surface characterization of yttria-stabilized tetragonal zro2 part 1. structural, morphological, and...

J. CHEM. SOC. FARADAY TRANS., 1995, 91(1), 113-123 113

Surface Characterization of Yttria-stabilized Tetragonal ZrO, Part 2$-Adsorption of CO

C. Morterra,* G. Cerrato, V. Bolis and C. Lamberti Department of Inorganic, Physical and Materials Chemistry, University of Turin, Via P. Giuria 7, I- 10 125 Turin, Italy L. Ferroni and L. Montanaro Department of Materials Science and Chemical Engineering, Turin Polytechnic, Turin, Italy

Two preparations of tetragonal zirconia (t-ZrO,) stabilized with 3 mol% Y,O, have been characterized, and their features compared with those of a 2 mol% t-ZrO, preparation and of a monoclinic zirconia (m-ZrO,) preparation.

At ambient temperature, 60 adsorption occurs only on surface cationic Lewis acid sites located in crystallographically defective configurations. The various t-ZrO, preparations present several types of such defective sites, depending on the temperature of the sintering stage. The spectral features of the ambient-temperature interaction of t-ZrO, with CO are complemented, on quantitative grounds, by gas-volumetric and micro- calorimetric data. At high firing temperatures, amorphous phases start segregating from the t-ZrO, crystallites, causing the ambient-temperature adsorptive capacity towards CO to decline.

Upon CO adsorption at low temperaure (ca. 77 K), weaker adspecies form prevalently. These are ascribed to CO adsorption on cationic Lewis acid sites located in regular patches of low-index crystal planes, and to CO H-bonded to surface OH groups.

Different t-ZrO, preparations and differently sintered samples exhibit different particle sizes and various pro- portions of crystal surface defect sites vs. regular-face terminations : these differences are reliably monitored by CO adsorption patterns observed via iR spectroscopy, both at 300 and ca. 77 K.

During the last decade, powdery microcrystalline preparations of both monoclinic (m-ZrO,) and tetragonal zirconia (t-ZrO,) have been receiving increasing attention, owing to their wide range of applications from the field of heterogeneous catalysis to that of neo-ceramic tools.

The monoclinic modification is most frequently used in heterogeneous catalysis, as here the crystalline form does not seem to play a major role, and m-ZrO, preparations exhibit high surface area and high thermal stability (at least up to ca. 1500 K). Several papers have dealt with the structural and surface chemical characterization of m-ZrO, microcrystalline preparations (e.g. see ref. 1-7, and references therein).

t-ZrO, preparations are instead required for neo-ceramic materials. The t-ZrO, phase is stable either at high temperatures (above ca. 1473 K), or at temperatures below 1300 K if the crystallites were prepared under certain conditions' and/or their average size is below ca. 300 A.9 t-ZrO, is thus most often stabilized by the addition of small amounts of divalent (e.g. Ca2 +) or trivalent cations (e.g. Y3 ').

This series of papers deals with the surface physico- chemical characterization of some Y-stabilized t-ZrO, preparations. The aim is to find out whether any correlation exists between the structural and morphological features of t-ZrO, crystallites, isolated in some early stages of the thermal sintering process, and their surface chemical behaviour. Part 1 of this series" described the evolution with firing temperature of the size and shape of the crystallites of three Y- stabilized t-ZrO, preparations, and the parallel modification of the surface functionalities. This paper reports some aspects of the surface acidic features of two of the three Y-stabilized t-ZrO, preparations dealt with in Part 1. The acid features have been studied mainly by following the ambient- temperature interaction of the t-ZrO, systems with CO by IR spectroscopy and microcalorimetry, but some spectroscopic

t Part 1: Ref. 10.

data will also be reported concerning the high-coverage adsorption of CO attained at low temperatures (ca. 77 K).

Experimental Materials

Two yttria-stabilized t-ZrO, preparations, termed TO3 and TZ-3Y, respectively, have been examined in detail, and occasionally an m-ZrO, preparation, termed ZRP, was used for comparison. Their main features are as follows: (i) The TO3 specimens, which contain 3 mol% of Y 2 0 3 as phase- stabilizing agent, were prepared by the sol-gel method, as described in detail in ref. 10. After the preparation, aliquots of the powdery TO3 specimens were fired in air for 3 h at Tl = 873, 1073, 1173 and 1323 K; the resulting materials are designated as T03873, T031073, To31173 and T031323, respectively. Another t-ZrO, preparation, termed T02, prepared by the same procedure as for the parent TO3 preparation and containing 2 mol% Y203 , was also used for comparison pur- poses.

The BET surface area of the T03,, preparations varies continuously between ca. 80 and ca. 35 m2 g-' in the firing temperature range considered, as already described in ref. 10. (A somewhat faster decrease of surface area was noted in the case of the TO2 specimens after firing at TI > 1073 K.)

X-Ray diffraction (XRD) data indicate that, after firing at ca. lo00 K, the TO3 preparation is entirely (micro-)crystalline, i.e. there are no broad structureless features to be ascribed to amorphous ZrO,, and the crystal phase is virtually pure (micro-)crystalline t-ZrO, , lo whereas for TI 2 lo00 K the amount of bulk m-ZrO, phase increases slowly, but still remains well below ca. 15%.

Transmission electron micrographs (TEM) are reported in detail in Part 1," and indicate that, after firing at Tl 2 1173 K, the low-Y,O,-content preparations like TO2 are unstable, and an amorphous phase starts to segregate out, which

114 J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91

rapidly and thickly coats the t-ZrO, crystallites. The amorphous phase was reported in ref. 10 to be much richer in Y203 than the starting materials. This segregation phenomenon is virtually absent in the case of the more stable TO3 preparations, at least for firing at temperatures up to ca. 1300 K. After firing at T1 2 1323 K, the segregation of an amorphous Y-rich phase also begins to occur in the case of the TO3 preparations, although quite slowly and apparently to a limited extent.

