Title: Oxide supported Rh catalysts in automotive NO reduction chemistry: The roles of atomically dispersed Rh, water, and dynamic restructuring
Advisor: Phil Christopher
At a time when global warming has led to more aggressive regulations on emissions control around the world, the demand for automotive pollution-mitigating catalytic converters has grown. The three-way catalyst (TWC) in catalytic converters, composed of platinum group metal active sites, has played a critical role in automotive pollution mitigation. Specifically, rhodium (Rh) is crucial in TWCs to reduce nitric oxides (NOx) emissions produced by burning hydrocarbon fuel. Rh is a unique catalyst in its ability to reduce NO at high rates with a high selectivity towards N2, while remaining robust.
Despite extensive investigation, NO reduction mechanisms under automotive conditions remain unclear. Particularly, it is unknown which sites are responsible for the unselective reduction of NO to NH3 under light-off conditions, or in CO-rich feeds. This is important for further optimization of TWCs as NH3 is currently an unregulated emission from automobiles, despite it being a strong greenhouse gas. While Rh is highly selective towards N2 at steady state conditions, the current understanding in literature rarely incorporates realistic automotive operating conditions where large concentrations of CO and H2O are present. Additionally, most studies have been performed over nanoparticle surfaces at higher weight loadings of Rh, >1%, despite the fact that the actual Rh weight loading in the TWC is lower than ~0.3%, where a mix of nanoparticles, small clusters, and atomically dispersed species are likely to be present.
In this work, we focused on identifying the distinct properties and reactivity of atomically dispersed Rh active sites on oxide supports for NO reduction under automotive conditions. We developed and optimized approaches to synthesize atomically dispersed Rh catalysts on various oxide supports using principles of strong electrostatic adsorption. Probe molecule Fourier-transform infrared spectroscopy (FTIR) was utilized in correlation with temperature programed desorption, aberration corrected scanning transmission electron microscopy, and Density Functional Theory calculations to provide insights into the structure and local coordination of the Rh species. Probe molecule FTIR proved to be particularly useful in characterizing the distribution of Rh species in a sample as probe molecules bond with different stoichiometries and vibrational stretching frequencies on different Rh species ranging from atomically dispersed to nanoparticles.
Using a series of well characterized samples with various Rh structures, we observed via light off and kinetic measurements that atomically dispersed Rh active sites result in distinct NO reduction reactivity as compared to Rh nanoparticles. Specifically, under dry environments, NO is reduced by CO at higher rates on Rh nanoparticles as compared to atomically dispersed Rh species. However, at low NO conversion, Rh nanoparticles more selectively produce N2O, while atomically dispersed Rh species favor N2. Interestingly, the addition of H2O to the feed significantly promotes the NO consumption rate for atomically dispersed Rh active sites and results in 100% selectivity to NH3 at relevant operating temperatures. Alternatively, Rh clusters are less reactive than atomically dispersed species in the presence of H2O, and the reaction primarily leads to N2 formation. Thus, Rh structure has a strong effect on NO reduction chemistry in catalytic converter conditions, both in terms of NO consumption rate and selectivity. Our results suggest that TWC formulations and emission control strategies should be targeted to avoid the formation of atomically dispersed Rh species to mitigate low temperature NH3 formation. More broadly, the results highlight the potential role (sometimes negative) of atomically dispersed metal species in industrial catalytic processes.