In Situ ATR-FTIR Spectroscopic Study of Adsorption of Perchlorate

At pHs of 11.3 and 10.6 a surface excess of the [Co(en)3]3+ cation was readily ..... Abruna, H. D. Electrochemical Interfaces/Modern Techniques for In...
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Langmuir 1999, 15, 4595-4602

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In Situ ATR-FTIR Spectroscopic Study of Adsorption of Perchlorate, Sulfate, and Thiosulfate Ions onto Chromium(III) Oxide Hydroxide Thin Films Jens Degenhardt and A. James McQuillan* Department of Chemistry, University of Otago, P.O. Box 56, Dunedin, New Zealand Received November 17, 1998. In Final Form: March 25, 1999 A thin chromium oxide hydroxide colloid film has been used as a model of the passive stainless steel surface for studies of anion adsorption from aqueous solutions. The adsorption of perchlorate, sulfate, and thiosulfate ions has been investigated by in situ attenuated total reflection infrared (ATR-IR) spectroscopy. Surface charge was monitored from the surface excess concentrations of tetramethylammonium ions and of perchlorate ions using the infrared spectroscopic STIRS technique. The colloid film showed a high positive charge at low pH and a low negative charge at high pH. The adsorption of sulfate was only observed for a positive surface charge. The infrared spectrum of adsorbed sulfate was significantly altered by the interfacial electric field, but there was no evidence of sulfate coordination to surface Cr(III) ions. Thiosulfate gave adsorption behavior analogous to sulfate. Adsorption isotherms for sulfate and for thiosulfate were determined from infrared spectral data, and Langmuir binding constants of (2.3 ( 0.4) × 105 and (1.4 ( 0.4) × 105 M-1 were obtained for the respective adsorbates. The lack of chemical binding of sulfate and of thiosulfate to the chromium oxide hydroxide surface may be part of the basis for the corrosion-promoting properties of these ions at stainless steel surfaces.

Introduction Stainless steels are produced by adding chromium to ordinary steel and owe their high corrosion resistance to a thin passivating oxide film on the surface. This surface film which has a thickness of a few nanometers consists mainly of amorphous chromium(III) hydroxide and chromium(III) oxide.1,2 However, stainless steels can undergo pitting corrosion under certain conditions. Considerable interest has been paid in electrochemical studies of the pitting behavior of stainless steel on the influence of anions such as chloride, sulfate,3 and thiosulfate.4-7 Some electrochemical studies3-5,8 have been supported by in situ adsorption measurements to determine the role these processes may play in the breakdown of passivity. The adsorption of anions onto metal surfaces from aqueous solutions may be studied by electrochemical (e.g. capacitance, impedance), optical (e.g. ellipsometry, second harmonic generation), spectroscopic (e.g. UV-visible, infrared (IR), Raman), and radiotracer methods.9,10 However, many of these methods are difficult to successfully apply to the stainless steel/aqueous solution interface and only some of them are able to reveal * Corresponding author. E-mail: [email protected]. (1) Maurice, V.; Yang, W. P.; Marcus, P. J. Electrochem. Soc. 1996, 143, 1182. (2) Ryan, M. P.; Newman, R. C.; Thompson, G. E. Philos. Mag. B 1994, 70, 2, 241. (3) Varga, K.; Baradlai, P.; Barnard, W. O.; Myburg, G.; Halmos, P.; Potgieter, J. H. Electrochim. Acta 1997, 42, 25. (4) Park, J. O.; Verhoff, M.; Alkire, R. Electrochim. Acta 1997, 42, 3281. (5) Thomas, A. E.; Kolics, A.; Wieckowski, A. J. Electrochem. Soc. 1997, 144, 586. (6) Garner, A. Corros. Australas. 1988, February, 16. (7) Newman, R. C.; Wong, W. P.; Garner, A. Corrosion-NACE 1986, 42, 489. (8) Beaglehole, D., Webster, B., Werner, S. J. Colloid Interface Sci. 1998, 202, 541. (9) Bard, A. J., Faulkner, L. R. Electrochemical Methods; Wiley: New York, 1980. (10) Abruna, H. D. Electrochemical Interfaces/Modern Techniques for In Situ Interface Characterisation; VCH: New York, 1991.

