nitrogen-containing tio2 photocatalysts
TRANSCRIPT
Nitrogen-containing TiO2 photocatalysts
Part 1. Synthesis and solid characterization
C. Belver a, R. Bellod a, A. Fuerte b, M. Fernandez-Garcıa a,*a Instituto de Catalisis y Petroleoquımica, CSIC, Campus Cantoblanco, 28049 Madrid, Spain
b Departamento de Energıa, CIEMAT, Av. Complutense, 28040 Madrid, Spain
Available online 11 April 2006
Abstract
A series of N-substituted Ti isopropoxide precursors were synthesized by using three different amine-type ligands. The resulting Ti-complexes
were characterized by nuclear magnetic resonance (NMR) and used to obtain solid precipitates by a reverse microemulsion method. These N-rich
solid precipitates were subjected to three calcination treatments differing in the gas atmosphere allowed to contact the solid, yielding nanosized
materials. A thermogravimetric analysis of the solid precipitates, combined with a mass spectrometry/infrared study of the evolving gaseous
products, were able to show the influence of preparation parameters, e.g. Ti-precursor nature and treatment conditions, in the decomposition
process of the solid precursors and the formation of the final nanoparticulated solid catalysts. Both parameters affect the interaction between solid
oxygen species (O2�; OH�) and N,C-containing fragments present in the solid precursors. The chemical (e.g. nitrogen content), structural (phase,
cell parameters and volume, and defect structure) and morphological (BET area and primary particle size) properties of the catalytic final solids
were studied as a function of the preparation conditions.
# 2006 Elsevier B.V. All rights reserved.
Keywords: Photocatalysis; Microemulsion method; NMR; TG-MS; DRIFTS; TiO2 anatase materials; N-doping and impurity; Visible light absorption; Pollutant
mineralization and degradation
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Applied Catalysis B: Environmental 65 (2006) 301–308
1. Introduction
Photocatalytic destruction of organic pollutants in the
presence of TiO2 appears as a viable decontamination process
of widespread application, no matter the state (gas or liquid) or
chemical nature of the process target [1,2]. However, its
technological application seems limited by several factors,
among which the most restrictive one is the need of using an
ultraviolet (UV) excitation source. The efficient use of solar
light or, in other words, of the visible region of the spectrum,
may then appear as an appealing challenge for developing the
future generation of photocatalytic materials [3,4].
One new approach to produce visible light activated TiO2
photocatalysts is by doping with anions, such as N3�, C4�, S4�
or halides (F�, Cl�, Br�, I�) [5]. It was first suggested that these
species substitute the oxygen lattice on TiO2 and lead to a band
DOI of related article: 10.1016/j.apcatb.2006.02.016.
* Corresponding author. Tel.: +34 91 5854939; fax: +34 91 5854760.
E-mail address: [email protected] (M. Fernandez-Garcıa).
0926-3373/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcatb.2006.02.007
gap narrowing, resulting in high visible absorption. Many
works described the enhanced photocatalytic properties of
titania by nitrogen doping [6–9], although special care must be
taken with the synthesis method [10–12]. The nitrogen
substitutional doping of TiO2 is through to be a method for
narrowing the band gap by changing the valence band structure
without a change in the position of conduction band.
