Marta Fernández-García

New Pd/CexZr1−xO2/Al2O3 three-way catalysts prepared by microemulsion: Part 1. Characterization and catalytic behavior for CO oxidation

The state of a series of palladium catalysts supported on ceria–zirconia and ceria–zirconia/alumina in the calcined state and under a reactive CO+O2 atmosphere has been characterized by a combination of electron transmission microscopy, infrared and electron paramagnetic resonance spectroscopies, and the results obtained used to interpret the catalytic behavior displayed by the samples in the CO oxidation reaction. The Ce/Zr promoter enhances the catalytic properties of the noble metal by favoring the formation of the metallic state and through creation of anionic vacancies in the Ce–Zr component even from room temperature. The optimum promoting effect in the CO oxidation is observed for palladium particles in contact with three-dimensional-like Ce–Zr particles due to the appropriate nature and high number of the anion vacancies present at their surface. Compared with classical ceria-based systems, CO oxidation is here maximized for Pd-supported on the bulk material and not on mixed ceria-containing/alumina catalysts. These differences are discussed on the basis of the effect of the ceria and ceria–zirconia promoter particle size on the nature and number of active sites.

New Pd/CexZr1−xO2/Al2O3 three-way catalysts prepared by microemulsion: Part 2. In situ analysis of CO oxidation and NO reduction under stoichiometric CO+NO+O2

Abstract The light-off behavior for CO oxidation and NO reduction under stoichiometric CO+NO+O2 gas mixtures of a series of palladium catalysts supported on ceria–zirconia, ceria–zirconia/alumina and alumina has been examined by means of catalytic activity tests, and by DRIFTS, XANES and EPR spectroscopies. The results show the promoting effect of the Zr–Ce mixed oxide on both reactions. The extent of promotion depends on the characteristics of the promoter entities present for each catalyst and on the nature of the reaction. As observed in the absence of NO, CO oxidation was mainly enhanced by contact between Pd and 3D aggregated promoter entities, although an inhibiting effect of NO on CO oxidation, attributed to passivation of the interface sites due to formation of oxidized states of Pd, was observed at low temperatures. In contrast, the higher activity for NO reduction over catalysts supported on ceria–zirconia/alumina suggests that most active sites for that reaction involve Pd interacting with dispersed promoter entities.

Nature and catalytic role of active silver species in the lean NOx reduction with C3H6 in the presence of water

A study of the lean NOx reduction activity with propene in the presence of water over Ag/Al2O3 catalysts with different silver loadings (1.5–6 wt%) has been done using X-ray diffraction, ultraviolet–visible spectroscopy, transmission electron microscopy, and in situ diffuse reflectance infrared and X-ray absorption spectroscopies under reaction conditions. The catalysts were prepared by an impregnation method employing EDTA complexes that allow highly dispersed silver phases to be obtained, which are stabilized under reaction conditions by strong interactions with the support. It is shown that the active species corresponds to silver aluminate-like phases with tetrahedral local symmetry. The role of silver in the reaction mechanism is shown to be mainly in the activation of NOx and propene species. In particular, the silver entities have been found to offer a new reaction path for propene activation which involves generation of acrylate species as a partially oxidized active intermediate. Differences between two active catalysts containing 1.5 and 4.5 wt% of Ag suggest that optimization of the SCR activity can be related to the oxygen lability of the tetrahedral silver aluminate-like phase present in the catalyst. As postulated previously, the high nonselective propene oxidation activity of the highest loaded sample (with 6 wt% Ag) appears to be related to formation of metallic silver surface states at low reaction temperatures which are active for NO dissociation.  2003 Elsevier Science (USA). All rights reserved.

Redox interplay at copper oxide-(Ce, Zr)Ox interfaces: influence of the presence of NO on the catalytic activity for CO oxidation over CUO/CeZrO4

A catalyst composed of copper oxide supported on cerium–zirconium mixed oxide (CuOx/CeZrO4) has been studied with respect to its activity for CO oxidation under stoichiometric conditions employing either oxygen or oxygen with a small amount of NO as oxidant. The nature of copper oxide entities, as well as the redox properties of the catalyst following interactions with CO and O2–NO, has been studied by XPS, EPR, and static NO adsorption infrared spectroscopy while in situ DRIFTS has been employed to follow processes occurring at the catalyst surface under reaction conditions. Characterization of the copper oxide species in both fully oxidized and partially reduced states reveals that they are significantly affected by interactions with the underlying support. On the basis of catalytic activity results and in combination with analysis of the evolution of particular Cu+ carbonyls, comparing the CO oxidation reaction in the presence and absence of NO, it is proposed that the two basic factors affecting the catalytic performance of this type of system are the facility for achieving a partially reduced state for the copper oxide phase at the interfacial zone and the redox properties of the CuOx/CeZrO4 interface.

Visible light-activated nanosized doped-TiO2 photocatalysts

The report shows that highly doped anatase-like binary mixed oxides constitute a route to improve the performance of nanosized titania photocatalysts under sunlight excitation.

Light-off behaviour of PdO/γ-Al2O3 catalysts for stoichiometric CO-O2 and CO-O2-NO reactions: A combined catalytic activity-in situ DRIFTS study

Abstract Three PdO/ γ -Al 2 O 3 catalysts differing in Pd loading (between 0.05 and 1 Pd wt%) have been examined with regard to their light-off catalytic activity for CO oxidation and NO reduction reactions under stoichiometric conditions. Catalytic activity results are explained on the basis of DRIFTS analysis of the adsorbed species present under reaction conditions. It is shown that differences between the catalysts (apart from the expected increasing activity with the Pd loading for both reactions) are considerably greater for NO reduction than for CO oxidation reactions. This is explained by structural differences between the active metallic Pd particles formed during the particle nucleation/growth process that takes place during the course of the light-off run upon interaction with the reactant mixture. On the basis of differences in the natures and relative intensities of adsorbed CO and NO species present during competition for atop and bridging sites over the Pd particles, the correlation (within the Pd loading studied) between NO reduction capability and Pd loading is attributed to the increasing NO dissociation efficiency as the relative size of the particles formed during the course of the reaction is increased.

