A general and low-cost synthetic route to high-surface area metal oxides through a silica xerogel template

Article abstract of DOI:10.1016/j.jpcs.2004.09.010

Porous metal oxides with a large surface area are synthesised by means of a procedure based on the templating approach. An inexpensive porous silica xerogel synthesised at moderate temperatures (～100 °C) in order to preserve the silanol superficial groups was used as template. In a first step, the silica porosity was filled with a concentrated solution containing a metallic salt. Then, the impregnated sample was calcined in air at a temperature of 600 °C. Under these conditions, the metal oxides were synthesised within the confined space provided by the silica pores. Finally, the product was recovered after dissolution of the silica framework in 2 M NaOH solution. The materials obtained by this procedure are made up of aggregates of nanoparticles and/or 3D solid structures containing confined pores. In this work, the synthetic route proposed is illustrated by the preparation of various binary metal oxides (i.e. Fe₂O₃, Cr₂O₃, NiO, CeO₂, Mn₂O₃, Co₂O₃ and Al₂O₃). The BET surface areas measured for these materials are in the range of 100-270 m² g^-1. The proposed method is not restricted to the binary metal oxides. It can also be used in the preparation of other inorganic materials such as metal sulphides or mixed metal oxides.

Full text of DOI:10.1016/j.jpcs.2004.09.010

Journal of Physics and Chemistry of Solids 66 (2005) 741–747
www.elsevier.com/locate/jpcs
A general and low-cost synthetic route to high-surface area
metal oxides through a silica xerogel template
*
Antonio B. Fuertes
´
Department of Chemistry and Materials, Instituto Nacional del Carbon (CSIC), P.O. Box 73, 33080 Oviedo, Spain
Received 28 June 2004; revised 15 September 2004; accepted 29 September 2004
Abstract
Porous metal oxides with a large surface area are synthesised by means of a procedure based on the templating approach. An inexpensive
porous silica xerogel synthesised at moderate temperatures (w100 8C) in order to preserve the silanol superficial groups was used as
template. In a first step, the silica porosity was filled with a concentrated solution containing a metallic salt. Then, the impregnated sample
was calcined in air at a temperature of 600 8C. Under these conditions, the metal oxides were synthesised within the confined space provided
by the silica pores. Finally, the product was recovered after dissolution of the silica framework in 2 M NaOH solution. The materials obtained
by this procedure are made up of aggregates of nanoparticles and/or 3D solid structures containing confined pores. In this work, the synthetic
route proposed is illustrated by the preparation of various binary metal oxides (i.e. Fe2O3, Cr2O3, NiO, CeO2, Mn2O3, Co2O3 and Al2O3). The
BET surface areas measured for these materials are in the range of 100–270 m2 gK1. The proposed method is not restricted to the binary
metal oxides. It can also be used in the preparation of other inorganic materials such as metal sulphides or mixed metal oxides.
q 2004 Elsevier Ltd. All rights reserved.
Keywords: A. Oxides, Nanostructures; B. Chemical synthesis; C. X-ray diffraction
1. Introduction
templates have been used for this purpose, i.e. soft
templates (i.e. surfactants) and hard templates (i.e. porous
solids such as silica or carbon). When hard templates are
employed, the synthesis of the porous material takes place
in a confined space formed by the porosity of the
template. The use of a reduced space is of great
importance because it restricts the growth of the material
during the synthesis (i.e. by sintering) and facilitates the
formation of nanostructures (i.e. nanoparticles, nanocylin-
ders, etc.). Given that the synthesis of metal oxides
usually requires high temperatures that induce the
formation of large particles through sintering, the use of
the template technique to prepare porous metal oxides is
highly appropriate. Recently, several papers describing the
preparation of HSMO by this technique have appeared.
Two types of hard templates have been employed, i.e.
active carbons [19–21] and mesostructured silica materials
(MSM) [22–24]. Schwickardi et al. [20] described the use
of commercial active carbons as templates to prepare
different types of HSMO. Dong et al. [21] reported the
preparation of monodisperse and porous spheres of
Recently, the preparation of high-surface area metal
oxides (HSMO) has received widespread attention because
they can be used in a high number of applications including
catalysis [1–4], adsorption [5–7], as electrodes in electro-
chemical cells [8–10], as chemical sensors [11,12], etc.
HSMO are currently prepared by procedures such as the
precipitation of precursors, sol–gel techniques, crystal-
lization, supramolecular chemistry, thermolysis, etc. An
extensive survey of this subject has been recently reported
by Landau [13]. Some of the synthetic procedures are quite
complicated and/or involve the use of expensive reagents.
Others give materials which have poor structural
characteristics.
In recent years, a wide variety of porous materials (i.e.
silica, carbon, metals, polymers, etc.) have been obtained
by means of the template technique [14–18]. Two types of
* Tel.: C34 985 11 9090; fax: C34 985 29 7662.
E-mail address: abefu@incar.csic.es
0022-3697/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2004.09.010

