Room temperature spontaneous conversion of OCS to CO2 on the anatase TiO2 surface

Article abstract of DOI:10.1039/c4cc01992j

High-resolution FT-IR spectroscopy combined with quantum chemical calculations was used to study the chemistry of OCS-disproportionation over the reduced surface of isotopically labelled, nanocrystalline TiO₂. Analysis of the isotopic composition of the product gases has revealed that the reaction involves solely OCS molecules from the gas-phase. Using quantum chemical calculations we propose a plausible mechanistic scenario, in which two reduced Ti³⁺ centres mediate the reaction of the adsorbed OCS molecules.

Full text of DOI:10.1039/c4cc01992j

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Room temperature spontaneous conversion of
OCS to CO on the anatase TiO surface†
2
2
Cite this: Chem. Commun., 2014,
0, 7712
5
a
ab
bc
bc
a
Svatopluk Civis,* Martin Ferus, Judit E. Sˇ poner,* Jirı Sˇ poner, Ladislav Kavan
ˇ
´
a
Received 17th March 2014,
Accepted 9th May 2014
and Marketa Zukalova
´ ´
DOI: 10.1039/c4cc01992j
www.rsc.org/chemcomm
5
High-resolution FT-IR spectroscopy combined with quantum with three under-coordinated (fivefold) Ti ions. Essentially, two
chemical calculations was used to study the chemistry of OCS- of these Ti ions trap the two electrons and change their formal
4
+
3+
disproportionation over the reduced surface of isotopically labelled, oxidation state from Ti to Ti . The resulting spin multiplicity
nanocrystalline TiO . Analysis of the isotopic composition of the can be a closed-shell singlet or an open-shell singlet and a
2
product gases has revealed that the reaction involves solely OCS triplet. The corresponding electronic structure is characterised
molecules from the gas-phase. Using quantum chemical calculations by several defect states in the band gap. These states are
5
–8,16–21
we propose a plausible mechanistic scenario, in which two reduced localised on both the Ti interstitial and substitution sites.
Ti centres mediate the reaction of the adsorbed OCS molecules.
3+
Previously, we have studied the oxygen atom exchange
between gas phase molecules and a reduced titania surface, as
Titanium dioxide, TiO , is one of the most versatile oxide well as the photocatalytic effects on this material, using isotopic
2
1
–13
21–23
materials and is used in a wide variety of applications.
The reactions of gaseous carbonyl sulphide (OCS) are of logues:
potential importance in atmospheric chemistry. OCS is a pre- have extended this approach to examine the interaction of OCS
dominant sulphur-containing compound in the troposphere with Ti O
a mixing ratio of 500 pptv). Approximately 0.64 Tg per year of
labelling.
For this purpose, we synthesised anatase isotopo-
2
,24,25
16
17
18
2 2 2
Ti O , Ti O and Ti O . In the present study, we
1
6
1
8
2
and describe the reactivity at the molecular level.
1
4
(
Similarly to our previous studies focused on the reactivity of
reduced titania with CO
1
5
11,25–27
OCS in the troposphere is transported to the stratosphere,
2
,
(which is a homologue of
where it can be oxidised or photodissociated via reactions with OCS), High resolution Fourier transform infrared spectroscopy
O atoms or OH radicals to form sulphate aerosols. The question (HR-FTIR) was used to analyse the products of the reaction.
of whether OCS can be converted to some other less harmful
species by reactions like catalytic dismutation is an on-going interaction of OCS with nanocrystalline Ti O
research challenge. In this paper we describe the conversion of by the ratio of OCS/TiO , i.e., by the number of interacting
OCS to CO and CS which is a model reaction of this kind. We show molecules relative to the number of defects in the titania.
that the reaction occurs efficiently over the surface of partly reduced (a) Number of bulk defects much higher than the number of
n-doped) nanocrystalline TiO (anatase) acting as a catalyst.
