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Photocatalytic behaviour of sulphated TiO2 for phenol degradation
Applied Catalysis B: Environmental 45 (2003) 39–50
Photocatalytic behaviour of sulphated TiO2 for phenol degradation G. Colón∗ , M.C. Hidalgo, J.A. Nav´ıo Instituto de Ciencia de Materiales de Sevilla, Centro Mixto Universidad de Sevilla-CSIC, Americo Vespucio s/n, 41092 Sevilla, Spain Received 13 October 2002; received in revised form 3 March 2003; accepted 3 March 2003
Abstract Photocatalytic oxidation of phenol was performed over sulphated TiO2 prepared by a sol–gel method. Comparison with non-sulphated TiO2 and sulphated Degussa was also made. Wide structural and surface characterisation of catalysts was carried out in order to establish a correlation between the effect of sulphation process on the TiO2 photocatalytic properties. Sulphation process clearly stabilizes TiO2 catalyst phase against sintering, maintaining anatase phase and relatively high surface area values with respect non-sulphated TiO2 . In spite of sulphate loading should be higher at low temperatures, better conversion values for phenol oxidation has been obtained for samples calcined at 700 ◦ C. For this calcination temperature, the optimisation of structural and surface area values lead to better photocatalytic performance in spite of sulphate presence and therefore acidic properties of samples should be dismissed. Sulphated TiO2 samples prepared by sol–gel presented higher photocatalytic conversions for temperatures higher than 600 ◦ C. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Sulphated TiO2 ; Photocatalysis; Phenol oxidation
1. Introduction The worldwide interest in solid acid catalysis that has developed over the last few years has focused to the exceptional properties of sulphated metal oxides. Since Arata discovered ZrO2 /SO4 2− which catalyses n-butane skeletal isomerization at room temperature [1], ZrO2 /SO4 2− and promoted ZrO2 /SO4 2− have been investigated extensively [2]. As with most catalytic processes, a potentially way to improve photocatalytic performance is to increase the number and strength of surface acid sites [3–7]. It has been reported that several TiO2 prepared from TiCl4 hydrolysis and showing different surface acidity, exhibited improved photocatalytic activity for gas phase ethanol photooxidation as acidity increased [7]. At the same time, it is ∗ Corresponding author. E-mail address: [email protected] (G. Col´on).
well known that doping titanium dioxide with metal oxides such as WO3 , the surface acidity properties were improved, increasing this way its photocatalytic activity [3]. Recently, TiO2 /SO4 2− solid superacid has been used as a catalyst for a variety of organic reactions [8–10]. Furthermore, it seems that sulphated TiO2 could have interesting photocatalytic properties in certain reactions [11]. In this sense, it has been reported that sulphated TiO2 Degussa P25 presented higher photocatalytic activity than TiO2 /SO4 2− for heptane, TCE, ethanol and acetaldehyde photodegradation. At the same time, deactivation for the photocatalytic oxidation of these compounds results lower for Degussa sulphated photocatalysts. These authors also reported that the active sites in sulphated Degussa were more actives than those on TiO2 /SO4 2− . Sulphated TiO2 is expected to be more acidic than either TiO2 or Degussa P25 since several researchers have shown TiO2 /SO4 2− to be extremely strong solid acid
0926-3373/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0926-3373(03)00125-5
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catalyst [12–14]. The strong acid sites on TiO2 /SO4 2− play an important role in photocatalytic oxidation reaction, suggesting that strong acid sites increase the adsorption strengths and therefore coverages of different organics, which contributes to the improved photocatalytic activity [11]. Moreover, Fu et al. [15] studied the structure of TiO2 /SO4 2− and its activity for the photodegradation of CHBr3 , C6 H6 and C2 H4 in air. For catalysts studied in that work, conversion of CH3 Br over sulphated TiO2 was six times higher that of TiO2 . They also reported that deactivation of TiO2 was faster than TiO2 /SO4 2− . They concluded that the improved rate for sulphated titanium dioxide was due to a greater surface area as well as larger fraction of anatase phase, which is more active than rutile for the photocatalytic application. Both conclusions, the improvement of acid properties and, on the other hand larger surface area and an appropriate structural feature, are not clear enough, and perhaps a combination of both explanations should be considered. However, these interesting results refer in both cases to the photocatalytic behaviour in gas phase photoreactions. No other results can be found in literature about the behaviour of these sulphated systems on liquid photocatalysis. In the present paper we deal with the preparation of sulphated TiO2 from sol–gel method. We have studied the effect of sulphate impregnation process and thermal treatment of photocatalysts in the final photocatalytic properties of TiO2 . The knowledge of effect of sulphate on the precursor as well as the further thermal treatment with photocatalytic activity of these systems will be considered. Comparison with Degussa P25 sulphated and unsulphated TiO2 will be made. 2. Experimental
(50 ml/g) for 1 h. Then, the suspension was filtered and powder was dried in oven at 120 ◦ C overnight. Sulphated and non-sulphated TiO2 samples were calcined in air at temperatures ranged between 400 and 800 ◦ C for 2 h. Degussa P25 was used as reference. In this case, the preparation of sulphated P25 sample was achieved following the same procedure as sol–gel TiO2 . BET surface area measurements were carried out by N2 adsorption at 77 K using a Micromeritics 2000 instrument. Pore volumes were determined using the cumulative adsorption of nitrogen by the BJH method. Thermal evolution of the samples under dynamic conditions was studied by differential thermal and thermogravimetric analysis (DTA and TGA). These curves were obtained simultaneously in static air with a SII Seiko instrument Exstar 6000, model TG/DTA 6300 at a heating rate of 10 ◦ C/min. Calcined alumina was used as reference material. X-ray diffraction (XRD) patterns were obtained using a Siemens D-501 diffractometer with Ni filter and graphite monochromator. The X-ray source was Cu K␣ radiation. Anatase/rutile fractions were calculated by taking into account the relative diffraction peak intensities. From the line broadening of corresponding X-ray diffraction peaks, we have calculated the mean crystallite size according to the Warren and Averbach equation (peaks were fitted by using a Voigt function): D=
λ × 180 π × cosθ × L
where L is the line width at medium height, λ the wavelength of the X-ray radiation 0.15406 nm and θ the diffracting angle. UV-Vis spectra were recorded in the diffuse reflectance mode (R) and transformed to a magnitude proportional to the extinction coefficient (K) through the Kubelka–Munk function, F(R∝ ).
