The unique interplay between copper and zinc during catalytic carbon dioxide hydrogenation to methanol

Abstract

In spite of numerous works in the field of chemical valorization of carbon dioxide into methanol, the nature of high activity of Cu/ZnO catalysts, including the reaction mechanism and the structure of the catalyst active site, remains the subject of intensive debate. By using high-pressure operando techniques: steady-state isotope transient kinetic analysis coupled with infrared spectroscopy, together with time-resolved X-ray absorption spectroscopy and X-ray powder diffraction, and supported by electron microscopy and theoretical modeling, we present direct evidence that zinc formate is the principal observable reactive intermediate, which in the presence of hydrogen converts into methanol. Our results indicate that the copper-zinc alloy undergoes oxidation under reaction conditions into zinc formate, zinc oxide and metallic copper. The intimate contact between zinc and copper phases facilitates zinc formate formation and its hydrogenation by hydrogen to methanol.

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

“…[22] Furthermore, and irrespective of the difference in palladium particle size and distribution, the 2Pd-ZnO-i sample shows very similar behavior during operando XAS study (Figure S3). In contrast to our previous study, [27] where we have observed changes in Cu K-edge XANES that result from an oxidative desegregation of zinc from a copper-zinc alloy phase upon switching to a CO2/H2 mixture, the position of the palladium white line remains constant, not only during carbon dioxide hydrogenation conditions, but even after switching to pure carbon dioxide (Figure 2c). Since the white line is sensitive to the oxidation state of palladium, this suggests that palladium remains in a metallic state and that the formate species, as observed by IR (Figures S1-S2 and Figure 2b), are not bound to the palladium.…”

“…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.…”

Operando high-pressure investigation of size-controlled CuZn catalysts for the methanol synthesis reaction

“…It has been shown that Cu-based catalysts containing ZnO are more active than their ZnO-free counterparts 8 and that ZnO both enhances the dispersion of the Cu nanoparticles (NPs), and acts as an electronic promoter 8,9 through a strong metal-support interaction [8][9][10] . This promoting effect of Zn has been attributed to the Zn being present in a number of different configurations: the formation of graphitic ZnO x layers on Cu NPs 11 , metallic Zn atoms within the Cu NP surface 12 , CuZn alloy formation 13 , Zn-decoration of stepped Cu surfaces 1 , or the formation of ZnO at the interface with Cu in single crystals 3,14 . A strong correlation between the Zn coverage on Cu NPs and the methanol synthesis activity was also reported 2 .…”

Influence of a Cu–zirconia interface structure on CO2 adsorption and activation

“…[1][2][3] Due to the highly oxidized state, thermodynamic stability, and unreactive nature of CO 2 , economical, active, and selective catalysts are mandatory and the chemical conversion and the economical utilization of CO 2 is a notable scientific and technical challenge. 1 Numerous experimental and computational studies have shown that CO 2 reduction takes place at a metal-oxide interface, [4][5][6][7][8][9][10] which is also an active domain for many other industrially important catalytic reactions 11 such as the water-gas-shift reaction 12,13 and CO oxidation 14,15 just to mention but a few. These reactions have been reported to take place over a variety of metal-oxide interfaces with diverse chemical nature and composition e.g., Au-TiO 2 , [15][16][17] Cu-ZnO, 4,9 Rh-ZrO 2 , 13,18 FeO-Pt, 19 Pd-Co 3 O 4 , 20,21 Pt-SiO 2 22,23 , and others 24 .…”