Reactions of titanium oxides with water molecules. A matrix isolation FTIR and density functional study

Article abstract of DOI:10.1016/S0009-2614(01)00675-3

Reactions of titanium dioxides with water molecules have been studied by matrix isolation infrared and density functional theoretical calculations. In solid argon, titanium dioxide molecules reacted with water to form OTi(OH)₂ molecules spontaneously on annealing. The potential energy surface along the TiO₂+H₂O→OTi(OH)₂ reaction path was calculated. Although titanium dioxide-water complex was predicted to be stable, this complex was not observed due to low energy barrier and high exothermicity for the hydrolysis reaction. Evidence is also presented for the formation of OTi-OH₂ complex.

Full text of DOI:10.1016/S0009-2614(01)00675-3

27 July 2001
Chemical Physics Letters 343 52001) 178±184
www.elsevier.com/locate/cplett
Reactions of titanium oxides with water molecules. A matrix
isolation FTIR and density functional study
Limin Shao, Luning Zhang, Mohua Chen, Hao Lu, Mingfei Zhou *
Department of Chemistry and Laser Chemistry Institute, Fudan University, Shanghai 200433, China
Received 7 May 2001; in ®nal form 30 May 2001
Abstract
Reactions of titanium dioxides with water molecules have been studied by matrix isolation infrared and density
functional theoretical calculations. In solid argon, titanium dioxide molecules reacted with water to form OTiꢀOH†
2
molecules spontaneously on annealing. The potential energy surface along the TiO2 ‡ H2O ! OTiꢀOH† reaction path
2
was calculated. Although titanium dioxide±water complex was predicted to be stable, this complex was not observed
due to low energy barrier and high exothermicity for the hydrolysis reaction. Evidence is also presented for the for-
mation of OTi±OH2 complex. Ó 2001 Published by Elsevier Science B.V.
1. Introduction
each reaction step comprised the breaking of one
M@O bond and the formation of two OHgroups.
The reactions were found to involve stable water
complexes as intermediates. As far as we know,
there is no experimental report on reactions be-
tween water and molecular transition metal oxides.
In the present study, we report a matrix isolation
FTIR spectroscopic and density functional theo-
retical study on reactions between titanium dioxide
molecules and water molecules.
The interaction of water with titanium dioxide
is of fundamental importance in a wide variety of
®elds such as heterogeneous catalysis and phot-
ocatalysis, corrosion science and environmental
and geochemistry. Supported titanium oxide is
currently intensively studied as photocatalyst in
water cleavage and decontamination. TiO2 is also
widely used as a pigment in paints and as a catalyst
support [1,2]. Accordingly, water adsorption on
titanium oxide surface has been well studied both
experimentally and theoretically [3].
2. Experimental and theoretical methods
Recently, water adsorption and hydrolysis on
molecular transition metal oxides 5TiO2 and CrO3)
and oxyhydroxides 5ScO5OH), VO2ꢀOH† and
MnO3ꢀOH†) has been studied by means of quan-
tum chemistry [4,5]. In the investigated reactions,
The experimental setup for pulsed laser ablation
and matrix infrared spectroscopic investigation
has been described previously [6]. The 1064 nm
Nd:YAG laser fundamental 5Spectra Physics,
DCR 150, 20 Hz repetition rate and 8 ns pulse
width) was focused onto the rotating TiO2 or Ti
targets, and the ablated species were co-deposited
with H2O or O2=H2O mixtures in excess argon
* Corresponding author. Fax: +86-21-65102777.
E-mailaddress: mfzhou@fudan.edu.cn 5M. Zhou).
0009-2614/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V.
PII: S 0 0 0 9 - 2 6 1 4 5 0 1 ) 0 0 6 7 5 - 3

