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Synthesis and characterization of mesoporous and nano-crystalline phosphate zirconium oxides
Journal of Alloys and Compounds 483 (2009) 425–428
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis and characterization of mesoporous and nano-crystalline phosphate zirconium oxides J.M. Hernández Enríquez a,∗ , L.A. Cortez Lajas a , R. García Alamilla a , A. Castillo Mares a , G. Sandoval Robles a , L.A. García Serrano b a
Instituto Tecnológico de Cd. Madero, División de Estudios de Posgrado e Investigación, Juventino Rosas y Jesús Urueta S/N, Col. Los Mangos, 89440 Cd. Madero, Tam., Mexico Instituto Politécnico Nacional, Escuela Superior de Ingeniería Textil, Av. Instituto Politécnico Nacional s/n, Edificio #8, Col. Linda Vista, Delegación Gustavo A. Madero, 07738 México, D.F., Mexico b
a r t i c l e
i n f o
Article history: Received 30 August 2007 Received in revised form 25 July 2008 Accepted 6 August 2008 Available online 11 December 2008 Keywords: Nanostructured materials Oxide materials Sol–gel processes Crystal growth Crystal structure
a b s t r a c t In this work the preparation and characterization of the materials such as zirconia (ZrO2 ) and zirconia promoted with phosphate ion (ZrO2 –PO4 3− ) is presented. Pure zirconium hydroxide [Zr(OH)4 ] was synthesized by the sol–gel method using precursors such as zirconium n-butoxide and 1-butanol maintaining a pH 8 during the synthesis. Zr(OH)4 was impregnated with 15 wt.% of the acid agent. Both were calcined in a dynamic air atmosphere for 3 h at 400, 500 and 600 ◦ C. The supports were characterized by thermal analysis, X-ray diffraction, nitrogen physisorption as well as infrared spectroscopy. The results showed a positive effect on the physicochemical properties of the catalytic supports after Zr(OH)4 impregnation with the dopping agent (H3 PO4 ). Phosphate zirconium oxides remained thermically stable after calcination. It was observed that the dopping agent remained firmly attached to the zirconium oxide surface, inhibiting the particle growth and delaying the syntherization of the material and the apparition of the monoclinic phase, obtaining mesoporous and nano-crystalline materials (crystallite size 1.0–6.5 nm) with high surface areas (210–329 m2 /g) and tetragonal structure defined for the calcination temperature of 600 ◦ C. © 2008 Published by Elsevier B.V.
1. Introduction Zirconia is an important material that has attracted enormous interest in catalytic process such as paraffin isomerization, hydrogenation of olefins, alcohol dehydrogenation and other technological uses [1–4]. The use of the zirconia as a support material or catalyst often requires a high accessible surface area as well as a large and well-developed porous texture. However, zirconia oxides generally have surface areas of 50 m2 /g or less, which is rather low compared with conventional supports such as SiO2 or Al2 O3 . Higher surface areas are attainable with amorphous zirconia (200–300 m2 /g), but this was usually achieved at the expense of much lower thermal stability [5–8]. On the other hand, zirconia exists in three crystallographic phases: monoclinic, tetragonal and cubic. Among these phases, the tetragonal phase is preferable for some acid-catalyzed reactions (paraffin isomerization). Efforts have been made to partially or fully stabilize the tetragonal phase with enhanced surface area by some additives such as Y2 O3 or
SO4 2− [9,10]. For that, in this work it is presented the synthesis of zirconium oxides by the sol–gel method, modified this one with the presence of the phosphate ion (PO4 3− ) to study the possible influence of this parameter as a synthesis variable on the thermal, textural and structural properties. 2. Experimental 2.1. Supports preparation The Zr(OH)4 preparation was carried out by the sol–gel method dissolving zirconium n-butoxide in 1-butanol taking place the hydrolysis and condensation of this by a slow drop addition of a water/1-butanol solution maintaining a pH 8 during the synthesis. Later, the gel was aged for 72 h and dried at 120 ◦ C for 24 h. The impregnation of zirconium hydroxide (Zr(OH)4 ) with the acid agent (PO4 3− ) took place using the wet incipient technique adding the necessary amounts of a phosphoric acid solution (H3 PO4 , 2N) that contained the PO4 3− ion to obtain a 15 wt.% of the acid agent present in the support. The modified hydroxides were dried at 120 ◦ C for 24 h, calcined afterward in a dynamic air atmosphere at 400, 500 and 600 ◦ C for 3 h. For pure zirconia calcined at 400, 500 and 600 ◦ C its nomenclature is: Z400, Z500 and Z600 and for phosphate zirconia calcined at 400, 500 and 600 ◦ C its nomenclature is: ZP400, ZP500 and ZP600. 2.2. Supports characterization
∗ Corresponding author. Tel.: +52 833 215 8544; fax: +52 833 215 8544. E-mail address: [email protected] (J.M. Hernández Enríquez). 0925-8388/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.jallcom.2008.08.094
The thermogravimetric and differential thermal analysis took place in a thermogravimetric balance TA Instruments STD 2960 Simultaneous DSC-TGA; samples
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Fig. 1. TG–DTG profiles of Zr(OH)4 .
