WAXS Time-Resolved Phase Speciation of Chlorine LDH

Article pubs.acs.org/cm

XAS/WAXS Time-Resolved Phase Speciation of Chlorine LDH Thermal Transformation: Emerging Roles of Isovalent Metal Substitution Hudson W.P. Carvalho,†,‡,∥ Sandra H. Pulcinelli,† Celso V. Santilli,† Fabrice Leroux,§ Florian Meneau,‡ and Valérie Briois*,‡ †

Instituto de Quimica UNESP, Rua Prof. Francisco Degni, 55, 14800-900 Araraquara-SP, Brazil Synchrotron SOLEIL, L’Orme des Merisiers, BP 48, 91192 Saint-Aubin, France § Institut de Chimie de Clermont Ferrand, ICCF-UMR 6296, 63177, Aubière, France ‡

S Supporting Information *

ABSTRACT: The XAS/WAXS time-resolved method was applied for unraveling the complex mechanisms arising from the evolution of several metastable intermediates during the degradation of chlorine layered double hydroxide (LDH) upon heating to 450 °C, i.e., Zn2 Al(OH) 6 ·nH2 O, ZnCuAl(OH)6·nH2O, Zn2Al0.75Fe0.25(OH)6·nH2O, and ZnCuAl0.5Fe0.5(OH)6·nH2O. After a contraction of the interlamellar distance, attributed to the loss of intracrystalline water molecules, this distance experiences an expansion (T > 175− 225 °C) before the breakdown of the lamellar framework around 275−295 °C. Amorphous prenucleus clusters with crystallo-chemical local order of zinc-based oxide and zincbased spinel phases, and if any of copper-based oxide, are formed at T > 175−225 °C well before the loss of stacking of LDH layers. This distance expansion has been ascribed to the migration of ZnII from octahedral layers to tetrahedral sites in the interlayer space, nucleating the nano-ZnO or nano-ZnM2O4 (M = Al or Fe) amorphous prenuclei. The transformation of these nano-ZnO clusters toward ZnO crystallites proceeds through an agglomeration process occurring before the complete loss of layer stacking for Zn 2 Al(OH) 6 ·nH 2 O and Zn 2 Al 0 . 7 5 Fe 0 . 2 5 (OH) 6 ·nH 2 O. For ZnCuAl(OH) 6 ·nH 2 O and ZnCuAl0.5Fe0.5(OH)6·nH2O, a cooperative effect between the formation of nano-CuO and nano-ZnAl2O4 amorphous clusters facilitates the topochemical transformation of LDH to spinel due to the contribution of octahedral CuII vacancy to ZnII diffusion. KEYWORDS: layered double hydroxide, WAXS, quick-EXAFS, thermal decomposition

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INTRODUCTION Layered double hydroxides (LDHs) belong to a wide class of lamellar trioctahedrical compounds consisting of hydrotalcitelike positively charged layers, in which the divalent cations (MII) are partially replaced by the trivalent ones (MIII) and the resulting additional charge is counter-balanced by anions (Aq−) located in between these adjacent layers. These compounds are described by the general chemical formula [MII1−xMIIIx(OH)2][Ax/qq−·nH2O], in which the x/q ratio may vary from 0.17 to 0.33 depending on the combination of di- and trivalent elements, and n denotes the number of interlayer water molecules.1,2 The variety of chemical compositions of LDHs makes them very attractive for a wide range of applications such as catalysts, anion exchangers, stabilizers, scavengers, and biological carriers for drug immobilization among others.1−6 Many of these LDH applications require postsynthesis thermal treatment of the layered compounds. In fact, LDH compounds decompose into a mixture of oxides of rather high specific surface area and of very small crystal size, which are thermally stable and form metal crystallites by reduction.7 Moreover, when the calcined compound is brought into contact with © 2013 American Chemical Society

water solutions containing several anions, the original structure is reconstructed, giving rise to the so-called memory effect.7 Several studies aiming to understand the thermal decomposition steps of these compounds were reported in the literature.8 Usually, LDH is thermally decomposed through four main steps: the first one corresponds to the evaporation of weakly adsorbed water molecules, while the second one is due to the evaporation of the water molecules located between the layers, i.e., in the interlamellar space. Sometimes it happens that a mass-loss plateau occurs as a second thermal event that some authors attribute to the beginning of the collapse of the layered structure giving rise to a decrease of the interlayer space and making difficult the exit of water molecules.9 The third step occurs when the layers decompose upon the dehydroxylation process leading the structure to fully collapse and successively (or concomitantly as observed here) the oxide to crystallize. Received: April 24, 2013 Revised: June 13, 2013 Published: June 13, 2013 2855

dx.doi.org/10.1021/cm401352t | Chem. Mater. 2013, 25, 2855−2867

Chemistry of Materials

Article

The fourth and final step is usually attributed to the Aq− anion decomposition. The thermal decomposition induces at intermediate temperatures (T ∼ 300 °C) the formation of ill-known amorphous phases acting as a reservoir. Indeed they yield to the first crystalline phases that can be then easily identified from X-ray Powder Diffraction (XRPD) patterns.10 Due to the stoichiometry of the initial LDH, in which divalent cations are in excess compared to trivalent ones, the first crystalline phase detected by XRPD around 400 °C is the divalent metal oxide. The temperature of appearance depends on the nature of the divalent cations, for instance zincite ZnO is formed at lower temperatures than the periclase MgO, this explaining the better efficiency of the reconstruction method for the later-based hydrotalcite. At much higher temperatures (T ∼ 800 °C), the crystalline MIIMIII2O4 spinel is observed by XRPD. The question of the identification of the phases containing the trivalent metals in the temperature range between the formation of the amorphous phases at ∼300 °C and their detection by XRPD at 800 °C was addressed in several works involving local order techniques, such as Nuclear Magnetic Resonance (NMR) and X-ray Absorption Spectroscopy (XAS).8,10 However, most of these studies dealt with ex situ characterizations of LDH carried out at only a few decomposition temperatures, typically 400−450, 600, and 750−800 °C. Controversial results dealing with the formation of a spinel structure involving the trivalent cations were presented in several papers.8,10 Taking advantage of the time resolution provided by QuickXAS and Wide Angle X-ray Scattering (WAXS) techniques available at a third generation Synchrotron Radiation (SR) facility, this paper deals with the in-depth analysis of the thermal decomposition of hydrotalcite-like materials containing ZnII, CuII, AlIII, and FeIII with a special emphasis on the role played by isovalent metal substitution into the Zn−Al based chlorine-LDH. The in situ monitoring by the SR techniques of the LDH thermal decomposition is focused on the physical− chemical transformations occurring at intermediate temperatures (T ≤ 450 °C). The structural results, obtained with a typical time resolution of a few seconds, will be compared to the thermogravimetric analysis carried out between Room Temperature (RT) and 450 °C using the same heating rate.

