Catalytic Performance of Zeolites on Urea Thermolysis and Isocyanic

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Catalytic Performance of Zeolites on Urea Thermolysis and Isocyanic Acid Hydrolysis Weijuan Yang,* Zhenchao Chen, Junhu Zhou, Zhenyu Huang, and Kefa Cen State Key Laboratory of Clean Energy Utilization, Institute for Thermal Power Engineering, Zhejiang University, Hangzhou 310027, Zhejiang, People's Republic of China ABSTRACT: Urea thermolysis and isocyanic acid hydrolysis over three kinds of zeolites are investigated experimentally and the main decomposition products, NH3 and HNCO, are focused on. The results gained using thermogravimetric analysis with heating rates of 2 °C/min from 50
500 °C show NH3 releases mainly in 133
250 °C and the main thermal product above 250 °C is HNCO. NH3 release process appears double-peaked while HNCO triple-peaked. Zeolites shift urea decomposition to lower temperature and shorten the process. The experiments of urea thermolysis over zeolites were done in the fixed reactor at programmed temperature and constant temperature. Although zeolites enhance the production of both NH3 and HNCO, the yield ratio of NH3 increases to 1.1 from 0.9 when adding zeolites but the yield ratio of HNCO is always below 0.8. Under zeolites’s catalytic effect, the peak of NH3 release and the second peak of HNCO release become stronger. Moreover, zeolites can result in the HNCO peaks integrating into a stronger peak at 500 °C. The total yield of NH3 and HNCO increases about 0.1
0.2 with zeolites and the catalytic effect is more obvious at low temperature. In the experiment of thermolysis with a urea-water spray, over 96% urea could decompose to NH3 and HNCO when the temperature is over 550 °C and the residence time is more than 1.0 s. Zeolites show good catalytic performance on HNCO hydrolysis to NH3 and the conversion of HNCO to NH3 increases with increasing temperature and reaches above 80% at 250 °C and can touch 100%. The catalytic effect on urea thermolysis and HNCO hydrolysis decreases in the order H
Y > H-β > H-ZSM5, which might be due to the amount of acidic sites on the catalysts. The apparent activation energy of the hydrolysis reaction is so low that the overall hydrolysis reaction rate on catalysts is mainly determined by external and internal mass-transfer limitations.

1. INTRODUCTION In recent decades, many researchers have been engaged in studying urea thermolysis since urea is widely used in flue gas denitrification as a reducing agent, especially in automobile diesel applications.1
3 In contrast to ammonia, urea is an innocuous raw material and favorable, as it is a nonvolatile source of ammonia. No severe transportation, handling, or storage is required. It is generally accepted that urea decomposes to equimolar amounts of ammonia (NH3) and isocyanic acid (HNCO) which are real reagents reacting with NO.4,5 NH2 CONH2 f NH3 þ HNCO

ð1Þ

Isocyanic acid formed from reaction 1 is quite stable in the gas phase but easily reacts with gas phase water on the surface of special metal oxides such as Selective Catalytic Reduction (SCR) catalysts described in reaction 2.6,7 HNCO þ H2 O f NH3 þ CO2 8
11

ð2Þ

the byproducts of urea According to previous studies, pyrolysis include some high molecular compounds, such as cyanuric acid (CYA), ammelide, ammeline, and melamine. These polymeric complexes do not decompose completely unless the temperature exceeds 320 °C. They are believed to be one of the potential causes for the stoichiometric imbalance of urea consumption. If they deposit on the catalysts, then the catalytic performance will be deactivated. It is clear from the literature that most SCR experiments at a lab-scale have been focused on NH3
SCR, but little is known r 2011 American Chemical Society

