3642
J. Phys. Chem. C 2007, 111, 3642-3650
Influence of the Divalent Cation on the Thermal Activation and Reconstruction of Hydrotalcite-like Compounds Javier Pe´ rez-Ramı´rez,*,†,‡ So` nia Abello´ ,† and Niek M. van der Pers§ Laboratory for Heterogeneous Catalysis, Institute of Chemical Research of Catalonia (ICIQ), AV. Paı¨sos Catalans 16, 43007, Tarragona, Spain, Catalan Institution for Research and AdVanced Studies (ICREA), Pg. Lluı´s Companys 23, 08010, Barcelona, Spain, and Department of Materials Science and Engineering, Faculty of 3mE, Delft UniVersity of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands ReceiVed: August 2, 2006; In Final Form: NoVember 13, 2006
The influence of the divalent cation on the thermal decomposition and subsequent reconstruction of Mg3Al, Ni3-Al, and Mg2-Ni-Al hydrotalcite-like compounds has been investigated by in situ XRD. Diffraction studies were complemented by ICP-OES, AAS, TEM, N2 adsorption, FT-IR, TGA, and TPD-MS characterizations. The decomposition mechanism, as for the evolvement of chemical species and phase transitions, is not influenced by the divalent cation in the brucite-like sheets. An intermediate dehydrated layered phase is formed at 423-473 K in all samples, which is transformed into the corresponding mixed oxide at 573-623 K. The characteristic platelet-like morphology in all as-synthesized hydrotalcites is preserved in Mg-Al oxide, while uniform nanoparticles are generated in Ni-Al oxide. Mg-Ni-Al oxide exhibits intermediate morphological features between the binary samples. The divalent cation stipulates the ability of the resulting oxide to recover the original layered structure. In contrast with the facile reconstruction of MgAl oxide, no sign of recovery was observed in Ni-Al oxide upon exposure to water vapor at room temperature. In the latter sample, even rehydration of the intermediate layered phase was not fully reversible. Addition of nickel to the binary Mg-Al sample dramatically reduces the ability of the resulting mixed oxide to recover the hydrotalcite structure. This is tentatively attributed to the intimate mixing of the various cations in the oxide phase. Relevant kinetic parameters of the reconstruction process were obtained by fitting the in situ XRD data with the Avrami-Erofe’ev model. The rate coefficient for reconstruction of the ternary Mg-NiAl oxide was reduced by 1 order of magnitude with respect to the binary Mg-Al oxide.
1. Introduction Synthetic hydrotalcite-like compounds (HTlc’s) are layered double hydroxides with the general formula [M2+1-xM3+x(OH)2][Xm-]x/m‚nH2O. These materials consist of brucite-type octahedral layers, in which M3+ cations partially substitute for M2+ cations. The positive charge resulting from this substitution is balanced by Xm- anions (often carbonate) and water molecules located in the interlayer space (Figure 1).1,2 The practical impact of HTlc’s is remarkable, with applications as sensors, flame retardants, anion exchangers, selective sorbents, and precursors for catalysts or catalyst supports.3,4 Thermal decomposition of hydrotalcites is often practiced to obtain high-surface-area and well-dispersed multimetallic mixed oxide catalysts.5 This treatment is also intermediate in the functionalization of the clay by intercalation of anions in the interlayer. This approach makes use of the memory effect, a unique property by which the oxide is retrotopotactically transformed into the original hydrotalcite structure in aqueous solutions or humid atmospheres.3,6 In this manner, the compensating anion (e.g., CO32-) in the as-synthesized hydrotalcite is first decomposed and the calcined product is reconstructed in aqueous solutions containing the desired anion. The memory property has been applied in (i) the removal of anions in * Corresponding author. Fax: +34 977 920 224. E-mail:
[email protected]. † ICIQ. ‡ ICREA. § Delft University of Technology.
