Langmuir 199410, 3587-3597
3587
In Situ Fourier Transform Infrared Spectroscopic Evidence for the Formation of Several Different Surface Complexes of Oxalate on Ti02 in the Aqueous Phase Stephan J. Hug* and Barbara Sulzberger Swiss Federal Institute for Environmental Science and Technology, CH-8600DuebendorL Switzerland Received December 13, 1993. I n Final Form: June 25, 1994@ With a modified attenuated total reflection infrared absorption (ATR-IR)method, vibrational spectra and adsorption isotherms of sulfate, acetate, and oxalate on Ti02 (Degussa P-25, mostly anatase) were measured. For oxalate, the dependence of the spectral shape on the surface coverage is consistent with the formation of several surface complexes. The method uses a horizontal attenuated total reflection (HATR)element coated with a layer ofhighly dispersed solid material and allows quantitative in situ FTIR measurements of adsorption as a function of solution parameters (adsorbate concentration, pH, ionic strength, etc.). Sets of FTIR spectra obtained under varying conditions are analyzed using singular value decomposition (SVD)and global analysis with acid-base and various adsorption isotherm expressions. This allows for a separation of spectral contributions from dissolved and adsorbed species and of species with distinct structures andlor different adsorption equilibrium constants. The vibrational spectra of adsorbed oxalate suggest that several forms of inner spherically bound surface complexes are formed at lower pH values. This can be explained by the formation ofprotonatedand unprotonated surface complexes and by the heterogeneity ofthe Ti02 used (Degussa P-25 anatase contains ca. 15-30% rutile). We compare the spectra of surface-bound oxalate species with those in aqueous solutions and discuss possible structural assignments.
Introduction Mineral surfaces can store and release solutes by adsorptioddesorption processes and can catalyze chemical and photochemical reactions that do not readily occur in the aqueous phase. These processes are of great importance in environmental and in many technical processes such as catalysis3 and photochemical detoxification of wastewater^.^^^ For a detailed understanding of the reactions in these heterogeneous systems, it is important to investigate the interaction of adsorbates with the surface at a molecular level. It is of special interest whether an adsorbate is adsorbed by specific chemical interaction, e.g. is coordinated inner spherically with surface sites (chemisorbed), or whether it is less specifically adsorbed by electrostatic or hydrophobicinteractions with the surface (physisorbed). Inner-sphere coordination has a large effect on the chemical reactivity of the adsorbate and makes thermal and photochemically induced charge transfer between surface and adsorbate possible. Reaction rates and pathways are expected to be strongly dependent on the specific molecular structure and the nature ofthe chemical bond between surface and adsorbate. For example, on semiconductor particles like TiOz,inner-sphere complexes can act as direct electron donors for photochemically produced valence band holes, while physisorbed species are oxidized in secondary reactions with surface generated hydroxyl radicals. The two pathways occur with different rates and lead to different products.6 Oxalate has been Abstract published in Advance ACS Abstracts, September 1, 1994. (1)Stumm, W.; Morgan, J. J. Aquatic Chemistry, 2nd ed.; Wiley & Sons: New York, 1981. (2) Stumm, W. Chemistry of the Solid-Water Interface; Wiley & Sons: New York, 1992. (3)Schiavello, M. Photocatalysis andEnvironment;Kluwer Academic Publishers: Dordrecht, 1988. (4) Bahnemann, D.; Cunningham, J.; Fox,M. A.; Pelizetti, E.; Pichat, N.; Serpone, N. Aquatic and Surface Photochemistry; Helz, G. R., Zepp, R. G., Crosby, D. G., Eds.; Lewis Publishers: Boca Raton, FL, 1994; pp 261. (5)Turchi, Craig S.;Ollis, David S. J. Catal. 1990,122,178. @