about the activity of the catalyst for the preceding hydrolysis of HNCO in the Urea-SCR process. Yim et al.12 conducted flow reactor studies of urea decomposition and found urea completely decomposed into NH3 and HNCO at 350 °C with residence time 0.1s. But the aluminum tube reactor could catalyze the hydrolysis of HNCO which was in favor of the urea decomposition. Fang et al.9 researched the urea thermolysis with and without vanadia-based SCR catalysts. The results implied that the major function of the catalyst was to accelerate the urea decomposition to the final products by eliminating the ammonia consumption process. Metal-exchanged zeolites showed high activities and selectivities toward urea-SCR and were extensively investigated as potential alterative to vanadia-based SCR catalysts.13
15 Furthermore, most of the HNCO hydrolysis experiments were conducted on anatase but not zeolites. Hauck et al.16,17 investigated the catalytic HNCO hydrolysis on anatase and found that dissociative adsorption of HNCO on TiO2 forms Ti-NCO and hydrogen bonded OH species. Piazzesi et al.18,19 found that the present NH3 and NO2 had a negative effect on the hydrolysis owing to the competitive adsorption on the catalyst. Czekaj et al.20,21 summarized the hydrolysis reaction pathway as follows: water attacks the
NCO group f formation of

Received: April 4, 2011 Accepted: May 27, 2011 Revised: May 24, 2011 Published: May 27, 2011 7990

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Table 1. Properties of Zeolites materials SBET (m2 3 g
1) pore volume (cm3 3 g
1)

characteristic ratio

H-ZSM5

277

0.19

SiO2/AL2O3 = 16.14

H
Y

330.1

0.21

SiO2/AL2O3 = 7.68

H-β

374

0.29

SiO2/AL2O3 = 15.53

carbamic acid f formation of a carbamate complex f CO2 desorption f formation of NH3. There is little information available in the literature about kinetic data of urea decomposition and HNCO hydrolysis over zeolites. In the present work, the decomposition of urea over zeolites was investigated and the main decomposition products, NH3 and HNCO, were focused on since they were the real reagents in the urea-SNCR or urea-SCR application. First, the thermal decomposition of urea was studied by means of simultaneous thermogravimetric (TG), differential scanning calorimeter (DSC), and online mass spectrometry (MS) evolved gas analysis. The pyrolysis of urea into NH3 and HNCO over a fixed reactor was examined versus various parameters including reaction temperature, heating rate and reactor residence time, and combined HNCO hydrolysis over zeolites bed was investigated. The investigated zeolites were H-ZMS5, H
Y, and H-β, as shown in Table 1. The Brunauer
Emmett
Teller (BET) surface area and pore volume were measured by nitrogen adsorption porosimetry (Autosorb-1C, Quantachrome). The ratios of SiO2: Al2O3 were measured by X Ray Fluorescence (XRF, Rigaku). Figure 1. Schematic of the experimental apparatus.

2. EXPERIMENTAL METHODS 2.1. Materials. Urea with purity g99% used in this work was

obtained from SCRC (Sinopharm Chemical Reagent Co., Ltd.) and zeolites were provided by NKUC (Nankai University catalyst Co., Ltd., Tianjin China). Zeolites were calcined in argon at 600 °C for 6 h before using. The mixtures of zeolites and urea were prepared by impregnating 1 g zeolite powder with 3.08 g 32.5 wt % urea water solution to get a urea to zeolite ratio of 1:1, and then the mixture was dehydrated in an oven at 40 °C for 72 h. 2.2. Methods. Simultaneous TG/DSC-MS experiments were performed using a Netzsch Model STA 449F3 Jupiter instrument with a silicon carbide (SiC) furnace connected to an Aeolos quadrupole mass spectrometer QMS403C (0
300 a.m.u) by a special coupling system. The stainless steel capillary transfer line was kept at 300 °C to avoid condensation. A series of experiments were performed using 10 mg sample powder. The heating rate was 2 °C/min and the flow rate of 99.9999% argon gas was set to 80 mL/min. The MS data were acquired by using the single ion mode (SIM) to monitor the mass/charge ratios 17(NH3) and 43(HNCO) with highest sensitivity. Ammonia temperature programmed desorption (NH3-TPD) experiment was carried out with 100 mg of sample in an AutoChem II 2920 analyzer of Micromeritics. Adsorption of NH3 was performed at 100 °C until saturation. Before saturation, the sample was treated in situ at 500 °C for 1 h in flowing He to remove water vapor and impurities. After adsorption weakly bound species were desorbed by purging the sample with He. TPD was completed by increasing the temperature up to 600 °C at a rate of 10 °C/min. The decomposition of dry urea was studied at temperature programmed and constant temperature in the fixed bed reactor with the apparatus as shown in Figure1(a). The flow rate of N2

