CuO/La0.5Sr0.5CoO3 nanocomposites in TWC

Abstract

In this contribution several La0.5Sr0.5CoO3 based nanocomposites have been prepared and tested for application as Three-Ways Catalysts (TWC), aiming to develop Platinum Group Metal (PGM)-free catalysts. To reach this objective we designed and realized nanocomposites in which active CuO nanoparticles are deposited on La0.5Sr0.5CoO3. This perovskite is active in oxidation and is characterized by high oxygen anion mobility; copper is active in reduction: catalytic bifunctionality is thus built-in via a tailor-made and controlled nano-composition. The supporting perovskite was prepared following the “citrate” route. The deposition was carried out by means of the Ammonium-Driving-Deposition precipitation (ADP) to highly disperse CuO on La0.5Sr0.5CoO3. In a precedent paper we focused on nanocomposites obtained using LaCoO3 as a support because this perovskite is active in oxidation. Sr-doped LaCoO3, in addition, is characterized by a more relevant presence of oxygen vacancies and mobility and the desire of comparing these systems is to better investigate the different role played by all these aspects on the interaction between highly dispersed CuO nanoparticles and perovskite and on the catalytic activity.The copper amount on the nanocomposite surface does not increase linearly with the nominal composition reaching a plateau: migration below the surface is observed for the nanocomposite with 30 wt.% of Cu.The surface composition of the perovskite is modified by the copper deposition which causes the decrease of A-cations surface segregation and enhances the presence of cobalt suggesting a certain synergy; the reducibility of the perovskite is also greatly favored by deposition.Both model reactions (CO oxidation and CO assisted NO reduction) and reactions with a synthetic automotive exhaust mixture, including 10% steam, and oxygen, were carried out. We compared the results with the ones obtained in similar reactions with CuO/LaCoO3. Different interaction and synergy were observed with respect to CuO/ La0.5Sr0.5CoO3. Sr-doping, in fact, enhances oxygen mobility affecting the reducing character of the nanodispersed CuO and thus the reactivity under different conditions.The deposition of copper oxide significantly increases the activity of the nanocomposites in CO oxidation (about 100% conversion at 200 °C) and in CO + NO (50% conversion at 250 °C, more than 80% at 400 °C) reactions. When compared with the corresponding CuO/LaCoO3, the more significant difference has been observed in nanocomposites poorer in CuO, which became highly active at lower temperature. On simulated gasoline engine exhaust the nanocomposites always improve the oxidation activity compared to the parent perovskite, while the NO reduction is quantitative in the absence of O2.The activity on a mixture simulating actual gasoline-engine exhaust proves that ADP synthesis provides materials with a higher activity compared to wet impregnation (WI), thanks to a higher dispersion of copper. NO reduction in fuel-rich conditions is activated at approx. 300 °C, (400 °C on WI sample), when significant amount of O2 is still in the mixture. This feature completes the good performance in absence of noble critical metals that are promising facts to develop PGM-free catalysts for the automotive industry.