Investigation of the Hydrolysis of Isocyanic Acid in Urea Scr: Catalyst Screening and Low-Temperature Kinetics

INVESTIGATION OF THE HYDROLYSIS OF ISOCYANIC ACID IN UREA SCR: CATALYST SCREENING AND LOW-TEMPERATURE KINETICS

G. Piazzesi, M. Elsener, O. Kröcher, A. Wokaun

Several oxide catalysts were tested for the hydrolysis of isocyanic acid under typical diesel exhaust conditions. Among the tested catalysts, TiO 2 and ZrO 2 have been shown to be the most active catalysts. The kinetics of the HNCO hydrolysis over TiO 2 at low temperatures was studied and reaction orders and the activation energy were determined. The effect of NO2 on the catalytic activity of TiO 2 was also investigated, revealing that, at low temperatures, the formation of ammonium nitrate inhibits the reaction.

1 INTRODUCTION The addition of NO 2 to the inlet gas caused a consid- erable decrease of the TiO 2 catalytic activity at low The Selective Catalytic Reduction with harmless urea temperatures (< 200°C). It was proven that, at these (urea SCR) is currently the most attractive method to temperatures, NO 2 reacts with NH 3 to form solid am- reduce NO x emissions from heavy-duty diesel engines monium nitrate that deposits on the catalyst surface [1]. In this process urea is first thermally decomposed blocking the active sites. Since ammonium nitrate is to isocyanic acid (HNCO) by fast heating, then, HNCO not stable at temperatures far above 200°C, the cata- is catalytically hydrolyzed to NH 3: lytic activity was completely restored by thermal treat-

NH 2-CO-NH 2 → NH 3 + HNCO ment at 450°C in the absence of NO 2. These results are in good agreement with the findings of Koebel et HNCO + H 2O → NH 3 + CO 2 al. [4] for vanadia-based SCR catalyst. The aim of this work was to develop a hydrolysis catalyst for isocyanic acid, active under typical diesel ex- 100 haust conditions and to study the kinetics and mecha- 90 nism of this poorly studied reaction. Since NO 2 will be 80 used in most SCR systems for the enhancement of 70 the SCR reaction, its effect on the hydrolysis activity of 60 TiO 2 also needs to be studied. 50 yield [%] yield 3 40

2 EXPERIMENTAL NH 30 Isocyanic acid was produced by depolymerization of 20 cyanuric acid [2]. The activity tests were performed in 10 a fixed-bed glass micro-reactor and the reaction prod- 0 ucts were quantified by FTIR spectroscopy. 0 100 200 300 400 500 T [°C] Several powder catalysts were tested: Al 2O3 (Wacker), Al2O3 (BET = 104.5 m2/g) TiO 2 (Wacker), SiO 2 (Grace), SiO 2-Al 2O3 mixed oxides SiO2 (BET = 350 m2/g) (Grace), Fe-zeolite (Umicore AG) and ZrO 2 (calcina- 15% Al2O3 - 85% SiO2 (BET = 573 m2/g) tion of ZrO(OH) 2 from Wacker at 550°C). 24% Al2O3 - 76%SiO2 (BET = 552 m2/g) Fe-Zeolite (BET = 301.9 m2/g) The kinetic study over TiO 2 was carried out at 125 < T ZrO2 (BET = 67.4 m2/g) < 150°C. The bed with a mass of 150 mg was diluted TiO2 (BET = 57 m2/g) by inert SiC at a ratio TiO 2 : SiC = 1 : 2. Fig. 1: Ammonia yield as a function of temperature. The effect of NO 2 on the catalytic activity was studied Tests performed with 150 mg of powder catalysts, flow by combined adsorption and temperature pro- rate = 200 l N/h, inlet flow composition: ≈ 1000 ppm grammed desorption (TPD) measurements [3]. HNCO, 5% H 2O, 10% O 2 and N 2 balance.

3 RESULTS 4 ACKNOWLEDGEMENT

Among the catalysts tested, TiO 2 and ZrO 2 were found Financial support by the Wacker-Chemie GmbH is to be the most active for the hydrolysis of isocyanic gratefully acknowledged. acid (Figure 1). By means of a differential analysis, intrinsic rate data 5 REFERENCES were obtained for the HNCO hydrolysis over TiO 2 at [1] M. Koebel, M. Elsener, M. Kleemann, Catal. To- low temperatures and under typical diesel exhaust day 59 , 335 (2000). conditions. Careful experiments and calculations con- firmed that the reaction is running in the kinetically [2] M. Kleemann, M. Elsener, M. Koebel, A. Wokaun, controlled regime under the experimental conditions Ind. Eng. Chem. Res. 39 , 4120 (2000). applied. The reaction orders were 0.901 ± 0.047 with [3] J. Després, Ph.D. Thesis No. 15006 , ETH Zürich, respect to HNCO and 0.172 ± 0.046 with respect to (2003). H2O. The activation energy was calculated from tests at 100 < T < 200°C with 150 mg of TiO 2 ( EA ≈ 40 [4] M. Koebel, G. Madia, M. Elsener, Catal. Today kJ/mol). 73 , 239 (2002).