Single Atom Dynamics in Chemical Reactions

Abstract

Metrics & MoreArticle Recommendations CONSPECTUS: Many heterogeneous chemical reactions involve gases catalyzed over solid surfaces at elevated temperatures and play a critical role in the production of energy, healthcare, pollution control, industrial products, and food. These catalytic reactions take place at the atomic level, with active structures forming under reaction conditions. A fundamental understanding of catalysis at the single atom resolution is therefore a major advance in a rational framework upon which future catalytic processes can be built. Visualization and analysis of gas-catalyst chemical reactions at the atomic level under controlled reaction conditions are key to understanding the catalyst structural evolution and atomic scale reaction mechanisms crucial to the performance and the development of improved catalysts and chemical processes. Increasingly, dynamic single atoms and atom clusters are believed to lead to enhanced catalyst performance, but despite considerable efforts, reaction mechanisms at the single atom level under reaction conditions of gas and temperature are not well understood. The development of the atomic lattice resolution environmental transmission electron microscope (ETEM) by the authors is widely used to visualize gas−solid catalyst reactions at this atomic level. It has recently been advanced to the environmental scanning TEM (ESTEM) with single atom resolution and full analytical capabilities. The ESTEM employs high-angle annular dark-field imaging where intensity is approximately proportional to the square of the atomic number (Z). In this Account, we highlight the ESTEM development also introduced by the authors for real time in situ studies to reliably discern metal atoms on lighter supports in gas and high temperature environments, evolving oxide/metal interfaces, and atomic level reaction mechanisms in heterogeneous catalysts more generally and informatively, with utilizing the wider body of literature. The highlights include platinum/carbon systems of interest in fuel cells to meet energy demands and reduce environmental pollution, in reduction/oxidation (redox) mechanisms of copper and nickel nanoparticles extensively employed in catalysis, electronics, and sensors, and in the activation of supported cobalt catalysts in Fischer−Tropsch (FT) synthesis to produce fuels. By following the dynamic reduction process at operating temperature, we investigate Pt atom migrations from irregular nanoparticles in a carbon supported platinum catalyst and the resulting faceting. We outline the factors that govern the mechanism involved, with the discovery of single atom interactions which indicate that a primary role of the nanoparticles is to act as reservoirs of low coordination atoms and clusters. This has important implications in supported nanoparticle catalysis and nanoparticle science. In copper and nickel systems, we track the oxidation front at the atomic level as it proceeds across a nanoparticle, by directly monitoring Z-contrast changes with time and temperature...

Atom-by-atom analysis of sintering dynamics and stability of Pt nanoparticle catalysts in chemical reactions

“…Spherical aberration correctors have led to greatly improved probe resolution [20]. Methods to identify single atoms are reported in the literature [19,21] where single atoms appear as white dots in HAADF images and are quantified by intensity profiles and calibration of intensities [22]. MEMS heating holders, such as from DENSsolutions used here, provide much improved temperature control and stage stability.…”

“…As the nature of the active reaction site of the metal nanoparticle, along with the processes by which the particle sinters and deactivates, have been shown to change both in magnitude and mechanism with the reaction environment [4,22,[27][28][29][30], it is of fundamental importance to activate, react, observe and analyse nanoparticle systems in-situ, in real time, under controlled continuous gas flow reaction conditions at operating temperatures. Reacting supported single platinum atoms can now be resolved and analysed in controlled flowing reducing gas environments at operating temperatures in the ESTEM using HAADF imaging (ESTEM-HAADF) [3,31].…”

Structural Regulation and Support Coupling Effect of Single‐Atom Catalysts for Heterogeneous Catalysis

“…Recently, a few experimental and theoretical studies suggest that SACs are likely to undergo structural reconstruction under the harsh environments from synthesis or catalysis. [128][129][130][131][132] Tracking the structure evolution via in situ or operando characterization techniques is highly beneficial for identifying the true active sites and clarifying the catalytic mechanism under realistic conditions. Moreover, understanding the SACs dynamics in terms of the location, geometric and electronic structure, and reaction pathway can generate new insights into www.advancedsciencenews.com the structure-activity relationship important in guiding the rational design of advanced heterocatalysts.…”

Quo Vadis Micro-Electro-Mechanical Systems for the Study of Heterogeneous Catalysts Inside the Electron Microscope?

“…Aperture-based ETEMs [54,[61][62][63][64][65][66], where differential pumping systems are used to confine gasses near the sample, can offer better resolutions and contrasts as they do not deal with the additional scattering brought on by the silicon nitride windows. This makes differentially pumped ETEM superior for the imaging of light element materials, such as graphene [67] or light organic molecules [68].…”