Perturbing the properties of layered double ...

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www.rsc.org/materials | Journal of Materials Chemistry

Perturbing the properties of layered double hydroxides by continuous coprecipitation with short residence time† S! onia Abell" o,a Sharon Mitchell,a Marta Santiago,a Georgiana Stoicaa and Javier P"erez-Ram"ırez‡*ab

Downloaded by Universitat Rovira I Virgili on 23 November 2012 Published on 11 June 2010 on http://pubs.rsc.org | doi:10.1039/C0JM00088D

Received 20th January 2010, Accepted 19th April 2010 First published as an Advance Article on the web 11th June 2010 DOI: 10.1039/c0jm00088d A previous work (S. Abell" o and J. P"erez-Ram"ırez, Adv. Mater., 2006, 18, 2436) revealed an unanticipated variation in the textural properties of Mg–Al hydrotalcite, prepared by continuous coprecipitation with short residence time, s ¼ 1 s which, at that time, was not fully understood. Herein, we report the generalisation of such variation in physical properties to layered double hydroxides (LDHs) of different composition (Ni–Al, Mg–Al, and Mg–Fe hydrotalcite-like compounds). In particular stable colloidal suspensions and, on drying, impervious LDH particles have been prepared using the in-line dispersion precipitation (ILDP) method with s ¼ 1 s. This is thought to be a consequence of variation in the mechanism of inter-crystallite interactions with decreasing crystallite size. The resulting materials are characterised using multiple techniques and are compared to analogous materials attained at longer residence times (s ¼ 12 s). We show that despite the apparent compositional similarity and structural isomorphicity of the precipitates, their textural and morphological properties and their thermal stability differ strongly. Thermal activation of the LDHs, however, resulted in the development of comparable textural properties in the corresponding oxides, independent of the residence time.

Introduction Coprecipitation is one of the most frequently applied methods in research laboratories and industry to prepare precursors of ceramics, adsorbents, catalysts, and catalyst supports for a huge variety of applications.1 In contrast to solid-state routes, this wetchemical technique may be tailored to yield high-purity, ultrafine powders with a wide range of compositions, achieving excellent dispersion of component species.2 The solids derived from (co)precipitation typically possess an intrinsic porosity as a consequence of the external surface of the small particles and due to interparticulate voids. Porous precursors are often thought to be desirable to obtain a final product, e.g. after calcination, with inherited porosity characteristics. The large associated surface areas are an attractive feature for applications where a high throughput activity is sought.3 Coprecipitation is generally practised in batch mode and involves feeding a solution containing the desired cationic salts and a solution containing the precipitating agent into a stirred tank reactor at constant or variable pH. A major drawback of batch processing is the variation in both the residence time of the precipitating particles and of the concentration of the reacting

a

Institute of Chemical Research of Catalonia (ICIQ), Avinguda dels Pa€ısos Catalans 16, Tarragona, 43007, Spain b Catalan Institution for Research and Advanced Studies (ICREA), Passeig de Llu"ıs Companys 23, Barcelona, 08010, Spain † Electronic supplementary information (ESI) available: Additional TEM and stereomicroscopic images. See DOI: 10.1039/c0jm00088d ‡ Current address: Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, HCI E 125, Wolfgang-Pauli-Strasse 10, CH-8093 Zurich, Switzerland. E-mail: [email protected]; Fax: +41 44 633 1405; Tel: +41 44 633 7120

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species within the reactor, which are neither spatially nor temporally constant when the reagent solutions are being introduced in the course of the preparation. This inevitably gives rise to variance in the properties of the resulting solids.1,4,5 Many operating variables which are known to influence the properties of the resulting precipitate (e.g. temperature, time, supersaturation, pH, reactor geometry, way of mixing, additives, and aging conditions) have been extensively investigated for both batch and continuous precipitation methods.4,6,7 New reactor designs including the sliding surface mixing reactor,8 the vortex reactor,9 and the impinging jet,10 have been developed in order to improve control over the precipitation process (e.g. the mode of contact of the reacting species and their efficient mixing). An ability to decouple the different mechanistic steps occurring during particle nucleation and growth would permit manipulation of the structure of the product at both the atomic and the macroscopic scales and hence the effective tailoring of its properties. Due to the importance of the particle size distribution on the performance of layered materials in many of their current applications, the obtainment of control over this feature has been the subject of much attention. The attainment of small particle sizes, for example, is thought to be advantageous due to the high activities often observed as a consequence of the increased presence of surface defects in such samples.11 A short nucleation step and subsequent slow controlled growth is thought to be optimal for the preparation of monodisperse particles. Duan et al.12,13 coprecipitated fine-grained hydrotalcite-like compounds with a narrow particle size distribution (diameters in the range of 60–80 nm) using a colloid mill, where a rapid mixing and nucleation process takes place, followed by a separate aging process. While such devices may be employed to obtain a narrow This journal is ª The Royal Society of Chemistry 2010


