Effect of metal deposition on the photocatalytic activity of titania-silica for the removal of 2-propanol diluted in water
- 494 Downloads
The influence of transition metals, such as Ag, Cr, and Co, on the photocatalytic properties of titania-silica (TiO2-SiO2) prepared via both impregnation (imp) and photo-assisted deposition (PAD) methods has been investigated. The decomposition of 2-propanol under UV-light irradiation was used to probe the effect of transition metals deposition on the activity of titania-silica. PAD-Ag/TiO2-SiO2 prepared by the PAD method was highly active compared to the original TiO2-SiO2, while imp-Cr/TiO2-SiO2 prepared by conventional impregnation method showed a weak tendency toward the decomposition of 2-propanol.
KeywordsTiO2-SiO2 Transition metals Photo-assisted deposition Impregnation Photocatalytic activity
Environmental pollution has increased public concern nowadays and the decontamination of polluted water and air by photocatalysis has been attracting a lot of attention for its efficiency and promising economy. Titanium dioxide in the form of anatase is the most studied photocatalyst, but it can only work under ultraviolet (UV) light because of its large band gap . The use of supported titanium dioxide has allowed the enhancement of the photodegradation rates in comparison with net titania [2, 3]. Doping titania with metal ions may enhance its photoactivity [4, 5] or may even enable its sensitization under visible light . Titania-silica is one of the most active mixed oxide catalysts for liquid–phase oxidations; it has become an important industrial catalyst. Ever-increasing domains for clean oxidation processes with high product selectively to avoid by-products and waste formation has lead to considerable research efforts in this area. Numerous applications deal with transition metals on carrier oxides and most 3d metals show some oxidation activity. However, product selectively in general is often low and homogenous reactions always compete with reactions on the catalyst surface [6, 7, 8, 9, 10]. A number of studies have shown that titania-silica intimate mixtures exhibit enhanced UV photocatalytic activity compared with pure titania. With this background, we undertake a target to combine the futures of intimately mixed titania-silica with the incorporation of transition metal ions. Indeed, several transition metal ions allow a band gap shift to the visible light range when impregnated on large band gap materials [11, 12]. The photodeposition process involves the reduction of metal ions by conduction band electrons, the anodic process being the oxidation of water by valance band holes. Oxidizable additives are generally added to improve the rate of deposition, such as acetate, formaldehyde, methanol, or 2-propanol, but these caused the aggregation of transition metals on the surface of the catalysts, which lead to a decrease in the activity. Our principle work in this paper focused on enhancing the photocatalytic activity. We synthesized transition metals deposited on titania-silica by TM/TiO2-SiO2 (TM = transition metals [Ag, Co, and Cr], present in the form of oxides) using photo-assisted deposition (PAD) and impregnation (imp) techniques. To observe the destruction of 2-propanol on a local and global scale, a photocatalyst with high reactivity under UV light is required. A number of characterizations have been carried out elaborately in attempts to obtain this goal.
Preparation of TiO2-SiO2
The synthesis method is focused on the first step on the prehydrolysis of aqueous ammonia (NH4OH, 25%) to obtain titania-silica mixed oxides. Titanium butoxide orthotitanate (TBOT, ≥98%) and tetraethyl orthosilicate (TEOS, ≥98%) were selected as the source of titania and silica, respectively. Here, we give a typical procedure for the preparation of titania–silica mixed oxide containing 92.8 mol% silica. Firstly, 2.25 ml TBOT was mixed with 5.1 ml acetylacetone and 10.0 ml anhydrous EtOH. Secondly, 17 ml TEOS was mixed with a solution containing 10.8 ml distilled water, 23.9 ml aqueous ammonia, and 20.0 ml anhydrous EtOH under stirring. After a few minutes, the solution changed to opaque white due to the formation of dense Si sol. The above solutions were simultaneously added into the beaker containing 10.0 ml anhydrous EtOH under stirring. After all operations were finished, the samples were aged for 72 h at room temperature. Finally, the samples obtained were evaporated to dryness, and then calcined at 823 K for 5 h in air.
Preparation of Ag, Co, and Cr/TiO2-SiO2
The TM loaded on TiO2-SiO2 (PAD-TM/TiO2-SiO2, 4 wt% as TM metal, where TM are Ag, Co, and Cr) were prepared using the PAD method: TM metals were deposited on TiO2-SiO2 from aqueous solution of silver nitrate, cobalt nitrate, and chromium nitrate under UV light irradiation.