2.1. Catalysts preparation and characterisation 2.2. Catalytic runs TiO2 system was prepared by a sol–gel method using titanium tetraisopropoxide (TIP) as precursor in isopropanol (3.9 ml TIP in 200 ml iPrOH). Hydrolysis of the isopropanol–TIP solution was achieved by adding certain volume of bidistilled water (200 ml). Precipitated were then filtered and dried at 120 ◦ C overnight. Sulphation of TiO2 was performed by impregnating the fresh powders in a H2 SO4 1 M solution
Catalytic runs were performed in a Pyrex immersion well reactor with 450 ml of reaction volume. Irradiation of the reaction solutions was carried out by using a medium pressure 400 W Hg lamp supplied by Applied Photophysics. Oxygen flow was employed in all experiments as oxidant and simultaneously to produce a homogenous suspension of the catalyst in the
G. Col´on et al. / Applied Catalysis B: Environmental 45 (2003) 39–50
solution. In order to achieve a surface equilibrium between photocatalysts and phenol, before each experiment the catalysts (1 g/l) were settled in suspension with the reagent mixture for 10 min. Initial phenol concentration was in all cases 50 ppm and was following by means of UV-Vis spectroscopy using characteristic 270 nm band [16,17]. The pH of the suspension was 7 in all cases. About 2 ml of suspension was removed and filtered (Millipore Millex25 0.45 m membrane filter) previously to UV-Vis spectra. Photon efficiencies were calculated using the following expression [18,19]: ξ=
−d[C]/dt × 100 P0
being the photon flux (P0 ) of our lamp equal to 2.6 × 10−7 Einstein/s L, and −d[C]/dt was calculated from the slope of the conversion curves at the first stage of the reaction (up to 30 min). 3. Results and discussions 3.1. Photocatalysts characterization Table 1 summarises the surface data for sulphated and non-sulphated TiO2 samples calcined at different
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temperatures. Sulphated TiO2 samples exhibit relatively high surface areas with respect to non-sulphated ones. It is clear that sulphate presence stabilizes the surface area values. Even after calcination at 700 ◦ C specific surface area presents a rather high value of 20 m2 /g. The effect of sulphate group is more evident as we calcine at higher temperatures. Thus at 400 ◦ C both series present similar surface area values. Calcination at higher temperature produces a much drastic decrease in the SBET values for non-sulphated series. Sulphate impregnation of Degussa P25 (Table 1) and further calcination lead also to a significant decrease in the SBET values, but not so drastic as for TiO2 . In fact, sulphation process takes place in this case upon crystalline TiO2 in contrast with that prepared from sol–gel route. Therefore, effect of sulphation, in terms of improvement of surface area, is not achieved when sulphation step takes places upon the crystalline oxide. Regarding to the pore size distribution (Fig. 1), it is worth of noting a similar evolution for pore size distribution as for SBET already mentioned. It can be notice that sulphated TiO2 exhibits also certain stabilization of the initial pore distribution. Non-sulphated TiO2 pore sizes shift towards higher pore diameter values rapidly after calcination at 400 ◦ C. Meanwhile, sulphated one suffers this shift more slightly upon calcination. Similar fact was observed for SBET values.
Table 1 Surface and structural properties of sulphated and non-sulphated TiO2 Sample
SBET (m2 /g)
Average pore diameter (Å)
Anatase molar fraction
Crystallite size (nm)
Sulphated TiO2 400 ◦ C 500 ◦ C 600 ◦ C 700 ◦ C 800 ◦ C
90 88 41 20 6
40 45 + 250 150 + 300 320 –
100 100 100 93 0
5.7 8.9 21.2 33.7 –
Non-sulphated TiO2 400 ◦ C 98 28 500 ◦ C 600 ◦ C 17 700 ◦ C 6 800 ◦ C –
45 + 250 350 330 500 –
99 99 82 10 0
10.2 22.0 31.9 35.6 –
Degussa P25 400 ◦ C
47
350
77
20.9
Sulphated Degussa P25 400 ◦ C 36 700 ◦ C 13
400 500
77 2
22.0 25.0
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Fig. 1. Pore size distribution for TiO2 /SO4 2− , TiO2 and Degussa/SO4 2− series.
