Zr-substituted La2CeNiO7 pyrochlore catalysts via topotactic exsolution: Oxygen vacancy-mediated bifunctional activity for stable combined steam and dry methane reforming

Abstract

In the combined steam and dry reforming of methane (CSDRM) for converting greenhouse gases to syngas, Ni catalysts are prone to rapid deactivation due to sintering and coke formation. Nanoparticle formation via exsolution has attracted interest as a potential solution. However, conventional exsolution on perovskite-based catalysts presents two main challenges: the majority of Ni remains within the lattice, leading to reduced catalytic activity, and the formation of cation defects results in structural instability. To address these issues, the topotactic exsolution method was applied to the pyrochlore structure, as it compensates for cation defects by substitution with external cations and facilitates the formation of additional Ni nanoparticles. In this study, we developed catalysts by introducing Zr on the surface of Ni containing La2CeNiO7 pyrochlore via applying topotactic exsolution. Furthermore, we proposed changes in the exsolution mechanism induced by the non-isovalent ions (Zr4+ and Ni2+) and optimized the support properties for high activity and stability in the CSDRM reaction. The incorporation of Zr not only promotes the formation of more Ni nanoparticles but also enhances the strong metal-support interaction, dispersion of nanoparticles and oxygen vacancy concentration. However, an excess amount of Zr reduces oxygen vacancies and increases nanoparticle size, thereby diminishing catalyst stability. The La2CeNiO7 based Ni catalyst substituted with the optimal amount of 10 wt% Zr exhibited no deactivation and maintained the CH4 conversion of 94.4 % and the CO2 conversion of 85.9 %, close to the equilibrium, which was not achieved in previous studies during CSDRM reaction at 800 degrees C for 100 h.