Changing the characteristics and properties of zeolite Y and nano-anatase in the formation of a nano-anatase/Y composite with improved photocatalytic and adsorption properties
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Zeolite Y and the NTD/Y nanocomposite, which were synthesized in situ (the addition of zeolite Y to the reaction mixture in the course of the synthesis of NTD by the sulfate method), were studied by a variety of methods. The decrease in the particle size (scanning electron microscopy) and the water content in pores (X-ray powder diffraction study, the full-profile Rietveld method, IR spectroscopy, differential scanning calorimetry), the increase in OH groups content and the decrease in the water content on the surface of zeolite (X-ray photoelectron spectroscopy) in the composition of NTD/Y compared to the initial zeolite Y were all established. A larger specific surface area of NTD/Y (Brunauer–Emmet–Teller method) compared to the initial zeolite Y is due to the fact that zeolite Y in the nanocomposite contains a smaller amount of water because of the synthesis conditions and the presence of nanocrystalline NTD on the surface of zeolite particles. It was found that NTD/Y nanocomposite exhibits a higher photocatalytic activity in the model decomposition reaction of methyl orange under UV and adsorption capacity for the extraction of P(V) and As(V) ions from aqueous media compared to the initial zeolite and pure NTD obtained under the same conditions, which differs from NTD/Y by the larger particle size, the smaller specific surface and the smaller content of OH groups and water on the surface. The role of Bronsted and Lewis centers in the realization of properties is discussed.
KeywordsZeolite Y Nanosized titanium dioxide Nanocomposite Crystalline structure and microstructure Adsorption and photocatalytic properties
Zeolites are aluminosilicates having a high porous specific surface area and the controlled pore, channel, and cavity sizes. Zeolites Y have different forms: NaY zeolite synthesized from sodium aluminosilicate gels (Breck 1974), NH4+ and HY zeolites prepared from zeolite Y by ion exchange with NH4Cl (Liu et al. 1992) or by ion exchange of the initial compensating cations with ammonium ions or polyvalent metal cations followed by the thermal treatment (Serykh 2014), and USY zeolite characterized by a large bulk Si/Al ratio (Si/Al > 5), which was subjected to several hydrothermal treatments by Park et al. (2003). Different forms of zeolites Y are characterized by high acid resistance, thermal stability, and catalytic activity in cracking, isomerization, alkylation, and so on. Due to high reactivity, these zeolites are of high demand in oil refining and petrochemistry, in particular, as an active component of cracking catalysts. Besides, zeolites Y of different compositions and modules have adsorption (Galhotra et al. 2009; Klein et al. 1994) and antibacterial properties (Kubota et al. 2008).
Nanosized titanium dioxides (NTD) are photostable and environmentally safe, have high quantum yield for the UV photocatalytic oxidation, and exhibit adsorption properties due to the formation of highly reactive surface hydroxyl groups (Demina et al. 2014; Dadachov 2006). In addition, NTD show a bactericidal effect against pathogenic microorganisms (Ismagilov et al. 2009).
According to the literature data, NTD/Y nanocomposites combine the properties of the initial components: photocatalytic (Domoroshchina et al. 2017; Kuwahara et al. 2012; Wang et al. 2008, 2015; Kamegawa et al. 2013; Jansson et al. 2015; Maraschi et al. 2014), adsorption (Domoroshchina et al. 2017; Kravchenko et al. 2016; Zendehdel et al. 2014), and bactericidal (Domoroshchina et al. 2017).
Photocatalytic properties of nanocomposites NTD/Y on the decomposition of organic pollutants from the gas phase and the aqueous medium are most studied: photocatalytic degradation of 2-propanol in water (Kuwahara et al. 2012; Kamegawa et al. 2013), C.I. Basic Violet 10 in suspension (Wang et al. 2008), marbofloxacin and enrofloxacin in water (Maraschi et al. 2014), Methyl Orange dye in water (Domoroshchina et al. 2017; Wang et al. 2015), formaldehyde and trichloroethylene in gas phase under UV (Jansson et al. 2015) and the degradation of 2-propanol in water under visible light (Kamegawa et al. 2013).