Figure 1. Representation of the hydrotalcite structure with carbonate as the compensating anion.
contaminated water,7 (ii) the immobilization of amino acids and enzymes to produce drugs,8 and (iii) the insertion of new functionalities and active catalytic sites.9 A representative example in the latter group is meixnerite, the Mg-Al hydrotalcite analogue with OH- groups in the interlayer space, which is an efficient solid-base catalytic system for a wide spectrum of relevant reactions in organic synthesis.10-12 Besides the above benefits, the memory of hydrotalcites can be also detrimental to the catalytic activity and stability, particularly in applications where HTlc-derived mixed oxides operate in wet streams at moderate temperatures.13 The recrystallization of the hydrotalcite leads to segregation of the oxide phase upon recalcination, thus
10.1021/jp064972q CCC: $37.00 © 2007 American Chemical Society Published on Web 02/14/2007
Thermal Decomposition and Reconstruction of HTlc’s inducing the formation of inhomogeneous, low-surface-area, and poorly dispersed oxide catalysts.14-16 Detailed memory-related studies are scarce and are practically limited to the Mg-Al system,17-23 although other combinations of metals such as Zn-Al,24 Mg-Fe,25,26 Cu-Zn-Al,16 and CuCo-Zn-Al27 also exhibit the property to reconstruct. Sato et al.28 originally hypothesized that reconstruction involves the insertion of protons or hydroxyl ions at defect sites of the unstable Mg1-xAl2x/3( )x/3O rock-salt phase obtained by hydrotalcite decomposition. However, Rajamathi et al.20 supported a dissolution-precipitation mechanism from the obtainment of hydrotalcite when MgO and Al2O3 were soaked in a carbonate solution at 338 K for 5 days. The memory effect is not a universal property of HTlc-derived oxides. For example, calcined Co-Al, Ni-Fe, and Ni-Al hydrotalcites (the latter is known as takovite) do not easily recover the original structure.29-32 A deep understanding of the thermal decomposition and reconstruction of hydrotalcites is essential for optimization of the activation procedure according to practical requirements. A hurdle to gaining detailed insights into these processes is that studies have been typically practiced ex situ, i.e., arresting the reaction at a certain time, exposing to atmosphere, handling, and analyzing the product thereby obtained. In situ methodologies are highly preferred, since they guarantee that the sample being analyzed is truly representative of the reaction matrix.33-35 In situ studies have been increasingly applied to investigate the evolution of hydrotalcite-like compounds (mainly Mg-Al) upon thermal decomposition.18,21-23,33,34,36,37 High-temperature X-ray diffraction (XRD) is the technique of first choice to assess phase transitions on heating.18 Other techniques have been used in order to get insights into changes of metal coordination (27Al MAS NMR, XAFS)19,22 and chemical species at the solid surface (FT-IR, Raman), as well as monitoring the evolution of gasphase products (thermogravimetric analysis (TGA)-mass spectrometry (MS)).23,36 In contrast, very few studies monitoring changes in the solid during the reconstruction process using in situ techniques are available. Exceptionally, Millange et al.21 reported pioneering work using in situ energy-dispersive X-ray diffraction to investigate the retrotopotactic transformation of calcined Mg-Al hydrotalcite in sodium carbonate solution. The development of in situ methodologies is important for improved mechanistic and kinetic insights into the memory effect in HTlc’s, as recently shown for the Mg-Al system.38 Our next goal is to investigate this process in relation to the composition of the brucite-like sheets. This paper studies the impact of the divalent cation on the thermal activation and subsequent rehydration of HTlc’s in a water-saturated nitrogen flow. In situ XRD studies, complemented by other characterization techniques, were conducted over Mg-Al, Ni-Al, and Mg-Ni-Al hydrotalcites. These materials were specifically selected due to the reported ability (Mg-Al) or inability (Ni-Al) of the resulting mixed oxides to recover the original layered structure. The ternary Mg-Ni-Al system made it possible to assess the degree of resemblance in thermal decomposition and memory characteristics vis-a`-vis the binary systems. The reconstruction process has been modeled and kinetic parameters have been obtained. 2. Experimental Section 2.1. Materials Preparation and Characterization. MgAl, Ni-Al, and Mg-Ni-Al hydrotalcite-like compounds with nominal molar ratios of Mg:Al and Ni:Al ) 3:1 and Mg:Ni:Al ) 2:1:1 were prepared by coprecipitation at constant pH using the in-line dispersion-precipitation (ILDP) method.39,40 Aque-