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crystal size distribution, colloid mills are known to suffer from problems related to their specific geometry such as obstruction, and the absence of pH control at the feed point mixing position. Using a T-shaped micromixer, Schur et al.14 improved the definition of the coprecipitation and aging steps in the continuous preparation of Cu/ZnO composite catalysts, by avoiding the further addition of reagent materials following initial contact of the base and acid solutions. Based on the reported flow rates and reactor size, the reaction time studied was around 12 s. More recently, the same authors coupled this method with a subsequent spray drying process to study the coprecipitation of ZnO– Al2O3 systems.15 The rapid quenching of the resulting slurries offered additional improved control over the post-precipitation drying step, enabling collection of the freshly precipitated precursor with limited time for particle growth. The nanostructured materials obtained were reported to have higher surface areas due to the smaller particle sizes. The development of similar microreactor methodologies, however, remains in its early stages due to the low solid concentrations required in order to prevent clogging. Regardless of the specific microreactor design, the efficient mixing of the reactant feeds appears vital for the obtainment of monodisperse particle size distributions and the improvement of compositional homogeneity. In an earlier communication, some of us reported the in-line dispersion precipitation (ILDP) method for the preparation of Mg–Al layered double hydroxides (LDHs) with tuneable porous and morphological properties.16 This was accomplished by performing the precipitation in continuous mode using a miniaturised reaction chamber (Fig. 1a) attached to

a high-shear homogeniser, to ensure rapid mixing, with in-line pH control at the reactor outlet. The residence time was controlled by adjusting the flow of the feed solutions, permitting alteration of the reaction time between 1–75 s. Using a Scherrerbased analysis, we demonstrated that reduction of the residence time to 10 s yielded smaller hydrotalcite crystallites, which, correspondingly possessed a higher surface area, compared to the products formed at longer residence times or by conventional batch precipitation. This trend, however, was found to be discontinuous and a further increase in surface area was not observed on reduction of the residence time to 1 s. Under such conditions, the small crystallites produced were thought to tightly associate, resulting in large polycrystalline assemblies with close to zero porosity.16 Despite highlighting a singular regime of potential interest, the unexpected behaviour observed at short residence times was beyond the scope of the original investigation. Therefore, these observations were not further pursued at that time. To our knowledge, the effect of the residence time during continuous coprecipitation has not been evaluated in sufficient depth, and few studies are devoted to the properties of precipitates synthesised with short reaction times (s < 10 s). This variable is of crucial importance as it directly impacts on the complex and interconnected crystal formation mechanisms of nucleation, growth, agglomeration, and ripening, and thus on the characteristics of the final product. Here, we explore the perturbation of the properties of LDHs with hydrotalcite-like structure and different compositions (Ni–Al, Mg–Al, and Mg–Fe), prepared using the ILDP method, on reduction of the residence time to 1 s (sometimes denoted as ‘flash’ precipitation). The products obtained are compared with LDH phases prepared with s ¼ 12 s, whose properties are more representative of those typically reported for hydrotalcite-like materials prepared in batch mode, and are taken as a benchmark. The composition, morphology, and texture of the as-synthesised samples are extensively characterised using a range of techniques in order to gain further insight into the markedly different nature of the particles obtained and their mechanism of formation. Several noteworthy observations are made, including the rapid obtainment of stable LDH colloidal dispersions and the isolation of impervious LDH particles, which have increased thermal stability. These changes appear to be predominantly linked to variation in inter-crystallite arrangement and occur independently of the composition studied.

Experimental Synthesis of hydrotalcite-like compounds

Fig. 1 Photographs of (a) the ILDP reactor, (b) the Ni–Al hydrotalcite slurries prepared with s ¼ 1 s and 12 s, collected right after precipitation, and (c) the products dried at 353 K for 12 h. The distinct settling characteristics of the precipitate slurries and visual appearance of the solids are evidenced.

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Hydrotalcite-like compounds containing different combinations of divalent and trivalent metal cations (M2+-M3+ ¼ Ni–Al, Mg– Al, and Mg–Fe) and having a nominal molar M2+/M3+ ratio of 3 were prepared by coprecipitation at pH 10 using the ILDP method.16 Aqueous solutions of the metal nitrates (0.75 M of Mg(NO3)2$H2O or Ni(NO3)2$H2O and 0.25 M of Al(NO3)3$9H2O or Fe(NO3)3$9H2O) and the precipitating agent (NaOH and Na2CO3, 1 M of each) were continuously fed at 303 K by means of peristaltic pumps (Watson-Marlow 520S) into the precipitation chamber. The chamber, shown in Fig. 1a, has J. Mater. Chem., 2010, 20, 5878–5887 | 5879