Fig. 3. TG–DTG profiles of Zr(OH)4 –PO4 3− .
were analyzed in air flow (10 ml/min) with a heating rate of 10 ◦ C/min in the temperature range of 25–900 ◦ C. Nitrogen physisorption was used to determine the surface areas of the materials at the temperature of liquid nitrogen (−196 ◦ C) in a Quantachrome Autosorb-1 instrument; previous to the analysis, samples were degasified for 2 h at 350 ◦ C. X-ray diffraction patterns were obtained in a Bruker Advance D800 diffractometer with Cu K␣ radiation ( = 1.5406 Å) and a graphite secondary beam monochromator; the intensities of the diffraction lines were obtained in the 2 range between 20◦ and 70◦ with a step of 0.02◦ and a measuring time of 2.7 s per point. Infrared spectroscopy was carried out in a Fourier transforms spectrometer (PerkinElmer Spectrum One) with transparent pills containing the sample to be analyzed and KBr as binder (90 wt.%) using a number of scanning of 16 and a resolution of 4 cm−1 .
ZrO2 [14]. When the sample was annealed above 500 ◦ C, no further weight loss was observed. In the DTA curve, however, the shift of the base line shows that the thermal energy was produced during the calcination, which can be related to transformation from the tetragonal into the monoclinic phase. On the other hand, the TG–DTG profiles of the Zr(OH)4 –PO4 3− also presented the same weight loss stages (Fig. 3). The origin of these stages and peaks are similar to the explanation for the respective curves of the Zr(OH)4 , however, the only difference between the samples was the minor total weight loss for the Zr(OH)4 –PO4 3− . This fact could be due to thermal stability of the phosphate ions to zirconium linkages is much higher than that hydroxyl groups, it has also been determined into Zr(OH)4 –SO4 2− [14]. Then removal of the phosphate ions of the zirconia’s surface needs a higher temperature, it is impossible at lower temperature which delays the formation of a new crystalline phase and the growth crystallite too. The endothermic peak that appears at 489 ◦ C on the DTA curve for the Zr(OH)4 –PO4 3 is related about that (Fig. 4). This point can be demonstrated by XRD results of the phosphate zirconia.
3. Results and discussion 3.1. Thermal analysis (TGA–DTA) Figs. 1 and 2 show the TG–DTG and TG–DTA profiles of the Zr(OH)4 , respectively. Three weight loss stages were observed in the temperature range of 25–500 ◦ C. The largest one located between 25 and 200 ◦ C can be attributed to the loss of physically adsorbed water and the evaporation of 1-butanol, this weight loss stage also is related with the first endothermic peak in the DTA profile located at 50 ◦ C [11]. The second stage was highly exothermic (200–350 ◦ C), originating from the combustion of the organic compounds in the sample (alcoxi groups) two peaks located in the DTA profile at 258 and 285 ◦ C [12,13]. The last weight loss stage observed between 350 and 450 ◦ C, and the third exothermic peak registered in the DTA curve at 403 ◦ C corresponded to the decomposition of Zr(OH)4 into
Fig. 2. TG–DTA profiles of Zr(OH)4 .
3.2. X-ray diffraction (XRD) The XRD patterns of the samples ZrO2 and ZrO2 –PO4 3− annealed at 400, 500 and 600 ◦ C are shown in Figs. 5 and 6, respectively. The results show that there is a difference between pure and acid zirconia and also a change in the crystallite size. In the first case, tetragonal and monoclinic phases coexisted [15,16]. Their concen-
Fig. 4. TG–DTA profiles of Zr(OH)4 –PO4 3− .
