Abstract
A new class of porphyrin(Pp)/Fe co-loaded TiO2 composites opportunely prepared by impregnation of [5,10,15,20-tetra(4-tert-butylphenyl)] porphyrin (H2Pp) or Cu(II)[5,10,15,20-tetra(4-tert-butylphenyl)] porphyrin (CuPp) onto Fe-loaded TiO2 particles showed high activities by carrying out the degradation of 4-nitrophenol (4-NP) as probe reaction in aqueous suspension under heterogeneous photo-Fenton-like reactions by using UV-visible light. The combination of porphyrin-Fe-TiO2 in the presence of H2O2 showed to be more efficient than the simple bare TiO2 or Fe-TiO2.
1. Introduction
Nowadays, due to the increasing presence of refractory molecules in the wastewater streams, it is important to develop new technologies to degrade such recalcitrant pollutant molecules into smaller innocuous ones. For this reason efficient oxidation processes operating under environmentally friendly conditions are needed [1]. As well known, Fenton chemistry encompasses reactions of hydrogen peroxide in the presence of iron to generate highly reactive species such as the hydroxyl radical and possibly others.
In the last few years, Fenton-like reactions, in combination with other advanced oxidation processes, are assuming fundamental and practical perspectives in water treatment processes [2, 3]. The combination of various technologies, in fact, is often effective to achieve a complete mineralization of the pollutant(s) present in the starting effluents because many stable products of environmental concern can be persistent after the treatment by Fenton reaction.
Recently, the utilization of TiO2 as catalyst for the photooxidation of organic pollutants in water is becoming a relevant topic in view of a possible application in economically advantageous and environmentally friendly processes not only performed with the aim to abate pollutants but also for synthetic purposes [4–8].
Various advanced oxidation technologies have been used in the presence of TiO2, H2O2, and irradiation to enhance the efficiency of the overall photodegradation process [9–12]. Also, in the last years, dye-sensitized TiO2-based materials have been employed for improving the efficiency of energy light conversion towards photocatalytic processes [13–19].
In this work the design of novel composites metal free or Cu-porphyrin/Fe co-loaded TiO2 as well as their application as catalytic systems for photoassisted heterogeneous Fenton-like reactions has been reported. In particular, we demonstrated that the presence of porphyrins and Fe species co-loaded onto the TiO2 surface along with H2O2 in the reacting medium is beneficial for 4-nitrophenol (4-NP) photodegradation in aqueous medium.
2. Experimental
2.1. Materials
4-Nitrophenol, used without further purification, Fe(NO3)3·9H2O, and hydrogen peroxide solution (30% wt.) were purchased from Aldrich. Solutions were prepared dissolving the required quantity of 4-NP in water obtained by a New Human Power I water purification system.
TiO2 in the microcrystalline phase of anatase, specific surface area 8 m2 g−1, was kindly provided by Tioxide Huntsman.
The synthesis of the [5,10,15,20-tetra(4-tert-butylphenyl)]porphyrin (H2Pp) and of the Cu(II) [5,10,15,20-tetra(4-tert-butylphenyl)]porphyrin (CuPp) was carried out as reported previously [20].
2.2. Preparation of the Hybrid H2Pp-Fe-TiO2 and CuPp-Fe-TiO2 Catalysts
The 1% wt Fe-TiO2 powder, successively indicated as Fe-TiO2, was prepared by wet impregnation of TiO2 with aqueous solutions of Fe(NO3)3·9H2O by an incipient wetness impregnation followed by a drying process at 393 K and final calcination at 350°C for 5 h as described in a previous work [21].
Fe-loaded TiO2 powder impregnated with functionalized metal-free porphyrin and Cu(II)-porphyrin Fe-TiO2, successively indicated as H2Pp-Fe-TiO2 and CuPp-Fe-TiO2, used as photocatalytic systems, were prepared by impregnation of Fe-TiO2 powders with 6 μmol/g of sensitizer (H2Pp or CuPp) per gram of TiO2. The opportune amount of sensitizers was dissolved in 15 ml of CHCl3 (or CH2Cl2), and 2 g of finely ground Fe-TiO2 was added to this solution.
