Adela Muñoz-Páez

The hydration of Cu2+: Can the Jahn-Teller effect be detected in liquid solution?

The long elusive structure of Cu(II) hydrate in aqueous solutions, classically described as a Jahn-Teller distorted octahedron and recently proposed to be a fivefold coordination structure [Pasquarello et al., Science 291, 856 (2001)], has been probed with x-ray-absorption spectroscopy by performing a combined theoretical and experimental analysis. Two absorption channels were needed to obtain a proper reproduction of the x-ray-absorption near-edge structure (XANES) region spectrum, as already observed in other Cu(II) complexes [Chaboy et al., Phys. Rev. B 71, 134208 (2005)]. The extended x-ray-absorption fine-structure (EXAFS) spectrum was analyzed as well within this approach. Quite good reproductions of both XANES and EXAFS spectra were attained for several distorted and undistorted structures previously proposed. Nevertheless, there is not a clearly preferred structure among those including four-, five-, and sixfold coordinated Cu(II) ions. Taking into account our results, as well as many more from several other authors using different techniques, the picture of a distorted octahedron for the Cu(II) hexahydrate in aqueous solution, paradigm of the Jahn-Teller effect, is no longer supported. In solution a dynamical view where the different structures exchange among themselves is the picture that better suits the results presented here.

Sodium-doped V2O5/TiO2 systems: An XRD, DTA, TG/DTG, IR, V-UV, TPR, and XANES study

Abstract Vanadia-titania samples doped with different amounts of sodium have been prepared by impregnation of titania (P-25) with aqueous solutions of ammonium vanadate and calcination at 770 K, and have been characterized by X-ray diffraction, differential thermal analysis, thermogravimetric analysis, Fourier Transform-infrared spectroscopy, visible-ultraviolet (diffuse reflectance) spectroscopy, temperature-programmed reduction, and X-ray absorption near-edge structure spectroscopy. The results obtained indicate that doping the vanadia-titania system with increasing amounts of sodium leads initially to formation of VNaO compounds with a Na/V ratio close to 0.5 that are reduced at a higher temperature than V 2 O 5 , formed in the absence of sodium. If the amount of sodium is increased up to 3%, formation of Na 3 VO 4 has been found.

The 'Invisible' Metal Particles in Catalysis

Abstract An easy, reliable and straightforward method to determine the sizes of small metal particles in supported metal catalyst which are invisible for most techniques (chemisorption, XRD, HRTEM) is presented. The technique we consider more appropriate is EXAFS, because it detects metal-metal bonds even before metal atoms are forming particles. Due to this capability it has become a routine technique in catalysis, although it requires an elaborate data analysis procedure. In the particular case of supported metal catalyst, this procedure can be simplified because nearly everything is known about the investigated structure, the metal particles. With the appropriate fitting strategies, the main contribution to the EXAFS spectrum, the metal-metal bonds, can be emphasized, and within it, most part of the fit parameters are known. The only unknown parameter is the coordination number for each metal shell. Once this value is obtained, the number of metal atoms per particle can be calculated and from that, metal particle diameter can be deduced. An example of this procedure for a Pt Al 2 O 3 catalyst is shown.

Liquid EXAFS cells for measurements in transmission and fluorescence mode of corrosive samples

The design of two small, compact and portable cells for EXAFS measurements of liquids is presented. One of the cells has been optimized for transmission measurements and the other for fluorescence. The sample chamber of both cells has been designed metal free to avoid corrosion by acids or bases. Materials and construction of the cell are such that their cost is quite low. The cells have been tested by recording EXAFS spectra of ionic aqueous solutions.

Study of metallic components of historical organ pipes using synchrotron radiation X-ray microfluorescence imaging and grazing incidence X-ray diffraction

A comparative study of the composition and microstructure of two different brass alloys from reed pipes, one from a Spanish baroque organ and the other from a modern one, was carried out. This study allowed us to determine the procedure followed to produce the brass used to make ancient reed pipes. Moreover the distribution and correlation of lead and other trace elements present into the main component of the brass, the copper and zinc phases, of the historical tongues and shallots were established. This chemical composition was compared with that of a tongue from a twentieth-century organ. The whole study was accomplished using a combination of laboratory and synchrotron radiation techniques. X-ray fluorescence was the technique used to obtain elemental and chemical imaging of the main phases and the trace elements at a sub-micrometer scale.