molecular details of adsorption. An alternative approach is to recognize that the adsorption properties of stainless steel are primarily determined by the behavior of the passive surface film and that studies of the behavior of such a material should provide model adsorption behavior for stainless steel surfaces. In the current study, colloidal chromium(III) oxide hydroxide (COH) was chosen to model stainless steel surfaces because this amorphous material appears to have properties similar to that of the noncrystalline protective film on stainless steel.1,2 A thin film of COH particles was deposited onto an IR internal reflection element, and the in situ adsorption of anions from aqueous solutions was monitored by attenuated total reflection (ATR) infrared spectroscopy. The strong absorption of water and the low spectroscopic sensitivity expected from surfaces has previously deterred the application of infrared spectroscopy to solid/aqueous interfacial systems. The technique used in this work overcomes these difficulties by sampling a porous hydrous colloid film of only a few micrometers in thickness and of high surface area. Recently diffuse reflectance and in situ ATR infrared methods have been used to investigate the adsorption behavior of phosphate,11,12 sulfate,13,14 carbonate,15 oxalate,16 and organic anions17-21 onto various other metal oxides and hydroxides. However, the diffuse reflectance work12,13 was carried out after removal of the aqueous phase and the adsorption behavior may not be the same for surfaces immersed in aqueous solutions. In this paper we report the results of an in situ ATR-IR study of the adsorption behavior of perchlorate, sulfate, and thiosulfate ions on thin chromium(III) oxide hydroxide colloid films from aqueous (11) Tejedor-Tejedor, M. I.; Anderson, M. A. Langmuir 1990, 6, 602. (12) Persson, P.; Nilsson, N.; Sjo¨berg, S. J. Colloid Interface Sci. 1996, 177, 263. (13) Persson, P.; Lo¨vgren, L. Geo. Cosmochim. Acta 1996, 60, 2789. (14) Hug, S. J. J. Colloid Interface Sci. 1997, 188, 415. (15) Dobson, K. D.; McQuillan, A. J. Langmuir 1997, 13, 3392. (16) Hug, S. J.; Sulzberger, B. Langmuir 1994, 10, 3587. (17) Couzis, A.; Gulari, E. Langmuir 1993, 9, 3414. (18) Sperline, R. P.; Song, Y.; Freiser, H. Langmuir 1992, 8, 2183.

10.1021/la981616t CCC: $18.00 © 1999 American Chemical Society Published on Web 05/29/1999

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solutions over a wide range of pH. The surface charge properties of the colloid films have been established and related to the adsorption behavior of the anions. The infrared spectra of the adsorbed anions have revealed the predominantly electrostatic nature of their adsorption. Adsorption isotherms for sulfate and thiosulfate have been analyzed in terms of the Langmuir model and adsorption binding constants determined. The anion adsorption behavior has been discussed in relation to the passivity of stainless steel surfaces. Experimental Section Materials. Purified water (Milli-Q, Millipore) was used for all solutions. The 1 M stock solutions (stored under N2) were prepared from NaOH (Merck, pure), HClO4 (Riedel de Hae¨n, 70% in water, analytical grade), tetramethylammonium hydroxide (TMAOH) (Merck, 20% solution, for synthesis), and HCl (BDH, 35.4% solution, Analar). All solutions containing lower concentrations were freshly prepared by dilution prior to each experiment. Chrome alum (CrK(SO4)2‚12H2O, BDH, ANALAR), was used for the preparation of the chromium hydroxide sol. For the adsorption experiments, tetramethylammonium perchlorate (TMAP) (ICN Pharmaceuticals), Na2S2O3‚5H2O (BDH, ANALAR) and Na2SO4‚10H2O (Riedel de Hae¨n, analytical grade) were dissolved in MilliQ water to give 1 × 10-2 M stock solutions from which the desired concentrations were prepared. KCl (BDH, ANALAR) was used in the adsorption isotherm experiments. Preparation of the Chromium Oxide Hydroxide Films. The COH sol was prepared by heating a 4 × 10-4 M solution of chrome alum to 85 °C and keeping it at this temperature for 24 h under vigorous stirring.22 The sol was then allowed to cool to room temperature and the colloid particles separated using a 0.22 µm membrane filter (Millipore, type GS). The particles were resuspended in 10 mL water and centrifuged three times to remove excess sulfate adsorbed to the hydroxide. The washed COH particles were suspended in 10 mL water and the pH adjusted to 3.0 with 1 M HClO4 to stabilize the sol for storage. The total chromium concentration of the sol prepared in this way was 0.023 M. The COH thin films were prepared by overnight evaporation in air at ambient temperature of 100 µL of aqueous COH sol on top of a 45° ZnSe single internal reflection ATR prism. To ensure a clean and contaminant-free surface the ZnSe prism was polished with 0.015 µm Al2O3 (BDH, highly pure for polishing) on a polishing cloth (Buehler microcloth) and then thoroughly washed with water prior to placing the sol on the prism surface. Once dried in air the films were stable on ZnSe to solutions over the pH range from 12 to about 3. Procedures. All solutions were degassed immediately prior to use by sonication under vacuum to remove dissolved carbon dioxide in particular. During experiments the solutions were kept under argon to prevent uptake of atmospheric carbon dioxide. Any pH adjustments were carried out in an inert gas atmosphere after degassing. All infrared spectra were recorded using the film-coated ZnSe prism and a Harrick prism liquid cell accessory in a BioRad Digilab FTS 60 spectrometer fitted with a DTGS detector. All spectra were from 64 scans at 4 cm-1 resolution with a total acquisition time of about 90 s. Solutions were fed by a peristaltic pump (Masterflex pump system, Tygon tubing) into a glass flow cell mounted with an O-ring onto the COH coated side of the ATR prism.23 The flow rate was ∼3 mL min-1. Unless otherwise stated the background spectrum for all adsorbate spectra is that from water on the COH film. At the start of each experiment the COH film was washed with 1 × 10-2 M sodium hydroxide solution for 45 min. This procedure removed the remaining sulfate arising from the starting material and surface carbonates from adsorption of atmospheric carbon dioxide during the film-drying process. All solution spectra were taken using ATR with water on the ZnSe prism for the background. X-ray diffraction experiments were carried out in a Philips PW 1050 diffractometer at 40 kV/30 mA. Cu KR radiation was (19) Sperline, R. P.; Song, Y.; Freiser, H. Langmuir 1994, 10, 37. (20) Sperline, R. P.; Song, Y.; Freiser, H. Langmuir 1997, 13, 3237.