Adsorption properties of NTD/Y for the extraction of the following dyes were studied: methylene blue (Zendehdel et al. 2014) and phenosafranine dye (Easwaramoorthi and Natarajan 2009), which are adsorbed on the surface of NTD/Y particles, and amaranth dye (Alwash et al. 2013), in relation to which NTD/Y does not exhibit adsorption activity. The adsorption of P(V) ions from aqueous media in the presence of NTD/Y nanocomposite was studied by Domoroshchina et al. (2017) and Kravchenko et al. (2016).
Nanocomposites NTD/zeolite can be conditionally divided into two groups, depending on the level and nature of the interaction of zeolite and NTD. Group I includes nanocomposites that are a mixture of NTD and zeolite with altered microstructural characteristics (NTD—polycrystalline, nanocrystalline or amorphous and its localization; the morphology and size of all levels of zeolite and NTD particles) in the nanocomposite composition. Group II includes nanocomposites, the formation of which changed the characteristics of the crystal structure of NTD or/and zeolite. The characteristics of each group separately, or of group I and group II together can be realized in one composite and the physical–chemical properties of the nanocomposite (the composition of the surface; specific surface area, pore size, etc.) are changed.
The level and nature of the interaction of components depend on the methods and conditions for obtaining nanocomposites and determine their properties.
Ion exchange between the Na+ and NH4+ forms of zeolite Y in an aqueous solution of (NH4)2(TiNO)(C2O4)2 followed by calcination at 400–550 °C (nanocrystalline NTD in NTD/Y) (Liu et al. 1992; Zhang et al. 1995; Alwash et al. 2013; Easwaramoorthi and Natarajan 2009);
Wet impregnation of zeolite NaY (Alwash et al. 2013; Kuwahara et al. 2012; Wang et al. 2008) or HY (Kamegawa et al. 2013) with a TiO2 sol followed by evaporation of the liquid, drying, and calcination at 350–600 °C (amorphous (Alwash et al. 2013) or nanocrystalline (Wang et al. 2008) NTD in NTD/Y);
Mechanical mixing of NTD, which was synthesized by the sol–gel method (Maraschi et al. 2014), via the hydrolysis of titanium isopropoxide (Xu and Langford 1997) or a solution of titanium tetraisopropoxide in ethanol (Ito et al. 2014), with HY zeolite (Maraschi et al. 2014) or NaY zeolite (Xu and Langford 1997; Ito et al. 2014); the separation of the precipitate, washing, drying, and annealing at 350–450 °C [nanocrystalline NTD in NTD/Y (Xu and Langford 1997; Ito et al. 2014)];
A sol–gel method using titanate n-butyl acetate followed by annealing at 400 °C (Easwaramoorthi and Natarajan 2009; Wang et al. 2015), the degree of crystallinity of the composite decreases with an increase in TiO2 loading (Easwaramoorthi and Natarajan 2009); amorphous NTD in NTD/Y (Wang et al. 2015);
Method in situ using TiOSO4.xH2SO4.yH2O as precursor (nanocrystalline NTD in NTD/Y) (Kravchenko et al. 2016);
Modified method of cold impregnation: co-dispersion of NTD (Hombifine N with anatase or sample with η-phase) and zeolite Y in a dilute KOH solution with a subsequent microwave treatment (amorphous NTD in η-phase/Y, nanocrystalline NTD in Hombifine N/Y) (Kravchenko et al. 2016).
