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Incorporation of cerium ions by sonication in Ni–Mg–Al layered double hydroxides
Applied Clay Science 48 (2010) 542–546
Contents lists available at ScienceDirect
Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
Note
Incorporation of cerium ions by sonication in Ni–Mg–Al layered double hydroxides Octávio R. Macedo Neto a, Nielson F.P. Ribeiro b, Carlos A.C. Perez b, Martin Schmal b, Mariana M.V.M. Souza a,⁎ a b
Escola de Química — Universidade Federal do Rio de Janeiro (UFRJ), Centro de Tecnologia, Bloco E, sala 206, CEP 21941-909, Rio de Janeiro, RJ, Brazil NUCAT/PEQ/COPPE/UFRJ, Centro de Tecnologia, Bloco G, sala 128, CEP 21945-970, Rio de Janeiro, RJ, Brazil
a r t i c l e
i n f o
Article history: Received 5 August 2009 Received in revised form 19 February 2010 Accepted 23 February 2010 Available online 1 March 2010 Keywords: Layered double hydroxide Hydrotalcite-like compounds Mixed oxides Coprecipitation Sonication
a b s t r a c t The incorporation of Ce in Ni–Mg–Al layered double hydroxides (LDHs) was studied by coprecipitation and under sonication. Mixed oxides were obtained by calcination. X-ray diffraction (XRD) patterns of the as-synthesized samples showed formation of well-crystallized LDHs and CeO2. Rietveld refinement was employed to calculate the content of cerium that was incorporated in the LDH structure. Sonication increased the incorporation of Ce in the structure. The specific surface areas of cerium-containing LDHs prepared by the conventional method were similar to that of free-Ce sample. Synthesis under sonication significantly increased the specific surface area. After calcination both specific surface areas and pore volumes increased compared to the corresponding LDH. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Layered double hydroxides (LDHs), or hydrotalcite-like compounds, have many industrial applications as catalysts, catalyst precursors, adsorbents and ion exchangers (Cavani et al., 1991). These compounds may be applied in catalytic fields as such or after thermal decomposition. Heating induces dehydration, dehydroxylation and loss of compensating anions, forming mixed oxides with basic properties, high specific surface area, homogeneous dispersion of the metal ions and a better resistance to sintering than the corresponding supported catalysts (Tichit et al., 1995; Casenave et al., 2001). The acid–base and/or redox properties of LDHs and their derived mixed oxides depend on the chemical composition, preparation methods and treatment conditions. Ni–Mg–Al mixed oxides derived from LDHs have been extensively studied as catalysts in different reactions, such as hydrogenation of acetonitrile (Lebedeva et al., 1999), partial oxidation of methane and light paraffins (Basile et al., 1998; Schulze et al., 2001), reforming of methane (Tsyganok et al., 2003; Fonseca and Assaf, 2005) and aqueous-phase reforming of ethanol (Cruz et al., 2008). Cerium oxide (CeO2) is widely used as a promoter in various redox reactions due to its reducibility and high oxygen storage capacity, besides preventing the metal sintering and decreasing coke formation on nickel catalysts (Yao and Yao, 1984; Trimm, 1997). The introduction of Ce ions in the hydroxide layer of LDHs is difficult because of its large ionic radius, when compared to Mg2+ and Al3+. Das et al. (2006) incorporated cerium ions in Mg–Al LDH.
