Rapid and facile synthesis method of Ti-MCM-48  and CeTi-MCM-48  materials and their photocatalytic performance

Rapid and facile synthesis method of Ti-MCM-48  and CeTi-MCM-48  materials and their photocatalytic performance
M. Filipa, Ramona Enea, C. Munteanua, M. Mureseanub and Viorica Parvulescua*
a”Ilie Murgulescu” Institute of Physical Chemistry, Romanian Academy, Splaiul Independentei 202, 060021, Bucharest, Romania
b University of Craiova, Department of Chemistry, 107 I Calea Bucuresti, 200144 Craiova, Romania
Email: Viorica Parvulescu * – vpirvulescu@icf.ro
* Corresponding author
Abstract
MCM-48 mesoporous silica was synthesized under basic conditions using cetyltrimethylammonium bromide (CTAB) as template and used as support to obtain iTi-MCM-48 and iCeiTi-MCM-48 photocatalysts. Information on the porosity, morphology and structure of the obtained samples were given by N2 adsorption-desorption, X-ray diffraction, scanning and transmission electron microscopy (SEM, TEM) and ultraviolet–visible diffuse reflectance spectroscopy (UV– Vis DRS). Photocatalytic properties were evaluated in photocatalytic oxidation of aqueous phenol solution (20 mg L-1).
Keywords
MCM-48; mesoporous silica; hydrothermal treatment; surfactant; Ti-MCM-48, CeTi-MCM-48; impregnation; photocatalyst; band gap energy, phenol, photodegradation.
1. Introduction
Mesoporous materials, reported by Mobil Company in 1992 [1], have attracted considerable attention due to their high surface areas and large pore size an play a significant role in the fields of catalysis, adsorption, medication, etc. So far, various kinds of mesoporous silica so-called MCM-41,1,2 MCM-48,1,2 SBA-1,3 MSU-1,4 KIT-1,5,6 FSM-16,7 etc. have been synthesized using surfactant micelles as the structure-directing agent and different preparation methods have been developed to synthesize mesoporous materials, such as hydro-thermal synthesis, sol–gel, anode corrosion and synthesis under ambient temperature [2]. In the preparation of supported catalysts, the selection of a suitable support and/or preparation method are of great importance to obtain catalysts with high activity, thus mesoporous silica has gained intense interest in the field of nanomaterials and catalysis being an ideal support candidate for the preparation of high-dispersion catalysts, based on its unique advantages over conventional support materials providing enough surface reactive sites and nanoscale channel wall facilitating transfer of photogenerated charges to surface, are of great potential to act as photocatalysts [3, 4]. However, is always necessary to activate mesoporous silicates by introducing transition-metal ions into the mesoporous frameworks because these materials are composed of Si and O atoms and are not sensitive to either ultraviolet (UV) or visible light. Furthermore, a large number of papers with preparations of various mesoporous metal-SiO2 materials (Ti, Fe, Ce, Cr, Pt, Pd, Co) have been reported [5-7] via direct hydrothermal method or post-synthesis wet impregnation and their catalytic performance have been explored [8, 9]. The photocatalytic activity of TiO2 supported on hexagonal mesoporous silica was reported to be significantly increased in comparison with TiO2 alone. Ti-MCM-41 has an increased photocatalytic activity for oxidizing phenol in comparison with the activity of pure TiO2 or composite Ti-Si oxides [10]. Furthermore, doping TiO2 with cerium oxide (CeO2) have received a great deal of interest because of special f and d electron orbital structures and the unique UV-absorbing ability, high thermal stability, high electrical conductivity and large oxygen storage capacity of CeO2 which and made these materials were used in many catalytic processes for the photocatalytic degradation of Reactive Red 195, 2,4-dichlorophenoxyacetic acid or photodegradation of rhodamine B dye by under visible light etc [11-14]. The incorporation of a second metal to a catalyst changes the structural properties, adsorption, reduction characteristics, product distribution improving catalytic activity, selectivity, and stability of the mono-metallic catalysts. [15, 16].
2. Experimental
2.1. Materials and chemicals
Tetraethyl orthosilicate, TEOS (98%, Aldrich) was used as silica source and cetyltrimethylammonium bromide, CTAB (98%, Aldrich) as the structure directing agent, ethanol (99.8%) and ammonium hydroxide, NH4OH (29%) from Fermont were used to carried out the synthesis of mesoporous silica. Titanium (IV) isopropoxide and cerium nitrate hexahydrate were used as metal precursors.
2.2. Preparation of mesoporous MCM-48 silica