J.M. Hernández Enríquez et al. / Journal of Alloys and Compounds 483 (2009) 425–428
427
Fig. 7. Surface area versus crystallite size related to ZrO2 and ZrO2 –PO4 3− samples.
Fig. 5. XRD patterns of ZrO2 calcined at 400, 500 and 600 ◦ C.
temperature increases, the diffraction peaks tend to sharpen, and this suggests a further enlargement of the crystallite size (Table 1). However, the calcination at higher temperatures (600 ◦ C) did not change the XRD pattern phase of the zirconia except for the peaks becoming sharper and stronger. 3.3. Surface area measurement (BET)
Fig. 6. XRD patterns of ZrO2 –PO4 3− calcined at 400, 500 and 600 ◦ C.
tration changed with the annealing temperature. At 400 ◦ C the main phase is tetragonal, but when the temperature was increased, the tetragonal phase concentration decreased with an increase in concentration on monoclinic phase and also a fast crystallite growth was observed (Table 1). At 600 ◦ C the monoclinic phase concentration was 100 wt.%. In the second case, the transformation of tetragonal into monoclinic phase is delayed due to doping agent (PO4 3− ion). Before at 600 ◦ C, the broad diffraction patterns were observed, indicating a characteristic of tetragonal zirconia nanoparticles with a low crystallinity [11,13,17]. When the calcination
Table 1 and Fig. 7 report the surfaces areas and the crystallite size of the ZrO2 and ZrO2 –PO4 3− as a function of the calcination temperature. The first striking difference appears from the comparison between the areas of the two series of supports. The area pertaining to the ZrO2 –PO4 3− calcined at 600 ◦ C is about 11 times larger than the one of the ZrO2 calcined at the same temperature. Further, also the trend with the temperature of the two series of supports is different. Pure zirconia shows a sharp and continuous decrease in the surface area with the temperature. Phosphate zirconia, instead, exhibits an almost invariant pattern and the calcination temperature only provokes a slight decrease in area. The present larger areas of the ZrO2 –PO4 3− are to be related, in the first instance, to the presence of the PO4 3− ion that improves the thermal stability into ZrO2 and these results also suggest that the interaction of the acid agent with the zirconia surface inhibits the strong syntherization on the material. This fact can be demonstrated by the TG–DTA analysis. In general, the samples of smaller grain size showed a higher surface area. 3.4. Infrared spectroscopy (FT-IR) FT-IR spectra for the zirconia and phosphate zirconia are shown in Figs. 8 and 9, respectively. In the different FT-IR spectra of the unmodified zirconia it is possible to observe in the region between
Table 1 Synthesis variables and textural properties of ZrO2 and ZrO2 –PO4 3− . Material
Calcination temperature (◦ C)
Acid agent
Surface area (m2 /g)
Crystallite size (nm)
Z400 Z500 Z600 ZP400 ZP500 ZP600
400 500 600 400 500 600
– – – H3 PO4 H3 PO4 H3 PO4
144 73 18 329 256 210
4.3 6.9 16.2 1.0 1.1 6.5
Crystallite size determined by X-ray diffraction.
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organic compound bands tend to disappear and the Zr–O signals increasing their intensity. On the other hand, the FT-IR spectra of modified zirconia (ZrO2 –PO4 3− ) exhibit peaks in the region of 1300–850 cm−1 , where pure ZrO2 does not show any bands. The spectra only show a broad-sharp band with peak maxima at 1050 cm−1 characteristic phosphorous–oxygen (P–O) stretching modes in the phosphate ion [21]. But it is not possible to determine the exact structure of this species from this data. However, by analogy with the structures determined for ZrO2 –SO4 2− , the formation of similar structure for ZrO2 –PO4 3− is expected. 4. Conclusions
Fig. 8. FT-IR spectra of ZrO2 calcined at 400, 500 and 600 ◦ C.
The results showed a positive effect on the physicochemical properties of the catalytic supports after Zr(OH)4 impregnation with the dopping agent (H3 PO4 ). Phosphate zirconium oxides remained thermically stable after calcination. It was observed that the dopping agent remained firmly attached to the zirconium oxide surface, inhibiting the particle growth and delaying the syntherization of the material and the apparition of the monoclinic phase, obtaining mesoporous and nano-crystalline materials (crystallite size, 1.0–6.5 nm) with high surface areas (210–329 m2 /g) and tetragonal structure defined for the calcination temperature of 600 ◦ C. These materials are considered as promising supports for potential catalytic applications. Acknowledgements The authors acknowledge Instituto Tecnológico de Cd. Madero for the facilities used in carrying out this work and CONACYT for its financial support through scholarship 181668. The authors also acknowledge the financial assistance via project COSNET-479.04P. References
Fig. 9. FT-IR spectra of ZrO2 –PO4 3− calcined at 400, 500 and 600 ◦ C.
