Progress and challenges in structural, in situ and operando characterization of single-atom catalysts by X-ray based synchrotron radiation techniques

Operando characterization technique innovations in single-atom catalyst-derived electrochemical CO2 conversion

Single-atom catalysts (SACs) are the perfect epitome of cutting-edge innovation in catalysis, offering distinct active sites in the form of isolated, individual atoms. SACs amalgamate the advantages of homogeneous and heterogeneous catalysis, in turn enhancing the activity, selectivity, and stability during catalytic processes. The latest progressions in in situ/operando characterization techniques have simplified the understanding of the catalyst heterogeneity and structure sensitivity of SACs by enabling the real-time observation of SACs under a coexistent working environment to provide insights into their structure and catalytic efficiency. Operando techniques are the backbone for investigating the stability and activity of SACs during energy conversions. Techniques such as operando X-ray absorption spectroscopy (XAS), polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS), and near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) have been employed to study SACs under different reaction conditions. Furthermore, in situ visualization microscopy techniques have made notable progress in the imaging of ongoing catalytic reactions on SACs and revealed enigmatic effects such as facet-resolved catalytic ignition and anisotropic surface oxidation. Therefore, in this review, we have compiled the developments in the significant operando characterization techniques for SAC-derived electrochemical carbon dioxide reduction reaction (eCO2RR).

Interfacial reactions and speciation identification during arsenic treated with nanoscale zerovalent iron (nZVI) in water: A review

This perspective briefly summarized the progress of inorganic arsenic (As) treated with nanoscale zerovalent iron (nZVI) in water over the past two decades. The intrinsic interfacial reaction between As and nZVI encompassed multiple effects, such as complexation, oxidation, reduction, and co-precipitation, ascribed to core-shell structure of nZVI and environmental behavior of As in water. Surface complexation occurred via ligand exchange of arsenate anions with Fe-OH groups on the iron oxide shell. However, interfacial oxidation of As(III) to As(V) was attributed to form a Fe(III) oxide-Fe(II)-As(III) ternary surface complex under anoxic conditions, as well as generate reactive oxygen species (e.g., H2O2, •OH) from iron reacted with O2 under oxic conditions. Reduction of As(III) to As(0) was followed by subsurface accumulation near the Fe(0) core. Advanced characterization techniques, including high-resolution X-ray photoelectron spectroscopy, in situ X-ray absorption spectroscopy, spherical aberration-corrected scanning transmission electron microscope, and density functional theory combined with quick-scanning extended X-ray absorption fine structure, have unraveled the multi-tiered distributions of As on nZVI at atomic scale. This review highlights critical gaps in understanding As-Fe redox dynamics and advocates for future research to engineer nZVI with tailored surface properties for enhanced As sequestration.

Exploring the properties, types, and performance of atomic site catalysts in electrochemical hydrogen evolution reactions

Atomic site catalysts (ASCs) have recently gained prominence for their potential in the electrochemical hydrogen evolution reaction (HER) due to their exceptional activity, selectivity, and stability. ASCs with individual atoms dispersed on a support material, offer expanded surface areas and increased mass efficiency. This is because each atom in these catalysts serves as an active site, which enhances their catalytic activity. This review is focused on providing a detailed analysis of ASCs in the context of the HER. It will delve into their properties, types, and performance to provide a comprehensive understanding of their role in electrochemical HER processes. The introduction part underscores HER's significance in transitioning to sustainable energy sources and emphasizes the need for innovative catalysts like ASCs. The fundamentals of the HER section emphasizes the importance of understanding the HER and highlights the key role that catalysts play in HER. The review also explores the properties of ASCs with a specific emphasis on their atomic structure and categorizes the types based on their composition and structure. Within each category of ASCs, the review discusses their potential as catalysts for the HER. The performance section focuses on a thorough evaluation of ASCs in terms of their activity, selectivity, and stability in HER. The performance section assesses ASCs in terms of activity, selectivity, and stability, delving into reaction mechanisms via experimental and theoretical approaches, including density functional theory (DFT) studies. The review concludes by addressing ASC-related challenges in HER and proposing future research directions, aiming to inspire further innovation in sustainable catalysts for electrochemical HER.

