Search

Redox and catalytic performance in total methane oxidation of a nanostructured ceria–terbia catalyst supported on magnesia is presented and compared to that of a pure ceria catalyst supported on MgO. The investigated material, Ce0.5Tb0.5Ox (3% mol.)/MgO, features several remarkable properties: a quite low total molar loading of the two lanthanide elements, high reducibility as well as very high oxygen storage capacity at low temperatures and higher catalytic activity than MgO-supported CeO2. In terms of lanthanide atomic content, the catalytic performance of Ce0.5Tb0.5Ox (3% mol.)/MgO largely improves compared to that of bulk type ceria and ceria–magnesia solid solutions. Such a behavior implies proper optimization of the usage of lanthanide elements. A second contribution to atomic economy in the catalyst design relates to the fact that the novel formulation demonstrates a good stability in the redox and catalytic performance against very high temperature treatments. An investigation of the structure of both the fresh and high temperature-aged catalysts at the atomic scale, by means of complementary aberration corrected microscopy techniques, reveals the occurrence of a variety of highly dispersed, exotic, lanthanide-containing nanostructures, which span from isolated, atomically dispersed, Ln species to nanometer-sized CeTbO2−x patches, extended CeTbO2−x bilayers and 3D CeTbO2−x nanoparticles. Nanoanalytical results evidence the mixing of the two lanthanides at atomic levels in these nanostructures. The combined effects of nanostructuring, mixing of the lanthanides at the atomic level and interaction with the MgO oxide are the roots of the improvement in functional, redox and catalytic properties of the novel Ce0.5Tb0.5Ox (3% mol.)/MgO catalyst. [ABSTRACT FROM AUTHOR]

Ceria‐zirconia mixed oxides are widely used as catalysts and catalytic supports. Nevertheless, due to economic and geo‐strategic concerns, reducing and optimizing the lanthanide content in these formulations while preserving or even improving their excellent redox and catalytic properties has become a challenge for current research. In this context, the design of core@shell nanostructures arises as a feasible alternative to achieve the aforesaid goal. In the present work, a new nanostructured ZrO2@CeO2 system based on spherical and mesoporous zirconia cores coated by a well‐dispersed nanometer‐thick ceria layer has been prepared by a novel synthetic approach in 2 different steps: (1) synthesis of the zirconia cores by a sol‐gel method and (2) coating with a thin ceria shell by controlled chemical precipitation. The resulting nanocomposite was characterized by several techniques, which clearly reveal its perfect core@shell nanostructure and enhanced reducibility as compared with pure ceria. [ABSTRACT FROM AUTHOR]

The selective oxidation of veratryl alcohol (VA), a model compound of lignin, with oxygen molecules to produce veratraldehyde (VAld) was studied over monometallic Au, Pd, and bimetallic Au:Pd nanoparticles supported on a Ce0.62Zr0.38O2 mixed oxide for the first time. These bimetallic Au-Pd catalysts with Au:Pd molar ratios from 0.4 to 4.3 were synthesized by the sol-immobilization method. Furthermore, all the catalysts were characterized by inductively coupled plasma-atomic emission spectroscopy (ICP-AES), N2 physisorption, X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) imaging, energy dispersive X-ray spectroscopy (EDXS), and temperature programmed reduction (TPR) techniques. A synergistic effect between gold and palladium was observed over all the bimetallic catalysts in a wide range of studied Au:Pd ratios. Remarkably, the optimum Au:Pd ratio for this reaction was 1.4 with a turnover frequency of almost six times larger than for the monometallic gold and palladium catalysts. Selectivity to veratraldehyde was higher than 99% for the monometallic Au, Pd, and all the bimetallic Au-Pd catalysts, and stayed constant during the reaction time. [ABSTRACT FROM AUTHOR]

