Aerogel processing of MTi2O5 (M = Mg, Mn, Fe, Co, Zn, Sn) compositions using single source precursors: Synthesis, characterization and photocatalytic behavior

Article abstract of DOI:10.1016/j.molcata.2004.11.008

Oxide semiconductor photocatalysts based on TiO₂ (band gap of 3.2 eV) are normally used to destroy organics under UV irradiation. Currently we have been focused on the aerogel synthesis of nanocrystalline photocatalysts that will destroy organic compounds such as acetaldehyde using visible and UV radiations. Herein we report the results of our attempts to synthesize mixed metal oxides of the type MTi₂O₅ (M = Mg, Mn, Fe, Co, Zn, and Sn). The preparation method involves the formation of a single source precursor by the condensation reaction of Ti(OPrⁿ)₄ with M(OAc)₂, followed by their (M[O-Ti(OPrⁿ)₃] ₂) hydrolysis and drying under supercritical conditions by a modified aerogel process. Product compositions, oxide structure types, and morphology were characterized by powder X-ray diffraction, scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDXA), and BET surface area measurements. Only for Mg, the stoichiometry MgTi₂O₅ with pseudo-brookite structure type was obtained, while the composites MTiO ₃/TiO₂ were obtained for M = Fe, Co and Zn. Photocatalytic behavior of these composite oxides were tested by the decomposition of gaseous acetaldehyde under UV light (320 nm < λ < 400 nm). Of these, CoTiO₃/TiO₂ showed an interesting catalytic behavior; it decomposed acetaldehyde in the dark, at room temperature. Other mixed metal oxides such as MgTi₂O₅, and composites such as FeTiO ₃/TiO₂ and ZnTiO₃/TiO₂ decomposed acetaldehyde only under UV light (not under visible light or in the dark). The photocatalytic activity was compared with the reference degussa P25 TiO ₂ photocatalyst.

Full text of DOI:10.1016/j.molcata.2004.11.008

Journal of Molecular Catalysis A: Chemical 229 (2005) 145–150
Aerogel processing of MTi O (M = Mg, Mn, Fe, Co, Zn, Sn)
2
5
compositions using single source precursors: synthesis,
characterization and photocatalytic behavior
a
b
b
b,∗
P.N. Kapoor , S. Uma , S. Rodriguez , K.J. Klabunde
a
Department of Chemistry, University of Delhi, Delhi 110007, India
b
Department of Chemistry, Kansas State University, Manhattan, KS 66506, USA
Received 3 August 2004; received in revised form 9 November 2004; accepted 9 November 2004
Available online 7 January 2005
Abstract
Oxide semiconductor photocatalysts based on TiO
2
(band gap of 3.2 eV) are normally used to destroy organics under UV irradiation.
Currently we have been focused on the aerogel synthesis of nanocrystalline photocatalysts that will destroy organic compounds such as
acetaldehyde using visible and UV radiations. Herein we report the results of our attempts to synthesize mixed metal oxides of the type
MTi
2 5
O (M = Mg, Mn, Fe, Co, Zn, and Sn). The preparation method involves the formation of a single source precursor by the conden-
n
n
sation reaction of Ti(OPr )
4
2 3 2
with M(OAc) , followed by their (M[O Ti(OPr ) ] ) hydrolysis and drying under supercritical conditions by
a modified aerogel process. Product compositions, oxide structure types, and morphology were characterized by powder X-ray diffraction,
scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDXA), and BET surface area measurements. Only for Mg,
the stoichiometry MgTi
Co and Zn. Photocatalytic behavior of these composite oxides were tested by the decomposition of gaseous acetaldehyde under UV light
320 nm < λ < 400 nm). Of these, CoTiO /TiO showed an interesting catalytic behavior; it decomposed acetaldehyde in the dark, at room
temperature. Other mixed metal oxides such as MgTi , and composites such as FeTiO /TiO and ZnTiO /TiO decomposed acetaldehyde
2 5 3 2
O with pseudo-brookite structure type was obtained, while the composites MTiO /TiO were obtained for M = Fe,
(
3
2
2
O
5
3
2
3
2
only under UV light (not under visible light or in the dark). The photocatalytic activity was compared with the reference degussa P25 TiO
photocatalyst.
