Ana B. Hungría

Inverse CeO2/CuO Catalyst As an Alternative to Classical Direct Configurations for Preferential Oxidation of CO in Hydrogen-Rich Stream

A novel inverse CeO(2)/CuO catalyst for preferential oxidation of CO in H(2)-rich stream (CO-PROX) has been developed on the basis of a hypothesis extracted from previous work of the group (JACS 2007, 129, 12064). Possible separation of the two competing oxidation reactions involved in the process (of CO and H(2), respectively) is the key to modulation of overall CO-PROX activity and is based on involvement of different sites as most active ones for each of the two reactions. Achievement of large size CuO particles and adequate CeO(2)-CuO interfacial configurations in the inverse catalyst apparently allows appreciable enhancement of the catalytic properties of this kind of system for CO-PROX, constituting an interesting alternative to classic direct configurations so far explored for this process. Reasons for such behavior are analyzed on the basis of operando-XRD, -XAFS, and -DRIFTS studies.

Nanotomography in the chemical, biological and materials sciences

Nanotomography is a technique of growing importance in the investigation of the shape, size, distribution and elemental composition of a wide variety of materials that are of central interest to investigators in the physical and biological sciences. Nanospatial factors often hold the key to a deeper understanding of the properties of matter at the nanoscale level. With recent advances in tomography, it is possible to achieve experimental resolution in the nanometre range, and to determine with elemental specificity the three-dimensional distribution of materials. This critical review deals principally with electron tomography, but it also outlines the power and future potential of transmission X-ray tomography, and alludes to other related techniques.

Single-Step Process To Prepare CeO2 Nanotubes with Improved Catalytic Activity

CeO(2) nanotubes have been grown electrochemically using a porous alumina membrane as a template. The resulting material has been characterized by means of scanning electron microscopy (SEM), X-ray energy dispersive spectroscopy, high-angle annular dark-field scanning transmission electron microscopy tomography, high-resolution electron microscopy (HREM), and electron energy loss spectroscopy. According to SEM, the outer diameter of the nanotubes corresponds to the pore size (200 nm) of the alumina membrane, and their length ranges between 30 and 40 microm. HREM images have revealed that the width of the nanotube walls is about 6 nm. The catalytic activity of these novel materials for the CO oxidation reaction is compared to that of a polycrystalline powder CeO(2) sample prepared by a conventional route. The activity of the CeO(2) nanotubes is shown to be in the order of 400 times higher per gram of oxide at 200 degrees C (77.2 x 10(-2) cm(3) CO(2) (STP)/(gxs) for the nanotube-shaped CeO(2) and 0.16 x 10(-2) cm(3) CO(2) (STP)/(gxs) for the powder CeO(2)).

3 D Characterization of Gold Nanoparticles Supported on Heavy Metal Oxide Catalysts by HAADF‐STEM Electron Tomography

Living on the edge: Three-dimensional reconstructions from electron tomography data recorded from Au/Ce(0.50)Tb(0.12)Zr(0.38)O(2-x) catalysts show that gold nanoparticles (see picture; yellow) are preferentially located on stepped facets and nanocrystal boundaries. An epitaxial relationship between the metal and support plays a key role in the structural stabilization of the gold nanoparticles.

3-D characterization of CdSe nanoparticles attached to carbon nanotubes

The crystallographic structure of CdSe nanoparticles attached to carbon nanotubes has been elucidated by means of high resolution transmission electron microscopy and high angle annular dark field scanning transmission electron microscopy tomography. CdSe rod-like nanoparticles, grown in solution together with carbon nanotubes, undergo a morphological transformation and become attached to the carbon surface. Electron tomography reveals that the nanoparticles are hexagonal-based with the (001) planes epitaxially matched to the outer graphene layer.

First‐Principles Design of Highly Active and Selective Catalysts for Phosgene‐Free Synthesis of Aromatic Polyurethanes

We thank the Generalitat Valenciana (GV/2009/063 and PROM-ETEO 2088/130) and the Spanish MICINN (MAT2011-28009, and Consolider Ingenio 2010-MULTICAT: CSD2009-00050) for financial support. We thank Red Espanola de Supercomputacion (RES) for computational resources. D.C. thanks Spanish MICINN for a postgraduate scholarship, and S.L. thanks ITQ for a post-doctoral fellowship.

