Is There a Negative Thermal Expansion in Supported Metal Nanoparticles? An In-Situ X-ray Absorption Study Coupled with Neural Network Analysis

Interactions with their support, adsorbates and unique structural motifs are responsible for the many intriguing properties and potential applications of supported metal nanoparticles (NPs). At the same time, they complicate the interpretation of experimental data. In fact, the methods and approaches that work well for the ex situ analysis of bulk materials may be inaccurate or introduce artifacts in the in situ analysis of nanomaterials. Here we revisit the controversial topic of negative thermal expansion and anomalies in the Debye temperature reported for oxide-supported metal NPs. In situ X-ray absorption experimental data collected for Pt NPs in ultrahigh vacuum and an advanced data analysis approach based on an artificial neural network demonstrate that Pt NPs do not exhibit intrinsic negative thermal expansion. Similarly as for bulk materials, in the absence of adsorbates the bond lengths in metal NPs increase with temperature. The previously reported anomalies in particle size-dependent Debye temperatures can also be linked to the artifacts in the interpretation of conventional X-ray absorption data of disordered materials such as NPs

Operando Insights into Nanoparticle Transformations during Catalysis

Nanostructured materials play an important role in today’s chemical industry acting as catalysts in heterogeneous thermal and electrocatalytic processes for chemical energy conversion and the production of feedstock chemicals. Although catalysis research is a long standing discipline, the fundamental properties of heterogeneous catalysts like atomic structure, morphology and surface composition under realistic reaction conditions, together with insights into the nature of the catalytically active sites, have remained largely unknown. Having access to such information is however of outmost importance in order to understand the rate-determining processes and steps of many heterogeneous reactions and identify important structure-activity/selectivity relationships enabling knowledge-driven improvement of catalysts. In the last decades, in situ and operando methods have become available to identify the structural and morphological properties of the catalysts under working conditions. Such investigations have led to important insights into the catalytically-active state of the materials at different length scales, from the atomic level to the nano-/micrometer scale. The accessible operando methods utilizing photons range from vibrational spectroscopy in the infrared and optical regime to small-angle X-ray scattering (SAXS), diffraction (XRD), absorption spectroscopy (XAFS) and photoelectron spectroscopy (XPS), whereas electron-based techniques include scanning (SEM) and transmission microscopy (TEM) methods. In this work, we summarize recent findings of structural, morphological and chemical nanoparticle transformations during selected heterogeneous and electrochemical reactions, integrate them into the current state of knowledge, and discuss important future developments

The global forum on migration and development

The paper explores the proposed rationale of the Global Forum on Migration and Development that was launched by Kofi Annan in 2006 as UN Secretary General, as an informal inter-governmental discussion space. First, it identifies the series of claims in Annan’s speech to the High-Level Dialogue that he convened in New York: that international migration must be managed; that to proceed from the present situation of entrenched disagreements and mistrust requires constructive structured communication; that the Global Forum can provide this and is a feasible way forward, unlike proposals for binding international conventions; and that through processes of growing mutual education and mutual acceptance the Forum can be fruitful. Implied are notions of building trust and community amongst the “migro-crats”, the public policymakers in the global networks of migration. Second, the paper monitors how the hypotheses had fared by the time of the second Forum conference, in Manila in 2008, by discourse analysis of its concluding report. The Manila meeting’s declaration of a “focus on the person” appeared in reality to a large extent mean a focus on the “migro-crats” and their interactive processes of mutual education and team-building that are intended to produce practical cooperation. To clarify this strategy and draw out its mindset and assumptions, the paper presents a series of tools for discourse analysis that may be more widely useful in migration studies and for participation in migration policy debate

