A new class of porphyrin(Pp)/Fe co-loaded TiO2 composites opportunely prepared by impregnation of [5,10,15,20-tetra(4-
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 [
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 [
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 [
Various advanced oxidation technologies have been used in the presence of TiO2, H2O2, and irradiation to enhance the efficiency of the overall photodegradation process [
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.
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-
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 [
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
The mixture was stirred for 3-4 h, and the solvent was removed under vacuum.
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 CuK
The set-up used for the photocatalytic experiments is reported in Figure
Set-up used for the photocatalytic experiments.
Emission spectrum of SANOLUX HRC UV-vis lamp.
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
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.
Syntheses of the metal-free porphyrin [5,10,15,20-tetra(4-
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 [
Further, the novel composites used as the photocatalysts in this work were prepared by impregnation of the Fe-TiO2 powder with 6
Analysis of SEM picture (Figure
SEM micrographs of (a) BareTiO2, (b) Fe-TiO2, and (c) CuPp-Fe-TiO2.
Figure
XRD patterns of TiO2 powder samples compared with the bare sample.
Figure
Diffuse reflectance spectra of bare TiO2 and samples obtained by impregnation of bareTiO2 with Fe or CuPp-Fe-TiO2.
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 (
The band gap values (
A plot of the modified Kubelka-Munk function
Modified Kubelka-Munk function versus energy of absorbed light of the bare TiO2 (blue line), Fe-TiO2 (brown line), and CuPp-Fe-TiO2 (green line).
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.
In this work, for the first time the synergistic effect of
Figure
Degradation of 4-NP as a function of irradiation time in the presence of different photocatalysts, H2O2 and air bubbling. Experimental conditions: [4-NP] = 20 mgL−1; [H2O2] = 4.9 mM; catalyst amount = 0.4 gL−1; reaction volume = 300 mL; pH = 6.2; lamp: UV-vis lamp SANOLUX, 300 W.
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 [
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 (
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 (
As shown in Figure
The Pp transfers electron into the conduction band of TiO2 according to (
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
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
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 (
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
Degradation of 4-NP as a function of irradiation time in the presence of H2Pp-Fe-TiO2 as photocatalyst and H2O2, under dinitrogen or pure dioxygen bubbling.
In addition to the role described previously [
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.
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.