Low-Temperature Selective Catalytic Reduction of NOx with Urea

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Kinetics, Catalysis, and Reaction Engineering

Low-Temperature Selective Catalytic Reduction of NOx with Urea Supported on Carbon xerogels Di Yin, Jun Li, Jitong Wang, Licheng Ling, and Wenming Qiao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00748 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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Industrial & Engineering Chemistry Research

Low-Temperature Selective Catalytic Reduction of NOx with Urea Supported on Carbon xerogels Di Yin,a, † Jun Li,a, † Jitong Wang,a Licheng Ling,a Wenming Qiao*,a,b a

State Key Laboratory of Chemical Engineering, East China University of Science and

Technology, Shanghai, 200237, China b

Key Laboratory of Specially Functional Polymeric Materials and Related Technology (East

China University of Science and Technology), Ministry of Education, China *E-mail: [email protected] (Wenming Qiao). Tel.: +86-21-64253730.

Abstract

Low-temperature selective catalytic reduction (SCR) of NOx on urea-supported carbon xerogels is studied. The kinetics results show that the orders of reaction for NO and O2 are 1 and 0.5 for NO reduction, while the orders of reaction for NO2 and O2 are 1 and 0.03 for NO2 reduction, respectively. The apparent activation energies of NO and NO2 reduction are calculated to be -14.0 and 0.79 kJ/mol, respectively. Possible mechanisms are proposed, based on the hypothesis that there exist two kinds of active sites on the surface of carbon xerogel, including non-reductive and reductive ones. Spillover of the NO3 species generated on the non-reductive ones into the reductive ones which are subsequently oxidized by NO3, and the migration of NO3

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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species from non-reductive ones into the oxidized carbon sites are considered to be of great importance for the successive urea-SCR process and the formation of steady-state NOx removal period.

Keywords: NOx, selective catalytic reduction, urea/carbon catalyst, low temperature 1. Introduction NOx (primarily consists of NO and NO2) are the main contaminants in environment, resulting in acid precipitation, photochemical fog, ozone hole and even global warming1. A substantial amount of study has been consumed to acquire approaches for taking control of NOx emissions. Among these, the selective catalytic reduction (SCR) is extensively applied for clearing NOx away from the combustion operations, and the most important commercial catalyst for the SCR reaction by far is V2O5/TiO2 catalyst with NH3 as the reductant2,3, owning to its excellent activity, selectivity, toxin immunity, and catalyst working life. However, it still suffers from some unavoidable limitations, such as high temperatures required for the process and secondary pollution resulted from the release of unreacted ammonia4. Nowadays, an important environmental concern posed by the retention of NOx in the atmosphere, particularly in crowded traffic areas and confined indoor parking areas, has received much attention. Because it is denounced for imposing the respiratory disease and other serious health problems on the habitants living near such areas5. In spite of great success of the aforementioned V-based catalysts for removing NOx from stationary source, it seems to be not active enough when these catalysts are operated under ambient conditions. Moreover, NH3 is not a suitable reducing agent applied in such areas with dense population when considering its flammability and toxicity6. As


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a result, there is an urgent necessity to acquire more efficient and environmental friendly NOx elimination technologies that could proficiently operate at environmental condition. The currently available strategies for removal of NOx at room temperature mainly include the absorption of NOx in alkaline solutions7,8, non-thermal plasma (NTP) technologies9,10 and adsorption of NOx on porous materials11-14. However, there is still a challenging work for efficient and cost-effective removal of NO through the wet process, and NOx purified by NTP technologies has been proven to be quite energy intensive. Plenty of interest has been drawn by the adsorption of NOx on porous carbon materials because of their excellent removal capability and convenient availability without creating any secondary pollution issues, thus it has been regarded as an alternative method for NOx removal at room temperature. It is generally understood that, not only can NOx be physically adsorbed in the pore systems of carbonaceous materials, but also a reactive adsorption between the carbon surface and NOx molecules could take place in the oxidizing atmosphere. The mechanism of NO oxidation over porous carbon is a complicated process and continues to be discussed after two decades. Adapa et al.17 suggested Eley-Rideal (E-R) and Langmuir-Hinshelwood (L-H) mechanisms for catalytic NO oxidation. Here, in the L-H mechanism, adsorbed NO reacts with dissociated oxygen activated by carbon. However, for the E-R mechanism, NO adsorbed in carbon micropores can directly react with gaseous oxygen. Both mechanisms depend on the development of alike reactive intermediates, including C*-NO, C*-NO2, C*-NO3, and C*-NO-NO3. Claudino et al.15 suggested that NO oxidation on the porous carbon is a micropore filling procedure with NO as the adsorbed species. That is to say, the narrow micropores in porous carbon serve as nano-reactors for NO oxidation. Mochida et al.11 studied the kinetics of NO oxidation over activated carbon fiber in a wide range of NO concentration from 10-380 ppmv. They found that the kinetics are second-order for NO in


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the range of 20-380 ppmv, but first-order kinetics is suggested when NO concentration was below 20 ppmv. The second-order kinetics might designate that NO dimer is an intermediate in the NO oxidation on the carbon surface. Therefore, micropores or active sites was able to concentrate NO to generate the dimer on the carbon surface, which is consistent with the micropore filling mechanism proposed by Claudino et al.15 NO2 is generated through the oxidation of dimer, and delays on the carbon surface till the adsorption is saturated. However, in the case of very low concentration of NO, < 20 ppmv, diminishes the probability of dimer formation before the oxidation, giving first-order kinetics, which could be a result of L-H mechanism or E-R mechanism proposed by Adapa et al.17 However, carbon based adsorbents exhibit low selectivity generally, and could waste their elimination capability easily due to the saturated adsorption which taken place on the carbon surface18. Consequently, this process involves the difficult tasks of regeneration of the exhausted sorbents and further disposal of the pollutants. These concerns have greatly limited the industrial applicability of NOx adsorption on carbonaceous materials in industry. In the year of 2004, an effective solution to the above problem was first proposed by Shirahama et al19-20. They used a carbon adsorbent as porous support to load urea and subsequently remove NOx at room temperature. In their studies of urea-supported activated carbon fiber (ACF) for removing NOx in the presence of O2, they found that the gaseous NOx was first transformed into the adsorbed intermediates on the accessible carbon surface, and then these intermediates could be converted into nontoxic N2 by the neighboring urea molecules during SCR operation. This process completely eliminates the secondary pollution owing to a combinatory cooperation of porous NOx adsorbent with solid-state reducing agent, making this technology become a worth studying subject

