Impaginato 75 1. Introduction Ethylene gas (C2H4) is the ripening hormone of several fruits and vegetables (Hussain et al., 2011; Keller et al., 2013). Ethylene is used to promote the uniform ripening of immature fruits such as bananas, but in most cases it doesn’t work as beneficial ele- ment both in food and horticultural industries. Few ppm of ethylene in the storage atmosphere induce very fast ripening in postharvest which causes unde- sirable product losses (Keller et al., 2013). Thus it is desired to remove or degrade ethylene from the storage environment in order to preserve the postharvest products and keep them fresh for a longer time period. The important role of ethylene as growth regulator of climacteric products was inten- sively investigated in the last 50 years and now the ripening process associated to high ethylene produc- tion is well known. Various methods to remove ethyl- ene have been developed: ventilation, or controlled atmosphere, ethylene oxidation using potassium per- manganate. The photocatalytic ethylene oxidation thus represents an innovative way to extend posthar- vest life of the climacteric fruits and vegetables (Park et al., 2009; Keller et al., 2013; Ye et al., 2013). Among the catalysts able to induce ethylene photo- oxidation, TiO2 is promising and attractive due to low cost, non-toxicity and high efficiency. The photocat- alytic properties of nanostructured TiO2 have already been highlighted in several applications, such as pho- tocatalytic degradation of both air and water organic pollutants, and self-cleaning surface protection (Pal et al., 2014). Anchoring TiO2 nanocrystals on suitable meso- porous substrate brings relevant practical advan- tages. The immobilization on support materials slows or totally inhibits nanoparticles coarsening, preserv- ing their higher surface area. Secondly, the substrate greatly improves handling, processing and recover of TiO2 nanopowder. Mesoporous silica, as a catalyst support over nano-sized TiO2, represent a good choice due to high specific surface area and improved thermal stability of the TiO2 anatase crys- talline phase. Some recent works also demonstrated higher photocatalytic activity of TiO2/SiO2 mixed oxide composites (Fu et al., 1996; Zhan et al., 2014). Adv. Hort. Sci., 2016 30(2): 75-80 DOI: 10.13128/ahs-19132 Photo-oxidation of ethylene over mesoporous TiO2/SiO2 catalysts A. Licciulli 1 (*), R. Nisi 1, S. Pal 1, A. M. Laera 1, P. Creti 2, A. Chiechi 3 1 Dipartimento di Ingegneria dell’Innovazione, Università di Salento, Via Arnesano, 73100 Lecce, Italy. 2 CNR Istituto Microelettronica e Microsistemi, Sezione di Lecce, 73100 Lecce, Italy. 3 Salente SRL, Via Monteroni 147, 73100 Lecce, Italy. Key words: gaseous phase photocatalysis, liquid-crystal template, modified storage environment, mixed oxides. Abstract: Mesoporous TiO2/SiO2 catalysts were prepared in order to increase the post-harvest life of climacteric fruits and vegetables reducing ethylene concentration by photo-oxidation. TiO2/SiO2 powders were synthesized by sol-gel method using titanium isopropoxide Ti(OiPr)4 and tetraethoxysilane Si(OC2H5)4 as source of metal oxides. Mesoporous SiO2 framework was used as catalyst support of nanostructured TiO2 to enhance the photocatalytic efficiency. Different TiO2/SiO2 molar ratios were prepared through sol-gel process. A liquid-crystal template route allowed to obtain the mesoporous silica structure, and contemporary TiO2 insertion in the silica framework. The X-ray diffraction (XRD) analy- sis demonstrated that silica insertion in TiO2 framework inhibits the anatase to rutile phase transformation at higher sin- tering temperature. The photocatalytic efficiency of the catalysts was measured by the photo-oxidation of ethylene gas under UV light irradiation. Complete photo-oxidation of ethylene was observed after 24 h of reaction time. Results show that the silica framework increases the surface area of the composites and make crystalline anatase phase more stable at higher temperature. (*) Corresponding author: antonio.licciulli@unisalento.it Received for publication 16 