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an effective volume of ca. 6 cm3 and is stirred at 13,500 rpm using a high-shear homogeniser (IKA UTL25 Ultra Turrax). The average residence time of the slurry in the reactor (s ¼ Vreactor/Fliquid) is determined by the flow of the acid and base solutions. In this work, experiments were conducted with s ¼ 1 s and 12 s. An in-line probe (Mettler Toledo InLab 410) measured the pH of the slurry directly at the outlet of the precipitation chamber and was connected to one of the pumps for pH control. The precipitate formed under steady-state operation conditions was immediately collected by direct filtration of the slurry under reduced pressure without aging, and washed with distilled water, enabling its rapid isolation within a few minutes. The solid was finally dried at 353 K for 12 h. In specific cases, the emerging precipitate was subsequently aged at room temperature for 15 h using magnetic stirring (500 rpm). The as-synthesised hydrotalcites were calcined in static air at 450 K and 723 K for 2 h, using a ramp rate of 5 K min"1. Techniques The chemical composition of the solids and the filtrates was determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES) in a Polyscan 61E Thermo Jarrell Ash spectrophotometer. Before analysis, the solids were dissolved in a 3:1 HCl:HNO3 solution. Powder X-ray diffraction patterns were acquired in a Siemens D5000 diffractometer with BraggBrentano geometry using Ni-filtered Cu-Ka radiation. Data were collected in the 2q range of 5–70# with an angular step of 0.05# and a counting time of 8 s per step. Confocal scanning laser microscopy (CSLM) was measured in a Nikon TE2000-E microscope equipped with an Ar–Kr/Ar air-cooled laser and three detection channels for fluorescence/reflectance. For CSLM analysis, the particles were embedded in Spurr’s epoxy resin and semi-thin sections of 0.7 mm were cut with a Reichert Ultracut E ultramicrotome from Leica Microsystems. These sections were mounted on a slide and stained with a drop of Rhodamine G (Panreac) to preserve fluorescence. Optical microscopy images of particle cross-sections were taken by serial-sectioning in the z-direction every 0.1 mm. AFM imaging was undertaken using a MultiMode AFM operated by a Nanoscope IIIa controller, interfaced with a Quadrex extender module (Veeco Instruments), equipped with a video microscope to allow for sample positioning. Samples were prepared by dispersion in methanol onto a freshly cleaved mica surface and subsequent drying at 313 K. Topographical images were collected in contact mode in air using triangular Si3N4 cantilevers (Veeco Instruments), which have a nominal spring constant of 0.06 N m"1 and a tip radius of curvature of 10 nm. The tip–sample contact force was minimised by optimising the deflection setpoint, and images were collected at scan rates of 0.5 or 1 Hz. Height, deflection, and friction data were collected simultaneously. Transmission electron microscopy (TEM) was carried out in a JEOL JEM1011 microscope operated at 100 kV. A few droplets of the sample suspended in ethanol were placed on a carbon-coated copper grid followed by evaporation at ambient conditions. The skeleton density of the solids was measured by helium pycnometry at 295 K in a Quantachrome Pentapycnometer. Prior to the measurement, the sample was dried at 393 K for 12 h. N2 isotherms at 77 K were measured on a Quantachrome Autosorb 5880 | J. Mater. Chem., 2010, 20, 5878–5887

1-MP analyser. Prior to the analysis, the samples were degassed in vacuum at 393 K for 16 h. Mercury intrusion porosimetry was measured on Pascal 140 and 440 porosimeters (Thermo Electron), which operate in the pressure range of 0.01–400 MPa. Prior to the intrusion experiments, the samples were degassed (10"1 Pa) at 393 K for 16 h. Thermogravimetry was measured in a Mettler Toledo TGA/SDTA851e microbalance. Analyses were performed in air (50 cm3 min"1) ramping the temperature from 298 to 1173 K at 5 K min"1. The evolution of gases during the calcination of the hydrotalcites was studied by temperatureprogrammed desorption coupled to mass spectrometry (TPDMS). The sample (50 mg) was loaded in a quartz fixed-bed reactor (10 mm i.d.) and heated in dry air (50 cm3 min"1) from 298 to 973 K at 5 K min"1 and masses m/z 18 (H2O) and m/z 44 (CO2) were monitored with a quadrupole mass spectrometer (Pfeiffer OmniStar GSD 301O). In situ Fourier transform infrared spectroscopy was carried out in a Thermo Nicolet 5700 spectrometer (Thermo Scientific) using a SpectraTech Collector II diffuse reflectance (DRIFT) accessory equipped with a hightemperature chamber, ZnSe windows, and a mercury cadmium telluride (MCT) detector. Spectra were automatically recorded every 80 s using KBr (Aldrich, IR spectroscopy grade) under reaction conditions as the background. The range 650– 4000 cm"1 was covered by co-addition of 32 scans at a nominal resolution of 4 cm"1. Optical stereomicroscopy was carried out in a Zeiss Stemi SV11 microscope. Samples were mounted on a FP82HT Hot Stage connected to the microscope and heated in static air from 298 to 650 K at 5 K min"1.

Results Outward appearance of the precipitates Hydrotalcite-like compounds with a different combination of divalent (M2+ ¼ Mg2+, Ni2+) and trivalent (M3+ ¼ Al3+, Fe3+) metal cations and a ratio of M2+/M3+ ¼ 3 were synthesised by the ILDP method using residence times of s ¼ 1 s or 12 s and isolated by direct filtration under reduced pressure. An immediate indication of the significance of residence time as a variable during synthesis is noted on observation of the precipitate slurries. As illustrated for the Ni–Al sample (Fig. 1b), the slurry obtained with s ¼ 12 s instantly settles while with s ¼ 1 s, the solids remain dispersed in the solution phase, appearing to form a colloidal sol. If collected, these sols are stable with little evidence of sedimentation even after storage for 1 year. On direct filtration and drying, further visible differences are noted between the solid products (Fig. 1c). The solids obtained with s ¼ 1 s are shiny and translucent and are found to be extremely difficult to crush when ground manually. In contrast, for s ¼ 12 s the resulting materials are dull with a fine grain texture and are easily convertible to a powder form. The latter feature is typical for samples prepared at longer residence times, either by continuous or by batch precipitation. The differences in solution behaviour, mechanical strength, and optical properties provide further evidence for the influence of residence time on the characteristics of the isolated solid phases. These observations were general in all the samples synthesised with short residence time, independent of the particular metal combination studied. This journal is ª The Royal Society of Chemistry 2010

