Gallia‐ Versus Alumina‐Supported Cu: Dynamics of Ga in Catalysts for Green Methanol Production

The transformation of CO$_2$ and green hydrogen into methanol presents a sustainable route for chemical and fuel production. Conventional methanol synthesis catalysts, such as Cu/ZnO/Al$_2$O$_3$, employ Al$_2$O$_3$ as a structural promoter, while Ga$_2$O$_3$ has recently emerged as a promising alternative. This study compares Cu-based catalysts supported on Al$_2$O$_3$ (CA) and Ga$_2$O$_3$ (CG), prepared via coprecipitation of layered double hydroxide precursors with identical molar Cu:M (M = Al or Ga) ratio of 70:30. Using in situ and operando X-ray absorption spectroscopy and X-ray powder diffraction, we investigate the structural and redox dynamics of Ga during activation and CO$_2$ hydrogenation. Gallium from its precursor state undergoes several phase transitions. At elevated temperatures, Ga exhibits redox activity, transitioning from Ga$^{3+}$ to metallic Ga$^0$ and forming CuxGay alloys at 480 °C, followed by de-alloying and re-oxidation at even higher temperatures. Our results suggest that the beneficial role of Ga reported in literature arises from metal-oxide interfacial effects rather than bulk alloying. Excess Ga$_2$O$_3$ leads to low conversion levels and pronounced deactivation compared to the Al$_2$O$_3$-supported Cu catalyst and thus should be prevented. ... mehrThese findings highlight the importance of controlling promoter loading and dynamic behavior in catalyst design to optimize activity, stability, and selectivity for CO$_2$-to-methanol conversion.