February 2016 Accepted for publication 5 April 2016 Copyright: © 2016 Author(s). This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Adv. Hort. Sci., 2016 30(2): 75-80 76 The mesoporous SiO2 is able to reduce the recombi- nation of photogenerated electron-hole pair, while higher surface area increases the efficiency in the heterogeneous photocatalysis. Over the past thirty years a successful synthetic strategy, named liquid-crystal template, leading to mesoporous materials has been developed allowing to finely tune the porosity (Corma et al., 2006; Xiao et al., 2008). This methodology involves supramolec- ular aggregates of ionic surfactants that act as tem- plating agents, which are able to direct and control the crystallization of the inorganic nanoparticles (Beck et al., 1992). After the surfactant removal by thermal annealing, mesoporous solids with high sur- face area can be obtained. The morphological and structural properties of the products can be finely tuned by varying process parameters such as pH, temperature, reagent concentrations, surfactants and silica sources. In this work, TiO2/SiO2 binary composites obtained through a liquid-crystal template methodol- ogy have been tested as catalyst in UV light assisted ethylene oxidation at room temperature for the application in modified atmosphere packaging of cli- macteric fruits and vegetables. A titania precursor was added in the reaction medium containing the surfactant and the silica source allowing the one-pot formation of TiO2 and SiO2. Structure and morpholo- gy of the obtained TiO2/SiO2 binary composites with several weight ratios were investigated by using Brunauer-Emmett-Teller (BET) surface area, X-ray dif- fraction (XRD), Fourier transform infrared spec- troscopy (FTIR) and Field emission scanning electron microscopy (FESEM) measurements. 2. Materials and Methods Synthesis of TiO2/SiO2 binary oxides All the reagents involved in the catalyst synthesis were purchased from Sigma Aldrich and were used without any further modification. The starting col- loidal solution was obtained by dissolving 28 g of Pluronic F127 (EO106 PO70 EO106) in 150 g of distilled water through vigorous stirring. The complete poly- mer dissolution was reached after addition of a solu- tion of 2M HCl (600 g). Then the required amount of tetraethoxysilane [Si(OC2H5)4, TEOS] was added drop wise and continued to stirring for an hour. At this stage, titanium tetraisopropoxide [Ti(OiPr)4, TTIP] was slowly added and stirred for overnight. The obtained reaction mixture was heated at 100°C for 24 h and after that, a xerogel was obtained by centrifuging. The xerogel was then dried and calcined at 550°C for 6 h with the heating and cooling rate of 1°C/min to remove the organic content. Following this proce- dure, binary oxide with the TiO2/SiO2 weight ratio of 9/1, 8/2, and 7/3 were prepared. For comparison, a sample of pure TiO2 was also prepared with the same experimental condition. Throughout the text, these samples are represented as TSBA-10, TSBA-91, TSBA- 82, and TSBA-73, respectively. Characterization Crystalline phases of the TiO2/SiO2 powder sam- ples were characterized by X-ray diffraction (XRD) performed on a Rigaku Ultima X-ray diffratometer using CuKα radiation (λ=1.5406 Å) operating at 40 kV/30 mA with the step size of 0.02°. FTIR spectra of the obtained composites were carried out with a JASCO FTIR-6300 over the range 4000-400 cm-1 with a resolution of 4 cm-1 and accumulating 256 scans for each measurement adopting KBr disc method. Specific surface area of the samples was measured by BET method using a Quantachrome NOVA 2200e surface analyzer. FESEM measurements were per- formed with a Zeiss scanning electron microscope. Photocatalytic experimental set up Photo-oxidation of ethylene gas under UV irradia- tion in the presence of TiO2/SiO2 composite powder samples was performed inside a photocatalytic reac- tor consisting of a quartz tube of 3L irradiated by 4 UV light sources (Osram Puritec HNS15WG13, 15W, emitting at 254 nm). The pictures of the catalytic reactor have been reported in figure 1. The