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Structure and composition Despite the contrasting visual appearance observed, the compositional and structural characteristics of samples prepared with different residence times are found to be similar. Chemical composition analysis by ICP-OES gave metal ratios close to 3, the nominal ratio present during synthesis (Table 1). The content of divalent and trivalent cations in the filtrate was negligible, indicating that the metals precipitate rapidly (within 1 s) in contact with the basic solution. Considering the short length of synthesis, this indicates the achievement of a high level of product homogeneity, particularly in the cases of the Ni–Al and Mg–Al LDHs. Mg–Fe LDHs prepared with differing residence times exhibit a larger deviation in product composition. As no metal cations were detected in the filtrates, however, the origin of the deviation is unclear and could also reflect inaccuracies in the concentrations of the feed stream or in preparation for ICP-OES analysis. Although the sol–gel-like nature of the slurry obtained with s ¼ 1 s might be suggestive of the formation of an amorphous phase, the X-ray diffraction patterns of the precipitates collected after 1 and 12 s appear identical (Fig. 2, right). A single crystalline phase with reflections assigned to those expected for a hydrotalcite-type structure (powder diffraction file 89–460 from ICDD) is observed in both cases. No reflections associated with the single metal hydroxides phases were visible. The observed line broadening and relative intensities of reflections are similar for each metal combination studied. Table 1 shows the calculated lattice parameters of the as-synthesised samples assuming the R-3M space group (c0 ¼ 3d003 and a0 ¼ 2d110).19 Due to the differences in ionic radii between the M2+ and M3+ present in the LDHs, a linear relationship between LDH composition and the lattice parameter, a0 (Vegard’s law, assuming ordered cation arrangement within the brucite-like lattice) is often expected. In particular, this type of analysis has been effectively applied to Mg–Al LDHs18 but also may be applied to Ni–Al17 containing materials (Shannon ionic radii $ Ni2+ ¼ 0.69 A, $ Al3+ ¼ 0.54 A). $ 20 Calculation of Mg2+ ¼ 0.72 A, the composition based on previously reported relationships (Table 1) indicates metal ratios in close agreement with the results from bulk chemical analysis. This suggests that even under rapid precipitation conditions the metals are incorporated homogeneously within the LDH structure. This conclusion was further supported by EDX analysis of the Mg–Al samples. The molar Mg/Al ratio, averaged over 10 different particles was

Fig. 2 N2 isotherms at 77 K (left) and powder X-ray diffraction patterns (right) of the hydrotalcite phases coprecipitated with s ¼ 1 s and 12 s.

measured as 2.6 $ 0.1 and 2.7 $ 0.2 for samples prepared with residence times of s ¼ 1 and 12 s respectively. Consequently, the residence time appears to have little influence on the composition of the LDH phases obtained. A Scherrer-based analysis was applied to approximate the crystallite dimensions reflecting the sizes of the coherent diffraction domains in the crystalline products (Table 1). It should be noted that this type of analysis is subject to considerable errors (both instrumental and theoretical) and that the values obtained are only an estimate and may not correspond to the exact crystallite size. The results may, however, be informative for showing trends in crystal sizes and to provide an indication of the extent of any disorder present. Analysis of the

Table 1 Characterisation of the composition, structure, and porosity of hydrotalcites prepared with different residence times Sample

s/s

M2+/M3+ a/-

c0/nm

a0/nm

M2+/M3+ b/-

D003(D110) c/nm

rHe/g cm"3

Vp, He/cm3 g"1

Vp, N2/cm3 g"1

SBET/m2 g"1

Ni–Al

1 12 1 12 1 12

2.98 2.96 2.72 2.80 3.5 2.83

2.35 2.37 2.35 2.34 2.38 2.36

0.304 0.304 0.306 0.306 0.311 0.311

3.3 d 3.2 d 2.7 e 2.7 e —f —f

5 (8) 4 (8) 4 (8) 6 (11) 7 (14) 8 (11)

2.45 2.83 1.84 1.99 —g —g

0 0.35 0 0.43 0 0.30

0 0.17 0 0.19 0 0.27

0 52 0 48 0 61

Mg–Al Mg–Fe a

Molar metal ratio in solid by ICP-OES. b Metal ratio calculated from diffraction data. c Average crystallite size estimated by Scherrer analysis applied to the (003) or (110) reflection, respectively. d Based on results of Kannan et al.17 e Based on results of Kaneyoshi and Jones.18 f No previously reported structural relationship. g Not determined.


