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Applied Surface Science xxx (2005) xxx–xxx
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Magnesia formed on calcination of Mg(OH)2
prepared from natural bischofite
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I.F. Mironyuk a, V.M. Gun’ko a,*, M.O. Povazhnyak a, V.I. Zarko a,
V.M. Chelyadin b, R. Leboda c, J. Skubiszewska-Zie˛ba c, W. Janusz c
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Institute of Surface Chemistry, 17 General Naumov Street, 03164 Kiev, Ukraine
Pricarpatsky Stefanyk University, 57 Shevchenko Street, Ivano-Frankovsk, Ukraine
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Maria Curie-Sklodowska University, 20031 Lublin, Poland
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Received 1 December 2004; accepted 5 June 2005
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Abstract
Calcination of magnesium hydroxide, which was prepared from natural bischofite MgCl26H2O, leading to dehydration
2(BBMgOH) ! BBMg–O–MgBB + H2O, is accompanied by transition of phase not only to MgO but also to MgOx at x < 1
(assigned to Mg4O3) at moderate temperatures. At higher temperatures, MgOx is completely transformed into MgO. Magnesium
hydroxide and oxide heated at different temperatures were studied using the TEM, XRD, IR, PCS, TG-DTA, nitrogen and argon
adsorption methods. The electronic structure of MgO and Mg4O3 was studied using the ab initio quantum chemical method with
periodic conditions. According to TEM images, the morphology of particles changing from Mg(OH)2 laminae to aggregates of
interpenetrated MgO cubelets and foils depend strongly on the calcination temperature. Significant changes in surface area are
observed mainly at 325–470 8C on desorption of a major portion of eliminated water corresponding to 28.4 wt.% at its total
amount of 30.9 wt.%. Pore size distribution (PSD) is sensitive to treatment conditions and the main PSD peaks shift towards
larger pore size with elevating temperature. The characteristics of the surface hydroxyls as well as of the bulk Mg–O bonds
depend on heating conditions, as noticeable changes are observed in the XRD patterns and the IR spectra of the samples
undergoing the mentioned transformation of phase Mg(OH)2 ! MgOx ! MgO.
# 2005 Published by Elsevier B.V.
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Keywords: Bischofite; Magnesium hydroxide; Magnesia; MgOx; TEM; XRD; IR; PCS; TG-DTA; Argon adsorption; Nitrogen adsorption;
Periodic ab initio calculation; Water desorption; Transition of phase; Particle morphology; Surface hydroxyls
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1. Introduction
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* Corresponding author. Tel.: +380 44 422 9627;
fax: +380 44 424 3567.
E-mail address: [email protected] (V.M. Gun’ko).
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Synthesis of nanomaterials with metal oxides
possessing the particle morphology and the physicochemical properties strongly different from those of
solid materials is of importance for many applications
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0169-4332/$ – see front matter # 2005 Published by Elsevier B.V.
doi:10.1016/j.apsusc.2005.06.020
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the addition of reagents, calcination temperature, etc.
[16,17]. The modification of the acid–base properties
of magnesia is usually carried out by mixing it with
other oxides, metallic ions or noble metals, which
has revealed as a very effective way of tailoring the
activity towards many organic processes. Calcination
of gel with Mg(OH)2 at 372 8C gives the weight loss of
32.3%, higher than theoretical value of 30.9% [18].
Multidimensional magnesium oxide structures with
cone-shaped branching were produced using a simple
chemical vapour deposition method. The dominant
structures in the product include two-dimensional
assemblies and three-dimensional complex configurations [19]. Thus, the Mg(OH)2 ! MgO transformation can result in formation of materials of varied
textures and surface physicochemical properties.
However, the processes of preparation of highly
disperse magnesia from row natural minerals utilized
to form magnesium hydroxide as an intermediate
compound to produce MgO are complex and have
been studied only partially, e.g. on treatment of
seawater [20]. Therefore, the aim of this work is to
study changes in the morphology, the structural and
adsorption properties of magnesia characterized by
different phase composition (MgO + MgOx) dependent on temperature of dehydration of Mg(OH)2
produced from natural bischofite (MgCl26H2O plus
such impurities as Br, Fe, Mn, Si, Al, Ti, Cu, Ba, B, Au
and Ca).
