WSAC Catalyst Surface

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Article Cite This: Ind. Eng. Chem. Res. 2018, 57, 2873−2881

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Catalytic Oxidation of Hydrogen Sulfide on Fe/WSAC Catalyst Surface Modification via NH3‑NTP: Influence of Gas Gap and Dielectric Thickness Kai Li,†,§ Sijian Liu,†,§ Xin Song,†,§ Chi Wang,‡ Ping Ning,*,† Maohong Fan,§ and Xin Sun† †

Faculty of Environmental Science and Engineering and ‡Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650500, P. R. China § Department of Chemical Engineering, University of Wyoming, Laramie, Wyoming 82071, United States ABSTRACT: Surface modification of the Fe/WSAC catalyst with NH3-NTP could enhance its catalytic oxidation ability toward H2S. Influences of discharge gaps and dielectric thicknesses were indicated. The H2S catalytic oxidation performance of Fe/WSAC catalyst was increased first and then decreased with increasing of discharge gap or dielectric thickness. The SEM images showed significant changes in the surface topography after treatment at low discharge gaps or large dielectric thicknesses, after which the surface was clear and smooth. The BET results showed that the surface area increased after the plasma treatment at low discharge gas gaps or large dielectric thicknesses. From the XPS spectra, the nitrogen content increased after the plasma treatment, and the deconvolution of the O 1s lattice oxygen peak revealed the oxygen content to increase at short discharge gas gaps. NH3-plasma treatment is an efficient modification method for the improvement of the catalytic oxidation performance of activated carbon catalysts.

1. INTRODUCTION Activated carbon materials are widely employed owing to their highly porous structure, huge surface area, and flexible adsorption performance characteristics.1 The use of different methods to modify activated carbon materials and obtain specific adsorption or catalytic properties is of high interest. The modification methods of activated carbon include impregnation, acid treatment,2 base treatment,3 microwave radiation, and surface oxidation.4,5 Nevertheless, in recent years, new simpler surface modification methodologies have been developed in the field of activated carbon. An example of such a novel method for the modification of the physical and chemical surface properties is nonthermal plasma (NTP), which has the unique ability of initiating both physical and chemical reactions without affecting the matrix properties.6,7 Thus, NTP surface modification technologies have been applied in the field of materials preparation and pretreatment. Compared to other methods (glow discharge, corona discharge, rf microwave discharge, etc.) for the generation of NTP, dielectric barrier discharge (DBD) is characterized by being rapid, simple, and low energy consuming. During plasma treatments, different discharge gases will produce different effects on the material surface.8−11 Numerous studies12−15 have shown that the introduction of nitrogen-containing substances such as ammonia not only helps improve the adsorption properties of activated carbon but also helps their desulfurization efficiency. Thus, ammonia-DBD plasma was employed in this research. Moreover, several parameters of the DBD process © 2018 American Chemical Society

(input voltage, treatment time, gas gap, and dielectric thickness) can affect the discharge characteristics. The effects of the voltage and treatment time were investigated in our previous research,16 H2S catalytic oxidation activity increased first and then decreased, with increasing output voltage and treatment time. And the optimal output voltage and treatment time were 6.8 kV and 10 min, respectively. However, the influence of the gas gap and dielectric thickness on the catalyst activity has rarely been reported in the literature. The discharge gas gap and dielectric thickness can have a significant effect on DBD-modified catalysts. The discharge gas gap affects the generation of plasma by varying the number of electrons collisions,17−20 and the dielectric thickness may prevent the spark discharge.19,21 Discharge gas gap and dielectric thickness play an important role in DBD systems, being able to influence the discharge mode and intensity. Therefore, research on catalysts modified by DBD is very important for the development of surface modifications by plasma technology. In this work, the effect of plasma treatment for surface characterization of Fe/WSAC catalyst on the hydrogen sulfide (H2S) catalytic oxidation capacity was studied, and the influence of plasma modification at different discharge intensities (i.e., gas gap and dielectric thickness values) was Received: Revised: Accepted: Published: 2873

