An Intelligent Catalyst: The Self-Regenerative Palladium–Perovskite Catalyst for Automotive Emissions Control

Abstract

An innovative ÔÔintelligent catalystÕÕ that exhibits a greatly improved durability as a result of the self-regenerative function of Pd nanoparticles has been developed. Self-regeneration was realized through a cycle between solid solution and segregation of Pd in a perovskite crystal, without any auxiliary treatment. That is, Pd atoms move back and forth between the inside (as Pd cations in the lattice) and the outside (as Pd nanoparticles) of the perovskite crystal in synchronization with the fluctuations between reductive and oxidative (redox) atmospheres that occur in real automotive exhaust gases. As a result of this cyclic redox reaction, the growth of Pd nanoparticles can be suppressed during the entire lifetime of the vehicle. This self-regenerative function provides a new and useful tool for the development of future automotive catalysts. The mechanism of the self-regenerative function is described, and it is shown by ex situ and in situ X-ray absorption fine-structure spectroscopic analyses that the self-regenerative function occurs at an extremely high speed over a wide range of temperatures. In addition, technical solutions for overcoming the weaknesses of general perovskite catalysts for practical use are described.

Structured Perovskite-Based Catalysts and Their Application as Three-Way Catalytic Converters—A Review

“…Recently, the use of LaFe 0. 95 Pd 0.05 O 3 [14] for the exhaust after treatment of gasoline engines and the demonstration that La 0.9 Sr 0.1 CoO 3 can rival with noble metal-based catalysts for the treatment of diesel vehicle exhaust [15] confirmed the suitability of and rejuvenated the interest for this class of catalysts. Over the past several years, a continuous effort was undertaken to understand and improve the catalytic properties of perovskite-type oxides in automotive applications.…”

“…NO dissociatively adsorbs on a surface oxygen vacancy (□) (Equation (11)). The recombination of two adsorbed nitrogen species leads to the formation of N 2 (Equation (12)) while parallel pathways produce undesired N 2 O (Equation (13)) and isocyanate species (Equation (14)) [57]. The oxygen vacancy is restored by the reducing species in the gas phase (e.g., CO (Equation (15))).…”

“…Therefore, palladium reversibly moves in and out of the perovskite matrix in response to the cyclic redox fluctuations of the exhaust gas composition. These reversible structural changes, which are fast (only a few seconds at 600 °C [14]) and occur even at low temperature (100-400 °C) [72,73], suppress the growth of Pd particles compared to Pd/Al 2 O 3 . To illustrate this phenomenon, the behaviors of two catalysts of similar initial activity are compared after ageing in a real exhaust at 900 °C for 100 h. The reference Pd/γ-Al 2 O 3 catalyst loses about 10% of its activity ( Figure 6) owing to the sintering of noble metal particles (D > 100 nm).…”

“…On the same line, the "self-regenerative" perovskite-type oxide catalyst LaFe 0. 95 [14]. Though the thermal stability of perovskite-type oxides is high, their sulfur resistance is limited.…”

“…The low sulfur tolerance of perovskite-type oxides can be overcome by appropriately designing the structured catalyst. A double layer configuration where the active perovskite-type oxide phase is located in the inner layer can avoid interactions between the active phase and sulfur-containing compounds and hence catalyst deactivation [14]. TWC converters containing a noble metal-based perovskite-type oxide with the double layer configuration shows better performance, over a wide range of space velocities, than a standard catalyst with a 1.5 times higher noble metal loading.…”