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Characterization of phosphated zirconia by XRD, Raman and IR spectroscopy
Colloids and Surfaces A: Physicochemical and Engineering Aspects 141 (1998) 227–235
Characterization of phosphated zirconia by XRD, Raman and IR spectroscopy Gamal A.H. Mekhemer Chemistry Department, Faculty of Science, Minia University, El-Minia 61519, Egypt Received 10 September 1997; accepted 11 March 1998
Abstract Phosphated zirconia samples were prepared by an impregnation method, using an aqueous solution of diammonium hydrogen phosphate and two different zirconia precursors (monoclinic ZrO and Zr(OH ) ). The dried impregnated 2 4 samples were calcined at 770, 870 and 970 K for 1 h in static air. The samples of phosphated zirconia obtained were characterized by X-ray diffraction, Raman and IR spectroscopy and N adsorption. The results revealed that the 2 presence of phosphate ions causes the tetragonal ZrO phase to form at low temperature (≤970 K ) and the surface 2 area to stabilize, when Zr(OH ) was the precursor used. However, using monoclinic ZrO as precursor no such effects 4 2 were observed. At least two different surface phosphate species were observed, namely pyrophosphate and [(Zr–O) 2 PO H ] species. © 1998 Elsevier Science B.V. All rights reserved. 2 Keywords: Zirconia; Phosphated zirconia; X-ray diffractometry; Nitrogen adsorption; Zirconium hydroxide; Raman and IR spectroscopy
1. Introduction Recent research interest in zirconia, ZrO , has 2 greatly enhanced the prospects for applying this material in catalysis, automotive gas sensors, etc. [1]. Its complex chemical properties, which include simultaneously reducing, oxidizing, acidic and basic properties, make ZrO a particularly attrac2 tive material for catalysis [2]. It has become well established that the performance of a heterogeneous catalyst depends not only on the intrinsic catalytic activity of the components but also on its texture and stability. One of the most important factors in controlling the texture and strength of a catalyst involves the correct choice of a support and the preparation of the support in the appropriate form [3,4]. Studies carried out by Rijnten [5], Crucean et al. [6 ] and by Tulier et al. [7] have
shown that, besides sintering phenomena, the phase(s) formed on heat treatment, the extent of crystallite growth, and the rate of phase transformation are also factors of importance in understanding the textural stability of ZrO supports. 2 Phosphates have been claimed to play the role of support stabilizers [8]. This behaviour may be thought of as being one particular aspect of a more general phenomenon, by which the presence of anionic groups at the surface of metal oxides tends to broaden the temperature range of existence of some metastable crystal phases (typical in the case of tetragonal ZrO , that remain stable at 2 temperatures as high as #1000 K when carrying surface sulfate [9,10]. The introduction of small amounts of phosphorus compounds (phosphates) onto the oxide markedly enhances its acidic properties, regardless of the type of phosphate com-
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pound used, e.g. (NH ) HPO , (NH )H PO or 42 4 4 2 4 H PO [11]. Catalytic activities for ring opening, 3 4 isomerization of cyclopropane and isomerization of butene have been observed for crystalline zirconium phosphates [12]. Vibrational spectroscopy (IR absorption and Raman scattering) is a powerful tool for the investigation of the surface structure of phosphated zirconias [13]. Qualitative approaches have been used for the interpretation of the experimental data obtained [14,15]. Hence, the objective of this paper is to observe the effects of phosphate ions on structural properties of monoclinic zirconia and zirconium hydroxide calcined at different temperatures. To accomplish this objective XRD, Raman, IR spectroscopy and N adsorption were used to 2 characterize the test materials.
the content of phosphate was found to decrease from 6 to #4 wt.% for the samples of PZrO (T ) 2 upon calcination at up to 970 K. 2.2. Apparatus and techniques 2.2.1. X-ray powder diffractometry (XRD) XRD of phosphated zirconias was carried out using a Siemens–Guinier diffractometer operated at 40 kV and 33 mA, with Cu Ka radiation (l= ˚ ). The patterns obtained were matched 1.54056 A with relevant ASTM Cards for identification. The particle size D (nm) of phosphated zirconia was calculated from the pattern-resolved peak for the (111: ) reflections of the monoclinic phase and from the (111) reflections of the tetragonal phase, using the Schererr relationship [16 ]: D=Kl/b cos h
2. Experimental 2.1. Materials Zirconium hydroxide, type XZ0632/03 (Zr(OH ) ), was obtained from MEL, Manchester 4 ( UK ). Pure zirconia, ZrO , was prepared by 2 calcination of the hydroxide at 870 K for 1 h. Two series of phosphated zirconias were obtained. The first series (designated PZrO (T )) was prepared x from Zr(OH ) as a precursor, whereas the second 4 (designated PZrO (T )) was prepared using ZrO 2 2 (T=calcination temperature). The phosphated species were loaded by impregnation of diammonium hydrogen phosphate solution, (NH ) HPO 42 4 (Merck), onto the respective precursor, using minimal amounts of water to give eventually 6 wt.% PO 3−. Excess water was removed by evaporation. 4 The resulting materials were dried at 380 K for overnight, and the phosphated zirconias were then obtained by calcination of the dried materials at different temperatures (770, 870 and 970 K ) for 1 h. Pure Zr(OH ) was similarly calcined, and 4 products are designated ZrO (T ). The phosphate x content of the calcination products were approximated from the intensity of nP–O bands in the IR and Raman spectra. Accordingly, samples of PZrO (T ) were found to maintain the initial x content of phosphate after calcination, whereas
where K is the crystallite shape constant (#1), l is the radiation wavelength, b is the corrected line breadth and h is the Bragg angle. 2.2.2. Specific surface area Surface area measurements of test sample were carried out by BET analysis of N adsorption 2 isotherm at 77 K [17]. These isotherms were obtained using a Sorptomatic Carlo Erba-1800 (Italy) adsorption equipment. Test materials were outgassed to the pressure of 3×10−5 mbar for 2 h at 470 K, prior to adsorption measurements. 2.2.3. Laser Raman spectroscopy (LRa) LRa measurements were performed using the 488 nm line of an Ar ion laser and a computercontrolled Dilor OMARS-89 spectrometer equipped with a model IRY12 optical multichannel analyser (OMA). The laser beam power was typically 30 mW and the resolution was set at 5±1 cm−1. Test samples were pressed into pellets, and the spectra were recorded over two frequency ranges: 1500–1200 cm−1 for phosphate vibrations, and 1200–100 cm−1 for Zr–O vibrations. 2.2.4. Infrared spectroscopy (IR) IR measurements were made with a Bruker IFS-88 FT-IR spectrometer equipped with a liquid-nitrogen-cooled MCT detector. A specially
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designed transmission cell [18], allowing in situ pretreatment at temperatures up to 770 K and recording spectra in vacuum (≤10−5 mbar) or in a static gas atmosphere, was employed. Approximately 20 mg cm−2 of test materials were pressed into self-supporting wafers and mounted inside the cell for pretreatment and measurement. The pretreatment conditions were as follows; heating in a stream of O at 723 K for 1 h and 2 subsequent evacuation at the same temperature for 1 h. The spectra were recorded at 300 K with a resolution of 2 cm−1.