Characterization of Active Sites/Entities and Redox/Catalytic Correlations in Copper-Ceria-Based Catalysts for Preferential Oxidation of CO in H2-Rich Streams

This article reviews work done at authors’ laboratories about catalysts based on combinations between copper and ceria for preferential oxidation of CO in H2-rich streams (CO-PROX). The main focus of this review is the characterization of active sites for the process on the basis of spectroscopic analysis of the systems under reaction conditions (operando techniques). On such a basis, it is exposed the state of the art in this field in connection with results obtained in other laboratories.

Interconnected mesopores and high accessibility in UVM-7-like silicas

Nanoparticulated bimodal mesoporous silicas (NBS) have proved to constitute adequate supports in a variety of applications requiring enhanced accessibility to the active sites. Mass-transfer kinetics seems to be highly favoured in UVM-7-derived NBS materials. To understand the mass-diffusion phenomena throughout UVM-7-like supports requires well-grounded knowledge about their pore architecture. 3-D reconstructions of the UVM-7 mesostructure carried out by electron tomography reveal the existence of a true hierarchic connectivity involving both inter- and intra-nanoparticle pores. This connectivity makes self-supported nanoparticulated mesoporous bimodal carbon replicas of the supports feasible to obtaining by nanocasting. Both the temperature-induced mobility of gold nanodomains and the fast and efficient enzyme adsorption in UVM-7-like silicas are examples of non-constrained diffusion processes happening inside such an open network.

Nanoconfinement of Ni clusters towards a high sintering resistance of steam methane reforming catalysts

This study reports an improvement of the stability of steam reforming catalysts at relatively low temperatures, such as for pre-reforming, and reforming of biomass derived compounds, by enhanced stabilization of Ni nanoparticles through spatial confinement in a mixed oxides matrix. We revealed a simple approach of three dimensional engineering of Ni particles by means of self-assembly of Ni atoms inside the nanoribbon of hydrotalcite-derived mixed oxides. Taking advantage of Transmission Electron Microscopy (TEM), together with electron tomography, the three dimensional (3D) structure of the catalyst was investigated at a nanometer scale, including the Ni particle size, shape, location and spatial distribution, as well as pore size and morphology of the mixed oxides. Porous nano-ribbons were formed by high temperature treatment, adopting the layer structure of the hydrotalcite-like materials. Ni particles formed by selective reduction of mixed oxides embedded in the nano-ribbons with connected pore channels, allowing good access for the reactants. These spatially confined and well distributed Ni particles increased catalyst stability significantly compared to the Ni particles supported on the support surfaces in a commercial catalyst during the steam methane reforming.

Advanced Electron Microscopy Investigation of Ceria–Zirconia-Based Catalysts

The potentials of advanced transmission and scanning transmission electron microscopy in nanostructural studies of ceria–zirconia mixed oxides are overviewed. The crystallographic criteria that allow us to discriminate the different CeO2–ZrO2 polymorphs and the nanocrystal size range within which they can be applied are discussed. The combined use of high resolution electron microscopy (HREM) and high‐angle annular dark field scanning transmission electron microscopy (HAADF‐STEM) to detect disorder–order transitions in the cation sublattice of this family of oxides and the size limit down to which each of them can be used for that purpose are also analyzed. Criteria to discriminate, on the basis of HREM images, the oxygen arrangement of the so called κ‐Ce2Zr2O8 phase from that of an oxidized pyrochlore Ce2Zr2O8 phase are presented and applied to the interpretation of experimental HREM images. HAADF‐STEM tomography studies and detailed analysis of HAADF‐STEM images establish both the crystallographic and compositional features of the surfaces of the mixed oxides that give enhanced redox activity. These findings allow deeper understanding of the influence of different thermal ageing pretreatments on the redox behavior of this family of mixed oxides. Novel characterization data do evidence that this model does not only fruitfully apply to binary ceria–zirconia oxides but also to more complex ternary oxides containing terbium.