Fabrication of Honeycomb-Structured Porous Surfaces Decorated with Glycopolymers

We prepared breath figure patterns on functional surfaces by the surface segregation of a statistical glycopolymer, (styrene-co-2-(D-glucopyranosyl) aminocarbonyloxy ethyl acrylate (S-HEAGl). The synthesis of the statistical glycopolymer is prepared in a straightforward approach by conventional free radical copolymerization of styrene and the unprotected glycomonomer. Blends of this copolymer and high-molecular-weight polystyrene were spin coated from THF solutions, leading to the formation of surfaces with both controlled functionality and topography. AFM studies revealed that both the composition of the blend and the relative humidity play key roles in the size and distribution of the pores at the interface. Thus, the topographical features obtained on the polymer surfaces during film preparation by the breath figure methodology varied from 200 to 700 nm. Moreover, this approach leads to porous films in which the hydrophilic glycomonomer units are oriented toward the pore interface because upon soft annealing in water the holes are partially swelled. The self-organization of the glycopolymer within the pores was additionally confirmed by the reaction of carbohydrate hydroxyl groups with rhodamine isocyanate. Equally, we demonstrate the bioactivity of the anchored glycopolymers by means of the lectin binding test using concanavalin A (Con A).

Effect of g-C3N4 loading on TiO2-based photocatalysts: UV and visible degradation of toluene

g-C3N4 was introduced into a highly active anatase TiO2-based photocatalyst using a simple yet effective impregnation method. The corresponding composite materials were obtained with a carbon nitride weight content ranging from 0.25 to 4 wt.%, and their surface, morphological and structural properties were characterized using BET and porosity measurements, X-ray diffraction, infrared, UV-vis and photoluminescence spectroscopy, and transmission electron microscopy. They were subsequently tested for toluene photoelimination under UV and sunlight-type illumination conditions. The photocatalytic performance of the materials was analysed quantitatively through calculation of the quantum yield of the reaction under all illumination conditions essayed. We observed that the addition of carbon nitride enhances the photoactivity and selectivity to CO2 of samples containing 0.5 to 1 wt.% g-C3N4. Physicochemical characterization presented evidence that the enhanced behaviour is related to the intimate contact between the two components of the photocatalyst system, which provides two new (i.e. not present in the parent system) charge carrier handling routes affecting photocatalytic properties upon UV and visible light illumination.

Synthesis and aqueous solution properties of stimuli-responsive triblock copolymers

Two series of poly(di(methylamino)ethyl methacrylate)--poly(di(ethylene glycol)methyl ether methacrylate)--poly(di(methylamino)ethyl methacrylate), pDMAEMA--pDEGMEMA--pDMAEMA, and poly(di(ethylamino)ethyl methacrylate)--poly(di(ethylene glycol)methyl ether methacrylate)--poly(di(ethylamino)ethyl methacrylate), pDEAEMA--pDEGMEMA--pDEAEMA, triblock copolymers have been synthesized using copper-mediated living radical polymerization. Kinetic studies showed that the polymerizations were controlled yielding copolymers with MWt similar to predicted values with narrow polydispersities. UV/Vis experiments in aqueous solution indicate that the incorporation of pDMAEMA hydrophilic segments into the pDEGMEMA polymeric chains causes an increase in the lower critical solution temperature (LCST). Conversely, the incorporation of pDEAEMA hydrophobic segments has the reverse effect. It has been demonstrated that the cloud points of the copolymers increase when blocks pDMAEMA and pDEAEMA are protonated. At acidic pH and above 35-40 °C, the pDEGMEMA inner blocks become insoluble and, consequently, upon contact with water they form micellar aggregates constituted of a pDEGMEMA hydrophobic core with a pDMAEMA or pDEAEMA hydrophilic shell. At pH = 8.3 and 25 °C, the pDMAEMA--pDEGMEMA--pDMAEMA triblock copolymers form ordered aggregates of larger size. In addition under these conditions, in aqueous solution the pDEGMEMA form aggregates of smaller size than those found in the case of copolymers with pDMAEMA. Surface-tension measurements suggest that homopolymers, as well as copolymers, are surface active. The pDEGMEMA polymers present low values of the critical aggregation concentration (cac), due to their amphiphilic properties. The combination of pDEGMEMA thermosensitive segments with pDMAEMA and pDEAEMA blocks sensitive to pH and temperature has given rise to the formation of intelligent materials with responses to both stimuli. Their behavior depends on the MWt and the composition of the copolymers.

Role of Pt in Pt/Ba/Al2O3 NOx storage and reduction traps

The stability of stored NOx under air, lean propene–air mixtures and stoichiometric air–propene mixtures has been studied for Pt/Ba/Al2O3 catalysts under temperature programmed and constant λ conditions to simulate, with a simplified gas composition, the conditions experienced by a NOx storage and reduction catalyst during the warm-up period. The stability of stored NOx was mainly influenced by gas-phase oxygen concentration. Although the presence of propene lowered the onset temperature of NOx desorption, increasing the partial pressure of propene in the system had little influence on the stored NOx stability. Under the conditions of the experiments, the majority of NOx was desorbed before the onset of propene combustion. Results suggest that NOx desorption from the alumina and baria components undergo reverse spillover onto the platinum surface which inhibits the activation of propene thereby increasing the light-off temperature to produce CO2.