The partly reduced form of titania can be prepared by the
Two different experiments were conducted to explore the
1
6
18
2
. They differed
2
2
2
1
6
16
(
2
OCS gas molecules – low pressure experiment (1.2 Torr of OCS).
1
8
One gram of vacuum-calcinated Ti O (450 1C) was placed
2
1
6
thermal treatment of TiO in a vacuum. The formation of initial in the IR optical cell with 1.2 Torr of OCS (99%). The spectra
2
O vacancies in the surface is followed by the depletion of oxygen were obtained every 120 seconds using an InSb detector, in the
2
from the internal layers of titania. If a neutral oxygen is removed spectral range of the main CO –OCS infrared absorption bands
ꢀ
1
from the lattice, two extra electrons remain and fill the empty (1800–4000 cm ). The isotopic composition was determined
states of the titanium ions, forming a point defect in conjunction from calibration measurement of pure standard gases. The
evolution of product composition in the low pressure reaction
1
6
18
a
of OCS with solid Ti O
(0.000146 mol OCS and 0.0125 mol
J. Heyrovsk ´y Institute of Physical Chemistry, v. v. i., Academy of Sciences of the
Czech Republic, Dolej ˇs kova 3, CZ-18223 Prague 8, Czech Republic.
E-mail: svatopluk.civis@jh-inst.cas.cz
2
1
8
Ti O respectively) is depicted in Fig. 1. During the reaction,
2
1
6
the partial pressure of OCS significantly decreased, while new
products appeared: C O , C
b
c
Institute of Biophysics, v. v. i., Academy of Sciences of the Czech Republic,
Kr a´ lovopolsk a´ 135, CZ-612 65 Brno, Czech Republic
16
16,18
18
O and C O , with traces of
2 2
2
1
8
OCS. After 5 days, the reaction gas mixture consisted of 45.8%
CEITEC – Central European Institute of Technology, Masaryk University,
Campus Bohunice, Kamenice 5, CZ-62500 Brno, Czech Republic
1
6
18
16,18
16
C O
2
, 42.2% C O
2
, 3.2% C
2
O and 8.8% OCS, with traces
1
8
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/c4cc01992j of OCS. Gaseous CS
2
was not observed.
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Fig. 1 Evolution of the composition of the reaction mixture in the reaction
18
16
of Ti
2
O (s) + OCS (g).
Based on the analysis of the products, two possible scenarios
can be proposed:
1
8
(I) Conversion of OCS to CO
2
with the involvement of
O
from titania:
1
8
16
16,18
16,18
Ti O (s) + 2 OCS(g) - C
O (g) + Ti
2
O (s) + CS
2
2
2
(1)
(
II) Disproportionation of OCS to CO without the involve-
2
1
8
ment of O from titania:
16
Fig. 2 Spectra of the reaction mixture formed from 1.2 Torr OCS kept
over solid Ti O are depicted in panel A at the beginning of the experiment
Ti18O2
18
2
1
6
16
2
OCSðgÞ ƒ ƒƒ ! C O2ðgÞ þ CS2
(2)
(
a) and after five days (b). All OCS has been completely exchanged to CO
and equilibrium has been achieved. Panel B shows details of the v + v and
+ v bands of all CO isotopologues. For comparison, the spectrum of
2
,
1
3
2
,20,25,26
In previous studies,
we have observed a quick oxygen
2
v
2
3
2
1
6
isotopic exchange between gaseous C O and solid n-doped CO2 isotopologues was simulated using the Winproof program and para-
2
1
8
28
Ti O
2
. This reaction pathway consists of two steps:
meters of the absorption lines taken from the HITRAN database.
