In Situ-Determined Catalytically Active State of LaNiO3 in Methane Dry Reforming

Quantitative in situ X-ray diffraction in combination with catalytic tests in dry reforming of methane (DRM) has been performed to unveil the strong structural dynamics of LaNiO3 catalysts during the DRM reaction. Structure-activity correlations reveal polymorphic changes of the rhombohedral LaNiO3 structure first into cubic LaNiO3 and further into transient oxygen-deficient triclinic LaNiO2.7 and monoclinic LaNiO2.5. These changes occur up to 620 degrees C and already cause considerable DRM activity. Another intermediate structure, the Ruddlesden-Popper phase La2NiO4 with moderate DRM activity, is formed in parallel with the decomposition of monoclinic LaNiO2.5. The decay of La2NiO4 directly goes along with the appearance of crystalline metallic Ni and monoclinic La2O2CO3 and a drastic enhancement of DRM activity. The formation of monoclinic La2O2CO3 and decomposition of La2NiO4 proceed exactly alike up to 670 degrees C with the accumulation of metallic Ni. At 670 degrees C and up to 750 degrees C, monoclinic La2O2CO3 is directly transformed into hexagonal La2O2CO3, and no further Ni exsolution is observed. Only above 750 degrees C, hexagonal La2O3 is observed and apparently formed directly from a drastically accelerated decomposition of monoclinic La2O2CO3 alongside another small increase in metallic Ni. Our direct structure-activity correlation unambiguously shows that the active phase in DRM is a mixture of metallic Ni in contact with monoclinic La2O2CO3. The roles of the latter phase are twofold: acting as the CO2-activated species and stabilizing the metallic Ni particles. Naturally, this implies a perfect carbon removal ability of the metallic Ni/La2O2CO3 interface, which directly relates to an enhanced coking resistance and, most probably, long term stability. Heating LaNiO3 in hydrogen yields a similar sequence of structural transformations with the striking difference of the missing transient La2NiO4 structure, corroborating its crucial role in the formation of the DRM-active Ni/monoclinic La2O2CO3 interface. The final structural fate is a metallic Ni/hexagonal La2O3 phase mixture. Exemplified for the DRM reaction and the initial LaNiO3 structure, only the knowledge about the sheer complexity of the structural dynamics allows the unequivocal assignment of participating structures and phases to their respective catalytic performance, and therefore, allows definite conclusions about the formation and the properties of the final active phase.