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On the other hand, for sulphated Degussa P25, calcination at 700 ◦ C do not change considerably the initial average pore diameter, being in this case significantly higher than those observed for sol–gel TiO2 . Thus, sulphate groups produce an important stabilization effect on the surface features of TiO2 when sulphation is applied upon the fresh precursor. In the case of Degussa, sulphation treatment produces a detrimental effect, decreasing the surface area values. From XRD for different TiO2 series it can be observed that sulphate pre-treatment of the fresh precursor trends to stabilize the anatase phase upon calcination (Table 1). Anatase to rutile transition seems to shift towards higher temperature in the case of sulphated TiO2 . Thus, sulphated TiO2 calcined at 700 ◦ C still present significantly higher molar anatase/rutile ratio than non-sulphated one. From these results, it can be inferred that phase transition would take place at temperatures higher than 700 ◦ C, while for non-sulphated series this phase transition should take place between 600 and 700 ◦ C. It is well reported that sulphate and phosphate species [20] reinforce the bonding between zirconia fragments and delay the crystallization [21]. Therefore, additionally to the surface area stabilization it is clear that sulphation produces a clear crystalline stabilization of the anatase phase [22]. Regarding sulphated Degussa P25 (Table 1), sulphation process does not produces any significant change in the phase composition of TiO2 . Sulphated and non-sulphated commercial TiO2 present an anatase molar fraction of 77%, very close to the nominal 80% characteristic of this commercial TiO2 . Calcination at 700 ◦ C leads to almost complete rutilization of TiO2 . Sulphate groups in this case seem not to stabilize anatase fraction as in the case of sol–gel prepared sulphated TiO2 . Sol–gel prepared TiO2 series (sulphated and non-sulphated) present mean crystallite sizes in the nanometer range even at higher calcination temperature. Stabilization of the nanoparticles is more evident for sulphated series for which the increase of the crystallite size takes place at temperatures higher than 600 ◦ C. this crystallite size evolution could be related to the anatase stabilization at higher temperature. This stabilization of anatase phase as well as the improvement of surface area could be explained by taking into account the presence of sulphate groups anchored on the TiO2 precursor before calcination.
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It is well known that sulphate groups are stable in the oxide structure during calcination up to 600 ◦ C [23–26]. The acidic properties of sulphated metal oxides were modified by thermally decomposing the sulphate to different extents [23]. It is reported that the amount of ammonia desorbed from surface sites of either medium or high acidic strength changes parallel with the decreasing sulphate content and also the decreasing surface area, which occurs concomitantly with sulphate loss [24]. Fig. 2 represents the thermal evolution of sulphated TiO2 . As approximation, from TG curves of calcined samples it can be also assumed that the weight loss between 400 and 600 ◦ C could be due to the decomposition of strongly bonded sulphate species [26], though other species can be also decomposed in this range. A small endothermic effect can be observed for TiO2 /SO4 2− located around 550 ◦ C. This endotherm could be associated with the sulphate decomposition [26]. For sulphated Degussa P25, this endothermic effect is significantly more evident, indicating that in principle sulphate species are bonded to the surface with higher adsorption strength. Therefore, sulphate content in samples calcined at temperatures higher than 600 ◦ C should be considered as almost negligible. If we measure the weight loss from 400 ◦ C to the corresponding calcination temperature (Fig. 3) we can follow the evolution of SO4 2− groups during calcination. From this result, it can be deduced that the presence of sulphate on the TiO2 (sol–gel prepared and Degussa P25) could be only considered for samples calcined at temperatures lower than 700 ◦ C. Calcination at higher temperatures could lead to the practical elimination of the adsorbed SO4 2− . This is an important point to be taken into account, since the particular surface and structural features of sulphated series have been made by considering the presence of sulphates on the structure. The optical absorption properties of different TiO2 samples are also related with the crystalline structure. In Fig. 4 we represent the evolution of calculated band-gap values with the calcination temperature. It can be also observed the correlation between the crystalline phase present and the band-gap values. From these results it can be noted that absorption edge shifts to higher wavelength (in other words, towards the visible range), as the rutile phase is appearing. Since sulphated TiO2 series exhibits certain anatase stabilization, the band-gaps present higher values.
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Fig. 2. Thermal analysis curves (TG, DTG and DTA) for TiO2 /SO4 2− , TiO2 and Degussa/SO4 2− photocatalysts.
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Fig. 3. Weight loss from 400 ◦ C and each corresponding temperature calculated from TG curves.
From these results, we can estimate that anatase phase present a band-gap value around 3.6 eV, meanwhile rutile phase band-gap is located around 2.9 eV.
3.2. Phenol photooxidation Photocatalytic properties of sol–gel prepared TiO2 (sulphated and non-sulphated) were tested for the
Fig. 4. Band-gap values (eV) for TiO2 /SO4 2− and TiO2 calcined at different temperatures. It is also indicated the anatase molar fraction (A: anatase) present in each case.