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intense (003) reflection (a measure of the crystal dimension in the stacking direction, c) and of the (110) reflection (a measure of the crystal dimension in the a-b plane) indicate that the size of the crystalline domains are not equivalent in all crystallographic directions, with aspect ratios of approximately 2. Products prepared by ‘flash’ coprecipitation give sizes of approximately D003 (D110) ¼ 5 (8) nm, 4 (8) nm, and 7 (14) nm for Ni–Al, Mg–Al, and Mg–Fe hydrotalcites, respectively. In general the estimated average size slightly increases for the products obtained with longer residence times except for Ni–Al, where the D003 estimate is observed to decrease. As previously observed in Ni–Al LDHs,21 this is assigned to a non-uniform line broadening in the diffraction patterns and is thought to be related to the occurrence of turbostratic disorder, that is, a lack of positional relationship between consecutive layers within the structure. Morphology A closer examination of the products’ morphology, at both macro- and nanoscopic scales, using confocal laser scanning microscopy (CLSM), atomic force microscopy (AFM), and transmission electron microscopy (TEM), provided further evidence of the suspected differences in the particle properties of the hydrotalcites synthesised with different residence times. These analyses were systematically conducted over the Ni–Al hydrotalcites. The sample prepared with s ¼ 12 s exhibits many morphological features commonly associated with typical hydrotalcite-like phases. Optical sectioning through the particles at different heights (Fig. 3a, right) reveals a rough surface, which is indicative of the mesoporous character typical of LDHs prepared by coprecipitation due the occurrence of edge-face interaction of unsymmetrical crystallites. The AFM image of this

sample (Fig. 3b, right) shows distinct particles whose appearance is also similar to that commonly reported, of (hexagonal) platelike morphology.22 The particles observed have lateral dimensions of ca. 0.5–1 mm and typical thicknesses/heights of ca. 10–20 nm. The surfaces of the particles themselves are not planar, appearing to be formed by aggregation of smaller subunits (ca. 40–50 nm in length and 20–30 nm in width). TEM images provide further evidence of the polycrystalline nature of the particle aggregates. The smaller ‘subunits’ do not appear to have a preferred positional relationship and have lateral lengths and thickness in the ranges of 15–50 nm and 3–20 nm, respectively. Both plate-like and fibrous features are visible (Fig. 3c, right), morphologies which are typical of hydrotalcites prepared by conventional batch coprecipitation.23,24 Comparison with the images of the Ni–Al hydrotalcite obtained with s ¼ 1 s evidences stark differences with respect to s ¼ 12 s. CLSM micrographs show that the exposed surface is much smoother in the former specimen, which suggests a tighter crystal packing arrangement within the structure. As supported by density analysis (see below), the white dots visible in the images (Fig. 3a, left) are attributed to the presence of occluded solvent/pores in these particles. In this case, the application of AFM reveals more about the nature of the structure of the materials obtained with s ¼ 1 s. Strikingly, the height image recorded from a particle surface (Fig. 3b, left) supports the idea of a tight association of small particle subunits leading to the formation of large agglomerates of low porosity. Regularly shaped elongated surface features are visible. These features show greater alignment (in Fig. 3b, left, along the y axis) than the similarly sized features observed in the product obtained with s ¼ 12 s (Fig. 3c, right, inset) which do not appear to show any preferential orientation. Analysis of the height profiles measured over the sample surface (inset in Fig. 3b) indicates that these rod-like features are uniformly spaced with an approximate separation of 20 nm. Observation of this sample by TEM confirmed the tightly aggregated polycrystalline nature of these particles (Fig. 3c, left), which are typically of dimensions less than 10 nm. Further TEM micrographs of this material may be found in Fig. SI1 of the ESI†. Porosity

Fig. 3 Images of the Ni–Al hydrotalcites coprecipitated with s ¼ 1 and 12 s by (a) CSLM, (b) AFM, and (c) TEM. In AFM, the position of the height profile (inset) is marked with the horizontal line.

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Variation in the residence time is also found to strongly influence the porosity of the resulting hydrotalcite-like compounds, thus supporting the idea of differences in the modes of particle aggregation. Results of skeletal density analysis, as determined by helium pycnometry, show that the Ni–Al hydrotalcite phase obtained with s ¼ 12 s has a density of rHe ¼ 2.83 g cm"3, in agreement with that of the naturally occurring mineral takovite (Ni6Al2(OH)16(CO3,OH)$4H2O, r ¼ 2.8 g cm"3).25 The sample synthesised with s ¼ 1 s, however, has a significantly lower density (rHe ¼ 2.45 g cm"3), despite the apparent similarity in crystal structure. The same trend in He density was observed for the Mg–Al hydrotalcites (Table 1). Sample composition, atomic structure, and the presence of occluded porosity are all factors which could lead to differences in the measured skeletal density between samples. As the solids show little compositional variation and the PXRD data indicates that hydrotalcite is present as This journal is ª The Royal Society of Chemistry 2010


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the only crystalline phase, with similar broadening of the reflections observed between samples, the low apparent density of the samples prepared with s ¼ 1 s is tentatively attributed to the presence of occluded pores within the structure (supporting the observations by CLSM). However, the presence of a higher degree of amorphicity could also lead to similar observations. The N2 adsorption isotherms at 77 K of the samples prepared with s ¼ 12 s exhibit a form typical of hydrotalcite-like phases (Fig. 2, left).26 The type IIb isotherms, which exhibit H3 hysteresis, are characteristic of materials with slit-shaped pores between aggregates of particles of plate-like morphology and inherent associated mesoporosity.27 The total pore volume (Vp, N2) and BET surface area (SBET) range from 0.17–0.30 cm3 g"1 and 48– 60 m2 g"1, respectively (Table 1). It should be noted that although