229
3. Results and discussion
during the calcination of PZrO (T ) series, the x presence of PO 3− delays the formation of the 4 oxo bonds, which may explain the observed loss of surface area as a function of temperature. In addition to the formation of oxo bonds on calcination temperatures two possible processes have been identified as being responsible for the changes occurring in the surface area of the samples on calcination in static air are crystallite growth and the accompanying phase transformation (vide infra) on the one hand and sintering between crystallites on the other [3–7]. It has also been established that the monoclinic crystallites exhibit a more marked growth on calcination than do the tetragonal ones.
3.1. Surface area
3.2. XRD
The surface area of pure and phosphated zirconias (PZrO (T )) as a function of calcination temx perature is given in Table 1. The main trend reflected by these results is a decrease of the surface area as the calcination temperature increases, except for the PZrO (T ) series. The PZrO (T ) 2 x samples developed higher surface areas than pure zirconia. In contrast, the PZrO (T ) series shows 2 no temperature effects on the surface area. Powers and Gray [19] have demonstrated that the loss of water at the expense of double hydroxy bridges results in formation of oxo bridges at greater than 350 K. In the PZrO (T ) series oxo bridges are 2 formed prior to the addition of PO 3− species, 4 which help to maintain the surface area. Thus,
XRD patterns obtained for the ZrO (T ) samples x are given in Fig. 1, and the results thereby conveyed are summarized in Table 1. It is obvious that heating at 570 K results in an amorphous material, whereas heating at 670 K leads to a mixture of tetragonal (t-ZrO ) and monoclinic 2 (m-ZrO ) zirconia. The crystallization to mono2 clinic ZrO increases with calcination temperature, 2 which is evident from the corresponding diffraction peaks being more intense and sharp at 970 K. The particle size is shown to increase with temperature ( Table 1). XRD patterns of the PZrO (T ) series, given in x Fig. 2, indicate a tetragonal ZrO structure for 2 PZrO (770) and (870). This is also found to be x
Table 1 Surface area and bulk characteristics of phosphated zirconia Sample
Surface a area±2 (m2 g−1)
Crystalline bulk b phases
Particle size D±1 (nm)
ZrO (770) x ZrO (870) x ZrO (970) x PZrO (770) x PZrO (870) x PZrO (970) x PZrO (770) 2 PZrO (870) 2 PZrO (970) 2
85 40 25 165 105 75 34 34 34
m-ZrO (d) and t-ZrO (t) 2 2 m-ZrO (d) and t-ZrO (t) 2 2 m-ZrO (s) 2 t-ZrO (s) 2 t-ZrO (s) 2 t-ZrO (s) 2 m-ZrO2 (d) and t-ZrO (t) 2 m-ZrO (d) and t-ZrO (t) 2 2 m-ZrO (s) 2
13.0 20.3 30.5 11.9 15.6 20.0 16.7 22.7 24.6
a Determined by BET analysis of N adsorption isotherms. 2 b As XRD determined, where d=dominant, t=trace, s=sole.
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Fig. 1. X-ray powder diffractograms for zirconias obtained at different calcination temperatures.