⁎ Corresponding author. Tel.: + 55 21 25627598; fax: + 55 21 25627596. E-mail address: [email protected] (M.M.V.M. Souza). 0169-1317/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2010.02.015
Lucrédio et al. (2007) prepared Ni–Mg–Al LDHs and used anion exchange of the Ce-EDTA complex to incorporate cerium ions. Only a part of the cerium chelate was introduced in the interlayer space. Daza et al. (2008) also used the method with Ce-EDTA complexes, showing, however, that the complex was not intercalated. Conventionally, LDHs are synthesized by coprecipitation from metal nitrates at a fixed pH under stirring, followed by a long aging and/or hydrothermal treatment to improve the crystallinity. The substitution of the conventional aging step by microwave irradiation has been described in the literature, with substantial reduction of the crystallization time (Fetter et al., 1997; Rivera et al., 2006). Kooli et al. (1997) described the use of ultrasound to intercalate vanadate ions in Mg–Al LDH. Climent et al. (2004) have performed an extensive study of the application of ultrasound irradiation during the coprecipitation step for LDH synthesis. Mg–Al LDHs prepared under sonication presented an increase in the specific surface area and the number of defect sites in the solid, leading to a higher basicity. However, to our knowledge, no one reported the use of ultrasound to incorporate large cations, such as cerium ions, into the hydroxide layer of LDHs. In the present study, we investigated the preparation of LDHs containing Ni, Mg, Al and Ce in the hydroxide layer by conventional coprecipitation and under sonication. 2. Experimental 2.1. Synthesis The LDHs were prepared by coprecipitation from aqueous solutions at room temperature, always containing 20 mass% of NiO. 200 mL of solution
O.R. Macedo Neto et al. / Applied Clay Science 48 (2010) 542–546
A containing the corresponding nitrates ((Ni+Mg)/(Al+Ce)=3.0 and [Ni+Mg+Al+Ce]=1.5 M) were slowly dropped (1 mL min− 1) under vigorous stirring into 200 mL of solution B containing appropriated 3+ amounts of Na2CO3 and NaOH (CO2− =0.375 and OH−/M3+ =6.3). 3 /M The mixture A+B was then maintained under stirring for 1 h. The gel formed was aged under constant pH (10) for 18 h at 60 °C. The solid was filtered, washed with distilled water (90 °C) until pH 7 was reached and dried at 120 °C overnight. For the samples prepared under sonication, the addition of solution A onto solution B was carried out with simultaneous ultrasound irradiation of the mixture (ultrasound bath Unique, model Ultracleaner 14000). The total time under sonication was 260 min. This procedure was repeated at three different temperatures: 25, 60 and 90 °C, for the sample without cerium salt addition. The gels were not aged; they were directly filtered, washed and dried. The Ni–Mg–Al–Ce mixed oxides were produced by calcination of the LDHs under dry air (120 mL min− 1), using a heating rate of 10 °C min− 1, from room temperature to 500 °C and maintaining at this temperature for 6 h. The materials were labeled as NixCeLDH for the conventional coprecipitation series and NixCeLDHUS for sonication series, where x = 2 or 9 depending on Al/Ce molar ratio of the synthesis gel, 2.33 or 9, respectively. For comparison, samples without cerium (NiLDH or NiLDHUS) were also prepared. After calcination, the mixed oxides were labeled as NixCeLDHc or NixCeLDHUSc. 2.2. Characterization The chemical composition of the materials was determined by X-ray fluorescence (XRF) using a Rigaku spectrometer (Rix 3100). X-ray powder diffraction (XRD) patterns were recorded in a Rigaku Miniflex II diffractometer with monochromator, CuKα radiation (40 kV and 40 mA), 2θ range from 2 to 90º, with steps of 0.05° and 4 s. The textural properties, such as the BET specific surface area, pore volume and average pore diameter (BJH method), were determined by N2 adsorption–desorption at −196 °C in a Micromeritcs ASAP 2000. Prior to the analysis the samples were outgassed for 20 h at 200 °C. 3. Results and discussion 3.1. Structural characteristics The chemical composition of the synthesized samples is presented in Table 1. The M2+/M3+ molar ratios were similar to the nominal values. The Al/Ce ratios were below the nominal values, which can be associated with a small loss of Al cations during the coprecipitation and washing steps. The results of the XRD measurements obtained for NiLDHUS samples prepared by sonication at three different temperatures are presented in Fig. 1. Only the LDH phase was formed (JCPDS 41-1428). The intensity of the X-ray reflections increased with increasing
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Fig. 1. X-ray diffractograms of the NiLDHUS synthesized under sonication at: (a) 25 °C, (b) 60 °C and (c) 90 °C.
