X-Ray diffraction
Figure 1 shows XRD patterns of Mordenite Fe3O4-Fe2O3 samples with different thermal treatments in the air atmosphere. The XRD peaks can be indexed to pure mordenite Zeolite orthorhombic structure crystallographic XRD pattern JCPDS 43–0171, without peaks of other chemical agents. Samples of Zeolite composites present high crystallinity. Also, composite samples show a tendency to decrease crystallinity as a function of increasing the temperature of treatment, which could be related to dealumination in the zeolite.
Scanning electron microscopy and elemental analysis
In Fig. 2, SEM images reveal different structures of studied samples. Figure 2a of Mordenite pristine shows particles < 500 nm in diameter with less order than studied samples with iron oxides. Images of Figs. 2b-2d reveal that the surface of the particles is rough, and the particles get aggregated to a varied extent for the samples of mordenite composite with Fe3O4-Fe2O3 and composite with different thermal treatments of 100, 200, and 300°C in air atmosphere. All composite samples show varied diameters of particles in the cumulus structures, that, aggregated, are related to the thermal treatment. Particle size increases with the temperature of thermal treatment as shown in agglomerate particle indicium.
To know the chemical composition of the mordenite Fe3O4-Fe2O3 composites, elemental chemical analysis was performed on the catalysts synthesized by means of EDS. Results are presented in Table 1. It shows the presence of Al, Si, O, Na and Fe for all synthesized samples with similar proportions with an indication of a typical dealumination process as a function of the temperature of the thermal treatment.
Table 1
Elemental analysis of mordenite Fe3O4-Fe2O3 composites with different thermal treatments for 3h in air atmosphere.
Element | Without thermal treatment | 100°C | 200°C | 300°C |
| W % | At % | W % | At % | W % | At % | W % | At % |
Si | 32.4 | 22.8 | 32.1 | 22.7 | 33.7 | 23.9 | 33.2 | 23.4 |
Al | 4.5 | 3.7 | 4.3 | 3.6 | 3.4 | 2.9 | 3.8 | 3.2 |
Na | 3.6 | 3.8 | 3.8 | 4.1 | 3.9 | 4.2 | 3.1 | 3.3 |
O | 54.7 | 67.9 | 55.2 | 68.9 | 53.5 | 66.9 | 34.9 | 68.2 |
Fe | 4.8 | 1.8 | 4.6 | 1.7 | 5.5 | 2.1 | 5.0 | 1.9 |
Following, to know the Fe distribution in the mordenite support, an elemental mapping for the sample of mordenite Fe3O4-Fe2O3 composite with thermal to 100°C for 3 h in air atmosphere was obtained and results are presented in Fig. 3. Al, Si, O, Na and Fe presence is revealed, with Fe atoms well distributed on the whole mordenite support. This homogenous distribution of Fe atoms is very convenient since they are used as the catalytic active center for the photodegradation of contaminants.
UV-Vis optical diffuse reflectance
The optical properties of materials are related to the electronic structure. The calculated results of diffuse reflectance and band gap energy of studied samples are presented in Fig. 4. As shown in Fig. 4a in the diffuse reflectance spectra, mordenite pristine has high diffuse reflectance for wavelengths > 400 nm, corresponding to visible light, as opposed to mordenite composite samples with Iron oxides, which present low diffuse reflectance for wavelengths > 400 nm corresponding to important visible light absorption, from green to violet. Low optical diffuse reflectance for visible light of mordenite Fe3O4-Fe2O3 composites can be used for the photodegradation reaction of contaminants excited by visible light. Direct optical band gap energy (Eg) was estimated for mordenite pristine and mordenite Fe3O4-Fe2O3 composite with different thermal treatments in air atmosphere using an equation proposed by Kubelka and Munk in 1931:
$$\frac{K}{S}=\frac{(1-R)}{2R}=F\left(R\right)$$
where S, R, K and F are the scattering, reflectance, absorption coefficients and Kubelka-Munk function, respectively. Eg and the absorption coefficient are related through the well-known Tauc relation [40]:
$${\left(\alpha h\nu \right)}^{n}=A(h\nu -{E}_{g})$$
where α is the linear absorption coefficient, \(\nu\) is light frequency, h is the Planck constant, and A is the proportionality constant. The power of the parenthesis, n, is taken equal to 2 for direct band gap materials. When incident radiation scatters in a perfectly diffuse manner, the absorption coefficient K becomes equal to 2α. In this case, considering the scattering coefficient S as constant concerning wavelength, the Kubelka-Munk function is proportional to the absorption coefficient α. Applying the last equation, we obtain the relation [41]:
$${\left[F\left(R\right)h\nu \right]}^{2}=A(h\nu -{E}_{g})$$
The \({\left[F\left(R\right)h\nu \right]}^{2}\) vs. \(h\nu\) (photon energy) graph is plotted, and the energy band gap of the powder sample is extracted easily.