Figure 1. SEM images of dried and gold-coated COH particles at the edge of dried sol drop: (a) low magnification, scale bar 5 µm; (b) higher magnification, scale bar 500 nm. used, and scans were performed between 3 and 75° angle of incidence (2θ). The sample for this experiment was prepared by evaporation of the chromium hydroxide sol onto glass with bare glass as a control. SEM images were taken with a Cambridge S360 electron microscope. The samples for electron microscopy were prepared by evaporation of 100 µL of the COH sol onto aluminum and then gold coated by vapor deposition. The DTA-TG data were obtained with a NETZSCH STA 409C instrument in air (10 dm3 h-1) at a heating rate of 10° min-1.

Results and Discussion Film Characterization. Figure 1 shows SEM images of COH colloid films at two different magnifications. As described by Matijevic et al.22,24,25 the heating of a dilute chrome alum solution leads to the formation of a monodisperse COH colloid with spherical particles of a narrow size distribution. The average particle diameter measured from the SEM images was 430 nm with the smallest particle being 330 nm and the largest particle measuring 500 nm. The standard deviation σn-1 (n ) 40) of the diameter was 35 nm. The dried film consisted of a patchy multilayer of particles and was about 2.5 µm thick. It was similar in appearance to that formed by sol evaporation on ZnSe. Evaporation of the COH sol does not form a continuous film as has been previously found for other hydrous metal oxides using the same method.26 Chromium hydroxide precipitated from heated solutions, as well as chromium hydroxide precipitated from cold solutions by base addition, which has undergone an aging process is a noncrystalline material consisting of polymeric chains with the empirical formula Cr(OH)3‚ H2O.22,27-30 In accordance with these findings the COH colloids prepared for the current project was found to be amorphous to Cu KR X-ray radiation. (21) Biber, V.; Stumm, W. Environ. Sci. Technol. 1994, 28, 763. (22) Sprycha, R.; Matijevic, E. Langmuir 1989, 5, 479. (23) Dobson, K. D.; Connor, P. A.; McQuillan, A. J. Langmuir 1997, 13, 2614.

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Figure 2. ATR-IR spectrum of air-dried COH film and background spectrum of bare ZnSe prism.

Figure 2 shows the infrared spectrum of a COH film. The strong absorptions between 1200 and 900 cm-1 are due to adsorbed sulfate which has its origin in the chrome alum. The nature of this sulfate in the colloid film is considered later in this paper. The large band that appears at 1630 cm-1 and the broad feature above 3000 cm-1 arise mainly from to the -OH bending and stretching modes of water molecules, respectively. Surface hydroxyl groups are also expected to contribute to -OH stretching absorptions in the 3000 cm-1 region.31 This indicates that the air-dried film still contains water adsorbed or trapped in capillaries within the substrate. The presence of loosely adsorbed water was confirmed by DTA-TG measurements. In the temperature range 70-250 °C a mass loss combined with an endothermic process was observed which is an indication of the desorption of adsorbed or trapped water.28 Removal of Sulfate by Washing. The spectra in Figure 2 show intense absorption signals at 1115, 1064, and 978 cm-1 due to the large amount of adsorbed sulfate in the COH film prepared from an acidic sol. Following a procedure developed for other oxide films15,23,44 the sulfate (24) Matijevic, N. W.; Lindsay, A. D.; Kratohvil, S.; Jones, M. E.; Larson, R. I.; Cayey, N. W. J. Colloid Interface Sci. 1971, 36, 273. (25) Matijevic, E Acc. Chem. Res. 1981, 14, 22. (26) Connor, P. A.; Dobson, K. D.; McQuillan, A. J. Langmuir 1995, 11, 4193. (27) Avena, M. J.; Giacomelli, C. E.; De Pauli, C. P. J Colloid Interface Sci. 1996, 180, 428. (28) Ratnasamy, P.; Le´onard, A. J. J. Phys. Chem. 1972, 13, 1838. (29) von Meyenburg, U.; Sˇ iroky´, O.; Schwarzenbach, G. Helv. Chim. Acta 1973, 56, 1099. (30) Giovanoli, R.; Stadelmann, W.; Feitknecht, W. Helv. Chim. Acta 1973, 56, 839. (31) Kittaka, S.; Sasaki, T.; Fukuhara, N.; Kato, H. Surf. Sci. 1993, 282, 255. (32) Giacomelli, C. E.; Avena, M. J.; Camera, O. R.; De Pauli, C. P. J. Colloid Interface Sci. 1995, 169, 149. (33) Davydov, A. A. Infrared Spectroscopy of Adsorbed Species on the Surface of Metal Oxides; Wiley: New York, 1990. (34) Connor, P. A. Ph.D. Thesis, University of Otago, New Zealand, 1997. (35) Hall, H. T.; Eyring, H. J. Am. Chem. Soc. 1950, 72, 782. (36) Dobson, K. D.; Connor, P. A.; McQuillan, A. J. Langmuir 1997, 13, 2614. (37) Kotrly´, S., Sˇ ucha, L. Handbook of Chemical Equilibria in Analytical Chemistry; Ellis Horwood: Chichester, U.K., 1985. (38) Nakamoto, K.; Fujita, J.; Tanaka, S.; Kobayashi, M. J. Am. Chem. Soc. 1957, 79, 4904.

