Influence of Ba addition on thermal stability and catalytic activity of La_2-xBa_xCuO₄ catalysts

Full text: The replacement of noble metals by transition metals in TWC is highly advisable for both environmental and economic reasons. In this work the catalytic reduction of NO by CO was studied using mixed oxides La_2-xBa_xCuO₄ (x = 0.0 or 0.4) as catalysts, prepared by the coprecipitation method and characterized by X-ray diffraction (XRD), O₂ temperature programmed desorption (O₂-TPD), H₂ temperature programmed reduction (H₂-TPR) and X-ray absorption near-edge structure (XANES). The NO reduction by CO was studied in laboratory microreactor as well as at the D06A-DXAS beamline of the Brazilian Synchrotron Light Laboratory (LNLS) for obtaining information about the evolution of the oxidation state of the catalysts active phases under reaction conditions. LaCu and LaBaCu fresh catalysts showed good activity in the NO reduction by CO reaction. The Ba addition caused structural changes creating oxygen vacancies on the surface and in the bulk of the catalyst that promoted the reducibility and activity of the fresh catalysts mainly at low temperatures. Another important role of barium was the improvement of the catalyst stability, maintaining their reducibility even after thermal aging. Based on in-situ XANES results, LaBaCu900 presented the higher reducibility with Cu⁺ and Cu° formation during the reaction. The use of a reducing pretreatment provided active sites formation, justifying the significant improvement in catalytic activity. The N₂O generation was also observed in all catalysts (fresh and aged) but in lower yields when compared with noble metal-based catalysts. The decrease in the activation energy of the pretreated catalysts at 500 °C under H₂ flow indicates a change in the active sites (presence of reduced copper species since the beginning of the reaction). (author)