(ii) TZ-3Y is a widely used powdery ceramic material of commercial origin,? containing 3 mol% Y 2 0 3 as stabilizing agent and fired at T < 1273 K (manufacturer's specifications). This is designated as TZ-3Y, which does not carry any sub- script numeral 7'' whenever the starting material was fired in air or in vucuo at temperatures below ca. 1200 K. [In fact, for most of the present work, the starting TZ-3Y was fired in air or oxygen only at ca. 800 K, i.e. at a temperature much lower than the highest temperature experienced in the preparation (nominally, ca. 1173 K), and this was done in order to get rid of the abundant organic contaminants added as binder during the preparation.] Whenever the oven firing treatment of TZ-3Y was carried out at TI 2 1200 K, the symbol TZ- 3YT, is used to designate the samples.

The BET surface area of the starting TZ-3Y material is 18 m2 g-', and declines slowly to ca. 15 m2 g-' for firing at T' = 1323 K. XRD data (see ref. 10) indicate that the starting TZ-3Y preparation, nominally prefired at 7'' z 1173 K, con- tains ca. 12% of the m-ZrO, phase, and becomes virtually pure t-ZrO, upon firing at Tl > 1300 K.

(iii) As a reference material for the m-ZrO, phase, samples of a virtually pure microcrystalline preparation, previously termed ZRPT,, have been used after oven firing at TI 2 873 K and after vacuum activation at various temperatures T, , as described below. The reference ZRP material, which was prepared by the hydrolysis of Zr isopropoxide as described by Bensitel et aE.,, has been thoroughly characterized in previous

Methods

After the preparation and the preliminary firing steps, t-ZrO, specimens were stored in air. Samples suitable for in situ FTIR experiments were prepared in the form of self- supporting pellets (ca. 30 mg ern-,), and transferred into the vacuum system (residual P G lo-' Torr) for in situ treatment. The pellets were vacuum activated and oxidized at ca. 800 K to remove surface contaminants, rehydrated at ambient temperature in the vacuum system with saturated water vapour, and eventually vacuum activated (normally for 2 h) at the chosen temperature ( T,). The activation temperature, T, , is indicated as a numeral following the sample symbol (e.g. T03,0,3673 stands for a pelleted sample of preparation T03, oven prefired at 1073 K, surface cleaned in uucuo and in oxygen at ca. 800 K, thoroughly rehydrated at ambient temperature, and eventually vacuum activatesat 673 K prior to adsorption experiments).

FTIR spectra were run at a resolution of 2 cm-' on Bruker spectrophotometers (IFS 113v and IFS 88), equipped with an MCT detector. The ro-vibrational contribution of the gaseous CO phase was computer subtracted from the CO adsorption spectra. Spectral integrations and band resolutions were carried out, when needed, on unsmoothed segments of the absorbance spectra using either a Pascal program by Bruker (Simband), in which one sets only the

f TSK zirconia powder, type TZ-3Y (3% Y,O, lot. N. Z306341P), produced and distributed by Toyo Soda Manufacturing Co., Tokyo, Japan.

number of spectral components and the desired accuracy, or a home-made Fortran program, which is based on Minuit minimization capabilities' and has been described else- where. ' ,* '

Heats of adsorption of CO have been measured at 303 K by means of a standard Tian-Calvet microcalorimeter (Setaram, France), connected to a vacuum/gas volumetric apparatus that enables the simultaneous determination of the heat released and of the amounts adsorbed during the adsorption process. A stepwise adsorption procedure was followed, as described e1se~here.l~

A few HRTEM micrographs, supplementary to the TEM data presented and discussed in detail in ref. 10, were obtained with a JEOL JEM 2000 EX (200 kV, LaB, filament, top-entry stage).

Results and Discussion The ambient-temperature adsorption of CO followed by IR spectroscopy is commonly regarded as a versatile tool for the characterization of the strongest Lewis acid sites present at the surface of oxidic systems without d electrons. In fact, on these systems (and this is certainly the case for ZrO,, as long as no reduction to Zr3+ occurs) CO is adsorbed at 300 K in small amounts on highly uncoordinated surface cationic centres, acting as strong Lewis acid sites, through a a-charge released from the 5 0 lone-pair orbital of CO. The charge release brings about an increase of the vco vibrational mode of the CO molecule (vco = 2143 cm-' for the free gaseous molecule): the increase measures the charge-withdrawing capacity of the adsorbing centres, and thus their Lewis acidic strength." Complementary to CO adsorption at ambient temperature, the spectroscopic study of CO adsorption at low temperatures (ca. 77 K), which reaches high coverages (almost a full monolayer), yields information on all of the types of less acidic (and normally more abundant) coordi- natively unsaturated (cus) surface cationic centres, while the interaction of CO with the strongest acid centres is observed less clearly than at 300 K as a consequence of the overwhelm- ing amount of CO on the weaker sites and of the resulting strong adsorbate-adsorbate perturbations.

Only some preliminary spectroscopic results have been reported so far for the CO/t-ZrO, system,'6 whereas the system CO/m-ZrO, has been investigated in recent years in more detail (e.g. see ref. 6 and 14, and references therein).

t-ZrO, Materials fired at T, < 1170 K IR Data: Adsorption at ca. 300 K The ambient-temperature uptake of CO on T03,,, is negli- gible for samples vacuum activated at 5523 K. On these systems the thermal treatment causes only the desorption of undissociated coordinated water lo and the liberation of surface cus cationic sites that are still too few and too weak to coordinate CO. After activation at 2523 K, the elimi- nation of surface hydroxy groups starts occurring to an appreciable extent, and there is formation of stronger and more abundant cus Zr4+ centres. On these systems, CO uptake at ambient temperature becomes observable in the IR spectrum, with the formation of a strong broad absorption in the 2220-2160 cm-' range.