temperature from 25 to 60 °C, accompanied by an increase in the crystallinity. A further increase of the synthesis temperature to 90 °C did not significantly change the XRD pattern. Thus, all other synthesis under sonication was carried out at 60 °C. Climent et al. (2004) also observed an increase in the intensity of X-ray reflections when increasing temperature of Mg–Al LDH synthesis under sonication, from 0 to 50 °C. X-ray diffraction patterns of the as-synthesized samples are shown in Fig. 2. LDHs exhibited sharp and symmetrical reflections at 11.7°, 23.2°, 60.6° and 61.8° (ascribed to the diffraction by the (003), (006), (110) and (113) planes) and broad and asymmetric reflections at 34.7°, 38.8° and 46.0° (ascribed to the diffraction by the (102), (105) and (108) planes), characteristic of a well-crystallized LDH. The absence of other phases in NiLDH samples suggests that Ni2+ isomorphically replaced Mg2+ cations in the brucite-like layers due to the similar ionic radii of Ni2+ (0.72 Å) and Mg2+ (0.65 Å) (Cruz et al., 2008; Rodrigues et al., 2003). The samples containing cerium also exhibited additional reflections of CeO2 (JCPDS 34-394). The crystallinity of the LDH samples decreased with the increase in the cerium content because of the difference in the ionic radius of Ce3+ (1.02 Å), in agreement with the results of Das et al. (2006). The XRD patterns of the cerium samples prepared under sonication showed a small shift of the LDH reflections
Table 1 Chemical composition and structural characteristics of the as-synthesized samples. Sample
Ni/Mga (molar)
Al/Cea (molar)
(Ni + Mg)/ (Al + Ce)a (molar)
a (Å)
c (Å)
d (nm)c
NiLDH NiLDHUS Ni9CeLDH Ni9CeLDHUS Ni2CeLDH Ni2CeLDHUS
0.23 0.23 0.30 0.25 0.28 0.28
– – 7.71 8.53 1.98 2.21
3.3 (3.0)b 2.7 (3.0)b 2.7 (3.0) 3.0 (3.0) 3.2 (3.0) 3.0 (3.0)
3.0519 3.0520 3.0517 3.0600 3.0620 3.0710
23.4051 23.3780 23.4034 23.6545 23.8352 23.9324
7.0 6.1 6.3 5.3 5.1 3.4
a b c
(0.20) (0.20) (0.22) (0.22) (0.25) (0.25)
(9.00) (9.00) (2.33) (2.33)
The values in parentheses are nominal values. (Ni + Mg)/Al molar ratio. Crystallite sizes calculated by the Scherrer equation.
Fig. 2. X-ray diffractograms of the as-synthesized samples. * → LDH; Δ → CeO2.
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Fig. 3. Rietveld analysis of Ni9CeLDHUS. (I) Bragg's position of LDH phase and (II) Bragg's position of the CeO2 phase.
to lower angles due to the change of the lattice parameters a and c (see below). Rietveld refinement was applied to the XRD patterns to calculate the content of cerium incorporated in the LDH structure. The structure model was taken from Bellotto et al. (1996) and cerianite from AMCSD (American Mineralogist Crystal Structure Database). The results of the Rietveld analysis for Ni9CeLDHUS are displayed in Fig. 3. The quantitative phase analysis was performed using 25% of ZnO as internal standard to evaluate the presence of amorphous phases. For Ni9CeLDH, which contained approximately 10 mass% of CeO2, 4.0% of CeO2 was incorporated in the LDH structure when the conventional method was used; this content increased to 5.1% for the sample prepared under sonication. For Ni2CeLDH, which contained 23 mass% of CeO2, these values were 8.2 and 12.7%, for the conventional and sonication methods, respectively. It is well known that the sonochemical effect of ultrasounds arises from acoustic cavitation, that is the formation, growth and implosive collapse of the bubbles in a liquid (Peters, 1996). Cavitational collapse leads to microjet and shock-wave impacts on the surface, together with interparticle collisions, resulting in rapid mass transfer, particle size reduction, crystal defects and metal activation. These mechanical effects of jets and shock-waves must be responsible for the higher incorporation of cerium ions in the brucite-like layers. Based on the rhombohedral symmetry of LDHs, the lattice parameters a (cation–cation distance in the brucite-like layer) and c = 3c′(thickness of one brucite-like layer and one interlayer) were calculated (Table 1). The parameter a was the same for NiLDH samples, and also for Mg–Al hydrotalcite (Cruz et al., 2008), showing the isomorphous substitution of Mg2+ by Ni ions in the brucite layers. With incorporation of cerium ions the parameter a increased, mainly when sonication was used. The parameter a was closely related to the higher incorporation of cerium ions in the brucite-like layer during sonication, as revealed by the Rietveld analysis. The larger ionic radius of Ce3+ caused an expansion in the cation–cation distance. There was also an increase in the parameter c with cerium incorporation, which can be associated with the low polarizing ability of Ce3+ compared to Al3+, decreasing the electrostatic interaction between the positively charged hydroxide layers and the interlayer anions (Das et al., 2006). The samples prepared under sonication presented smaller crystallites sizes, calculated by the Scherrer equation (Table 1), which was directly related to a reduced aggregation of the particles during sonication. After calcination of the cerium free sample at 500 °C, the characteristic lamellar structure disappeared and the XRD patterns (Fig. 4) showed only the presence of MgO as periclase phase (JCPDS 45-946), where Ni was included as Mg–Ni–O solid solution, causing a
Fig. 4. X-ray diffractograms of the mixed oxides. ● → Mg–Ni–O; □ → CeO2.