L. Shao et al. / Chemical Physics Letters 343 &2001) 178±184
179
onto a 11 K CsI window, which was mounted on a
cold tip of a closed-cycle helium refrigerator 5Air
Products, Model CSW202) for 1 h at a rate of 2±4
mmol/h. Typically, 5±10 mJ/pulse laser power was
used. H2O and D2O were subjected to several
freeze±pump±thaw cycles before use. O2 5Shanghai
BOC, 99.6%) and 18O2 5Isotec Inc., >97%) were
used without further puri®cation. Infrared spectra
were recorded on a Bruker IFS113V spectrometer
at 0:5 cmÀ1 resolution using a DTGS detector.
Matrix samples were annealed at di€erent tem-
peratures, and selected samples were subjected to
broadband photolysis using a high pressure mer-
cury lamp.
1010±900 and 770±400 cmÀ1 regions are shown in
Fig. 1, with the product absorptions listed in Table
1. Co-deposition of laser-ablated Ti atoms with
H2O=O2=Ar produced strong TiO2, O4À
5953:8 cmÀ1) [14,15] and very weak TiO absorp-
tions. New absorptions at 996.6, 753.6, 677.0,
483.2 and 414:9 cmÀ1 were observed on annealing
as with the TiO2 ‡ H2O=Ar experiments. Very
weak absorptions due to Ti ‡ H2O reaction were
also observed on broadband photolysis [16]. In the
spectra using a H216O ‡ 18O2 sample, the TiO2,
À
TiO and O4 absorptions were shifted as observed
before [13], and the 996.6, 753.6, 677.0, 483.2 and
414:9 cmÀ1 were shifted to 955.5, 745.1, 659.1,
480.9 and 414:3 cmÀ1
, respectively. When a
Quantum chemical calculations were performed
using the GAUSSIAN 98 program [7]. The three-
parameter hybrid functional according to Becke
with additional correlation corrections due to Lee,
Yang and Parr was utilized 5B3LYP) [8,9]. The
6-311++G5d,p) basis sets were used for Hand O
atoms, the all electron basis set of Wachters±Hay
as modi®ed by Gaussian was used for Ti atom
[10,11]. Reactants, various possible transition
states, intermediates and products were optimized.
The vibrational frequencies were calculated with
analytic second derivatives, and zero point vibra-
tional energies 5ZPVE) were derived. Transition
state optimizations were done with the synchro-
nous transit-guided quasi-Newton5STQN) method
[12], followed by the vibrational frequency calcu-
lations showing the obtained structures to be true
saddle points.
D2O ‡ 16O2 sample 5with about 40% HDO con-
tamination) was used, the 996:6 cmÀ1 band went
to 995:2 cmÀ1, and both the 753.6 and 677:0 cmÀ1
bands split into two bands at 747.8, 740:6 cmÀ1
and 671.0, 666:9 cmÀ1, as shown in Fig. 2.
The 996.6, 753.6, 677.0, 483.2 and 414:9 cmÀ1
bands are assigned to di€erent modes of the
OTiꢀOH†2 molecule. These bands were observed in
both TiO2 ‡ H2O=Ar and Ti ‡ H2O=O2=Ar ex-
periments, and increased together on annealing
and photolysis, suggesting di€erent vibrational
modes of the same molecule. The 996:6 cmÀ1 band
showed very small deuterium isotopic shift 5about
1:4 cmÀ1). This band split into two bands at 955.5
3. Results and discussion
Laser ablation of a bulk TiO2 target in pure
argon produced TiO2 5946.8, 917:0 cmÀ1) and TiO
5990:2 cmÀ1) as the major products [13]. When
laser-ablated TiO2 was co-deposited with 0.25%
H2O in argon, new product absorptions at 996.6,
753.6, 677.0, 483.2 and 414:9 cmÀ1 were produced
on sample annealing. These absorptions also in-
creased on broadband photolysis and on subse-
quent 30 K annealing.
Fig. 1. Infrared spectra in the 1010±900 and 770±400 cmÀ1
regions from co-deposition of laser-ablated Ti atoms with 0.2%
H2O ‡ 0:5% O2 in argon. 5a) 1 h sample deposition at 11 K,
5b) after 28 K annealing, 5c) after 20 min broadband photolysis,
and 5d) after 30 K annealing.
Experiments were also done using metal Ti as
target and H2O ‡ O2 mixtures in excess argon as
reagent gas, and the representative spectra in the

