Marcos Fernández-García

Advanced Nanoarchitectures for Solar Photocatalytic Applications

UV-Visible ار راد ن .د TiO2 ( تیفرظ راون مان هب نورتکلا یاراد یژرنا زارت لماش VB و ) رگید زارت ی یژرنا اب ( ییاناسر راون مان هب نورتکلا زا یلاخ و رتلااب VB یم ) .دشاب ت ود نیا نیب یژرنا توافت یژرنا فاکش زار ، پگ دناب هدیمان یم .دوش هک ینامز زا نورتکلا لاقتنا VB هب VB یم ماجنا دریگ ، TiO2 اب ودح یژرنا بذج د ev 2 / 3 ، نورتکلا تفج کی دیلوت یم هرفح .دیامن و نورتکلا هرفح ی نا اب هدش دیلوت یم کرتشم حطس هب لاقت ثعاب دناوت شنکاو ماجنا اه یی ددرگ . TiO2 دربراک ،دراد یدایز یاه هلمج زا یم ناوت اوه یگدولآ هیفصت یارب (CO2) و بآ و ... نآ زا هدافتسا درک .

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.

Understanding the antimicrobial mechanism of TiO2-based nanocomposite films in a pathogenic bacterium

Titania (TiO2)-based nanocomposites subjected to light excitation are remarkably effective in eliciting microbial death. However, the mechanism by which these materials induce microbial death and the effects that they have on microbes are poorly understood. Here, we assess the low dose radical-mediated TiO2 photocatalytic action of such nanocomposites and evaluate the genome/proteome-wide expression profiles of Pseudomonas aeruginosa PAO1 cells after two minutes of intervention. The results indicate that the impact on the gene-wide flux distribution and metabolism is moderate in the analysed time span. Rather, the photocatalytic action triggers the decreased expression of a large array of genes/proteins specific for regulatory, signalling and growth functions in parallel with subsequent selective effects on ion homeostasis, coenzyme-independent respiration and cell wall structure. The present work provides the first solid foundation for the biocidal action of titania and may have an impact on the design of highly active photobiocidal nanomaterials.

XANES analysis of catalytic systems under reaction conditions

ABSTRACT The article gives an overview of the XANES technique contribution to the analysis of multicomponent catalysts. The theoretical basis of the technique, the interpretation of the energy position and intensity of XANES features, and the numerical methods developed to interpret XANES data on catalytic systems are described and discussed in the first part. In the second part, the most recent XANES studies of catalytic samples are reviewed, giving particular attention to the solid state chemistry information extracted under real, in-situ conditions. This concerns structural and redox properties of the systems during preparation stages, activation treatments like reduction, oxychlorination, or sulfidation, and reaction conditions. Additional effort was made to describe the use of XANES to gain insight on electronic properties of catalysts. The influence of all these redox, geometrical and electronic properties on catalytic behavior, particularly as a function of particle size and active metal-support interaction, has also been examined on the light of XANES data.

The behavior of mixed-metal oxides: Physical and chemical properties of bulk Ce1−xTbxO2 and nanoparticles of Ce1−xTbxOy

The physical and chemical properties of bulk Ce(1-x)Tb(x)O(2) and Ce(1-x)Tb(x)O(y) nanoparticles (x<or =0.5) were investigated using synchrotron-based x-ray diffraction (XRD), x-ray adsorption near edge spectroscopy (XANES), Raman spectroscopy (RS), and first-principles density-functional (DF) calculations. DF results and Raman spectra point to a small tetragonal distortion after introducing terbium in ceria. The results of XRD show a small contraction (< or = 0.08 A) in the cell dimensions. The presence of Tb generates strain in the lattice through the variation of the ionic radii and creation of crystal imperfections and O vacancies. The strain increases with the content of Tb and affects the chemical reactivity of the Ce(1-x)Tb(x)O(y) nanoparticles towards hydrogen, SO(2), and NO(2). DF calculations for bulk Ce(1-x)Tb(x)O(2) and Ce(8-n)Tb(n)O(16) (n=0, 1, 2, or 4) clusters show oxide systems that are not fully ionic. The theoretical results and XANES spectra indicate that neither a Ce<-->Tb exchange nor the introduction of oxygen vacancies in Ce(1-x)Tb(x)O(y) significantly affect the charge on the Ce cations. In contrast, the O K-edge and Tb L(III)-edge XANES spectra for Ce(1-x)Tb(x)O(y) nanoparticles show substantial changes with respect to the corresponding spectra of Ce and Tb single oxide references. The Ce(0.5)Tb(0.5)O(y) compounds exhibit a much larger Tb(3+)/Tb(4+) ratio than TbO(1.7). A comparison with the properties of Ce(1-x)Zr(x)O(y) and Ce(1-x)Ca(x)O(y) shows important differences in the charge distribution, the magnitude of the dopant induced strain in the oxide lattice, and a superior behavior in the case of the Ce(1-x)Tb(x)O(y) systems. The Tb-containing oxides combine stability at high temperature against phase segregation and a reasonable concentration of O vacancies, making them attractive for chemical and catalytic applications.