small increase in the lattice parameter of MgO phase. These results indicate that there was no segregation of a spinel-phase, in accordance with the literature (Rodrigues et al., 2003; Takehira et al., 2004). For the samples containing cerium ions, Ce species were present in the Mg–Ni–O phase, which was confirmed by Rietveld analysis (not shown), and the CeO2 phase remained unchanged.
3.2. Textural characteristics The specific surface area, pore volume and average pore diameter of LDHs and derived mixed oxides are presented in Tables 2 and 3, respectively. The specific surface areas of cerium-containing LDHs prepared by the conventional method were similar to that of NiLDH, showing that the incorporation of cerium ions did not greatly influence the textural properties. Sonication significantly increased the specific surface area, and this increase was more pronounced for NiLDH. This increase of almost four times was much higher than expected for Mg–Al LDH synthesized under microwave irradiation (Benito et al., 2006; Bergadà et al., 2007). Thus, ultrasound not only accelerated the crystal formation but dispersed small particles as well as it reduced the aggregation during nucleation and crystal growth, giving rise to a large increase in the specific surface area.
Table 2 Textural properties of the as-synthesized samples. Sample
SBET (m2 g− 1)
Vp (cm3 g− 1)
dp (Å)
NiLDH NiLDHUS Ni9CeLDH Ni9CeLDHUS Ni2CeLDH Ni2CeLDHUS
62 241 56 135 69 159
0.22 0.36 0.18 0.26 0.19 0.28
29.9 68.7 36.9 43.8 42.5 56.2
Table 3 Textural properties of the calcined samples. Sample
SBET (m2 g− 1)
Vp (cm3 g− 1)
dp (Å)
NiLDHc NiLDHUSc Ni9CeLDHc Ni9CeLDHUSc Ni2CeLDHc Ni2CeLDHUSc
216 298 242 242 232 234
0.28 0.35 0.35 0.34 0.34 0.33
32.6 34.9 40.8 42.4 42.4 46.6
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Calcination increased the specific surface area and pore volume compared to the corresponding LDHs. The increase of the specific surface area was much higher for the samples prepared by the conventional method. The presence of cerium only caused a slight increase in the specific surface area of the mixed oxides prepared by the conventional method, as observed by Lucrédio et al. (2007). This result is in contrast to the results of Das et al. (2006), who observed a steady decrease in the specific surface area of mixed oxides with increasing
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cerium content. The sonication did not have a great influence in textural properties of mixed oxides, in agreement with Climent et al. (2004), who observed an increase of only 20% in the specific surface area of the Mg–Al mixed oxide derived from LDH synthesized under sonication, compared to the conventionally prepared sample. The N2 isotherms (Fig. 5) were of type IV for all samples, typical of mesoporous materials, with the hysteresis loop associated with capillary condensation in the mesopores. The hysteresis type showed that the aggregates of plate-like particles formed nonuniform slitshaped pores (Carja et al., 2001). For NiLDH, there was a significant difference in the isotherm shape at high relative pressures (P/P0 N 0.8) when sonication was applied. Thus, ultrasound also changed the shape and/or size of the pores. In fact, all samples prepared under sonication had increased average pore diameters (Table 2), and this increase was higher for NiLDH. For the cerium-containing samples, sonication shifted the isotherms to higher adsorbed volumes due to the increased specific surface area.
4. Conclusions Ni–Mg–Al–Ce LDHs were prepared by conventional coprecipitation and sonication. Sonication significantly reduced the preparation time. X-ray diffraction patterns revealed the formation of LDH and CeO2 phases, with a part of the cerium ions incorporated in the metal hydroxide layer. Sonication increased the incorporation of the cerium ions in LDH structure. The materials were mesoporous in nature and the specific surface areas of LDHs prepared under sonication were much higher than those obtained by the conventional coprecipitation. The mixed oxides presented an increase in the specific surface area due to the destruction of the lamellar structure and formation of Mg–Ni–O solid solutions.
References
Fig. 5. N2 adsorption–desorption isotherms of (A) NiLDH, (B) Ni9CeLDH and (C) Ni2CeLDH samples.
