Surface hydration of aqueous γ-Al2O3 studied by Fourier transform Raman and infrared spectroscopy—I. Initial results

0584-8539/93 56.00+0.00 Q 1993 Pergatnon Press Ltd

Spccnodrimico Acfa. Vol. 49A, No. 516. pp. 691-705, 1993 Printed in Gnat Britain

Surface hydration of aqueous y-A&O, studied by Fourier transform Raman and infrared spectroscopy-I. Initial results CHRIS DYER and PATRICK J. HENDRA Department of Chemistry, The University, Highfield, Southampton SO9 SNH, U.K.

and WILLIS FORSLING*

and MAINE RANHEIMJZR

Department of Inorganic Chemistry, LuieH University, S-951 87 LnieB, Sweden (Received 14 July 1992; accepted 3 November 1992)

Abstract-The hydration of y-AirOr has been studied by Fourier transform (FT) Raman and infrared (IR) spectroscopy, and by X-ray diffraction (XRD). The initial findings are presented, along with a discussion of the possible canses for the major spectral changes that occnr after hydration. The aims of the study and ongoing research are described.

BACKGROUND

or more correctly aluminium oxide (A1203), finds application in a wide variety of processes in the chemical industry. Perhaps the most obvious is its use as either a catalyst or catalyst support in the petrochemical industry. Another less well-known application uses y-alumina as the dielectricum in electrolytic capacitors [l]. The exact nature of the alumina depends strongly on the method of production, as this controls the phase. Typical methods of production include thermal and anodic oxidation of the bulk metal, as well as dehydration of the various hydrated alumina minerals and hydroxides. The surface structure of each different phase is quite distinct; this is reflected by the difference in bulk crystallographic structure. In aqueous suspension, the nature of the surface strongly affects the reactivity at the solid/water interface. The surface reactions of y-A&O3 in aqueous solution are of special interest to the capacitor industry. In an electrolytic capacitor the dielectricum consists of a layer of y-A&O3 or y’-A1203 with a thickness of 100-800 nm. The oxide film is formed by anodic oxidation of ultra pure aluminium foil using aqueous electrolyte solutions in special formation tanks. The homogeneity and stability of the oxide layer are reflected in the service life and electric properties of the capacitor. To increase the oxide stability and minimize surface hydration reactions leading to the formation of aluminium hydroxides, the anodized foil is treated with ortho-phosphate. Surface adsorption of phosphates seems to prevent degradation of the oxide layer both in aqueous solution and the inorganic electrolytes used in the manufacture of capacitors. Very little is actually known about the mechanism for phosphate adsorption at y-A&O3 as a function of pH and temperature. As a part of an inter-disciplinary research programme in material science, the surface reactions of aqueous +41203 have been studied by means of potentiometric titrations and FTIR/FI-Raman spectroscopy. This is the Grst step in the combined chemical/spectroscopic study of hydroxide/phosphate competition at the y-A12031water interface. This paper reports the first results from vibrational studies of y-A&O3 equilibrated in aqueous solution at different pHs. ALUMINA,

Oxide mineral surfaces react with water to produce hydroxide functional groups (hydroxylation). Furthermore the mineral surfaces are hydrated; molecular water is * Author to whom correspondence should be addressed. u(A) 49:5/e-6

691

CHRISDYER et al.

692

adsorbed to the hydrophilic hydroxide groups resulting in a multilayer of water molecules at the interface with different properties to bulk liquid water [2]. Once the hydroxylation and hydration have taken place the adsorption and desorption of protons can be described using the Briinsted acid/base concept. Dissolved ions or molecules are also adsorbed forming surface coordination complexes, where the surface may be ascribed Lewis acid/base qualities [3]. In an aqueous y-A&O3 suspension the hydroxylated surface is supposed to be formed by water dissociation as a result of surface reactions where OH- bonds to =Al, and H+ to =O surface sites, producing ionizable surface hydroxide groups (= represents the surface). Thus the acid/base properties of hydroxylated and hydrated alumina surfaces can be interpreted in terms of proton adsorption and desorption at the aluminol (cAlOH) functional groups according to the following reaction scheme: =AlOH + H++ =AlOH’ =AlOH; =AlOH+