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[1,2]. Magnesia (MgO) powders are widely used for
preparation of heat-resistant ceramics and special
cements [3]. MgO is also utilized as a heterogeneous
catalyst in synthesis of organics [4]. Disperse
magnesia can be also applied as a drug carrier, a
filler of lacquer-paint and polymer materials as well as
for many other purposes [1–6]. Roughly disperse
magnesia can be produced on calcination of magnesite
or dolomite or by treatment of MgS or other similar
compounds [2–7]. Highly disperse magnesia can be
prepared on calcination of natural compounds, such as
4MgCO3Mg(OH)24H2O or magnesium hydroxide
Mg(OH)2 [2–7]. Similar materials could be produced
by treatment of brucite at 520–550 K in vacuo [7,11–
15]. It should be noted that polycrystalline magnesia is
useful as a model system because its morphological
changes on heating are well documented [8–15]. The
TEM and AFM methods were successfully applied to
image the evolution of the morphology of polycrystalline MgO formed from Mg(OH)2 on sintering
procedures and compared with more regular MgO
samples [7]. The ordered and periodic structure on
surfaces of polycrystalline MgO powders were
observed in air by means of the AFM method. The
transformation Mg(OH)2 ! MgO is accompanied by
fragmentation of the original laminae into parallel
foils of MgO with 1–1.5 nm thickness, developed
along (1 1 1) planes and by the appearance of
aggregates of interpenetrated MgO cubelets of 1–
1.5 to 2–3 nm. The resulting aggregates, formed by
topotactic Mg(OH)2 ! MgO transformation, were
maintaining the gross shape of the original Mg(OH)2
microcrystals [7]. The effect of successive annealing
at higher temperature causes an increment of the MgO
terraces from 2–3 to 10 nm [7,11,14,15]. The average
edge length of the MgO cubes corresponds to
approximately 7 nm in agreement with the high value
of surface area of 200 m2/g [7]. The increased values
of dimension and roughness as compared to Mg(OH)2
can be explained with the transition to a new crystalline habitus, caused by the well known topotactic
transformation Mg(OH)2 ! MgO with release of
H2O. Annealing at 800 8C results in a further
enlargement of MgO aggregates with a decreased
mean height and in lower values of surface average
roughness [7]. Textural and acid–base properties of
MgO depend, to a great extent, on the synthesis
conditions, such as pH, gelifying agent, sequence of
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2. Experimental
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2.1. Materials
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Natural bischofite MgCl26H2O (Poltava Deposit,
Ukraine) was used as the raw material. It was mixed
with distilled water to concentration of 200 g/dm3.
This solution includes also 1.0 g MgCO3 and 0.16 g/
dm3 of Br and such impurities as Fe, Mn, Si, Al, Ti,
Cu, Ba, B, Au and Ca at the total content lower than
0.0185%. Mg(OH)2 was prepared using a mixture of
the bischofite solution with NaOH at pH 10.5–11.0
and room temperature. The precipitated product was
washed-off by distilled water and dried at 110 8C for
5 h. The structural characteristics of the product
were studied after its heating at different temperatures.
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2.2. TG-DTA
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where c1 is a constant, s the half-width of a desorption
DTG peak, Tmax the temperature of the maximal water
desorption, Q (T) the temperature dependence of the
surface coverage by the hydroxyl groups, n the reaction
order (n = 1 for molecularly adsorbed–desorbed water
and n = 2 for associatively desorbed water), Emin and
Emax the minimal and maximal values of the activation
energy Ea and Rg is the gas constant. Eq. (1) solved
using a regularization procedure describes temperature
dependence of the rate A (T) of water desorption related
to the differential TG (DTG) data. The physical model
of the TPD experiments used as a base of the kernel in
Eq. (1) was described in details elsewhere [21].
2.3. Transmission electron microscopy
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2.4. X-ray powder analysis
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The morphology of the materials (Fig. 1) was studied
using a JEM-100CXII electron microscope (accelerating voltage 100 kV and resolution 0.204 nm).
X-ray diffraction (XRD) patterns were recorded
using a DRON-4-07 (LOMO, St. Petersburg) dif-
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Thermal dehydration of Mg(OH)2 to form MgO
was studied using thermogravimetric (TG) measurements with differential thermal analysis (DTA) at 20–
1000 8C and a constant heating rate of 0.125 K/s using
a Q-1550D (Paulik, Paulik & Erdey, MOM, Budapest)
apparatus. Samples (loaded in quartz ampoules)
similar to those studied by the TG-DTA method were
prepared under the same conditions of calcination in
an electric furnace. These samples were heated at
different temperatures in the 20–1000 8C range,
cooled to room temperature, placed in a desiccator
without contact with air for storage and then used in
different measurements.
