Structural evolution of a Cu/ZnO/Al₂O₃ catalysts revealed by operando TEM

Abstract text (incl. figure legends and references)

Introduction:

Cu–ZnO/Al₂O₃ catalysts have been used industrially since the mid-1960s for both the water-gas shift (WGS) reaction and methanol synthesis. Methanol produced from CO₂ and H₂ is believed to play a key role as an energy storage molecule [1]. The catalyst is activated by reduction in situ, which promotes the formation of a partially reduced ZnO_x overlayer over Cu NPs through the strong metal-support interaction (SMSI). The SMSI has been observed in the spent catalyst after reaction [2], but has so far scarcely been investigated operando.

Materials and Methods:

The catalyst (Cu–ZnO/Al₂O₃) was synthesized by calcination of a zincian malachite precursor with a Cu:Zn ratio of 70:30 and 3 mol % Al, following a protocol published previously [3]. A sample was mounted in a DENS Climate environmental TEM holder, which was connected to our homebuilt gas feed setup.

Selected area electron diffraction (SAED) temperature series were corrected for astigmatism and centered with sub-pixel accuracy[4], enabling us to track small lattice parameter changes and phase fractions from Rietveld refinement with the fast time resolution of SAED.

Results:

The calcined sample is first activated (pH₂ = 79 mbar, 10% H2, 4K.min-1) during a 2-step process involving the segregation of CuO NPs from a (Cu,Zn) carbonate phase from 110C to 200C, then reduction and sintering of these NPs from Cu^II to Cu⁰. A Cu^I intermediate is detected as well between 200C and 250C, with all 3 oxidation states co-existing at these temperatures until only Cu⁰ remains after 2h at 250C.

The SMSI ZnO_x overlayer only partially covers few particles after the reduction is complete after 2h at 250C. Cooling the sample to 50C leads to a much fuller overlayer on a much larger NPs population, indicating that the SMSI overlayer is not stable at high temperatures and forms reversibly. This is compounded by results in MeOH synthesis conditions (H₂:CO₂:He = 3:1:0.5) and reverse water gas shift conditions (H2:CO2:He = 1:1:0.5), shedding tremendous insight into how Cu wetting by ZnO_x is mediated by temperature as well as H₂ and CO₂ partial pressures (Figure 1.a) through a delicate balance of Cu-Zn alloy formation and reoxidation on Cu NPs surfaces [5]. Increasing the temperature to 400C showed a sudden increase of the Cu lattice parameter, which then leveled off to the expected Cu lattice parameter over 30 minutes, demonstrating the potential of operando SAED to track alloying and reoxidation in real time and revealing the time constant of these phenomena (Figure 1.b).

Figure 1. a) Evolution of the catalyst throughout activation and operating conditions. Cooling after reduction leads to a full ZnO_x coverage on the Cu NP. This overlayer is thicker and more stable as the CO₂ fraction increases disappears reversibly as the temperature increases. b) Evolution of the Cu lattice parameter in RWGS conditions. The increase and decrease corresponds to Cu-Zn and re-oxidation.

References

A. Olah, A. Goeppert, G. K. Surya Prakash, in "Beyond Oil and Gas: The Methanol Economy", Wiley-VCH, Weinheim, 2006. Lunkenbein, T., Schumann, J. et al., Angewandte Chemie54, 15 (2015). Schumann, J., Lunkenbein, T. et al. ChemCatChem6, 10 (2014). Frisch, B., Wu, M., Ultramicroscopy 235 (2022) Amann, P., Klötzer, B. et al., Science 376, 6593 (2022).