How Ti Doping Improves the Catalytic Methane Dry Reforming of Nanoporous Reduced LaNiO3 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 xNi = 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 xNi = 0.75 sample, which yield the La2O3 phase relevant for DRM activity at low Ti doping levels. A common impurity phase between xNi = 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 xNi = 0.75, where intermediate lanthanum oxycarbonate formation occurs. For xNi = 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 xNi = 0.75) to a more reactive-oxygendominated one for samples xNi ≤ 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.