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The cationic chemical composition was determined by inductive coupled plasma atomic emission spectroscopy (ICP-AES) analysis carried out by the Service Central d’Analyse du CNRS, Solaize, France. Thermogravimetric analyses were carried out under air with a flow of 100 mL min−1 and a heating rate of 10 °C min−1, using SDT Q600TA Instruments. SR-Based Technique Characterization. Quick-EXAFS Measurements. The X-ray absorption spectra were collected in the QuickEXAFS mode at the SAMBA beamline of the SOLEIL Synchrotron.12 At this experimental station, the white beam is collimated and vertically focused by two Pd-coated silicon cylindrical bendable mirrors. The beam is monochromatized by a Si(111) channel-cut crystal. The oscillation frequency of the channel cut was set to 1 Hz, corresponding to the acquisition of two spectra every 1 s. Only the set of spectra recorded with increasing Bragg angle was considered in the analysis. Data were recorded in transmission mode and the X-ray attenuation measured by ionization chambers with nitrogen filling gas. For improving the signal-to-noise ratio, the data collected at RT were averaged over 150 spectra (2.5 min). The samples were pressed in pellets, placed into a homemade oven,13 and heated under an air atmosphere from RT until 450 °C at 10 °C·min−1. As 10 successive collected spectra are strictly superimposable during the thermal decomposition, indicating that no significant local order change occurs, they were merged to be used as one temperature point in the subsequent data analysis. The change in temperature occurring during this 10 s is of 1.6 °C. Zn and Cu K edge Quick-EXAFS data were collected on the same sample during the same thermal treatment by using a wide amplitude oscillation (2.25°) of the Quick-EXAFS monochromator suitable for covering both edges. Fe K edge QuickEXAFS data were collected in a separate experiment by repeating the thermal decomposition of a sample belonging to the same preparation batch as that characterized at the Zn and/or Cu K edges. EXAFS and XANES Data Analysis. The analysis of the X-ray absorption data was carried out using the software package Athena and Artemis.14 For the XANES analysis, a linear background was fitted to the pre-edge region and subtracted from the spectra which were normalized in a consistent way. Data were calibrated in energy using the maximum of the first derivative of a metallic reference foil recorded simultaneously with the data related to the samples. For the EXAFS analysis, first a pre-edge background was removed using a linear function. A postedge background using the Autobk algorithm was applied with a cutoff Rbkg = 1 and k-weight = 2 in order to isolate the EXAFS oscillations χ(k). Then the EXAFS data were Fourier transformed using a k2-weighting Kaiser−Bessel window. EXAFS fitting of distances and Debye−Waller factors was performed with the Artemis interface to IFeFFIT using least-squares refinements.15 The S02 amplitude reduction parameter which takes multielectronic effects into account and the energy shift E0 were first calibrated by fitting relevant crystalline references. The reliability of the fit was assessed by a residual factor RF that was minimized. PCA and MCR-ALS Analysis. Determination of the Number of Components. During the thermal decomposition of chlorine-LDH, complex chemical transformations involving several intermediate species occur. In order to determine the number of relevant species present during the decomposition, a Principal Component Analysis (PCA) of the normalized Quick-XANES spectra was carried out using the SixPACK software.16 PCA is a robust method which allows us to decompose a series of spectra recorded at a given edge into a minimal number of spectral components17,18 under the constraint of orthonormality of the components. It is important to note that in any case, the PCA components so-determined are XAS spectra representative of pure chemical species. The minimal number of components required for reproducing satisfactorily the set of data is indicative of the number of different chemical species containing the absorbing atom involved in the set of data. This approach has been successfully used for the analysis of intermediate species in several time-resolved XANES studies.19,20 In our case, PCA was applied to the time-resolved data recorded at the three K edges. It is noteworthy that the number of components determined by PCA is in good agreement

EXPERIMENTAL SECTION

LDH Preparation. The chlorine-LDHs were prepared by a titration-coprecipitation method.11 In the first step, acid solutions of MIICl2 and MIIICl3 (MII = ZnII and CuII; MIII = AlIII and FeIII) were obtained by dissolving the respective salts (MII/MIII molar ratio = 2) with deionized water; then this solution, under magnetic stirring and N2 flow, was titrated with 1 M NaOH aqueous solution. The coprecipitation pH was stabilized at 9. The resultant solution was kept stirring overnight, then centrifuged and washed with deionized water three times. Finally, as-prepared chlorine-LDHs were dried at 60 °C over 24 h under air. The samples were named according to the nominal cationic chemical composition: ZA = [Zn0.66Al0.33(OH)2.00][Cl0.33]nH2O, ZCA = [Zn0.33Cu0.33Al0.33(OH)2.00][Cl0.33]nH2O, ZAF = [Zn 0 . 6 6 A l 0 . 2 5 Fe 0 . 0 8 ( OH ) 2 . 0 0 ][Cl 0 . 3 3 ]n H 2 O, and ZCAF = [Zn0.33Cu0.33Al0.165Fe0.165(OH)2.00][Cl0.33]nH2O. Characterizations of As-Prepared LDH. The X-ray powder diffraction (XRPD) patterns of the as-prepared chlorine-LDH samples were recorded with a Siemens D5000 powder diffractometer using the Cu Kα radiation (λ = 1.5418 Å) monochromatized by a graphite monocrystal at a resolution of 0.01°. 2856



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calculated from the equation d00l = 2π/qmax, where qmax is the scattering vector corresponding the 00l peak position. The average crystallite size, D (Å), and the microstrains, ε, along the c direction were deduced from the full width at half-maximum (fwhm) of the diffraction peak observed in the WAXS patterns (B) corrected from the instrumental broadening (b) by using a well-crystallized silver behenate standard reference (β2 = B2 − b2).26 The simultaneous evolution of the crystallite size and microstrains calculated from the basal distance deduced from the (003) peak, and from its second harmonic, the (006) peak, were calculated using relation 2:27

with a more empirical method based on the determination of a set of isosbestic points in the time-resolved data. Identification of the Species. The constituents of a phase mixture can be easily identified if during the phase transformation the constituent mostly formed can be isolated from the set of QuickXANES data or can be deduced from complementary techniques like XRPD or WAXS. Herein, considering the information given by WAXS, the spectra of nanometric-sized ZnO, ZnAl2O4, and CuO compounds were recorded as references and considered as pure components in the further data treatment, together with the pristine chlorine-LDH recorded at RT. In the case of Zn K edge data, for which four components were determined by PCA, we used the method based on the inspection of isosbestic points shared by the data recorded during the thermal treatment for isolating the so-called “dehydrated LDH” compound. At the Cu K edge, because the transformation is not complete and one of the metastable species is not easily identified from the set of Quick-XANES data, the Multivariate Curve Resolution with Alternating Least Squares (MCR-ALS) method21 was used to recover the Quick-XANES spectrum of pure species in the mixture of phases. The underlying idea behind MCR-ALS is the bilinear decomposition of the matrix D(Nsnapshots, Nenergy) representing the experimental data (herein Quick-XANES spectra) at the energies (Nenergy) of the mixtures (Nsnapshots) obtained in this work at successive temperature values of the heating process according to relation 1: T

D = CS + E

β 2 cos2 θc λ

2

=

⎛ θc ⎞2 ⎛ 1 ⎞2 2 ⎜ ⎟ + 16ε sin⎜ ⎟ ⎝D⎠ ⎝λ⎠

(2)

in which θc is the Bragg angle.

