The Roles of Sulfur-Containing Species in the Selective Catalytic

Article pubs.acs.org/IECR

The Roles of Sulfur-Containing Species in the Selective Catalytic Reduction of NO with NH3 over Activated Carbon Yuran Li,* Yangyang Guo, Jin Xiong, Tingyu Zhu,* and Junke Hao Beijing Engineering Research Center of Process Pollution Control, National Engineering Laboratory for Cleaner Hydrometallurgical Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing, 100190, China S Supporting Information *

ABSTRACT: In the selective catalytic reduction (SCR) of NO with NH3 over activated carbon (AC), deactivation occurs over time in the presence of SO2. This work distinguishes the multiple roles of SO2 in the gas phase versus the solid deposition product and clarifies the effects of the physicochemical properties of AC on NO conversion. The deposition products were detected using temperature-programmed desorption (TPD) coupled with mass spectrum (MS) analysis and Fourier transform infrared (FTIR) spectrometry. The results showed that the activated carbon loses less de-NOx activity when it has more CO- and CO2containing groups with decomposition temperatures over 900 K. The Raman spectra revealed that the disorder of the microcrystalline structure of the graphite has a positive linear correlation with NO conversion regardless of the presence of functional groups. The deposition products were analyzed by Gaussian-Lorentz deconvolution of the TPD spectra, and it was discovered that the sulfur-containing species included sulfate and strongly adsorbed SO2/SO3; the NH3-containing species included NH4HSO4 and freely adsorbed NH3; and the ratios of SO2/SO3, NH4HSO4 and NH3 were approximately 31 mol %, 42 mol %, and 26 mol %, respectively. NH4HSO4 does not notably inhibit NO conversion, even with a high loading amount. The inhibitory effect of gaseous SO2 on NO conversion is reversible, and this inhibitory effect is greater than that caused by the loss of functional groups. Increasing the disorder of the microcrystalline structure of the graphite and reducing the gaseous SO2 were identified as ways to improve activated carbon activity for NO conversion.

1. INTRODUCTION Coal-fired power plants use selective catalytic reduction (SCR) of NO by NH3 over a V2O5−WO3/TiO2 catalyst as an effective method to control NOx. Generally, the SCR reactors are located upstream of the economizer and of flue gas desulfurization in order to meet the optimum working temperature of 300−450 °C.1,2 For other industrial flue gases, such as sintering flue gas or coke-oven flue gas, the effluent gas temperature is normally below 200 °C. However, SCR technology cannot be adopted without reheating, so low temperature de-NOx technology for this industry becomes extremely urgent under strict emission standards, such as those in China.3,4 With regard to the sulfur poisoning of low-temperature catalysts,5 activated carbon (AC) is an ideal material, either as a catalyst or as a support for metal oxides, for low temperature de-NOx and the simultaneous removal of SO2 and heavy metals in flue gas.6−9 The SCR activity over AC gradually declines in the presence of SO2. During the solid phase, the ammonium sulfate deposited on the catalyst surface has two effects on the reduction of NO.10−14 The sulfate ions substantially improve the Brønsted acid on the catalyst surface, strengthening the NH3 adsorption and activation onto the surface. However, ammonium sulfate is deposited on the active sites and blocks the pores of the AC, inhibiting the reaction. In the gas phase, SO2 has an inhibitory effect on NO © XXXX American Chemical Society

conversion due to competitive adsorption onto the same active sites.15,16 The factor that exerts the more significant influence on this deactivation is not yet known. Various AC materials also affect NO conversion.17−20 Correlations between NO conversion and physical properties, such as specific surface area and pore size distribution, are not apparent, and conflicting conclusions have been drawn by different investigators.21−24 Limited work has been reported on the microcrystalline structure of AC except for that of original coal.16 Some investigators have recognized that surface chemistry plays a decisive role.25 Oxygen functional groups are generally considered to enhance NH3 adsorption, and a certain amount of CO2-containing groups is required for NH3 adsorption. In addition, closely situated CO-containing groups may be involved in NO adsorption and may further react with the adsorbed NH3.26−30 The adsorbed NH3 could react with NO or be activated by the active phases. When activated carbon is treated thermally to restore its activity, the structural change and the loss of the functional Received: Revised: Accepted: Published: A

August 24, 2016 October 20, 2016 November 10, 2016 November 10, 2016 DOI: 10.1021/acs.iecr.6b03255 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

relative humidity (RH) of 80%. After being mixed in the mixing vessel, the gas was fed into the reactor. When the AC samples first adsorbed 1000 ppm of SO2 or 500 ppm of NH3 for 120 min and then adsorbed other gas components, the atmosphere was labeled (SO2)* ← NH3 + NO + O2 or (NH3)* ← NO + O2. This process is sequential adsorption. The effluent gas was continuously detected using a quadruple mass spectrometer (MS, GAM200, IPI). The SO2 and NO components were identified using the major mass ions of 64 and 30, respectively. NH3 was detected by a Fourier transform infrared (FTIR) spectrometer (Nicolet 6700, Thermo) with a wavelength of 954−980 cm−1 because NH3 and H2O have the same ionic mass and cannot be distinguished by mass spectrometry. The NO conversion rate, expressed as η, was calculated according to eq 1, with an error of less than 3% from the mass flow controllers and the MS. The difference in NO conversion rates expressed as Δη refers to the NO conversion rate in the original AC minus the NO conversion rate on the heated AC, as shown in eq 2.

