M.A. Cauqui

Some contributions of electron microscopy to the characterisation of the strong metal–support interaction effect

Abstract The contribution of electron microscopy techniques to establishing the existence and actual nature of the metal–support interaction effects occurring in a variety of supported metal catalysts is reviewed. Data pertaining to systems based on both classic reducible supports like TiO 2 , CeO 2 or some ceria-based mixed oxides, and several others generally considered as non-reducible oxides, like La 2 O 3 or SiO 2 , are presented and discussed. The specific temperature and chemical conditions under which strong metal–support interaction phenomena are onset or reverted in each case are also analysed. The whole set of data presented and discussed here clearly shows that the electron microscopy techniques have made an outstanding contribution to the characterisation of the strong metal/support interaction effects exhibited by different metal/oxide systems. Likewise, it is demonstrated that this powerful family of techniques has very much helped to discriminating between true SMSI-like phenomena, as originally defined by Tauster et al., and several other apparent effects which, though at a first sight show some of the chemical, nano-structural and/or catalytic characteristics of the SMSI effect, have a neatly different origin.

Cerium-terbium mixed oxides as alternative components for three-way catalysts: A comparative study of Pt/CeTbOx and Pt/CeO2 model systems

Abstract By using a combination of oxygen buffering capacity (OBC) and oxygen storage capacity (OSC) measurements, the redox behaviour of a Pt/CeTbOx catalyst is compared to that of a classic model TWC system: Pt/CeO2. The results reported here show that the redox efficiency of the Pt/CeTbOx catalyst is much better, especially at low temperature operation conditions such as those occurring during the cold start of engines. The catalyst containing terbium also shows lower ‘light-off’ temperatures for both methane and carbon monoxide oxidation.

Role of physicochemical characteristics in the uptake of TiO2 nanoparticles by fibroblasts.

The relation between the physico-chemical properties of nanoparticles (NPs) and the degree of cellular uptake is incompletely elucidated. In this study, we investigated the influence on the cellular uptake of a wide range of fully characterized TiO2 NPs. L929 fibroblasts were exposed for 24 h to clinically relevant concentrations of nano-TiO2 and the degree of their association was assessed by ultrahigh resolution imaging microscopy (URI), scanning (SEM) and transmission (TEM) electron microscopy, as well as inductivity coupled plasma-mass spectroscopy (ICP-MS). The role of actin polymerization, a central feature of active internalization, was also studied and the results indicated that the internalization of TiO2 NPs involves a combination of actin-dependent uptake of large agglomerates as well as non actin-dependent uptake of small agglomerates. SEM and TEM revealed that the agglomerates of all NPs types were attached to the cellular membrane as well as internalized and confined inside cytoplasmic vesicles. URI and ICP-MS demonstrated that the particle association with cells was dose-dependent. The highest association was observed for spherical particles having mixed anatase-rutile crystallographic phase and the lowest for spindle-shaped rutile particles. ICP-MS revealed that the association was size-dependent in the order 5>10>40 nm for anatase spherical nanoparticles.

Metal-support interaction phenomena in rhodium/ceria and rhodium/titania catalysts: Comparative study by high-resolution transmission electron spectroscopy

Abstract The high resolution transmission electron microscopy (HRTEM) study of Rh/CeO 2 and Rh/TiO 2 catalysts reduced at temperatures ranging from 473 K to 773 K suggests that the metal-support interaction phenomena they show are different. In contrast to that found for Rh/TiO 2 , over Rh/CeO 2 no metal decoration effects could be observed. Likewise, the chemisorptive and catalytic behaviour of Rh/ CeO 2 does not exhibit drastic changes with the reduction temperature. The HRTEM images, however, reveal the occurrence of quite singular rhodium-ceria interaction phenomena.

Influence of reduction treatment on the structural and redox behaviour of ceria, La/Ce and Y/Ce mixed oxides

A methodology based on reoxidation at 298 K with O2(5%)/He pulses followed by quantitative TPO runs allowed us to study the redox state reached by CeO2 and Ln3+ -containing ceria‐based mixed oxide samples Ln3+:La3+ or Y3+) after their reduction in a flow of hydrogen. The temperature range covered in the reduction treatments was from 623 up to 1223 K. During the severe reduction treatments hexagonal sesquioxide phases segregate from all the samples. The reduction temperature at which segregation occurs follows the trend La/Ce < Y/Ce < CeO2. The TPO and H2-TPD traces of reduced samples which showed evidence of segregation in XRD exhibit characteristic features which can also be used as an indication of this effect. The discussion is focussed on the overall and detailed understanding of the reoxidation process and on the analysis of the H2-TPD profiles.

