SO2 Poisoning and Recovery of Copper-Based Activated Carbon Catalysts for Selective Catalytic Reduction of NO with NH3 at Low Temperature

catalysts

Article SO2 Poisoning and Recovery of Copper-Based Activated Carbon Catalysts for Selective Catalytic Reduction of NO with NH3 at Low Temperature

Marwa Saad * , Agnieszka Szymaszek , Anna Białas , Bogdan Samojeden and Monika Motak *

Faculty of Energy and Fuels, AGH University of Science and Technology, Al. Mickiewicza 30, 30-059 Kraków, Poland; [email protected] (A.S.); [email protected] (A.B.); [email protected] (B.S.) * Correspondence: [email protected] (M.S.); [email protected] (M.M.); Tel.: +48-12-617-2123 (M.M.)

Received: 31 October 2020; Accepted: 3 December 2020; Published: 5 December 2020

Abstract: A series of materials based on activated carbon (AC) with copper deposited in various amounts were prepared using an incipient wetness impregnation method and tested as catalysts for selective catalytic reduction of nitrogen oxides with ammonia. The samples were poisoned with SO2 and regenerated in order to analyze their susceptibility to deactivation by the harmful component of exhaust gas. NO conversion over the fresh catalyst doped with 10 wt.% of Cu reached 81% of NO conversion at 140 ◦C and about 90% in the temperature range of 260–300 ◦C. The rate of poisoning with SO2 was dependent on Cu loading, but in general, it lowered NO conversion due to the formation of (NH4)2SO4 deposits that blocked the active sites of the catalysts. After regeneration, the catalytic activity of the materials was restored and NO conversion exceeded 70% for all of the samples.

Keywords: SCR; activated carbon; copper; DeNOx; SO2 poisoning

1. Introduction

Selective catalytic reduction (SCR) of nitrogen oxides with ammonia (NH3-SCR) is one of the most efficient methods of the abatement of NOx emitted by stationary sources. Since the 1970s, the most common commercial catalyst for the technology has been V2O5-TiO2 promoted by MoO3 or WO3 [1,2]. Despite high catalytic activity above 350 ◦C, the material has some considerable limitations, such as very narrow temperature window, toxicity of vanadium and high catalytic activity in oxidation of SO2 to SO3, causing the emission of a secondary contamination [3–6]. Additionally, the catalyst can oxidize NH3 to N2O which is one of the greenhouse gases [7]. Therefore, there is an urgent need for the novel catalyst to be made of environmentally friendly components and exhibit sufficient activity in the low temperature range (around 250 ◦C or lower) to place it downstream of the desulphurization unit and electrostatic precipitator [8,9]. Another harmful pollutant emitted to the atmosphere as the result of fuel burning is sulphur dioxide (SO2)[2,10,11]. This gas has a detrimental effect on human health and quality of the environment. What is important is that its presence in the exhausts can cause corrosion as well [12,13]. In the industrial applications of NH3-SCR, the catalyst can be easily deactivated by sulphur compounds formed in the combustion chamber due to their presence in fuel [14]. Thus, it lowers the activity of DeNOx catalysts [3,5,11,15]. In the scientific literature, several poisoning mechanisms by SO2 have been reported [16]. Du et al. [3] postulated that the main reason for the deactivation was the formation of ammonium sulphates that plug pores of the catalyst in the temperature range of 200–300 ◦C. On the other hand, Xu et al. [11] suggested that SO2 adsorbs on the active sites of SCR catalysts and forms metal sulphates that interrupt the redox cycle of NO reduction. The poisoning effect occurs mainly

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in the low temperature range (below 300 ◦C) of SCR. It is particularly important for the industrial installations, due to the fact that SO2 oxidation over V2O5-TiO2 increases linearly with the amount of NOx in the combustion zone. As a result, the sulphur trioxide produced can react with the steam and form corrosive sulphuric acid at an increased temperature [5]. Due to the presented interaction of SO2 with the surface of the commercial SCR catalyst and the formation of by-products during the process below 300 ◦C, there is a high demand for the novel catalyst that would be active and resistant to poisoning in the low temperature range of the reaction. Many catalysts including modified layered clays [6,17,18], hydrotalcites [10,19] or zeolite- supported metal oxides [20,21] exhibited high catalytic activity in the abatement of NO emission using the SCR method. The special attention of many researchers has been recently directed to the application of modified activated carbon (AC) as NH3-SCR catalysts [22–25]. This is mainly due to its high specific surface area, well-developed system of pores, and hydrothermal durability [26,27]. The positive influence of activated carbon used as a catalyst support for the simultaneous removal of both NOx and SO2 was confirmed by many researchers [28,29]. Liu et al. [29] tested the catalytic activity of activated carbon modified with aluminum, copper, iron, zinc or manganese in NOx and SOx removal. Among the analyzed materials, Cu supported on microwave-activated coconut shell AC occurred to be the most efficient in simultaneous removal of the considered contaminations. The catalytic tests indicated that at 175 ◦C (low-temperature SCR) NO and SO2 conversion reached 52.5% and 68.3%, respectively. Additionally, many researchers reported that the amount of SO2 adsorbed on the surface of activated carbon is related to the initial modification of the surface of the support and its pore structure [30–33]. It was proved that after some specific treatment the materials are resistant to SO2 in the gas stream. Samojeden and Grzybek [13] deposited nitrogen groups on the surface of activated carbon and analyzed SO2 sorption capacity of the resulting materials. The modification of the activated carbon facilitated adsorption of sulphur oxide without the degradation of the structural features of the samples. According to the above considerations and based on recent knowledge, we have decided to prepare a series of activated carbon-based catalysts loaded with various contents of copper and test the fresh materials in low-temperature NH3-SCR reaction. Additionally, keeping in mind that the industrial exhausts always contain some amounts of other contaminations, such as SO2, the samples obtained were poisoned with sulphur oxide and subjected to the catalytic tests under the same conditions. Furthermore, after exposure to SO2, the catalysts were regenerated using thermal treatment and the NH3-SCR reaction over these materials was repeated.