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A.B. Fuertes / Journal of Physics and Chemistry of Solids 66 (2005) 741–747
inorganic materials (i.e. oxides and phosphates) using
submicrometric porous carbon spheres as templates.
However, the use of active carbons as templates has
certain limitations due to the fact that, at the high
treatment temperatures employed during the synthesis,
reactions may occur between the infiltrated salts and the
carbon. When the heat treatment is performed in air, the
carbon may be rapidly oxidised (ignition) even at
relatively low temperatures due to the catalytic effect of
infiltrated salts. On the other hand, some metallic salts
may be reduced to metal when the heat treatment takes
place under inert conditions. For these reasons, a more
inert hard template such as porous silica could be more
appropriate. Tian et al. [22,23] recently reported the
fabrication of ordered porous inorganic materials (i.e.
metal oxides, sulphides) by using various MSMs (i.e.
SBA-15, SBA-16 and FDU-1) as templates. The authors
emphasize the importance of preserving the silanol groups
on the silica surface to attain a good degree of
impregnation. With this objective they devised a special
technique (microwave digestion) to eliminate the surfac-
tant employed in the synthesis of MSM so as not to alter
the presence of the silanol groups within the silica pores.
Although, the procedure proposed by Tian et al. [22]
yields high-surface area metal oxides, it requires the use
of expensive surfactants as templating agents for the
synthesis of MSM and a sophisticated method to remove
the surfactant. In short, this synthetic approach is too
complex and costly for practical application. A low-cost
route for manufacturing HSMO could be provided by
using porous silica templates synthesised without the aid
of surfactants (i.e. silica gels). Accordingly, in this work
we propose a new approach based on the use of
inexpensive silica xerogel as templating agent. The
xerogel is obtained from a synthesis mixture formed
exclusively by a silica source (sodium silicate), HCl and
water. Furthermore, because the templated material is
obtained without the aid of structure-directing agents (i.e.
surfactants) and the synthesis takes place at a moderate
temperature (100 8C), the surface silanol groups can
easily be preserved. This procedure is illustrated here
with the synthesis of various binary metal oxides.
filtered, washed several times with water and then with
acetone and dried at room temperature.
For a typical synthesis of the templated metal oxides,
hydrated metal nitrates were dissolved in ethanol (around
0.8–1.2 g nitrate/g ethanol). Afterwards, the silica xerogel
was impregnated with this solution until incipient wetness
was attained. The impregnated sample was dried at 80 8C.
The impregnation-drying cycle was repeated up to three
times for a high loading of the metal nitrate into the silica
porosity. The impregnated samples were then calcined in air
at 600 8C (2 8C/min) for 4 h. The metal oxide products were
obtained after dissolution of the silica framework in 2 M
NaOH solution.
The samples were characterised by nitrogen adsorption at
K196 8C by using a Micromeritics ASAP 2010 volumetric
adsorption system. The BET surface area was deduced from
the isotherm analysis in the relative pressure range of 0.04–
0.20. The total pore volume was calculated from the amount
of nitrogen adsorbed at a relative pressure of 0.99. The PSD
was calculated by means of the Kruk–Jaroniec–Sayari
method [26] applied to the adsorption branch. X-ray
diffraction (XRD) analysis was carried out in a Siemens
D5000 instrument operating at 40 kV and 20 mA and using
Cu Ka radiation (lZ0.15406 nm). The structure of the
materials was characterised by transmission electron
microscopy (TEM).
3. Results and discussion
The porosity of the silica xerogel used as template is
made up of mesopores of quite uniform size within the
2–15 nm range. This is illustrated in Fig. 1 which shows the
nitrogen sorption isotherm and PSD (inset) of this material.
The values obtained for the BET surface area and pore
volume are 510 m2 gK1 and 0.8 cm3 gK1, respectively. The
TEM image (Fig. 1, inset) obtained for the silica xerogel
shows a disordered pore network, which is characteristic of
this type of material [27]. The FT-IR spectrum of the silica
xerogel (not shown) exhibits a broad peak in the 3000–
3800 cmK1 region, suggesting an abundance of silanol
groups.
An energy dispersive X-ray analysis (EDX) of the
synthesised metal oxides revealed that these samples only
contain traces of silica. The identification of the product
obtained by the templated technique was carried out by
X-ray diffraction (XRD) analysis at the wide-angle range
(2qZ10–908). The XRD patterns for several templated
oxides are shown in Fig. 2. In general, they contain broad
diffraction peaks, indicating that the oxide is made up of
very small particles. The size of these nanoparticles was
estimated from the broadening of the XRD peaks according
to the Scherrer formula. The values deduced are indicated in
Table 1. The as-synthesised iron oxide exhibits an X-ray
diffraction profile consisting of two very broad low-intensity
reflections (Fig. 2a, lower). This spectrum is characteristic
2. Experimental section
The synthesis of silica xerogel was carried out in a two-
step process according to a procedure reported elsewhere
[25]. In a typical synthesis, the silica source (Sodium
silicate, Aldrich, 27% SiO2C14% NaOH) was added under
stirring to an aqueous solution containing HCl. The molar
composition of the synthesis mixture was: sodium silicate/
HCl/H2OZ1/6/194. In the first stage, the solution was
stirred in a closed Teflon vessel for 20 h at room
temperature. In the second stage, the mixture was
maintained for 2 days at 100 8C. The resulting gel was

