Spray-Flame Synthesis of LaMnO3+δ Nanoparticles for Selective CO Oxidation (SELOX)

LaMnO3+delta nanoperovskites were prepared via the continuous and scalable spray-flame synthesis (SFS) technique from metal nitrate-based solutions by using either ethanol (EtOH) as solvent or a mixture of ethanol (50 vol %) and 2-ethylhexanoic acid (50 vol %) (EtOH/2-EHA). Solutions based on pure EtOH generated a mixture of several phases and a broad and multimodal particle size distribution, which is attributed to a combination of gas-to-particle and droplet-to particle formation of particles. The product contained a bimodal distribution of the orthorhombic (Pnma II) LaMnO3 perovskite-like phase and additional, unwanted phases such as La2O3 and sub-20 nm Mn-rich amorphous/poorly crystalline particles. The incorporation of 2-EHA led to high surface area (>100 m(2) g(-1)), small, and crystalline LaMnO3+delta nanoparticles with sizes ranging between 4 and 15 nm in the presence of few sub-200 nm particles (<10 wt %). This sample is mainly composed of the orthorhombic Mn4+ rich (Pnma I) LaMnO3+delta phase, and it counts with a very high specific surface area that makes it highly promising for catalytic applications. FTIR and UV-VIS spectroscopy of the precursor solutions revealed the oxidation of the Mn2+ precursor in advance of the particle formation process along with the esterification of the solvent mixture. It is assumed that the observed liquid-phase oxidation supports the formation of Mn4+-rich perovskites. According to O-2-TPD and H-2-TPR measurements, the EtOH/2-EHA sample presented a much higher formation of adsorbed active oxygen species and higher reducibility than the EtOH-made material, leading to a superior performance for both the catalytic oxidation of CO and the selective oxidation (SELOX) of CO.