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photocatalytic oxidation of phenol. In order to determine the effect of sulphate treatment on the photocatalytic properties of TiO2 , we have also performed similar catalytic runs with sulphated Degussa P25 calcined at 400 and 700 ◦ C. In Fig. 5, we show the conversion plots for different TiO2 samples calcined at temperatures between 400 and 800 ◦ C. Surprisingly, for TiO2 /SO4 2− series final conversions increases samples calcined up to 700 ◦ C. It is worth noting that in spite of this high temperature the photocatalytic behaviour of this sample is the best of its series. Comparing these results with non-sulphated TiO2 series, it is clear that calcination at temperatures higher than 600 ◦ C produces a detrimental effect upon conversion values. It can be clearly pointed out that sulphated TiO2 present higher conversion values especially for samples calcined at higher temperatures (700 and 600 ◦ C), for which surface area is lower and sulphate content is supposed to be almost negligible as it was already mentioned. Consequently, in principle there is no direct relationship between the surface features or the presence of surface sulphate and therefore the acidic properties of TiO2 with the improvement of the phenol conversion. Calcination of TiO2 /SO4 2− at 800 ◦ C destroys the photocatalytic activity of TiO2 . The drastic decrease in the conversion for this sample can be explained by taking into account that calcination at 800 ◦ C leads to the complete rutilization of TiO2 , as well as a low surface area values. On the other hand, sulphation process on Degussa TiO2 produces worse in its photocatalytic activity. In this case, sulphation process seems to ruin the initial features of Degussa P25. No beneficial effect can be observed by the fact of the increase of surface acidity as other authors have reported for gas phase photocatalysis. In Fig. 6, we represent the evolution of photon efficiency, calculated from the conversion slope for different TiO2 photocatalysts. If we consider the photon efficiency per gram of catalyst (Fig. 6a) it is worth noting that better efficiencies have been observed for TiO2 /SO4 2− with respect to non-sulphated TiO2 . Sulphated Degussa photon efficiency values are in all cases worst than TiO2 and TiO2 /SO4 2− . Best photocatalytic performance is achieved for TiO2 /SO4 2− calcined at 700 ◦ C. As calcination temperature rises up, photon efficiency for this series increases till that temperature. Although this higher calcination treatment, TiO2 /SO4 2− exhibits similar
photon efficiency than non-sulphated Degussa P25 (33% versus 38%, respectively). Non-sulphated TiO2 presents higher photon efficiency at low calcination temperature, decreasing as calcination is higher than 600 ◦ C. Although the better approximation to the real situation should consider the efficiency per gram, since all photocatalytic experiments were carried out using the same amount of catalyst, we can also observe the evolution of the photon efficiency per surface area (Fig. 6b). Thus comparison of the photon efficiency per surface unit could indicate us the efficiency and activity for each photocatalyst’s surface. In this case, as non-sulphated TiO2 exhibit lower surface areas, photon efficiency values follow an increasing growth till calcination temperature of 600 ◦ C. Surface contribution is higher for catalysts calcined at temperatures lower than 600 ◦ C, since SO4 2− stabilizes the surface area and therefore the photon efficiencies per area are higher for those catalysts with lower specific surface area. However photon efficiency per m2 at 700 ◦ C for sulphated TiO2 is still higher, indicating that the surface contribution is not the reason for the higher conversion observed for TiO2 /SO4 2− calcined at this temperature. The photoactivity of the surface for TS700 is significantly higher with respect to TS catalyst with higher surface area. Thus, it could be TS700 presents the more active surface. This higher activity could be related to structural features (anatase stabilisation as well or the absence of crystalline defects). From these results, it can be noticed that sulphate pre-treatment improves the photocatalytic behaviour of TiO2 by affecting the structural and surface properties. In previous works [11,13], other authors reported the increasing of photocatalytic conversions due to the enhancement of the acid properties of sulphated TiO2 . In our case, the presence of sulphate seems to improve the photocatalytic activity of TiO2 in two ways: (i) Stabilizing the anatase crystalline phase at higher temperature and at the same time eliminating the crystal defects present in the structure. These defects could be the responsible of the recombination process and therefore of a low efficiency. Sulphated TiO2 calcined at 700 ◦ C presents high degree of crystallization only in the anatase form, with small crystallite size. (ii) Sulphate pre-treatment affect also to the surface area value. The presence of sulphate in
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Fig. 5. Conversion plots for phenol photodegradation for TiO2 /SO4 2− , TiO2 and Degussa/SO4 2− .
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Fig. 6. Photon efficiencies for phenol photodegradation for TiO2 /SO4 2− , TiO2 and Degussa/SO4 2− : (a) per gram and (b) per square meter.
the fresh precursor seems to avoid sintering process and the following collapse of surface area. In principle, no direct relationship with acidic properties due to the presence of adsorbed SO4 2− groups can be made. In this sense, we have performed other sulphate treatment by using less concentrated H2 SO4
solution (0.1 M). In the case of sol–gel sulphated TiO2 (Fig. 7), concentration of H2 SO4 does not affect significantly to the conversion profile of sample calcined at 700 ◦ C. On the contrary, it seems that for sulphated Degussa P25 (Fig. 8), the detrimental effect of sulphate is clear when increase the sulphate presence on the TiO2 .
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Fig. 7. Effect of H2 SO4 concentration used for the sulphation process on the conversion values for TiO2 /SO4 2− calcined at 700 ◦ C.
Fig. 8. Effect of H2 SO4 concentration used for the sulphation process on the conversion values for Degussa/SO4 2− calcined at 400 ◦ C.
4. Conclusions We have obtained a sulphated TiO2 photocatalyst by a sol–gel method, and then calcined at different
temperatures. Sulphated TiO2 exhibit higher photon efficiencies than non-sulphated TiO2 and sulphated Degussa P25. The improvement of the photocatalytic efficiency is more evident as temperature arises till
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700 ◦ C. In principle this better photocatalytic behaviour cannot be explained by terms of surface acidity due to the presence of sulphate groups. Since the elimination of SO4 2− takes place from 400 up to 600 ◦ C, it can be expected that at temperatures higher than 600 ◦ C the occurrence of sulphate could be neglected. The performed photocatalytic activities exhibited by the sulphated series with respect to non-sulphated one may be explained by considering that sulphation as photocatalyst pre-treatment. This pre-treatment would affect to the TiO2 crystallization by stabilizing the anatase phase at higher temperature and keeping at the same time a rather high specific surface area. Thus, by calcining at higher temperature we are able to eliminate as much as possible the recombination process occurring in the crystal defects of the structure. At lower temperature (400 and 500 ◦ C), the presence of sulphate seems to be a negative fact since photon efficiencies of sulphated TiO2 are lower than non-sulphated one. Therefore no clear benefit can be assumed by the supposed increase of surface acidity by sulphation. Especially in the case of sulphated Degussa P25, there is clearly no evidence of such acidic properties improvement. Moreover, sulphation process leads to a drastic decrease of the photocatalytic properties of Degussa P25 for the reaction considered. The improvement of the photocatalytic activity is therefore related to the optimisation of the redox step in the photocatalytic process instead of the acidity properties that could favour the adsorption of the organic substrate.
Acknowledgements This research was financed by Ministerio de Ciencia y Tecnolog´ıa (project ref. BQU2001-3872C02-01). Partial support by the Junta de Andaluc´ıa (P.A.I. group reference FQM181) is also acknowledged.