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RESULTS The nominal and experimental elemental compositions of the different chlorine-LDH samples are presented in Table 1. The Table 1. Nominal and Experimental LDH Chemical Composition sample

(1)

ZA

This bilinear eq 1, which is generally valid for spectroscopic data governed by the Beer−Lambert law like XAS data, is solved iteratively by an Alternating Least Squares (ALS) method to obtain the matrices of individual pure spectra ST and of the concentration profiles C of the components which best fit the experimental data. The matrix E contains the residuals. The ALS optimization starts using initial estimates of either the C or the ST matrix, herein the ST matrix. Furthermore, the MCR-ALS method imposes C and ST to follow physically and chemically meaningful constraints (rather than mathematical or statistical constraints as for PCA) like non-negativity of XAS absorbance and concentration, unimodality (profiles without double peaks), and closure (the concentrations of all the components are equal to a constant value) for instance. It is noteworthy that this MCR-ALS technique quite common for electronic spectroscopy like UV−vis, or vibrational spectroscopy like Raman or FT-IR21 is more rarely used for XAS,22−25 probably because the mathematical routines are not yet implemented in the most common softwares used for XAS data analysis. The use of the method implemented in the Unscrambler chemometrics software package (CAMO software AS, Oslo, Norway) was very helpful in this work to identify the metastable copper constituents formed upon thermal degradation of the ZCA and ZCAF samples. Determination of the Concentration Profiles. A least-squares combinatorial fitting of the normalized time-resolved XANES spectra using the chemical species previously identified as pure components was carried out using the Athena software. Linear Combinations (LC), in the energy range −20 to 120 eV relative to the edge position, E0, were carried out with all the combinations involving the standards; e.g., for four components, binary, ternary, and quaternary fits are successively performed by the automatic “Combinatorics” routine installed in Athena. Based on the R-factor value, which measures the mismatch between the experimental spectrum and the LC, we retain the good combination. The results of the LC give the compositional fractions of the components during the thermal evolution suffered by the LDH. WAXS Measurements. Wide Angle X-ray Scattering (WAXS) data collection was performed at the SWING beamline at SOLEIL. The white beam delivered by the U20 undulator source was monochromatized (λ = 0.8266 Å) by a Si double crystal and focused by a KB mirror system. The measurements were performed using a twodimensional CCD detector located at 0.51 m of the sample. Data were recorded at RT as well as during the chlorine-LDH thermal decomposition (from RT to 450 °C at 10 °C·min−1) using the LINKAM THM 600 cell. The interlamellar distance of LDH was

ZCA ZAF ZCAF

nominal composition (Zn0.66)(Al0.33) (OH)2.00Cl0.33·nH2O (Zn0.33 Cu0.33)(Al0.33) (OH)2.00Cl0.33·nH2O (Zn0.66)(Al0.25Fe0.08) (OH)2.00Cl0.33·nH2O (Zn0.33Cu0.33)(Al0.165Fe0.165) (OH)2.00Cl0.33·nH2O

experimental composition (Zn0.66)(Al0.34) (OH)2.00Cl0.34.0.94H2O (Zn0.32Cu0.34)(Al0.33) (OH)2.00Cl0.34.0.95H2O (Zn0.64)(Al0.27Fe0.09) (OH)2.00Cl0.36.0.70H2O (Zn0.32Cu0.34)(Al0.16Fe0.18) (OH)2.00Cl0,34.0.94H2O

atomic ratio of different metals was obtained by ICP-AES, whereas the water and hydroxyl ratios were determined by thermogravimetric analysis. A good agreement between nominal and experimental compositions has been obtained, even for the chlorine-LDH containing three and four different metal atoms, underlining the presence of a solid state solution between such an intricate intralayer cation chemical composition. Furthermore, the crystallization of hydrotalcite-like materials with hexagonal (R3̅m) structure was evidenced by XRPD diffraction patterns of the as-prepared chlorine-LDHs (Figure S1, Supporting Information). Long Range Order Thermal Evolution. The chemical transformations affecting the LDH long-range order upon thermal decomposition were first investigated by in situ WAXS. Figure 1 displays the WAXS patterns recorded during the thermal decomposition of the ZA, ZCA, and ZAF chlorineLDH compounds. As the temperature increases, similar transformations are observed. At the early stages, the stacked structure is first degraded, as indicated by the progressive loss of the (003) and (006) diffraction signals, together with the hydroxide-layer long-range order, as evidenced by the disappearance of the (012) and (101) diffraction peaks. The vanishing temperatures of the (00l) peaks depend on the LDH chemical composition, as gathered in Table 2. The crystallization of divalent metal oxide phases, i.e., ZnO and, if any, CuO, then occurs at a later stage. Irrespective of the chlorineLDH composition, the onset of this crystallization occurs at the same temperature: 266 ± 1 °C (Table 2). It is important to note that the onset crystallization temperature of the first divalent oxide is always lower than the temperature of the complete degradation of the chlorine-LDH stacked structure. The difference between both temperatures is equal to 12 °C for ZAF, 16 °C for ZCA, and 30 °C for ZA, evidencing a significant 2857



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lized the octahedral atomic layer of LDH. Additionally, the isovalent substitution of Zn by Cu in ZCA gives rise to a significant delay in the crystallization of ZnO compared to ZA or to ZAF since the (101) ZnO peak for ZCA appears at about 293 °C compared to 265/266 °C for ZA and ZAF. Finally, neither an aluminum-based oxide crystalline phase nor an iron one could be evidenced in the reported WAXS patterns. Only a diffuse broad peak (Figure 2) is observed in the q range

Figure 2. Comparison of the WAXS patterns recorded at 450 °C for ZA, ZCA, and ZAF.

between 2.56 and 2.63 Å−1 for these samples measured at 450 °C. This feature could be ascribed to the formation of a ZnM2O4 nanocrystalline spinel (M = Al or Al/Fe), since the most intense diffraction peak (311) of this phase is expected in this q range (as for ZnAl2O4 JCPDF/ASTM 05-0669). The dTG curves of the studied chlorine-LDHs are presented in Figure 3a and compared to the structural parameters determined from the WAXS: (b) LDH interlamellar distance, (c) crystallite size in the c direction, and (d) microstrain along the c direction. Irrespective of the LDH samples, the dTG curves display three decomposition steps named S1, S2, and S3, tentatively attributed in the literature to the loss of interparticle pore and surface water (T < 140 °C), to the loss of gallery intercalated water (140−200 °C), and to dehydroxylation (200−300 °C) of the layers,28 respectively. Moreover, as shown in Figure S2 of the Supporting Information, above the decomposition temperature (>450 °C), the TGA curves of all chlorine-LDH show a two-step release of chloride species. Upon heating, only a slight variation of the interlamellar distance is observed as expected for chlorine-LDH, attributed to the strong electrostatic interactions between the interlayer chlorine anion and the layers.24 As expected in the free water evaporation step, the interlamellar distance is essentially invariant during heating up to 100 °C. At the opposite, the loss of intercalated water (S2) gives rise to a small decrease of the interlamellar distance, while a small expansion is observed at the beginning of the S3 step. This effect has been related by some authors to the relaxation of electrostatic interaction

Figure 1. WAXS patterns as a function of temperature, (a) ZA, (b) ZCA, and (c) ZAF. Miller indices: red = chlorine-LDH phase, blue = ZnO wurtzite, magenta = CuO tenorite.

decrease of the temperature of extinction of the (00l) peaks upon isovalent substitution of ZnII or AlIII by CuII or FeIII atoms, respectively. This feature indicates that the heterogeneous chemical composition induced by substitution destabi-

Table 2. Temperature of LDH Degradation and Divalent Oxide Formation

a

LDH

Td(003) (°C)a

Ta(101)ZnO (°C)b

Ta(002/−111)CuO (°C)b

ZA (Zn0.66Al0.33) ZCA (Zn0.33Cu0.33Al0.33) ZAF (Zn0.66Al0.25Fe0.08)

295 283 278

265 293 266

267

Td: temperature at which the peak disappears. bTa: temperature at which the peak appears. 2858



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Figure 3. Comparison of the dTG curves (a) with the evolutions as a function of the temperature of the structural parameters determined by WAXS: (b) LDH interlamellar distance, (c) average crystallite size in the c direction, and (d) microstrain along the c direction. WAXS data are not available for the ZCAF LDH.

