Speaker
Description
The hydrogenation of CO$_2$ to methanol occurs with high efficiency on Cu/ZnO-based catalysts. However, the nature of the Cu−Zn interaction and especially the role of Zn in Cu/ZnO catalysts are still not fully understood. In the industrial Cu/ZnO/Al$_2$O$_3$ catalyst, Zn was found to migrate onto the Cu surface during the reaction, thus forming a Cu−ZnO interface that is crucial for a high catalytic activity.[1] However, whether a Cu−Zn alloy or a Cu−ZnO structure is formed, the transformation of this interface under working conditions and how this influences the catalytic performance needs further investigation.
Cu$_2$O nanocubes (NCs) have been shown to be a good model system to investigate the interaction of Cu and ZnO.[2-4] In this work, we prepared cubic Cu$_2$O NCs modified with a ZnO shell of various thicknesses, supported on SiO$_2$ or ZrO$_2$. In this way, an intimate contact between Cu and ZnO is created even before the start of the reaction. The evolution of the catalyst’s structure and composition during the CO$_2$ hydrogenation reaction were investigated by means of spectroscopy (XPS, XAS), diffraction (XRD), and microscopy methods (SEM, STEM). [5]
We found that the initial Zn loading affects the structure and oxidation state of Zn, which, in turn, influences the catalytic performance. High Zn loadings result in a stable ZnO shell and lead to an increased methanol production when compared to a Zn-free catalyst. However, a too high loading caused the initially amorphous ZnO layer to crystallize, which inhibits the catalyst’s activity. Low Zn loadings, in contrast, lead to the presence of metallic Zn species (and potential Cu-Zn alloy formation) and show no significant improvement over the bare (Zn-free) Cu NCs regarding their catalytic performance. It therefore appears, that there needs to be a minimum content of Zn to promote the Cu catalyst and that there is an optimum ZnO shell thickness that leads to a disordered ZnO layer on top of Cu, resulting in the highest activity.
References
-
T. Lunkenbein et al., Formation of a ZnO Overlayer in Industrial Cu/ZnO/Al$_2$O$_3$ Catalysts Induced by Strong Metal–Support Interactions. Angewandte Chemie 127.15 (2015): 4627-4631.
-
D. Kordus et al., Enhanced Methanol Synthesis from CO$_2$ Hydrogenation Achieved by Tuning the Cu–ZnO Interaction in ZnO/Cu$_2$O Nanocube Catalysts Supported on ZrO$_2$ and SiO$_2$. Journal of the American Chemical Society 146.12 (2024): 8677-8687.
-
M. Rüscher et al., Tracking heterogeneous structural motifs and the redox behaviour of copper–zinc nanocatalysts for the electrocatalytic CO$_2$ reduction using operando time resolved spectroscopy and machine learning. Catalysis Science & Technology 12.9 (2022): 3028-3043.
-
A. Herzog et al., Time-resolved operando insights into the tunable selectivity of Cu–Zn nanocubes during pulsed CO$_2$ electroreduction. Energy & Environmental Science 17 (2024): 7081-7096.
-
D. Kordus et al., Shape-dependent CO$_2$ hydrogenation to methanol over Cu$_2$O nanocubes supported on ZnO. Journal of the American Chemical Society 145.5 (2023): 3016-3030.