A.B. Fuertes / Journal of Physics and Chemistry of Solids 66 (2005) 741–747
743
Fig. 1. Nitrogen sorption isotherm, pore size distribution (inset) and TEM image (inset) for the silica xerogel used as template.
of the poorly crystalline iron(III) oxide ferrihydrite [28].
This material is an excellent adsorbent for the elimination of
traces of metallic ions (i.e. Zn (II), As (V), etc.) in
contaminated water [29]. Moreover, ferrihydrite can be
partially transformed into magnetic nanoparticles of
g-Fe2O3 (maghemite) by means of heat treatment at
temperatures of around 350 8C [30]. Accordingly, the
sample obtained here in this way is attracted by a magnet
and its XRD pattern contains the peaks that are character-
istic of g-Fe2O3 (Fig. 2a, upper). Thus, this technique would
be useful for making magnetic nanoparticles with a large
surface area. The aluminium oxide obtained by means of
this procedure exhibits an amorphous framework as was
deduced by XRD. The other oxides show XRD patterns that
are characteristic of the crystalline phases corresponding to
the products listed in Table 1.
large particles, which enclose framework-confined pores.
Accordingly, the porosity of the metal oxides may consist
of two types of pores; i.e. pores arising from the voids
between the nanoparticles (textural pores) and pores
confined within the solid framework (structural pores).
The TEM images obtained for different templated metal
oxides illustrate this feature (Fig. 3). Thus, Fig. 3a clearly
evidences the existence of structural pores in the iron
oxide (ferrihydrite) sample. This structural porosity seems
to be an inverse replica of the silica xerogel framework
(Fig. 1, inset). Moreover, taking into account the density
of Fe2O3 (5.2 g cmK3), the prepared iron oxide has a
surface to framework volume ratio of w1.4!107 cmK1
,
which is close to that deduced for a mesoporous carbon
obtained from the same silica template [25]. This clearly
suggests that the structure of our iron oxide is a faithful
replica of that of the silica template. The other TEM
images in Fig. 3 shows that these products are made up of
aggregates of nanoparticles with sizes below 20 nm. The
sizes estimated from the TEM images are in good
agreement with those obtained from XRD peak broad-
ening (Table 1).
The physical properties of the metal oxides prepared
by means of the templated technique are presented in
Table 1. These materials exhibit BET surface areas of 96–
270 m2 gK1 and total pore volumes of 0.2–0.8 cm3 gK1
.
The values calculated for the surface areas are larger than
those reported for similar materials prepared by more
complex techniques at similar calcination temperatures
(i.e. w600 8C) [13]. The metal oxides synthesised by the
template method consist of aggregated nanoparticles or
A more precise image of the structure of the synthesised
materials can be obtained by means of N2 sorption
experiments. The existence of structural and/or textural