Nevertheless, an open question in this way is to know how
the absorption shift towards the visible part occurs. Quantum
chemical calculations suggest that the narrowing of the band
gap is due to a mixing of the N 2p and O 2p states [13,14], being
identified the dominant transitions at the absorption edge those
from N 2pP to Ti dxy, instead of from O 2pP as occurred in TiO2
[15]. Nevertheless, Irie et al. [16] suggest that the N 2p levels
are separated (not mixed) from the valence band (formed by O
2p states), forming an isolated narrow band responsible for the
visible light sensitivity of the oxynitride powders. By other
side, some authors point out that the N-doping of TiO2 is rather
similar in properties to impurity sensitization, inducing the
formation of localized states in the band gap [17,18]. In this
sense, the doped nitrogen may exist as NO-related species
C. Belver et al. / Applied Catalysis B: Environmental 65 (2006) 301–308302
Table 1
Main synthesis details of Ti–N samples and T references
Sample/reference Ti:ligand
molecular ratio
Treatment
M1 1:1 Ar/O2 20–450 8CM2 1:1 Ar 20–200 8C; Ar/O2 200–450 8CM3 1:1 Ar 20–450 8C; Ar/O2 450 8C2M1 1:2 Ar/O2 20–450 8C2M2 1:2 Ar 20–200 8C; Ar/O2 200–450 8C2M3 1:2 Ar 20–450 8C; Ar/O2 450 8CN3 1:1 Ar 20–450 8C; Ar/O2 450 8CPh3 1:1 Ar 20–450 8C; Ar/O2 450 8CT1 – Ar/O2 20–450 8CT3 – Ar 20–450 8C; Ar/O2 450 8CTaq3 1:3 Ar 20–450 8C; Ar/O2 450 8C
which may be present alone or together with Ti–N bondings,
the former (NO-type) created by a N–O interaction where N
is close to tetrahedral-like interstitial positions and the latter
(Ti–N bond) formed by substitutional replacement of O anions
by N anions. By other side, doping with nitrogen not only
modifies the electronic properties of titania, but also can induce
the formation of oxygen-defect sites having critical influence in
catalytic activity [18,19]. Therefore, at least two types of
species could exist on Ti–N catalysts. N-doping of TiO2-based
materials affect the photocatalytic activity of TiO2-based
systems because can act as charge trapping or recombination
centers depending on their presence at the surface/bulk and
other structural/electronic details.
Here, we will describe the synthesis of N-containing TiO2-
based materials by modifying a titanium(IV) isopropoxide
precursor with a series of N-containing molecules. Three
different molecules will serve as ligands; the first one 2-
methoxyethylamine, the second N,N,N0,N0-tetramethylethyle-
nediamine and the third one 1,2-phenylenediamine. They are
able to substitute one or several propoxide ligands forming
different Ti precursors which would be utilized to obtain TiO2-
based photocatalysts by using a reverse micelle microemulsion
method. There are, to our knowledge, two recent reports using a
N-containing Ti precursor from TiCl4 to obtain Ti–N catalysts
after calcination [9,11]. However, the use of a titanium(IV)
isopropoxide may be able to eliminate a potential interference
from Cl� ions. This synthetic approach could have, on the other
hand, several advantages over those already described in the
literature and mostly based on the post-treatment of TiO2
materials with NH3 or other (like triethylamine) molecules and/
on N ion implantation [6–10,12]. These beneficial effects are
essentially related to the simultaneous potential control of the
chemical, e.g. nitrogen and associated vacancy species content,
and morphological, e.g. particle size, properties of the final
solid, allowing correct comparisons between different speci-
mens. In this paper, we will describe the synthesis method and
the resulting catalytic solids, while in the following paper (part
2), we will analyze the catalytic properties of these systems.
Apart from using three different N-containing ligands, we will
study the influence of the calcination treatment temperature and
atmosphere, as these variables have been shown to be important
in obtaining adequate photocatalytic materials (see [18] and
references cited therein).
2. Experimental
Titanium(IV) isopropoxide (Aldrich) was first subjected
to a reaction with different N-containing amine-type ligands:
2-methoxyethylamine (called M, hereafter), CH3OCH2CH2
NH2 (Aldrich); N,N,N0,N0-tetramethylethylenediamine (N),
(CH3)2NCH2CH CHCH2N(CH3)2 (Aldrich); and 1,2-pheny-
lenediamine (Ph), C6H4(NH2)2 (Aldrich). In a typical expe-
riment, the corresponding N-containing ligand was added to
a 0.05 M solution of Ti(iPrO)4 in dry isopropanol, in a Slenchk
type flask under nitrogen atmosphere. After stirring at room
temperature for 1.5 h, the solvent was evaporated, under
reduced pressure, until obtaining a concentrated solution of
the N-containing Ti precursor in isopropanol (1.9 M approxi-
mately). This solution was added dropwise to an inverse
microemulsion containing an aqueous solution dispersed in
n-heptane (10/85, v/v), using Triton X-100 (Aldrich) as
surfactant and hexanol (Aldrich) as cosurfactant [20]. The
resulting mixture was stirred for 24 h, centrifuged, rinsed with
methanol and calcined using three different treatments always
consisting on a ramp at 1 8C min�1 with 2 h plateaus at 200 and
450 8C, being the latter the final temperature of treatment.
