Operando SEM and XPS insights of self-sustained oscillatory dynamics of ethylene to syngas over nickel catalysts

Abstract text (incl. figure legends and references)

As scientists, it is our duty to pave the way for the development of a sustainable economy based on renewable energy resources and to reduce greenhouse gas emissions with an expected market value of 48Bn $ by 2027 [1], the catalyst—whether heterogeneous, homogeneous, or enzymatic—represents one of the pillars on which modern society is built. However, apart from the simplest model reactions, the scientific community is still unable to fully comprehend the underlying mechanism of catalysis and the atomistic details of the active state.

The objective is to obtain mechanistic insights into the role of surface species present in the catalytic process of partial oxidation of ethylene to syngas on a model polycrystalline nickel foil. Using a combination of operando scanning electron microscopy (OSEM) for real-time observation of structural dynamics and ambient pressure x-ray photoelectron spectroscopy (APXPS) we correlate the state of the catalyst with its activity. Although challenging to study, the self-sustained oscillating reaction mode enables detailed analysis of the successive elementary steps of the catalytic act (Figure S1). The obtained insights are envisioned to expand the research on the self-regenerating operation of real-world catalysts.

We were able to extend the functionality of a TFS Quattro S ESEM by developing a mobile in-situ that enabled the ambient pressure x-ray photoemission spectroscopy and operando scanning electron microscopy studies to be performed under identical conditions. This unit is composed of a modified sample holder that can achieve working temperatures of up to 1250C in reactive gas atmospheres, an automated heating unit and an automated gas feeding system.

By maintaining a constant gas composition in isothermal conditions, a synchronized self-sustained oscillation mode was identified, in which the reaction`s product distribution reflects the catalyst morphology and chemical state. We were able to pinpoint the catalytic behavior down to competing species that influence catalyst deactivation with excellent spatial resolution and to discriminate between local and non-local mechanisms that impact the collective dynamics usually reported using traditional approaches.

The combination of spatially resolved real-time imaging [2], [3] with integral surface-sensitive spectroscopy enables the chemical identification of surface species and simultaneous visualization of their evolution as a function of reaction conditions. We were able to provide insight into structure sensitivity and coupling phenomena between reactive gas-phase and catalyst states that are related to the emergence of catalytic activity in a synchronized self-sustained oscillatory mode. The here described structure-activity correlations are not accessible through integral characterization techniques or by studying steady-states alone, highlighting the importance of real-time, laterally resolved direct observation of active catalysts.

Figure S1: Time series of a Ni surface during ethylene to syngas conversion. HFOV is 33.5µm.

[1] Catalyst Market Size, Share & Trends Analysis Report By Raw Material 2020 – 2027

[2] Danilatos,Gerasimos D. "Environmental scanning electron microscopy and microanalysis." Microchimica Acta 114.1 (1994): 143-155

[3] Sandoval-Diaz, Luis, et al. "Visualizing the importance of oxide-metal phase transitions in the production of synthesis gas over Ni catalysts." Journal of Energy Chemistry (2020).