Al layered double hydroxides obtained by the sol–gel method

Microporous and Mesoporous Materials 80 (2005) 213–220 www.elsevier.com/locate/micromeso

Synthesis and characterization of Zn/Al and Pt/Zn/Al layered double hydroxides obtained by the sol–gel method Didier Tichit a,*, Olivier Lorret a,b, Bernard Coq a, Federica Prinetto b, Giovanna Ghiotti b a

Laboratoire de Mate´riaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRS-ENSCM-UM1, Institut C. Gerhardt FR 1878, 8 rue Ecole Normale, F-34296 Montpellier Cedex 5, France b Dipartimento di Chimica IFM, Universita` di Torino and INSTM Consortium, UdR Torino, Via P. Giuria 7, I-10125 Torino, Italy Received 24 September 2004; received in revised form 10 December 2004; accepted 13 December 2004 Available online 22 January 2005

Abstract The influence of the nature of the Zn, Al and Pt precursors, and of the temperature of precipitation and aging have been studied in connection with the preparation of Zn/Al and Pt/Zn/Al layered double hydroxides (LDH) by the sol–gel method. Whatever the precursors the XRD analysis shows that LDH is formed at the expense of ZnO when the precipitation and aging temperatures decrease from 353 K to 273 K. Moreover, chemical composition and TG analysis suggest the presence of weak amounts of hydrozincite, hydrozincite-like and Al(OH)3 phases. When the precursors are Zn acetate-2-hydrate or Al acetylacetonate the amount of LDH reaches a maximum of 50 mol% at 273 K. At variance, about 90 mol% of LDH is obtained when using Zn acetylacetonate and Al isopropoxide as precursors, which are precipitated at 273 K. This proportion is slightly improved for the Pt-containing sample prepared under the same conditions. The specific surface areas of the different samples obtained after calcination at 723 K increase with their LDH content, reaching values of 110–120 m2 g
1. They make them particularly attractive for catalytic applications.
2005 Elsevier Inc. All rights reserved. Keywords: Hydrotalcite; Sol–gel; Precursors; Zn/Al-LDH; Platinum

1. Introduction Layered double hydroxides (LDH or anionic clays) can xþ III be described by the generic formula ½MII 1
x Mx ðOHÞ2  n
[Ax/n] , mH2O. Their structure is formed by positively charged layers containing in required amounts (0.25 6 x 6 0.33) divalent (MII: Mg, Zn, Ni, Co, . . .) and trivalent cations (MIII: Al, Ga, Cr, . . .) with an octahedral coordi

nation. Compensating anions ðAn
: CO2
3 ; NO3 ; Cl ;
OH ; . . .Þ are located in the interlayer spaces along with water molecules. Due to features of the LDH compounds, *

Corresponding author. Tel.: +33 04 67 16 34 77; fax: +33 04 67 14 43 22. E-mail address: [email protected] (D. Tichit). 1387-1811/$ - see front matter
2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2004.12.015

thermal activation allows to obtain mixed oxides (e.g. Mg(Al)O) or spinel-like structures presenting synergetic effects between the elements and strongly basic character, depending on the chemical composition [1–4]. Moreover, in the case of LDH containing noble or transition metals, dedicated activation treatments can give rise to highly dispersed metal nanoclusters [4–6]. This led to an extensive development of these materials for various applications such as anion exchangers [7], host structures for the synthesis of nano-composite materials [8] and precursors of catalysts [4,5,9]. In this latter case the attractive interest comes from the control of the crystallinity and texture, as well as the intrinsic acid–base and redox properties, which can be fine tuned by the composition, the synthesis method and the activation process [4]. Co-precipitation of