for removal of NOx from the atmospheric


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environment21-27. Wang et al22. investigated the kinetics of SCR reaction of NO with urea loaded on a pitch-based spherical activated carbon (PSAC), and suggested that NO oxidation to NO2 catalyzed by the micropores in PSAC was the rate-controlling step in the urea-SCR of NO. This result was further corroborated by others' reports that the urea-SCR activity of NO could be significantly improved by an enhanced NO oxidation. These studies, nevertheless, are virtually focused on employing the microporous carbons as the support, which restricts the urea-loading capacity seriously. Furthermore, the literature related to the mechanism and kinetics of SCR reaction of NO2 with urea, to our best knowledge, is rather scarce. In our previous work27, both microporous carbon sphere and mesoporous carbon sphere were introduced as the supports to load urea for NOx removal. Compared with the microporous carbon support, the mesoporous carbon support exhibited much larger urea loading capacity as well as higher SCR activity of NO2, but unfortunately, the mechanism of this phenomenon was not clarified due to other focus concerned. In addition, some important issues, for instance, the effect of pore structure on the NOx removal capacity and the role of O2 in these processes are still unclear. Hence, systemic and continual investigation into the thorough understanding of this urea-SCR is extremely urgent. Carbon xerogels are innovative porous carbon materials that have gotten significant consideration in the field of adsorption, gas purification and energy storage28. The use of carbon xerogels as the support for NOx elimination, up to now, nevertheless, is extremely restricted. Lately, well-developed mesoporous carbon xerogels were creatively prepared by our group via colloidal silica assisted sol-gel process29-31. The obtained carbon xerogels exhibited highly opened 3D mesoporous network structure that would not only allow easy access and well dispersion of the loading substance, but also accelerate the mass transmission of the reactants


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and products across the pore channels, making them appropriate as the support to load urea for NOx elimination. Herein, carbon xerogels were prepared by a united hard templating and sol-gel approach, with resorcinol-formaldehyde polymer as carbon precursor and colloidal silica nanoparticles as hard templates, which were afterward activated via a physical activation approach with CO2 as activation agent at 950 °C for varying activation times. The obtained carbons were then used as the supports to load a certain amount of urea for the initial studies, and the best performing candidate would be further characterized, with the intention of better evaluate the effect of the structural properties and the urea loadings on the NOx removal capacity. The effect of NO/NO2, O2 feed concentrations as well as reaction temperatures on the SCR activity was investigated to ascertain the kinetic parameters involving the apparent activation energies and the orders of reaction for the urea-SCR of both NO and NO2. Special attention was paid to the role of oxygen in the urea-SCR processes, which was illustrated by transient response experiments and temperature-programmed desorption (TPD) technology. The development of surface functional groups on carbon at changing reaction phases was also characterized by Fourier transform infrared (FTIR) spectra, and an updated mechanism regarding elementary reactions of these processes was proposed based on the experimental observations, which ought to supply a helpful guidebook to their pragmatic applications. 2. Experimental section 2.1 Preparation of Carbon Xerogels The carbon xerogels were manufactured based on a colloidal silica nano-casting path, which involved mingling of colloidal silica nanoparticles with resorcinol-formaldehyde resin sol to


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achieve a complex hydrogel, and then via the ambient drying, carbonization and silica template removal (as shown in Scheme S1). The comprehensive synthesis procedure was provided in our former publication29-31. In the present work, the mole ratio of resorcinol (R) to formaldehyde (F) for the precursors was fixed at 1:2, and the total resin concentration was set at 15 g/100 mL together with the colloidal silica. In order to modify the pore structure of carbon xerogels, the original samples were treated by CO2 activation at 950 °C for 1-3 h. Finally, the as-acquired samples were designated as RF-x, where x signified the activation time, specifically, RF-0 was the virgin carbon xerogel without activation. 2.2 Preparation of Urea-Supported Catalysts Urea was loaded on RF-x via the incipient wetness impregnation method with water solution of urea based on the next steps. The supports were immersed into urea solution, kept at room temperature for 24 h, and afterward dehydrated at 60 °C for 24 h in a vacuum dryer. The as-achieved urea/carbon composites were denoted as yU/RF-x, where y signified the mass percentage of urea on the support. In this work, 10 wt.% of urea was loaded on the pristine carbon xerogel and its activated samples. In addition, RF-2 with 30 wt.% and 60 wt.% urea loading was also prepared, respectively. 2.3 Material Characterization The morphologies of the samples were viewed using a transmission electron microscopy (TEM, JEOL 2100F), and a scanning electron microscopy (SEM, JEOL 7100F). The thermogravimetric analysis (TA Instruments SDT-Q600) was performed under a nitrogen flow rate of 100 mL·min-1. The samples were heated to objective temperature under a heating rate of 10 °C·min-1. The surface species on the samples were also identified with Fourier transform


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infrared spectroscopy (FTIR, Nicolet Instrument 5SXC Spectrometer). The samples derived from different reduction periods were initial finely ground and afterward diluted with KBr under a concentration of 1 wt% of sample in KBr. And the FTIR spectra were recorded via accumulating 400 scans on a spectra resolution of 4 cm-1. Nitrogen adsorption/desorption isotherms of the samples were measured at 77 K with Quadrasorb SI gas sorption

analyzer. Before the measurements, the carbon samples were

degassed in vacuum at 423 K for 12 h, and the urea/carbon composites were degassed at 333 K for 12 h. Brunauer-Emmett-Teller method was used to calculate the BET surface area (SBET). The total pore volume (Vt) was evaluated from the adsorbed amount at the relative pressure of 0.996. The micropore surface area (Smic) and micropore volume (Vmic) were attained via t-plot method. The pore size distributions of mesoporosity were achieved from desorption branch by using the Barrett-Joyner-Halenda (BJH) model, and the pore size distributions of microporosity were achieved from adsorption branch by using the Quenched Solid Density Functional Theory (QS-DFT) model. 2.4 Denitration Experiments Dynamic tests were performed to estimate capabilities of the catalysts for both NO and NO2 removal. A certain amount of catalysts was packed into a quartz flow tube reactor (inner diameter of 8 mm) and a K-type thermoelectric couple was incorporated close to the reactor to measure the sample temperature. An approximated gas mixture for NO (or NO2) reduction experiments was diluted from gas cylinders of verified 5000 ppmv NO/N2 (or NO2/N2) and pure O2 with nitrogen to the desired concentration of 100 - 1000 ppmv NO (or NO2) and 0-20 vol % O2 through mass flow controllers. In every test, the total flow rate was kept at 100 mL·min-1 and


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the reaction temperature was regulated from 30 to 90 °C. The concentrations of NO and NO2 in the inlet and outlet gas were measured via an ECO PHYSICS CLD62 NO/NOx analyzer with a minimum detection level as low as 0.5 ppmv. The conversion of NOx during the tests was computed via the following equation: NOx conversion = (CNO,in - CNO,out- CNO2,out)/CNO,in × 100%

(1)

