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In-Situ Infrared Spectroscopic Studies of Adsorption Processes on Boehmite Particle Films: Exchange of Surface Hydroxyl Groups Observed upon Chelation by Acetylacetone Scott A. Dickie and A. James McQuillan* Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand Received June 25, 2004. In Final Form: September 24, 2004 Adsorption processes on poorly crystalline boehmite (PCB) particle films have been studied using attenuated total reflection infrared spectroscopy. This method allows the in-situ investigation of wet surface chemical processes. Thin films of aggregated particles of PCB that are stable between pH 4 and 11 have been prepared by drying aqueous PCB dispersions. Carbonate adsorbs to the PCB films during the film formation process but can be removed without impact on the film by washing with alkali at pH 10. The adsorption of acetylacetone (acac) to the surface of PCB has been studied at the solid/liquid and solid/gas interfaces. The concomitant changes in the OH deformations of hydroxyl groups present on the surface has been observed. The IR absorption of surface hydroxyl groups involved in adsorption of a bidentate chelating ligand have been spectroscopically isolated through their interaction with acac.
Introduction Alumina is an important material used widely in a number of industrial applications, particularly as a catalyst support in the ceramics industry,1 and as a coating for pigments.2-4 The predominant crystal phase of alumina present in pigment coatings is believed to be boehmite (γ-AlOOH)2, and thus its surface chemistry is of great interest. Boehmite is a stable phase and shows little inclination to transform into other aluminum oxide crystal phases on the time-scale of most laboratory experiments.5 Boehmite can be prepared with a wide range of crystallite size.6 Samples of boehmite with a small crystallite size and high surface area are often termed pseudoboehmite or poorly crystalline boehmite (PCB), although the distinction is largely arbitrary as no difference in structure exists except for the extent of long range order within the sample. As crystallite size decreases, water content increases and can reach levels up to 30 wt%.7 Boehmite is made up of layers of edge-shared AlO6 octahedra, as shown in Figure 1. The layers are held together by H-bonding between hydroxyl groups present on adjacent layers. We will adopt the previously used terminology8 in naming the different hydroxyl groups present: µi sites have ligand atoms that bridge ‘i’ metals; ηi sites have ‘i’ nonbridging ligand atoms bonding to the metal. The µ2-OH groups on the (010) basal plane of the layers are involved in interlayer H-bonding or exposed on the terminal faces of the crystal. The edges and corners * Author to whom correspondence should be addressed. Fax: +64 3 4797906. E-mail:
[email protected]. (1) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Pergamon Press: Oxford, 1986. (2) Morterra, C.; Cerrato, G.; Visca, M.; Lenti, D. M. J. Mater. Chem. 1992, 2, 341. (3) Loyd, T. B.; Fowkes, F. M.; Brand, J. R.; Dizikes, L. J. J. Coat. Technol. 1992, 64, 91. (4) Losoi, T. J. Coat. Technol. 1989, 61, 57. (5) Peryea, F. J.; Kittrick, J. A. Clays Clay Miner. 1988, 36, 391. (6) Tettenhorst, R.; Hofmann, D. A. Clays Clay Miner. 1980, 28, 373. (7) Fitzgerald, J. J.; Piedra, G.; Dec, S. F.; Seger, M.; Maciel, G. E. J. Am. Chem. Soc. 1997, 119, 7832. (8) Nordin, J. P.; Sullivan, D. J.; Phillips, B. L.; Casey, W. H. Geochim. Cosmochim. Acta 1999, 63, 3513.
