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

Maxim Zabilskiy,Maarten Nachtegaal,Mark A Newton,Jeroen A Van Bokhoven,Frank Krumeich,Vitaly L Sushkevich

doi:10.1002/anie.202103087

Abstract

Pd/ZnO catalysts show good activity and high selectivity to methanol during catalytic CO2 hydrogenation. The Pd‐Zn alloy phase has usually been considered as the active phase, though mechanistic studies under operando conditions have not been conducted to verify this. Here, we report a mechanistic study under realistic conditions of methanol synthesis, using in situ and operando X‐ray absorption spectroscopy, X‐ray powder diffraction, and time‐resolved isotope labeling experiments coupled with FTIR spectroscopy and mass spectrometry. Pd‐Zn alloy‐based catalysts, prepared through reduction of a heterobimetallic PdIIZnII acetate bridge complex, and which do not contain zinc oxide or any PdZn/ZnO interface, produce mostly CO. The Pd‐Zn phase is associated with the formation of CO, and does not provide the active sites required to produce methanol from the direct hydrogenation of carbon dioxide. The presence of a ZnO phase, in contact with a Pd‐Zn phase, is essential for efficient methanol production.

Highlights

Despite decades of effort, a commercially viable synthesis of methanol from carbon dioxide, has yet to be realised, and the development of an appropriate catalyst formulation is still very much in progress
Taking into account recent works,[25,26,27,28] 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
In contrast to copper-zinc alloys, we have shown through the application of operando XAS, that nanoparticulate palladium–zinc is stable and does not undergo oxidative disruption under the conditions required for the direct hydrogenation of carbon dioxide to methanol

Summary

Introduction

Gentzen et al.,[24] 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, methanol synthesis typically utilizes a significantly more reducing feedstock (H2/ CO2 = 3). Taking into account recent works,[25,26,27,28] 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. Angewandte Chemie International Edition published by Wiley-VCH GmbH existence of zinc oxide with palladium or palladium–zinc phases is required to form methanol with high activity and selectivity in this industrially relevant reaction

Results and Discussion

Conclusion

Conflict of interest