was controlled by the MFC. The electric furnace provided a required temperature and the reactor with a sample cup was put in it. The reactors and sample cup were made of quartz to minimize any catalytic surface reactions. The sample cup which was filled with small granular pellets of samples had a height of 8 mm and outer diameter of 6 mm. In the temperature programmed tests, two different heating rates were used, 10 and 15 °C/min, respectively, with a temperature range of 25
500 °C. About 10 mg urea was used in each test and an N2 flow of 100 mL/min was used as a sweep gas. At the exit of the furnace, the sweep gas flow was diluted by a flow of 400 mL/min N2 to achieve reasonable retention time for FTIR gas phase analysis. For the constant temperature, 400 and 500 °C were adapted with 5 mg urea loading. A flow of 900 mL/min diluted gas was used to lower the concentrations of NH3 and HNCO. The urea water solution pyrolysis combined with HNCO hydrolysis was conducted with a total mass flow of 680 mL/min in the apparatus as shown in Figure 1(b). Two furnaces and two reactors were designed, one for urea solution pyrolysis and the other for HNCO catalytic hydrolysis. A desired amount of urea solution was continuously fed into the horizontal reactor using a homemade nozzle connected to a syringe pump and the flow rate delivered was dependent on both the diameter of the syringe used and the rotational rate of the leadscrew. The residence time of the gas in the reactor was a function of the sweep gas flow rate, the temperature, and the pressure. The pressure effect was neglected due to the low flow rate. The residence time was estimated to be between 1.03s and 2.77s in ideal conditions. The feed rate of 2 wt % urea solution was 0.05 mL/min to keep the volume fraction of urea in diluted gas 500 ppm. About 0.1 g of zeolites with 0.16 mL volume was filled in the fixed-bed vertical reactor at 150
400 °C, and the particle size of the catalysts was 7991

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250 μm. The Hourly Space Velocity (GHSV) amounted to 2.55  105 h
1 and was guaranteed by the flow rate of dilution gas. At the final exit of the reactor, the gases were analyzed with a frustrated total internal reflection (FTIR) spectrometer (Gasmet DX4000, Calcmet software) equipped with a heated multiple pass gas cell (path length = 5 m, volume = 400 mL, and T = 180 °C). The instrument was calibrated for the analyses of NH3, H2O, and HNCO with 2% precision.

3. RESULTS AND DISCUSSION 3.1. TGA/DSC-MS of Dry Urea. The TGA/DSC-MS profiles of pure urea pyrolysis as a function of temperature at the heating rate of 2 °C/min are shown in Figure 2. The thermolysis process can be subdivided into four stages based on the TGA profile. The first stage is a weight loss for drying the sample ( H-β > H-ZSM5, which might be due to the amount of acidic site of the catalysts. 3.2. Dry Urea Pyrolysis. Figure 4 shows the NH3 and HNCO concentration in the thermolysis of pure urea and urea mixed with zeolites in the temperature programmed process. Some peaks of NH3 and HNCO from urea mixed with zeolites shift toward higher temperature compared to pure urea. This is due to different conditions of heat transfer and the sample. The heat transfer gets worse while the mixtures have greater volume and weight. Thus, it might take more time for the mixtures to reach the same temperature compared to pure urea. Also, the adsorption of gaseous reaction products on the catalyst might shift the

Figure 2. TGA/DSC-MS results of neat urea in dry Ar (initial weight:10 mg; heating rate: 2 °C/min; flow rate 80 mL/min). 7992

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Figure 3. TGA/MS results of pure urea and urea mixed with zeolites in dry Ar (initial weight:10 mg; heating rate: 2 °C/min; flow rate 80 mL/min).

