasing in intensity upon acetone adsorption.

cm-1) and free OH groups in silanol pairs (∼3716 cm-1). Unavoidably, the latter interaction perturbs also the H-bond present in the silanol pairs (∼3645 cm-1), so that around that spectral position differential spectra will contain both a downward contribution (H-bond interactions that are eliminated/ modified) and an upward contribution (H-bond interactions that are formed). This conflicting action hinders the appearance, in the adsorption pattern of Figure 4, of an isosbestic point (visible in the spectral patterns of A673 and A1100), masks, in the

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Figure 6. Absorbance differential spectra in the νOH spectral range of A1100, relative to the adsorption of the first acetone doses [section (a)] and to the last steps of acetone desorption [section (b)]. All OH spectra were normalized against the background νOH spectrum of the bare A1100 sample [i.e., spectrum (3) of Figure 3]. In adsorption differential spectral patterns, bands relative to species that (upon adsorption) form or increase are pointing up, bands relative to species that decrease are pointing down; in desorption differential spectral patterns, bands relative to species that (upon desorption) form or increase will decrease their pointing down intensity, and bands relative to species that decrease will reduce their pointing up intensity]. In section (a), acetone adsorption equilibrium pressures (Torr) are (1) 0.1, (2) 0.2, (3) 0.3, (4) 0.5, (5) 0.7, (6) 1.0, (7) 2.0, (8) 3.0, (9) 4.0, and (10) 5.0. In section (b), adsorbed acetone evacuation times (sec) are (I) 20, (II) 40, (III) 60, (IV) 120, (V) 240, and (VI) 480. Inset to section (a): Blown-up absorbance spectral pattern of free surface OH groups, decreasing in intensity upon acetone adsorption.

higher-ν section of the H-bonded OH band, the real extent of the upward spectral construction of the OH/acetone interaction, and renders confuse the desorption spectral pattern (that, for this reason, is not reported). (ii) The construction on A303 of a broad upward band (ν < ∼3650 cm-1) due to H-bonded OH species implies the early appearance of a higher-ν component at ∼3500 cm-1 (see the low-laying upward arrow in Figure 4, curve 1), accompanied first and then overwhelmed by a lower-ν component centered at ∼3400 cm-1 (upper upward arrow in Figure 4, curve 6). The high-ν OH component at ∼3500 cm-1 represents the first step of the adsorption process also on A673 [Figure 5, section (a)] and, far more clearly, represents the last fraction of adsorbed acetone to be desorbed from both the medium-dehydrated A673 and the highly dehydrated A1100 [see section (b) of Figure 5 and 6]. The high-ν component at ∼3500 cm-1 so turns out to be characteristic of the H-bonding interaction present in the more stable form of chemisorbed acetone (i.e., the species characterized by the highest and virtually constant qdiff ≈ -75 kJ mol-1). Since for H-bonded OH species smaller ∆νOH shifts with respect to the free OH band (∼3745 cm-1; -∆νOH ≈ 250 cm-1) mean weaker interactions,21,25 the only possibility for the higher-ν component at ∼3500 cm-1 to represent, at the same time, the strongest form of acetone adsorption is that the acetone molecule is actually held with two (weaker) H-bondings. This is a first experimental evidence for the early formation, on adequately hydroxylated silica, of bifurcated acetone complexes, so far proposed as favored surface acetone complexes only on the basis of theoretical calculations.4 The distance between two free OH groups, either terminal/geminal or contained in silanol pairs, was calculated to be ∼5.3 Å.4 (iii) Surface free OH groups forming H-bondings with adsorbed acetone are consumed quite slowly (see the blownup spectral patterns in the insets to Figure 4-6): at pe) 5 Torr (p/p° ≈ 0.017), all AT systems still present some residual free

Crocella` et al.

Figure 7. Absorbance spectra in the νCO spectral range of acetone adsorbed on A303, under equilibrium pressures up to 130.0 Torr (p/p° ≈ 0.46). A very thin-pellet sample was used in this case, to avoid spectral saturation under high acetone pressures. Section (a): spectra as obtained, linearized in the 1800-1650 cm-1 interval. Upper brokenline trace: spectrum of 90 Torr gaseous acetone in the same IR cell. Section (b): spectra 1-4 and 7-17 of section (a), after interactive subtraction of the contribution of the gaseous phase. Acetone equilibrium pressures (Torr) are: 1 (0.5), 2 (1.0), 3 (1.5), 4 (2.0), 5 (5.0), 6 (7.0), 7 (10.0), 8 (13.0), 9 (20.0), 10 (24.0), 11 (30.0), 12 (40.0), 13 (50.0), 14 (70.0), 15 (90.0), 16 (110.0), and 17 (130.0).