This paper describes the results obtained when ultraviolet laser treatment was performed as a surface treatment prior to adhesive bonding for two aeronautical carbon fibre-reinforced plastics based on an epoxy resin prepreg. Different laser- processing parameters were employed, and their effect on the surfaces was analysed through morphological characterisation and wettability studies. X-ray photoelectron spectroscopy measurements were performed to determine the cleaning and activation effects of the treatment. The strength of the bonded joint was studied for laser-treated and manually ground samples. Samples processed under the selected laser conditions exhibited better adhesive behaviour than the manually treated samples, thereby suggesting that ultraviolet laser treatment could be used as an alternative method for surface activation of aeronautical composites based on epoxy resins. [ABSTRACT FROM AUTHOR]

A series of gold-ruthenium bimetallic catalysts of increasing Au:Ru molar ratios supported on a Ce0.62Zr0.38O2 mixed oxide are prepared and their structural and chemical features characterized by a combination of macroscopic and atomic-scale techniques based on scanning transmission electron microscopy. The influence of the temperature of the final reduction treatment used as activation step (350-700 °C range) is also investigated. The preparation method used allows catalysts to be successfully prepared where a major fraction of the metal nanoparticles is in the size range below 5 nm. The structural complexities characteristic of this type of catalysts are evidenced, as well as the capabilities and limitations of both the macroscopic and microscopic techniques in the characterization of the system of metal nanoparticles. A positive influence of the addition of Ru on both the resistance against sintering and the catalytic performance of the starting supported Au catalyst is evidenced. [ABSTRACT FROM AUTHOR]

A novel CeO2/MgO catalyst with low ceria loading has been synthesized. This catalyst showed unique redox properties compared with conventional high and low surface area CeO2. Advanced (scanning) transmission electron microscopy techniques revealed the presence of a variety of highly dispersed ceria nanostructures: isolated CeOx entities, CeO2 clusters, as well as fairly small (<5 nm) CeO2 nanoparticles. More interestingly, this CeO2/MgO catalyst showed outstanding stability in its redox response against high temperature aging treatments. Thus, after reduction in hydrogen at 950ºC and further oxidation at 500ºC, CeO2 reduction effects took still place at low temperatures, and no significant loss of oxygen storage capacity (OSC) was detected. Unique ceria-bilayer nanostructures were found and characterized in the aged catalyst. Their peculiar structural and chemical properties seem to be responsible for the large improvement observed in the stability of the redox response. [ABSTRACT FROM AUTHOR]

In this study, AA5083 samples were butt welded under a conduction regime with high-power diode laser (HPDL). Various mixtures composed of Ar and CO 2 were used as a shielding gas. The influence of the shielding gas composition on the microstructure and on the properties of laser welds was analyzed. The weld beads were deeply characterized by metallographic/microstructural studies, X-ray diffraction (XRD), X-ray energy dispersive spectrometry (X-EDS) chemical analyses, X-ray photoelectron spectra (XPS), microhardness, and tensile strength. The corrosion resistance of laser-remelted surfaces with different CO 2/Ar ratios was also estimated by means of electrochemical tests. The addition of CO 2 to the shielding gas results in a better weld penetration and oxidizes the weld pool surface. This addition also promotes the migration of Mg toward the surface of weld beads and induces the formation of magnesium aluminates spinel on the welds. The best corrosion resistance result is achieved with 20 pct CO 2. The overall results indicate that the addition of small percentage of CO 2 to Ar leads to improvements of the mechanical and corrosion properties of the aluminum welds. [ABSTRACT FROM AUTHOR]

Abstract: Two alumina-supported Ce/Pr mixed oxides, with different alumina promoters, La and Si, have been investigated in this paper. A gradual decrease of the Oxygen Storage Capacity (OSC) was observed after reducing the samples at progressively higher temperatures. However, depending on the nature of the alumina promoter, the temperature at which the loss of OSC started to be evident changed, being higher in the La-doped sample. A structural characterization study by X-ray diffraction (XRD) of the samples submitted to different reduction and reoxidation treatments in a range of temperatures was performed. From this study a correlation between the nature of the observed phases, the alumina promoter, and the deactivation behaviour could be stated. FTIR study is of particular interest because the observed correlation between the reduction degree and the position and intensity of the FTIR band is altered when deactivation phenomena appear. [Copyright &y& Elsevier]