Fig. 2. X-ray powder diffractograms phosphated ZrO , 2 PZrO (T ) series obtained at different calcination temperatures. x
the case for PZrO (970), except for a few weak x peaks assignable to a minor proportion of m-ZrO . The crystallite size of the metastable 2
Fig. 3. X-ray powder diffractograms for phosphated ZrO , 2 PZrO (T ) series obtained at different calcination temperatures. 2
tetragonal zirconia increases with temperature ( Table. 1). It is known [1] that pure ZrO assumes 2 three polymorphic structures: monoclinic (at 770–1470 K ), tetragonal (1470–1970 K ) and cubic (above 1970 K ) structures. Thus the above results reveal that the presence of phosphate ions in Zr(OH ) stabilizes the tetragonal structure at low 4 temperatures (at 770–970 K ). A similar effect has been shown to occur using additives of Ca2+, Y3+ and Mg2+ compounds [20,21]. Diffractograms obtained for PO 3−/ZrO pre4 2 pared from the crystalline oxide (PZrO (T ) series) 2 at different calcination temperatures are displayed in Fig. 3. XRD patterns of these materials reflected nothing but the structure of the crystalline oxide. Only m-ZrO phases with weak features of tetra2 gonal phase are detectable. These results conclude that the treatment of crystalline ZrO with 2 PO 3− ions does not influence the crystallization 4 of ZrO . The particle size of PZrO (T ) series 2 2 increases with temperature similarly to the ZrO (T ) series. x Mercera et al. [14] have shown that the tetragonal ZrO sinters to form increasingly larger 2 crystallites as the temperature increases. At a cer-
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tain temperature a critical crystallite size is reached, and hence, transformation from tetragonal to monoclinic crystallites occurs, the crystallite size of the latter being larger than the tetragonal crystallites. Thus the observed increase in the particle size of all series ( ZrO (T ), PZrO (T ) and x x PZrO (T )) with temperature ( Table 1) can be 2 related to sintering effects on the t- and m-ZrO 2 structures. 3.3. LRa spectra According to the results presented in Section 3.2 as well as reported data [22–24], crystalline ZrO 2 is known to exist in monoclinic, tetragonal and cubic modifications. LRa spectra of monoclinic, tetragonal and amorphous zirconias have been discussed earlier [25] and also recently by Mercera et al. [14]. Relevant Raman active vibrations of the various zirconia modifications are summarized in Table 2. The spectra observed for the ZrO (T ) x series are shown in Fig. 4. The spectrum of ZrO (770) displays bands and shoulders at 100, x 182, 218, 266, 304, 335, 375, 475, 533, 555 and 623 cm−1. According to Table 2, these bands are characteristic of the monoclinic (dominant) and tetragonal ZrO (trace). The spectra of 2 ZrO (870) and (970) exhibit the same band posix tions as those of m-ZrO , the sole difference being 2 the intensification of bands due to m-ZrO . The 2 spectra of the ZrO (T ) series are indicative of the x predominance of the monoclinic structure at all calcination temperatures. This spectral behaviour is quite consistent with XRD results of these materials ( Fig. 1 and Table 1). Table 2 Reported LRa frequencies for the various modifications of ZrO 2 Modification
Raman shift (cm−1)
Reference
Amorphous m-ZrO 2
550–600 (broad) 98–102, 180–189, 220, 225, 300, 335, 380, 475, 535, 555, 615, 635 148, 260–270, 315–330, 460–475, 608, 640 490
[23] [7]
t-ZrO 2 c-ZrO 2 c=cubic ZrO . 2
[6 ] [6 ]
Fig. 4. LRa spectra of pure ZrO at different calcination 2 temperatures.
LRa spectra of the PZrO (T ) series in the frex quency range of Zr–O vibration are shown in Fig. 5(a). The spectrum of PZrO (770) exhibits x weak bands at 632, 465, 312, 264, 180 and 147 cm−1. According to Table 2, these bands are characteristic of the t-ZrO . The spectrum of 2 PZrO (870) exhibits similar bands but with higher x intensities. This means that t-ZrO is more stable 2 at 870 K than 770 K. The absence of a band at 490 cm−1, and the presence of five strong bands for t-ZrO , indicate that the presence of phosphate 2 ions on ZrO calcined at 870 K crystallizes into 2 the tetragonal phase rather than the cubic one. The stability of t-ZrO increases with calcination 2 temperature. The spectrum of PZrO (970) exhibits x the same band positions as those of the tetragonal phase, the sole difference being the intensification of bands characteristic of the tetragonal modification. These results are consistent with XRD data of PZrO (T ) series. Raman spectra of this series x indicate that the presence of PO 3− ions stabilize 4 the tetragonal phase at low temperatures, which is indeed the result obtained from XRD. Raman spectra of the PZrO (T ) series in the 2 frequency range of Zr–O vibration are given in
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(a)
(a)
(b)
(b)
Fig. 5. LRa spectra of phosphated ZrO , PZrO (T ) series 2 x obtained at different calcination temperatures (a) in the Zr–O region and (b) in the P–O region.
Fig. 6. LRa spectra of phosphated ZrO , PZrO (T ) series 2 2 obtained at different calcination temperatures (a) in the Zr–O region and (b) in the P–O region.
Fig. 6(a). The spectrum of PZrO (770) exhibits 2 bands similar to the bands of ZrO (770). In view x of Table 2, these bands are assignable to m-ZrO . 2 The spectrum of PZrO (870) is also similar to that 2
of PZrO (770), the difference being that the inten2 sity of the bands assigned to the monoclinic phase at 870 K is higher than at 770 K. The intensity of the bands assigned to the monoclinic phase
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G.A.H. Mekhemer / Colloids Surfaces A: Physicochem. Eng. Aspects 141 (1998) 227–235 Table 3 LRa data relevant to the structure of phosphated zirconia Structure
Symmetry
nP–O (cm−1)
References
(ZrO) PO H 2 2 or ZrO PO H 2 2 ZrP O on amorphous ZrO 2 7 2 ZrP O on t-ZrO 2 7 2 ZrP O on m-ZrO 2 7 2
C
918 (ir), 886 (r)
[5,26 ]
1020–1190 (ir,r) 750 (r), 995 (r), 1048 (r), 1095 (ir), 1113 (ir, r) 750 (r), 948 (ir), 975 (r), 1020 (r), 1048 (r), 1095(ir), 1185 (ir) 943 (ir), 965 (r), 1058 (r), 1090 (ir), 1151 (ir)
[5,26–28] [5,26–28] [5,26–28]
2v
C 2v – – –
r=Raman active; ir=IR active.
