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

Abstract

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 NH3 under light-off conditions or in CO-rich feed. By varying Rh catalyst structure from atomically dispersed species to clusters on Al2O3 and CeO2, we provide evidence that in the reduction of NO by CO with co-fed H2O, low-temperature NH3 formation occurs on atomically dispersed Rh sites, whereas high-nuclearity Rh clusters are selective to N2.

Single‐atom Automobile Exhaust Catalysts

“…to be active to reduce the NO with CO via the totally different reaction pathways compared to Rh NPs. Very recently, Christopher and co‐workers raised a question if the low‐temperature NH 3 formation from NO reduction in the presence of CO and H 2 O is due to single atom Rh? [158] They provided the evidence showing that single atom Rh on γ‐Al 2 O 3 is selective for the low‐temperature unselective reduction (USCR) of NO to NH 3 , whereas the Rh clusters are responsible for SCR process to N 2 (Figure 15). As a result, avoiding the formation of single atom Rh is recommended for the SCR process.…”

“…Illustration of the NO reduction over different Rh structures on oxide supports between 150°C and 300°C. Reproduced with permission from ref [158],. Copyright 2020 American Chemical Society.…”

The Hydrothermal Stability and the Properties of Non- and Strongly-Interacting Rh Species over Rh/γ, θ-Al2O3 Catalysts

“…The modern commercial TWCs contain Pt, Pd, and Rh for removing carbon monoxide (CO), hydrocarbons (HCs), and nitrogen oxides (NO x ). Rh is a crucial ingredient, aimed at eliminating NO x pollution [1,3]. The global consumption of Rh has increased rapidly in past decades and about 81% of Rh usages are in auto catalyst manufacturing in 2012 [4].…”

“…Feed stream: 1% CO, 1000 ppm HCs (C 3 H 6 :C 3 H 8 = 2:1), 1000 ppm NO, 0.917% O 2 , 12% CO 2 , 3% H 2 O, and N 2 balance. Space velocity: 6,000,000 cm3 …”

“…Feed stream: 1000 ppm CO, 1000 ppm NO, 3% H2O, and N2 balance. Space velocity: 7,800,000 cm3 •gcat −1 •h −1 for Rh/γ-Al2O3.…”