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Langmuir 1998, 14, 352-362
Photoinduced Reactions in Porous Systems: Reactions at the Solid-Liquid Interface1 Robert J. Kavanagh and J. K. Thomas* Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556 Received June 5, 1997. In Final Form: November 5, 1997 Transient absorption spectroscopy is used to observe photoinduced reactions at a solid SiO2-liquid interface. Several different types of conditions are arranged for the reactants: (1) where both reactants are in the liquid contained in the SiO2 pore, (2) where one reactant is adsorbed to the SiO2 surface, with the other in the liquid, and (3) where both reactants are at the interface and where the surface captures one of the products of the photoreaction in the liquid. Pore size in the nanometer range plays a major role in the outcome of the photochemistry. Studies in category 1 show that the rate constants decrease with decreasing pore size. In category 2 the rate of approach of the liquid-borne reactants to the surface is efficient and can be explained by simple diffusion theory. On the contrary, the rate of capture by the surface of cations produced in the liquid phase is significantly less efficient than that of neutral species studied in category 2. Locating both reactants at the SiO2 surface can lead to efficient reactions, but rapid back-reaction of the products lead to low yields of products compared to the bulk liquid phase. The results of the unique conditions imposed by a surface on conventional reactions are discussed in terms of what is established in bulk solution.
Introduction 2
Photoinduced reactions of molecules on silica and alumina surfaces2fg,3 have been the focus of numerous studies. A general finding is that the kinetics of the reactions are more complex than those in bulk solvent. This is not unexpected due to the heterogeneous nature of the surfaces, and possibly due to fractal effects.2jk,4 For the most part, the observed chemistry can be described by reasonable models involving expected properties of the oxide surfaces. The photochemistry in these systems occurred at solid-vacuum or solid-gas interfaces. Less has been published on the photochemistry at the solidliquid interface. This situation is conveniently achieved by filling the pores of the solid silica material with solvents. Several conditions may then prevail. Condition 1 is where both reactants are in the solvent phase condition 2 is where one reactant is in the solvent phase and the other is at the liquid-solid interface, and condition 3 is where both * Author to whom correspondence should be addressed. (1) The authors wish to thank the NSF for support of this work. We also give our special thanks to Dr. Guohong Zhang for much help in this work. (2) (a) Turro, N. J. Modern Molecular Photochemistry; Benjamin: New York, 1978; p 36. (b) Liu, Y. S.; Ware, W. R. J. Phys. Chem. 1993, 97, 5980-5986. (c) Liu, Y. S.; de Mayo, P.; Ware, W. R. J. Phys. Chem. 1993, 97, 5987-5994. (d) Liu, Y. S.; de Mayo, P.; Ware, W. R. J. Phys. Chem. 1993, 97, 5995-6001. (e) Bauer, R. K.; de Mayo, P.; Natarajan, L. V.; Ware, W. R. Can. J. Chem. 1984, 62, 1279-1286. (f) Wilkinson, F.; Kessler, R. W. J. Chem. Soc., Faraday Trans. 1 1981, 77, 309. (g) Oelkrug, D.; Honnen, W.; Wilkinson, F.; Willsher, C. J. J. Chem. Soc., Faraday Trans. 1 1987, 83, 2081-2095. (h) Krasnansky, R.; Koike, K.; Thomas, J. K. J. Phys. Chem. 1990, 94, 4520-4527. (i) Liu, X.; Mao, Y.; Ruetten, S. A.; Thomas, J. K. Sol. Energy Mater. Sol. Cells 1995, 38, 199. (j) Albery, W. J.; Bartlett, P. N.; Wilde, C. P.; Dament, J. R. J. Am. Chem. Soc. 1985, 107, 1854. (k) Leheny, A. R.; Turro, N. J.; Drake, J. M. J. Phys.Chem. 1992, 96, 8498. (l) Leheny, A. R.; Turro, N. J.; Drake, J. M. J. Chem. Phys. 1992, 97 (5), 3736. (3) (a) Mao, Y.; Thomas, J. K. Langmuir 1992, 8, 2501; J. Chem. Soc., Faraday Trans. 1992, 88, 3079. (b) Pankasem, S.; Thomas, J. K. J. Phys. Chem. 1991, 95, 7385. (c) Pankasem, S.; Thomas, J. K. J. Phys. Chem. 1991, 95, 6990 (d) Pankasem, S.; Thomas, J. K. Langmuir 1992, 8, 501. (4) (a) Samuel, J.; Ottolenghi, M.; Avnir, D. J. Phys. Chem. 1991, 95, 1890 and references cited therein. (b) Klafter, J.; Drake, J. M.; Levitz, P.; Blumen, A.; Zumofen, G. J. Lumin. 1990, 45, 34.
