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Monitoring the structural and textural changes of Ni-Zn-Al hydrotalcites under heating
Thermochimica Acta 687 (2020) 178594
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Monitoring the structural and textural changes of Ni-Zn-Al hydrotalcites under heating
T
E. Meza-Fuentesa,*, J. Rodriguez-Ruizb, C. Solano-Poloa,b, M.C. Rangelc, A. Farod a
Grupo de Estudios en Materiales y Combustibles, Campus San Pablo, Universidad de Cartagena, Cartagena, Colombia Grupo de Investigación en Procesos de la Industria Petroquímica, SENA-Centro para la Industria Petroquímica, Cartagena, Colombia c Instituto de Química, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, Agronomia, 90650-001 Porto Alegre, RS, Brazil d Instituto de Química. Universidade Federal do Rio de Janeiro, Ilha do Fundão, 21941-909, Rio de Janeiro, Brazil b
ARTICLE INFO
ABSTRACT
Keywords: Hydrotalcite decomposition Nickel oxide Activation energy
The structural changes of nickel-aluminum and nickel-zinc-aluminum hydrotalcites under heating were studied. The temperature and the activation energy required for hydrotalcites decomposition and NiO formation depend on the kind and on the amount of cations. Upon heating, the cell parameters and the crystal sizes changed, aluminum and zinc delaying crystal sintering and zinc leading to the smallest crystals of nickel oxide. The increase of aluminum and/or zinc content decreases the crystals size of hydrotalcites but zinc decreased the specific surface area. The zinc-containing solids produced oxides at lower temperatures while the zinc-free solids were the most thermally stable, possibly due to the increase of a parameter caused by the larger ionic radius of zinc. All solids were mesoporous with interparticles mesopores, whose shape depends on the composition of the solids and the conditions of collapse of the hydrotalcite type structure.
1. Introduction Hydrotalcite-like compounds (HTLc) are anionic clays also referred as layered double hydroxides (LDH). These solids are made of sheets with brucite-like [Mg(OH)2] structures, where Mg2+ cations are located in the center of octahedra and coordinated to six OH groups. The octahedra share sides forming infinite layers placed on each other. The isomorphous partial substitution of Mg2+ by Al3+ ions (giving rise to hydrotalcites) creates a positive charge in the sheets that must be compensated by anions incorporated in interlayer spaces, neutralizing and stabilizing the structure [1]. Hydrotalcites can be represented by the formula [M(II)1-xM(III)x(OH)2](A nx/n )·mH2O, where M(II) is a divalent cation such as Mg2+, Ni2+, Zn2+, Co2+, Mg2+ or Cu2+; M(III) is a trivalent cation such as Al3+, Cr3+ or Fe3+, and A nx/n is an anion like (CO3)2−, (NO3)-, (SO4)2−, (OH)-, Cl-, Br-, I-, (ClO4)-, (WO4)2-, (CrO4)2-, (W7O24)6-, [Fe(C2O4)3]- and [Fe(CN)6]4-, with x values ranging from 0.20 to 0.33 to obtain the hydrotalcite structure [1–6]. These compounds crystallize as rhombohedral systems with R-3 m symmetry. The crystallographic parameters a and c can be calculated from (h k l) planes using X-ray diffraction patterns and the Eqs. 1 and 2 [7,8]. The a parameter (lattice edge) depends on the radius of ions, while the c parameter (interlayer spacing) is related to the size and number of ions, as well as to the orientation and strength of
⁎
electrostatics forces between the interlayer anions and the layers [1]. a = 2d(1 c = d(0
1 0) 0 3)
(1) (2)
Hydrotalcites can be prepared by low cost methods and show chemical properties that depend on the cations and anions in the structure. This allows to obtain solids for different applications in several areas such as catalysis, pharmacy, agriculture, environmental remediation, corrosion prevention and others [9–13]. One of the main advantages of hydrotalcites comes from the easiness of exchanging interlayer anions with other ions in aqueous solutions. In addition, due to the memory effect, these solids are able to reconstruct the HTLc structure with new compensation anions in aqueous solutions, under heating at moderate temperatures. This property can also be used for the efficient removal of anionic contaminants in effluents, such as nitrate, chloride, sulfate, chromate and other anions containing metals, like uranium and manganese [14–18]. By calcining hydrotalcites, solids composed of oxide or mixed oxide nanocrystals can be produced. They generally have high specific surface areas, being promising for catalytic applications like methane and alcohols dry reforming, water gas shift reaction, ammonia decomposition, biodiesel production, carbon dioxide methanation and others [19–29]. Therefore,
Corresponding author. E-mail address: [email protected] (E. Meza-Fuentes).
https://doi.org/10.1016/j.tca.2020.178594 Received 2 September 2019; Received in revised form 10 March 2020; Accepted 15 March 2020 Available online 19 March 2020 0040-6031/ © 2020 Elsevier B.V. All rights reserved.
Thermochimica Acta 687 (2020) 178594
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hydrotalcites have been widely used as catalyst precursors and in previous research [24,25,30], it has been noted that nickel is a promising candidate to replace copper in water gas shift catalysts at low temperatures. Aiming to contribute to an efficient description of the preparation process of these catalysts, the structural and textural changes of Ni-Al-Zn hydrotalcites under heating were studied in this research.
Table 1 Molar ratios for metals and carbonate presents in hydrotalcites-like solids.
2. Experimental 2.1. Samples preparation
Sample
Molar ratio (Ni/ Al)
Molar ratio (Ni/ Zn)
Molar ratio (Al/(CO3)2−)
N0.75A0.25 N0.66A0.33 N0.50Z0.25A0.25 N0.37Z0.37A0.25
2.96 2.16 2.08 1.55
—— —— 2.00 1.06
2.09 2.00 2.09 2.09
(BJH method) were measured by nitrogen adsorption/desorption at -196.15 °C, using a Micromeritics ASAP 2010 instrument. Before analyses, 0.2 g of the sample was heated up to 200 °C under vacuum (2 mmHg) to remove adsorbed volatiles.
Four hydrotalcites were prepared by the low-supersaturation method at constant pH, as described elsewhere [24,25]. For the preparation of hydrotalcites the number of moles of nickel nitrate [Ni (NO3)2·6H2O] was kept constant at 0.05, varying the number of moles of aluminum nitrate [Al(NO3)3·9H2O], zinc nitrate [Zn(NO3)2·6H2O], potassium hydroxide (KOH) and potassium carbonate (K2CO3) to obtain the solids N0.75A0.25 (Ni/Al = 3), N0.66A0.33 (Ni/Al = 2), N0.50 Z0.25A0.25 (Ni/Al = 2 and Ni/Zn = 2) and N0.37 Z0.37A0.25 (Ni/Al = 1.5 and Ni/Zn = 1). In the preparation, calculated amounts of the metal nitrates were dissolved in 200 ml of deionized water and added simultaneously with 200 ml of a second solution containing the potassium carbonate and potassium hydroxide, to a beaker containing 400 ml of deionized water. The mixing process of the two solutions was conducted with a peristaltic pump and performed under vigorous agitation maintaining a pH of 8.3-8.4. The mixture was stirred at 65 °C for additional 2 h and the precipitated solid was filtered, washed with distilled water and dried at 60 °C, for 24 h.