J. Phys. Chem. C, Vol. 111, No. 9, 2007 3643 ous solutions of the respective metal nitrates Mg(NO3)2‚6H2O, Ni(NO3)2‚6H2O, and Al(NO3)3‚9H2O and the precipitating agent (NaOH + Na2CO3) were continuously fed at room temperature by means of peristaltic pumps into a homemade microreactor with an effective volume of ca. 6 cm3. An in-line probe measured the pH of the slurry directly at the outlet of the microreactor and was connected to one of the pumps to keep constant pH 10. The microreactor was stirred at 13 500 rpm by means of a high-speed disperser, and the residence time was fixed at 36 s. The resulting slurry was aged at 298 K in a glass vessel for 12 h under mechanical stirring (500 rpm). Finally, the material was filtered, thoroughly washed with deionized water to remove Na+ and NO3- ions, and dried at 353 K for 12 h. The chemical composition of the as-synthesized samples was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES; Perkin-Elmer Plasma 400) and atomic absorption spectroscopy (AAS; Hitachi Z-8200). Transmission electron microscopy (TEM) was measured in a JEOL JEM1011 microscope operated at 80 kV and equipped with an SIS Megaview III CCD camera. The samples were mounted on a carbon-coated copper grid by placing a few droplets of a suspension of the ground solid in chloroform, followed by evaporation at ambient conditions. N2 adsorption-desorption isotherms at 77 K were measured on a Quantachrome Autosorb 1-MP gas adsorption analyzer. Prior to analysis the samples were degassed at 393 K for 16 h. The BET model was used to derive the specific surface area of the samples.41 2.2. Decomposition and Reconstruction Studies. Thermal analysis was carried out in a Mettler Toledo TGA/SDTA851e microbalance equipped with a 34-position sample robot. Analyses were performed in N2 flow (50 cm3 STP min-1) using ca. 3 mg of sample placed in 70 µL R-Al2O3 crucibles. The temperature was increased in the range of 323-973 K using a heating rate of 5 K min-1. The evolution of the gases during decomposition of the hydrotalcites was studied by TPD-MS (temperature-programmed desorption coupled to mass spectrometry). To this end, ca. 100 mg of the as-synthesized material was placed in a quartz fixed-bed reactor (10 mm i.d.) and the temperature was raised from 298 to 973 K at 5 K min-1 in N2 flow (50 cm3 STP min-1). Masses m/z 18 (H2O) and m/z 44 (CO2) were continuously monitored by use of a quadrupole mass spectrometer (Pfeiffer OmniStar GSD 301O). In situ X-ray diffraction experiments were performed at ambient pressure in a Bruker-AXS D5005 theta-theta diffractometer equipped with a Bruker-AXS MRI high-temperature chamber and a diffracted beam graphite monochromator using Cu KR radiation. A thin layer of sample (ca. 30 mg) was mounted on the Pt90-Rh10 heater strip by placing a few droplets of a suspension of finely ground sample in ethanol followed by drying at ambient conditions. To minimize the specimen displacement effect, the specimen installation was carefully reproduced and the exact position of the heater strip was systematically checked in every experiment. Diffractograms were acquired for the Bragg-Brentano geometry in the range of 2θ ) 5°-70° with a step size of 0.1° and a counting time per step in the range of 1-6 s. In situ XRD patterns during thermal decomposition of the materials in N2 flow (100 cm3 STP min-1) were recorded isothermally at intervals of 50 K in the range of 303-723 K after 10 min equilibration at each temperature. The heating rate used was 5 K min-1. The gas flow was positioned across the specimen, leading to good solidgas contact. In situ reconstruction studies were conducted at room temperature over the hydrotalcites previously heated in
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TABLE 1: Characterization Data of the As-Synthesized Hydrotalcites molar ratio in solid sample
Mg/Al
Mg-Al-as Mg-Ni-Al-as Ni-Al-as
2.8 2.4
a
cell parameters (nm)
crystallite sizea (nm)
Ni/Al
c
a
003
110
Vp (cm3 g-1)
SBETb (m2 g-1)
0.5 2.6
2.33 2.33 2.33
0.306 0.305 0.304
6.4 4.2 3.4
23 8.6 8.7
0.29 0.24 0.52
69 76 222
Scherrer method. b BET method.