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Contents lists available at ScienceDirect
Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
Note
Incorporation of cerium ions by sonication in Ni–Mg–Al layered double hydroxides Octávio R. Macedo Neto a, Nielson F.P. Ribeiro b, Carlos A.C. Perez b, Martin Schmal b, Mariana M.V.M. Souza a,⁎ a b
Escola de Química — Universidade Federal do Rio de Janeiro (UFRJ), Centro de Tecnologia, Bloco E, sala 206, CEP 21941-909, Rio de Janeiro, RJ, Brazil NUCAT/PEQ/COPPE/UFRJ, Centro de Tecnologia, Bloco G, sala 128, CEP 21945-970, Rio de Janeiro, RJ, Brazil
a r t i c l e
i n f o
Article history: Received 5 August 2009 Received in revised form 19 February 2010 Accepted 23 February 2010 Available online 1 March 2010 Keywords: Layered double hydroxide Hydrotalcite-like compounds Mixed oxides Coprecipitation Sonication
a b s t r a c t The incorporation of Ce in Ni–Mg–Al layered double hydroxides (LDHs) was studied by coprecipitation and under sonication. Mixed oxides were obtained by calcination. X-ray diffraction (XRD) patterns of the as-synthesized samples showed formation of well-crystallized LDHs and CeO2. Rietveld refinement was employed to calculate the content of cerium that was incorporated in the LDH structure. Sonication increased the incorporation of Ce in the structure. The specific surface areas of cerium-containing LDHs prepared by the conventional method were similar to that of free-Ce sample. Synthesis under sonication significantly increased the specific surface area. After calcination both specific surface areas and pore volumes increased compared to the corresponding LDH. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Layered double hydroxides (LDHs), or hydrotalcite-like compounds, have many industrial applications as catalysts, catalyst precursors, adsorbents and ion exchangers (Cavani et al., 1991). These compounds may be applied in catalytic fields as such or after thermal decomposition. Heating induces dehydration, dehydroxylation and loss of compensating anions, forming mixed oxides with basic properties, high specific surface area, homogeneous dispersion of the metal ions and a better resistance to sintering than the corresponding supported catalysts (Tichit et al., 1995; Casenave et al., 2001). The acid–base and/or redox properties of LDHs and their derived mixed oxides depend on the chemical composition, preparation methods and treatment conditions. Ni–Mg–Al mixed oxides derived from LDHs have been extensively studied as catalysts in different reactions, such as hydrogenation of acetonitrile (Lebedeva et al., 1999), partial oxidation of methane and light paraffins (Basile et al., 1998; Schulze et al., 2001), reforming of methane (Tsyganok et al., 2003; Fonseca and Assaf, 2005) and aqueous-phase reforming of ethanol (Cruz et al., 2008). Cerium oxide (CeO2) is widely used as a promoter in various redox reactions due to its reducibility and high oxygen storage capacity, besides preventing the metal sintering and decreasing coke formation on nickel catalysts (Yao and Yao, 1984; Trimm, 1997). The introduction of Ce ions in the hydroxide layer of LDHs is difficult because of its large ionic radius, when compared to Mg2+ and Al3+. Das et al. (2006) incorporated cerium ions in Mg–Al LDH.
