Sensing catalytic conversion: Simultaneous DRIFT and impedance spectroscopy for in situ monitoring of NH3–SCR on zeolites

Abstract In order to meet the legislative emission requirements for NOx-containing exhaust gases, SCR catalysts, in particular zeolites, are used. To improve catalysts and the catalytic processes, an in-depth understanding of the reaction mechanisms is required as well as an analysis of the physicochemical properties of the SCR catalysts, preferably in real-time. Here, we introduce a setup combining impedance spectroscopy and infrared spectroscopy in diffuse reflection mode for in situ measurements on zeolites under SCR-related conditions. This setup allows for the first time to simultaneously monitor both, the proton conductivity of zeolites and the vibration modes of the molecules involved in the catalytic conversion of NO by NH3. We studied both, pure and Fe-promoted H-form zeolites, as sensors and model catalysts at the same time, and found out that weakly bound NH3 is dominating the proton conductivity of both zeolites in a temperature range below the desorption temperature of NH3. When a part of the weakly bound NH3 is consumed by the SCR reaction, proton conductivity and thus the sensing effect gets dominated by strongly bound NH3. This allows applying impedance spectroscopy to assess the degree of NH3 loading and the state of the SCR conversion process in zeolite catalyst.