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Photocatalytic behaviour of sulphated TiO2 for phenol degradation G. Colón∗ , M.C. Hidalgo, J.A. Nav´ıo Instituto de Ciencia de Materiales de Sevilla, Centro Mixto Universidad de Sevilla-CSIC, Americo Vespucio s/n, 41092 Sevilla, Spain Received 13 October 2002; received in revised form 3 March 2003; accepted 3 March 2003
Abstract Photocatalytic oxidation of phenol was performed over sulphated TiO2 prepared by a sol–gel method. Comparison with non-sulphated TiO2 and sulphated Degussa was also made. Wide structural and surface characterisation of catalysts was carried out in order to establish a correlation between the effect of sulphation process on the TiO2 photocatalytic properties. Sulphation process clearly stabilizes TiO2 catalyst phase against sintering, maintaining anatase phase and relatively high surface area values with respect non-sulphated TiO2 . In spite of sulphate loading should be higher at low temperatures, better conversion values for phenol oxidation has been obtained for samples calcined at 700 ◦ C. For this calcination temperature, the optimisation of structural and surface area values lead to better photocatalytic performance in spite of sulphate presence and therefore acidic properties of samples should be dismissed. Sulphated TiO2 samples prepared by sol–gel presented higher photocatalytic conversions for temperatures higher than 600 ◦ C. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Sulphated TiO2 ; Photocatalysis; Phenol oxidation
1. Introduction The worldwide interest in solid acid catalysis that has developed over the last few years has focused to the exceptional properties of sulphated metal oxides. Since Arata discovered ZrO2 /SO4 2− which catalyses n-butane skeletal isomerization at room temperature [1], ZrO2 /SO4 2− and promoted ZrO2 /SO4 2− have been investigated extensively [2]. As with most catalytic processes, a potentially way to improve photocatalytic performance is to increase the number and strength of surface acid sites [3–7]. It has been reported that several TiO2 prepared from TiCl4 hydrolysis and showing different surface acidity, exhibited improved photocatalytic activity for gas phase ethanol photooxidation as acidity increased [7]. At the same time, it is ∗ Corresponding author. E-mail address: [email protected] (G. Col´on).
well known that doping titanium dioxide with metal oxides such as WO3 , the surface acidity properties were improved, increasing this way its photocatalytic activity [3]. Recently, TiO2 /SO4 2− solid superacid has been used as a catalyst for a variety of organic reactions [8–10]. Furthermore, it seems that sulphated TiO2 could have interesting photocatalytic properties in certain reactions [11]. In this sense, it has been reported that sulphated TiO2 Degussa P25 presented higher photocatalytic activity than TiO2 /SO4 2− for heptane, TCE, ethanol and acetaldehyde photodegradation. At the same time, deactivation for the photocatalytic oxidation of these compounds results lower for Degussa sulphated photocatalysts. These authors also reported that the active sites in sulphated Degussa were more actives than those on TiO2 /SO4 2− . Sulphated TiO2 is expected to be more acidic than either TiO2 or Degussa P25 since several researchers have shown TiO2 /SO4 2− to be extremely strong solid acid
0926-3373/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0926-3373(03)00125-5
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catalyst [12–14]. The strong acid sites on TiO2 /SO4 2− play an important role in photocatalytic oxidation reaction, suggesting that strong acid sites increase the adsorption strengths and therefore coverages of different organics, which contributes to the improved photocatalytic activity [11]. Moreover, Fu et al. [15] studied the structure of TiO2 /SO4 2− and its activity for the photodegradation of CHBr3 , C6 H6 and C2 H4 in air. For catalysts studied in that work, conversion of CH3 Br over sulphated TiO2 was six times higher that of TiO2 . They also reported that deactivation of TiO2 was faster than TiO2 /SO4 2− . They concluded that the improved rate for sulphated titanium dioxide was due to a greater surface area as well as larger fraction of anatase phase, which is more active than rutile for the photocatalytic application. Both conclusions, the improvement of acid properties and, on the other hand larger surface area and an appropriate structural feature, are not clear enough, and perhaps a combination of both explanations should be considered. However, these interesting results refer in both cases to the photocatalytic behaviour in gas phase photoreactions. No other results can be found in literature about the behaviour of these sulphated systems on liquid photocatalysis. In the present paper we deal with the preparation of sulphated TiO2 from sol–gel method. We have studied the effect of sulphate impregnation process and thermal treatment of photocatalysts in the final photocatalytic properties of TiO2 . The knowledge of effect of sulphate on the precursor as well as the further thermal treatment with photocatalytic activity of these systems will be considered. Comparison with Degussa P25 sulphated and unsulphated TiO2 will be made. 2. Experimental
(50 ml/g) for 1 h. Then, the suspension was filtered and powder was dried in oven at 120 ◦ C overnight. Sulphated and non-sulphated TiO2 samples were calcined in air at temperatures ranged between 400 and 800 ◦ C for 2 h. Degussa P25 was used as reference. In this case, the preparation of sulphated P25 sample was achieved following the same procedure as sol–gel TiO2 . BET surface area measurements were carried out by N2 adsorption at 77 K using a Micromeritics 2000 instrument. Pore volumes were determined using the cumulative adsorption of nitrogen by the BJH method. Thermal evolution of the samples under dynamic conditions was studied by differential thermal and thermogravimetric analysis (DTA and TGA). These curves were obtained simultaneously in static air with a SII Seiko instrument Exstar 6000, model TG/DTA 6300 at a heating rate of 10 ◦ C/min. Calcined alumina was used as reference material. X-ray diffraction (XRD) patterns were obtained using a Siemens D-501 diffractometer with Ni filter and graphite monochromator. The X-ray source was Cu K␣ radiation. Anatase/rutile fractions were calculated by taking into account the relative diffraction peak intensities. From the line broadening of corresponding X-ray diffraction peaks, we have calculated the mean crystallite size according to the Warren and Averbach equation (peaks were fitted by using a Voigt function): D=
λ × 180 π × cosθ × L
where L is the line width at medium height, λ the wavelength of the X-ray radiation 0.15406 nm and θ the diffracting angle. UV-Vis spectra were recorded in the diffuse reflectance mode (R) and transformed to a magnitude proportional to the extinction coefficient (K) through the Kubelka–Munk function, F(R∝ ).
