J.A. Pérez-Omil

Lanthanide salts as alternative corrosion inhibitors

Abstract Lanthanum and samarium nitrates and chlorides have been investigated as corrosion inhibitors of AISI 434 SS in sodium chloride solutions, at room temperature. Electrochemical techniques allowed to evaluate the degree of protection and the cathodic nature of the inhibitors. Scanning electron microscopy and energy dispersive spectrometry were used to analyze the composition of the protective films formed after full-immersion tests.

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.

Bridging the Gap between CO Adsorption Studies on Gold Model Surfaces and Supported Nanoparticles

An in-depth understanding of CO adsorption on highly dispersed gold nanoparticles (AuNPs) is critically important to fully interpret the catalytic behavior of supported gold systems in processes such as CO oxidation, PROX (selective oxidation of CO in presence of a large excess of H2), [5–7] or LT-WGS (low-temperature water gas-shift) reactions. Despite its relevance, the quantitative data and fundamental information presently available on the CO–Au interaction mainly comes from studies carried out on model single-crystal and thin-film surfaces under experimental conditions far from those at which catalytic assays on supported gold systems are typically run. Probably because of the very weak and singular nature of the CO–Au interaction, which on the basis of both theoretical and experimental studies is generally acknowledged to take place on low-coordination surface sites, and the additional contribution of the support, a few studies have been carried out that were aimed at estimating the amount of CO adsorbed at low temperature on powdered or model supported AuNPs. To our knowledge, however, none of these have arrived at a detailed quantitative description of this process under conditions close to those occurring in real catalytic reactions. To bridge this gap, we have developed an approach in which AuNP size distributions, as determined from HAADFSTEM (high-angle annular dark-field scanning transmission electron microscopy) and quantitative CO adsorption data, as determined from volumetric adsorption at 308 K, under CO partial pressures ranging from 6.65 10 Pa to 3.99 10 Pa, are jointly analyzed with the help of a nanostructural model for the gold particles. This model could be deduced from the analysis of images recorded in a parallel HRTEM (highresolution transmission electron microscopy) study. As discussed herein, this approach gives a substantial experimental support to the extension to supported gold catalysts of the chemical principles governing the CO adsorption on model surfaces. We investigated two catalyst samples, 2.5 wt% Au/ Ce0.62Zr0.38O2 (Au/CZ) and 1.5 wt% Au/Ce0.50Tb0.12Zr0.38O2 x (Au/CTZ), which have significantly different gold particle size distributions. Two consecutive CO volumetric adsorption isotherms were recorded on the gold catalysts and the corresponding supports. Prior to running the second isotherms, samples were evacuated (residual pressure Pres< 1.33 10 4 Pa) for 30 min at 308 K. By processing the volumetric data in a similar way to earlier studies (for details, see the Supporting Information), the amounts of CO adsorbed on the AuNPs, on the surface cations of the supports (weak adsorption), and on the surface anions of the supports, which mainly consist of strongly chemisorbed carbonate species, could be determined from the difference of the two isotherms. The amount of CO adsorbed on the AuNPs at PCO= 1.33 10 4 Pa (100 Torr) was used as a measurement of the saturation coverage. The corresponding data are reported in Table 1 and the Supporting Information, Figure S1. Gold particle size distributions were determined for each of the investigated catalysts from the analysis of series of experimental ultra-high-resolution HAADF-STEM images (Supporting Information, Figure S2). In accordance with the physical principles lying behind the mechanism of image formation, this technique is particularly suitable for obtaining reliable metal particle size distributions in oxide-supported metal catalysts. Moreover, as recently shown, this technique can be fruitfully applied to a very fine characterization of AuNPs dispersed on mixed oxides of heavy elements, as is the case of those investigated herein. (Size distributions for Au/CZ and Au/CTZ catalysts are shown in the Supporting Information, Figure S2). [*] M. L pez-Haro, Dr. J. J. Delgado, J. M. Cies, E. del Rio, S. Bernal, Dr. M. A. Cauqui, Dr. S. Trasobares, Dr. J. A. P rez-Omil, Dr. J. J. Calvino Departamento de Ciencia de los Materiales e Ingenier a Metalfflrgica y Qu mica Inorg nica Facultad de Ciencias, Universidad de C diz Campus Rio San Pedro, 11510-Puerto Real, C diz (Spain) Fax: (+34)956-016-288 E-mail: jose.calvino@uca.es

Chemical Imaging at Atomic Resolution as a Technique To Refine the Local Structure of Nanocrystals

