Perovskite-type metal oxides are a class of mixed oxides with unique catalytic properties and can be implemented as catalysts without additional functionalization by metals. In addition they can serve as supports for catalytically active metal phases. The resulting interface between the perovskite-type metal oxide and the metal nanoparticles has been found to have a similar nature as that between reducible metal oxides and metals. As opposed to other metal oxides, researchers have shown that precious metals can be segregated reversibly from perovskite-type metals oxide lattices, which is a temperature limited process that enhances the resistance against precious metal sintering.
Analysis through various X-ray based methods on Pd containing perovskite-type metal oxides after isothermal oxidation and reduction treatments has indicated that the fraction of Pd segregating at the perovskite metal oxide surface on reduction, and the fraction dissolving in the perovskite metal oxide lattice on reoxidation increases with increasing temperature. Several researchers have so far tried to explain this.
It is likely that not all of the reduced metal is accessible for catalysis. This is attributed to the stabilization of metals in high oxidation states within the perovskite metal oxide lattices and not all perovskite compositions show the reversible metal segregation at suitable temperatures. A significant benefit of the incorporation of the active metal into a perovskite-type oxide lattice and exploiting the reversible metal segregation is that the active metal contents can be significantly reduced to yield an active but moreover highly redox stable catalyst.
Patrick Steiger and colleagues at Paul Scherrer Institut, EPFL and Zurich University of Applied Sciences in Switzerland, rationalized the required parameters to initiate overall structural reversibility in this form of materials applying a probe reaction catalyzed by nickel, that is carbon dioxide hydrogenation. Their research work is published in ChemSusChem.
The authors explored the parameters of reversible segregation behavior for a typically used catalyst metal (nickel) to avoid nickel sintering that is common on most metal/support type catalyst materials. The researchers used the temperature-programmed reduction, X-ray absorption spectroscopy, and X-ray diffraction, catalytic tests as well as electron microscopy to determine the extent and limits of reversible nickel segregation from the LaFe1-xNixO3±δ host lattice. The authors exclusively reduced nickel at a reduction temperature of 600 °C and segregated it to the oxide surface at which it developed catalytically active nickel metal particles.
The researchers observed that the extent of nickel reduction from the selected perovskite was dependent on the perovskite B-site composition. The reduction extent also increased from about 35% in the LaFe0.95Ni0.05O3±δ perovskite to about 50% in the LaFe0.8Ni0.2O3±δ. They realized complete structural reincorporation after oxidation at 650 °C for about 2 h. Reversible nickel segregation was observed to lead to active as well as highly redox stable Nickel catalyst. Nickel particle growth was completely suppressed.
The proposed process offers great potential to reverse particle sintering under conditions that are commonly applied to regenerate coked catalysts. This could enhance the catalytic lifetime as well as cost efficiency.
Patrick Steiger, Renaud Delmelle, Debora Foppiano, Lorenz Holzer, Andre Heel, Maarten Nachtegaal, Oliver Kröcher, and Davide Ferri. Structural Reversibility and Nickel Particle stability in Lanthanum Iron Nickel Perovskite-Type Catalysts. ChemSusChem 2017, 10, 2505 – 2517.
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