Urea Decomposition and Implication for NO_x Reduction with Cu‐Zeolite and Vanadia‐Selective Catalytic Reduction

Abstract

Understanding urea decomposition is critical to achieve highly efficient selective catalytic reduction (SCR). The urea decomposition process in an exhaust pipe and in Cu‐zeolite and vanadia‐SCR (V‐SCR) was studied in engine test cells. The unconverted urea at the SCR inlet can be substantial at lower temperatures. HNCO and NH3 are two dominant products at the SCR inlet. Urea and HNCO continue to decompose in SCR catalysts, with a rate much faster than in the homogeneous stream. The HNCO hydrolysis process is extremely efficient in Cu‐zeolite SCR and the abundant NH3 from urea overdosing can improve the NOx conversion efficiency. While for V‐SCR, the HNCO hydrolysis reaction can become the rate‐limiting step (especially after aging), abundant urea at low temperatures impairs NOx reduction.

In Situ Pore-Making and Heteroatom Doping of Carbon Nanofibers Electrode for High Performance Vanadium Redox Flow Battery

“…2(a), a considerable weight loss (∼7 wt%) of the PAN-10% urea nanofibers around 150 ℃ in air was observed, which should be attributed to the first-step decomposition of urea, the decomposition products were NH 3 and isocyanic acid (HNCO). 38 The weight loss around 260 ℃ corresponded to a linear structure converting to a stable ladder-like structure of polyacrylonitrile. 39 The weight loss of PAN-10% urea nanofibers during the stabilization process was more than that of pristine PAN, indicating an acceleration effect of urea on the structure transformation.…”

“…After the stabilization process, almost all of the urea in the fibers was converted to HNCO, which would further decompose into NH 3 and CO 2 during the carbonization process. 38 The generated NH 3 and CO 2 were served as the nitrogen and oxygen source to realizing the hetero-atom doping of the fiber. Furthermore, CO 2 would etch the fiber surface at higher temperature to enrich the defects of the porous carbon nanofibers.…”

Urea Conversion for Low‐Temperature Selective Catalytic Reduction in a Swirled Diesel Exhaust Gas Configuration

“…Furthermore, Cu zeolite SCR catalysts were recently reported to reveal substantial urea conversion activity in heavy-duty diesel engine exhaust already above approx. 195 °C, whereas VWT is less active [12]. It was also indicated that between 150 °C and 170 °C the reaction of molten urea, existing in bare form or deposited on SCR catalysts, can be limited by mass transport of the water needed.…”

Effects of Catalyst Physicochemical Properties on the Catalytic Activity of Cu Zeolite Selective Catalytic Reduction at Different Inlet Gas Conditions

“…Through the combination of time-resolved X-ray absorption spectroscopy and transient experiments, an ammonia inhibition effect on the rate-limiting copper reoxidation at low temperatures was captured. Wu et al 38 found that the unconverted urea at the SCR inlet could be substantial at lower temperatures, which impaired NOx reduction. It was recommended to adopt different low-temperature dosing strategies for different SCR catalysts.…”