Generation of Nano-Catalyst Particles by Spinodal Nano-Decomposition in Perovskite

Abstract

A new mechanism of nano-catalyst generation based on the spinodal nano-decomposition in self-regenerating perovskite catalysts for automotive-emissions control is proposed. To demonstrate existence of the spinodal nano-decomposition in real perovskite catalysts, we performed first-principles calculations to evaluate the free energy of La(Fe 1−x Pd x )O 3 and La(Fe 1−x Rh x )O 3 . The result indicates appearance of a spinodal region in the phase diagram of each material. Formation of nano-catalyst particles in the perovskite host matrix is crucial for the self-regeneration of perovskite catalyst. Based on the spinodal nano-decomposition model, possible materials are designed for new three-way catalyst with no contents of precious metal.Self-regenerating Pd-, Rh-, and Pt-doped perovskite catalysts for automotive-emissions control are attracting much interest due to their unique functionalities. [1][2][3][4][5][6][7][8][9][10][11] In a conventional catalyst made of fine precious-metal particles supported on a solid like an almina, agglomeration of the metal particles is inevitable because of high temperature conditions in the redox environment. This is the very reason for deterioration of automotive three-way catalysts. In the self-regenerating perovskite catalysts, interestingly, the deterioration is strongly suppressed. Therefore, consumption of the precious metal is greatly reduced, providing a highly efficient solution for supply problems. The reason for non-deterioration is thought to be reformation of a precious-metal doped perovskite lattice in the NO x -reduction environment from segregated nano-particles of precious-metal created in the CO-and C x H-oxidation environment. In this regeneration model, precious-metal atoms are assumed to move into and out of the perovskite host matrixes. Growth of precious-metal grains is suppressed due to this repeated motion of precious metal atoms between a solid solution and metallic nano-particles during three-way catalytic reactions.In some doped perovskite structures, it is known that diffusion of oxygen happens rather frequently. At the same time, we may expect motion of metal atoms via a process exchanging metallic atoms. The process would be enhanced, if oxygen vacancies are created in the perovskite host crystal. This scenario might support the above model of self-regenerating perovskite catalysts. For realization of high efficiency in the three-way catalytic functions with * E-mail address: hkizaki@aquarius.mp.es.osaka-u.ac.jp 1/10Submitted to Applied Physics Express no deterioration, however, we should consider another model of the self-regenerating catalyst. The new model should explain some mysteries known experimentally in the perovskite catalyst. The self-regeneration can happen, only when the host perovskite lattice structure maintains its essential structure. We need to know why the redox reaction keeps its cycle in a redox environment changing in a frequency of about a few hertz (1 ∼ 4 Hz). Thus we need to understand motion of precious metal atoms in a rat...

Oxidative Methane Reforming with an Intelligent Catalyst: Sintering‐Tolerant Supported Nickel Nanoparticles

“…According to previous investigations of perovskite‐supported precious metal systems, a possible mechanism including the rapid diffusion of the metal,5a the nanoscale spinodal decomposition,5g and the generation of surface phases5c might account for the cyclical re‐dispersion function observed here. The diffusion of metallic Ni particles is expected for a host perovskite with a large cell volume and sufficient structural defects.…”

Spinodal nanodecomposition in semiconductors doped with transition metals

“…As one more example of possible functionalities worth mentioning is the case of catalysts for automotive-emissions control. Here, by using decomposed alloy with spatially separated NCs containing the relevant metal, e.g., Pt, the destructive process of metal agglomeration could be much reduced (Hamada et al, 2011;Katz et al, 2011;Kizaki and Katayama-Yoshida, 2013;Kizaki et al, 2008).…”

Differences in the Nature of Active Sites for Methane Dry Reforming and Methane Steam Reforming over Nickel Aluminate Catalysts

“…These materials are of great interest for methane steam reforming and methane dry reforming reactions due to their resistance to sintering and the decreased formation of carbonaceous deposits. 16,28,30 The reactivity of AB 2 O 4 /ABO 3 materials has been ascribed to many mechanisms: ability of individual atoms within the bulk structure to move reversibly in and out lattice, 16,23 nanospinodal decomposition, 24,39 reactivity of oxygen contained within the material, 15,18,22,27,28,31,34 formation of metal nanoparticles within the bulk structure framework, 8,9,19,30,40,41 or formation of nickel oxide particles supported by the nickel aluminate structure. 20 Several methods are commonly used to prepare mixed oxide catalysts: solid state reaction, 35 co-precipitation, 6,14,31,42,43 sol-gel method, 13,20,44 combustion synthesis, 19 alkoxide method, 17,18,24 and Pechini synthesis.…”

“…It was found that Pd was able to move out of the perovskite lattice under reducing conditions and back in its original position in an oxidizing atmosphere giving it high catalytic activity and a long lifetime. In a separate study by Kizaki et al, La(Fe 1-x Pd x )O 3 perovskites prepared by the alkoxide method were found to have excellent stability and activity for automotive emissions control due to nanospinodal decomposition 24. In nanospinodal decomposition, the perovskite decomposes into LaFeO3 and PdFeO 3 at high temperatures, but the crystal structure itself is maintained.…”