The value of Eg for studied samples was obtained by plotting \({\left[F\left(R\right)h\nu \right]}^{2}\) as function of \(h\nu\) (Fig. 4b) and extrapolation of the linear portion of the curve. Obtained results are presented in Fig. 3b. The band gap values estimated for the mordenite pristine and mordenite Fe3O4-Fe2O3 composites with different thermal treatments in the air atmosphere are ~ 3.4 eV and ~ 2.3 eV, respectively. These results agree with reports of other authors using iron oxide nanoparticles with different organic modifiers [34] or using iron oxide nanoparticles [34, 42]. Moreover, the band gap energy value estimated for the mordenite iron oxides composites show the possibility to promote the generation of free charge carriers by using visible light, improving the photodegradation process.
X-ray photoelectron spectroscopy
Survey XPS was obtained to confirm the chemical composition of mordenite Fe3O4-Fe2O3 composite catalysts. In Fig. 5a can see six signal peaks located at 1072 eV, 103 eV, 532 eV, 75 eV, 285 eV and 712 eV. They correspond to Na 1s, Si 2p, O 1s, Al 2p, C 1s, and Fe 2p3/2, respectively. Also, a high-resolution spectrum of the Fe 2p region was obtained for mordenite composite synthesized without thermal treatment and those with thermal treatment at 100, 200 and 300°C for 3 h in air. They were obtained with the purpose to detect changes in oxidation, according to the chemical equation:
$${Fe}_{3}{O}_{4}+ {O}_{2}\to {Fe}_{2}{O}_{3}\bullet {Fe}_{3}{O}_{4}$$
Figure 5b-e shows high-resolution spectra and deconvolution into six peaks of mordenite Fe3O4-Fe2O3 composite with different thermal oxidation treatments (without treatment, 100°C, 200°C, and 300°C) in air atmosphere. Fe 2p high resolution spectrum is composed of two spectral bands located at 725.3 eV, and 711.9 eV, corresponding to 2p1/2, and 2p3/2 of Fe3+ species, respectively. Also, the other two peaks at a binding energy of 723.8 eV and 710.6 eV are attributed to 2p1/2 and 2p3/2 of the Fe2+ species, respectively. The remaining two weak peaks at 719 eV and 733 eV are satellite peaks. These results and assignments agree with reports of other authors for Fe3O4 and Fe2O3 samples, indicating the successful formation of iron oxide compounds in the mordenite matrix [43–45]. Table 2 presents oxidation states of deconvolution estimates of species percent in studied samples. They were calculated through the integral of deconvoluted signals in individual XPS peaks. Fe3+ peak signal increases with temperature treatment, from 46 at.% to 55 at.%, which could be related to an oxidation process. At the same time, the Fe2+ peak signal decreases with temperature treatment, from 54 at.% to 45 at.%. Then, as a result of thermal treatment, Fe2+ in Fe3O4 partially becomes Fe3+ in Fe2O3 in studied mordenite composite samples, allowing to obtain Fe2O3 and Fe3O4 in different relative concentrations in the same sample through a simple oxidative thermal process.
Table 2
Atomic percent oxidation states of iron oxide species in mordenite Fe3O4-Fe2O3 composite samples as a function of temperature in oxidative thermal treatment.
The temperature in thermal treatment | at.% Fe2+ | at.% Fe3+ |
Without thermal treatment | 54 | 46 |
100°C | 48 | 52 |
200°C | 47 | 53 |
300°C | 45 | 55 |
Surface area analysis
Figure 6 shows N2 adsorption-desorption isotherms of mordenite Fe3O4-Fe2O3 composite a) without thermal treatment, and with thermal treatment in air atmosphere at b) 100°C, c) 200°C, d) 300°C. They exhibit low adsorption at low relative pressure and a hysteresis characteristic of interparticle mesopores of the aggregates present in the zeolites [46–48]. Table 3 shows the textural parameters of the catalysts synthesized. Adsorbed amount slowly decreases with the temperature of thermal treatment, as well as the total pore volume. The BET surface area, SBET, of catalysts diminishes with respect to temperature treatment. Pore diameters increase as a function of temperature, indicating a sintering process, maybe due to a thermal dealumination process as observed above for surface area data. The textural properties are related to organic residue adsorption application and your possible elimination for the remediation process[49, 50].