Where CNO,in is the inlet NO concentration(ppmv), CNO,out and CNO2,out are the effluent NO and NO2 concentrations(ppmv), respectively. Particularly, NOx conversion would maintain a certain level for a period of time during NOx reduction process, which was defined as the steadystate NOx conversion21,22. 2.5 Transient Response and TPD Experiments Transient response experiments were performed to find out the role of O2 in the urea-SCR of both NO and NO2, which were operated in the aforesaid quartz flow tube reactor upon switching off and on O2. In a standard procedure , about 0.3 g of RF-2 with 30 wt.% of urea loading was packed into the reactor, and then a feed gas including 500 ppmv NO (or NO2), 20 vol % O2, and balance N2 gone through the sample bed at 30 °C with a gas flow rate of 100 mL·min-1. While the urea-SCR arrived at the steady-state of NOx conversion (approximately 5 h), O2 was switched off and substituted using a makeup N2 of the equal flow rate to sustain the total flow rate unchanging. O2 of the original flow rate was brought back to the feed gas to substitute the makeup N2, after the NOx conversion arrived at a new steady-state. Temperature-programmed desorption (TPD) experimentations were likewise accomplished in the quartz flow tube reactor in which 0.3 g of RF-2 was loaded with a K-type thermoelectric couple incorporated close to the reactor to measure the temperature. Four types of simulated


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mixture including 500 ppmv NO, 500 ppmv NO2, 500 ppmv NO + 20 vol % O2 and 500 ppmv NO2 + 20 vol % O2 with a balance of nitrogen were used as the adsorption gas in the test, respectively. In a typical process, the sample that was loaded in the reactor was primarily treated in-situ in a N2 current at 150 °C for 1 h, and afterward cooled back to 30 °C in the identical current. Next, one type of simulated mixture was altered to the catalyst bed in the reactor at 30 °C for 2 h, which was subsequently purified with nitrogen at the equal temperature for 1 h to eliminate physisorbed NOx. Finally, a TPD test was operated instantly in a nitrogen flow at a heating rate of 5 °C·min-1 from 30 °C up to 300 °C. In this section, the total gas flow rate was sustained at 100 mL·min-1 for each procedure, and the concentrations of NO and NO2 in the flue gas during the TPD procedure were detected by ECO PHYSICS CLD62 NO/NOx analyzer. 3. Results and Discussion 3.1 Characterization of the samples

Figure 1. Typical SEM (a) and TEM (b) images of RF-0.


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Figure 1 shows the typical morphology of the pristine carbon xerogel derived by SEM and TEM. The SEM image in Figure 1a reveals that the RF-0 consists of honeycomb-like porous structure with rather randomly distributed and spherical mesopores that are considered to be reversely replicated from the silica template29-31. This structure is further confirmed by the TEM image (Figure 1b), in which highly ramified amorphous carbon matrix with interconnected and disordered mesoporous structure could be clearly observed. These significant features are convinced to facilitate the uniform dispersion of supported urea and permit reactants' effortless transportation into the interior active sites, and significantly, create a condition most beneficial for the interaction between urea molecules and adsorbed NOx species.


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Figure 2. Nitrogen adsorption-desorption isotherms (a, b), BJH pore size distributions (c, d) and DFT pore size distributions (e, f) of RF-x (a, c, e) and RF-2 with various amount of urea loadings (b, d, f). The N2 adsorption-desorption isotherms and derived pore size distributions of RF-x and RF2 with various loadings of urea were presented in Figure 2. The isotherms of pristine carbon


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xerogel and its activated samples (Figure 2a) are similar which display type-IV curves with an H2-type hysteresis loop, indicating their mesoporous characteristic31. In addition, it seems that activation of carbon xerogels with CO2 does not powerfully influence their mesoporous structure, since their BJH pore size distributions centralized at 5-20 nm are similar as shown in Figure 2c. However, with the increase of the activation time, the peak of the micropore size distributions evaluated using QS-DFT model (Figure 2e) shifts to large pore size obviously. After the impregnation of RF-2 with urea, the samples exhibit the as-expected decrease in the nitrogen adsorption amount as shown in Figure 2b, suggesting that the urea molecules could be dispersed into the open framework of RF-2 at nanoscale dimensions. However, the urea loaded samples still maintain the mesoporous structure with the average pore size lain in 10-12 nm. Table 1. Pore parameters of RF-x and RF-2 with different amount of urea loadings. SBET

Smic

Vt

Vmic

D

(m2·g-1)

(m2·g-1)

(cm3·g-1)

(cm3·g-1)

(nm)

RF-0

816

290

1.40

0.13

10.7

RF-1

1153

500

1.86

0.21

10.7

RF-2

1421

635

2.18

0.28

10.7

RF-3

1329

601

1.93

0.26

11.2

10U/RF-2

1295

384

2.17

0.17

9.4

30U/RF-2

561

24

1.58

-

9.4

60U/RF-2

492

19

1.14

-

11.9

Samples


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The details of pore parameters of the samples are summarized in Table 1. It could be observed that, increasing the activation time from 0 to 2 h, the micropore surface area and micropore volume of the carbon xerogels increase gradually from 290 to 635 m2·g-1 and 0.13 to 0.28 cm3·g-1, respectively. Notably, the increment of both BET surface area and total pore volume is much larger than that of the micropore surface area and the micropore volume during the activation process. It should be deduced that not only do some micropores create in the thick mesopore walls, but also new mesopores are generated as a result of the gasification of the thin carbon framework with the high-temperature calcination of CO2. However, when the activation time is 3 h, the activated carbon xerogel shows obviously inferior pore volume as well as surface area in comparison with those of RF-2. This should be due to the excessive activation of the materials, causing the expansion and merging of the existing pores. After the impregnation with urea, the BET surface area and total pore volume of the catalysts are in the range of 492-1421 m2·g-1 and 1.14-2.18 cm3·g-1, respectively, decreasing with the increase of the urea loading amount. Especially, after 60 wt.% urea loading, the porosity of RF-2 support decreases drastically as a result of a large number of urea molecules filling the pores, however, the catalyst still maintain a sufficient amount of porosity and interfacial area, being favorable for the kinetic diffusion and adsorption of NOx. 3.2 Effect of pore structure on NOx removal


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Figure 3. The relationship between different carbon supports and their steady-state NOx conversion of both NO and NO2 reduction. (Sample: 10U/RF-x, m = 0.33 g. Reaction condition: NO or NO2, 500 ppmv; O2, 20 vol %; N2, balance; reaction temperature = 30 °C; particle diameter = 0.5-0.8 mm.) Figure 3 shows the relationship between different carbon supports and their steady-state NOx conversion of both NO and NO2 reduction. It is observed that, associated with Table 1, the urea-SCR activity of both NO and NO2 is improved with the increase of microporous porosity in the supports. It could be attributed to the importance of microporous porosity for NO and NO2 adsorption, specially, the micropores are considered to serve as the main nano-reactors for NO adsorption and oxidation32. Among these supports, RF-2 which gives the highest microporous volume of 0.28 cm3·g-1 and the largest BET surface area of 1421 m2·g-1 exhibits the largest removal activity of both NO and NO2 with 10 wt. % urea loading, making it the best candidate. 3.3 Effect of urea loading amount on NOx removal


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Figure 4. Breakthrough curves of NO (a) and NO2 (b) over RF-2 with different amount of urea loadings. (Sample: RF-2, m=0.3 g; 10U/RF-2, m = 0.33 g; 30U/RF-2, m = 0.39 g; 60U/RF-2, m = 0.48 g. Reaction condition: NO (a), 500 ppmv; NO2 (b), 500 ppmv; O2, 20 vol %; N2, balance; reaction temperature = 30 °C; particle diameter = 0.5-0.8 mm.) In addition to the pore structure of the supports, the urea-loading amount is another important index for this “consuming catalyst”, which determines the regeneration period as well as the replacement cost. Figure 4 shows the influence of urea loading amounts on both NO and NO2 reductions. The removal time of both processes is prolonged obviously with the increase of urea loading amounts. For NOx reduction over RF-2 without urea loading, the NOx conversion sharply decreased from the initial values (96% for NO, and 91% for NO2) to zero in 12 h, and there was no steady-state NOx removal period observed in the absence of urea. It is worth noting that the steady-state NOx conversion of NO reduction (Figure 4a) is sharply declined with the increase of the urea loading amounts, on the contrary, the steady-state NOx conversion of NO2 reduction (Figure 4b) is slightly increased with the increase of urea loading amounts. It should be ascribed to that increasing urea loadings could result in some pore blockages but increase the interaction chance between NOx and urea molecules.


