TiO-assisted photodegradation of environmentally relevant organic compounds in waste water

Li Ming, Zhong Huiqiong, Zhang Lizhen, Yang Jun
(Department of Chemistry, Jinan University, Guangzhou 510632, China)

AbstractNanosized TiO/SiO photocatalysts in pure anatase phase with crystallite size of 6 to 22 nm were prepared via hydrothermal method using titanium sulfate. These catalysts were characterized by using BET, XRD measurements. The experimental results show that the addition of silica gel could effectively improve the thermal stability of TiO and inhibit the transformation of anatase to rutile phase and the growth of titanium dioxide crystals during the calcination treatment. The nanosized 40 % TiO/SiO-700 catalyst shows enhanced photocatalytic activity and has the highest rate constant of 0.046 min-1, comparing with 0.033 min-1 for P-25. After 90 min irradiation of UV light, nearly no absorption band was observed in the range of 200-800 nm in the presence of 40 % TiO/SiO-700 catalyst, whereas for P-25, a broad band appeared in range of 200-360 nm. The probable reason is that TiO/SiO catalyst has a relatively large collecting capability of degradation intermediates as well as target substrate from the solution phase onto the catalyst, thereby preventing dissolution of toxic substances in solution.

Since the discovery of photocatalytic splitting of water on TiO electrodes by Fujishima and Honda [1], there has been an explosion of research related to the fabrication of oxide semiconductor photocatalysts for environmental application [2-3]. Among various oxide semiconductor photocatalysts, titania is believed to be the most promising one due to its high photocatalytic activity, chemical/photoerosion stability, low cost and without risks for the environment or humans [4-5]. The efficiency of some commercial TiO in the treatment of exhaust gas and wastewater contaminated with organic and inorganic pollutants has been fully proved. However, when naked TiO are used, problems occur. Firstly, small particles trend to agglomerate into large particles, making against on catalyst performance. Secondly, the separation and recovery of catalyst is difficult [6] Therefore, many papers have reported titanium dioxide supported on various matrix, such as silica, active carbon, zeolite, etc. Among them, the titania-silica system was regarded as a potential candidate for photocatalyst that could be explained as a synergetic effect.
Very recently, we have investigated the synthesis of TiO/SBA-15 composites by a post-synthesis step [7]. However, in most cases, the preparation of silica-modified titanium dioxide usually employed Ti-alkoxide rather than Ti-salt as a starting material. Here, we describe a facile method for the synthesis of high photocatalytic silica-modified anatase TiO using titanium sulfate as starting material.

2.1 Materials
In the present work, commercial silica gel was used as the catalyst support (supplied by Qingdao Chemical Industries). TiO particles were obtained from Degussa (P-25). Ti(SO was purchase from Aldrich.
2.2 Synthesis
In a typical synthesis, 2.40 g of titanium sulfate was dissolved into 25 ml deionized water with vigorous stirring, then a required amount of silica gel was added into the above solution under stirring for 30 min and the mixture was further sonicated for 30 min. After that, the mixture was transferred to a Teflon-lined autoclave, and the hydrothermal reaction was carried at 100 C for 2 h. The resulting products were centrifugated, washed, dried at 80 C overnight and calcined at a set temperature. Throughout the subsequent discussion, the samples will be labeled by indicating the titania content, support and calcined temperature: x % TiO/SiO-T, where x % was the amount of TiO loading by weight and T was the calcination temperature.
2.3 Characterization
XRD patterns of the samples were determined on a XD-2 powder X-ray diffractometer over the scan range 2=20-70 for wide angle XRD. Crystallite size was calculated using Scherrer’s formula d=0.91cos. BET surface area of photocatalysts was measured by N physisorption using Micromeritics ASAP 2020 system.
2.4 Photocatalytic reaction
The experiments were carried out in a cylindrical double wall jacket glass reactor containing 0.050 g of catalyst sample and 100 ml of MB solution with a concentration of 20 mg/l. The mixture was first stirred for 20 min in dark at room temperature, then the reaction solution was placed perpendicularly to a 125 W medium pressure mercury lamp by water cooling, and the solution was bubbled with air under magnetic agitation. Samples were collected at regular interval and centrifugated to analyze by diffusive reflective UV-Vis spectrophotometry to determine the concentration of MB.

