How Ti Doping Improves the Catalytic Methane Dry Reforming of Nanoporous Reduced LaNiO ₃ Perovskites

High Resolution Image Download MS PowerPoint Slide We studied the effect of Ti substitution on nickel B-sites in LaNiO 3 to unravel the influence of Ti doping on structural stability, Ni exsolution, and methane dry reforming (DRM) properties. Ni can be substituted by Ti down to compositions of x Ni = 0.25 without compromising both phase and structure purity. At even higher Ti doping levels, formation of the pyrochlore-type La 2 Ti 2 O 7 phase occurs. Ti substitution has a significant influence on the stability under reducing conditions and the appearance of specific intermediate structures relevant for DRM operation. Full decomposition is only observed for LaNiO 3 and the x Ni = 0.75 sample, which yield the La 2 O 3 phase relevant for DRM activity at low Ti doping levels. A common impurity phase between x Ni = 0.75 and 0.25 is La 2 TiO 5, which acts as a Ti and La sink and hinders the formation of La 2 O 3 . For higher Ti doping levels, hydrogen reduction increases the amount of La 2 Ti 2 O 7 . A common denominator of all samples after hydrogen reduction is the full leaching of all nominally available Ni from the perovskite. The self-activation properties during DRM operation strongly depend on the Ti substitution level. Self-activation with either full or partial decomposition is only possible for LaNiO 3 and x Ni = 0.75, where intermediate lanthanum oxycarbonate formation occurs. For x Ni = 0.50, the remaining perovskite structure is stable, but Ni exsolution nevertheless occurs, triggering DRM activity. Successive Ti doping invokes a change in the DRM mechanism from oxycarbonate-based at low Ti amounts (LaNiO 3 and x Ni = 0.75) to a more reactive-oxygen─dominated one for samples x Ni ≤ 0.50, as indicated by X-ray photoelectron spectra. Ti doping also allows to economize the amount of Ni for DRM applications─ it can be lowered to a quarter of the initial amount referenced to pure LaNiO 3 without compromising DRM activity.