The behavior of mixed-metal oxides: Structural and electronic properties of Ce1−xCaxO2 and Ce1−xCaxO2−x

Synchrotron-based time-resolved x-ray diffraction (TR-XRD), x-ray absorption near edge spectroscopy (XANES), Raman spectroscopy (RS), and first-principles density functional (DF) calculations were used to study the structural and electronic properties of Ce–Ca mixed-metal oxides. The XRD results and DF calculations show that doping with calcium induces relatively minor variations (<0.05 A) in the cell dimensions of ceria. However, the presence of Ca leads to slightly distorted tetragonal structures, a substantial strain in the lattice of the oxide and a tendency to form O vacancies in an ideal Ce1−xCaxO2 solid solution. The two latter effects can be a consequence of the large number of oxygen neighbors that Ca is forced to have in Ce1−xCaxO2 and differences in the electronic charges of calcium and cerium cations. The Ce1−xCaxO2−x systems are not fully ionic. Cation charges derived from the DF calculations indicate that these systems obey the Barr model for charge redistribution in mixed-metal oxides. The ...

Combining Time-Resolved Hard X-ray Diffraction and Diffuse Reflectance Infrared Spectroscopy To Illuminate CO Dissociation and Transient Carbon Storage by Supported Pd Nanoparticles during CO/NO Cycling

A novel combination of time-resolved hard X-ray diffraction, diffuse reflectance infrared spectroscopy, and mass spectrometry reveals how Pd nanoparticles dissociate CO and store atomic carbon and how this carbon storage dynamically changes the population of linear and bridging CO species at the surface of the Pd nanoparticles.

High Activity of Ce1−xNixO2−y for H2 Production through Ethanol Steam Reforming: Tuning Catalytic Performance through Metal–Oxide Interactions

The importance of the oxide: Ce{sub 0.8}Ni{sub 0.2}O{sub 2-y} is an excellent catalyst for ethanol steam reforming. Metal-oxide interactions perturb the electronic properties of the small particles of metallic nickel present in the catalyst under the reaction conditions and thus suppress any methanation activity. The nickel embedded in ceria induces the formation of O vacancies, which facilitate cleavage of the OH bonds in ethanol and water.

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.

EPR study on oxygen handling properties of ceria, zirconia and Zr–Ce (1 : 1) mixed oxide samples

The characteristics of ceria, zirconia and Zr–Ce (1 : 1 molar ratio) samples prepared by using inverse microemulsions have been studied to obtain information on the modifications produced in the oxygen handling properties of the single oxides by the formation of the mixed oxide. The structural characteristics of the samples are compared on the basis of XRD and Raman spectroscopy data, which show that the Zr–Ce sample forms a homogeneous mixed oxide structure corresponding to the (pseudocubic) tetragonal phase t″ while CeO2 presents, as expected, a cubic fluorite structure and ZrO2 is formed mainly by a tetragonal phase with only traces of the thermodynamically most stable monoclinic phase. Evaluation by EPR of the type and intensity of superoxide radicals generated upon oxygen adsorption on the outgassed samples reveals a higher surface reducibility in the mixed oxide sample, associated vacancies defects being generated on it in higher amounts and at lower temperatures than on pure ceria. It is also shown that both Ce and Zr ions participate in the oxygen chemisorption process on the mixed oxide sample, actually giving rise to much larger amounts of O2-–Zr4+ species than the pure ZrO2 sample; the latter species show a higher thermal stability than O2-–Ce4+ species.