2
©
2004 Elsevier B.V. All rights reserved.
Keywords: Sol–gel method; Nanocomposites; Photocatalysts; Acetaldehyde decomposition
1
. Introduction
ries some of the mixed metal oxides with high surface areas
2
(>600 m /g), have been studied for the decomposition of ac-
Titanium dioxide is the most widely used photocatalyst
etaldehydeundervisibleandUVlightirradiation. Cr, Co, Mn,
V, Ni, and Fe incorporated titania–silica aerogels were ef-
fective for the photocatalytic decomposition of acetaldehyde
under visible light irradiation (λ > 420 nm) [3]. We further
showed efficient visible and UV photocatalytic activities for
acetaldehyde decomposition based on microporous titanium
due to energy and environmental considerations [1]. How-
ever, titania can operate only under UV light irradiation, since
the light energy must be higher than its band gap (3.2 eV).
Currently many research efforts are aimed at the discovery of
photocatalysts that work under visible light irradiation [1,2].
One such effort involves the usage of mixed metal oxides for
the photodecomposition of volatile organics. In our laborato-
silicate [ETS-10, (Na,K)2TiSi O13], and transition metal in-
5
corporated ETS-10 [4].
Composite semiconducting materials such as TiO2/SnO2
[5], TiO2/WO3 [6], ZnO/SnO2 [7], have also been used as
UV photocatalysts. These oxides with their different band
∗
Corresponding author. Tel.: +1 785 5326849; fax: +1 785 5326666.
E-mail address: kenjk@ksu.edu (K.J. Klabunde).
1
381-1169/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2004.11.008

1
46
P.N. Kapoor et al. / Journal of Molecular Catalysis A:Chemical 229 (2005) 145–150
positions provide wider charge separation, thereby prevent-
ing the recombination of holes and electrons. Coating a thin
layer of MgO particles on TiO2 particles was found to en-
hance the photocatalytic oxidation of 2-chlorophenol, 2,4-
dichlorophenol and 4-aminobenzoic acid [8]. TiO2/Fe2O3
binary mixed oxides (1:1 composition) prepared by sol–gel
impregnation, exhibiting absorption in the visible spec-
tral region (570–600 nm) showed photocatalytic activity for
the aqueous degradation of o-cresol under UV light [9].
Nanocomposites of ZnFe2O4/TiO2 (with a Zn/Ti molar ratio
of 0.05) showed increased activity for the photodecompo-
sition of phenol as compared to pure TiO2 [10]. Increased
photodecomposition of phenol by TiO2/SiO2 has also been
noted and was attributed to the adsorption sites provided by
SiO2 [11]. Furthermore, intimately mixed metal oxides such
as CaO Al2O3 and MgO Al2O3 were shown to have in-
creased adsorption for paraoxon (diethyl-p-nitrophenyl phos-
phate) [12].
2. Experimental
2.1. Synthesis
Metal acetates, Co(OAc) , Mn(OAc) , Fe(OAc) , Zn-
2
2
2
(OAc) were purchased from Aldrich and used as received.
2
Samples of Mg(OAc) ·4H O, also from Aldrich were re-
2
2
fluxed overnight with excess acetic anhydride to remove
water, and the insoluble Mg(OAc) was collected by filtra-
tion, washed with dry toluene and dried at 50 C under vac-
uum overnight. Ti(OPr ) and decalin were purchased from
Aldrich and used as received.
2
◦
n
4
n
2.2. Synthesis and hydrolysis of Mg[O Ti(OPr ) ]
3
2
n
The starting materials, M[O Ti(OPr ) ] (M = divalent
metal), have been synthesized earlier by the thermal con-
densation of Ti alkoxide with metal (M) acetate [17]. A sim-
4
2
We wanted to investigate some of the titanium contain-
ilar synthetic procedure has been followed. The Mg analog
n
ing mixed metal oxides MTi2O (M = Mg, Mn, Fe, Co, Zn,
was prepared by refluxing two moles of Ti(OPr ) (4.94 g,
5
4
and Sn) as photocatalysts. We preferred utilizing high surface
area nanostructured mixed metal oxides in order to enhance
surface adsorption and photocatalysis. While synthesizing
mixed metal oxides by aerogel processing, single source pre-
cursors, which would possess both (Ti and M) the elements,
are essential to control stoichiometry. Although a mixture of
homometallic alkoxides could be used, single source precur-
sors have the advantage of avoiding the mismatch between
the hydrolysis rates of the different nonmetal precursors.
