Mechanistic Investigation of Novel Niobium-Based Materials as Enhanced Oxygen Storage Components and Innovative CO Oxidation Catalyst Support for Environmental Emission Control Systems
Creators: Leung, Emi
Publisher: name: Columbia University
Year: 2016
Subjects: subject: Niobium
subject: Catalysts
subject: Gas--Storage
subject: Automobiles--Motors--Exhaust gas--Environmental aspects
subject: Environmental engineering
subject: FOS: Environmental engineering
subject: Chemical engineering
subject: FOS: Chemical engineering
subject: Materials science

description: Nb-doped ZrO₂-CeO₂-Y₂O₃ solid solution (Nb-ZrCeYO) is studied as a possible oxygen storage component in three way automobile exhaust catalysts. It shows enhanced oxygen storage (OS) capacity with a higher extent of reduction at temperatures within the typical operating range of three-way catalyst compared with solid solutions without Nb. However, after several days of exposure to ambient air, the OS behavior of the Nb-doped samples shows significant degradation. Degradation is slowed for samples stored in evacuated environments (i.e. vacuum sealed glass tubes). NbOₓ segregation to the surface under oxidizing conditions is hypothesized as the cause of the degradation. This hypothesis is consistent with the temperature programmed reduction data. The addition of small amounts of Pt to the aged samples restores the enhanced initial performance advantages. It is postulated that electrons supplied by metallic Pt mimic reducing conditions, which are known to re-disperse surface NbOₓ species into the bulk solid solution, leading to stable, time-independent OS performance. However, the small advantage caused by Nb addition over the current technology is insignificant for the TWC application. Therefore, we focus on other environmental applications such as CO oxidation by Nb-containing catalysts with the specific objective of enhanced CO oxidation activity by formation of Cu¹⁺ species supported on Nb₂O₅.
      The preparation of a Cu(1)Nb(2)Oₓ results in a solid solution crystallized in three different phases: CuO, Nb₂O₅, and CuNb₂O₆. The solid solution shows enhanced low temperature CO oxidation (&lt;155˚C) activity compared to the reference CuO solid solution. Analysis by hydrogen-temperature programmed reduction (H2-TPR) indicates there are two different Cu species in the Nb-containing solid solution: highly dispersed Cu species and bulk CuO. The existence of an interaction between Cu and Nb ions is hypothesized for the enhanced low temperature CO oxidation activity by formation of Cu⁺¹. This hypothesis is consistent with XPS data, indicating the existence of more catalytically active Cu¹⁺/⁰ and Cu²⁺ species in the Nb₂O₅ sample, where the reference bulk CuO oxide shows only the less active Cu²⁺ species. 
      Impregnation of Cu-containing precursor salts on the Nb₂O₅ support leads to enhanced CO oxidation activity: The Cu supported Nb₂O₅ sample shows improved CO oxidation activity compared with the reference Cu supported on Al₂O₃. An isothermal aging test shows high stability of the Cu¹⁺ species on the Nb₂O₅ support at 155˚C for 20 hours in air. Studies of the optimization of the Cu supported Nb₂O₅ leads one to conclude that low surface coverage of NbOx on Al₂O₃ is the reason why these samples shows lower CO oxidation activity. The optimal amount of Cu species on the Nb₂O₅ support is 6%, where activity is similar to 1%Pt/Al₂O₃, the state of the art CO oxidation catalyst in industry, but a phase transformation of Nb₂O₅ occurring at 800˚C, leads to a loss in the enhanced CO activity. A gradual loss in surface area is observed for samples aged at higher temperatures, indicating support sintering as the main cause of the performance deterioration. Stable performance at low temperatures makes CuOₓ/Nb₂O₅ a potential candidate for stationary abatement applications, which operate at temperatures &lt;400˚C. Advanced aging would be necessary to qualify it for specific applications. A kinetic model for CO oxidation of CuOₓ/Nb₂O₅ is also developed.