On the Redox Mechanism of Low‐Temperature NH₃‐SCR over Cu‐CHA: A Combined Experimental and Theoretical Study of the Reduction Half Cycle

Abstract

Cu-CHA is the state-of-the-art catalyst for the Selective Catalytic Reduction (SCR) of NOx in vehicle applications.Although extensively studied, diverse mechanistic proposals still stand in terms of the nature of active Cu-ions and reaction pathwaysinSCR working conditions.Herein we address the redoxm echanism underlying Low-Temperature (LT) SCR on Cu-CHA by an integration of chemical-trapping techniques,transient-response methods,operando UV/Vis-NIR spectroscopyw ith modelling tools based on transient kinetic analysis and density functional theory calculations.W es how that the rates of the Reduction Half-Cycle (RHC) of LT-SCR displayaquadratic dependence on Cu II ,t hus questioning mechanisms based on isolated Cu II-ions.W ep ropose,i nstead, aCu II-pair mediated LT-RHC pathway,inwhichNOoxidative activation to mobile nitrite-precursor intermediates accounts for Cu II reduction. These results highlight the role of dinuclear Cu complexes not only in the oxidation part of LT-SCR, but also in the RHC reaction cascade.

Theoretical Study on the NOx Selective Catalytic Reduction on Single-Cu Sites and Brønsted Acid Sites in Cu-SSZ-13

“…The involved barriers are clearly too high for this cycle to be active (>250 kJ/mol) (see Figure ), at least for the mechanism investigated in this work. This is in agreement with the previous investigations that propose that Cu dimers are required for NO activation. ,,,,,− …”

“…40 With an increasing number of NH 3 ligands, the Cu(NH 3 ) n + complex is exceedingly weakly bound to the zeolite framework and thus becomes mobile at elevated temperatures. 15,26,34,40,60,61 This situation is poorly described by the harmonic approximation, which can to some extent be circumvented using MD simulations or approximations to the entropies of weakly bound adsorbates. 34 Here, we focus on the reactivity of Cu with only a single NH 3 adsorbed, and the free energy profile in Figure 2b is referenced to that.…”

Coupling Electrocatalytic Nitric Oxide Oxidation over Carbon Cloth with Hydrogen Evolution Reaction for Nitrate Synthesis

“…[1] Exhaust gases containing NO are dominantly emitted into the atmosphere directly.T he fast reaction between NO and O 2 leads to the generation of nitrogen dioxide and then reacts with water to produce nitric acid, resulting in serious environmental crisis. [2] In order to eliminate the pollution of NO,t he selective catalytic reduction (SCR) process has been explored to convert the harmful NO gas into harmless N 2 by the reaction with ammonia, [3] as illustrated in Scheme 1. However,further improvements are needed in the optimization of operating conditions and the avoidance of valuable chemicals sacrifice.…”

Mechanism and Enhancement of the Low-Temperature Selective Catalytic Reduction of NOx with NH3 by Bifunctional Catalytic Mixtures

“…Indeed, such a step is highly exergonic (ΔG 2 = −407 kJ mol −1 ), thus favoring the overall energetics of the "NO → HONO → N 2 + H 2 O" cascade (ΔG = ΔG 1 + ΔG 2 = −380 kJ mol −1 ). Given that HONO is unstable and readily decomposes at 473 K, 28,53,54 a more efficient scavenging by adsorbed NH 3 and a lower extent of HONO degradation are expected in the case of the intimate mixture (panel e) because, in this case, the distance for HONO to travel to reach the NH 3 adsorbates on V−W/Ti is shorter as compared to panel c, which thus accounts for the much more prominent synergistic enhancement observed over panel e (Figure 3a). In addition to the activity enhancement, the improved N 2 selectivity recorded on the mixture of V−W/Ti and Mn/Ce (Figure 3b) is also compatible with the HONO mechanism.…”