curve to higher temperatures. The second peak of evolved NH3 disappears due to the effect of zeolites. Both curves of evolved NH3 and HNCO show higher peaking with a fast heating rate. But the residence time of gaseous products reduce, which is beneficial to prevent the formation of intermediates. HNCO release is accelerated by zeolites and the release time is significantly shortened. It seems that the releases of HNCO are divided into two parts. The first peak comes from the urea thermolysis and the second mainly comes from the decomposition of polymeric complexes. The second peak becomes higher and shifts toward low temperature due to the catalytic effect of zeolites. Table 2 summarizes the amount of yield NH3 and HNCO from the gas phase. The amount of NH3 and HNCO is obtained by the integration of the evolved gas curve versus total gas flow

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rate. According to reaction 1, when urea completely decomposes to equimolar amounts of NH3 and HNCO, the yield ratio of NH3 or HNCO equals 1.0, respectively, and the total is 2.0. It is implied that urea decomposed incompletely since the ratios sum is less than 2.0 in Table 2 and the HNCO yield is far from the ideal value. The NH3 ratio over 1.0 means that some HNCO hydrolyzes to NH3 with zeolite catalyst. Any kind of zeolite, especially H
Y, shows a promoted effect on both NH3 and HNCO production, and the addition of zeolites has positive effect on urea thermolysis. At the outlet of the horizontal reactor, a little powder with a pale color is found deposited on the reactor surface. The powder might be smuggled by the hot sweep gas, which might be the reason why the yield ratio of HNCO is lower than the data obtained by Lundstr€om et al.,23 who performed urea thermolysis under flow reactor conditions using DSC and FT-IR. The NH3 and HNCO concentrations from urea decomposition at constant temperature are shown in Figure 5.When the reactor reached the desired temperature, a cup with a sample was quickly put into the reactor. Then the test began and lasted for 10 min. The mass balance for NH3 and HNCO are presented in Table 3. The shoulders of the curve become narrow and the peak reaches a high value due to the high temperature. Zeolites make the two peaks of HNCO unite at 500 °C. The union of HNCO peaks implies that the two stages of HNCO release, from urea decomposition and from polymeric complexes decomposition, are connected and integrated. The yield of HNCO increases when temperature increases to 500 °C. The high temperature and heat flux are in favor of the decomposition of CYA, ammelide, and ammeline. For the pure urea, the yield of NH3 changes very little. Just as in the results from TGA experiments, the evolved NH3 mainly comes from the second stage (urea decomposition). NH3 will be completely released under the temperature of 400 °C. Zeolites improve urea pyrolysis and the ratios sum increased about 0.1
0.2. The total of ratios shows that over 90% of urea decomposes completely to NH3 and HNCO at 500 °C, which are effective in the process of SCR or SNCR. 3.3. Urea-Water Solution Pyrolysis. Figure 6 shows the results of urea solution pyrolysis tests whose conditions are listed in Table 4. At 550 and 600 °C, virtually complete conversion was observed independently of the applied gas flow rate. Below 550 °C, urea decomposition was incomplete and the conversion increased with decreasing gas flow rate. Obviously, urea thermolysis is faster at higher temperature. In addition to the rate of the chemical reaction, the evaporation rate of water from the aerosols is also increased. Given that urea thermolysis only starts after the evaporation of water from the aerosols, the effective residence time of urea, which is available for the thermolysis reaction, is also increased at higher temperature. The total yield ratio of NH3 and HNCO increases from 1.02 to 1.41 at 450 °C, while the residence time increases from 1.25 to 2.77s. At high temperatures over 550 °C, the urea solution decomposes completely within 1 s and the residence time impacts the results very little. The ratio of NH3 to HNCO increases from approximately 1 to 1.3 with increasing temperature. The result of the ratio is consistent with the ratio obtained by Aoki et al.,24 which is 1.1
1.5. Gentemann and Caton25 had found little HNCO converted to NH3 in a quartz tube even at the high temperature and long residence time. The HNCO hydrolysis experiments were conducted. The horizontal reactor was set at 550 °C and residence time was controlled 2.43s to ensure the complete pyrolysis of urea solution. The pyrolysis products plus dilution gas directly flowed into the 7993