Fig. 1-3 report some spectral features of the adsorption of CO onto a T03873 system, activated in vacuo at three temperatures in the interval 673-1173 K. The vco us. (A,),,, plots of Fig. l(c)-3(c) account for the spectral shifts caused by inductive effects produced by adsorbed (charge-releasing) CO on adsorbing (charge-releasing) CO. In the case of several oxidic systems similar to the present one the vco us. (Aco)t,t

J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91 115

a, C

-e E s

2220 2200 2180 2160 wavenumber/cm-'

g ,,t 3

c I

E

n $ E, Q)

m 3

2200/

jgO~ 21 80;

0 2 4 6 8 1 0 1 2 1 absorbance (arb. units)

Fig. 1 Spectral features after adsorption of CO at 300 K onto T03*,,673. (a) Spectral pattern upon adsorption for Pco in the range 1.0 x 10-'-1.3 x 10' Torr. (b) Optical adsorption isotherms (integrated absorbance us. Pco). (0) Total isotherm; (A) segment of the isotherm of species (CO), (see text); (0) segment of the isotherm of the remaining absorption (0) - (A). (c) Spectral position of the resolved (CO), component (A), as a function of the total integrated absorbance of adsorbed CO [v(,,,~ us.

a, 0

m -e f ? s

wavenum ber/cm - ' pc,/Torr

221 0

r I 5 2200

E,

'r- P

a,

$ 2190

21 80 3

I , i , l , l . I , I . I ' i ,

2 4 6 8 1 0 1 2 1 absorbance (arb. units)

1

Fig. 2 Spectral features after adsorption of CO at 300 K onto T03,,,873. (a) Spectral pattern upon adsorption for P,, in the range 1.0 x 10-'-1.3 x 10' Torr. (b) Optical adsorption isotherms (integrated absorbance us. P,,). (0) Total isotherm; (A) isotherm of the resolved species (CO), ; (0) isotherm of the resolved species (CO),. (c) Spectral position of the resolved CO components (CO), (A) and (CO), (0) as a function of the total integrated absorbance of adsorbed CO [v(co)i us. (Aco)tot].

wavenumber/cm- ' Pco/Torr

21 80 3 0 2 4 6 8 1 0 1 2

absorbance (arb. units) 4

Fig. 3 Spectral features after adsorption of CO at 300 I( onto T03,,,1073. (a) Spectral pattern upon adsorption for Pco in the range 1.0 x 10-'-1.3 x lo2 Torr. (b) Optical adsorption isotherms (integrated absorbance us. P,,). (0) Total isotherm; (A) isotherm of the resolved species (CO),; (0) isotherm of the resolved species (CO),. (c) Spectral position of the resolved CO components (CO), (A) and (CO), (e), as a function of the total integrated absorbance of adsorbed CO [ v ( , ~ ) ~ us. (Aco)tot].


plots were found to be linear, whenever well defined CO adsorbed species were formed, and to yield useful information on some properties of the relevant adsorption sites. For more details see ref. 6, 15 and 17.

Starting with the adsorption pattern on To3873673 [Fig. l(a)J, it can be noted that the band of adsorbed CO most likely comprises several closely overlapped components. In fact, at very low CO coverages there is only a sharp and quasi-symmetrical CO band (see curves 1 and 2 in Fig. l(a); v,,, = 2206.5 cm-'; Avl12 x 10 cm-I), whereas at increasing coverages the overall CO absorption shifts to lower wavenumbers to an extent much greater than can be accounted for by the adsorbate-adsorbate inductive effects, becomes asym- metrical on the low-wavenumber side and its overall breadth greatly increases. [For instance, in curve 3 in Fig. l(a), the overall A v I I 2 is 26 cm-'-1 It is thus revealed that the strong Lewis sites at the surface of unsintered TO3 t-ZrO, are heterogeneous, consistent with the low crystalline order and irregular particle morphology reported for this system."

A computer spectral deconvolution of band patterns like that of Fig. l(a) is fairly easy to carry out, but a 'meaningful' band resolution is virtually impossible, as no partial resolution of the spectral components can be guessed by visual inspection, and therefore the setting of starting and/or fixed spectral parameters for the computer simulation (q. number of components, type of profile, bandwidth of the components) is bound to be largely arbitrary. Still, if one con- siders that in the initial steps of adsorption a single symmetrical band is formed at high frequency, due to the most acidic sites, the spectral simulation of the first curves in Fig. l(a) can be attempted on a two-band basis. The following can be noted: (i) In the very first stages of CO uptake, most of the CO absorption is actually due to a single band of rather constant width ( A V ~ / ~ = 9-11 cm-') and shape (over 85% gaussian). Its frequency presents a steep linear dependence on the overall CO coverage, as shown by the plot of Fig. l(c). The frequency of this CO band, extrapolated to zero coverage, is very high (singlet frequency vo x 2207.5 cm- '), indicating a very high acidity for the relevant adsorbing sites, compatible only with surface cations in highly defective structural and/or coordinative configuration. (Note that on m- ZrO,, no CO vo frequencies above ca. 2202 cm-' were ever observed, except when in the presence of strong inductive effects from strong electron-withdrawing surface contaminants like, for instance, sulfates.2.6.)

(ii) At still fairly low CO coverages (P, x 2-4 Torr), the highest-frequency CO component seems not to grow any more [see the segment of individual adsorption isotherm marked by triangles in Fig. l(b)]. At higher CO pressures, the band loses its identity: the vco us. plot is no longer linear, the bandwidth increases at each incremental CO dose and so does the shape and breadth of the remaining part of the overall CO absorption. At this point, the spectral resolution strategy based on a two-band hypothesis is no longer tenable, while any resolution in terms of more than two bands would be arbitrary and would have little physical meaning.

