Unraveling the Hydrolysis of Z<sub>2</sub>Cu<sup>2+</sup> to ZCu<sup>2+</sup>(OH)<sup>−</sup> and Its Consequences for the Low-Temperature Selective Catalytic Reduction of NO on Cu-CHA Catalysts
Wenshuo Hu, Umberto Iacobone, Federica Gramigni, Yu Zhang, Xiaoxiang Wang, Shaojun Liu, Chenghang Zheng, Isabella Nova, Xiang Gao, Enrico Tronconi
Abstract
As the state-of-the-art catalyst for the selective catalytic reduction (SCR) of NOx, Cu-CHA has been extensively investigated in both its practical and fundamental aspects. Among the latter, how Z2Cu2+, an active site for SCR, participates in low-temperature (LT) SCR reactions remains debated. Here, we propose a scheme involving the hydrolysis of Z2Cu2+ to ZCu2+(OH)-, a thermodynamically and kinetically favorable process under LT-SCR conditions, based on multiple pieces of evidence from a probe reaction (transient CO oxidation), transient Cu2+ reduction kinetic runs, in situ FTIR spectroscopy, and first-principles calculations. Such an integrated investigation reveals unambiguously that the hydrolysis of Z2Cu2+ to ZCu2+(OH)- occurs facilely in the presence of NH3, which may thus reconcile the identical quadratic kinetics of Z2Cu2+/ZCu2+(OH)- reduction with the inactivity of Z2Cu2+ in the formation of Cu2+ pairs. Accordingly, we highlight that NH3-assisted hydrolysis plays a critical role in LT-SCR and should be taken into account especially when discussing SCR reaction details over Z2Cu2+.