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Figure 3. ATR-IR spectra of desorption of sulfate from COH film: (a) loosely bound ionic sulfate removed with water; (b) strongly bound monodentate sulfate removed with 0.01 M NaOH; (c) 0.05 M Na2SO4 aqueous solution on ZnSe prism. Background spectra were of (a, b) water on COH film and of (c) water on ZnSe prism.

was completely removed with an aqueous alkali wash before the start of each experiment. Washing the film with water alone was not sufficient to remove all surface sulfate. Figure 3 shows the effect on the adsorbed sulfate of washing for 45 min with (a) water and (b) 1 × 10-2 M sodium hydroxide solution. The spectrum of an aqueous sodium sulfate solution is shown in Figure 3c for comparison. During the water wash, negative bands appeared at 1112 and 983 cm-1 due to weakly bound sulfate which is readily removed from the film. When the film was subsequently washed with the sodium hydroxide solution further absorption losses associated with more strongly bound sulfate were recorded at 1122, 1059, and 982 cm-1. No further absorption losses were observed after ∼45 min of alkaline washing, and the infrared spectrum of the film subsequently dried under argon indicated that sulfate removal was complete. The nature of the sulfate removed from the films by washing will be discussed in a later section. pH Dependence of Surface Charge: STIRS Experiment. Surface titration by internal reflection spectroscopy (STIRS) was first used to monitor, with in situ IR spectroscopy, the variation of surface charge with pH and to determine the isoelectric point of TiO2 films.23 The method uses constant ionic strength mixtures of TMAOH, TMAP, and perchloric acid to obtain solutions over a wide pH range and which contain very weakly complexing anions and cations. At high pH, metal oxide surfaces have net negative charge. Under such conditions if a solution containing tetramethylammonium ions (TMA+) is present, a surface excess TMA+ concentration produces a measurable enhancement of the TMA+ spectrum compared with its bulk solution spectrum. As the (39) Rudolph, W.; Irmer, G. J. Solution Chem. 1994, 23, 663. (40) Finholt, J.; Anderson, R.; Fyfe, J.; Caulton, K. Inorg. Chem. 1965, 4, 43. (41) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 5th ed.; Wiley: New York, 1997. (42) Freedman, A. N.; Straughan, B. P. Spectrochim. Acta 1971, 27A, 1455. (43) Gabelica, Z. Chem. Lett. 1979, 1419. (44) Duffy, N. W.; Dobson, D.; Gordon, K. C.; Robinson, B. H.; McQuillan, A. J. Chem. Phys. Lett 1997, 266, 451.

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Figure 4. (a) ATR-IR spectrum of 0.05 M aqueous TMAP solution. (b) STIRS spectra from COH film at different pH from (TMA)OH/TMAP/HClO4 mixtures with TMA+ and ClO4- concentrations of 1 × 10-3 M. Background spectra are shown of (a) water on ZnSe prism and (b) water on COH film. STIRS spectra are offset for clarity.

pH is lowered, the TMA+ signals become weaker until the isoelectric point is reached and no enhancement of TMA+ concentration at the surface can be observed. For pH lower than the isoelectric point the surface has a positive charge resulting in a surface excess of anions such as the perchlorate ion. Surface excess concentrations have been monitored23 from the absorbances of the ν15 mode of TMA+ at 1484 cm-1 (F2, antisymmetric CH3 deformation) and of the ν3 mode of perchlorate at 1104 cm-1 (T2, antisymmetric Cl-O stretch). For the present STIRS experiments the solutions over the alkaline pH range were prepared by mixing 1 × 10-3 M solutions of TMAOH and TMAP while those over the acidic pH range were obtained from mixtures of TMAP and HClO4 solutions. Thus [TMA+] and [ClO4-] were maintained at 1 × 10-3 M over the alkaline and acidic pH ranges, respectively, and surface excesses of ions could readily be detected without any significant contribution from bulk solution absorptions. The resulting spectral data are shown in Figure 4. The spectrum of an aqueous TMAP solution is also shown for comparison. Electrokinetic measurements22,24,32 of this chromium hydroxide colloid have shown isoelectric points of 7-8.5. Thus at the initial pH of 11.5 the surface is expected to have a significant negative charge, but no excess TMA+ concentration was observed in any of the measurements. This is a surprising result. The chromium hydroxide used in the present experiments, which was dried and then rehydrated, may not contain sufficient acidic protons to produce the high negative surface charge observed previously for a chromium hydroxide colloid prepared similarly22,24,32 but not dried as in the present work. A possible cause could be cross-linking reactions within the polymeric chromium hydroxide during the drying procedure. In this case the chromium hydroxide would only produce an appreciable negative surface charge when Cr-O-Cr links were broken and when OH- was adsorbed from solution. Due to the slow kinetics of the Cr3+ ligand exchange and the high stability of the polymer,