increases with temperature, as also observed in the Raman spectrum of PZrO (970). These results are 2 consistent with the corresponding XRD results. From these results it is to be concluded that the stability of the monoclinic phase in the PZrO (970) series increases with calcination tem2 perature. These results also demonstrate that addition of phosphate to crystalline ZrO has no effect 2 on the crystalline structure. LRa spectra of the PZrO (T ) series in the frex quency region of phosphate vibrations are given in Fig. 5(b). The spectrum of PZrO (770) exhibits x bands at 879, 936, 995, 1015 and 1036 cm−1, which are assignable to (Zr–O) PO H or ZrO PO H (see 2 2 2 2 Table 3). The spectrum of PZrO (870) exhibits x similar bands in addition to a band at 1060 cm−1. According to Table 3 these bands can be assigned to two different surface phosphate species, pyrophosphate (P O 4−) and ( Zr–O) 2 7 2 PO H (or ZrO PO H ). After calcination at the 2 2 2 high temperature of 970 K, the spectra reveal at least two surface phosphate species on the surface. These Raman spectra are consistent with IR spectra reported by Busca et al. [13]. Raman spectra of the PZrO (T ) series in the 2 frequency region of P–O vibrations are given in Fig. 6(b). The spectra of these materials exhibit similar band positions to the PZrO (T ) series. x These bands are characteristic of the presence of more than one type of surface phosphate species, such as P O 4− and (Zr–O) PO H or 2 7 2 2 ZrO PO H (see Table 3). Thus, one may suggest 2 2
that the structures of the surface phosphate on the PZrO (T ) series are similar to those observed 2 on the PZrO (T ) series at high temperatures. x 3.4. IR spectra IR spectra of the PZrO (T ) series are given in x Fig. 7(a) and (b). The IR spectrum of PZrO (770) in the nOH region (Fig. 7(a)) exhibits x four OH bands at 3771, 3744, 3660 and 3643 cm−1. The strong band at 3771 cm−1 is typical for isolated ZrOH groups [29], whereas the weak band at 3744 cm−1 is probably caused by silica impurity [29]. The other two bands at 3660 and 3643 cm−1 are due to free and hydrogenbonded surface P–OH groups, respectively [13]. In the region of phosphate vibrations ( Fig. 7(b)), the spectrum of PZrO (770) exhibits bands at x 1445, 1425, 1369, 1350 and 1208 cm−1. These bands were observed in the IR spectra of a-zirconium phosphate [13,30]. The two weak bands at 1445 and 1425 cm−1 may be due to impurities of surface carbonate. The spectrum of PZrO (870) is similar to that of PZrO (770). The x x band displayed at 1208 cm−1 in the spectrum of PZrO (970) is shifted to the higher frequency of x 1240 cm−1 and a weak band emerged at 1306 cm−1. The new bands are assignable to nP–O–P groups. The results conclude that two different surface phosphate species are formed on ZrO , consistent with the corresponding LRa 2 results.
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(a)
(a)
(b)
(b)
Fig. 7. IR spectra of phosphated ZrO , PZrO (T ) series 2 x obtained at different calcination temperatures (a) in the hydroxyl stretching region and (b) in the P–O region.
Fig. 8. IR spectra of phosphated ZrO , PZrO (T ) series 2 2 obtained at different calcination temperatures (a) in the hydroxyl stretching region and (b) in the P–O region.
IR spectra of the PZrO (T ) series in the nOH 2 region ( Fig. 8) display three bands at 3771, 3743 and 3666 cm−1. These bands are due to isolated ZrOH groups, silanol groups and POH groups,
respectively. In the spectra in the phosphate region ( Fig. 8(b)) the spectra show several bands characteristic of the P O 4− and ( Zr–O) PO H (or 2 7 2 2 ZrO PO H ) species (see Table 3). 2 2
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4. Conclusion The physico-chemical characterization of the present phosphated ZrO samples indicate that the 2 presence of phosphate ions in Zr(OH ) leads, 4 upon calcination, to phosphated zirconias assuming metal stable tetragonal ZrO structure. In 2 contrast, the precursor of phosphate ions in m-ZrO does not alter the bulk structure of the 2 oxide. This results may be related to the fact that the production of zirconia from PO 3−/Zr(OH ) 4 4 is preceded by thermal decomposition of the hydroxide, which allows the phosphate species to influence the material’s bulk crystallization. The likely formation of pyrophosphate species may play an important role in the stabilization of tetragonal zirconia at low temperature, since it assumes a tetragonal-to-monoclinic phase change.
References [1] [2] [3] [4]
[5] [6 ] [7] [8] [9] [10]
T. Yamaguchi, Catal. Today 20 (1994) 199. K. Tanabe, Mater. Chem. Phys. 13 (1985) 347. D.L. Trimm, A. Stanislaus, Appl. Catal. 21 (1986) 215. D.L. Trimm, Design of Industrial Catalysts (Chemical Engineering Monographs 11), Elsevier, Amsterdam, 1980, p. 91. H.Th. Rijnten, Thesis, Delft University of Technology, 1971. E. Crucean, B. Rand, Trans. Br. Ceram. Soc. 78 (1979) 58. P. Tulier, J.A. Dalmon, G.A. Martin, P. Vergnon, Appl. Catal. 29 (1987) 305. K. Gishti, A. Iannibello, S. Marengo, G. Morelli, P. Tittarelli, Appl. Catal. 12 (1984) 381. K. Arata, Adv. Catal. 37 (1990) 165. C.J. Norman, P.A. Goulding, P.J. Moles, in: Acid–Base
