The transformation of CO₂ and green hydrogen into methanol presents a sustainable route for chemical and fuel production. Conventional methanol synthesis catalysts, such as Cu/ZnO/Al₂O₃, employ Al₂O₃ as a structural promoter, while Ga₂O₃ has recently emerged as a promising alternative. This study compares Cu-based catalysts supported on Al₂O₃ (CA) and Ga₂O₃ (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 CO₂ hydrogenation. Gallium from its precursor state undergoes several phase transitions. At elevated temperatures, Ga exhibits redox activity, transitioning from Ga^3+ to metallic Ga⁰ 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 Ga₂O₃ leads to low conversion levels and pronounced deactivation compared to the Al₂O₃-supported Cu catalyst and thus should be prevented. These findings highlight the importance of controlling promoter loading and dynamic behavior in catalyst design to optimize activity, stability, and selectivity for CO₂-to-methanol conversion.
Baumgarten et al. (Thu,) studied this question.