Degenhardt and McQuillan

Cr-O bonds are unlikely to break under the relatively mild conditions and the rather short time scale of the experiment. This behavior could be typical for chromium oxides and hydroxides with a high degree of condensation since for R-chromia no acidic surface -OH groups could be detected when ammonia was adsorbed from the gas phase.33 On the other hand very small amounts of TMA+ were found to adsorb at high pH values onto chromium hydroxide films prepared from sols that were obtained by precipitation from chromium(III) salt solutions with base.34 Such sols are very likely to contain a crystalline modification of chromium hydroxide with a higher water content than the one used in the current study.28,35 The first sign of a perchlorate surface excess concentration can be detected once the pH drops below 5.0. As in the previously reported STIRS data for adsorption on TiO2, there is no change in the spectrum of adsorbed perchlorate compared with that of perchlorate in aqueous solution. The perchlorate signal indicates the expected increase in positive surface charge with decreasing pH. The pH limit for the experiment was set to 3.0 to avoid damage of the film by the acid. The results of the STIRS experiment do not allow a precise isoelectric point to be determined, but the absence of a surface excess perchlorate signal at pH > 5 suggests that the isoelectric point lies in this pH range. As expected, this is at a higher pH than the isoelectric point found for a TiO2 colloid film using the same method.36 To detect any small negative surface charge of the chromium hydroxide film at high pH, the TMA+ ion in the STIRS experiment was substituted by [Co(en)3]3+ from [Co(en)3]Cl3. The higher charge of this coordination complex cation is more suitable for detecting a small negative surface charge. At the same time it is sufficiently inert (β3 ) 48.6837) to not undergo chemical reactions during the adsorption process. At pHs of 11.3 and 10.6 a surface excess of the [Co(en)3]3+ cation was readily detected on the film in absorptions at 1585, 1470, 1164, and 1060 cm-1 (spectra not shown). These absorptions were unchanged when compared with the spectrum of a [Co(en)3]Cl3 aqueous solution, indicating that the interaction between surface and the cation is predominantly electrostatic. Thus use in STIRS experiments of highly charged inert ions allows low surface charges to be observed. Adsorption of Sulfate on Chromium Oxide Hydroxide. The free sulfate ion has Td symmetry with four fundamental vibrational modes.41 Only ν1 (A1, symmetric S-O stretch) and ν3 (F2, antisymmetric S-O stretch) fall within the spectral range investigated in this work. Only the triply degenerate ν3 mode is infrared active, but symmetry distortions may cause the ν1 Raman active mode to become weakly infrared active38 and have other spectral consequences. Slight changes in the appearance of the sulfate spectrum compared with that of free sulfate were observed in the spectrum of Co(NH3)6]2(SO4)3‚5H2O by Nakamoto et al.38 The changes are due to the perturbing effect of the [Co(NH3)6]+ cation on the sulfate tetrahedra. In dilute aqueous solution the infrared absorptions of the ν3 and ν1 modes are observed at 1099 cm-1 (s) and at 977 cm-1 (vw), respectively, as shown in Figure 5a. When sulfate coordinates to metal ions, in coordination complexes and in adsorption to metal oxide surfaces from solution, characteristic band splittings can be observed. Monodentate binding lowers the symmetry of the molecule to C3v and causes the ν3 mode to split into two modes with symmetries A1 and E. Both of them and the ν1 mode are infrared active, and three bands are observed in the infrared spectrum. In bidentate sulfato complexes the sulfate symmetry is lowered further to C2v and the ν3 mode is split into three infrared active modes with symmetries

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Figure 6. Comparison of sulfate infrared spectra (not on same absorbance scale): ‚‚‚, 5 × 10-2 M aqueous Na2SO4 solution; ---, sulfate adsorbed on COH film from 1 × 10-3 M solution, low surface coverage; s, sulfate adsorbed on COH film from 1 × 10-3 M solution, high surface coverage. Figure 5. (a) ATR-IR spectrum of 0.05 M aqueous Na2SO4 solution. (b) Modified STIRS spectra from COH film with SO42concentration of 1 × 10-3 M and pH range from the presence of H2SO4 or NaOH. Background spectra are shown of (a) water on ZnSe prism and (b) water on COH film. Spectra are offset for clarity.