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Catalysis II, Sapporo, Japan, 2–4 December, 1993), in: H. Hattori, M. Misono, Y. Yono (Eds.), Studies in Surface Science and Catalysis, vol. 90, Elsevier, Amsterdam, 1994, p. 269. [11] F. Abbattista, A. Delastro, G. Gozzelino, D. Mazza, M. Vallino, G. Busca, V. Lorenzelli, J. Chem. Soc., Faraday Trans. 86 (1990) 3653. [12] K. Segawa, S. Nakata, S. Asaoka, Mater. Chem. Phys. 17 (1987) 180. [13] G. Busca, V. Lorenzelli, P. Galli, A. La Ginestra, P. Patrono, J. Chem. Soc., Faraday Trans. 1 83 (1987) 853. [14] P.D.L. Mercera, J.G. Van Ommen, E.B.M. Doesburg, A.J. Burggraaf, J.R.H. Ross, Appl. Catal. 57 (1990) 127. [15] A. Feinberg, C.H. Perry, J. Phys. Chem. Solids 42 (1981) 513. [16 ] P. Scherrer, Goettinger Nachricten 2 (1918) 98. [17] B.C. Lippens, B.G. Linsen, J.H. de-Boer, J. Catal. 3 (1964) 32. [18] T. Beutel, Ph.D., University Munch, Germany, 1994. [19] D.A. Powers, H.B. Gray, Inorg. Chem. 12 (1973) 2721. [20] S. Somiya, N. Yamamoto, H. Yanagida ( Eds.), Science and Technology of Zirconia II, American Ceramic Society, 1988. [21] E.C. Subbarao, in: A.H. Heuer, L.W. Hobbs ( Eds.), Science and Technology of Zirconia, American Ceramic Society, 1981. [22] C.H. Perry, F. Lu, D.W. Liu, B. Alzab, J. Raman Spectrosc. 21 (1990) 577. [23] D.K. Smith, H.W. Newkirk, Acta Crystallogr. 18 (1965) 983 and Refs. therein [24] Ch. Schild, A. Wokaun, R.A. Koepper, A. Baiker, J. Catal. 130 (1991) 657. [25] C.M. Phillipi, K.S. Mazdiyasni, J. Am. Ceram. Soc. 54 (1971) 254. [26 ] D. Spielbauer, Ph.D. Thesis, University Munch, Germany, 1995. [27] K. Segawa, Y. Kurusu, Y. Nakajima, M. Kinoshita, J. Catal. 94 (1985) 491. [28] S.E. Horsley, D.V. Nowell, D.T. Stewart, Spectrochim. Acta A 30 (1974) 535. [29] A.A. Tsyganenko, V.N. Filimonov, J. Mol. Struct. 19 (1973) 40. [30] B. Marchon, A. Novak, J. Chem. Phys. 78 (1983) 2105.
Characterization of phosphated zirconia by XRD, Raman and IR spectroscopy Gamal A.H. Mekhemer Chemistry Department, Faculty of Science, Minia University, El-Minia 61519, Egypt Received 10 September 1997; accepted 11 March 1998
Abstract Phosphated zirconia samples were prepared by an impregnation method, using an aqueous solution of diammonium hydrogen phosphate and two different zirconia precursors (monoclinic ZrO and Zr(OH ) ). The dried impregnated 2 4 samples were calcined at 770, 870 and 970 K for 1 h in static air. The samples of phosphated zirconia obtained were characterized by X-ray diffraction, Raman and IR spectroscopy and N adsorption. The results revealed that the 2 presence of phosphate ions causes the tetragonal ZrO phase to form at low temperature (≤970 K ) and the surface 2 area to stabilize, when Zr(OH ) was the precursor used. However, using monoclinic ZrO as precursor no such effects 4 2 were observed. At least two different surface phosphate species were observed, namely pyrophosphate and [(Zr–O) 2 PO H ] species. © 1998 Elsevier Science B.V. All rights reserved. 2 Keywords: Zirconia; Phosphated zirconia; X-ray diffractometry; Nitrogen adsorption; Zirconium hydroxide; Raman and IR spectroscopy
1. Introduction Recent research interest in zirconia, ZrO , has 2 greatly enhanced the prospects for applying this material in catalysis, automotive gas sensors, etc. [1]. Its complex chemical properties, which include simultaneously reducing, oxidizing, acidic and basic properties, make ZrO a particularly attrac2 tive material for catalysis [2]. It has become well established that the performance of a heterogeneous catalyst depends not only on the intrinsic catalytic activity of the components but also on its texture and stability. One of the most important factors in controlling the texture and strength of a catalyst involves the correct choice of a support and the preparation of the support in the appropriate form [3,4]. Studies carried out by Rijnten [5], Crucean et al. [6 ] and by Tulier et al. [7] have
shown that, besides sintering phenomena, the phase(s) formed on heat treatment, the extent of crystallite growth, and the rate of phase transformation are also factors of importance in understanding the textural stability of ZrO supports. 2 Phosphates have been claimed to play the role of support stabilizers [8]. This behaviour may be thought of as being one particular aspect of a more general phenomenon, by which the presence of anionic groups at the surface of metal oxides tends to broaden the temperature range of existence of some metastable crystal phases (typical in the case of tetragonal ZrO , that remain stable at 2 temperatures as high as #1000 K when carrying surface sulfate [9,10]. The introduction of small amounts of phosphorus compounds (phosphates) onto the oxide markedly enhances its acidic properties, regardless of the type of phosphate com-
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pound used, e.g. (NH ) HPO , (NH )H PO or 42 4 4 2 4 H PO [11]. Catalytic activities for ring opening, 3 4 isomerization of cyclopropane and isomerization of butene have been observed for crystalline zirconium phosphates [12]. Vibrational spectroscopy (IR absorption and Raman scattering) is a powerful tool for the investigation of the surface structure of phosphated zirconias [13]. Qualitative approaches have been used for the interpretation of the experimental data obtained [14,15]. Hence, the objective of this paper is to observe the effects of phosphate ions on structural properties of monoclinic zirconia and zirconium hydroxide calcined at different temperatures. To accomplish this objective XRD, Raman, IR spectroscopy and N adsorption were used to 2 characterize the test materials.