Synthesis of MCM-48 was carried out as follows: a mixture of 1.2g CTAB, 50 mL deionized H2O and 25 mL ethanol was stirred 2h at 40oC. After that, NH4OH were added to reach pH at 11. Then, 1.6 mL of TEOS was poured into the solution immediately under vigorous stirring. Then, the mixture was thermal threated at 100(C for 2 hours. The solid product was recovered by filtration and dried at room temperature overnight. The inorganic material was calcined for CTAB removal at 550 (C for 8 h at a heating rate of 2oC min-1.
The molar ratio: 1.00 SiO2 : 0.41 CTAB : 11 NH4OH : 53 EtOH : 409.17 H2O
Synthesis of Ti-MCM-48: The samples, with molar ratio Si/Ti=25 were obtained by impregnation using an alcoholic solution of titanium (IV) isopropoxide. Then the pure-silica MCM-48 was impregnated in the above solution at the room temperature. The impregnated sample was dried and calcined at 550(C at a heating rate of 2(C/min for 6 h in air.
Synthesis of CeTi-MCM-48: The same procedure as for Ti-MCM-48 with the difference that the molar ratio is Ce/Ti=1 and the ceria precursor from Ce(NO3)3 in aqueous solution was used. The drying procedure and calcination was the same as for iTi-MCM-48
2.3. Characterization of materials
Small-angle XRD data were acquired on a PANalytical’s X-ray diffractometer using Cu Kα (λ = 0.15418 nm). The samples were scanned in the range 2θ = 0.4–8.0ș. N2 adsorption/desorption isotherms were measured with a Micromeritics-Tristar 3000. Before measurements, the samples were outgassed at 573 K for 6 h. The chemical microanalyses by scanning electron microscopy (SEM) were performed on a FEI Quanta 3D FEG scanning microscope and transmission electron microscopy (TEM) characterization was recorded on a Tecnai 10 Philips. Several images at various positions for each sample were obtained to gain better knowledge of the surface morphology.
2.4. Photocatalytic activity test
Photocatalytic activity of the composites was assessed by photodegradation of a phenol solution and was carried out in at room temperature using a stationary quartz reactor with UV (60W, filter =365, 254 nm) lamp, 6mL aqueous solution of phenol 20 mg L-1 and 0,003g of sample. Prior to irradiation, this solution was magnetically stirred in the dark for about 30 min to reach the adsorption equilibrium so that the loss of compound due to adsorption can be taken into account. Photocatalytic experiments were performed for 240 min. All experiments were conducted at room temperature and the samples were filtered through a 0.45 μm Millipore membrane filter with pore size before the analysis. The separation and quantification of phenol were achieved by using a Surveyor Thermo Electron HPLC system (Thermo Scientific). The Hypersil Gold C18 column with 5 (m packing (150 mm ( 4.6 mm i.d.) was used for the analysis. The determinations were made in isocratic conditions, at 250C using a mixture of methanol/water (40:60 v/v) as mobile phase. The volume injected was 5 μL and the flow rate of the mobile phase was 1mL/min. The wavelength was set at 225 nm to integrate the peaks. A good resolution was obtained for phenol (3.14 min). Calibration curves were constructed in the range of 1-150 ppm. Linearity was achieved and the correlation coefficient was 0.999. All analyzes were performed in triplicate always with reproducibility within 3 %. For separation and quantification of phenol degradation products, a reverse phase Hypersil Gold C18 column with 5 (m packing (250 mm ( 4.6 mm i.d.) was used. A mobile phase composition of 15%MeOH/85%H2O/1% glacial acetic acid was utilized. The mobile phase was delivered at a flow rate of 1mL/min and the wavelength was set at 270 nm. The reaction intermediates were confirmed by co-injection of commercial standards under the same operation conditions.
3. Results and Discussion
3.1. Characterization
XRD
The XRD patterns of the calcined samples shown in Fig. 1 indicates that CeTi-MCM-48 and Ti-MCM-48 developed patterns consistent with MCM-48. All samples exhibited the intense {211} peak and weak {220}, {420}, and {332} peaks which suggest the Ia3d cubic phase of MCM-48 mesoporous materials. Compared with pure MCM-48, the XRD patterns of metal incorporated MCM-48 shifted slightly to lower angles with a decrease in the peak strength.There result an enlargement in the unit cell parameter as the metal cations are incorporated, resulting in a larger M–O bond distance. This result is confirmed by the increase of the d spacing of bimetallic materials [17].
Fig. 1. Low-angle XRD patterns of MCM-48, iTi-MCM-48 and iCeiTi-MCM-48
According to the formula, , the unit cell a0 was calculated using d211-spacing values [18] and listed in Table 1.The wall thickness values were calculated by following Eq: [19].
Surface area measurements
Table 1. Textural properties of MCM-48 and metal doped MCM-48.
Sample
a0
(nm)
Surface area
(m2/g)
Mesopore Volume
(cm3/g)
Db
(nm)
Ec
(nm)

