Cu/ZnO/AlO catalysts have been used industrially since the mid-1960s for both the water-gas shift (WGS) reaction and methanol synthesis. Methanol produced from CO and H is believed to play a key role as an energy storage molecule. The catalyst is commonly activated by reduction in situ, which is widely believed to promote the formation of a partially reduced ZnO overlayer over Cu nanoparticles through the strong metal-support interaction (SMSI).[1] However, the chemical nature and stability of this overlayer in operating conditions is still heavily debated. In this study, we follow the structural and morphological evolution of a calcined Cu/ZnO/AlO catalyst by operando TEM at 1 bar throughout its activation in diluted H as well as in both methanol synthesis conditions (H:CO=3:1) and reverse water-gas shift (rWGS) conditions (H:CO=1:1).
The calcined sample is first activated (p = 79 mbar, 4Kmin) during a two-step process involving the further segregation of CuO nanoparticles from a (Cu,Zn) carbonate phase from 110°C to 200°C, and a subsequent reduction and sintering of these nanoparticles from CuII to Cu0. A CuI intermediate is detected as well between 200°C and 250°C, with all 3 oxidation states co-existing at these temperatures until only Cu0 remains after 2h at 250°C. This is determined from the analysis of the selected area electron diffraction (SAED) data, which allows us to track the phase fraction of all three components throughout the reduction step, taking advantage of the fast time resolution (1s) of operando SAED. HO formation is detected via mass spectrometry throughout both activation steps.
The presence of an overlayer on the reduced Cu nanoparticles is of particular interest and, importantly, only a sparse coverage can be observed in the reduced catalyst at 250°C with particles showing either open metallic Cu surface or island-like ZnO domains. However, the overlayer expands upon cooling to 50°C leading to a higher overall coverage of the Cu surfaces. This phenomenon is reversible and ZnO can be observed to wet and dewet Cu surfaces repeatedly upon cooling and heating. Furthermore, the overlayer showed an increased thickness and stability across the same 50-250°C temperature range when the feed was switched to methanol reaction conditions (p = 489 mbar, p = 167mbar). However, in the rWGS feed (p = 305 mbar, p = 306 mbar), thickness, coverage and stability of the overlayer increased once more, in spite of the lower partial H pressure. This shows that the overlayer dynamics are not solely dictated by reducing conditions, but are also mediated by CO which adjusts the stoichiometry of defective ZnO.
Finally, we leverage recent methodological developments in the Rietveld refinement of SAED to investigate the deactivation of the catalyst at high temperatures (250-400°C).
We are currently starting to collaborate with the Theory department (Dr. Christoph Scheurer) in order to advance the existing structural models and establish a machine learning approach, in which our operando data act as structural observables.
References
[1] T. Lunkenbein et al. (2015), Angewandte Chemie International Edition, 54, 4544-4548.