The activation of a 2.6 wt% Au/CeZrO catalyst prepared by deposition-precipitation with urea is investigated. At 298 K, the activity for CO oxidation of the as-prepared sample is very low; it is significantly increased when heated at 418 K, under the reaction mixture, and much strongly when pre-treated under flowing O(5%)/He, at 523 K. As revealed by XPS, FTIRS, HAADF-STEM, and HRTEM studies, the activation process consists of the transformation of the initial urea-containing Au(III) precursor deposited on the support into a highly dispersed metal phase (Au nano-particle mean size: 1.8 nm). [ABSTRACT FROM AUTHOR]

LaAlO3 perovskites, as such and with 25% molar Al substitution by Cu, Co, or Ga, have been prepared by sol-gel methods and tested as heterogeneous catalysts in the gas-phase conversion of ethanol. LaAlO3 presented a significant acidic character, with high formation of ethylene by ethanol dehydration. B-site substitutions increased the basicity of the catalysts, favoring the dehydrogenation of ethanol to acetaldehyde. The most reducible Cu- and Co-substituted materials, characterized by easier formation of surface oxygen vacancies, promoted the self-condensation of acetaldehyde by the Tishchenko mechanism, with formation of acetone and odd-carbon number products. Aldol coupling of acetaldehyde, favored on pure and Ga-substituted LaAlO3, led to the formation of butadiene and hexadiene. The role of Ga insertion, favoring both dehydrogenation of ethylene and dehydration of higher alcohols, corresponds to an amphoteric character. The formation of olefins and diolefins on all catalysts suggests that LaAl-based materials present the most acidic character among La-perovskites. [ABSTRACT FROM AUTHOR]

Despite the increasing economic incentives and environmental advantages associated to their substitution, carbon-rich fossil fuels are expected to remain as the dominant worldwide source of energy through at least the next two decades and perhaps later. Therefore, both the control and reduction of CO2 emissions have become environmental issues of major concern and big challenges for the international scientific community. Among the proposed strategies to achieve these goals, conversion of CO2 by its reduction into high added value products, such as methane or syngas, has been widely agreed to be the most attractive from the environmental and economic points of view. In the present work, thermocatalytic reduction of CO2 with H2 was studied over a nanostructured ceria-supported nickel catalyst. Ceria nanocubes were employed as support, while the nickel phase was supported by means a surfactant-free controlled chemical precipitation method. The resulting nanocatalyst was characterized in terms of its physicochemical properties, with special attention paid to both surface basicity and reducibility. The nanocatalyst was studied during CO2 reduction by means of Near Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS). Two different catalytic behaviors were observed depending on the reaction temperature. At low temperature, with both Ce and Ni in an oxidized state, CH4 formation was observed, whereas at high temperature above 500 °C, the reverse water gas shift reaction became dominant, with CO and H2O being the main products. NAP-XPS was revealed as a powerful tool to study the behavior of this nanostructured catalyst under reaction conditions. [ABSTRACT FROM AUTHOR]

Nano-structural and nano-analytical studies show that the dramatic difference in CO oxidation activity observed between two Au/Ce0.50Tb0.12Zr0.38O2−x samples prepared by deposition–precipitation with urea and further activated under oxidising or reducing conditions is due to the poisoning effect of a very thin layer of carbon grown on the pre-reduced catalyst. [ABSTRACT FROM AUTHOR]

Three nanostructured catalysts with low total rare earth elements (REEs) content (i.e., 15 mol.%) were prepared by depositing CeO2 or Ln3+-doped CeO2 (Ln3+ = Y3+ or La3+; Ln/Ce = 0.15) on the surface of ZrO2 nanoparticles, as nanometre-thick, fluorite-type clusters. These samples were subjected to successive reduction treatments at increasing temperatures, from 500 to 900 °C. A characterisation study by XPS was performed to clarify the diffusion process of cerium into the bulk of ZrO2 crystallites upon reduction to yield CexZr1−xO2−δ surface phases, and the influence of the incorporation of non-reducible trivalent REE cations, with sizes smaller (Y3+) and larger (La3+) than Ce4+ and Ce3+. For all nanocatalysts, a reduction treatment at a minimum temperature of 900 °C was required to accomplish a significant cerium diffusion. Notwithstanding, the size of the dopant noticeably affected the extent of this diffusion process. As compared to the undoped ZrO2-CeO2 sample, Y3+ incorporation slightly hindered the cerium diffusion, while the opposite effect was found for the La3+-doped nanocatalyst. Furthermore, such differences in cerium diffusion led to changes in the surface and nanostructural features of the oxides, which were tentatively correlated with the redox response of the thermally aged samples. [ABSTRACT FROM AUTHOR]