2. Results and Discussion

2.1. X-ray Diffraction Analysis X-ray diffraction (XRD) patterns of the fresh, poisoned and regenerated samples are presented in Figure1. It can be noticed that for the samples 5U and 5U1Cu, there are di ffraction maxima described by Miller’s indices (002), and (100) and located at 2θ ~25◦ and 43◦, respectively. The maxima reflect the graphite crystallite structure [01-075-1621] and confirm the disordered nature of activated carbon [30]. Additionally, there are no reflections related to copper or copper oxide on the surface of the sample. Thus, it can be concluded that 1 wt.% of Cu loading is not sufficient to form crystallites of metal oxides. Similar results was obtained by Tseng and Wey [34] who did not observe any XRD reflections for copper content lower than 3% as such very small crystals could not be detected by XRD. On the other hand, after introduction of 5 wt.% of copper, the characteristic maxima of CuO [00-045-0937] at 2θ ~35.2◦ and 38.5◦ appear in the pattern. These maxima arise from (111) and (022) planes, respectively. It suggests the formation of copper oxide particles due to a higher amount of Cu on the catalyst surface [35–37]. Therefore, increasing Cu concentration to 5 wt.% results in the formation of a bulk copper oxide phase. Additionally, for the samples poisoned with SO2, intensity of reflections ascribed to copper species significantly decreased. After thermal regeneration, some of them were recovered Catalysts 2020, 10, 1426 3 of 21

which suggests that SO2 could cause reversed reconstruction of the active phase. While the copper loading was increased to 10 wt.%, new reflections assigned to highly crystalline copper oxide phase appeared. The additional diffraction maxima at 2θ ca. 48.7◦ and 67.9 ◦ attributed to CuO and 29.5◦, 36.5◦, 43.5◦, 62.0◦, 65.7◦ and 74.5◦ assigned to Cu2O [01-077-0199] confirm the formation of higher amount of crystalline copper oxide [35,38]. The maxima are described by Miller’s indices (202), (113), (110), (111), (200), (220), (310) and (311), respectively. Also, new reflections assignable to face centre cubic crystal structure of copper [01-070-3039] which were observed at 2θ ca. 43.7◦ and 50.4◦ are described by Miller’s indices (111) and (200), respectively. After poisoning with SO2, the shape of XRD patterns of 5U10Cu did not change noticeably. Nonetheless, the intensity of diffraction lines of CuO and Cu2O significantly decreased. However, no reflections assigned to the surface metal sulphates were detected. This suggests that their amount could be below the detection limit of the apparatus. Additionally, the sizes of the copper oxide crystals decreased upon poisoning. Similar results were obtained by Liu et al. [38] who observed that the intensity of the copper crystals decreased after poisoning, and according to Ma et al.[39], after poisoning more copper sulphates and a side product such as copper sulphide (CuS) were formed in place of copper oxide which was a poorly ordered material and had a lower crystallinity than CuO [40]. The average crystal sizes for the crystallites of Catalysts 2020, 10, x FOR PEER REVIEW 4 of 25 CuO and Cu2O deposited on the selected samples, estimated using Scherrer’s equation, are presented in Table1.

5U fresh 5U poisoned A 5U regenerated

Intensity (arb. un.) (arb. Intensity (0 0 2) (0 (1 0 0)

10 20 30 40 50 60 70 80 2θ (°)

5U1Cu fresh B 5U1Cu poisoned 5U1Cu regenerated (0 0 2) 0 (0 (1 0 0) (1

Intensity (arb. un.) (arb. Intensity

10 20 30 40 50 60 70 80 2θ (°)

Figure 1. Cont.

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5U5Cu fresh C 5U5Cu poisoned 5U5Cu regenerated C (1 0 0) CuO (0 2 2) C (0 0 2) CuO (1 1 1)

Intensity (arb. un.) (arb. Intensity

10 20 30 40 50 60 70 80 2θ (°)

5U10Cu fresh 5U10Cu poisoned D 5U10Cu regenerated O (1O 1 1) 2 CuO (1 1 1) O (2 0 0) Cu 2 O (3 1 0) 2 O (2 2 0) O (3 1 1) 2 2 Cu CuO (-2 0 2) 0 (-2 CuO Cu CuO (1 1 3) Cu (2 0 0) 0 (2 Cu O (1 1 0) CuO (0 2 2) Cu (1 1 1) 1 (1 Cu 2 Cu Cu C (0 0 2) Cu

Intensity (arb. un.) Intensity

10 20 30 40 50 60 70 80 2θ (°) Figure 1. X-ray diﬀraction patterns of fresh, poisoned and regenerated: (A) 5U; (B) 5U1Cu; (C ) 5U5Cu; (D) 5U10Cu. Figure 1. X-ray diffraction patterns of fresh, poisoned and regenerated: (A) 5U; (B) 5U1Cu; (C) 5U5Cu; Table 1. Sizes of CuO and Cu2O crystals present on activated carbon (AC) surface, calculated by (D) 5U10Cu. Scherrer’s equation.

The lateral size (La) and stacking height (Lc) parametersForm of Copper were Oxide calculated using the Scherrer Type of equations andSample are listed in Table 2 [41]: CuO Cu2O the Sample Miller’s Crystalite𝐾𝜆 Size Miller’s Crystalite Size Indices𝐿 = (nm) Indices (nm) (1) 𝐵() cos 𝜃 fresh 21 23 𝐾 𝜆 5U5Cu poisoned(111) 17(200) 19 𝐿 = (2) regenerated𝐵() cos 19 𝜃 20 fresh 30 30 5U10Cu poisoned(111) 24(200) 27 Table 2. Theregenerated lateral size (La) and stacking height 26(Lc) parameters for selected samples. 28

Sample 𝑳𝒄(nm) 𝑳𝒂(nm) 5U fresh 4.51 11.81 5U poisoned 4.51 11.80 5U regenerated 4.50 11.80

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The lateral size (La) and stacking height (Lc) parameters were calculated using the Scherrer equations and are listed in Table2[41]:

Kcλ Lc = (1) B(002) cos θ

Kaλ La = (2) B(100) cos θ

Table 2. The lateral size (La) and stacking height (Lc) parameters for selected samples.

Sample Lc(nm) La(nm) 5U fresh 4.51 11.81 5U poisoned 4.51 11.80 5U regenerated 4.50 11.80

For calculation of La and Lc, we applied the Scherrer constant Kc equal to 0.9 for the (002) diffraction line and Ka equal to 1.77 for the (100) diffraction line, where λ (0.15 nm) is the wavelength of the X-ray used; β002 and β100 are full widths at half maximum of the diffraction lines (002) and (100). The changes in La and Lc are very small and not meaningful. The positions of the XRD reflections of the support shift with poisoning, which, according to Jenkins and Snyder [42], may be the result of microstrain. The effect will generally be a distribution of reflections around the unstressed peak location, and a crude broadening of the peak in the resultant pattern.