OH species. (For instance, highly dehydrated A1100 on which (almost) only 1:1 acetone/OH interactions absorbing at ∼3400 cm-1 are expected to form, at pe ) 5 Torr still possesses some 20% of the starting free OH population). OH spectral data obtained with a thin sample (not shown for brevity) indicate that only under a pressure of ∼10-12 Torr (p/p° ≈ 0.035-0.040) virtually all free OH groups become involved in acetone uptake. This datum legitimates the rough extrapolations proposed, in a previous section, on the basis of the still growing shape of gasvolumetric isotherms at pe ≈ 8 Torr (see Figure 1a). (iv) On A303, that possesses both free and H-bonded silanols (curve 1 of Figure 3), the speed at which the integrated intensity (I) of the former species declines upon adsorption (see the inset to Figure 4) turns out to be definitely lower than the speed at which the overall amount of adsorbed acetone increases. [Overall acetone uptake can be evaluated by the integrated intensity of the νCO band envelope, as shown by the spectral patterns in Figure 7 (to be discussed below), and by the relevant spectral features reported in Table 1SM of the Supporting Information)]. This is a first clear indication that acetone does not adsorb only at free OH groups, as it has been considered so far, but can also “replace” (some of) the mutual H-bonding interactions between vicinal OH groups. νCO Spectral Range. Let us now consider the spectral region of the analytical νCO mode: (1) Centered at ∼1740 cm-1, gaseous acetone presents a strong, broad, and complex absorption, as expected of a A1 mode of a relatively light asymmetric-top molecule of C2V symmetry.26 A segment of the spectrum of gaseous acetone, obtained in conditions equivalent to adsorption ones, is reported in the upper part of Figure 7a. (2) Liquid acetone presents a broad and complex main absorption, with an evident band maximum at ∼1717 cm-1 25,26 and a pronounced high-ν shoulder at ∼1747 cm-1. (3) Dilute acetone solutions (∼3.5 vol %) in nonpolar CCl4 present a sharp peak centered at 1716 cm-1 and a high-ν shoulder at ∼1747 cm-1 of much decreased relative intensity with respect to the liquid. (4) Dilute acetone solutions (∼3.5 vol %) in polar CHCl3 present a somewhat broader main band centered at ∼1710 cm-1 (acetone and chloroform interact specifically by forming a weak Hbonding) and at ∼1750 cm-1 a weak satellite band of further decreased relative intensity with respect to the liquid.

Adsorption of Acetone on Silica The strength of acetone interaction with surface sites will be mainly evaluated by the ∆νCO frequency change with respect to either the liquid or the solutions in a nonpolar solvent (∼1717 cm-1), rather than with respect to the gas (as often done4), because the passage to a condensed phase brings about per se a drastic red-shift of the νCO mode. Figure 7 reports the spectral pattern relative to acetone uptake onto a thin pellet of A303 (see figure caption), run under equilibrium pressures between 0.5 Torr (p/p° ≈ 0.002) and 130 Torr (p/p° ≈ 0.46). The pattern is shown before [section (a)] and after subtraction of the gas contribution [section (b), mostly relative to high coverages]. It is noted that: (i) For pressures up to ∼7 Torr (e.g., see curve 6), the spectral contribution of the gaseous phase is negligible, as an IR cell of very small optical path was employed. (ii) At low pressures, the νCO band is made up of two severely overlapped components: specific acetone uptake yields two H-bonded species, as expected on the basis of what reported in the previous discussion. The two νCO components are initially almost constantly positioned at ∼1710 and ∼1696 cm-1, respectively, and present relative intensities that vary rapidly with acetone pressure. [See also the two low-lying resolved spectral sets shown in Figure 1SM of the Supporting Information, in which few examples of bands-envelope resolution are reported. The two band sets correspond to acetone equilibrium pressures of 1.5 and 7 Torr, respectively]. The higher-ν adspecies is ascribed to 1:1 acetone/OH complexes (species B), as it forms more slowly and presents a smaller red-shift (-∆νCO ≈ 7 cm-1), whereas the lower-ν adspecies is ascribed to the stronger 1:2 acetone/OH complexes postulated in the νOH section (species A), as it forms first and presents a larger red-shift (-∆νCO ≈ 21 cm-1). The assignment will be confirmed by desorption rates (Figure 9). (iii) Under relatively high acetone pressures, definitely higher than those corresponding to the first appearance of the gas spectral features, in section (b) a third adsorbed species shows up as a shoulder, apparently centered at ∼1715 cm-1 and actually characterized by a main νCO peak located, with increasing coverage, at 1724-1719 cm-1 (blue-shifted by 7-2 cm-1) and a weak high-ν satellite band at ∼1750 cm-1. [These spectral features are better evidenced by computer resolved spectra, an example of which is shown in the upper part of Figure 1SM of the Supporting Information, as well as by band-resolved spectral data, a complete set of which, relative to the adsorption pattern of Figure 7, is reported in Table 1SM of the Supporting Information]. These features are ascribed to the gradual build-up of a physically adsorbed (liquid-like) acetone phase, called species C. Its late appearance confirms that, when dealing with specifically adsorbed A and B species (p e ∼8 Torr), a physisorbed phase is indeed virtually absent. As long as they remain the sole species appreciably present at the surface, the chemisorbed species A and B exhibit virtually constant spectral position and bandwidth. But in the presence of the physically adsorbed component C, they tend to present lower-ν peak maxima and larger half-band widths, as a consequence of a typical solvent effect brought about by a more and more abundant liquid-like phase. [These aspects are quite evident in Table 1SM of the Supporting Information. Also, some mass-normalized absorbance νCO spectral patterns relative to the low-pressure (i.e., no physisorption) section of acetone adsorption runs on AT systems, and to some of the relevant BT desorption runs, are reported in Figure 2SM of the Supporting Information].


Figure 8. Mass-normalized optical adsorption isotherms [integrated absorbance in the 1750-1640 wavenumbers range (cm-1) vs equilibrium pressure (Torr)] relative to BT acetone uptake on A303 (a), A673 (b), and A1100 (c). Star symbols: overall isotherms; triangles: A acetone species; squares: B acetone species. Insets to section (b): Left-hand frame: Langmuir plot presentation [pe/na (equilibrium pressure over integrated absorbance; Torr cm × 102) vs equilibrium pressure (Torr)] of A acetone species adsorbed on A303 (lower trace) and A673 (upper trace), respectively. Right-hand frame: Langmuir plot presentation [pe/ na (equilibrium pressure over integrated absorbance; Torr cm × 102) vs equilibrium pressure (Torr)] of B acetone species adsorbed on A303 (lower trace), A673 (middle trace), and A1100 (upper trace), respectively.