2.1. Catalysts preparation and characterisation 2.2. Catalytic runs TiO2 system was prepared by a sol–gel method using titanium tetraisopropoxide (TIP) as precursor in isopropanol (3.9 ml TIP in 200 ml iPrOH). Hydrolysis of the isopropanol–TIP solution was achieved by adding certain volume of bidistilled water (200 ml). Precipitated were then filtered and dried at 120 ◦ C overnight. Sulphation of TiO2 was performed by impregnating the fresh powders in a H2 SO4 1 M solution
Catalytic runs were performed in a Pyrex immersion well reactor with 450 ml of reaction volume. Irradiation of the reaction solutions was carried out by using a medium pressure 400 W Hg lamp supplied by Applied Photophysics. Oxygen flow was employed in all experiments as oxidant and simultaneously to produce a homogenous suspension of the catalyst in the
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solution. In order to achieve a surface equilibrium between photocatalysts and phenol, before each experiment the catalysts (1 g/l) were settled in suspension with the reagent mixture for 10 min. Initial phenol concentration was in all cases 50 ppm and was following by means of UV-Vis spectroscopy using characteristic 270 nm band [16,17]. The pH of the suspension was 7 in all cases. About 2 ml of suspension was removed and filtered (Millipore Millex25 0.45 m membrane filter) previously to UV-Vis spectra. Photon efficiencies were calculated using the following expression [18,19]: ξ=
−d[C]/dt × 100 P0
being the photon flux (P0 ) of our lamp equal to 2.6 × 10−7 Einstein/s L, and −d[C]/dt was calculated from the slope of the conversion curves at the first stage of the reaction (up to 30 min). 3. Results and discussions 3.1. Photocatalysts characterization Table 1 summarises the surface data for sulphated and non-sulphated TiO2 samples calcined at different
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temperatures. Sulphated TiO2 samples exhibit relatively high surface areas with respect to non-sulphated ones. It is clear that sulphate presence stabilizes the surface area values. Even after calcination at 700 ◦ C specific surface area presents a rather high value of 20 m2 /g. The effect of sulphate group is more evident as we calcine at higher temperatures. Thus at 400 ◦ C both series present similar surface area values. Calcination at higher temperature produces a much drastic decrease in the SBET values for non-sulphated series. Sulphate impregnation of Degussa P25 (Table 1) and further calcination lead also to a significant decrease in the SBET values, but not so drastic as for TiO2 . In fact, sulphation process takes place in this case upon crystalline TiO2 in contrast with that prepared from sol–gel route. Therefore, effect of sulphation, in terms of improvement of surface area, is not achieved when sulphation step takes places upon the crystalline oxide. Regarding to the pore size distribution (Fig. 1), it is worth of noting a similar evolution for pore size distribution as for SBET already mentioned. It can be notice that sulphated TiO2 exhibits also certain stabilization of the initial pore distribution. Non-sulphated TiO2 pore sizes shift towards higher pore diameter values rapidly after calcination at 400 ◦ C. Meanwhile, sulphated one suffers this shift more slightly upon calcination. Similar fact was observed for SBET values.
Table 1 Surface and structural properties of sulphated and non-sulphated TiO2 Sample
SBET (m2 /g)
Average pore diameter (Å)
Anatase molar fraction
Crystallite size (nm)
Sulphated TiO2 400 ◦ C 500 ◦ C 600 ◦ C 700 ◦ C 800 ◦ C
90 88 41 20 6
40 45 + 250 150 + 300 320 –
100 100 100 93 0
5.7 8.9 21.2 33.7 –
Non-sulphated TiO2 400 ◦ C 98 28 500 ◦ C 600 ◦ C 17 700 ◦ C 6 800 ◦ C –
45 + 250 350 330 500 –
99 99 82 10 0
10.2 22.0 31.9 35.6 –
Degussa P25 400 ◦ C
47
350
77
20.9
Sulphated Degussa P25 400 ◦ C 36 700 ◦ C 13
400 500
77 2
22.0 25.0
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Fig. 1. Pore size distribution for TiO2 /SO4 2− , TiO2 and Degussa/SO4 2− series.
