Sulfate-promoted titania photocatalyst for high efficiency gas phase toluene degradation

Article abstract of DOI:10.1246/cl.2005.336

A regenerable, halide-free, sulfate-promoted titania photocatalyst was developed for the high-efficiency, room temperature gas phase photodegradation of toluene at higher than trace concentrations. Copyright

Full text of DOI:10.1246/cl.2005.336

336
Chemistry Letters Vol.34, No.3 (2005)
Sulfate-promoted Titania Photocatalyst for High Efficiency Gas Phase Toluene Degradation
ꢀ
´
Elodie Barraud, Florence Bosc, Nicolas Keller, and Valerie Keller
´ ´
´
LMSPC (Laboratoire des Materiaux, Surfaces et Procedes pour la Catalyse) UMR 7515 du CNRS-ELCASS (European Laboratory
for Catalysis and Surface Science), Louis Pasteur University, 25, rue Becquerel, BP 08, 67087 Strasbourg Cedex 2, France
(Received December 24, 2004; CL-041592)
A regenerable, halide-free, sulfate-promoted titania photo-
0.15
0.10
0.05
0.00
120
100
80
60
40
20
0
catalyst was developed for the high-efficiency, room tempera-
ture gas phase photodegradation of toluene at higher than trace
concentrations.
0.2
0.1
0.0
100%
14 nm
21
5
3.5
2.5
Molarity, M
We report on the use of a halide-free sulfate promoted tita-
nia photocatalyst for the high efficiency room temperature pho-
todegradation of gaseous toluene at high concentration level un-
der a continuous flow stream. The use of sulfated TiO2 was re-
cently reported as being non beneficial for the gas phase toluene
oxidation,1 but a more systematic control of the synthesis is re-
ported here and resulted in a higher efficiency.
Within the target to meet strict anti-pollution legislation re-
quired by the environmental pressure, the photocatalytic oxida-
tion is very promising for the purification of contaminated
wastewater containing organic pollutants and the removal of
air contaminants at room temperature.2 Its application to gaseous
toluene only leads to moderate conversions with apparent deac-
tivation.3,4 A recent approach consists of adding chloride at the
TiO2 surface by HCl pretreating, leading to highly reactive
0.1
1
10
100%
15 nm
40%
46 nm
100%
38 nm
100%
24 nm
400
500
600
700
800
Calcination temperature / °C
Figure 1. Influence of the calcination temperature of 0.5 M-sul-
fated-titania on the S/Ti atomic surface ratio ( ), the BET sur-
face area ( ), the mean particle size derived from the XRD peak
broadening using the Scherrer calculation and the anatase molar
content in wt % of the resulting materials. Inset: influence of the
sulfation solution molarity on the S/Ti atomic surface ratio (
and the overall sulfate content in wt %. (indicated below the
points) for a calcination at 500 ꢃC.
)
and oxidizing chlorine radicals which act in the same way as
4
ꢁ
OH radicals and thus react with adsorbed aromatics. However,
the benefit is only effective for low concentrations (ca. 14 ppm,
i.e. 50 mg/m3) and during the very early stage of the process, de-
pletion of chloride radicals occurring on stream.
tase mean particle size of 14 nm. Higher calcination temperature
led to a drastic decrease in surface area down to 8 m2/g with a
strong increase in the particle size, corresponding to the material
sintering. By contrast, a low calcination temperature (e.g.