---*=AlOH + H+ =AlO-

Ku1 = {=AlOH~H+}/{=AlOH+)

+ H+

Ku,={=AlO-XH+}/{=AlOH}

log Ku1 =7.2 and log &=9.5

[4],

in which {i} denotes the thermodynamic activity of species i. The activity coefficients for charged species adsorbed on charged surfaces include the electrical potential energy of the ions in the surface field. Besides the surface reactions, hydration may lead to bulk solid phase transitions. Depending upon the conditions the reaction products between aluminium and water have been reported to be bayerite, gibbsite and nordstrandite (Al(OH),), boehmite and diaspore (AlOOH) and y-AIZO, and corundum (A&O,). Transformations among these phases may occur as a result of recrystallization and dehydration processes [5]. y-A&O3 is produced by thermal and anodic oxidation of aluminium and is also formed by dehydration of boehmite at temperatures between 450 and 600°C and pseudoboehmite at temperatures below 350°C. The general structure is of spine1 type with a cubic closepacked oxygen lattice. The unit cell consists of 32 oxygens with 21 l/3 Al atoms and 2 2/3 vacant sites distributed among the positions of four-fold and six-fold oxygen coordination. The oxygen lattice of y-A&O, is fairly well ordered but the tetrahedral Al lattice is strongly disordered. In contrast, Al(OH)3 is structurally defined as an approximately hexagonal close packed hydroxyl lattice with aluminium atoms in two-thirds of the available octahedral holes. The basic structure can be described as a “sandwich” of aluminium atoms between a double layer (AB) of hydroxyl groups. Within the sandwich the Al(OH)6 octahedra are joined along edges while sheets of hydroxyl groups in adjacent sandwiches are held together by hydrogen bonding. EXPERIMENTAL

The reagents used in this study are as follows: (1) High purity gamma-alumina was bought from Mandoval Ltd with the following specifications: Purity > 99.99%. Mean particle size
Surface hydration of aqueous y-Alz03-I.

Initial results

693

.O

Fig. 1. Raman specta of upper and lower layers from centrifuged suspension. Teflon peaks are marked with an x .

assumed stable to oxidation. During that time hydroxylation and hydration processes at the particle surfaces were completed. This was verified by obtaining stable glass electrode potentials. These measurements reflect the equilibrium conditions in the system. The application of these ideas to mineral surface reactions in aqueous solution is relatively new, and finding many applications [6]. The surface reactions involve proton exchange between the mineral surface and the bulk aqueous solution. On the assumption that the solubility of the solid phase is negligible in the pH range studied, the pH value is determined by the buffer capacity of the hydroxo groups formed. The number of these groups is considerable (l-5/nm*) [7], and both earlier published results as well as ongoing titrations in this system indicate an equilibration time of 4-5 weeks. During this period the pH changes regularly and steadily, to reach a fixed equilibrium value. Aqueous suspensions of y-A1209 with exactly the same solid concentrations (2Og/l) were prepared to 60°C. This enabled a comparison to be made between the rate of hydration at 60°C and at room temperature. Twenty cm3 from the stock suspension and 30 cm3 0.167 M NaCl (or pure water) were mixed in a vessel to obtain the desired concentration of aluminium surface sites ( = 1.6 X 10m3M). (This value is obtained from potentiometric titration experiments, which will be described in a future publication [S).) The pH of each sample was adjusted using 0.1 M HCl or 0.1 M NaOH and the suspensions left for approximately 12 h under stirring to obtain equilibrium. The experimental procedure corresponds exactly to that used in the potentiometric titrations. After a final pH measurement, the solid part was filtrated and dried in an oven at 50°C for 24 h, then Raman and IR spectra obtained directly from the solid phase without further processing. An alternative method of separating the solid phase from the supematant was tried, which although successful highlighted a serious flaw in the experimental method. When centrifuged, the solid phase separated out into two distinct layers of slightly different colour. After drying, the Raman spectra of the two phases were obtained and compared (Fig. 1). The differences are obvious, and accounted for by comparing the two spectra with the Raman spectrum of poly(tetrafluoroethene), PTFE, known as Teflon. The peaks due to FTFE are marked with a cross. The vigorous stirring had stripped a small quantity of Teflon from the stirring bar and deposited it into the suspension, from which it became attached to the hydrated surface. No estimation of the amount deposited was attempted. The attachment to the surface was observed to be strongest in acidic conditions, since the Raman spectrum of the acidified filtrate contained the strongest Teflon bands (Fig. 2). This can be rationalized if the microscopic particles of Teflon are assumed to have a small negative charge, which attracts them to the positively-charged acidified surface. New samples hydrated for 4 months

694

CHRISDYER et al.

