Mechanistic Understanding of Cu-CHA Catalyst as Sensor for Direct NH-SCR Monitoring: The Role of Cu Mobility.

The concept to utilize a catalyst directly as a sensor is fundamentally and technically attractive for a number of catalytic applications, in particular, for the catalytic abatement of automotive emission. Here, we explore the potential of microporous copper-exchanged chabazite (Cu-CHA, including Cu-SSZ-13 and Cu-SAPO-34) zeolite catalysts, which are used commercially in the selective catalytic reduction of automotive nitrogen oxide emission by NH (NH-SCR), as impedance sensor elements to monitor directly the NH-SCR process. The NH-SCR sensing behavior of commercial Cu-SSZ-13 and Cu-SAPO-34 catalysts at typical reaction temperatures (i.e., 200 and 350 °C) was evaluated according to the change of ionic conductivity and was mechanistically investigated by complex impedance-based in situ modulus spectroscopy. Short-range (local) movement of Cu ions within the zeolite structure was found to determine largely the NH-SCR sensing behavior of both catalysts. Formation of NH-solvated, highly mobile Cu species showed a predominant influence on the ionic conductivity of both catalysts and, consequently, hindered NH-SCR sensing at 200 °C. Density functional theory calculations over a model Cu-SAPO-34 system revealed that Cu reduction to Cu by coadsorbed NH and NO weakened significantly the coordination of the Cu site to the CHA framework, enabling high mobility of Cu species that influences substantially the NH-SCR sensing. The in situ spectroscopic and theoretical investigations not only unveil the mechanisms of Cu-CHA catalyst as sensor elements for direct NH-SCR monitoring but also allow us to get insights into the speciation of active Cu sites in NH-SCR under different reaction conditions with varied temperatures and gas compositions.