Unravelling the Zn‐Cu Interaction during Activation of a Zn‐promoted Cu/MgO Model Methanol Catalyst

Abstract

We report on an inverse model Cu/MgO methanol catalyst modified with 5 % zinc oxide at the Cu surface to element‐specifically probe the interplay of metallic copper and zinc oxide during reductive activation. The structure of copper and zinc was unraveled by in situ X‐ray diffraction (XRD) and in situ X‐ray absorption spectroscopy (XAS) supported by theoretical modelling of the extended X‐ray absorption fine structure and X‐ray absorption near‐edge structure spectra. Temperature‐programmed reduction in H2 during in situ XAS showed that copper was reduced starting at 145 °C. With increasing reduction temperature, zinc underwent first a geometrical change in its structure, followed by reduction. The reduced zinc species were identified as surface alloy sites, which coexisted from 200 °C to 340 °C with ZnO species at the copper surface. At 400 °C Zn−Cu bulk‐alloyed particles were formed. According to in situ XRD and in situ XAS, about half of the ZnO was not fully reduced, which can be explained by a lack of contact with copper. Our experimental results were further substantiated by density functional theory calculations, which verified that ZnO with neighboring Cu atoms reduced more easily. By combining these results, the distribution, phase and oxidation state of Zn species on Cu were estimated for the activated state of this model catalyst. This insight into the interplay of Cu and Zn forms the basis for deeper understanding the active sites during methanol synthesis.

Versatile in situ/operando Setup for Studying Catalysts by X‐Ray Absorption Spectroscopy under Demanding and Dynamic Reaction Conditions for Energy Storage and Conversion

“…A peak at 2.2 Å in the FT‐region developed for both catalysts, indicating some brass alloying for both catalysts at 230 °C (Figure 7). [32] …”

“…We have also recently reported that only ZnO that is in direct contact with copper and able to form a surface CuÀ Zn alloy can be reduced. [32] Its fraction is low both for CZA and CZG.…”

“…A peak at 2.2 Å in the FT-region developed for both catalysts, indicating some brass alloying for both catalysts at 230 °C (Figure 7). [32] The activity data in terms of reactions rates of methanol formation recorded during the operando study by gas chromatographic analysis over catalysts CZG and CZA are depicted in Figures 5 (a) and (b). Note that GC measurements gave quantitative results every 4 min, resulting in the time-resolved profile shown in Figure 5.…”

Solvent Additive-Induced Deactivation of the Cu–ZnO(Al2O3)-Catalyzed γ-Butyrolactone Hydrogenolysis: A Rare Deactivation Process

“…Reduction of bulk ZnO occurs only at higher temperatures, but the baseline drift starting at 350 °C can be assigned to the start of the ZnO reduction (ZnO domains in interaction with Cu) and the decomposition of a residual carbonate phase. 39 , 40 The quantification of the TPR profile reveals that all Cu(II) was reduced, with a H 2 /M molar ratio of 1.16. The higher value than unity is due to the partial reduction of ZnO; ZnO species in close interaction with Cu can form a Cu–Zn brass.…”

“…Moreover, the role of Zn may also be considered as it has been claimed as having a role in the active site for methanol synthesis. 40 , 46 , 55 − 57 …”