groups may inhibit the NO conversion. This work explores whether its physical properties or functional groups affect NO conversion. After thermal treatment, knowing which properties will be preserved and will contribute to de-NOx activity will help to select the AC. As SO2 coexists with NO, it is of interest to know whether the inhibitory factor for NO conversion is derived from the gas- or solid-phase sulfur-containing species.

2. EXPERIMENTAL SECTION 2.1. Materials and Characterization. Five commercial AC samples made from coal, bamboo, coconut, and shell were selected and labeled AC-1 to AC-5. AC-1 and AC-3 to AC-5 were all from the Gongyi AC plant in Henan province, and AC-2 was from the Xinhua AC plant in Shanxi province. AC-2 and AC-3 showed different properties due to the various parent coal and preparation methods. The samples were dried at 383 K for 12 h and then crushed. The particles were sieved through a 0.38−0.83 mm screen before the experiments. The original AC samples were heated in N2 at 1173 K for 2 h to remove the functional groups, and heated AC samples were thus obtained. The pore properties of the samples were determined using N2 adsorption at 77 K in an automatic surface area and porosity analyzer (Autosorb iQ, Quantachrome). The surface area (SBET) was calculated from the N2 adsorption isotherms using the multipoint Brunauer−Emmett−Teller (BET) equation. The total pore volume (Vt) was determined from the amount of N2 adsorbed at p/p0 = 0.95. The pore size distributions were calculated using the Quenched Solid Density Functional Theory (QSDFT) split model, with results shown in the Supporting Information, Figure 1S. Acid−base titration was used to quantitatively determine the basic and acidic functional groups.31 A total of 0.50 g of each sample was placed in a vial with 50 mL of 0.1 mol/L HCl solution, sealed, and shaken for 24 h at 293 K. The mixture was then filtered. A total of 20 mL of the filtrate was titrated with 0.1 mol/L NaOH solution. The number of basic sites was calculated from the amount of HCl solution that reacted with the carbon. The same process was used to measure the acidic sites, except 50 mL of 0.1 mol/L NaOH solution was used. One sample, AC-3, was examined extensively due to its excellent de-NOx performance, as described in sections 3.4−3.6. Two samples of NH4HSO4/AC-3 and (NH4)2SO4/AC-3 were prepared with the pore-volume impregnation method using an aqueous solution containing NH4HSO4 or (NH4)2SO4. A total of 10.0 g of AC-3 was slightly immersed in 11.5 mL of deionized water with 25 mg NH4HSO4 or (NH4)2SO4 at 333 K for 12 h and then dried at 333 K for 24 h. To investigate the SO2 and NH3 desorption from the ammonium bisulfate and ammonium sulfate, the loading was 2.5 mg/g AC for each sample. To investigate the effects of the deposition products on NO conversion, the loading amounts were 95 and 200 mg/g AC for the NH4HSO4/AC-3. 2.2. De-NOx Performance Evaluation. The SCR reaction of NO on AC was investigated in a fixed-bed quartz tube reactor of 20 mm in diameter and 520 mm in height. For each experiment, 1.0 g of AC with a size of 0.38−0.83 mm was loaded onto the sintered sieve plate in the middle of the reactor. The reaction temperature was 423 K. The reaction time was 120 min. The gas flow rate at the standard state was 300 mL/min. The gaseous hourly space velocity (GHSV) was approximately 11180 h−1. The blended gas consisted of 1000 ppm of SO2, 500 ppm of NO, 500 ppm of NH3, 5 vol % O2, with a balance of Ar. Water vapor was generated using N2 from a bubbling container with a

η% =

[NO]in − [NO]out × 100 [NO]in

(1)

Δη = ηori − ηheat

(2)

The differential thermogravimetric (DTG) curves of the samples were measured on a TG apparatus (Versa Therm HM, Thermo Cahn). Approximately 300 mg of the sample was heated in Ar from room temperature to 900 K at a heating rate of 10 K/min. The oxygen functional groups were analyzed by temperatureprogrammed desorption (TPD) coupled with MS at a heating rate of 10 K/min. The effluent CO2 and CO gases were identified by their major mass ions of 44 and 28, respectively. The fine structural details of the AC were detected by a Raman spectrometer (Labramaramis, HORIBA JobinYvon) using a 514.5 nm Ar line as the excitation source. Raman spectra from 0 to 4000 cm−1 at approximate 0.5 cm−1 intervals were measured for 15 s using backscattering geometry filtered with a 64 cm single monochromator.