If we now turn our attention to the data relative to CO uptake on the T036,31073 specimen of Fig. 3 (and skip for the moment T03,,,873 of Fig. 2), it can be seen that for P, > 4 Torr [see, for instance, curve 1 in Fig. 3(a)] the overall CO absorption is visibly resolved into two components. This simplified shape of the overall CO band is due to changes of relative intensity, spectral position and bandwidth within the individual CO components brought about by the onset of the thermal sintering process. Consequently, the overall CO absorption can be computer resolved with some confidence into terms of two components. The resulting

individual isotherms are shown in Fig. 3(b), and the relevant vco us. (Aco)t,t plots are presented in Fig. 3(c). The following can be noted: (i) The two CO adspecies behave quite differently: the higher-frequency component, (CO), , (obviously corresponding to more acidic Lewis sites) saturates at fairly low CO coverages, and is far less abundant than the lower- frequency component, (CO), , for which a saturation plateau is not yet reached at CO pressures as high as 130 Torr. This behaviour, also observed in the case of m-Zr0,,4*6-'4 seems to suggest that the slowly saturating CO band at lower frequencies can be assigned to CO uptake onto uncoordinated cationic sites on extended patches of regular crystal planes, and the assignment of the rapidly saturating band at higher frequencies to CO uptake onto highly uncoordinated cationic sites located in crystallographically defective configurations. (This assignment would be justified on the basis of the coin- like shape of the crystals, shown by the electron micrographs discussed in ref. 10.)

plot [see the plots in Fig. 3(c)] is linear and fairly steep in the whole range of CO coverage explored. This indicates the presence of two individual CO adspecies. In fact, the breadth and per- centage gaussian profile of the two bands also change very little with coverage. The frequency of both species is affected appreciably, and continuously, by the overall surface concentration of the charge-releasing CO adspecies. Note, in partic- ular, that for one of the two species [ ie . (CO),] the effect also continues after reaching saturation. The extrapolated vo frequency of the high-v species (ca. 2202 cm-') is appreciably lower than the vo of the individual CO species that was isolated in the earliest stages of CO uptake onto T03,,,673 (vo x 2207.5 cm-I), suggesting that the latter was probably due to CO uptake onto some different and even more defective adsorbing centres that were selectively annealed in the early stages of the sintering process.

The latter hypothesis is consistent with some of the features of CO uptake onto T03,,,873, reported in Fig. 2. In fact, the vco us. (Aco)tot plots of this system are both linear for P, 2 4 Torr [see the position marked with the arrow in Fig. 2(b) and (c)] and extrapolate to the same vo values observed for the T0367,1073 system (ca. 2202 and ca. 2192 cm-'); only the slopes of the straight lines are somewhat different, as the surface area is different for the two adsorbing systems. For P, 5 4 Torr, a different segment of straight line is obtained for the high-frequency species (CO), [the segment is reported as a broken line in Fig. 2(c)], and that segment extrapolates to the same vo (ca. 2207 cm-') observed in the early stages of CO uptake onto T03,,,673. This suggests that, in TO3 systems fired at the lowest temperatures, there are actually two distinct types of strong Lewis acid sites to be ascribed to crystallographically defective configurations.

(ii) For both CO components the vco us.

IR Data: Adsorption at ca. 77 K In order to check the tentative assignment proposed for the various CO adspecies observed at 300 K on TO3 systems, some CO adsorption experiments have been carried out at ca. 77 K. At low temperatures, higher CO coverages can be reached, and this allows the adsorption occurring on less acidic sites located in extended patches of regular crystal planes to be isolated with more confidence. In fact, the relevant bands, centred at lower frequencies, usually exhibit high intensity and large frequency shifts with coverage, owing to diffuse adsorbate-adsorbate interactions (e.g. see ref. 18-20). As mentioned before, the drawback of working at low temperatures is that, in the presence of high amounts of more weakly adsorbed species, the spectral features of the more strongly adsorbed species, which are far less abundant, are severely perturbed and observed much less clearly. Fig. 4(a)


2200 21 60 21 20 wavenumber/cm - '

T

2200 2160 2120 wavenumber/cm -'

Fig. 4 Spectral patterns of CO adsorbed at ca. 77 K on some t-ZrO, systems (CO pressures in the range 1.0 x 10-3-2.5 x lo1 Tom). (a) T03,,,773; (b) TO3,,731023; (c) TZ-3Y 1073; (d) TO333231073; TO2,3,31073.

shows that: (i) The early stages of CO uptake at 77 K onto TCU8,,773 bring about the formation of the two CO adspecies which are also observed at ambient temperature (CO), and (CO),. [Actually, in the early stages of vacuum activation, the rapidly saturating species, (CO), , was shown to be complex and resolvable into two components.] The bands (CO), and (CO), are indicated in Fig. 4(u) as bands a and b.

(ii) When the species b has almost reached its saturation (with an intensity considerably higher than that attained at 300 K), a third CO adspecies, termed c in the figure, appears at ca. 2185 cm-' and then increases in intensity and shifts to lower wavenumbers with coverage, reaching an ultimate position of ca. 2175 cm-'.

On the basis of the spectral behaviour, the (CO), species is certainly ascribable to cus Zr4+ centres located on flat portions of low-index crystal planes. This indicates that all of the CO adspecies observable upon CO uptake at 300 K, characterized by vo 2 2190 cm- I , should be ascribed to up to three different families of cus Zr4+ centres located in crystallographically defective configurations. This behaviour is quite different from that reported for m-ZrO, preparations of similar crystal size and surface area:7 there, the filling of the cus Zr4+ centres located on regular crystal planes started at

ambient temperature, and only one rapidly saturating family of sites located in defective configurations on the crystal could be isolated.

(iii) The presence of a fourth well resolvable CO species, located at even lower frequencies (species d, vco x 2158 cm- I), indicates the presence still of abundant surface hydroxy groups, to which CO is weakly hydrogen bonded at high CO coverages."

Volumetric and Calorimetric Data The adsorption of CO at ca. 300 K onto the materials dealt with in the previous section has also been checked from a quantitative point of view, in order to evaluate the population of the CO adspecies and the relevant adsorption energy distributions.