of the crystal contain η1, η2, and η3 sites of which the η2 and η3 sites are available for surface bidentate chelation. The IR absorptions of OH groups located on the external crystal surfaces of PCB have been shown to be spectroscopically distinct from those situated in the bulk of the crystal. Changes in the IR signal of these surface hydroxyls is observed when the amount of physisorbed water is altered.9-11 The ability to observe changes in the environment of these surface hydroxyls through their IR absorptions provides the opportunity to witness processes occurring at the surface, such as chemisorption or physisorption, that would remove surface OH groups or alter their environment, respectively. There have been several spectroscopic studies on the aqueous surface chemistry of boehmite. The two main techniques used to study surface processes at the boehmite/ water interface have been attenuated total reflection infrared (ATR-IR) spectroscopy12-15 and NMR.8,16 Casey et al. have used various NMR techniques to study dissolution processes of mineral oxides. The polyoxocation AlO4Al12(OH)24(H 2O)127+ has been used as a model for alumina surfaces as its structure closely resembles the surfaces of some Al-hydr(oxide) surfaces. They observed discrete signals for the η-OH and µ-OH groups in the 17O NMR spectra.16 The adsorption of fluoride to bayerite and boehmite has also been investigated with 19F NMR.8 At least three chemically distinct fluoride sites were found on boehmite with the major peak being due to adsorption of fluoride onto η-OH sites. ATR-IR studies on boehmite and other alumina’s have typically used the wet paste method and have addressed the mode of binding of several (9) Morterra, C.; Emanuel, C.; Cerrato, G.; Magnacca, G. J. Chem. Soc., Faraday Trans. 1992, 88, 339. (10) Wang, S.-L.; Johnston, C. T.; Bish, D. L.; White, J. L.; Hem, S. L. J. Colloid Interface Sci. 2003, 260, 26. (11) Lewis, D. G.; Farmer, V. C. Clay Miner. 1986, 21, 93. (12) Axe, K.; Persson, P. Geochim. Cosmochim. Acta 2001, 65, 4481. (13) Laiti, E.; Oehman, L.-O. J. Colloid Interface Sci. 1996, 183, 441. (14) Nordin, J.; Persson, P.; Laiti, E.; Sjoeberg, S. Langmuir 1997, 13, 4085. (15) Nordin, J.; Persson, P.; Nordin, A.; Sjoeberg, S. Langmuir 1998, 14, 3655. (16) Phillips, B. L.; Casey, W. H.; Karisson, M. Nature (London) 2000, 404, 379.
10.1021/la048423s CCC: $27.50 © 2004 American Chemical Society Published on Web 11/17/2004
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Figure 1. A diagram depicting a cross section of a PCB crystal with the different oxygen sites labeled. The µ3-O (edge only) and µ4-O groups are present on the (010) basal plane, while µ2-OH groups are involved in interlayer H-bonding and exposed on crystal surfaces. Important for bidentate chelation are η2-OH groups found on the corners and edges of the crystal. Dangling bonds (- - -) indicate a continuation of the PCB structure.
ligands, including phosphate,17 phenylphosphonic acid,13,18 oxalate,12 acetate,19 and o-phthalate.14,20 ATR-IR spectroscopy is able to provide information about the binding mode of ligands usually by comparing the IR spectra from adsorbed species with corresponding solution species and coordination complexes. This approach has to date yielded limited information on adsorption sites. An alternative and more-versatile ATR-IR method is to use immobilized particle films for adsorption studies.21-23 The aim of this paper is to establish a versatile ATR-IR methodology using immobilized particle films for the investigation of boehmite surface chemical processes. PCB has been used as its high surface area provides enhanced spectral sensitivity for surface studies. In doing so, we have addressed the adsorption of carbonate to the boehmite surface and have developed a procedure for carbonate removal without destabilizing the PCB film. Acetylacetone (acac) has been chosen to investigate the effect chelation has on the surface hydroxyls of a PCB sample as it is a classic bidentate chelating ligand in coordination chemistry. It binds in a chelating fashion forming sixmembered rings with virtually all transition and maingroup metal ions.24,25 Investigation of the binding of acac onto the PCB surface has been carried out with a view to probing different types of surface hydroxyl groups. Experimental Section Acetylacetone (Merck, 99+%), NaHCO3 (BDH, AR), and NaOH (AR) were used as received. All water used in experiments was deionized (Millipore, MilliQ RG, resistivity18 MΩ cm). Solution (17) Laiti, E.; Persson, P.; Oehman, L.-O. Langmuir 1996, 12, 2969. (18) Persson, P.; Laiti, E.; Ohman, L.-O. J. Colloid Interface Sci. 1997, 190, 341. (19) Persson, P.; Karlsson, M.; Ohman, L.-O. Geochim. Cosmochim. Acta 1998, 62, 3657. (20) Persson, P.; Nordin, J.; Rosenqvist, J.; Lovgren, L.; Ohman, L.-O.; Sjoberg, S. J. Colloid Interface Sci. 1998, 206, 252. (21) Connor, P. A.; Dobson, K. D.; McQuillan, A. J. Langmuir 1995, 11, 4193. (22) Connor, P. A.; McQuillan, A. J. Langmuir 1999, 15, 2916. (23) McQuillan, A. J. Adv. Mater. (Weinheim, Ger.) 2001, 13, 1034. (24) Wilkinson, G. a. M. Comprehensive Coordination Chemistry; Pergamon Press: Pergamon: 1987; Vol. 2, Ligands. (25) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds: Part B, 5th ed.; John Wiley and Sons: New York, 1997.