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D. Tichit et al. / Microporous and Mesoporous Materials 80 (2005) 213–220

mineral salts in aqueous medium followed by hydrothermal treatments of the gels constitutes the conventional elaboration process of LDH materials [6,10]. However, alternative routes have been recently explored with the aim of a better control of the structural and textural properties. This has been achieved, for instance, through microwave irradiation or ultrasonic treatment in the course of the classical co-precipitation route from inorganic precursors [11,12]. Interestingly, some few reports have demonstrated that LDH can also be obtained by the sol–gel method with MII = Mg and Ni [13], and MIII = Al, Ga or In [14]. It is worthy to note that the solids thus obtained exhibited specific surface areas generally larger than those obtained using a conventional co-precipitation procedure [13–15], but with controversial results regarding the basicity and the MII/MIII ratios [13,14,16]. The synthesis of Zn-containing LDH was extensively studied due to the attractive properties of these materials as sensors and catalysts. These studies addressed, for instance, the preparation of a large composition range [17], the re-hydration of calcined materials [18], the synthesis of nano-composites by intercalation of polymeric guest molecules [8] or the one-step formation of Zn/Cr [19], Zn/Al and Zn/Fe LDH [20]. The classical co-precipitation route yields always highly crystalline materials, giving rise (after calcination) to mixed oxides of low specific surface areas, and of very low basic strength [3,21,22], which are detrimental features for catalytic applications. However, in spite of their interest, Zn-containing LDH have not been yet synthesized by the sol–gel method. Actually, Al doped ZnO have been prepared by sol–gel methods to obtain thin films for various electrical and optical applications [23–29]. However, their Al content, below 10 at.%, was too low for obtaining the LDH structure. This work was thus aimed at studying the preparation of Zn/Al and Pt/Zn/Al LDH of larger surface area for catalytic applications, using the sol–gel method. This latter material was chosen as representative of noble metal containing LDH whose interest as precursors of catalysts has been already highlighted [30,31].

2. Experimental 2.1. Synthesis of solids Several samples have been prepared using different Zn and Al precursors, i.e. Zn acetylacetonate (Zn(acac)2 Æ xH2O) or Zn acetate-2-hydrate (Zn(acet)2 Æ 2H2O), and Al acetylacetonate (Al(acac)3) or Al isopropoxide (Al(OPri)3). In all cases the same general procedure was followed. Typically 6 · 10
2 mol of Zn(acac)2 Æ xH2O or Zn(acet)2 Æ 2H2O was dissolved at 273, 298 or 353 K under vigorous stirring in 400 cm3 ethanol by addition

of the required amount of HCl (Carlo Erba, 35% RPE) to reach the complete dissolution (for example, 3 cm3 at 353 K, 10 cm3 at 273 K). The solution was then added dropwise for about 10 min to 2 · 10
2 mol of Al(acac)3 or Al(OPri)3 (Zn/Al = 3 mol/mol) dissolved in 100 cm3 ethanol, containing ca. 0.5 cm3 HCl. The solution was maintained at 273, 298 or 353 K for 3 h under stirring. The pH of the mixture was then adjusted to ca. 10 by dropwise addition of NaOH (1 M, 2 cm3 min
1). A gel was thus formed which is maintained for aging for 15 h at the same temperature under continuous stirring. After centrifugation the gel was then washed 4 times with ethanol, then washed repeatedly with deionized water (Na < 100 ppm) and dried overnight at 353 K. The same procedure above described was followed for the preparation of the Pt-containing samples with 4 · 10
4 mol of Pt acacetylacetonate (Pt(acac)3) or H2Pt(OH)6 added to 5.96 · 10
2 mol of Zn(acac)2 Æ xH2O dissolved in 400 cm3 ethanol and 10 cm3 HCl at 273 K and vigorous stirring. The synthesis parameters of the different samples including the nature of the precursors and the temperature of preparation and aging are given in Table 1. The labelling of the samples, e.g. Zn(acac)Al(OPri)-273 indicates that the precursors Zn(acac)2 Æ xH2O and Al(OPri)3 have been used with a temperature of synthesis of 273 K then aged for 15 h at 273 K. 2.2. Characterization Elemental analyses of the as-prepared solids were performed at the Service Central d
Analyse du CNRS (Solaize, France). X-ray powder diffraction patterns (XRD) were obtained with a CGR Theta 60 diffractometer using monochromatized Cu-Ka1 radiation (k = ˚ , 40 kV and 50 mA). The specific surface areas 1.542 A were determined by nitrogen adsorption (BET method) at 77 K with a Micromeritics ASAP 2100 apparatus on samples calcined in synthetic air at 723 K and outgassed at 523 K and 10
4 Pa. TG-DSC experiments were carried out with a Setaram TG-DSC-111 apparatus, with fully programmable heating and cooling sequences, sweep gas valves switchings, and data analysis. About 50 mg of an as-prepared sample were placed in a platinum crucible and dried at 393 K in a He stream (flow rate: 20 cm3 min
1). Heating was performed from 393 to 873 K (heating rate: 1 K min
1) in a He stream (flow rate: 20 cm3 min
1).

