Laser Raman Spectroscopic Investigation on the Conversion of Molten Urea into NH3 for the Selective Catalytic Reduction of NO x in Oxygen-Rich Exhaust Gases

The selective catalytic reduction (SCR) is widely used for the NOx removal from the oxygen-rich exhaust gases of diesel passenger cars and trucks. SCR implies the reduction of NOx by NH3 originated from AdBlue, which is an aqueous solution of urea (32.5 wt.%) stored on-board of the vehicles. However, urea is not completely converted at low exhaust temperatures potentially resulting in by-products and deposits influencing the SCR performance. In this context, the present paper deals with the conversion of molten urea remaining from AdBlue evaporation. A novel reaction chamber equipped with laser Raman spectroscopy (LRS) for quantitative in situ analysis was constructed to examine the conversion of liquid urea as well as formation of side-products. Validation of the analytical set-up was made by ex situ tools including elemental analysis and infrared spectroscopy. The in situ LRS studies performed in diesel model exhaust between 150 and 170 °C evidenced accelerating conversion of the molten urea with increasing temperature, while substantial production of biuret occurred. Additionally, the influence of SCR catalysts such as V2O5/WO3/TiO2, TiO2, and Fe/BEA zeolite on the urea decomposition and by-product formation was evaluated. The in situ LRS investigations showed enhanced NH3 selectivity particularly for TiO2 associated with its pronounced hydrolysis activity in urea conversion. However, estimation of chemical and diffusion kinetics suggested that both hydrolysis and biuret production were strongly affected by mass transport of NH3 and H2O thus limiting the effect of the SCR catalysts upon the reaction of urea.