Figure 2. SEM micrograph of a typical PCB particle film. pH measurements were made with a Mettler-Toledo MP220 meter to a precision of (0.1. PCB (Disperal P2) was supplied by Sasol GmbH with a particle size of ca. 10 nm, a stated surface area of ca. 260 m2 g-1, and pHpznpc ) 9 as measured by electrophoresis. This material contains 3-4% nitric acid for dispersion purposes. Dispersions of 1 mg mL-1 were prepared in MilliQ water by sonication then pH adjusted to ca. 7 by addition of a NaOH solution. The pH adjustment was required for the formation of particle films stable in the pH 4-11 range under the flow conditions employed. The particle film was formed by placing 200 µL of the dispersion onto a clean ZnSe ATR prism then partially dried using a water pump vacuum to produce an ∼2 cm2 film. Figure 2 shows a scanning electron micrograph of a typical particle film prepared on an aluminum substrate. The film consists of densely packed aggregated particles and has a relatively uniform surface topography. Atomic force microscope studies of a similarly prepared wet particle film showed the thickness to be ∼1 µm. A Harrick FastIR single internal reflection accessory containing a 45° ZnSe prism interfaced via a rubber O-ring to a polymethyl methacrylate flow cell was used to collect spectral data. A schematic of the flow cell is shown in Figure 3. ZnSe prisms were cleaned prior to use by light polishing using 0.05 µm γ-alumina (BDH) on a wet polishing micro cloth (Buehler). The solutions were delivered to the flow cell using a Masterflex cartridge pump and Masterflex Tygon tubing at a flow rate of ca. 2 mL min-1.
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Figure 3. A schematic diagram of the experimental setup used in ATR-IR experiments.
Figure 4. Infrared spectrum of a partially dried particle film of PCB on a ZnSe ATR prism. A BioRad Digilab FTS 60 equipped with a KBr beam splitter, DTGS detector, and WinIR 4.0 software or a Digilab FTS 4000 equipped with a CsI beam splitter, DTGS detector, and Win IR Pro software were used to collect and analyze spectra. ATR-IR spectra were obtained from 64 scans at 4 cm-1 resolution and not corrected for frequency dependence. Spectra are shown as recorded and were not further modified by subtraction or baseline correction. Reported spectra of adsorbed species were generally recorded after the system had reached equilibrium under constant solution conditions. Nitrogen gas (BOC) contained not more than 25 ppm H2O. A N2 gas flow of variable humidity was prepared at 25 °C by metered mixing of a dry N2 flow (silica gel) and a water-saturated N2 flow (Dreschler bottle). The humidity of the N2 flow was measured using a Vaisala HMP45A humidity probe. Variable pressures of acetylacetone vapor in N2 were similarly prepared. CO2 was removed from the initial N2 feed using NaOH pellets.