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shown to significantly enhance photochemical hydrogen peroxide production by iron particles7 and to act as a photoreductant in the dissolution of hematites (both important processes in cloud and fog droplets). On microcrystalline mineral particles, more than one surface structure might contribute to the observed overall adsorption and chemical reactivity on the surfaces of one material. A detailed structural understanding of surface adsorption is a necessary first step for the modeling and understanding of surface reactions. While surface analytical methods employing vacuum techniques, such as Auger and X-ray photoelectron spectroscopy (AES and XPS),S are able to provide information about structure and electronic properties of certain adsorbates on clean single crystal mineral surfaces, they are not applicable for the in situ study of adsorption on small particles in liquid phases. Methods that can be applied in air or aqueous environments are scanning tunneling and atomic force microscopy(STM and AFM),1° extended X-ray absorption fine structure (EXAFS),ll electron spin resonance (ESR),lz ultraviolet and visible absorption (UV-vis), and I m a m a n . STWAFM is well suited for the characterization of atomic surface structure of flat samples. Adsorbates can be studied only when surface diffusion is slow on the time scale of the measure(6) Richard, C. J . Photochem. Photobiol., A 1993,72,179. (7) Pehkonen, S. D.; Siefert, R.; Erel, Y.; Webb, S.; Hoffmann, M. R. Environ. Sci. Technol. 1993,27,2056. (8)Siffert, C.; Sulzberger, B. Langmuir 1991,7,1625. (9) Hochella, M. F., Jr. In Spectroscopic Methods in Mineralogy and Geology; Hawthorne, Frank C., Ed.; Reviews in Mineralogy, Book Crafters: Chelsea, MI, 1988; Vol. 18, p 573. (10) (a)Hansma, P. K.; Tersoff, J. J.Appl. Phys. 1987,61,R1. (b) Eggleston, C. M.; Hug, S. J. In Physikalisch-Chemische Untersuchungsmethoden in den Geowissenschaften;Pavicevic, M., Amthauer; G., Eds.; Springer Verlag: Berlin, in press. (11)Brown, G. E., Jr.; Calas, G.; Waychunas, G. A.; Petiau, J . In Spectroscopic Methods in Mineralogy and Geology; Hawthorne, Frank C., Ed.; Reviews in Mineralogy, Book Crafters: Chelsea, MI, 1988;Vol. 18, p 431. (12) Calas, G.In Spectroscopic Methods in Mineralogy and Geology; Hawthorne, Frank C., Ed.; Reviews in Mineralogy, Book Crafters: Chelsea, MI, 1988; Vol. 18, p 513.
0 1994 American Chemical Society
Hug and Sulzberger
3588 Langmuir, Vol. 10, No. 10, 1994 ment.13 EXAFS requires surface adsorbates with atoms that exhibit significant X-ray absorption cross sections and is not well suited for adsorbates composed of carbon and hydrogen atoms. EPR is restricted to radicals and paramagnetic species. W-vis absorption and particularly fluorescence are sensitive methods if a surface complex exhibits characteristic absorption and/or fluorescence p1-0perties.l~Many surface complexes,however, are nonfluorescent and absorb weakly in the W - v i s region, with the absorption of the surface species masked by the absorption of the bulk. Vibrational transitions of organic adsorbates as measured in IR and Raman spectra occur a t different energies than bulk transitions and are sensitive to the molecular structure, which makes IR and Raman spectroscopic methods applicable for a wide range of organic adsorbates. A problem in vibrational spectroscopy of aqueous systems is the strong IR absorption by water. Attenuated total reflection (ATR) elements provide conveniently short path lengths that make subtraction of the aqueous background absorption possib1e.l5-l7 However, for various reasons, a reliable subtraction of this background absorption, which is normally by afactor 100-1000 larger than the absorptions of interest, is often problematic in aqueous suspensions of solid particles. In the background section we discuss these problems in more detail and show how they can be overcome by employing coated horizontal ATR elements. In conjunction with advanced component analysis and global fitting of parametrized sets of spectra a t all wavelengths, overlapping spectral contributionscan be separated. Af'ter the Experimental Section, we present results obtained for adsorption of sulfate, acetate, and oxalate on Ti02 powder. The formation of several surface complexes formed by adsorption of oxalate on anatase will be discussed in terms of the anatase surface properties and the possible structures of oxalate surface complexes.
