Perovskite-type nanomaterials are attractive and promising oxides for environmental- (e.g., oxidation of CO) and energy-relevant (e.g., electrochemical water splitting to produce H₂/O₂) catalytic reactions. For these applications, two of the most sought characteristics of perovskite compounds – high specific surface area and phase purity – are hardly attained by conventional synthesis methods. Envisioning the cost-efficient and one-step production of high surface-area perovskite materials, the spray-flame synthesis of three model compounds LaFeO₃, LaCoO₃ and LaMnO₃ for catalytic CO oxidation is investigated in this thesis. Using metal nitrates as low-cost precursors, the effect of two solvents – ethanol and 2-ethylhexanoic acid (2-EHA) – on the chemistry of the precursor solutions and on the particle-size distribution, specific surface area and phase composition of the produced materials is analyzed.

The use of ethanol as solvent for the spray-flame synthesis led to the formation of perovskite (LaFeO₃, LaCoO₃ and LaMnO₃) compounds with multimodal particle size distributions from which, large particles (> 100 nm) were identified as lanthanum-rich – especially in the LaCoO₃ case – and the source of secondary phases as La₂O₃ or La₂CoO₄. ATR-FTIR analyses of the ethanol-based solutions allowed us to identify the formation of species such as HNO₃ at low temperatures, which point to the hydrolysis of the metal ions – especially in the solution containing iron nitrate – and the formation of hydroxide species. The use of ethanol/2-EHA based solutions was effective to generate perovskite materials with high specific surface areas, with narrow particle size distributions – only few sub-200 nm particles were identified – and proved also to be effective to reduce the concentration of secondary phases (e.g., La₂O₃) in the final products. Ethanol/2-EHA mixtures have been used before especially for the spray-flame synthesis of single oxides (e.g., Fe_xO_y, Co_xO_y, and Al₂O₃) and they have been related to the generation of droplet micro-explosions or the formation of metal carboxylates. In this thesis, the chemical interaction among ethanol, 2-EHA, and the corresponding metal nitrates is further investigated. Based on ATR-FTIR analyses, esterification between ethanol and 2-EHA to produce ethyl 2-ethylhexanoate and water was identified and in the case of Mn-based solutions, oxidation of Mn²⁺ was induced by the addition of 2-EHA at low temperatures, causing the formation of Mn⁴⁺-rich LaMnO_3+d nanoparticles from spray-flame synthesis. Both esterification and oxidation of manganese are novel aspects related to the use of ethanol/2-EHA-based solutions of metal nitrates for the spray-flame synthesis of perovskites. Furthermore, esterification was identified to be catalyzed by the transition metal ion present in solution with the iron-containing solution presenting a remarkable activity to produce the ester compound and water. Using ethanol/2-EHA-based solutions, the effect of the droplet sizes by modifying the dispersion gas flow rate (5, 6, 7, and 8 slm) and the identified esterification reaction on the particle-size distribution of the three spray-flame synthesized model perovskites are also analyzed in this dissertation.

The spray-flame synthesized LaFeO₃, LaCoO₃ and LaMnO₃ perovskite samples based on different mixtures of ethanol/2-EHA are analyzed for the catalytic oxidation of CO. For this reaction, LaCoO₃ samples were identified to present the highest activity followed by LaMnO₃ and LaFeO₃. The LaMnO₃ samples were selected to perform a more detailed catalytic analysis including the selective oxidation (SELOX) of CO and the characterization with ex situ techniques as H₂-TPR and O₂-TPD.

This thesis provides relevant information for the spray-flame synthesis of perovskites and by extension, mixed-oxide systems when low-cost metal nitrates are used as precursors. It is shown how chemical/physical effects related to the solutions of precursors play important roles in the spray-flame synthesis of nanomaterials and most importantly, it is presented how these solutions can be modified to improve the quality of the synthesized materials.