Calculation of a distribution function of activation
energy of water desorption f (Ea) was carried out using
integral equation:
c1 T½QðTÞn
ðT Tmax Þ2
AðTÞ ¼ pffiffiffiffiffiffiffiffiffiffi exp 2s 2
2ps 2
Z Emax
Ea
f ðEa Þexp (1)
dEa
Rg T
Emin
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Fig. 1. TEM images of particles of (a) Mg(OH)2 and MgO heated at
(b) 600 8C and (c) 900 8C. Electronograms are shown in the right
upper corner.
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fractometer with a Co anode and a Fe filter. Analysis
of the crystalline structure was carried out according
to the JCPDS Database (International Center for
Diffraction Data, PA, 2001). Computer simulation of
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Table 1
Textural characteristics of Mg(OH)2 and magnesium oxide heated at 600 and 900 8C
Sample
T (8C)
SBET (m2/g)
Smic (m2/g)
Smes (m2/g)
Smac (m2/g)
Vmic (cm3/g)
Vmes (cm3/g)
Vmac (cm3/g)
Mg(OH)2
MgO
MgO
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900
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0.001
0
0.218
0.199
0.010
0.468
0.599
0.635
Note: Micropores correspond to R < 1 nm, mesopores to 1 < R < 25 nm and macropores to R > 25 nm and T is the calcination temperature.
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The IR spectra were recorded by means of a
Specord M-80 (Carl Zeiss) spectrophotometer. The
samples stirred with KBr (1:100) were pressed in
plates of 20 mm 5 mm in size.
2.6. Textural characteristics
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X Z
¼
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fV;i ðRÞdR
rmin
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rk;i ð pÞ
wi
ti ð p; RÞ fV;i ðRÞdR
R s sf =2
(2)
where ci = cslit, ccyl and csph are the weight factors
determining contributions of slitlike and cylindrical
pores and gaps between spherical particles, respectively, to the volume filled by adsorbate (WS), wi a
constant (equal to 1 for slit-shaped pores, 2 for
cylindrical pores and 1.36 for gaps between spherical
particles packed in aggregates with the cubic lattice) in
modified Kelvin equation, rmin and rmax the minimal
and maximal half-widths (or pore radii), respectively,
rk ( p) is determined with the modified Kelvin equation, the thickness of the adsorbed layer t ( p, R) can be
computed with the modified BET equation and
ssf = (ss + sf)/2 is the average collision diameter of
surface and fluid (nitrogen) atoms [24–27]. The desorption data were utilized to compute the f (R) distributions with Eq. (2) and modified regularization
procedure under non-negativity condition ( f (R) at
any R) at a fixed regularization parameter a = 0.01.
Eq. (2) was solved at f V,i (R) 6¼ f V,j (R) with multimodal self-consistent (subsequent for f V,i (R) at different i regularization with respect to slitlike and
cylindrical pores and gaps between spherical particles.
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To study the textural characteristics of the materials,
low-temperature (77.4 K) nitrogen adsorption–desorption isotherms were recorded using a Micromeritics
ASAP 2405N adsorption analyser. The specific surface
area (Table 1, SBET) was calculated according to the
standard BET method [22,23]. Additionally, the
specific surface area was estimated using the argon
adsorption from an Ar–He mixture at 77.4 K with
chromatographic control (mean error 6%). The Ar
adsorption was also studied using a Micromeritics
Gemini 2360 adsorption analyser. The total pore
volume Vp was evaluated by converting the volume
of nitrogen adsorbed at p/p0 0.98 ( p and p0 denote the
equilibrium pressure and the saturation pressure of
nitrogen at 77.4 K, respectively) to the volume of liquid
nitrogen per gram of the material.