20C0.0

I 1800

I (aa

I 1700

I 16m

I lsm

I

lal

I

1900

I 1200

I wo

I IOm

III 900

200

700

I 6m

I !mo

m

I

I am

2

0

Fig. 2. Raman spectra of stirred samples from acidic, neutral and alkaline suspensions.

without stirring were prepared and the subsequent experiments have shown no Teflon contamination. Potentiometric titrations confirmed that equilibrium condition has been attained in these unstirred suspensions. Both the Raman and IR spectra were recorded using a Perkin-Elmer PE176OX Fourier transform infrared (FUR) spectrometer equipped with a near-infrared (NIR) Raman bench. Data storage and manipulation were carried out using the Perkin-Elmer IR Data Manager (IRDM) software package, version 3.3. In the Raman experiment, the scattering was excited with intensity-stabilized (0.1% rms) 1064nm emission from a Spectron SL 301 neodymium-doped yttrium aluminium garnet (Nd:YAG) laser, and the scattered light collected with 180” backscattering-geometry lens optics. The spectra presented are typically 320 accumulations at 4 cm-’ resolution. The Indium Gallium Arsenide (InGaAs) detector and integral preamplifier were cooled to 77 K (with liquid nitrogen) as this yielded a four-fold gain in signal-to-noise performance. The interference filters used to reject light at the excitation wavelength allowed collection of Stokes Raman scatter greater than 5200 cm-’ shift. The laser reference wavenumber was set to 93% cn-’ in all experiments and the mirror drive speed chosen as 0.1 cm/s. All the Raman spectra presented in this paper have been corrected for instrumental response as a function of wavelength. This was done primarily to avoid gross misinterpretation of relative peak heights, but also to rectify the profile of the strong background exhibited by y-A1203. The correction method used was that of PET-I-Yet al. [9]. The emission source was an electric tube furnace, loaded with crushed ceramic at a temperature of 1373 f 5 K (integral thermocouple rated to 1550 K). A recorded Raman spectrum can be considered to be a product of the “true” spectrum arising from the sample and the “response” spectrum of the instrument, which is strongly wavelengthdependent. This is primarily due to the beamsplitter/detection system/rejection filter combination. Multiplication of the recorded spectrum by the correction function derives a more faithful representation of the true spectrum. The effect of correction is shown in Fig. 3. This sometimes startles new users of FT-Raman spectrometers. A slight degradation in S:N performance is almost unavoidable upon correction. This is because the correction function, being experimentallyderived, contains noise which “contaminates” the spectra on multiplication. However, no spectral smoothing or blanking was carried out at any stage. The samples used in this series of experiments were all fine white powders. They were loaded into the standard solid sampling accessory and illuminated with 600 + 25 mW (integral laser power meter). Typically 100 mg of sample was used. Higher laser power yielded improved S:N ratios, but at the expense of a perceptible heating of the sample holder, which was not evident as a thermal background. To avoid sample degradation, the power was reduced to 600 mW.

Surface hydration of aqueous y-A1203--I. Initial results

695

.O

2

Fig. 3. Raman spectra of Si02, corrected and uncorrected.

For the IR experiments, spectra were collected by the diffuse-reflectance technique, using the Perkin-Elmer DRIFT accessory. A sample dilution of 2.5% by weight in oven-dried, spectro-

scopic grade potassium bromide (KBr) with refractive index 1.559 and particle size 5-20 w was used in all cases. Two hundred scans at 4 cm-’ resolution were collected for all samples. For the X-ray diffraction experiments, dif’fractograms were obtained from 600-700mg of powder sample using a Philips diffractometer (diffraction control unit PW 1710/00). The wavelengths used were 1.54056 and 1.54435 A (Cu Kal and Kaz, respectively). Structures were indexed with the help of the JCPDS Alphabetical Index of Inorganic Compounds.