reactants are at the liquid-solid interface. A restriction is imposed on the reactions due to the nanoscale dimensions of the pores which constrict the solvents in pools or threads. The purpose of this study is to investigate photoinduced reactions in the inner spaces of porous solids where the nanometer spaces are filled by solvents. The studies are carried out in pores that vary in diameter from 40 to 150 Å. It is first pertinent to consider the nature of the solvent close to the oxide wall and to estimate how much solvent is localized at the wall. A study of adsorption properties carried out by Unger5 clarifies this somewhat, where the energetics of adsorption of different molecules to different silica gel surfaces were studied. In this study, the strongest adsorption was found for surfaces where a hydrogen-bonding type interaction was present, such as between methanol and the native hydroxylated surface or between methanol and a derivatized surface with a hydroxyl group. The adsorption of benzene is weaker in all cases and weakest of all when benzene is adsorbed to a derivatized surface, where the silanols are replaced by alkyl groups. Surface adsorption of solvents by silica gel is discussed extensively in the literature.6 These studies demonstrate that the properties of the solvent on the surface adsorbed layer are significantly changed compared to the bulk properties. Early studies by Dalton and Iler6a,7 indicated that a monolayer of water molecules is immobilized at the silica surface through hydrogen bonding to the surface silanol groups. Studies by Peschel and Aldfinger,6a,8 measuring the change in the viscosity of water between (5) (a) Unger, K. Agnew. Chem., Int. Ed. Engl. 1972, 11, 267-278. (b) Porous Silica, its properties and use as a support in column liquid chromatography; Unger, K. K., Ed.; J. Chromatography, 16; Elsevier Scientific Publishing Company: Dordrecht, 1979. (6) (a) Iler, R. K. The Chemistry of Silica, Solubility, Polymerization, Colloid and Surface Properties and Biochemistry; John Wiley & Sons: New York, 1979. (b) Zhuravlev, L. T. Colloids Surf., A: Physicochem. Eng. Aspects 1993, 74, 71. (7) Dalton, R. L.; Iller, R. K. J. Phys. Chem. 1956, 60, 955. (8) Peschel, G.; Aldfinger, K. H. J. Colloid Interface Sci. 1970, 34, 505.
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Photoinduced Reactions in Porous Systems
spherical and flat fused silica surfaces, detected changes even when the surfaces were held up to 160 nm apart. This suggests multilayer adsorption. These effects are not limited to water as solvent. Recently studies have been carried out on the orientational dynamics of supercooled liquids in restricted geometries by Awschalom and Warnock.9 A sub-picosecond timeresolved birefringence technique was applied to both polar and nonpolar liquids confined in porous sol-gel glasses. The range of pore sizes probed was 10-187 Å. Distinctly different responses were found for solvents in the presence of porous systems. By studying different confining geometries, the surface response was found to become more pronounced with decreasing pore radius, which in turn is due to the surface to total volume ratio becoming larger. The thickness of the surface layer was estimated from their calculations to be approximately 12 Å and was found to be independent of pore size. The viscosity of this surface layer was estimated to be three times greater than the bulk solvent viscosity. By modifying the surface and using more weakly adsorbing species, it was confirmed that this behavior was due to adsorption. It was concluded that a polar liquid interacting strongly with a substrate forms a viscous surface boundary layer with a high effective viscosity. This is related to other studies where decreasing pore size has been shown to markedly reduce the freezing point of solvents held within the porous network.6a,10,11 This is due to the high curvature experienced by the small solvent droplets in the porous solid. With the above concepts in mind, this study examines reactions at the solid-liquid interface and the differences of nanometer dimensions on reactions induced in the small solvent pools trapped in the porous structure. Experimental Section Chemicals. Silica gels (Davisil grade 634, 100-200 mesh, 60 Å, surface area 480 m2/g, pore volume 0.75 cm3/g; Davisil grade 644, 100-200 mesh, 150 Å, surface area 300 m2/g, pore volume 1.15 cm3/g, both 99+% pure; Merck grade 10181, 35-70 mesh, 40 Å, surface area 675 m2/g, pore volume 0.68 cm3/g; Merck grade 10184, 70-230 mesh, 100 Å, surface area 300 m2/g) were obtained from Aldrich and were used after washing with distilled and deionized water. Cyclohexane, benzene, and acetonitrile were 99+% anhydrous, spectrophotometric grade received from Aldrich; methanol was Mallinckrodt Spectro-grade. All solvents were further purified and dried using molecular sieve 3A pellets obtained from Aldrich. Pyrene was purified by liquid chromatography (silica gel as adsorbent, cyclohexane as eluant), followed by recrystallization from cyclohexane. 1-Pyrenebutyric acid (PBA) was recrystallized from a 50:50 methanol-benzene solution. All were Aldrich products. Nitromethane, 1-nitropropionic acid, and phthalic anhydride were obtained from Aldrich and were used as received; all were 99+% pure. Nitrogen (prepurified) and oxygen were obtained from Mittler; all were used as received. Sample Preparation. Silica gel was dried overnight (6-10 h) at 150 °C to remove physisorbed water. The dried silica gel was cooled in a dessicator and then immediately transferred to vials containing dried solvent required for the study in question (9) (a) Shafer, M. W.; Awschalom, D. D.; Warnock, J. J. Appl. Phys. 1987, 61, 5438. (b) Awschalom, D. D.; Warnock, J. Orientational Dynamics of Supercooled Liquids in Restricted Geometries. In Molecular Dynamics in Restricted Geometries; Klafter, J. Drake, J. M., Eds.; Wiley Interscience: New York, 1989. (10) (a) Jackson, C. L.; McKenna, G. B. J. Chem. Phys. 1990, 93, 9002 and references cited therein. (b) Brun, M.; Eyraud, P.; Eyraud, L.; Richard, M.; Eyraud, C. C. R. Acad. Sci. Paris 272 (8 mars 1971). (c) Eyraud, C.; Brun, M.; Eyraud, L.; Lallemand, A.; Eyraud, P. C. R. Acad. Sci. Paris 273 (11 octobre 1971). (d) Jackson, C. L.; McKenna, G. B. J. Non-Cryst. Solids 1991, 131, 224. (e) Patrick, W. A.; Kemper, W. A. J. Phys. Chem. 1938, 42, 369. (f) Hodgson, C.; McIntosh, R. Can. J. Chem. 1960, 38, 958. (11) Kavanagh, R. J.; Thomas, J. K. Unpublished work.