these values are slightly lower than those previously reported for Mg–Al hydrotalcites prepared by ILDP with s ¼ 12 s,16 they cannot be directly compared as a different stirring speed was employed. In great contrast, the hydrotalcites prepared with s ¼ 1 s showed no N2 uptake within the whole range of relative pressures, yielding materials with zero detectable surface area and pore volume. Analysis of porosity by N2 adsorption is limited to the study of pores up to 100 nm in size due to condensation of nitrogen. In order to confirm the absence of porosity (e.g. larger mesopores and/or macropores) in these samples, mercury porosimetry was used. Fig. 4 illustrates the results obtained for the Ni–Al hydrotalcites. No distinctive Hg intrusion was observed in the hydrotalcite products prepared with s ¼ 1 s over the pressure range 0.01–400 MPa, which covers pore sizes ranging from 4 nm to 120 mm. On the other hand, the Ni–Al, s ¼ 12 s shows intrusion in the range 10–400 MPa. Application of the Washburn equation to the latter sample indicated a pore size distribution centred at 100 nm (inset of Fig. 4), which is in agreement with the BJH pore size distribution obtained from N2 adsorption data (not shown). The combination of short residence time during continuous coprecipitation and the abrupt quenching of the precipitate induce fast nucleation, leading to an excellent packing of very small crystallites, and ultimately providing impervious layered materials. Thermal activation

Fig. 4 Hg intrusion curves at 298 K of the Ni–Al hydrotalcites with s ¼ 1 and 12 s. Inset: pore size distribution determined by the Washburn equation.

Thermogravimetric analysis of the samples is shown in Fig. 5a and 5b. The solids, as illustrated for the Ni–Al sample, exhibit two broad weight losses associated with multiple weight loss steps; loss of physisorbed water, loss of interlayer water, loss of interlayer anions and dehydroxylation of the metal hydroxide layers.28,29 The total weight loss (%36%) and the relative

Fig. 5 Characterisation of Ni–Al hydrotalcites coprecipitated with s ¼ 1 s (— —) and 12 s (-- --) upon calcination in air: (a) thermogravimetric profiles, (b) derivative of the weight loss, (c) evolution of product gases measured by mass spectrometry, and (d) in situ infrared spectra at selected temperatures.


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contribution of the individual weight loss steps were almost identical, in agreement with the similarity in chemical composition observed between the samples prepared with different residence times. This relates not only to the molar M2+/M3+ ratio determined by ICP-OES (Table 1), but also to the relative proportions of hydroxyls, carbonates, and interlayer water present in the hydrotalcites. The residence time is, however, found to influence the thermal stability of the resulting materials. The temperatures at which weight losses occur, initially due to loss of interlayer water and at higher temperatures due to the simultaneous decarbonation and dehydroxylation of the material, are shifted to higher temperatures in the sample coprecipitated with s ¼ 1 s. Particularly, the collapse of the hydrotalcite structure occurs at a higher temperature (ca. 50 K) in the hydrotalcites prepared with s ¼ 1 s (e.g. 628 K vs. 578 K for the Ni–Al sample). TPD-MS data acquired during calcination (Fig. 5c) confirmed the nature of the gasses evolved. Masses associated with H2O and CO2 were observed, which showed a maximum release at the same temperatures as those corresponding to the weight losses observed by thermogravimetry. The impeded release of water and carbon dioxide upon hydrotalcite decomposition in the samples prepared with s ¼ 1 s is very likely linked to the absence of accessible porosity in these materials. Further insight into the thermal decomposition of hydrotalcite samples precipitated with short residence times was attained from an in situ DRIFTS study. Fig. 5d compares the infrared spectra of the Ni–Al hydrotalcites recorded at different temperatures. Heating to 423 K led to a reduction in intensity of the bands at 3500 and 1650 cm"1, associated with the loss of physically adsorbed water in both samples.30 By 473 K, the band at 1650 cm"1, together with that at 3050 cm"1, which is related to hydrogen bonding between interlayer H2O and CO32" ions in the interlayer space, are no longer visible in either sample. However, and in agreement with thermogravimetric data, the release of water from the sample coprecipitated with s ¼ 1 s was delayed with respect to that with s ¼ 12 s, as shown by the more intense 1650 cm"1 band at 423 K in the former specimen. At this temperature, the n3 mode of the carbonate starts to split into two new bands at 1540 and 1370 cm"1 in Ni–Al, s ¼ 1 s, while this doublet is more intense in Ni–Al, s ¼ 12 s. This phenomenon, which is a consequence of the reorganisation or grafting of the carbonate species within the interlayer,31 additionally suggests that the transformation of the layered structure towards the mixed metal oxide occurs at a higher temperature in the sample precipitated under ‘flash’ conditions. This is corroborated in the spectra at 673 K, where the sample precipitated with s ¼ 1 s still displays a higher intensity of the bands related to hydroxyls and both carbonate components than the sample with s ¼ 12 s, i.e. confirming the higher thermal stability of the sample prepared with short residence time. Optical stereomicroscopy of the Ni–Al coprecipitated with s ¼ 1 s and calcined at different temperatures indicates that the green hydrotalcite particle experiences multiple fractures by thermal effect and a dark particle is finally attained due to the mixed oxide formation beyond 623 K (Fig. SI2, ESI†). This phenomenon might be a consequence of the stresses induced in the crystal structure upon heating, which is related to the hindrance of gaseous species to escape from the impervious hydrotalcite particles. 5884 | J. Mater. Chem., 2010, 20, 5878–5887