3. Result and discussion From chemical analysis, it was found that the Ni/Al or Zn and Al/ carbonate ratios were close to the expected values from experimental methodology (Table 1). Also, the solids showed X-ray diffraction patterns with well-defined peaks corresponding to an HTLc phase (JCPDS 89-0460), signals assigned to other structure not being detected, as shown in Fig. 1. The main difference among the diffractograms is the shift of the peak related to (1 1 0) plane, caused by the variation of Ni2+ and Zn2+ contents. As seen in Fig. 1, the decrease of nickel content shifted the peak to higher angles whereas the addition of zinc shifted the peak to lower angles, this effect increasing with zinc amount. This result evidences that zinc increases the a parameter, a fact that can be related to its larger ionic radius [1,25]. Upon heating, the hydrotalcites produced different phases, as illustrated by the X-ray diffractograms obtained in 25−500 °C range (Figs. 2 and 3). Both the shape and the position of the peaks were changed during heating. The peak related to the (0 0 3) crystallographic plane was gradually shifted to higher angles and become related to (0 0 l) crystallographic planes between 150−250 °C. This can be assigned to the decrease of c parameter caused by (a) water molecules loss; (b) rearrangement and interaction of carbonate anions between the layers followed by their loss; (c) loss of hydroxyl groups in positive layers and to the (d) formation of a hexagonal structure with the (0 0 1) plane [32,33] at 200 °C (N0.75A0.25, N0.66A0.33, and N0.50 Z0.25A0.25 samples) and at 150 °C (N0.37 Z0.37A0.25 sample). In the case of N0.37Z0.37A0.25 sample, the peaks assigned to (0 0 l) planes were shifted to higher angles at 150 °C, while for the other samples this occurred at 200 °C, as it shows in Figs. 2 and 3. This means
2.2. Characterization techniques Samples were characterized by atomic absorption spectroscopy (AAS), X-ray diffraction (XRD), diffuse reflectance infrared with Fourier transform spectroscopy (DRIFT) and thermogravimetry (TG). In addition, specific surface area and porosity measurements were performed on samples previously calcined at 500 °C to observe the effect of temperature on the textural properties of the solids obtained from hydrotalcites. Elemental chemical analyses were carried out by atomic absorption spectroscopy on a Unicam 969 Solar instrument, on samples previously dissolved in an acid solution. The carbonate content in the solids was determined indirectly through the carbon measurement in LECO Model 200CS apparatus, equipped with an infrared detector. X-ray diffraction patterns were recorded in a Shimadzu XDR-600 instrument using CuKα radiation (λ = 1.54059 Å), generated at 40 kV and 40 mA. The experiments were carried out in situ in the X-ray diffractometer using a heating chamber from room temperature up to 500 °C, using a 0.02° step in the 5–80° (2θ) range. The values of a and c parameters were obtained using Equations 1 and 2, as well as the data obtained from X-ray diffraction patterns by integration of the peaks corresponding to the planes (0 0 3) and (1 1 0) using a Psd Voigt model and Origin 2018 software. Infrared spectra were recorded in situ using a Perkin-Elmer Spectrum One, equipped with a DRIFT chamber, working at 2 cm−1 resolution, in the 4000−400 cm−1 region, from room temperature to 500 °C. The thermogravimetry analyses were performed in a TA Instruments SDT Q600 model, from 30 to 800 °C and heating rates of 2.0, 5.0, 10.0 and 20 °C min−1 under air flow (50 mL·min−1), to determine the activation energy (Ea) related to the collapse of the HTLc structure and the formation of the first particles of nickel oxide (NiO). The activation energy was calculated by the Kissinger method from the slopes of Ln` (β/T2dm) vs 1/Tdm plots, where β is the heating rate and Tdm is the temperature (K) at which the collapse and formation of NiO was recorded [31]. The specific surface area (BET method) and average pore diameter
Fig. 1. X-ray diffraction patterns for the samples based on hydrotalcites containing nickel (N), aluminum (A) and zinc (Z). 2
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Fig. 2. X-ray diffraction patterns for (a) N0.75A0.25, (b) N0.66A0.33, (c) N0.50Z0.25A0.25 and (d) N0.37Z0.37A0.25 samples heated at different temperatures. N = nickel; A = aluminum and Z = zinc.
that the N0.37Z0.37A0.25 sample is less thermally stable than the other samples, in agreement with the increase of a parameter caused by the highest amount of zinc. Therefore, the changes in the shift and shape of peaks in the 25−250 °C range is related to the partial decomposition of hydrotalcites, as a consequence of the loss of interlayer water and decomposition of hydroxyl and carbonate groups. This leads to a decrease of a and c parameters and finally to the collapse of HTLc structure, to give rise to the first crystals of NiO, at 300 °C. The changes of HTLc structure and its collapse under heating can be followed in Fig. 4, which shows the variation of c parameter with temperature. In the case of zinc-free solids, the most significant changes occur between 100 and 200 °C for N0.75A0.25 sample and between 150 and 200 °C for N0.66A0.33 sample. They are related to the contraction of HTLc structure as a consequence of the interlayer water molecules which leave the structure, as well as to the decomposition of carbonate anions and hydroxyl groups. The interlayer spaces decreased 12.0 and
11.6 % for N0.75A0.25 and N0.66A0.33, respectively, from room temperature up to 250 °C. The N0.37Z0.37A0.25 sample was the least thermally stable, in this solid the (0 0 3) peak is not observed at 250 and the c parameter could not be calculated at this temperature. The largest decrease in the c parameter value occurred between 100 and 150 °C, whereas there was no change in this parameter at higher temperatures. For the N0.50Z0.25A0.25 sample, the decrease of c parameter was 13 % and the final value was close to that of N0.66A0.33 sample. This means that the decrease of c parameter with temperature did not depend on carbonate anions amount and on zinc in HTLc structure. In the case of a parameter only minor changes can be observed under heating, as it shows in Fig. 5, making emphasis that the estimated values for this parameter are very close to the step used (0.02°) during the recording of X-ray diffractograms. In the graphics it is evidenced that the largest decrease of this parameter occurred between 200 and 3
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Fig. 3. X-ray diffraction patterns in 7-27 (2θ) range for (a) N0.75A0.25, (b) N0.66A0.33, (c) N0.50Z0.25A0.25 and (d) N0.37Z0.37A0.25 samples heated at different temperatures. N = nickel; A = aluminum and Z = zinc.
Fig. 4. c parameter for the samples as a function of temperature in the 25250 °C range. The values were calculated by Equation 2 using (0 0 3) or (0 0 1) plane. N = nickel; A = aluminum and Z = zinc.
Fig. 5. a parameter for the samples as a function of temperatures in the 25250 °C range. The values were estimated from (1 1 0) plane using Equation 1. N = nickel; A = aluminum and Z = zinc.
250 °C. This is mainly caused by the decomposition of hydroxyl groups and by the migration of species in the layers to form the phases observed at 300 °C. In the case of N0.66A0.33 sample. This parameter changed only at temperatures above 200 °C.
The changes noted for N0.75A0.25, N0.50Z0.25A0.25 and N0.37Z0.37A0.25 samples showed two significant decreases at the same temperature ranges, being more significant for N0.37Z0.37A0.25 sample, which shows 4
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Fig. 7. DRIFT spectra for the N0.75A0.25 sample at different temperatures. N = nickel and A = aluminum.
