High Turnover Frequency CO–NO Reactions over Rh Overlayer Catalysts: A Comparative Study Using Rh Nanoparticles

A comparative turnover frequency (TOF) study for structure-sensitive CO–NO reactions between overlayer (thin-film) and nanoparticle Rh catalysts was performed using a combined experimental and theoretical approach. Two types of honeycomb catalysts were prepared: one by the arc-plasma deposition of a 3 nm Rh overlayer having a (111) preferential orientation atop a 20-μm Fe–Cr–Al metal foil and the other by a conventional wet coating of Rh/ZrO2 powders comprised of Rh nanoparticles onto a cordierite honeycomb. The reaction rate of the overlayer was found to be more superior to the nanoparticles, despite a smaller surface area, as the TOF was 14-fold greater on the overlayer than on the nanoparticle. In situ infrared spectroscopy suggested that a hollow-site NO formed onto the overlayer in contrast to the bridge- and on-top-NO adsorptions on the nanoparticles. The energy barriers for surface reactions of adsorbed NO molecules were analyzed using a density functional theory based molecular dynamics approach for a Rh(111) slab and a Rh55 cluster to model the overlayer and the nanoparticle. The nanoparticle was more favorable to the dissociation of bridge NO compared with the hollow NO on the overlayer despite needing to overcome much greater barriers for N–N recombination, suggesting that the surface of the nanoparticles was predominately covered by N and O atoms, where N–N recombination was the rate-limiting step. Conversely, the Rh(111) overlayer, which offered not only moderate or comparable energy barriers for NO dissociation and N–N recombination but also a lower energy barrier migration for N atoms on the surface, enabled a high-TOF reaction. The proposed mechanism was rationalized by comparing the results with the empirical kinetics of CO–NO reactions.