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Table 2 shows the material balance of reacted NOx to supported urea over RF-2 during the steady-state NOx removal period. The mole ratio of reacted NOx to supported urea over RF-2 with variable urea loading amounts is approximately equal to 2 for both processes, which provides a number of significant evidences in the mechanism resemblance between the ureaSCRs of NO and NO2. Although the results indicate that urea is almost completely consumed at breakthrough time of these two reactions, much higher utilization efficiency of the urea molecules could be observed during NO2 reduction due to the much higher steady-state NOx conversion. Interestingly, with increase of urea loadings from 10 to 60 wt.%, the utilization efficiency of the supported urea is slightly improved during NO2 reduction. It could be ascribed to the great pore volume combined with appropriate range of pore size of the carbon support that allow the easy entrance of urea into the deeper pore channels, resulting in the uniform dispersion of urea molecules on the carbon surface. Moreover, these structures are beneficial to conquer the kinetic resistance for gas diffusion, which is regard as a precondition for efficient urea-SCR of NOx at ambient temperatures. Table 2. Mole ratio of reacted NOx to supported urea over RF-2. 10U/RF-2 Material balance

30U/RF-2

60U/RF-2

NO-

NO2-

NO-

NO2-

NO-

NO2-

SCR

SCR

SCR

SCR

SCR

SCR

Amount of sample (g)

0.3

0.3

0.3

0.3

0.3

0.3

Actual urea content on sample (%)

9.9

9.9

30.0

30.0

58.3

58.3


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Actual amount of urea on sample 0.49

0.49

1.50

1.50

2.91

2.91

9.6

8.6

34.1

24.7

71.2

41.7

1.29

1.15

4.56

3.31

9.53

5.58

78.4

89.3

64.9

91.5

53.2

93.4

1.01

1.03

2.96

3.03

5.07

5.21

2.02

2.06

1.97

2.02

1.74

1.79

(mmol) Breakthrough time (BTT) (h) Total amount of supplied net NOx until BTT (mmol) Ratio of reacted NOx to supplied net NOx (%) Amount of removed NOx (mmol) Mole ratio of reacted NOx to urea over catalysts

3.4 Effect of NOx concentration on urea-SCRs


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Figure 5. Effect of inlet NOx concentration on steady-state NOx conversion (a) and on reaction rate (b). (Sample: 30U/RF-2, m = 0.39 g. Reaction condition: NO or NO2, 100-1000 ppmv; O2, 20 vol %; N2, balance; reaction temperature = 30 °C; particle diameter = 0.07-0.2 mm.) In order to further clarify the intrinsic kinetics behavior of the urea-SCR of NOx, primary trials are performed to guarantee the elimination of external and internal diffusion effects on both NO and NO2 removal processes. As shown in Figure S1, while the total flow rate exceeds 100 ml·min-1, the effect of external diffusion on the two processes can be eliminated. In addition, the internal diffusion effect on the two processes should be negligible when the particle diameter of the catalysts is smaller than 0.2 mm, according to Figure S2. Therefore, the total flow rate of 100 mL·min-1 and the crunched samples with a particle diameter in the range of 0.07-0.2 mm are used to ensure the obtained kinetics consequences were free from mass transmission's influences in the present work. Figure 5 shows the effect of inlet NOx concentration on steady-state NOx conversion and on reaction rate for both processes. It could be observed from Figure 5a that, increasing NO concentration from 100 to 1000 ppmv, the steady-state NOx conversion gradually increases from 65.6% to 69.2%. On the contrary, the steady-state NOx conversion for NO2 removal decreases slightly from 97.8% at 100 ppmv NO2 to 92.6% at 1000 ppmv NO2, the recession should be due to the enhanced oxidation of carbon surface and the aggravated disproportionation of adsorbed NO2 at high NO2 concentration, resulting in more produced NO into the gas phase. As shown in Figure 5b, higher inlet NOx concentration extraordinarily accelerates the rates of these reactions. The orders of steady-state urea-SCR for NO and NO2 are both calculated to be about 1.Here , the incredibly parallel kinetics results of these two reactions deeply verifies

the mechanism

similarity between the urea-SCRs of NO and NO2.


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3.5 Effect of O2 concentration on urea-SCRs

Figure 6. Effect of feed O2 concentration on steady-state NOx conversion (a) and on reaction rate (b). (Sample: 30U/RF-2, m = 0.39 g. Reaction condition: NO/NO2, 500 ppmv; O2, 0-20 vol %; N2, balance; reaction temperature = 30 °C; particle diameter = 0.07-0.2 mm.) Figure 6 illustrates the effect of feed O2 concentration on steady-state NOx conversion and on reaction rate for both processes. It is found that, according to Figure 6a, increasing the feed O2 concentration from 0 vol % to 20 vol %, the steady-state NOx conversion of NO removal increases rapidly from 0% to 68.3%, and the steady-state NOx conversion of NO2 removal increases gently from 84.3% to 93.9%. The result reveals that O2 plays a positive role for both reactions. As shown in Figure 6b, higher feed O2 concentration also increases the reaction rates of these two reactions. The orders of these two reactions for oxygen were determined to be about 0.5 for steady-state urea-SCR of NO and about 0.03 for steady-state urea-SCR of NO2. Herein, the kinetics results reveal that the urea-SCR of NO depends more strongly on oxygen concentration compared with the urea-SCR of NO2. Moreover, the former process almost certainly contains the adsorption and subsequent decomposition of molecular oxygen into two


20

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atomic oxygen on the carbon surface, because the reaction rate is in direct proportion to the square root of oxygen concentration32,33. While the order for oxygen in the later reaction is too small, strongly implying that oxygen takes part in this reaction mainly in adsorbed species. 3.6 Effect of reaction temperature on urea-SCRs