3. Results and discussion The chemical compositions of the various samples are reported in Table 1, and the XRD patterns are shown in Fig. 1. The XRD patterns of both series of samples,

D. Tichit et al. / Microporous and Mesoporous Materials 80 (2005) 213–220

215

Table 1 Synthesis, conditions chemical composition and some morphological features of the samples prepared Sample

T synt. (K)

Chemical composition (wt%) Zn

Al

C

Zn(acac)Al(acac)-353 Zn(acac)Al(acac)-298 Zn(acac)Al(acac)-273 Zn(acac)Al(OPri)-353 Zn(acac)Al(OPri)-298 Zn(acac)Al(OPri)-273 Zn(acet)Al(acac)-273 Pt(OH)Zn(acac)Al(OPri)-273 Pt(OH)Zn(acac)Al(OPri)-273 (non-aged) Pt(acac)Zn(acac)Al(OPri)-273

353 298 273 353 298 273 273 273 273 273

63.30 55.17 49.97 52.18 56.60 47.20 76.40 44.56 48.40 60.80

2.43 7.00 8.14 6.72 7.73 7.11 0.80 6.53 6.30 4.10

0.69 0.74 1.07 0.89 0.99 1.28 0.31 2.45 1.99 1.14

a

Cl

3.57 – 1.02

Zn/Al

a (nm)

S.S.a (m2 g
1)

Pt

0.55 0.20 200 ppm

10.75 3.25 2.53 3.20 3.25 2.75 39.4 2.82 3.17 6.12

0.3067 0.3070 0.3071

41 74 99 47 81 110 51 122 106 55

BET specific surface areas determined on samples calcined at 723 K and outgassed at 523 K.

(112)

(103)

(110)

g

(110) (113)

(018)

(104) (015)

Intensity (a.u.)

(101) (012)

(006)

(003)

(102)

(002)

(101)

(100)

obtained either from Zn(acac)2 Æ xH2O and Al(acac)3 or Zn(acac)2 Æ xH2O and Al(OPri)3, show the peaks characteristic of ZnO and LDH phases, in relative amounts depending on the temperature of synthesis and aging from 273 to 353 K. Indeed the (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2) diffraction peaks, coinciding with the ASTM data for zincite-type ZnO (ASTM 5664), clearly appear at 0.282, 0.260, 0.248, 0.191, 0.163, 0.148 and 0.138 nm, respectively, for the samples synthesized at 353 K. In contrast, the (0 0 3), (0 0 6), (1 0 1), (0 1 2), (1 0 4), (0 1 5) and (0 1 8) diffraction peaks, typical of the LDH structure, at 0.757, 0.382, 0.280, 0.259, 0.247, 0.229 and 0.194 nm, respectively, dominate the XRD profiles of the samples synthesized at 273 K. The reflection of this LDH phase was indexed for a hexago-

f

e d

c b a

10

20

30

40

50

60

70

2θ /degrees

Fig. 1. Diffration patterns of (a) Zn(acac)-353, (b) Zn(acac)Al(acac)298, (c) Zn(acac)Al(acac)-273, (d) Zn(acac)Al(OPri)-353, (e) Zn(acac)Al(OPri)-298, (f) Zn(acac)Al(OPri)-273 and (g) Zn(acet)Al(acac)-273.

