Ginesa Blanco

Modification of the oxygen storage capacity of CeO2–ZrO2 mixed oxides after redox cycling aging

Abstract High surface area CeO2–ZrO2 mixed oxides were treated at 900–950°C either under wet air or under successive reducing and oxidizing atmospheres in order to study the evolution of the oxygen storage capacity (OSC) of these solids after different aging treatments. Several complementary methods were used to characterize the redox behavior: temperature programmed reduction (TPR) by H2, TPO, magnetic susceptibility measurements to obtain the Ce3+ content, FT-IR spectroscopy of adsorbed methanol and a method to compare the oxygen buffering capacity (OBC) of the oxides. All the results confirm that the mixed oxides exhibit better redox properties than pure ceria, particularly after aging. The enhancement in the OSC at moderate temperature has to be related to a deeper penetration of the reduction process from the surface into the under-layers. Redox cycling aging promotes the reduction at low temperature of all the mixed oxides, the improvement being much more important for low surface area aged samples. The magnitude of this effect does not depend on the BET surface areas which have similar values after cycling. This underlines the critical influence that the preparation and activation procedure have on the final OSC behaviors of the ceria–zirconia mixed oxides.

Designing Novel Hybrid Materials by One-Pot Co-condensation: From Hydrophobic Mesoporous Silica Nanoparticles to Superamphiphobic Cotton Textiles

This work reports the synthesis and characterization of mesoporous silica nanoparticles (MSNs) functionalized with tridecafluorooctyltriethoxysilane (F13) and their in situ incorporation onto cotton textiles. The hybrid MSNs and the functional textiles were prepared by a one-pot co-condensation methodology between tetraethylorthosilicate (TEOS) and F13, with hexadecyltrimethylammonium chloride (CTAC) as the template and triethanolamine as the base. The influence of the F13 to TEOS molar ratio (1:10, 1:5 and 1:3) on the nanoparticle morphology, porosity, degree of functionalization, and hydro/oleophobic properties is discussed. The hybrid nanosilicas presented high colloidal stability and were spherical and monodispersed with average particle size of ∼45 nm. They also showed high surface areas, large pore volumes, and a wormhole-type mesoporous structure. The increase in the organosilane proportion during the co-condensation process led to a more radially branched wormhole-like mesoporosity, a decrease in the surface area, pore volume, and amount of surface silanol groups, and an enrichment of the surface with fluorocarbon moieties. These changes imparted hydrophobic and oleophobic properties to the materials, especially to that containing the highest F13 loading. Cotton textiles were coated with the F13-MSNs through an efficient and less time-consuming route. The combination between surface roughness and mesoporosity imparted by the MSNs, and the low surface energy provided by the organosilane resulted in superhydrophobic functional textiles. Moreover, the textile with the highest loading of fluorocarbon groups was superamphiphobic.

Influence of high temperature treatments under net oxidizing and reducing conditions on the oxygen storage and buffering properties of a Ce0.68Zr0.32O2 mixed oxide

Abstract TPR, TPO, O2 pulses and magnetic balance experiments have been performed to study the effect of two different aging treatments, in oxidizing atmosphere and in reducing conditions, on the redox behavior of a Ce0.68Zr0.32O2 mixed oxide. It has been observed that, although the samples resulting from both treatments have similar textural properties, their redox behavior is significantly different. In particular, aging in H2 improves the oxygen storage capacity (OSC) at low temperatures by decreasing the temperature at which reduction takes place. Also, the oxygen buffering capacity (OBC) is enhanced after aging under reducing conditions. In addition, the study of the re-oxidation of the samples reveals that the rate of the O2 uptake increases if compared with the fresh sample or that aged in air. High resolution electron microscopy and Raman spectroscopy data have also been used to obtain information on the possible origin of such effects.

Cerium-terbium mixed oxides as alternative components for three-way catalysts: A comparative study of Pt/CeTbOx and Pt/CeO2 model systems

Abstract By using a combination of oxygen buffering capacity (OBC) and oxygen storage capacity (OSC) measurements, the redox behaviour of a Pt/CeTbOx catalyst is compared to that of a classic model TWC system: Pt/CeO2. The results reported here show that the redox efficiency of the Pt/CeTbOx catalyst is much better, especially at low temperature operation conditions such as those occurring during the cold start of engines. The catalyst containing terbium also shows lower ‘light-off’ temperatures for both methane and carbon monoxide oxidation.