December 7, 2017 February 2, 2018 February 14, 2018 February 14, 2018 DOI: 10.1021/acs.iecr.7b05079 Ind. Eng. Chem. Res. 2018, 57, 2873−2881

Article

Industrial & Engineering Chemistry Research

Figure 1. Dielectric barrier discharge treatment system: (1) NH3 cylinder; (2) gas pressure reducing valve; (3) mass flow control box; (4) mass flow controller; (5) valve; (6) dielectric; (7) plasma region; (8) quartz sand; (9) high voltage electrode; (10) ground electrode; (11) catalysts; (12) plasma generator; (13) regulator; (14) absorbed bottle.

region (7), where the catalyst is treated. The electrode and gap had a coaxial cylindrical structure. The DBD reactor was connected to a high-frequency plasma generator (CTP-2000P) comprising an inner high-voltage electrode (stainless steel stick), a dielectric (quartz), and a ground electrode (aluminum alloy). NH3 plasma was generated at a discharge output voltage of 6.8 kV and input voltage of 30 V with a frequency of 7.8 kHz. The samples (0.2 g of catalyst in all experiments) were placed in the chamber for the same amount of treatment time (10 min). In this research, the discharge gas gap was varied at 3.5, 4.5, 5.5, 6.5, and 7.5 mm (at a fixed dielectric thickness of 1.5 mm), denoted respectively as Dg-3.5mm, Dg-4.5mm, Dg5.5mm, Dg-6.5mm, and Dg-7.5mm. The dielectric thickness was 1, 1.5, and 2 mm (at a fixed discharge gas gap of 5.5 mm), denoted respectively as Dt-1mm, Dt-1.5mm, and Dt-2mm. 2.3. Materials Characterization. Fourier transform infrared spectra (FTIR) were recorded on a Thermal Fisher IS50 spectrometer equipped with a temperature-controllable diffuse reflection chamber and a high-sensitivity MCT detector. The sample was placed in a micro sample holder. All the spectra were recorded by accumulating 32 scans at a resolution of 4 cm−1 with 333.15 K. A FEI field emission scanning electron microscope (SEM, HITACHI S4800) was used to observe the sample surface topography. The electron acceleration voltage employed was 5 kV. The surface area and pore size distribution of the samples were determined by the nitrogen adsorption−desorption method on a NOVA2000e (Quantachrome Instruments) surface area analyzer. The samples were initially degassed at 573.15 K for 3 h, after which the adsorption isotherms were obtained by dosing nitrogen at 77 K. The results were analyzed by the Horvath−Kawazoe (HK) and BET methods. X-ray photoelectron spectroscopy (XPS) (PHI 5500) analysis was carried out with Al Kα radiation from an Al target and power of 200 W. The spectra were recorded on a

investigated. A variety of characterization methods (BET, XPS, SEM, and FTIR) were employed to examine the materials and support our conclusions.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Yunnan Province is China’s major walnut producing areas. If a large number of walnut shells (about 250 000 t/a) are discarded or burned, it would cause serious waste of resources. As an inexpensive precursor, walnut shell has high fixed carbon content, low ash content. Meanwhile, activated carbon derived from walnut shell has high surface area and high pore volume.22 Therefore, walnut shell was used to prepare catalysts in this research. Walnut shell was carbonized at 700 °C under nitrogen to prepare walnut-shell activated carbon (WSAC, sieved at 40−60 mesh size). WSAC was mixed with KOH powder at a mass ratio of KOH/WSAC = 2:1 in a quartz crucible, placed in a tube furnace, and calcined under nitrogen at 700 °C for 1 h. After cooling, WSAC was washed with distilled water until neutral pH and dried in a fan dryer at 100 °C for 6 h. The WSAC sample was loaded with 5 wt % Fe by a co-precipitation method. The colloid solution was prepared from an analytical grade Fe(NO3)3·9H2O solution and a Na2CO3 solution. The activated carbon catalyst was added to the colloid solution and treated with ultrasound for 40 min. After drying at 100 °C, the WSAC loaded with Fe was calcined at 300 °C for 3 h under nitrogen. Finally, the activated carbon was dipped in a KOH solution and introduced in an ultrasonic apparatus for 40 min. The activated carbon was finally dried and the sample was denoted as Fe/WSAC. 2.2. NH3 Plasma Treatment. Figure 1 shows the DBD treatment system. *First, the catalysts were placed in the plasma region (7). Second, the discharge gap was varied by adjusting the diameter of the electrode (9), and the introduction of ammonia gas was controlled by the mass flow controller (5). Finally, the input power was controlled by a plasma generator (12). The plasma is generated in the plasma 2874