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Figure 4. NH3 and HNCO production from samples under a 100 mL/min N2 sweep flow and 400 mL/min N2 dilution flow with a heating rate of 10 °C/min and 15 °C/min.

vertical reactor packed with zeolites. The gases at the inlet of the vertical reactor contained 550 ppm NH3 and 450 ppm HNCO. As Figure 7 shows, the catalysts have great impact on the HNCO hydrolysis. The conversion of HNCO is defined as follows:

Table 2. Total Yield Ratios of NH3 and HNCO at Temperature Programmed sample load (mg)

X HNCO ¼ ðHNCOin
HNCOout Þ=HNCOin  100% ð3Þ where XHNCO is the HNCO conversion, HNCOin is the volume fraction of HNCO at the entrance of the vertical reactor while HNCOout is that for the outlet of the reactor. Among the three types of zeolites, H
Y zeolite has the highest activity, which is in accordance with the TGA results and it gains higher HNCO conversion, especially below 250 °C. Piazzesi et al.26 studied the isocyanic acid hydrolysis over Fe-ZMS5 and found the hydrolysis activity was related to the Lewis acidity of the catalyst. Chen et al.27 investigated the hydrolysis of HNCO over γ-Al2O3 and found that the addition of CuO lowers the catalytic activity and acid sites. Roughly, the catalytic activity correlates with the acidity of the zeolites. The amount of adsorbed NH3 can be regarded as an indicator for the acidic strength of the catalysts. It can be noticed from Figure 8 that for H-β and H-ZSM5 catalysts two peaks can be observed, which show a maximum in desorption rate around 190 and 380 °C, while one peak for H
Y around 190 °C. As the desorption temperature is associated with the strength of the acid sites where NH3 is adsorbed, the weak acid sites decrease in the similar order

urea

10 °C/min, yield

15 °C/min, yield

(mol/mol urea)

(mol/mol urea) load (mg) NH HNCO total NH HNCO total 3 3

pure urea

10

10

0.874

0.583

1.457 0.951

0.587

1.538

urea/

20

9.7

1.133

0.608

1.741 1.149

0.632

1.781

20 20

9.8 9.5

1.165 1.156

0.708 0.68

1.873 1.138 1.836 1.167

0.719 0.676

1.857 1.843

H-ZSM5 urea/H
Y urea/H-β

H
Y > H-β > H-ZSM5 with the catalytic activity. Since the activity of H
Y was higher than those of the catalysts, it could be deduced that the catalyst with more acid on the weak acid center was favorable for the hydrolysis of HNCO. The stability of isocyanate (-NCO) groups, which comes from the dissociation of HNCO on the catalysts, decreases with increasing temperature, and they are very easily removed from the surface above 250 °C. Moreover, the hydrolysis of the
NCO species with water to ammonia is accelerated by increasing temperature. This is why the HNCO conversion in Figure 7 increases sharply below 250 °C and very little above 300 °C. Also the oxidation of NH3 by 7994

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Figure 5. NH3 and HNCO production from samples under a 100 mL/min N2 sweep flow and 900 mL/min N2 dilution flow at 400 and 500 °C.

Table 3. Total Yield Amounts of NH3 and HNCO at Constant Temperature sample load (mg)

urea

400 °C, yield ratio

500 °C, yield ratio

(mol/mol urea)