A1, B1, and B2. The ν1 mode is also infrared active, and thus, a total of four bands are observed in the spectrum. The adsorption of sulfate from aqueous solution to the COH film was studied once the sulfate originating from the COH sol preparation was removed by the alkaline wash treatment. Figure 5b shows the results of a modified STIRS experiment. The pH range was achieved by mixing solutions of NaOH, Na2SO4, and H2SO4 keeping the sulfate concentration constant at 1 × 10-3 M in all solutions. The first detectable surface excess due to sulfate adsorption appears at pH ) 8.0. Adsorbed sulfate shows two bands in the spectrum with peaks at 1116 cm-1 (s) and at 982 cm-1 (vw) and a subsidiary shoulder on the 1116 cm-1 band at lower wavenumber. As the pH decreases and the positive surface charge of the substrate increases, the absorbances of the two bands increase proportionally. A shift in the maximum of the ν3 mode from 1104 cm-1 at high pH (low surface coverage) to 1115 cm-1 at low pH (high surface coverage) is observed, but the general shape of the band remains unchanged. At very low pH three very small negative bands at 1378, 1466, and 1567 cm-1 are also observed. They originate from the removal of small amounts of surface carbonate at low pH. Surface carbonate arises from the incomplete exclusion of atmospheric carbon dioxide for which the chromium hydroxide has a high affinity. This assignment was confirmed by adsorption experiments with aqueous sodium bicarbonate solutions. Figure 6 shows a direct comparison among solution sulfate and sulfate at low and sulfate at high surface coverage. The spectra are not on the same scale. The differences between the spectrum of the sulfate ion in solution and that of sulfate adsorbed to the COH colloid film may suggest that the adsorption is due to other than electrostatic forces. Due to its higher negative charge, sulfate is attracted to a smaller positive surface charge and adsorbs at higher pH than observed for perchlorate. The sulfate ν3 mode has shifted somewhat to higher wavenumbers and has broadened on adsorption, which indicates a distortion of the tetrahedral sulfate ion in the electric field close to the positively charged COH surface.

The sulfate ion with its higher 2- charge appears to be affected more strongly by the interfacial electric field than the perchlorate ion, which only bears a 1- charge. The general appearance of the absorption band, however, is still close to that of sulfate in aqueous solution, and Td symmetry is largely retained. Any chemical binding would cause a distortion of the sulfate ion with a lowering of the symmetry and a characteristic splitting of the bands of the ion. Instead, only the very strong triply degenerate ν3 mode and a very weak ν1 mode are observed. Adsorption of anions to hydrous metal oxide surfaces is expected to be closely related to association between the anions and the corresponding cations in aqueous solution. Rudolph and Irmer39 have interpreted the asymmetry of the ν3 mode and the presence of a weak ν1 mode in the infrared spectrum of a concentrated aqueous cadmium sulfate solution as evidence for ion pairing. Although they observed a shift of the overall maximum of the ν3 band to lower wavenumbers during ion pairing, the general trend of the development of a band at lower frequency is still similar to what is observed during sulfate adsorption onto chromium hydroxide. When comparing Cr3+ and Cd2+, one has also to consider the higher electric field experienced by a sulfate ion in the vicinity of a 3+ rather than a 2+ counterion. As a hard acid Cr3+ would have a higher tendency than the soft acid Cd2+ to coordinate to one of the oxygens of sulfate. Therefore the spectrum of sulfate from the Cr3+SO42- ion pair is expected to be closer to that of monodentate bound sulfate than that from the Cd2+SO42- ion pair. Support for the assignment of sulfate adsorption on COH colloidal films as being primarily due to electrostatic forces comes from the infrared spectral data of ionic sulfate in coordination complexes.38 The sulfate S-O stretch absorptions in [Co(NH3)6]2(SO4)3‚5H2O are at 973 cm-1 (vw) and 1130-1140 cm-1 (vs) with the latter band consisting of an overlapping doublet. In contrast, the spectrum of sulfate as a monodentate ligand in [Co(NH3)5SO4]Br shows38 three discrete bands at 970 (m), ∼1040 (s), and ∼1130 cm-1 (s) while that of sulfate as a bidentate ligand in general consists of four separate bands. The spectrum of the air-dried COH film and the spectral features lost during the film washing procedures can now be discussed. The infrared spectrum of the air-dried film shown in Figure 2 shows three distinctive bands at 1115, 1064, and 978 cm-1. This band splitting is consistent with monodentate bound sulfate which originates from the

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Figure 7. ATR-IR spectra of development of monodentate binding in adsorbed sulfate during drying of the COH sol film in an Ar stream. The background is that from a bare ZnSe prism. Spectra are offset for clarity.

starting material of the sol (chrome alum). The band positions are very close to those given40,41 at 1122, 1059, and 978 cm-1 for [Cr(OH2)5SO4Cl]‚0.5H2O which contains a monodentate sulfate ligand. The absorption losses at 1112 and 983 cm-1 of the more weakly attached sulfate, which was removable by a water wash, correspond closely to those of an ionic sulfate. The absorptions at 1122, 1059, and 982 cm-1 during film washing with 1 × 10-2 M NaOH solution correspond closely to those of monodentate bound sulfate. To further probe the nature of sulfate adsorbed on the hydrous COH film, the infrared spectrum of the film was monitored while the film was drying under a stream of argon. The resultant evolution of the spectra are shown in Figure 7. Initially the spectrum indicates the presence of predominantly ionic sulfate with one strong absorption band with a maximum at 1115 cm-1 (ν3 mode) and a very weak signal at 980 cm-1 (ν1 mode). The first spectrum on the time scale exhibits a strong underlying absorption at below 1000 cm-1 which is due to bulk water on the film. As the background for the spectra in Figure 6 is not water on the colloid film but a bare ZnSe prism, bulk water is still visible in the spectrum. The removal of the aqueous phase can be monitored using the stretching and bending vibrational modes of water at 1630 cm-1 and above 3000 cm-1. As the film dries, the sulfate bands broaden and split and eventually the spectrum resembles that of the dry film (Figure 2) which was discussed earlier. It is also significant that there was no spectral evidence of sulfate coordination to a COH sample which had been alkali washed to remove its original sulfate followed by immersion in an aqueous sulfate solution for several weeks. Therefore it can be concluded that solution sulfate in contact with hydrous COH has little tendency to form monodentate complexes. However, when the aqueous phase is removed, coordination readily takes place and significant changes in the infrared spectrum become apparent. With respect to sulfate adsorption, COH behaves differently from the hematite investigated with in situ IR by Hug.14 Sulfate was found to give monodentate coordination to hematite surface Fe3+ ions when adsorbed from aqueous solutions and when dried a bidentate binding developed. Using potentiometric titration and diffuse reflectance IR spectroscopy, Persson et al.13 found that sulfate was bound to goethite as an outer sphere complex.