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A.B. Fuertes / Journal of Physics and Chemistry of Solids 66 (2005) 741–747
Fig. 2. XRD patterns of various templated metallic oxides: (a) Fe2O3; (b) Mn2O3; (c) CeO2; (d) NiO.
porosities is evidenced by the analysis of N2 sorption
isotherms. Fig. 4 shows the isotherms and the PSDs (insets)
for various representative metal oxides. The N2 isotherm
obtained for iron oxide/ferrihydrite (Fig. 4a) exhibits a
capillary condensation step at p/pow0.6–0.8. This clearly
indicates that the material contains framework-confined
mesopores (structural), the pore size being centred at
w6.5 nm (Fig. 4a, inset). Fig. 1a provides a TEM image
that illustrates this porous structure. By applying the as-plot
analysis to the N2 adsorption data obtained for Fe2O3
(ferrihydrite), it was estimated that around 70% of the pore
volume corresponds to the structural mesopores, whereas
Table 1
Structural properties of high-surface area metal oxides
Product (XRD)
SBET (m2 gK1
)
Vt (cm3 gK1 a
)
Structural porosity
Vs (cm3 gK1 c
Crystallite size (nm)b
)
Pore size (nm)d
Fe2O3
Ferrihydrite
Maghemite
Cr2O3
270
176
118
96
0.45
0.44
0.57
0.39
0.35
0.26
0.21
0.78
0.32
0.32
–
6.5
11
–
–
–
17
11
7
NiO
0.12
0.04
0.08
0.14
–
6.9
4.6
4.6
8.6
–
CeO2
141
117
98
Mn2O3
Co2O3
18
10
–
Al2O3
153
a
Total pore volume from N2 adsorption at p/poZ0.99.
Estimated from XRD peak broadening by using the Scherrer formula.
Volume of structural pores.
b
c
d
Maximum PSD for the structural pores.

A.B. Fuertes / Journal of Physics and Chemistry of Solids 66 (2005) 741–747
745
Fig. 3. TEM images of the various synthesised metallic oxides: (a) Fe2O3 (ferrihydrite); (b) Co2O3; (c) Al2O3; (d) NiO.
the rest can be assigned to interparticle voids (textural
porosity). A similar balance structural/textural porosity is
obtained for Fe2O3 (maghemite). This material exhibits in
relation with ferrihydrite, a lower specific surface area and a
larger pore size (see Table 1). The cobalt oxide shows a
similar balance between structural and textural porosity.
The N2 sorption isotherm of CeO2 (Fig. 4b) shows a
capillary condensation step at p/pow0.4–0.5, which reflects
the existence of structural mesopores. These mesopores
exhibit a narrow PSD centred at w4.6 nm (Fig. 4b, inset).
Unlike iron oxide, the structural porosity of CeO2 only
represents around 10% of the total pore volume, the rest
corresponding to textural porosity (interparticle voids).
Except for Cr2O3 and Al2O3, the other oxides exhibit some
degree of structural porosity, suggesting the presence of 3D
porous metal oxide structures. The values for structural pore
size and pore volume estimated from the as-plot analysis are
indicated in Table 1. The aluminium oxide shows an
interesting structure as deduced from the N2 sorption
isotherm (Fig. 4d). Thus, it exhibits a pronounced capillary
condensation step at a relative pressure of w0.9, which is
indicative of large, quite uniform large pores as is evidenced
by the PSD (Fig. 4d, inset). This shows that the porosity in
the aluminium oxide is made up of pores centred at
w37 nm. It is also clear from the TEM image in Fig. 3c,
which shows that this material is made up of the aggregation
of uniform nanoparticles with sizes within the 10–20 nm
range.
In addition to binary metal oxides, the template
procedure can be extended to the preparation of multi-
component metal oxides and other compounds such as metal
sulphides. Thus, our laboratory has prepared various porous
mixed oxides like CuO–CeO2, which are currently under
study as catalysts for CO-oxidation. In principle, the only
limitation to the use of this technique concerns those
compounds, which are affected by the agent used to remove
the silica template. As an example, ZnO is dissolved by the
NaOH solution and consequently it cannot be prepared by
means of this procedure.
4. Conclusions
In summary, a novel synthetic route to obtain porous
metal oxides that exhibit a large surface area is reported.
The method presented is based on the templating
approach. An inexpensive porous silica xerogel that
contains abundant superficial silanol groups is used as
template. This facilitates the filling of the silica porosity
by the infiltrating solution. The metal oxides are
synthesised in a confined space provided by the silica
porosity. As a result, the solid formed will be made up of

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A.B. Fuertes / Journal of Physics and Chemistry of Solids 66 (2005) 741–747
Fig. 4. Nitrogen sorption isotherms and PSDs of structural pores (insets) of several representative metallic oxides synthesised by means of the templating
technique. (a) Fe2O3 (ferrihydrite); (b) CeO2; (c) Cr2O3; (d) Al2O3.
aggregates of nanoparticles and/or 3D solid structures
containing confined pores. The solid therefore exhibits
a textural porosity arising from the interparticle voids
and a structural porosity made up of the framework-
confined pores. The synthetic route proposed in this work
is illustrated by the preparation of several binary metal
oxides. However, this method can be extended to other
inorganic materials such as metal sulphides, mixed metal
oxides, etc.
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