Reducibility of ceria–lanthana mixed oxides under temperature programmed hydrogen and inert gas flow conditions

Abstract The present paper deals with the preparation and characterization of La/Ce mixed oxides, with La molar contents of 20, 36 and 57%. We carry out the study of the structural, textural and redox properties of the mixed oxides, comparing our results with those for pure ceria. For this aim we use temperature programmed reduction (TPR), temperature programmed desorption (TPD), nitrogen physisorption at 77 K, X-ray diffraction and high resolution electron microscopy. The mixed oxides are more easy to reduce in a flow of hydrogen than ceria. Moreover, in an inert gas flow they release oxygen in higher amounts and at lower temperatures than pure CeO2. The textural stability of the mixed oxides is also improved by incorporation of lanthana. All these properties make the ceria–lanthana mixed oxides interesting alternative candidates to substitute ceria in three-way catalyst formulations.

Influence of reduction treatment on the structural and redox behaviour of ceria, La/Ce and Y/Ce mixed oxides

A methodology based on reoxidation at 298 K with O2(5%)/He pulses followed by quantitative TPO runs allowed us to study the redox state reached by CeO2 and Ln3+ -containing ceria‐based mixed oxide samples Ln3+:La3+ or Y3+) after their reduction in a flow of hydrogen. The temperature range covered in the reduction treatments was from 623 up to 1223 K. During the severe reduction treatments hexagonal sesquioxide phases segregate from all the samples. The reduction temperature at which segregation occurs follows the trend La/Ce < Y/Ce < CeO2. The TPO and H2-TPD traces of reduced samples which showed evidence of segregation in XRD exhibit characteristic features which can also be used as an indication of this effect. The discussion is focussed on the overall and detailed understanding of the reoxidation process and on the analysis of the H2-TPD profiles.

Fully Reversible Metal Deactivation Effects in Gold/Ceria–Zirconia Catalysts: Role of the Redox State of the Support

Gold nanoparticles supported on reducible oxides are highly interesting catalytic materials. In particular, they are known to exhibit exceptional activity for CO oxidation, lowtemperature water–gas shift (LT-WGS), and selective oxidation of CO in the presence of a large excess of hydrogen (PROX). 20–24] Despite the extraordinary research effort devoted to these catalysts, there are still some key questions about the nanostructural constitution and chemical properties of the gold sites involved in the above reactions that require further clarification. The relationship between the redox state of the support and the chemical properties of gold nanoparticles is one of these major open questions. 5] Its understanding, however, is critically important to fully interpret catalysis by Au/reducible oxide systems, particularly in the case of processes like LTWGS, 19] PROX, and hydrogenation reactions, which typically occur under net reducing conditions. To gain further information on this issue, we investigated CO adsorption on an Au/Ce0.62Zr0.38O2 (Au/CZ) sample subjected to different redox pretreatments. In our experimental approach, FTIR spectroscopic and volumetric adsorption techniques were combined with studies on ultimate oxygen storage capacity (OSC), metal dispersion, as determined by high-resolution TEM (HRTEM) and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), and X-ray photoelectron spectroscopy (XPS). By this approach, the influence of the redox state of the support on the CO adsorption capability of Au nanoparticles could be established on a quantitative basis. In contrast with earlier proposals suggesting that gold catalysts do not exhibit a strong metal/support interaction (SMSI) effect, the results presented and discussed herein indicate that the behavior of our Au/CZ shows close resemblances with those of a number of noble metal/reducible oxide systems which are acknowledged to show this effect. Our experimental approach also allowed us to show that the absorption coefficient of the n[CO(Au)] band depends on the redox state of the support. The implications of this finding for the use of integrated absorption data as a quantitative tool for characterizing the changes of CO adsorption capability occurring in gold nanoparticles supported on reducible oxides is discussed. Figure 1 shows the n(CO) FTIR spectra recorded on the same sample disk successively submitted to a) oxidizing treatment at 523 K (Au/CZ-O523); b) pretreatment (a) followed by reduction at 473 K (Au/CZ-O523-R473); and

[VO(acac)2] hybrid catalyst: from complex immobilization onto silica nanoparticles to catalytic application in the epoxidation of geraniol