Results and Discussion IR Spectrum of PCB Particle Film. The infrared spectrum of PCB has been well studied26,27 and is characterized by prominent OH stretching and bending modes associated with the interlayer hydrogen bonds of the structure. Figure 4 shows the infrared spectrum of a PCB particle film prepared as described earlier in this paper. The asymmetric and symmetric stretches of the interlayer OH groups are seen at 3294 and 3094 cm-1, respectively, along with the corresponding bending modes at 1153 and 1067 cm-1 and a band at 732 cm-1 from the coupled in-plane OH deformation.26 Residual water is evident in the broad band underlying the interlayer OH stretch modes between 3700 and 3000 cm-1 and the associated bending mode at 1639 cm-1. The librational modes of water also contribute to a progressively increasing absorption below ∼1000 cm-1. A minor peak is also seen at ∼900 cm-1 which has been attributed to the OH (26) Kiss, A. B.; Keresztury, G.; Farkas, L. Spectrochim. Acta, Part A 1980, 36, 653. (27) Stegmann, M. C.; Vivien, D.; Mazieres, C. Spectrochim. Acta, Part A 1973, 29, 1653.
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Figure 5. Infrared difference spectra arising from desorption and readsorption of carbonate on a PCB film. Spectra of a PCB film after exposure to 1 × 10-4 mol L-1 NaOH for (a) 10 and (b) 30 min from a background spectrum of a PCB film in water. Spectrum of adsorbed carbonate from film exposure to 1 × 10-4 mol L-1 NaHCO3 (pH 7) for (c) 10 and (d) 30 min from a background spectrum of a PCB film with adsorbed carbonate removed (b).
deformation of surface hydroxyl groups.9,10 Minor absorptions of the boehmite films are evident at ∼1490 and 1389 cm-1 due to small amounts of carbonate and nitrate, respectively. Removal of Adsorbed Carbonate by Alkaline Washing. Immersion of the vacuum-dried boehmite films in water resulted in the rapid loss of the small amount of nitrate present. When the pH of the solution was increased to 10, adsorbed carbonate was progressively removed, as shown in Figure 5, spectra i and ii. The adsorption of carbonate onto metal oxides has been frequently studied.9,28-32 There are a variety of different carbonate adsorption modes, which are influenced by the metal oxide and experimental conditions. Negative peaks appear at 1501 and 1388 cm-1 due to the asymmetric and symmetric O-C-O stretching modes of adsorbed carbonate. Such bands have been attributed to carbonate bound in a monodentate fashion to the surface of PCB.9 Under the conditions employed in this work, this was the only species observed. To obtain a “clean surface” for further adsorption experiments, a washing procedure was adopted that involved flowing of a 1 × 10-4 mol L-1 NaOH solution over the film for a period of 45 min. This procedure removes almost all the surface bound carbonate resulting from contact with the atmosphere during formation of the film. Confirmation of the origin of these bands and the reversible nature of the adsorption is shown in Figure 5 spectra iii and iv which result from the exposure of the washed film to 1 × 10-3 mol L-1 NaHCO3 solution adjusted to pH 7. The peak at 1640 cm-1 is due to the H2O bending mode. The 1200-900 cm-1 region contains features that arise from at least two different sources: (a) Monodentate carbonate has a characteristic weak C-O stretching absorption at 1080-1040 cm-1. (b) There is also a contribution arising from changes in the interlayer OH bending mode between 1100 and 1050 cm-1 that is not yet well understood and is being further investigated. All films used in subsequent experiments have been subjected to washing with 1 × 10-4 mol L-1 NaOH for 30 min followed by washing with deionized water. Adsorption of Acetylacetone to PCB. β-diketones exist as two different tautomers. They are able to (28) Villalobos, M.; Leckie, J. O. J. Colloid Interface Sci. 2001, 235, 15. (29) Van Geen, A.; Robertson, A. P.; Leckie, J. O. Geochim. Cosmochim. Acta 1994, 58, 2073. (30) Su, C.; Suarez, D. L. Clays Clay Miner. 1997, 45, 814. (31) Russell, J. D.; Paterson, E.; Fraser, A. R.; Farmer, V. C. J. Chem. Soc., Faraday Trans. 1 1975, 71, 1623. (32) Wijnja, H.; Schulthess, C. P. Soil Sci. Soc. Am. J. 2001, 65, 324.