Fig. 6. Crystals size of nickel oxide for solids heated in the 300-500 °C range. The values were calculated the Scherrer equation from (2 0 0) plane. N = nickel; A = aluminum and Z = zinc.
that the transformations of HTLc phase occurred more drastically in this solid, probably by the faster decomposition of hydroxyl groups to produce small crystals of nickel oxide, a fact that is favored by the highest zinc content in this sample. These changes of hydrotalcite lattice parameters under heating are related to chemical processes during which carbonate anions and the positive layers were decomposed, giving rise to the first crystals of nickel oxide (NiO) for all materials and of zinc oxide (ZnO) for N0.50Z0.25A0.25 and N0.37Z0.37A0.25 samples, as detected from X-ray diffraction. It is also expected that amorphous aluminum oxides were produced [24]. Fig. 6 shows the crystal sizes for nickel oxide as a function of temperature. It can be noted that nickel oxide was produced for all samples at 300 °C. As expected, the crystals size increased with temperature, as a consequence of crystals sintering. The biggest crystals were found for the nickel-richest sample (N0.75A0.25) in which aluminum seems not to be efficient in avoiding sintering. The easiness of nickel oxide for sintering is well-known and reported by several authors [34–39]. However, the increase of aluminum content and concomitant decrease of nickel (N0.66A0.33 sample) led to a decrease of sintering, in accordance with the role of aluminum in avoiding sintering [24,34]. By adding zinc to the samples and decreasing the nickel amount, the NiO sintering process was less pronounced until 450 °C for N0.50Z0.25A0.25 and N0.37Z0.37A0.25 solids, with a strong increase of this phenomenon at 500 °C in the case of N0.37Z0.37A0.25 solid. The smallest crystals were produced for the sample with the highest amount of zinc and the lowest amount of nickel (N0.37Z0.37A0.25 sample). The thermal behavior of the samples containing nickel and aluminum, monitored by infrared spectroscopy confirmed the presence of carbonate anions, water molecules and hydroxyl groups, as illustrated in Figs. 7 and 8. The broad band observed at 3400−3600 cm−1 is characteristic of ν(OeH) stretching vibration of water molecules and of hydroxyl groups at the positive layers [40,41]. This band decreased with temperature increase because of the loss of water molecules at low temperatures, which was followed by decomposition of hydroxyl groups in the layers, mainly above 300 °C. The N0.75A0.25 sample showed a band at 2972 cm−1 which can be associated to aluminum hydroxides, such as gibbsite, boehmite or diaspore [36]. The δ(H··O··H) bending mode generates a band at 1650 cm−1 related to physisorbed water and at 1750 cm−1 for interlayer water [6,40–44]. The first absorption is observed in the spectra at room temperature but this band disappears at 100 °C. For the N0.75A0.25 and N0.66A0.33 samples, it is related to interlayer water and appears between 25−300 °C. Above 100 °C, the ν3 (1365 cm−1) band, associated to carbonate, splitted into two other ones at 1375−1524 cm−1 (N0.75A0.25) and at
Fig. 8. DRIFT spectra for the N0.66A0.33 sample at different temperatures. N = nickel and A = aluminum.
1360−1524 cm−1 (N0.66A0.33). This indicates a change in the carbonate symmetry upon dehydration of hydrotalcite and a decrease of the distance between the positive layers [6,40–44]. The band at 1530 cm−1, generated by the interaction of carbonate anions with hydroxyl groups between the layers [(CO3)2-···OH], was observed between 100 and 350 °C, due to the decrease of the interlayer distance. However, the band to 1365 cm−1 was observed up to 450 °C, a fact that can be related to carbonate species that remained occluded in the solid after the structure collapse. The bands at 810 cm−1 and in the 350−450 °C range are attributed to the Al-O stretching, present in transition aluminas [44]. The thermogravimetry curves of hydrotalcites showed two stages in which the weight loss decreased, as shown in Fig. 9. The first stage is related to the structural collapse caused by the loss of water from interlayers and to the partial decomposition of hydroxyl groups and of carbonate anions [1,5,44,45]. For this process the N0.66A0.33 sample showed the highest temperature (250 °C) at which the collapse of the HTLc structure begins, followed by the N0.75A0.25 sample (245 °C). These findings agree with the results of XRD, which showed the partial decomposition HTLc structure at 250 °C for both samples. In zinc-containing materials, it is observed that the first stage of mass loss occurred at temperatures lower than those exhibited by zinc-free solids [6]. The least stable solid is the N0.37Z0.37A0.25 sample, which collapsed at 192 °C, followed by the N0.50Z0.25A0.25 sample (220 °C), confirming the XRD results. After the first weight loss, a second stage was observed, which lasted up to temperatures above 350 °C. This stage is related to the final 5
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decomposition of the HTLc structure, producing nickel and zinc oxides. The weight loss at higher temperatures is related to the formation of aluminum oxide and to the exit of the reminiscent molecules that have been occluded in the solids during the structure collapse. Table 2 shows the activation energy for the collapse of structure and for the nickel oxide formation. It can be noted that the N0.75A0.25 sample showed the lowest activation energy for the collapse of the HTLc structure, explaining why the process was the fastest one, occurring in the shortest range of temperature (108 °C). At the collapse temperature, the solid showed higher value of c parameter than N0.66A0.33 and N0.50Z0.25A0.25 samples. This facilitates the exit of water molecules from interlayer spaces and produced by the decomposition of hydroxide groups, as well as of carbon dioxide produced by initial decomposition of carbonate anions. Therefore, nickel oxide was produced at the lowest temperature (353 °C) requiring the lowest value of activation energy (108 kJ·mol−1). The decomposition of N0.66A0.33 and N0.50Z0.25A0.25 samples was slower (125 °C and 140 °C, respectively) because the activation energies were higher (78 and 79 kJ·mol−1). The c parameters were very close as well as the activation energies. The production of nickel oxide occurred at 375 and 360 °C, for N0.66A0.33 and N0.50Z0.25A0.25 samples, respectively, being facilitated by the smaller crystals of zinc-containing solid, which showed a lower activation energy (149 kJ·mol−1) than the other sample (157 kJ·mol−1). The N0.37Z0.37A0.25 sample collapsed at 192 °C, requiring an activation energy of 67 kJ·mol−1. As concluded by XRD, the lowest stability of this solid is related to the increase of a parameter as consequence of its highest amount of zinc. As consequence, nickel oxide was produced at low temperature (358 °C) requiring low activation energy (124 kJ·mol−1). The effect of heating on textural properties of the final solids was studied by adsorption/desorption isotherms for samples calcined at 500 °C. It was noted that the zinc-free solids (N0.75A0.25 and N0.66A0.33 samples; Sg = 154 and 200 m2/g, respectively) showed higher specific
Fig. 9. Thermogravimetry curves for hydrotalcite-like solids. N = nickel; A = aluminum and Z = zinc. Table 2 . Activation energy (Ea) associated to collapse of HTLc structure and NiO crystals formation. N = nickel; A = aluminum and Z = zinc. Sample
Ea of structure collapse (kJ· mol−1)
Ea of NiO formation (kJ· mol −1 )
N0.75A0.25 N0.66A0.33 N0.50Z0.25A0.25 N0.37Z0.37A0.25
56 78 79 67
108 157 149 124
Fig. 10. Nitrogen adsorption/desorption isotherms for calcined samples. N = nickel; A = aluminum and Z = zinc. 6
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Fig. 11. Pore size distribution for calcined samples. N = nickel; A = aluminum and Z = zinc.
Fig. 12. Transformations that occur during heating of hydrotalcite-like materials. 7
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surface areas than those based on zinc (N0.50Z0.25A0.25 and N0.37Z0.37A0.25 sample; Sg = 112 and 127 m2/g, respectively). The adsorption-desorption isotherms (Fig. 10) for N0.75A0.25, N0.66A0.33 and N0.50Z0.25A0.25 samples are typical of mesoporous solids. The hysteresis loop observed at high values of relative pressures indicates interparticle mesopores. Additionally, the N0.50Z0.25A0.25 sample showed type H1 hysteresis, which is typical of materials with porous formed by agglomerates of uniform spheres with a regular arrangement that originate an average pore diameter of 4.45 nm in this solid. For N0.75A0.25 (4.45 nm; pore diameter) and N0.66A0.33 (4.17 nm; pore diameter) samples, the H2 type hysteresis loops were noted, that are typical of systems of interconnected pores or of ink-bottle type pores [46,47]. These materials additionally presented a narrower and more symmetrical distribution of pores than that recorded by zinc-containing materials, as shown in Fig. 11. Additionally, is not observed an important contribution of pores with diameters greater than 7 nm. The N0.37Z0.37A0.25 sample showed a typical adsorption-desorption isotherm of solids with slit-shaped interparticle mesopores, which are formed by the irregular overlapping of particles in the form of plates [46]. This due probably to collapse of hydrotalcite structure at low temperatures, hindering the agglomeration of oxide particles formed in the calcination process, which led to the formation of interparticles pores with larger diameter (10.2 nm). This solid registered the widest distribution of pores, with diameters greater than 50 nm. Fig. 12 shows a hypothetical scheme for the transformations that is believed to occur for the solids upon heating. Initially (25−120 °C), the loss of interlayers water leads to the decrease of c parameter. This process is accelerated at higher temperatures (150−200 °C) where the loss of carbonate and hydroxyl groups begins, favored by the approximation of the positive layers. The HTLc structure collapses due to the decrease of cell parameters and then the first nickel oxide and zinc oxide particles appear and subsequently grow with increasing temperature, giving rise to particles with different sizes. For the solids containing nickel and aluminum the particle agglomeration favored the formation of interconnected pores, while the addition of zinc led to a material with spherical particles that originate capillary pores (N0.50Z0.25A0.25) and a material made of agglomerate plate-like particles and slit-shaped pores (N0.37Z0.37A0.25).