180
L. Shao et al. / Chemical Physics Letters 343 &2001) 178±184
Table 1
Infrared absorptions ꢀcmÀ1† from co-deposition of laser-ablated titanium atoms with H2O ‡ O2 mixtures or laser-ablated TiO2 with
H2O in excess argon
H216O=16O2
H216O=18O2
D216O=16O2
Assignment
1010.5
996.6
990.2
985.7
968.8
961.8
953.8
946.8
917.0
753.6
677.0
483.2
414.9
1010.5
955.5
948.3
944.1
968.8
H2TiO 5Ti@O stretching)
995.2
990.2
985.7
OTiꢀOH† 5Ti@O stretching)
2
TiO
TiO site
HTiO 5Ti@O stretching)
OTi±OH2 5Ti@O stretching)
O4À
901.8
904.3
881.6
745.1
659.1
480.9
414.3
953.8
946.8
917.0
740.6
666.9
TiO2 5sym-OTiO stretching)
TiO2 5asy-OTiO stretching)
OTiꢀOH† 5asym-TiOHstretching)
2
OTiꢀOH† 5sym-TiOHstretching)
2
OTiꢀOH† 5out-of-plane TiOHbending)
2
OTiꢀOH† 5in-plane TiOHbending)
2
and 951:5 cmÀ1 in the Ti=H216O ‡ 18O2=Ar spec-
tra. The 955:5 cmÀ1 band is the isotopic counter-
part, which gave an isotopic 16O/ 18O ratio of
1.043. This ratio is slightly lower than the diatomic
TiO ratio 1.0446, indicating that this band is
mainly due to a Ti±O stretching vibration. The
951:5 cmÀ1 band is unusually strong, and is mostly
due to the overtone of the out-of-plane TiOH
bending vibration observed at 480:9 cmÀ1 in Fermi
resonance with the 955:5 cmÀ1 band. In the
H216O ‡ 16O2 case, the 996:6 cmÀ1 Ti±O funda-
and the interaction is diminished. The 753.6 and
677:0 cmÀ1 bands are antisymmetric and sym-
metric Ti±OHstretching vibrations. In the
H216O ‡ 18O2 experiment, these two bands shifted
to 745.1 and 659:1 cmÀ1, which correlate to the
18OTi 516OH)518OH) molecular absorptions. In the
HDO ‡ D2O ‡ O2 experiments, both the 753.6
and 677:0 cmÀ1 bands split into doublet at 747.8
and 740:6 cmÀ1, and 671.0 and 666:9 cmÀ1. These
four absorptions are due to antisymmetric and
symmetric Ti±OHstretching vibrations of the
mental is above the overtone by about 30:2 cmÀ1
,
OTi5OH)5OD) and OTiꢀOD† molecules, respec-
2
tively. The 483:2 cmÀ1 band shifted to 480:9 cmÀ1
with H216O ‡ 18O2, and is assigned to the out-of-
plane TiOHbending vibration. The 414 :9 cmÀ1
band is probably due to the in-plane TiOHbend-
ing vibration.
DFT calculations support the above assign-
ments. As shown in Fig. 3, the OTiꢀOH†2 molecule
was predicted to have a 1A1 ground state with
planar C2v symmetry. The calculated vibrational
fundaments are listed in Table 2. The above
characterized Ti±O, Ti±OHstretching and bending
vibrations were calculated at 1055:2 cmÀ1 5291 km/
mol), 784:3 cmÀ1 5282 km/mol), 678:6 cmÀ1 561
km/mol), 537:8 cmÀ1 5349 km/mol) and 460:1 cmÀ1
5190 km/mol). As can be seen in Table 2, the cal-
culated isotopic frequencies are also in good
agreement with the observed values.
Fig. 2. Infrared spectra in the 1010±860 and 770±620 cmÀ1
regions from co-deposition of laser-ablated Ti atoms with
H2O ‡ O2 mixtures in excess argon. 5a) H216O ‡ 18O2, sample
deposition, 5b) after 28
K annealing of sample 5a), 5c)
Weak absorptions at 961:8 cmÀ1 in the TiO2 ‡
HDO ‡ D2O ‡ O2, sample deposition, and 5d) after 28 K an-
H2O=Ar experiments produced on annealing and
nealing of sample 5c).

