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S C I E N T I F I C H I G H L I G H T S

9 6 H I G H L I G H T S 2 0 2 4 I

X-ray characterization unveils origin of high catalytic performance of LaNiO3 perovskites for CO2 upcycling

Using advanced X-ray techniques, this study revealed the transformation and catalytic performance of exsolved LaNiO₃ perovskite catalysts for CO₂ methanation. In-situ analyses, including X-ray absorption spectroscopy, uncovered Ni particle exsolution and strong metal-support interactions, which enhance CO₂ conversion and stability compared to conventional Ni/La₂O₃ catalysts.

Global warming from greenhouse gas emissions has driven interest in CO₂ upcycling, including converting it to methane using renewable H₂. Nickel-based catalysts are favoured for CO₂ methanation due to their cost- effectiveness and selectivity for methane, though their performance is limited at lower temperatures (T < 300°C) and by Ni particle sintering. La₂O₃ supports are often used to stabilize Ni nanoparticles and reduce sintering. Additionally, alternative preparation methods such as exsolution, which allows Ni nanoparticles to emerge directly from the oxide lattice, have attracted attention due to their ability to enhance strong metal-support interactions (SMSI), although these interactions are not yet fully understood.

Exsolution offers control over the formation and stabilization of Ni nanoparticles, which remain anchored within the support. Limited studies on perovskite catalysts suggest that exsolved Ni catalysts show promise for CO2 hydrogenation [1]. However, in-situ characterization of exsolved perovskite catalyst formation and reactivity remains scarce. Probing the catalyst’s evolution under real reaction conditions is essential for these highly reactive lanthanum-based perovskites, as they readily react with water and CO2, complicating ex-situ/post-mortem characterization.

In this study, LaNiO₃-derived catalysts activated under H2 at temperatures ranging from 400°C to 600°C were compared to traditional Ni/La₂O₃ systems. The transition from LaNiO₃ to the active phase was monitored through in- situ and ex-situ techniques, revealing 4-6 nm Ni particles embedded within a La₂O₃ matrix (Figure 80a). Catalytic testing of the activated LaNiO₃ catalysts (Figure 80b) showed superior CO₂ conversion and CH₄ selectivity relative to Ni/La₂O₃ catalysts with similar atomic composition. In particular, LaNiO₃-600 achieved CO₂ conversion near thermodynamic limits, with CH₄ selectivity reaching 99.7% at 350°C. Durability tests confirmed its stability,

maintaining high CH₄ selectivity (>99%) with minimal deactivation, while Ni/La₂O₃ exhibited lower performance.

In-situ X-ray absorption spectroscopy (XAS) performed at beamline BM23, under 1 bar 20% H₂/He, was used to study the fresh LaNiO3 catalyst during H2-activation. Linear combination analysis at the Ni K-edge, using reference spectra of NiO, Ni, La2NiO4, and LaNiO3, quantified the evolution of Ni phases during reduction. The Ni K-edge spectra (Figure 81a) indicated a two-step transformation of LaNiO₃: at 400°C, partial reduction to La₂NiO₄ and Ni0 occurred; by 500°C, the reduction completed with a fully metallic Ni phase embedded in La2O3. Initial formation of atomically dispersed Ni was followed by agglomeration into nanoparticles with a bulk-like electronic structure. While previous studies under inert gases (He) [2] showed NiO formation at initial stages, direct Ni0 formation occurred under H2. Furthermore, the reduction occurred at lower temperatures in H2 (below 500°C) than in inert atmospheres (above 550°C).

Advanced photoelectron spectroscopy (AP-XPS/HAXPES) with tender and soft X-rays (at BESSY II, Germany) provided additional surface information on LaNiO3 transformation relative to Ni/La2O3. The Ni0 signal, absent in the calcined sample, appeared at 400°C and intensified at 600°C, indicating increasing Ni0 concentration as LaNiO₃ reduced to Ni0 and La₂O₃. Depth-dependent measurements revealed SMSI in the Ni/La2O3 reference, where significant Ni encapsulation by La was observed. LaNiO₃-600, however, maintained a higher surface Ni0 concentration, potentially explaining its superior CO₂ methanation performance.

Following the observation of La₂O₂CO₃ formation during long-term CO2 methanation tests, AP-XPS measurements (at SOLEIL, France) were conducted under reaction conditions (Figure 81b). These showed that while Ni remained metallic, La species formed. The LaNiO3 catalyst, initially converted to La2O3 upon H2 activation, subsequently transformed into La-hydroxide (La(OH)3), while (La2(CO3)3) dominated on supported Ni/La2O3. The stable and inert La2(CO3)3 hindered CO2 methanation by

Fig. 80: a) Energy-dispersive X-ray (EDX) spectroscopy mapping of the exsolved LaNiO3 catalyst, highlighting the distribution

of key elements. b) Catalytic performance comparison between LaNiO3-based catalysts and the Ni/La2O3 reference, demonstrating enhanced CO₂ conversion and CH₄ selectivity.