Drafting the manuscript: E.Meza-Fuentes, M.C. Rangel, A. Faro, J. Rodriguez-Ruiz, C. Solano-Polo. revising the manuscript critically for important intellectual content E.Meza-Fuentes, M.C. Rangel, A. Faro, J. Rodriguez-Ruiz Category 3 Approval of the version of the manuscript to be published (the names of all authors must be listed): E.Meza-Fuentes, M.C. Rangel, A. Faro, J. Rodriguez-Ruiz, C. SolanoPolo. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements EMF, JRR, CSP, thank the University of Cartagena and SENA for financial support. The authors acknowledge the financial support of CNPq, FINEP, PETROBRAS and SENA. References [1] F. Cavani, F. Trifiro, A. Vaccari, Hydrotalcite-type anionic clays: preparation, properties and application, Catal. Today 11 (1991) 173–301, https://doi.org/10. 1016/0920-5861(91)80068-K. [2] E.L. Crepaldi, J. Barros Valim, Hidróxidos duplos lamelares: síntese, estrutura, propriedades e aplicações, Quím, Nova 21 (1998) 300–311, https://doi.org/10. 1590/S0100-40421998000300011. [3] G. Brown, M.C. Gastuche, Mixed magnesium-aluminium hydroxides. II. structure and structural chemistry of synthetic hydroxycarbonates and related minerals and compounds, Clay Miner. 7 (1967) 193–201, https://doi.org/10.1180/claymin. 1967.007.2.06. [4] M. Crivello, C. Pérez, E. Herrero, G. Ghione, S. Casuscelli, E. Rodríguez-Castellón, Characterization of Al–Cu and Al–Cu–Mg mixed oxides and their catalytic activity in dehydrogenation of 2-octanol, Catal. Today 107-108 (2005) 215–222, https:// doi.org/10.1016/j.cattod.2005.07.168. [5] J. Rodriguez-Ruiz, A. Pajaro-Payares, E. Meza-Fuentes, Síntesis y caracterización estructural de hidrotalcitas de Cu-Zn-Al, Rev. Colomb. Quím 45 (2016) 33–38, https://doi.org/10.15446/rev.colomb.quim.v45n3.61381. [6] E. Meza-Fuentes, J. Rodriguez-Ruiz, M. Rangel, Characteristics of NiO present in solids obtained from hydrotalcites based on Ni/Al and Ni-Zn/Al, DYNA 86 (2019) 58–65, https://doi.org/10.15446/dyna.v86n210.78559. [7] V. Rives, S. Kannan, Layered double hydroxides with the hydrotalcite-type structure containing Cu2+, Ni2+ and Al3+, J. Mater. Chem. 10 (2000) 489–495, https://doi. org/10.1039/A908534C. [8] J. Pérez-Ramirez, G. Mul, J.A. Moulijn, In situ Fourier Transform infrared and laser Raman spectroscopy study of the thermal decomposition of Co-Al and Ni-Al hydrotalcites, Vib. Spectrosc. 27 (2001) 75–88, https://doi.org/10.1016/S09242031(01)00119-9. [9] K. Goh, T. Lim, Z. Dong, Application of layered double hydroxides for removal of oxyanions: a review, Water Res. 42 (2008) 1343–1368, https://doi.org/10.1016/j. watres.2007.10.043. [10] G. Lee, J. Kang, N. Yan, Y. Suh, J. Jung, Simple preparation method for Mg-Al hydrotalcites as base catalysts, J. Mol. Catal. A Chem. 423 (2016) 347–355, https:// doi.org/10.1016/j.molcata.2016.07.018. [11] N. Duong, T. Hang, A. Nicolay, Y. Paint, Corrosion protection of carbon steel by solvent free epoxy coating containing hydrotalcites intercalated with different organic corrosion inhibitors, Prog. Org. Coat 101 (2016) 331–341, https://doi.org/ 10.1016/j.porgcoat.2016.08.021. [12] A. Vanaamudan, B. Chavada, P. Padmaja, Adsorption of reactive blue 21 and reactive red 141 from aqueous solutions onto hydrotalcite, J. Environ. Chem. Eng. 4 (2016) 2617–2627, https://doi.org/10.1016/j.jece.2016.04.039. [13] L. Deng, Z. Shi, X. Peng, S. Zhou, Magnetic calcinated cobalt ferrite/magnesium aluminium hydrotalcite composite for enhanced adsorption of methyl orange, J. Alloy, Compd 688 (2016) 101, https://doi.org/10.1016/j.jallcom.2016.06.227. [14] M. Sanches-Cantu, M. Hernandez-Torres, A. Castillo-Navarro, E. Cadena-Torres, E. Rubio-Rosas, J. Gracia-Jimenez, F. Tzompantzi, Evaluation of hydrotalcite-like compounds with distinc interlaminar anions as catalysts precursors in methylene blue photodegradation, Appl. Clay Sci. 135 (2017) 1–8, https://doi.org/10.1016/j. clay.2016.08.028. [15] S. Eiby, D. Tobler, S. Nedel, A. Bischoff, B. Christiansen, A. Hansen, Competition between chloride and sulphate during the reformation of calcined hydrotalcite, Appl. Clay Sci. 132-133 (2016) 650–659, https://doi.org/10.1016/j.clay.2016.08. 017. [16] M. Kato, M. Azimi, S. Fayaz, M. Shah, M. Hoque, N. Hamajima, S. Ohnuma, T. Ohtsuka, M. Maeda, M. Yoshinaga, Uranium in well drinking water of Kabul,
4. Conclusions Nickel-aluminum and nickel-zinc-aluminum based hydrotalcites go on structural and textural changes upon heating that include the exit of water and decomposition of carbonate and hydroxide groups. These processes lead to the decrease of cell parameters and then to structure collapse, producing nickel oxide, zinc oxide and aluminum oxide. The zinc-free solids were the most thermally stable while the addition and the increase of zinc favors the decomposition. This was assigned to the increase of a parameter caused by the big ionic radius of zinc. The crystals size also changed during heating, aluminum and zinc delaying crystal sintering and zinc leading to the smallest crystals of nickel oxide. The increase of aluminum and/or zinc decrease the crystals size but zinc decreased the specific surface area of calcined solids. All solids have interparticles mesopores with pore shapes that depend on the composition of the materials and of the collapse of hydrotalcite type structure. Authorship contributions Category 1 Conception and design of study: E.Meza-Fuentes, M.C. Rangel, A. Faro, J. Rodriguez-Ruiz; acquisition of data: E.Meza-Fuentes, J. Rodriguez-Ruiz, C. SolanoPolo. analysis and/or interpretation of data: E.Meza-Fuentes, M.C. Rangel, A. Faro, J. Rodriguez-Ruiz, C. Solano-Polo. Category 2 8
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Monitoring the structural and textural changes of Ni-Zn-Al hydrotalcites under heating
T
E. Meza-Fuentesa,*, J. Rodriguez-Ruizb, C. Solano-Poloa,b, M.C. Rangelc, A. Farod a
Grupo de Estudios en Materiales y Combustibles, Campus San Pablo, Universidad de Cartagena, Cartagena, Colombia Grupo de Investigación en Procesos de la Industria Petroquímica, SENA-Centro para la Industria Petroquímica, Cartagena, Colombia c Instituto de Química, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, Agronomia, 90650-001 Porto Alegre, RS, Brazil d Instituto de Química. Universidade Federal do Rio de Janeiro, Ilha do Fundão, 21941-909, Rio de Janeiro, Brazil b
ARTICLE INFO
ABSTRACT
Keywords: Hydrotalcite decomposition Nickel oxide Activation energy
The structural changes of nickel-aluminum and nickel-zinc-aluminum hydrotalcites under heating were studied. The temperature and the activation energy required for hydrotalcites decomposition and NiO formation depend on the kind and on the amount of cations. Upon heating, the cell parameters and the crystal sizes changed, aluminum and zinc delaying crystal sintering and zinc leading to the smallest crystals of nickel oxide. The increase of aluminum and/or zinc content decreases the crystals size of hydrotalcites but zinc decreased the specific surface area. The zinc-containing solids produced oxides at lower temperatures while the zinc-free solids were the most thermally stable, possibly due to the increase of a parameter caused by the larger ionic radius of zinc. All solids were mesoporous with interparticles mesopores, whose shape depends on the composition of the solids and the conditions of collapse of the hydrotalcite type structure.
