Langmuir 1989,5, 96-100
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Surface Chemistry of Zirconia Polymorphs W. Hertl Research & Development Division (SP-FR-6), Corning Glass Works, Corning, New York 14831 Received October 15, 1987. I n Final Form: June 15, 1988 An FTIR spectroscopic study was carried out to determine and compare the different surface chemical properties of the amorphous, tetragonal, and monoclinic phases of zirconia powders. When zirconia is prepared by precipitation from solution it can exist in any of the three polymorphs, depending on the thermal history. Ammonia and pyridine reactions showed that all three polymorphs have both Lewis and Bronsted acid activity. Zr-NHz groups form on tetragonal zirconia. CO produced formates on monoclinic- and tetragonal-phase zirconias. A terminal CO band was observed only on amorphous-phase zirconia and indicated that the Lewis acid site carries a +2 charge. The reactions with COz showed that the surface basicity of all three polymorphs can involve OH groups to form HC03-. C032- is also detectable on tetragonal-phase zirconia. Bidentate carbonates are observed on monoclinic ZrOz, and at elevated temperatures unidentate carbonates form. DzO exchanged completely with the highest frequency monoclinicand tetragonal-phase OH groups. The monoclinic- and tetragonal-phase hydroxyls reacted readily with alcohol to form alkoxy1 groups, which oxidized at elevated temperatures to form carboxylate groups. The amorphous-phase zirconia did not react with alcohol.
Introduction Zirconia is an important material due to its interesting thermal and mechanical properties. The bulk properties of zirconia have been extensively studied. However, there is little published information about the surface chemical properties of zirconia, and it is these properties which are important in processing, lubrication, catalysis, etc. Zirconia, which is prepared by precipitation from solution, can exist in any of three metastable morphologies depending on the thermal treatment. In the available literature, the phase is often not specified and the thermal history is not always clear. A study was undertaken to survqy the surface chemical properties of the amorphous, tetragonal, and monoclinic phases of zirconia. Chemical probes and FTIR were used for most of these studies, which included D20exchanges, pyridinejammonia to identify Lewis acid sites (surface zirconium ions) and Bransted acid sites (H+ donor sites), CO to determine the charge on the surface cations, C 0 2 to measure the basicity of surface oxygen ions, and alcohol reactions to measure OH reactivity. Differences in the surface chemical properties were observed on the three different polymorphs.
Experimental Section 1. Zirconia. The zirconia powders were obtained from Magnesium Elektron, Inc., Flemington, NJ. X-Ray diffraction was used to identify the specific phases. a. Monoclinic Phase. Grade E-10 powder had BET surface area = 14 m2/g.
b. Amorphous Phase. Grade SC-100 powder had an initial BET surface area = 32 m2/gm which decreased with modest heating. The spectra showed that the powder contained some nitrate groups, presumably as a residue from the synthesis method. c. Tetragonal Phase. The tetragonal-phase zirconia was prepared by a vacuum thermal treatment of the amorphous powder and had a surface area = 5 m2/gm. Below 375 "C the zirconia remained amorphous, but between 375 and 700 "C under vacuum the amorphous zirconia transformed to the tetragonal phase. Above 700 "C (500 "C in air) the zirconia was converted to the monoclinic phase. The stable phase of zirconia is monoclinic up to 1170 "C, where it transforms to the tetragonal phase. However, when finely divided zirconia is formed by precipitation from solution, ignition yields a powdered oxide that crystallizesto the tetragonal phase.' (1) (1) Wright, A. F.; Nunn, S.; Brett, N. H. in Advances in Ceramics; Clauasen, N., Heuer, A. H., Eds.; American Ceramic Society: Washington, D.C.,1983; Vol. 12, p 784.