DOI: 10.1021/acs.iecr.7b05079 Ind. Eng. Chem. Res. 2018, 57, 2873−2881

Article

Industrial & Engineering Chemistry Research

Figure 2. Experimental setup: (1) nitrogen cylinder; (2) H2S cylinder; (3) gas pressure reducing valve; (4) mass flow controller; (5) mass flow control box; (6) value; (7) premix tank; (8) tube furnaces; (9) reactor; (10) catalyst; (11) heating control; (12) tube furnace control; (13) residual gas analyzer; (14) data acquisition unit; (15) absorbed bottle.

Figure 3. Influence of the (A) gas gap and (B) dielectric thickness on the Fe/WSAC activity toward H2S removal (reaction conditions: 500 ppm of H2S, GHSV 60 000 h−1, and reaction temperature 333.15 K).

of each catalyst sample was a combination of the H2S breakthrough capacity and breakthrough time, as calculated by the formula below:

concentric hemispherical analyzer in constant-pass energy mode at 71.55 eV with a step size of 0.1 eV for the C 1s, N 1s, and O 1s photoelectron lines. Survey scans in the range of 0−1300 eV were recorded at a pass energy of 89.45 eV with a step size of 1.0 eV. 2.4. H2S Removal Efficiency Measurements. In this paper, a standard dynamic test was used to evaluate the capacity for H2S catalyticoxidation. The experimental setup is shown in Figure 2, in which 99.99% N2 and 5% H2S were mixed in a premixing tank to prepare a 500 ppm of H2S gas mixture. Additional O2 was not introduced in this system, and H2S catalytic oxidation was by oxygen-containing functional group of the Fe/WSAC catalyst. The mixture was controlled by a mass flow controller (MFC) at 60 mL min−1. Finally, the gas mixture was passed through a column of the catalyst sample in a fixed-bed quartz reactor at a gas hourly space velocity (GHSV) of 60 000 h−1. The reaction temperature was stabilized by a series of furnaces at 333 K. The reaction was carried out under atmospheric pressure. The outlet gas was monitored every 30 min with a flame photometric detector (FPD) on a GC at 423 K. The measurements were stopped at H2S concentrations above 100 ppm. The catalytic oxidation capacity

H 2S breakthrough capacity =

H 2S(inlet) − H 2S(outlet) H 2S(outlet)

3. RESULTS AND DISCUSSION 3.1. Influence of the Gas Gap and Dielectric Thickness on the NH3-NTP Modifications. In this section, the effects of different gas gaps and dielectric thicknesses on the H2S catalytic oxidation capacity were described. The Fe/WSAC catalysts were modified by NH3-NTP at different gas gaps and dielectric thicknesses (at 30 V input voltage for 10 min). The same dielectric thickness (1.5 mm) was employed for all the samples treated at different gas gaps, and the same gas gap (5.5 mm) was used for all the samples treated at different dielectric thicknesses. Figure 3 shows the H2S catalytic oxidation properties of the Fe/WSAC catalysts reinforced with plasma treatment under different treatment conditions. The test results revealed that the Fe/WSAC catalyst modified by NH3-NTP at a 2875