Unraveling Dynamic Structural Evolution of Single Atom Catalyst via In Situ Surface-Enhanced Infrared Absorption Spectroscopy

Metal-nitrogen-carbon (M-N-C) single-atom catalysts (SACs) have been widely applied in catalyzing electrochemical redox reactions. However, their long-term catalytic stabilities greatly limit their practical applications. This work investigates the dynamic evolution of two model Cu-N-C SACs with different Cu-N coordinations, namely the Cu1/Npyri-C and Cu1/Npyrr-C, in electrochemical CO reduction reaction (CORR), based on a collection of in situ characterizations including in situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy, in situ X-ray absorption spectroscopy, quasi-in situ electron paramagnetic resonance spectroscopy and in situ ultraviolet-visible spectroscopy, complemented by theoretical calculations. Our findings reveal that the Cu nanoparticle formation rate over Cu1/Npyrr-C is more than 6 times higher than that over Cu1/Npyri-C during the electrochemical CORR. Quasi-in situ electron paramagnetic resonance and in situ UV-vis spectroscopy measurements demonstrate that hydrogen radicals can be in situ produced during electrochemical CORR, which will attack the Cu-N bonds in the Cu-N-C SACs, causing leaching of Cu2+ followed by subsequent reduction to form Cu nanoparticles. Kinetic calculations show that Cu1/Npyri-C displays a better catalytic stability than Cu1/Npyrr-C resulting from the stronger Cu-Npyri bonds. This study deepens the understanding of the deactivation mechanism of SACs in electrochemical reactions and provides guidance for the design of next-generation SACs with enhanced durability.

Graphene quantum dots as supramolecular linkers in metal organic frameworks for constructing metal atom electrocatalysts

A new strategy is developed to stabilize single cobalt sites by covalent linking between metal and N-doped graphene quantum dots (N-GQDs) confined within the pores of zeolitic imidazolate frameworks (ZIFs). The incorporation of N-GQDs not only facilitates the formation of abundant active sites but also promotes their accessibility.

Modulation of Single-Iron-Atom Coordination Environment Toward Three-Electron Oxygen Reduction for Photocatalytic CH4 Conversion to CH3OH

By modifying the coordination environment of single-Fe-atom active site, effective regulation of the photocatalytic oxygen reduction pathway can be achieved to attain high activity for photocatalytic oxidation of CH4 to CH3OH in an aqueous solution. A comprehensive investigation is conducted to study the impact of different coordination numbers of single Fe atoms on photocatalytic CH4 oxidation reaction over carbon nitride. Among which, Fe1/C3-xN4 with a Fe-N3 coordination exhibit an exceptional photocatalytic performance in CH4 oxidation, reaching a remarkable methanol yield of 928.27 µmol gcat -1, much higher than Fe1/C3N4 and Fe1/C3N4-x (308.47 and 473.26 µmol gcat -1, respectively). Based on a collection of in situ characterizations and time-dependent density functional theory calculations, it is determined that Fe1/C3-xN4 with an optimal coordination number possesses the optimized electronic configuration that enables three-electron oxygen reduction to generate hydroxyl radicals for photocatalytic conversion of CH4 to CH3OH.

Single Atom Cocatalysts in Photocatalysis

Single-atom (SA) cocatalysts (SACs) have garnered significant attention in photocatalysis due to their unique electronic properties and high atom utilization efficiency. This review provides an overview of the concept and principles of SA cocatalyst in photocatalysis, emphasizing the intrinsic differences to SAs used in classic chemical catalysis. Key factors that influence the efficiency of SAs in photocatalytic reactions, particularly in photocatalytic hydrogen (H2) production, are highlighted. This review further covers synthesis methods, stabilization strategies, and characterization techniques for common SAs used in photocatalysis. Notably, "reactive deposition" method, which often shows a self-homing effect and thus achieves a maximum utilization efficiency of SA cocatalysts, is emphasized. Furthermore, the applications of SA cocatalysts in various photocatalytic processes, including H2 evolution, carbon dioxide reduction, nitrogen fixation, and organic synthesis, are comprehensively reviewed, along with insights into common artifacts in these applications. This review concludes by addressing the challenges faced by SACs in photocatalysis and offering perspectives on future developments, with the aim of informing and advancing research on SAs for photocatalytic energy conversion.