The gas-phase hydrogenation of benzoic acid was studied over Au supported on CeO2 and Ce0.62Zr0.38O2 (CZ). HAADF-STEM has established formation of nanoscale (mean = 1.5-2 nm) Au particles, which is consistent with CO adsorption measurements. Incorporation of Au facilitated partial support reduction during TPR to 573 K where the presence of Zr increased oxygen mobility, resulting in a greater degree of Ce4+ reduction (to Ce3+) in Au/CZ, as demonstrated by oxygen storage capacity and XPS measurements. Hydrogenation of an aqueous benzoic acid feed generated benzaldehyde and benzyl alcohol with a higher rate over Au/CZ that is attributed to the action of oxygen vacancies, which activate the carboxyl function for hydrogen attack. A parallel/consecutive kinetic model has been applied to quantify catalytic selectivity. A concerted (single step) conversion is proposed for Au/CeO2 that involves bridging interaction of the benzoate with Ce cations and Au nanoparticles with hydrogen addition. A stepwise conversion on Au/CZ is achieved via a Mars and van Krevelen mechanism with benzoic acid activation at an oxygen vacancy and reaction with surface hydrogen to generate benzaldehyde as a reactive intermediate that is converted to benzyl alcohol via nucleophilic CO attack. Switching from an aqueous to ethanolic feed increased rate due to greater oxygen vacancy availability with higher selectivity to benzaldehyde and appreciable toluene formation over Au/CZ. [ABSTRACT FROM AUTHOR]

Abstract: This work presents results of multitechnique (HRTEM, EDX, XRD, XPS, ICP, BET and EELS) studies on structure evolution of nanocrystalline ceria-based Ce1−x Yb x O2−(x/2) mixed oxides with increasing Yb content. Nano-sized (∼5nm) mixed oxides were prepared by precipitation in water-in-oil (W/O) microemulsion method. No phase separation or/and segregation were observed for studied samples after heating in oxygen up to 950°C. Realistic models of the intermediate structures (defected F- and C-type) for mixed oxides and mechanism of CeO2 →Ce1−x Yb x O2−(x/2) →Yb2O3 structural changes have been proposed and tested (HRTEM images and diffraction patterns simulation). Reducibility of Ce1−x Yb x O2−(x/2) mixed oxides and its changes with increasing of Yb content was measured and correlated with the structure features. The highest reducibility of Ce ions was observed for Ce0.5Yb0.5O1.75, which has a structure intermediate between F and C-type, with maximum number of defects and the highest average strain. [Copyright &y& Elsevier]

Abstract: We investigate a series of three La2O3/Al2O3 samples, with a lanthana loading close to the theoretical monolayer, obtained by calcination at 773K, 973K, or 1173K from a common precursor. The samples were characterized by N2 adsorption at 77K, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), a variety of nano-analytical and nano-structural electron microscopy techniques, temperature programmed desorption (TPD-CO2), and volumetric CO2 adsorption techniques. By combining the information obtained from all these studies, a detailed description of their texture, nano-structure, lanthana distribution, and chemical properties could be gained. Three different forms of supported lanthana have been identified, and their relative weight evaluated quantitatively. As revealed by CO2 adsorption, these forms show strong, weak, or not measurable surface basicity, respectively. Upon increasing the calcination temperature, a progressive inter-conversion of the co-existing lanthana forms with inherent loss of surface basicity is observed. The effect, though moderate, is particularly noticeable on the sample calcined at 1173K. The amount of CO2 irreversibly chemisorbed on the different samples, including the alumina support, correlates well with their catalytic activity for the Meerwein–Ponndorf–Verley reaction of cyclohexanone with 2-propanol. This behavior is discussed with reference to that expected for pure lanthana. We conclude that our supported lanthana samples represent an advantageous alternative to pure La2O3 as highly basic catalytic materials. [Copyright &y& Elsevier]