2.2. Fourier Transform Infrared Spectroscopy Fourier transform infrared (FT-IR) spectra of the fresh, poisoned, and regenerated catalysts are presented in Figure2. In the spectra of non-poisoned samples, the characteristic peaks at 1120, 1560, 1 1 1715 and 3430 cm− can be distinguished [43,44]. The peak at 1120 cm− corresponds to C–N or N–H 1 1 bonds in aliphatic groups, while the peaks at 1560 cm− and 1715 cm− can be assigned to the stretching 1 vibration of carboxylic anions [45]. The peak at at 3430 cm− conﬁrms the complexation of –OH groups on the AC surface [46–48]. Therefore, the samples doped with copper exhibit hydrophilic properties. FT-IR did not detect any peaks related to copper. However, literature suggests that the absorption peaks 1 in the infrared spectrum of Cu(OH)2 at low frequencies below 700 cm− are due to Cu–O vibrations, 1 1 a i.e., peaks at 505 cm− , 592 cm− associated with the Cu–O vibrations of monoclinic CuO [49], 1 1 1 while bands centered at 3572 cm− , 3315 cm− correspond to the hydroxyl ions [50], and 1121 cm− , 1 1360 cm− related to Cu–OH vibration of the orthorhombic phase of Cu(OH)2 [49]. In this work, it is diﬃcult to identify bands derived from copper oxides/hydroxides, due to much strongest signals 1 arising from the support i.e., C–N or N–H bonds at 1120 cm− in all tested samples (even in unpromoted sample). Another possibility is the vibration of Cu–O was not registered due to its low intensity [51]. 1 Poisoning with SO2 resulted in the appearance of new peaks at 1040, 1165, 1300 and 1400cm− . 1 2 1 The band at 1040 cm− can be assigned to the presence of SO4 − ions, while the one at 1165 or 1300 cm− 1 are attributed to the vibrations of S–O and S=O bonds, respectively [42,43,52,53]. The peak at 1400 cm− 2 can be assigned to SO3 − species present on the poisoned samples [44,54]. It indicates that recovery of the poisoned catalyst at elevated temperature can partly restore the groups initially present on the surface of activated carbon. Moreover, regeneration of the materials resulted in the disappearance of most of the peaks assigned to sulphur species, suggesting that the applied route of recovery was appropriately chosen. Catalysts 2020, 10, 1426 6 of 21

2.3. Thermogravimetric Analysis Thermogravimetric analysis over modified activated carbon was carried out in order to examine thermal behavior of the catalysts in the temperature range of 20–800 ◦C (cf. Table3). The results obtained are presented in Figure3. Weight loss of each material during every stage of the analysis is listed in Table3. In case of all of the samples, three main stages of decomposition can be distinguished. For 5U, the temperature ranges of decomposition are as it follows: 20–80 ◦C, 80–460 ◦C, and 460–800 ◦C. During the first stage total moisture was evaporated from the surface of AC and during the second stage, the weight loss of the materials increased only slightly. However, above 460 ◦C the materials lost significant amount of their initial weight during combustion of activated carbon [43]. At the end of the experiment, the weight loss of the fresh, poisoned and regenerated sample was 33.35%, 57.60%, and 51.44%, respectively. Hence, it can be noticed that poisoning of activated carbon with Catalysts 2020, 10, x FOR PEER REVIEW 7 of 25 SO2 decreased its thermal stability. Additionally, regeneration procedure did not restore it to the initial level.

5U fresh A 5U poisoned 5U regenerated

Transmitance (%) Transmitance 1400 1165 1040 1300 1560 1715 3430 1120

4000 3750 3500 3250 2000 1750 1500 1250 1000 750 500 Wavelength (cm-1)

5U1Cu fresh B 5U1Cu poisoned 5U1Cu regenerated

1120 Transmitance (%) Transmitance 1300 1400 1165 1040 1715 3430 1560 1120

4000 3750 3500 3250 2000 1750 1500 1250 1000 750 500 Wavelength (cm-1)

Figure 2. Cont.

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5U5Cu fresh C 5U5Cu poisoned 5U5Cu regenerated 1120

Transmitance (%) Transmitance 1400 1165 1300 1040 1715 3430 1560 1120

4000 3750 3500 3250 2000 1750 1500 1250 1000 750 500 Wavelength (cm-1)

5U10Cu fresh D 5U10Cu poisoned 5U10Cu regenerated

1120 Transmitance (%) Transmitance 1400 1300 1165 1040 1560 1715 3430 1120

4000 3750 3500 3250 2000 1750 1500 1250 1000 750 500 Wavelength (cm-1)

Figure 2. Fourier transform infrared spectra of fresh, poisoned and regenerated: (A) 5U; (B ) 5U1Cu; (C) 5U5Cu; (D) 5U10Cu. Figure 2. Fourier transform infrared spectra of fresh, poisoned and regenerated: (A) 5U; (B) 5U1Cu; (C) 5U5Cu; (D) 5U10Cu. Table 3. Weight loss of the samples during thermogravimetric analysis (TGA) in the particular Poisoning with SO2 resulted in the appearance of new peaks at 1040, 1165, 1300 and 1400cm−1. temperatureThe band range.at 1040 cm−1 can be assigned to the presence of SO42− ions, while the one at 1165 or 1300 cm−1 are attributed to the vibrations of S–O and S=O bonds, respectively [42,43,52,53]. The peak at 1400 Sample Weight Loss in the Particular Temperature Range (%) cm−1 can be assigned to SO32− species present on the poisoned samples [44,54]. It indicates that recoverySamples of the withoutpoisoned Cu catalyst 20–80 at ◦elevatedC 80–460 temperature◦C 460–800 can partly◦C restore Total weightthe groups loss (%)initially present on the5U surface fresh of activated 12.77 carbon. Moreover 4.51, regeneration 18.07 of the materials 35.35 resulted in the 5U poisoned 22.47 6.32 28.81 57.60 5U regenerated 18.08 6.82 26.54 51.44

Samples with Cu 20–80 ◦C 80–380 ◦C 380–800 ◦C Total weight loss (%) 5U1Cu fresh 16.73 6.08 36.35 59.16 5U1Cu poisoned 26.16 8.90 58.30 93.36 5U1Cu regenerated 20.15 9.21 36.59 65.95 5U5Cu fresh 21.50 5.99 32.19 59.68 5U5Cu poisoned 29.64 9.25 57.58 96.47 5U5Cu regenerated 23.39 6.46 34.11 63.96 5U10Cu fresh 23.09 6.19 34.28 63.56 5U10Cu poisoned 27.69 9.43 56.77 93.89 5U10Cu regenerated 25.27 7.07 33.31 65.65 CatalystsCatalysts 20202020, 10,, x10 FOR, 1426 PEER REVIEW 8 of 2110 of 24

100 A 5U fresh 5U poisoned 5U regenerated 80°C 90 460°C

70 Relative mass (%) Relative

100 200 300 400 500 600 700 800 Temperature (°C)

100 B 5U1Cu fresh 5U1Cu poisoned 90 80°C 5U1Cu regenerated

80 380°C

70 570°C

Relative mass (%) 40

0 100 200 300 400 500 600 700 800 Temperature (°C)

100 C 5U5Cu fresh 5U5Cu poisoned 90 80°C 5U5Cu regenerated 380°C 80

60 570°C

40 Relative mass (%) 30

0 100 200 300 400 500 600 700 800 Temperature (°C) Figure 3. Results of thermogravimetric analysisFigure obtain 3. Conted. for fresh, poisoned, and regenerated: (A) 5U; (B) 5U1Cu; (C) 5U5Cu; (D) 5U10Cu.

