Correlated Operando Electron Microscopy and Photoemission Spectroscopy in Partial Oxidation of Ethylene over Nickel

The production of syngas from light hydrocarbons is a viable way of converting under-utilized hydrocarbon sources into valuable products until a full transition to renewable energy sources is achieved. However, current heterogeneous catalysts for syngas production suffer from deactivation, either by coking or oxidation. Here, we report on the behavior of model nickel catalysts within the context of ethylene partial oxidation and observe the catalyst-environment interaction as a function of reactant feed and temperature. Using a combination of operando microscopy and spectroscopy and focusing on a reaction regime characterized by synchronized self-sustained oscillatory dynamics, we are able to gain additional insights into the dynamic interplay between reactive species and active catalyst surfaces of varying reactivity. Real-time secondary electron imaging coupled with online mass spectrometry and thermal data shows that the oscillatory behavior is characterized by a highly active half-period during which the surface of the nickel catalyst is metallic and a less active half-period during which the surface is oxidized. Complementing the direct surface imaging, operando X-ray photoelectron spectroscopy provides missing information about the alternating chemical state of the catalyst surface in the oscillating reaction regime. It reveals that changes in the gas phase composition (C2H4/O2 ratio) alter the population of reaction intermediates (e.g., carbides) on the nickel surface, which in turn drives the selectivity of the reaction toward different products. The observed chemical dynamics involve changes in gas-phase composition, rate-dependent heat of reaction, the chemical state of the catalyst, and the formation of reaction products, all of which are interconnected. Ultimately, the complex oscillations and catalytic behavior are attributed to a multistep mechanism that involves complete ethylene oxidation, dry and wet reforming of ethylene, and the reverse water gas shift reaction.