In situ X-ray Absorption Spectroscopy of Metal/Nitrogen-doped Carbons in Oxygen Electrocatalysis

Metal/nitrogen-doped carbons (M-N-C) are promising candidates as oxygen electrocatalysts due to their low cost, tunable catalytic activity and selectivity, and well-dispersed morphologies. To improve the electrocatalytic performance of such systems, it is critical to gain a detailed understanding of their structure and properties through advanced characterization. In situ X-ray absorption spectroscopy (XAS) serves as a powerful tool to probe both the active sites and structural evolution of catalytic materials under reaction conditions. In this review, we firstly provide an overview of the fundamental concepts of XAS and then comprehensively review the setup and application of in situ XAS, introducing electrochemical XAS cells, experimental methods, as well as primary functions on catalytic applications. The active sites and the structural evolution of M-N-C catalysts caused by the interplay with electric fields, electrolytes and reactants/intermediates during the oxygen evolution reaction and the oxygen reduction reaction are subsequently discussed in detail. Finally, major challenges and future opportunities in this exciting field are highlighted.

In situ X-ray emission and high-resolution X-ray absorption spectroscopy applied to Ni-based bimetallic dry methane reforming catalysts

The promoting effect of cobalt on the catalytic activity of a NiCoO Dry Methane Reforming (DMR) catalyst was studied by a combination of in situ Kβ X-ray Emission Spectroscopy (XES) and Kβ-detected High Energy Resolution Fluorescence Detected X-ray absorption spectroscopy (HERFD XAS). Following the calcination process, Ni XES and Kβ-detected HERFD XAS data revealed that the NiO coordination in the NiCoO catalyst has a higher degree of symmetry and is different than that of pure NiO/γ-Al₂O₃. Following the reductive activation, it was found that the NiCoO/γ-Al₂O₃ catalyst required a relatively higher temperature compared to the monometallic NiO/γ-Al₂O₃ catalyst. This finding suggests that Co is hampering the reduction of Ni in the NiCoO catalyst by modulation of its electronic structure. It has also been previously shown that the addition of Co enhances the DMR activity. Further, the Kβ XES spectrum of the partly reduced catalysts at 450 °C reveals that the Ni sites in the NiCoO catalyst are electronically different from the NiO catalyst. The in situ X-ray spectroscopic study demonstrates that reduced metallic Co and Ni are the primary species present after reduction and are preserved under DMR conditions. However, the NiCo catalyst appears to always be somewhat more oxidized than the Ni-only species, suggesting that the presence of cobalt modulates the Ni electronic structure. The electronic structural modulations resulting from the presence of Co may be the key to the increased activity of the NiCo catalyst relative to the Ni-only catalyst. This study emphasizes the potential of in situ X-ray spectroscopy experiments for probing the electronic structure of catalytic materials during activation and under operating conditions.

In Situ X-ray Microscopy Reveals Particle Dynamics in a NiCo Dry Methane Reforming Catalyst under Operating Conditions

Herein, we report the synthesis of a γ-Al₂O₃-supported NiCo catalyst for dry methane reforming (DMR) and study the catalyst using in situ scanning transmission X-ray microscopy (STXM) during the reduction (activation step) and under reaction conditions. During the reduction process, the NiCo alloy particles undergo elemental segregation with Co migrating toward the center of the catalyst particles and Ni migrating to the outer surfaces. Under DMR conditions, the segregated structure is maintained, thus hinting at the importance of this structure to optimal catalytic functions. Finally, the formation of Ni-rich branches on the surface of the particles is observed during DMR, suggesting that the loss of Ni from the outer shell may play a role in the reduced stability and hence catalyst deactivation. These findings provide insights into the morphological and electronic structural changes that occur in a NiCo-based catalyst during DMR. Further, this study emphasizes the need to study catalysts under operating conditions in order to elucidate material dynamics during the reaction.

Elucidation of Structure-Activity Correlations in a Nickel Manganese Oxide Oxygen Evolution Reaction Catalyst by Operando Ni L-Edge X-ray Absorption Spectroscopy and 2p3d Resonant Inelastic X-ray Scattering

Herein, we report the synthesis and electrochemical oxygen evolution experiments for a graphene-supported Ni₃MnO₄ catalyst. The changes that occur at the Ni active sites during the electrocatalyic oxygen evolution reaction (OER) were elucidated by a combination of operando Ni L-edge X-ray absorption spectroscopy (XAS) and Ni 2p3d resonant inelastic X-ray scattering (RIXS). These data are compared to reference measurements on NiO, β-Ni(OH)₂, β-NiOOH, and γ-NiOOH. Through this comparative analysis, we are able to show that under alkaline conditions (0.1 M KOH), the oxides of the Ni₃MnO₄ catalyst are converted to hydroxides. At the onset of catalysis (1.47 V), the β-Ni(OH)₂-like phase is oxidized and converted to a dominantly γ-NiOOH phase. The present study thus challenges the notion that the β-NiOOH phase is the active phase in OER and provides further evidence that the γ-NiOOH phase is catalytically active. The ability to use Ni L-edge XAS and 2p3d RIXS to provide a rational basis for structure-activity correlations is highlighted.

Mechanisms of chloroform and carbon tetrachloride toxicity in primary cultured mouse hepatocytes

Mechanisms of chloroform (CHCl3) and carbon tetrachloride (CCl4) toxicity to primary cultured male B6C3F1 mouse hepatocytes were investigated. The cytotoxicity of both CHCl3 and CCl4 was dose- and duration-dependent. Maximal hepatocyte toxicity, as determined by lactate dehydrogenase leakage into the culture medium, occurred with the highest concentrations of CHCl3 (5 mM) and CCl4 (2.5 mM) used and with the longest duration of treatment (20 hr). CCl4 was approximately 16 times more toxic than CHCl3 to the hepatocytes. The toxicity of these compounds was decreased by adding the mixed function oxidase system (MFOS) inhibitor, SKF-525A (25 microM) to the cultures. The addition of diethyl maleate (0.25 mM), which depletes intracellular glutathione (GSH)-potentiated CHCl3 and CCl4 toxicity. The toxicity of CHCl3 and CCl4 could also be decreased by adding the antioxidants N,N'-diphenyl-p-phenylenediamine (DPPD) (25 microM), alpha-tocopherol acetate (Vitamin E) (0.1 mM), or superoxide dismutase (SOD) (100 U/mL) to the cultures. These results suggest that: in mouse hepatocytes, both CHCl3 and CCl4 are metabolized to toxic components by the MFOS; GSH plays a role in detoxifying those metabolites; free radicals are produced during the metabolism of CHCl3 and CCl4; and free radicals may be important mediators of the toxicity of these two halomethanes.