A.B. Hungría

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.

Confinement effects in quasi-stoichiometric CeO2 nanoparticles

This paper deals with the analysis of structural and electronic effects of size in quasi-stoichiometric CeO2 nanoparticles prepared by a microemulsion method. The preparation method yields highly controlled materials in terms of particle size distribution and chemical oxidation state, with the presence of Ce(III) species only below an average particle size of ca. 8 nm. The rather low quantity of Ce reduced ions produces marked differences with confinement effects previously reported in the literature. A steady behavior of the fluorite lattice parameter is observed as a function of size in the 5–10 nm range. In this range, the bandgap displays a small decrease of ca. 0.1 eV, with significant differences from the behavior expected on the basis of the effective mass approximation. These structural and electronic properties are rationalized on the basis of the characterization of the materials.

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.

Behavior of bimetallic Pd–Cr/Al2O3 and Pd–Cr/(Ce,Zr)Ox/Al2O3 catalysts for CO and NO elimination

A series of Pd–Cr bimetallic catalysts supported on a (Ce,Zr)Ox/Al2O3 mixed support or Al2O3 alone have been characterized using a combination of X-ray diffraction, electron paramagnetic resonance, and Raman spectroscopy and employing in situ diffuse reflectance infra-red Fourier transform and X-ray near-edge structure spectroscopy to analyze the redox and chemical processes taking place during the course of the CO+NO+O2 reaction. The catalytic behavior of these bimetallic systems was strongly affected by the nature of the support. In the case of the alumina support, evidence for interactions between Pd and Cr was observed in the calcined state. Under reaction conditions, formation of a mixed oxide phase containing Pd(I) and Cr(III) appears to be responsible for the improvement in CO oxidation and for the detrimental effect in the NO reduction process with respect to a monometallic, Pd reference system. In the case of the (Ce,Zr)Ox/Al2O3 mixed support, the addition of Cr was less influential in terms of catalytic activity. Results for the latter are rationalized mainly on the basis of the absence of interaction between the metal components in the calcined state and the presence of surface entities containing Cr alone when metallic-like Pd particles are formed during the course of reaction.

Effects of thermal pretreatment on the redox behaviour of Ce0.5Zr0.5O2: isotopic and spectroscopic studies

The temperature programmed reduction (TPR) of a single phase Ce0.5Zr0.5O2 solid solution is investigated by using 1H2/Ar and 2H2/Ar mixtures as reducing agents. The effects of mild (700 K) or severe (1273 K) pre-oxidation (MO, SO) on the TPR profile are analysed. It is observed that after SO pretreatment hydrogen activation/scrambling occurs at about 600 K and the reduction profile is characterised by a single reduction peak at 920 K. In contrast, a TPR/MO pretreatment of such SO sample leads to a facile hydrogen activation/scrambling and low temperature reduction profiles, the main reduction peak being observed at ca. 650 K. Using 1H2 and 2H2 as reducing agents, an isotopic effect is observed for the reduction processes with a peak occurring above 700 K, while equal activation energies are measured for those occurring below 700 K. This suggests that under the TPR conditions the surface reduction steps (activation of the reducing agent at the surface, surface reduction steps and water evolution) are not rate limiting for the low temperature reduction process, which is controlled by bulk diffusion, while some surface step (probably water formation) is limiting for the high temperature reduction. Studies by EPR on the nature of the oxygen species adsorbed on variously pretreated specimens and by FTIR of the methoxy species formed upon methanol adsorption on the same materials reveal that the main changes induced by high temperature calcination are related to segregation of ceria- and zirconia-type islands, with some surface enrichment in cerium probably occurring as well. Only subtle differences between SO and MO pretreated samples can be detected at either surface or bulk (XRD experiments) levels, in contrast with the significant modifications produced by the different pretreatments on the reduction characteristics of the sample.

Influence of Sn4+ on the structural and electronic properties of Ti1−xSnxO2 nanoparticles used as photocatalysts

Ti(1-x)Sn(x)O(2) nanocrystalline materials employed for photocatalysis have been characterised by means of X-ray diffraction, Raman, X-ray absorption (XANES and EXAFS) and UV-Vis spectroscopy and high resolution transmission electron microscopy. Single-phase samples with anatase or rutile type structures and similar tin contents permitted a separate study of the effect of Sn(4+) ions on these crystalline forms, whereas materials composed of anatase and rutile mixtures were used to investigate the distribution of the dopant cations when both phases coexist. The results obtained from the single-phase doped TiO(2) samples indicate that the presence of tin causes a different effect when doping anatase or rutile in both their structural and electronic properties. While a random substitution of Sn(4+) for Ti(4+) seems plausible for the rutile phase, some kind of gradient in Sn(4+) concentration is possible in anatase. On the other hand, when anatase and rutile coexist, effects of doping are visible in both phases. Regarding chemical composition, a homogeneous distribution of tin was found in both calcined and hydrothermal multiphase samples. Photocatalytic experiments show that both tin-doping and coexistence of different phases have a beneficial effect on the activity of the catalysts.

Thermal behavior of (Ce,Zr)Ox/Al2O3 complex oxides prepared by a microemulsion method

A study is reported on the thermal behavior of a (Ce,Zr)Ox/Al2O3 specimen prepared by a microemulsion method and subjected to aging treatments. A similarly prepared classical CeOx/Al2O3 support was used as a reference. Attention is paid to the 1273–1373 K region of treatment, where previous experience demonstrates that this type of material suffers deactivation processes. The structural/morphological characterization of the complex support was performed using X-Ray diffraction (XRD), transmission electron microscopy (TEM) and Raman spectroscopy. Surface and redox properties were also examined using electron paramagnetic resonance (EPR) and temperature programmed reduction (TPR). The (Ce,Zr)Ox/Al2O3 sample contains a mixed oxide component that essentially maintains its structural stability after aging treatment under air at 1273 K although an abrupt loss of the structural homogeneity, leading to segregation into two distinct phases, is produced upon aging at 1373 K. Ce–Zr promoter phases detected during the study maintain the t″ tetragonal symmetry and display a moderate surface composition evolution (small with respect to bulk samples) irrespective of the aging treatment. This is attributed to (Ce,Zr)Ox–Al2O3 interactions, which induce beneficial effects on both components, the Ce–Zr promoter and the alumina carrier. TPR experiments show that the aged (Ce,Zr)Ox/Al2O3 materials display a low temperature reduction peak around 650 K, which was previously observed only on activated bulk materials subjected to redox cycling, and may be of importance in the oxygen handling properties of these materials.