XAFS analysis of the potasium-silica interaction in rice husks

Abstract The Si K-edge X-ray absorption spectra of three samples obtained from rice husk have been recorded to investigate changes in medium and short range order that could explain the drastic differences in surface area of the samples, from 1 to 260 m2/g. The recorded differences are similar to those detected in densified silica, where they have been associated with changes in medium range order. Similar changes have been observed in K2OSiO2 glasses where the addition of potassium oxide leads to the disruption of SiO4 network, and the formation of chain like structure of non-bridging oxygen atoms, as increasing the quantity of potassium oxide. This suggests that this behavior is due to the strong interaction between silica and the potassium ions contained in the rice husks, which leads to a dramatic decrease of the specific surface area if K+ ions are not removed before the annealing of the sample.

Optimized end station and operating protocols for reflection extended x-ray absorption fine structure "ReflEXAFS… investigations of surface structure at the European Synchrotron Radiation Facility beamline BM29

The development of the capability to engineer the surface properties of materials to match specific requirements demands high quality surface characterization techniques. The ideal tool should provide chemically specific structural characterization as well as surface sensitivity and depth profiling. Ideally the characterization method should also be applicable to systems both with and without long range order. X-ray absorption spectroscopy fine structure, when using the standard transmission detection system, provides all this information with the significant exception of surface sensitivity. In contrast, by detecting the reflected instead of the transmitted beam, it encompasses all these requirements because when the incident beam impinges onto a sample surface at glancing angles, in conditions close to the total reflection, only the outermost regions of the system under study are sampled. Such a technique provides information about the local structure as a function of depth as well as thin layer structure in the case of layered samples. Although it is potentially the ideal tool to study surface modified materials, experimental difficulties have hampered its widespread use in the fields of surface and materials sciences. As a solution to the experimental challenges, we provide a detailed description of an appropriate experimental station, the sample requirements, the measuring protocols, and software routines needed to optimize the collection of the data. To illustrate the capabilities of the technique the results obtained for a model multilayer sample are presented and analyzed under the total external reflection approximation.

The interplay of the 3d9 and 3d10L electronic configurations in the copper K-edge XANES spectra of Cu(II) compounds.

A theoretical analysis of the X-ray absorption near-edge structure spectra at the Cu K-edge in several divalent copper [Cu(II)] compounds showing a distorted nearest-neighborhood around copper is presented. The experimental spectra of CuO and KCuF(3) have been compared with computations performed in the framework of the multiple-scattering theory. The results show that ab initio single-channel multiple-scattering calculations are not able to reproduce the experimental spectra. On the contrary, the experimental spectra can be accounted for by using two excitation channels and the sudden limit of the multichannel multiple-scattering theory. The comparison between experimental data and computations indicates that both 3d(9) and 3d(10)L electronic configurations are needed to account for the absorption process in these systems, suggesting that this is the general case for the K-edge XANES of divalent copper compounds.

EXAFS investigation of the second hydration shell of metal cations in dilute aqueous solutions

Abstract EXAFS spectra of aqueous dilute solutions (0.1 M) containing Cr3+, Rh3+, Ga3+ and Zn2+ ions are presented. From the EXAFS fit parameters it is deduced that always the first hydration shell is formed by six water molecules with MO distances between 1.95 and 2.06 A. A second hydration shell is also observed in all cases, although the amplitude of this contribution increases with the stability of aquocomplexes. This can be quantified by the correlation between the Debye-Waller factors for the first and second hydration shells, and the first-shell water release constants of the hydrated cations.

Study of the reversibility on the local La3+ environment after thermal and drying treatments in lanthanum-exchanged smectites

Abstract Changes produced by thermal treatments (300°C, 500°C, and 700°C) and after an “in situ” drying treatment on the hydration sphere of the lanthanum ions located in the interlamellar space of montmorillonites have been investigated. X-ray absorption spectroscopy (XAS) spectra for these materials have been recorded with this purpose at room and liquid nitrogen temperature by using a special cell where the temperature and the atmosphere could be controlled. The dry atmosphere employed during the drying treatment produced either drastic or no significant effects on the La L III -edge EXAFS spectra, depending upon the previous thermal treatment given to the sample. These results have allowed us to postulate a dehydration reaction mechanism for these samples and to show how XAS measurement is a very useful tool to be employed in the study of the local environments adopted by the exchangeable cations in clays.