3850 and 3100 cm−1 bands of absorption related with stretch vibrations of O–H groups. Bands at 3850, 3740 and 3680 cm−1 have been assigned to isolated O–H groups bound to single cations Zr4+ (type I) and to bridging O–H (type II) coordinate to more than one cation. Two bands at 3400 and 1620 cm−1 were attributed to dehydration of water [16,18]. Three absorption bands at 1340, 1220 and 992 cm−1 together with another little bands located in the range of 1450–900 cm−1 were observed for pure zirconia calcined at 400 and 500 ◦ C, and also related with traces of the organic compounds on zirconia. For these calcination temperatures, the spectra have two small bands at 2890 and 2870 cm−1 , produced by the symmetric and asymmetric flexion vibration of the C–H bonds associated to the CH2 groups connected to aliphatic groups [19]. All the spectra show bands located at 754, 580 and 500 cm−1 which are assigned to Zr–O bonds [20]. When the calcination temperature increases, the
[1] N. Yamamoto, S. Sato, R. Takahashi, K. Inui, J. Mol. Catal. A 243 (2006) 52–59. [2] M. Yamasaki, H. Habazaki, K. Asami, K. Izumiya, K. Hashimoto, Catal. Commun. 7 (2006) 24–28. [3] K. Arata, H. Matsuhashi, M. Hino, H. Nakamura, Catal. Today 81 (2003) 17–30. [4] E. López, J.G. Hernández, I. Schifter, E. Torres, J. Navarrete, A. Gutiérrez, T. López, P.P. Lottici, D. Bersani, Appl. Catal. A. 193 (2000) 215–225. [5] R. Silva, J.M. Hernández, A. Castillo, J.A. Melo, R. García, M. Picquart, T. López, Catal. Today 107–108 (2005) 838–843. [6] Ch.L. Chen, T. Li, S. Cheng, N. Xu, Ch.Y. Mou, Catal. Lett. 78 (2002) 223–229. [7] A. Pintar, M. Besson, P. Gallezot, Appl. Catal. B 30 (2001) 123–139. [8] J.M. Grau, C.R. Vera, J.M. Parera, Appl. Catal. A 172 (1998) 311–326. [9] F. Chen, K. Zhu, L. Huang, Y. Chen, F. Kooli, Mater. Res. Bull. 41 (2006) 10–18. [10] B. Tyagi, M.K. Mishra, R.V. Jasra, Catal. Commun. 7 (2006) 52–57. [11] Y. Cao, J.Ch. Hu, Z.S. Hong, J.F. Deng, K.N. Fan, Catal. Lett. 81 (2002) 107–112. [12] Y. Sun, L. Yuan, W. Wang, Ch.L. Chen, F.S. Xiao, Catal. Lett. 87 (2003) 57–61. [13] Y.W. Suh, J.W. Lee, H.K. Rhee, Catal. Lett. 90 (2003) 103–109. [14] J.A. Wang, M.A. Valenzuela, J. Salmones, A. Vázquez, A. García, X. Bokhimi, Catal. Today 68 (2001) 21–30. [15] Y.Y. Huang, B.Y. Zhao, Y.Ch. Xie, Appl. Catal. A 171 (1998) 75–83. [16] J.A. Moreno, G. Poncelet, Appl. Catal. A 210 (2001) 151–164. [17] S. Melada, M. Signorreto, S. Ardizzone, C. Bianchi, Catal. Lett. 75 (2001) 1999–2204. [18] Q.H. Xia, K. Hidajat, S. Kawi, J. Catal. 205 (2002) 318–331. ˜ [19] C.L. Li, O. Novaro, X. Bokhimi, E. Munoz, J.L. Boldú, J.A. Wang, T. López, R. Gómez, N. Batina, Catal. Lett. 65 (2000) 209–216. [20] F. Babou, G. Coudurier, C. Vedrine, J. Catal. 152 (1995) 341. [21] G.D. Yadav, J.J. Nair, Micropor. Mesopor. Mater. 33 (1999) 1–48.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Synthesis and characterization of mesoporous and nano-crystalline phosphate zirconium oxides J.M. Hernández Enríquez a,∗ , L.A. Cortez Lajas a , R. García Alamilla a , A. Castillo Mares a , G. Sandoval Robles a , L.A. García Serrano b a
Instituto Tecnológico de Cd. Madero, División de Estudios de Posgrado e Investigación, Juventino Rosas y Jesús Urueta S/N, Col. Los Mangos, 89440 Cd. Madero, Tam., Mexico Instituto Politécnico Nacional, Escuela Superior de Ingeniería Textil, Av. Instituto Politécnico Nacional s/n, Edificio #8, Col. Linda Vista, Delegación Gustavo A. Madero, 07738 México, D.F., Mexico b
a r t i c l e
i n f o
Article history: Received 30 August 2007 Received in revised form 25 July 2008 Accepted 6 August 2008 Available online 11 December 2008 Keywords: Nanostructured materials Oxide materials Sol–gel processes Crystal growth Crystal structure