Fig. 5 reports, as a function of CO coverage, the volumetric adsorption isotherms and the heat of adsorption plots for the samples T038,3673 and T03873 1073, i.e. for the materials described in Fig. 1 and 3, respectively. The surface-area-normalized volumetric isotherms of Fig. 5(u) compare well with the corresponding total optical isotherms, and confirm that at the highest CO pressure reached the adsorption process is far from completion. The curve related to the adsorption on T03,,,1073 lies below that related to T03,,,673 in the


co coverage/moIecuIes nm-2

"I

n l v

0 0.2 0.4 0:: 0:8 1.0 nJprnol m-

Fig. 5 Volumetric and calorimetric features of CO adsorption onto T03,,, T, . (a) Surface-area-normalized adsorption isotherms of CO uptake at 303 K on T03,,,673 (upper curve) and TO3,,,1073 (lower curve). (b) Differential adsorption heats, as a function of CO coverage, for CO uptake at 303 K on T03,,,673 (upper plots) and T03,,,1073 (lower plots). Empty symbols correspond to primary adsorption isotherms, full symbols to secondary isotherms.

whole range of pressures examined, indicating that the overall population of acid sites per unit surface area that can be revealed by CO at ca. 300 K has been lowered by the thermal treatment at 1073 K. This is in agreement with the spectroscopic evidence. At Pco x 20 Torr, the two isotherms become virtually parallel, and the difference (ca. 0.08 CO molecules per nm2, corresponding to ca. 25% of the overall uptake at that pressure) allows estimation of the fraction of the most energetic sites which have been annealed in the earliest stages of thermal sintering. [This estimate is approx- imate, as the lower-lying isotherm corresponds to a material dehydrated at higher temperature, so that more of the weakly adsorbed species, (CO),, can be present which partly com- pensates for the decrease in the number of strongly adsorbed species, (CO), .]

The heat vs. coverage plots of Fig. 5(b) indicate that in both materials the CO-adsorbing surface is quite heterogeneous, as the differential heat curves decrease steeply, and tend to a common plateau value of ca. 23-28 kJ mol-'. This steeply decreasing heat corresponds with the IR spectral observation of various CO adspecies at different frequencies (intrinsic structural heterogeneity), as well as the decreasing frequency of each CO adspecies with CO coverage brought about by adsorbate-adsorbate perturbations (induced heterogeneity).

The zero-coverage heat values, qo (estimated by extrapo- lating the curve passing through the middle point of the experimental histogram^),'^ are significantly different in the two cases: q o x 65 and 50 kJ rno1-I for T03,,,673 and T03,,, 1073, respectively, meaning that the adsorption energies of the most energetic and first covered fraction of sites are quite different. This is consistent with the spectroscopic data of Fig. 1 and 3: the vo frequency of the most energetic CO adspecies is ca. 2207 cm-' for T03,,,673 and ca. 2202 cm-' for T03,,,1073, while many of the most energetic sites responsible for the (CO), band are eliminated by the thermal treatment at 1073 K.

To summarize, the data presented in this first section devoted to materials fired at relatively low temperatures indicate that: (i) On unsintered TO3 preparations, there are abundant and heterogeneous cus cationic sites characterized by high coordinative unsaturation and high Lewis acidity. This is reflected in the high vo values, high adsorption heats (4 = 40-60 kJ mol-') and fast saturation with CO coverage.

(ii) The family of strongly acidic centres, ascribed to crystallographically defective configurations, is complex : at least two species are partly resolved by CO uptake at 300 K, and present vo singlet frequencies at ca. 2207 and ca. 2202 cm-', respectively. (iii) The strongly acidic centres are gradually and selectively annealed upon sintering, starting with those characterized by the highest vo . (iv) Most of the CO uptake at 300 K occurs on another type of Lewis acid site, still ascribable to defective crystal configurations. These sites are characterized by slow saturation with CO coverage, lower adsorption heats ( q = 20-40 kJ mol- ') and lower CO singlet frequency (yo x 2192 cm-I). (v) The adsorption of CO on cus Zr4+ sites located on regular crystal planes, characterized by lower CO frequencies and lower adsorption heats, can be observed only at low temperatures.

t-ZrO, Materials fired at T, 2 1170 K IR Data The spectral features of the room-temperature adsorption of CO onto T03,,,, and TZ-3Y, isolated at comparable dehy- dration stages, are shown in Fig. 6 and 7, respectively. The following can be noted: (i) The overall adsorption onto T03,,,, is still resolvable in terms of two individual CO components, whose frequencies vary linearly with overall CO coverage [see Fig. qc)] and whose singlet vo frequencies are still ca. 2202 and ca. 2192 cm-' as in the case of the TO3 systems fired at lower temperatures. The CO species absorb- ing at higher frequencies (vo = 2202 cm- ') still saturates in the earliest stages of adsorption, and its amount relative to the lower-frequency species has further decreased, indicating that the incipient sintering process has further selectively annealed the most defective crystal terminations.

(ii) CO uptake onto the TZ-3Y preparation (Fig. 7) yields only one component, whose spectral features (Av,,, x 17 cm-l; vo x 2192 cm-') are virtually coincident with those of the lower-v species, (CO),, observed at 300 K on the To31173 systems. This indicates that, no matter what the preparative procedure of the t-ZrO, systems, the cus Zr4+ configurations obtained are always the same, and only the relative concentrations of them vary with preparation pro- cedures and conditions. The preparation procedure and the more severe thermal treatments adopted with the TZ-3Y


2220 2200 2180 2160 wavenum ber/cm -

20 40 60 80 100 120 1 P,,P-orr

n E

2190

P t

21 80 3 0 2 4 6

absorbance (arb. units) Fig. 6 Spectral features after adsorption of CO at 300 K onto T03,,,,773. (a) Spectral pattern upon adsorption for Pco in the range 1.0 x 10-‘--1.3 x lo2 Torr. (b) Optical adsorption isotherms (integrated absorbance us. P C J . (0) Total isotherm; (A) isotherm of the resolved species (CO), ; (8) isotherm of the resolved species (CO),. (c) Spectral position of the resolved CO components (CO), (A) and (CO), (a), as a function of the total integrated absorbance of adsorbed CO [ Y ( ~ ) ~ us.

system are likely to eliminate all of the most defective crystal terminations, which is consistent with what is expected of a presintered material of rather low surface area (ca. 18 m2

(iii) In the T03,1,3 system, the specific adsorption activity towards CO at 300 K seems to have declined dramatically: on passing from To31073 to To31173 (cf Fig. 3 and 6), the overall intensity of the CO absorption underwent a five-fold decrease (e.g. at 100 Torr CO, the integral absorbance (Aco)tot is CQ. 10 cm-l on the former system [Fig. 3(b)] and is ca. 2 cm-I on the latter), while the surface area decreased by only some 30% (i.e. from ca. 48 to ca. 33 m2 g-’, see Fig. 1 of ref. 10). This confirms that, in the case of t-ZrO,, all of the CO adspecies observable at 300 K are adsorbed on discrete families of cus Zr4+ centres located at crystal defect sites, whereas none of the cus Zr4+ sites located in ‘regular’ crystallographic configurations reach a Lewis acidity sufficient for chemisorbing CO at 300 K.