nal lattice with a R-3m rhombohedral symmetry, generally used for the description of this structure. One can conclude that the LDH phase is formed at the expense of ZnO as the synthesis temperature decreases. It should be noted that when NaOH is added to the alcoholic solution of Zn and Al precursors, formation of the gel occurs with a concurrent increase of temperature revealing the exothermic character of this reaction. As the relative amount of LDH phase increases on decreasing the synthesis temperature, the Zn/Al molar ratio of the samples decreases, going from 10.75 to 2.53 in the series obtained using Al(acac)3 (Table 1). Conversely, for materials prepared using the Al(OPri)3 precursor the Zn/Al molar ratio of 3 ± 0.2 is close to the nominal value in solution. On that account, the influence of Zn and Al precursors on the materials synthesized at 273 K can be identified from XRD patterns. Those of Zn(acac)Al(acac)-273 and Zn(acet)Al(acac)-273 samples (Fig. 1 traces c and g, respectively) show that higher amounts of LDH phase are obtained by using Zn(acac)2 Æ xH2O instead of Zn(acet)2 Æ 2H2O. According to the XRD pattern of Zn(acet)Al(acac)-273, exhibiting mainly the peaks of ZnO, a dramatic increase of the Zn/Al molar ratio of this sample confirms that Zn(acec)2 Æ 2H2O precursor likely precipitates under these conditions into a zincitetype phase. On the other hand, XRD patterns of Zn(acac)Al(acac)-273 and Zn(acac)Al(OPri)-273 samples (Fig. 1 traces c and f, respectively) show that Al(OPri)3 leads mainly to the formation of the LDH phase. The lattice c parameter of the LDH phase of 2.270 nm in all samples (calculated as three times the d003 value) is very close to the values of 2.265, 2.283 and 2.299 nm reported for Zn/Al [17], Zn/Cr [32] or Zn/Ga [33] LDH respectively, containing CO2
3 as compensating anions. The lattice a parameter for the LDH phase of Zn(acac)Al(OPri)-273 of 0.3067 nm (calculated as twice the position of the d110 peak) is close to the value of 0.3072 nm reported for Zn–Al LDH with an Zn/ Al atomic ratio ca. 2 [17]. The higher Zn/Al ratio observed clearly indicates that an additional Zn-containing

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phase is present beside the LDH. If it was ZnO, it would amount to almost 20%. This disagrees with the shape of the XRD pattern (Fig. 1 trace f), and likely suggests the presence of another undetected Zn-containing phase, as it will be discussed further. Due to the superposition of the (1 0 0), (0 0 2) and (1 0 1) XRD peaks of ZnO with the (1 0 1), (0 1 2) and (1 0 4) diffraction peaks of LDH, their relative intensities obviously vary with the synthesis temperature in the two series of Zn(acac)Al(acac) and Zn(acac)Al(OPri) samples. However, a rough estimation of the mean crystallite sizes of ZnO could be tentatively proposed from its most intense (1 0 1) line using the Scherrer
s formula. For the LDH phase the mean crystallite size will be obtained from the (0 0 3) line. As expected, the crystallite size of ZnO increases as the temperature of aging increases from 298 to 353 K. It goes from 9 to 18 nm in the Zn(acac)Al(OPri) series, and from 11 to 21 nm in the Zn(acac)Al(acac) series. The mean crystallite size of the LDH phase, which also increases with temperature, goes from 9 to 17 nm in the Zn(acac)Al(OPri) series, and is larger in the Zn(acac)Al(acac) series going from 20 to 46 nm. From the chemical compositions reported in Table 1, it is noted that only Zn(acac)Al(OPri)-273 shows significant amounts of residual Cl, coming from HCl used in the synthesis, in spite of several washing steps, whereas the other samples contain Cl in trace amounts (<200 ppm). This suggests the involvement of Cl in a stable crystallographic phase. The calculation of a reliable formula for the LDH in the samples is thus hindered by the presence of several companion phases. However, by making a reasonable hypothesis, the chemical compositions allow us to do a rough estimate of the nature and compositions of the different phases present in the samples. Zn(acac)Al(OPri)-273 exhibits the characteristic XRD pattern of a LDH (Fig. 1, trace f). Several features might be considered concerning this sample: (1) The lattice a parameter is in agreement with a value of x, the molar fraction in aluminium, of 0.31 [17]. The chemical analysis leads to a value of x = 0.27,