Figure 2. In situ XRD patterns during thermal decomposition of hydrotalcites in N2. The abbreviations “as”, “dh”, and “ca” make reference to as-synthesized, dehydrated, and calcined samples. Crystalline phases: 0, hydrotalcite; 9, dehydrated layered phase; 2, MgO; 3, NiO; /, heater strip.
N2 at 473 K for 10 min (dehydrated layered phase) or at 723 K for 5 h (oxide phase). After the decomposed samples were cooled to 303 K in nitrogen atmosphere, the gas was moisturized with water by use of an Ansyco SYCOS-H humidity generator and introduced to the chamber at a total flow of 100 cm3 STP min-1. XRD patterns were continuously recorded with time. The temperature and relative humidity inside the chamber in the vicinity of the sample were monitored by a Novasina HygroDat100 sensor. Under the conditions applied in the humidity generator, the water content in the N2 carrier for reconstruction experiments was ca. 2 vol % H2O (relative humidity of 90%). In this paper, the differently treated hydrotalcites are denoted as Mg-Al-x, Ni-Al-x, and Mg-Ni-Al-x, where “x” refers to the as-synthesized sample (“as”), dehydrated sample (“dh”, obtained by thermal treatment of “as” at 473 K), calcined sample (“ca”, obtained by thermal treatment of “as” at 723 K), rehydrated sample (“rh”, obtained by exposure of “dh” to water vapor at 303 K), and reconstructed sample (“rc”, obtained by exposure of “ca” to water vapor at 303 K). 3. Results and Discussion 3.1. As-Synthesized Samples. Table 1 shows the molar metal ratios in the solids as determined by ICP-OES (Al) and AAS (Mg, Ni) analyses. The XRD patterns of the as-synthesized samples at 303 K in Figure 2 reveal the hydrotalcite-type structure as the only crystalline phase (powder diffraction file 89-460 from ICDD). Characteristic reflections of the three basal (003), (006), and (009) planes can be identified at ca. 11.4°, 22.8°, and 34.8° 2θ, respectively, and the reflections associated
with the nonbasal (110) and (113) planes can be identified at ca. 60.7° and 61.8° 2θ, respectively. The cell parameters c and a of the rhombohedral structure were determined from the position of the (003) and (110) diffraction lines, respectively, assuming a 3R stacking of the layers (Table 1). The obtained values are in good agreement with the literature.3 The average crystallite size in the c- and a-directions was estimated by the Scherrer method42 using the (003) and (110) reflections, respectively. The dimension in the a-direction was larger than that in the c-direction, as can be expected from the plate-like shape of hydrotalcite crystals.3 The presence of nickel in the brucite-like sheets leads to a reduced crystallite size, in agreement with a previous work comparing Mg-Al and NiAl hydrotalcites.43 The TEM micrographs in Figure 3 evidence the often encountered whisker-like morphology of these layered materials.30,44 The estimated lateral length of these platelets, where a number of crystallites are aggregated, is in the range of 20-50 nm with a thickness of 5-20 nm, depending on the divalent cation in the structure. In good correspondence with XRD, the platelet dimensions in Ni-Al were considerably smaller than those in Mg-Al hydrotalcite. N2 adsorption was carried out to determine the porous properties of the as-synthesized samples. The isotherms were characteristic of materials with slit-shaped pores between aggregates of platy particles.43,45 The total pore volume and BET surface area of Mg-Al and Mg-Ni-Al hydrotalcites were very similar, being ca. 2 and 3 times higher in takovite, respectively (Table 1). The enhanced porous characteristics of Ni-Al hydrotalcite can be mainly attributed to interparticle porosity due to the decreased particle size.