Figure 7. Effect of reaction temperature on steady-state NOx conversion (a) and on Ln r [r: reaction rate;] (b). (Sample: 30U/RF-2, m = 0.39 g. Reaction condition: NO or NO2, 500 ppmv; O2, 20 vol %; N2, balance; reaction temperature = 30-90 °C; particle diameter = 0.07-0.2 mm.) Figure 7 illustrates the relationship between reaction temperature and steady-state NOx conversion and the derived reaction rate for both processes. It could be observed from Figure 7a that, with the growth of reaction temperature from 30 to 90 °C, the steady-state NOx conversion for the NO removal decreases noticeably from 68.3% to 27.5%.However, the steady-state NOx conversion for the NO2 removal increases slightly from 93.9% to 98.9%. The two opposite trends apparently indicate that the rate-limiting steps for these two reactions are different. As can be seen in Figure 7b, the apparent activation energies of the two reactions were estimated according to the Arrhenius equation to be -14.0 and 0.79 kJ/mol for the steady-state urea-SCR of


21


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NO and NO2, respectively. The previous reaction's negative apparent activation energy implies that the adsorption of the reactants on the carbon surface could be the rate-controlling step for urea-SCR procedure of NO. Meanwhile, the small positive apparent activation energy of the latter reaction indicates that reaction of NO2 with urea on carbon surface could be the ratecontrolling step for urea-SCR procedure of NO2, corresponding to previous reports22,27. 3.7 The role of O2 in urea-SCR of NOx

Figure 8. Transient response upon switching off and on O2 during steady-state urea-SCR of NO (a) and NO2 (b) on 30U/RF-2. (Sample: 30U/RF-2, m = 0.39 g. Reaction condition: NO/NO2, 500 ppmv; O2, 20 vol %; N2, balance; reaction temperature = 30 °C; particle diameter = 0.07-0.2 mm.) The role of gaseous O2 for these two reactions was further illustrated by transient response experiment and TPD technology. Transient response during steady-state urea-SCR of NO and NO2 on 30U/RF-2 are shown in Figure 8. For urea-SCR process of NO (Figure 8a), the NOx conversion first declines sharply from a steady-state value of 68.3% to about 10% as soon as O2 is switched off, followed by a continual but slow decrease toward zero. Thereafter, the NOx


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conversion could shoot up to the original value in a very few minutes after reintroduction of O2. The result obviously designates that the gaseous oxygen is likely to be the determining factor in the urea-SCR of NO, the adsorption and dissociation of molecular oxygen on carbon surface is considered indispensable for the generation of crucial reactive nitrogen intermediates. For urea-SCR process of NO2 (Figure 8b), the NOx conversion first declines sharply from a steady-state value of 93.9% to 86.6% when O2 is shut off, and then it flattens out with a slight decline to a steady value of 84.3% in 3.5 h. After reintroduction of O2, the NOx conversion could also increase gradually to the original value within about one hour. Notice that 84.3% of NO2 is still reduced by supported urea in the absence of oxygen, mainly due to the production of the important NO3 intermediates through the disproportionation of the adjacent adsorbed NO2 on the carbon surface34. When the urea-supported catalysts are exposed in the O2 containing stream, the NO2 molecules could also occupied most of the active sites on carbon surface involving some O2 adsorption active sites due to their high polarity, but O2 adsorbed on the limited active sites still promotes the urea-SCR of NO2. It is important to mention that the NOx conversion for urea-SCR of both NO and NO2 recover to their original values after reintroduction of O2, implying that the active sites for O2 adsorption could be regenerated, and the produced NOx species may spill over to the vicinal urea supported on the carbon surface. TPD technology is also used to examine the species adsorbed on carbon surface and further identify the important intermediates for the reactions. The TPD tests are run for RF-2 samples after exposure of RF-2 to various simulated mixture for 2 h. The TPD profiles of NO and NO2 are illustrated in Figure S3. The release of NO and NO2 was detected in the temperature range of 30-300 °C, in agreement with the previous reports35-37. As can be seen, the distribution of NO and NO2 profiles after exposure of RF-2 to various simulated mixture except 500 ppmv NO


23


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without O2 are similar, suggest similar adsorbed NOx species produced during NO oxidation and NO2 adsorption. Consistent with the literatures, the releasing NO and NO2 during TPD could be ascribed to the disintegration of –C–NO2 and –C–ONO2, respectively36,37. The amount of NO, NO2, and total NOx desorbed from RF-2 during TPD after exposure of RF-2 to various simulated mixture are listed in Table 3. It is worth noting that the gross amount of adsorbed NOx after NO adsorption with 20 vol % O2 is significantly larger than that after NO adsorption without O2, suggesting that the adsorbed NOx species, especially –C–ONO2 intermediate, should be generated with the aid of oxygen. However, after NO2 adsorption with or without O2, the total desorbed NOx for these two processes is almost identical. While the amount of adsorbed NO2 after NO2 adsorption with O2 is noticeably larger than that after NO2 adsorption without O2, implying supplied oxygen could accelerate the conversion of –C–NO2 into –C–ONO2 species on the carbon surface. Table 3. Amount of NO, NO2, and total NOx desorbed from RF-2 during TPD after exposure of RF-2 to various simulated mixture (mmol·g-1). 500 ppmv NO

500 ppmv NO

500 ppmv NO2

+ 20 vol % O2

500 ppmv NO2 + 20 vol % O2

NO

0.17

1.37

1.43

1.09

NO2

0.01

1.10

1.11

1.65

Total NOx

0.18

2.47

2.54

2.74

3.8 Mechanism consideration of urea-SCR of NOx


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Figure 9. FTIR spectra of RF-2, and 30U/RF-2 after exposure of 30U/RF-2 to the feed gas of 500 ppmv NO + 20 vol % O2 (a) and 500 ppmv NO2 + 20 vol % O2 (b) for various times, respectively. Although the TPD tests indicate that various surface species could be formed on RF-2 during NOx adsorption in the oxidizing atmosphere, the forms of absorbed species on ureasupported RF-2 need to be further investigated. And then, the 30U/RF-2 catalysts after NO reduction for 5 h, 34 h, 39 h and after NO2 reduction for 5 h, 24 h, 29 h are selected to be characterized by Fourier transform infrared (FTIR) technology. Figure 9 presents the FTIR spectra of RF-2, and 30U/RF-2 after exposure to different gas mixture for various times. In the spectrum of pristine RF-2, several bands at 3440, 1580 and 1120 cm-1 are observed. The bands at 3440 and 1120 cm-1 could be assigned to stretching and bending vibration of phenolic –OH on aromatic rings, respectively38. The band at 1580 cm-1 is probably ascribed to C=O stretching vibration in conjugative systems including quinone, diketone, and ketoester

39

. After urea supported on RF-2, new bands appear at 1664 and 1444

cm-1, which represent –NH2 bending and asymmetric N–C–N stretching vibration, respectively40.