(mol/mol urea) load (mg) NH HNCO total NH HNCO total 3 3

neat urea

5

5

1.594 0.981

0.809 1.79

urea/

10

4.7 1.061 0.606

0.935 0.659

1.667 1.08

0.792 1.872

10 10

4.8 1.101 0.694 4.6 1.105 0.698

1.795 1.142 1.803 1.127

0.769 1.911 0.756 1.883

H-ZSM5 urea/H
Y urea/H-β

oxygen gas is investigated. In the presence of O2 in the dilution gas stream (10% O2), the NH3 concentration is not decreased and the oxidation of NH3 to N2 or NO over zeolites catalyst does not happen, although Yim et al. found that the NH3 oxidation happened over Cu-ZSM5 above 250 °C. It is supposed that the oxidation would be related to the Cu addition. Although the water was present in additional excess in all of the experiments, the Pseudofirst order rate constants of the HNCO hydrolysis were calculated according to the rate equating to reactions under differential conditions in a plug flow reactor: k¼


V lnð1
XHNCO Þ½cm3 =g 3 s W

ð4Þ

Figure 6. NH3 and HNCO production from urea solution.

Where V is the total flow rate at actual temperature and pressure (the effect of pressure is neglected), and W is the catalyst weight. The Pseudofirst order rate constants obtained in this way for the HNCO hydrolysis are shown in Figure 9. The different temperature dependence of the two rate constants could be clearly seen. When the temperature was over 250 °C, the apparent activation energy of HNCO hydrolysis was approximately 18.4 kJ/mol for H
Y, 19.9 kJ/mol for H-β, while 26.1 kJ/mol 7995

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Table 4. Experimental Condition for Urea Solution Pyrolysis flow gas (mL/min)

residence time and total yield ratio (mol/mol urea) 450 °C

500 °C

550 °C

600 °C

sweep

dilution

time (s)

yield

time (s)

yield

time (s)

yield

time (s)

yield

a

400

280

1.25

1.02

1.16

1.55

1.09

1.98

1.03

2.03

b

280

400

1.78

1.11

1.66

1.64

1.56

1.94

1.47

2.06

c

180

500

2.77

1.41

2.59

1.79

2.43

1.99

2.29

2.05

Figure 7. The conversion of HNCO (0.1 g catalyst; inlet flow composed 450 ppm HNCO, 550 ppm NH3, 8% H2O, and N2 balance; flow rate = 680 mL/min).

Figure 8. NH3-TPD profile (100 mg of catalyst; purging gas: He; flow rate = 0.05 L/min; heating ramp = 10 °C/min).

for H-ZSM5. The activation energy was so low that it suggested that the HNCO hydrolysis reaction was controlled by external diffusion. However, the energy of activation increased to 31.8 kJ/mol for H
Y, 32.4 kJ/mol for H-β while 37.3 kJ/mol for H-ZSM5 in the low temperature region (150
250 °C). It was assumed that the hydrolysis reaction was mainly controlled by internal mass transfer within the catalyst pores.

Figure 9. Temperature dependence of the rate constants for HNCO hydrolysis.

4. CONCLUSIONS The thermolysis and hydrolysis of urea over zeolites (H
Y, H-ZSM5, H-β) at 50
500 °C was experimentally investigated. TGA/DSC and online MS evolved gas analysis indicate that urea first decomposes to NH3 and HNCO in the process of pure urea thermolysis and the process can be subdivided into four stages. The primary NH3 comes from the first stage of 133
250 °C and the main thermal product above 250 °C is HNCO. The addition of zeolites into urea shifts the urea thermolysis to lower temperature and shortens the process. Urea thermolysis over zeolites under fixed bed reactor conditions was researched using FT-IR. The yield ratio of NH3 remains above 0.9 and increases to 1.1 when adding zeolites but the yield ratio of HNCO is always below 0.8. Zeolites are in favor of accelerating the urea decomposition to NH3 and HNCO by eliminating the high molecular compounds. Under zeolites’s catalytic effect, the peak of NH3 release and the second peak of HNCO release become stronger. Moreover, zeolites can result in the HNCO peaks integrating into a stronger peak. The total yield of NH3 and HNCO increase about 0.1
0.2 with zeolites and the catalytic effect is more obvious at lower temperatures. The catalytic effect decreases in the order H
Y > H-β > H-ZSM5, and this might be due to the weak acid sites of the catalysts. It is gained that 95% of urea decomposes completely to NH3 and HNCO at 500 °C, which are effective in the process of SCR or SNCR. In the experiment of thermolysis with a urea-water spray, over 96% urea could decompose to NH3 and HNCO when the temperature is over 550 °C and the residence time is more than 1.0 s. The high yield of HNCO shows that the HNCO is 7996