This hypothesis is confirmed by comparison of the spectral patterns in Fig. *a) and (b), relative to CO adsorption at ca. 77 K on (unsintered) T03,,,773 and on (partly sintered) T0311731023: in the latter system the species (C0)b is much more weakly adsorbed (more than can be expected on the

g-9.

basis of the decrease in surface area), whereas the species (CO), (due to cus Zr4+ sites located on regular crystal planes, and totally absent at 300 K) reaches an intensity comparable with that of the corresponding band on the unsintered system. This means that, in the earliest stages of sintering, most of the defective terminations have been annealed, and the size of the patches of regular crystal planes has increased considerably [see, for instance, the HRTEM detail in Fig. 8(a)]. This size increase is also responsible for the large downwards spectral shift undergone with CO coverage at ca. 77 K by the frequency of the (CO), species: it ranges from ca. 2185 to ca. 2165 cm-’, and indicates the presence of an extended ‘regular’ network of mutually perturbing dipoles. In the case of T03,,,773 of Fig. qa), the frequency of (CO), varies much less, i.e. from ca. 2185 to ca. 2175 cm-’, indicating the presence of much smaller flat regular patches on which dipoles can create mutual perturbations.

(iv) On the CO/TZ-3Y system, shown in Fig. 7, the only CO species formed at ambient temperature [(CO),; vo x 2192 cm-’1 exhibits an intensity almost twice as large as that observed on T03,,73, despite a surface area that is approx- imately one half of that of the T03,,,, system. This apparent contradiction should be ascribed to the different morphology

20 2200 2180 2160 wavenumberlcm-’

20 40 60 80 100 120 1 P,,/Torr

(c)

2200 -- r I

E

n $

21 80 I , I I I

1 0 2 4 6 absorbance (arb. units)

Fig. 7 Spectral features after adsorption of CO at 300 K onto TZ-3Y. (a) Adsorption spectral pattern for CO on TZ-3Y 773 (Pco in the range 1.0 x 10-’-1.3 x lo2 Torr). (b) Optical adsorption isotherm of species (CO), (integrated absorbance us. Pm). (c) Spectral position of the species (CO), , as a function of the total integrated absorbance of adsorbed CO [ v ( ~ ) ~ us. (Ac0)&j.

1 20 J. CHEM. SOC. FARADAY TRANS., 1995, VOL. 91

Fig. 8 HRTEM micrographs of (a) T03,, ,3 and (b) TZ-3Y

prevalently exhibited by TZ-3Y [compare, for instance, the HRTEM detail in Fig. 8(a) and (b) ] : most of the particles of TZ-3Y are huge aggregates of smaller crystallites, arranged intimately in a disordered fashion. This morphology, compared with that of the single crystallites prevalently exposed by T03, allows a larger amount of crystal defects on the one hand (as confirmed by the high intensity of CO spectra at 300 K), and on the other, a much more limited extent of mutual dipole-dipole interactions among species (CO), formed at ca. 77 K on regular planes, as confirmed by the smaller downwards shift with CO coverage undergone by the (CO), band presented in Fig. 4(c) (Av x 7 cm- I).

The last firing step explored in this work is 1323 K. The characterization work dealt with in ref. 10 showed that, on reaching this sintering stage, the morphological features of the TZ-3Y preparation do not change appreciably (both the size and texture of the particles remain almost unchanged, as

does the surface area of the material, 15 m2 g-I). In contrast, the surface area of TO3 declines to 28 m2 g-', while the increase of the average particle size is accompanied by the commencement of segregation around the particles of a thin amorphous phase, in which the concentration of Y 2 0 , is higher than in the bulk of the material. This segregation phenomenon was found to be much more prevalent, and to begin at lower firing temperatures, in the case of T02, in which the t-ZrO, phase is less stable.

The adsorption of CO on TZ-3YI3,, (not shown in the figures) confirms that this system is fairly stable. In fact, both at 300 and at ca. 77 K, the adsorptive features towards CO do not change appreciably with respect to what is reported above for the starting TZ-3Y system [shown in Fig. 7 and 4(cll.

The surface situation monitored by CO uptake on To31323 is somewhat different from that of the same system


in the previous step of the thermal sintering process. In fact, CO adsorption at 300 K reveals only a small residual amount of sites belonging to the sole family of cus Zr4+ centres at crystal defect sites responsible for the species (CO), (which is still slow to saturate and has a singlet frequency at vo x 2193 cm-'). These data are presented as A,, vs. P,, and vco vs. A,, plots in Fig. 9; no CO spectral pattern is reported in the figure, as it does not differ (except for the intensities) from the one-band pattern shown in Fig. 7(a) for TZ-3Y.