400 ꢃC) resulted in a low surface area of 49 m2/g with no differ-
ence in the particle size, which could be attributed to the low
cristallinity of the sulfated material with a different pore struc-
ture. Indeed, incorporation of sulfates into oxides has already
been reported to delay its crystallization compared to non-sulfat-
ed materials.5 Sulfation of titania shifted towards higher temper-
atures the anatase ! rutile phase transition, as rutile phase ap-
peared after calcination at 600 ꢃC on similar sulfate-free sol–
Sulfated titania (SO42ꢂ–TiO2) catalysts are prepared follow-
ing the two-step method used by Arata for sulfated oxides.4 It
consists first of the sol–gel preparation of an amorphous Ti(OH)4
hydroxide by putting the Ti[OCH(CH3)2]4 precursor into water
and ethanol at a basic pH, kept at 9 by dropwise addition of am-
monia. After drying overnight at 22 ꢃC and further at 110 ꢃC, the
hydroxide is sulfated by sulfuric acid in water (0.25–5 M con-
centrations using 1.5 mL for 1 g of solid) and then calcined in
air between 400 and 800 ꢃC for 5 h.
For a 0.5 M sulfation solution, Figure 1 shows a maximum
of surface sulfation for materials calcined at 500 ꢃC, the S/Ti
atomic surface ratio being determined by XPS. Atomic absorp-
tion spectroscopy showed a content in sulfate species of 3.5
wt % in agreement with thermal gravimetry analysis (TGA),
which displayed a similar total weight loss in air starting at
550 ꢃC and instability of sulfate species at temperatures higher
than 550 ꢃC (not shown). Higher calcination temperatures led
to a decrease in the S/Ti atomic surface ratio down to zero at
800 ꢃC, due to progressive desulfation of the catalyst. Perform-
ing Arþ bombardment on 0.5 M-sulfated-titania led to remove
the top-layers and resulted in a drastic decrease in the S/Ti
atomic surface ratio down to zero, proving the exclusive surface
gel TiO2 compared to 800 ꢃC for SO42ꢂ–TiO2, in agreement
6
with the work of Colon et al.7 Inset of Figure 1 shows the in-
crease in S/Ti atomic surface ratio with increasing the sulfation
solution molarity, following a non-linear behavior from 0.1 to
0.2 as a function of the molarity, whereas the overall sulfate con-
tent increased from 2.5 to 21 wt %. This behaviour is character-
istic of SO42ꢂ–TiO2 with strong sulfate surface anchorage at low
concentration, and for which complete bulk sulfation requires a
2ꢂ
high molarity solution as reported by Ecormier et al. for SO4
–
ZrO2.8 Low molarity led exclusively to surface sulfation, while
increasing the molarity solution led progressively to the forma-
tion of polysulfates and finally to bulk sulfation as supposed with
the 5 M-sulfated sample, for which no drastic decrease in the
S/Ti atomic surface ratio was observed after Arþ bombardment
compared to low molarity sulfation solution materials. The
2ꢂ
location of SO4 species.
A maximum in BET surface area was obtained for a calcina-
tion temperature of 500 ꢃC, i.e. 117 m2/g, together with an ana-
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.3 (2005)
337
0.5 M-SO42ꢂ–TiO2 catalyst calcined at 500 ꢃC can thus be seen
as a high surface area partially sulfated material with strong sul-
fate anchorage and a S/Ti atomic surface ratio of 0.128.
Photocatalysis was conducted in a cylindrical concentric
tubular photoreactor.9 The reactant mixture flows between the
internal and the external concentric tubes, set 4 mm apart. Illumi-
nation is provided by a 8-W black light tube located inside the
inner tube of the reactor. 440 mg of photocatalyst is evenly coat-
ed on the internal side of the external tube by evaporating a cat-
alyst-containing aqueous slurry to dryness. The toluene content
is set at ꢄ110 ppm (ꢄ400 mg/m3), with a relative humidity ratio
of 30%9 in 200 cm3/min flowing air. Before photocatalysis, the
catalyst is exposed to the polluted air stream with no illumina-
tion until dark-adsorption equilibrium is reached. Afterwards
the UV illumination is switched on. The on-stream efficiency
of SO42ꢂ–TiO2 depending on the calcination temperature and
the sulfation solution molarity were compared to the widespread
commercial TiO2 P25 (Degussa) in Figure 2.
moval efficiency, indicating that a partial sulfation of TiO2, i.e.