For example, homogeneous, high surface area nanocrys-
talline or amorphous forms of mixed metal spinel oxides
MAl2O4 (M = Mg, Ca, Mn, Co, Fe, and Zn) have been
successfully synthesized [13] using a modified aerogel
method, from bimetallic oxo-bridged alkoxides [(RO2Al
O M O Al(OR)2]. Final products MAl2O4 were obtained
by the thermal decomposition of the hydroxides [(HO)2Al
O M O Al(OH)2]. Nanosized BaTiO3, BaZrO3, and
BaTi0.5Zr0.5O3 have been prepared by single-source alkox-
ide, semi-alkoxide routes [14]. Through sol–gel chemistry,
NdAlO3 embedded in an Al2O3 matrix was obtained [15],
0.0174 mol) and 1 mol of Mg(OAc) (1.24 g, 0.0087 mol) in
50 ml decalin in a flask connected to a fractionating column.
2
The solution was refluxed and n-propyl acetate was removed
◦
continuously from 90 to 100 C (a sample of this n-propyl
acetate was confirmed by GC–MS; total yield of ester is
about 1.83 g). Eventually, the reflux temperature was raised
◦
to the boiling point of decalin (190 C). The refluxing was
continued for 5 h with the slow removal of decalin. The re-
maining decalin was removed by vacuum distillation and a
pale yellow clear solution formed was dried under vacuum
◦
at 50 C for overnight. Finally a pale yellow viscous paste of
n
Mg[O Ti(OPr ) ] was obtained (total yield 4.40 g).
3
2
2.3. Hydrolysis and supercritical drying of
n
Mg[O Ti(OPr ) ]
3
2
The yellow paste from the above preparation was dis-
solved in 30 ml toluene and 30 ml of isopropanol. The clear
pale yellow solution obtained was refluxed, cooled, and was
added, with stirring, 3 ml of water in 10 ml of isopropanol.
i
i
◦
using a single source precursor [NdAl3(OPr )12(Pr OH)]. Ti-
The white gel obtained was stirred continuously at 40 C
i
tanium iron isopropoxide [FeCl{Ti2(OPr )9}] has been used
overnight. The gel was placed in a high-pressure autoclave,
which was sealed, pressured to 100 psi with nitrogen, and
as a single source precursor to prepare TiO2/Fe2TiO com-
5
◦
posites by the sol–gel route [16].
heated with stirring at 265 C for 4 h (the pressure rose to
In this article we discuss the results of our investigation
on the synthesis and characterization of mixed metal oxides
900 psi). The solvent was vented over several minutes at this
temperature. After cooling, a gray white fluffy solid (yield
1.83 g) was obtained. Calcinations of these powders were car-
ried out at different temperatures depending upon the prod-
ucts obtained. Similar procedures were also used for prepa-
ration of other alkoxide precursors and they are summarized
in Table 1.
with stoichiometry, MTi2O (M = Mg, Mn, Fe, Co, Zn, and
5
Sn) using an aerogel method from a single source precur-
sor. Only for Mg, the stoichiometry MgTi2O with pseudo-
5
brookite structure type was obtained, while the composites
MTiO3/TiO2 were obtained for M = Fe, Co and Zn. We tested
the photocatalytic behavior of the products for the decompo-
sition of gaseous acetaldehyde. Although our primary goal
was to obtain photocatalysts, we surprisingly discovered a
catalyst, CoTiO3/TiO2, that would decompose acetaldehyde
in the dark at room temperature.
2.4. Characterization
Textural characterization of the samples was performed on
a Nova 1200 gas sorption analyzer (Quantachrome Corp.).