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Figure 6. Diagram of acac forming an enol-type complex with a metal
Figure 8. Infrared spectra of acac adsorbed onto a PCB film from a 1 × 10-4 mol L-1 acac solution at pH 7. (i) 10 min and (ii) 60 min. The background spectrum is from an alkalinewashed film under MilliQ water. Table 1. Wavenumbers and Assignments of the Principal Infrared Absorption Bands of Acetylacetone Adsorbed to a PCB Particle Film in the Al(acac)3 Complex and Deprotonated in Aqueous Solution
Figure 7. Infrared spectra of (a) 1 mol L-1 acac in a 1 mol L-1 NaOH and (b) 1 mol L-1 aqueous acac solution. Background spectra are from MilliQ water.
coordinate to metal atoms through the two oxygen atoms (enol form) or through the γ carbon atom (keto form). The two types are easily distinguished from IR spectra due to the delocalization of the double and single bonds in the enol form (Figure 6),33 while the keto form retains distinct single and double bonds. Figure 7 shows the aqueous solution IR spectra of the acetylacetonate ion (spectrum i) and acetylacetone (spectrum ii). The acetylacetonate ion is used as a model for the enol form of acac which is involved in coordinative adsorption. The large difference in the IR spectra between the enol and keto forms of acac allows coordinative adsorption to be readily recognized. In solution at pH 7, the keto form is almost exclusively present, making the carbonyl peaks of the keto form at 1723 and 1697 cm-1 particularly useful in observing the presence of uncoordinated acac. Figure 8 shows the infrared spectra of acac adsorbed onto a PCB particle film at different stages during adsorption from a solution of 1 × 10-4 mol L-1 at pH 7. Large absorptions are present at 1532, 1502, and 1396 cm-1, while smaller absorptions are present at 1593 and 1299 cm-1. The spectrum also contains a feature at ∼1090 cm-1 which is analogous to the feature seen in Figure 5 upon the adsorption of carbonate to the PCB surface. The absence of bands in the ∼1700 cm-1 region indicates that the acac is binding to the PCB surface through its oxygen atoms, forming an enol-type complex. Acac typically forms six-membered ring chelates with Al3+ ions. The difference between the spectra of the acetylacetonate ion and the adsorbed acac indicates a significant alteration to the geometry of the acetylacetonate ion upon binding. Such a large change in the IR spectrum is generally consistent with the formation of an inner-sphere complex. The IR spectra of these types of inner-sphere adsorbed ligands often resemble those of their corresponding coordination (33) Bock, B.; Flatau, K.; Junge, H.; Kuhr, M.; Musso, H. Angew. Chem., Int. Ed. Engl. 1971, 10, 225.
assignments34,35 ν(CdC) νs(CsO) + νas(CsCsC) ν(CdO) ν(CdO) νas (CsCsC) + νas(CsO) ν(CdO) + δ(CsH) δd(CH3) δs(CH3) δ(CsH) ν(CdC) + ν(CsCH3) δ(CH3) Fr(CH3) ν(C-CH3)+ ν(CdO)
ν/cm-1 of acac adsorbed
ν/cm-1 of Al(acac)3
1593
1590
ν/cm-1 of acetylacetonate ion 1547
1532 1502
1545 1530 1492 1466
1425
1396
1387 1387
1362
1299
1288
938
1191 1028 935
1313 1188 999, 965
complexes,12,21 and the infrared spectum of adsorbed acac differs little from that of Al(acac)3.34 Table 1 contains the wavenumbers of the peak absorptions of acac adsorbed on PCB, Al(acac)3, and of the acetylacetonoate ion. The following points are evident upon comparison of the spectra of the adsorbed species to that of the coordination complex. The ν(CdC) mode appears in a very similar position to that of the Al(acac)3 complex at 1590 cm-1, while the two ν(CdO) modes are shifted down by ∼10 and 30 cm-1, respectively. The ν(CdO) + δ(C-H) combination band at 1466 cm-1 is not visible in the spectrum of the adsorbed acac but could be obscured by other absorptions in that area. The large absorption at 1396 cm-1 matches closely that of the δd(CH3) and δs(CH3), which both absorb at 1387 cm-1 in the Al(acac)3 complex. At lower wavenumbers, only very weak absorptions are seen for the adsorbed acac, while multiple peaks are present for the Al(acac)3 complex. The increase in the relative intensity of the 1502 cm-1 peak at higher coverages may be due to surface crowding effects. There is enough similarity in the adsorbed acac and Al(acac)3 spectra to indicate that acac is binding in a distorted six-membered ring arrangement. (34) Nakamoto, K. Infrared Spectra of Inorganic and Coordination Compounds; John Wiley and Sons: New York, London, 1963.