This work reports the preparation of a novel hybrid nanocatalyst through the immobilization of oxidovanadiumIV acetylacetonate ([VO(acac)2]) onto silica nanoparticles functionalized with 3-aminopropyltriethoxysilane (APTES) and its catalytic application in the allylic epoxidation of geraniol. All the nanomaterials were characterized by chemical analysis, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). The parent and amine-functionalized silica nanoparticles were also analyzed by 29Si and 13C solid-state nuclear magnetic resonance and the supported [VO(acac)2] nanomaterial was characterized by magnetic susceptibility measurement. The APTES organosilane was grafted onto the silica nanoparticles with an efficiency of 90%, through mono- and bidentate covalent grafting. The [VO(acac)2] complex was covalently anchored on the surface of the APTES-functionalized silica nanoparticles with 25% of efficiency. Magnetic susceptibility measurements in combination with XPS indicated that the oxidation state of the metal center (VO2+) was preserved upon the immobilization reaction. The catalytic performance of the novel hybrid [VO(acac)2] nanocatalyst was evaluated in the epoxidation of geraniol, at room temperature, using tert-butyl hydroperoxide as an oxygen source. The nanocatalyst presented 100% of substrate conversion, 99% of selectivity towards the 2,3-epoxygeraniol product and was stable upon reuse in further four cycles.

XPS analysis and microstructural characterization of a Ce/Tb mixed oxide supported on a lanthana‐modified transition alumina

In the framework of a research project aimed at developing alternative materials for application in three-way catalysis, this work reports on the preparation and characterization of a ceria-terbia mixed oxide supported on a lanthana-modified transition alumina. This complex multicomponent oxide system has been characterized by combining x-ray diffraction (XRD), high-resolution electron microscopy (HREM), temperature-programmed desorption mass spectrometry (TPD-MS), Fourier transform infrared spectroscopy (FTIR) and x-ray photoelectron spectroscopy (XPS). Following a dry impregnation procedure, the lanthana-containing phase was deposited first, the ceria-terbia oxide being dispersed on the modified alumina in a subsequent step. The structural and chemical characterization studies carried out on the La 2 O 3 /Al 2 O 3 binary system provide no evidence of a crystalline lanthana-containing phase. Neither lanthanum oxide nor the phases resulting from its aging in air could be identified. Likewise, no lanthana-alumina mixed oxide phases could be deduced from analysis of the HREM micrographs, selected-area electron diffraction (SAED) or XRD patterns. As deduced from the H 2 O and CO 2 TPD studies, the chemisorptive properties of the La 2 O 3 /Al 2 O 3 are significantly different from those of the alumina and the bulk lanthana. We conclude, accordingly, that the supported lanthana consists of a highly dispersed oxide in strong interaction with the alumina support. By contrast, after deposition and further calcination of the cerium-terbium nitrate precursors, the presence of fluorite microcrystals of average size 5 nm could be identified both by XRD and HREM. The XPS studies carried out on the alumina support, La 2 O 3 /Al 2 O 3 and CeTbO x /La 2 O 3 /Al 2 O 3 , as well as a series of reference systems consisting of aged-in-air lanthana, CeLaO x and CeTbO x mixed oxides, have revealed the presence in the ternary system (CeTbO x /La 2 O 3 /Al 2 O 3 ) of at least two different types of chemical environments for the lanthanum ions. With the help of the La 3d 5/2 spectra recorded for the different reference systems, we conclude that these two environments correspond to highly dispersed lanthana strongly interacting with OH- and/or CO 3 2- groups, and to La 3+ species incorporated in the Ce/Tb mixed oxide phase.

Oxygen buffering capacity (OBC) of praseodymium-modified CeO2: influence of the Pr distribution in the ceria host lattice

This work reports on the redox behaviour of different ceria-praseodymia mixed oxides with two different chemical compositions (Ce/Pr molar ratios of 4:1 and 9:1), The samples consist of bulk oxides and ceria-supported samples with varying mixed oxide loading (equivalent to 7 or 12 mixed oxide layers). As revealed by x-ray diffraction and high-resolution electron microscopy, the samples consist of homogeneous fluorite-like mixed oxides. Their redox properties were characterized by means of the oxygen buffering capacity (OBC) technique, which quantitatively evaluates the capacity of the samples for attenuating fast oscillations of the oxygen partial pressure in the He stream flowing through the sample. These oscillations are induced by injecting with a high frequency pulses of oxygen into the inert gas stream. Although the XPS analyses performed on both a bulk and a supported sample showed Ce/Pr molar ratios very close to each other, the bulk mixed oxide exhibits significantly better OBC values. Also remarkable is that an ageing treatment at 1223 K improves the OBC response of the supported mixed oxides. In accordance with the XPS data, the enhancement of their redox properties runs parallel to an increase in the Ce/Pr surface ratio. These results are discussed in terms of the role played by the depth profile distribution of Pr in the different oxide samples.