Absorption-spectroscopic sensor system for spatially and temporally resolved analyses of urea decomposition inside SCR catalysts

The state-of-the-art approach to experimentally characterize the gas phase composition inside structured catalysts is to extract gas using capillary probes and analyze it ex situ . However, the limited selectivity and temporal resolution of existing measurement systems hinder the modeling and optimization of selective catalytic reduction (SCR) systems. This study proposes an extractive measurement system, which uses a capillary to sample gas at defined positions inside the catalyst-coated honeycomb monolith, and employs a highly selective tunable diode laser absorption spectroscopy (TDLAS) spectrometer to analyze it. By minimizing the internal volume of the spectrometer and positioning it on the low-pressure side of the pump employed for gas sampling, H 2 O, NH 3 , and CO 2 mole fraction measurements with a temporal resolution below 1 s are achieved. In proof-of-concept measurements, the sensor system is employed to investigate the decomposition of urea-water solution (UWS) in a generic SCR test bench. It is shown that it yields reproducible results and provides new insights into the dynamic effects governing urea thermolysis and subsequent hydrolysis of isocyanic acid.