RESULTSAND

DISCUSSION

Eflects of hydration The X-ray diffractograms and the vibrational spectra of unhydrated and hydrated y-A&O3 are shown in Figs 4-6. The vibrational spectra and XRDs confirm the result of the electrochemical experiments. A reaction has certainly occurred and the effect shown to a greater or lesser extent in all three spectral methods. XRD data shows the formation of a phase containing not only y-A1,03, but also Al(OH)3 in the bayerite polymorph. The JR spectra show that after hydration in neutral water the region around 36OOcm-I, which was previously a featureless strong band, has acquired several sharp but weak features. Weak shoulders on the intense 3600 cm-’ band are visible at 3726 and 3695 cm-‘. A sharp narrow band at 3657cm-’ is followed by a rather broad, medium-intensity band at 3620 cm-‘, and another at 3548 cm-‘, which also has a weak shoulder at 3557 cm-‘. A further medium-intensity broad feature at 347Ocm-’ precedes an asymmetric broad doublet of medium intensity at 3439 and 3422cm-‘. The centre of the doublet is 3431 cm-‘, in agreement with the literature values (see Ref. [5], and references therein). Other features clearly emerge at lower frequency, also in agreement with the literature. An interesting comparison can be made between the hydrated alumina IR spectrum and that obtained from deuterated alumina. The sharp, weak features around 3500 cm-’ have migrated to around 2600 cm-’ (Fig. 7). This is in reasonable agreement with the value predicted from the simple theory of isotopic exchange shifts in OH/OD stretching vibrations. This shows that, in principle, deuteration can be used in this system as an aid to identifying OH modes. The Raman spectra show the biggest change on

6%

CHRISDYER 2.0 18 I

6

et al.

1 i

0.8.

.I

20

30

40

50

60

70

Fig. 4. XRD data of y-Al,O, hydrated for 4 months. Peaks corresponding to alumina are marked A and those due to bayerite marked B. x denotes reference aluminium peaks.

hydration. From a rather featureless, high-intensity background, with only one weak band at around 260 cm-‘, several strong bands emerge below 1000 cm-‘. The strongest band is situated at the 545 cm-’ shift, broadened to lower shift by weak shoulders, estimated to lie at 532 and 524 cm-‘. The medium-intensity band at 434 cm-’ is narrow, whilst that at 386 cm-’ is quite broad. The strong 323 cm-’ is almost as intense as the 545 cm-’ line, but much narrower. The same is true of the 297 cm-’ feature. A weak narrow band at 280 cm-’ shift precedes the medium-intensity, rather broad asymmetric doublet at 251 and 241 cm-‘. This is surprising, as it is quite usual to only observe intense background scattering from aluminas. This effect is well documented and its cause much discussed [lo, 111; the strong, sharp and relatively simple pattern superimposed on this

Fig. 5. IR spectra of hydrated (lower spectrum) and unhydrated (upper spectrum) alumina.

Surface hydration of aqueous y-Al,O,-I.

Initial results

697

,

Fig. 6. Raman spectra of hydrated (upper spectrum) and unhydrated (lower spectrum) alumina.

high background must therefore be due to a new phase. In this system, the phase is bayerite. It should be noted that the suspensions from which the samples were prepared contained Cl-. This was in order to make the hydration conditions used for the spectroscopic studies closely match those used in the potentiometric titrations. When a comparison was made with spectra from samples prepared in deionized water only, no discernible differences were found after several re-trials. We conclude that Cl- plays no significant part in the reaction scheme for the surface.

O.JIo-

03

0.25

0.M

0.15

0.10

0.05

O.OCO_

ux0.

0

I 3Ka

I

1

I

35m

34m

52a

I

xm

I

2m

I

2500

I 2400

I

2x0

U-l

Fig. 7. A comparison of the IR spectra of hydrated (lower spectrum) and deuterated (upper spectrum) alumina between 4WO and 2CQO cm-‘.

M 1.0

CHRISDYER et al.

698

0 CM-1

Fig. 8. Raman spectra of alumina samples from acidic, neutral and alkaline suspensions (upper, central and lower spectra, respectively).

