Photodeposition of precious metals onto mesoporous TiO2 nanocrystals with enhanced their photocatalytic activity for methanol oxidation
Graphical abstract
Highlights
► Precious metals have been photodeposited onto mesoporous TiO2. ► The prepared photocatalysts have been compared with TiO2 P-25. ► Photocatalytic activity behaves quite differently depending on types of precious metals. ► The photocatalytic efficiency is in the order Pd > Pt > Au. ► The photonic efficiency of mesoporous TiO2 has improved two times by addition of Pd.
Introduction
TiO2 is an interesting material for photocatalytic applications and it is regarded as the most efficient and environmentally benign photocatalyst [1], [2], [3], [4], [5]. It has been widely used as a photocatalyst for the removal of hazardous organic substances and as an electrode material for dye-sensitized solar cells due to its strong oxidizing and reducing ability under UV light irradiation [6], [7], [8]. The photocatalytic activity of a semiconductor is largely controlled by (i) the light absorption properties, e.g., light absorption spectrum and coefficient, (ii) reduction and oxidation rates on the surface by the electron and hole, (iii) and the electron–hole recombination rate [9]. TiO2 in a mesoporous structure is interesting in the sense that it generally has a high surface area in a continuous structure rather than in discrete particles [10]. This continuity can be expected to make the electron transfer within the material easier, resulting in higher activity [11], [12], [13], [14], [15], [16]. The modification of the TiO2 with precious metals can alter the charge-transfer properties between TiO2 and the surrounding environment, thus improving the performance of TiO2 nanomaterials and a good candidate material for photocatalytic reactions [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. Depending on the preparation conditions, photodeposition procedures typically yield small metal deposits ranging from a few to around 20 nm in diameter [21]. The size and dispersion of the metal deposits on the TiO2 particles will, however, be critical for the control of their photocatalytic activity. Given the strong effect of deposit size and dispersion, the development of preparation methods providing a close control over the deposit size is essential [22], [23], [24]. Murdoch et al. [25] have demonstrated that Au particles in the size range 3–30 nm on TiO2 are very active in hydrogen production from ethanol. It was found that Au particles of similar size on anatase nanoparticles delivered a rate two orders of magnitude higher than that recorded for Au on rutile nanoparticles. Surprisingly, it was also found that Au particle size does not affect the photoreaction rate over the 3–12 nm range. In addition, a high uniformity of both, precious metals and TiO2, in size and shape also appears to be essential for achieving enhanced photocatalytic activities [25], [26], [27], [28], [29]. On the other hand, the generation of hydroxyl (OH) radicals plays a key role in the generally accepted mechanism of the heterogeneous photocatalytic degradation of organic pollutants in aqueous suspensions of TiO2 [9], [30]. In order to quantify the rate of OH generation, it has been proposed to use a well-known OH scavenger, i.e., CH3OH. Photooxidation of CH3OH is a process in which photogenerated holes oxidize hydroxide ions adsorbed at the surface of TiO2 nanoparticles to produce highly oxidizing (OH) radicals, which subsequently attack adsorbed pollutant molecules. This primary step of HCHO formation initiates a series of degradation reactions that ultimately lead to mineralization of the pollutants. It has been reported that a considerable fraction of the HCOOH formed on the photocatalyst surface is decomposed into CO2 and H2O on the spot without being released from the surface [31], [32].
In our previous work, we have prepared the precious metals doped TiO2 networks assembled through a novel in situ preparation method starting from suitable precursors in the presence of a triblock copolymer employed as the structure directing agent [11], [12], [13], [14], [15], [16]. In this contribution, we are focusing on photodeposition of precious metals onto mesoporous TiO2 networks. The newly developed photocatalysts have been compared with TiO2 P-25 (Evonik AG) by the determination of the rate of HCHO formation during the photocatalytic degradation of CH3OH in aqueous suspensions to determine the corresponding photonic efficiencies.
Section snippets
Experimental
Materials: The block copolymer surfactant EO106-PO70EO106 (F-127, EO = CH2CH2O, PO = CH2(CH3)CHO), MW 12,600 g/mol, Ti(OC(CH3)3)4 (TBOT), HCl, CH3OH, C2H5OH, CH3COOH and HAuCl4, H2PtCl6 and K2PdCl4 were purchased from Sigma–Aldrich.
Preparation of mesoporous TiO2: Mesoporous TiO2 nanocrystals were synthesized through a simple sol–gel process in the presence of the F127 triblock copolymer as structure directing agent. To minimize possible variables, the molar ratio of each reagent in the starting
Photodeposition Au, Pt and Pd onto mesoporous TiO2
Au, Pt and Pd/TiO2 photocatalysts were prepared by the photo-reduction method. 0.5 g mesoporous TiO2 was mixed with HAuCl4, H2PtCl6 or K2PdCl4 solution containing the equivalent amount of Au3+, Pd2+/or Pt4+ followed by the addition of methanol (1% (v/v) methanol/H2O) to obtain 0.5 wt% precious metals/TiO2. The suspension was magnetically stirred for 12 h under UV illumination. After irradiation, the samples have been centrifuged, washed then dried at 110 °C overnight to obtain Au, Pd, and Pt/TiO2.
Structural investigations
Mesostructured TiO2 nanocrystals were prepared using Pluronic F127 as the structure-directing agent. These nanoparticles are quite stable and grow slowly due to the slow introduction of water from the ambient environment and the esterification of acetic acid [38]. The use of surfactants enables the controlled synthesis of uniform TiO2 nanocrystals with very small particle size ∼10 nm. The synthesis pathway has been selected in order to have a TiO2 platform with uniform size and shape. The
Conclusions
Precious metals have been photodeposited onto mesoporous TiO2 and compared with TiO2 P-25 for photooxidation of methanol. The results indicate that the photonic efficiencies ξ of TiO2 P-25 and undoped mesoporous TiO2 are 4.62 and 8.63%, respectively. While the photonic efficiency of doped mesoporous TiO2 was drastically increased to 13.9, 15.6 and 17.8% by photodeposition of Au, Pt and Pd nanoparticles, respectively. Photocatalytic activity behaves quite differently depending on types of
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