The mixture was stirred for 3-4 h, and the solvent was removed under vacuum.
2.3. Characterizations
The morphology of the Fe-TiO2 photocatalysts was studied by using a scanning electron microscopy (SEM) Zeiss Evo 40. X-ray diffraction patterns of all of the samples were performed by using a powder diffractometer (model Ultima+ Rigaku) equipped with CuKα radiation from 20° to 80°. The accelerating voltage and current used were 40 kV and 26 mA, respectively. The diffuse reflectance spectra (DRS) of photocatalysts were recorded in the range 200–800 nm by using a Varian CARY 100 Scan UV-vis spectrophotometer equipped with a diffuse reflectance integration sphere.
2.4. Photocatalytic Measurements
The set-up used for the photocatalytic experiments is reported in Figure 1 and consists of a 500 ml glass Pyrex reactor containing 4-NP solution/photocatalyst suspension placed in the center of a wood box and irradiated from the top with a 300 W UV-visible lamp (SANOLUX HRC) emitting in the wavelength range 300–900 nm. The lamp was housed in the upper window of the box at 14 cm distance from the reactor, and the radiant flux measured by a DELTA OHM Photo-Radiometer HD 9221, equipped with a sensor LP 9221 PHOT, was 340 W/m2 in the 200–950 nm range. The emission spectrum of the lamp is reported in Figure 2. Oxygenation was ensured by bubbling air in the suspension during the experiments.
The novel hybrid composite photocatalysts based on the metal free and Cu porphyrins onto the Fe-loaded TiO2 have been used to test the degradation of 4-NP as a probe pollutant molecule.
The removal of 4-NP during the reaction processes has been evaluated as the ratio of the concentrations / versus time. and were calculated measuring the absorbance values and of 4-NP at 317 nm at time and at the initial time , respectively, by means of a UV-vis spectrophotometer (Cary 100 Scan, VARIAN).
The extent of mineralization of the 4-NP was determined on the basis of total organic carbon measurement using a TOC analyzer (IL550 TOC-TN, HACH-LANGE).
The amount of Fe3+ in solution was measured according to the UNI-EN-ISO 11885 method using an ICP spectrometer THERMO SCIENTIFIC iCAP 6000 SERIES.
3. Results and Discussion
3.1. Synthesis and Characterization of the Photocatalysts
Syntheses of the metal-free porphyrin [5,10,15,20-tetra(4-tert-butylphenyl)porphyrin], H2Pp, and its Cu(II) complex, CuPp, were performed according to a procedure already reported in the literature [20].
Also, the Fe-TiO2 composite, used as the support for the sensitizers H2Pp and CuPp, was prepared by a wet impregnation process followed by dryness and calcination as described in a previous work [21].
Further, the novel composites used as the photocatalysts in this work were prepared by impregnation of the Fe-TiO2 powder with 6 μmol/g of sensitizers (H2Pp or CuPp) per gram of Fe-TiO2 as described in the experimental section, and they were indicated, respectively, as H2Pp-Fe-TiO2 and CuPp-Fe-TiO2.
Analysis of SEM picture (Figure 3) shows that the Fe-TiO2 (Figure 3(b)) and CuPp-Fe-TiO2 (Figure 3(c)) samples have a higher number of irregular shaped particles than bare TiO2 (Figure 3(a)). However, the sizes of the Fe-loaded particles, consisting of aggregates of tiny crystals, are smaller compared to that of the bare TiO2 sample. The presence of Fe3+ ions seems to hamper the growth of TiO2 particles.
(a)
(b)
(c)
Figure 4 shows the X-ray diffractograms of selected samples. It can be noticed that no modification of the starting anatase phase of the bare TiO2 supports occurred after the impregnation treatments as no additional lines attributable to the presence of other phases can be observed.
Figure 5 shows the diffuse reflectance spectra in air of the bare TiO2, Fe-TiO2, and CuPp-Fe-TiO2 recorded in the range 200–800 nm.
The spectrum of bare TiO2 clearly shows an absorption starting at about 380 nm which is typical of bare titania in the anatase phase.