but the ZnO peaks in the XRD pattern are not detected, and the presence of a Zn-containing phase different from ZnO could therefore be assumed; (2) The Cl content is dramatically higher than in the other samples; (3) The molar ratio (2CO3 + Cl)/Al > 1 disagrees with
an involvement of both CO2
3 and Cl as compensating anions in the LDH phase only. In order to propose a composition of this sample one could tentatively ascribe the carbon content to the carbonate compensating anions of the LDH, due to their well known affinity for this structure, and the Cl content to the hydrozincite-like compound Zn5Cl2(OH)8. The occurrence of this latter phase was indeed currently reported in similar syntheses [20,21]. Aluminium involved in the LDH must equilibrate the negative charge provided by the carbonate anions. The remaining small excess of Al is probably involved in an amorphous alumina phase. The compositions thus derived are reported in Table 2. Hydrozincite-type phase and alumina are not detected by XRD due to their low concentrations. Moreover, the hydrozincite exhibits a lamellar structure whose XRD pattern could be superimposed on that of LDH, and the alumina phase is probably amorphous. The value x = 0.31 in the LDH formula is close to that expected from the lattice a parameter. Generally speaking, the optimum value of x in the LDH family ranges from 0.2 to 0.33 [6,10]; however, there is a specific behaviour for the Zn-containing LDH. Pure lamellar phases are indeed reported only for x = 0.33 and 0.37 in Zn/Al [17], x = 0.33 in Zn/Ga [33] and in Zn/Cr samples [19,32]. The Zn/Cr LDH compounds have been extensively studied to clarify their peculiar ability to be synthesized with x = 0.33. This was related to the behaviour of chromium ions in solution. It has been suggested that mixed Zn–Cr complexes are formed from condensation between hexaquozinc and de-protonated chromium complexes leading to [Zn2Cr(OH)6]2+ layers of the LDH [34]. From XRD and EXAFS experiments, a cationic order in the layers has been put into evidence, in complete agreement with this structural pathway

Table 2 Proposed compositions of the various samples prepared Sample

Proposed composition

Zn(acac)Al(acac)-353 Zn(acac)Al(acac)-298 Zn(acac)Al(acac)-273 Zn(acac)Al(OPri)-353 Zn(acac)Al(OPri)-298 Zn(acac)Al(OPri)-273 Pt(OH)Zn(acac)Al(OPri)-273 Pt(OH)Zn(acac)Al(OPri)-273 (non aged) Pt(acac)Zn(acac)Al(OPri)-273

0.23 0.34 0.51 0.43 0.43 0.87 0.95 0.80 0.44

[Zn0.66Al0.34(OH)2(CO3)0.165 Æ mH2O] + 0.75ZnO + 0.02 [Zn5(CO3)2(OH)6] [Zn0.66Al0.34(OH)2(CO3)0.165 Æ mH2O] + 0.54ZnO + 0.12Al(OH)3 [Zn0.66Al0.34(OH)2(CO3)0.165 Æ mH2O] + 0.38ZnO + 0.11Al(OH)3 [Zn0.66Al0.34(OH)2(CO3)0.165 Æ mH2O] + 0.47ZnO + 0.10Al(OH)3 [Zn0.66Al0.34(OH)2(CO3)0.165 Æ mH2O] + 0.46ZnO + 0.11Al(OH)3 [Zn0.69Al0.31(OH)2(CO3)0.157 Æ mH2O] + 0.05 [Zn5Cl2(OH)8] + 0.05Al(OH)3 [Zn0.64Pt0.0037Al0.32(OH)2(CO3)0.203 Æ mH2O] + 0.02 [Zn5Cl2(OH)8] + 0.03 [Zn5(CO3)2(OH)6] [Zn0.66Pt0.0014Al0.33(OH)2(CO3)0.165 Æ mH2O] + 0.03 [Zn5(CO3)2(OH)6] + 0.17ZnO [Zn0.66Al0.33(OH)2(CO3)0.165 Æ mH2O] + 0.01 [Zn5(CO3)2(OH)6] + 0.55ZnO