Similar to 5U, thermal stability results obtained for the samples modified with copper can also be divided into three stages. In the temperature range of 20–80 °C total moisture was evaporated from the surface of the samples. It can be observed that the addition of copper shifted the temperature range of the second stage to lower values, from 460 °C (for 5U) to 380 °C. Nevertheless, during that stage only negligible weight loss was detected. Above 380 °C, a third stage of decomposition was initiated due to rapid combustion of activated carbon and desorption of carbonic complexes from its surface. Additionally, the mass loss of the samples was higher in comparison to 5U and it gradually

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increased with the increasing amount of the Cu introduced in the case of fresh, poisoned and Catalysts 2020regenerated, 10, 1426 samples. 9 of 21

100 D 5U10Cu fresh 5U10Cu poisoned 90 5U10Cu regenerated 80°C 80 380°C 70

60 570°C

Relative mass (%) 40

0 100 200 300 400 500 600 700 800 Temperature (°C)

Figure 3. Results of thermogravimetric analysis obtained for fresh, poisoned, and regenerated: (A) 5U; (B) 5U1Cu;It (canC) 5U5Cu; be observed (D) 5U10Cu. that the Cu-doped samples poisoned with SO2 exhibited considerably higher and more rapid decomposition during thermogravimetric analysis (TGA) in comparison to the fresh Similarmaterials. to 5U, thermalAdditionally, stability for results the obtainedpoisoned for Cu-containing the samples modified materials, with copperthe decomposition can also be is finished at dividedapproximately into three stages. 570 In the °C. temperature The extensive range weight of 20–80 loss◦C totalis the moisture result wasof carbon evaporated burning from and the vaporization of surface ofthe the adsorbed samples. sulphur It can be observedoxide from that the the support addition and of copper thermal shifted decomposition the temperature of copper range sulphates. The of the second stage to lower values, from 460 ◦C (for 5U) to 380 ◦C. Nevertheless, during that stage mechanism observed during TGA is in agreement with the generally accepted sequence of CuSO4 only negligible weight loss was detected. Above 380 C, a third stage of decomposition was initiated decomposition, described by Equation (3)◦ [55]: due to rapid combustion of activated carbon and desorption of carbonic complexes from its surface. Additionally, the mass loss of the samples was higher in comparison to 5U and it gradually increased CuO · CuSO → 2CuO+ SO (3) with the increasing amount of the Cu introduced in the case of fresh, poisoned and regenerated samples. There is another possible decomposition reaction according to Van and Habashi [56]. It can be observed that the Cu-doped samples poisoned with SO2 exhibited considerably higher and moreCuO·CuSO rapid decomposition4 can decompose duringthermogravimetric in an inert atmosphere analysis to (TGA) CuO, in SO comparison2 and O2. toDue the to fresh the presence of O2, materials.SO Additionally,2 could be oxidised. for the poisoned C is a reducer Cu-containing and thus materials, could reduce the decomposition SO3 back to isSO finished2, accelerating at the carbon approximatelyloss over 570 380◦C. °C. The From extensive the point weight of lossview is of the the result catalysts of carbon tested burning in the andSCR vaporization reaction, in the temperature of the adsorbedrange where sulphur the oxide reaction from is the carried support out and and thermal where decomposition activated carbon of copper could sulphates. be possibly applied, the The mechanismmodified observed ACs shows during no TGA significant is in agreement weight with loss. the generally accepted sequence of CuSO4 decomposition,Regeneration described by of Equation the samples (3) [55]: modified with copper resulted in the partial recovery of thermal stability. However, the thermal stability is still lower than that of the fresh materials. Nevertheless, CuO CuSO4 2CuO + SO3 (3) the applied regeneration method· successively→ removed sulphate species from the catalyst’s surface Thereand is anotherthe susceptibility possible decomposition to thermal reaction decomposition according tomight Van and be Habashiattributed [56 ].to CuO theCuSO presence of traces of · 4 can decomposesulphur in confirmed an inert atmosphere by the energy-dispersive to CuO, SO2 and OX-ray2. Due sp toectroscopy the presence (EDX) of O2 ,analysis SO2 could presented be in Table 4. oxidised. C is a reducer and thus could reduce SO3 back to SO2, accelerating the carbon loss over 380 ◦C. From theTable point 4. of Results view of of theenergy-dispersive catalysts tested X-ray in the spectrosco SCR reaction,py (EDX) in the elemental temperature analysis range of the fresh, where the reaction is carried out and where activatedpoisoned, carbon and regenerated could be possibly catalysts. applied, the modified ACs shows no significant weight loss. Regeneration of the samples modifiedSample with copper resultedApproximate in the partial Weight recovery (%) of thermal stability. However, the thermal stability is still lower than that ofCu the fresh materials.S Si Nevertheless, the applied regeneration method successively removedfresh sulphate species0 from the0 catalyst’s2.55 surface and the susceptibility to thermal decomposition5U mightpoisoned be attributed to0 the presence2.04 of traces1.80 of sulphur confirmed by the energy-dispersive X-ray spectroscopyregenerated (EDX) analysis0 presented 2.04 in Table1.804 . fresh 2.02 0 0 5U1Cu poisoned 2.44 1.53 0 regenerated 2.13 0.50 0 fresh 5.54 0 0 5U5Cu poisoned 6.70 0.99 0 regenerated 6.15 0.23 0 5U10Cu fresh 7.49 0 0

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Table 4. Results of energy-dispersive X-ray spectroscopy (EDX) elemental analysis of the fresh, poisoned, and regenerated catalysts.

Sample Approximate Weight (%) Cu S Si fresh 0 0 2.55 5U poisoned 0 2.04 1.80 regenerated 0 2.04 1.80 fresh 2.02 0 0 5U1Cu poisoned 2.44 1.53 0 regenerated 2.13 0.50 0 fresh 5.54 0 0 5U5Cu poisoned 6.70 0.99 0 regenerated 6.15 0.23 0 fresh 7.49 0 0 5U10Cu poisoned 8.56 0.56 0 regenerated 7.56 0.13 0

2.4. Scanning Electron Microscopy (SEM) and EDX Analysis

2.4.1. SEM Analysis Scanning electron microscopy (SEM) and EDX analysis were performed in order to observe the changes of the surface morphology and amounts of surface elements of the samples before and after poisoning with SO2. As illustrated in Figure4a, the surface of the support (5U) before modiﬁcation with Cu is smooth and uniform. Relating to Figures4 and5, it can be noticed that introduction of copper resulted in visible changes in the surface morphology, such as the formation of more porous texture of the catalysts Larger aggregates appeared on the surface of carbon with higher loading of copper 5U10Cu at a magniﬁcation of 20,000 times, as shown in Figure5. Catalysts 2020, 10, x FOR PEER REVIEW 13 of 25

Figure 4. Pictures of scanning electron microscopy (SEM) for samples: (a) fresh 5U; (b) poisoned 5U; Figure 4. Pictures( ofc) regenerated scanning 5U; (d electron) fresh 5U1Cu; microscopy (e) poisoned 5U1Cu; (SEM) (f) regenerated for samples:5U1Cu; (g) fresh ( 5U5Cu;a) fresh (h) 5U; (b) poisoned 5U; (c) regeneratedpoisoned 5U; 5U5Cu; (d) fresh (i) regenerated 5U1Cu; 5U5Cu; (e ()k) poisonedfresh 5U10Cu; (l) 5U1Cu; poisoned 5U10Cu; (f) regenerated (m) regenerated 5U1Cu; (g) fresh 5U10Cu. 5U5Cu; (h) poisoned 5U5Cu; (i) regenerated 5U5Cu; (k) fresh 5U10Cu; (l) poisoned 5U10Cu; (m) regenerated 5U10Cu.