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Industrial & Engineering Chemistry Research quite stable in the gas phase even in the humid gas and difficult to convert to NH3. In the presence of zeolites, the hydrolysis of HNCO occurs quickly. The conversion of HNCO to NH3 increases with increasing temperature and reaches above 80% at 250 °C. The H
Y zeolite shows a better catalytic performance due to the high amount of acid sites. The apparent activation energy of the hydrolysis reaction is so low that the overall hydrolysis reaction rate on catalysts is mainly determined by external and internal mass-transfer limitations. More experiment and analysis are expected to be carried out to investigate how the zeolites accelerate the thermolysis of urea and catalyze the HNCO hydrolysis.

’ AUTHOR INFORMATION Corresponding Author

*Phone: þ86-571-87952885; Fax: þ86-571-87951616; Email: [email protected].

’ REFERENCES (1) Koebel, M.; Elsener, M.; Madia, G. Reaction pathways in the selective catalytic reduction process with NO and NO2 at low temperatures. Ind. Eng. Chem. Res. 2001, 40 (1), 52–59. (2) Koebel, M.; Elsener, M.; Madia, G. Recent advances in the development of urea-SCR for automotive applications. SAE Technical Paper Series 2001, 2001
01–3625. (3) Sluder, C. S.; Storey, J. M. E.; Lewis, S. A.; Lewis, L. A. Low temperature urea decomposition and SCR performance. SAE Technical Paper Series 2005, 2005
01–1858. (4) Koebel, M.; Elsener, M.; Kleemann, M. Urea-SCR: a promising technique to reduce NOx emissions from automotive diesel engines. Catal. Today 2000, 59 (3
4), 335–345. (5) Alzueta, M. U.; Bilbao, R.; Millera, A.; Oliva, M.; Iba~ nez, J. C. Interactions between nitric oxide and urea under flow reactor conditions. Energy Fuels 1998, 12 (5), 1001–1007. (6) Koebel, M.; Strutz, E. O. Thermal and hydrolytic decomposition of urea for automotive selective catalytic reduction systems:thermochemical and practical aspects. Ind. Eng. Chem. Res. 2003, 42 (10), 2093–2100. (7) Kleemann, M.; Elsener, M.; Koebel, M.; Wokaun, A. Hydrolysis of isocyanic acid on SCR catalysts. Ind. Eng. Chem. Res. 2000, 39 (11), 4120–4126. (8) Schaber, P. M.; Colson, J.; Higgins, S.; Thielen, D.; Anspach, B.; Brauer, J. Thermal decomposition (pyrolysis) of urea in an open reaction vessel. Thermochim. Acta 2004, 424 (1
2), 131–142. (9) Fang, H. L.; DaCosta, H. F. M. Urea thermolysis and NOx reduction with and without SCR catalysts. Appl. Catal. B: Environ. 2003, 46 (1), 17–34. (10) Schaber, P. M.; Colson, J.; Higgins, S.; Dietz, E.; Thielen, D.; Anspach, B.; Brauer, J. Study of the urea thermal decomposition (pyrolysis) reaction and importance to cyanuric acid production. Am. Lab. 1999, 31, 13–21. (11) Stradella, L.; Argentero, M. A study of the thermal decomposition of urea, of related compounds and thiourea using DSC and TGEGA. Thermochim. Acta 1993, 219, 315–323. (12) Yim, S. D.; Kim, S. J.; Baik, J. H.; Nam, I.; Mok, Y. S.; Lee, J.-H.; Cho, B. K.; Oh, S. H. Decomposition of urea into NH3 for the SCR process. Ind. Eng. Chem. Res. 2004, 43 (16), 4856–4863. (13) Iwasaki, M.; Yamazaki, K.; Banno, K.; Shinjoh, H. Characterization of Fe/ZSM-5 DeNOx catalysts prepared by different methods: Relationships between active Fe sites and NH3-SCR performance. J. Catal. 2008, 260 (2), 205–216. € (14) Brandenberger, S.; KrOcher, O.; Tissler, A.; Althoff, R. The state of the art in selective catalytic reduction of NOx by ammonia using metal-exchanged zeolite catalysts. Catal. Rev.: Sci. Eng. 2008, 50 (4), 492–531.

