Tracking the phase changes in micelle-based NiGa nanocatalysts for methanol synthesis under activation and working conditions

The hydrogenation of CO₂ into high energy density fuels such as methanol, where the required H₂ is obtained from renewable sources, is of utmost importance for a sustainable society. In recent years, NiGa alloys have attracted attention as promising catalyst material systems for the hydrogenation of CO₂ into methanol at ambient pressures. They thus represent an energy-saving alternative to the Cu-based catalysts employed in today's catalytic industry that require high pressures for the CO₂ hydrogenation. However, the underlying reaction mechanisms for the NiGa system are still under debate. One of the challenges here is to unravel the evolution and coexistence of the different species in the heterogeneous NiGa catalyst system under activation and reaction conditions. To shed light on their evolution under activation in H₂ and their catalytic roles under CO₂ hydrogenation working conditions on well-defined Ni₃Ga₁ and Ni₅Ga₃ nanoparticle (NP) catalysts, we employed a multi-probe approach. It included advanced machine learning-based analysis of operando X-ray absorption spectroscopy data combined with operando powder X-ray diffraction and near ambient pressure X-ray photoelectron spectroscopy measurements, as well as reactivity studies using bed-packed mass flow reactors. In addition, we employed atomic force microscopy and scanning transmission electron microscopy for structural characterization. Under H₂ activation at 1 bar total pressure, we concluded the formation of metallic Ni, starting for Ni₃Ga₁ at 300 °C, and for Ni₅Ga₃ at 400 °C. At higher temperatures, the formation of NiGa alloys follows. The α’-Ni₃Ga₁ alloy phase is predominantly formed for the Ni₃Ga₁ NPs, while the coexistence of α’-Ni₃Ga₁, δ-Ni₅Ga₃ and Ga₂O₃ phases is observed for the Ni₅Ga₃ NPs after the H₂ activation. The formation of the Ga₂O₃ phase also results in the presence of excess metallic Ni. Under CO₂ hydrogenation reaction conditions, Ga partially oxidizes again to form a Ga₂O₃-rich particle shell for both NP compositions, yet, to a larger extent for the Ni₃Ga₁ NPs, which, in turn, feature a higher amount of excess Ni. We reveal that metallic Ni is responsible for the high selectivity of the Ni₃Ga₁ NPs towards the production of methane in our catalytic tests. In contrast, the Ni₅Ga₃ NPs display a strong selectivity toward methanol production (>92%), more than one order of magnitude higher than that for the Ni₃Ga₁ NPs, which we ascribe to the presence of the δ-Ni₅Ga₃ phase.