DOI: 10.1021/acs.iecr.7b05079 Ind. Eng. Chem. Res. 2018, 57, 2873−2881

Article

Industrial & Engineering Chemistry Research 5.5 mm gas gap and a 1.5 mm dielectric thickness sustained 100% H2S conversion for 210 min. However, 100% H2S conversion was only sustained for 30 min over the Fe/WSAC catalyst without NH3-NTP treatment. One reason behind the enhancement of the catalytic oxidation capacity after plasma treatment may be certain changes in the pore size distribution11 that improve the H2S absorption; another reason may be that the oxygen functional groups formed during the plasma treatment23,24 promote the oxidation of H2S. According to the gas discharge theory,17−20 the number of collisions of electrons drifting from the cathode to the anode is equivalent to pd (where “p” denotes the pressure and “d” the gas gap). The collisions determine the degrees of electronic avalanche and ionization, which would result in different effects after modification. Consequently, the gas gap has an important role in plasma treatments. Due to the increased number of electron collisions upon increasing the gas gap, the discharge intensity is more powerful and the interactions between the reactive species and the catalyst surface are stronger. However, excessive interactions may be disadvantageous for the catalyst surface, and thus the optimal gas gap must be determined. As shown in Figure 3A, Dg-5.5mm was concluded to afford the best catalytic oxidation capacity compared to the other samples, and the influence of the gas gap on the H2S catalytic oxidation capacity was found to present a parabolic shape, and it was concluded that 5.5 mm was the optimal gas gap. The role of the insulating dielectric in the DBD system is to ensure that no spark or arc is generated in the discharge gap.19,21 The strength of the discharge is influenced by the dielectric thickness. As shown in Figure 3B, Dt-1.5mm afforded the best catalytic oxidation capacity. The H2S removal efficiency displayed a growing trend at dielectric thicknesses below 1.5 mm; however, at dielectric thicknesses above 1.5 mm, the H2S removal efficiency exhibited a decreasing trend. The catalytic oxidation capacity of Dt-2.0mm was lower because the 2 mm barrier thickness results in a lower production efficiency of plasma active species.25 Another reason may be that thicker dielectrics result in weaker discharges between the two DBD electrodes.26 In contrast, the catalytic oxidation of H2S over the catalyst modified by NH3-NTP at Dt-1.0mm was inferior than that at Dt-1.5mm, which may be due to thinner dielectrics generating higher electric fields, dramatic discharges, and highdensity plasma active species. All the above would severely impact the catalyst surface and possibly destroy the pore structure.27 From the above results, gas gap and dielectric thickness have a direct impact on the discharge effect of nonthermal plasma, and the surface modifications obtained at optimal gas gap and dielectric thickness values could enhance the removal efficiency of H2S over Fe/WSAC catalysts. It was then envisioned that plasma treatment at different discharge gas gaps and dielectric thicknesses would have different effects on the physical and chemical properties of the catalyst. In order to confirm our conjecture, BET and XPS analyses were conducted. 3.2. BET Results. In the previous section, we mentioned that plasma surface modifications were able to transform the pore structure of catalysts. Thus, in order to observe the effects of different discharge conditions on the pore structure of the Fe/WSAC catalyst, the samples were characterized by BET analysis. The results of the surface area and pore characterization analyses are listed in Table 1. The BET surface area of Fe/ WSAC was determined to be 556.7 m2 g−1 and exhibited a

Table 1. Surface Area and Pore Characterization of the Samples sample

BET surface area (m2/g)

average pore width (Å)

total pore volume (cm3/g)

