EPR study of the radicals formed upon UV irradiation of ceria-based photocatalysts

Spanish National Research Council, Madrid, Madrid, Spain
Journal of Photochemistry and Photobiology A Chemistry (Impact Factor: 2.5). 07/2002; 150(1):213-221. DOI: 10.1016/S1010-6030(02)00092-8


EPR measurements reveal remarkable differences on the type of radicals produced after UV illumination of TiO2, CeO2 and 0.8% CeO2/TiO2 photocatalysts. Photoactivation of the TiO2 sample in vacuum results in the formation of Ti4+–O− species and a small amount of Ti3+ centers. In the presence of adsorbed oxygen, irradiation of this material also generates Ti4+–O3− radicals. In the case of the CeO2/TiO2 catalyst, the ceria component is present in a highly dispersed state, as indicated by XRD and UV–Vis diffuse reflectance spectra (DRS) results. Accordingly, the only type of Ce4+–O2− adducts generated on the CeO2/TiO2 sample are indicative of the presence of two-dimensional patches of ceria on the anatase surface. On the other hand, photoactivation of the CeO2/TiO2 sample in the presence of oxygen also leads to the formation of some Ti4+–O− and Ti3+ centers. In the case of the CeO2 sample, superoxide radicals are observed upon irradiation in vacuum and subsequent oxygen adsorption. Further irradiation of this material in the presence of oxygen increases the amount of Ce4+–O2− radicals and simultaneously generates new species, which are tentatively assigned to Ce4+–O2H radicals. Photocatalytic activity was tested for toluene oxidation, and the results obtained show that the photodegradation rate is slightly lower for CeO2/TiO2 than for the TiO2 sample. However, the selectivity towards benzaldehyde (6–13%) is comparable for both materials. In the case of CeO2, the photo-oxidation rate is an order of magnitude lower than for TiO2, although mineralization of toluene is almost complete. Photoactivity results are discussed in connection with the characteristics of the radicals observed.

Download full-text


Available from: Javier Soria, Jan 15, 2014
60 Reads
  • Source
    • "This implies that there was an interaction between Ce and Zr in the mixed oxides, and the formation of Ce x Zr 1Àx O 2 solid solution is possible. On the other hand, the shifting of the adsorption band is the result of either the rearrangement in the electronic adsorption site [43] or the strain development [44], with respect to those of the pure phase. Other bands in the mixed oxides are attributed to the various charge transitions . "
    [Show abstract] [Hide abstract]
    ABSTRACT: We tested 3 wt% gold (Au) catalysts on CeO2–ZrO2 mixed oxides, prepared by co-precipitation (CP) and the sol–gel (SG) technique, for steam reforming of methanol (SRM). Uniform Ce1−xZrxO2 solid solution was dependent on the Zr/Ce ratio, where the incorporation of Zr4+ into the Ce4+ lattice with a ratio of 0.25 resulted in smaller ceria crystallites and better reducibility, and was found to be efficient for SRM activity. The catalytic activity was suppressed when the ratio was ≥0.5, which led to the segregation of Zr from solid solution and sintering of Au nanoparticles. It was found that the CP technique produced better catalysts than SG in this case. For the bimetallic catalysts, the co-operation of Au–Cu supported on Ce0.75Zr0.25O2 (CP) exhibited superior activities with complete methanol conversion and low CO concentration at 350 °C. Furthermore, the size of the alloy particle was strongly dependent on the pH level during preparation.
    International Journal of Hydrogen Energy 02/2013; 38(3):1348-1362. DOI:10.1016/j.ijhydene.2012.10.117 · 3.31 Impact Factor
  • Source
    • "), WO 3 (Waldner et al., 2007; Sayama et al., 2010; Saepurahmanet et al., 2010; Cao et al., 2011), WS 2 (Jing and Guo, 2007), Fe 2 O 3 (Chen et al., 2001; Bandara et al., 2001; Pal and Sharon, 1998), V 2 O 5 (Akbarzadeh et al., 2010; Teramura et al., 2004a,b), CeO 2 (Lin and Yu, 1998; Coronado et al., 2002; Ji et al., 2009; Song et al., 2007), CdS (Bessekhouad et al., 2004; Reuterġardh and Langphasuk, 1997; Tang and Huang, 1995), ZnS (Torres-Martínez et al., 2001), and CuO (Lim and Kim, 2004; Sathishkumar et al., 2011; Nezamzadeh-Ejheieg and Hushmandrad, 2010). "
    Applied Computational Fluid Dynamics, 03/2012; , ISBN: 978-953-51-0271-7
  • Source
    • "\g[ = 1/3(g x + g y + g z ); g = 3(g y -g x )/(2g z -g y -g x ); b, in bulk; s, on surfaces. Reference 1, this study; 2, Hochstrasser et al. (1969); 3, Bernarek and Schlick (1991); 4, Kappers et al. (1978); 5, Coronado et al. (2002); 6, Murata et al. (2003); 7, Zhao et al. (2004); 8, Wang and Lunford (1971); 9, Priest et al. (1991) of the spin-orbit coupling for oxygen and the energy difference between the pp u and pr u * orbitals. Scaling the value of (g \ -g e ) observed for F 2 -by the ratio of the spinorbital coupling for F to O yields g \ & 2.014 and g || & g e = 2.0023 for the isolated O 2 3-ion. "
    [Show abstract] [Hide abstract]
    ABSTRACT: Single-crystal W-band electron paramagnetic resonance (EPR) spectra of an electron-irradiated quartz, measured at room temperature, 110 and 77K, disclose three previously reported hole centers (#1, G and an ozonide radical). The W-band EPR spectra of these three centers clearly resolve six magnetically nonequivalent sites each, whereas previous X- and Q-band EPR studies reported Centers #1 and the ozonide radical to consist of only three symmetry-related components and interpreted them to reside on twofold symmetry axes in the quartz structure. The calculated g matrices of Center #1 and the ozonide radical show that deviations from twofold symmetry axes are <10°, which are probably attributable to distortion related to neighboring charge compensating ions. The W-band EPR spectra of Center G not only result in improved g matrices but also allow quantitative determination of the nuclear hyperfine (A) and quadrupole (P) matrices of its 27Al hyperfine structure that was incompletely resolved before. In particular, the g-maximum and g-minimum principal axes of Center G are approximately along two pairs of O–O edges of the SiO4 tetrahedron, while the unique A principal axis is approximately along a Si–Si direction. These new spin-Hamiltonian parameters suggest that Center G most likely involves trapping of a hole between two oxygen atoms related to a silicon vacancy and stabilized by an Al3+ ion in the neighboring tetrahedron (hence an O2n−–Al3+ defect, where n is either 1 or 3).
    Physics and Chemistry of Minerals 04/2008; 35(2):103-115. DOI:10.1007/s00269-007-0203-5 · 1.54 Impact Factor
Show more