The interaction of oxygen with the surface of CeO2–TiO2 mixed systems: an example of fully reversible surface-to-molecule electron transfer

The interaction of oxygen with the surface of CeO2-TiO2 mixed oxides prepared via sol gel was investigated by means of electron paramagnetic resonance (EPR). Upon admission of molecular oxygen onto the surface of the as prepared materials (which underwent final oxidative calcination) the formation of superoxide O-2(-) ions is observed without the need for preliminary annealing in a vacuum and consequent oxygen depletion. The superoxide species is symmetrically adsorbed ("side-on" structure) on the top of a Ce4+ ion. Surprisingly the electron transfer is fully reversible at room temperature having the typical behavior shown by molecular oxygen carriers, which, however, link to oxygen in a completely different manner ("end-on" structure). We suggest that the active sites are Ce3+ ions present in the stoichiometric cerium titanate which forms during the synthesis. The features of these Ce3+ ions must be different from those of the same ions formed in CeO2 by reductive treatments, which show a different reactivity to O-2. The observation reported here opens up innovative perspectives in the field of heterogeneous catalysis and in that of sensors as the total reversibility of the electron transfer is observed in a significant range of oxygen pressure

Photoactivity properties of ZnO doped with cerium ions: an EPR study

open4noIn the present study, we investigated the effect of cerium doping in zinc oxide matrix, used as photoactive material. Cerium ions into the matrix of ZnO can act like a 'trap' for the electrons, lowering the e(-)/h+ recombination rate and so increasing the photocatalytic efficiency of the ZnO.We synthesised doped samples using a simple precipitation route. The amount of dopant used was, 1 and 10% molar. The samples have been studied via x-ray diffraction measurements for the structural characterisation; UV-visible diffuse reflectance was used for the optical analysis; Branauer-Emmett-Teller (BET) model for the measurement of the surface area. Finally, the samples have been analysed via electron paramagnetic resonance (EPR) spectroscopy for the electronic characterisation and for testing their photoactivity. The spin trapping technique was also use to measure the amount of stable radical adducts formed via reaction of OH. radicals with molecules of the DMPO ( 5,5-dimethyl-1-pyrroline-N-oxide) spin probe.openCerrato, Erik; Gionco, Chiara; Paganini, Maria Cristina; Giamello, ElioCerrato, Erik; Gionco, Chiara; Paganini, Maria Cristina; Giamello, Eli

Cerium doped zirconium dioxide as a potential new photocatalytic material. the role of the preparation method on the properties of the material

partially_open6noSmall amounts of Ce4+ ions dispersed in the bulk of ZrO2 give to this material unexpected properties of photoactivity under visible light. Three different samples have been synthesized following different methods and using different precursors and all show some degree of photoactivity. This has been monitored via EPR spectroscopy in terms of charge separation (electrons and holes formation) under polychromatic light having lambda > 420 nm. This behaviour, unimaginable for bare zirconia due to the large band gap of this oxide (around 5 eV), is possible because of the presence in the solid of empty 4f Ce states located in the middle of the band gap which act as intermediate levels in a double excitation mechanism. The doped oxide here described can be considered an example of third generation photoactive material operating under visible light. (C) 2015 Elsevier B.V. All rights reserved.openGionco, C.; Paganini, M.C.; Chiesa, M.; Maurelli, S.; Livraghi, S.; Giamello, E.Gionco, C.; Paganini, M. C.; Chiesa, M.; Maurelli, S.; Livraghi, S.; Giamello, E