a b s t r a c t In this work the preparation and characterization of the materials such as zirconia (ZrO2 ) and zirconia promoted with phosphate ion (ZrO2 –PO4 3− ) is presented. Pure zirconium hydroxide [Zr(OH)4 ] was synthesized by the sol–gel method using precursors such as zirconium n-butoxide and 1-butanol maintaining a pH 8 during the synthesis. Zr(OH)4 was impregnated with 15 wt.% of the acid agent. Both were calcined in a dynamic air atmosphere for 3 h at 400, 500 and 600 ◦ C. The supports were characterized by thermal analysis, X-ray diffraction, nitrogen physisorption as well as infrared spectroscopy. The results showed a positive effect on the physicochemical properties of the catalytic supports after Zr(OH)4 impregnation with the dopping agent (H3 PO4 ). Phosphate zirconium oxides remained thermically stable after calcination. It was observed that the dopping agent remained firmly attached to the zirconium oxide surface, inhibiting the particle growth and delaying the syntherization of the material and the apparition of the monoclinic phase, obtaining mesoporous and nano-crystalline materials (crystallite size 1.0–6.5 nm) with high surface areas (210–329 m2 /g) and tetragonal structure defined for the calcination temperature of 600 ◦ C. © 2008 Published by Elsevier B.V.
1. Introduction Zirconia is an important material that has attracted enormous interest in catalytic process such as paraffin isomerization, hydrogenation of olefins, alcohol dehydrogenation and other technological uses [1–4]. The use of the zirconia as a support material or catalyst often requires a high accessible surface area as well as a large and well-developed porous texture. However, zirconia oxides generally have surface areas of 50 m2 /g or less, which is rather low compared with conventional supports such as SiO2 or Al2 O3 . Higher surface areas are attainable with amorphous zirconia (200–300 m2 /g), but this was usually achieved at the expense of much lower thermal stability [5–8]. On the other hand, zirconia exists in three crystallographic phases: monoclinic, tetragonal and cubic. Among these phases, the tetragonal phase is preferable for some acid-catalyzed reactions (paraffin isomerization). Efforts have been made to partially or fully stabilize the tetragonal phase with enhanced surface area by some additives such as Y2 O3 or
SO4 2− [9,10]. For that, in this work it is presented the synthesis of zirconium oxides by the sol–gel method, modified this one with the presence of the phosphate ion (PO4 3− ) to study the possible influence of this parameter as a synthesis variable on the thermal, textural and structural properties. 2. Experimental 2.1. Supports preparation The Zr(OH)4 preparation was carried out by the sol–gel method dissolving zirconium n-butoxide in 1-butanol taking place the hydrolysis and condensation of this by a slow drop addition of a water/1-butanol solution maintaining a pH 8 during the synthesis. Later, the gel was aged for 72 h and dried at 120 ◦ C for 24 h. The impregnation of zirconium hydroxide (Zr(OH)4 ) with the acid agent (PO4 3− ) took place using the wet incipient technique adding the necessary amounts of a phosphoric acid solution (H3 PO4 , 2N) that contained the PO4 3− ion to obtain a 15 wt.% of the acid agent present in the support. The modified hydroxides were dried at 120 ◦ C for 24 h, calcined afterward in a dynamic air atmosphere at 400, 500 and 600 ◦ C for 3 h. For pure zirconia calcined at 400, 500 and 600 ◦ C its nomenclature is: Z400, Z500 and Z600 and for phosphate zirconia calcined at 400, 500 and 600 ◦ C its nomenclature is: ZP400, ZP500 and ZP600. 2.2. Supports characterization
∗ Corresponding author. Tel.: +52 833 215 8544; fax: +52 833 215 8544. E-mail address: [email protected] (J.M. Hernández Enríquez). 0925-8388/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.jallcom.2008.08.094
The thermogravimetric and differential thermal analysis took place in a thermogravimetric balance TA Instruments STD 2960 Simultaneous DSC-TGA; samples
426
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Fig. 1. TG–DTG profiles of Zr(OH)4 .