3. RESULTS AND DISCUSSION 3.1. Effect of physical structure on NO conversion. The properties of the five AC samples are shown in Table 1. The Table 1. Characterization of the Five AC Samples sample no.

raw material

SBET (m2/g)

Vt (mL/g)

acidic sites (mmol/g)

basic sites (mmol/g)

AC-1 AC-2 AC-3 AC-4 AC-5

bamboo coal-sx coal-hn shell coconut

396 95 796 606 1230

0.423 0.096 0.407 0.363 0.568

0.7686 0.2432 0.7940 0.4403 0.4546

0.0067 0.0384 0.0963 0.0919 0.2078 ̀

specific surface areas and total pore volumes were related to the NO conversion rates, and the results are shown in Figure 2S. The results showed that neither parameter had a linear or regular relationship with the NO conversion rate, which was consistent with a previous report.25 Raman spectroscopy was then used to reveal the graphite microcrystalline structure of the AC. These results are shown in Figure 1. The two sharp Raman peaks, the G (graphite)-band and the D (disorder)-band, appeared at approximately 1595 and 1340 cm−1, respectively.32 For the original samples, AC-1 and B

DOI: 10.1021/acs.iecr.6b03255 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article


3.2. Effect of Functional Groups on NO Conversion. The acidic and basic functional groups of the five AC samples are given in Table 1. AC-1 and AC-3 had more acidic sites, and AC-5 had more basic sites. The acidic and basic sites showed a minimal direct relationship with the NO conversion rate. As shown in Figure 3, the acidic sites appeared to have an obvious pattern in

Figure 3. Relationships between basic and acidic sites in AC and the difference in NO conversion rates. Conditions: 1.0 g of AC, 300 mL/ min gas flow rate, 423 K, 120 min, 500 ppm of NO, 500 ppm of NH3, 5 vol % O2 and Ar balance.

the difference of NO conversion rates; however, the basic sites did not. More acidic sites greatly reduced the NO conversion rate except for AC-3. The basic sites amounted to less than 0.2 mmol/ g compared with the acidic sites, which ranged from 0.2 to 0.8 mmol/g. The decomposition of the various types of oxygen functional groups corresponds to the release of CO and CO2 at various temperature ranges.34,35 The results are shown in Figure 4. AC-1 contained more carboxyl, anhydride, and phenolic hydroxyl groups, while AC-3 contained more lactone and carbonyl groups. The TPD spectra of CO and CO2 were separated into several desorption peaks by the Gaussian−Lorentz deconvolution method. The five desorption temperature peaks for CO2 corresponded to strong carboxylic acid, weak carboxylic acid, anhydride, and two types of lactones, respectively.34,35 The desorption temperature peaks for CO included phenol, carbonyl, and a small number of anhydride groups. The fit curves for the AC samples are shown in Figure 5. The peak positions and integral areas for the desorption temperature peaks are shown in Tables S1 and S2. The correlation between the area integration of the functional groups and the difference in NO conversion was initially compared based on the total desorbed CO2 and CO, as shown in Figure 6a. CO2 shows a better linear relation with NO conversion, thus, to tell which specific acidic oxygen groups have a direct relation to the NO conversion, the deconvolution of the CO2 peak is shown in Figure 6b,c. Strong and weak carboxylic acids correspond to peaks 1 and 2 and show little correlation with the differences in NO conversion. The anhydride and lactone groups correspond to peaks 3−5 and show a positive linear relationship with the differences in NO conversion for the samples, except for AC-3. However, for CO-containing groups, each individual peak showed a minimal relationship with the differences in NO conversion. Carboxylic acids, anhydrides, lactones, and phenols are acidic groups, while carbonyl oxygen is neutral or basic. The carboxyl, anhydride, and phenol groups on the AC mainly enhanced the NH3 adsorption, followed by the SCR reaction.36,37 As the

Figure 1. Raman spectra of original AC (a) and heated AC (b).

AC-4 showed a relatively higher G-band peak than the others. Compared with the original AC, the heated AC promoted D peak intensity, especially in the AC-3 sample. The ratio of the band intensity (Id/Ig) is typically used in amorphous carbons to indicate the sp2 cluster size or the relative content of the sp3 bonding structures.33 The ratio also reflects the degree of disorder in the graphite microcrystalline structure, and a greater number of structural defects in the AC indicates a poorly organized structure.33 The results in Figure 2 show that

Figure 2. Relationships between the Id/Ig value and NO conversion rate. Conditions: 1.0 g of AC, 300 mL/min gas flow rate, 423 K, 120 min, 500 ppm of NO, 500 ppm of NH3, 5 vol % O2 and Ar balance.

the overall Id/Ig values of the heated AC were larger than those of the original AC. This indicates that thermal treatment caused the microcrystalline structure of the AC to become more disordered. The Id/Ig value has a positive linear correlation with NO conversion in the original and treated AC, indicating that the disorder of the graphite microcrystalline structure promotes the SCR reaction because the additional structural defects provide adsorption sites for NH3 and/or NO. C