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Figure 9. Infrared spectra of a PCB film under N2 flow of humidity (i) 73%, (ii) 57%, (iii) 43%, (iv) 27%, and (v) 0%. The background spectrum was from a film under water saturated N2 at 25 °C.
Along with the growth of adsorbed acac peaks in Figure 8, a distinct absorption loss centered at 874 cm-1 is seen. This absorbance loss scales over time with the increase in absorbance seen for the adsorbed acac bands, indicating that the two changes are linked. The wavenumber of the 874 cm-1 band corresponds well to those assigned to the deformation of surface OH groups on PCB.9-11 The loss of absorption at 874 cm-1 upon binding of acac is attributed to the removal of surface OH groups upon exchange with an incoming acac molecule. For a bidentate chelating ligand such as acac to bind coordinatively requires access to an aluminum ion with two or more exchangeable OH groups (η2 or η3 sites). Such aluminum ions are found on the corners and edges of crystals rather than on the planar surfaces where the OH groups are typically of the bridging variety. If bidentate chelation were to occur on µ2-OH sites, it would require the aluminum ion to raise its coordination number by one or an existing Al-O bond to be broken. It is therefore logical that the absorption loss at 874 cm-1 arises from bidentate adsorption at η2 or η3 sites. Dehydration of PCB and Influence on Hydroxyl Absorptions. Metal oxides generally have some adsorbed H2O under normal atmospheric conditions. To probe the influence of this water on the IR spectrum of PCB, a film was “cleaned” using the previously described alkaline washing procedure followed by N2-saturated water for 5 min and then the bulk water was removed with watersaturated N2 vapor. Progressive dehydration was achieved by stepwise decreases in the humidity of the N2 flowed at 10 L min-1. Figure 9 shows the changes in the IR spectrum from an initial humidity of 100%, as the film was progressively dehydrated. The most prominent features are the loss of water absorptions at 3600-3000 cm-1, 1640 cm-1, and the vibrational mode which results in an apparent baseline change below 1000 cm-1. The loss of water is accompanied by decreases in the interlayer OH group absorptions of boehmite at 3256, ∼3090, 1149, 1067, and 716 cm-1. This appears to suggest that dehydration of the film results in a breakdown of interlayer H-bonding. However, absorptions of OH groups in H-bonds undergo changes in intensity with environmental influences.36 The loss of (35) Ernstbrunner, E. E. J. Chem. Soc. A 1970, 1558. (36) Shuster, P. Z., G and Sandorfy, C The Hydrogen Bond/Theory; North-Holland: Amsterdam, New York, Oxford, 1976; Vol. 1. (37) Okada, K.; Nagashima, T.; Kameshima, Y.; Yasumori, A.; Tsukada, T. J. Colloid Interface Sci. 2002, 253, 308. (38) Melia, T. P.; Merrifield, R. J. Appl. Chem. 1969, 19, 79.
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Figure 10. Infrared spectra of a PCB film under N2 flow of humidity (i) 42%, (ii) 30%, (iii) 20%, (iv) 15%, (v) 10%, (vi) 7.5%, (vii) 5%, and (viii) 3%. The background spectrum was from a film under N2 flow of 44% humidity at 25 °C.