3.1 XRD analysis

The XRD patterns of nanosized TiO/SiO samples with different TiO contents and 40 % TiO/SiO samples calcined at different temperatures were shown in Fig. 1 and Fig. 2, respectively. It is seen that along with the increase of TiO amount, the (101) reflection due to anatase was found to increase.
Fig. 1 XRD patterns of TiO/SiO with different TiO contents. (Calcined at 700 C)
Fig. 2 XRD patterns of as-synthesized 40% TiO/SiO and calcined at different temperatures.

As Fig 2 shown, for the catalyst containing 40 % of TiO, anatase phase is the main phase found in as-synthesized catalyst without calcination. After calcination, in the temperature range of 500 to 700 C,no rutile phase can be found and anatase phase is the only phase, and the refection of anatase (101) increases with the increment of calcination temperature. Beyond 700 C, the gradual transformation of anatase phase to rutile phase occurred, and for sample calcined at 900 C, only 23.7 % anatase phase transformed into rutile, indicating that anatase TiO grains embedded silica have a relatively thermal stability. In our previous work [7], it was found that the addition of silica SBA-15 inhibited the transformation of anatase phase to rutile phase and the growth of titania crystal grain.
3.2 Textural properties
The specific surface areas, pore volumes and average particle sizes of the TiO/SiO samples with different silica gel addition and 40 % TiO/SiO samplescalcined at different temperatures are shown in Table 1. The specific surface area of the samples decrease from 349.2 m/g (naked SiO) to 25.1 m/g (100 % TiO) with the increase of TiO and it is not significantly decreased when TiO content is not more than 20 %, which is probably due to that a little amount of TiO can not result in dramatically change in silica structure. Similar phenomenon was reported by Zhao et al[8], and in their work, Ti-Si mixed oxides were prepared by sol-gel one step hydrolysis method. For sample containing 40 % TiO, with the increase of calcination temperature, the BET surface areas, pore volumes decrease and average particle sizes gradually increase from 6 nm at 500 C to 14 nm at 900 C. It suggests that the addition of silica can improve thermal stability of samples and inhibit the growth of titania crystallite. Similar result was reported by Chen and co-workers for silica gel supported TiO particles prepared by acid-catalyzed sol-gel method[9]. Contrast to this, Chen et al[10]prepared silica-doped titania photocatalyst through sol-gel method and reported that the optimum amount of silica can effectively prevent the growth of titania, but too little or too much silica cannot prevent the growth of titania grains during the heat treatment.

Table 1. Textural properties and average TiO crystal size of the silica-modified photocatalysts

Catalyst BET surface area (m/g) Structure Pore volume
Crystal size (Anat. nm) Rate constant(min-1
P-25 54.0 Anatase/Rutile   21 0.033
SiO-700 349.2   0.79    
40% TiO/SiO
334.9 Anatase 1.03   0.007
40% TiO/SiO-500 310.2 Anatase 0.81   0.017
40% TiO/SiO-600 289.8 Anatase 0.54   0.037
40% TiO/SiO-700 275.9 Anatase 0.52 11 0.046
40% TiO/SiO-800 265.3 Anatase/Rutile 0.56 12 0.030
40% TiO/SiO-900 235.3 Anatase/Rutile 0.48 14 0.025
20% TiO/SiO-700 347.7 Anatase 0.74   0.038
60% TiO/SiO-700 200.1 Anatase 0.39 17 0.042
80% TiO/SiO-700 134.0 Anatase 0.33 22 0.034
100% TiO-700 25.1 Anatase/Rutile 0.12 28 0.027

3.3 Photodegradation of methylene blue
3.3.1 Effect of TiO content
100 ml of methylene blue (MB) solution (0.02 g/l) was photocatalytically degraded in the presence of 0.05 g of TiO/SiO composites with different TiO contents and 0.02 g Degussa P-25 TiO, the concentrations of methylene blue versus reaction time were shown in Fig. 3. During the initial 20 min without photo irradiation, in the presence of 40 % TiO/SiO-700, 21 % MB was decolored because of the adsorption of MB onto catalyst surface. After that, the concentration of MB remains almost unchanged, indicating that the adsorption of MB onto catalyst reaches equilibrium after 20 min stirring in the dark. The adsorption capacities for MB degradation decrease from 29 % to 0.4 % when TiO content increases from 20 % to 100 %.