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On the other hand, for sulphated Degussa P25, calcination at 700 ◦ C do not change considerably the initial average pore diameter, being in this case significantly higher than those observed for sol–gel TiO2 . Thus, sulphate groups produce an important stabilization effect on the surface features of TiO2 when sulphation is applied upon the fresh precursor. In the case of Degussa, sulphation treatment produces a detrimental effect, decreasing the surface area values. From XRD for different TiO2 series it can be observed that sulphate pre-treatment of the fresh precursor trends to stabilize the anatase phase upon calcination (Table 1). Anatase to rutile transition seems to shift towards higher temperature in the case of sulphated TiO2 . Thus, sulphated TiO2 calcined at 700 ◦ C still present significantly higher molar anatase/rutile ratio than non-sulphated one. From these results, it can be inferred that phase transition would take place at temperatures higher than 700 ◦ C, while for non-sulphated series this phase transition should take place between 600 and 700 ◦ C. It is well reported that sulphate and phosphate species [20] reinforce the bonding between zirconia fragments and delay the crystallization [21]. Therefore, additionally to the surface area stabilization it is clear that sulphation produces a clear crystalline stabilization of the anatase phase [22]. Regarding sulphated Degussa P25 (Table 1), sulphation process does not produces any significant change in the phase composition of TiO2 . Sulphated and non-sulphated commercial TiO2 present an anatase molar fraction of 77%, very close to the nominal 80% characteristic of this commercial TiO2 . Calcination at 700 ◦ C leads to almost complete rutilization of TiO2 . Sulphate groups in this case seem not to stabilize anatase fraction as in the case of sol–gel prepared sulphated TiO2 . Sol–gel prepared TiO2 series (sulphated and non-sulphated) present mean crystallite sizes in the nanometer range even at higher calcination temperature. Stabilization of the nanoparticles is more evident for sulphated series for which the increase of the crystallite size takes place at temperatures higher than 600 ◦ C. this crystallite size evolution could be related to the anatase stabilization at higher temperature. This stabilization of anatase phase as well as the improvement of surface area could be explained by taking into account the presence of sulphate groups anchored on the TiO2 precursor before calcination.
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It is well known that sulphate groups are stable in the oxide structure during calcination up to 600 ◦ C [23–26]. The acidic properties of sulphated metal oxides were modified by thermally decomposing the sulphate to different extents [23]. It is reported that the amount of ammonia desorbed from surface sites of either medium or high acidic strength changes parallel with the decreasing sulphate content and also the decreasing surface area, which occurs concomitantly with sulphate loss [24]. Fig. 2 represents the thermal evolution of sulphated TiO2 . As approximation, from TG curves of calcined samples it can be also assumed that the weight loss between 400 and 600 ◦ C could be due to the decomposition of strongly bonded sulphate species [26], though other species can be also decomposed in this range. A small endothermic effect can be observed for TiO2 /SO4 2− located around 550 ◦ C. This endotherm could be associated with the sulphate decomposition [26]. For sulphated Degussa P25, this endothermic effect is significantly more evident, indicating that in principle sulphate species are bonded to the surface with higher adsorption strength. Therefore, sulphate content in samples calcined at temperatures higher than 600 ◦ C should be considered as almost negligible. If we measure the weight loss from 400 ◦ C to the corresponding calcination temperature (Fig. 3) we can follow the evolution of SO4 2− groups during calcination. From this result, it can be deduced that the presence of sulphate on the TiO2 (sol–gel prepared and Degussa P25) could be only considered for samples calcined at temperatures lower than 700 ◦ C. Calcination at higher temperatures could lead to the practical elimination of the adsorbed SO4 2− . This is an important point to be taken into account, since the particular surface and structural features of sulphated series have been made by considering the presence of sulphates on the structure. The optical absorption properties of different TiO2 samples are also related with the crystalline structure. In Fig. 4 we represent the evolution of calculated band-gap values with the calcination temperature. It can be also observed the correlation between the crystalline phase present and the band-gap values. From these results it can be noted that absorption edge shifts to higher wavelength (in other words, towards the visible range), as the rutile phase is appearing. Since sulphated TiO2 series exhibits certain anatase stabilization, the band-gaps present higher values.
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Fig. 2. Thermal analysis curves (TG, DTG and DTA) for TiO2 /SO4 2− , TiO2 and Degussa/SO4 2− photocatalysts.
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Fig. 3. Weight loss from 400 ◦ C and each corresponding temperature calculated from TG curves.
From these results, we can estimate that anatase phase present a band-gap value around 3.6 eV, meanwhile rutile phase band-gap is located around 2.9 eV.
3.2. Phenol photooxidation Photocatalytic properties of sol–gel prepared TiO2 (sulphated and non-sulphated) were tested for the
Fig. 4. Band-gap values (eV) for TiO2 /SO4 2− and TiO2 calcined at different temperatures. It is also indicated the anatase molar fraction (A: anatase) present in each case.
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photocatalytic oxidation of phenol. In order to determine the effect of sulphate treatment on the photocatalytic properties of TiO2 , we have also performed similar catalytic runs with sulphated Degussa P25 calcined at 400 and 700 ◦ C. In Fig. 5, we show the conversion plots for different TiO2 samples calcined at temperatures between 400 and 800 ◦ C. Surprisingly, for TiO2 /SO4 2− series final conversions increases samples calcined up to 700 ◦ C. It is worth noting that in spite of this high temperature the photocatalytic behaviour of this sample is the best of its series. Comparing these results with non-sulphated TiO2 series, it is clear that calcination at temperatures higher than 600 ◦ C produces a detrimental effect upon conversion values. It can be clearly pointed out that sulphated TiO2 present higher conversion values especially for samples calcined at higher temperatures (700 and 600 ◦ C), for which surface area is lower and sulphate content is supposed to be almost negligible as it was already mentioned. Consequently, in principle there is no direct relationship between the surface features or the presence of surface sulphate and therefore the acidic properties of TiO2 with the improvement of the phenol conversion. Calcination of TiO2 /SO4 2− at 800 ◦ C destroys the photocatalytic activity of TiO2 . The drastic