absorbance at 1067 cm-1 is about 10% of the original band absorbance. This observation favors an interpretation of the absorbance loss as being due to environmental influences rather than the disruption of H-bonding. The perturbation of this absorption has two possible causes: (a) The crystallite size of PCB is such that only 3-4 Aloctahedral layers are present in each crystal. It is possible that adsorbed water is able to influence the nature of H-bonding in the interlayer through the intervening bonding. (b) Hoffman and Tettenhorst,6 along with Tsukada et al.,37 have reported that interlayer H2O is present in boehmite. Their X-ray diffraction (XRD) studies of synthetic boehmite samples prepared with varying degrees of crystallinity showed an expansion of d020 with decreasing crystallinity. This increase in d spacing was also accompanied by an increase in the amount of water present. They proposed that incorporation of water distributed locally in the interlayer and coordinated to Al-OH groups leads to an increase in the average d spacing. The observed changes in the IR signal from the bulk OH groups may therefore be related to a change in the amount of interlayer water during hydration and dehydration. During dehydration of the PCB, the most-striking spectral changes occur below 40% humidity. To provide a clearer view of the changes in the 1100-600 cm-1 region, spectra in the 44-3% humidity range are shown in Figure 10. Losses at 1064 and 718 cm-1 due to interlayer OH modes are evident as noted previously. At 30% humidity a peak at 883 cm-1 emerges and increases in intensity as it shifts to lower wavenumber with further decrease in humidity. Between 10 and 7.5% humidity, the peak at 883 cm-1 is lost while a larger peak at 835 cm-1 emerges and continues to increase in intensity as the humidity is decreased to 3%. The peak at 883 and later at 835 cm-1 appears to grow at the expense of two broad absorptions at ∼990 and ∼910 cm-1. The growth of the peak at 835 cm-1 is also accompanied by the appearance of a peak at 3670 cm-1 (Figure 9). Analogous peak shifts upon dehydration of PCB have been observed previously in separate accounts by Morterra et al.9 and Johnston et al.10 and were assigned to the bending and stretching modes, respectively, of OH groups present on the surface of PCB crystals. The peak shift is caused by the removal of surface water, resulting in the liberation of the OH groups from H-bonding. When the humidity drops below 10%, complete monolayer water coverage is lost,10 resulting in non-Hbonded surface OH groups. The infrared absorptions of these surface OH groups become much sharper and shift
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Langmuir, Vol. 20, No. 26, 2004 11635 Table 2. Wavenumbers of PCB Surface Hydroxyl Deformations
Figure 11. Infrared spectra of (a) acac coordinatively adsorbed to the PCB surface from a partial pressure of (i) 10, (ii) 29, (iii) 48, and (iv) 95 Pa.38(b) acac coordinatively and physically adsorbed to the PCB surface from a partial pressure of (iv) 95, (v) 190, and (vi) 480 Pa.38 The background spectrum is of a PCB film dehydrated by dry N2.
to lower and higher wavenumber (bending and stretching modes, respectively) upon removal of H-bonding. In light of the earlier assignment of the 874 cm-1 peak in Figure 8 to the OH deformation of η2 and or η3 groups, it would be expected that the same absorption loss would be evident in Figure 9 upon removal of H-bonding from these groups. However, there is a large intensity difference between the peak at 874 cm-1 in Figure 8 and the resultant 835 cm-1 peak in Figure 9. The large intensity difference between these two peaks can be explained by the relative proportions of surface η-OH and µ-OH groups on a PCB crystal. We believe that the growth of stronger peaks at 883 and later at 835 cm-1 will strongly influence the apparent peak wavenumber in the underlying broad absorption loss observed at ∼910 cm-1, obscuring any associated absorption loss present at lower wavenumbers. It is therefore likely that the center of the ∼910 cm-1 band is at somewhat lower wavenumbers, and any absorption arising from a dehydration-induced downshift of η-OH absorptions would be obscured by the much stronger signal from the 835 cm-1 band. Adsorption of Gaseous Acetylacetone to Dehydrated PCB. To investigate the adsorption of acac onto a dehydrated PCB film, a similar experimental arrangement to that used for the dehydration studies was employed. A film was prepared and washed as previously described followed by dehydration using a 0% humidity N2 flow. The dehydrated film was exposed to varying partial pressures of acac at 25 °C. Figure 11a shows the adsorption of acac onto the PCB surface at increasing partial pressures. The spectrum is dominated by large absorptions at 1603, 1532, 1464, and 1404 cm-1, while