In NTD/Y nanocomposites synthesized by the ion exchange reaction (Liu et al. 1992; Zhang et al. 1995; Alwash et al. 2013), nanocrystalline NTD occupies zeolite cavities (group II), as it was demonstrated by X-ray photoelectron spectroscopy (XPS), IR spectroscopy, X-ray powder diffraction, scanning (SEM) and transmission electron microscopy (TEM). In NTD/Y synthesized by wet impregnation (Alwash et al. 2013; Jansson et al. 2015) or mechanical mixing techniques (Maraschi et al. 2014), amorphous (Alwash et al. 2013), and presumably, nano-anatase (Jansson et al. 2015) are present only on the surface of the zeolite (group I) [Energy-dispersive X-ray spectroscopy (EDS), X-ray powder diffraction]. The application of the sol–gel method results in the formation of samples containing amorphous NTD both on the surface and within the zeolite framework (groups I + II) [IR spectroscopy, X-ray powder diffraction, SEM, Brunauer–Emmett–Teller method (BET)] (Wang et al. 2015).
The location of NTD particles in the NTD/Y nanocomposite depends not only on the method of synthesis and additional treatment (microwave and/or high-temperature annealing), which facilitates the incorporation of nanocrystalline NTD into the zeolite framework (group II) (Liu et al. 1992; Zhang et al. 1995; Alwash et al. 2013), but also on the amount of the introduced NTD and its characteristics, in particular, the particle size and the state of the starting NTD (amorphous or nanocrystalline). An increase in the NTD loading leads to the filling of the zeolite pores and the partial location of amorphous NTD on the zeolite surface (groups I + II) (Wang et al. 2015), whereas smaller NTD particles promote a more uniform distribution of NTD over the zeolite surface (group I) (Maraschi et al. 2014) or the insertion of nanocrystalline NTD into the zeolite framework (group II) (Liu et al. 1992; Zhang et al. 1995). The amorphous state of the starting NTD facilitates a more uniform distribution of nanoparticles on the zeolite surface (group I) compared to nanocrystalline Degussa P25 (Maraschi et al. 2014).
Analysis of the literature data indicates the relationship between the photocatalytic activity of NTD/Y and the modification of NTD (Domoroshchina et al. 2017), particles size (Maraschi et al. 2014) and the position of NTD in the nanocomposite (on the surface of the zeolite or in its pores) (Easwaramoorthi and Natarajan 2009), the zeolite modulus (Alwash et al. 2013), the specific surface (Zhang et al. 2013) and hydrophilicity/hydrophobicity (Alwash et al. 2013) of the nanocomposite NTD/Y as well as the relationship between adsorption properties of nanocomposite and its hydrophilicity/hydrophobicity (Alwash et al. 2013) and the position of NTD in NTD/Y (Easwaramoorthi and Natarajan 2009).
The goal of the present study is to study the structural behavior of zeolite Y and NTD in the in situ synthesized NTD/Y nanocomposite and substantiate the observed change in NTD/Y properties compared to the initial Y and NTD.
Zeolite Y, which was used as a template for the NTD/Y nanocomposite, has the protonated form HNaY (partial replacement of Na+ with H+) with the formula Na(H)58[Al58Si134O384](H2O)240 (Si/Al = 2.3). The zeolite Y was synthesized in the Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, by a procedure described by Ginter et al. (1992).
The NTD/Y nanocomposites were synthesized in situ (Kravchenko et al. 2016) by the addition of zeolite Y (0.25 g) to the reaction mixture in the course of the synthesis of NTD via short-term hydrolysis of titanyl sulfate, TiOSO4.xH2SO4.yH2O, (7.35 g) with the subsequent heating to 95(± 2) °C, separation of the resulting solid phase by vacuum filtration, washing with water and acetone, and drying in a drying oven for 30 min at t = 90 °C.
