Synthesis of Different CeO₂ Structures on Mesoporous Silica and Characterization of Their Reduction Properties

Abstract

An investigation was performed to establish the effects of precursor composition on the structure of ceria deposited onto mesoporous silicas, MCM-41 and SBA-15. The structure of the deposited ceria was characterized by XRD, TEM, Raman and UV-visible spectroscopy, XANES, and EXAFS. Using Ce(acac) 3 as the precursor resulted in formation of 3 nm diameter ceria particles located predominantly in the mouths of the support pores. Amorphous agglomerates, but with domains of comparable size to those obtained using Ce(acac) 3 , were obtained using Ce(O i Pr) 4 . Much smaller domains and a high degree of dispersion were obtained using Ce(O t Bu) 4 as the precursor. The structure of the deposited ceria affects its reduction-reoxidation properties. Reduction and reoxidation are almost fully reversible for the nanoparticles. Only surface reduction is observed up to 723 K, whereas partial bulk reduction takes place at higher reduction temperatures. In case of the well-dispersed, layerlike Ce species on the silica surface, an extent of reduction similar to that of the nanoparticles is achievable, but upon heating the samples in 10% H 2 /He to 923 K the reduction becomes partially irreversible. The latter effect might be explained by a formation of cerium silicates that prevent the complete reoxidation of the cerium species. ' INTRODUCTIONDispersion of a transition metal oxide on a high surface area support such as Al 2 O 3 or SiO 2 makes it possible to combine the catalytic properties of the transition metal oxide with the high surface area and thermal and mechanical stability of the support. 1 Good illustrations of the manner in which the transition metal oxide is introduced onto the support and the physical properties of the deposited oxide are given for the cases of TiO x , 1-9 VO x , 5,[9][10][11][12][13][14] or MoO x 10,11,15-21 dispersed on high surface area supports. A notable consequence of this approach is the formation of catalytically active sites which are not present on either the support or the bulk transition metal oxide. For example, neither TiO 2 nor SiO 2 is active for the liquid-phase epoxidation of alkenes, but TiO 2 /SiO 2 exhibits high activity and selectivity for this reaction. 6,[22][23][24] The appearance of such novel catalytically active sites has been attributed to the formation of a twodimensional oxide overlayer whose physical properties differ from those of the corresponding bulk oxide. Dispersed transition metal oxides have also been shown to stabilize the dispersion of small metal particles and to enhance the activity of supported metal oxo species. For example, Au particles have been shown to be much more stable on TiO x /SiO 2 than on SiO 2 25 and isolated vanadate species dispersed on TiO x /SiO 2 exhibit turnover frequencies that are up to 60-fold higher than when dispersed on SiO 2 . 7 CeO 2 is a highly reducible oxide exhibiting oxygen storage and release properties that are important for the operation of threeway, automotive exhaust catalysts. 26 It can also stabilize the dispersion of n...

The effect of sunlight induced surface defects on the photocatalytic activity of nanosized CeO2 for the degradation of phenol and its derivatives

“…The major peaks in the PL spectra of the pristine and sunlight exposed CeO 2 appeared at 453 nm, 470 nm, 483 nm, 493 nm and 550 nm (Figure 3b). The mild variation in the observed values and that reported in the literature [43,44], may be attributed to variation in synthetic route and calcination conditions. Instead of discussing the origin, we concentrated on the variation in the intensity of these emission peaks for pristine and sunlight exposed CeO 2 .…”

Tin-grafted TiO2 with enhanced activity for photocatalytic hydrogen generation from aqueous methanol solutions

“…Depending on the surface concentration, two different oxide species exist: isolated species at a low content and agglomerated species with excess loading [27]. Alkoxide precursors are usually preferred over acetylacetonates when the aim is the synthesis of isolated sites by means of grafting, because previous work has indicated that acetylacetonates might lead to the formation of agglomerates already at lower loadings [28,29]. Owing to different chemical states, the catalytic behaviors of the two species for specific reactions may differ greatly.…”

Synthesis of micron-sized porous CeO2SiO2 composite particles for ultraviolet absorption

“…The sample fabricated with 1.5 wt% CeO 2 in the precursor showed a 30% enhancement of its UV absorption capacity as compared to commercial TiO 2 nanoparticles with a wavelength of 300 nm. This can be attributed to the porous structure of the CeO 2 ASiO 2 composite particles for UV absorption, which can prevent potential damage from UV light and restrain the unnecessary surface activity of the CeO 2 [15,16]. It should be noted that the aerosol process caused the porous CeO 2 ASiO 2 composite particles with more than 0.75 wt% of CeO 2 in the precursor to show high UV absorption ability.…”

“…When CeO 2 ASiO 2 composite particles are prepared, the SiO 2 nanoparticles enhance the coverage on the skin and give the skin a natural color. The CeO 2 ASiO 2 composite particles not only enhance the thermal stability but also increase the surface area [15,16]. The use of these particles is known to be an efficient way to improve the activity of UV absorption as well.…”