Spectral resolution of the closely overlapped νCO components A and B was carried out as described in the Experimental [examples of resolution are shown in Figure 1SM of the Supporting Information] and lead to resolved optical adsorption isotherms, reported in Figure 8, and desorption spectral profiles, shown in Figure 9. Figure 8 indicates the following: (i) The stronger-held species A starts first and remains far predominant in the whole early adsorption stage on A303, whereas its maximum amount reduces to less than 1/5 on A673, and to less than 1/40 (though still present) on A1100. A fast decreasing trend with activation temperature is the obvious consequence of a lower and lower probability of having silanol couples at the proper distance to yield bifurcated acetone adspecies. (ii) The (νCO)A band grows fast in intensity and reaches an asymptotic value for pressures as low as ∼3 Torr (p/p° ≈ 0.01) on all AT systems. Langmuir plots (left-hand inset to Figure 8b), reported for A303 and A673 in terms of equilibrium pressure (pe) over integrated absorbance I (for each species, I is proportional to the adsorbed amount), indicate that the growth of the (νCO)A band follows Langmuir’s algorithm up to very close to the asymptote for A303, and up to the asymptote for A673. This justifies the virtually constant isosteric heat postulated, for species A, in the calorimetric section (Figure 2). (iii) Optical isotherms relative to the (νCO)B band of the three AT systems grow not so steeply in the whole early adsorption stage and, at pe ) 5 Torr, are still quite far from saturation. Data relative to the thin A303 pellet of Figure 7 [deducible from absorbance figures reported in Table 1SM of the Supporting Information] indicate that saturation of species B is obtained,

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Figure 9. BT desorption profiles [integrated absorbance in the 1750-1640 wavenumbers range (cm-1) vs evacuation time (sec)] relative to the two acetone species A and B, as indicated on the curves, adsorbed on A303 (a) and A673 (b), respectively.

on highly hydrated A303, at pe as high as ∼40 Torr (p/p° ≈ 0.14). This pressure figure is actually much higher than that roughly estimated above, for highly dehydrated A1100, on the basis of the overall quantitative isotherm of Figure 1a. (iv) Surprisingly, the initial part of the (νCO)B band isotherms is almost coincident for A303/A673, and only some 15% lower for A1100. This indicates that, as long as there are abundant free OH groups, supposed to be the favored acetone adsorbing sites, they possess a (nearly) constant sticking probability so that the initial uptake of (1:1) species B depends almost exclusively on acetone pressure, that in the perfect gas approximation represents the gas phase concentration. (v) The right-hand inset to Figure 8b shows that, for pe up to ∼5 Torr (for species B it represents only the very initial part of the overall process), also the isotherms of species B follow Langmuir’s algorithm, meaning that the stronger initial fraction of 1:1 OH/acetone interactions is energetically rather homogeneous. The slope of the Langmuir regime for species B extrapolates saturation I figures of 43, 35, and 36 cm-1 for A303, A673, and A1100, respectively. But these data are purely indicative, as the saturation plateau of species B lies well outside the Langmuir-regime part of the isotherms. Desorption paths in Figure 9 confirm the reversibility of both specific acetone adspecies. Vacuum elimination of (νCO)B band is almost immediate (at BT, it requires ∼1 min on all AT systems), whereas the elimination of (νCO)A band requires definitely longer evacuation times (∼30 min for highly hydrated A303). The lower desorption rate of species A is consistent with its higher molar adsorption enthalpy, as previously evaluated from qdiff plots (Figure 2), and also indicated by ab initio calculations.4 QuantitatiWe and Thermodynamic Information. Sites Population. Comparison between total quantitative isotherms (Figure 1a) and total-and-resolved spectroscopic isotherms (Figure 8) should allow to evaluate the surface concentrations of A and B sites. This could be easily done by direct comparison between the upper curve of Figure 1a (overall uptake on A303) and the upper curve of Figure 8a (total integrated νCO absorbance on

Crocella` et al.

Figure 10. Conversion of the resolved optical adsorption isotherms of Figure 8 into resolved quantitative adsorption isotherms [adsorbed amounts na (µmol m-2) vs adsorptive equilibrium pressure pe (Torr)], by comparison with the quantitative gas-volumetric isotherms of Figure 1a and by assuming, for the νCO band of adsorbed acetone species A and B, the same molar extinction coefficient. Section (a): A303; section (b): A673; section (c): A1100. Star symbols: overall isotherms; triangles: acetone adspecies A; squares: acetone adspecies B.