Thermal Decomposition and Reconstruction of HTlc’s
J. Phys. Chem. C, Vol. 111, No. 9, 2007 3645
Figure 3. TEM of the as-synthesized hydrotalcites and products derived from thermal activation.
3.2. Thermal Decomposition. Dehydration of the Interlayer Space. The in situ XRD patterns upon thermal decomposition of the as-synthesized hydrotalcites in the temperature range of 303-723 K are shown in Figure 2. Up to 473 K, all samples experienced a progressive shift of the (003) reflection at 11.4° 2θ to higher angles. This is related to dehydration of the material by gradual removal of interlayer water. The intensity of the (006), (009), and (113) diffraction lines at ca. 22.8°, 34.8°, and 61.8° 2θ significantly decreases from 323 to 373 K. At 423 K, the (006) vanishes, the (009) remains as a weak reflection, and the (113) is present as a shoulder, so that the (110) reflection looks asymmetric. Besides, a very broad reflection centered around 36° 2θ emerges. The diffractograms at 423 and 473 K were very similar in all cases. The occurrence of the three broad reflections at ca. 13.8°, 36°, and 60.7° 2θ (marked by solid squares in Figure 2) and the simultaneous disappearance at 623 K for Mg-Al and Mg-Ni-Al hydrotalcites and 573 K for takovite are evidence for assigning them to the same dehydrated structure. The latter intermediate phase has been exclusively noticed in the Mg-Al system.18,19,21,23,33 Our results conclude that the decomposition of hydrotalcite-like compounds goes through this characteristic phase (denoted as “dh”) independently of the nature of the divalent cation in the structure. The broadening of the diffraction lines in the intermediate dehydrated layered phase suggests an important disorder in the stacking of the layers as compared to the as-synthesized hydrotalcite. The position of the (110) reflection is not affected in the temperature range of 303-473 K, revealing that the characteristic aparameter of the hydrotalcite is not altered on dehydration. The dehydration of the as-synthesized samples can be quantified by determination of the interlayer space from the basal reflection at 11°-14° 2θ. The interlayer space is defined as the distance between two hydroxyl groups in adjacent layers. As illustrated in Figure 1, this can be computed as the difference between the c-spacing from XRD (0.77 nm) and the thickness of the brucite sheet (0.48 nm).3 Values of ca. 0.29 nm were obtained in the Mg-Al, Ni-Al, and Mg-Ni-Al HTlc’s, which
Figure 4. Variation of interlayer space with temperature during dehydration of the hydrotalcites, and the thereof derived degree of shrinking.
are in excellent agreement with the interlayer space reported by Allmann.1 As shown in Figure 4, and similar for the three samples, the interlayer space is practically constant in the range of 303-323 K, continuously decreasing in the temperature range of 323-473 K down to ca. 0.16 nm in the fully dehydrated samples. This is equivalent to shrinking of the interlayer space in the dehydrated structure by 40-45% compared to the original hydrotalcite. The value of 0.16 nm is close to the thickness of the carbonate anion (0.14 nm), located with its C3 axis
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Figure 5. TGA and TPD-MS profiles during decomposition of the hydrotalcites in N2 at 5 K min-1. The dashed profiles represent the derivative of the weight loss.
