Selective Catalytic Reduction over Cu/SSZ-13: Linking Homo- and Heterogeneous Catalysis

Abstract

Active centers in Cu/SSZ-13 selective catalytic reduction (SCR) catalysts have been recently identified as isolated Cu and [Cu(OH)] ions. A redox reaction mechanism has also been established, where Cu ions cycle between Cu and Cu oxidation states during SCR reaction. While the mechanism for the reduction half-cycle (Cu → Cu) is reasonably well-understood, that for the oxidation half-cycle (Cu → Cu) remains an unsettled debate. Herein we report detailed reaction kinetics on low-temperature standard NH-SCR, supplemented by DFT calculations, as strong evidence that the low-temperature oxidation half-cycle occurs with the participation of two isolated Cu ions via formation of a transient [Cu(NH)]-O-[Cu(NH)] intermediate. The feasibility of this reaction mechanism is confirmed from DFT calculations, and the simulated energy barrier and rate constants are consistent with experimental findings. Significantly, the low-temperature standard SCR mechanism proposed here provides full consistency with low-temperature SCR kinetics.

Recent Progress in Atomic-Level Understanding of Cu/SSZ-13 Selective Catalytic Reduction Catalysts

“…Furthermore, complexity is also evident in the nature of the catalytically active Cu species, as first recognized from SCR kinetics. For example, Figure 10 displays steady state NO x light-off curve for a Cu/SSZ-13 catalyst with relatively low Cu loading, allowing low-and high-temperature reaction regimes to be clearly resolved [19,38]. Importantly, the NO x conversion decrease with increasing temperature between~250 and 350 • C indicates a temperature-dependent change in the nature of active Cu sites, which has now been repeatedly demonstrated via in situ/operando XAS.…”

“…In probing this low Cu-loading regime, critical new insights have been provided for understanding the standard SCR mechanism. Of special note is that reaction rates at low temperatures are limited by O 2 activation; notably, for the process by which Cu(I) is re-oxidized back to Cu(II) to finish redox cycling of the active sites [19,21]. In particular, a few important experimental findings led to the surprising proposal that rate-limiting, low-temperature O 2 activation requires participation of two Cu(I) sites.…”

“…The realization that linear [Cu(NH 3 ) 2 ] + is the most stable and most mobile Cu(I) ion complex makes this species the best candidate for O 2 activation. Based on this, Gao et al [19] used DFT computer simulations to first propose the following pathway as an essential elemental step for the O 2 activation, that is, the oxidation half-cycle of SCR:…”

“…This group's subsequent Science paper [21] provided many additional important details on the rates of complexed Cu-ion transport and the formation of the binuclear Cu complex. Significantly, Reaction (9) and especially the [Cu(NH 3 ) 2 ] + diffusion and Cu-O 2 -Cu binding steps, have been proven to be thermodynamically feasible from theoretical simulations [19,21]. At the lowest Cu loadings, formation of the Cu-O 2 -Cu intermediate is the rate-limiting, essentially controlled by the entropy costly diffusion of [Cu(NH 3 ) 2 ] + .…”

“…Now after 3 years, new and exciting results have appeared in literature, especially with respect to the developing molecular-level understanding of the nature of the active Cu species under realistic operating (i.e., situ/operando) conditions, as well as the SCR reaction mechanisms [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. As such, it seems timely to review these new fundamental understandings.…”