⁎ Corresponding author. Tel.: + 55 21 25627598; fax: + 55 21 25627596. E-mail address: [email protected] (M.M.V.M. Souza). 0169-1317/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2010.02.015
Lucrédio et al. (2007) prepared Ni–Mg–Al LDHs and used anion exchange of the Ce-EDTA complex to incorporate cerium ions. Only a part of the cerium chelate was introduced in the interlayer space. Daza et al. (2008) also used the method with Ce-EDTA complexes, showing, however, that the complex was not intercalated. Conventionally, LDHs are synthesized by coprecipitation from metal nitrates at a fixed pH under stirring, followed by a long aging and/or hydrothermal treatment to improve the crystallinity. The substitution of the conventional aging step by microwave irradiation has been described in the literature, with substantial reduction of the crystallization time (Fetter et al., 1997; Rivera et al., 2006). Kooli et al. (1997) described the use of ultrasound to intercalate vanadate ions in Mg–Al LDH. Climent et al. (2004) have performed an extensive study of the application of ultrasound irradiation during the coprecipitation step for LDH synthesis. Mg–Al LDHs prepared under sonication presented an increase in the specific surface area and the number of defect sites in the solid, leading to a higher basicity. However, to our knowledge, no one reported the use of ultrasound to incorporate large cations, such as cerium ions, into the hydroxide layer of LDHs. In the present study, we investigated the preparation of LDHs containing Ni, Mg, Al and Ce in the hydroxide layer by conventional coprecipitation and under sonication. 2. Experimental 2.1. Synthesis The LDHs were prepared by coprecipitation from aqueous solutions at room temperature, always containing 20 mass% of NiO. 200 mL of solution
O.R. Macedo Neto et al. / Applied Clay Science 48 (2010) 542–546
A containing the corresponding nitrates ((Ni+Mg)/(Al+Ce)=3.0 and [Ni+Mg+Al+Ce]=1.5 M) were slowly dropped (1 mL min− 1) under vigorous stirring into 200 mL of solution B containing appropriated 3+ amounts of Na2CO3 and NaOH (CO2− =0.375 and OH−/M3+ =6.3). 3 /M The mixture A+B was then maintained under stirring for 1 h. The gel formed was aged under constant pH (10) for 18 h at 60 °C. The solid was filtered, washed with distilled water (90 °C) until pH 7 was reached and dried at 120 °C overnight. For the samples prepared under sonication, the addition of solution A onto solution B was carried out with simultaneous ultrasound irradiation of the mixture (ultrasound bath Unique, model Ultracleaner 14000). The total time under sonication was 260 min. This procedure was repeated at three different temperatures: 25, 60 and 90 °C, for the sample without cerium salt addition. The gels were not aged; they were directly filtered, washed and dried. The Ni–Mg–Al–Ce mixed oxides were produced by calcination of the LDHs under dry air (120 mL min− 1), using a heating rate of 10 °C min− 1, from room temperature to 500 °C and maintaining at this temperature for 6 h. The materials were labeled as NixCeLDH for the conventional coprecipitation series and NixCeLDHUS for sonication series, where x = 2 or 9 depending on Al/Ce molar ratio of the synthesis gel, 2.33 or 9, respectively. For comparison, samples without cerium (NiLDH or NiLDHUS) were also prepared. After calcination, the mixed oxides were labeled as NixCeLDHc or NixCeLDHUSc. 2.2. Characterization The chemical composition of the materials was determined by X-ray fluorescence (XRF) using a Rigaku spectrometer (Rix 3100). X-ray powder diffraction (XRD) patterns were recorded in a Rigaku Miniflex II diffractometer with monochromator, CuKα radiation (40 kV and 40 mA), 2θ range from 2 to 90º, with steps of 0.05° and 4 s. The textural properties, such as the BET specific surface area, pore volume and average pore diameter (BJH method), were determined by N2 adsorption–desorption at −196 °C in a Micromeritcs ASAP 2000. Prior to the analysis the samples were outgassed for 20 h at 200 °C. 3. Results and discussion 3.1. Structural characteristics The chemical composition of the synthesized samples is presented in Table 1. The M2+/M3+ molar ratios were similar to the nominal values. The Al/Ce ratios were below the nominal values, which can be associated with a small loss of Al cations during the coprecipitation and washing steps. The results of the XRD measurements obtained for NiLDHUS samples prepared by sonication at three different temperatures are presented in Fig. 1. Only the LDH phase was formed (JCPDS 41-1428). The intensity of the X-ray reflections increased with increasing
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Fig. 1. X-ray diffractograms of the NiLDHUS synthesized under sonication at: (a) 25 °C, (b) 60 °C and (c) 90 °C.