the content of phosphate was found to decrease from 6 to #4 wt.% for the samples of PZrO (T ) 2 upon calcination at up to 970 K. 2.2. Apparatus and techniques 2.2.1. X-ray powder diffractometry (XRD) XRD of phosphated zirconias was carried out using a Siemens–Guinier diffractometer operated at 40 kV and 33 mA, with Cu Ka radiation (l= ˚ ). The patterns obtained were matched 1.54056 A with relevant ASTM Cards for identification. The particle size D (nm) of phosphated zirconia was calculated from the pattern-resolved peak for the (111: ) reflections of the monoclinic phase and from the (111) reflections of the tetragonal phase, using the Schererr relationship [16 ]: D=Kl/b cos h
2. Experimental 2.1. Materials Zirconium hydroxide, type XZ0632/03 (Zr(OH ) ), was obtained from MEL, Manchester 4 ( UK ). Pure zirconia, ZrO , was prepared by 2 calcination of the hydroxide at 870 K for 1 h. Two series of phosphated zirconias were obtained. The first series (designated PZrO (T )) was prepared x from Zr(OH ) as a precursor, whereas the second 4 (designated PZrO (T )) was prepared using ZrO 2 2 (T=calcination temperature). The phosphated species were loaded by impregnation of diammonium hydrogen phosphate solution, (NH ) HPO 42 4 (Merck), onto the respective precursor, using minimal amounts of water to give eventually 6 wt.% PO 3−. Excess water was removed by evaporation. 4 The resulting materials were dried at 380 K for overnight, and the phosphated zirconias were then obtained by calcination of the dried materials at different temperatures (770, 870 and 970 K ) for 1 h. Pure Zr(OH ) was similarly calcined, and 4 products are designated ZrO (T ). The phosphate x content of the calcination products were approximated from the intensity of nP–O bands in the IR and Raman spectra. Accordingly, samples of PZrO (T ) were found to maintain the initial x content of phosphate after calcination, whereas
where K is the crystallite shape constant (#1), l is the radiation wavelength, b is the corrected line breadth and h is the Bragg angle. 2.2.2. Specific surface area Surface area measurements of test sample were carried out by BET analysis of N adsorption 2 isotherm at 77 K [17]. These isotherms were obtained using a Sorptomatic Carlo Erba-1800 (Italy) adsorption equipment. Test materials were outgassed to the pressure of 3×10−5 mbar for 2 h at 470 K, prior to adsorption measurements. 2.2.3. Laser Raman spectroscopy (LRa) LRa measurements were performed using the 488 nm line of an Ar ion laser and a computercontrolled Dilor OMARS-89 spectrometer equipped with a model IRY12 optical multichannel analyser (OMA). The laser beam power was typically 30 mW and the resolution was set at 5±1 cm−1. Test samples were pressed into pellets, and the spectra were recorded over two frequency ranges: 1500–1200 cm−1 for phosphate vibrations, and 1200–100 cm−1 for Zr–O vibrations. 2.2.4. Infrared spectroscopy (IR) IR measurements were made with a Bruker IFS-88 FT-IR spectrometer equipped with a liquid-nitrogen-cooled MCT detector. A specially
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designed transmission cell [18], allowing in situ pretreatment at temperatures up to 770 K and recording spectra in vacuum (≤10−5 mbar) or in a static gas atmosphere, was employed. Approximately 20 mg cm−2 of test materials were pressed into self-supporting wafers and mounted inside the cell for pretreatment and measurement. The pretreatment conditions were as follows; heating in a stream of O at 723 K for 1 h and 2 subsequent evacuation at the same temperature for 1 h. The spectra were recorded at 300 K with a resolution of 2 cm−1.
229
3. Results and discussion
during the calcination of PZrO (T ) series, the x presence of PO 3− delays the formation of the 4 oxo bonds, which may explain the observed loss of surface area as a function of temperature. In addition to the formation of oxo bonds on calcination temperatures two possible processes have been identified as being responsible for the changes occurring in the surface area of the samples on calcination in static air are crystallite growth and the accompanying phase transformation (vide infra) on the one hand and sintering between crystallites on the other [3–7]. It has also been established that the monoclinic crystallites exhibit a more marked growth on calcination than do the tetragonal ones.
3.1. Surface area
3.2. XRD
The surface area of pure and phosphated zirconias (PZrO (T )) as a function of calcination temx perature is given in Table 1. The main trend reflected by these results is a decrease of the surface area as the calcination temperature increases, except for the PZrO (T ) series. The PZrO (T ) 2 x samples developed higher surface areas than pure zirconia. In contrast, the PZrO (T ) series shows 2 no temperature effects on the surface area. Powers and Gray [19] have demonstrated that the loss of water at the expense of double hydroxy bridges results in formation of oxo bridges at greater than 350 K. In the PZrO (T ) series oxo bridges are 2 formed prior to the addition of PO 3− species, 4 which help to maintain the surface area. Thus,
XRD patterns obtained for the ZrO (T ) samples x are given in Fig. 1, and the results thereby conveyed are summarized in Table 1. It is obvious that heating at 570 K results in an amorphous material, whereas heating at 670 K leads to a mixture of tetragonal (t-ZrO ) and monoclinic 2 (m-ZrO ) zirconia. The crystallization to mono2 clinic ZrO increases with calcination temperature, 2 which is evident from the corresponding diffraction peaks being more intense and sharp at 970 K. The particle size is shown to increase with temperature ( Table 1). XRD patterns of the PZrO (T ) series, given in x Fig. 2, indicate a tetragonal ZrO structure for 2 PZrO (770) and (870). This is also found to be x
Table 1 Surface area and bulk characteristics of phosphated zirconia Sample
Surface a area±2 (m2 g−1)
Crystalline bulk b phases
Particle size D±1 (nm)
ZrO (770) x ZrO (870) x ZrO (970) x PZrO (770) x PZrO (870) x PZrO (970) x PZrO (770) 2 PZrO (870) 2 PZrO (970) 2
85 40 25 165 105 75 34 34 34
m-ZrO (d) and t-ZrO (t) 2 2 m-ZrO (d) and t-ZrO (t) 2 2 m-ZrO (s) 2 t-ZrO (s) 2 t-ZrO (s) 2 t-ZrO (s) 2 m-ZrO2 (d) and t-ZrO (t) 2 m-ZrO (d) and t-ZrO (t) 2 2 m-ZrO (s) 2
13.0 20.3 30.5 11.9 15.6 20.0 16.7 22.7 24.6
a Determined by BET analysis of N adsorption isotherms. 2 b As XRD determined, where d=dominant, t=trace, s=sole.