experi- mental set up was equipped with flowmeters, vacu- um pump, diaphragm pump for gas recirculation, and thermocouples for the temperature control inside the reactor. The TiO2/SiO2 powder samples (3 g) were placed in the quartz tube on a rectangular quartz ves- sel. Before filling the reactor with ethylene gas/air atmosphere, it was kept in vacuum to ensure the purity inside the chamber. Then a gas mixture of air and ethylene (1 equivalent %) was introduced in the reactor with a controlled flow system. All the experi- ments were performed at atmospheric pressure and the purged gas was continuously circulated in a closed circuit while maintaining room temperature inside the reactor. Ethylene photo-oxidation was monitored at regular intervals by using an Agilent Gas Chromatograph 7820 A equipped with a capillary column (HP-PLOT/Q). Licciulli et al. - Photo-oxidation of ethylene over mesoporous TiO2/SiO2 catalysts 77 3. Results and Discussion To investigate the effect of silica framework and mesoporosity in the catalyst nanostructure, BET spe- cific surface area measurements were performed on the powder samples. BET measurements showed an increase in specific surface area with the increase in silica content. The sample SBA of pure silica repre- sents pores with an average diameters of 3.4 nm and a specific surface area of ~553 m2/g, while the pure TiO2 (TSBA-10) has pores with average diameter of 16.7 nm and a specific surface area of ~59 m2/g. A great ratio of surface/volume is a crucial parameter to optimize photocatalytic efficiency of nanostruc- tered semiconductors. Intermediate values of specif- ic surface area and average diameters between those recorded for pure silica and TiO2, have been obtained on the binary TiO2/SiO2 oxides (Fig. 2). The phase transformation of the TiO2/SiO2 cata- lysts was investigated by XRD and the spectra of each sample are presented in figure 3. The average crys- talline size was estimated according to the Scherrer’s equation, accounting the most intense diffraction peaks of the corresponding anatase and rutile phases. D= kλ/βcosθ (1) where D is the average crystallite size, k is the shape factor (0.9), λ is the wavelength of X-ray radiation, β is the full line width at half-maxima (FWHM) of the main diffraction peak and θ is the Bragg angle of the corresponding diffraction peak (Pal et al., 2014). The weight fraction of anatase and rutile contents in the samples were calculated according to the fol- lowing equations: Fig. 1 - Pictures of the experimental set-up reactor for evaluat- ing photocatalysis efficiency. Fig. 2 - Pore size distribution curve of (A) the mesoporous silica (SBA) and (B) TiO2 (TSBA-10). Fig. 3 - XRD spectra of the TiO2/SiO2 samples with different TiO2/SiO2 weight ratio. Adv. Hort. Sci., 2016 30(2): 75-80 78 WA=1/[1+1.26(IR110/IA101)] (2) WR=1/[1+0.8(IA101/IR110)] (3) where WA and WR are the weighted fraction of anatase and rutile in the mixed phase, and IA101 and IR110 are the integrated intensity of corresponding anatase (101) and rutile (110) diffraction peaks, respectively (Pal et al., 2014). Phase composition and average crystallite size of the corresponding crystals are summarized in Table 1, where it is observed that TSBA-73, TSB-82 and TSBA-91 samples consist of pure anatase phase whereas in case of TSBA-10, there is a small amount of rutile phase formation (~11 wt%). These data clearly indicate that the meso- porous silica framework helps to stabilize the anatase crystalline phase and prevent the thermodynamically favoured phase transformation in to the rutile phase, as reported by Fu et al. (1996). FTIR measurements were carried out to investi- gate the bonding of SiO2 and TiO2 network and they are showed in figure 4. As a reference of pure silica, an FTIR spectrum of SBA is also presented. All the spectra, except for the sample TSBA-10 (100 wt% TiO2), show intense and distinct vibrational bands at 1092, 805 and 472 cm-1 which are assigned to the asymmetric stretching of Si-O-Si (1092 cm-1), Si-O-Si symmetric stretching (805 cm-1), Si-O-Si bending vibration mode (474 cm-1). The broad band