P.N. Kapoor et al. / Journal of Molecular Catalysis A:Chemical 229 (2005) 145–150
147
Table 1
Preparative conditions, product stoichiometry and surface areas of the materials
Starting materials
stoichiometry
Surface area after
supercritical drying (m /g)
Surface area after heat treatment in
air (m /g) (heating conditions)
Product stoichiometry
(from XRD and EDAX)
2
2
n
◦
Two moles of Ti(OPr )4 and
mol of magnesium
acetate
Two moles of Ti(OPr )4 and
mol of manganese(II)
acetate
78
51 (500 C/4 h)
MgTi2O5
1
n
◦
204
126 (500 C/4 h)
Amorphous
1
◦
2
5 (700 C/4 h)
Mn2O3 and TiO2 (rutile)
Amorphous
n
◦
Two moles of Ti(OPr )4 and
mol of iron(II) acetate
297
237
157 (500 C/4 h)
1
n
◦
Two moles of Ti(OPr )4 and
20 (500 C/4 h)
Amorphous
1
mol of cobalt(II) acetate
◦
1
6 (700 C/4 h)
CoTiO3/TiO2 (rutile)
Amorphous
n
◦
Two moles of Ti(OPr )4 and
1
320
146
152 (500 C/4 h)
mol of zinc acetate
◦
2
7 (700 C/4 h)
ZnTiO3/TiO2 (anatase
and rutile)
Amorphous
n
◦
Two moles of Ti(OPr )4 and
58 (500 C/4 h)
1
mol of tin(II) acetate
◦
4
3 (700 C/4 h)
Unidentified by powder
X-ray diffraction
The specific surface areas were calculated according to
the Brunauer–Emmett–Teller (BET) method. X-ray powder
diffraction experiments were conducted on a Scintag-XDS-
tration of the reactant acetaldehyde and the product carbon
dioxide.
2
000 and Bruker D8 Advance powder X-ray diffractome-
ters with Cu K␣ radiation. Scans were made in the 2θ range
3. Results and discussion
◦
5
–60 with a step size of 0.025. The surface morphology of
the calcined samples was visualized using a Hitachi S-3500N
Scanning Electron Microscope. Visible absorption spectra of
the samples were recorded on a Cary 500 Scan UV–vis NIR
Spectrophotometer with an integrating sphere attachment for
their diffuse reflectance in the range 200–800 nm.
3
.1. Characterization
One of the important aspects of this work is the successful
synthesis of MTi2O nanostructured materials by a modified
5
aerogel procedure. This approach is concerned with hydroly-
sis, gel formation, supercritical drying, and vacuum or air de-
hydration. However, only in the case of Mg, the composition
2
.5. Photocatalysis studies
MgTi2O with a pseudo-brookite structure was obtained after
calcinations at 500 C in air (Fig. 1a). The powder diffrac-
5
◦
The experimental set up for the photocatalytic oxidation
of acetaldehyde includes a light source, a static reactor, and
3
a circulating water bath. The static reactor is a 305 cm glass
container with a quartz lid to access light radiation. A typical
experiment was carried out as follows. The required amount
of the powdered sample (about 60–80 mg) was uniformly
spread over a quartz basket. The sample was later placed
in the air filled reactor. The temperatures for all the exper-
iments were carried out at 298 K. The reactor was closed
and stirred continuously after the introduction of 100 ␮l of
liquid acetaldehyde. The samples were illuminated with a
1
000 W high pressure Hg lamp. A combination of two filters,
VIS-NIR long pass filter (400 nm) and a colored glass filter
>420 nm) was utilized for the purpose of allowing only vis-
(
ible radiation. For UV illumination (320 nm < λ < 400 nm),
most of the IR and visible light was cut off. Gaseous
samples (35 ␮l) were extracted and analyzed by GC–MS
Fig. 1. Powder X-ray diffraction patterns of (a) MgTi2O5, (b) mixture of
Mn2O3 and TiO2, (c) CoTiO3/TiO2, and (d) ZnTiO3/TiO2 after calcinations
(see Table 1).