A303), by assuming that the two acetone adspecies possess the same molar extinction coefficient and contribute evenly to the build-up of the overall spectroscopic isotherm. The assumption is, in principle, not correct. Still, experimental and spectroscopic arguments can be proposed to argue that this simplifying assumption should be acceptable in numerical terms. [Some of these experimental/spectroscopic arguments are reported in the section Text 1SM of the Supporting Information]. On the basis of equal extinction coefficients for species A and B, six (approximate) resolved quantitative isotherms, relative to acetone uptake on AT systems, have been calculated, and are reported in Figure 10. The following indications can be derived: (i) On A1100, no residual H-bonding interactions between surface silanols are left (curve 3 of Figure 3), and only free-OH/ acetone interactions (i.e., the only interactions considered possible in previous literature) can occur. The (1:2) species A is very scarce (∼0.08 µmol m-2), and accounts for only ∼0.16 µmol OH per m2 (i.e., up to ∼0.1 OH groups per nm2). At pe ) 5 Torr, the amount of (1:1) B complexes is ∼0.8 µmol m-2, and considering that some 35% of free OH groups have not interacted yet with acetone (the figure was calculated by comparing the absorbance (I) of the 3747 cm-1 OH band in curve 10 of the inset to Figure 6 with the starting absorbance (I0) of the same OH band in spectrum 3 of Figure 3), an overall OH amount of up to ∼1.4 µmol m-2 (i.e., ∼0.85 OH groups per nm2) is deduced for A1100. (ii) On A673, only a small minority of surface OH groups is still involved in OH-OH H-bonding interactions (spectrum 2 of Figure 3). So, also in this case, virtually all surface OH groups should be available for free-OH/acetone interactions. The saturation concentration of (1:2) A complexes is ∼0.4 µmol m-2, and accounts for a maximum of ∼0.8 µmol OH per m2 (i.e., ∼0.5 OH groups per nm2). The amount of (1:1) B

Adsorption of Acetone on Silica complexes is definitely larger: at pe ) 5 Torr it is ∼1.0 µmol m-2, and since some 15% of surface OH groups are still free at that pressure [ratio between I (3745 cm-1) in curve 10 of the inset to Figure 5 and I0 (3747 cm-1) in curve 2 of Figure 3], an overall amount of up to ∼2.2 µmol free OH groups per m2 (i.e., up to ∼1.35 OH groups per nm2) can be inferred for the A673 system. (iii) On A303, an appreciable fraction of surface OH population is involved in mutual H-bonding interactions and, according to the previous literature,3-6 should not be able to interact with acetone. The saturation concentration of (1:2) complexes A is fairly large (∼1.57 µmol m-2) and accounts for ∼3.15 µmol OH per m2 (i.e., ∼1.8 OH groups per nm2). At pe ) 5 Torr, the amount of (1:1) complexes B is ∼1 µmol m-2, and considering that only some 10% of free OH groups has not interacted yet with acetone [ratio between I (3740 cm-1) in curve 10 of the inset to Figure 4, and I0 (3744-3716 cm-1) of the free-OH envelope of curve 1 in Figure 3], an overall amount of ∼4.25 µmol free OH groups per m2 (i.e., ∼2.5 OH groups per nm2) are deduced for the A303 system. (Note that, using the saturation values of species B given by Langmuir’s plots in the right-hand inset to Figure 8b, overall OH concentrations only marginally higher than those calculated above are obtained, even if the isotherms of species B do follow Langmuir’s algorithm only at the very beginning). A comparison of these rough OH quantitative data with the data reported recently by Zhuravlev for most amorphous silicas8 and, some years ago, for Aerosil silicas by Mathias et al.7 is not straightforward. Still, some conclusions can be drawn: In view of the often verified variability of the starting OH species population at the surface of different batches of the same Aerosil material (e.g., see ref 9), the result obtained for A673 (medium dehydration stage, almost all free OH species) is in good agreement with literature datum.7 The OH datum obtained for A1100 (very high dehydration stage, all free OH species) agrees with Zhuravlev’s “general” data8 and is definitely lower than the “specific” and somewhat controversial datum reported by Mathias et al.7 The datum obtained for A303 (∼2.5 OH groups per nm2) is, apparently, the most “wrong” one. In fact, this figure coincides with the maximum overall OH population reported by Mathias et al.7 and by others9 as characteristic of all highly hydrated pyrogenic silicas, while (at least) some 50% of the background OH groups of A303 are actually involved in H-bonding interactions of the OH-OH type. We obtain so a second clear evidence that not only free OH groups, but also H-bonded OH species of all types (i.e., both OH pairs and longchain interacting OH) must participate in the H-bonding interaction(s) with acetone. On the other hand, should this not be the case, and only free OH species be able to interact with acetone, the passage from A303 to A673 should bring about an appreciable increase of the overall acetone uptake, as suggested by the trend of (resolved) free OH band in Figure 3 as well as by the trend of free OH population reported by Mathias et al. for Aerosil silicas.7 On the contrary, a sharp ∼30% decrease of overall acetone uptake is actually observed (Figures 8 and 10). It is so deduced that, under relatively high pressures, acetone uptake on nonporous silica produces a sort of ligand displacement (or “replacement”), substituting pre-existing OH-OH H-bonding interactions with OH-carbonyl ones. At the end of the specific first-layer uptake, most (if not all) surface OH species will be H-bonded to acetone carbonyl groups rather than to adjoining OH groups. This aspect has never been considered before, on either experimental or theoretical ground.

J. Phys. Chem. C, Vol. 113, No. 37, 2009 16525 TABLE 1: Thermodynamic Features of Adsorbed Acetone

species A species B

Keq

-∆adsG° (kJ/mol)

-∆adsH° (kJ/mol)

-∆adsS° (J/molK)