L. Shao et al. / Chemical Physics Letters 343 &2001) 178±184
181
Fig. 3. Optimized geometric parameters for TiO, TiO2, O2Ti±OH2, OTiꢀOH†2, OTi±OH2, TiꢀOH† and transition states 5TS1, TS2).
2
were diminished on broadband photolysis, and
were tentatively assigned to the Ti±O stretching
vibration of the OTi±OH2 complex. This complex
absorption was not observed in the Ti ‡ H2O ‡
O2=Ar experiments, as the TiO absorption in these
experiments is very weak. As shown in Fig. 3, DFT
calculations predicted the OTi±OH2 complex to
have a 3A00 ground state that correlates to 3D
ground state TiO. The Ti±O stretching vibration
was calculated at 1009:3 cmÀ1, red-shifted by
about 33:4 cmÀ1 compared with the diatomic TiO
vibration calculated at 1042:7 cmÀ1. Experimen-
tally, we observed a 28:4 cmÀ1 red-shift.
In both the TiO2 ‡ H2O=Ar and Ti ‡ H2O‡
O2=Ar experiments, the primary products after
sample deposition are TiO2 and TiO. The
OTiꢀOH† absorptions increased on annealing,
2
indicating that the OTiꢀOH† molecules are
2
formed by reaction of TiO2 with water in solid
argon, reaction 51). This reaction was predicted to
be exothermic by about 70.9 kcal/mol at the
B3LYP/6-311++G5d,p) level of theory. The

182
L. Shao et al. / Chemical Physics Letters 343 &2001) 178±184
Table 2
Calculated vibrational frequencies ꢀcmÀ1†, and intensities 5km/mol) for the structures described in Fig. 3
Molecule
Frequency 5intensity, mode)
TiO
TiO2
1042.75220)
1026.1539, a1), 978.95470, b2), 339.7515, a1)
O2Ti±OH2
3869.95167, a00), 3783.45a0), 1634.15114, a0), 1001.2555, a0), 966.85488, a00), 554.151, a00), 466.95263, a0),
388.4552, a0), 305.2510, a0), 259.1546, a00), 106.75133, a0), 77.750.1, a00)
3907.551, a1), 3904.35329, b2), 1055.25291, a1), 784.35282, b2), 678.6561, a1), 537.85349, b1), 515.150, a2),
474.0572, b2), 460.15190, a1), 225.0519, b2), 192.853, a1), 53.3530, b1)
3906.15127), 3892.75197), 1011.65279), 774.45269), 659.5553), 535.45340), 512.952), 469.2579),
457.45186), 217.1517), 188.553), 52.6529)
16OTiꢀ16OH†
2
18OTi 516OH)518OH)
16OTiꢀ16OD†
2846.752), 2843.45225), 1055.15288), 766.55309), 666.9581), 414.25244), 389.9523), 388.650), 351.9594),
202.6520), 178.055), 49.6526)
2
OTi±OH2
3794.5587, a0), 3695.65185, a00), 1611.65178, a0), 1009.35285, a0), 442.658, a00), 338.9570, a0), 307.1574, a0),
96.0531, a0), 31.653, a00)
TiꢀOH†
3973.450, rg), 3971.35379, rl), 762.25357, rl), 649.950, rg), 457.65404, pl), 454.050, pg), 40.5510, pl)
3878.05113), 1893.8578), 1263.9571), 1030.75287), 910.65267), 662.0597), 605.2599), 501.45126),
409.55147), 277.8533), 116.2546), 1393.1i51659)
2
TS1
TS2
3926.05138), 973.45157), 829.25138), 622.4577), 466.65130), 430.45146), 241.0558), 143.6523), 443.0i531)
density functional calculations [4]. We have also
calculated the potential energy surface along the
OTiꢀOH†2 absorptions increased on annealing also
suggest that reaction 51) requires negligible acti-
vation energy.
TiO2 ‡ H2O ! OTiꢀOH† reaction path, and the
2
1
1
1
result is shown in Fig. 4. Our calculation results
are in good agreement with those of Johnson and
Panas. Water reacts with TiO2 to form a stable
O2Ti±OH2 complex. Formation of this complex
was predicted to be exothermic by 28.0 kcal/mol,
TiO2ꢀ A1† ‡ H2Oꢀ A1† ! OTiꢀOH†2ꢀ A1†;
DE ˆ À70:9 kcal=mol:
ꢀ1†
The reactions of water with TiO2 molecules were
recently addressed by Johnson and Panas by using
Fig. 4. Potential energy surface along the TiO2 ‡ H2O ! OTiꢀOH† reaction path. Energies given are in kcal/mol and are relative to
2
1
1
the ground state reactants: TiO2ꢀ A1† ‡ H2Oꢀ A1†.

