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.

将 CO ₂氢化成高能量密度燃料（例如甲醇），其中所需的 H ₂来自可再生资源，这对于可持续社会至关重要。近年来，NiGa合金作为用于在环境压力下将CO ₂加氢成甲醇的有前途的催化剂材料系统而受到关注。因此，它们代表了当今催化行业使用的铜基催化剂的节能替代品，这些催化剂需要高压处理 CO ₂氢化。然而，NiGa 系统的潜在反应机制仍在争论中。这里的挑战之一是在活化和反应条件下解开多相 NiGa 催化剂体系中不同物种的演变和共存。为了阐明它们在 H _{2 中}活化下的演化以及它们在 CO ₂加氢工作条件下对明确定义的 Ni ₃ Ga ₁和 Ni ₅ Ga ₃纳米颗粒 (NP) 催化剂的催化作用，我们采用了多探针方法。它包括基于操作数X 射线吸收光谱数据的高级机器学习分析，并结合操作数粉末 X 射线衍射和近环境压力 X 射线光电子能谱测量，以及使用填充床质量流反应器的反应性研究。此外，我们采用原子力显微镜和扫描透射电子显微镜进行结构表征。

在1 bar 总压力下的 H ₂活化下，我们得出金属 Ni 的形成，从300 °C 的Ni ₃ Ga ₁开始，以及在 400 °C 的Ni ₅ Ga ₃开始。在更高的温度下，NiGa合金的形成随之而来。Ni ₃ Ga ₁ NPs主要形成α'-Ni ₃ Ga ₁合金相，而Ni观察到α'-Ni ₃ Ga ₁、δ-Ni ₅ Ga ₃和Ga ₂ O ₃相共存H ₂后的₅ Ga ₃ NP激活。Ga ₂ O ₃相的形成也导致过量金属Ni的存在。在 CO ₂加氢反应条件下，对于两种 NP 组合物，Ga 再次部分氧化以形成富含Ga ₂ O _{3 的}粒子壳，但对于 Ni ₃ Ga ₁ NPs 的程度更大，反过来，Ni ₃ Ga ₁ NPs 具有更高的含量过量的 Ni。我们发现，在我们的催化测试中，金属 Ni 是 Ni ₃ Ga ₁ NPs 对甲烷产生的高选择性的原因。相比之下，Ni ₅ Ga ₃NPs 对甲醇生产表现出很强的选择性 (>92%)，比 Ni ₃ Ga ₁ NPs高一个数量级以上，我们将其归因于 δ-Ni ₅ Ga ₃相的存在。