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After initial 5 h of urea-SCR of both NO and NO2, an absorption band is found at 1381 cm-1, which is ascribed to asymmetric stretching vibration of NO3 species39. Meanwhile, an escalation in the intensity of the band at 1580 cm-1 is also observed which implies an escalation in the quantity of C=O functional groups as a result of carbon surface oxidation. After 34 h of ureaSCR of NO when NO breaks through the catalyst bed and NO2 just starts to release, as shown in Figure 9a, the band at 1381 cm-1 increases slightly. Surprisingly, weak bands at 1664 and 1444 cm-1 can also be recognized on the exhausted catalyst, implying that a few of urea molecules still remain on the carbon surface. After 24 h of urea-SCR of NO2 when NO2 breaks through the catalyst bed, as shown in Figure 9b, the band at 1381 cm-1 increases significantly, meanwhile a small band is appeared at 1355 cm-1 as a shoulder, probably resulting from an adsorbed form of NO2 in multi-layer or liquid state41. The bands at 1664 and 1444 cm-1 disappear as expected, implying the completely consumption of urea. Moreover, a new band at 3150 cm-1 appears in the spectrum that could be ascribed to –OH stretching vibration of carboxylic groups42, indicating further oxidation of carbon surface. After 39 h of urea-SCR of NO, the band at 1381 cm-1 increases significantly, the bands at 1664 and 1444 cm-1 finally disappear and a new weak band at 3150 cm-1 appears. And after 29 h of urea-SCR of NO2, the bands at 3150, 1381 and 1355 cm-1 increase obviously, and moreover, the band at 1355 cm-1 becomes more intense with reaction time than the band at 1381 cm-1. Note that, in this urea/carbon composite system, the solid-state urea is always considered to be fixed on the carbon surface, and NOx molecules seem not to be reduced by urea directly in gas phase. Thus, the interaction between NOx molecules and carbon surface is especially important for building the mechanism of the urea-SCRs. Shirahama et al34. proposed that there are two kinds of active site that one adsorbs NOx molecules strongly and the other adsorbs NOx


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molecules weakly on carbon surface. Zhang et al37. also found that the zigzag and armchair edge sites of activated carbon could be easily oxidized to form oxygen-containing functional groups during NO + O2 or NO2 adsorption, which was convinced to be closely associated with transient NO oxidation kinetics without impacting steady-state kinetics. In this work, excellent agreement of the experiment data would be attained by presuming that there are two kinds of active site on the carbon surface, including non-reductive sites (–C) and reductive sites (–C*) with different reactivity for NOx adsorption35,36,43. This hypothesis is specially supported by the FTIR results that adsorbed NO3 intermediates and oxidation of carbon surface are observed simultaneously in the initial urea-SCR period. As for the urea-SCR of NO, oxygen is apparently essential to this process according to the transient response results. In addition, the TPD result further indicates that NO adsorption on spotless RF-2 carbon is nearly ignorable when oxygen is absence. Thus, the strong reliance of physisorbed NO on oxygen gas arrives at the inference that NO could co-adsorb physically with O2 on the non-reductive sites32. Then the adsorbed O2 dissociates into two oxygen atoms that rapidly oxidize the adjacent adsorbed NO to produce adsorbed NO2 species. In addition, the subsequent disproportionation of the NO2 species gives adsorbed NO3 species with release of NO into the gas phase. Based on the experiment data, the produced NO3 species is assumed to spill over from the non-reductive sites into reductive sites, oxidize these sites to generate oxygen-containing functionalities with the release of NO23,44,45. Afterwards, the newly formed NO3 species could migrate to the oxidized carbon surface and release the vacant non-reductive sites for successive NO adsorption. The urea molecules that highly dispersed on the carbon surface exhibit very high reductive activity towards the NO3 species and finally reduce the NOx


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species to produce N2, H2O and CO2. Therefore, a possible mechanism including several elementary reactions for urea-SCR of NO occurred on carbon surface is postulated as follow: –C + NO → –C–NO

(2)

2 –C + O2 → 2 –C–O

(3)

–C–NO + –C–O → –C–NO2 + –C

(4)

–C–NO2 + –C–NO2 → –C–ONO2 + –C + NO

(5)

–C–ONO2 + 2 –C* → –C + 2 –C*–O + NO

(6)

–C–ONO2 + –C*–O → –C–O + –C*–ONO2

(7)

–C/C*–ONO2 + CO(NH2)2 + NO → –C/C*–O + 2 N2 + 2 H2O + CO2

(8)

It is important to mention that NO3 species could be the oxygen source for oxygencontaining functionalities which are incorporated into the carbon skeleton during NO + O2 adsorption. The oxidation of reductive carbon surface by NO3 species and the migration of them on the oxidized sites is found to be of great importance for the successive urea-SCR of NO, and the mechanism of NO reduction should be updated according to these findings. The mechanism of urea-SCR of NO is illustrated in Figure 10(a). As for urea-SCR of NO2, NO2 is suggested to directly adsorb on the reductive sites and rapidly oxidizes these sites to produce oxygen-containing functionalities with NO release simultaneously36,43. Therefore, the important NO3 intermediates are assumed to be generated via the disproportionation of adsorbed NO2 groups on the non-reductive sites and the direct adsorption of NO2 on the oxidized carbon sites. Finally, the supported urea molecules could react


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with the adjacent NO3 species and weakly adsorbed NO2 to produce N2, H2O and CO2. Therefore, a possible mechanism including several elementary reactions for urea-SCR of NO2 occurred on carbon surface is postulated as follow: –C + NO2 → –C–NO2

(9)

2 –C + O2 → 2 –C–O

(10)

–C* + NO2 → –C*(NO2) → –C*–O + NO

(11)

–C–NO2 + –C–NO2 → –C–ONO2 + –C + NO

(12)

–C–O/–C*–O + NO2 → –C–ONO2/–C*–ONO2

(13)

–C–ONO2 + –C*–O → –C–O + –C*–ONO2

(14)

–C/C*–ONO2 + CO(NH2)2 + –C*(NO2) →2 –C/C*–O + 2 N2 + 2 H2O + CO2

(15)

Note that, NO2 should be the major oxygen source for oxygen-containing functionalities that are incorporated into the carbon skeleton during NO2 adsorption. In addition, most of NO3 species are considered to be produced by the route in which NO2 directly adsorbed on the oxidized carbon sites. It is important to mention that, the urea-SCR activity of NO2 is always much higher than that of NO in the experimental conditions. Meanwhile, the oxidation of the carbon surface is also found to be more furious during NO2 reduction. Moreover, the supported urea could be completely consumed at breakthrough time (24 h) during NO2 reduction, but it is still observed at the corresponding breakthrough time (34 h) during NO reduction. However, with additional 5 hours after NO2 releases (39 h during NO reduction), the supported urea disappears eventually. All these results are in conformity with the suggested mechanisms. The mechanism of urea-SCR of NO2 is illustrated in Figure 10(b).


