(224b) Direct Decomposition of NO Using Specialized Metal Oxides in a Novel Chemical Looping Reactor System

NO is a potent pollutant that is produced via the flame combustion of fossil fuels. Several strategies for mitigating NO emissions employ a reducing agent that selectively reacts with NO over a catalyst. This technology is known as selective catalytic reduction (SCR) and is commercially used with ammonia as the reducing agent. Alternatively, NO removal can be achieved by direct NO decomposition as it is thermodynamically unstable as compared to its decomposition products. This eliminates the use of a reducing agent and incentivizes the direct decomposition process, however, this reaction has a high activation energy barrier, thus imposing kinetic limitations. The catalysts for direct NO decomposition also suffer from loss in reactivity in the presence of O₂ and CO₂, which are typically present in NO containing flue gas streams.

This study introduces a unique reaction system that counters the drawbacks of the direct catalytic NO decomposition system. The proposed system employs a chemical looping reactor that utilizes a specialized oxygen uncoupling metal oxide for NO decomposition. The flue gas stream containing NO reacts with the metal oxide in a reactor, producing N₂ and oxidizing the metal oxide. The oxidized metal oxide is then regenerated at a higher temperature, releasing O₂ and creating oxygen vacancies that are essential for NO decomposition in the first reactor. Thus, as the metal oxide does not get consumed in the overall chemical looping system, this technology mimics the direct decomposition reaction with a key difference that it produces N₂ and O₂ products in two separate streams. This enables the chemical looping system to be governed by a different reaction mechanism than the catalytic system. Several metal oxides have been experimentally screened for their activity towards NO decomposition and stability towards oxygen uptake in a thermogravimetric analyzer (TGA). The TGA weight profiles also gave an insight into the change in reaction kinetics of NO decomposition upon addition of supports and dopants. The effect of CO₂ inhibition and reaction temperature have also been tested for two different composite metal oxides. These results have been supported with solid characterization techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Raman Spectroscopy. Further, ab initio atomistic simulations and density functional theory calculations were performed to understand the reaction mechanism and supplement the experimental results. Finally, fixed bed reaction tests have been performed to investigate the nature of NO breakthrough curves on different metal oxide systems. Findings from this study lay the groundwork for this novel NO abatement process and will facilitate the advancement of this technology.