wavenumber/cm-1
assignment of surface OH deformations
∼910 ∼874 ∼835 ∼795
µ2-OH in condensed medium η OH groups in condensed medium free µ2-OH free η OH group
smaller absorptions are also present at 1066 and 1029 cm-1. As the partial pressure is increased (Figure 11a) the absorption of existing bands increases along with the emergence of peaks at 1292, 1262, 1242, 930, 907, and 726 cm-1. The spectrum is fairly consistent with that seen for the acac coordinatively adsorbed from aqueous solution apart from minor shifts in some bands and the emergence of several weaker absorptions. At higher partial pressures of acac (Figure 11b, spectra v and vi), the appearance of physically adsorbed acac is signaled by the emergence of peaks at 1724 and 1701 cm-1, which match those of the CdO stretch of the keto tautomer of acac. The spectrum becomes complex due to the contributions from the different forms of acac present on the surface. Figure 11a contains a broad loss in absorption centered at 795 cm-1 upon coordinative adsorption of acac. The magnitude of this absorption loss correlates with the growth in coordinative adsorption of acac and suggests that the two processes are linked. Once the presence of adsorbed molecular acac becomes apparent (Figure 11b, spectra v and vi), there is a corresponding loss in absorbance at 833 cm-1. This is likely to be due to the broadening of the signal from surface hydroxyls upon H-bonding with molecular acac present on the surface. Thus, it appears that initial adsorption of acac results in the formation of a bidentate surface complex and a loss of absorbance at 795 cm-1, while the subsequent physisorption of acac leads to a loss at 833 cm-1. The two different types of adsorption appear to affect different surface OH groups. Lewis and Farmer11 assigned three peaks at 3695, 3665, and 3570 cm-1 observed in IR spectra of boehmite under reduced pressure as belonging to different surface hydroxyls. The peaks at 3665 and 3570 cm-1 were assigned to hydroxyls on the (010) and (100) faces, respectively, and were found to be unaffected by treatment with H3PO4. The peak at 3695 cm-1 was suggested as arising from hydroxyls at crystal ends due to its disappearance upon treatment with H3PO4. Hydroxyl deformation modes shift to lower wavenumber when the corresponding OH stretching modes shift to higher wavenumber. The behavior of the 3695 cm-1 peak therefore correlates with the behavior of the 795 cm-1 band seen in Figure 11a. We assign the 795 cm-1 band to η2 and η3 OH groups found at edges and corners of PCB crystals. Peak wavenumbers and assignments for the different types of surface hydroxyls are given in Table 2. Summary 1. The use of immobilized particle films in conjunction with ATR-IR spectroscopic methods provides a versatile means of studying adsorption processes at solid/aqueous and solid/gas interfaces. CO2 readily adsorbs to PCB but can be removed by alkaline solution washing. It appears that changes in hydration and/or in ligand adsorption alter the environment of interlayer OH groups in the PCB crystal, although further work is required to clarify this process. 2. The large difference in the IR spectra of acac adsorbed from aqueous solution and the acetylacetonate ion indi-
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cates that acac forms an inner-sphere surface complex with boehmite surface Al3+ ions. Comparison of the IR spectra of adsorbed acac with that of the Al(acac)3 complex suggests that a distorted six-membered chelate ring is formed. 3. The removal of water from a PCB surface results in µ2-OH groups having IR absorptions at ∼910 cm-1 being liberated from H-bonding, resulting in a reduction of the IR absorption bandwidth and a shift to a lower-wavenumber peak at 835 cm-1. 4. The coordinative adsorption of acac to a PCB film in aqueous solution results in a concomitant loss of an IR absorption at 874 cm-1. Similarly, the initial coordinative adsorption of gaseous acac to a dehydrated PCB film results in an IR absorption loss at 795 cm-1. Subsequent
Dickie and McQuillan
physisorption by acac results in a much larger absorption loss at 833 cm-1. The IR absorption losses due to coordinative adsorption at 874 and 795 cm-1 are assigned to the removal of η2-OH and/or η3-OH groups in the presence and absence, respectively, of a condensed phase. The absorptions of η2-OH and/or η3-OH groups occur at ∼30 cm-1 lower wavenumbers than surface µ2-OH groups in both the presence and absence of a condensed phase. Acknowledgment. We thank the Foundation for Research Science and Technology along with Resene Paints Ltd for funding, Colin Gooch of Resene Paints for advice, and Sasol GmbH for the supply of the PCB sample. LA048423S
