Direct observations of dynamic PtCo interactions in fuel cell catalyst precursors at the atomic level using E(S)TEM

The role of high-resolution transmission electron microscopy and aberration corrected scanning transmission electron microscopy in unraveling the structure-property relationships of Pt-based fuel cells electrocatalysts

Platinum-based fuel cell electrocatalysts are structured on a nano level in order to extend their active surface area and maximize the utilization of precious and scarce platinum. Their performance is dictated by the atomic arrangement of their surface layers atoms via structure-property relationships. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) are the preferred methods for characterizing these catalysts, due to their capacity to achieve local atomic-level resolutions. Size, morphology, strain and local composition are just some of the properties of Pt-based nanostructures that can be obtained by (S)TEM. Furthermore, advanced methods of (S)TEM are able to provide insights into the quasi-in situ, in situ or even operando stability of these nanostructures. In this review, we present state-of-the-art applications of (S)TEM in the investigation and interpretation of structure-activity and structure-stability relationships.

Structural and Valence State Modification of Cobalt in CoPt Nanocatalysts in Redox Conditions

Platinum is the primary catalyst for many chemical reactions in the field of heterogeneous catalysis. However, platinum is both expensive and rare. Therefore, it is advantageous to combine Pt with another metal to reduce cost while also enhancing stability. To that end, Pt is often combined with Co to form Co-Pt nanocrystals. However, dynamical restructuring effects that occur during reaction in Co-Pt ensembles can impact catalytic properties. In this study, model Co₂Pt₃ nanoparticles supported on carbon were characterized during a redox cycle with two in situ approaches, namely, X-ray absorption spectroscopy (XAS) and scanning transmission electron microscopy (STEM) using a multimodal microreactor. The sample was exposed to temperatures up to 500 °C under H₂, and then to O₂ at 300 °C. Irreversible segregation of Co in the Co₂Pt₃ particles was seen during redox cycling, and substantial changes of the oxidation state of Co were observed. After H₂ treatment, a fraction of Co could not be fully reduced and incorporated into a mixed Co-Pt phase. Reoxidation of the sample increased Co segregation, and the segregated material had a different valence state than in the fresh, oxidized sample. This in situ study describes dynamical restructuring effects in CoPt nanocatalysts at the atomic scale that are crucial to understand in order to improve the design of catalysts used in major chemical processes.

Visualizing single atom dynamics in heterogeneous catalysis using analytical in situ environmental scanning transmission electron microscopy

Progress is reported in analytical in situ environmental scanning transmission electron microscopy (ESTEM) for visualizing and analysing in real-time dynamic gas-solid catalyst reactions at the single-atom level under controlled reaction conditions of gas environment and temperature. The recent development of the ESTEM advances the capability of the established ETEM with the detection of fundamental single atoms, and the associated atomic structure of selected solid-state heterogeneous catalysts, in catalytic reactions in their working state. The new data provide improved understanding of dynamic atomic processes and reaction mechanisms, in activity and deactivation, at the fundamental level; and in the chemistry underpinning important technological processes. The benefits of atomic resolution-E(S)TEM to science and technology include new knowledge leading to improved technological processes, reductions in energy requirements and better management of environmental waste. This article is part of a discussion meeting issue 'Dynamic in situ microscopy relating structure and function'.

Recent Progress of In Situ Transmission Electron Microscopy for Energy Materials

In situ transmission electron microscopy (TEM) is one of the most powerful approaches for revealing physical and chemical process dynamics at atomic resolutions. The most recent developments for in situ TEM techniques are summarized; in particular, how they enable visualization of various events, measure properties, and solve problems in the field of energy by revealing detailed mechanisms at the nanoscale. Related applications include rechargeable batteries such as Li-ion, Na-ion, Li-O₂ , Na-O₂ , Li-S, etc., fuel cells, thermoelectrics, photovoltaics, and photocatalysis. To promote various applications, the methods of introducing the in situ stimuli of heating, cooling, electrical biasing, light illumination, and liquid and gas environments are discussed. The progress of recent in situ TEM in energy applications should inspire future research on new energy materials in diverse energy-related areas.

Single Atom Dynamics in Chemical Reactions

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. Regeneration of deactivated catalysts is key to prolong catalyst life. We discuss and review analyses of dynamic redox cycles for the redispersion of nickel nanoparticles with single atom resolution. In the FT process, pretreatment of practical cobalt/silica catalysts reveals higher low-coordination Co⁰ active sites for CO adsorption. Collectively, the ESTEM findings generate structural insights into catalyst dynamics important in the development of efficient catalysts and processes.