Pore size distributions (PSDs) (differential PSD
f V (R) dVp/dR) of the studied materials were
calculated using modified overall adsorption equation
proposed by Nguyen and Do [24] for slit-shaped pores
of carbons and modified for other materials with
cylindrical pores and pores as gaps between spherical
particles [25–27]. The nitrogen desorption data were
utilized to compute the f V (R) distribution functions
using a modified regularization procedure CONTIN
[28] under non-negativity condition ( f V (R) 0 at any
R) with a fixed regularization parameter a = 0.01. A
mixture of model pores, such as slit-shaped pores (as
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2.5. Infrared spectroscopy
Mg(OH)2 and MgO particles include lamellar and foil
structures (Fig. 1)), cylindrical pores (which can be
characteristic for intermediate structures formed on
calcination of the material) and gaps between
spherical particles (since nanoparticles observed in
Fig. 1c can be roughly approximated by equivalent
spherical particles forming random aggregates) was
applied. In this case, the isotherm equation can be
given in the form of overall equation:
X
WS ¼
ci W i
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the crystalline structure of MgOx was performed using
the Powder Cell 2.3 program package.
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m
PD ¼ 22
G
(4)
where m2 = (D2*–D*2)q4, D* is the average value of
the diffusion coefficient, G = Dq2 and q = q = 4pn/
l (sin(u/2)) is the scalar of the wave scattering vector
[29,30]. Monodisperse particles correspond to
PD < 0.02 and PD between 0.02 and 0.08 represents
a narrow distribution. An average size of colloidal
particles value can be estimated in respect to the
scattering intensity,
P
Ni di6
dPCS ¼ Pi
(5)
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i Ni di
3. Results and discussion
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Particle size distribution of magnesia samples was
studied using a Zetasizer 3000 (Malvern Instruments)
apparatus based on the photon correlation spectroscopy (PCS) (wavelength l = 633 nm, scattering
angle Q = 908 and software Version 1.3) [29,30].
Deionised distilled water (pH 6.67) and oxide
samples (5 g/dm3 water) were utilized to prepare
suspensions sonicated for 6 min using an ultrasonic
disperser (Sonicator Misonix, power 500 W and
frequency 22 kHz). To characterize the particle size
distribution, polydispersity (PD) can be used as a
measure of its non-uniformity and PD can be written
as follows:
The morphology of the studied samples changes on
their calcination from lamellar magnesium hydroxide
particles (100–200 nm in length and 3–15 nm in
thickness shown in Fig. 1a) to more compacted
magnesia structures (Fig. 1b and c). Observed thin
platelets, foils and cubelets of magnesia were formed
on calcination of the washed-off material due to
transformation of original Mg(OH)2 laminae into
MgOx and MgO caused by dehydration 2(BBMgOH)
! BBMg–O–MgBB + H2O (leading to particle
strengthening) and parallel fragmentation of laminae.
These morphological changes reflect in the nitrogen
adsorption–desorption isotherms (Fig. 2) and pore size
distribution (Fig. 3) as well as in the structural
parameters (Table 1). The isotherm shape suggests
that the studied materials are meso- and macroporous
since the initial raising of the curves is very low and
the nitrogen adsorption sharply increases only at p/
p0 > 0.9. The calcination at 600 8C leads to enlargement of the hysteresis loop at near the same curve at p/
p0 < 0.9. However, the calcination at 900 8C
diminishes the nitrogen adsorption over the total
pressure range and the hysteresis loop becomes
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2.7. Photon correlation spectroscopy
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The electronic structure of MgO and Mg4O3 was
calculated using Hartree-Fock (HF) theory with the 6–
31G (d, p) basis set and periodic conditions using the
NWChem 4.5 Program Package [31].
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fn ðRi Þ ¼ 0:5ð f ðRi Þ þ f ðRi1 ÞÞðRi Ri1 Þ:
2.8. Quantum chemical calculations
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particle number,
P
N i di
dN ¼ Pi
i Ni
(6)
and particle volume (or weight)
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Ni di4
dV ¼ Pi
3
i Ni di
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For a pictorial presentation of the pore size distributions, the f (R) functions were re-calculated to incremental PSDs (IPSDs).
(7)
with inequality dN dV dPCS (the equal sign for
monodisperse particles).
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Fig. 2. Nitrogen adsorption–desorption isotherms for Mg(OH)2 and
MgO heated at 600 and 900 8C.
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Fig. 5. TG (1) and DTA (2) curves of calcination of Mg(OH)2.
Fig. 3. Pore size distributions of Mg(OH)2 and MgO heated at 600
and 900 8C.
