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

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

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

Identifying structure-selectivity correlations in the electrochemical reduction of CO₂: a comparison of well-ordered atomically-clean and chemically-etched Cu single crystal surfaces

Despite significant theoretical efforts, the identification of the active sites for the electrochemical reduction of CO2 (CO2RR) to specific chemical products has remained elusive. This is partially due to insufficient experimental data gathered on clean and atomically well-ordered electrode surfaces. Here, ultrahigh vacuum based preparation methods and surface science characterization techniques are used together with gas chromatography to demonstrate that subtle changes in the preparation of well-oriented Cu(100) and Cu(111) single crystal surfaces drastically affect their CO2RR selectivity. Copper single crystals with clean, flat, and atomically ordered surfaces are theoretically predicted to yield hydrocarbons. However, these were found experimentally to favour the production of H2. Only when roughness and defects are introduced, for example through an electrochemical etchingor a plasmatreatment, significant amounts of hydrocarbons are generated. These results clearly indicate that structural and morphological effects are the key factors determining the catalytic selectivity of CO2RR

Identifying structure-selectivity correlations in the electrochemical reduction of CO₂: a comparison of well-ordered atomically-clean and chemically-etched Cu single crystal surfaces

Despite significant theoretical efforts, the identification of the active sites for the electrochemical reduction of CO2(CO2RR) to specific chemical products has remained elusive. This is partially due to insufficient experimental data gathered on clean and atomically well-ordered electrode surfaces. Here, ultrahigh vacuum based preparation methods and surface science characterization techniques are used together with gas chromatography to demonstrate that subtle changes in the preparation of well-oriented Cu(100) and Cu(111) single crystal surfaces drastically affect their CO2RR selectivity. Copper single crystals with clean, flat, and atomically ordered surfaces are theoretically predicted to yield hydrocarbons. However, these were found experimentally to favour the production of H2. Only when roughness and defects are introduced, for example through an electrochemical etching or a plasma treatment, significant amounts of hydrocarbons are generated. These results clearly indicate that structural and morphological effects are the key factors determining the catalytic selectivity of CO2RR

Growth Dynamics and Processes Governing the Stability of Electrodeposited Size-Controlled Cubic Cu Catalysts

The renewable energy-powered conversion of industrially generated CO2 into useful chemicals and fuels is considered a promising technology for the sustainable development of our modern society. The electrochemical reduction of CO2 (CO2RR) is one of the possible conversion processes that can be employed to close the artificial carbon cycle, mimicking nature’s photosynthesis. Nevertheless, to enable green catalytic processes, selectivity for the desired products must be achieved. In the case of CO2RR, the selectivity is strongly dependent on the electrocatalyst structure. Here, we explore the phase space of synthesis parameters required for the electrodeposition of Cu cubes with {100} facets on glassy carbon substrates and elucidate their influence on the size, shape, coverage, and uniformity of the cubes. We found that the concentration of Cl– ions in solution controls the cube size, shape, and coverage, whereas the ratio of the reduction versus oxidation time and number of cycles in the alternating potential electrodeposition protocol can be used to further tune the cube size. Cyclic voltammetry experiments were complemented with in situ electrochemical scanning electron microscopy to follow the growth dynamics and ex situ transmission electron microscopy and electron diffraction. Our results indicate that the cube growth starts from nuclei formed during the first cycle, followed by a layered deposition and partial dissolution of new material in subsequent cycles