MCM-48
8.29
1605.1
0.82
2.23
1.56

Ti-MCM-48
8.55
1107.26
0.60
2.72
1.41

CeTi-MCM-48
8.28
918.46
0.47
2.73
1.32

a unit cell; b Pore diameter; cWall thickness.
The structural parameters of various samples deduced from the nitrogen sorption isotherms of various samples are summarized in Table 1. Compared with pure MCM-48, the incorporation of the heteroatom resulted in higher pore diameter and unit cell (a0). BET surface and pore size of the materials diminished with the addition of incorporated metals. This may be due to the penetration of Ti and Ce into the pores and shrinkage of surface area and result could be explained by some destruction of metallic MCM-48 pore structure, which is in agreement with the XRD’s results.
The isotherms and the pore size distribution of MCM-48, Ti-MCM-48 and CeTi-MCM-48 samples are shown in Fig. 2. All adsorption isotherms were type IV, typical of mesoporous materials that exhibit distinct increase in P/P0 range from 0.25 to 0.35, showing that the three samples had uniform pore size distribution and large pore volume. Moreover, the N2 adsorption–desorption isotherm of MCM-48 exhibited more distinct increase than that of Ti-MCM-48 and CeTi-MCM-48, which indicated that the mesoporous ordering of MCM-48 sample was better than that of metal doped MCM-48, and a more uniform pore distribution were obtained for MCM-48 mesoporous material.
Fig. 2. Nitrogen adsorption-desorption isotherms of MCM-48, Ti-MCM-48 and CeTi-MCM-48
The N2 adsorption–desorption isotherm of CeTi-MCM-48 exhibited less increase than that of Ti-MCM-48, which indicated that the mesoporous ordering of CeTi-MCM-48 sample was worse than that of Ti-MCM-48. The pore size of MCM-48 mesoporous material was smaller than that of Ti-MCM-48 sample that may be due to the ionic radius of Ti that introduced into the framework is bigger than that of Si, which led to the structure collapse in Ti-MCM-48, so the average pore size of Ti-MCM-48 increased.
Fig. 3. SEM images of Ti-MCM-48 (a), CeTi-MCM-48 (b) and (c) MCM-48
Fig. 3 shows the SEM micrographs of pure MCM-48, Ti-MCM-48 and CeTi-MCM-48. The spherical shape of MCM-48 was shown in Fig. 3c. Ti-MCM-48 and CeTi-MCM-48 morphologies shown in Fig. 3b and c had some distortion from the pure MCM-48 due to the larger size of Ce and Ti. Because of the existence of both Ce and Ti, bimetallic CeTi-MCM-48 exhibited, even more, distortion in the structure.
Fig. 4. TEM images of Ti-MCM-48 (a) and CeTi-MCM-48 (b)
The structural order of iCeiTi-MCM-48 observed by TEM indicated the projection along (100) direction [20], showing the highly ordered cubic structure with uniform mesopores (see Fig. 4a and b). Fortunately, the introduction of Ce and Ti atoms did not destruct the mesoporous structure.
UV-VIS analysis
Diffused UV absorption spectra shows a single peak with a maximum at 250 nm attributed to the ligand to metal charge transfer from O2- to Ti4+ in tetrahedral geometry. CeTi-MCM-48 sample shows strong absorption in ultraviolet region and a broad adsorption in visible region ((= 450 nm) owing to the photosensitizing effect of Ce3+. It is worthy of note that the nanohybrid containing cerium and titanium exhibits a much stronger absorption in both ultraviolet and visible-light regions than Ti-MCM-48 sample.
Fig. 5. Optical absorption edges and diffused UV absorption spectra (inset) of calcined Ti-MCM-48 and CeTi-MCM-48
The band gap energy of the above samples could be calculated by using the equation (αh()n = k(h(–Eg), where α is the absorption coefficient, k is the parameter that related to the effective masses associated with the valence and conduction bands, n is 1/2 for a direct transition, h( is the absorption energy and Eg is the band gap energy [21]. Plotting (αh()1/2 versus h( based on the spectral response in Fig. 5 gives the extrapolated intercept corresponding to the Eg value.
Tabel 2. Band gap energy data of photocatalysts Ti-MCM-48 and CeTi-MCM-48
Sample
Ti-MCM-48
CeTi-MCM-48

Band gap energy (eV)
3.41
3.15


As shown in Table 2, the optical band gap energies of the CeTi-MCM-48 display a red-shifts compared to Ti-MCM-48. The difference of band gap energy between these samples result in a different photocatalytic activity, which is connected with their electron configurations. When the band gap energy corresponding to larger wavelengths becomes smaller, the electron-hole pairs are produced more easily and photocatalytic activity increases [Mureseanu, M., Parvulescu, V., Radu, T., Filip, M., Carja G., Journal of Alloys and Compounds, 648 (2015), 864–873.]
3.2. Photocatalytic activity
4. Conclusions
Ce, Ti doped MCM-48 mesoporous materials were synthesized by a rapid and facile synthesis method, and the effects of different Ce, Ti-doping methods on the structure of pure-silica MCM-48 materials were investigated in this paper. The mesoporous ordering of CeTi-MCM-48 and Ti-MCM- 48 became poor compared with MCM-48. In N2 adsorption–desorption study, the isotherms of the samples MCM-48, Ti-MCM-48 and CeTi-MCM-48 were all typical type IV isotherms, certifying that the three samples had mesoporous structure. However, the pore structure had different changes by different Ti and Ce doping elements. The photocatalytic properties were evaluated by phenol photodegradation under UV irradiation. Thus, these photocatalysts are expected to serve as potential catalysts for advanced oxidative degradation processes of organic pollutants.
Acknowledgments: Authors, M.F, C.M and.V.P. thank the financial support received from Romanian Academy (research programs 2.6.16 IPC) and INFRANANOCHEM PROJECT POS-CCE O 2.2.1, no. 19/01.03.2009.
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