Article


investigated in the following sections due to its excellent de-NOx performance. 3.4. Effects of the Presence of Gaseous SO2 on NO Conversion. In this section, only the AC-3 sample was investigated for NO conversion rates under various conditions, as shown in Figure 8. In the absence of SO2, the NO conversion rate initially decreased and then slowly increased to reach equilibrium at 75% after 120 min. In the presence of SO2, the NO conversion rate reached a minimum value and then stabilized at 21%. It was clear that SO2 inhibited NO conversion. Preadsorbed SO2 slightly increased NO conversion, which indicates that the acidic surface increased NH3 adsorption.11 Preadsorbed NH3 showed the same function as the gaseous NH3 on the SCR of NO, while the relatively low NO conversion rate indicated that the amount of preadsorbed NH3 is smaller than the amount of real-time adsorbed NH3 under the same adsorption time of 120 min. The NO conversion rate decreased rapidly from 100% to a low point and then ascended slowly for the SCR reaction of NO. NO initially adsorbed on the AC surface, as the original NO breakthrough curves climbed rapidly, and then the adsorbed NO reacted with NH3. This dynamic reaction promoted more NO reduction, and then the effluent NO concentration gradually decreased. This phenomenon could be caused by the catalytic sites on AC, the oxygen groups and the defects in the carbon edges, which require more time to reach equilibrium, hence the continuing rise in the NO conversion rate after 120 min. To investigate whether the inhibitory effect was from gaseous SO2 or from the solid phase products, SO2 was added and removed stepwise. Figure 9 shows the experimental results. In contrast to Figure 8, the initial decrease stage of the NO conversion rate as in Figure 9 was hidden over time and not observed. After the NO conversion climbed to an equilibrium of 78%, SO2 was introduced. The NO conversion immediately decreased and gradually plateaued at approximately 23%. Upon SO2 shutoff, the NO conversion sharply increased to above 78% in 6 h. The NO catalytic reaction on the AC continued after 3 h, and NO reduction was very stable on the AC. The inhibitory effect of SO2 on NO conversion is reversible and impermanent. In contrast, for the SCR of NO over manganese oxides supported on a functionalized multiwalled carbon nanotube catalyst at 473 K, when H2O and SO2 were added and removed stepwise, the SCR activity could only be partially recovered.38 3.5. Analysis of Deposition Products. The TPD-MS method was used to identify the various SO2 and NO species deposited on the AC surface. The desorption curves of NO species are shown in Figure 10. In the absence of NH3, the peak of NO desorption was the highest at a maximum centered at 402 K. However, the peaks were notably lower, and the peak temperatures increased to 430−452 K whether NH3 was present or preadsorbed. The NO desorption peak centered at approximately 402−452 K corresponds to the physically adsorbed NO.15 The melting point for NH4NO3 is 442.75 K, and it decomposes when below its boiling point of 483.15 K.39 Therefore, the NO-containing species include physically adsorbed NO and NH4NO3. The amount of NO-containing species was very small and could be neglected in the SCR reaction. Whether SO2 was preadsorbed on the AC or present in the atmosphere, the peaks and temperature of NO desorption decreased because a portion of NO was desorbed by SO2. The desorption curves of SO2 are shown in Figure 11. The SO2 desorption displays two peak temperatures centered at 595 and

Figure 4. TPD-MS profiles of CO2 (a) and CO (b) of the AC samples (The corresponding relationship between the functional groups and temperature is based on refs 34 and 35).

activated carbon had more carboxyl, anhydride, and phenol groups with decomposition temperatures below 900 K, the NO conversion was largely eliminated after the AC was thermally treated. When the AC had more lactone and carbonyl groups with high decomposition temperature, the NO conversion rate decreased slightly. 3.3. Comparison of the Effects of SO2 and Characteristics of AC on NO Conversion. As shown in Figure 7, the NO conversion rates varied from 57% to 73% for the five original AC samples and from 28% to 67% for the heated AC samples. The NO conversion rates decreased more for AC-1 and AC-4 than for others. This is because these two samples have more functional groups with decomposition temperatures below 900 K, such as carboxylic acids, anhydrides, and phenols; in particular, carboxylic acids decompose below 700 K. Another characteristic of AC-1 and AC-4 is a high degree of graphitization, with few structural defects. In stark contrast, the NO conversion rates decreased slightly for AC-3. AC-3 has high contents of lactone and carbonyl or quinone, which have decomposition temperatures above 900 K, and it has a higher degree of disorder in the graphite microcrystalline structure and a greater number of structural defects. To prepare high-performance AC for the SCR of NO, the disorder of the graphite microcrystalline structure and the contents of lactone and carbonyl groups, but not the total specific surface area and acidic functional groups, should be increased. The presence of SO2 also reduced the NO conversion of the original AC to 19% - 32%. This means that the inhibitory effect of the SO2 gas on NO conversion is larger than that from the loss of the functional groups. To slow the decrease of NO conversion, the SO2 gas in the atmosphere should be reduced. AC-3 was D


Article


Figure 5. Deconvolution spectra of CO2 (a) and CO (b) from five AC samples: peak 1, strong carboxyl; peak 2, weak carboxyl; peak 3, anhydride; peak 4, lactone; peak 5, lactol.

