Hydrolysis of Isocyanic Acid on SCR Catalysts

Abstract

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).

Formation of HNCO, HCN, and NH3 from the pyrolysis of bark and nitrogen-containing model compounds

“…In fact, before N 2 O was recognized as a problem, urea was injected into hot combustion gases in order to reduce NO emissions (the SNCR process). However, this practice led to high emissions of N 2 O [4]; this is probably because HNCO is formed by the pyrolysis of urea [5].…”

“…The linear amides (Fig. 2) urea [5], nylon 6,6 [19], and the proteins poly-L-leucine and poly-L-proline [6] all form HNCO under pyrolysis. Urea lacks the functional group -CHR-NH-, which nylon 6,6, as well as poly-L-leucine and poly-L-proline, has.…”

“…Consequently, urea does not form HCN as the other linear amides do. All of the above-mentioned linear amides form ammonia under pyrolysis [5,6,19]. Urea has primary amine groups, nylon 6,6 and poly-L-leucine mainly contain secondary amine groups (Fig.…”

Concept of Vaporized Urea Dosing in Selective Catalytic Reduction

“…The mixer is placed between the urea injector and the SCR brick. It is made of fixed-geometry metal blades to help urea droplets break up [23][24][25]. The demanded value of urea-mass injected was realized in either open or closed-loop transfer functions.…”

Urea-SCR Temperature Investigation for NOx Control of Diesel Engine

“…In fact, merely 20% or less of the urea decomposes to HNCO and NH3 in the gas phase at 330 ºC, and only about 50% decomposes at 400 ºC [2,3]. HNCO has been shown to be very stable in the gas phase, requiring an oxide surface to catalyze its decomposition to NH3 [4]. Therefore, only about 25% of the desired NH3 is available at the catalyst inlet at 400 ºC.…”