induced by the progressive dehydroxylation of the sheets.28 The onset of the S2 step coincides with the decrease of the crystallite size (Figure 3c) and the increase of the microstrain along the c axis (Figure 3d). This feature confirms that the S1 step involves the release of free water, and the subsequent S2 step corresponds to the release of intercalated water, the departure of the latter affecting strongly the long-range order. Local Range Order Thermal Evolution. The evolutions of Quick-XANES spectra at the Zn, Cu, and Fe K edges measured in situ during chlorine-LDH thermal decomposition are presented in Figure 4 for selected samples. The evolutions are characteristic of the local order changes occurring around the selected absorbing atom. A clear shift of the Cu-rising K edge to lower energy is evidenced upon heating, indicating a reduction process of copper oxidation state. At the opposite, the edge positions for the Zn and Fe K edge data are essentially invariant, indicating that upon heating these metals remain divalent and trivalent, respectively. Evolution of the Local Order around Zn. The QuickXANES spectra of the four chlorine-LDH recorded at RT are presented in Figure 5. They are characteristic of a common local order around Zn (according to the EXAFS fitting) with a second coordination shell at 3.11 Å ± 0.02 Å composed of divalent (Zn, Cu) and trivalent (Al, Fe) ions. The average coordination number of divalent and trivalent secondneighboring atoms determined by EXAFS analysis (Table S1 and Figure S3, Supporting Information) follows the ratio 2:1. Moreover, the Quick-XANES spectra of the chlorine-LDH recorded at 450 °C are also displayed in Figure 5 and reveal a local fingerprint with a shape similar to the recorded QuickXANES spectrum of a nanosized ZnO phase reference.

Nevertheless, we note a slight but significant difference between the intensities of the resonances of the nano-ZnO reference and the intensities of the LDH spectra recorded at 450 °C. This suggests that an additional species to the nano-ZnO is formed upon heating at 450 °C. The presence of an ill-crystallized phase tentatively assigned to ZnM2O4 has been suggested from the WAXS results for the Zn-based phases calcined at 450 °C. Accordingly, for all the chlorine-LDH samples calcined at 450 °C, a satisfactory simulation was achieved by LC considering the spectra of the nano-ZnO reference and of a nano-ZnAl2O4 spinel reference compounds presented Figure 5. The spinel was prepared by thermal treatment at 450 °C of the as-prepared [(Zn0.08Cu0.58Al0.33OH)2.00][Cl0.33]·nH2O sample. It is noteworthy that this as-prepared sample does not correspond to a chlorine-LDH phase, since the formation of an ill-crystallized prespinel ZnAl2O4 has been evidenced from XRPD. Examples of LC obtained for the Zn-based chlorine-LDH calcined at 450 °C are presented in Figure S4 of the Supporting Information, and the nano-ZnO and nano-ZnAl2O4 compositional fractions so-obtained are gathered in Table S2 in the Supporting Information. Upon thermal treatment at 450 °C under the heating conditions used herein, about 10 to 16 ± 3% of nanospinel is formed. Taking into account the formation of these two Zn-based species during the thermal decomposition of all chlorine-LDH samples, the spectra characteristic of the as-prepared LDH, nano-ZnO, and nano-ZnAl2O4 spinel reference phases were included during the PCA analysis of the set of time-resolved data. Figure 6a displays the first five eigenvectors, or weighted PCA components, obtained for the calcination of the ZCA sample. Only the fifth component appears to be in the noise 2859



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Figure 5. Zn K edge Quick-XANES spectra recorded for the RT pristine chlorine-LDHs and the LDH samples heated at 450 °C. For comparison purposes, the spectra of nano-ZnO and nano-ZnAl2O4 references are also presented.

Figure 4. Evolutions of Quick-XANES spectra at the Zn, Cu, and Fe K edges measured in situ during chlorine-LDH thermal decomposition. (a) ZCA at the Zn K edge, (b) ZCA at the Cu K edge, and (c) ZAF at the Fe K edge. Figure 6. PCA analysis of the temperature evolution of the ZCA compound: (a) The first five components determined from PCA calculation weighted by eigenvalues; (b) comparison of the R-factor values resulted from a two-component fit, a three-component fit, and a four-component fit.

and has been neglected in the target testing of the normalized spectra. The time-resolved data were then fitted with linear combinations using the first two components, then the first three ones and finally all four. Figure 6b compares the fit residuals obtained for each set of fits. Fits obtained with two and three components display the same trend with a huge maximum in the temperature range centered around the dTG minimum between S2 and S3 events, indicating that this number of components is inadequate to describe the whole set of data. The fit quality over the full temperature range is greatly improved by the addition of the fourth PCA component. This result is verified for all the Zn-based LDH samples for which

the screening tests are quite similar with first four eigenvalues for the components higher than 1.29 In conclusion, PCA analysis points out that four distinct phases containing Zn atoms are present during the thermal decomposition: the pristine chlorine-LDH phase, a nano-ZnO phase, a nano prespinel ZnAl2O4 phase, and a fourth intermediate species predominantly observed around 200 °C (Figure 6b). 2860



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Moreover, the close examination of the data revealed the presence of two sets of isosbestic points, indicating that the asprepared chlorine LDH phase is transformed into an intermediate species itself transformed into the final phase at 450 °C. Figure 7 displays the XANES of the intermediate

Figure 8. Cu K edge Quick-XANES spectra recorded for the ZCA and ZCAF pristine chlorine-LDHs, for the same LDH samples heated at 450 °C, and for the same LDH samples heated at 450 °C and after cooling down at 50 °C.

cannot be satisfactorily reproduced by a linear combination involving these standards.32,33 Nevertheless, we have evidenced by WAXS that this phase partially contains crystalline CuO (Figure 1b). Using the MCR-ALS method on the full set of Quick-XANES data, we isolate the three components presented in Figure 9. Two of them are identified as the as-prepared ZCA

Figure 7. Zn K edge Quick-XANES spectra characterizing the chlorine-LDH samples after the loss of surface and gallery embedded water. These spectra were determined by the inspection of isosbestic points describing the set of time-resolved data. It deals with the first set of isosbestic points associated with the transformation of the pristine chlorine-LDH into the so-called “dehydrated” LDH.

species determined from the isosbestic points. It is interesting to note that this intermediate phase is observed in the temperature range corresponding to the end of the dehydration step, i.e., matching the temperature range of the dTG minimum between S2 and S3 (Figure 3a). Accordingly, the temperature at which the intermediate species are dominant depends on the LDH chemical composition. This finding is in good agreement with the observation reported by Kanezaki concerning the temperature dependence on the formation of the metastable phase resulting from the elimination of interlayer species from Mg/Al/CO3-LDH with different Mg/Al ratios.30 Evolution of the Local Order around Cu. The QuickXANES spectra of the ZCA and ZCAF chlorine-LDH recorded at RT, 450 °C, and after cooling down at 50 °C are compared in Figure 8. The RT spectra are quite similar for both samples and characteristic of the Cu local order in chlorine-LDH samples presenting Jahn−Teller distorted CuII octahedrons as clearly deduced from the EXAFS analysis for the ZCA RT data (Table S1 and Figure S3, Supporting Information). The EXAFS parameters are well in line with those reported for the [CuCrCl] LDH sample.31 From the comparison of the spectra recorded at 450 and 50 °C (after cooling down), it appears that for both LDH compositions, the phases formed at high temperature are not stable upon cooling: both samples evolving toward the same phase, identified by the EXAFS analysis as a nanocrystalline CuO phase. Quick-XANES data recorded at 450 °C for both LDHs display a rising edge significantly shifted to lower energy, indicating that upon heating a reduction of copper occurs. Interestingly the energy of the rising edge is shifted to higher value upon cooling, underlining the thermal reversibility of the copper oxidation state. The Quick-XANES spectrum of the ZCA phase decomposed at 450 °C does not correspond to any spectra of standard copper oxide-based phases (CuO, Cu2O, or CuAl2O4) and