decrease in the conversion for this sample can be explained by taking into account that calcination at 800 ◦ C leads to the complete rutilization of TiO2 , as well as a low surface area values. On the other hand, sulphation process on Degussa TiO2 produces worse in its photocatalytic activity. In this case, sulphation process seems to ruin the initial features of Degussa P25. No beneficial effect can be observed by the fact of the increase of surface acidity as other authors have reported for gas phase photocatalysis. In Fig. 6, we represent the evolution of photon efficiency, calculated from the conversion slope for different TiO2 photocatalysts. If we consider the photon efficiency per gram of catalyst (Fig. 6a) it is worth noting that better efficiencies have been observed for TiO2 /SO4 2− with respect to non-sulphated TiO2 . Sulphated Degussa photon efficiency values are in all cases worst than TiO2 and TiO2 /SO4 2− . Best photocatalytic performance is achieved for TiO2 /SO4 2− calcined at 700 ◦ C. As calcination temperature rises up, photon efficiency for this series increases till that temperature. Although this higher calcination treatment, TiO2 /SO4 2− exhibits similar
photon efficiency than non-sulphated Degussa P25 (33% versus 38%, respectively). Non-sulphated TiO2 presents higher photon efficiency at low calcination temperature, decreasing as calcination is higher than 600 ◦ C. Although the better approximation to the real situation should consider the efficiency per gram, since all photocatalytic experiments were carried out using the same amount of catalyst, we can also observe the evolution of the photon efficiency per surface area (Fig. 6b). Thus comparison of the photon efficiency per surface unit could indicate us the efficiency and activity for each photocatalyst’s surface. In this case, as non-sulphated TiO2 exhibit lower surface areas, photon efficiency values follow an increasing growth till calcination temperature of 600 ◦ C. Surface contribution is higher for catalysts calcined at temperatures lower than 600 ◦ C, since SO4 2− stabilizes the surface area and therefore the photon efficiencies per area are higher for those catalysts with lower specific surface area. However photon efficiency per m2 at 700 ◦ C for sulphated TiO2 is still higher, indicating that the surface contribution is not the reason for the higher conversion observed for TiO2 /SO4 2− calcined at this temperature. The photoactivity of the surface for TS700 is significantly higher with respect to TS catalyst with higher surface area. Thus, it could be TS700 presents the more active surface. This higher activity could be related to structural features (anatase stabilisation as well or the absence of crystalline defects). From these results, it can be noticed that sulphate pre-treatment improves the photocatalytic behaviour of TiO2 by affecting the structural and surface properties. In previous works [11,13], other authors reported the increasing of photocatalytic conversions due to the enhancement of the acid properties of sulphated TiO2 . In our case, the presence of sulphate seems to improve the photocatalytic activity of TiO2 in two ways: (i) Stabilizing the anatase crystalline phase at higher temperature and at the same time eliminating the crystal defects present in the structure. These defects could be the responsible of the recombination process and therefore of a low efficiency. Sulphated TiO2 calcined at 700 ◦ C presents high degree of crystallization only in the anatase form, with small crystallite size. (ii) Sulphate pre-treatment affect also to the surface area value. The presence of sulphate in
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Fig. 5. Conversion plots for phenol photodegradation for TiO2 /SO4 2− , TiO2 and Degussa/SO4 2− .
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Fig. 6. Photon efficiencies for phenol photodegradation for TiO2 /SO4 2− , TiO2 and Degussa/SO4 2− : (a) per gram and (b) per square meter.
the fresh precursor seems to avoid sintering process and the following collapse of surface area. In principle, no direct relationship with acidic properties due to the presence of adsorbed SO4 2− groups can be made. In this sense, we have performed other sulphate treatment by using less concentrated H2 SO4
solution (0.1 M). In the case of sol–gel sulphated TiO2 (Fig. 7), concentration of H2 SO4 does not affect significantly to the conversion profile of sample calcined at 700 ◦ C. On the contrary, it seems that for sulphated Degussa P25 (Fig. 8), the detrimental effect of sulphate is clear when increase the sulphate presence on the TiO2 .
G. Col´on et al. / Applied Catalysis B: Environmental 45 (2003) 39–50
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Fig. 7. Effect of H2 SO4 concentration used for the sulphation process on the conversion values for TiO2 /SO4 2− calcined at 700 ◦ C.
Fig. 8. Effect of H2 SO4 concentration used for the sulphation process on the conversion values for Degussa/SO4 2− calcined at 400 ◦ C.
4. Conclusions We have obtained a sulphated TiO2 photocatalyst by a sol–gel method, and then calcined at different
temperatures. Sulphated TiO2 exhibit higher photon efficiencies than non-sulphated TiO2 and sulphated Degussa P25. The improvement of the photocatalytic efficiency is more evident as temperature arises till
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700 ◦ C. In principle this better photocatalytic behaviour cannot be explained by terms of surface acidity due to the presence of sulphate groups. Since the elimination of SO4 2− takes place from 400 up to 600 ◦ C, it can be expected that at temperatures higher than 600 ◦ C the occurrence of sulphate could be neglected. The performed photocatalytic activities exhibited by the sulphated series with respect to non-sulphated one may be explained by considering that sulphation as photocatalyst pre-treatment. This pre-treatment would affect to the TiO2 crystallization by stabilizing the anatase phase at higher temperature and keeping at the same time a rather high specific surface area. Thus, by calcining at higher temperature we are able to eliminate as much as possible the recombination process occurring in the crystal defects of the structure. At lower temperature (400 and 500 ◦ C), the presence of sulphate seems to be a negative fact since photon efficiencies of sulphated TiO2 are lower than non-sulphated one. Therefore no clear benefit can be assumed by the supposed increase of surface acidity by sulphation. Especially in the case of sulphated Degussa P25, there is clearly no evidence of such acidic properties improvement. Moreover, sulphation process leads to a drastic decrease of the photocatalytic properties of Degussa P25 for the reaction considered. The improvement of the photocatalytic activity is therefore related to the optimisation of the redox step in the photocatalytic process instead of the acidity properties that could favour the adsorption of the organic substrate.
Acknowledgements This research was financed by Ministerio de Ciencia y Tecnolog´ıa (project ref. BQU2001-3872C02-01). Partial support by the Junta de Andaluc´ıa (P.A.I. group reference FQM181) is also acknowledged.
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