1. Introduction Hydrotalcite-like compounds (HTLc) are anionic clays also referred as layered double hydroxides (LDH). These solids are made of sheets with brucite-like [Mg(OH)2] structures, where Mg2+ cations are located in the center of octahedra and coordinated to six OH groups. The octahedra share sides forming infinite layers placed on each other. The isomorphous partial substitution of Mg2+ by Al3+ ions (giving rise to hydrotalcites) creates a positive charge in the sheets that must be compensated by anions incorporated in interlayer spaces, neutralizing and stabilizing the structure [1]. Hydrotalcites can be represented by the formula [M(II)1-xM(III)x(OH)2](A nx/n )·mH2O, where M(II) is a divalent cation such as Mg2+, Ni2+, Zn2+, Co2+, Mg2+ or Cu2+; M(III) is a trivalent cation such as Al3+, Cr3+ or Fe3+, and A nx/n is an anion like (CO3)2−, (NO3)-, (SO4)2−, (OH)-, Cl-, Br-, I-, (ClO4)-, (WO4)2-, (CrO4)2-, (W7O24)6-, [Fe(C2O4)3]- and [Fe(CN)6]4-, with x values ranging from 0.20 to 0.33 to obtain the hydrotalcite structure [1–6]. These compounds crystallize as rhombohedral systems with R-3 m symmetry. The crystallographic parameters a and c can be calculated from (h k l) planes using X-ray diffraction patterns and the Eqs. 1 and 2 [7,8]. The a parameter (lattice edge) depends on the radius of ions, while the c parameter (interlayer spacing) is related to the size and number of ions, as well as to the orientation and strength of
⁎
electrostatics forces between the interlayer anions and the layers [1]. a = 2d(1 c = d(0
1 0) 0 3)
(1) (2)
Hydrotalcites can be prepared by low cost methods and show chemical properties that depend on the cations and anions in the structure. This allows to obtain solids for different applications in several areas such as catalysis, pharmacy, agriculture, environmental remediation, corrosion prevention and others [9–13]. One of the main advantages of hydrotalcites comes from the easiness of exchanging interlayer anions with other ions in aqueous solutions. In addition, due to the memory effect, these solids are able to reconstruct the HTLc structure with new compensation anions in aqueous solutions, under heating at moderate temperatures. This property can also be used for the efficient removal of anionic contaminants in effluents, such as nitrate, chloride, sulfate, chromate and other anions containing metals, like uranium and manganese [14–18]. By calcining hydrotalcites, solids composed of oxide or mixed oxide nanocrystals can be produced. They generally have high specific surface areas, being promising for catalytic applications like methane and alcohols dry reforming, water gas shift reaction, ammonia decomposition, biodiesel production, carbon dioxide methanation and others [19–29]. Therefore,
Corresponding author. E-mail address: [email protected] (E. Meza-Fuentes).
https://doi.org/10.1016/j.tca.2020.178594 Received 2 September 2019; Received in revised form 10 March 2020; Accepted 15 March 2020 Available online 19 March 2020 0040-6031/ © 2020 Elsevier B.V. All rights reserved.
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hydrotalcites have been widely used as catalyst precursors and in previous research [24,25,30], it has been noted that nickel is a promising candidate to replace copper in water gas shift catalysts at low temperatures. Aiming to contribute to an efficient description of the preparation process of these catalysts, the structural and textural changes of Ni-Al-Zn hydrotalcites under heating were studied in this research.
Table 1 Molar ratios for metals and carbonate presents in hydrotalcites-like solids.
2. Experimental 2.1. Samples preparation
Sample
Molar ratio (Ni/ Al)
Molar ratio (Ni/ Zn)
Molar ratio (Al/(CO3)2−)
N0.75A0.25 N0.66A0.33 N0.50Z0.25A0.25 N0.37Z0.37A0.25
2.96 2.16 2.08 1.55
—— —— 2.00 1.06
2.09 2.00 2.09 2.09
(BJH method) were measured by nitrogen adsorption/desorption at -196.15 °C, using a Micromeritics ASAP 2010 instrument. Before analyses, 0.2 g of the sample was heated up to 200 °C under vacuum (2 mmHg) to remove adsorbed volatiles.
Four hydrotalcites were prepared by the low-supersaturation method at constant pH, as described elsewhere [24,25]. For the preparation of hydrotalcites the number of moles of nickel nitrate [Ni (NO3)2·6H2O] was kept constant at 0.05, varying the number of moles of aluminum nitrate [Al(NO3)3·9H2O], zinc nitrate [Zn(NO3)2·6H2O], potassium hydroxide (KOH) and potassium carbonate (K2CO3) to obtain the solids N0.75A0.25 (Ni/Al = 3), N0.66A0.33 (Ni/Al = 2), N0.50 Z0.25A0.25 (Ni/Al = 2 and Ni/Zn = 2) and N0.37 Z0.37A0.25 (Ni/Al = 1.5 and Ni/Zn = 1). In the preparation, calculated amounts of the metal nitrates were dissolved in 200 ml of deionized water and added simultaneously with 200 ml of a second solution containing the potassium carbonate and potassium hydroxide, to a beaker containing 400 ml of deionized water. The mixing process of the two solutions was conducted with a peristaltic pump and performed under vigorous agitation maintaining a pH of 8.3-8.4. The mixture was stirred at 65 °C for additional 2 h and the precipitated solid was filtered, washed with distilled water and dried at 60 °C, for 24 h.