1785 258

18.550 13.758

75 50

189 122

Adsorption Entropy. As suggested by one of the reviewers, a further step in the energetic characterization of the acetonesilica interaction is now possible, as a consequence of the Langmuir character of (virtually all) species A uptake and of the earliest part of species B uptake. After conversion of the optical Langmuir plots reported in the insets to Figure 8b into corresponding quantitative Langmuir plots (not shown), slope and intercept of the straight-line regimes allow to obtain Keq and, consequently, ∆adsG°. Calculated Keq and ∆adsG° values are reported in columns 2 and 3 of Table 1. It should be noted that these figures concern the adsorption of all species A, but only the very first and more energetic fraction of species B and are relative to the “easier” system A673, but very similar values (differences below 5%) were obtained also for the resolved A and B components of A303. In order to apply the T-constant equation [∆G° ) ∆H° T∆S°] and evaluate ∆ads S° for the two acetone adspecies, reasonable ∆adsH° figures deriving from the two regimes (i.e., lower and higher coverages) qdiff plots of Figure 2 have been adopted, and are reported in column 4 of Table 1. In particular, the virtually constant qdiff ≈ -75 kJ/mol value has been used to represent the more energetic species A (prevailing at low coverages and on more hydrated systems), whereas a qdiff ≈ -50 kJ/mol value been used to represent the average contribution of the most energetic fraction of the heterogeneous species B (i.e., the little initial portion of species B uptake that behaves according to Langmuir’s algorithm). ∆adsS° figures for both acetone adspecies have been so obtained, and are reported in the last column of Table 1. An over 50% increase of ∆adsS° on passing from one-Hbond species B to two-Hbond species A seems reasonable, in view of the fairly different level of immobilization produced in the two cases. As for the absolute values of ∆adsS° obtained, they seem quite realistic if compared with other acetone entropy figures: Absolute molar entropy of gaseous acetone [(S°)gas] is 295.1 J/(mol K) if calculated via DFT27 and 295.5 J/(mol K) if obtained by adding ∆vapS° to the std molar entropy of the liquid [(S°)liq ) 200 J/mol K]. A limit ∆adsS° of ∼-295 J/(mol K) would thus correspond to the complete loss of all freedom degrees available for the gas at ambient temperature (i.e., basically translational and rotational). Statistical thermodynamics predicts, for all gases (including acetone) transformed upon adsorption into a sort of 2-dimensional gas, a ∆adsS° ≈ -166 J/mol K.28 The strong interaction of acetone with ice surface has been found to imply a ∆adsS° of ∼-236 J/mol K28, or ∼-250 J/mol K.29 For the localized coordinative interaction of acetone with carbon-supported Cu, ∆adsS° ) -213 J/(mol K), and with Ni ∆adsS° ) -190 J/(mol K), as compared with the far less specific interaction with the plain active carbon support on which ∆adsS° ) -92 J/mol K.30 Part II. Acetone on Mesoporous Silica (Spherical MCM41). The overall behavior of the MCMT/acetone system is, as expected, quite similar to that of the AT/acetone one. Still, some remarkable differences have been observed and will be reported as schematically as possible. For brevity, main attention will be focused on the emblematic behavior of medium-dehydrated MCM673, with only some comparisons with highly hydrated MCM303 when necessary. Moreover, some of the calorimetric results obtained with narrow-pores MCMT will be compared

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Figure 11. Section (a). Quantitative isotherms [surface area normalized adsorbed amounts na (µmol m-2) vs adsorptive equilibrium pressure pe (Torr)] relative to the adsorption at 298 K of acetone vapor onto MCM303 (squares) and MCM673 (circles). Section (b). Surface area normalized calorimetric isotherms [released integral heats -Qint (J m-2) vs adsorptive equilibrium pressure pe (Torr)] relative to the adsorption at 298 K of acetone vapor onto MCM303 (squares) and MCM673 (circles). In both isotherm plots, empty symbols represent primary adsorption runs, and solid symbols the secondary adsorption ones. Note that the primary calorimetric adsorption run is virtually coincident for the two specimens, and is therefore reported only for MCM673 (empty circles). Section (c). Differential molar adsorption heats -qdiff (kJ mol-1), obtained by graphical differentiation based on experimental points, are plotted as a function of surface area normalized adsorbed amounts na (µmol m-2). Squares: MCM303; circles: MCM673; triangles: MCM-R673 (reference mesoporous system). Empty symbols represent primary adsorption runs, solid symbols the secondary adsorption ones.

with the corresponding ones relative to the more typical “conventional” MCM-R673. QuantitatiWe Data. Normalized adsorption isotherms, reported in Figure 11a, indicate the following: (i) Overall acetone specific adsorptivity, that on nonporous silica (Figure 1a; maximum overall uptake: ∼3 µmol m-2) was found to be definitely lower than the potential monolayer capacity, in the case of spherical mesoporous MCM-41 turns out to be even lower. This indicates an even lower surface density of specific adsorbing sites (silanols). (ii) Acetone adsorbing capacity declines fast with activation temperature confirming, also for mesoporous MCMT, its dependence on the overall surface OH population. The indication will be corroborated by IR data. (iii) The Langmuirlike adsorption isotherms present a very steep initial part (up to pe ≈ 3 Torr) and for pe as high as 6-8 Torr both isotherms seem to have almost reached saturation. (iv) Unlike Aerosil, the MCMT/acetone system presents a small but evident irreversibility, somewhat larger for the adsorbent activated at lower temperature. Figure 11b reports the calorimetric isotherms and indicates the following: (i) Unlike adsorbed amounts, the normalized adsorption heats released by MCMT are comparable to those of Aerosil (Figure 1b; maximum released heat: ∼0.2 J m-2), meaning that molar enthalpies of adsorption on the mesoporous system should be expected to be definitely larger. Differential adsorption heat plots will confirm it. (ii) Primary calorimetric