Table 3
Textural properties of mordenite Fe3O4-Fe2O3 composite with different thermal oxidation treatments.
Catalyst | SBET (cm2/g) | Vpore (cm3/g) | Dpore (nm) |
Without thermal treatment | 296 | 0.063 | 4.6 |
100°C | 290 | 0.060 | 4.7 |
200°C | 274 | 0.055 | 4.9 |
300°C | 267 | 0.051 | 5.0 |
Photocatalyst properties
Photocatalytic methylene blue (MB) degradation by the synthesized composite catalyst was evaluated with visible light excitation. Figure 7 presents the results of UV–visible absorption spectra after photodegradation of MB with visible light for different times of a) mordenite pristine, mordenite Fe3O4-Fe2O3 composites b) without 3h in air thermal treatment, and with 3h in air thermal treatment at c) 100°C, d) 200°C and e) 300°C. f) Mordenite Fe3O4-Fe2O3 composite after 3h thermal treatment at 100°C together with H2O2. Studied catalysts with iron oxides present MB degradation with visible light exposition. Absorbance curves show an important contribution from the adsorption effects of methylene blue in the dark before visible light irradiation exposure on the mordenite composites, organic residues adsorbed is the typical behavior in synthetic zeolites with a high Si/Al rate and is mentioned in the introduction section [22–24]. Figure 8 exhibits relative photocatalytic efficiency and kinetics of photocatalytic degradation of MB with visible light of studied catalysts. mordenite pristine and mordenite Fe3O4-Fe2O3 composite with different thermal treatments in air atmosphere samples. Sample with thermal treatment at 100°C for 3 h in an air atmosphere presents the best photocatalytic activity with almost 70 % MB dgradation after 120 min, without adding a co-catalyst (Fig. 8a). The Fenton effect was proved by adding 2.5 ml of H2O2 to the mordenite composite during the MB photocatalytic degradation [51, 52]. Figures 8a and 8b show how adding H2O2 to samples improves photocatalytic efficiency and kinetics of photocatalytic degradation of MB with visible light, reaching ~ 90 % MB degradatin after 120 min. On the other hand, mordenite pristine and mordenite Fe3O4-Fe2O3 composite with thermal treatment at 300°C do not present considerable photocatalytic activity. Therefore, the samples with iron oxides composites to increasing the temperature of the thermal treatment produced more oxidation in the samples and favored Fe2O3 formation in comparison with Fe3O4, modification of the surface area, crystallinity and morphology, then related with the properties for photocatalytic applications.
The relative concentration as a function of time curves based on the formula for first order kinetic reaction [53, 54] was matched to study the kinetics of MB photodegradation according to the equation:
$$-ln\frac{C}{{C}_{0}}=kt$$
where C0 and C are respective initial and real-time MB concentrations, and k is the first-order degradation rate constant with visible light. Matched lines are shown in Fig. 8b and obtained kinetic constants are depicted in Table 4. The rate constant increases from 0.0043 to 0.00688 min− 1 from no thermal treatment to 100°C thermal treatment, about a 60% increase. Also, the rate constant increases about four times from 0.0043 to 0.016 min− 1 after 100°C thermal treatment and a little portion of hydrogen peroxide addition on the mordenite composites samples. Considering the rate constant reaction of MB degradation of this catalyst, an extrapolation indicates it could eliminate 99%, approximately, after 180 min of visible irradiation.
Table 4
Kinetic constants for MB photodegradation of mordenite Fe3O4-Fe2O3 samples.
Catalyst | R2 | k (min− 1) |
Without thermal treatment | 0.91 | 4.35 x10− 3 |
100°C | 0.98 | 6.88 x10− 3 |
200°C | 0.97 | 2.01 x10− 3 |
300°C | 0.96 | 5.72 x10− 4 |
100°C + H2O2 | 0.98 | 1.64 x10− 2 |
Mordenite pristine | 0.95 | 6.68 x10− 4 |
The stability of the catalyst was investigated by monitoring the catalytic activity during successive cycles of degradation and results are shown in Fig. 9a, where C0 and C are the initial and real-time MB concentrations. Mordenite Fe3O4-Fe2O3 sample with thermal treatment at 100°C for 3 h in an air atmosphere exhibits a very stable photocatalytic performance after five cycles of test without significant loss of activity. Additionally, Fig. 9b shows what thanks to the magnetic properties of the catalysts synthesized could be possible to recover the catalyst through a magnetic field, allowing its reusability in applications of wastewater photodegradation.