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smaller and narrower, i.e. the mesoporosity decreases,
however, the macroporosity increases (Table 1, Vmac).
Therefore, the main IPSDV peaks at R > 10 nm shift
towards larger R values (Fig. 3) for samples calcined at
higher temperatures. Additionally, the PSD intensity
of narrow mesopores at R < 10 nm decreases for the
calcined samples. This result corresponds to a
diminution of Vmes and Smes (Table 1) and an increase
in Vmac with elevating temperature of the calcination.
The maximal Smes and SBET values (Table 1) are
observed for a sample heated at a non-maximal
temperature (Fig. 4). These morphological changes
are caused by fragmentation of the initial laminae and
formation of aggregates of interpenetrated MgO
cubelets (which are denser than laminae) and
enhancement of their sintering on heating (Fig. 1).
As a whole, an increase in the dehydration degree
leads to non-linear changes in the specific surface area
Fig. 4. Specific surface area as a function of Mg(OH)2 calcination
temperature for fresh samples.
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(Fig. 4) and a sharp increase in SBET is observed after
heating of Mg(OH)2 at T > 325 8C and a maximum is
at T = 470 8C corresponding to completion of the main
stage of the dehydration (Fig. 5, elimination of
28.4 wt.% of water) and transition of phase
Mg(OH)2 ! MgO. Subsequent elevating of temperature leads to a strong diminution of the surface area
and it is equal to 40 m2/g after calcination at 1000 8C.
A complex shape of the IPSDV (Fig. 3) can be caused
by several types of interparticles space in the
Mg(OH)2 and MgO powders corresponding to gaps
between adjacent non-porous primary particles (crystallites) in the same aggregate and particles from
neighbouring aggregates and complex random shape
of aggregates (Fig. 1). It should be noted that the
deviation of the pore shape from the model of slitshaped pores [26] is negative, especially for Mg(OH)2.
This result suggests the presence of individual laminae
in the powder in agreement with the TEM images
(Fig. 1a). Notice that the specific surface area of
samples after their storage during several months
(Table 1) differs from that of fresh samples (Fig. 4).
This difference could be caused by both the difference
in the used methods of the measurements, treatment
conditions of samples before the measurements and
certain changes in the oxide structure due to long-term
storage after calcination. However, the results
obtained using different methods for fresh and aged
samples are in qualitative agreement.
The PCS measurements of the particle size
distribution in the aqueous suspension show that
Mg(OH)2 particles in contrast to magnesia heated at
600 and 900 8C rapidly form the sediment due to
strong hydrogen binding of these particles into large
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Fig. 6. Particle size distributions for magnesia samples (calcined at
600 and 900 8C) in the aqueous suspensions at CMgO = 0.5 wt.% and
pH 10.56 and 10.50, respectively.
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a significant endothermic effect over this temperature
range caused by the dehydration and the phase
transformation. Associative desorption of water on
condensation of the surface hydroxyls requires a
significant energy due to essential changes in the
lattice structure of the material [8–15]. The distribution function of the activation energy f (Ea) (Fig. 7)
corresponds to relatively high Ea values for associatively desorbed water over this temperature range due
to additional energy needed for transition of phase
Mg(OH)2 ! MgOx and MgO with changes in the
lattice (Figs. 1 and 8 and Tables 2 and 3). Desorption
of water at T > 470 8C gives the weight loss of only
2.5 wt.%. Consequently, the main structural changes
linked to water desorption occur at T < 470 8C that is
in agreement with the literature [7–15]. Consequently,
certain impurities present in the studied materials
produced from natural bischofite do not practically
affect the morphological changes on the transformation Mg(OH)2 ! MgO.
The XRD data show that formation of MgO over
the temperature range of the main dehydration of
Mg(OH)2 (325–470 8C) is accompanied by appearance of a MgOx phase (Fig. 8). The content of this
phase decreases on heating at T > 470 8C and only
MgO is observed at T > 600 8C. The morphology of
particles heated at 325–470 8C corresponds to both
lamellar and foiled structures (primary particles of 20–
100 nm in size form loose aggregates) (Fig. 1).