Figure 9. Cu K edge XANES spectra of components determined by MCR-ALS analysis (red lines). For comparison purposes, the experimental spectra of the pristine chlorine-LDH, the nano-CuO standard, and the CuCl32− complex (black lines) are presented.

chlorine-LDH (component 1) and a nano-CuO phase (component 2). The spectrum of the third component displays a rising edge at lower energy than those of the first two divalent components, well in line with the formation of a monovalent copper-based species. Furthermore, the component 3 spectrum displays a step-like shape with several small structures, presenting strong similarities, in particular the lack of a white line, with the ones reported in the literature for CuClx(x−1)− complexes.34−36 For comparison purposes, the spectrum of a solution containing mainly CuCl32− and prepared as a reference compound by dissolving anhydrous CuCl into HCl (pH = 1) solution in the presence of NaCl (3 M ionic strength)37 is reported in Figure 9. It is very likely that the component 3 2861



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spectrum is a mixture of several CuClx(x−1)− species. As a matter of fact, the EXAFS spectrum recorded for ZCA heated at 450 °C was best fitted with ∼1 chloride ligand at 2.22 Å and ∼1 oxygen atom at 1.96 Å, as reported in Table S3 in the Supporting Information. The Cu−O distance is the typical bond length found for divalent copper phases, such as CuO,38 whereas the Cu−Cl distance is in good agreement with the distance reported in the literature for trigonal chloride coordination as encountered in CuICl32−.34 But it is noticeable that this value is also close to the arithmetic average for linear Cu I−Cl (2.13 Å) 39 and tetrahedral Cu I−Cl (2.34 Å) coordination.34 Then the Cu−Cl distance found for ZCA heated at 450 °C could also be considered as additional evidence of the presence of a mixture of CuClx(x−1)− species. This mixture of species will be noted hereafter as the CuIClx-like phase. At the end of the heating (450 °C), the ZCA sample is composed of 86 ± 2% CuIClx and 14 ± 2% nano-CuO, as determined by linear combination. Moreover, the use of the same components, determined by MCR-ALS for the ZCA chlorine-LDH, to fit by LC the Quick-XANES spectrum of the ZCAF chlorine-LDH decomposed at 450 °C gives excellent agreement with 51% ± 2% CuIClx and 22 ± 2% nano-CuO. The presence of iron inside the ZCAF LDH significantly decreases the reduction phenomenon suffered by copper upon heating up to 450 °C. Note that only three components are necessary to reproduce by LC fitting the set of data recorded at the Cu K edge: the pristine chlorine-LDH, the nano-CuO, and the Cu I Cl x component determined by MCR-ALS. Unlike the results reported at the Zn K edge, no dehydrated species are isolated from the XANES data set at the Cu K edge. Actually, this does not mean that dehydration does not happen around Cu during thermal evolution but only that the structural changes occurring around Cu during the interlayer water loss do not lead to strong changes of the shape of XANES spectra. This is clearly evidenced by comparing the XANES spectra of the pristine chlorine-LDH and of the sample recorded at the temperature at which the dehydrated LDH species was isolated from the Zn K edge data set (Figure S5, Supporting Information). From the Cu side, the main event in the dehydration process is associated more with a reduction reaction concomitantly to a change of ligand rather than a dehydrated Cu-based subhydroxide phase. Evolution of the Local Order around Fe. The QuickXANES spectra of the ZAF and ZCAF chlorine-LDH recorded at RT and 450 °C are compared in Figure 10. The spectra of the as-prepared chlorine-LDH are characteristic of Fe embedded in the hydrotalcite-like structure10 (Table S1 and Figure S3, Supporting Information). After heating at 450 °C, the position of the rising edge is essentially invariant, indicating that no change of the iron oxidation state occurs for both LDHs upon heating. Actually the XANES recorded at 450 °C are very close to the shape of the XANES spectra recorded for the maghemite (γ-Fe2O3) or for the spinel ZnFe2O4 reference compounds (Figure 10), for which only slight but significant differences exist. Both reference compounds are spinel-like structures, but in the ZnFe2O4 case, octahedral sites are only occupied by FeIII whereas in the maghemite one, trivalent iron atoms are spread out in the spinel structure occupying both tetrahedral and octahedral sites. It results from these differences that the white line of the ZnFe2O4 compound will be significantly more intense than the one of the maghemite, whereas the pre-edge

Figure 10. Fe K edge Quick-XANES spectra recorded for the ZAF and ZCAF pristine chlorine-LDHs, for the same LDH samples heated at 450 °C and for γ-Fe2O3 and ZnFe2O4 references.

structure will be concomitantly less intense. Considering a Molecular Orbitals (MO) scheme for discussing the origin of transitions in 3d K edge XANES within the electric dipole approximation, the white line is ascribed to transitions from the 1s core level Atomic Orbital (AO) toward MO involving 4p empty AO levels, whereas the pre-edge is associated with a transition toward 3d AO hybridized with 4p AO.40 The increase of the intensity of the white line for a compound embedded in octahedral symmetry compared to a compound embedded in a tetrahedral one arises from a stronger participation of the 4p AOs of the metal in the MO toward the transition occurs, due to (i) the decrease of the orbital overlap between the metal and the ligand resulting from a longer distance in Oh than in Td and (ii) the nonmixing of the 4p AO with 3d AO in pure Oh symmetry.41,42 The comparison of both iron-based chlorineLDHs heated at 450 °C evidences similar variation of white line and pre-edge intensities suggesting a higher proportion of iron atoms in the octahedral site in the phase derived from ZAF than in the one derived from ZCAF. As a matter of fact, LC fitting of the ZCAF sample cooled down to 50 °C gives a satisfactory agreement with 64% γ-Fe2O3 and 36% ZnFe2O4, whereas the ZAF sample is only composed of the ZnFe2O4 reference but with a slight mismatch to the white line intensity and the first EXAFS oscillation shape. We assume that this slight discrepancy between ZAF and the fitting curve results from the formation of a Zn(Al2−xFex)O4 spinel solid solution, difficult to unravel from the other components.

