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

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

Thermally Stable Nanoparticles on Supports

This patent describes a new synthesis method for the large scale and low-cost production of size-selected nanoparticles with uniform 2D arrangement on a surface and nanowire patterned substrates with tunable width and interwire distance in a single preparation step. The nanoparticles prepared using our modified synthesis procedure exhibit an enhanced thermal stability and resistance against coarsening/sintering and desoreption at high temperature [at least 1060?C for Pt/TiO2(110)] thanks to the presence of a “polymeric glue” at the nanoparticle/support interface. Such thermally stable particles are can not only be stabilized on single crystal surfaces such as TiO2(110) but also on real-world catalytic supports such as nanocrystalline powders (anatase TiO2, CeO2, etc.). A second major advantage of our invention is the the possibility of using the strong nanoparticle/support interactions present in these low dimensional systems to create patterned surfaces at the nanoscale. As an exampl

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

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

Method For Forming Thermally Stable Nanoparticles on Supports

This patent describes a new synthesis method for the large scale and low-cost production of size-selected nanoparticles with uniform 2D arrangement on a surface and nanowire patterned substrates with tunable width and interwire distance in a single preparation step. The nanoparticles prepared using our modified synthesis procedure exhibit an enhanced thermal stability and resistance against coarsening/sintering and desoreption at high temperature [at least 1060?C for Pt/TiO2(110)] thanks to the presence of a “polymeric glue” at the nanoparticle/support interface. Such thermally stable particles are can not only be stabilized on single crystal surfaces such as TiO2(110) but also on real-world catalytic supports such as nanocrystalline powders (anatase TiO2, CeO2, etc.). A second major advantage of our invention is the the possibility of using the strong nanoparticle/support interactions present in these low dimensional systems to create patterned surfaces at the nanoscale. As an exampl

Formation, thermal stability, and surface composition of size-selected AuFe nanoparticles

The surface composition of isolated Au0.5Fe0.5 nanoparticles (NPs) synthesized by micelle encapsulation and supported on TiO2(110) has been investigated. The study reveals that phase-segregated structures are present after annealing at 300 degrees C. A subsequent thermal treatment at 700 degrees C resulted in the formation of a AuFe alloy. At this temperature, a state characteristic of Fe was identified at the NPs\u27 surface. Annealing at 900 degrees C resulted in the disappearance of the Fe surface state, which is attributed to Au segregation to the surface. The initial hexagonal NP arrangement on the TiO2(110) surface was preserved up to 900 degrees C. At 1000 degrees C, Au desorption was observed

In situ and operando electron microscopy in heterogeneous catalysis - insights into multi-scale chemical dynamics

This review features state-of-the-art in situ and operando electron microscopy (EM) studies of heterogeneous catalysts in gas and liquid environments during reaction. Heterogeneous catalysts are important materials for the efficient production of chemicals/fuels on an industrial scale and for energy conversion applications. They also play a central role in various emerging technologies that are needed to ensure a sustainable future for our society. Currently, the rational design of catalysts has largely been hampered by our lack of insight into the working structures that exist during reaction and their associated properties. However, elucidating the working state of catalysts is not trivial, because catalysts are metastable functional materials that adapt dynamically to a specific reaction condition. The structural or morphological alterations induced by chemical reactions can also vary locally. A complete description of their morphologies requires that the microscopic studies undertaken span several length scales. EMs, especially transmission electron microscopes, are powerful tools for studying the structure of catalysts at the nanoscale because of their high spatial resolution, relatively high temporal resolution, and complementary capabilities for chemical analysis. Furthermore, recent advances have enabled the direct observation of catalysts under realistic environmental conditions using specialized reaction cells. Here, we will critically discuss the importance of spatially-resolved operando measurements and the available experimental setups that enable (1) correlated studies where EM observations are complemented by separate measurements of reaction kinetics or spectroscopic analysis of chemical species during reaction or (2) real-time studies where the dynamics of catalysts are followed with EM and the catalytic performance is extracted directly from the reaction cell that is within the EM column or chamber. Examples of current research in this field will be presented. Challenges in the experimental application of these techniques and our perspectives on the field's future directions will also be discussed

Piece by Piece-Electrochemical Synthesis of Individual Nanoparticles and their Performance in ORR Electrocatalysis

The impact of individual HAuCl4 nanoreactors is measured electrochemically, which provides operando insights and precise control over the modification of electrodes with functional nanoparticles of well‐defined size. Uniformly sized micelles are loaded with a dissolved metal salt. These solution‐phase precursor entities are then reduced electrochemically—one by one—to form nanoparticles (NPs). The charge transferred during the reduction of each micelle is measured individually and allows operando sizing of each of the formed nanoparticles. Thus, particles of known number and sizes can be deposited homogenously even on nonplanar electrodes. This is demonstrated for the decoration of cylindrical carbon fibre electrodes with 25±7 nm sized Au particles from HAuCl4‐filled micelles. These Au NP‐decorated electrodes show great catalyst performance for ORR (oxygen reduction reaction) already at low catalyst loadings. Hence, collisions of individual precursor‐filled nanocontainers are presented as a new route to nanoparticle‐modified electrodes with high catalyst utilization