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Figure 8. (a) ATR-IR spectrum of a 0.05 M aqueous Na2S2O3 solution. (b) STIRS spectra from COH film of 1 × 10-3 M S2O32solution, pH range from the presence of HCl or NaOH. Background spectra are from (a) water on ZnSe prism and (b) water on COH film. Spectra are offset for clarity.

Below pH ) 5 the adsorbed sulfate on goethite was found to be protonated. In the present study neither bidentate coordination to Cr3+ nor protonation reactions of adsorbed sulfate were observed. This apparently different behavior for sulfate adsorption on goethite and on the COH film may partly be due to the almost complete removal of the aqueous phase for the diffuse reflectance measurements. Adsorption of Thiosulfate on Chromium Oxide Hydroxide. The free thiosulfate ion has a C3v symmetry with six fundamental modes of vibration of which only ν4 (E) and ν1 (A1) fall within the spectral region investigated. These two infrared active modes are associated with the antisymmetric and the symmetric S-O stretches and are observed at 1115 and 996 cm-1 for dilute aqueous solutions as shown in Figure 8a. The S-O stretching vibrations are useful indicators for the binding mode of thiosulfate,41 and characteristic shifts and splittings are expected on coordination to metal ions and on adsorption to metal oxide surfaces. Monodentate coordination of thiosulfate through one sulfur atom shortens the S-O bond and shifts the antisymmetric S-O stretch up between 1130 and 1175 cm-1. If one sulfur coordinates by bridging between two metal centers, the antisymmetric S-O stretching band shifts to greater than 1175 cm-1. Band positions lower than 1130 cm-1 indicate coordination through oxygen.42,43 The pH dependence of thiosulfate adsorption to a COH film was investigated with a procedure similar to that of the modified STIRS experiment used for the sulfate adsorption. Solutions were all 1 × 10-3 M in Na2S2O3 but also contained NaOH or HCl to obtain a range of pH from 10.8 to 3.2. Figure 8b shows the spectra from the modified STIRS experiment for thiosulfate adsorption to the COH film. As observed for sulfate adsorption, the first signs of thiosulfate adsorption are detected at pH just below 8. The two thiosulfate bands increase proportionally with decreasing pH, which indicates that there is no difference in the binding mode at low and high surface coverage. Thiosulfate exhibits a considerable broadening of the antisymmetric S-O stretching mode with adsorption on COH as was observed for sulfate adsorption. The peak of the antisymmetric S-O stretching band shifts from 1114 cm-1 in aqueous solution to 1104 cm-1

Chromium(III) Oxide Hydroxide Thin Films

Langmuir, Vol. 15, No. 13, 1999 4601

in the adsorbate and develops a shoulder on the higher wavenumber side around 1140 cm-1. The half-height width of the band after adsorption is approximately twice the half-height width of the free solution species. However, no complete band splitting is observed for thiosulfate when adsorbed onto COH from aqueous solution. Therefore it appears that the adsorption is predominantly ionic. The downshift of the antisymmetric S-O stretch absorption could be due to a chemical interaction through oxygen, and the band broadening may be from a band developing around 1140 cm-1 caused by a Cr-S interaction or crystal lattice interactions. Chemical interaction through sulfur and oxygen would suggest a structure with mixed S/O binding similar to that proposed by Freedman and Straughan for PbS2O3(s) which has two antisymmetric S-O stretching bands at 1140 and 1116 cm-1.42 Cr3+ is a hard acid and would be expected to favor oxygen coordination rather than through the terminal sulfur which is a soft base. The spectra of a COH film with adsorbed thiosulfate were obtained while it was drying under an argon stream. Unlike sulfate, thiosulfate did not develop coordination which results in band splitting, but the intensity of the 1140 cm-1 shoulder increased markedly on drying. Thus the interaction between COH and thiosulfate appears to be predominantly ionic in nature, but some degree of chemical interaction cannot be ruled out. Adsorption Isotherms for Sulfate and for Thiosulfate on Chromium Oxide Hydroxide. The adsorption isotherms for sulfate and for thiosulfate were determined in 0.01 M KCl solution containing 1 × 10-3 M HCl at pH 3.0. The Langmuir equation was chosen to calculate the binding constants because it has been previously applied to such adsorption experiments from aqueous solutions.44-47 Starting from the Langmuir equation

ϑ)

Kc 1 + Kc

Figure 9. Adsorption isotherm data of sulfate adsorbed to COH film from aqueous 0.01 M KCl solution, pH adjusted to 3.0 with HCl: (a) absorbance vs concentration isotherm plot; (b) linearized plot according to Langmuir model (absorbance at 1116 cm-1).