Genética bioquímica del calamar argentino, Illex Argentinus

Previously reported results on natural populations of the Argentinean squid, Illex argentinus, urged further studies using electrophoretically detectable loci. The muscles of 202 adult squids caught in the Argentinean Sea were examined. Electrophoretic and staining methods for the 46 enzymes studied are given in detail. Forty-seven loci were detected (20 polymorphic), 45 of which are considered useable in population genetics screening. The expression of the loci coding for these enzymes is described and interpreted, and the presented results will serve as a basis for a more detailed examination of genetic variation in Argentinean squid populations.Estudios biológicos previos realizados en el calamar argentino, Illex argentinus, han sugerido la conveniencia de profundizar en el conocimiento de la estructura poblacional de la especie, utilizando el análisis genético de proteínas detectables electroforéticamente. Con esta finalidad se analizó el músculo esquelético de un total de 202 individuos capturados en el Mar Argentino. Se utilizó la técnica de electroforesis horizontal en gel de almidón al 11%. Se estudiaron 46 sistemas enzimáticos distintos. La expresión de los loci para estos enzimas son descritos e interpretados en detalle. Un total de 47 loci fueron detectados (20 polimórficos), de los cuales 45 se consideran utilizables para el análisis de la estructura poblacional de la especie

Potentialabhängige Morphologie von Kupferkatalysatoren während der Elektroreduktion von CO₂, ermittelt durch In‐situ‐Rasterkraftmikroskopie

Eine effiziente Charakterisierung der Katalysatoren im Realraum und unter realistischen Bedingungen der elektrochemischen CO2‐Reduktion (CO2RR) gelang durch elektrochemische AFM. Die Entwicklung von Strukturmerkmalen konnte von der Mikrometer‐ bis hin zur atomaren Skala aufgelöst werden. Auf einer Cu(100)‐Modelloberfläche treten während der CO2RR in 0,1 m KHCO3 ausgeprägte nanoskalige Oberflächenmorphologien auf, wobei sich granulare Strukturen potentialabhängig in glatt geschwungene Berg‐und‐Tal‐Oberflächen oder rechteckige Terrassenstrukturen umwandeln. Mit stärker negativem Potential steigt die Dichte der unterkoordinierten Cu‐Zentren während der CO2RR. Durch atomar aufgelöste In‐situ‐Bildgebung wird bei bestimmten kathodischen Potentialen spezifische Adsorption nachgewiesen, die die Katalysatorstruktur beeinflusst. Die Ergebnisse verdeutlichen die komplexen Abhängigkeiten zwischen Morphologie, Struktur, Defektdichte, angelegtem Potential und Elektrolyt bei Kupfer‐CO2RR‐Katalysatoren

Structural Evolution of Ga-Cu Model Catalysts for CO₂ Hydrogenation Reactions

We studied the initial stages of Ga interaction with the Cu(001) surface and environment-induced surface transformations in an attempt to elucidate the surface chemistry of the Cu–Ga catalysts recently proposed for CO2 hydrogenation to methanol. The results show that Ga readily intermixes with Cu upon deposition in vacuum. However, even traces of oxygen in the gas ambient cause Ga oxidation and the formation of two-dimensional (“monolayer”) Ga oxide islands uniformly covering the Cu surface. The surface morphology and the oxidized state of Ga remain in H2 as well as in a CO2 + H2 reaction mixture at elevated pressures and temperatures (0.2 mbar, 700 K). The results indicate that the Ga-doped Cu surface under reaction conditions exposes a variety of structures including GaOx/Cu interfacial sites, which must be taken into account for elucidating the reaction mechanism

Enhanced thermal stability and nanoparticle-mediated surface patterning: Pt/TiO2(110)

This letter reports (i) the enhanced thermal stability (up to 1060 degrees C) against coarsening and/or desorption of self-assembled Pt nanoparticles synthesized by inverse micelle encapsulation and deposited on TiO2(110) and (ii) the possibility of taking advantage of the strong nanoparticle/support interactions present in this system to create patterned surfaces at the nanoscale. Following our approach, TiO2 nanostripes with tunable width, orientation, and uniform arrangement over large surface areas were produced