The N2 adsorption isotherms at 77 K and derived porous properties of the Ni–Al samples prepared with s ¼ 1 and 12 s after calcination to 450 K and 723 K, are shown in Fig. 6 and Table 2, respectively. No significant changes are observed following sample dehydration, the N2 isotherms of the samples heated to 450 K being similar to those of the as-synthesised materials. This suggests that the gradual crystal macro-fracturing observed by optical stereomicroscopy for the s ¼ 1 s does not

Fig. 6 N2 isotherms at 77 K of Ni–Al hydrotalcites coprecipitated with s ¼ 1 and 12 s and derived calcination products.

Table 2 Porous properties of the calcined hydrotalcites determined by N2 adsorption at 77 K Sample

s/s

Tcalc/K

Vp, N2/cm3 g"1

SBET a/m2 g"1

Ni–Al

1 12 1 12 1 12 1 12 1 12

450 450 723 723 450 450 723 723 450 450

0 0.15 0.19 0.22 0 0.09 0.06 0.36 0 0.34

7 56 193 203 5 153 102 235 0 57

Mg–Al

Mg–Fe


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Fig. 7 TEM micrographs of the Ni–Al hydrotalcites calcined at 723 K.

result in large variation in the porosity of the material. In contrast, thermal decomposition at 723 K, i.e. after dehydroxylation/decarbonation, yields materials in which the original particles are transformed into Ni–Al mixed metal oxides, which exhibit comparable high total surface areas (SBET ¼ 193 and 203 m2 g"1 for s ¼ 1 and 12 s, respectively) and similar morphologies of increased particle uniformity observed by TEM (Fig. 7). The transformation from lamellae to nodular particles in the mixed oxide derived from Ni–Al hydrotalcite was reported elsewhere.32 These morphological differences are exclusive for the Ni–Al counterpart, as many LDHs are commonly reported to retain their specific plate-like morphology upon calcination.32,33 The N2 isotherms are also equivalent, showing porosity in the mesopore range (hysteresis loop type H2) (Fig. 6). The resulting porosities of the s ¼ 1 s and s ¼ 12 s samples in the Mg– Al and Mg–Fe counterparts after calcination at 723 K were not found to exhibit such a close resemblance (Table 2), but in all cases, the porosity is greatly increased from that of the starting hydrotalcites. These results suggest that for Ni–Al samples the development of porosity occurs mainly during the dehydroxylation/decarbonation step, being more or less independent of the porosity (determined by the crystal/particle arrangement) of the as-synthesised LDHs prepared with different residence times.

Discussion Mechanism of LDH formation The thermodynamic driving force which governs the precipitation of a solid from the liquid phase is that of supersaturation, which is related to the relative concentration and solubility of a chemical compound and influences both the mechanism and kinetics of crystallite nucleation and growth.34 Precipitation of the LDH phase is initiated by combination of basic and metal salt containing solutions, which are introduced into the reactor by two separate feeds. Control of the supersaturation during this step may have a significant influence on the properties of the resulting solid phase. In the ILDP method, a high-shear homogeniser is employed in order to provide intense mixing conditions, facilitating the contact between the two feed solutions, and reducing the occurrence of concentration gradients. Hence, the variation in the degree of supersaturation both locally in the feed points and globally within the whole reactor is avoided. Due to the low product solubility and high supersaturation conditions employed, LDH nucleation is thought to occur instantly. Although product crystallisation inevitably leads to the consumption of supersaturation, which might result in This journal is ª The Royal Society of Chemistry 2010

a dependence of the average supersaturation on the feed point flow rates, this effect is thought to be small and is generally ignored by most modelling approaches as a knowledge of the kinetics of nucleation and growth are required.35 Variation of the residence time, therefore, is predominantly thought to influence the extent of crystallite growth. The successive reduction of the residence time (and hence effective reaction time) in the ILDP method has uncovered a new regime of phases of hydrotalcite-like structure whose physical properties are distinct from those of LDHs prepared by coprecipitation syntheses with longer reaction times (or standard batch coprecipitation). Characterisation of these phases, particularly by the application of different imaging techniques, has shown that the distinct morphology and porosity, lower density, and higher thermal stability observed in samples with s ¼ 1 s are likely to be a consequence of the differences in their macrostructure. The particles appear to be composed of uniformly sized crystallites, which are tightly packed. This is noteworthy as it implies that the nucleation of crystalline LDH phases occurs rapidly under these conditions (303 K, pH 10, high-shear stirring), in the absence of crystallisation of other metal hydroxide impurities. The fact that no other phases are formed even at short residence times supports previous works,19,36 which report that the LDH phase is the thermodynamically favoured product at pH 10. When comparing the formation of LDH phases prepared with different residence times, this means that the principal variation during synthesis is between the conditions of growth (or aging) of the LDH crystallites within the reactor. It is important to remark that no post-aging operation was performed over the different slurries. Properties which are found to have a dependence on the residence time, therefore, are likely to be dependent on the macrostructure (particle sizes and arrangement) of the resulting phases. Mechanism of LDH aggregation Furthermore, these results offer an insight into the complex mode of crystallite aggregation and morphological development of the LDH phase. As both product slurries are subjected to a rapid quenching, i.e. without a subsequent aging operation, and immediately washed and dried equivalently, it is thought that the differences observed in sample macrostructure must originate from differences in the particles formed within the reactor, i.e. differences in the particle size. The dense packing observed in the dried product formed with s ¼ 1 s indicates that (a) colloidally sized nucleating LDH crystallites are able to associate and align on a very short time scale and (b) the method of product isolation may have a significant influence on the product macrostructure. In this case, for example, drying of the thick paste obtained on direct filtration and washing of the released slurry yields a tightly packed material which is found to be composed of translucent particles. These results support previous suggestions of a change in the mode of favoured interactions between nanoparticulate LDHs.37 The increased relative mechanical strength and absence of porosity in the samples obtained is consistent with a preference for the occurrence of stronger face-to-face interactions, and hence to a reduction in the interparticle voids associated with weaker faceto-edge interactions of plate-like particles.37,38 J. Mater. Chem., 2010, 20, 5878–5887 | 5885

