Synthesis, Structure, and Gas-Surface Interaction in RM₂O_{5-δ} Mullite-Type Oxides as Nitric Oxide (NO) Oxidation Catalysts

To efficiently mitigate the pollutants in exhaust from diesel engines, including NOₓ and particulate matter, it is important to oxidize NO to NO₂, a reaction that currently uses platinum group metals (PGMs) as catalysts. Due to the scarcity and high costs of PGMs, it is valuable to develop catalysts based on Earth-abundant elements. Mullite-type oxides was shown to have high activity for NO oxidation by Wang et.al. This dissertation is focused on studying synthesisstructure-property relationships in RM₂O_{5-δ} (R = Y, Sm, Gd, Pr, Bi; M = Mn, Fe) mullite-type oxides with the objective of optimizing their properties for NO oxidation. Synthesis conditions such as precipitation pH and calcination temperatures were systematically varied to study their impact on surface composition and NO adsorption property of SmMn₂O₅. Through this study, surface area and surface Mn/Sm ratio were determined as the primary parameters that affect NO adsorption, with crystalline phase having a secondary effect. A distinct R-site dependent mullitetype phase formation and thermal stability was observed for R = Y, Gd, Sm, Pr or Bi in RMn₂O₅ and Fe substituted R(Mn₁₋ₓFeₓ)₂O_{5-δ} compounds. The RMn₂O₅ compounds displayed high thermal stability; however, increasing Fe incorporation in the mullite-type structure was found to decrease the stability of mullite-type phase. Furthermore, with 100% Fe, no mullite-type phase was formed for R = Y and Sm, but a sub-group transition to BiFe₂O_{4.5} O9-phase was found for R = Bi. The influence of crystalline phase on catalytic activity was further examined by comparing SmMn₂O₅ mullite-type and SmMnO₃ perovskite phases. The superior catalytic performance of SmMn₂O₅ was attributed to its ability to regenerate active sites with nitrate dissociation at low temperature. Finally, hydrothermal method was demonstrated as a promising low-temperature synthesis route to obtain pure-phase RMn₂O₅ mullite-type oxides with high surface area. Furthermore, PrMn₂O₅ mullite-type oxide obtained by this method has been shown to be a potential low temperature catalyst with higher NO conversion efficiency when compared to SmMn₂O₅.