Fig. 3. TG–DTG profiles of Zr(OH)4 –PO4 3− .
were analyzed in air flow (10 ml/min) with a heating rate of 10 ◦ C/min in the temperature range of 25–900 ◦ C. Nitrogen physisorption was used to determine the surface areas of the materials at the temperature of liquid nitrogen (−196 ◦ C) in a Quantachrome Autosorb-1 instrument; previous to the analysis, samples were degasified for 2 h at 350 ◦ C. X-ray diffraction patterns were obtained in a Bruker Advance D800 diffractometer with Cu K␣ radiation ( = 1.5406 Å) and a graphite secondary beam monochromator; the intensities of the diffraction lines were obtained in the 2 range between 20◦ and 70◦ with a step of 0.02◦ and a measuring time of 2.7 s per point. Infrared spectroscopy was carried out in a Fourier transforms spectrometer (PerkinElmer Spectrum One) with transparent pills containing the sample to be analyzed and KBr as binder (90 wt.%) using a number of scanning of 16 and a resolution of 4 cm−1 .
ZrO2 [14]. When the sample was annealed above 500 ◦ C, no further weight loss was observed. In the DTA curve, however, the shift of the base line shows that the thermal energy was produced during the calcination, which can be related to transformation from the tetragonal into the monoclinic phase. On the other hand, the TG–DTG profiles of the Zr(OH)4 –PO4 3− also presented the same weight loss stages (Fig. 3). The origin of these stages and peaks are similar to the explanation for the respective curves of the Zr(OH)4 , however, the only difference between the samples was the minor total weight loss for the Zr(OH)4 –PO4 3− . This fact could be due to thermal stability of the phosphate ions to zirconium linkages is much higher than that hydroxyl groups, it has also been determined into Zr(OH)4 –SO4 2− [14]. Then removal of the phosphate ions of the zirconia’s surface needs a higher temperature, it is impossible at lower temperature which delays the formation of a new crystalline phase and the growth crystallite too. The endothermic peak that appears at 489 ◦ C on the DTA curve for the Zr(OH)4 –PO4 3 is related about that (Fig. 4). This point can be demonstrated by XRD results of the phosphate zirconia.
3. Results and discussion 3.1. Thermal analysis (TGA–DTA) Figs. 1 and 2 show the TG–DTG and TG–DTA profiles of the Zr(OH)4 , respectively. Three weight loss stages were observed in the temperature range of 25–500 ◦ C. The largest one located between 25 and 200 ◦ C can be attributed to the loss of physically adsorbed water and the evaporation of 1-butanol, this weight loss stage also is related with the first endothermic peak in the DTA profile located at 50 ◦ C [11]. The second stage was highly exothermic (200–350 ◦ C), originating from the combustion of the organic compounds in the sample (alcoxi groups) two peaks located in the DTA profile at 258 and 285 ◦ C [12,13]. The last weight loss stage observed between 350 and 450 ◦ C, and the third exothermic peak registered in the DTA curve at 403 ◦ C corresponded to the decomposition of Zr(OH)4 into
Fig. 2. TG–DTA profiles of Zr(OH)4 .
3.2. X-ray diffraction (XRD) The XRD patterns of the samples ZrO2 and ZrO2 –PO4 3− annealed at 400, 500 and 600 ◦ C are shown in Figs. 5 and 6, respectively. The results show that there is a difference between pure and acid zirconia and also a change in the crystallite size. In the first case, tetragonal and monoclinic phases coexisted [15,16]. Their concen-
Fig. 4. TG–DTA profiles of Zr(OH)4 –PO4 3− .