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Figure 5. SEMSEM pictures pictures of fresh 5U10Cu at 20,000 times magnification. magniﬁcation.

The porosity of the materials might be the result result of decomposition decomposition of metal nitrates and emission of NO x,,N N22 andand O O22 uponupon the the calcination calcination procedure procedure perfor performedmed during during active active phase phase deposition. deposition.

2.4.2. Energy-Dispersive Energy-Dispersive X-ray Spectroscopy (EDX) Analysis The approximate elemental analysis of the fresh, poisoned and regenerated catalysts is listed in Table4 4.. AllAll ofof thethe obtainedobtained outcomesoutcomes ofof thethe analysisanalysis areare inin agreementagreement withwith findingsfindings reportedreported inin thethe literature, for for example example by by Khan Khan et et al. al. [57] [57 ]or or Kooti Kooti and and Matouri Matouri [58]. [58 ].In InEDX EDX spectra spectra (not (not included included in thein the paper), paper), the the presence presence of ofcarbon, carbon, nitrogen nitrogen and and oxygen oxygen were were confirmed confirmed by by the the presence presence of peaks around 0.27 keV, 0.4 keV, 0.50.5 keV,keV, respectively.respectively. Additionally,Additionally, aa peakpeak characteristiccharacteristic of silicon can be observed inin thethe spectra spectra at at 1.5 1.5 keV. keV. Doping Doping with with copper copper resulted resulted in the in appearance the appearance of new of peaks new located peaks locatedat binding at binding energies energies of 0.85, 0.94, of 0.85, 8.04 0.94, and 8.948.04 keVand which8.94 keV correspond which correspond to CuL1, CuLa, to CuL1, CuKa CuLa, and CuKb,CuKa andrespectively. CuKb, respectively. The lower amount The lower of Siamount impurities of Si (ash)impurities were detected.(ash) were detected. After poisoning with SO2,, new new peaks peaks of of sulphur sulphur SKa SKa and and SKb SKb appeared appeared at the binding energies of 2.37 and 2.57 keV, respectively. Additionally, according to the EDX EDX results results presented presented in in Table Table 44.,., deposition of sulphur on the catalyst surface was ob observedserved to be inversely pr proportionaloportional to the content of copper. The increase of the copper loading resulted in lower S deposition on the catalyst surface. Thus, Thus, the results confirmedconfirmed that modificationmodification with coppercopper provided aa noticeablenoticeable enhancementenhancement inin SOSO22 tolerance of the catalysts. Regeneration resulted in in the the release release of of a a small small amount amount of of sulphur. sulphur. Nevertheless, Nevertheless, some some sulphate aggregates did not decompose, even at the applied regeneration temperaturetemperature ofof 300300 ◦°C.C. 2.5. Catalytic Tests 2.5. Catalytic Tests The catalytic activity of the fresh, poisoned and regenerated catalysts was studied applying the The catalytic activity of the fresh, poisoned and regenerated catalysts was studied applying the temperature range of 140–300 ◦C, which stands for the low temperature range of NH3-SCR. The results temperature range of 140–300 °C, which stands for the low temperature range of NH3-SCR. The of NH3-SCR catalytic tests are presented in Figures6–10. results of NH3-SCR catalytic tests are presented in Figures 6–10.

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100 5U fresh A 5U poisoned 90 5U regenerated 80

40 NO conversion (%)

0 140 160 180 200 220 240 260 280 300 Temperature (°C)

350 5U fresh B 5U poisoned 300 5U regenerated

250

200

150

O concentration (ppm) 100 2 N

0 120 140 160 180 200 220 240 260 280 300 320 Temperature (°C)

Figure 6. ResultsCatalystsFigure 2020 of ,6. 10catalytic Results, x FOR PEERof catalytic tests REVIEW carriedtests carried out out overover fresh, fresh, poisoned, poisoned, and regenerated and regenerated 5U: (A) NO16 of 5U:25 (A) NO conversion; (B) N2O concentration. conversion; (B)N2O concentration.

100 5U1Cu fresh A 5U1Cu poisoned 90 5U1Cu regenerated 80

40 NO conversion (%) conversion NO

0 140 160 180 200 220 240 260 280 300 Temperature (°C)

350 5U1Cu fresh B 5U1Cu poisoned 300 5U1Cu regenerated

250

200

150 O concentration (ppm) O concentration 2

N 100

0 140 160 180 200 220 240 260 280 300 320 ° Temperature ( C)

Figure 7. Results of catalytic tests carried out over fresh, poisoned, and regenerated 5U1Cu: (A) NO

conversion; (B)NFigure2O concentration.7. Results of catalytic tests carried out over fresh, poisoned, and regenerated 5U1Cu: (A) NO conversion; (B) N2O concentration.

Catalysts 2020, 10,Catalysts 1426 2020, 10, x FOR PEER REVIEW 17 of 25 13 of 21

100 A 90

40 NO conversion (%) conversion NO

20 5U5Cu fresh 5U5Cu poisoned 10 5U5Cu regenerated

0 140 160 180 200 220 240 260 280 300 Temperature (°C)

350 5U5Cu fresh B 5U5Cu poisoned 300 5U5Cu regenerated

250

200

150 O concentration (ppm) Oconcentration

2 100 N

0 120 140 160 180 200 220 240 260 280 300 320 Temperature (°C)

Figure 8. ResultsCatalysts 2020 of, 10 catalytic, x FOR PEER tests REVIEW carried out over fresh, poisoned, and regenerated 5U5Cu:18 of 25 (A) NO Figure 8. Results of catalytic tests carried out over fresh, poisoned, and regenerated 5U5Cu: (A) NO

conversion; (Bconversion;)N2O concentration. (B) N2O concentration.

100 A

40 NO conversion (%) NO conversion

20 5U10Cu fresh 5U10Cu poisoned 10 5U10Cu regenerated

0 140 160 180 200 220 240 260 280 300 Temperature (°C)

350 5U10Cu fresh B 5U10Cu poisoned 300 5U10Cu regenerated

250

200

150 O concentration (ppm) 2 100 N

0 120 140 160 180 200 220 240 260 280 300 320 Temperature (°C)

Figure 9. Results of catalytic tests carried out over fresh, poisoned, and regenerated 5U10Cu: (A) NO Figure 9. Results of catalytic tests carried out over fresh, poisoned, and regenerated 5U10Cu: (A) NO conversion; (B)N2O concentration. conversion; (B) N2O concentration.

