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Abstract:
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 H2 activation and their catalytic roles under CO2 hydrogenation working conditions on 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 conclude 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. Contrary, 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.