(gas chromatograph equipped with a mass selective detec-
tor GCMS-QP5000 from Shimadzu) to follow the concen-

1
48
P.N. Kapoor et al. / Journal of Molecular Catalysis A:Chemical 229 (2005) 145–150
tion pattern matched very well with that of the reported PDF
JCPDS Powder Diffraction File) number 35-0796. MgTi2O5
diffraction patterns. Usually iron oxide in the presence of
TiO2 in air tends to stabilize in a 3+ state with a compo-
(
withapseudo-brookitestructurehassofarbeenpreparedonly
by high temperature quenching (>700 C) [18,19]. In the case
sition Fe2TiO , in the pseudo-brookite structure [8,20,21].
5
◦
Preparation of FeTi2O normally requires much more strin-
5
of Co, and Zn, the products obtained even after heating at
gent conditions, such as heating the mixture of Fe, Fe2O3,
and TiO2 at 1170–1300 C in sealed quartz tubes containing
◦
◦
7
00 C was a mixture of MTiO3 and TiO2 (mostly rutile)
(
Fig. 1). For Mn, the powder X-ray pattern indicated the for-
200 mbar of He [21]. Sn in a 4+ oxidation state has been
substituted for Ti in the pseudo-brookite structure [22], and
there are no known phases with Sn in a 2+ oxidation state
with titanium. This probably explains the unidentified pow-
der X-ray pattern obtained by us, while heating the precursor
in vacuum. Product formation, calcinations conditions along
with the surface areas obtained for the different samples are
listed in Table 1.
mation of Mn2O3 and TiO2 (anatase and rutile) (Fig. 1b). The
reason might be the instability of the compositions, CoTi2O5,
MnTi2O , and ZnTi2O in the pseudo-brookite structure. The
5
5
formation of solid solutions between MnTi2O and CoTi2O5
5
were known without the formation of the end members [20].
It is important to mention that these earlier preparations were
done by the traditional ceramic method. For Fe and Sn, cal-
cinations of the hydroxylated products at 500 C in air re-
sulted in amorphous powders as shown by the X-ray powder
◦
Surface areas of the as prepared materials after supercriti-
2
cal extraction were 150–300 m /g, except for the Mg compo-
Fig. 2. Scanning electron micrographs of (a) CoTiO3/TiO2 and (b) ZnTiO3/TiO2.

P.N. Kapoor et al. / Journal of Molecular Catalysis A:Chemical 229 (2005) 145–150
149
2
Table 2
sition for which the surface area was 78 m /g (Table 1). The
a
−1
Pseudo-first-order rate constants (k, min ) for the decrease of acetalde-
hyde by MgTi2O5 and the composites MTiO3/TiO2 (M = Fe, Zn) under UV
irradiation (320 nm < λ < 400 nm)
surface areas decrease after calcinations and also with in-
crease in calcination temperatures. Even so, the surface area
obtained for MgTi2O (51 m /g) is notable, since a similar
surface area cannot be achieved by the traditional ceramic
2
5
−1
Sample
k (min
)
Degussa P25 TiO2
MgTi2O5
0.01(3)
◦
method of preparing MgTi2O at temperatures >700 C [18].
5
0.004(5)
0.001(4)
0.003(4)
The morphology as indicated by the SEM images (Fig. 2)
for the MTiO3/TiO2 (M = Co, Zn) composites indicate uni-
form crystallites. The ratios of Co/Ti and Zn/Ti as indicated
by the EDAX analyses were 0.51 and 0.52, respectively. This
FeTiO3/TiO2
ZnTiO /TiO
3
2
a
Calculated from the plots of ln(C0/C) vs. time (t), where C0 and C denote
the gas phase concentrations of acetaldehyde at t = 0 and t = t, respectively.
ratio matched very well the expected total composition of
◦
MTi2O . The Fe sample that was calcined at 500 C, which
any photocatalytic activity under visible light. Comparison
of the pseudo-first-order rate constants under UV light for
5
was amorphous, also exhibited a ratio of Fe/Ti, 0.45. The re-
sults from EDAX coupled with powder X-ray diffraction re-
sults clearly indicate that we have crystalline solid MgTi2O5
and the formation of MTiO3/TiO2 composites for M = Fe,
Co, and Zn.