Background Attenuated total reflection (ATR)-IRspectroscopy has become the method of choice for IR work in aqueous systems, as it easily provides the short path lengths needed in a strongly absorbing medium. Studying adsorption directly to the surface of the ATR element is possible,l8 but the choice of materials with the required optical properties is too limited for environmentally relevant studies. Aqueous suspensions of highly dispersed minerals have been successfully used to obtain qualitative and semiquantitative IR spectra of surface ad50rbates.l~ However, there are several factors which complicate quantitative measurements in suspensions with changing solution parameters: (1)The surface adsorbate concentration in suspensions is small compared to the concentration of HzO, so that the subtraction of the large HzO absorptions makes the measurements extremely sensitive to changes in the concentration and dispersitivity of the particles.20 (2) The dispersitivity of the particles can (13) Eggleston, C.M.; Stumm, W.Geochim. Cosmochim.Acta 1993, 57,4843. (14) Hering, J. G.;Stumm, W.Langmuir 1991,7 , 1567. (15) Harrick, N. J.Internal Reflection Spectroscopy; Wiley & Sons: New York, 1967. (16) Harrick, N. J. Internal Reflection Spectroscopy-Review and Supplement; Harrick Scientific Co.: Ossining: NY, 1985. (17) Mirabella, F.M. Jr. Appl. Spectrosc. Rev. 1986,21,45. (18) Sperline, R.P.;Muralidharan, S.; Freiser, H. Langmuir 1987, 3, 198. (19) (a) Tejedor-Tejedor, M. I . ; Anderson, M. A. Langmuir 1986,2, 203. (b) Tejedor-Tejedor, M. I . ; Anderson, M. A. Langmuir 1990, 6, 602. (c) Tejedor-Tejedor, M. I.; Yost, E. C.; Anderson, M. A. Langmuir 1990, 6,979. (d) Tunesi, S.;Anderson, M.A. Langmuir 1992,8, 487.
change drasticallywith pH and surface coverage. Particles are known to coagulate and to flocculate when surface charge is neutralized by adsorption of oppositely charged molecules.2 (3) The surface charge of the ATR element and of the suspended mineral particles is dependent on pH and on the surface coverage with adsorbate. This affects the concentrations ofthe particles in the proximity of the ATR element by electrostatic interactions. The problem was discussed in detail by Tickanen et aLZ0 In order to avoid the problems of changing dispersitivity and to increase the surface probed by the evanescent light, it is advantageous to coat the ATR element with a stable layer ofhighly dispersed material. In this way, the probed surface area in the immediate vicinity ofthe ATR element is optimized and the particles are immobilized, thus preventing a change in the dispersitivity. We found that such layers of colloidal T i 0 2 (Degussa P-25, particle size 20-30 nm) on ZnSe could be prepared by drying aqueous suspensions on the surface of the ATR element and by removing excess not-well-adheringparticles with a gentle stream of HzO. Thin layers (1-3 pm thick) adhered sufficiently well to the ZnSe surface to allow a flow of aqueous solution over the particulate layer in which solute concentration and/or pH could be varied in a continuous manner. We will show that varying pH and concentration did not change the dispersitivity of the deposited particulate layer, so that the IR absorption of the surface adsorbates could be quantitatively studied as a function of solution parameters. The coating ofATR elements with stable layers of model biomembrane films has been described before.21 Coating of ATR elements with layers of mineral particles for the systematic study of surfaceaqueous phase equilibria (in situ) and our quantitative spectral analysis applied to these systems are, to our knowledge, new. Ideally, disperse coatings should be of similar thickness as the penetration depth of the probing light. This optimizes the signal while keeping the layer thin, allowing for a rapid exchangebetween the particles and the solution. The penetration depth of the evanescent wave from the ATR element into a homogeneous solution is expressed by Harrick,15J6 as
(la) where nl is the refractive index of the ATR material, nz is the refractive index of the probed medium, n21= ndnl, 8 is the internal reflection angle, and 11 = Lvacuum/nl. More useful for quantitative IR measurements is the effective path length de, which expresses the equivalent path length in a transmission measurement which results in the same absorption. Expressions for the limiting cases where the thickness of the layer ( d ) is very large (semiinfinite) compared to d, (d >> d,) and for thin films (d S) with one or two hydroxyl groups and formation of protonated and unprotonated mononuclear and bridging complexes. The reactions are written with mono-deprotonated oxalic acid, the predominant species a t pH 3.