Abstract: The redox behaviour under flowing 5%CO/He and 5%H2/Ar of two Ce0.62Zr0.38O2 mixed oxides is investigated. The samples, hereafter referred to as CZ-MO and CZ-SO, were obtained by application to the same starting material two well established thermal ageing routines. These routines ensured the preparation of oxide samples with significantly different chemical and structural properties. To evaluate their redox behaviour, ultimate oxygen storage capacity (OSC) data were determined from stepwise thermo-gravimetric experiments at temperatures ranging from 473K to 1173K. In good agreement with earlier studies from the literature, both CO-OSC and H2-OSC data show that CZ-MO exhibits a superior reducibility throughout the whole range of temperatures. If CO-OSC and H2-OSC data are compared, we may notice that, in qualitative agreement with the thermodynamic predictions, CO-OSC values are significantly larger than those of H2-OSC at T ≤773K. Above this reference temperature CO-OSC and H2-OSC show much closer values. Though ultimate OSC data are generally assumed to be determined by the thermodynamics of the oxide-reductant system, it is shown that the CO-OSC values recorded at the lowest temperatures are not consistent with this assumption. As in the case of H2-OSC, the low-temperature CO-OSC data are actually determined by kinetic factors. [Copyright &y& Elsevier]

Three nanostructured catalysts with low total rare earth elements (REEs) content (i.e., 15 mol.%) were prepared by depositing CeO 2 or Ln 3+ -doped CeO 2 (Ln 3+ = Y 3+ or La 3+ ; Ln/Ce = 0.15) on the surface of ZrO 2 nanoparticles, as nanometre-thick, fluorite-type clusters. These samples were subjected to successive reduction treatments at increasing temperatures, from 500 to 900 °C. A characterisation study by XPS was performed to clarify the diffusion process of cerium into the bulk of ZrO 2 crystallites upon reduction to yield Ce x Zr 1-x O 2-δ surface phases, and the influence of the incorporation of non-reducible trivalent REE cations, with sizes smaller (Y 3+ ) and larger (La 3+ ) than Ce 4+ and Ce 3+ . For all nanocatalysts, a reduction treatment at a minimum temperature of 900 °C was required to accomplish a significant cerium diffusion. Notwithstanding, the size of the dopant noticeably affected the extent of this diffusion process. As compared to the undoped ZrO 2 -CeO 2 sample, Y 3+ incorporation slightly hindered the cerium diffusion, while the opposite effect was found for the La 3+ -doped nanocatalyst. Furthermore, such differences in cerium diffusion led to changes in the surface and nanostructural features of the oxides, which were tentatively correlated with the redox response of the thermally aged samples.

The selective oxidation of veratryl alcohol (VA), a model compound of lignin, with oxygen molecules to produce veratraldehyde (VAld) was studied over monometallic Au, Pd, and bimetallic Au:Pd nanoparticles supported on a Ce 0.62 Zr 0.38 O₂ mixed oxide for the first time. These bimetallic Au-Pd catalysts with Au:Pd molar ratios from 0.4 to 4.3 were synthesized by the sol-immobilization method. Furthermore, all the catalysts were characterized by inductively coupled plasma-atomic emission spectroscopy (ICP-AES), N₂ physisorption, X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) imaging, energy dispersive X-ray spectroscopy (EDXS), and temperature programmed reduction (TPR) techniques. A synergistic effect between gold and palladium was observed over all the bimetallic catalysts in a wide range of studied Au:Pd ratios. Remarkably, the optimum Au:Pd ratio for this reaction was 1.4 with a turnover frequency of almost six times larger than for the monometallic gold and palladium catalysts. Selectivity to veratraldehyde was higher than 99% for the monometallic Au, Pd, and all the bimetallic Au-Pd catalysts, and stayed constant during the reaction time.