Effect of pH adjustment

A major interest in the surface chemistry of aqueous y-A120, is the pH-dependent speciation. In this series of experiments, the y-A1203 was hydrated for 4 months, then the pH adjusted and the suspension was allowed to equilibrate for 12 h. As indicated by Figs 8 (Raman) and 9 (IR), very small changes are evident in the acidic and basic samples, when compared with the neutral species. Of course the pH cannot be changed drastically because the pH range for surface stability is reasonably small. The lack of major changes is not altogether surprising, since the hydration time is very long compared to the pH adjustment time. Furthermore the acid-base properties of the surface, which are

0.

I I ~03BB 3wo

I WD

I

I

sza m

I

2eoo

I

m

I MO

I I I I I 2alo~,m mm 1700 !4m

I ml

I 1400

I

I

I

aca t?ooIicfJ

I

lamp

U-1

Fig. 9. IR spectra of alumina samples from acidic, neutral and alkaline suspensions (upper, central and lower spectra, respectively).

1

Surface hydration of aqueous y-A1203-I.

Fig. 10. Raman spectrum of hydrated y&O3

Initial results

699

after heat treatment.

obtained from the potentiometric titrations, only involve proton adsorption/desorption reactions, which may not be immediately obvious from vibrational spectra. Different behaviour may be exhibited by systems where the suspension itself is made in acidic or basic conditions, as opposed to merely adjusting the pH after equilibration. However, such systems have not yet been studied electrochemically. Effect of high-temperature drying and vacuum treatment All samples were dried at 50°C for 24 h prior to storage in a desiccator. A small amount of each sample was then heated to 2OO”C,again for 24 h. The spectra (Fig. 10 Raman and Fig.11 IR) show the disappearance of the AI formed on hydration. 0.500

OAF

0.

.O

Fig. 11. IR spectrum of hydrated y-A&O3 after heat treatment.

CHRIS DYER et al.

700

I

I

1100

.O

I lCC0

I 900

1

1

Boo

1

ml

Km

I 5m

I ml

I

4a

0

Fig. 12. Raman spectrum of hydrated y-AI,OI, after vacuum treatment.

However, such behaviour is not exhibited by samples exposed to high vacuum for (<10m9 Torr) for 24 h (Fig. 12 Raman and Fig. 13 IR). The spectra still show the Al(OH)3 vibrational features. The bayerite formed on hydration is either converted to boehmite (AlOOH) by the heat treatment or reverts to A1203. These are the two simplest explanations although the phase transition behaviour of the A&O,-H,O system is quite complex [12]. In fact, depending on the exact nature of the system, eta-A&O, or boehmite are the two likely products. Since boehmite gives excellent IR and Raman spectra (Figs 14 and 15, respectively) we conclude that the dehydration products must be A&O3 and not AlOOH.

0.45 1 0.

0.

I I I I I m,o&?alm15400m5ma?a2m2a2a0

I

I

I

I

I I I, lKl0 leoo 1700 1 2Uii.O

I

I

16a 15Ql WI

a-1

Fig. 13. IR spectrum of hydrated Y-A1203after vacuum treatment.

I II 1.3001x0

I flrn ml

!

.O

Surface hydration of aqueous y-A120,-I.

Initial results

0.70

OS-1

Fig. 14. IR spectrum of boehmite.

It must be stressed that this conversion to bayerite and back to alumina only occurs at the interface layers. The bulk of y-A&O9remains unreacted. This is simple to demonstrate. The broad, intense IR peaks around 3600 cm-’ and 1630cm-’ are due to the 0.2 moles of water required to stabilize y-A&O,. These peaks remain constant throughout the hydration/dehydration and only disappear >9OO”C.Combining this data with the XRD data from hydrated y-A1203(Fig. 4) where the amount of the new phase is shown to be small compared to ~-A1103,we can be sure that the transformation that occurs is primarily an interface and not a bulk effect.

I

WI.0

iim

I

Cm-J

I

nxl

I

em

I

mo

I

0m

I

sm

Ol-1

Fig. 15. Raman spectrum of boehmite.