Catalysts 2020, 10, x FOR PEER REVIEW 19 of 25 Catalysts 2020, 10, 1426 14 of 21

t = 220oC 5U 5U1Cu 5U5Cu 5U10Cu 100

40 NO conversion (%)

0 fresh poisoned regenerated

Figure 10. Comparison of the catalytic activity of 5U, 5U1Cu, 5U5Cu, 5U10Cu at 220 ◦C. It can be noted that in the case of all of the tested materials, increasing the reaction temperature Figure 10. Comparison of the catalytic activity of 5U, 5U1Cu, 5U5Cu, 5U10Cu at 220°C. accelerates NO conversion. Even for AC not modiﬁed with copper, the catalytic activity was very high,It since can thebe noted material that exhibited in the case 50% of all of NOof the conversion tested materials, at about increasing 140 ◦C. Thus, the reaction the sample temperature can be a promisingaccelerates catalyst NO conversion. for a low-temperature Even for AC SCRnot modifi process.ed Allwith of copper, the copper-doped the catalytic samples activity exhibitedwas very satisfactoryhigh, since the NO material conversion exhibited and the 50% sequence of NO ofconversion catalytic activityat about for 140 the °C. considered Thus, the sample fresh materials can be a atpromising 220 ◦C was catalyst as follows: for a low-temperature 5U10Cu > 5U5Cu SCR> 5U1Cu process.> 5U.All Hence,of the copper- it increaseddoped with samples the content exhibited of Cusatisfactory introduced. NO Therefore,conversionthe and results the sequence indicate of that catalytic copper activity loading for plays the considered an important fresh role materials in the improvementat 220 °C was of as NO follows: conversion 5U10Cu during > 5U5Cu SCR. > Since 5U1Cu concentration > 5U. Hence, of it N increased2O was below with 100 the ppmcontent for freshof Cu 5U,introduced. it appears Therefore, that non-doped the results support indicate is the that most copper selective loading among plays all of an the important considered role materials. in the Therefore, the major product of the SCR process over 5U is predicted to be N produced according to improvement of NO conversion during SCR. Since concentration of N2O was2 below 100 ppm for fresh the5U, reaction it appears described that non-doped by Equation support (4): is the most selective among all of the considered materials. Therefore, the major product of the SCR process over 5U is predicted to be N2 produced according to 4NO + 4NH + O 4N + 6H O (4) the reaction described by Equation (4): 3 2 → 2 2

On the other hand, the increased4NO + 4NH concentration + O →4N of N+2O 6H in theO exhausts for copper-doped samples(4) mightOn be the the other result hand, of non-selective the increased reactions concentration of SCR, of such N2O as in ammoniathe exhausts oxidation for copper-doped in the temperature samples rangemight of be 260–300 the result◦C[ of6 ,non-selective59]. reactions of SCR, such as ammonia oxidation in the temperature rangeAfter of 260–300 poisoning °C [6,59]. with SO 2, the catalytic activity decreased considerably in the case of all of the catalystsAfter in poisoning comparison with to SO the2, freshthe catalytic samples. activity It can decreased be noted, thatconsiderably for 5U about in the 50% case of of the all initialof the activitycatalysts was in lostcomparison upon poisoning. to the fresh As shownsamples. in FigureIt can 9be, NO noted, conversion that for for5U that about sample 50% displayedof the initial a continuousactivity was decline lost upon to 21% poisoning. at 140 ◦ AsC and shown 31% in at Figu 300 re◦C. 9, This NO isconversion probably for related that tosample the formation displayed of a sulphatecontinuous species decline on theto 21% external at 140 surface °C and of the31% catalyst at 300 °C. [11 ,This13]. Accordingis probably to related the studies to the carried formation out by of Yusulphate et al. [60 species], sulphate on the species external improve surface ammonia of the cata adsorptionlyst [11,13]. due According to their acidic to the nature. studies Furthermore, carried out whenby Yu sulphates et al. [60], react sulphate with ammonia species onimprove the catalyst ammonia surface, adsorption ammonium due sulphateto their salts,acidic suchnature. as NHFurthermore,4HSO4 or (NH when4)2 Ssulphates2O7 are formed react with under ammonia the conditions on the ofcatalyst the SCR surface, process. ammonium The saltsare sulphate believed salts, to havesuch aas significant NH4HSO influence4 or (NH4 on)2S2 theO7 are lower formed catalytic under activity the conditions in NO conversion, of the SCR since process. they plug The the salts pores are ofbelieved the catalyst to have [16 ].a Allsignificant of the materials influence doped on the with lowe copperr catalytic exhibited activity higher in NO resistance conversion, to poisoningsince they withplug SO the2. pores For 5U1Cu of the thecatalyst conversion [16]. All of of NO the declined materials from doped 89% with to 45% copper at 220 exhibited◦C, while higher for 5U10Cu resistance at theto poisoning same temperature with SO2 it. For dropped 5U1Cu from the100% conversion to 97%. of Thus, NO declined it should from be emphasized 89% to 45% that at 220 the °C, addition while offor copper 5U10Cu not at only the same activates temperature the catalyst it dropped in NO conversion,from 100% to but 97%. it also Thus, enhances it should the be resistance emphasized to SOthat2 poisoning. the addition of copper not only activates the catalyst in NO conversion, but it also enhances the resistanceAs already to SO2 mentioned, poisoning. the main two reasons of deactivation of Cu-doped catalysts used for SCR isAs the already formation mentioned, of copper the main and two ammonium reasons of sulphates deactivation [10,14 of]. Cu-doped The effect catalysts of the formation used for SCR of thoseis the compounds formation of can copper be explained and ammonium by the di ffsulphateserence of [10,14]. desorption The temperatureeffect of the betweenformation ammonia of those compounds can be explained by the difference of desorption temperature between ammonia (150–

Catalysts 2020, 10, 1426 15 of 21

(150–400 ◦C) and sulphur oxide (>400 ◦C). Due to the fact that (NH4)2SO4 decomposes at 150–400 ◦C, 2 SO4 − moieties can easily interact with metallic sites freed after NH3 desorption and form metal sulphates and sulphites [5]. Jangjou et al. [61] postulated that ammonium and copper sulphates are interchangeable, depending on the availability of ammonia in the gas phase. Similar indications were found in the experiments in the following work due to the change of copper sulphates formed during poisoning into ammonium sulphates upon NH3 exposure during SCR tests according to Equation (5):

1 1 CuSO + 3NH (NH ) SO + N + H + Cu (5) 4 3(g) → 4 2 4 2 2 2 2

Furthermore, after poisoning N2O concentration in the exhausts increased. It indicates that some amounts of NH3 preferentially reacted with NO or O2 to form N2O in the presence of sulphur species (according to Equations (6) and (7)):

2NH + 2O N O + 3H O (6) 3 2 → 2 2 4NO + 4NH + 3O 4N O + 6H O (7) 3 2 → 2 2 Regeneration resulted in the recovery of the catalytic activity of all of the materials to some extent. The regenerating eﬀect is especially visible in the temperature range of 280–300 ◦C for the samples doped with copper. This points to the fact that the presence of copper not only prevents excessive deactivation by SO2, but also has an impact on the restitution of the satisfactory catalytic performance. In order to clearly display the dependency of catalytic activity and ability to regeneration, the comparison of the catalytic activity obtained at 220 ◦C for the fresh, poisoned and regenerated materials is presented in Figure 10.