Ceria nanocubes (NC) modified with increasing concentrations of praseodymium (5, 10, 15, and 20 mol %) have been successfully synthesized by a hydrothermal method. The as-synthesized Pr-modified ceria nanocubes exhibit an enhanced oxidase-like activity on the organic dye TMB within a wide range of concentrations and durations. The oxidase activity increases with increasing Pr amounts in Pr-modified ceria nanocubes within the investigated concentration range. Meanwhile, these Pr-modified ceria nanocubes also show higher reducibility than pure ceria nanocubes. The kinetics of their oxidase mimetic activity is fitted with the Michaelis-Menten equation. A mechanism has been proposed on how the Pr incorporation could affect the energy level of the bands in ceria and hence facilitate the TMB oxidation reaction. The presence of Pr 3+ species on the surface also contributes to the increasing activity of the Pr-modified ceria nanocubes present higher oxidase activity than pure ceria nanocubes.

Novel photochromic hybrid nanomaterials were prepared through the immobilization of the lacunary Keggin-type phosphomolybdate (TBA4H3[PMo11O39]·xH2O, denoted as PMo11) and sandwich-type lanthano phosphomolybdates (K11[Ln(III)(PMo11O39)2]·xH2O, denoted as Ln(PMo11)2, where Ln(III) = Sm, Eu, Gd, Tb and Dy) onto positively-charged functionalized silica nanoparticles. The functionalized silica nanoparticles were prepared by a one-step co-condensation route between tetraethyl orthosilicate and dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride, presenting an average particle size of 95 ± 26 nm, a spherical morphology and a pore diameter of 13.7 nm. All characterization techniques proved the successful immobilization of the phosphomolybdates. The photochromic properties of the resulting hybrid nanomaterials in the solid state were evaluated by UV-Vis spectroscopy and colorimetry. All materials revealed promising photochromic properties under UV irradiation (λ = 254 nm). The lacunary phosphomolybdate anchored onto the silica nanoparticles, C18-SiO2@PMo11, showed the best photoswitching properties, with the color changing from green to dark-blue (ΔE* = 26.8). Among the Ln(PMo11)2-based hybrid nanomaterials, those containing higher Mo loadings--Eu(III)- and Tb(III)-based samples--presented more significant color changes from green to dark-blue (ΔE* = 18.8-18.9). These results revealed that the optical properties of the as-prepared hybrid nanomaterials did not depend directly on the type of Ln(III) cation, but only on the amount of Mo, which was the target element responsible for the photochromic behavior.

Tm-doped TiO2 nanoparticles were synthesized using a water-controlled hydrolysis reaction. Analysis was performed in order to determine the influence of the dopant concentration and annealing temperature on the phase, crystallinity, and electronic and optical properties of the resulting material. Various characterization techniques were utilized such as X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy and UV-vis spectroscopy. For the samples annealed at 773 and 973 K, anatase phase TiO2 was obtained, predominantly internally doped with Tm(3+). ICP-AES showed that a doping concentration of up to 5.8 atom % was obtained without reducing the crystallinity of the samples. The presence of Tm(3+) was confirmed by X-ray photoelectron spectroscopy and UV-vis spectroscopy: the incorporation of Tm(3+) was confirmed by the generation of new absorption bands that could be assigned to Tm(3+) transitions. Furthermore, when the samples were annealed at 1173 K, a pyrochlore phase (Tm2Ti2O7) mixed with TiO2 was obtained with a predominant rutile phase. The photodegradation of methylene blue showed that this pyrochlore phase enhanced the photocatalytic activity of the rutile phase.

Nano-structural and nano-analytical studies show that the dramatic difference in CO oxidation activity observed between two Au/Ce0.50Tb0.12Zr0.38O2-x samples prepared by deposition-precipitation with urea and further activated under oxidising or reducing conditions is due to the poisoning effect of a very thin layer of carbon grown on the pre-reduced catalyst.