The challenging problem of mapping the chemical composition of cation columns in individual nanocrystals at atomic resolution is addressed by using a method based on aberration-corrected electron microscopy, core-loss electron energyloss spectroscopy, and simulations. The potential of this novel approach to provide unique structural information, which is the key to rationalizing macroscopic behavior, is illustrated with the analysis of ceria–zirconia mixed oxides, which are nanomaterials with substantial technological impact. Metal nanoparticles supported on this family of oxides are currently materials of interest as catalysts in a variety of chemical transformations in the area of environmental protection, such as low-temperature water-gas shift, selective oxidation of CO in the presence of large amounts of hydrogen, or three-way catalysis. Strong variations in the chemistry of ceria–zirconia mixed oxide catalysts have been observed after they have undergone redox cycles involving reduction treatments at high temperatures ( 1173 K) then oxidation at mild temperatures ( 823 K). In particular their reducibility is significantly enhanced after such aging treatments. Scanning transmission electron microscopy (STEM) techniques have provided crucial information to account for these changes in the redox behavior. High-resolution electron microscopy (HREM) combined with high-angle annular dark-field (HAADF) imaging and tomography have revealed the occurrence of a disorder–order transformation in the cationic sublattice of these oxides, which tend to rearrange into a distribution characteristic of the so called pyrochlore phase. This phase is an archetype structure for A2B2O7 (A= + 3 cation, B=+ 4 cation) compounds and can be considered a fluorite superstructure. The structural transformation takes place during the reduction step of the cycle, in which the fully reduced mixed oxide with Ce/Zr molar ratio 1:1 adopts the Ce2Zr2O7 stoichiometry. Nevertheless, HAADF studies have clearly shown that, in the case of ceria–zirconia mixed oxides, this cation-ordered arrangement is preserved even after full reoxidation, that is, in the oxide with Ce2Zr2O8 stoichiometry, whenever the oxidation temperature does not exceed 823 K. Electron-microscopy studies have also revealed another remarkable feature of the ceria–zirconia aged oxides with the pyrochlore-type cation sublattice: the occurrence of compositional heterogeneities at the nanometer scale. Taking these observations into account and also considering that the disorder–order transition may not be completed in the time scale and under the temperature conditions used in the redox-cycling treatments, the important question arises whether these heterogeneities are in fact occurring on a finer scale, that is, at the atomic level. Such heterogeneities, compatible with the HREM and HAADF observations, will strongly influence the details of the counterpart oxygen sublattice and, consequently, the chemical and catalytic response of these oxides. To date, the atomic-column by atomic-column compositional analysis of the oxidized pyrochlore required to justify such a possibility has not been accomplished. Herein, using the capabilities of an aberration-corrected Nion UltraSTEM microscope (operated at 100 kV) we not only provide the first direct chemical evidence of the cationic order present in the Ce2Zr2O8 oxidized pyrochlore but we also show how atomicresolution electron energy-loss spectroscopy (EELS) mapping, based on core–shell ionization, can be combined with EELS image simulation to detect quite subtle local deviations in the cation sublattice from the completely ordered structure. This information provides a much more accurate structural description of the active catalyst nanocrystals, which must be considered to model both their oxygen-exchange capabilities and, eventually, their catalytic performance. [*] Dr. S. Trasobares, Dr. M. L pez-Haro, Dr. J. A. Perez-Omil, Dr. J. J. Calvino Departamento de Ciencia de los Materiales e Ingenier a Metalfflrgica y Qu mica Inorg nica Facultad de Ciencias, Universidad de C diz Campus Rio San Pedro, 11510-Puerto Real, C diz (Spain) Fax: (+34)956-016286 E-mail: susana.trasobares@uca.es Homepage: http://www.uca.es/tem-uca

Structure of highly dispersed metals and oxides: exploring the capabilities of high‐resolution electron microscopy

The potential applicability of high-resolution electron microscopy (HREM), in combination with image analysis and image simulation tools, to retrieve structural information from nanometre-sized particles present in oxide-supported metal and oxide catalysts is analysed. Specifically, the possibilities and limitations of this technique to determine features such as the size, morphology and chemical nature of the particles, their surface structure and their structural relationship with the support are considered through the discussion of several examples. The interpretation of a series of HREM images of Pt and Rh catalysts supported on cerium oxides after treatments under different redox environments illustrates the case of highly dispersed metals. In addition, the results obtained in this study provide an approximate picture of the evolution of metal– support interaction effects in this family of catalysts, which is closely related to three-way catalysts (TWCs). The results of a nanostructural investigation of two catalyst systems, one consisting of MgO-supported neodymia clusters and the second of vanadium– magnesium oxide also supported on MgO, provide the examples for supported oxide catalysts. These find application in oxidation reactions. For the former, the growth of neodymia in the form of rounded patches in a parallel orientation relationship with the support has been observed. For the vanadia-containing catalysts, the formation of a weakly ordered MgV2O4 spinel surface phase on the MgO support crystallites, after exposure to typical reaction conditions in the oxidative dehydrogenation of propane, has been confirmed. The structural relationship at the spinel/MgO interface has also been established. Copyright

Imaging nanostructural modifications induced by electronic metal-support interaction effects at Au||cerium-based oxide nanointerfaces.