3. Result and discussion From chemical analysis, it was found that the Ni/Al or Zn and Al/ carbonate ratios were close to the expected values from experimental methodology (Table 1). Also, the solids showed X-ray diffraction patterns with well-defined peaks corresponding to an HTLc phase (JCPDS 89-0460), signals assigned to other structure not being detected, as shown in Fig. 1. The main difference among the diffractograms is the shift of the peak related to (1 1 0) plane, caused by the variation of Ni2+ and Zn2+ contents. As seen in Fig. 1, the decrease of nickel content shifted the peak to higher angles whereas the addition of zinc shifted the peak to lower angles, this effect increasing with zinc amount. This result evidences that zinc increases the a parameter, a fact that can be related to its larger ionic radius [1,25]. Upon heating, the hydrotalcites produced different phases, as illustrated by the X-ray diffractograms obtained in 25−500 °C range (Figs. 2 and 3). Both the shape and the position of the peaks were changed during heating. The peak related to the (0 0 3) crystallographic plane was gradually shifted to higher angles and become related to (0 0 l) crystallographic planes between 150−250 °C. This can be assigned to the decrease of c parameter caused by (a) water molecules loss; (b) rearrangement and interaction of carbonate anions between the layers followed by their loss; (c) loss of hydroxyl groups in positive layers and to the (d) formation of a hexagonal structure with the (0 0 1) plane [32,33] at 200 °C (N0.75A0.25, N0.66A0.33, and N0.50 Z0.25A0.25 samples) and at 150 °C (N0.37 Z0.37A0.25 sample). In the case of N0.37Z0.37A0.25 sample, the peaks assigned to (0 0 l) planes were shifted to higher angles at 150 °C, while for the other samples this occurred at 200 °C, as it shows in Figs. 2 and 3. This means
2.2. Characterization techniques Samples were characterized by atomic absorption spectroscopy (AAS), X-ray diffraction (XRD), diffuse reflectance infrared with Fourier transform spectroscopy (DRIFT) and thermogravimetry (TG). In addition, specific surface area and porosity measurements were performed on samples previously calcined at 500 °C to observe the effect of temperature on the textural properties of the solids obtained from hydrotalcites. Elemental chemical analyses were carried out by atomic absorption spectroscopy on a Unicam 969 Solar instrument, on samples previously dissolved in an acid solution. The carbonate content in the solids was determined indirectly through the carbon measurement in LECO Model 200CS apparatus, equipped with an infrared detector. X-ray diffraction patterns were recorded in a Shimadzu XDR-600 instrument using CuKα radiation (λ = 1.54059 Å), generated at 40 kV and 40 mA. The experiments were carried out in situ in the X-ray diffractometer using a heating chamber from room temperature up to 500 °C, using a 0.02° step in the 5–80° (2θ) range. The values of a and c parameters were obtained using Equations 1 and 2, as well as the data obtained from X-ray diffraction patterns by integration of the peaks corresponding to the planes (0 0 3) and (1 1 0) using a Psd Voigt model and Origin 2018 software. Infrared spectra were recorded in situ using a Perkin-Elmer Spectrum One, equipped with a DRIFT chamber, working at 2 cm−1 resolution, in the 4000−400 cm−1 region, from room temperature to 500 °C. The thermogravimetry analyses were performed in a TA Instruments SDT Q600 model, from 30 to 800 °C and heating rates of 2.0, 5.0, 10.0 and 20 °C min−1 under air flow (50 mL·min−1), to determine the activation energy (Ea) related to the collapse of the HTLc structure and the formation of the first particles of nickel oxide (NiO). The activation energy was calculated by the Kissinger method from the slopes of Ln` (β/T2dm) vs 1/Tdm plots, where β is the heating rate and Tdm is the temperature (K) at which the collapse and formation of NiO was recorded [31]. The specific surface area (BET method) and average pore diameter
Fig. 1. X-ray diffraction patterns for the samples based on hydrotalcites containing nickel (N), aluminum (A) and zinc (Z). 2
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Fig. 2. X-ray diffraction patterns for (a) N0.75A0.25, (b) N0.66A0.33, (c) N0.50Z0.25A0.25 and (d) N0.37Z0.37A0.25 samples heated at different temperatures. N = nickel; A = aluminum and Z = zinc.
that the N0.37Z0.37A0.25 sample is less thermally stable than the other samples, in agreement with the increase of a parameter caused by the highest amount of zinc. Therefore, the changes in the shift and shape of peaks in the 25−250 °C range is related to the partial decomposition of hydrotalcites, as a consequence of the loss of interlayer water and decomposition of hydroxyl and carbonate groups. This leads to a decrease of a and c parameters and finally to the collapse of HTLc structure, to give rise to the first crystals of NiO, at 300 °C. The changes of HTLc structure and its collapse under heating can be followed in Fig. 4, which shows the variation of c parameter with temperature. In the case of zinc-free solids, the most significant changes occur between 100 and 200 °C for N0.75A0.25 sample and between 150 and 200 °C for N0.66A0.33 sample. They are related to the contraction of HTLc structure as a consequence of the interlayer water molecules which leave the structure, as well as to the decomposition of carbonate anions and hydroxyl groups. The interlayer spaces decreased 12.0 and
11.6 % for N0.75A0.25 and N0.66A0.33, respectively, from room temperature up to 250 °C. The N0.37Z0.37A0.25 sample was the least thermally stable, in this solid the (0 0 3) peak is not observed at 250 and the c parameter could not be calculated at this temperature. The largest decrease in the c parameter value occurred between 100 and 150 °C, whereas there was no change in this parameter at higher temperatures. For the N0.50Z0.25A0.25 sample, the decrease of c parameter was 13 % and the final value was close to that of N0.66A0.33 sample. This means that the decrease of c parameter with temperature did not depend on carbonate anions amount and on zinc in HTLc structure. In the case of a parameter only minor changes can be observed under heating, as it shows in Fig. 5, making emphasis that the estimated values for this parameter are very close to the step used (0.02°) during the recording of X-ray diffractograms. In the graphics it is evidenced that the largest decrease of this parameter occurred between 200 and 3
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Fig. 3. X-ray diffraction patterns in 7-27 (2θ) range for (a) N0.75A0.25, (b) N0.66A0.33, (c) N0.50Z0.25A0.25 and (d) N0.37Z0.37A0.25 samples heated at different temperatures. N = nickel; A = aluminum and Z = zinc.
Fig. 4. c parameter for the samples as a function of temperature in the 25250 °C range. The values were calculated by Equation 2 using (0 0 3) or (0 0 1) plane. N = nickel; A = aluminum and Z = zinc.
Fig. 5. a parameter for the samples as a function of temperatures in the 25250 °C range. The values were estimated from (1 1 0) plane using Equation 1. N = nickel; A = aluminum and Z = zinc.
250 °C. This is mainly caused by the decomposition of hydroxyl groups and by the migration of species in the layers to form the phases observed at 300 °C. In the case of N0.66A0.33 sample. This parameter changed only at temperatures above 200 °C.
The changes noted for N0.75A0.25, N0.50Z0.25A0.25 and N0.37Z0.37A0.25 samples showed two significant decreases at the same temperature ranges, being more significant for N0.37Z0.37A0.25 sample, which shows 4
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Fig. 7. DRIFT spectra for the N0.75A0.25 sample at different temperatures. N = nickel and A = aluminum.