D. Tichit et al. / Microporous and Mesoporous Materials 80 (2005) 213–220

c

Intensity (a.u)

and a value of x = 0.33 [35]. Otherwise, a careful study of reconstructed Zn/Al LDH with x values ranging between 0.5 and 0.16, has shown that in all cases the re-hydrated phases exhibit a molar fraction x = 0.33 [18]. The slightly lower value of x than the optimum found in Zn(acac)Al(OPri)-273 (0.31 versus 0.33) could be accounted for the presence of trace amounts of ZnO. The XRD patterns of the other samples show the presence of significant amounts of both ZnO and LDH. Taking into account the above results, and assuming that all carbon is involved in the carbonate species as compensating anions of the LDH with x = 0.33, the compositions reported in Table 2 could be proposed. After the assignment of Zn and Al to LDH, the remaining is considered to belong to ZnO and alumina phases, respectively. The amounts of LDH in the two Zn(acac)Al(OPri) and Zn(acac)Al(acac) series of samples, i.e. 35–75 mol%, are in agreement with the XRD patterns. From a general point of view, the XRD patterns show that ZnO is formed at the expense of LDH when the temperature of synthesis increases, and the rough evaluation of compositions suggests that amorphous alumina is also probably formed. Therefore, Zn and Al cations are more likely involved in the pure hydroxides or oxides rather than co-precipitated in the Zn/Al LDH structure when the synthesis is performed above 273 K. The titration curves in aqueous media of Zn– Al mixtures reveal that the plateau corresponding to the precipitation of Zn/Al LDH phase is poorly defined and close to the precipitation of Zn(OH)2 and Al(OH)3 [19,20]. This is a different behaviour with respect to other well known systems, e.g. Mg/Al or Ni/Al, for which the precipitation of LDH gives rise to a well defined plateau at an intermediate pH between those of the pure hydroxides. In the course of the synthesis by the sol–gel method, the pure hydroxides are preferentially formed leading to a mixture of ZnO, formed by dehydration of Zn(OH)2, and alumina. This is due to the very different reactivity of the Zn and Al precursors. It is known that the sols obtained by hydrolysis of Zn and Al precursors have low stabilities and that precipitation or gelation easily occurs; for that reason, amines are currently added to stabilize these gels [24]. The chemical analysis of the Pt-containing samples (Table 1) shows that only trace amounts of Pt (200 ppm) have been introduced using the Pt(acac)3 precursor. Surprisingly, even if the final material contains only traces amounts of Pt, the presence of Pt(acac)3 has influenced the synthesis process of the material. The Zn/Al ratio is indeed much larger than in the Pt-free sample, i.e. Zn(acac)Al(OPri)-273. The outcome is the occurrence of a well crystallized ZnO-type phase in a significant amount (Fig. 2, trace c), and a decrease of the specific surface from 110 to 55 m2 g
1 in Pt(acac) Zn(acac)Al(OPri)-273 (Table 1). A composition of this

217

b

a 10

20

30

40

50

60

70

2θ /degrees

Fig. 2. Diffraction patterns of: (a) Pt(OH)Zn(acac)Al(OPri)-273, (b) Pt(OH)Zn(acac)Al(OPri)-273 (non aged) and (c) Pt(acac)Zn(acac)Al(OPri)-273.