−
1
MgTi O (0.004 min ) and the composites FeTiO /TiO
2
3
2
5
−
1
−1
(0.001 min
)
and ZnTiO /TiO2 (0.003 min )
3
were
found to be lower when compared to degussa P25 TiO
(0.01 min ) (Table 2).
2
−
1
Inspection of the UV–vis spectra for the different products
Fig. 3), clearly indicate that the cobalt and iron composites
However, we obtained interesting results for the cobalt
sample. CoTiO /TiO decomposed acetaldehyde at room
(
3
2
have stronger visible absorption in the range of 450–650 nm.
A similar visible absorption behavior has been observed for
TiO2/Fe2O3 mixed metal oxides and also for iron doped tita-
nia [8,23]. The broad peak between 500 and 700 nm observed
for the cobalt sample is mainly due to the presence of CoTiO3
temperature in the dark as indicated by the increase in CO2
concentration and the decrease in acetaldehyde concentration
with time (Fig. 4). The activity in the dark was higher than de-
gussa TiO which decomposes acetaldehyde only under UV
2
light. A balance between the consumed acetaldehyde con-
[24]. Note that in the case of Zn and Mg, the absorption edge
centration and the amount of CO formed was not achieved,
2
is moved below 350 nm as compared to that of degussa P25
TiO2.
since CO was the only dominant gaseous product of the re-
action of acetaldehyde oxidation. A trace amount of acetic
2
acid was the only other product seen in the gaseous phase.
The gas phase concentrations of other organic compounds
were probably below GC–MS detection limits. The acetic
3
.2. Photocatalytic studies
We investigated the photocatalytic activities of MgTi2O5
◦
acid, being less volatile (bp 118 C), probably was strongly
and the MTiO3/TiO2 composites (M = Fe, Co, and Zn) for the
gas phase decomposition of acetaldehyde. MgTi2O and the
adsorbed on the surface of the catalyst. The concentration of
acetic acid observed in the gaseous phase, might not corre-
spond to the total concentration of acetic acid produced in
the reaction [25].
The catalytic activity in the absence of light is due to the
presence of Co, and a similar behavior has previously been
5
composites MTiO3/TiO2 (M = Fe and Zn) decomposed ac-
etaldehyde only under UV irradiation (320 nm < λ < 400 nm).
The pseudo-first-order rate constants, calculated from the
plots of ln(C0/C) versus time (t), where C0 and C denote
the gas phase concentrations of acetaldehyde at t = 0 and
t = t, respectively, are listed in Table 2. In spite of the strong
visible absorption shown by the Fe sample, it did not show
reported by us in the case of CoOx loaded SiO2 xerogels
2+
[
25]. The mechanism was explained by the reaction of Co
3+
•−
to form superoxo complexes (LxCo
O O ) in the pres-
ence of oxygen. This complex formation is considered to be
responsible for the chain initiation
Co3 O O + CH3CHO
→
+
•−
3+
•
Co
O OH + CH3C O.
Alternately, Co3+ can react with acetaldehyde to form an
acetyl radical, which then proceeds to form the acetylperoxy
radical with oxygen. This chain initiation was then explained
as the reason for the activity of the cobalt catalyst.
In summary, we have attempted to prepare MTi2O5
(M = divalent metal) oxides by aerogel approach. We suc-
ceeded in preparing MgTi2O by this method at temperatures
5
◦
as low as 500 C, while the composites MTiO3/TiO2 have
been found for M = Fe, Co, and Zn. Cobalt containing mate-
rial showed an interesting catalytic activity, by decomposing
Fig. 3. UV–vis diffuse reflectance spectra of materials under study.

1
50
P.N. Kapoor et al. / Journal of Molecular Catalysis A:Chemical 229 (2005) 145–150
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Acknowledgement
This work was supported by MURI project (DAAD19-01-
[25] I.N. Martyanov, S. Uma, S. Rodrigues, K.J. Klabunde, Langmuir, in
1
0619) from the U.S. Army Research office.
press.