Calcination at T < 600 8C changes the structure but
the lamellar type partially remains. Formation of the
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aggregates and flocks (Fig. 6). Secondary particles of
MgO prepared at 600 8C, corresponding to the phase
composition with MgO and MgOx, have a broader
distribution at the polydispersity PD = 0.0786 (i.e. this
distribution is narrow) than that of the sample calcined
at 900 8C. The latter has a very narrow distribution at
PD = 0.002 corresponding to monodisperse particles.
The effective diameter (184 nm) of aggregates of the
sample calcined at 900 8C is in a good agreement
with the size of secondary particles observed by the
TEM (Fig. 1c). Preservation of these aggregates on
sonication reveals that the binding of primary particles
is strong in contract to samples treated at lower
temperatures. Notice that the pH value of the
suspension at CMgO = 0.5 wt.% is equal to 10.56 and
10.50 for the samples calcined at T = 600 and 900 8C,
respectively. Consequently, the basicity of the first
sample (MgO + Mg4O3) can be slightly higher than that
of pure MgO.
The total amount of water eliminated on thermal
dehydration of the magnesium hydroxide corresponding to 30.85 wt.% (Fig. 5) is in agreement with the
theoretical value. Heating of the hydroxide from 20 to
325 8C leads to the weight loss of 1.3 wt.% (mainly
elimination of molecularly adsorbed water). This
reveals that the amount of water weakly bound (i.e.
molecularly adsorbed) to the surface is relatively low
because of very low microporosity and low mesoporosity of the samples (Fig. 3) because water can be
effectively adsorbed rather in narrow pores than in
macropores. The main amount (28.4 wt.%) of
desorbed water is realised on heating at 325–
470 8C. The DTA curve (Fig. 5, curve 2) demonstrates
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Fig. 7. Distribution function of the activation energy of water
desorption calculated on the basis of DTG data.
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Table 2
Structural characteristics of magnesium hydroxide, oxide and dioxide
Parameter
Syngony
Type
Space group
Lattice constants (nm)
Mg–O bond length (nm)
Material
Mg(OH)2 [32]
MgO [33]
MgO2 [34]
Trigonal
CdI2
3̄m1
a = 0.3142, c = 0.4766
0.21625
Cubic
NaCl
Fm3m
a = 0.42112
0.21062
Cubic
FeS2
Pa3
a = 0.48441
0.19928
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
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426
DP
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MgO nanocrystallites possessing a different lattice in
comparison with Mg(OH)2. These processes cause an
increase in surface area. Calculation of the thickness
of the MgO layer h = 2/(rS) gives 1.7 nm in agreement
with the literature [7–15] at maximal value
S = 316 m2/g shown in Fig. 4. Dehydration of
Mg(OH)2 and fragmentation of crystallites (Fig. 1)
can cause deficit of oxygen atoms with formation of
MgOx (x < 1) crystallites. Computer simulation of
MgOx using the Powder Cell 2.3 program package
shows that the best fitting of the MgOx diffractograms
corresponds to formation of oxygen vacancies in the
positions (1/2, 1/2, 0) and (1/2, 1/2, 1) of the cell
shown in Fig. 10. This structure corresponds to Mg4O3
with the characteristic 2Q values, relative intensity of
the main lines and interplanar spacing dh k l determined from the experimental data and modelling
results shown in Table 4. Notice that similar neutral
oxygen vacancies may not lead to changes in the cubic
lattice. An increase in calcination temperature
T > 470 8C leads to sintering of foiled particles with
condensation of residual hydroxyls, liquidation of the
O2 deficit and transformation of Mg4O3 into MgO
(Figs. 1 and 8). This process can occur without a
strong endothermic effect (Fig. 5) because of the same
type of cubic lattice. Notice that primary particles
sintered into secondary particles (Fig. 1c) are durable
EC
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Table 3
Structural characteristics of materials prepared on calcination of Mg(OH)2 at different temperatures
Calcination temperature (8C)
20
440
600
900
Material
CO
422
cubic lattice of MgO is completed at 900 8C (Fig. 1c).
The MgO polycrystallites form prismatic non-porous
particles of 20–50 nm in size. These primary particles
form aggregates of a random structure (Fig. 1).
Changes in the lattice constants and the Mg–O bond
length are observed on different stages of the
Mg(OH)2 dehydration (Tables 2 and 3). The values
of the lattice constants a and c corresponding to
Mg(OH)2 increases by 1.2 and 3.8%, respectively, on
heating from 20 to 440 8C. The Mg–O bond length
increases from 0.2152 to 0.2194 nm.