perpendicular to the brucite-like sheets.46 This indicates the tight confinement of the compensating anion in the gallery of the “dh” phase. Various works with the extensively studied Mg-Al hydrotalcite have indicated that certain dehydroxylation of the brucitelike layers occurs concurrently with the removal of interlayer water in the formation of the dehydrated phase.23,47 To determine the degree of dehydroxylation and decarbonation in the “dh” samples depending on the metals in the brucite-like layers, TGA and TPD-MS experiments were conducted. As shown in Figure 5, the first weight loss of ca. 17% below 500 K has been typically attributed to interlayer water removal,19 which is supported by the first peak with m/z 18 in MS analysis during thermal decomposition. The second m/z 18 peak due to incipient dehydroxylation presents certain overlapping with the first one around 500 K. Extrapolation of the tail of the second H2O peak to the baseline has been used to estimate the temperature at which dehydroxylation is initiated (see dashed lines in Figure 5). This leads to ca. 473 K in all the samples, in excellent agreement with previous in situ XAFS and 27Al MAS NMR studies.19,22 Below 473 K, dehydroxylation is minor and no decarbonation occurs, either, since CO2 evolves above 523 K (m/z 44 in Figure 5). Consequently, the changes monitored up to 473 K are purely a consequence of the removal of interlayer water and the dehydration of the hydrotalcite structure is not affected by the metal composition of the brucite-like sheets. Transformation of the Intermediate Layered Phase into the Mixed Oxide. The dehydrated phase is stable up to 523-573 K. Above these temperatures, extensive dehydroxylation and decarbonation cause the formation of the corresponding mixed oxide (Figure 2). The presence of a single weight loss in the temperature range of 500-723 K in all samples and the coincidence of the maxima in the MS profiles of H2O and CO2 (Figure 5) support the widely reported coupling of dehydroxylation and decarbonation processes.22 However, formation of H2O starts around 500 K in all cases, while CO2 evolves at 523 K for the Ni-Al sample and above 573 K for the Mg-Al and Mg-Ni-Al samples. Dehydroxylation is originated by condensation of the brucite-like layers, leading to oxidic domains in the samples. The latter likely accelerate carbonate decomposi-
tion, ultimately pairing in temperature with dehydroxylation. The stability of Ni-Al-dh is lower than that of Mg-Al-dh and Mg-Ni-Al-dh, in view of the disappearance of the characteristic reflections at 573 K in the former sample as compared to at 623 K in the magnesium-containing samples. In agreement, the maxima of the H2O and CO2 profiles in Figure 5 appear at a lower temperature in Ni-Al (574 K) than in Mg-Ni-Al (635 K) and Mg-Al (653 K). The lower thermal stability of Ni-Al hydrotalcite as compared to Mg-Al hydrotalcite is wellknown.48,49 The diffractograms of the calcined samples in Figure 2 show characteristic reflections of the rock-salt MgO (periclase) and NiO (bunsenite) structures, with ICDD powder diffraction files 45-0946 and 47-1049, respectively. Thorough works by Clause et al.30,31,50 attempted to characterize these complex oxidic phases resulting from HTlc decomposition. At calcination temperatures close to 723 K used here, both systems were described as Al-doped MgO or NiO with an inhomogeneous distribution of aluminum (namely residing on the surface of the oxide). This leads to MgO or NiO particles decorated by aluminate-type patches.31 The surface enrichment by Al was regarded as vital in order to explain the remarkable thermal stability of the Al-modified nickel and magnesium oxides, as compared to the corresponding aluminum-free counterparts. Morphology Changes upon Thermal ActiVation. In situ analyses have concluded the basically identical decomposition mechanism of the HTlc’s, as for the evolvement of chemical species and phase transitions. We have examined possible morphological alterations on heating. Early work by Reichle,5 supported later by several authors,30,43,44 stated that the fiberlike morphology of hydrotalcite-like compounds illustrated in Figure 3 for the “as” samples is not altered by the thermal treatment. This holds true for the dehydrated and calcined MgAl samples, which virtually look like the as-synthesized material. However, in contrast with previous works,30,43,44 this invariability cannot be generalized for the nickel-containing samples. The platelets in takovite transform into uniform NiO-rich nanoparticles (
