Use of a µ-Scale Synthetic Gas Bench for Direct Comparison of Urea-SCR and NH3-SCR Reactions over an Oxide Based Powdered Catalyst

Abstract

Abstract:The selective catalytic reduction (SCR) of NOx by NH3 has been extensively studied in the literature, mainly because of its high potential to remediate the pollution of diesel exhaust gases. The implementation of the NH3-SCR process into passenger cars requires the use of an ammonia precursor, provided by a urea aqueous solution in the conventional process. Although the thermal decomposition and hydrolysis mechanisms of urea are well documented in the literature, the influence of the direct use of urea on the NOx reduction over SCR catalysts may be problematic. With the aim to evaluate prototype powdered catalysts, a specific synthetic gas bench adjusted to powdered material was developed, allowing the use of NH3 or urea as reductant for direct comparison. The design of the experimental setup allows vaporization of liquid urea at 200 °C under 10 bar using an HPLC pump and a micro injector of 50 μm diameter. This work presents the experimental setup of the catalytic test and some remarkable catalytic results towards further development of new catalytic formulations specifically dedicated to urea-SCR. Indeed, a possible divergence in terms of DeNOx efficiency is evidenced depending on the nature of the reductant, NH3 or urea solution. Particularly, the evaluated catalyst may not allow an optimal NOx conversion because of a lack in ammonia availability when the urea residence time is shortened. This is attributed to insufficient activity of isocyanic acid (HNCO) hydrolysis, OPEN ACCESSCatalysts 2015, 5 1536 which can be improved by addition upstream of an active solid for the hydrolysis reaction such as ZrO2. Thus, this µ-scale synthetic gas bench adjusted to powdered materials enables the specific behaviour of urea use for NOx reduction to be demonstrated.

Study of the remarkable reactivity of HNCO/urea with NO2 in the NOx SCR by urea process over an oxide-based catalyst

“…Note also that without catalyst, no evolution of the gas composition was recorded during the SCR experiments, whatever the studied temperature. 3 The studied catalyst, denoted aZr, was provided by Solvay. This material was developed for the urea-SCR process for vehicles equipped with a Diesel particulate filter (DPF).…”

“…Supplementary tests were also performed with the addition of 150 mg ZrO2 (Solvay) upstream the SCR catalyst. ZrO2 addition was demonstrated to allow the recovery of the activity in standard SCR condition (NO2/NOx = 0), 3 due to its catalytic activity in HNCO hydrolysis. 10…”

“…In order to evaluate powdered catalysts using the "real" reaction mixture, an innovative laboratory bench adapted to powdered catalysts (100 mg) was developed, allowing the use of NH3 or urea (aqueous solution) as NOx reductant specis. 3 The urea residence time between the injection zone and the catalytic bed is a key parameter of the deNOx efficiency. For short urea residence time, a significant decrease of the NOx reduction efficiency was observed over a prototype SCR catalyst provided by Solvay, when switching from gaseous NH3 to urea aqueous solution, especially in "standard-SCR" condition.…”

“…They showed that the decrease in NOx reduction with urea can be attributed to a lack in NH3 availability due to limitations in HNCO hydrolysis. 3 Then, it was assumed that HNCO is largely present at the catalyst surface for the shorter urea residence time, even if traces of urea cannot be excluded.…”

Automotive Emission Control Catalysts

“…This arrangement, however, results in additional complexities in designing catalytic systems to ensure efficient urea decomposition and hydrolysis into NH 3 molecules. Based on the results obtained from well-controlled micro-scale reactor experiments, Seneque et al describe potential limitations of urea as an NH 3 carrier when urea residence time is not sufficient [8]. They further show that the observed lack of NH 3 availability can be attributed to the insufficient SCR catalyst activity for isocyanic acid (HNCO: urea decomposition product) hydrolysis, which can be complemented by adding an active hydrolysis catalyst (e.g., ZrO 2 ) upstream of the SCR catalyst.…”