720 K, corresponding to the strongly adsorbed SO2 or the oxidized SO3 and sulfate.15 The integral area reflects the amount

desorbed. The deconvolution results of the desorption spectra, as obtained from the Gaussian−Lorentz method, are shown in E


Article


Figure 7. Effects of SO2 and functional group loss on NO conversion rate. Conditions: 1.0 g of AC, 300 mL/min gas flow rate, 423 K, 120 min, 500 ppm of NO, 500 ppm of NH3, 1000 ppm of SO2 (when used), 5 vol % O2 and Ar balance.

Figure 8. Effect of SO2 gas and preadsorbed SO2 on NO conversion rate. Conditions: 1.0 g of AC-3, 300 mL/min gas flow rate, 423 K, 500 ppm of NO, 500 ppm of NH3, 1000 ppm of SO2 (when used), 5 vol % O2 and Ar balance. (SO2)* ← NH3 + NO + O2: a sequential adsorption of first SO2 and then NH3 + NO + O2. (NH3)* ← NO + O2: a sequential adsorption of first NH3 and then NO + O2).

Figure 6. Relationship between differences in NO conversion rate and CO2 and CO containing functional groups (a) and various CO2 containing groups (b,c) of AC. Conditions: 1.0 g of AC, 300 mL/min gas flow rate, 423 K, 120 min, 500 ppm of NO, 500 ppm of NH3, 5 vol % O2 and Ar balance. Figure 9. Effect of stepwise addition-removal of SO2 gas on the NO conversion rate. Conditions: 1.0 g of AC-3, 300 mL/min gas flow rate, 423 K, 500 ppm of NO, 500 ppm of NH3, 1000 ppm of SO2 (when used), 5 vol % O2 and Ar balance).

Table 2. In the atmosphere of SO2 + NH3 + NO + O2 (SCR reaction), the deposition of sulfur included 57.5 mol % sulfate and 42.5% strongly adsorbed SO2/SO3. Compared to the atmosphere of SO2 adsorption, the total molar amount of desorbed SO2 increased by 1.68-fold in the atmosphere of the SCR reaction, of which the strongly adsorbed SO2/SO3 increased by 5.2-fold and the sulfate increased by 90.4%. This indicates that the SCR reaction can significantly promote the deposition of SO2-containing species, especially the strongly adsorbed SO2/ SO3. For the AC preadsorbed with SO2, the deposition was still mainly sulfate, approximately 86.7 mol %. The total molar

amount of desorbed SO2 decreased to 49.1%, of which the strongly adsorbed SO2/SO3 decreased to 63.3%, and the sulfate decreased to 45.5%. This means that half of the SO2-containing species deposited on the AC were consumed after the introduction of NO and NH3 and then migrated into the gas phase. NH3 reacts mainly with the strongly adsorbed SO2/SO3. The melting point for NH4HSO4 is 420.05 K and the boiling point is 763.15 K. (NH4)2SO4 begins to decompose below F


Article


Figure 10. NO desorption curves after AC-3 adsorption in various atmospheres: (SO2)* ← NH3 + NO + O2, a sequential adsorption of first SO2 and then NH3 + NO + O2; (NH3)* ← NO + O2, a sequential adsorption of first NH3 and then NO + O2.

Figure 12. Simultaneous SO2 and NH3 desorption curves for various parent AC-3 samples.

Unlike the NH4HSO4/AC-3 sample, the adsorbed AC-3 began to release NH3 at 450 K. This NH3 was mainly from the free NH3 on the adsorbent surface, which was verified by an NH3 desorption curve with a desorption peak temperature of 510 K. On the basis of the deconvolution results of the desorption spectra, the deposition products were mainly NH4HSO4, composing approximately 61.7 mol %, and free NH3. In general, the deposition products included the strongly adsorbed SO2/ SO3, NH4HSO4, and freely adsorbed NH3, composing 31 mol %, 42 mol %, and 26 mol %, respectively. 3.6. Effects of the Presence of Ammonium Sulfate on NO Conversion. It can be predicted that the amount of the deposition products will accumulate over the reaction time. The product samples were prepared though the SCR reaction in an atmosphere of SO2 + NH3 + NO + O2 over AC-3 for various reaction times. The DTG profiles of the samples are shown in Figure 13. The weight loss calculated from the integral area above

Figure 11. SO2 desorption curves after AC-3 adsorption in various atmospheres: (SO2)* ← NH3 + NO + O2, a sequential adsorption of first SO2 and then NH3 + NO + O2.