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Stability of LDH colloids and mechanism of LDH growth Reduction in the residence time to 1 s led to the formation of stable colloidal dispersions of LDH nanoparticles for all compositions studied. This is noteworthy as stable colloidal LDH suspensions are thought to be promising precursors for several applications including thin film deposition, catalyst impregnation, potential liquid crystalline behaviour, etc.38–40 There are, however, few reported methods for the obtainment of colloidal dispersions of LDHs. The majority of approaches are related to the sol–gel synthesis and typically require careful control of both solvent and metal precursor reactivity in attempts to control the rates of metal hydrolysis.38,39 In agreement with a previous patent which describes the employment of high-shear mixing zones for the formation of colloidal suspensions,40 the ILDP method, in the short residence time regime, provides an attractive alternative method which is rapid, able to operate under aqueous conditions, and employing cheap, commercially available metal salts. The stability of the colloidal suspension and the absence of metal ions in the filtrates collected on removal of the colloidal particles suggest that, under these conditions, the particles undergo limited growth after isolation from the feed streams. This is substantiated by the absence of porosity in the samples prepared with s ¼ 1 s that have been subjected to a post-aging treatment for 15 h at room temperature, that is, the aging operation does not alter the properties of the solids precipitated under ‘flash’ conditions. Extension of the residence time is known to result in further growth16 of the crystallites (as evidenced here by the observation of particle sedimentation, and confirmed by the imaging and diffraction data). In addition to the growth of individual crystals with longer exposure to the feed streams inside the reactor, a greater occurrence of crystal intergrowth may also occur (e.g. due to an increased likelihood of heterogeneous nucleation) This, combined with an increased preference for edge-to-face interactions, might explain the observed separation of the particles into the architectures more typically associated with hydrotalcite-like phases (e.g. hexagonal platelets, sand-rose, etc.).

Conclusions Coprecipitation with short residence times (s ¼ 1 s) yields colloidally sized hydrotalcite-like materials possessing singular properties. The resulting LDH phases are characterised by the absence of porosity, greatly contrasting with the well-developed mesoporosity obtained by standard coprecipitation with longer residence times in continuous or batch mode. This phenomenon has been demonstrated for LDH phases of different compositions and is attributed to the restricted growth time and to the rapid assembly of crystallites within the solids precipitated. This also leads to significant differences in particle properties (e.g. mechanical, optical, density) with respect to samples attained under conventional precipitation, while the composition and atomic structure are nearly identical. Despite the differences observed between the precursors, thermal activation leads to a large increase in their textural properties irrespective of the initial porosity and the morphological properties of the calcined samples are very similar. However, the high level of crystallite/ 5886 | J. Mater. Chem., 2010, 20, 5878–5887

particulate aggregation attained with short residence time and the absence of accessible porosity in the material, which hamper the release of water and carbon dioxide upon hydrotalcite decomposition, are responsible for its higher apparent thermal stability. These unique properties open new doorways both in the current and potential applications of hydrotalcite-like compounds. The preparation of impervious thin films and coatings, for example, or the fabrication of LDH nanocomposites with enhanced properties may be envisaged. Finally, it should be pointed out that this work has focused on the coprecipitation of hydrotalcite-like phases and general conclusions can be extrapolated for these types of layered compounds with different cationic compositions. However, judging whether other inorganic structures suffer similar size dependent behaviours requires further investigation and will be the subject of future work. In any case, although other practical issues in the use of miniaturised reactors such as clogging still need to be addressed, the variation of the residence time during continuous coprecipitation far below the threshold investigated here (& 1 s) remains at an early stage. Moving into the microand millisecond time-scale may provide a better understanding of the materials ancestors, i.e. nuclei, crystallites, etc., offering key advantages for the synthesis of materials with tailored properties.

Acknowledgements This research was supported by the Spanish MICINN (CTQ2006-01562/PPQ, CTQ2009-09824/PPQ, ConsoliderIngenio 2010 grant CSD2006-0003, 2009 SGR 461, PTQ05-0100980, and AP2005-5147) and the ICIQ Foundation. Dr C. Gardner is thanked for helpful instruction and discussion related to AFM.

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