Treatments differ in the specific point/temperature where O2
(20 vol.%) is added to the Ar, being incorporated from the
beginning (treatment called ‘‘1’’), at the onset of the first (‘‘2’’)
and second plateau (‘‘3’’). Reference samples (called T,
hereafter) were produced by using the Ti isopropoxide as
precursor. An additional reference was obtained by a post-
treatment with the N ligand of a TiO2 nanoparticle (Taq) in
order to have a comparison with samples already studied in
the literature. Table 1 summarizes the names of the samples
and references and main synthesis parameters.1H and 13C NMR spectra of N-containing Ti precursors were
taken on a Varian XR300; chemical shifts are given in ppm with
tetramethylsilane used as internal standard. Bidimensional
homonuclear (COSY) and heteronuclear (HMQC) chemical
shift correlation experiments were carried out in a Varian
INOVA-500 spectrometer. On account of the very poor
solubility of these titanium complexes in the deuterated
solvents, their 1H NMR spectra were not of sufficient quality,
and only in the case of the 2-methoxyethylamine ligand, a
reasonable signal was obtained (see below).
Chemical composition was analyzed by using inductively
coupled plasma and atomic absorption (ICP–AAS). The BET
surface areas were measured by nitrogen adsorption–desorption
isotherms at 77 K with a Micromeritics TriStar 3000 equip-
ment. XRD profiles were obtained with a Seifert diffractometer
using Ni-filtered Cu Ka radiation. Particle sizes were cal-
culated from XRD patterns using the Willianson–Hall method
which takes into account, the strain and particle size contribu-
tions to the XRD peak broadening [21]. Lattice parameters
were calculated by fitting and refining the anatase unit cell with
the help of the PLV program [22].
Thermogravimetric measurements were performed by using
a thermogravimetric-mass spectrometer (TG-MS) system
C. Belver et al. / Applied Catalysis B: Environmental 65 (2006) 301–308 303
from Perkin-Elmer TG-7. Samples were treated in He at room
temperature (RT) before subjected to a heating ramp of
10 8C min�1 from 25 to 600 8C under He or 20% O2/He
mixture. The evolving gaseous products were detected by mass
spectrometry, scanning the m/z values of 15, 16, 17, 18, 27, 28,
29, 30, 32, 39, 40, 41, 44, and 46.
The infrared analyses were carried out during the calcination
treatments described before by using a Perkin-Elmer 1725X
FTIR spectrometer, with a multiple reflection transmission cell
(Infrared Analysis Inc.), directly connected to the gaseous
effluents from the calcination reactor.
3. Results and discussion
The synthesis of the N-containing Ti precursor was
followed by NMR in the case of a Ti:M 1:1 precursor. For
comparison purposes, spectroscopic data (1H NMR and 13C
NMR) of the reactants [titanium(IV) isopropoxide and 2-
methoxyethylamine] and the Ti:M 1:1 precursor are sum-
marised in Table 2. Homonuclear (COSY) and heteronuclear
(HMQC) bidimensional NMR experiments were required to
unequivocally assign the signals to the corresponding atoms.
The differences between the chemical shift of the titanium
complex (Ti:M 1:1) and free ligand (e.g. coordination shift,
Dd = dcomplex � dligand) are also included in this Table 2. It can
be observed that all proton signals experience a low-field shift
(Dd = 0.02–0.64 ppm), being this more dramatic for protons
that are closer to the titanium atom (DdNH2>Dda >Ddb >Ddc).
This de-shielding effect is induced for the coordination of
the new ligand (methoxyethylamine) to the titanium atom.
