Redox phase transitions and reaction-induced strain in an oscillating reaction: an operando SEM approach to dry reforming of methane on Ni catalyst

Abstract text (incl. figure legends and references)

Introduction

Chemical dynamics, represented by structural, morphological, or compositional changes of the solid during heterogeneous reactions¹, have been revealed by several in situ techniques including environmental scanning electron microscopy (ESEM)^2,3. However, it is still not entirely clear how these dynamics affect catalytic observables, such as activity and selectivity. Here, we visualize the chemical dynamics of a metallic Ni catalyst at conditions of dry reforming of methane (DRM) by operando SEM to investigate their influence on activity. We found that redox Ni/NiO transitions drive the chemical conversion. Below 810 °C, the catalyst is oxidized and inactive, but it is reduced and active to syngas (H₂ + CO mixture) production above this temperature. Furthermore, the system achieved a state of continuous phase transitions which generated periodic self-sustained oscillations. Analysis of these processes and of the spent catalyst hints the Mars-van Krevelen nature of DRM and the action of in situ strain as a feedback element of the oscillations. Likely, these observations are of general occurrence in catalysis and may impact novel mechanochemical reactor designs.

Objective

Investigate the correlation of chemical dynamics with catalysis.

Materials & Methods

A commercial ESEM equipped with a laser heating stage², a quartz tube reactor³, and a gas feeding station was used. The catalyst was a Ni foam. DRM was studied in a mixture of Ar:CO₂:CH₄ in the temperature interval between 700-900°C. ESEM images were recorded every 18s, and gaseous compositions were measured online by mass spectrometry. Results were complemented by near ambient pressure X-ray photoelectron spectroscopy (NAP XPS) measurements and high-resolution transmission electron microscopy (HRTEM) imaging of a focused ion beam lamella of the spent catalyst.

Results & Discussion

Fig. 1a,b show that bright NiO phases grew at the catalyst under DRM feed at 700-810°C. Subsequently, Fig. 1c,d show that the bright features disappeared (810-825°C). NAP XPS of Ni2p core levels reveal the reduction of NiO into Ni at 810°C (Fig. 1e). Catalytically, syngas production activated during this reduction (Fig. 1f), and further increased with increasing temperatures. The catalyst was later held in the interval of the phase transition (810-825°C). An oscillatory, self-sustained behavior spontaneously initiated (Fig. 1g).

The periodic signals represent cycles of NiO growth and subsequent reduction with a strong influence on activity. High NiO coverage lead to small production rates. Subsequently, the system resets its active reduced state, closing an oscillation. A closer analysis shows that the oscillations are temporal separations of the steps of a Mars-van Krevelen cycle. First, CO₂ oxidizes the Ni. Subsequently, CH₄ reacts with NiO. The HRTEM image of the spent catalyst (Fig. 2) indicates strain accumulation at the inner layers of the solid, which may be feeding back mechanically the oscillatory reaction.

Conclusion

We highlight the non-static nature of working catalysts with video footage of the dynamics and their correlation to catalytic observables during oscillatory DRM. Our approach helps achieving basic knowledge about heterogeneous reaction mechanisms and the trade-off of chemical and mechanical effects.

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