3. Materials and Methods

3.1. Catalyst Preparation The commercial AC was provided by Gryfskand (Hajnówka, Poland). The initial material was oxidized using 5.0 M solution of nitric acid (HNO3) (Avantor Performace Materials Poland S.A., Gliwice, Poland) at 90 ◦C for 2 h. Afterwards, the material was filtered, washed with distilled water to pH ~ 7.0 and dried at 110 ◦C overnight. Acid treatment was followed by the introduction of nitrogen groups (N-groups) by the incipient wetness impregnation with urea solution (5 wt.%). Then the material was 3 dried at 110 ◦C for 24 h and thermally treated in the flow of 2.25 vol.% of O2 in He (gas flow 100 cm 1 min− ) at 350 ◦C for 2 h. The synthesized samples of modified activated carbon were ground and sieved to obtain the fraction of 0.25–1.0 mm. The active metal was deposited on the surface of AC by incipient wetness impregnation using aqueous solutions of Cu(NO3)2 (Avantor Performace Materials Poland S.A., Gliwice, Poland). Appropriate concentrations of Cu2+ cations were used in order to introduce 1 wt.%, 5 wt.%, and 10 wt.% of the metallic phase on each sample. Finally, Cu-doped AC was dried and calcined in the stream of 2.25 vol.% of O2 in He at 250 ◦C for 1 h. The analyzed fresh materials are listed in Table5.

Table 5. List of the analyzed fresh samples and their codes.

No. Sample Code Cu-Loading 1 5U 0 wt.% 2 5U1Cu 1 wt.% 3 5U5Cu 5 wt.% 4 5U10Cu 10 wt.% Catalysts 2020, 10, 1426 16 of 21

In order to compare the catalytic performance of the fresh and contaminated modified AC doped with Cu, a set of the resulting materials was poisoned with SO2 by the exposition on the gas mixture of 1000 ppm of SO2 in helium at 180 ◦C for 30 min. Subsequently, the catalytic activity of the poisoned Catalysts 2020, 10, x FOR PEER REVIEW 21 of 25 samples was tested in NH3-SCR. Afterwards, another set of the samples was poisoned by SO2, but in that case, the flow of sulphur oxide was stopped after 30 min and the materials were straightforwardly that case, the flow of sulphur oxide was stopped after 30 min and the materials were regenerated in He atmosphere by increasing the temperature in the reactor from 200 to 300 C and straightforwardly regenerated in He atmosphere by increasing the temperature in the reactor◦ from keeping each catalyst in the final temperature for 1 h. The procedure of the activity tests is presented in 200 to 300 °C and keeping each catalyst in the final temperature for 1 h. The procedure of the activity Figure 11. tests is presented in Figure 11.

FigureFigure 11. 11. Experimental procedureprocedure of of the the catalytic catalytic tests te oversts fresh,over fresh, poisoned, poisoned, and regenerated and regenerated catalysts. 3.2.catalysts. Catalysts Characterization

3.2. CatalystsThe structure Characterization of the catalysts was analyzed using X-ray diffraction technique. The X-ray diffraction patterns were obtained using Empyrean (Panalytical) diffractometer (Panalytical, Almelo, The structure of the catalysts was analyzed using X-ray diffraction technique. The X-ray UK). The instrument was equipped with copper-based anode (Cu-Kα LFF HR, λ = 0.154059 nm). diffraction patterns were obtained using Empyrean (Panalytical) diffractometer (Panalytical , Almelo, The diffractograms were collected in the 2θ range of 2.0–80.0 (2θ step scans of 0.02 and a counting UK). The instrument was equipped with copper-based anode (Cu-K◦ α LFF HR, λ = 0.154059◦ nm). The time of 1 s per step). The average size of copper oxide crystals was calculated using Scherrer’s formula diffractograms were collected in the 2θ range of 2.0–80.0° (2θ step scans of 0.02° and a counting time (Equation (8)) [41]: of 1 s per step). The average size of copper oxide crystalsKλ was calculated using Scherrer’s formula (Equation (8)) [41]: Dhkl = (8) βhklcosθ Recorded XRD data were analysed using𝐾𝜆 X’pert Highscore plus (with database) (Malvern 𝐷 = (8) Panalytical, Malvern, UK). References codes𝛽 are𝑐𝑜𝑠𝜃 given in square brackets. RecordedFT-IR was XRD performed data towere examine analysed the characteristic using X’pe chemicalrt Highscore groups plus present (with in thedatabase) analyzed (Malvern materials. 1 Panalytical,The spectra Malvern, were recorded UK). onReferences Perkin Elmer codes Frontier are given spectrometer in square brackets. in the region of 4000–400 cm− with a 1 resolutionFT-IR was of 4 cmperformed− . Before to each examine measurement, the characteristic the analyzed chemical sample groups was mixed present with in KBrthe atanalyzed the ratio materials.of 1:100 and The pressed spectra into were a disk.recorded on Perkin Elmer Frontier spectrometer in the region of 4000– 400 cmThe−1 with changes a resolution in weight of of 4 thecm− catalysts1. Before ineach the measurement, temperature range the analyzed of 20–800 sample◦C and inwas the mixed atmosphere with 3 1 KBrof nitrogen at the ratio (100 of cm 1:100min and− )pressed were analyzed into a disk. using TGA. The experiment was performed using SDT Q600The V20.9 changes Build 20in instrument.weight of the catalysts in the temperature range of 20–800 °C and in the atmosphereThe morphology of nitrogen and(100 chemicalcm3 min−1 composition) were analyzed of theusing synthesized TGA. The catalystsexperiment were was examined performed by usingSEM SDT that Q600 was carriedV20.9 Build using 20 JEOL instrument. JSM 6360LA microscope at a magnification of 5000 and 20,000. EDSThe was morphology carried out usingand chemical Shimadzu composition 7000 diffractometer of the synthesized with an catalysts energy dispersive were examined X-ray analyzer.by SEM thatThe was elemental carried analysisusing JEOL was JSM performed 6360LA bymicroscope selecting at random a magnification points (micro-area) of 5000 and of 20,000. the sample EDS was at a carriedworking out distance using Shimadzu of 10mm at7000 15–20 diffractometer kV (beam voltage) with an in energy a vacuum dispersive chamber X-Ray at spot analyzer. size of 4.7The to elemental5.3 nm (INCA analysis conventional was performed units). by The selecting acquisition random time points of the (micro-area) spectrums wasof the 300 sample s, the at beam a working current distancewas 86 uAof 10mm and the at detector15–20 kV dead (beam time voltage) was of in about a vacuum 30%. Thechamber elemental at spot composition size of 4.7 to of 5.3nm the examined (INCA conventionalcatalysts was units),. analyzed The using acquisition the ZAF time method of the [62 spectrums]. was 300 s, the beam current was 86 uA and the detector dead time was of about 30%. The elemental composition of the examined catalysts 3.3. Catalytic Tests was analyzed using the ZAF method [62].