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Figure 10. Reaction schemes of urea-SCR over RF of NO (a), and of NO2 (b) Conclusion In summary, various amount of urea impregnated on carbon xerogels with different porous texture are prepared as the catalysts, and the urea-SCR of NOx over these catalysts is systematically studied at room temperature with a particular goal on kinetic and mechanism viewpoints . We find that increasing the microporous porosity of the support promotes the ureaSCR activity of both NO and NO2, possibly due to improved adsorption capacity. Large amount of urea loading may block some pore channels but increase the interact chance between NOx and


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urea, thus restricts NO reduction but improves NO2 reduction. The kinetic results exhibit that the existence of gaseous oxygen is beneficial to the urea-SCR of both NO and NO2 .However, a furiously competitive adsorption between NO2 and O2 could occur on the carbon surface during NO2 reduction. It is found that the co-adsorption of NO and O2 in micropores of carbon should be the rate-controlling step in urea-SCR of NO, while surface reaction of NO2 with urea should be the rate-controlling step in urea-SCR of NO2, which is proved by both the estimated apparent activation energies and the kinetics results of these two reactions. Founded upon those observations, we propose that there are two kinds of active site, including non-reductive and reductive site, existed on the carbon surface. And different reactive adsorption capacities of NOx on these sites lead to the difference in the urea-SCR mechanism between NO and NO2. The possible mechanisms regarding elementary reaction occurred on the carbon surface are suggested, and those mechanisms are further confirmed by the oxygen transient experiment, NOx-TPD and FTIR results. It is found that the oxidation of the reductive carbon surface by NO3 or NO2 species and the migration of NO3 species on the oxidized carbon surface are considered to be of significant importance for the successive urea-SCRs of NOx. This work updates the ureaSCR mechanism available in the literature that gives pretty thoughtful information for the practical application of this low-temperature urea-SCR technology. Acknowledgment The authors are grateful for financial supports from NSFC (U1710252, 21506061), Young Elite Scientists Sponsorship Program by CAST, the China Petroleum Science and Technology Innovation Fund (2015D-5006-0405), Fundamental Research Funds for the Central Universities and Shanghai Rising-Star Program (17QB1401700).


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Author Information Corresponding Author * E-mail: [email protected] (Wenming Qiao). Tel.: +86-21-64253730. Author Contributions † These authors contributed equally.

Supporting Information 1. Preparation route of carbon xerogels 2. Elimination of the external and internal diffusions 3. TPD profiles These materials are available free of charge via the Internet at http://pubs.acs.org.

Reference (1) Bosch, H.; Janssen, F. Catalytic Reduction of Nitrogen Oxides. A Review on the Fundamentals and Technology. Cat. Today. 1987, 2, 369-531. (2) Nova, I.; Ciardelli, C.; Tronconi, E.; Chatterjee, D.; Bandlkonrad, B. NH3-SCR of NO over a V-based catalyst: Low-T redox kinetics with NH3 inhibition. AlChE. J. 2006, 52,(9), 3222-3233. (3) Nova, I.; Ciardelli, C.; Tronconi, E.; Chatterjee, D.; Weibel, M. Unifying redox kinetics for standard and fast NH3-SCR over a V2O5-WO3/TiO2 catalyst. AlChE. J. 2010, 55, (6), 1514-1529. (4) Shen, M.; Li, C.; Wang, J.; Xu, L.; Wang, W.; Wang, J. New insight into the promotion effect of Cu doped V2O5/WO3–TiO2 for low temperature NH3-SCR performance. Rsc. Adv. 2015, 5, (44), 35155-35165.


32

Page 33 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(5) Maruo, Y. Y.; Ogawa, S.; Ichino, T.; Murao, N.; Uchiyama, M. Measurement of local variations in atmospheric nitrogen dioxide levels in Sapporo, Japan, using a new method with high spatial and high temporal resolution. Atmos. Environ. 2003, 37, (8), 1065-1074. (6) Shwan, S.; Olsson, L.; Skoglundh, M.; Jansson, J. Kinetic modeling of Fe-BEA as NH3-SCR catalyst-effect of phosphorous. AlChE. J. 2015, 61, (1), 111-123. (7) Pradhan, M. P.; Joshi, J. B. Absorption of NOx gases in aqueous NaOH solutions: Selectivity and optimization. AlChE. J. 2010, 45, (1), 38-50. (8) Patwardhan, J. A.; Joshi, J. B. Unified model for NOx absorption in aqueous alkaline and dilute acidic solutions. AlChE. J. 2010, 49, (11), 2728-2748. (9) Stere, C. E.; Adress, W.; Burch, R.; Chansai, S.; Goguet, A.; Graham, W. G.; Rosa, F. D.; Palma, V.; Hardacre, C. Ambient Temperature Hydrocarbon Selective Catalytic Reduction of NOx Using Atmospheric Pressure Nonthermal Plasma Activation of a Ag/Al2O3 Catalyst. Acs. Catal. 2015, 4, (2), 666-673. (10) Kim, G. T.; Bo, H. S.; Lee, W. J.; Park, J.; Min, K. K.; Sang, M. L. Effects of applying nonthermal plasma on combustion stability and emissions of NOx and CO in a model gas turbine combustor. Fuel. 2017, 194, 321-328. (11) Mochida, I.; Kawabuchi, Y.; Kawano, S.; Matsumura, Y.; Yoshikawa, M. High catalytic activity of pitch-based activated carbon fibres of moderate surface area for oxidation of NO to NO 2 at room temperature. Fuel. 1997, 76, (6), 543-548. (12) And, R. Q. L.; Yang, R. T. Carbon Nanotubes as a Superior Sorbent for Nitrogen Oxides. Ind. Eng. Chem. Res. 2001, 40, (20), 4288-4291. (13) Wang, M. X.; Huang, Z. H.; Shimohara, T.; Kang, F.; Liang, K. NO removal by electrospun porous carbon nanofibers at room temperature. Chem. Eng. J. 2011, 170, (2), 505-511.


33


Page 34 of 37

(14) Sousa, J. P. S.; Pereira, M. F. R.; Figueiredo, J. L. NO oxidation over nitrogen doped carbon xerogels. Appl. Catal. B. Environ. 2012, 125, (33), 398-408. (15) Claudino, A.; Soares, J. L.; Moreira, R. Adsorption equilibrium and breakthrough analysis for NO adsorption on activated carbons at low temperatures. Carbon, 2004, 42(8-9): 1483-1490. (16) Mochida, I.; Shirahama, N.; Kawano, S.; Korai, Y.; Yasutake, A.; Tanoura, M.; Fujii, S.; Yoshikawa, M. NO oxidation over activated carbon fiber (ACF). Part 1. Extended kinetics over a pitch based ACF of very large surface area. Fuel. 2000, 79, (14), 1713-1723. (17) Adapa, S.; Gaur, V.; Verma, N. Catalytic oxidation of NO by activated carbon fiber (ACF). Chem. Eng. J. 2006, 116, (1), 25-37. (18) Kaneko, K. Anomalous Micropore Filling of No On Fe2O3-Dispersed Activated Carbon Fibers. Langmuir. 1988, 39, (3), 183-192. (19) Shirahama, N.; Mochida, I.; Korai, Y.; Choi, K. H.; Enjoji, T.; Shimohara, T.; Yasutake, A. Reaction of NO2 in air at room temperature with urea supported on pitch based activated carbon fiber. Appl. Catal. B: Environ. 2004, 52, (3), 173-179. (20) Shirahama, N.; Mochida, I.; Korai, Y.; Choi, K. H.; Enjoji, T.; Shimohara, T.; Yasutake, A. Reaction of NO with urea supported on activated carbons. Appl. Catal. B: Environ. 2005, 57, (4), 237-245. (21) Zhi, W.; Wang, Y.; Wang, D.; Chen, Q.; Qiao, W.; Liang, Z.; Ling, L. Low-Temperature Selective Catalytic Reduction of NO with Urea Supported on Pitch-Based Spherical Activated Carbon. Ind.Eng.Chem.Res. 2010, 49, (14), 6317-6322. (22) Wang, Z.; Wang, Y.; Long, D.; Mochida, I.; Qiao, W.; Liang, Z.; Liu, X.; Yoon, S. H.; Ling, L. Kinetics and Mechanism Study of Low-Temperature Selective Catalytic Reduction of