>S-(OH)(OH,)+
+ HA-
4
>S-A
+ 2H20
(9)
+ HA- >S-AH+ + 2H20 (10) >S-(OH) + HA- >S-A- + H20 (11) >S-(OH2)+ + HA>S-AH + H20 (12) >S-OH + >S-(OH2)+ + HA- -.>S2A+ 2H20 (13) >S-(OH2)22+ + >S-(OH)(OH2)+ + HA- -. >S-(OH2):+
+
-t
-+
>SA2+ 4- 4H20 (14) At low pH, the protonated surface complex is formed in equilibrium with the unprotonated surface complex. Calculations with the above pKa values and with surface complexation constants in the range of 106-107,assuming reactions 11and 12, have shown that at pH 3.0 formation of protonated and unprotonated surface complexescan be expected while a t pH values higher than 6 only the unprotonated form is present. The situation changes with the pKa values of the oxide. Calculations with hematite, for example (with pKa values of 7 and 9))predict that the protonated oxalate complex is not formed. Preliminary experiments with the analogous adsorption of oxalate on hematite a t pH 3.0 show IR spectra whose spectral shape is much less dependent on surface coverage. We will not present detailed surface speciation calculations for the Ti02 system for two reasons: (1) Different sites are expected to be present on microcrystalline anatase on different crystal faces and the microscopic pKa values of these sites are not known. (2) Degussa P-25 is not a pure anatase preparation. We have chosen this preparation because it is widely used for adsorption and catalysis studies, affording comparability of our results with previous work. Studies on pure phases in connection with more detailed speciation calculations and spectral analysis will follow in a later study. Tentative Structural Assignments of the IR Spectra. (1) Aqueous Species. The vibrations in oxalate are strongly coupled and a n analysis in terms of the group frequencies of the carboxylic group in regard to coordination3l is not applicable. The spectrum of aqueous oxalate (Ox2-, with two strong lines at 1570 and 1308 cm-l) is in agreement with spectra reported by Begun and FletcheF (two strong lines a t 1579 and 1308 cm-l in saturated aqueous CSZOXsolutions, 1576 and 1312 cm-' in DzO). A D M symmetry was assigned to oxalate in consistency with the observed Raman and IR absorptions. In the two protonated oxalate species, HOx- and HzOx, the symmetry is lowered and more IR lines are observed. The lines a t 1725 and 1738 cm-l arise from v,(C=O) vibrations in agreement with the position of these vibrations in the Me(0x)l-3 compound^.^^-^^ We have currently no assignment for the peaks at 1620 and 1621 cm-'. (31)Deacon, G.B.;Phillips, R. J. Coord. Chem. Rev. 1980,33,227. (32)Begun, G.N.; Fletcher, W. H. Spectrochim. Acta 1963,19,1343. (33)Fujita, J.; Martell, A. E.; Nakamoto, K. J . Chem. Phys. 1962,36, 331.