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

Oxygen buffering capacity (OBC) of praseodymium-modified CeO2: influence of the Pr distribution in the ceria host lattice

This work reports on the redox behaviour of different ceria-praseodymia mixed oxides with two different chemical compositions (Ce/Pr molar ratios of 4:1 and 9:1), The samples consist of bulk oxides and ceria-supported samples with varying mixed oxide loading (equivalent to 7 or 12 mixed oxide layers). As revealed by x-ray diffraction and high-resolution electron microscopy, the samples consist of homogeneous fluorite-like mixed oxides. Their redox properties were characterized by means of the oxygen buffering capacity (OBC) technique, which quantitatively evaluates the capacity of the samples for attenuating fast oscillations of the oxygen partial pressure in the He stream flowing through the sample. These oscillations are induced by injecting with a high frequency pulses of oxygen into the inert gas stream. Although the XPS analyses performed on both a bulk and a supported sample showed Ce/Pr molar ratios very close to each other, the bulk mixed oxide exhibits significantly better OBC values. Also remarkable is that an ageing treatment at 1223 K improves the OBC response of the supported mixed oxides. In accordance with the XPS data, the enhancement of their redox properties runs parallel to an increase in the Ce/Pr surface ratio. These results are discussed in terms of the role played by the depth profile distribution of Pr in the different oxide samples.

Preparation of rhodium catalysts dispersed on TiO2SiO2 aerogels

Abstract TiO2 SiO2 aerogels have been used as supports to disperse rhodium. Three different ways have been followed for the preparations: (a) a classic TiO2 SiO2 aerogel, obtained from a mixture of alkoxides and water dissolved in ethanol, was impregnated with a rhodium nitrate solution; (b) a TiO2 SiO2 sonogel, obtained by hydrolysis of alkoxides in the absence of alcohol, under the action of ultrasound, was impregnated with a rhodium nitrate solution; and (c) a mixture of alkoxides and water, including rhodium nitrate in the reaction vessel, was exposed to the action of ultrasound, thus leading to a ternary Rh TiO2 SiO2 sonogel. The behaviour of these three catalysts has been compared with that of a Rh/TiO2 SiO2 system (d), obtained by conventional impregnation methods, starting from a commercial silica support. The samples prepared by impregnation of aerogels (a,b) present high levels of rhodium dispersion and show an increase of the catalytic activities in parallel with the pretreatment temperature in flow of hydrogen. On the contrary, the sample prepared including rhodium before gelling (c) presents a poor metallic dispersion, is not active for benzene hydrogenation, and has the capability of uptaking large amounts of hydrogen at ambient temperature. Both patterns of behaviour are greatly different from that characteristic of conventional Rh/TiO2 SiO2 catalyst (d).

Influence of the preparation procedure on the chemical and microstructural properties of lanthana promoted Rh/SiO2 catalysts: A FTIR spectroscopic study of chemisorbed CO

Abstract In this work, the influence of the preparation procedure on the chemical and microstructural properties of a series of Rh/La 2 O 3 /SiO 2 catalysts has been investigated. The catalysts were characterized by using HREM, H 2 and CO volumetric adsorption and FTIR spectroscopy of chemisorbed CO. From our study, significant effects were observed for the following preparative variables: (a) the lanthana loading; (b) the way of introducing the metal and promoter, either consecutive (lanthana first and then the rhodium salt) or by co-impregnation; and (c) the calcination temperature (873 K or 1173 K) applied for preparing the La 2 O 3 /SiO 2 intermediate phase onto which the noble metal was deposited.

Chemical and microstructural investigation of Pt/CeO2 catalysts reduced at temperatures ranging from 473 to 973 K

The combined application of chemical techniques and HREM shows that platinum deactivation in Pt/CeO2 catalysts steadily increases with the reduction temperature (Tr: 473–1173 K). Though no suppression of the H2 adsorption was observed, the TPD-H2 traces indicate the occurrence of significant chemical changes with Tr. As revealed by HREM, the metal decoration starts at Tr: 973 K, a temperature well above those at which the chemical effects can be observed.