The TM loaded on TiO2-SiO2 (imp-TM/TiO2-SiO2, 4 wt% as TM) were prepared using the impregnation method. The TM were deposited by the impregnation of TiO2-SiO2 with aqueous solution of silver nitrate, cobalt nitrate, and chromium nitrate. The samples were dried at 378 K and reduced by H2 (20 ml min−1) at 473 K for 2 h.
X-ray diffraction (XRD) was used to identify the crystal phases of the prepared catalyst. These studies were performed by using a Rigaku RINT-2000 wide-angle X-ray diffractometer system using Cu Kα radiation to the assess the crystallinity of the catalyst and a power of 40 kV × 30 mA. The intensity data were collected at 298 K. TM/titania-silica were ran from 20 to 80° (2θ). The powders were characterized by a UV-vis spectrophotometer (Shimadzu UV-2450) at 295 K in the range 200–800 nm. BaSO4 was used as the standard in these measurements. The morphology and particle size of the prepared samples were examined via a transmission electron microscope (TEM, Hitachi H-9500 operated at 300 kV).
Photocatalytic reactions by TM/TiO2-SiO2
The photocatalytic activity of TM/TiO2-SiO2 was evaluated by the photodegradation of 2-propanol aqueous solution. All photocatalytic reactions were performed under UV light irradiation with continuous oxygen and maintaining constant mechanical stirring for 1 h under a dark atmosphere. Twenty-five milliliters of 2-propanol (2.6 × 10−3 mol dm−3) was mixed with 50 mg of calcined catalysts in a quartz cell. The UV light was provided by a 100 W high-pressure mercury lamp (wavelength λ > 250 nm). The catalysts and solution were separated by filtration and then the resulting solution was analyzed by gas chromatography (GC, GC-14B, Shimadzu).
Results and discussion
Fig. 3B shows the UV-vis spectra of the TiO2-SiO2, PAD-Co/TiO2-SiO2, and imp-Co/TiO2-SiO2 catalysts. As shown in Fig. 3B, the TiO2-SiO2 exhibits absorption in the UV light range, while the PAD-Co/TiO2-SiO2 and imp-Co/TiO2-SiO2 catalysts exhibit absorption in the UV light at high wavelength. Figure 3C shows the UV-vis spectra of the TiO2-SiO2, PAD-Cr/TiO2-SiO2, and imp-Cr/TiO2-SiO2 catalysts. As shown in Fig. 3C, the TiO2-SiO2 exhibit absorption in the UV light range, which the PAD-Cr/TiO2-SiO2 and imp-Cr/TiO2-SiO2 catalysts exhibit absorption in the UV light at high wavelength.
Photocatalytic degradation of 2-propanol
The photocatalytic degradation of 2-propanol is used as a probe reaction to test the catalytic activity of the catalysts. It is divided into two sections as follows.
From the results noted, the photocatalytic activity of PAD-Ag/TiO2-SiO2 is higher than that of imp-Ag/TiO2-SiO2. This is attributed to the aggregation of Ag in the case when PAD-Ag/TiO2-SiO2 is smaller than that of imp-Ag/TiO2-SiO2, which is confirmed by UV-vis and TEM results and is in agreement with an earlier report . The photocatalytic activity of PAD-Cr/TiO2-SiO2 is higher than that of imp-Cr/TiO2-SiO2.
Different transition metals (Ag, Co, and Cr) deposited on TiO2-SiO2 was synthesized by photo-assisted deposition (PAD) and impregnation (imp) methods. Using the PAD method in the deposition of transition metals afforded well controlled size and good dispersion on tetrahedrally coordinated Ti-oxide of the support. Also, the direct interaction between nano-sized transition metals and the photo-excited tetrahedrally coordinated Ti-oxide realized by the PAD method under UV-light irradiation gives one the possibility to design unique and active nano-sized metal catalysts. The characterization results showed enhancement of the photocatalytic activity. Such photocatalytic activity of TM/TiO2-SiO2 in UV light was tested for the degradation of 2-propanol. The PAD-Ag/TiO2-SiO2 catalyst exhibited the highest photocatalytic activity and smallest particle size with high dispersion.
- 9.M. Taramasso, G. Perego and B. Notari, US Patent 4, 410, 501 (1983)Google Scholar