Impact of Active Site Variations on Catalytic Nitric Oxide Reactions

Abstract

In this study, I explored the intricate dynamics of active site variations in catalysts and their consequential effects on the catalytic reaction of nitric oxide (NO). The primary objective of this research was to elucidate the relationship between active site modifications and their impacts on the efficiency of NO reactions, aiming to advance the design and application of more effective catalytic systems. The core of the thesis presents a series of studies that systematically investigate the effects of altering active sites on the NO reaction process. Initially, my research focused on aging as one of the possible changes in active sites in Three-Way Catalysts (TWCs). Aging in TWCs can significantly influence NO reactions through the formation of by-products such as ammonia (NH3) and nitrous oxide (N2O). I elucidated the mechanisms of NH3 and N2O formation using commercially aged Pd/Rh-based TWCs. The emissions of NH3 and N2O over these catalysts are closely dependent on the individual reductant species within a complex reaction network. Under simulated full mixture feed composition containing CO, H2, and a hydrocarbon (HC) mixture of C3H6 and C3H8 as reductants, NH3 formation was maximized under rich conditions (0.97 ≤ λ < 1.00) at high temperatures ranging from 350 to 500 ℃. N2O formation was more pronounced under stoichiometric conditions (λ = 1.00) at lower temperatures from 250 to 350 ℃. By conducting simple feed tests with a single reductant (H2, CO, or C3H6) and NO, we determined the role of individual reductant species in NH3 and N2O emissions characterized from full feed conditions. To minimize the aging effects on Pd/Rh-based TWCs, I conducted a study on substituting part of the Pd with Pt. This research comprehensively investigated the impact of Pt substitution in commercial Pd/Rh-based TWC formulations with respect to catalytic performance. TWC performance was systematically evaluated under realistic simulated exhaust conditions including fuel-rich, stoichiometric, and fuel-lean (0.99 ≤ λ ≤ 1.01) scenarios. Pt-substituted TWCs outperformed their Pd-based counterparts in CO, C3H6, and C3H8 oxidation, and NO reduction reactions under all tested conditions. Furthermore, the Pt-substituted TWCs exhibited significant stability upon hydrothermal aging at 1,050 ℃, retaining higher N2 selectivity after aging compared to Pd-based TWCs. Beyond TWCs, direct NO decomposition (DND) offers a promising alternative to selective catalytic reduction (SCR) for emission control, as it does not require the injection of reductants such as urea/NH3. The most common change in active sites during the DND reaction is deactivation. My research aimed to address this by exploring the mechanisms of deactivation and regeneration strategies using Rh-supported on various metal oxides such as PrOX, TiO2, Al2O3, and CeO2. We discovered that catalytic activity on Rh/PrOX and Rh/TiO2 was initially very high but decreased over time due to strong poisoning by adsorbed oxygen. This deactivation was positively regenerated by reductive treatments with CO. In addition to changes in active sites due to aging and poisoning, the performance of catalysts can also be altered through interactions in mixed catalyst systems. To enhance the understanding of this, I conducted research on maximizing photocatalytic NO oxidation performance by doping Ag2S and mixing with Na-ZSM-5. These modifications improved visible light absorbance and NO oxidation performance, addressing mass transfer limitations and enhancing NO adsorption and subsequent oxidation. Finally, I also focused on variations in active sites that can occur during the manufacturing process. I proposed a synthesis method that involved sulfuric-functionalization on TiO2 followed by Pt impregnation. This process enabled control over Pt dispersion and phase through the formation and alteration of active metal-amine-sulfate bonds. The characteristics and performance of the synthesized catalysts were evaluated through DRIFTS, Raman, XPS, TEM, XRD, and NO oxidation performance tests. Overall, this studies provide fundamental insights into the behavior of active sites in catalysts, significantly advancing our ability to enhance catalytic systems for NO reactions and beyond. This study plays a crucial role in understanding catalytic transformations for various applications.