Crocella` et al. isotherms of the two MCMT specimens are virtually coincident (only one primary uptake curve is shown in the figure), meaning that primary adsorption enthalpies become larger the higher the activation temperature. (iii) Also for calorimetric isotherms the initial section (up to pe ≈ 3 Torr) is very steep, and the adsorption process presents some irreversibility that, unlike adsorbed amounts (Figure 11a), is definitely larger for the adsorbent activated at higher temperature. Graphical differentiation of quantitative/calorimetric data yields differential molar adsorption heat plots, reported in Figure 11c. Most differential heats of both MCMT systems lie well above those obtained for nonporous silica (Figure 2b). IR spectroscopic data will indicate that the strength of CO/OH H-bondings involved in the two silica/acetone systems does indeed present some differences but not very large. The enthalpic datum thus suggests that acetone adsorption within the narrow mesoporous network of MCMT must involve also other forces like, for instance, diffuse van der Waals forces of all three types,10,31 as expected of the interaction between a polar adsorbate and an adsorbent carrying polar functionalities. But the same forces would not be (so) active in the early stages of acetone uptake on the “open” and “flat” Aerosil surface. Moreover, as suggested by one of the reviewers, the unusually high acidity of acetone H atoms may appreciably contribute to the overall adsorbate/adsorbent interaction within narrow pores and not on an “open” surface. In order to confirm that the very small size of mesopores (18 Å diameter) is actually responsible for the high energies of interaction observed, a gas-volumetric/microcalorimetric analysis has been carried out also on the larger-pores reference MCMR673 system. Differential adsorption heats, shown in Figure 11c (lowest curves, triangles), are quite similar for primary and secondary adsorption run (virtually reversible adsorption), and indicate that the energetic response of MCM-R673 is only somewhat higher than that of the nonporous silica system: a strong initial interaction (q° ≈ 150-200 kJ mol-1) is followed by a flat qdiff plateau (∼80-100 kJ mol-1) for coverages up to ∼1.8 µmol/m2. This coverage value, attained for pe ≈ 9 Torr, is much larger than the coverages reached by MCMT systems, and corresponds to a pressure/coverage condition fairly similar to that of the corresponding nonporous A673 system. As for the narrow-pores MCMT system, it is noted that the primary runs point to very high q° figures, which justify the partial irreversibility of acetone uptake. This is particularly evident for MCM673: Figure 11b (upper curves, circle symbols) shows that the qdiff difference between primary and secondary adsorption run is quite high. For MCM303, a large qdiff difference between primary and secondary run is limited to the first ∼0.2 µmol m-2 (∼0.12 acetone molecules per nm2), and corresponds to the very early and steep stage of the adsorption isotherm. But in the case of MCM673, large qdiff differences concern most of the adsorption process: up to ∼0.4 µmol m-2 (∼0.24 acetone molecules per nm2), corresponding to the onset of the flat (saturation) part of the overall adsorption isotherm. IR Spectroscopic Data. Adsorption Patterns. Figure 12 presents, for the two MCMT specimens, the νOH spectrum recorded before adsorption (broken lines) and after contact with 3 Torr acetone (solid lines). The following points are noted: (i) The spectrum of bare MCM303 (section a) is dominated by a broad and very complex band due to OH groups mutually interacting by H-bonding, whereas from the spectrum of bare MCM673 (section b) almost all H-bonded OH species have been eliminated.


Figure 12. Absorbance IR spectra in the O-H stretching spectral region (νOH; 3850-3100 cm-1) of MCM303 [section (a)] and MCM673 [section (b)] run before acetone adsorption (broken-line traces) and after contact with 3 Torr acetone (solid-line traces). Solid-line spectrum of section (a) is not reported in the ∼3470-3270 cm-1 range, because the presence of the first νCO overtone band and of Fermi resonance effects involving OH deformation modes renders that spectral segment quite complex in the case of MCM673 [see the solid-line spectrum in section (b)] and impossible to read in the case of the MCM303 system. Insets: absorbance spectral patterns in the νCO spectral range, relative to the early stages of acetone adsorption on the two MCMT samples. Acetone equilibrium pressures (Torr) are: (a) 0.005, 0.02, 0.06, 0.2, 0.3, 0.5, and 3.0 (b) 0.01, 0.03, 0.07, 0.1, 0.2, 0.3, and 0.5.

(ii) The dramatic increase of the free νOH stretching band (∼3750 cm-1) produced in the spectrum of bare MCM673, as compared with the large overall decrease of acetone adsorbing capacity shown in Figure 11a, renders even more evident than for Aerosil the fact that H-bonding acetone/silanol interactions do not concern only free OH species but also mutually H-bonded OH species. (iii) Spectra relative to the adsorption at pe as low as 3 Torr (p/p° ≈ 0.01) indicate that almost all free OH groups have been already involved in interaction with the adsorbate. Consistently, Figure 11a shows that at that pressure overall adsorbed amounts are not far from the asymptotic part of the isotherm. The νCO band of adsorbed acetone (spectral patterns of the early stages of acetone uptake are presented as insets in Figure 12), indicates the following: (i) As for Aerosil, the νCO band is made up of two closely overlapped components, the assignment for which is the same proposed previously: the low-ν species A to the interaction with two OH groups, and the higher-ν species B to the interaction with one OH group. (ii) On MCM303, species A is far more predominant in the earliest adsorption stages and is centered at ∼1690 cm-1, i.e., some 5 cm-1 below the position occupied on A303. This ∆νCO difference is not negligible for an aliphatic ketone carbonyl,25,32,33 and should be considered at least in part responsible for the stronger interaction indicated by qdiff plots. (iii) On MCM303, also species B absorbs at slightly lower ν than on A303 (red-shifted by 4 cm-1), and when for pe g 0.5 Torr it becomes predominant, it also becomes much broader and requires two closely overlapped peaks for a proper computer simulation. [This aspect, indicated by the two frequencies reported on the uppermost spectrum in Figure 12a, is clearly shown by the resolved spectral features of acetone uptake on MCM303, reported in Table 2SM of the Supporting Information]. The heterogeneity of adspecies B is likely to be connected with the porous nature of the adsorbent, and may possibly indicate that 1:1 interactions distinguish (single) OH groups inside and outside the narrow meso-pores.