X-ray powder diffraction analysis. The X-ray powder diffraction patterns were collected in a rotation mode on a HZG-4 diffractometer (a plane graphite monochromator, CuKα radiation, diffracted beam) in a step scan mode (the count time was 10 s per step, the step size was 0.02°, the 2θ angle range was 2°–50° or 80°). The qualitative phase analysis of samples with nanocomposites was carried out using the database PCPDFWIN PDF-2 and (Treacy and Higgins 2007). The sizes of NTD crystallites in NTD/Y (D, nm) are determined from the following formula: D = 0.9λ/βcosθ, where λ is the wavelength, CuKα = 1.54051 Å, 2θ ~ 25°, and β is the integrated peak width. The standard deviation is 5%.
High-accuracy synchrotron measurements of Y and NTD/Y samples were performed on the high-resolution powder diffraction beamline ID22 at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). The beamline is equipped with a water-cooled Si 111 silicon double-crystal monochromator and a Si 111 analyzer (Fitch 2004). A powdered sample was loaded into a 1-mm diameter thin-walled borosilicate glass capillary, which was rotated at a rate of 1200 revolutions per minute for better counting statistics. The NIST Si 640c reference material was used as a standard for calibration of the goniometer and the refinement of the X-ray wavelength [λ = 0.399927(13) Å]. The X-ray diffraction data were collected in the 2θ angle range of 1–27° with a step of 0.002°. The structural characteristics of the samples were determined by the Rietveld refinement using the MRIA program (Zlokazov and Chernyshev 1992). Diffraction profiles were modeled by the Voigt function (Toraya 1986) taking into account anisotropic line broadening (Popa 1998).
The X-ray powder diffraction patterns were processed and the crystallite size of the NTD component of the nanocomposite was calculated (D = 0.9λ/βcosθ, where λ = 1.54051 Å, 2θ ~ 25°, β is the integral peak width, the standard deviation is 5%) using the program for the processing of X-ray diffraction patterns of nanosized and amorphous compounds and calculations of characteristics of the substructure (Kuz’micheva et al. 2016).
IR spectra were recorded on a Bruker EQUINOX 55 Fourier-transform infrared spectrometer with a 2 cm−1 resolution in the 400–4000 cm−1 frequency range.
Scanning electron microscopy (SEM) studies were carried out on a high-resolution JSM 7500F electron microscope using the procedures described by Obolenskaya et al. (2012).
The sorption capacities with reference to nitrogen were measured by the Brunauer–Emmet–Teller (BET) method using the procedures described by Kravchenko et al. (2016).
X-ray photoelectron spectroscopy (XPS) was recorded on a PHOIBOS 150 MCD spectrometer (Al Ka radiation) under vacuum (3 × 10−9 Torr) at a pass energy of 30 and 10 eV for panoramic spectrum and separate lines, respectively. The power of the source was 225 W. The step size was 1 eV for panoramic spectrum and 0.1 eV for separate lines. All binding energies were calibrated with contaminant carbon (C 1 s = 284.6 eV) as a reference.
Differential scanning calorimetry (DSC). The thermal effects observed under dynamic heating conditions were measured on a Netzsch DSC 204 Phoenix differential scanning calorimeter in a dry gas atmosphere (argon) at a flow rate of 100 mL/min in the temperature range of 20–600 °C at a temperature scan rate of 10 °C/min under an inert atmosphere. The weight of powdered samples was 7–10 mg. The samples were placed in standard aluminum crucibles with a pierced lid. The results were processed using the Netzsch Proteus software.
Photocatalytic activity (PCA) was studied on the model decomposition reaction of methyl orange (MeO, CAS no. 547-58-0) organic dyes according to (Domoroshchina et al. 2017).
Adsorption properties. Study of the adsorption capacity of the samples was performed according to (Kravchenko et al. 2016) on an aqueous solution with concentration of phosphorus 10 mg/l (Cinitial) prepared from the standard solutions of P(V) and As(V).