Fig. 6. Crystals size of nickel oxide for solids heated in the 300-500 °C range. The values were calculated the Scherrer equation from (2 0 0) plane. N = nickel; A = aluminum and Z = zinc.
that the transformations of HTLc phase occurred more drastically in this solid, probably by the faster decomposition of hydroxyl groups to produce small crystals of nickel oxide, a fact that is favored by the highest zinc content in this sample. These changes of hydrotalcite lattice parameters under heating are related to chemical processes during which carbonate anions and the positive layers were decomposed, giving rise to the first crystals of nickel oxide (NiO) for all materials and of zinc oxide (ZnO) for N0.50Z0.25A0.25 and N0.37Z0.37A0.25 samples, as detected from X-ray diffraction. It is also expected that amorphous aluminum oxides were produced [24]. Fig. 6 shows the crystal sizes for nickel oxide as a function of temperature. It can be noted that nickel oxide was produced for all samples at 300 °C. As expected, the crystals size increased with temperature, as a consequence of crystals sintering. The biggest crystals were found for the nickel-richest sample (N0.75A0.25) in which aluminum seems not to be efficient in avoiding sintering. The easiness of nickel oxide for sintering is well-known and reported by several authors [34–39]. However, the increase of aluminum content and concomitant decrease of nickel (N0.66A0.33 sample) led to a decrease of sintering, in accordance with the role of aluminum in avoiding sintering [24,34]. By adding zinc to the samples and decreasing the nickel amount, the NiO sintering process was less pronounced until 450 °C for N0.50Z0.25A0.25 and N0.37Z0.37A0.25 solids, with a strong increase of this phenomenon at 500 °C in the case of N0.37Z0.37A0.25 solid. The smallest crystals were produced for the sample with the highest amount of zinc and the lowest amount of nickel (N0.37Z0.37A0.25 sample). The thermal behavior of the samples containing nickel and aluminum, monitored by infrared spectroscopy confirmed the presence of carbonate anions, water molecules and hydroxyl groups, as illustrated in Figs. 7 and 8. The broad band observed at 3400−3600 cm−1 is characteristic of ν(OeH) stretching vibration of water molecules and of hydroxyl groups at the positive layers [40,41]. This band decreased with temperature increase because of the loss of water molecules at low temperatures, which was followed by decomposition of hydroxyl groups in the layers, mainly above 300 °C. The N0.75A0.25 sample showed a band at 2972 cm−1 which can be associated to aluminum hydroxides, such as gibbsite, boehmite or diaspore [36]. The δ(H··O··H) bending mode generates a band at 1650 cm−1 related to physisorbed water and at 1750 cm−1 for interlayer water [6,40–44]. The first absorption is observed in the spectra at room temperature but this band disappears at 100 °C. For the N0.75A0.25 and N0.66A0.33 samples, it is related to interlayer water and appears between 25−300 °C. Above 100 °C, the ν3 (1365 cm−1) band, associated to carbonate, splitted into two other ones at 1375−1524 cm−1 (N0.75A0.25) and at
Fig. 8. DRIFT spectra for the N0.66A0.33 sample at different temperatures. N = nickel and A = aluminum.
1360−1524 cm−1 (N0.66A0.33). This indicates a change in the carbonate symmetry upon dehydration of hydrotalcite and a decrease of the distance between the positive layers [6,40–44]. The band at 1530 cm−1, generated by the interaction of carbonate anions with hydroxyl groups between the layers [(CO3)2-···OH], was observed between 100 and 350 °C, due to the decrease of the interlayer distance. However, the band to 1365 cm−1 was observed up to 450 °C, a fact that can be related to carbonate species that remained occluded in the solid after the structure collapse. The bands at 810 cm−1 and in the 350−450 °C range are attributed to the Al-O stretching, present in transition aluminas [44]. The thermogravimetry curves of hydrotalcites showed two stages in which the weight loss decreased, as shown in Fig. 9. The first stage is related to the structural collapse caused by the loss of water from interlayers and to the partial decomposition of hydroxyl groups and of carbonate anions [1,5,44,45]. For this process the N0.66A0.33 sample showed the highest temperature (250 °C) at which the collapse of the HTLc structure begins, followed by the N0.75A0.25 sample (245 °C). These findings agree with the results of XRD, which showed the partial decomposition HTLc structure at 250 °C for both samples. In zinc-containing materials, it is observed that the first stage of mass loss occurred at temperatures lower than those exhibited by zinc-free solids [6]. The least stable solid is the N0.37Z0.37A0.25 sample, which collapsed at 192 °C, followed by the N0.50Z0.25A0.25 sample (220 °C), confirming the XRD results. After the first weight loss, a second stage was observed, which lasted up to temperatures above 350 °C. This stage is related to the final 5
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decomposition of the HTLc structure, producing nickel and zinc oxides. The weight loss at higher temperatures is related to the formation of aluminum oxide and to the exit of the reminiscent molecules that have been occluded in the solids during the structure collapse. Table 2 shows the activation energy for the collapse of structure and for the nickel oxide formation. It can be noted that the N0.75A0.25 sample showed the lowest activation energy for the collapse of the HTLc structure, explaining why the process was the fastest one, occurring in the shortest range of temperature (108 °C). At the collapse temperature, the solid showed higher value of c parameter than N0.66A0.33 and N0.50Z0.25A0.25 samples. This facilitates the exit of water molecules from interlayer spaces and produced by the decomposition of hydroxide groups, as well as of carbon dioxide produced by initial decomposition of carbonate anions. Therefore, nickel oxide was produced at the lowest temperature (353 °C) requiring the lowest value of activation energy (108 kJ·mol−1). The decomposition of N0.66A0.33 and N0.50Z0.25A0.25 samples was slower (125 °C and 140 °C, respectively) because the activation energies were higher (78 and 79 kJ·mol−1). The c parameters were very close as well as the activation energies. The production of nickel oxide occurred at 375 and 360 °C, for N0.66A0.33 and N0.50Z0.25A0.25 samples, respectively, being facilitated by the smaller crystals of zinc-containing solid, which showed a lower activation energy (149 kJ·mol−1) than the other sample (157 kJ·mol−1). The N0.37Z0.37A0.25 sample collapsed at 192 °C, requiring an activation energy of 67 kJ·mol−1. As concluded by XRD, the lowest stability of this solid is related to the increase of a parameter as consequence of its highest amount of zinc. As consequence, nickel oxide was produced at low temperature (358 °C) requiring low activation energy (124 kJ·mol−1). The effect of heating on textural properties of the final solids was studied by adsorption/desorption isotherms for samples calcined at 500 °C. It was noted that the zinc-free solids (N0.75A0.25 and N0.66A0.33 samples; Sg = 154 and 200 m2/g, respectively) showed higher specific
Fig. 9. Thermogravimetry curves for hydrotalcite-like solids. N = nickel; A = aluminum and Z = zinc. Table 2 . Activation energy (Ea) associated to collapse of HTLc structure and NiO crystals formation. N = nickel; A = aluminum and Z = zinc. Sample
Ea of structure collapse (kJ· mol−1)
Ea of NiO formation (kJ· mol −1 )
N0.75A0.25 N0.66A0.33 N0.50Z0.25A0.25 N0.37Z0.37A0.25
56 78 79 67
108 157 149 124
Fig. 10. Nitrogen adsorption/desorption isotherms for calcined samples. N = nickel; A = aluminum and Z = zinc. 6
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Fig. 11. Pore size distribution for calcined samples. N = nickel; A = aluminum and Z = zinc.
Fig. 12. Transformations that occur during heating of hydrotalcite-like materials. 7
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surface areas than those based on zinc (N0.50Z0.25A0.25 and N0.37Z0.37A0.25 sample; Sg = 112 and 127 m2/g, respectively). The adsorption-desorption isotherms (Fig. 10) for N0.75A0.25, N0.66A0.33 and N0.50Z0.25A0.25 samples are typical of mesoporous solids. The hysteresis loop observed at high values of relative pressures indicates interparticle mesopores. Additionally, the N0.50Z0.25A0.25 sample showed type H1 hysteresis, which is typical of materials with porous formed by agglomerates of uniform spheres with a regular arrangement that originate an average pore diameter of 4.45 nm in this solid. For N0.75A0.25 (4.45 nm; pore diameter) and N0.66A0.33 (4.17 nm; pore diameter) samples, the H2 type hysteresis loops were noted, that are typical of systems of interconnected pores or of ink-bottle type pores [46,47]. These materials additionally presented a narrower and more symmetrical distribution of pores than that recorded by zinc-containing materials, as shown in Fig. 11. Additionally, is not observed an important contribution of pores with diameters greater than 7 nm. The N0.37Z0.37A0.25 sample showed a typical adsorption-desorption isotherm of solids with slit-shaped interparticle mesopores, which are formed by the irregular overlapping of particles in the form of plates [46]. This due probably to collapse of hydrotalcite structure at low temperatures, hindering the agglomeration of oxide particles formed in the calcination process, which led to the formation of interparticles pores with larger diameter (10.2 nm). This solid registered the widest distribution of pores, with diameters greater than 50 nm. Fig. 12 shows a hypothetical scheme for the transformations that is believed to occur for the solids upon heating. Initially (25−120 °C), the loss of interlayers water leads to the decrease of c parameter. This process is accelerated at higher temperatures (150−200 °C) where the loss of carbonate and hydroxyl groups begins, favored by the approximation of the positive layers. The HTLc structure collapses due to the decrease of cell parameters and then the first nickel oxide and zinc oxide particles appear and subsequently grow with increasing temperature, giving rise to particles with different sizes. For the solids containing nickel and aluminum the particle agglomeration favored the formation of interconnected pores, while the addition of zinc led to a material with spherical particles that originate capillary pores (N0.50Z0.25A0.25) and a material made of agglomerate plate-like particles and slit-shaped pores (N0.37Z0.37A0.25).