2ꢂ
the presence of TiO2 and well-dispersed SO4 with optimized
2ꢂ
contact between SO4 and TiO2 domains, is required for ob-
taining a high efficiency. The main role of TiO2 is the reactant
adsorption, the band gap excitation and the transformation, while
the sulfates could play a double role: (i) better desorption of the
eꢂ-rich sp2-bound carbon aromatic poisons on the eꢂ-rich sul-
fates, thus limiting deactivation; (ii) positive electron trap ef-
fect12 hindering the eꢂ-hole recombination which is one of the
most limiting photocatalytic factors, thus increasing the photoca-
talytic efficiency. Sulfating TiO2 already led to an electron trans-
fer from bulk TiO2 to sulfate, causing a delocalisation and a new
electron distribution on the TiO2 surface.13 An irreversible shift
in Ti2p binding energies for SO42ꢂ–TiO2 under the XPS electron
beam confirmed that sulfation induced an irreversible charge
transfer. A similar electron trap effect was observed on WO3/
2ꢂ
TiO2.9 Excessively high SO4 surface coverage decreases the
number of TiO2 surface sites available for the reaction while a
2ꢂ
low SO4 content reduces the connection between sulfated
and non-sulfated TiO2 zones, and thus limits both positive roles.
The optimum calcination temperature of 500 ꢃC can result from
the highest surface area and a good cristallinity, obtained without
desulfation of the material, thus leading to higher adsorption ca-
pacity and photoefficiency.
The stability in air of the sulfates at the TiO2 surface up to
550 ꢃC (not shown TG-DTA) allowed regeneration of the photo-
catalyst to be performed by combusting into CO2 in air for 2 h at
450 ꢃC the strongly adsorbed sp2-bound carbon intermediates,
without destroying the sulfate-containing surface and modifying
the anatase-to-rutile ratio.
100
80
60
40
20
0
We think that the halide-free sulfated TiO2 catalyst is thus
a regenerable and highly efficient alternative for improving the
on-stream gas phase toluene photodegradation for which the
commercial TiO2 and other alternatives lack of efficiency.4
0
20
40
60
80
100
120
140
Time on toluene stream / min
Figure 2. Photocatalytic activity in terms of outlet toluene con-
centration as a function of time on stream over the commercial
Degussa P25 reference catalyst ( ), sol–gel TiO2 (F) and
0.5 M-sulfated titania catalysts calcinated at 400 ( ), 500 ( ),
600 ( ), 800 ꢃC ( ), and SO42ꢂ–TiO2 calcinated at 500 ꢃC
using 0.25 M ( ), 1 M ( ) and 5 M ( ) solutions.
References
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2
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3
4
P25 titania only reached 90% of conversion and was rapidly
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5
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9
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(not shown FTIR), as reported by Marci et al.10
2ꢂ
Taking into account the surface area of the 0.5 M-SO4
–
TiO2 calcinated at 500 ꢃC and the saturation coverage of sulfate
species of 2.4 * 1018 atom/m2,11 the sulfate equivalent mono-
layer coverage corresponds to 4.4 wt % SO42ꢂ. Thus, the opti-
mum efficiency photocatalysts correspond to a near monolayer
10 G. Marci, M. Addamo, V. Augugliaro, S. Coluccia, E.
Garcia-Lopez, V. Loddo, G. Marta, L. Palmisano, and M.
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2ꢂ
surface coverage of the TiO2 surface by SO4 species, the
0.5 M- and 1 M-SO42ꢂ–TiO2 corresponding to 3.5 and 5 wt %
SO4 respectively. Lower and higher coverages, obtained by
varying the sulfation solution molarity decreased the toluene re-
2ꢂ
13 S. M. Jung and P. Grange, Catal. Lett., 76, 27 (2001).
Published on the web (Advance View) February 5, 2005; DOI 10.1246/cl.2005.336