Figure 13. Mass-normalized optical adsorption isotherms [integrated absorbance in the 1750-1640 wavenumbers range (cm-1) vs equilibrium pressure (Torr)] relative to BT acetone uptake on MCM673. Star symbols: overall isotherm; triangles: A acetone species; squares: B acetone species. Inset: Langmuir plot presentation [pe/na (equilibrium pressure over integrated absorbance; Torr cm × 103) vs equilibrium pressure (Torr)] of A (lower trace) and B acetone species (upper trace) adsorbed on MCM673.

(iv) Also in the case of MCM673 the resolved A and B bands are red-shifted by some 5 cm-1 with respect to A673, and again justify at least in part the far more energetic MCM-41/acetone interaction. [Table 3SM of the Supporting Information reports the main spectral features of the acetone adsorption pattern on MCM673]. The low-ν 1:2 component A is somewhat prevalent at the beginning of the process and soon reaches saturation, when the higher-ν 1:1 component B starts prevailing. The latter component is here resolvable in terms of only one peak of fairly constant νmax, meaning that single OH groups of one type only remain after activation at 673 K. QuantitatiVe Aspects. Computer resolution of adsorption spectral patterns on MCMT leads to resolved optical adsorption isotherms. For brevity, only the set of optical isotherms relative to MCM673 is shown in Figure 13. It is evident that (i) the bifurcated acetone species A is still very abundant and, for pe > 1 Torr, becomes saturated (I ≈ 40 cm-1), so that the still slightly growing shape of the overall isotherm is entirely ascribable to the residual slow increase of species B, and (ii) in the very narrow pressures range explored, both resolved optical isotherms seem to fit Langmuir’s algorithm, as shown by the Langmuir plots reported in the inset (as usual, the integral absorbance I of the resolved A and B bands was used to represent the relevant adsorbed amounts na). Still, this Langmuirlike behavior must be considered rather fortuitous, in view of the complex forces system acting in this case, and evidenced by calorimetric data. Also for this reason, no attempts have been done to evaluate, on the basis of Langmuir plots, ∆adsS° values for the two acetone adspecies A and B (as was successfully done, on the contrary, in the case of non porous silica). The slope of the Langmuir plot of species B yields a saturation value of ∼32 cm-1: this figure, necessarily approximate, suggests that (unlike nonporous silica A673) at pe ≈ 3 Torr also species B is not far from its asymptote, and that the latter remains unexpectedly lower than the asymptote of the 1:2 species A. This datum, combined with the scarce overall OH population of MCM-41 (as indicated by acetone uptake data in Figure 11a), suggests that silanols are probably clustered at the surface of mesoporous silica, and/or the curved nature of the internal surface of narrow meso-pores facilitates the formation of bifurcated acetone species also after dehydration at relatively high temperatures. If the total and resolved optical isotherms of MCM673 (Figure 13; maximum pe ) 3 Torr) are compared with the corresponding (total) quantitative primary adsorption isotherm of Figure 11a,

16528


Figure 14. BT desorption profiles [integrated absorbance in the range 1750-1640 wavenumbers (cm-1) vs evacuation time (s)] relative to the two acetone species A and B (as indicated on the curves) adsorbed on MCM303 (a) and MCM673 (b), respectively. Insets: absorbance spectral patterns in the νCO spectral range, relative to the last steps of BT acetone desorption from the two MCMT samples. Top-to-bottom, BT evacuation times (min) are: (a) 20, 60, and 90 (b) 2, 5, 20, 60, and 90.

and we assume again the same molar extinction coefficient for the two νCO components, a (rough) evaluation of MCM673 OH populations can be tried. The asymptotic absorbance of species A (∼40 cm-1) yields an adsorbed acetone amount of ∼0.31 µmol m-2 (i.e., ∼0.19 molecules per nm2) and, on account of the 1:2 acetone/OH interaction, an OH density of ∼0.62 µmol m-2 (corresponding to ∼0.38 OH groups per nm2). As for the 1:1 species B, the Langmuir’s plot monolayer capacity (32 cm-1) yields an OH density of ∼0.25 µmol m-2 (i.e., ∼0.15 OH groups per nm2), bringing the overall OH population of MCM673 to some 0.53 OH groups per nm2. As anticipated, it is indeed a modest residual OH concentration, if compared with the already relatively low OH concentration of the corresponding nonporous A673 system (∼1.35 OH groups per nm2). For the highly hydrated MCM303 system, the evaluation of OH population from resolved νCO band patterns and the highlying primary isotherm in Figure 11a is bound to be even more inaccurate, due to the double nature of the 1:1 species B and the fact that, consistently, the overall optical isotherm B does not follow at all Langmuir’s algorithm. Saturation absorbance of the (νCO)A band is found at ∼44 cm-1, leading to an acetone amount of ∼0.45 µmol m-2 (∼0.27 acetone molecules per nm2) and to an OH population of ∼0.9 µmol m-2 (∼0.54 OH groups per nm2). For acetone species B, the saturation absorbance is grossly evaluated, by either graphical extrapolation or an empirical algorithm, at (up to) ∼60 cm-1, corresponding to a maximum acetone amount of ∼0.62 µmol m-2 (i.e., ∼0.37 acetone molecules per nm2) and to an equivalent concentration of OH groups. The overall OH population of (nominally) fully hydrated MCM303 would thus be of the order of only ∼1.5 µmol m-2 (i.e., ∼0.9 OH groups per nm2), a concentration figure unexpectedly low and comparable with that of nonporous silicas activated at T g 1150 K.