Textural changes (such as the increase in the
surface area on heating and dehydration of Mg(OH)2
(Fig. 4)) can be caused by two factors: decrease in the
size of primary particles because of elimination of
significant amounts of water and fragmentation of
primary particles. Both processes lead to decrease in
the size (d) of particles determining surface area
(S 1/d), i.e. an increase in S despite an increase in
the material density (since r = 2.36–2.40 g/cm3 for
Mg(OH)2 and 3.65 g/cm3 for MgO). The magnesium
hydroxide is lamellar with planes of oxygen and
magnesium ions (Fig. 9) and each Mg2+ ion is 0–6-fold
coordinated and interacting with three oxygen ions
from one O2 layer and three ones from another O2
layer. On calcination, the fragmentation of the thin
Mg(OH)2 layers can be promoted by formation of
Mg(OH)2
Mg(OH)2
MgO
M4O3
MgO
MgO
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Structural parameter
Lattice constants (nm)
Mg–O bond length (nm)
a = 0.3144, c = 0.4678
a = 0.3182, c = 0.4856
a = 0.42314
a = 0.4026
a = 0.42294
a = 0.42225
0.21518
0.21938
0.21157
0.2013
0.21147
0.21112
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Fig. 8. XRD patterns of: (a) Mg(OH)2, (b) MgOx heated at 440 8C
and (c) MgO 600 8C.
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Fig. 9. Model of unit cell of magnesium hydroxide and a model
fragment of a structural layer.
492
case of Mg(OH)2, a broad band is at nMgO =
1400 cm1 but nMgO = 1440 cm1 for MgO and
1497 cm1 for Mg4O3. The oxygen vacancies in
MgOx (Fig. 10b) cause enhancement of the strength of
remaining Mg–O bonds and the size of the unit cell
decreases (Tables 2–4). The surfaces of MgO and
MgOx are well hydrated and the IR spectra show the
O–H stretching vibrations of the free surface hydroxyls at 3700–3720 cm1 (Fig. 11), e.g. an intensive
band at nOH = 3704 cm1 for Mg(OH)2. A broad band
at 3200–3600 cm1 characteristic for crystalline
hydrates of magnesium salts, hydroxides and different
EC
TE
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477
478
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as they remain after strong sonication of their
suspension for several minutes (Fig. 6). This effect
can be caused by the process Mg4O3 ! MgO leading
to chemical binding of individual cubelets.
Certain (but small) differences in the lattice
constants and the Mg–O bond length (Tables 2–4)
in comparison with the published data [7–15,32,33]
can be caused by different levels of the hydration of
the samples and the effects of impurities because the
samples studied here were prepared from natural
bischofite. Diminution of the a value of the MgO
lattice (Table 3) with elevating temperature of the
calcination occurs due to more complete dehydration
and relaxation of the lattice. A decrease in the Mg–O
bond length from 0.21518 nm in Mg(OH)2 to
0.21157 nm in MgO and 0.2013 nm in Mg4O3 should
be accompanied by an increase in the wavenumbers of
the Mg–O stretching vibrations (nMgO) as well as the
Mg–O–Mg deformation vibrations (Fig. 11). In the
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Fig. 10. Models of cells of (a) MgO and (b) Mg4O3.
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I.F. Mironyuk et al. / Applied Surface Science xxx (2005) xxx–xxx
Table 4
Parameters obtained from the XRD data for different lines and computer simulations of MgOx
510
511
I (a.u.)
dh k l (nm)
45.6
52.6
78.1
95.2
100.8
125.8
100
58.4
37.4
57.8
15.1
11.8
0.23098
0.20202
0.14208
0.12121
0.11617
0.10055
45.4
52.8
78.04
95.2
100.9
125.8
100
70.6
60.8
74.8
29.2
25.9
0.23202
0.20094
0.14208
0.12116
0.11601
0.10046
hydrated metal oxides [1,2,35–37] is due to the O–H
stretching vibrations of adsorbed water molecules and
the surface hydroxyls disturbed by the hydrogen
bonds. A band at 3656 cm1 (Fig. 11) is linked to
slightly disturbed hydroxyls which may locate in zone
of surface defects or between adjacent laminae. The
intensity of these bands decreases on the sample
calcination due to associatively desorption of water
molecules
2(BBMgOH) ! BBMg–O–MgBB + H2O.