1
8
16
16,18
16,18
Ti O
2
(s) + C O
2
(g) - Ti
O
2
(s) + C
O
2
(g)
(3)
(4)
1
8
1
8
16,18
16,18
18
2
(b) The number of Ti O bulk defects is comparable to (or
Ti O (s) + C
O (g) - Ti
O + C O (g)
2
2
2
2
1
6
less than) the number of OCS molecules – atmospheric
The experimental conditions in the set-up described were pressure experiment (760 Torr of OCS).
similar to our previous study designed to investigate the isotopic In parallel to the previous low pressure OCS experiment, a
1
6
exchange. The molar ratio between the gas phase molecules reference high pressure experiment was carried out using
1
8
18
16
and the solid phase (Ti O ) species was approximately 1 : 1000. 0.0125 mol of Ti O
2
and 0.017 mol of OCS. An identical cell
2
16
18
According to our previous observations, C O
at the active sites (surface oxygen vacancies) of Ti O
exchanged the O atoms from the titania framework. The C
molecule appeared in the reaction mixture as an intermediate was analysed in a 30 m multipass cell using a high resolution
see reaction (3)), and it was present in the gaseous mixture in Bruker IFS 120 spectrometer after one and six months. Initially,
low concentrations. Assuming that mechanism I is effective in our only traces of C O were detected (see the weak band v in Fig. 3,
O2 would rather be the primary product than an panel A, spectrum b). After the first month, the concentration of
intermediate of the OCS conversion. In mechanism I, the pathway C O
2 2
immediately reacted was filled with 1 g of vacuum-calcined Ti O powder and blown
18
2
and mutually with carbonyl sulphide until atmospheric pressure was reached.
18
16,18
O
2
To achieve a higher detection limit, 1 ml of the reaction mixture
(
1
6
2
3
16,18
case, C
16
16
2
gradually grew to 2%, and after six months it reached 37%
2
is absent, because a subsequent (Fig. 3, panel A, spectrum c). The OCS molecule was present at
16
16
producing isotopologue C O
isotopic exchange step would involve the formation of C O
18
18
2
from a high concentration and reacted at the surface of the Ti O
2
,
16,18
C
O according to reaction (4). This scheme is in obvious where it filled the oxygen vacancies, according to reaction (2).
2
1
8
disagreement with the spectra depicted in Fig. 2, panel B, where The resulting CS
2
was adsorbed onto the Ti O
2
surface in the
1
6
16,18
18
20
all C O
2
, C
O and C O isotopologues have been identified. In same manner as water, hindering the subsequent isotopic
2 2
16
16
18
16
contrast to mechanism I, in mechanism II, C O is characterized exchange between C O and Ti O . Only C O from the parent
2
2
2
2
16
16
by a disproportionation reaction according to eqn (2). The C O
OCS remained in the mixture. The CS2 was present in the
may subsequently undergo isotopic exchange reactions with the gas phase at a 12% concentration (compare Fig. 3, panel A, CS
labelled titania via eqn (3) and (4), which leads to the formation of standard spectrum a and spectrum c).
2
2
18
all possible CO
2
isotopologues. The latter scenario is in accordance
2 2
The adsorption of CS on the Ti O surface changed the colour
with our observations.
of the sample from the initial white-yellow to an ochre-brown after
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Table 1 The concentrations of the individual gases of the samples formed
16
16
in the cell containing OCS and Ti
the experiment
2
O (A450) at the beginning of
1
6
CS
(%)
2
Molecule
OCS (%)
C
O
2
Spectrum
(Fig. 4)
1
6
Sample description
(%)
Gas phase after 6 months
Exposed for 4 days to
12
22
52
30
37
49
c
d
7
60 Torr of nitrogen
Exposed for 1 day to
60 Torr of oxygen
Exposed for 2 days to
60 Torr of oxygen
36
50
14
0
49
50
e
f
7
7
was no longer detected. Moreover, the TiO sample returned to
2
white. The results from the quantitative analysis of the reaction
mixture are summarised in Table 1.