Crocella` et al. Desorption Profiles. Figure 14 reports the BT optical desorption profiles for the resolved adspecies A and B, from MCM303 [section (a)] and MCM673 [section (b)], respectively. Two interesting aspects are evidenced: (i) Also at BT, that is unavoidably higher than the temperature of quantitative-calorimetric experiments, there is clear evidence for a relatively small amount of acetone that is not thoroughly desorbed after evacuation times as long as 1.5 h (see the spectral patterns in the insets to Figure 14, relative to the last steps of the evacuation/desorption process). Desorption patterns also indicate that the small irreversible amount is entirely due to the stronger-held species A in the case of MCM303 [section (a)], whereas it is partly due also to species B in the case of MCM673 [section (b)]. This partial irreversibility is not unexpected, considering the high qdiff values caused by the very small-size mesoporous framework of spherical MCM-41, actually not so different from microporosity. (ii) The desorption rate of acetone complexes A is definitely lower for MCM303 (see the top profile A) than for MCM673 (bottom profile A), even if the overall starting amount of species A is similar in the two cases, and despite the fact that the average qdiff is higher for acetone uptake on MCM673 than on MCM303. This is clear evidence for the important role played, in desorption processes, by kinetic effects. In fact, acetone desorption from MCM303 is expected to be a far more difficult process, due to the ample and complex system of H-bondings (involving both CO/OH and OH/OH interactions) existing in the small pore channels, and monitored by the strong and broad νOH band present in the 3700-3000 cm-1 range of Figure 12a. 4. Conclusions Gas-phase acetone adsorption on silica is a rather complex phenomenon that, for small-size mesoporous silica, turns out to be even more complex. The main reason for this complexity resides in the heterogeneous nature of the adsorption process, which is of both structural and energetic origin. The specific (chemisorption) part of the adsorption process is made up of two forms of interaction with surface silanols, one of which (termed species A) is more energetic and implies the formation of two H-bonds per acetone carbonyl group. The existence of this acetone adspecies, first postulated on a theoretical ground, has been demonstrated with evident spectroscopic arguments. Species A, energetically favored and quite homogeneous (uptake according to Langmuir’s algorithm on both nonporous and mesoporous silica), forms as long as there are at the surface silanol pairs properly spaced to hold an acetone molecule in the bifurcated form. This possibility was found, with spectroscopic evidence, to persist longer at the surface of a narrow mesoporous silica of the MCM-41 family than on nonporous silica, even if the overall surface hydration of the former system is fairly low. More thoroughly dehydrated mesoporous systems (i.e., activation temperatures higher than 673 K) have not been tried, due to the temperature-vulnerable nature of the ordered porous network. The less energetic acetone adspecies B, due to the H-bonding interaction with single silanols, was found to be far more heterogeneous in the case of nonporous silica, where Langmuir’s algorithm is followed only in a short initial portion of adsorption isotherms, than in the case of medium-activated (narrow) mesoporous silica, where saturation is reached almost as fast as for species A, and Langmuir’s algorithm is apparently obeyed almost to saturation. Highly hydrated silica specimens (nominally, “fully” hydrated silicas) are usually little considered in the literature, whereas

Adsorption of Acetone on Silica here they have been examined in some detail in the case of both nonporous and mesoporous silica. The study of their interaction with acetone led us to two main conclusions: (i) The overall starting surface OH concentration is fairly low (namely, much lower than the nominal monolayer capacity), and this aspect is particularly evident in the case of the (narrow) mesoporous spherical MCM-41 system. As a consequence of this low-concentration starting point, also silicas activated at higher temperatures will present OH populations far lower than expected, and this brings about low and fast declining overall specific adsorbing capacities toward acetone. (ii) Despite the low initial surface OH concentration, the starting surface hydrated layer is made up of OH species both free (from H-bonding) and involved in mutual H-bonding interactions, and this is so for both nonporous and mesoporous silica. Unlike what reported in both theoretical and experimental literature, there is clear spectroscopic and quantitative evidence that acetone can specifically interact by H-bonding with both types of surface silanols. In particular, in the case of mutually H-bonded hydroxyls, acetone uptake brings about a sort of surface ligand-displacement or replacement effect. On mesoporous silicas of the MCM-41 family, acetone forms with surface silanols H-bonds somewhat stronger than on nonporous silica activated in the same conditions. This is demonstrated by lower carbonyl stretching frequencies and by a partial irreversibility of adsorbed acetone. But in the case of the narrow-pores spherical MCM-41 system, unusually high differential adsorption heats obtained for both types of specific acetone uptake cannot be explained only in terms of stronger CO-OH H-bonds, and the presence of other types of (nonspecific) interaction must be thought to be operative in the case of acetone adsorption within the narrow cavities of the highly ordered quasi-microporous MCM-41 preparation. Acknowledgment. This work is part of the Ph.D. research activity of one of the authors (V.C.) and has been partly financed with funds of the Italian Ministry MIUR (Project PRIN-2006, Prot. 2006 032335). Supporting Information Available: Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Morterra, C. et al., in preparation. (2) Griffits, D.; Marshall, M. K.; Rochester, C. H. J. Chem. Soc. Faraday Trans. 1 1974, 70, 400.

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