Drafting the manuscript: E.Meza-Fuentes, M.C. Rangel, A. Faro, J. Rodriguez-Ruiz, C. Solano-Polo. revising the manuscript critically for important intellectual content E.Meza-Fuentes, M.C. Rangel, A. Faro, J. Rodriguez-Ruiz Category 3 Approval of the version of the manuscript to be published (the names of all authors must be listed): E.Meza-Fuentes, M.C. Rangel, A. Faro, J. Rodriguez-Ruiz, C. SolanoPolo. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements EMF, JRR, CSP, thank the University of Cartagena and SENA for financial support. The authors acknowledge the financial support of CNPq, FINEP, PETROBRAS and SENA. References [1] F. Cavani, F. Trifiro, A. Vaccari, Hydrotalcite-type anionic clays: preparation, properties and application, Catal. Today 11 (1991) 173–301, https://doi.org/10. 1016/0920-5861(91)80068-K. [2] E.L. Crepaldi, J. Barros Valim, Hidróxidos duplos lamelares: síntese, estrutura, propriedades e aplicações, Quím, Nova 21 (1998) 300–311, https://doi.org/10. 1590/S0100-40421998000300011. [3] G. Brown, M.C. Gastuche, Mixed magnesium-aluminium hydroxides. II. structure and structural chemistry of synthetic hydroxycarbonates and related minerals and compounds, Clay Miner. 7 (1967) 193–201, https://doi.org/10.1180/claymin. 1967.007.2.06. [4] M. Crivello, C. Pérez, E. Herrero, G. Ghione, S. Casuscelli, E. Rodríguez-Castellón, Characterization of Al–Cu and Al–Cu–Mg mixed oxides and their catalytic activity in dehydrogenation of 2-octanol, Catal. Today 107-108 (2005) 215–222, https:// doi.org/10.1016/j.cattod.2005.07.168. [5] J. Rodriguez-Ruiz, A. Pajaro-Payares, E. Meza-Fuentes, Síntesis y caracterización estructural de hidrotalcitas de Cu-Zn-Al, Rev. Colomb. Quím 45 (2016) 33–38, https://doi.org/10.15446/rev.colomb.quim.v45n3.61381. [6] E. Meza-Fuentes, J. Rodriguez-Ruiz, M. Rangel, Characteristics of NiO present in solids obtained from hydrotalcites based on Ni/Al and Ni-Zn/Al, DYNA 86 (2019) 58–65, https://doi.org/10.15446/dyna.v86n210.78559. [7] V. Rives, S. Kannan, Layered double hydroxides with the hydrotalcite-type structure containing Cu2+, Ni2+ and Al3+, J. Mater. Chem. 10 (2000) 489–495, https://doi. org/10.1039/A908534C. [8] J. Pérez-Ramirez, G. Mul, J.A. Moulijn, In situ Fourier Transform infrared and laser Raman spectroscopy study of the thermal decomposition of Co-Al and Ni-Al hydrotalcites, Vib. Spectrosc. 27 (2001) 75–88, https://doi.org/10.1016/S09242031(01)00119-9. [9] K. Goh, T. Lim, Z. Dong, Application of layered double hydroxides for removal of oxyanions: a review, Water Res. 42 (2008) 1343–1368, https://doi.org/10.1016/j. watres.2007.10.043. [10] G. Lee, J. Kang, N. Yan, Y. Suh, J. Jung, Simple preparation method for Mg-Al hydrotalcites as base catalysts, J. Mol. Catal. A Chem. 423 (2016) 347–355, https:// doi.org/10.1016/j.molcata.2016.07.018. [11] N. Duong, T. Hang, A. Nicolay, Y. Paint, Corrosion protection of carbon steel by solvent free epoxy coating containing hydrotalcites intercalated with different organic corrosion inhibitors, Prog. Org. Coat 101 (2016) 331–341, https://doi.org/ 10.1016/j.porgcoat.2016.08.021. [12] A. Vanaamudan, B. Chavada, P. Padmaja, Adsorption of reactive blue 21 and reactive red 141 from aqueous solutions onto hydrotalcite, J. Environ. Chem. Eng. 4 (2016) 2617–2627, https://doi.org/10.1016/j.jece.2016.04.039. [13] L. Deng, Z. Shi, X. Peng, S. Zhou, Magnetic calcinated cobalt ferrite/magnesium aluminium hydrotalcite composite for enhanced adsorption of methyl orange, J. Alloy, Compd 688 (2016) 101, https://doi.org/10.1016/j.jallcom.2016.06.227. [14] M. Sanches-Cantu, M. Hernandez-Torres, A. Castillo-Navarro, E. Cadena-Torres, E. Rubio-Rosas, J. Gracia-Jimenez, F. Tzompantzi, Evaluation of hydrotalcite-like compounds with distinc interlaminar anions as catalysts precursors in methylene blue photodegradation, Appl. Clay Sci. 135 (2017) 1–8, https://doi.org/10.1016/j. clay.2016.08.028. [15] S. Eiby, D. Tobler, S. Nedel, A. Bischoff, B. Christiansen, A. Hansen, Competition between chloride and sulphate during the reformation of calcined hydrotalcite, Appl. Clay Sci. 132-133 (2016) 650–659, https://doi.org/10.1016/j.clay.2016.08. 017. [16] M. Kato, M. Azimi, S. Fayaz, M. Shah, M. Hoque, N. Hamajima, S. Ohnuma, T. Ohtsuka, M. Maeda, M. Yoshinaga, Uranium in well drinking water of Kabul,
4. Conclusions Nickel-aluminum and nickel-zinc-aluminum based hydrotalcites go on structural and textural changes upon heating that include the exit of water and decomposition of carbonate and hydroxide groups. These processes lead to the decrease of cell parameters and then to structure collapse, producing nickel oxide, zinc oxide and aluminum oxide. The zinc-free solids were the most thermally stable while the addition and the increase of zinc favors the decomposition. This was assigned to the increase of a parameter caused by the big ionic radius of zinc. The crystals size also changed during heating, aluminum and zinc delaying crystal sintering and zinc leading to the smallest crystals of nickel oxide. The increase of aluminum and/or zinc decrease the crystals size but zinc decreased the specific surface area of calcined solids. All solids have interparticles mesopores with pore shapes that depend on the composition of the materials and of the collapse of hydrotalcite type structure. Authorship contributions Category 1 Conception and design of study: E.Meza-Fuentes, M.C. Rangel, A. Faro, J. Rodriguez-Ruiz; acquisition of data: E.Meza-Fuentes, J. Rodriguez-Ruiz, C. SolanoPolo. analysis and/or interpretation of data: E.Meza-Fuentes, M.C. Rangel, A. Faro, J. Rodriguez-Ruiz, C. Solano-Polo. Category 2 8
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[17]
[18] [19]
[20]
[21] [22]
[23]
[24] [25]
[26]
[27] [28] [29]
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