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€ (15) Devadas, M.; KrOCher, O.; Elsener, M.; Wokaun, A.; Mitrikas, G.; S€oger, N.; Pfeifer, M.; Demel, Y.; Mussmann, L. Characterization and catalytic investigation of Fe-ZSM5 for urea-SCR. Catal. Today 2007, 119 (1
4), 137–144. (16) Hauck, P.; Jentys, A.; Lercher, J. A. Surface chemistry and kinetics of the hydrolysis of isocyanic acid on anatase. Appl. Catal. B: Environ. 2007, 70 (1
4), 91–99. (17) Hauck, P.; Jentys, A.; Lercher, J. A. On the quantitative aspects of hydrolysis of isocyanic acid on TiO2. Catal. Today 2007, 127 (1
4), 165–175. € (18) Piazzesi, G.; Elsener, M.; KrOCher, O.; Wokaun, A. Influence of NO2 on the hydrolysis of isocyanic acid over TiO2. Appl. Catal. B: Environ. 2006, 65 (3
4), 169–174. € (19) Piazzesi, G.; KrOCher, O.; Elsener, M.; Wokaun, A. Adsorption and hydrolysis of isocyanic acid on TiO2. Appl. Catal. B: Environ. 2006, 65 (1
2), 55–61. € (20) Czekaj, I.; KrOCher, O.; Piazzesi, G. DFT calculations, DRIFT spectroscopy and kinetic studies on the hydrolysis of isocyanic acid on the TiO2-anatase (101) surface. J. Mol. Catal A: Chem. 2008, 280 (1
2), 68–80. € (21) Czekaj, I.; Piazzesi, G.; KrOCher, O.; Wokaun, A. DFT modeling of the hydrolysis of isocyanic acid over the TiO2 anatase (101) surface: Adsorption of HNCO species. Surf. Sci. 2006, 600 (24), 5158–5167. (22) Eichelbaum, M.; Farrauto, R. J.; Castaldi, M. J. The impact of urea on the performance of metal exchanged zeolites for the selective catalytic reduction of NOxPart I. Pyrolysis and hydrolysis of urea over zeolite catalysts. Appl. Catal. B: Environ. 2010, 97, 90–97. (23) Lundstrom, A.; Andersson, B.; Olsson, L. Urea thermolysis studied under flow reactor conditions using DSC and FT-IR. Chem. Eng. J. 2009, 150 (2
3), 544–550. (24) Aoki, H.; Fujiwara, T.; Morozumi, Y.; Miura, T., Measurement of urea thermal decomposition reaction rate for NO selective noncatalytic reduction. In Fifth International Conference on Technologies and Combustion for a Clean Environment, Lisbon, Portugal, 1999; pp 115-118. (25) Gentemann, A. M. G.; Caton, J. A., Decomposition and Oxidation of Urea-water Solution as Used in Selective Non-catalytic Removal (SNCR) Processes. In 2nd Joint Meeting of the United States Section of the Combustion Institute Spring Technical Conference, Oakland, CA, 2001. € (26) Piazzesi, G.; Devadas, M.; KrOCher, O.; Elsener, M.; Wokaun, A. Isocyanic acid hydrolysis over Fe-ZSM5 in urea-SCR. Catal. Commun. 2006, 7 (8), 600–603. (27) Chen, Z.; Yang, W.; Zhou, J.; Lv, H.; Liu, J.; Cen, K. HNCO hydrolysis performance in urea-water solution thermohydrolysis process with and without catalysts. J. Zhejiang Univ. Sci. A 2010, 11 (11), 849–856.

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