We propose a plausible mechanism for the disproportionation
reaction using quantum chemical calculations (for details see the
1
6
Fig. 3 Spectra of the reaction mixture formed from 760 Torr of OCS kept
18
over solid Ti
compared with the spectrum of the CS
beginning of the experiment is marked (b). After one week, new bands of C
and CS emerge (c). The most intense v band of CS
are highlighted red and blue, respectively. The intensity of the turated Ti centres, which may give rise to a triplet electronic
O
2
are depicted in panel A. The experimental spectra are
2
standard (a). The spectrum in the
16
O
2
and the fundamental ESI†) on a minimal model consisting of two coordinatively unsa-
2
2
1
16
29–31
3+
OCS band
CS band increases due to desorption of this molecule into the gas phase in the
presence of nitrogen (d) and oxygen (e). The maximum desorption of CS was
achieved in the presence of 760 Torr of oxygen during the period of two days (f).
2
configuration, in agreement with ref. 7, 12 and 13. Note that we
also made numerous unsuccessful attempts to find a possible
reaction pathway assuming singlet electronic configuration. The
computed free energy profile of the reaction, along with the
optimized geometries, is depicted in Fig. 4 (Cartesian coordinates
of all optimized geometries are listed in the ESI†). The reaction
starts with the activation of the first OCS molecule upon binding to
2
16
A detailed view of the C O
2 2 3
and CS v bands is presented in the panels B and C,
respectively, which are shown in the ESI.†
6
months. This colour suggests the possible interaction of
sulphur-containing species with TiO . However, no shifts of
the characteristic Ti O vibration bands were found in the
Raman spectrum of the exposed sample, which was directly
measured several times in the glass cell that contained OCS.
This confirms that the structure of titania remained essentially
unchanged during the interaction with sulphur-containing
species. The further exploration of the titania surface using
XPS analysis is described in the ESI.†
2
3+
1
8
two adjacent Ti surface sites in the fashion shown in structure 1,
Fig. 4. This is followed by the attack of a second gas-phase OCS
molecule at the activated carbon of OCS, requiring an activation
2
1
6
ꢀ
1
energy of 43.2 kcal mol (structure 2 shows the transition state of
this reaction step) and leading to the formation of a thiocarbonate-
3+
like intermediate (structure 3) bound over two Ti centres. In
the rate determining step, this intermediate isomerises via a
four-centred transition state, shown in structure 4, which leads to
the formation of a dithiocarbonate-like intermediate (structure 5).
2
To quantify the adsorbed CS , we repeated the previous
1
6
experiment. The cell was again filled with OCS until atmo-
spheric pressure was reached. The gaseous mixture was mea-
sured after six months in a 30 m absorption cell using a HR FT
spectrometer in the spectral range of the strong fundamental
2
bands of CS (see Fig. 3). The cell was then evacuated and re-filled
with nitrogen until atmospheric pressure was reached. A small
sample was analysed after four days, and several characteristic
infrared bands (see Fig. 3, panel A, spectrum d) of desorbed CS
were observed. This four day desorption of nitrogen resulted in
2
1
6
18
2
2% CS and 49% C O at the Ti O surface. The quantitative
2 2 2
1
6
2
data are summarised in Table 1. The colour of the Ti O (A450)
powder sample remained yellow-brown. The evacuation proce-
dure was repeated, and the cell containing the still yellow-brown
1
8
Ti O
obtained after 1 and 2 days are depicted in Fig. 3, panel A, spectra
e) and (f). A significant increase in the intensity of the CS band
2
was filled with pure oxygen. The high resolution spectra
(
2
ꢀ1
near 1500 cm is apparent. After 2 days (in an O atmosphere),
2
Fig. 4 The calculated free energy profile of the disproportionation reac-
tion. Computations were carried out at the B3LYP level of theory using the
aug-cc-pVTZ basis set on all atoms except Ti, which was described with
the gas sample only contained the mixture of CS and CO : the
2
2
2
concentration of CS in the cell significantly increased to 36%
after 1 day and to 50% after 2 days and the concentration of the
30
the Stuttgart small core relativistic pseudo-potential. For computational
1
6
16
18
isotopologue C O
2
increased to 49 and 50%, respectively (see
details, see the ESI.† Colour coding: O light red, O dark red, S yellow,
1
6
Fig. 3 and panel B in the ESI†). At this point, the reactant, OCS, C grey, H white, Ti blue.