The bands at 1610–1640 cm1 in the IR spectra
(Fig. 11) correspond to water molecules adsorbed on
the hydroxide and magnesia surfaces. A broad band at
420–680 cm1 corresponds to deformation vibrations
of Mg–O–Mg for both Mg(OH)2 and MgO but for
Mg4O3, this band expands towards higher wavenumbers. Bands at 380 and 330 cm1 observed for
Mg(OH)2 correspond to libration vibrations of the
surface hydroxyls and adsorbed water molecules. The
IR spectra as well as the XRD data reveal certain
differences in the structural properties of MgO and
MgOx prepared from Mg(OH)2 at different temperatures.
To elucidate certain experimental data, theoretical
calculations were performed with periodic conditions
for the cubic lattice using the 6–31G (d, p) basis set at
a = 0.42225 nm for MgO and smaller a = 0.4026 nm
for Mg4O3. These calculations show that for the latter,
the atomic charge qMg decreases from 1.026 to 0.778
and qO changes from 1.026 to 1.041, i.e. the basic
properties of MgOx can be slightly higher than that of
MgO. The highest occupied orbital in Mg4O3 lies
higher than that of MgO and corresponds to 5.39 eV
similar to the levels localized near neutral oxygen
vacancies in MgO [38]. This orbital locates on
magnesium ions placed around the oxygen vacancy.
It is above the occupied levels in the upper valence
zone of MgO by approximately 3.2 eV. Calculations
with the unit cell Mg4O4H give the atomic charge of H
in the hydroxyl group equal to 0.369 and qO = 0.950.
In the case of the unit cell Mg4O3H, qH = 0.383 and
qO = 1.001. These values suggest that electrostatic
component of the O–H bond could be higher for
Mg4O3 than that for MgO. These results are in
agreement with the pH values of the suspensions as
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2u (degree)
EC
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dh k l (nm)
RR
507
I (a.u.)
CO
506
Calculated
2u (degree)
Fig. 11. IR spectra of magnesium hydroxide (curve 1) and oxides
heated at (curve 2: Mg4O3) 440 8C and (curve 3: MgO) 600 8C over
(a) low and (b) high wavenumbers.
UN
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111
200
220
311
222
400
Experimental
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4
5
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4. Conclusion
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The MgO material produced from natural bischofite on dissolution–washing–drying–calcination is
akin to that prepared from pure magnesium hydroxide
and described in the literature. Obtained results show
the formation of the Mg4O3 phase at moderate
temperatures on calcination of magnesium hydroxide.
The morphology of Mg4O3 particles slightly differs
from that of MgO and it is characterized by a higher
amount of surface hydroxyls. The aggregates of
Mg4O3 particles are less stable than that of MgO.
Quantum chemical calculations and the IR spectra
showing higher frequency of the O–H stretching
vibrations of a portion of the surface hydroxyls of the
Mg4O3 depict slightly higher basic properties of these
groups. Therefore, this material can possess higher
catalytic ability as a basic compound in comparison
with more ordered MgO.
RR
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Acknowledgments
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This research was supported by NATO (Grant No.
PST.CLG.979895). R.L. is grateful to the Foundation
for Polish Science for financial support. The quantum
chemical program package NWChem (Version 4.5
developed and distributed by Pacific Northwest
National Laboratory, P.O. Box 999, Richland, WA
99352, USA, and funded by the U.S. Department of
Energy) was used to obtain some of these results.
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well as with the IR spectra showing a shoulder with
slightly higher nOH = 3720 cm1 for the free surface
hydroxyls for Mg4O3 than that for MgO at
nOH = 3700 cm1 or Mg(OH)2 at nOH = 3704 cm1.
Calculations of the clusters (i.e. with no periodic
conditions) corresponding to the unit cells with the
optimized geometry by HF/6–31G (d, p) give
qH = 0.369 and qO = 0.952 (in OH) for Mg4O2(OH)2
and qH = 0.340 and qO = 0.947 (in OH) for
Mg4O(OH)2. The polarity of the OH groups from
the cluster calculations corresponds to the reverse order
in comparison with results obtained using periodic
conditions. These results show the importance of the
use of periodic conditions on calculations of the
electronic characteristics of magnesia crystallites.
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