temperature from 25 to 60 °C, accompanied by an increase in the crystallinity. A further increase of the synthesis temperature to 90 °C did not significantly change the XRD pattern. Thus, all other synthesis under sonication was carried out at 60 °C. Climent et al. (2004) also observed an increase in the intensity of X-ray reflections when increasing temperature of Mg–Al LDH synthesis under sonication, from 0 to 50 °C. X-ray diffraction patterns of the as-synthesized samples are shown in Fig. 2. LDHs exhibited sharp and symmetrical reflections at 11.7°, 23.2°, 60.6° and 61.8° (ascribed to the diffraction by the (003), (006), (110) and (113) planes) and broad and asymmetric reflections at 34.7°, 38.8° and 46.0° (ascribed to the diffraction by the (102), (105) and (108) planes), characteristic of a well-crystallized LDH. The absence of other phases in NiLDH samples suggests that Ni2+ isomorphically replaced Mg2+ cations in the brucite-like layers due to the similar ionic radii of Ni2+ (0.72 Å) and Mg2+ (0.65 Å) (Cruz et al., 2008; Rodrigues et al., 2003). The samples containing cerium also exhibited additional reflections of CeO2 (JCPDS 34-394). The crystallinity of the LDH samples decreased with the increase in the cerium content because of the difference in the ionic radius of Ce3+ (1.02 Å), in agreement with the results of Das et al. (2006). The XRD patterns of the cerium samples prepared under sonication showed a small shift of the LDH reflections
Table 1 Chemical composition and structural characteristics of the as-synthesized samples. Sample
Ni/Mga (molar)
Al/Cea (molar)
(Ni + Mg)/ (Al + Ce)a (molar)
a (Å)
c (Å)
d (nm)c
NiLDH NiLDHUS Ni9CeLDH Ni9CeLDHUS Ni2CeLDH Ni2CeLDHUS
0.23 0.23 0.30 0.25 0.28 0.28
– – 7.71 8.53 1.98 2.21
3.3 (3.0)b 2.7 (3.0)b 2.7 (3.0) 3.0 (3.0) 3.2 (3.0) 3.0 (3.0)
3.0519 3.0520 3.0517 3.0600 3.0620 3.0710
23.4051 23.3780 23.4034 23.6545 23.8352 23.9324
7.0 6.1 6.3 5.3 5.1 3.4
a b c
(0.20) (0.20) (0.22) (0.22) (0.25) (0.25)
(9.00) (9.00) (2.33) (2.33)
The values in parentheses are nominal values. (Ni + Mg)/Al molar ratio. Crystallite sizes calculated by the Scherrer equation.
Fig. 2. X-ray diffractograms of the as-synthesized samples. * → LDH; Δ → CeO2.
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Fig. 3. Rietveld analysis of Ni9CeLDHUS. (I) Bragg's position of LDH phase and (II) Bragg's position of the CeO2 phase.
to lower angles due to the change of the lattice parameters a and c (see below). Rietveld refinement was applied to the XRD patterns to calculate the content of cerium incorporated in the LDH structure. The structure model was taken from Bellotto et al. (1996) and cerianite from AMCSD (American Mineralogist Crystal Structure Database). The results of the Rietveld analysis for Ni9CeLDHUS are displayed in Fig. 3. The quantitative phase analysis was performed using 25% of ZnO as internal standard to evaluate the presence of amorphous phases. For Ni9CeLDH, which contained approximately 10 mass% of CeO2, 4.0% of CeO2 was incorporated in the LDH structure when the conventional method was used; this content increased to 5.1% for the sample prepared under sonication. For Ni2CeLDH, which contained 23 mass% of CeO2, these values were 8.2 and 12.7%, for the conventional and sonication methods, respectively. It is well known that the sonochemical effect of ultrasounds arises from acoustic cavitation, that is the formation, growth and implosive collapse of the bubbles in a liquid (Peters, 1996). Cavitational collapse leads to microjet and shock-wave impacts on the surface, together with interparticle collisions, resulting in rapid mass transfer, particle size reduction, crystal defects and metal activation. These mechanical effects of jets and shock-waves must be responsible for the higher incorporation of cerium ions in the brucite-like layers. Based on the rhombohedral symmetry of LDHs, the lattice parameters a (cation–cation distance in the brucite-like layer) and c = 3c′(thickness of one brucite-like layer and one interlayer) were calculated (Table 1). The parameter a was the same for NiLDH samples, and also for Mg–Al hydrotalcite (Cruz et al., 2008), showing the isomorphous substitution of Mg2+ by Ni ions in the brucite layers. With incorporation of cerium ions the parameter a increased, mainly when sonication was used. The parameter a was closely related to the higher incorporation of cerium ions in the brucite-like layer during sonication, as revealed by the Rietveld analysis. The larger ionic radius of Ce3+ caused an expansion in the cation–cation distance. There was also an increase in the parameter c with cerium incorporation, which can be associated with the low polarizing ability of Ce3+ compared to Al3+, decreasing the electrostatic interaction between the positively charged hydroxide layers and the interlayer anions (Das et al., 2006). The samples prepared under sonication presented smaller crystallites sizes, calculated by the Scherrer equation (Table 1), which was directly related to a reduced aggregation of the particles during sonication. After calcination of the cerium free sample at 500 °C, the characteristic lamellar structure disappeared and the XRD patterns (Fig. 4) showed only the presence of MgO as periclase phase (JCPDS 45-946), where Ni was included as Mg–Ni–O solid solution, causing a
Fig. 4. X-ray diffractograms of the mixed oxides. ● → Mg–Ni–O; □ → CeO2.