Additionally, signals corresponding to protons in positions
Table 21H and 13C RMN data for the Ti:M 1:1 precursor
1H, d (ppm) CH2 [a] CH2 [b] CH3 [c]
Methoxyethylamine 2.85 {t, 2H, J = 6.8} 3.39 {t, 2H, J = 6.8} 3.36 {s,
Ti(iPrO)4 – – –
Ti:M 1:1 3.36 {t, 2H, J = 6.1} 3.64 {t, 2H, J = 6.1} 3.38 {s,
Dd 0.51 0.25 0.02
13C, d (ppm) CH2 [a] CH2 [b] CH
Methoxyethylamine 41.8 74.9 58
Ti(iPrO)4 – – –
Ti:M 1:1 51.3 73.0 59
Dd 9.51 �1.94 0
Deuterated solvent: CDCl3; {multiplicity, number of protons, coupling constant (J
e and d also display a small shift; the introduction of the
N-containing ligand in the structure of the titanium complex
implies that isopropoxide groups are no longer equivalent by
symmetry. The integration of signals appearing in the 1H NMR
spectrum confirms that only one isopropoxide group has been
substituted by a M group (integration; a:b:c:d:e = 2:2:3:3:18).
Similar behavior is observed in the 13C NMR spectrum, where
the coordination shifts (Dd) for different carbon atoms vary
between 0.2 and 13.2 ppm. All these facts proof that the Ti:M
1:1 titanium complex is successfully formed under our
synthesis conditions.
Although we were not able to characterize by NMR
spectroscopy the precursor state when using the N and Ph
molecules as N-containing ligands due to problems with
solubility of these compounds in the deuterared solvents,
convincing evidence of the Ti:N and Ti:Ph precursor formation
can be obtained from the TG-MS study. Fig. 1 displays the
weight loss and derivative thermograma of solid precursors/
precipitates (obtained from the corresponding microemulsions)
under He. Solid and dotted lines correspond, respectively, to
materials treated with O2/Ar at RT or at 200 8C and dried,
respectively, at RT or at 200 8C under He for treatment 2
previously to the recording stage. Samples pre-treated at higher
temperatures (450 8C) do not show appreciable mass losses.
Samples Ti:M and Ti:N show a similar behavior, displaying
peaks at 86, 233, 300, 330 and 438 8C in the derivative plot
while Ti:Ph only shows a weak contribution around 86 8C,
mostly ascribable to water/CO2 evolution according to MS (see
below). Precursor Ti:M is affected by the pre-treatment at
200 8C, losing the peaks evolving at temperature below or near
200 8C with respect to its counterpart pre-treated at RT. This
CH [d] CH3 [e] NH2
3H} – – 1.38 {s, 2H}
4.47 {septet, 4H, J = 6.1} 1.23 {d, 24H, J = 6.1} –
3H} 4.92–4.50 {m, 3H} 1.58–1.12 {m, 18H} 2.02 {s, 2H}
�0.24 �0.12 0.64
3 [c] CH [d] CH3 [e] NH2
.7 – – –
64.0 25.3 –
.0 78.3–76.3 25.6–24.4 –
.28 �13.2 ��0.2
, Hz)}.
C. Belver et al. / Applied Catalysis B: Environmental 65 (2006) 301–308304
Fig. 1. Thermogravimetric analyses for indicated precursors: (A) percentage of weight loss; (B) derivative thermograma. Solid and dotted lines correspond,
respectively, to materials treated in O2/Ar at RT or at 200 8C and dried for 2 h under He previously to the recording stage. See text for details.
accounts for ca. 5% of its initial weight. Surprisingly, an almost
complete absence of contributions with a main peak at ca. 86 8Ccan be observed for Ti:N after pre-treatment at 200 8C. The
Ti:Ph precursors display a behavior only weakly dependent on
the pre-treatment, with weak contributions at 86 and 330 8C.