34

Page 35 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NO with Urea Supported on Pitch-Based Spherical Activated Carbon. Ind.Eng.Chem.Res. 2011, 50, (10), 6017-6027. (23) Jin, M.; Shimohara, T.; Shirahama, N.; Yasutake, A.; Yoshikawa, M.; Mochida, I.; Yoon, S. H. Removal of NO x from air through cooperation of the TiO2 photocatalyst and urea on activated carbon fiber at room temperature. Appl. Catal. B: Environ. 2011, 110, (41), 273-278. (24) Zeng, Z.; Lu, P.; Li, C.; Zeng, G.; Jiang, X.; Zhai, Y.; Fan, X. Selective catalytic reduction (SCR) of NO by urea loaded on activated carbon fibre (ACF) and CeO/ACF at 30 °C: The SCR mechanism. Environ. Technol. 2011, 33, (11), 1331-1337. (25) Lu, P.; Zeng, Z.; Li, C.; Zeng, G.; Guo, J.; Jiang, X.; Zhai, Y.; Fan, X. Room temperature removal of NO by activated carbon fibres loaded with urea and La2O3. Environ. Technol. 2012, 33, (9), 1029-1036. (26) Jiang, X.; Lu, P.; Li, C.; Zeng, Z.; Zeng, G.; Hu, L.; Mai, L.; Li, Z. Experimental study on a room temperature urea-SCR of NO over activated carbon fibre-supported CeO2-CuO. Environ. Technol. 2013, 34, (5), 591-598. (27) Li, J.; Yin, D.; Long, D.; Wang, J.; Ling, L.; Qiao, W. Design of a dual-bed catalyst system with microporous carbons and urea-supported mesoporous carbons for highly effective removal of NOx at room temperature. Rsc. Adv. 2016, 6, (33), 27272-27281. (28) Samant, P. V.; Gonçalves, F.; Freitas, M. M. A.; Pereira, M. F. R.; Figueiredo, J. L. Surface activation of a polymer based carbon. Carbon. 2004, 42, (7), 1321-1325. (29) Sun, F.; Liu, J.; Chen, H.; Zhang, Z.; Qiao, W.; Long, D.; Ling, L. Nitrogen-Rich Mesoporous Carbons: Highly Efficient, Regenerable Metal-Free Catalysts for Low-Temperature Oxidation of H2S. Acs. Catal. 2013, 3, (5), 862–870.


35


Page 36 of 37

(30) Chen, H.; Sun, F.; Wang, J.; Li, W.; Qiao, W.; Ling, L.; Long, D. Nitrogen Doping Effects on the Physical and Chemical Properties of Mesoporous Carbons. J. Phys. Chem. C. 2013, 117, (16), 8318–8328. (31) Sun, F.; Wang, J.; Chen, H.; Li, W.; Qiao, W.; Long, D.; Ling, L. High efficiency immobilization of sulfur on nitrogen-enriched mesoporous carbons for Li-S batteries. Acs. Appl. Mater. Inter. 2013, 5, (12), 5630-5638. (32) Zhang, W. J.; Rabiei, S.; Bagreev, A.; Zhuang, M. S.; Rasouli, F. Study of NO adsorption on activated carbons. Appl. Catal. B: Environ. 2008, 83, (1–2), 63-71. (33) Sendt, K.; Haynes, B. S. Density Functional Study of the Chemisorption of O2 Across Two Rings of the Armchair Surface of Graphite. J. Phys. Chem. C. 2007, 111, (14), 5465-5473. (34) Shirahama, N.; Moon, S. H.; Choi, K. H.; Enjoji, T.; Kawano, S.; Korai, Y.; Tanoura, M.; Mochida, I. Mechanistic study on adsorption and reduction of NO over activated carbon fibers. Carbon. 2012, 40, (2), 2605-2611. (35) Zhu, Z.; Liu, Z.; Liu, S.; Niu, H. Adsorption and reduction of NO over activated coke at low temperature. Fuel. 2000, 79, (6), 651-658. (36) Gao, X.; Liu, S.; Zhang, Y.; Luo, Z.; Ni, M.; Cen, K. Adsorption and reduction of NO2 over activated carbon at low temperature. Fuel Process.Technol. 2011, 92, 139-146. (37) Zhang, Z.; Atkinson, J. D.; Jiang, B.; Rood, M. J.; Yan, Z. Nitric oxide oxidation catalyzed by microporous activated carbon fiber cloth: An updated reaction mechanism. Appl. Catal. B: Environ. 2014, 148–149, 573-581. (38) Fanning, P. E.; Vannice, M. A. A DRIFTS study of the formation of surface groups on carbon by oxidation. Carbon. 1993, 31, (5), 721-730.


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(39) Biniak, S.; Szymański, G.; Siedlewski, J.; tkowski, A. Ś. j. m. The characterization of activated carbons with oxygen and nitrogen surface groups. Carbon. 1997, 35, (12), 1799-1810. (40) Li, G.; Jones, C. A.; Grassian, V. H.; Larsen, S. C. Selective catalytic reduction of NO2 with urea in nanocrystalline NaY zeolite. J. Catal. 2005, 234, (2), 401-413. (41) Zawadzki, J.; Wisniewski, M.; Skowronska, K. Heterogeneous reactions of NO2 and NO-O2 on the surface of carbons. Carbon. 2003, 41, (2), 235-246. (42) Kante, K.; Deliyanni, E.; Bandosz, T. J. Interactions of NO2 with activated carbons modified with cerium, lanthanum and sodium chlorides. J. Hazard. Mater. 2009, 165, (1–3), 704-713. (43) Tighe, C. J.; Twigg, M. V.; Hayhurst, A. N.; Dennis, J. S. Adsorption and Reaction of NO2 on Carbon Black and Diesel Soot at Near-Ambient Temperatures. Ind. Eng. Chem. Res. 2011, 50, (18), 10480-10492. (44) Baker, R. T. K.; Jr, J. J. C. Catalytic gasification of graphite by chromium and copper in oxygen, steam and hydrogen. Carbon. 1981, 19, (2), 75-82. (45) Baumgarten, E.; Schuck, A. Oxygen spillover and its possible role in coke burning. Appl. Catal. 1988, 37, (00), 247-257.

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Low-Temperature Selective Catalytic Reduction of NOx with Urea

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