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The relatively high activation energy of this reaction step, i.e.,
9 H. Chen, C. E. Nanayakkara and V. H. Grassian, Chem. Rev., 2012,
112, 5919–5948.
ꢀ
1
63.7 kcal mol , concurs with the experimentally observed low
1
0 D. C. Sorescu, W. A. Al-Saidi and K. D. Jordan, J. Chem. Phys., 2011,
reaction rates. In the next reaction step, this intermediate
135, 124701.
3
+
decomposes to CS and CO bound to two adjacent Ti centres 11 D. C. Sorescu, S. Civis and K. D. Jordan, J. Phys. Chem. C, 2014, 118,
2
2
1
628–1639.
(structure 7) in a quick reaction step via a transition state shown
1
2 M. Zhang, Q. Wang, C. Chen, L. Zang, W. Ma and J. Zhao, Angew.
in structure 6.
Chem., Int. Ed., 2009, 48, 6081–6084.
Finally, the high-energy intermediate 7 releases a CO
2
mole- 13 J. Choi, D. Kang, K. H. Lee, B. Lee, K. J. Kim and N. H. Hur, RSC Adv.,
013, 3, 9402–9407.
2
cule, which leads to the final product of the reaction, i.e., CS
2
1
1
4 M. Chin and D. D. Davis, J. Geophys. Res., 1995, 100, 8993–9005.
5 B. Wang, M. Shao, S. H. Lu, B. Yuan, Y. Zhao, M. Wang, S. Q. Zhang
and D. Wu, Atmos. Chem. Phys., 2010, 10, 5911–5923.
3
+
adsorbed over two Ti catalytic centres. Whereas the high
63.7 kcal mol ) activation energy of the rate-determining step
indicates slow kinetics of the entire conversion process, the
total free energy change is only 6.5 kcal mol . This explains
ꢀ
1
(
16 Y. Furubayashi, T. Hitosugi, Y. Yamamoto, K. Inaba, G. Kinoda,
Y. Hirose, T. Shimada and T. Hasegawa, Appl. Phys. Lett., 2005,
ꢀ
1
86, 252101.
why the reaction proceeds despite the low reaction rates, 17 M. A. Henderson, W. S. Epling, C. H. F. Peden and C. L. Perkins,
J. Phys. Chem. B, 2003, 107, 534–545.
practically without external heat administration.‡
1
8 M. Nolan, S. D. Elliott, J. S. Mulley, R. A. Bennett, M. Basham and
P. Mulheran, Phys. Rev. B: Condens. Matter Mater. Phys., 2008,
77, 235424.
2 2
In summary, we describe the conversion of OCS to CO and CS
over the surface of partly reduced titania. The experiments indicate
that the reaction does not involve lattice oxygen atoms. Quantum
chemical calculations suggest a reaction pathway, in which a pair
1
2
2
2
9 S. C. Roy, O. K. Varghese, M. Paulose and C. A. Grimes, ACS Nano,
2010, 4, 1259–1278.
0 S. Civis, M. Ferus, P. Kubat, M. Zukalova and L. Kavan, J. Phys. Chem.
C, 2011, 115, 11156–11162.
1 L. F. Liao, C. F. Lien, D. L. Shieh, M. T. Chen and J. L. Lin, J. Phys.
Chem. B, 2002, 106, 11240–11245.
3+
of Ti centres catalyses the disproportionation of two OCS mole-
cules. The main advantage of the chemistry presented here is that
it proceeds spontaneously, albeit on longer timescales.
2 Y. Yanagisawa, Energy Convers. Manage., 1995, 36, 443–446.
This work was supported by the Grant Agency of the Czech 23 S. Sato, J. Phys. Chem., 1987, 91, 2895–2897.