0743-7463/89/2405-0096$01.50/0
2. Reagents. All liquids were reagent grade. All gases were Lecture Bottle grade. Zirconium 1-propoxidewas obtained from Alfa Products, Danvers, MA. 3. Spectra. All spectra were recorded with a Perkin-Elmer Model 1800 double-beam Fourier transform infrared spectrophotometer. A Harrick Scientific Co. diffuse reflectance accessory (DRIFT) equipped with a heatable vacuum cell and ZnS windows was used to hold the powders. The spectra were acquired at 4-cm-' resolution by using a DTGS detector operating at ambient temperature. Peak to peak signal to noise ratio, measured on the background of all the spectra over a lOO-cm-' interval between 2100-2200 or 3000-3100 cm-l, was 11501 or better. Spectral changes are often more easily visualized by using difference spectra. These were obtained by taking spectra of the powder in the cell immediately before and after a given treatment. No reference beam compensation was used. A 1:l digital subtraction of the after treatment spectrum minus the before treatment spectrum was carried out with the software provided with the spectrophotometer. The frequency repeatability of FTIR machines permits the accurate mathematical substraction of spectra to be carried out. 4. Treatments. The powder treatmenh were carried out in the heatable vacuum cell. The cell was attached to a vacuum rack, which attained a vacuum better than Torr, and the desired gas or vapor was then metered into the cell. At the completion of the treatment the cell was evacuated, cooled, closed off, and placed in the reflectance accessory, and the spectrum was recorded. All spectra were obtained with the powders at ambient temperature. The D20 exchange was carried out in the vacuum cell by using three 5-min exposures to DzOvapor at room temperature. The powder was heated to 200 "C for the final evacuation. The DzO exchange was nearly complete after the three exposures. Additional or longer D20exposures at 25 or 200 "C resulted in very small additional exchange. This small continuing exchange is presumably due to D 2 0 diffusion into the bulk material.
Results and Discussion This section will be divided into the following topics: (1) hydroxyl groups and water, (2) ammonia and pyridine for detecting acid sites, (3) CO for detecting surface cation charge, (4) COz for detecting surface basicity of oxygen ions, and (5) alcohol reaction with the surface. Within each section the surface properties of the amorphous, monoclinic, and tetragonal zirconia polymorphs will be compared and contrasted. 1. Hydroxyl Groups. Evacuation removed a substantial amount of water which reabsorbed on exposure to the atmosphere. This behavior is t y p i d of most oxides. 0 1989 American Chemical Society
Langmuir, Vol. 5, No. 1, 1989 97
Surface Chemistry of Zirconia Polymorphs Table I. Bands Observed with Ammonia Adsorption" monoclinic tetragonal amorassign400 "C 150 "C 350 "C phous ment 150 "C OH 3700-37601 3677-37401 37364 37331 36591 3528 3528 NH, 3505 3503 NH on 3352-3389 3349 3360 3340-3390 3347 L.a. 3260 3256 3272 3268 3271 3154 NHI+ 3205 3205 vw 3164 vw 3160 3164 16241 16471 16281 H20 16381 1606 1604 1602 NH on 1604 L.a. 1566-1525 1550 1550 1563 NHZ 1550 1499 1430-1445 1406 NH4+ 1445 1395 1376 1188 1164 "L.a. = Lewis acid site. 1 and indicate intensity decrease or increase of band present prior to NHS treatment.
Hydroxyl band frequencies for the monoclinic phaseH and tetragonal phase3p4have been reported and in some cases discussed in Generally from two to four discrete hydroxyl bands are observed. When OD for OH exchange takes place the intensities of the OH/H20 bands in the 3400-3800-cm-' region decrease, and new bands due to OD appear in the 24002800-cm-' region. When the DzO exchanges were carried out on the three polymorphs, all the observed OH bands showed a t least partial exchange. The highest frequency OH bands on the monoclinic-phase (3769 cm-') and tetragonal-phase (3744 cm-') zirconias exchanged completely. Reported deuterium exchange experiments, using a series of deuteriated compounds, showed that the 3780-cm-' OH band is more reactive and labile than the 3680-cm-' Zr-OH band.7 The broad bands a t lower frequencies on the tetragonal-phase (3540 cm-'), and amorphous-phase (3533 cm-') zirconias were 40% and 60% exchanged, respectively. Little change was observed in the molecular water bending band a t 1620 cm-', indicating that these broad bands are due to mutually H-bonded OH groups rather than water. 2. Ammonia and Pyridine. Lewis acid sites (surface zirconium ions) are detectable when ammonia or pyridine forms a coordinate bond with the surface. Other sites are also detectable. Ammonia adsorption was carried out on the three zirconia polymorphs at various temperatures from ambient to 400 "C.After evacuation at 25 O C a large amount of ammonia was sorbed, and the high-frequency OH band was perturbed to a lower frequency, due to NH3 which was H-bonded to surface OH groups. On evacuation a t 150 "C the OH band reappeared, and most, but not all, of the NH3 was removed. In Figure 1 are given difference spectra of monoclinic-, tetragonal-, and amorphous-phase Zr02 before and after exposure to ammonia at 150 "C or higher. The NH3 bands observed are given in Table I along with the band assignments. a. Lewis Acid Sites. All three polymorphs showed the presence of Lewis acid sites (surface zirconium ions), which (2) Tsyganenko, A. A,; Filimonov, V. N. Spectrosc. Lett. 1972,5(12), 477. (3) Erkelens, J.; Rijnten, H. Th.; Eggink-DuBurck,S. H. Recueil 1972, 91, 1426. (4) Argon, P. A.; Fuller, E. L.; Holmes, H. F. J. Colloid Interface Sci. 1975,526), 553. (5) Tret'yakov,N. E.; Pozdnyakov, D. V.; Oranskaya, 0. M.; Filimonov, V. N. Russ. J . Phys. Chem. 1970,44(4),596. (6) He, M.-Y.;Ekerdt, J. G. J . Catal. 1984, 87, 381. (7) Yamamchi. T.: Nakano.. Y.:. Tanabe, K. Bull. Chem. SOC.Jpn. 1978, 52(9), 5482.' '
MONOCLINIC
I o 004A
w
0
z a m (r
u) 0
m a
I
AMORPHOUS
J
4' 0
3500
3000
FREQUENCY ( c r n - l )
Figure 1. Difference spectra before and after NH3 adsorption on monoclinic and amorphous zirconia at 150 "C and tetragonal zirconia at 350 O C . The band frequencies and assignments are given in Table I.