508.15 K.39 Therefore, the deposited sulfate may include NH4HSO4 and (NH4)2SO4. The desorption curves of SO2 and NH3 were obtained simultaneously to distinguish NH4HSO4 and (NH4)2SO4, and the results are shown in Figure 12. The adsorbed AC obtained from the SCR reaction in the presence of SO2 was compared with the NH4HSO4/AC-3 and (NH4)2SO4/AC-3 samples. In all three samples, the peak desorption temperatures for SO2 appeared at approximately 570 and 710 K. The three samples had similar desorption peaks for NH3 at 570−605 K. Distinctively, the(NH4)2SO4/AC-3 released more NH3 at 850−900 K. Pure NH4HSO4 crystallite appears as a unique weight loss peak centered at approximately 633 K.40 The (NH4)2SO4 decomposes first to generate NH 4 HSO 4 and NH 3 between 486 and 581 K. Then, (NH4)2S2O7 is generated, and NH3 is released above 603 K.41 The desorption curves of the adsorbed AC are similar to those of the NH4HSO4/AC. Therefore, the deposition products included NH4HSO4, in accordance with the results from previous studies.42

Figure 13. DTG curves of the adsorption products and the deposition product weight (Wdep) at various adsorption times. Conditions: 300 mg of sample, heating rate of 10 K/min, Ar.

Table 2. Deconvolution Results of the Desorption Spectra in Figure 11 curves

total peak area (A·K)

peak area I (A·K)

peak area II (A·K)

SO2 + O2 SO2 + NH3 + NO + O2 (SO2)* ← NH3 + NO + O2

4.48 × 10−10 1.20 × 10−09 2.28 × 10−10

8.31 × 10−11 5.13 × 10−10 3.05 × 10−11

3.65 × 10−10 6.95 × 10−10 1.99 × 10−10

G


Article


the amount of NO-containing species deposited on the AC is very small and can be ignored. The deposition products included the strongly adsorbed SO2/SO3, NH4HSO4, and freely adsorbed NH3 at molar ratios of 31%, 42%, and 26%, respectively. A long adsorption time stabilizes the deposition products. NH4HSO4 had no inhibitory effects on NO conversion, while an inhibitory effect appeared in the presence of water vapor.

450 K linearly increased with reaction time and reached 25 mg after 50 h. The deposition weight per unit time was approximately 0.5 mg/h·g AC, indicating that the SCR reaction was steady. The DTG curves displayed two peak temperatures above 450 K, which were consistent with the SO2 desorption curves. The main peak temperature increased from 580 K to 591 K, and the shoulder peak temperature increased from 704 K to 765 K as the adsorption time increased. The increase in the desorption temperature indicated that a lengthy adsorption time made the sulfate product more stable. The effect of the deposition weight on NO conversion is shown in Figure 14. The samples with a deposition weight below

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b03255. Adsorption desorption isotherms and pore volume distributions of the five AC samples; relationships between AC pore structure and NO conversion rate; deconvolution results of CO2 and CO from five AC samples (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*Tel./Fax: +86-10-82544823. E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21207132, 21477131) and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB05050502).

Figure 14. Effect of deposition weight on the NO conversion rate. Conditions: 1.0 g of parent AC-3 products, 300 mL/min gas flow rate, 423 K, 120 min, 500 ppm of NO, 500 ppm of NH3, RH = 80% (when used), 5 vol % O2 and Ar balance).

■

25 mg were the same samples described in the previous paragraph. The samples with a deposition weight of 95 and 200 mg were from impregnation methods. The reaction atmosphere did not include SO2 to avoid interference. The NO conversion rate showed a minor change on the adsorbed AC-3 compared to that of 78% on the fresh AC-3. With loading amounts of 95 and 200 mg obtained through the impregnation method, the NO conversion rates were still approximately 78%. The results showed that high loading amounts of NH4HSO4 did not notably inhibit NO conversion. This conclusion also verified the reversible inhibitory effect of SO2 on NO conversion. In the presence of water vapor with a RH of 80% and NH4HSO4 loading of 200 mg on 1.0 g of AC-3, the NO conversion rate decreased to 59%. Therefore, inhibitory effects may have essentially been caused by the competitive adsorption of H2O and NH3 due to the large difference in the orders of magnitude of content.43