Origin of Visible Light Photoactivity of the CeO2/ZnO Heterojunction

open6noThe synthesis of a mixed CeO2-ZnO oxide results in a photocatalyst active under visible light. The characterization of the new material shows that Ce does not enter as a dopant in ZnO but rather forms isolated CeO2 nanoparticles supported on the surface of larger particles of the more abundant zinc oxide phase. The as obtained material exhibits a band gap corresponding to UV light (similar to 3.3 eV), but nevertheless. it shows a relevant photoactivity under irradiation with photons with lambda > 420 nm (visible light). The working hypothesis is that visible light irradiation leads to a charge separation and stabilization of a fraction of the carriers connected with the formation of the CeO2/ZnO interface. This phenomenon has been investigated by means of several methods. A specific EPR-based approach allowed to monitor and quantify the charge separation following the formation of holes in the valence band (VB) of the two materials. More complex is detecting the nature of the excited electrons, as this involves the formation of EPR invisible Ce3+ ions by trapping the electrons into localized 4f states of Ce ions at the interface between the two oxides. DFT calculations provide a rational for some of the observed phenomena and a basis for the discussion of the band alignment of the two systems as a consequence of the formation of a heterojunction. The theoretical results show that indeed electrons can be excited at the interface from the VB of the two oxides into the Ce 4f states with photons of 2.3 eV, thus justifying the occurrence of a visible-light activity despite the higher band gap of the two materials.openCerrato, E.; Gionco, C.; Paganini, M.C.; Giamello, E.; Albanese, E.; Pacchioni, G.Cerrato, E.; Gionco, C.; Paganini, M. C.; Giamello, E.; Albanese, E.; Pacchioni, G

Photocatalytic performances of rare earth element-doped zinc oxide toward pollutant abatement in water and wastewater

The photocatalytic performance of pristine and rare earth elements (La, Ce, Pr, Er, Yb) doped zinc oxide was tested toward the abatement of a model pollutant in MilliQ water and wastewater matrices. ZnO doped with Ce, Er and particularly with Yb exhibited photoactivity higher than bare zinc oxide and the benchmark TiO2 P25, especially in wastewater matrix. Several electrochemical investigations were performed via chronopotentiometry and cyclic voltammetry aimed to shed light on the reasons why the diverse materials behaved differently. From the overall data a complex picture emerged, where there is not a single property of the materials evidently outperforming the others. Nonetheless, from the analysis of whole data a limited role of doping emerged for La and Pr, doping with Er improved the photocurrent, doping with Yb favored a better accumulation of photoelectrons, and doping with Ce promoted a faster electron transfer

Rare earth ions doped ZnO: Synthesis, characterization and preliminary photoactivity assessment

This work reports the effect of doping zinc oxide with lanthanide ions on structural, EPR and UV visible properties. Bare and doped samples were synthesized using the simple and green hydrothermal process. Different rare earth ions (RE = La, Ce, Pr, Er and Yb) with 1% molar ratio RE/Zn were used. The samples have been studied using X Ray Diffraction, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and UV visible diffuse reflectance spectroscopy. Finally, electron paramagnetic resonance (EPR) spectroscopy, was used to assess the materials photoactivity under UV irradiation, both in solid state, to see the charge carriers’ generation and in solution, evaluating the OH• radical formation using the DMPO (5,5-Dimethyl-1-Pyrroline-N-Oxide) spin trapping technique. The results suggest that the synthesized materials could be interesting systems for the photocatalytic abatement of emerging organic persistent pollutants in wastewater treatment plants

Control of Membrane Fouling in Organics Filtration Using Ce-Doped Zirconia and Visible Light

Membrane fouling has been a major issue in the development of more efficient water treatment processes. Specifically in surface waters filtration, organic matter, such as humic-like substances, can cause irreversible fouling. Therefore, this study evaluates the activity of a photocatalytic layer composed of Ce-doped zirconia nanoparticles in improving the fouling resistance during filtration of an aqueous solution of humic acid (HA). These nanoparticles were prepared by hydrothermal and sol-gel processes and then characterized. Before the filtration experiments, the photodegradation of HA catalyzed by Ce-doped zirconia nanoparticles in dispersion was studied. It was observed that the sol-gel prepared Ce-ZrO2 exhibited higher HA removal in practically neutral pH, achieving 93% efficiency in 180 min of adsorption in the dark followed by 180 min under visible-light irradiation using light-emitting diodes (LEDs). Changes in spectral properties and in total organic carbon confirmed HA degradation and contributed to the proposal of a mechanism for HA photodegradation. Finally, in HA filtration tests, Ce-ZrO2 photocatalytic membranes were able to recover the flux in a fouled membrane using visible-light by degrading HA. The present findings point to the further development of anti-fouling membranes, in which solar light can be used to degrade fouling compounds and possibly contaminants of emerging concern, which will have important environmental implications