form coordinate bonds with the unshared electrons of the ammonia. The presence of the asymmetric and symmetric NH vibrational bands near 3350 and 3260 cm-' along with the bending vibration band near 1600 cm-' is typical of coordinately bound ammonia."l* b. Bronsted Acid Sites (Proton Donors). The formation of NH4+ionic species shows the presence of proton donor sites. All three polymorphs showed very weak bands due to the presence of NH4+, and the monoclinic and amorphous phases also showed the disappearance of OH groups. Some of the NH4+ might also arise from the presence of residual strongly sorbed water, which was also observed to decrease. c. Covalent Bonding. Ammonia covalently bonds to Zr with the formation of bonded Zr-NH2 and hydroxyl groups.* ZrO NH3 ZrO(OH)Zr(NH,)O
+
-
Bands due to the NH2 group (3528/3503 cm-') were observed on the tetragonal zirconia along with the formation of the OH group (3733 cm-'). Due to concurrent reactions which consume OH groups, it was not possible to detect whether new OH groups were formed on the monoclinic and amorphous polymorphs. To obtain a quantitative estimate of the concentration of surface binding sites, monoclinic- and tetragonal-phase ZrOz were treated with NH,, evacuated a t 150 O C and analyzed for nitrogen. The calculated surface NH, populations were as follows: monoclinic ZrOz, 0.73; tetragonal ZrOz, 0.46 molecule/nm2. The spectra showed that the principal surface species is due to NH3 bonded to Lewis (8)Tsyaanenko, A. A.; Pozdnyakov, D. V.; Filimonov, V. N. J. Mol. Struct. lg'i5, 29, 299. (9) Kung, M. C.; Kung, H. H. Catal. Reu.-Sci. Eng. 1985,27(3), 425. (10)Little, L. H. Infrared Spectra of Adsorbed Species; Academic: New York, 1966.
Hertl
98 Langmuir, Vol. 5, No. 1, 1989 w
Y /
a
-i
a
J
*
a 002411
1
II I
1800
1600
1400
FREQUENCY (ern-')
Figure 2. Difference spectrum of monoclinic-phase zirconia before and after pyridine adsorption. The sample was evacuated at 150 O C . Bands due to aromatic CH vibrations also appear at 3061 and 3078 crn-l.