NH3-SCR catalytic tests over all samples were performed in a ﬁxed-bed ﬂow microreactor under 3.3.atmospheric Catalytic Tests pressure in the setup shown in Figure 12.

NH3-SCR catalytic tests over all samples were performed in a fixed-bed flow microreactor under atmospheric pressure in the setup shown in Figure 12.

Catalysts 2020, 10, x FOR PEER REVIEW 22 of 25 Catalysts 2020, 10, 1426 17 of 21

Figure 12. Catalytic setup.

The reaction mixture (800 ppm ofFigure NO, 80012. Catalytic ppm of setup. NH3, 3.5 vol.% of O2) was introduced to the microreactor through mass ﬂow controllers and helium was added as a balance to maintain the total 3 1 ﬂowThe rate reaction of 100 cmmixturemin −(800. The ppm catalytic of NO, unit800 ppm downstream of NH3, 3.5 of thevol.% reactor of O2 was) was used introduced to decompose to the microreactorpossibly formed through NO 2massto NO. flow The controllers concentrations and helium of residual was added NO as and a balance N2O (the to maintain by-product the oftotal the flowreaction) rate of were 100 analysedcm3 min− downstream1. The catalytic of unit the reactordownstream by the of FT-IR the reactor detector was (ABB used 2000 to decompose AO series). possiblyThe experimental formed NO NH2 3to-SCR NO. process The concentrations was conducted of inresidual the temperature NO and N range2O (the of 140–300by-product◦C and of the NO reaction)conversion were was analysed calculated downstream according of to the formulareactor by presented the FT-IR on detector Equation (ABB (9): 2000 AO series). The experimental NH3-SCR process was conducted in the temperature range of 140–300 °C and NO NOin NOout conversion was calculated accordingNO to conversionthe formula= presented− on Equation (9): (9) NOin 𝑁𝑂 − 𝑁𝑂 𝑁𝑂 = (9) where NOin–inlet concentration of NO, NOout–outlet𝑁𝑂 concentration of NO. where 𝑁𝑂– inlet concentration of NO, 𝑁𝑂 - outlet concentration of NO. 4. Conclusions

4. ConclusionsIn the study presented, the influence of SO2 on Cu-doped activated-carbon catalysts for NH3-SCR process was investigated. The experiments performed showed that even the samples poisoned In the study presented, the influence of SO2 on Cu-doped activated-carbon catalysts for NH3- SCRwith process sulphur was oxide investigated. exhibit satisfactory The experiments catalytic perf performanceormed showed in low-temperature that even the samples selective poisoned catalytic withreduction sulphur of NO.oxide The exhibit results satisfactory of XRD indicated catalytic that performance poisoning within low-temperature SO2 did not influence selective significantly catalytic the structural properties of the support. However, after contamination the crystallinity of the active reduction of NO. The results of XRD indicated that poisoning with SO2 did not influence significantly thephase structural deposited properties on AC considerably of the support. decreased. However, FT-IR after spectra contamination generated the for thecrystallinity samples confirmedof the active the phasepresence deposited of characteristic on AC considerably groups in activated decreased. carbon FT-IR and spectra the groups generated attributed for the to samples the modification confirmed of thethe presence support. of Poisoning characteristic resulted groups in the in activated appearance carbon of the and peaks the ofgroups sulphur attributed species. to After the modification regeneration, ofthe the peaks support. disappeared, Poisoning suggesting resulted thatin the thermal appearance treatment of wasthe peaks an appropriate of sulphur method species. to restoreAfter regeneration,structural properties the peaks of disappeared, the materials. suggesting Based on that TGA thermal results, treatment it can be was concluded an appropriate that the presencemethod toof restore Cu decreased structural thermal properties stability of the of materials. the catalysts. Based Additionally, on TGA results, the formation it can be andconcluded decomposition that the presenceof copper of sulphates Cu decreased could bethermal the reason stability for rapid of the weight catalysts. loss of Additionally, the samplesdoped the formation with Cu inand the decompositiontemperature range of copper of about sulphates 380–570 could◦C. be Moreover, the reason the for decay rapid of weight activity loss of theof the samples samples may doped have withpartially Cu in resulted the temperature from the range burn ofof carbonabout 380–570 during °C. the Moreover, treatment orthe tests. decay The of activity lower catalytic of the samples activity mayof the have samples partially could resulted have alsofrom resulted the burn from of carbon blocking during the the pores treatment of ACand or tests. inhibiting The lower free catalytic diffusion activityof the gasof the phase samples through could the have catalyst’s also resulted channels. from The blocking increased the formation pores of AC of N and2O afterinhibiting poisoning free of SO was probably caused by the formation of bulk copper oxide species and their aggregation diffusion2 of the gas phase through the catalyst’s channels. The increased formation of N2O after that is confirmed to favour NH oxidation to nitrous oxide. Thus, interaction with SO not only poisoning of SO2 was probably 3caused by the formation of bulk copper oxide species 2and their decreases the catalytic activity, but also increases the tendency to form by-products. In fact, sulphate aggregation that is confirmed to favour NH3 oxidation to nitrous oxide. Thus, interaction with SO2 notspecies only aredecreases highly the acidic catalytic and could activity, promote but also the in adsorptioncreases the oftendency ammonia to form resulting by-products. in increased In fact, NO sulphateconversion. species Nevertheless, are highly the acidic experiments and could confirmed promote that, the probably, adsorption in the of low ammonia temperature resulting NH3-SCR in increasedthe formation NO ofconversion. ammonium Nevertheless, or metal sulphates the expe has lowerriments activation confirmed energy that, than probably, the reaction in the between low NH adsorbed on sulphate species and gas-phase NO. Regeneration by thermal treatment allowed the temperature3 NH3-SCR the formation of ammonium or metal sulphates has lower activation energy initial catalytic activity of the analyzed materials to be recovered to satisfactory level and its efficiency than the reaction between NH3 adsorbed on sulphate species and gas-phase NO. Regeneration by thermalincreased treatment linearly allowed with the the increasing initial catalytic loading ofactivi copper.ty of the analyzed materials to be recovered to satisfactory level and its efficiency increased linearly with the increasing loading of copper.

Author Contributions: The experimental work was designed and supported by B.S., M.M. M.S. prepared the catalysts; performed the catalytic experiments, characterization of catalysts, and analysis of data, writing—

Catalysts 2020, 10, 1426 18 of 21

Author Contributions: The experimental work was designed and supported by B.S., M.M., M.S. prepared the catalysts; performed the catalytic experiments, characterization of catalysts, and analysis of data, writing—original draft preparation; A.B. supported the catalytic experiments and characterization of catalysts; review, supervision; A.S., writing—review and editing; B.S., review, supervision; M.M., supervision. All authors have read and agreed to the published version of the manuscript. Funding: This work was funded by Grant AGH 16.16.210.476. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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