The proof that the N-containing Ti precursors are formed
can be extracted from the strong similarities detected in the
evolving gaseous products detected by MS for solid precursors
treated at RT. In Fig. 2A, we depicted the 30 and 46 m/z signals
which due to its parallel behavior for each material are mostly
associated to NO and NO2 desorption (the m/z 30 secondary
contribution has been eliminated from the plot). The thermal
evolution of these species displays strong similarities among
our three precursors, with the sharing of peaks at ca. 220, 250,
300 and 350 8C. Owing that the Ti:M 1:1 complex is known to
be formed by NMR, this would indicate that the N-containing
Ti precursor is obtained to a certain extent in all cases. The
NO + NO2 signal intensity roughly shows the expected ratio for
the Ti:M and Ti:N precursors, indicating the similar 1:1
Ti:ligand molecular ratio reached in both cases and that Ti–M,
Ti–N bonds are not hydrolyzed in the microemulsion media
during the preparation procedure. We could thus speculate that
the Ti precursor has four ligands with presence of one N-
containing molecule in both cases, being tetra-coordinated in
the M case (NMR result) but five-coordinated in the N case. For
Ti:Ph we found, however, rather weak contributions, indicating
the failing of the precursor synthesis and/or a significant
hydrolysis of Ti–Ph bonds at the microemulsion media. Still
some residual Ti–N bonds are present in the Ti precursor.
The decomposition route of the solid precursors can be
analyzed by using the MS spectra showed in Fig. 2. In addition
to the NO/NO2 contributions (Fig. 2A), in Fig. 2B, C and D, we
plotted the evolution of the 16 (B), 28, 44 (C), and 40 (D) m/z
signals. Fig. 2B profile is assigned to ammonia fragment
evolution and again indicate the strong similarities displayed in
the decomposition of solid precursors coming from Ti:M and
Ti:N precursors. Ammonia is desorbed from the sample in
essentially the same range of NO-containing molecules,
indicating a common intermediate in the process. This will
be further discussed below by using IR data. Main products of
the treatments are H2O and CO-containing molecules. Fig. 2C
displays a strong similarity between the 28 and 44 m/z signals
indicating that CO and CO2 are the main desorption products
associated (the m/z 28 secondary contribution has been
eliminated from the plot). An alternative assignment to a N2/
N2O couple seems less likely as the N-coupling reactions
demands for the presence of atomic nitrogen, hardly created in
the temperature ramp treatment. Differences between the Ti:M
and Ti:N solid precursors concern the significantly lower CO2
quantity desorbed in the former case, leading to a higher
CO:CO2 ratio. Apart from that, 39, 40 and 41 m/z values give
evidence of the evolution of C3 (C3H8, C3H6, C3H4) and C2
(CH2CN) hydrocarbon (HC) chains. Presence of relatively
weak signals at 27 and 29 m/z values favors the assignment to
propine or C2 products, giving rise to the main signal at m/z 40
(Fig. 2D). However, a number of C3 and C2 HC compounds
seems to be formed in variable quantities in both cases, the Ti:M
and Ti:N solid precursors. Concerning the Ti:Ph decomposi-
tion, apart from the overall lower magnitude, we found practical
absence of ammonia and HC chains in the desorption products.
With these data, it can be envisaged a similar pathway for the M
and N thermal breaking which releases C2/C3 and ammonia/
NO fragments, suggesting the breaking of the molecules by
both C–C and C–N bonds. C–N bond seems to break at lower
temperature than C–C ones (Fig. 2A and D). In any case, this
may leave different C,N fragments at the solid after
calcinations, as will be shown in part 2. Assuming that the
CO:CO2 ratio differences are mainly associated with the
burning of the N-containing ligand, we may also suggest a
significantly higher involvement of O anions coming from the
Ti-oxo-hydroxide solid precursor through the treatment and a
higher number of (anion) vacancies for catalysts (final solid)
C. Belver et al. / Applied Catalysis B: Environmental 65 (2006) 301–308 305
Fig. 2. MS signals detected in the evolving gaseous products for Ti:M, Ti:N and Ti:Ph solid precursors pre-treated at RT: (A) 30, 46 m/z signals; (B) 16 m/z signal; (C)
28, 44 m/z signals; and (D) 40 m/z signal. Vertical lines are plotted to facilitate comparison among samples.
obtained from the Ti:N precursor. Finally, the Ti:Ph precursor
seems too voluminous to be present in significant quantities at
the solid precursor, although, as mentioned, we found evidence
of N-containing fragments during the TG-MS experiment.