I

4m

I

3m

2

I.0

702

,

CHRIS

I

I

I

I

rim

1X0.0

I

wo

Km

DYER

et cd.

I

700

BM

I

ml

I

I

I

XII

4m

TX)

2

.O

01-l

Fig. 16. Raman spectrum of y-A&O, hydrated at 60°C.

Effect of hydration at 60°C

Figures 16 (Raman) and 17 (IR) show the comparison between the spectra obtained after 4 months at 20°C and 10 days at 60°C. The band area of the Raman spectra from the 609°C is 2-25times greater than that of the room temperature sample, despite hydrating for one tenth of the time. The effect of heating on the rate of formation of Al(OH)3 is profound. XRD shows (Fig. 18) that the sample contains more bayerite than the sample from hydration at room temperature. The fact that the Raman spectra show the largest degree of change on hydration of y-A&O3 is surprising because alumina usually yields a high background, which obscures other details; it is further surprising because aluminates are generally regarded as poor

0.45

0.

0.35 c 0.X-

0.25-

0.W t 0.15-

0.W

0.W O.Mo

I I I I f 4ow~om3600m32uOm2fim2w024alm

III

I

1 lml 1 XOl.0

I lnm

I I III 1700 16ul lsm l#y) ml

Fig. 17. IR spectrum of y-A&O, hydrated at 60°C

I 12m

II trm

ml

9

.O

Surface hydration of aqueous y-A&O&.

Initial results

703

2.0 16 1.6

X

1.4 m. 1.2 ;; I.0

B

0.6 0.6

0

Fig. 18. XRD of y-A&O3 hydrated at 60°C. Peaks corresponding to alumina are marked A and those due to bayerite marked B. X denotes reference peaks.

Raman scattering species [13]. However, the oxidation of thin films of @1203 has been studied by THOMAS et al. [14] using the conventional Raman technique. They discuss their results with emphasis on the change in surface structure. The changes clearly visible in our spectra of hydrated )~-Al~0, must be due to the formation of hydrated aluminium species at the surface. The dissolution of the alumina is negligibly small in the pH range chosen; any bulk precipitated AlC& or Al(OH)3 would have to be in huge excess to yield the Raman spectra apparent here. A more reasonable explanation is that the amount of bayerite formed is very large relative to the volume of y-A1203. This is due to the large surface area of y-A&OS; 145 m*/g corresponds to approximately 3% of all Al sites being at the surface. It must also be noted that although the Raman spectra are of quite high quality, they are obtained as a result of 320 accumulations. Without the S:N averaging capability of the FT experiment, these features would have been extremely difficult to record. FT-Raman spectroscopy has been applied to amorphous alumina (151 and the results discussed in relation to fluorescence caused by NIR excitation compared with visible excitation. However, the results presented here are the first ET-Raman data on Y-A1203or Al(OH)3, and all the evidence suggests that the hydration of y-A&O3 produces a stable bayerite A1(OH)3 surface. The confident assignment of the Raman bands to bayerite is made possible because the XRD and IR data both identify the hydrated phase unambiguously. Bayerite is

Fig. 19. Raman spectra showing the comparison between a simple mixture of sodium phenylphosphate and hydrated alumina powders (upper spectrum), and phenylphosphate anions adsorbed onto hydrated alumina (lower spectrum). x denotes phenylphosphate peaks.

704

CHRISDYER et al.

.O

Fig. 20. Raman spectra showing the comparison between phenylphosphate anion adsorbed onto hydrated alumina before (lower spectrum) and after (upper spectrum) heat treatment at 200°C.

composed of (Al-O) octahedra in a regular stacked arrangement. It is a good example of a condensed octahedral environment, to use the nomenclature of TARTE [16]. It should now be possible to combine IR, Raman, and XRD data into a definitive analysis of the vibrational modes of bayerite. The IR and Raman data are complementary in this respect. The Raman spectra are uncomplicated at low shifts, consisting of sparse, narrow bands; this allows analysis of (Al-O) modes more easily than in the IR. However, the IR spectra at higher frequency show clearly the (OH) vibrational modes, whereas none are visible in the Raman. The identification of the surface layer as bayerite Al(OH)3 has important implications for the potentiometric titration experiments. It can now be seen that the proton adsorption/desorption reactions are not occurring at a hydratedlhydroxylated y-A&O3 surface, but on a completely different phase only formed from the y-A&OS.