An improvement of light absorption in the visible range can be observed for the Fe-TiO2 and CuPp-Fe-TiO2 metal loaded samples, due to the presence of both iron and porphyrin systems producing a modest shift of the band gap edge in the case of CuPp-Fe-TiO2 sample. Typical absorption bands centered at, respectively, 417 nm (Soret band) and 540 nm ( band), due to the presence of the porphyrinic moiety, have been observed. Hence, the presence of iron onto the TiO2 surface enhances the light absorption capability in the visible region which is a prerequisite for the better utilization of the visible light for the photocatalytic processes.
The band gap values () of such unsupported materials were determined from their diffuse reflectance spectra by using the Kubelka-Munk equation [22]. This equation is based assuming that the reflectance at any wavelength is defined as , where is the measured diffuse reflectance (%).
A plot of the modified Kubelka-Munk function versus the energy of absorbed light is shown in Figure 6. All materials are considered to be indirect semiconductors, as TiO2.
The results obtained afford band gap energies of 3.20, 3.09, and 3.05 eV for bare TiO2, Fe-TiO2 and CuPp-Fe-TiO2 samples, respectively. Iron-induced band gap narrowing of 0.11 eV was observed for Fe-loaded titania.
3.2. Photoactivity of the Photocatalysts
In this work, for the first time the synergistic effect of t-butyl-porphyrinic structures (H2Pp and Cu-Pp) supported onto the Fe-loaded TiO2 surface powders was studied for the photodegradation of 4-nitrophenol (4-NP) in aqueous suspension under UV-visible light irradiation in the presence of H2O2. The amount of iron loaded onto the TiO2 surface during the first wet impregnation step followed by dryness at 393 K and final calcination at 350°C used to prepare Fe-TiO2 (1% wt) [21], support for H2Pp and Cu-Pp porphyrins, as well as the amounts of porphyrins (6 μmol/g) [20] impregnated onto Fe-TiO2 to prepare H2Pp-Fe-TiO2 and CuPp-Fe-TiO2, were chosen by taking into account our previous work [20, 21]. In a typical experiment the photodegradation of 20 mg·L−1 solution of 4-NP was carried out at the initial value of in the presence of mM, catalyst amount = 0.4 g·L−1 and by bubbling air in a 300 mL batch photoreactor. A 300 W UV-visible lamp sketched in Figure 1 was used as irradiation system.
Figure 7 shows the changes in 4-NP concentrations occurring under these experimental conditions. The results obtained in the case of H2Pp-Fe-TiO2 or CuPp-Fe-TiO2—in the presence of hydrogen peroxide and under the experimental condition reported above—were more satisfactory than those performed by using bare TiO2 or Fe-TiO2 under UV-visible light irradiation. The total organic carbon (TOC) analyses showed complete mineralization of 4-NP after ca. 60 min of irradiation for both samples loaded with porphyrins (H2Pp-Fe-TiO2 and CuPp-Fe-TiO2). On the contrary residual amounts of TOC (40–50% of abatement) were found after the same irradiation time when bare TiO2 or Fe-TiO2 samples were used.
Interestingly, despite the fact that the observed initial photoreaction rate was higher when CuPp instead of H2Pp was used as sensitizer, the maximum of degradation was obtained by using H2Pp-Fe-TiO2 photocatalyst; in fact, 4-NP disappeared completely within 45 minutes of irradiation time.
Negligible photoactivity was observed for all of the samples when carried out under dark. This suggests that the photoexcitation, together with presence of H2O2, is essential for inducing the photodegradation of 4-NP processes.
The photostability and the reusability of the photocatalysts are important parameters for practical application. In this work we have observed that all the composites, freshly prepared, that is, Fe-TiO2, H2Pp-Fe-TiO2 and CuPp-Fe-TiO2, can be recycled at least three times without any appreciable decrease of photoactivity.
In the light of the above results the beneficial effect of porphyrin-based sensitizers for the photodegradation of 4-NP has been confirmed [20, 23].