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Fig. 1. X-ray powder diffractograms for zirconias obtained at different calcination temperatures.
Fig. 2. X-ray powder diffractograms phosphated ZrO , 2 PZrO (T ) series obtained at different calcination temperatures. x
the case for PZrO (970), except for a few weak x peaks assignable to a minor proportion of m-ZrO . The crystallite size of the metastable 2
Fig. 3. X-ray powder diffractograms for phosphated ZrO , 2 PZrO (T ) series obtained at different calcination temperatures. 2
tetragonal zirconia increases with temperature ( Table. 1). It is known [1] that pure ZrO assumes 2 three polymorphic structures: monoclinic (at 770–1470 K ), tetragonal (1470–1970 K ) and cubic (above 1970 K ) structures. Thus the above results reveal that the presence of phosphate ions in Zr(OH ) stabilizes the tetragonal structure at low 4 temperatures (at 770–970 K ). A similar effect has been shown to occur using additives of Ca2+, Y3+ and Mg2+ compounds [20,21]. Diffractograms obtained for PO 3−/ZrO pre4 2 pared from the crystalline oxide (PZrO (T ) series) 2 at different calcination temperatures are displayed in Fig. 3. XRD patterns of these materials reflected nothing but the structure of the crystalline oxide. Only m-ZrO phases with weak features of tetra2 gonal phase are detectable. These results conclude that the treatment of crystalline ZrO with 2 PO 3− ions does not influence the crystallization 4 of ZrO . The particle size of PZrO (T ) series 2 2 increases with temperature similarly to the ZrO (T ) series. x Mercera et al. [14] have shown that the tetragonal ZrO sinters to form increasingly larger 2 crystallites as the temperature increases. At a cer-
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231
tain temperature a critical crystallite size is reached, and hence, transformation from tetragonal to monoclinic crystallites occurs, the crystallite size of the latter being larger than the tetragonal crystallites. Thus the observed increase in the particle size of all series ( ZrO (T ), PZrO (T ) and x x PZrO (T )) with temperature ( Table 1) can be 2 related to sintering effects on the t- and m-ZrO 2 structures. 3.3. LRa spectra According to the results presented in Section 3.2 as well as reported data [22–24], crystalline ZrO 2 is known to exist in monoclinic, tetragonal and cubic modifications. LRa spectra of monoclinic, tetragonal and amorphous zirconias have been discussed earlier [25] and also recently by Mercera et al. [14]. Relevant Raman active vibrations of the various zirconia modifications are summarized in Table 2. The spectra observed for the ZrO (T ) x series are shown in Fig. 4. The spectrum of ZrO (770) displays bands and shoulders at 100, x 182, 218, 266, 304, 335, 375, 475, 533, 555 and 623 cm−1. According to Table 2, these bands are characteristic of the monoclinic (dominant) and tetragonal ZrO (trace). The spectra of 2 ZrO (870) and (970) exhibit the same band posix tions as those of m-ZrO , the sole difference being 2 the intensification of bands due to m-ZrO . The 2 spectra of the ZrO (T ) series are indicative of the x predominance of the monoclinic structure at all calcination temperatures. This spectral behaviour is quite consistent with XRD results of these materials ( Fig. 1 and Table 1). Table 2 Reported LRa frequencies for the various modifications of ZrO 2 Modification
Raman shift (cm−1)
Reference
Amorphous m-ZrO 2
550–600 (broad) 98–102, 180–189, 220, 225, 300, 335, 380, 475, 535, 555, 615, 635 148, 260–270, 315–330, 460–475, 608, 640 490
[23] [7]
t-ZrO 2 c-ZrO 2 c=cubic ZrO . 2
[6 ] [6 ]
Fig. 4. LRa spectra of pure ZrO at different calcination 2 temperatures.
LRa spectra of the PZrO (T ) series in the frex quency range of Zr–O vibration are shown in Fig. 5(a). The spectrum of PZrO (770) exhibits x weak bands at 632, 465, 312, 264, 180 and 147 cm−1. According to Table 2, these bands are characteristic of the t-ZrO . The spectrum of 2 PZrO (870) exhibits similar bands but with higher x intensities. This means that t-ZrO is more stable 2 at 870 K than 770 K. The absence of a band at 490 cm−1, and the presence of five strong bands for t-ZrO , indicate that the presence of phosphate 2 ions on ZrO calcined at 870 K crystallizes into 2 the tetragonal phase rather than the cubic one. The stability of t-ZrO increases with calcination 2 temperature. The spectrum of PZrO (970) exhibits x the same band positions as those of the tetragonal phase, the sole difference being the intensification of bands characteristic of the tetragonal modification. These results are consistent with XRD data of PZrO (T ) series. Raman spectra of this series x indicate that the presence of PO 3− ions stabilize 4 the tetragonal phase at low temperatures, which is indeed the result obtained from XRD. Raman spectra of the PZrO (T ) series in the 2 frequency range of Zr–O vibration are given in
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(a)
(a)
(b)
(b)
Fig. 5. LRa spectra of phosphated ZrO , PZrO (T ) series 2 x obtained at different calcination temperatures (a) in the Zr–O region and (b) in the P–O region.
Fig. 6. LRa spectra of phosphated ZrO , PZrO (T ) series 2 2 obtained at different calcination temperatures (a) in the Zr–O region and (b) in the P–O region.
