Low-Temperature Ammonia Production during NO Reduction by CO Is Due to Atomically Dispersed Rhodium Active Sites

Rhodium plays a key role in three-way catalysts to control NO x emissions in gasoline engines primarily through the reduction of NO by CO. Despite extensive investigation, there remains uncertainty about NO reduction mechanisms. Particularly, it is unknown which sites are responsible for the unselective reduction of NO to NH 3 under light-off conditions or in CO-rich feed. By varying Rh catalyst structure from atomically dispersed species to clusters on Al 2 O 3 and CeO 2, we provide evidence that in the reduction of NO by CO with co-fed H 2 O, low-temperature NH 3 formation occurs on atomically dispersed Rh sites, whereas high-nuclearity Rh clusters are selective to N 2 .