Ga-promoted Cu surface undergoes complex structural transformations in methanol synthesis

A team of researchers from the Department of Interface Science at the Fritz Haber Institute of the Max Planck Society has delved into the intriguing question of what occurs to a Ga-promoted Cu surface during the conditions required for methanol synthesis. Their findings reveal intricate structural changes in this bimetallic catalyst, potentially altering our conventional understanding of its active surface structure.

The conventional method of hydrogenating CO2 into methanol is accomplished efficiently using well-known Cu/ZnO/Al2O3 catalysts at high pressures, typically around 50–100 bar. However, this approach comes with safety risks, high energy consumption, and constraints on the CO2 concentration in the gas feed to maintain selectivity.

Hence, there is a strong desire for a new class of catalysts suitable for low-pressure methanol synthesis, particularly for future endeavors involving small-scale devices powered by solar-generated hydrogen at ambient pressure.

Recent revelations have highlighted the promising catalytic performance of intermetallic compounds and alloys containing Ga, even at atmospheric pressures. However, Ga’s promotional role in these catalysts remains enigmatic, mainly due to a lack of knowledge about the surface structures.

In this context, studies employing surface-sensitive techniques on well-defined model catalysts under reaction conditions offer vital insights into the dynamic nature of active sites, reaction intermediates, and ultimately, the reaction mechanism.

Researchers at the Fritz Haber Institute’s Department of Interface Science harnessed laboratory-based Near Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS) and Scanning Tunneling Microscopy (NAP-STM) to monitor the in-situ structural and chemical changes of Ga-Cu bimetallic surfaces during CO2 hydrogenation.

Their observations unveiled temperature- and pressure-dependent de-alloying of the bimetallic surface, resulting in Ga-oxide islands embedded within the Cu surface. Interestingly, despite having a stoichiometry close to Ga2O3, the most stable Ga-oxide, this oxide phase formed an ultrathin layer.

The common explanation for the promotional effect of metals like Ga, which are susceptible to oxidation, involves a structure model where a bulk oxide layer sits on top of the metal surface, and the reaction mechanism entails the spillover of intermediate species at the interface. However, this study unequivocally demonstrated two essential points: (i) Ga-oxide is integrated into the metal surface, and (ii) these Ga-oxide islands are ultrathin, likely of “monolayer” thickness.

This formation of an ultrathin Ga-oxide layer on metal surfaces due to the reaction is also anticipated in Ga-containing intermetallic compounds. Importantly, such two-dimensional oxide films differ significantly from their bulk counterparts in terms of structure and reactivity.

Consequently, the GaOx/Cu interface created during CO2 hydrogenation may expose catalytically active sites that have never been considered for this reaction before. This valuable information would have been impossible to acquire using the standard bulk-sensitive techniques typically used for characterizing powder catalysts.

The findings from this study, recently published in Nature Communications, illuminate the intricate surface structure of Ga-containing catalytic systems.

This level of insight into active catalysts can only be achieved through cutting-edge experimental techniques conducted under actual reaction conditions. Only by deciphering the atomic structure of the Ga-oxide layer(s) and its interaction with the transition metal during operation can we gain a deeper understanding of the methanol synthesis catalyst’s reaction mechanism.

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