Understanding the dynamic behavior of catalysts is key to improving their efficiency and selectivity. A recent study published in Nature Communications has shed light on the performance of copper (Cu) catalysts during ethylene oxidation, utilizing operando transmission electron microscopy (TEM) combined with online mass spectrometry (MS). The research identifies three primary temperature regimes within which the catalyst exhibits differing structures and catalytic behaviors, thereby influencing the selectivity of reaction products.
Electron microscopy technology has significantly advanced the study of catalysis, providing insights at the atomic level. Despite its advantages, traditional electron microscopy operates under high vacuum conditions, which often do not accurately reflect the states of catalysts under operational conditions. This study overcomes this limitation, allowing for real-time monitoring of Cu catalysts during ethylene oxidation.
The researchers identified three distinct states of the Cu catalyst: at low temperatures, Cu2O (copper oxide) displays selectivity for ethylene oxide (EO) and acetaldehyde (AcH) through the oxometallacycle (OMC) pathway. A dynamic state emerges at medium temperatures, where oscillations between Cu and Cu2O occur, leading to reduced activation energies for partial oxidation reactions. At high temperatures, metallic Cu is the dominant phase, favoring full oxidation reaction pathways.
Interestingly, the findings challenge prior conclusions drawn from ultra-high vacuum studies which suggested metallic copper was the ideal selective catalyst for epoxidation. Instead, the operando approach revealed the significant role of oxide phases during reaction conditions, asserting the need for studying catalysts under realistic operational circumstances.
The research involved active Cu nanoparticles (NPs) which were prepared and treated under specific atmospheric conditions and observed at varying temperatures ranging from 200 °C to 950 °C. Following oxidation-reduction treatments, the nanoparticles underwent structural changes, ranging from hollow structures at low temperatures to more complex morphologies at elevated temperatures due to the Kirkendall effect and particle fragmentation.
Real-time observations showed how the shape and size of the Cu particles fluctuated as temperature increased. Notably, the particles displayed morphological transitions and dynamic behaviors, particularly between 600 °C and 800 °C. The researchers employed selected-area electron diffraction (SAED) to confirm phase compositions at different temperatures, with Cu2O and Cu phases present during oscillation regimes.
Online mass spectrometry provided real-time data confirming the catalyst was active for both selective (EO and AcH) and full oxidation (CO2) products across the studied temperature ranges. Analysis of mass signals illustrated shifts toward full oxidation products at higher temperatures, with metallic copper being correlated with increased total oxidation activity.
Density functional theory (DFT) calculations complemented the experimental observations, providing theoretical insights on the kinetics of ethylene oxidation under various conditions. These simulations suggested pristine Cu2O was active for producing AcH and EO, particularly under low-temperature conditions, which provided important contributions to the observed behavior of the catalyst during experiments.
The study concludes by emphasizing the significance of operando TEM techniques for elucidation of structure-performance relationships, particularly for redox-active metal catalysts like copper. The oscillatory behavior observed during reaction conditions poses interesting questions about the interplay between kinetics and thermodynamics during catalytic reactions, as well as future avenues of research aimed at optimizing catalyst performance. Through these insights, the research paves the way for improved catalysts for industrial applications, particularly for the selective oxidation of ethylene.