The band of adsorbed CO is fairly weak (ca. 50% of the intensity for T03,,,, of Fig. 6), and part of its decrease is certainly expected and ascribable to the effect of further sintering. However, sintering is not the only cause of the intensity decrease; some role is thought to be played by the onset of segregation around the ZrO, crystallites of an amorphous phase, preventing the exposure of some of the surface Zr4+ cations. In fact, in the case of T02, in which a thicker and much more abundant amorphous phase becomes segregated at T1 >/ 1250 K, CO uptake is no longer observed at 300 K.16

The detrimental effect on CO uptake of the altered crys- tallite surfaces does not concern only the adsorption sites located at crystal defects of TO3 and T02. In fact, Fig. 4(d) and (e) concerning CO uptake at ca. 77 K show that: (i) on T03,,,,1073 (i.e. on a TO3 system dehydrated in conditions in which, on 'regular' TO3 specimens, virtually no surface hydroxy groups would remain) CO uptake onto cus Zr4+ sites located on extended low-index crystal planes [see the band (CO), in Fig. 7(d)] is fairly weak and a downwards frequency shift with CO coverage of only ca. 10 cm-' is seen, meaning that the dipole-dipole interactions among adsorbed CO molecules are perturbed and interrupted. Meanwhile, there is the formation of a fairly strong band due to CO interacting via H-bonding with OH groups [the band (CO), , vco = 2158 cm-'1, consistent with the observation reported in ref. 10 that the amorphous phase segregated by the Ton systems is probably rich in OH groups. (ii) The behaviour of the To31323 system is exaggerated in the T02,,,,1073 system, shown in Fig. qe): the presence of a much more abundant amorphous segregated phase causes there to be almost no CO uptake at all on the cus Zr4+ sites located on the regular crystal planes, but yields only a strong CO band of type d, due to a weak H-bonding interaction.

- m .- El 0

0 20 40 60 -50 100 120 7

P,,/Torr

Volumetric and Calorimetric Data Some of the t-ZrO, samples fired at b 1170 K have also been examined from a quantitative point of view. Fig. 10 reports the surface-area-normalized volumetric and calorimetric data relative to CO uptake onto T03,,,,873 and TZ-3Y 873.

Note that T03,,,,873 exhibits a population of acidic sites, active towards CO at CQ. 300 K, and much decreased with respect to T03,,,673 and T03,,,1073 (e.g. at P, = 60 Torr, 0.16 molecules nm-' are adsorbed on T031,,,873, whereas ca. 0.32 molecules nm-, are adsorbed on T038,3673 and ca. 0.24 on T03,,,1073). Still, this overall two-fold decrease turns out to be much less than expected on the basis of the intensities of the relevant IR bands shown in Fig. l(b) and 6(b). This indicates that much caution must be applied when trying to draw quantitative information from the spectroscopic data of adsorbed species, since equations of the Beer-Lambert law type can be applied in the case of heterogeneous systems only by assuming constant scattering properties of the material.', This is clearly not the case with t-ZrO, preparations brought to rather different sintering stages, i.e. possessing rather different average particle sizes and, consequently, rather different scattering loss profiles.

T03, 73 was also compared with the commercial reference preparation TZ-3Y, which was fired at (virtually) the same temperature and was vacuum activated in similar conditions. The isotherms in Fig. lqa) show that the adsorptive capacity of TZ-3Y is twice that of T03,,,,873, as also indicated by the IR data in Fig. 6 and 7. This confirms that quite reliable quasi-quantitative information can be derived from the spectroscopic data, when comparing materials brought to similar sintering stages and thus possessing similar scattering features.

The higher concentration of acidic sites capable of adsorbing CO at ca. 300 K exhibited by TZ-3Y 873 with respect to T03, 173873 should be ascribed to the different morphology of the particles of this preparation, as shown by the micrographs in Fig, 8. The polyaggregate morphology of TZ-3Y 873 allows this lower-area preparation to possess a higher overall concentration of crystallography defective configurations, as compared with T03'173 , which is mainly made up of medium-sized individual crystallites. This remarkable difference indicates that preparative parameters may be very

2200 c 1

0 E 2- 3 5 Q)

2 2190 %

21 80 1

I I I

2 4 6 absorbance (arb. units)

Fig. 9 Some features of the adsorption o f C 0 at 300 K on T03,32,773. (a) Optical adsorption isotherm, relative to the only species formed, (co), (integrated absorbance us. Pm). (b) Spectral position of the (CO), component, as a function of the total integrated absorbance of of adsorbed co cvfco)b m- (~co~to,l.


CO coverage/molecules nm-2 0.12 0.24 0.36 0.48 0.60

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0.8

N 0.6

- 0

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0.2

0

....

0 20 40 60 80 100 p c oirorr

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_.

--. aa 0.24 cn g

s 0

0.12 0

r

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Y,

z 7

40

'"t 01 0 0.2 0.4 0.6 0.8 1

nJpmol m-2 0

Fig, 10 Volumetric and calorimetric features of CO adsorption onto TZ-3Y 873 and T 0 3 , , ,,873. (a) Surface-area-normald adsorption isotherms of CO uptake at 303 K on TZ-3Y 873 (upper solid-line curve) and T03,,, ,873 (lower solid-line curve). The two broken-line curves are the isotherms relative to T03,,,673 and T03,,,1073, reported in Fig. 5fa). (b) Differential adsorption heats, as a function of CO coverage, for CO uptake at 303 K on TZ-3Y 873 (upper plot) and T 0 3 , 173873 (lower plot). Empty symbols correspond to primary adsorption isotherms, full symbols to secondary isotherms.

important in determining the morphology of polycrystalline powders, and morphology, in turn, determines the surface reactivity of chemically equivalent solids. This effect has been observed for other oxides (e.g. see the case of microcrystalline Zn021).

The heats of adsorption plotted in Fig. 10(b) as a function of CO coverage show that the two materials treated at ca. 1173 K possess fairly similar highest-energy sites (qo x 60 kJ mol- l), meaning that the small amount of (CO), species still formed on T03,,,,873 (see Fig. 6) does not account for much in terms of adsorption energies at zero CO coverage. Moreover, the TZ-3Y 873 preparation exhibits a modest heterogeneity of the adsorbing sites (the heat curve declines very slowly to ca. 40 kJ mol- l), consistent with the IR data of Fig. 7, where moderate downwards shifts of vco are observed with increasing CO coverage. In contrast, the heat values for T03,,,,873 decline much more with CO coverage, towards the same low 4 values (ca. 25 kJ mol-') already observed in the case of T 0 3 8 , ~ [see Fig. 5(b)]. This is in apparent contradiction with the similar frequency decline trend exhibited by the two preparations in Fig. qc) and 7(c), but we have to consider that the vco us. (Aco),,, plots are not normalized for the surface area, and the surface areas of the two preparations are quite different. Moreover, we cannot exclude that the onset of segregation of an amorphous phase in the case of T03,,,,873 may play some role in this respect, alter- ing the adsorptive behaviour of the system at high coverages, and simulating the presence of sites of low energy, not confirmed by the IR spectra. (Consider, in this respect, that the temperature of the adsorbing pellet in the IR beam is bound to be definitely higher than that of the sample in the calorimetric thermostat.)