(1)

where

K ) ϑ/c

(2)

ϑ ) A/A∞

(3)

by introducing

one obtains

c 1 c ) + A A∞ KA∞

(4)

In the above equations ϑ is the surface coverage, K is the adsorption binding constant, c is the solution concentration of the ion, A is the absorbance at a particular wavelength at a certain solution concentration, and A∞ is the absorbance at the same wavelength at complete surface coverage. By the plotting of c/A versus c (eq 4), the binding constant K can be obtained from the slope of the linear graph and its intercept. Figure 9 shows (a) the isotherm data and (b) the plot according to eq 4 for sulfate adsorption on the COH film. As can be seen from Figure 9b, the experimental data fit (45) Rodriguez, R.; Blesa, M. A.; Regazzoni, A. E. J. Colloid Interface Sci. 1996, 177, 122. (46) Moser, J.; Punchihewa, S.; Infetta, P. P.; Gra¨tzel, M. Langmuir 1991, 7, 3012. (47) Awatani, T.; McQuillan, A. J. J. Phys. Chem. B 1998, 102, 4110.

Figure 10. Adsorption isotherm data of thiosulfate adsorbed to COH film from aqueous 0.01 M KCl solution, pH adjusted to 3.0 with HCl: (a) absorbance vs concentration isotherm plot; (b) linearized plot according Langmuir model (absorbance at 1104 cm-1).

the Langmuir isotherm very well (R2 ) 0.9999) and a binding constant K ) (2.3 ( 0.4) × 105 M-1 was obtained. Figure 10 shows the isotherm data of thiosulfate adsorbed onto the COH film and the linear plot according to the Langmuir model. The agreement with the Langmuir model is also very good from the linearity (R2 ) 0.9994) of Figure 10b. The binding constant K ) (1.4 ( 0.4) × 105 M-1 is very similar to that calculated for sulfate adsorption on the same substrate. The similarity of binding constants strongly suggests that the interaction between the COH surface and each of the sulfate and thiosulfate ions is of a very similar type and concurrs with such a conclusion reached from consideration of the adsorbate spectra. Conclusions An in situ infrared spectroscopic method has been used to obtain molecular details of adsorption processes from aqueous solution onto chromium(III) oxide hydroxide. The method is based on the use of a thin film of monodisperse chromium(III) oxide hydroxide colloid particles as a model material for the passivating film on the stainless steel

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surface. The amorphous colloid film deposited on a ZnSe internal reflection element has been shown by the STIRS technique to have high positive charge at low pH but little negative charge at high pH, probably because of lack of acidic -OH groups. The colloid film was stable over a wide range of pH and provided high-quality infrared spectra of adsorbed anions. The infrared spectra of adsorbate species, the dependence of adsorption on pH, and the adsorption isotherm data have provided a basis for establishing the nature of anion adsorption. The adsorption of perchlorate was clearly due to charge as no spectral changes compared to the solution species were observed. For adsorbed sulfate and for adsorbed thiosulfate there are small but significant changes in absorption peak wavenumbers and bandwidths, but these changes do not correspond to coordination. Rather the spectra appear to indicate that when these more negatively charged species are adsorbed they are more strongly polarized by the interfacial electric field to the extent that some lowering of the symmetry of the anion results. Thus the adsorption of perchlorate, of sulfate, and of thiosulfate from aqueous solution to a model film of the stainless steel passive surface layer appears to occur simply as a response to the surface charge and without the formation of coordinative bonds to the Cr(III) surface ions. For sulfate this behavior contrasts with adsorption

Degenhardt and McQuillan

to haematite particle films where monodentate sulfate coordination to surface Fe(III) is observed.14 Sulfate and thiosulfate are generally regarded as aggressive ions which tend to promote pitting corrosion at the stainless steel surface.6,7 The present results may suggest that the corrosion-promoting properties of these ions may be related to their inability to coordinate to the Cr(III) surface ions and that anions which are able to coordinate will tend to maintain or enhance the passivity. We have also studied the adsorption of phosphate and carbonate onto COH colloid films, and their somewhat different adsorption behavior will be reported in our next paper. Acknowledgment. We thank Industrial Research Ltd., Wellington, New Zealand, for sponsoring the work and Dr. Barbara Webster of IRL for her scientific support. Thanks to Sue Johnstone, Dental School at the University of Otago, for the SEM images and Assoc. Prof. Dave Craw from the Department of Geology of the University of Otago for the X-ray work. Our thanks go as well to H. Junicke and Dr. Voigt at the Martin-Luther-Universita¨t, HalleWittenberg, Germany, for the thermal analysis. Thanks also to Dr. Charles Clark for the preparation of the [Co(en)3]Cl3 complex and to Dr. Kevin Dobson for his guidance. LA981616T