A variety of advanced (scanning) transmission electron microscopy experiments, carried out in aberration-corrected equipment, provide direct evidence about subtle structural changes taking place at nanometer-sized Au||ceria oxide interfaces, which agrees with the occurrence of charge transfer effects between the reduced support and supported gold nanoparticles suggested by macroscopic techniques. Tighter binding of the gold nanoparticles onto the ceria oxide support when this is reduced is revealed by the structural analysis. This structural modification is accompanied by parallel deactivation of the CO chemisorption capacity of the gold nanoparticles, which is interpreted in exact quantitative terms as due to deactivation of the gold atoms at the perimeter of the Au||cerium oxide interface.

Rational design of nanostructured, noble metal free, ceria–zirconia catalysts with outstanding low temperature oxygen storage capacity

On the basis of detailed previous knowledge about the correlation existing between the Oxygen Storage Capacity (OSC) of ceria–zirconia mixed oxides, the nature of redox pretreatments and the nanostructure of this family of complex materials, it has been possible to design a synthetic strategy allowing one to prepare ceria–zirconia and ceria–yttria-doped zirconia materials, with total ceria contents below 20%, featuring OSC values higher than those observed for catalysts which incorporate supported noble metal particles in their formulation, especially in the very low, 373–473 K, temperature range. These novel, noble-metal free and highly reducible, ceria–zirconia materials allow a much more efficient usage of ceria as a component of oxygen storage formulations, which is nowadays considered one of the key innovation targets in the field of lanthanide oxide based catalysts.

Electron Microscopy (HREM, EELS) Study of the Reoxidation Conditions for Recovery of NM/CeO2 (NM: Rh, Pt) Catalysts from Decoration or Alloying Phenomena

The reversibility of metal–support interaction effects in NM/CeO2 catalysts (NM: Rh, Pt) is investigated using high-resolution electron microscopy and electron energy loss spectroscopy. Reoxidation treatments at 773 K followed by a mild reduction at 473 K are not effective in recovering ceria-based systems from the decorated or alloyed states observed upon high-temperature reduction (Tred>973 K).

Contributions of Electron Microscopy to Understanding CO Adsorption on Powder Au/Ceria–Zirconia Catalysts

The influence of the highly dispersed gold phase on the CO-support interaction occurring in two 2.5 wt % Au/Ce(0.62)Zr(0.38)O(2) catalysts with medium (Au/CZ-MD) and high (Au/CZ-HD) metal dispersion is quantitatively assessed. For this purpose, we have followed an approach in which high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), computer modelling, volumetric adsorption and FTIR spectroscopy studies are combined. This approach has already been fruitfully applied to the investigation of the specific CO-metal adsorption in Au/ceria-zirconia catalysts. As deduced from the experimental studies reported herein, the presence of gold dramatically increases the amount of CO strongly chemisorbed on the support. Moreover, this amount is sensitive to the metal dispersion, thus suggesting the occurrence of a mechanism in which the CO molecules that are initially adsorbed on the gold nanoparticles are further transferred to the support by means of a spillover process. An annular model is proposed for the growth of the CO phase adsorbed on the ceria-zirconia mixed oxide in the presence of Au. By assuming this model, we have estimated the width of the annulus, Delta r, of the adsorbed CO grown around the Au nanoparticles in Au/CZ-MD and Au/CZ-HD catalysts. This value is found to be very close to Delta r approximately 2 nm in both cases, the coincidence lending some additional support to the model. To further confirm this proposal, we have investigated the influence of CO pre-adsorption on the D(2)-Au/CZ-MD interaction, at 298 K. As revealed by FTIR spectroscopy, the kinetics of the deuterium spillover is significantly disturbed by the pre-adsorbed CO, which is fully consistent with an annular model for the CO adsorption. We conclude from the global analysis of the results reported here and those already available on CO-Au adsorption that the appropriate combination of nanostructural, computer modelling and chemical techniques is a powerful tool allowing us to gain a comprehensive picture of the complex series of processes involved in the CO adsorption on this relevant family of gold catalysts.

In situ transmission electron microscopy investigation of Ce(IV) and Pr(IV) reducibility in a Rh (1%)/Ce0.8Pr0.2O2−x catalyst

In-situ Atomic Resolution Transmission Electron Microscopy studies carried out on a Rh/Ce0.8Pr0.2O(2-x) catalyst, under hydrogen in the temperature range 298-1223 K, show the occurrence of consecutive reduction of Pr4+ and Ce4+ ions, and the formation of an oxygen-deficient Ln16O30 (Ln: Ce, Pr) ordered phase.