The porphyrins used as sensitizers (Sens) can be excited by visible light to produce electron-hole pairs (an electron in the excited singlet or triplet state of Pps and a hole in the ground state of Pps; see (1) and (3)):
Photoexcitation with UV light of energy greater than the TiO2 band gap promotes an electron from the valence band to the conduction band and leaves an electronic vacancy or hole () in the valence band (2).
As shown in Figure 6 the band gap energies for bare TiO2, Fe-TiO2, and CuPp-Fe-TiO2 samples are, respectively, 3.20, 3.09, and 3.05 eV. Thus minor amount of energy is required for the generation of an electron-hole pair photoexcitation of the photocatalysts Fe-TiO2 and CuPp-Fe-TiO2.
The Pp transfers electron into the conduction band of TiO2 according to (3). TiO2 works as an electron trapper and hinders the hole-electron recombination. In addition, Pp rapidly transfers excited electrons to TiO2 and enhances the separation of holes and electrons, significantly improving the photoefficiency.
In a cooperative manner, loading with Fe3+ ion can enhance the photocatalytic activity due to the charge trapping effect of Fe3+, which prevents the recombination of and according to the following reactions:
In order to better establish the role of the iron ions to try the distinction between a heterogeneous or a homogeneous process we have measured the amount of Fe3+ in solution by ICP analyses. As result of these measurements, very low amounts of Fe3+ ions (1–3 ppb) were detected in solutions at the end of each experiment. These amounts can be considered negligible compared with 4 ppm of Fe3+ loaded onto TiO2 surface dispersed in the solution. Hence, although a possible contribution of the homogeneous Fenton reaction occurring in the process cannot be excluded, this contribution can be considered negligible compared with the contribution of the heterogeneous photo-Fenton process.
According to the crystal field theory, Fe2+ (d6) is relatively unstable compared to Fe3+ (d5). Therefore, a release of trapped electron becomes easy to return to Fe3+. However, the Fe2+/Fe3+ energy level lies close to the Ti3+/Ti4+ level. As a result of this proximity, the trapped electron in Fe2+ can be easily transferred to a neighbouring superficial Ti4+ and combines with the oxygen molecule to form and finally [24].
The heterogeneous photo-Fenton degradation of 4-NP occurring in the presence H2O2 can be attributed to the increase of the concentration of hydroxyl radicals generated by photolytic peroxidation efficiently generated as shown in the following equation (5):
In order to assess the role of dissolved O2 during the photocatalytic degradation process, N2 was bubbled through the suspension to remove O2 from the solution. Figure 8 shows the photodegradation of 4-NP under N2, air, or pure dioxygen bubbling. It is possible to observe that the degradation of 4-NP occurs also under dinitrogen atmosphere.
In addition to the role described previously [24], the presence of dioxygen could be also important during the process due to the possible generation of singlet oxygen (1O2) or species according to (10). The generation of 1O2 in a heterogeneous system, where porphyrins are present, has been highlighted by Zebger and coworkers [25]:
4. Conclusions
Novel porphyrin(Pp)/Fe co-loaded TiO2 composites prepared by impregnation of [5,10,15,20-tetra(4-tert-butylphenyl)] porphyrin (H2Pp) or Cu(II)[5,10,15,20-tetra(4-tert-butylphenyl)] porphyrin (CuPp) onto Fe-loaded TiO2 have been characterized.
The synergistic effect of these porphyrinic structures (H2Pp and Cu-Pp) and iron co-loaded onto TiO2 powders has been studied for the photodegradation of 4-NP in aqueous suspension under UV-visible light irradiation in the presence of H2O2. To the best of our knowledge this complex system porphyrin-Fe-TiO2 + H2O2, that showed to be more performant than the simpler bare TiO2, Fe-TiO2, porphyrin-Fe-TiO2, H2O2-TiO2, H2O2-Fe-TiO2 systems, has been studied for the first time.
Acknowledgments
The authors wish to thank the University of Salento, Apulia Region: Progetto “Ritorno al Futuro” and the Interuniversity Consortium Chemistry for the Environment (INCA). Dr. Manuel Fernandez is acknowledged for helping the authors to measure the emission spectrum of UV-vis lamp used in this work.