The treatment in O2/Ar at 200 8C gives precursors having
similar decomposition behaviors as those illustrated in Fig. 2
expect for the smaller amount of CO2 detected as product for all
precursors. An obvious difference with corresponding N-
containing Ti precursors pre-treated at RT is visible in Fig. 1;
the Ti:N precursor is more sensitive to the pre-treatment than
Ti:M and Ti:Ph precursors, being the latter rather insensitive. It
thus appears that the N-containing ligand already evolves under
the treatment in the former case, a fact which may be predicted
by the strong NO and CO2 signals displayed at low temperature
by this sample with respect to the others in Fig. 2. So, although
the N and M precursors may evolve similarly under continuous
thermal treatments, it appears that a plateau at 200 8C may
enhance differences between final solids.
The decomposition process was also studied by infrared
spectroscopy (Fig. 3) comparing the different calcination
processes (described in Table 1) for the Ti:M solid precursor.
Data on treatment ‘‘3’’ (RT—450 8C Ar; 450 8C 2 h Ar/O2; see
Table 1) do not give additional insights concerning the
precursor decomposition described above by using TG-MS,
nevertheless complementary information was obtained from
the other two treatments (Fig. 3A and B). At the beginning of
calcinations, only a small contribution of water is detected by
infrared analysis, with absorbance in the region 1800–
1400 cm�1. When the temperature increased above 200 8C,
new bands appear in the spectra. The most significant are
centered at 3700, 3050–2950, 1390 and 1100–1000 cm�1.
Comparing their positions with different infrared databases, we
can attribute these bands to bonded O–H stretching, C–H
stretching (from CH3 or CH2 groups), CH3 or CH2 deformation
and C–O stretching frequencies, respectively [23]. The
presence of these vibrations suggests the formation of R–OH
species, mainly associated with the presence of gaseous
CH3OH (although CH3CH2OH can also contribute) [24,25].
The vibrations related to C–H bonds are also common for
alkanes, as C3H8, which presence in large quantities is,
however, not likely on the basis of the thermal analysis results.
Two narrow bands, centered at 932 and 968 cm�1, associated
with the formation of NH3 can be observed above 220 8C. In
some spectra is also possible to see the N–H stretching and NH2
C. Belver et al. / Applied Catalysis B: Environmental 65 (2006) 301–308306
Fig. 3. Infrared spectra of gaseous effluents from treatments 1 (A) and 2 (B) of the Ti:M 1:1 solid precursor. See text for details.
bending vibrations at 3336 and 1628 cm�1, respectively
(Fig. 3A) [26]. At higher temperatures, we detected CO2
(668 and 2350 cm�1) and CO (2142 cm�1) molecules [26],
which appearance suggests again the relevance of oxygen (O2�;
OH�) anions from the Ti-oxo-hydroxide in the calcination
process. The detection of these species at higher temperature
indicates that the decomposition of the Ti:M precursor begins
with the breaking of the C–N and RO–C bonds, corroborating/
completing the results obtained by thermal analysis.
The infrared spectra suffer small changes when the
temperature reaches 300 8C. A new band appears centered at
1745 cm�1 which can be related to a C O stretching frequency.
Comparing the location of this band with different carbonyl
compounds, we can suggest the detection of saturated esters,
which C O spectral region are centered at 1750–1735 cm�1. A
detailed analysis of the bands observed in the spectra indicates
the detection of methyl acetate (CH3COOCH3) with small
bands at 2960 (nas CH3), 1440 (ns CH3), 1366 (d CH3), 1240
(CO–O stretching) and 1040 cm�1 (O–CH2) [23]. This
molecule should give dominant mass signals at 31 and 28 m/
z values but the significant number of entities contributing to
both disallow its unambiguous detection by the MS technique.