Surface Complexes of Oxalate on Ti02 Table 3. Conceivable Structures of Inner-Sphere Oxalate Complexes on Titanium Surface Sites
Possible dimerization of oxalic acid and association with H20 in aqueous solution complicates the vibrational assignments. The solution spectra show rather large differences from spectra of crystalline oxalic acid measured in KBr.36 The peaks a t 1230 and at 1240 cm-l can be compared to the v,(CO) S(0-C=O) and the peak a t 1409 cm-' to the v,(C-0) v&C-C) in the [Me(Ox),P compounds. The spectra F01 and F 0 2 of the aqueous [Fe(ox),]* (r = 1-3, z = -3 to +1)agree well with those measured by Fujita et al.33 It is pointed out by these authors that the spectra of Me(0x) and of Me(OxI2-3 complexes are very similar for the vibrations a t energies higher than 1200 cm-l. [Ti(Ox),P in aqueous solution could not be measured because the pH required to hold Ti4+in solution is too low for work with a ZnSe ATR element. (2) Surface Species. The spectra of oxalate adsorbed on the Ti02 surface (T02, T02, T03) show little resemblance to the spectra of any of the free aqueous oxalate species or to the spectra of crystalline H2Ox. The resemblance of the surface spectra with those of the aqueous [Fe(Ox),T complex give strong support for the formation of specificinner-sphere co-ordination complexes of oxalate with surface Ti4+sites. Definitive structural assignments based on these spectra alone are not possible, nevertheless, taking the approximate Langmuir KL'Sas a measure ofthe surface complex stability, and considering the pH dependence as discussed in the last paragraph, the spectra can be discussed in terms of a range of conceivable surface structures. We limit our discussion to spectral features that remain qualitatively unchanged within the variations of the FFG and LA isotherm models shown in Figure 9 and we do not consider the 1610-1660 cm-l region where the signal-to-noise ratio is worse due to the presence of residual water vapor and subtraction of the background water absorption is more critical. A selection of possible structures is given in Table 3. The most interpretable features in the spectrum are the
+
+
(34) Nakamoto, K. Infrared and Raman Spectra ofznorganic and Coordination Compounds, 4th ed.; Wiley & Sons: New York, 1986. (35) Sadtler collection of FTIR spectra, Sadtler Research Laboratories Inc.
Langmuir, Vol. 10, No. 10, 1994 3597 bands due to the C-0 stretching vibration, which is split into two lines (1680-1685 and 1711 c q - l ) in the aqueous [Fe(ox),]Z complex. The split is caused by coupling of the two C-0 groups. Thus, the presence ofthis split indicates the presence of two bonds with predominantly C=O character. The only !I'iOz-oxalate surface spectrum where this split is present (in the form of a shoulder a t 1711 cm-l) is spectrum T01. Of the structures given above, this applies to structures S l a , Slb, or S5. S l a and S l b cannot be distinguished without a n exact knowledge of the vibrational force constants. Formation of a bidentate five-ring chelate complex is consistent with the relatively large Langmuir equilibrium constant associated with the formation of this species. A monodentate structure as in S5 would bind very weakly and could not account for the strong adsorption observed. In the spectra TO2 and T03, the split into two C=O vibrations is missing, indicating that only one bond with C=O character remains. This would be consistent with the protonated structures S2 and S3. These assignments are supported by the pH dependence, where spectra TO2 and TO3 are observed only at lower pH values. Structure S3 would be expected to be more stable than S2, as formation of a five-ring is energetically preferable over formation of a four-ring. A structure like S6 is probably formed with acetate, which adsorbs very weakly, similar to other monocarboxylic acids.36 Structure 54 would not show vibrational bands in the spectral region of C-0 stretching vibrations. Formation of structures S l a andor S l b and of the protonated structures S2 and S3 seem thus plausible propositions to explain the observed spectra with their corresponding Langmuir adsorption constants a t pH 3 and their pH dependence.
Conclusions Several complexes are formed by adsorption of oxalate on Degussa-P25 TiOz. Their contributions vary as a function of solution concentration of oxalate and as function of solution pH. Direct evidence for the formation of structurally different surface complexesis an important step in the understanding of photocatalysis, because the reactivity and the pathways for product formation are determined by the structures of surface species. The method presented here made it possible, for the first time, to study adsorption from aqueous solution to a solid mineral powder, in situ, quantitatively as a function of solution parameters and to separate the overlapping spectra of different species (aqueous and adsorbed on the surface) in a systematic way. This method should be generally applicable for the study of adsorption on finely dispersed solids and should also be useful for the in situ study of thermal and photochemical reactions on surfaces. Acknowledgment. We thank Dr. Carrick Eggelston and Professor Werner Stumm, EAWAG, for helpful discussions and Dr. Eggleston for prereviewing this paper. (36) Tunesi, S.; Anderson, M. A. Langmuir 1988,8 , 487.