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DISCUSSION Additionally to the ZnO and CuO crystalline phases well characterized by WAXS, the in situ Quick-XAS monitoring of LDH thermal transformation up to 450 °C evidences the formation of several oxide species involving the trivalent and divalent metals composing the pristine chlorine-LDH, nano-Zn based spinels (ZnAl2O4 and ZnFe2O4) and γ-Fe2O3, but also an unexpected copper chlorine-based phase, CuIClx. Except for the spinel zinc based phases assumed to be formed from the WAXS results, the formation of CuIClx and γ-Fe2O3 crystalline phases could not be deduced from long-range order analysis. The fact that the CuIClx phase is unstable, being transformed upon cooling into nano-CuO, and that this phase is silent by XRPD fully explains why its identification as a product of thermal 2862



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The Zn K edge Quick-XANES monitoring clearly evidences the effect of the LDH dehydration by pointing out the intermediate dehydrated species formed at the end of the S2 thermal event of the dTG curves (Figure 3a). Although the local order around Zn, determined by EXAFS fitting (Table S4, Supporting Information), is unchanged compared to the pristine chlorine-LDH, the sensitivity of XANES to medium range order due to the long mean free path of the photoelectron in the region near the edge45 allows for screening the changes occurring in the layer gallery. The onset of the transformation of the dehydrated species into nano-ZnO and nanospinel (Figure 11) is concomitant with that of the dehydroxylation step (S3 in Figure 3a). At this stage, taking place between 175 and 225 °C, the formation of nanoZnO and nanospinel evidenced by Quick-XANES is concomitant with the increase of the interlamellar distance shown by WAXS (Figure 3b). This could result from the migration of zinc species from the original 6-fold coordination site in the layer to the 4-fold tetrahedral site toward the interlayer space increasing slightly the interlamellar distance. Such behavior has been described recently during the thermal decomposition of the crystalline structure of Co−Al LDH which was modeled in this temperature range as a structure formed by regular stacking of aperiodic layers with random distribution of divalent tetrahedral sites in the interlayer space.46 The persistence of the layered structure with the interlamellar distance increase, together with the phase speciation determined by QuickXANES, leads us to propose that in this temperature range the system is composed of rich dehydrated hydroxylated layer domains and rich zinc based oxide domains still anchored in the layer as prenucleus clusters of the crystalline phase. In this temperature range, no ZnO diffraction peak can be detected by WAXS revealing the amorphous feature of these clusters. This set of results is an experimental evidence of the hypothesis recently made by Valente et al. from calorimetric analysis of the thermal decomposition of MgAl LDH;47 i.e., the formation of oxide phases is a localized phenomenon induced by some local critical dehydroxylation extension. At the temperature corresponding to the maximum of the S3 dTG event (∼250 °C), 50% of the so-called “dehydrated” LDH species is mainly transformed into ZnO and eventually ZnAl2O4 amorphous prenucleus clusters. Finally, above 270−300 °C, EXAFS analysis shows that the octahedral zinc environment associated with the local order characteristic of the LDH layer is transformed into a 4-fold coordination with a mean distance at 1.99 ± 0.02 Å in good agreement with the structural tetrahedral feature expected for ZnO and ZnM2O4 crystalline phases (M = Al or Fe) (Table S4 and Figure S6, Supporting Information). It is important to note that the formation of amorphous prenucleus ZnO and ZnM2O4 clusters is observed by XAS well before the crystallization of the final product demonstrated by WAXS above 265−293 °C (Table 2, Figure 1) and 450 °C (Figure 2), respectively. Consequently, the almost invariant amount of tetrahedral zinc oxide-based species revealed by the plateau observed above 270 °C in the speciation plots of Figure 11 indicates that the nucleation and subsequent growth of crystalline ZnO do not occur through the addition of single metal atoms. This feature indicates that the crystallization of the ZnO phase occurs by direct attachment of the amorphous clusters through a coalescence process. Our observations agree with recent experimental and theoretical studies showing that the thermodynamically stable crystalline phase of several

decomposition of chlorine-LDH at intermediate temperatures has never been reported in the literature to the best of our knowledge. Furthermore, the formation of the crystalline oxide phases involving the trivalent metal is well reported in the literature but at temperatures above 750 °C.10,28,43,44 Since the thermal stability of the layered edifice (Table 2) strongly depends on the isovalent substitution of metals in the pristine chlorine-LDH, it is of prime importance to follow the temperature evolution of each metallic species upon heating. First, we will discuss the effect of the substitution of Al by Fe, then we will consider the effect of the substitution of Zn by Cu. Trivalent Metal Substitution and Phase Speciation. The speciation of Zn-based phases for ZA and ZAF is compared in Figure 11. The overall behaviors of Zn based

Figure 11. Compositional fractions of Zn based species determined by XAS at the Zn K edge during the thermal evolution of the (a) ZA and (b) ZAF LDHs (heating rate 10 °C min−1).

species for both LDH samples upon thermal treatment are quite similar, indicating that the isovalent substitution of AlIII by FeIII does not play a significant role in the thermal transformation kinetics of zinc-based phases. The first dehydration stage transforms the pristine chlorine-LDH into the dehydrated chlorine-LDH, and then the dehydroxylation of this dehydrated LDH leads to the formation of the zinc oxidebased networks. At 450 °C, the composition of both samples is quite similar from the point of view of zinc-based species (Table 2) with ∼85% of ZnO and ∼15% of ZnM2O4. Based on the iron K edge analysis, it is noteworthy that the spinel like structure for ZAF is a Zn(Al2−xFex)O4 solid solution. Obviously for ZA it deals with the ZnAl2O4 normal spinel. 2863



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minerals can be formed directly through the aggregation of nanometric building blocks such as amorphous clusters.48 The substitution of aluminum by iron gives rise first to a significant shift of the temperature at which 100% of the dehydrated LDH phase is present (Figure 11), 172 °C for ZA and 223 °C for ZAF, and then to a significant decrease of the temperature at which the lamellar stacking is lost (Table 2), 295 °C for ZA and 278 °C for ZAF. The former thermal stability event of the ZAF LDH phase with respect to the dehydration process can result from the larger average crystallite size and lower microstrain of the LDH phase along the c direction observed before 200 °C for ZAF compared to ZA (Figure 3c and d). This is a kinetic contribution associated with the larger mean diffusion distance required for the exit of water molecules from larger LDH crystallites. The decrease of thermal stability of the LDH stacking is well in line with the TG curves for which the S3 dehydroxylation step is significantly shifted at lower temperatures for ZAF than for ZA (or ZCA). We propose that the lower temperature of the S3 step found for ZAF arises from the decrease of the thermodynamic stability of ZAF compared to ZA. This stability decrease is related to the contribution to the free energy of the positive value of the enthalpy of mixing associated with the formation of regular solid solution between Fe and Al in the pristine chlorineLDH.49 Furthermore, the dehydroxylation of the dehydrated ZA phase gives rise to a concomitant formation of amorphous prenucleus ZnO and ZnAl2O4 clusters whereas for ZAF the formation of prenucleus ZnM2O4 solid solution clusters begins more than 40 °C above that of ZnO. This delay in the onset of formation of prenucleus ZnM2O4 clusters is fully confirmed with the thermal decomposition of ZCAF (Figure S7, Supporting Information). Once again this can be tentatively associated with the thermodynamic contribution arising from the positive enthalpy of mixing involved in the formation of the Zn(Al2−xFex)O4 spinel solid solution inducing a less negative free energy of mixing.49 Divalent Metal Substitution and Phase Speciation. Archetype evolutions of the zinc and copper based species monitored at each corresponding K edge by Quick-XANES are presented in Figure 12. Copper and zinc transformations of the ZCA chlorine-LDH (Figure 12) and of the ZCAF one (Figure S7, Supporting Information) are monitored simultaneously, thanks to the capability of the Quick-XAS monochromator12 used herein. Close to the CuO crystallization temperature (267 °C), the evolution of Cu-based species determined by Quick-XAS reveals that more than 50% of the Jahn−Teller distorted coordinated Cu atoms in the pristine chlorine-LDH are already transformed into square-planar coordinated Cu in the nanoCuO phase. As reported by Quick-XAS, the onset of demixing of copper to form nano-CuO occurs well before the crystallization onset, but also well before the disappearance of the (00l) peak in the thermal evolution of the WAXS, indicating that the stacking of the octahedral layers is still present (Table 2) at this stage. It proves that the extraction of Cu from the sheet leads to the formation of cationic vacancies within the layers. Accordingly, an increase of diffusion coefficient proportional to the Cu vacancy concentration is expected. As a matter of fact, a concomitant transformation of dehydrated chlorine-LDH in amorphous prenucleus ZnAl2O4 and ZnO clusters is observed for ZCA and ZA. However, the proportion of ZnAl2O4 clusters rapidly reaches a plateau around 15% for ZA (Figure 11), whereas the formation of about 30%