Results and discussion
Crystallographic parameters of zeolite Y and the NTD/Y nanocomposite
Biso (T), Å
Biso (O), Å
P (s.o.f.) (Ow1a)
P (s.o.f.) (Ow2a)
P (s.o.f.) (Ow3a)
P (s.o.f). (Ow4a)
χ 2 /Rwp/Rexp
In the course of the refinement, hard constraints were imposed on the interatomic T–O and O–O distances (< 2.7 Å), and therefore, the atomic coordinates of the zeolite framework changed only slightly.
In the first step of the refinement, it was assumed that no small molecules are present in the pores of zeolite Y (i.e., anhydrous zeolite). However, the value χ2 = 91 (quality of the refinement) shows that this assumption is not true.
The increase in Biso(T) indicates a decrease in the total occupation of this position, which is due to a decrease in the Si/Al modulus, which is indeed smaller (Si/Al = 2.3) than in the initial structure (Si/Al = 5) (Jeanjean et al. 1989). The refinement of the mixed occupation of Si/Al turned out to be uninformative due to the proximity of atomic numbers, and as a consequence, the atomic scattering curves. R factors were practically unchanged in the process of various models refinement.
As can be seen from the comparison of the unit cell parameters of zeolite Y and the NTD/Y nanocomposite (Table 1), the unit cell parameter for the nanocomposite is substantially smaller than that for zeolite Y, which can be attributed only to a decrease in the amount of water (the diameter of the water molecule is ~ 0.3 nm) in the zeolite pores with diameter 0.74 nm (Fig. 1). The synthesis of the nanocomposite under the above-mentioned conditions is not accompanied by the replacement of Al ions (r Al IV = 0.39 Å) with Ti ions (r Ti IV = 0.42 Å) in the T position (T = Si or Al). If this would be the case, this would lead to a substantial decrease in the displacement parameter Biso(T) and an increase in the unit cell parameter, which was not observed after the structure refinement (Table 1). Thus, there are water molecules in the pores of the initial zeolite Y (it is possible that a certain amount remains in the zeolite Y in the nanocomposite NTD/Y).
δ(O–Si–O) + δ(O–Al–O)
δ(TO4) (T = Si, Al), ν(TiO)
νs(SiOSi) + νs(SiOAl), ν(TiO)
614, 667, 718sh
1084, 1174, 1206
Presence of residual structure-forming organic additive [δ(CH2)]
δ(H2O) H–O–H bending vibrations of zeolite water molecules
Presence of residual structure-forming organic additive [ν(CO)]
H–O–H bending vibrations
O–H vibrations of zeolite water
In some studies (Zhang et al. 1995; Zhao et al. 2009; Smirnov and Van de Graaf 1996), this band was assigned to Ti–O–Si symmetric stretching vibrations assuming the replacement of Si atoms with Ti (Zhang et al. 1995); alternatively (Chong et al. 2015), this fact was attributed to the location of NTD on the surface of zeolite without its inclusion in the framework. The X-ray powder diffraction data combined with the IR spectroscopic data suggest that the band at 960 cm−1 in the spectrum of NTD/Y can be assigned to NTD particles on the zeolite surface (group I).
The intensity redistribution for the bands in the region of 400–800 cm−1 for NTD/Y is due to the overlapping of Ti–O stretching bands and vibrational bands of SiO4 and AlO4 tetrahedra (Alwash et al. 2013; Ba-Abbad et al. 2012). The observed shift of the bands in this region may be due to intermolecular interactions (Fedorova et al. 2015). The bands at ~ 460 and ~ 480 cm−1 correspond to bending vibrations of the six-membered rings (hexagonal prisms) of zeolite Y. A decrease in the intensity of the band at ~ 460 cm−1 and the disappearance of the band at 484 cm−1 in the IR spectrum of NTD/Y are apparently attributed to a change in the Si/Al ratio (Knops-Gerrits et al. 1997). This experimental fact does not contradict the results of a full-profile analysis of the initial zeolite Y and zeolite Y in the composition of the nanocomposite NTD/Y (Table 1).