small increase in the lattice parameter of MgO phase. These results indicate that there was no segregation of a spinel-phase, in accordance with the literature (Rodrigues et al., 2003; Takehira et al., 2004). For the samples containing cerium ions, Ce species were present in the Mg–Ni–O phase, which was confirmed by Rietveld analysis (not shown), and the CeO2 phase remained unchanged.
3.2. Textural characteristics The specific surface area, pore volume and average pore diameter of LDHs and derived mixed oxides are presented in Tables 2 and 3, respectively. The specific surface areas of cerium-containing LDHs prepared by the conventional method were similar to that of NiLDH, showing that the incorporation of cerium ions did not greatly influence the textural properties. Sonication significantly increased the specific surface area, and this increase was more pronounced for NiLDH. This increase of almost four times was much higher than expected for Mg–Al LDH synthesized under microwave irradiation (Benito et al., 2006; Bergadà et al., 2007). Thus, ultrasound not only accelerated the crystal formation but dispersed small particles as well as it reduced the aggregation during nucleation and crystal growth, giving rise to a large increase in the specific surface area.
Table 2 Textural properties of the as-synthesized samples. Sample
SBET (m2 g− 1)
Vp (cm3 g− 1)
dp (Å)
NiLDH NiLDHUS Ni9CeLDH Ni9CeLDHUS Ni2CeLDH Ni2CeLDHUS
62 241 56 135 69 159
0.22 0.36 0.18 0.26 0.19 0.28
29.9 68.7 36.9 43.8 42.5 56.2
Table 3 Textural properties of the calcined samples. Sample
SBET (m2 g− 1)
Vp (cm3 g− 1)
dp (Å)
NiLDHc NiLDHUSc Ni9CeLDHc Ni9CeLDHUSc Ni2CeLDHc Ni2CeLDHUSc
216 298 242 242 232 234
0.28 0.35 0.35 0.34 0.34 0.33
32.6 34.9 40.8 42.4 42.4 46.6
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Calcination increased the specific surface area and pore volume compared to the corresponding LDHs. The increase of the specific surface area was much higher for the samples prepared by the conventional method. The presence of cerium only caused a slight increase in the specific surface area of the mixed oxides prepared by the conventional method, as observed by Lucrédio et al. (2007). This result is in contrast to the results of Das et al. (2006), who observed a steady decrease in the specific surface area of mixed oxides with increasing
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cerium content. The sonication did not have a great influence in textural properties of mixed oxides, in agreement with Climent et al. (2004), who observed an increase of only 20% in the specific surface area of the Mg–Al mixed oxide derived from LDH synthesized under sonication, compared to the conventionally prepared sample. The N2 isotherms (Fig. 5) were of type IV for all samples, typical of mesoporous materials, with the hysteresis loop associated with capillary condensation in the mesopores. The hysteresis type showed that the aggregates of plate-like particles formed nonuniform slitshaped pores (Carja et al., 2001). For NiLDH, there was a significant difference in the isotherm shape at high relative pressures (P/P0 N 0.8) when sonication was applied. Thus, ultrasound also changed the shape and/or size of the pores. In fact, all samples prepared under sonication had increased average pore diameters (Table 2), and this increase was higher for NiLDH. For the cerium-containing samples, sonication shifted the isotherms to higher adsorbed volumes due to the increased specific surface area.
4. Conclusions Ni–Mg–Al–Ce LDHs were prepared by conventional coprecipitation and sonication. Sonication significantly reduced the preparation time. X-ray diffraction patterns revealed the formation of LDH and CeO2 phases, with a part of the cerium ions incorporated in the metal hydroxide layer. Sonication increased the incorporation of the cerium ions in LDH structure. The materials were mesoporous in nature and the specific surface areas of LDHs prepared under sonication were much higher than those obtained by the conventional coprecipitation. The mixed oxides presented an increase in the specific surface area due to the destruction of the lamellar structure and formation of Mg–Ni–O solid solutions.
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Fig. 5. N2 adsorption–desorption isotherms of (A) NiLDH, (B) Ni9CeLDH and (C) Ni2CeLDH samples.
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