J.M. Hernández Enríquez et al. / Journal of Alloys and Compounds 483 (2009) 425–428
427
Fig. 7. Surface area versus crystallite size related to ZrO2 and ZrO2 –PO4 3− samples.
Fig. 5. XRD patterns of ZrO2 calcined at 400, 500 and 600 ◦ C.
temperature increases, the diffraction peaks tend to sharpen, and this suggests a further enlargement of the crystallite size (Table 1). However, the calcination at higher temperatures (600 ◦ C) did not change the XRD pattern phase of the zirconia except for the peaks becoming sharper and stronger. 3.3. Surface area measurement (BET)
Fig. 6. XRD patterns of ZrO2 –PO4 3− calcined at 400, 500 and 600 ◦ C.
tration changed with the annealing temperature. At 400 ◦ C the main phase is tetragonal, but when the temperature was increased, the tetragonal phase concentration decreased with an increase in concentration on monoclinic phase and also a fast crystallite growth was observed (Table 1). At 600 ◦ C the monoclinic phase concentration was 100 wt.%. In the second case, the transformation of tetragonal into monoclinic phase is delayed due to doping agent (PO4 3− ion). Before at 600 ◦ C, the broad diffraction patterns were observed, indicating a characteristic of tetragonal zirconia nanoparticles with a low crystallinity [11,13,17]. When the calcination
Table 1 and Fig. 7 report the surfaces areas and the crystallite size of the ZrO2 and ZrO2 –PO4 3− as a function of the calcination temperature. The first striking difference appears from the comparison between the areas of the two series of supports. The area pertaining to the ZrO2 –PO4 3− calcined at 600 ◦ C is about 11 times larger than the one of the ZrO2 calcined at the same temperature. Further, also the trend with the temperature of the two series of supports is different. Pure zirconia shows a sharp and continuous decrease in the surface area with the temperature. Phosphate zirconia, instead, exhibits an almost invariant pattern and the calcination temperature only provokes a slight decrease in area. The present larger areas of the ZrO2 –PO4 3− are to be related, in the first instance, to the presence of the PO4 3− ion that improves the thermal stability into ZrO2 and these results also suggest that the interaction of the acid agent with the zirconia surface inhibits the strong syntherization on the material. This fact can be demonstrated by the TG–DTA analysis. In general, the samples of smaller grain size showed a higher surface area. 3.4. Infrared spectroscopy (FT-IR) FT-IR spectra for the zirconia and phosphate zirconia are shown in Figs. 8 and 9, respectively. In the different FT-IR spectra of the unmodified zirconia it is possible to observe in the region between
Table 1 Synthesis variables and textural properties of ZrO2 and ZrO2 –PO4 3− . Material
Calcination temperature (◦ C)
Acid agent
Surface area (m2 /g)
Crystallite size (nm)
Z400 Z500 Z600 ZP400 ZP500 ZP600
400 500 600 400 500 600
– – – H3 PO4 H3 PO4 H3 PO4
144 73 18 329 256 210
4.3 6.9 16.2 1.0 1.1 6.5
Crystallite size determined by X-ray diffraction.
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organic compound bands tend to disappear and the Zr–O signals increasing their intensity. On the other hand, the FT-IR spectra of modified zirconia (ZrO2 –PO4 3− ) exhibit peaks in the region of 1300–850 cm−1 , where pure ZrO2 does not show any bands. The spectra only show a broad-sharp band with peak maxima at 1050 cm−1 characteristic phosphorous–oxygen (P–O) stretching modes in the phosphate ion [21]. But it is not possible to determine the exact structure of this species from this data. However, by analogy with the structures determined for ZrO2 –SO4 2− , the formation of similar structure for ZrO2 –PO4 3− is expected. 4. Conclusions
Fig. 8. FT-IR spectra of ZrO2 calcined at 400, 500 and 600 ◦ C.
The results showed a positive effect on the physicochemical properties of the catalytic supports after Zr(OH)4 impregnation with the dopping agent (H3 PO4 ). Phosphate zirconium oxides remained thermically stable after calcination. It was observed that the dopping agent remained firmly attached to the zirconium oxide surface, inhibiting the particle growth and delaying the syntherization of the material and the apparition of the monoclinic phase, obtaining mesoporous and nano-crystalline materials (crystallite size, 1.0–6.5 nm) with high surface areas (210–329 m2 /g) and tetragonal structure defined for the calcination temperature of 600 ◦ C. These materials are considered as promising supports for potential catalytic applications. Acknowledgements The authors acknowledge Instituto Tecnológico de Cd. Madero for the facilities used in carrying out this work and CONACYT for its financial support through scholarship 181668. The authors also acknowledge the financial assistance via project COSNET-479.04P. References
Fig. 9. FT-IR spectra of ZrO2 –PO4 3− calcined at 400, 500 and 600 ◦ C.