around 3300-3600 cm-1 and the band at 976 cm-1 are attrib- uted to the stretching vibration of H-bonded silanols (Si-OH) with hydroxyl groups of the adsorbed water molecules. Another peak of the adsorbed water is located at the wavenumber of 1634 cm-1 due to bending vibration of O-H groups. Furthermore, from the spectra it can be observed that the band at 665 cm-1 is absent in SBA sample, whereas it is present in all samples containing TiO2 and it is therefore attribuited to the Ti-O bond. The appearance of broad band at 450-800 cm-1 range is also attributed to the presence of TiO2 It is also interesting to see that the vibrational band at 1092 cm-1, due to asym- metric stretching of Si-O-Si, is gradually decreasing with increasing TiO2 content in the mixed composite that confirms the variation of TiO2/SiO2 ratio. FESEM measurements on two different samples, one without silica support (TSBA-10) and the other with silica support (TSBA-82) were carried out to investigate the effect of silica insertion in the com- posite catalyst samples. FESEM images of these two samples are showed in figure 5. The lower magnifica- tion images (Fig. 5 a, c) of TSBA-10 and TSBA-82 sam- ples represent the overall structure of these powder samples. Whereas from the images in figure 5 (b, d), it is clearly visible the mesoporous structures. The crystallite size as estimated from the XRD spectra (Table 1) of the corresponding samples matches well with the size as observed in the FESEM images (Fig. 5 b, d). The photocatalytic efficiency of the powder sam- ples was evaluated by the photo-oxidation of ethyl- ene under UV light illumination. The percentage of ethylene photo-oxidation with reaction time under UV light corresponding to four catalysts is summa- rized in Table 2. For a better comparison, data reported in Table 2 have been presented as his- togram in figure 6. From Table 2 and figure 6, it is clear that the overall photo-oxidation efficiency of TSBA-82 sample is higher respect to the others. It also shows a superior catalytic activity in early reac- tion times. The higher catalytic activity of TSBA-82 sample can be explained owing to stable anatase crystalline phase, higher specific surface area and having optimum amount of mesoporous silica sup- port. Table 1 - Anatase (WA), and rutile (WR) phase content in differ- ent TiO2/SiO2 samples and their corresponding crys- talline sizes calculated from XRD data Sample Phase content Crystalline size (nm) WA WR TSBA-73 1 0 15.749 - TSBA-82 1 0 15.998 - TSBA-91 1 0 17.663 - TSBA-10 0.887 0.111 17.553 13.866 Fig. 4 - FTIR spectra of pure mesoporous silica and of the TiO2/SiO2 binary systems. Licciulli et al. - Photo-oxidation of ethylene over mesoporous TiO2/SiO2 catalysts 79 4. Conclusions TiO2/SiO2 catalysts, with different TiO2/SiO2 weight ratios, were synthesized by liquid crystal tem- plate technique, being SiO2 the mesoporous support Fig. 5 - FESEM images of (a, b) TSBA-10 and (c, d) TSBA-82 samples. Fig. 6 - Histogram showing the ethylene oxidation efficiency of four catalyst samples with the reaction time under UV light exposure. Table 2 - Percentage of oxidized ethylene for the catalysts TSBA-10, TSBA-91, TSBA-82, and TSBA-73 as a function of the UV light irradiation Time (h) Oxidized ethylene (%) TSBA-10 TSBA-91 TSBA-82 TSBA-73 0 0 0% 0% 0% 1 2% 0% 6% 8% 2 8% 5% 15% 6% 3 16% 15% 20% 8% 4 22% 22% 25% 11% 5 26% 31% 30% 13% 24 98% 97% 99% 89% Adv. Hort. Sci., 2016 30(2): 75-80 80 of TiO2 catalyst. XRD results prove that a more stable anatase phase with silica insertion even at higher cal- cination temperature is obtained. Increasing the sili- ca content also leads to a higher specific surface area. FESEM images showed the well-defined struc- ture of the catalyst samples. The photocatalysis experiments revealed that, after 24 hours of reaction under UV light, total oxidation of ethylene was achieved in case of TSBA-82 catalyst. The other cata- lysts, such as, TSBA-0 and TSBA-91 also showed inter- esting results but the highest activity was observed with the TSBA-82 sample. 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