Spray-flame synthesis of La(Fe, Co, Mn)O3 perovskite nanoparticles for CO oxidation

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., FexOy, CoxOy, 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 LaMnO3+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.



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