Figure 12. Compositional fractions of species during the thermal evolution of ZCA determined by XAS at the Zn and Cu K edges (heating rate 10 °C min−1).

of the ZnAl2O4 phase is observed for ZCA heated to 270 °C followed by a quite constant proportion up to 350 °C (Figure 12). This finding is in good agreement with the topotactic transformation of LDH to spinel facilitated by Zn diffusion through octahedral CuII vacancy to the tetrahedral site of spinel oxide. In order to confirm this kinetic effect, the role of the heating rate (5 °C·min−1 instead of 10 °C·min−1 used previously) was investigated on the phase speciation occurring for the thermal decomposition of ZCA and ZCAF. The comparison of ZCA speciation plot monitored simultaneously at the Zn and Cu K edges during heating at 10 °C·min−1 (Figure 12) and 5 °C·min−1 (Figure 13) evidences two temperature domains bordered by the vertical dashed line at 260 °C named in the following as low temperature LT (260 °C) domains. In the LT domain, only the curve corresponding to the nano-ZnO is dependent on the heating rate. At the opposite, the speciation curves of all species show correlated dependence with the heating rate on the HT domain. Concerning the LT domain, the heating rate independency of pristine LDH transformation coupled to the increasing amount of LDH dehydrated phase is in agreement with the absence of a kinetic limitation effect on the evaporation of water present in interparticle pores and intercalated in the gallery. The most important finding in the LT domain is the significant reduction of the amorphous prenucleus ZnO cluster formation observed by decreasing the heating rate while the amount of amorphous prenucleus-spinel ZnAl2O4 clusters and CuO ones formed at the same temperature is kept constant 2864



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from localized diffusive reaction fronts where a reaction product grows at the surface of a solid reactant.33,52 The most interesting phenomenon revealed upon decreasing the heating rate is the sequential reduction of copper species, CuII → CuI → Cu0 observed under air atmosphere. Although the transformation of CuCl2 in CuCl (at 480 °C) and of Cu2OCl2 in CuCl and CuO (at 470 °C) under argon is wellknown, the observed CuII reduction seems surprising in the presence of air. Nevertheless, a controllable transformation in copper valence states between CuO and CuCl was recently reported during heating at 300 °C of the mixture CuCl2·2H2O/ cyclodextrin under air.53 Our speciation data indicate that the thermal decomposition of CuIClx to metallic copper occurring above 340 °C for a 5 °C·min−1 heating rate is concomitant with the transformation of ZnO to ZnAl2O4 (Figure 13). This finding suggests a thermochemical synergy between these transformations, involving for instance, the contribution on the heat released by the formation of spinel to the endothermic process of reduction of CuIClx to metallic copper. Furthermore, it is important to mention that the nanometric size of the astransformed copper metallic particles estimated from EXAFS can inspire the design of new synthetic routes of functional materials based on the thermal decomposition of copper-based LDH precursors.

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CONCLUSIONS The combined in situ XAS and WAXS studies have enabled a detailed description of the physical−chemical transformations taking place during chlorine-LDH thermal decomposition. Indeed, this approach has allowed the in situ survey of metastable metallic phase speciation at the local and long-range atomic orders and the correlation of the structural changes with the three mass loss steps observed in the dTG trace. The TG/WAXS combined analyses have shown that the chlorine-LDH second decomposition step, extensively described in the literature as resulting from the evaporation of interlayer adsorbed water molecules, is accompanied by an interlamellar distance contraction. This step also has a slight effect over the shape and intensity of the Zn K-edge XAS spectral white line, which is related to the increase of the atomic disorder inside the layer galleries. The third decomposition step (well-known as layer dehydroxylation) provides an intricate sum of physical chemical transformations involving divalent and trivalent metals composing the pristine chlorine-LDH. In this stage, an expansion of the interlamellar distance of the dehydrated LDH is observed before the breakdown of the lamellar framework arising around 275−295 °C. The change of interlamellar distance is ascribed to the topotactic transformation of dehydrated LDH into nano-ZnO and nano zincbased spinel amorphous clusters resulting from the migration of octahedrally coordinated zinc atoms to tetrahedral sites in the interlayer space. Upon further heating, ZnO crystallization arises from the direct aggregation of the amorphous prenucleus ZnO clusters occurring a few degrees before the loss of stacking for ZA and ZAF, whereas the quantity of amorphous zinc-based spinel clusters does not increase anymore. The spinel phase remains ill-crystallized as suggested by WAXS. Even if the substitution of trivalent metals in the pristine chlorine-LDH (Al by Fe) did not change the speciation of zincbased phases at 450 °C, an increase of the thermal stability of the dehydrated LDH is evidenced by the observed decomposition delay (223 °C for ZAF compared to 175 °C for ZA).

Figure 13. Compositional fractions of species during the thermal evolution of the ZCA at the Zn and Cu K edges (heating rate 5 °C·min−1).

irrespective of the heating rate. This demonstrates that the departure of Cu atoms from the LDH, coupled to the shortrange diffusion of Zn atoms toward the Cu depleted region, gives rise to different localized divalent cation distributions. Accordingly, some localized regions achieve the atomic proportion Zn/Al = 2 like in the pristine LDH composition while others attain the ratio Zn/Al = 0.5 like in the spinel one. This picture is reasonably close to the TEM observation of the LDH (NiCoAlO4)/spinel metastable mosaic structure formed after 12 h of heating to 250 °C.50 In the HT domain, the significant shift to lower temperatures of the speciation curve for the Cu(I)Clx species is observed. The temperature at which the CuIClx species is predominant is shifted from 450 to 340 °C when lowering the heating rate from 10 to 5 °C·min−1, respectively. The formation of the CuIClx species results from the chlorination of both amorphous CuO clusters and Cu embedded in the chlorine-LDH. A subsequent modification of the speciation curves for these species occurs as well. The decomposition of nano-CuO arises without the plateau step well evidenced at a 10 °C·min−1 heating rate, and the decomposition of LDH occurs more abruptly. This is a strong indication that decreasing the heating rate allows the chlorine species embedded in the LDH gallery to diffuse toward the Cu sites promoting the chlorination reaction. Indeed, similar heating rate dependency has been observed for the formation of CuCl during the thermal decomposition of Cu2OCl2.51 The dependency of reaction kinetics on the heating rate for reactions involving gas production (CO2, Cl2, O2...) is generally interpreted as resulting 2865