Fig. 3 The photocatalytic performance of the TiO/SiO-700 samples with different TiO content. (a) 20% TiO, (b) 40% TiO, (c) 60% TiO, (d) 80% TiO, (e) 100% TiO, (f) P-25, (g) 40% TiO without photo irradiation.

The apparent rate constants of different photocatalysts were calculated and listed in Table. 1. It is seen that the photocatalytic activities obey the following order: 40 % TiO/SiO-700 > 60 % TiO/SiO-700 > 20 % TiO/SiO-700 > 80 % TiO/SiO-700 > 100 % TiO-700, indicating that the addition of silica significantly improves the photodegradation ability. It is observed that the 40 % TiO/SiO-700 exhibits the highest rate constant of 0.046 min-1. It is generally accepted that the photocatalytic activity of composite catalysts depends on the adsorption of silica gel or other carriers and activity of titanium dioxide, the amount of them certainly influences on it. When they reached to a suitable ratio and quantity, the catalyst should show excellent photocatalytic capacity [11-12]. In our present work, an optimum amount addition of 60 % silica can reach an optimum synergetic effect between supports and titanium dioxide, and effectively improve the adsorption ability and photocatalytic activity of catalysts, but too much or too less silica addition will lower the photoactivity of catalysts.
3.3.2 Effect of calcination temperature
The effect of calcination temperature on the 40 % TiO/SiO samples for MB degradation is presented in Fig. 4. The photoactivity of 40 % TiO/SiO samples obeys the following order: 700 C >600 C >800 C >900 C >500 C. The samples calcined at 500 C and 600 C have higher BET surface area than those calcined at higher temperatures, which suggests besides BET surface the crystallinity of catalysts also plays an important role in determination of photocatalytic activity. As shown in Fig. 2, it can be seen that the sample calcined at 500 C has the highest BET surface area and smallest particle size, but its crystallinity is worse than those calcined at higher temperatures, which is probably the reason for its low photoactivity.
Fig. 4 The photocatalytic activity of the 40% TiO/SiO samples calcined at different temperature.(a) As-synthesized, (b) 500 C, (c) 600 C, (d) 700 C, (e) 800 C, (f) 900
Fig. 5 The UV-Vis spectrum of MB solution by photocatalysis under UV-irradiation after 90 min

3.3.3UV-Vis spectra
In order to compare the degradation ability of 40 % TiO/SiO-700 with P-25, the UV-Vis spectra of methylene blue solutions were measured after 90 min UV radiation in the presence of catalysts, and results are shown in Fig. 5. It is seen that the blank MB solution exhibits a broad absorption band in the range of 530-700 nm. In the presence of the 40 % TiO/SiO, the intensity of the 660 nm absorption band decreased rapidly under UV light irradiation and almost disappeared after 90 min, and there is almost no absorption band in the region of 250-400 nm, while P-25 has a broad absorption band at 300 nm. This indicates that 40 % TiO/SiO-700 has excellent degradation ability for MB. The good explanation is that 40 % TiO/SiO-700 has a relatively high capability of collection of degradation intermediates as well as target substrate from the solution phase onto the catalyst, thereby preventing dissolution of toxic substances in solution, whereas when P-25 was used, reaction intermediates mostly dissolved in the solution phase [13]

Nanosized TiO/SiO catalysts were prepared by hydrothermal method. The experimental results show that the addition of silica gel could effectively improve the thermal stability of TiO/SiO composite and inhibit the transformation of anatase to rutile phase and the growth of titanium dioxide crystals during the calcination treatment. The nanosized 40 % TiO/SiO-700 catalyst shows excellent photocatalytic activity for MB degradation, its activity is higher than that of P-25 with the same TiO amount. After 90 min irradiation of UV light, nearly no absorption band was observed in the UV-Vis spectra in the presence of 40 % TiO/SiO-700 catalyst, whereas for P-25, a broad band appeared in range of 200-360 nm.

We gratefully acknowledge financial support from the Team Project of Guangdong Province Natural Science Foundation (Grant No 05200555).