sample can be proposed (Table 2), assuming that (i) caranions of the bon is involved as compensating CO2
3 LDH (x = 0.33), (ii) the remaining carbon amount may belong to the hydrozincite phase [Zn5(CO3)2(OH)6], and (iii) Zn excess belongs to a ZnO phase. The complete composition is consistent with the shape of the XRD pattern (Fig. 2, trace c). Remarkably, this composition is close to that of Zn(acac)Al(OPri)-298 or Zn(acac)Al(OPri)-353. At variance, when H2Pt(OH)6 was used as precursor and aging was carried out with the gel, all the Pt engaged during the synthesis is present in the final material. This material exhibits the XRD pattern of a pure LDH (Fig. 2, trace a) which does not allow to conclude about the location of Pt; the Pt content is too low to induce a detectable modification of the lattice a parameter. A significant amount of Cl is present with a (2CO3 + Cl)/Al molar ratio higher than unity; chlorine could belong to the hydrozincite-type phase Zn5(Cl)2(OH)8. After distribution of Zn among this latter phase and the LDH phase (with x = 0.33 and carbonates as compensating anions), a residual quantity of Zn still remains. As some carbon amount is also unaffected, the formation of the Zn5(CO3)2(OH)6 phase is proposed. On the other hand, a specific study of the non-aged gel carried out for this sample shows that the amount of Pt was two times lower, the content of ZnO larger (Fig. 2, trace b) and Cl has disappeared. There is no proof that Pt belongs to some crystallographic sites in the brucite-like layer. After attribution to the carbonate species in the LDH (x = 0.33) of the corresponding amount, the remaining carbon is proposed to be involved in a Zn5(CO3)2(OH)6 phase. The compositions thus obtained are reported in Table 2.

D. Tichit et al. / Microporous and Mesoporous Materials 80 (2005) 213–220

m/m) x 100 (1-

a 300

400

500

600

700

800

900

1000

1100

T/K Fig. 3. TG DTG profiles of (a) Zn(acac)Al(OPri)-353, (b) Zn(acac)Al(OPri)-298 and (c) Zn(acac)Al(OPri)-273.

Fig. 3 shows the TG-DTG profiles recorded for the Zn(acac)Al(OPri) samples series. Total weight losses and shapes of TG profiles might depend on the main phase content of these samples, i.e. ZnO for Zn(acac)Al(OPri)-353 and Zn(acac)Al(OPri)-298, and LDH for Zn(acac)Al(OPri)-273. The first weight loss around 313 K, very intense for Zn(acac)Al(OPri)-353 and Zn(acac)Al(OPri)-298, becomes a weak shoulder for Zn(acac)Al(OPri)-273. This weight loss amounting to 2.2% and 4.9% of the initial mass for the two first ZnO-rich samples, represents almost 35% of the total weight loss recorded at 1073 K (i.e. 7% and 13.7% of the initial mass, respectively). At variance, the first weight loss of 3.9% of the initial mass for Zn(acac)Al(OPri)-273 represents only 13% of its total weight loss. The weight loss around 313 K is generally assigned to the removal of weakly adsorbed water molecules on the external surface of the LDH particles [32,36]. In ZnO it is assigned to the evaporation of water and solvents [23,25]. As expected, these phenomena are of lower magnitude for LDH than for ZnO. Two weight losses occur between 373 K and 623 K, and of very different extent depending on the sample. Both peaks are very intense for Zn(acac)Al(OPri)-273. Accounting for the prevailing LDH structure of this sample, the peak at 450 K is assigned to the removal of interlayer water molecules. The second peak at 518 K is assigned to the simultaneous removal of water due to the dehydroxylation of the brucite-like layers, and the release of CO2 from the decomposition of compensating carbonate anions. The weight loss corresponding to these events represents 87% of the total weight loss for Zn(acac)Al(OPri)-273. As expected the intensities of both peaks

