Correlated Operando\nElectron Microscopy and Photoemission\nSpectroscopy in Partial Oxidation of Ethylene over Nickel

The production of\nsyngas from light hydrocarbons is a\nviable way\nof converting under-utilized hydrocarbon sources into valuable products\nuntil a full transition to renewable energy sources is achieved. However,\ncurrent heterogeneous catalysts for syngas production suffer from\ndeactivation, either by coking or oxidation. Here, we report on the\nbehavior of model nickel catalysts within the context of ethylene\npartial oxidation and observe the catalyst-environment interaction\nas a function of reactant feed and temperature. Using a combination\nof operando microscopy and spectroscopy and focusing on a reaction\nregime characterized by synchronized self-sustained oscillatory dynamics,\nwe are able to gain additional insights into the dynamic interplay\nbetween reactive species and active catalyst surfaces of varying reactivity.\nReal-time secondary electron imaging coupled with online mass spectrometry\nand thermal data shows that the oscillatory behavior is characterized\nby a highly active half-period during which the surface of the nickel\ncatalyst is metallic and a less active half-period during which the\nsurface is oxidized. Complementing the direct surface imaging, operando\nX-ray photoelectron spectroscopy provides missing information about\nthe alternating chemical state of the catalyst surface in the oscillating\nreaction regime. It reveals that changes in the gas phase composition\n(C<sub>2</sub>H<sub>4</sub>/O<sub>2</sub> ratio) alter the population\nof reaction intermediates (e.g., carbides) on the nickel surface,\nwhich in turn drives the selectivity of the reaction toward different\nproducts. The observed chemical dynamics involve changes in gas-phase\ncomposition, rate-dependent heat of reaction, the chemical state of\nthe catalyst, and the formation of reaction products, all of which\nare interconnected. Ultimately, the complex oscillations and catalytic\nbehavior are attributed to a multistep mechanism that involves complete\nethylene oxidation, dry and wet reforming of ethylene, and the reverse\nwater gas shift reaction.