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

We studied the effect of Ti substitution on nickel B-sites in LaNiO₃ 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 La₂Ti₂O₇ 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 LaNiO₃ and the x_Ni = 0.75 sample, which yield the La₂O₃ phase relevant for DRM activity at low Ti doping levels. A common impurity phase between x_Ni = 0.75 and 0.25 is La₂TiO₅, which acts as a Ti and La sink and hinders the formation of La₂O₃. For higher Ti doping levels, hydrogen reduction increases the amount of La₂Ti₂O₇. 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 LaNiO₃ 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 (LaNiO₃ 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 LaNiO₃ without compromising DRM activity.