acid sites. These values can be considered a reasonable estimate of the maximum Lewis acid site concentration. When the ammonia-treated sample was exposed to ambient air for 0.5 h and then evacuated a t 150 "C it was found that some water had readsorbed. About 40% of the bound ammonia had hydrolyzed and was removed by the evacuation. Some H2 pretreatments were carried out a t 150 "C on the monoclinic-phase zirconia to see if any detectable surface reduction takes place. It has been reported that when H2 is passed over ZrOz a t room temperature after degassing at >700 "C OH bands appear along with bands a t 1562 and 1371 cm-' assigned to Zr-H. On heating to 250 "C, the OH bands increased in intensity and the Zr-H bands disappeared." Calcination at only 500 "C followed by H2 exposure at 200 "C showed OH bands appearing but no Zr-H.12 After the Hz pretreatment the Zr02 was exposed to NH3 at 150 "C. No major differences in sorbed NH3 were observed between the Hz treated and untreated zirconias. The NH band area of bound NH3 (between 3311 and 3428 cm-') varied from 1.3 on the initial surface to 1.6 on the Hz-pretreated surface. Some minor differences were observed in the OH bands in the 3650-3775-cm-l region. Pyridine also adsorbs on Lewis acid site^.'^-'^ A difference spectrum of pyridine adsorbed on monoclinic-phase ZrOz is shown in Figure 2, where the bands are identified?$ At 25 O C a substantial amount of pyridine was H-bonded to the OH groups. This was largely removed by evacuation at 150 O C . The two strong bands a t 1444 and 1606 cm-' are diagnostic for pyridine coordinately bound to Lewis acid sites. The presence of a band due to PyH+ along with decreased OH intensity showed the presence of some surface Brmsted acidity (H+donation). Nakano et a1." report that pyridinium ions do not form at 100 "C on ZrOz but are observed when adsorption takes place a t 200 OC and higher. 3. Carbon Monoxide, If CO binds to metal ions it can give additional information about Lewis acid sites. No detectable reaction took place between CO and any of the ~~
~
(11)Kondo, J.; Abe, H.; Sakata, Y.;Maruya, K.; Domen, K.; Onishi, T. J. Chem. Soc., Faraday Tram1 1988, (84(2),611. (12)Abe, H.; Maruya, K.; Domen, K.; Onishi, T. Chern. Lett. 1984, 1875. (13)Mortarra, C. I.; Coluccia, S. I.; Chiorino, A,; Boccuzzi, F. J.Catal. 1978,54, 348. (14)Pohle, W.I.; Brauer, P. J. Catal. 1982, 77,511. (15)Pohle, W.;Fink, P. 2.Phys. Chern. (Munich) 1978, 109, 77. (16)Morterra, C. I.; Ghiotti, G.; Boccuzzi, F.; Coluccia, S. J. Catal. 1978,51, 299.
(17)Nakano,Y.;Iizuka, T.; Hattori, H.; Tanabe, K. J. Catal. 1979,57,
1.
4000
V
I
I 3000
1
2000 FREQUENCY (cm-')
I
1500
Figure 3. Difference spectra of three zirconia polymorphs before and after CO adsorption at 400 O C .
three polymorphs up to 150 OC. All the CO reactions here were carried out a t 400 "C. a. Monoclinic and Tetragonal Phases. Figure 3 gives the difference spectra of monoclinic- and tetragonal-phase zirconias before and after CO exposure. Adsorption of CO on hydroxylated and dehydroxylated oxides, including Zr02, was studied by Pozdnyakov and Filimonov.18 They found that the OH groups play an important role in the formation of surface formates above 250 "C. CO adsorption led to intense, possibly overlapping, bands in the ranges 1610-1580 and 1400-1350 cm-' and some absorption in the 2850-295O-cm-' range corresponding to CH. They assigned these bands to formate groups on the basis of comparisons with spectra of adsorbed formic acid. Kondo et a1.l' reacted CO with ZrOz at 100 "C for 10 h; bands appeared at 1586,1385, and 1361 cm-' which were assigned to the antisymmetric COz stretch, CH in-plane deformation, and symmetric COz stretch of the formate ion. The CH stretching bands in their spectra are only barely detectable. At 250 "C a similar spectrum with increased formate was obtained. The bands shown in the spectra in Figure 3 fall within the ranges assigned to formate groups, although absorption in the 3000-cm-l region is very weak. The band frequencies were similar, but not identical, on these two Zr02 polymorphs. The reactionle which gives the surface formate structure isle CO
+
ZrOH
-
0
'4
Zr-'C-H
dY
b. Amorphous Phase. Figure 3 also gives the difference spectrum of amorphous-phase Zr02 before and after CO exposure. Carbonyl bands appeared at 1883 and 2170 (18)Pozdnyakov, D. V.;Filimonov, V. N. Zh.Fiz. Khim. 1972,46(4), 1011. (19) Rethwisch, D. G.; Dumesic, J. A. Langrnuir 1986,2,73.
Surface Chemistry of Zirconia Polymorphs W
Langmuir, Vol. 5, No. 1, 1989 99 W
y p q
a m
0051
0
.I;
1800
1600 FREQUENCY (cm-1)
w
0
z a
m Lz
% m
400" REACTION
a
I400
Figure 4. Difference spectra of monoclinic-phasezirconia before and after C02 adsorption at 25 and 400 "C. cm-', along with possible carbonate bands at 1393 and 1650 cm-l, but no formate bands were observed. The band frequency due to the terminal CO can be used to deduce the oxidation state of the Zr surface siteg as shown: surface state
terminal CO frequencies, cm-'
M2+-CO M+-CO
>2170 2 120-2 160 2143