2
2
2
2
2
4 O. Frank, M. Zukalova, B. Laskova, J. Kuerti, J. Koltai and L. Kavan,
Republic (No. P108/12/0814) and by the Ministry of Education,
Youth and Sports of the Czech Republic (Nos LD14115 and
LD13060, COST CM1104).
Phys. Chem. Chem. Phys., 2012, 14, 14567–14572.
5 S. Civis, M. Ferus, M. Zukalova, L. Kavan and Z. Zelinger, Opt.
Mater., 2013, 36, 159–162.
6 L. Kavan, M. Zukalova, M. Ferus, J. Kuerti, J. Koltai and S. Civis,
Phys. Chem. Chem. Phys., 2011, 13, 11583–11586.
Notes and references
7 M. Ferus, I. Matulkova, L. Juha and S. Civis, Chem. Phys. Lett., 2009,
472, 14–18.
‡
The B3LYP-calculations for this system are of a rather qualitative
8 L. S. Rothman, I. E. Gordon, Y. Babikov, A. Barbe, D. C. Benner,
P. F. Bernath, M. Birk, L. Bizzocchi, V. Boudon, L. R. Brown,
A. Campargue, K. Chance, E. A. Cohen, L. H. Coudert, V. M. Devi,
B. J. Drouin, A. Fayt, J.-M. Flaud, R. R. Gamache, J. J. Harrison,
J.-M. Hartmann, C. Hill, J. T. Hodges, D. Jacquemart, A. Jolly,
J. Lamouroux, R. J. Le Roy, G. Li, D. A. Long, O. M. Lyulin,
C. J. Mackie, S. T. Massie, S. Mikhailenko, H. S. P. Mueller,
O. V. Naumenko, A. V. Nikitin, J. Orphal, V. Perevalov, A. Perrin,
E. R. Polovtseva, C. Richard, M. A. H. Smith, E. Starikova, K. Sung,
S. Tashkun, J. Tennyson, G. C. Toon, V. G. Tyuterev and G. Wagner,
J. Quant. Spectrosc. Radiat. Transfer, 2013, 130, 4–50.
character. However, none of the currently available multireference
codes are able to treat these systems at a higher level.
1
2
L. Kavan, Chem. Rec., 2012, 12, 131–142.
S. Civis, M. Ferus, M. Zukalova, P. Kubat and L. Kavan, J. Phys. Chem.
C, 2012, 116, 11200–11205.
3
P. K. Dutta, A. Ginwalla, B. Hogg, B. R. Patton, B. Chwieroth,
Z. Liang, P. Gouma, M. Mills and S. Akbar, J. Phys. Chem. B, 1999,
1
03, 4412–4422.
4
5
B. O’Regan and M. Graetzel, Nature, 1991, 353, 737–740.
C. Di Valentin, G. Pacchioni and A. Selloni, J. Phys. Chem. C, 2009,
1
13, 20543–20552.
29 J. S. Wells, M. Schneider and A. G. Maki, J. Mol. Spectrosc., 1988, 132,
6
7
Y.-F. Li and A. Selloni, J. Am. Chem. Soc., 2013, 135, 9195–9199.
422–428.
P. Scheiber, M. Fidler, O. Dulub, M. Schmid, U. Diebold, W. Hou, 30 M. Dolg, U. Wedig, H. Stoll and H. Preuss, J. Chem. Phys., 1987, 86,
U. Aschauer and A. Selloni, Phys. Rev. Lett., 2012, 109, 136103.
866–872.
Y. He, O. Dulub, H. Cheng, A. Selloni and U. Diebold, Phys. Rev. Lett., 31 S. Civi ˇs , A. Walters, M. Y. Tretyakov, S. Bailleux and M. Bogey,
009, 102, 106105. J. Chem. Phys., 1998, 108, 8369–8373.
8
2
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