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(2) You, Y. W.; Zhao, H. T.; Vance, G. F. J. Mater. Chem. 2002, 12, 907. (3) Guo, S. Z.; Zhang, C.; Peng, H. D.; Wang, W. Z.; Liu, T. X. Compos. Sci. Technol. 2011, 71, 791. (4) Choy, J. H.; Kwak, S. Y.; Park, J. S.; Jeong, Y. J.; Portier, J. J. Am. Chem. Soc. 1999, 121, 1399. (5) Choy, J. H.; Kwak, S. Y.; Jeong, Y. J.; Park, J. S. Angew. Chem., Int. Ed. 2000, 39, 4042. (6) Portier, J.; Choy, J. H.; Subramanian, M. A. Int. J. Inorg. Mater. 2001, 3, 581. (7) Cavani, F.; Trifiro, F.; Vaccari, A. Catal. Today 1991, 11, 173. (8) del Arco, M.; Malet, P.; Trujillano, R.; Rives, V. Chem. Mater. 1999, 11, 624. (9) Iglesias, A. H.; Ferreira, O. P.; Gouveia, D. X.; Souza, A. G.; de Paiva, J. A. C.; Mendes, J.; Alves, O. L. J. Solid State Chem. 2005, 178, 142. (10) Crespo, I.; Barriga, C.; Ulibarri, M. A.; Gonzalez-Bandera, G.; Malet, P.; Rives, V. Chem. Mater. 2001, 13, 1518. (11) Intissar, M.; Segni, R.; Payen, C.; Besse, J. P.; Leroux, F. J. Solid State Chem. 2002, 167, 508. (12) Fonda, E.; Rochet, A.; Ribbens, M.; Barthe, L.; Belin, S.; Briois, V. J. Synchrotron Radiat. 2012, 19, 417. (13) La Fontaine, C.; Barthe, L.; Rochet, A.; Briois, V. Catal. Today 2013, 205, 148. (14) Ravel, B.; Newville, M. J. Synchrotron Radiat. 2005, 12, 537. (15) Newville, M. J. Synchrotron Radiat. 2001, 8, 322. (16) Webb, S. M. Phys. Scr. 2005, T115, 1011. (17) Wasserman, S. R. J. Phys. IV 1997, 7, 203. (18) Wasserman, S. R.; Allen, P. G.; Shuh, D. K.; Bucher, J. J.; Edelstein, N. M. J. Synchrotron Radiat. 1999, 6, 284. (19) McPeak, K. M.; Becker, M. A.; Britton, N. G.; Majidi, H.; Bunker, B. A.; Baxter, J. B. Chem. Mater. 2010, 22, 6162. (20) Wang, Q.; Hanson, J. C.; Frenkel, A. I. J. Chem. Phys. 2008, 129. (21) Ruckebusch, C.; Blanchet, L. Anal. Chim. Acta 2013, 765, 28. (22) Marquez-Alvarez, C.; Rodriguez-Ramos, I.; Guerrero-Ruiz, A.; Haller, G. L.; Fernandez-Garcia, M. J. Am. Chem. Soc. 1997, 119, 2905. (23) Ressler, T.; Wong, J.; Roos, J.; Smith, I. L. Environ. Sci. Technol. 2000, 34, 950. (24) Conti, P.; Zamponi, S.; Giorgetti, M.; Berrettoni, M.; Smyrl, W. H. Anal. Chem. 2010, 82, 3629. (25) Nunes, C. A.; Resende, E. C.; Guimaraes, I. R.; Anastacio, A. S.; Guerreiro, M. C. Appl. Spectrosc. 2011, 65, 692. (26) Huang, T. C.; Toraya, H.; Blanton, T. N.; Wu, Y. J. Appl. Crystallogr. 1993, 26, 180. (27) Zak, A. K.; Majid, W. H. A.; Abrishami, M. E.; Yousefi, R. Solid State Sci. 2011, 13, 251. (28) Constantino, V. R. L.; Pinnavaia, T. J. Inorg. Chem. 1995, 34, 883. (29) Frenkel, A. I.; Kleifeld, O.; Wasserman, S. R.; Sagi, I. J. Chem. Phys. 2002, 116, 9449. (30) Kanezaki, E. Inorg. Chem. 1998, 37, 2588. (31) Roussel, H.; Briois, V.; Elkaim, E.; de Roy, A.; Besse, J. P. J. Phys. Chem. B 2000, 104, 5915. (32) Fernandez-Garcia, M.; Martinez-Arias, A.; Rodriguez-Ramos, I.; Ferreira-Aparicio, P.; Guerrero-Ruiz, A. Langmuir 1999, 15, 5295. (33) Kim, J. Y.; Rodriguez, J. A.; Hanson, J. C.; Frenkel, A. I.; Lee, P. L. J. Am. Chem. Soc. 2003, 125, 10684. (34) Brugger, J.; Etschmann, B.; Liu, W.; Testemale, D.; Hazemann, J. L.; Emerich, H.; van Beek, W.; Proux, O. Geochim. Cosmochim. Acta 2007, 71, 4920. (35) Berry, A. J.; Hack, A. C.; Mavrogenes, J. A.; Newville, M.; Sutton, S. R. Am. Mineral. 2006, 91, 1773. (36) Cook, N. J.; Ciobanu, C. L.; Brugger, J.; Etschmann, B.; Howard, D. L.; de Jonge, M. D.; Ryan, C.; Paterson, D. Am. Mineral. 2012, 97, 476. (37) Sharma, V. K.; Millero, F. J. J. Solution Chem. 1990, 19, 375. (38) Asbrink, S.; Waskowska, A. J. Phys.: Condens. Matter 1991, 3, 8173.

This behavior is related to the larger crystallite sizes of pristine LDH observed for ZAF, which give rise to a larger mean diffusion distance for the exit of water molecules from larger LDH crystallite. On one hand, a significant temperature decrease for the loss of lamellar stacking and, on the other hand, a significant temperature increase for the onset of formation of Zn-based spinel amorphous clusters are observed for ZAF compared to ZA. These behaviors are related to the positive configurational entropy contribution to the free energy of the system resulting from the formation of Zn(Al2−xFex)O4 spinel solid solution. The in situ WAXS evidenced that in the case of the substitution of ZnII by CuII, first CuO crystallites are formed about 18 °C before the loss of the stacking of layers and 28 °C before the crystallization of ZnO. The fact that the LDH stacking was preserved during the demixing of copper and the subsequent formation of amorphous prenucleus CuO clusters allows us to propose that the formation of copper vacancies within the octahedral layers favors the topochemical transformation of dehydrated LDH into metastable zinc spinel-like phases at a low temperature (∼200 °C). It is somehow reminiscent of the intralayer flexibility as reported for other topochemical reactions involving the oxidation process in CoFe-54 or CoNi-based55 LDH systems or observed in the case of intralayer cation substitution, the so-called diadochy phenomenon.56 The chemical nature of copper phases and the amount of zinc spinel phase obtained by calcination at 450 °C can be changed in a controlled way by tuning the heating rate. Indeed, heating at 10 °C·min−1 was shown to transform amorphous CuO clusters into a metastable CuIClx phase, whereas at 5 °C·min−1, the CuIClx phase is transformed into Cu metallic nanodomains.

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ASSOCIATED CONTENT

S Supporting Information *

RT XRPD, TG and dTG, EXAFS simulations, and structural parameters determined by EXAFS fitting of the pristine LDH. Examples of LC fitting at the Zn K edge and compositional fraction of Zn and Cu species for ZCAF. This information is available free of charge via the Internet at http://pubs.acs.org/

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AUTHOR INFORMATION

Corresponding Author

*Phone: +33 1 69 35 96 44. Fax: +33 1 69 35 94 56. E-mail: [email protected]. Present Address ∥

Karlsruhe Institute of Technology, Engesserstraße 20, 76131 Karlsruhe, Germany Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support received from the Brazilian agencies, namely, the CAPES, CNPq, and FAPESP and from the bilateral FAPESP-CNRS cooperation program (Project Number EDC24785). The authors want to thank SOLEIL for providing beamtime at the SAMBA and SWING beamlines and for financial support of the experiments.

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REFERENCES

(1) Leroux, F.; Besse, J. P. Chem. Mater. 2001, 13, 3507. 2866



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WAXS Time-Resolved Phase Speciation of Chlorine LDH

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