The band at 1628 cm−1 responsible for H–O–H bending vibrations of zeolite water molecules (Kostrikin 2015; Saikia and Parthasarathy 2010) is identical in the spectra of both Y and NTD/Y. The bands at 3150–3630 cm−1 are also associated with zeolite water and correspond to antisymmetric and symmetric stretching vibrations of isolated OH groups (Kostrikin 2015). The intensity of the band at ~ 3400 cm−1 in the spectrum of Y is equal to that for NTD/Y, but this band in the spectrum of NTD/Y is shifted to lower frequencies accompanied by a slight broadening. The shift of the maxima of (OH) stretching vibrations to lower frequencies in the spectrum of NTD/Y attests to the involvement of OH groups in the formation of hydrogen bonds of different strength (Kostrikin 2015). The band at 3620–3630 cm−1 is assigned to O–H vibrations of zeolite water located in the cavities of the framework (Kostrikin 2015). The absence of this band in the spectrum of NTD/Y is in good agreement with the results of the Rietveld refinement of the crystal structure (Table 1): a decrease in the amount of water in the pores of the zeolite Y component of the NTD/Y nanocomposite.
The local O1s spectrum of pure NTD (Fig. 6b) contains a wide asymmetric band formed by the imposition of two bands: ~ 530.1 and ~ 532.5 eV corresponding to Ti–O bonds (Wagner et al. 2003) and OH groups (Kuz’micheva et al. 2010), respectively.
The local spectra of the initial Y and NTD/Y (Fig. 6b) contain components ~ 530.1 eV (T–O or/and Ti–O bands) and ~ 532.5 eV (less conspicuous for Y), which indicates a larger number of OH groups on NTD/Y surface compared to the initial Y. The additional components ~ 537.1 eV on Y spectrum and ~ 536.7 eV on NTD/Y one (the shift is due to the sample charging) are responsible for surface-adsorbed water molecules (Wagner et al. 2003; Sanjines et al. 1994) and characterize the chemical interaction between the initial zeolite and NTD in the formation of NTD/Y (Zhang et al. 2013).
The endothermic effects in the temperature ranges of 20–200 and 490–600 °C are associated with desorption of water molecules. The asymmetry of the first endothermic peak for the starting zeolite Y (Fig. 7a) is attributed to the overlapping of two endothermic effects: a decrease in the amount of physically adsorbed surface water (68.4 °C), which is easily removed upon heating, and water of crystallization (in the case under consideration, zeolite water), which is released at temperatures below 300° (Shishelova et al. 2010) (the peak at 180 °C) (Fig. 7b).
In the DSC thermogram of the NTD/Y nanocomposite, the first endothermic peak corresponds to desorption of physically adsorbed surface water (the peak at 70.3 °C) (Fig. 7a). The area under the first endothermic peak of zeolite Y is larger than the area under the corresponding peak of NTD/Y; the thermal effect is 184.7 and 157 J/g for Y and NTD/Y, respectively. This is evidence that the amount of water (both surface water and crystallization or zeolite water) in zeolite Y is larger compared to the NTD/Y nanocomposite. These results are in agreement with the data of IR spectroscopy.
The weak endothermic effects at 490 °C for NTD/Y and at 520 °C for Y are apparently attributed to elimination of constitutional water, which is present in the crystal structure in the form of OH1− ions and which is liberated in the temperature range from 300 to 1000 °C.
At temperatures above 500 °C, the band at ~ 1600 cm−1 disappears in the IR spectra and the band at 3620–3630 cm−1, which corresponds to O–H stretching vibrations of zeolite water located in the cavities of zeolite, has a lower intensity (Dikii et al. 2009; Evdokimova and Lankin 2015).
The in situ synthesis of NTD/Y nanocomposites was performed at 95(± 2) °C, at which the crystallization (zeolite) water is not released from zeolite, but physically absorbed surface water can be eliminated. A decrease in the water content in NTD/Y compared to Y (Table 1) can be attributed to the synthesis conditions (heating to t = 93(± 2) °C).
