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Au-Pd supported on ceria-zirconia catalysts, with Au:Pd molar ratios in the 1:1-10:1 range, have been prepared using deposition precipitation of Au followed by impregnation with Pd. The resulting Au-Pd supported catalysts were activated by oxidation at 350, 450 and 700 °C. The oxidation at 700 °C led to the formation of large Au-Pd alloy nanoparticles showing lower catalytic activity for CO oxidation. The Au-Pd/CeZrO catalyst with the highest Au:Pd ratio, 10:1, exhibited the highest CO oxidation activity. Graphical Abstract: [Figure not available: see fulltext.] [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]

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]

Graphical abstract Highlights • Ceria-praseodymia promoted Ni-alumina catalysts for CO 2 methanation are prepared. • Influence of Ni loading, promoter composition and alumina modifier is analyzed. • The Ni content has the more pronounced influence, followed by the alumina dopant. • The effect of Ce/Pr molar ratio in the promoter seems to be almost negligible. • Ni(10 wt%)/Ce 0.8 Pr 0.2 O 2-δ /Al 2 O 3 -SiO 2 shows the best catalytic performance. Abstract Two series of ceria-praseodymia promoted Ni-alumina catalysts were prepared from two different commercial modified alumina supports (3.5 wt% SiO 2 -Al 2 O 3 and 4.0 wt% La 2 O 3 -Al 2 O 3) by the incipient wetness impregnation method in two successive steps. The resulting materials were characterized in terms of their physico-chemical properties by means of N 2 physical adsorption at −196 °C, powder X-ray diffraction (XRD) and temperature programmed reduction with H 2 (H 2 -TPR). Furthermore, the as-prepared catalysts were tested for the CO 2 methanation reaction in a fixed-bed reactor at atmospheric pressure, gas hourly space velocity (GHSV) of 72,000 cm3·(h·g cat)−1 and CO 2 /H 2 molar ratio of 1/4 over the temperature range from 25 up to 850 °C. The influence of the nominal Ni loading (3, 5 and 10 wt%), molar composition of the Ce/Pr mixed oxide promoter (80/20 and 60/40), and alumina modifier (silica and lanthana) on the catalytic performance was carefully analyzed. Among these three composition parameters, the alumina dopant and especially the Ni content appear to have by far a much more pronounced effect on both the CO 2 conversion and CH 4 selectivity as compared to the Ce/Pr mixed oxide composition. Specifically, from the catalytic tests the sample containing a 10 wt% Ni loading, a Ce/Pr mixed oxide promoter of 80/20 molar composition, and silica as modifier provides the highest catalytic activity in terms of CO 2 conversion and CH 4 selectivity. Such behaviour has been ascribed to a complex interplay between several factors, mainly the larger fraction of catalytically active β-type NiO species and the lesser concentration of strong basic sites on the catalyst surface, as well as the electron back donation effect from the surface Ni atoms to the adsorbed CO x species, which favours the C–O bond cleavage (i.e., the rate-determining step of the methanation reaction). These findings are expected to be very helpful in order to rationally design synthetic strategies that allow developing highly active and low cost Ni-based catalysts for the CO 2 methanation reaction. [ABSTRACT FROM AUTHOR]

[Display omitted] • Phen-doped ceria nanocubes prepared by a hydrothermal method under mild conditions. • Phen incorporation does not alter both crystalline structure and morphology. • Phen molecule coordinated to cerium atoms of the ceria lattice. • Band gap decrease observed with increasing phen content. • Enhanced optical properties related to intermediate N 2 p states due to phen. A novel facile method to prepare ceria nanocrystals with well-defined cubic morphology and enhanced optical properties is herein disclosed. Both the decrease in the band gap and the appearance of absorption edges above 400 nm, which redound in a significant absorption of visible light, are accomplished by simply incorporating in situ different amounts of a typical chelating bidentate ligand, 1,10-phenanthroline, during the synthesis of these ceria nanocubes. Such a remarkable effect has been tentatively connected with the ability of this nitrogen-containing heterocyclic compound to coordinate Ce3+ cations from the ceria precursor salt, thus yielding intermediate N 2 p states along the band gap of the oxide. [ABSTRACT FROM AUTHOR]

By depositing ceria on the surface of yttrium-stabilized zirconia (YSZ) nanocrystals and further activation under high-temperature reducing conditions, a 13% mol. CeO2/YSZ catalyst structured as subnanometer thick, pyrochlore-type, ceria-zirconia islands has been prepared. This nanostructured catalyst depicts not only high oxygen storage capacity (OSC) values but, more importantly, an outstandingly stable redox response upon oxidation and reduction treatments at very high temperatures, above 1000 ~C. This behavior largely improves that observed on conventional ceria-zirconia solid solutions, not only of the same composition but also of those with much higher molar cerium contents. Advanced scanning transmission electron microscopy (STEM-XEDS) studies have revealed as key not only to detect the actual state of the lanthanide in this novel nanocatalyst but also to rationalize its unusual resistance to redox deactivation at very high temperatures. In particular, high-resolution X-ray dispersive energy studies have revealed the presence of unique bilayer ceria islands on top of the surface of YSZ nanocrystals, which remain at surface positions upon oxidation and reduction treatments up to 1000 °C. Diffusion of ceria into the bulk of these crystallites upon oxidation at 1100 °C irreversibly deteriorates both the reducibility and OSC of this nanostructured catalyst. [ABSTRACT FROM AUTHOR]