3850 and 3100 cm−1 bands of absorption related with stretch vibrations of O–H groups. Bands at 3850, 3740 and 3680 cm−1 have been assigned to isolated O–H groups bound to single cations Zr4+ (type I) and to bridging O–H (type II) coordinate to more than one cation. Two bands at 3400 and 1620 cm−1 were attributed to dehydration of water [16,18]. Three absorption bands at 1340, 1220 and 992 cm−1 together with another little bands located in the range of 1450–900 cm−1 were observed for pure zirconia calcined at 400 and 500 ◦ C, and also related with traces of the organic compounds on zirconia. For these calcination temperatures, the spectra have two small bands at 2890 and 2870 cm−1 , produced by the symmetric and asymmetric flexion vibration of the C–H bonds associated to the CH2 groups connected to aliphatic groups [19]. All the spectra show bands located at 754, 580 and 500 cm−1 which are assigned to Zr–O bonds [20]. When the calcination temperature increases, the
[1] N. Yamamoto, S. Sato, R. Takahashi, K. Inui, J. Mol. Catal. A 243 (2006) 52–59. [2] M. Yamasaki, H. Habazaki, K. Asami, K. Izumiya, K. Hashimoto, Catal. Commun. 7 (2006) 24–28. [3] K. Arata, H. Matsuhashi, M. Hino, H. Nakamura, Catal. Today 81 (2003) 17–30. [4] E. López, J.G. Hernández, I. Schifter, E. Torres, J. Navarrete, A. Gutiérrez, T. López, P.P. Lottici, D. Bersani, Appl. Catal. A. 193 (2000) 215–225. [5] R. Silva, J.M. Hernández, A. Castillo, J.A. Melo, R. García, M. Picquart, T. López, Catal. Today 107–108 (2005) 838–843. [6] Ch.L. Chen, T. Li, S. Cheng, N. Xu, Ch.Y. Mou, Catal. Lett. 78 (2002) 223–229. [7] A. Pintar, M. Besson, P. Gallezot, Appl. Catal. B 30 (2001) 123–139. [8] J.M. Grau, C.R. Vera, J.M. Parera, Appl. Catal. A 172 (1998) 311–326. [9] F. Chen, K. Zhu, L. Huang, Y. Chen, F. Kooli, Mater. Res. Bull. 41 (2006) 10–18. [10] B. Tyagi, M.K. Mishra, R.V. Jasra, Catal. Commun. 7 (2006) 52–57. [11] Y. Cao, J.Ch. Hu, Z.S. Hong, J.F. Deng, K.N. Fan, Catal. Lett. 81 (2002) 107–112. [12] Y. Sun, L. Yuan, W. Wang, Ch.L. Chen, F.S. Xiao, Catal. Lett. 87 (2003) 57–61. [13] Y.W. Suh, J.W. Lee, H.K. Rhee, Catal. Lett. 90 (2003) 103–109. [14] J.A. Wang, M.A. Valenzuela, J. Salmones, A. Vázquez, A. García, X. Bokhimi, Catal. Today 68 (2001) 21–30. [15] Y.Y. Huang, B.Y. Zhao, Y.Ch. Xie, Appl. Catal. A 171 (1998) 75–83. [16] J.A. Moreno, G. Poncelet, Appl. Catal. A 210 (2001) 151–164. [17] S. Melada, M. Signorreto, S. Ardizzone, C. Bianchi, Catal. Lett. 75 (2001) 1999–2204. [18] Q.H. Xia, K. Hidajat, S. Kawi, J. Catal. 205 (2002) 318–331. ˜ [19] C.L. Li, O. Novaro, X. Bokhimi, E. Munoz, J.L. Boldú, J.A. Wang, T. López, R. Gómez, N. Batina, Catal. Lett. 65 (2000) 209–216. [20] F. Babou, G. Coudurier, C. Vedrine, J. Catal. 152 (1995) 341. [21] G.D. Yadav, J.J. Nair, Micropor. Mesopor. Mater. 33 (1999) 1–48.