A comparative study on TWC reactions over Rh thin films and supported Rh nanoparticles under lean conditions

On a Fe-Cr-Al metal foils honeycomb fully-covered by 7 nm-thick Rh film (Rh/SUS), the stoichiometric NO-CO-C3H6-O-2-H-2-CO2-H2O reaction exhibited high turnover frequencies (TOF) more than 25-fold greater compared with those for a reference supported Rh nanoparticle catalysts (Rh/ZrO2), which were coated on a cordierite honeycomb substrate using a conventional slurry coating. As a result, these two types of honeycombs showed comparable catalyst performance at the stoichiometric air-to-fuel ratios (A/F). The Rh/SUS significantly improved the NO conversion to N-2 under lean conditions (14.60 < A/F <= 15.10) as compared with Rh/ZrO2. Most of the metallic Rh surface of Rh/SUS remained unchanged compared with Rh/ZrO2 when the atmosphere was changed from stoichiometric (A/F = 14.6) to lean (A/F > 14.6), where oxidized Rh should be thermodynamically stable. Metallic Rh formed on Rh/SUS was gradually oxidized on exposure to excess O-2, whereas Rh on ZrO2 was readily oxidized to less-active Rh2O3. The resistance of metallic Rh against oxidation is considered to be a possible reason for the enhanced NO conversion efficiency under lean conditions.