The detection of ester compounds from 300 8C by IR
corroborates again the higher resistance of C–C bonds
compared with C–N bonds and indicates that the fragments
like those formed during the first steps of the calcination (as R–
OH) can react between them to form more complex species.
This fact also explain the small bands at 3087 and 887 cm�1
observed from 320 to 450 8C that can be due to the presence of
C3H6 (marked as (*) in Fig. 3), which detection implies the
C. Belver et al. / Applied Catalysis B: Environmental 65 (2006) 301–308 307
Fig. 4. XRD diffractograms for the Ti–N samples and T references indicated.
Table 4
XRD-derived anatase-type lattice parameters (a, c) and volume (V) and primary
particle size (t) of Ti–N samples and T references
Sample/reference a (A) c (A) V (A3) t (nm)a
M1 3.781 � 0.009 9.43 � 0.03 134.9 � 0.7 13.5
M2 3.78 � 0.04 9.46 � 0.01 135.8 � 0.3 10.1
M3 3.792 � 0.007 9.48 � 0.03 136.4 � 0.5 10.0
2M1 3.80 � 0.01 9.50 � 0.04 137.2 � 0.7 13.1
2M2 3.788 � 0.008 9.47 � 0.02 135.9 � 0.4 13.7
2M3 3.793 � 0.03 9.48 � 0.01 136.4 � 0.6 8.3
N3 3.78 � 0.02 9.48 � 0.06 135.7 � 0.1 30.3
Ph3 3.786 � 0.004 9.48 � 0.02 136.0 � 0.3 12.8
T1 3.793 � 0.001 9.486 � 0.005 136.49 � 0.09 13.3
T3 3.791 � 0.004 9.471 � 0.004 136.2 � 0.3 10.9
Taq3 3.79 � 0.09 9.45 � 0.03 136.1 � 0.6 19.8
a Standard error 30%.
formation of C3 hydrocarbons with C C bonds. By other side,
the infrared study only identifies ammonia as nitrogen
compound formed during the amine decomposition, without
evidence of the formation of NO (1883 cm�1), NO2 (1618,
1318, 750 cm�1) or N2O (2224, 1286, 589 cm�1) [25]. This and
the existence of a common intermediate for ammonia and NO-
type species suggested by the TG-MS study would indicate that
ammonia is first formed in the degradation process and that
NO-type species are secondary products. Differences between
TG and IR results may be grounded on a different contact time
(rate of the temperature ramp) which would mainly affect
production and/or evolution of secondary products.
In brief, ‘‘1’’ and ‘‘2’’ thermal treatments studied by
infrared spectroscopy yields rather similar gaseous products
(Fig. 3A and B). The main differences between them are
related to the temperature at which the species were observed;
as an example, during the treatment 1 (Ar/O2 20–450 8C)
ammonia appears at 200 8C while for treatment 2 (Ar/O2
200–450 8C) was detected at 250 8C. Other important diffe-
rence resides in the ester compounds because the treatment
2 produces lower amount of them (the C O stretching band
only has some intensity at 300–350 8C). The increase of
Table 3
Main characterization results of Ti–N samples and T references
Sample BET area (m2 g�1)a %Nb
M1 107.8 0.05
M2 104.2 0.06
M3 61.2 0.07
2M1 106.5 0.53
2M2 100.2 0.14
2M3 68.2 0.04
N3 46.5 0.07
Ph3 63.1 0.03
T1 107.3 –
T3 74.9 –
Taq3 69.7 0.005
a Standard error � 0.5 m2 g�1.b Standard error � 0.2%.
temperatures detected in absence of O2 gas indicates a different
degree of O2�/OH� involvement in the chemical reactions
occurring during calcinations treatments. This fact suggests
again that the calcination process suffered by the solid
precursor can leave different N,C fragments and oxygen
vacancies in the final solid.
The final solids formed under the three treatments display
the exclusive presence of anatase-type phases as detected by
using XRD (Fig. 4) and Raman spectroscopy (data not shown).