Langmuir, Vol. 14, No. 2, 1998 353 (or used to load the adsorbates). Pyrene and any required reactants were then added with the correct dilution for the reaction. Samples were then allowed to equilibrate for 4-5 h. The samples were transferred to 2-mm quartz-walled cuvettes for analysis. N2 saturation, if required, was carried out in the cuvettes using prepurified N2 immediately prior to analysis. The extent of adsorption of the probes to the surface compared to the solvent was measured by analyzing the probe content in the supernatant. A comparison of this value to that in the original solvent then gives the amount of probe in each phase. Pyrene was covalently bound to the surface in the manner given below.20 The SiO2 sample was heated at 150 °C with 1-pyrenyldiazomethane for 1 h. The product was washed several times with methanol to remove any unbound material. Instrumentation. Steady-state absorption studies were carried out on a Varian Cary 3 UV-vis spectrophotometer. The Varian Cary 3 is a double-beam double-dispersion UV-vis spectrophotometer centrally controlled by a IBM PC-AT compatible computer. It has a wavelength accuracy of (0.2 nm and a wavelength repeatability of (0.04 nm. Lamp change occurs automatically at a user selectable wavelength, usually 290-315 nm in these studies (instrument default 310 nm). Photometric accuracy is (0.003 at an absorbance of 1.000. Steady-state fluorescence studies were carried out using a SLM/Aminco SPF-500C spectrofluorimeter which has as excitation source a Xenon arc lamp operating at 250 W; the Nozone coating on this lamp limits the useful range of this instrument from 300 to 900 nm. The sample detector uses a Hamamatsu R928P photomultiplier tube; a 1200 grooves/mm ruled diffraction grating is used for the excitation monochromator, while a 1200 grooves/mm holographic grating is used for the emission monochromator. Wavelength resolution is (0.2 nm, with minimum band pass 0.1 nm; data is stored by an on-board computer and transferred to a IBM PC-AT compatible computer for further processing. The sample holder was modified for detection of fluorescence from adsorbate semitransparent solid-liquid samples; this holder allows incident light to reach the sample at a ∼45° angle to the sample back surface, with the emitted light being detected at 90° to the incident light. The configuration of instruments required for time-resolved fluorescence measurements is well discussed in the literature12 and is only summarized here. Monochromatic laser light is used to excite the sample at 90° to the detection optics; a portion of the excitation light is reflected to a photodiode which triggers data acquisition. The emitted light is collected by a series of lenses, passed through a suitable cutoff filter into a Bausch and Lomb diffraction grating monochromator (1350 grooves/mm), and detected by a Hamamatsu R1664U microchannel plate photomultiplier. The signal was amplified with a 7A29 amplifier in a Tektronix 7912AD programmable digitizer with a 7B10 time base. The digitized signal was then transferred to and stored on an IBM PC-AT compatible computer. Lasers available for excitation light sources were a Photochemical Research Associates (PRA) Nitromite LN100 laser and a PRA LN1000 nitrogen laser. The principal wavelength of both is 337.1 nm; pulses of 70 µJ, 300 ps fwhm and 2.5 mJ, 0.8 ns fwhm are provided, respectively. The instrument configuration for transient absorption measurements has also been described,12 and only a short summary follows. For these studies, sample excitation was achieved using a Laser Photonics nitrogen laser which yields a 5-mJ, 10-ns 337.1nm laser pulse. Analysis light is obtained using a Oriel 450W Xe lamp. The lamp is powered by a PRA model 302 power supply which provides steady power with fluctuation
