Brønsted-NH4+ mechanism versusnitrite mechanism: new insight into the selective catalyticreduction of NO by NH3

Abstract

The selective catalytic reduction (SCR) of NO by NH₃ over V₂O₅-based catalysts is used worldwide to control NO_x emission. Understanding the mechanisms involved is vital for the rational design of more effective catalysts. Here, we have performed a systematic density functional theory (DFT) study of a SCR reaction by using cluster models. Three possible mechanisms have been considered, namely (i) a Lewis acid mechanism, (ii) a Brønsted acid mechanism and (iii) a nitrite mechanism. Our calculations down-play the significance of mechanism (i) due to its high barrier as well as the incorrect reaction order. On the other hand, our calculations demonstrate that both mechanisms (ii) and (iii) can lead to a first order reaction with respect to NO with the predicted barriers being consistent with the experimental observations. Thus, we conclude: there exists two competitive pathways for SCR. Mechanism (ii) is dominant when the Brønsted acidity of the catalysts is relatively strong, while mechanism (iii) becomes important when Brønsted acidity is weak or absent. Importantly, we demonstrate that the latter two mechanisms share a common feature where N–N bond formation is ahead of N–H bond cleavage, in contrast to that in mechanism (i). Such a sequence provides an effective way to reduce the side reaction of ammonia combustion since the relatively strong N–N bond has already been formed.