The sorption capacities. The specific surface area of the NTD/Y nanocomposite (SBET = 552.8 m2/g) was found to be larger than that of the initial zeolite Y (490.6 m2/g). The factors responsible for changes in the specific surface area of the NTD/Y nanocomposite were considered (Wang et al. 2008, 2015; Maraschi et al. 2014). The specific surface area of the “amorphous NTD/Y” nanocomposite prepared by the sol–gel method is smaller than that of the initial zeolite Y. In the study (Wang et al. 2015), a decrease in the specific surface area of the nanocomposite was attributed to small sizes of amorphous NTD particles, which penetrate into the zeolite framework. With an increase in the NTD loading, the pores are filled with NTD particles (the size of the particles deposited on the surface of the zeolite estimated by SEM is 20–100 nm), and then the deposition of amorphous NTD occurs on the surface of zeolite, which is accompanied by an increase in the specific surface area of NTD/Y (Wang et al. 2015).
The SBET value and the pore volume of the “nano-anatase/Y” nanocomposite (prepared by means of the wet impregnation method) were found to decrease with an increase in the TiO2 loading (Wang et al. 2008). This is attributed to dispersion of NTD on the surface of zeolite and the fact that the zeolite pores are partially blocked by NTD particles, the size of which increases with an increase in the TiO2 loading due to their aggregation.
Decrease in the specific surface area of sample 1 (SBET = 135 m2/g) with the initial Degussa P25 (anatase + rutile);
Increase in the specific surface area of sample 2 (SBET = 228 m2/g) with the initial amorphous TiO2 and nano-anatase in NTD/Y upon annealing of the nanocomposite at 350 °C;
Increase in the specific surface area of sample 3 (SBET = 215 m2/g) with the initial nano-anatase and nano-anatase in NTD/Y.
For the in situ synthesized NTD/Y nanocomposite under consideration, a decrease in the amount of water molecules in the pores of NTD/Y is one of the factors responsible for an increase in its specific surface area compared to the initial zeolite Y. The total surface of all particles in the nanocomposite is also increased taking into account their surface roughness [172.6 m2/g for NTD/Y and 30–50 m2/g for the initial zeolite Y (Kravchenko et al. 2016)]. Besides, it cannot be ruled out that nanocrystalline NTD with the anatase structure in the NTD/Y nanocomposite contributes to an increase in the specific surface area, as it was observed for sample 3 (Maraschi et al. 2014) considered above.
Thereby, the NTD/Y nanocomposite differs from the initial Y zeolite with smaller particle sizes, a large number of active OH groups and lower water content on the surface, a smaller content of water molecules in the pores of the zeolite and a larger specific surface area. NTD/Y differs from NTD with a smaller particle size, larger specific surface area and a large number of active OH groups and water content on the nanocomposite surface.
Adsorption properties. The adsorption properties of NTD are due to the surface hydroxyl groups with a high reactivity, which are formed at crystallite sizes (coherent scattering regions) below 10 nm at their amount close to 100% (Dadachov 2006).
The adsorption capacity of zeolites is affected by its modulus (Si/Al): with a decrease in the modulus, the hydrophilic properties of the zeolite are enhanced (Alwash et al. 2013) and the total concentration of the Bronsted acid centers increases due to the formation of protons for the conservation of the system electroneutrality witch most likely determines the adsorption properties of zeolite Y and NTD/Y under investigation.
The adsorption capacity of NTD/Y nanocomposite increases compared to the initial zeolite Y: RP(V) = 98.07% and RAs(V) = 99.56% (Fig. 9a), moreover, the degree of these ions extraction by pure TiO2 (D = 5.3(3) nm; SBET = 39.7 m2/g) is smaller compared to NTD/Y: RP(V) = 94.62%, RAs(V) = 96.34%.