A four-step reaction mechanism is proposed for the H 2 dissociation over pure ceria and gallium-promoted mixed oxide materials, in a combined experimental and computational investigation. Two samples of cerium-gallium mixed oxides with Ce/Ga atomic ratios equal to 90/10 and 80/20 were studied by time-resolved diffuse reflectance infrared spectroscopy under H 2 (D 2 ) flow at isothermal condition in the range of 523–623 K. X-ray photoelectron spectrometry allowed to conclude that only Ce 4+ is reduced to Ce 3+ (Ga 3+ is not reduced), in agreement with density functional theory (DFT) results. The time evolution profiles of gallium hydride ( Ga H ) species, hydroxyl groups (OH) and Ce 3+ infrared signals were analyzed and kinetic rate parameters for each step were obtained by mathematical modeling. The values for activation energies were in agreement with those calculated by DFT, for the different elementary pathways. A small activation energy (∼4 kcal/mol) was found for H 2 dissociation found on Ga ⋯ O Ce sites assuming that the heterolytic cleavage of the H H bond is the rate determining step. On pure ceria, the experimental activation energy is ∼23 kcal/mol, showing that the addition of Ga 3+ cations boosts the splitting of H 2 . Interestingly, the reduction step of pure CeO 2 surface domains seems to proceed via a CeH/OH pair intermediate, according to DFT calculations. Moreover, 71 Ga NMR experiments indicate the possible presence of gallia nanodomains. It is proposed that the generation of Ga ⋯ O Ce sites in the perimeter of such surface gallia nanodomains is responsible for the enhanced reactivity of the mixed materials. The key role of this new type of sites to improve the efficiency of relevant catalytic reactions such as selective alkyne hydrogenation and light alkane dehydrogenation is then analyzed. [ABSTRACT FROM AUTHOR]

[Display omitted] • Synthesis of praseodymia nanorods and nanoparticles. • Excellent multi-funtional oxidase-like activities of praseodymia nanomaterials. • Colorimetric sensing of L-cysteine and fluoride using praseodymia nanorods. The ability to mimic protein-based oxidase with multi-functional inorganic nanozymes would greatly advance biomedical and clinical practices. Praseodymia (PrO x) nanorods (NRs) and nanoparticles (NPs) have been synthesized using hydrothermal and precipitation methods. Both PrO x catalysts with different morphologies exhibit significantly higher oxidase-like activities (Michaelis-Menten constant K m ≤ 0.026 mM) than commercial PrO x and most so-far-reported artificial enzymes. One of the substrates, dopamine, can be oxidized and further polymerized to generate polydopamine in acidic conditions. Akin to CeO 2 , which is a well-studied nanozyme, a different mechanism involving holes+, oxygen vacancies and oxygen mobility over PrO x catalysts has been proposed in this work. However, fluoride ions were found to impose opposite effects on the oxidase-mimicking activity of PrO x and CeO 2 , implying a promising path for the exploration of new nanozymes. In support of this, PrO x was further applied in colorimetric sensing of L-cysteine and fluoride with high sensitivity. [ABSTRACT FROM AUTHOR]

A series of bimetallic Au-Pd catalysts supported on Ce 0.62 Zr 0.38 O 2 mixed oxide has been synthesized using a simultaneous deposition-precipitation method. Different Au:Pd ratios were obtained maintaining Au loadings constant and varying Pd content. Elemental analysis, X-ray diffraction, N 2 physisorption , X-ray photoelectron spectroscopy, scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) and X-ray energy dispersive spectroscopy (XEDS) techniques have been employed to characterize these catalysts. The catalytic activities for selective oxidation of glycerol over these bimetallic catalysts have been evaluated, a synergistic effect being observed. Thus, the bimetallic Au-Pd catalyst with the lowest Pd content was the one exhibiting the highest catalytic activity among all the prepared catalysts. Oxidation at 700 °C of the bimetallic 2.2AuPd/Ce 0.62 Zr 0.38 O 2 catalyst led to an increase of the catalytic activity for the selective oxidation of glycerol, which seemed to be due to the formation of a larger fraction of bimetallic Au-Pd alloy-type nanoparticles after oxidation at higher temperatures. [ABSTRACT FROM AUTHOR]

Monometallic Au, Pd and bimetallic Au-Pd catalysts supported on a Ce 0.62 Zr 0.38 O 2 mixed oxide have been synthesized by the sol-immobilization method. An in-depth Scanning-Transmission Electron Microscopy (STEM) study has been performed to reveal the structural and chemical nature of the metal nanoparticle system present in these catalysts. Attention has been paid both to the evolution of the particle size distribution and the degree of Au-Pd interaction as a function of the treatment used to activate the catalysts. This characterization work has been complemented by results coming from other macroscopic techniques like Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES), N 2 physisorption, Temperature Programmed Oxidation (TPO) or X-ray Photoelectron Spectroscopy (XPS). The whole set of characterization data evidences the intrinsic structural complexity of this type of bimetallic systems, in which a fraction of monometallic Au, monometallic Pd and bimetallic nanoparticles of varying compositions coexist. This picture, which clearly contrasts with other much more homogeneous situations described in previous literature on bimetallic catalysts, also indicates the requirement of combining both atomic scale and macroscopic techniques to understand the structure of these catalysts. Concerning catalytic performance, a synergistic effect between Au and Pd has been observed in the selective oxidation of benzyl alcohol on ceria-zirconia. Moreover, the catalytic activity of these bimetallic Au-Pd catalysts is higher than that on other commonly used supports, such as activated carbon or carbon nanotubes. The treatments in oxidative and inert atmospheres at 250 °C improve the catalytic activity with respect to the fresh, un-pretreated, catalyst. Subsequent reduction of the oxidized bimetallic catalyst leads to an activity similar to that of the fresh catalyst, which is reduced at room temperature with H 2 bubbles. [ABSTRACT FROM AUTHOR]

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]

Review: 393 refs. [ABSTRACT FROM AUTHOR]