The BET area and N content of the solids are summarized in
Table 3 while cell parameters and unit cell volume and primary
particle size are included in Table 4. Crystallinity of the
samples is somewhat poor except for the N3 sample and T1
reference; the latter is rather crystalline as it does not contain
N,C impurities and is calcined in oxygen from the beginning
while the former suffers a strong sintering process during
calcination, leading to the higher primary particle size observed
(Table 4). The pre-treatment influence seems to affect
morphological properties, decreasing surface area when
oxygen is not included in the treatment gas mixture, probably
due to the lost of surface OH species as could be observed in
their correspondence DRIFTS spectra (not shown), but less
affecting primary particle size. Differences in secondary
particle size and porosity are therefore evident between
treatments 1/2 and 3. The ligand nature, on the other hand,
seems to have significant influence on primary particle size,
giving increasing size in the N > Ph > M order. Main structural
properties as cell volume and parameters seem only weakly
varying in our set of samples. The N content of the samples was
rather small in all cases, independently of the precursor nature
of even the Ti:ligand molecular ratio, which is exemplified for
the M case in Table 3. The comparison with the T reference
systems would indicate that except for primary particle size,
morphological properties (BET area), main structural proper-
ties and chemical composition (N content) are not strongly
sensitive to the N-containing ligand nature or Ti:ligand
molecular ratio variables. As will be shown, strong differences
are, however, encountered in the photocatalytic properties
under sunlight excitation in terms of the N-containing Ti-ligand
precursor nature and Ti:ligand molecular ratio which can be
rationalized on the basis of the defect nature and distribution of
C. Belver et al. / Applied Catalysis B: Environmental 65 (2006) 301–308308
the different solids formed during the ‘‘calcination’’ treatments
of the solid precipitates/precursors.
4. Conclusions
Several N-containing Ti isopropoxide precursors have been
synthesized and used to obtain solid precipitates by a reverse
microemulsion method. The M and N ligands seem to allow the
production of N-rich solid precursors by maintaining Ti-ligand
bonds after hydrolysis of the isopropoxide groups in the
microemulsion media, while the failure of the synthesis method
is detected in the Ph case.
The thermal evolution of the solid precursor phases was
followed by TG, while the corresponding gas phase evolution
was analyzed by MS and IR. Overall, we detected the breaking of
C–N bonds before C–C ones for the M, N (and Ph) derived
fragments present in the solid precipitates/precursors. The
presence of oxygen in the calcinations gas mixture seems to
mainly facilitate the solid transformation, lowering the
temperatures of N (ammonia; NOx) and C (CO, CO2, HCs
and esters) containing gaseous products. So, the different
temperature (RT, 200, and 450 8C) where O2 is allowed to contact
the solid precursors modifies the interaction between solid
oxygen (O2�; OH�) species and N,C-containing fragments. The
latter is also affected by the N-containing ligand nature as
indicated by the differences encountered in the CO:CO2 ratio
detected by MS between N and M samples pre-treated in equal
conditions. Both preparation parameters (Ti precursor and
treatment conditions) will leave different C,N fragments and
oxygen vacancies in the final, catalytic solids.
The resulting solids display exclusive presence of an
anatase-type phase according to XRD/Raman results. Main
structural characteristics as cell parameters and volume are not
sensitive to both the treatment (1–3) and the N-containing
ligand nature. The treatment characteristics (1–3) seem to affect
morphological properties, decreasing surface area when
oxygen is not present in the calcinations gas mixture and
weakly affecting primary particle size, which is 10 � 2.5 nm
throughout the M/2M series of samples. The ligand nature has a
strong influence in the primary particle size, leading to an
increasing size in the N > Ph > M order. Main synthetic
variables (Ti precursor and the thermal treatment) permit
therefore to control the chemical, structural and morphological
properties of the final solids, being possible to synthesized
optimum visible response photocatalysts. Their promising
photocatalytic properties could be related to the presence of
differences in the N,C fragments and/or oxygen vacancies
formed during the synthesis process. This is the subject of part 2
of the study.
Acknowledgements
We appreciate financial support by CYCIT project
CTQ2004-03409. C.B. thanks the Spanish Ministry of
Education for a ‘‘Juan de la Cierva’’ post-doctoral grant.
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