Cu−Zn Alloy Formation as Unfavored State for Efficient Methanol Catalysts

Abstract

The active sites of Cu/ZnO‐based catalysts, commercially applied for the hydrogenation of CO2 or CO2‐rich synthesis gas, are still subject of current debates. Generally, the discussion is focused on the nature of the interfacial contact between Cu and ZnO, particularly whether it is rather of oxidic (Cu−ZnO) or alloying (Cu−Zn) character. We report on kinetic investigations on a Cu/ZnO : Al high performance catalyst activated at different temperatures. Incrementally increasing temperature under reductive conditions leads also to increased CuZn‐alloy formation, analyzed by in‐situ X‐ray diffraction, in‐situ X‐ray absorption spectroscopy and high resolution transmission electron microscopy. The combination of the catalytic data and the complementary characterization techniques provide valuable insights on the relevant reaction sites for CH3OH formation. Our results highlight the complexity of the interfacial contact with evidence for Cu−ZnO reaction sites and clarify the negative impact of CuZn alloy formation on the nature of the active site.

Mechanistic Study of Carbon Dioxide Hydrogenation over Pd/ZnO‐Based Catalysts: The Role of Palladium–Zinc Alloy in Selective Methanol Synthesis

“…XAS revealed the absence of palladium-zinc de-alloying or any indication of the surface oxidation of palladium, which might be expected due to possible palladium-formate intermediates,d uring the catalytic cycle.I nstead, transient experiments show that the palladium-zinc phase is very stable under different gas atmospheres (hydrogen, CO 2 /H 2 mixture and carbon dioxide) and it may well be only responsible for hydrogen activation, while carbon dioxide activation, and its subsequent hydrogenation, occur on the zinc oxide similar to the Cu/ZnObased system. [25,26] However,a tt his point, it is not clear, whether the palladium-zinc alloy alone governs the activity and selectivity of the catalyst, or the interface between the alloy phase and bulk zinc oxide represents the active site for the selective transformation of carbon dioxide to methanol.…”

“…Instead, transient experiments show that the palladium-zinc phase is very stable under different gas atmospheres (hydrogen, CO2/H2 mixture and carbon dioxide) and it may well be only responsible for hydrogen activation, while carbon dioxide activation, and its subsequent hydrogenation, occur on the zinc oxide similar to the Cu/ZnObased system. [27,28] However, at this point, it is not clear, whether 10.1002/anie.202103087…”

“…Gentzen et al, [26] investigated the behavior of a Pd/ZnO system under operando conditions and observed formation of palladium-zinc alloy during dimethyl ether synthesis from carbon monoxide-rich gas mixtures having a carbon monoxide/hydrogen ratio 1:1, however, methanol synthesis typically utilizes a significantly more reducing feedstock (H2/CO2 = 3). Taking into account recent works, [27][28][29][30] which have questioned the role of copper-zinc alloy in methanol synthesis from carbon dioxide, an operando investigation of carbon dioxide hydrogenation over Pd/ZnO system is warranted to understand the true nature of the active sites and to verify the role that a palladium-zinc alloy phase may have in facilitating this reaction. We therefore report a combined high-pressure operando study of methanol synthesis over Pd/ZnO catalysts using XAS, XRD and FTIR techniques.…”

“…Taking into account recent works, [25][26][27][28] which have questioned the role of copper-zinc alloy in methanol synthesis from carbon dioxide,a no perando investigation of carbon dioxide hydrogenation over Pd/ZnO system is warranted to understand the true nature of the active sites and to verify the role that apalladium-zinc alloy phase may have in facilitating this reaction. We therefore report ac ombined high-pressure operando study of methanol synthesis over Pd/ZnO catalysts using XAS,XRD and FTIR techniques.W efind that, despite the fact that ap alladium-zinc alloy phase is present during reaction, methanol is not formed when the support is an inert oxide (e.g.Al 2 O 3 ,SiO 2 ).…”

Mechanistic Study of Carbon Dioxide Hydrogenation over Pd/ZnO‐Based Catalysts: The Role of Palladium–Zinc Alloy in Selective Methanol Synthesis

“…Taking into account recent works, [25–28] which have questioned the role of copper‐zinc alloy in methanol synthesis from carbon dioxide, an operando investigation of carbon dioxide hydrogenation over Pd/ZnO system is warranted to understand the true nature of the active sites and to verify the role that a palladium–zinc alloy phase may have in facilitating this reaction. We therefore report a combined high‐pressure operando study of methanol synthesis over Pd/ZnO catalysts using XAS, XRD and FTIR techniques.…”