CONCLUSIONS

FI-Raman spectroscopy has proven to be a very useful tool in the study of the hydroxylated y-A1,03 surface. The results presented here provide the starting point not only for a more in-depth and fundamental study of the vibrational spectra at all stages of hydroxylation, but also the basis for the study of phosphate/hydroxide competition on y-A1203. To show the direction of future work, we show the effect of adsorbing phenylphosphate anions onto the hydrated y-A1203. Pure sodium phenylphosphate is a good Raman scatterer with the most intense band at 1001 cm-‘, which is due to the phenyl ringbreathing vibration. Figure 19 shows the Raman spectrum of sodium phenylphosphate dry-mixed with hydrated Y-A&O~ powder, compared to hydrated Y-A&O~with phenylphosphate actually adsorbed at the surface. (Phenylphosphate peaks are marked with a cross.) The upshift in the ring-breathing mode to 1006cm- implies loss of n-electron density in the phenyl ring, which is a result of the bonding to the hydrated alumina bayerite surface. Confirmation that a reaction has indeed taken place is given in Fig. 20, which shows the Raman spectra of the adsorbed sample before and after heating to 200°C for 24 h. The bayerite has disappeared, yet the phenylphosphate is still in evidence weakly. In summary, we have shown that IR and Raman spectroscopy in combination

Surface hydration of aqueous y-AizOB-I.

Initial results

705

are very useful analytical methods for the study of the competition of hydration and phosphate ion adsorption at hydrated y-A1203surfaces. Acknowledgemen&-The authors want to thank Dr Elena Babouchkina for valuable help with the KRD measurements. The work forms part of a programme 8nanciaIIy supported by the Swedish Natural Science Research Council and the Swedish National Board for Technical Development. C.D. gratefully acknowledges financial support from the Office of Naval Research, United States Department of Defence.

REFERENCES

[l] K. S. Chari and B. Mathur, Thin Solid Films 891,271 (1981). [2] G. A. Parks, in MineruCWuter Interfke Geochemistry, Vol. 23 (Edited by M. F. Hocheila, Jr and A. F. White), pp. 133-176. BookCrafters (1990). [3] P. W. Schindler, in MineraCWater Interfizce Geochemistry, Vol. 23 (Edited by M. F. Hocheila, Jr and A. F. White), pp. 281-308. BookCrafters (1990). [4] P. W. Schindler and W. Stumm, in Aquatic Surface Chemi.stry (Edited by W. Stumm), pp. 83-110. Wiley-Interscience (1987). [5] R. S. Alwitt, in Oxides and Oxide Films (Edited by J. W. Diggie and A. K. Vijh). Marcel Dekker, Inc. (1976). [6] L. Wu, W. Forsling and P. W. Schindler, J. Colloid Interface Sci. 147, 178 (1991). [7] J. A. Davis and D. B. Kent, in MineraCWuter Interface Geochetnistry, Vol. 23 (Edited by M. F. Hocheila and A. F. White), pp. 177-260. BookCrafters (1990). [8] L. 0. Ghman, personal communication. [9] C. J. Petty, G. M. Warnes, P. J. Hendra and M. Judkins, Spectrochitn. Acta 47A, 1179 (1991). [lo] P. J. Hendra, J. D. M. Turner, E. J. Loader and M. Stacey, J. P&r. C/rem. 78,300 (1974). [ll] H. Jezirowski and H. KnGzinger, Chem. Phys. Lett. 42,162 (1976). [12] K. Wefers and G. M. Bell, Oxides and Hydroxides of Aluminium. Alcoa technical paper Nr 19, Alcoa research laboratories (1972). [13] R. S. Krishnan, Proc. Indian Acad. Sci. %A, 450 (1947). [14] P. V. Thomas, V. Ramakrishnan and V. K. Vaidyan, Thin Solid Films 170,35 (1989). [15] A. Mortensen, D. H. Christensen, 0. F. Nielsen and E. Pederson, J. Ruman Spectrosc. 22,47 (1991). [16] P. Tarte, Spectrochim. Actu 23A, 2127 (1%7).