REFERENCES

(1) Forzatti, P. Present status and perspectives in de-NOx SCR catalysis. Appl. Catal., A 2001, 222, 221. (2) Li, J.; Peng, Y.; Chang, H.; Li, X.; Crittenden, J. C.; Hao, J. Chemical poison and regeneration of SCR catalysts for NOx removal from stationary sources. Front. Environ. Sci. Eng. 2016, 10, 413. (3) Emission standard of air pollutants for sintering and pelletizing of iron and steel industry; GB 28662-2012; Ministry of Environmental Protection of P.R.C., General Administration of Quality Supervision, Inspection and Quarantine of P.R.C., 2012. (4) Emission standard of pollutants for coking chemical industry; GB 16171-2012; Ministry of Environmental Protection of P.R.C., General Administration of Quality Supervision, Inspection and Quarantine of P.R.C., 2012. (5) Shi, Y. J.; Shu, H.; Zhang, Y. H.; Fan, H. M.; Zhang, Y. P.; Yang, L. J. Formation and decomposition of NH4HSO4 during selective catalytic reduction of NO with NH3 over V2O5-WO3/TiO2 catalysts. Fuel Process. Technol. 2016, 150, 141. (6) Tsuji, K.; Shiraishi, I. Combined desulfurization, denitrification and reduction of air toxics using activated coke 1. Activity of activated coke. Fuel 1997, 76, 549. (7) Tsuji, K.; Shiraishi, I. Combined desulfurization, denitrification and reduction of air toxics using activated coke 2. Process applications and performance of activated coke. Fuel 1997, 76, 555. (8) Gao, X.; Liu, S.; Zhang, Y.; Du, X.; Luo, Z.; Cen, K. Low temperature selective catalytic reduction of NO and NO2 with NH3 over activated carbon-supported vanadium oxide catalyst. Catal. Today 2011, 175, 164. (9) Yang, W.; Liu, F.; Xie, L.; Lian, Z.; He, H. Effect of V2O5 additive on the SO2 resistance of a Fe2O3/AC catalyst for NH3-SCR of NOx at low temperatures. Ind. Eng. Chem. Res. 2016, 55, 2677. (10) Hou, Y.; Huang, Z.; Guo, S. Effect of SO2 on V2O5/ACF catalysts for NO reduction with NH3 at low temperature. Catal. Commun. 2009, 10, 1538.

4. CONCLUSIONS A disordered graphite microcrystalline structure of AC promotes NO conversion independent of the presence or absence of functional groups. When AC samples have more lactone and carbonyl or quinone groups with decomposition temperatures above 900 K, they have a high catalytic activity for the SCR of NO and can resist thermal regeneration. After the loss of all oxygencontaining groups, AC still has an NO conversion rate of more than 60%. Discovering which structural defects and carboncontaining groups are most useful will be interesting. The inhibitory effect of gaseous SO2 on NO conversion is reversible, and this inhibitory effect is larger than that caused by the loss of the functional groups on the AC. In the SCR reaction, H


Article

Industrial & Engineering Chemistry Research (11) García-Bordejé, E.; Pinilla, J. L.; Lázaro, M. J.; Moliner, R.; Fierro, J. L. G. Role of sulphates on the mechanism of NH3-SCR of NO at low temperatures over presulphated vanadium supported on carbon-coated monoliths. J. Catal. 2005, 233, 166. (12) García-Bordejé, E.; Pinilla, J. L.; Lázaro, M. J.; Moliner, R. NH3SCR of NO at low temperatures over sulphated vanadia on carboncoated monoliths: effect of H2O and SO2 traces in the gas feed. Appl. Catal., B 2006, 66, 281. (13) Lei, Z.; Han, B.; Yang, K.; Chen, B. Influence of H2O on the lowtemperature NH3-SCR of NO over V2O5/AC catalyst: an experimental and modeling study. Chem. Eng. J. 2013, 215−216, 651. (14) Pan, S.; Luo, H.; Li, L.; Wei, Z.; Huang, B. H2O and SO2 deactivation mechanism of MnOx/MWCNTs for low-temperature SCR of NOx with NH3. J. Mol. Catal. A: Chem. 2013, 377, 154. (15) Guo, Y.; Li, Y.; Zhu, T.; Ye, M. Investigation of SO2 and NO adsorption species on activated carbon and the mechanism of NO promotion effect on SO2. Fuel 2015, 143, 536. (16) Li, P.; Liu, Z.; Li, Q.; Wu, W.; Liu, Q. Multiple roles of SO2 in selective catalytic reduction of NO by NH3 over V2O5/AC catalyst. Ind. Eng. Chem. Res. 2014, 53, 7910. (17) Li, W.; Tan, S.; Shi, Y.; Li, S. Utilization of sargassum based activated carbon as a potential waste derived catalyst for low temperature selective catalytic reduction of nitric oxides. Fuel 2015, 160, 35. (18) Singh, S.; Nahil, M. A.; Sun, X.; Wu, C.; Chen, J.; Shen, B.; Williams, P. T. Novel application of cotton stalk as a waste derived catalyst in the low temperature SCR-deNOx process. Fuel 2013, 105, 585. (19) Wang, Z.; Wang, Y.; Long, D.; Mochida, I.; Qiao, W.; Zhan, L.; Liu, X.; Yoon, S. H.; Ling, L. Kinetics and mechanism study of lowtemperature selective catalytic reduction of NO with urea supported on pitch-based spherical activated carbon. Ind. Eng. Chem. Res. 2011, 50, 6017. (20) Gálvez, M. E.; Boyano, A.; Moliner, R.; Lázaro, M. J. Low-cost carbon-based briquettes for the reduction of NO emissions: Optimal preparation procedure and influence in operating conditions. J. Anal. Appl. Pyrolysis 2010, 88, 80. (21) Izquierdo, M. T.; Rubio, B.; Mayoral, C.; Andrés, J. M. Modifications to the surface chemistry of low-rank coal-based carbon catalysts to improve flue gas nitric oxide removal. Appl. Catal., B 2001, 33, 315. (22) Rubio, B.; Izquierdo, M. T.; Mayoral, C.; Bona, M. T.; Andres, J. M. Unburnt carbon from coal fly ashes as a precursor of activated carbon for nitric oxide removal. J. Hazard. Mater. 2007, 143, 561. (23) Jastrząb, K. Properties of activated cokes used for flue gas treatment in industrial waste incineration plants. Fuel Process. Technol. 2012, 101, 16. (24) Jastrząb, K. Changes of activated coke properties in cyclic adsorption treatment of flue gases. Fuel Process. Technol. 2012, 104, 371. (25) Samojeden, B.; Motak, M.; Grzybek, T. The influence of the modification of carbonaceous materials on their catalytic properties in SCR-NH3: A short review. C. R. Chim. 2015, 18, 1049. (26) Guo, Q.; Jing, W.; Hou, Y.; Huang, Z.; Ma, G.; Han, X.; Sun, D. On the nature of oxygen groups for NH3-SCR of NO over carbonat low temperatures. Chem. Eng. J. 2015, 270, 41. (27) Shen, B.; Chen, J.; Yue, S.; Li, G. A comparative study of modified cotton biochar and activated carbon based catalysts in low temperature SCR. Fuel 2015, 156, 47. (28) Izquierdo, M. T.; Rubio, B.; de Yuso, A. M.; Ballestero, D. Enhancement of nitric oxide removal by ammonia on a low-rank coal based carbon by sulphuric acid treatment. Fuel Process. Technol. 2011, 92, 1362. (29) Izquierdo, M. T.; Rubio, B.; Mayoral, C.; Bona, M. T.; Andres, J. M. Denitrification of stack gases in the presence of low-rank coal-based carbons activated with steam. Energy Fuels 2007, 21, 2033. (30) 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, 27272.