decrease as the LDH content of the samples decreases. Moreover, they are shifted to lower temperatures. The weight losses corresponding to these events represent 4.7% and 8.8% of the total weight loss for Zn(acac)Al(OPri)-353 and Zn(acac)Al(OPri)-298, respectively. An additional weak weight loss of 2.6%, recorded at 693 K for Zn(acac)Al(OPri)-273, could be assigned to the removal of chloride anions as already shown by Inacio et al. [37]. Finally, a peak is recorded at 863 K corresponding to a relative weight loss of 5.8%. It extends widely until 1073 K. From coupling the TG and MS analysis of the evolved gases, a final step of CO2 release has been proposed in this temperature range for Zn/Al-CO3 samples [38]. After calcination at 723 K, all samples exhibit the characteristic XRD patterns of ZnO. In both series of Zn(acac)Al(acac) and Zn(acac)Al(OPri) samples calcined at 723 K, the specific surface areas increase on decreasing the synthesis and aging temperature, i.e. on increasing the LDH content. Remarkably, the highest specific surface area (122 m2 g
1) is reached for the Pt-containing LDH, (Pt(OH)Zn(acac)Al(OPri)-273). In all cases the nitrogen sorption follow a type IIb adsorption isotherm, according to the classification proposed by Rouquerol et al. [39], with H3-type hysteresis loop for the desorption (in Fig. 4 the Zn(acac)Al(OPri)-273 isotherm is reported as an example). Comparatively to the type IV isotherms those of type IIb do not present a plateau at high P/P0 values, and are characteristic for clay minerals. This behavior comes from nitrogen physisorption taking place between the aggregates of platelets particles. Indeed, it has been largely shown that the lamellar morphology of the LDH is maintained after calcination up to 823 K

250

Volume adsorbed / cm3 g-1 STP

b

Relative Weight loss (%)

c

∇

DTG signal (a.u.)

10 %

218

200

150

100

50

0 0

0.2

0.4

0.6

0.8

1

Relativepressure (P/P0) Fig. 4. Nitrogen adsorption desorption isotherms at 77 K for Zn(acac)Al(OPri)-273.

D. Tichit et al. / Microporous and Mesoporous Materials 80 (2005) 213–220

creases. A material composed of ca. 90% of LDH phase is obtained at 273 K using Zn(acac)2 Æ xH2O and Al(OPri)3 precursors. In this case, hydrozincite or hydrozincite-like compounds are likely formed as remaining phases. The specific surface areas of the materials calcined at 723 K are larger than 110 m2 g
1, which make them attractive for catalytic applications.

°

Intensity (a.u.)

219

°° ^

Acknowledgments

°

°

^

O.L. acknowledges support from a grant of the ‘‘cotutelle de the`se France-Italie’’.

°

References

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20

30

40

50

60

70

2θ /degrees Fig. 5. XRD pattern of the re-hydrated Zn(acac)Al(OPri)-273 sample (s: ZnO; ^: LDH).

[40,41], making the reconstruction of the mixed oxides into the lamellar structure easier. These reconstruction properties of calcined Zn–Al LDH obtained by co-precipitation have already been studied focusing the attention on the influence of several main parameters. It has been shown that the LDH structure is regenerated after 18 h suspension of Zn–Al mixed oxide in water, with, however, the additional formation of ZnO in amounts increasing with the calcination temperature [18]. The reconstruction property of Zn(acac)Al(OPri)-273 calcined at 573 K was thus investigated by pouring the solid in decarbonated water for 18 h at 298 K. The XRD pattern reported in Fig. 5 shows that the LDH structure is regenerated with, in addition, a net increase in intensity of the peaks characteristic for ZnO. Kooli et al. [18] suggest that the re-hydrated Zn/ Al LDH phases, irrespective of their initial compositions, exhibit a Zn/Al ratio of 2. If this is the case, a ZnO amount of around 20% would be expected for Zn(acac)Al(OPri)-273. This hypothesis appears well supported by the shape of the XRD pattern.

4. Conclusions Zn/Al and Pt/Zn/Al LDH have been elaborated by the sol–gel method. Other companion phases are also present in various amounts, depending on the nature of the metal precursors, and the synthesis and aging temperature. Formation of LDH is improved at the cost of ZnO and Al(OH)3 when the synthesis temperature de-

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