Identifying the nature of the active sites in methanol synthesis over Cu/ZnO/Al2O3 catalysts

Daniel Laudenschleger,Martin Muhler,Holger Ruland

doi:10.1038/s41467-020-17631-5

Daniel Laudenschleger, Martin Muhler + Show 1 more

Open Access

https://doi.org/10.1038/s41467-020-17631-5

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Abstract

The heterogeneously catalysed reaction of hydrogen with carbon monoxide and carbon dioxide (syngas) to methanol is nearly 100 years old, and the standard methanol catalyst Cu/ZnO/Al2O3 has been applied for more than 50 years. Still, the nature of the Zn species on the metallic Cu0 particles (interface sites) is heavily debated. Here, we show that these Zn species are not metallic, but have a positively charged nature under industrial methanol synthesis conditions. Our kinetic results are based on a self-built high-pressure pulse unit, which allows us to inject selective reversible poisons into the syngas feed passing through a fixed-bed reactor containing an industrial Cu/ZnO/Al2O3 catalyst under high-pressure conditions. This method allows us to perform surface-sensitive operando investigations as a function of the reaction conditions, demonstrating that the rate of methanol formation is only decreased in CO2-containing syngas mixtures when pulsing NH3 or methylamines as basic probe molecules.

Highlights

The heterogeneously catalysed reaction of hydrogen with carbon monoxide and carbon dioxide to methanol is nearly 100 years old, and the standard methanol catalyst Cu/ ZnO/Al2O3 has been applied for more than 50 years
There is no characterisation method available for the Cu/ZnO/ Al2O3 catalyst typically operated above 200 °C and 50 bar[4], which is able to identify this oxidising effect, because the adjustment of both high-temperature and high-pressure conditions and surface sensitivity is technically impossible for typical single-crystal investigations[11], transient in situ experiments[17,18], adsorption and desorption studies[12,19], in situ infrared (IR) spectroscopy[11,20], N2O frontal chromatography[12,19], X-ray photoelectron spectroscopy (XPS)[11,12,15,19], ambient pressure XPS13, neutron scattering[13], microcalorimetry[21,22], scanning tunnelling microscopy[11], in situ IR reflection-absorption spectroscopy[11], high-resolution transmission electron microscopy (TEM)[14,23], in situ TEM24, electron energy loss spectroscopy[14], in situ X-ray absorption spectroscopy[16], or operando X-ray diffraction[25]
For the experiment with 0.05% NH3, no NH3 signal was detected, since it is converted to the methylamines monomethylamine (MMA), dimethylamine (DMA) and trimethylamine (TMA) (Supplementary Fig. 10) reacting with oxygen-containing intermediates of methanol synthesis such as formate[26,27]

Summary

Introduction

The heterogeneously catalysed reaction of hydrogen with carbon monoxide and carbon dioxide (syngas) to methanol is nearly 100 years old, and the standard methanol catalyst Cu/ ZnO/Al2O3 has been applied for more than 50 years. By applying this high-pressure pulse unit (HPPU), we are able to identify the nature of the active sites of the working Cu/ZnO/Al2O3 catalyst under industrially relevant conditions.

Results

Conclusion