(31) Boehm, H. P. Some aspects of the surface chemistry ofcarbon blacks and other carbons. Carbon 1994, 32, 759. (32) Wang, Y.; Wei, H.; Liu, P.; Yu, Y.; Zhao, Y.; Li, X.; Jiang, W.; Wang, J.; Yang, X.; Sun, C. Effect of structural defects on activated carbon catalysts in catalytic wet peroxide oxidation of m-cresol. Catal. Today 2015, 258, 120. (33) Shimodaira, N.; Masui, A. Raman spectroscopic investigations of activated carbon materials. J. Appl. Phys. 2002, 92, 902. (34) Figueiredo, J. L.; Pereira, M. F. R. The role of surface chemistry in catalysis with carbons. Catal. Today 2010, 150, 2. (35) Shafeeyan, M. S.; Daud, W. M. A. W.; Houshmand, A.; Shamiri, A. A review on surface modification of activated carbon for carbon dioxide adsorption. J. Anal. Appl. Pyrolysis 2010, 89, 143. (36) Huang, Z.; Hou, Y.; Zhu, Z.; Liu, Z. Study on the NO reduction by NH3 on a SO42‑/AC catalyst at low temperature. Catal. Commun. 2014, 50, 83. (37) Li, P.; Liu, Q.; Liu, Z. N2O and CO2 Formation during Selective Catalytic Reduction of NO with NH3 over V2O5/AC Catalyst. Ind. Eng. Chem. Res. 2011, 50, 1906. (38) Pourkhalil, M.; Moghaddam, A. Z.; Rashidi, A.; Towfighi, J.; Mortazavi, Y. Preparation of highly active manganese oxides supported on functionalized MWNTs for low temperature NOx reduction with NH3. Appl. Surf. Sci. 2013, 279, 250. (39) Liu, G.; Ma, L.; Liu, J.Handbook of Physical Properties of Chemistry and Chemical Engineering (Inorganic); Chemical Industry Press: Beijing, 2002; pp 352, 411. (40) Ye, D.; Qu, R.; Song, H.; Gao, X.; Luo, Z.; Ni, M.; Cen, K. New insights into the various decomposition and reactivity behaviors of NH4HSO4 with NO on V2O5/TiO2 catalyst surfaces. Chem. Eng. J. 2016, 283, 846. (41) Fan, Y.; Cao, F. Thermal decomposition kinetics of ammonium sulfate. J. Chem. Eng. Chin. Univ. 2011, 25, 341. (42) Li, P.; Liu, Q.; Liu, Z. Behaviors of NH4HSO4 in SCR of NO by NH3 over different cokes. Chem. Eng. J. 2012, 181−182, 169. (43) Klinik, J.; Samojeden, B.; Grzybek, T.; Suprun, W.; Papp, H.; Gläser, R. Nitrogen promoted activated carbons as DeNOx catalysts. 2. The influence of water on the catalytic performance. Catal. Today 2011, 176, 303.

I


The Roles of Sulfur-Containing Species in the Selective Catalytic

Recommend Documents