L. Shao et al. / Chemical Physics Letters 343 &2001) 178±184
183
Fig. 5. Potential energy surface along the TiO ‡ H2O ! TiꢀOH† reaction path. Energies given are in kcal/mol and are relative to the
2
3
1
ground state reactants: TiOꢀ D† ‡ H2Oꢀ A1†.
and is barrier-free. The complex has non-planar Cs
symmetry with coordination between Ti atom and
O atom of the H2O unit. As can be seen from
Table 2, the antisymmetric and symmetric TiO2
stretching vibrations were predicted to be red-
shifted by about 12.1 and 24:9 cmÀ1 compared
with those of TiO2. No evidence was found for this
complex in present experimental conditions.
with water molecule to form OTiꢀOH†2±H2O
complex and ®nally to TiꢀOH†4. Note that the
OTiꢀOH† absorptions are weak in present exper-
2
imental conditions, the subsequent reaction prod-
ucts probably are too weak to be observed.
In the TiO2 ‡ H2O=Ar experiments, laser ab-
lation of the TiO2 target also produced substantial
TiO molecules, and the OTi±OH2 complex was
formed via reaction 52):
In O2Ti±OH2 complex, a hydrogen atom could
transfer to one of the O atoms in the TiO2 unit to
00
3
1
3
TiOꢀ D†‡H2Oꢀ A1† ! OTi±OH2ꢀ A †;
DE ˆ À16:1 kcal=mol:
form the OTiꢀOH† molecule through a transition
2
ꢀ2†
state. The geometric parameters of this transition
state are also shown in Fig. 3. The hydrolysis re-
action from O2Ti±OH2 to OTiꢀOH† was exo-
This reaction is exothermic by about 16.1 kcal/
mol. One could imagine the formation of TiꢀOH†
2
thermic by about 42.9 kcal/mol, with an energy
barrier of 6.5 kcal/mol. As the water addition to
TiO2 is initially exothermic by 28.0 kcal/mol,
which signi®cantly surmounts the energy barrier
for the hydrolysis reaction, the formation of
2
from OTi±OH2 analogous to the O2Ti±OH2 re-
action. However, no evidence was found for the
formation of the TiꢀOH† molecule. Fig. 5 shows
2
the potential energy surface along the TiO ‡ H2O
OTiꢀOH† is spontaneous in solid argon, as evi-
! TiꢀOH† reaction path. The hydrolysis reaction
2
2
denced by the increase of the OTiꢀOH† absorp-
from OTi±OH2 to TiꢀOH† has an energy barrier
2
2
tions on annealing. This also suggests that the
O2Ti±OH2 complex is very short-lived, rapidly
of 22.9 kcal/mol, signi®cantly higher than the
energy barrier for the TiO2 hydrolysis reaction.
The transition state is even higher than the
rearranging to form the OTiꢀOH† molecule.
2
3
According to Johnson and Panas's calculations
[4], the OTiꢀOH† molecule could further react
separated reactants TiOꢀ D† ‡ H2O by about
6.8 kcal/mol.
2

184
L. Shao et al. / Chemical Physics Letters 343 &2001) 178±184
4. Conclusions
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2
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We greatly acknowledge support for this re-
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