Structure of Extremely Nanosized and Confined In-O Species in Ordered Porous Materials

Abstract

Perturbed-angular correlation, x-ray absorption, and small-angle x-ray scattering spectroscopies were suitably combined to elucidate the local structure of highly diluted and dispersed InOx species confined in porous of ZSM5 zeolite. This novel approach allow us to determined the structure of extremely nanosized In-O species exchanged inside the 10-atom-ring channel of the zeolite, and to quantify the amount of In2O3 crystallites deposited onto the external zeolite surface.The study of structural, magnetic and electronic properties of nanostructured, subnanostructured and, in the other extreme, highly diluted monoionic species in solids deserves increasing attention not only from a fundamental point of view but also for their technological applications [1]. In one extreme, highly-dispersed metallic exchanged-atoms in an infinite variety of compounds present, e.g., very important catalytic properties [2], which are not clearly correlated with the active species responsible for them since their physical entities are often unknown. The design of new catalysts with improved activity, selectivity and stability requires the complete knowledge of the local environment of the active centers and its correlation with the desired reaction. The complete characterization of this kind of structures is at present a challenging problem not only in catalysis but also in many fields of physics.An additional difficulty is the confinement of the diluted species inside porous materials. In general, experimental techniques based on energetic probes that strongly interact with the materials, may destroy them. On the other side, low energetic probes are unuseful since they have to pass through the walls around the "hidden" locations of the confined clusters or atoms and their kinetic energy is attenuated.The importance of the extended X-ray absorption fine structure (EXAFS) spectroscopy to study structural properties in crystalline solids [2], nanoclusters [3] and highly dispersed ionic species in catalysis [4] has been long acknowledged. EXAFS constitutes a powerful "atom selective" technique to extract direct information about type, number and distances of neighbors of the absorber atom [5,6]. However, in EXAFS analysis, if more than one species has the same type of bonds (same element and similar bond-lengths), the information from each one would be almost impossible to extract directly, unless some additional information is known.The perturbed-angular-correlation (PAC) technique requires a suitable probe-atom (native or foreign) to be in the system under study, being in this sense also "atom selective". PAC enables the precise determination -at the probe site -of the electric-field gradient tensor (EFG), which is extremely sensitive to the anisotropy of the electronic density near the nucleus, which in turn reflects the probe-neighboring coordination. PAC has been intensively applied to many fields in science during the past two decades with success [7] and, very recently, to the development of accurate ab initio calculations of th...

Achieving Exceptional Photocatalytic Activity and Selectivity through a Well‐Controlled Short‐Ordered Structure: A Case Study of NaxTaOy⋅n H2O

“…Contraction in the averageM ÀOd istance is often accompanied by an increasei nl attice disorder and ar eduction in the coordination number,a sd ocumented in amorphous In 2 O 3 films. [14] Al onger InÀOb ond length implies highercrystallinity of the In 2 O 3 sample, [15,16] which is in accordancew ith the SR-WAXS resultso ft he short-ordered Na x Ta O y ·n H 2 Os amples.…”

Ab Initio Study of Structural, Electronic, and Hyperfine Properties of n-type SnO2:Ta Semiconductor

“…In condensed-matter physics, hyperfine experimental techniques , have been extensively employed to investigate structural, electronic, and magnetic properties at the atomic scale in pure and doped systems . In particular, the Time-Differential Perturbed γ–γ Angular-Correlation (TDPAC) spectroscopy provides a precise characterization of the electric-field-gradient (EFG) tensor at diluted (ppm) radioactive probe atoms, and therefore can be employed to elucidate the subnanoscopic environment of impurities or native atoms in solids. − The EFG is a second-order symmetric tensor whose components are defined by V ij ( r⃗ ) = (∂ 2 V ( r⃗ ))/(∂ x i ∂ x j ), where V ( r⃗ ) is the Coulomb potential generated by the charge density surrounding a probe nucleus. In the principal axis system, the EFG tensor is traceless.…”

Ionic exchange and the local structure in the HfO2/Ho2O3 system studied by PAC spectroscopy

“…Different experimental techniques, such as Nuclear Quadrupole Resonance (NQR), Nuclear Magnetic Resonance (NMR), Mössbauer Spectroscopy (MS), and Time-Differential c-c Perturbed-Angular-Correlation (PAC) spectroscopy, can be employed to study local electronic and structural properties at suitable probe-atom sites [22][23][24]. In particular, PAC has been extensively applied to study doped materials from the point of view of solid-state physics, chemistry, and biology in order to elucidate the subnanoscopic environment(s) of constituent or impurity atoms in solids, and the nature of chemical bonding in different kind of molecules and compounds [4,5,[25][26][27][28][29][30][31][32][33][34].…”