Fig. 6(a). The spectrum of PZrO (770) exhibits 2 bands similar to the bands of ZrO (770). In view x of Table 2, these bands are assignable to m-ZrO . 2 The spectrum of PZrO (870) is also similar to that 2
of PZrO (770), the difference being that the inten2 sity of the bands assigned to the monoclinic phase at 870 K is higher than at 770 K. The intensity of the bands assigned to the monoclinic phase
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G.A.H. Mekhemer / Colloids Surfaces A: Physicochem. Eng. Aspects 141 (1998) 227–235 Table 3 LRa data relevant to the structure of phosphated zirconia Structure
Symmetry
nP–O (cm−1)
References
(ZrO) PO H 2 2 or ZrO PO H 2 2 ZrP O on amorphous ZrO 2 7 2 ZrP O on t-ZrO 2 7 2 ZrP O on m-ZrO 2 7 2
C
918 (ir), 886 (r)
[5,26 ]
1020–1190 (ir,r) 750 (r), 995 (r), 1048 (r), 1095 (ir), 1113 (ir, r) 750 (r), 948 (ir), 975 (r), 1020 (r), 1048 (r), 1095(ir), 1185 (ir) 943 (ir), 965 (r), 1058 (r), 1090 (ir), 1151 (ir)
[5,26–28] [5,26–28] [5,26–28]
2v
C 2v – – –
r=Raman active; ir=IR active.
increases with temperature, as also observed in the Raman spectrum of PZrO (970). These results are 2 consistent with the corresponding XRD results. From these results it is to be concluded that the stability of the monoclinic phase in the PZrO (970) series increases with calcination tem2 perature. These results also demonstrate that addition of phosphate to crystalline ZrO has no effect 2 on the crystalline structure. LRa spectra of the PZrO (T ) series in the frex quency region of phosphate vibrations are given in Fig. 5(b). The spectrum of PZrO (770) exhibits x bands at 879, 936, 995, 1015 and 1036 cm−1, which are assignable to (Zr–O) PO H or ZrO PO H (see 2 2 2 2 Table 3). The spectrum of PZrO (870) exhibits x similar bands in addition to a band at 1060 cm−1. According to Table 3 these bands can be assigned to two different surface phosphate species, pyrophosphate (P O 4−) and ( Zr–O) 2 7 2 PO H (or ZrO PO H ). After calcination at the 2 2 2 high temperature of 970 K, the spectra reveal at least two surface phosphate species on the surface. These Raman spectra are consistent with IR spectra reported by Busca et al. [13]. Raman spectra of the PZrO (T ) series in the 2 frequency region of P–O vibrations are given in Fig. 6(b). The spectra of these materials exhibit similar band positions to the PZrO (T ) series. x These bands are characteristic of the presence of more than one type of surface phosphate species, such as P O 4− and (Zr–O) PO H or 2 7 2 2 ZrO PO H (see Table 3). Thus, one may suggest 2 2
that the structures of the surface phosphate on the PZrO (T ) series are similar to those observed 2 on the PZrO (T ) series at high temperatures. x 3.4. IR spectra IR spectra of the PZrO (T ) series are given in x Fig. 7(a) and (b). The IR spectrum of PZrO (770) in the nOH region (Fig. 7(a)) exhibits x four OH bands at 3771, 3744, 3660 and 3643 cm−1. The strong band at 3771 cm−1 is typical for isolated ZrOH groups [29], whereas the weak band at 3744 cm−1 is probably caused by silica impurity [29]. The other two bands at 3660 and 3643 cm−1 are due to free and hydrogenbonded surface P–OH groups, respectively [13]. In the region of phosphate vibrations ( Fig. 7(b)), the spectrum of PZrO (770) exhibits bands at x 1445, 1425, 1369, 1350 and 1208 cm−1. These bands were observed in the IR spectra of a-zirconium phosphate [13,30]. The two weak bands at 1445 and 1425 cm−1 may be due to impurities of surface carbonate. The spectrum of PZrO (870) is similar to that of PZrO (770). The x x band displayed at 1208 cm−1 in the spectrum of PZrO (970) is shifted to the higher frequency of x 1240 cm−1 and a weak band emerged at 1306 cm−1. The new bands are assignable to nP–O–P groups. The results conclude that two different surface phosphate species are formed on ZrO , consistent with the corresponding LRa 2 results.
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(a)
(a)
(b)
(b)
Fig. 7. IR spectra of phosphated ZrO , PZrO (T ) series 2 x obtained at different calcination temperatures (a) in the hydroxyl stretching region and (b) in the P–O region.
Fig. 8. IR spectra of phosphated ZrO , PZrO (T ) series 2 2 obtained at different calcination temperatures (a) in the hydroxyl stretching region and (b) in the P–O region.
IR spectra of the PZrO (T ) series in the nOH 2 region ( Fig. 8) display three bands at 3771, 3743 and 3666 cm−1. These bands are due to isolated ZrOH groups, silanol groups and POH groups,
respectively. In the spectra in the phosphate region ( Fig. 8(b)) the spectra show several bands characteristic of the P O 4− and ( Zr–O) PO H (or 2 7 2 2 ZrO PO H ) species (see Table 3). 2 2
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4. Conclusion The physico-chemical characterization of the present phosphated ZrO samples indicate that the 2 presence of phosphate ions in Zr(OH ) leads, 4 upon calcination, to phosphated zirconias assuming metal stable tetragonal ZrO structure. In 2 contrast, the precursor of phosphate ions in m-ZrO does not alter the bulk structure of the 2 oxide. This results may be related to the fact that the production of zirconia from PO 3−/Zr(OH ) 4 4 is preceded by thermal decomposition of the hydroxide, which allows the phosphate species to influence the material’s bulk crystallization. The likely formation of pyrophosphate species may play an important role in the stabilization of tetragonal zirconia at low temperature, since it assumes a tetragonal-to-monoclinic phase change.
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