Photocatalytic properties. The photocatalytic activity (PCA) of NTD/Y in the model degradation reaction of Methyl Orange under UV light is higher compared to initial Y and pure NDT (Fig. 9b).
The photocatalytic activity of NTD under UV depends on the content of surface hydroxyl groups responsible for the adsorption of the dye on the surface and considered the main active species involved in the dye degradation process (Alwash et al. 2013; Grätzel 2003).
The role of zeolite in increasing PCA of nanocomposites with NTD is to delocalize the excited electrons of titanium dioxide, which allows minimizing the rate of electron–hole pairs’ recombination (Lazau et al. 2011).
According to (Salama et al. 2007), the PCA of nanocomposites increases compared to NTD and zeolites with increasing Lewis acidity centers, as has been revealed for NTD/MOR composites. By adjusting the Si/Al ratio, the nature of the M cations, the degree of substitution of Mn+ by H+ and the depth of removal of the hydration water, it is possible to vary the acidity of the zeolites within a very wide range, and hence to control their photocatalytic activity.
The high activity of the NTD/Y photocatalyst compared to the initial components (zeolite Y and NTD) is due to the high specific surface area of the nanocomposite and the high dispersity of the nano-anatase on its surface (Fig. 9b). This conclusion is consistent with the literature data (Liu et al. 1992; Zhang et al. 2013; Guesh et al. 2016).
It should be noted that incorporation of NTD particles into the zeolite pores during the synthesis of NDT/Y nanocomposite increases the photocatalytic activity to a greater extent than the presence of NTD on the zeolite surface in nanocomposite (Easwaramoorthi and Natarajan 2009). The synthesis of nanocomposite by the ion exchange method, and in some cases, high-temperature annealing contributes to the presence of NTD particles in the pores of zeolite, which has not been envisaged in this work.
Study by X-ray diffraction and IR spectroscopy demonstrated that in the in situ synthesized nanocomposite based on zeolite Y, nanocrystalline anatase is present on the surface of the zeolite and is not incorporated into the framework (X-ray powder diffraction, SEM).
A smaller amount of water (X-ray powder diffraction, IR spectroscopy, DSC) in the pores of the zeolite Y component of nano-anatase/Y compared to the initial Y was found. This process leads to an increase in the specific surface area. The size of the faceted particles of zeolite Y (NSEM ≤ 750 nm) exceeds the size of the spherical nanocomposite particles forming associates (NSEM ≤ 300 nm) on the surface of faceted shapes.
The presence of anatase nanoparticles with crystallite size D = 5.7(3) nm on the surface of zeolite particles, the increase in OH groups content and the decrease in the water content on NTD/Y surface compared to Y (XPS) and the decrease in the water content in NTD/Y pores compared to Y (X-ray powder diffraction, IR spectroscopy, DSC) were established.
The increase in photocatalytic activity in the model degradation reaction of MeO under UV light is due to a higher specific surface area and a smaller particle size in NTD/Y compared to Y and NTD, a larger content of hydroxyl groups and a smaller water content on NTD/Y surface compared to Y and a larger content of hydroxyl groups and a larger water content on NTD/Y surface compared to NTD.
The increase in adsorption capacity of nanocomposite nano-anatase/Y for the extraction of P(V) and As(V) ions from aqueous media compared to the initial Y and pure NTD is caused by a higher specific surface area of NTD/Y and a large content of hydroxyl groups on NTD/Y surface compared to pure NTD and Y.
All these facts indicate the predominant role of the specific surface area and the content of active groups on NTD/Y surface in the realization of photocatalytic activity in the decomposition reaction of MeO under UV and adsorption activity of nanocomposite.
This study was financially supported by RFBR (research project No. 15-03-01289). The X-ray diffraction part of this research was supported by the Ministry of Education and Science of the Russian Federation (Grant no. RFMEFI61616X0069). We also thank the ESRF for access to the beamline ID22 (experiment MA-3313).
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