J. Phys. Chem. 1996, 100, 8535-8542
8535
Hydrotalcite Decomposition Mechanism: A Clue to the Structure and Reactivity of Spinel-like Mixed Oxides Maurizio Bellotto, Bernadette Rebours,* Olivier Clause, and John Lynch Institut Franc¸ ais du Pe´ trole, B.P. 311, 92506 Rueil Malmaison Cedex, France
Dominique Bazin and Eric Elkaı1m LURE, Bat 209D, Centre UniVersitaire Paris-Sud, 91405 Orsay, France ReceiVed: January 2, 1996X
The structure of the phases obtained upon dehydration and decomposition of hydrotalcite-like compounds is investigated by several experimental techniques. A reaction mechanism is proposed encompassing a change in coordination of the M(III) cations during the dehydration step. The formation of a 3-dimensional structure occurs upon the subsequent decomposition of the interlayer anions and dehydroxylation of the octahedral layers. In the decomposed material the cations are trapped in the interstices of a regular oxygen cubic close packed lattice and exhibit a considerable disorder. Strains develop during the decomposition, which are likely related to the observed increase of surface area. The thermal stability of the decomposed materials is connected to the reduced cation diffusivity in the oxygen lattice.
Introduction The subject of this work is the decomposition mechanism of hydrotalcite-like compounds (HTlcs). It is the second of two articles dedicated to such compounds, where the first [J. Phys. Chem. 1996, 100, 8527] is concerned with their crystal chemistry. Extensive reference will be made to the first article as part I in the following. The industrial applications of HTlcs are widespread, ranging from ion exchange and waste water halogen scavenging to antacids in medicine. However, the interest of the authors is limited to the catalytic applications of materials derived from HTlcs. It is now well established that HTlcs are precursors for a wide range of mixed oxide catalysts (for a review see ref 1); moreover, HTlcs are also formed during the impregnation of γ-Al2O3 with solutions of salts of bivalent cations, even at neutral pH.2 What makes HTlcs such a general catalyst precursor is (a) their ability to accommodate a very large number of M(II),M(III) cation pairs (e.g., refs 1 and 3-5), (b) the interspersion of the cations on an atomic scalesno cation segregation occurs (e.g., ref 6 for Ni,Al), but rather cation ordering is observed (e.g., ref 7 and part I)sand (c) the formation of materials with high surface area upon decomposition (e.g. ref 1 and references therein). The first two properties are a result of hydrotalcite structure. All M(II),M(III) cation pairs which can fit in the octahedral sites of brucite-type layers are able to form HTlcs. The dimensions of the layer, its features, e.g., ordering of M(III) cations and distortion of the coordination octahedra, and the nature of the intralayer bonding depend on the chemical species which make up the structure as well as on the M(II)/M(III) ratio (see part I). However, the interspersion of M(II) and M(III) cations is always maintained. The last property also appears to be a characteristic of hydrotalcite structure8,9 and is probably linked to the decomposition mechanism. High specific surfaces are obtained regardless of the morphology of the starting material. The microstructural properties of the decomposing material, such as crystallite size, do not influence the microstructural properties of the decomposed material, such as surface area, even if the decomposition is recognized to be topotactic.10 * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, April 15, 1996.
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The materials obtained upon decomposition are very poorly crystallized and exhibit a high-temperature stability, both as such and after a reduction-activation step which is used for some catalyst types (e.g., Ni,Al).11,12 The nature of this stability has been postulated to be due either to the paracrystalline nature of the material13,14 or to the formation of multiphasic shell and core particles whose surface is stabilized by the interaction of the different phases.12,15 To choose between the different hypotheses is a difficult task, also considering that the distinction between a single phase with compositional modulations and two different phases is somewhat arbitrary. Compositional modulations may be induced by or related to the strain field causing the paracrystallinity, and the relations between strain field and particle size are set forth in ref 16. The aim of the present paper is thus not to solve the quarrel, but to investigate the structural changes occurring during the decomposition, to follow the modification of the cation coordination, and to try to put forward some ideas and possible explications of the observed thermal stability. In the following we will be concerned with the Mg,Al system. To enhance the diffraction contrast between the cations and to enable differential radial distribution function (DRDF, see part I) analysis and EXAFS experiments, Ga was substituted for Al in some samples. The decomposition reaction is a two-step process.17 Interlayer water is lost in the temperature range 423473 K, and an intermediate structure is formed which still retains a layered structure though with a reduced basal spacing. The collapse of the structure occurs in the temperature range 573673 K, together with decomposition of the anions, CO32- and NO3- in the present work, and condensation of the hydroxyls of the octahedral layers. A three-dimensional structure intermediate between rock salt and spinel is formed,15,18 which is the base product for catalyst fabrication. The structural investigation of the decomposed products and even more so of the intermediate dehydrated phase requires special care to avoid rehydration to the original hydrotalcite phase. Such rehydration is very fast, especially for the Mg,Al system.19 To cope with such a requirement, in-situ decomposition experiments have been performed wherever possible. In © 1996 American Chemical Society
8536 J. Phys. Chem., Vol. 100, No. 20, 1996 all other cases rehydration was prevented by avoiding any contact between the samples and the atmosphere. Experimental Section Sample Preparation and Chemical Analysis. The preparation of HTlcs has been reported elsewhere6,8,15 and has also been resumed in part I. The present investigation deals with some of the samples studied in part I, namely, the Mg,Al and Mg,Ga ones. The procedure of chemical analysis for both the cations and the anions has therefore already been reported there. The calcinations for the ex-situ experiments were performed in a muffle furnace for 8 h at temperatures ranging from 473 to 1173 K, with heating rates higher than 10 K/min. Thermal Analysis and Surface Area Measurements. Thermogravimetric analysis coupled with infrared spectroscopy of the evolved gases (TG-IR) was performed on all the samples, to identify the decomposition processes and to associate them with gas evolution. A DuPont thermobalance coupled with a Bomem IR spectrometer was used in flowing He atmosphere (250 mL/min) and with a heating rate of 5 K/min from room temperature to 873 K. The sample quantities ranged from 13 to 22 mg in the different measurements. Surface areas were measured with a Carlo Erba Sorptomatic Model 1700 instrument, using nitrogen absorption. The hightemperature measurements were performed after a pretreatment at the desired temperature, without any intermediate contact between the sample and the atmospheric air. 27Al MAS-NMR. Nuclear magnetic resonance (NMR) free induction decay (FID) measurements were performed on a Brucker MSL 400 instrument to investigate the Al coordination. The wide bore magnet was fitted with a high-speed magic angle spinning (MAS) Brucker probe. The samples were spun in 4-mm-diameter rotors at 13 kHz. At 9.4 T the resonance frequency of 27Al is 104.26 MHz. In order to assure quantitative measurements, one-pulse experiments were performed. The pulse length was limited to 2 µs; with such a narrow pulse (