Conclusions The surface reactivity of tetragonal zirconia (t-ZrO,) has been tested and found to exhibit major differences with respect to the better known monoclinic phase (m-ZrO,). Two yttria-stabilized t-ZrO, preparations and a commercial powdery ceramic material of similar chemical composition have been examined by means of IR spectroscopy and

adsorption microcalorimetry. CO was used as a probe molecule, in order to test the surface acidic properties of the solids.

The surface acidic properties of the t-ZrO, preparations turn out to depend primarily on: (i) the degree of sintering. This is determined by the firing temperature (Tl), and is evaluated by HRTEM and XRD observations: (ii) the preparation route and the amount of structure-stabilizing agent (Y203) added.

All of the t-ZrO, data have been compared with the corresponding features of a reference m-ZrO, preparation, previously investigated in some detail.

The sintering stage (Tl) determines the nature and extent of (the flat patches of) regular low-index crystal faces exposed, as well as the nature and amount of structural defects at the edges of the microcrystals.

The surface Lewis acidity, due to cus Zr4+ cations, is somehow peculiar to the t-ZrO, phase with respect to the m-ZrO, one, and depends greatly on the sample preparation procedure.

Up to five CO adspecies may form at the surface of t-ZrO, at 77 K, but only three of them (and namely the highest vco CO adspecies) are detectable at 300 K. These high-frequency CO adspecies correspond to the formation of CO complexes on very strong cus Zr4+ sites located at crystal defect sites, e.g. at the edges of the microcrystals.

Strong Lewis sites on Ton preparations are quite heterogeneous: up to three CO adspecies can be singled out, characterized by different heats of adsorption, saturation behaviour, and zero-coverage CO frequency (vo at ca. 2207, ca. 2202 and ca. 2193 cm-'). The strongest Lewis sites of T o n are selectively and progressively suppressed by the thermal sintering process.

TZ-3Y, which is mainly made up of large crystal poly- aggregates, is much more homogeneous in terms of surface acidity. In fact, it presents only the lowest vco adspecies (vo w 2193 cm-') located on crystallographic defects, though in amounts very large compared with the low residual surface area of this pre-sintered material.

The steepness of the differential heat curves and of vco plots indicates that the energy distribution of the strong acidic sites is highly heterogeneous on all t-ZrO, prep-


arations. This is due to structural reasons (intrinsic heterogeneity, due to different crystallographic configurations), as well as to the different strength of the surface inductive effects (induced heterogeneity, caused by diffuse dipole-dipole interactions). The morphology of the samples (i.e. size, shape, aggregation of the microcrystals) is found to play a major role in determining both the structural differences among strong acid sites, and the extent of the mutual perturbative effects felt by CO admolecules.

CO complexes, formed on cus Zr4+ centres of lower acidity and located on flat patches of regular crystal faces, (CO),, can be observed only at low temperatures, unlike what has previously been observed in the case of m-ZrO, preparations. The spectral shift undergone with CO coverage by the (CO), adspecies is a monitor of the range of adsorbate-adsorbate interactions at the surface of t-ZrO, , and is thus quite sensitive to the average size of the regular patches of crystal planes terminating the particles.

This research was partly financed with funds from the CNR (Rome), Progetto Finalizzato Materiali Speciali.

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C. Morterra, L. Orio and C. Emanuel, J. Chem. SOC., Faraday Trans., 1990,86,3013. C. Morterra, L. Orio, V. Bolis and P. Ugliengo, Muter. Chem. Phys., 1991,29,457. (a) R. Srinivasan, B. H. Davis, L. A. Rice and R. J. de Angelis, J. Muter. Sci., 1992, 27, 661; (b) R. Srinivasan, C . R. Hubbard, 0. B. Cavin and B. H. Davis, Chem. Muter., 1993,5,27. R. C. Garvie, J. Phys. Chem., 1965,69,1238; 1978,82,218. C. Morterra, G. Cerrato, L. Ferroni and L. Montanaro, Muter. Chem. Phys., 1994,37,243. F. James and M. Roos, Comput. Phys. Commun., 1975,10,343. C. Lamberti, S. Bordiga, G. Cerrato, C. Morterra, D. Scarano, G. Spoto and A. Zecchina, Comput. Phys. Commun., 1993, 74, 119. C. Lamberti, C. Morterra, S. Bordiga, G. Cerrato and D. Scarano, Vib. Spectrosc., 1993,4,273. V. Bolis, C. Morterra, M. Volante, L. Orio and B. Fubini, Lang- muir, 1990,6, 695. (a) C. Morterra, E. Garrone, V. Bolis and B. Fubini, Spectro- chim. Acta, Part A, 1987, 43, 1577; (b) V. Bolis, B. Fubini, E. Garrone and C. Morterra, J. Chem. SOC., Faruday Trans. I , 1989,85,1383. C. Morterra, G. Cerrato, L. Ferroni, A. Negro and L. Monta- naro, Appl. Surf Sci., 1993,65/66,257. (a) E. Garrone, V. Bolis, B. Fubini and C. Morterra, Langmuir, 1989, 5 892; (b) V. Bolis, C. Morterra, B. Fubini, P. Ugliengo and E. Garrone, Langmuir, 1993,9, 1521. C. Noda and G. E. Ewing, Surf; Sci., 1990,240, 181. D. Scarano, A. Zecchina and A. Reller, Surf. Sci., 1988,198, 11. G. Spoto, C. Morterra, L. Marchese, L. Orio and A. Zecchina, Vacuum, 1990,41,37. V. Bolis, B. Fubini, E. Giamello and A. Reller, J. Chem. SOC., Faraday Trans. 1, 1989,85,855.

Paper 4/04568H; Received 25th July, 1994

surface characterization of yttria-stabilized tetragonal zro2 part 1. structural, morphological, and...

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