Hydrolysis of Isocyanic Acid on SCR Catalysts

The hydrolysis of isocyanic acid (HNCO) was investigated on various oxidic catalysts, including TiO 2, V 2 O 5 /TiO 2, and V 2 O 5 −WO 3 /TiO 2 . The kinetics were studied using powdered samples in a microreactor. The conversions of HNCO are high even at the high space velocities (10 6 h -1 ) and low temperatures (150 °C) used in the tests. The highest rate for the hydrolysis of HNCO was found on pure TiO 2 powder, while the addition of vanadia and tungsta decreased the activity slightly. To compare the rate of HNCO hydrolysis with the rate of the SCR reaction, uncorrected first-order rate constants were calculated for both reactions using the integral equation for a plug flow reactor. For a catalyst sample containing both WO 3 and V 2 O 5, the rate constant for hydrolysis of HNCO at low temperatures is about 2 orders of magnitude higher than the respective rate constant for the SCR reaction, whereas at high temperatures, the rate constants are of comparable magnitude. The apparent activation energy of the hydrolysis reaction amounts to ∼13 kJ/mol, thus suggesting that the overall reaction rate is mainly limited by external and internal mass transfer. Experiments using a coated monolithic SCR catalyst containing V 2 O 5 −WO 3 /TiO 2 showed that the conversion of HNCO is not complete at the high space velocities necessary for mobile applications. Additionally, experiments were performed on a diesel engine test stand using a coated monolithic SCR catalyst and various reducing agents. The use of urea strongly affected the NO x conversion, especially at the highest catalyst temperature of 455 °C, leading to a substantially increased slip of (NH 3 + HNCO).