Fe/WSAC Dg-3.5mm Dg-4.5mm Dg-5.5mm Dg-6.5mm Dg-7.5mm Dt-1mm Dt-1.5mm Dt-2mm

556.7 552.6 581.7 585.5 576.6 579.5 560.1 585.5 580.4

18.5 17.8 18.1 18.7 18.1 18.4 18.6 18.7 18.4

0.2113 0.1935 0.2632 0.2743 0.2611 0.2662 0.2606 0.2743 0.2674

slight increase after plasma surface modification at different discharge gas gaps and dielectric thicknesses. In contrast, the average pore width did not change significantly after plasma treatment. However, the total pore volume increased after NH3-NTP surface modification. Since the discharge gas gap may affect the generation of plasma17−20 and the dielectric thickness is able to prevent the spark discharge,19,21 they should take suitable values to obtain the best effects from the DBD treatment. At a gas gap of 5.5 mm and dielectric thickness of 1.5 mm, the surface area (585.5 m2 g−1) and total pore volume (0.2743 cm3 g−1) were the highest. It must be noted that a larger surface area and pore volume would be beneficial for H2S catalytic oxidation. In order to investigate the influence of different discharge gas gaps and dielectric thicknesses on its microporous structure, the HK method was used to analyze the pore width distribution (as shown in Figure 4). From Figure 4, it can be seen that the NH3-NTP surface modification had a significant effect on the pores with widths in the range of 0.4−0.75 nm due to the etching action.28 As shown in Figure 4A, the pores of the samples modified at discharge gas gaps of 4.5, 5.5, 6.5, and 7.5 mm exhibited a sharp increase at widths in the range from 0.4 to 0.75 nm. The highest catalytic activity is typically achieved with micropores smaller than 1 nm,29 where the sulfide species may be deposited,30 resulting in good H2S removal efficiency of the catalysts after plasma modification. Figure 4B shows the pore width distribution of the samples modified at different dielectric thicknesses. From the results, the number of pores with widths of 0.45, 0.5, and 0.7 nm increased after plasma modification. However, it is interesting that the sample modified at Dt-1.5mm displayed the best behavior for H2S removal albeit with the lowest number of pores with widths of 0.45, 0.5, and 0.7 nm. Moreover, at a gas gap of 3.5 mm, the pore width distribution did not change visibly, but the H2S catalytic oxidation capacity of this sample was higher than that of the sample without plasma surface modification. This suggests that the changes in the pore structure are not the only factor enhancing the H2S catalytic oxidation capacity of the catalysts. Other changes, such as chemical functional groups after NH3-NTP modification at different discharge gas gaps and dielectric thicknesses, may also represent major influencing factors. XPS was subsequently used to confirm this hypothesis, as detailed in the next section. 3.3. XPS Results. In order to clarify how plasma modifications change the functional groups on the catalyst surface, XPS was used to characterize the O 1s, N 1s, K 2p, C 1s, and Fe 2p profiles in the Fe/WSAC samples before and after plasma modification under different conditions. As shown in Table 2, the nitrogen content of all the samples increased 2876

DOI: 10.1021/acs.iecr.7b05079 Ind. Eng. Chem. Res. 2018, 57, 2873−2881

Article

Industrial & Engineering Chemistry Research

Figure 4. Pore width distribution of the samples modified at different (A) gas gaps and (B) dielectric thicknesses.

Table 2. Element Content on the Surface of the Catalysts As Determined by XPS (Atom %) element

Fe/WSAC

Dg-3.5mm

Dg-5.5mm

Dg-7.5mm

Dt-1mm

Dt-2mm

C 1s K 2p N 1s Fe 2p O 1s

77.118 4.109 1.171 0.45 17.153

82.913 2.305 1.609 0.339 12.835

82.892 2.106 1.201 0.248 13.553

81.324 2.642 1.725 0.313 13.997

83.762 1.829 1.466 0.332 12.611

80.937 2.61 1.324 0.271 14.858

Figure 5. (A, left) O 1s and (B, right) N 1s XPS spectra of the samples.

catalyst pores. This in turn would result in greater surface activity and catalyst activity, thus promoting the desulfurization efficiency. Moreover, potassium species may enhance the adsorption capacity due to effects on the micropore development.33 In our previous research, we confirmed that the introduction of potassium is beneficial for the desulfurization activity.16 For all the O 1s spectra, the 20%/80% Lorentzian/Gaussian sum and Shirley background were used for peak deconvolution. As shown in Figure 5A, all the samples show three different peaks, 530.8 eV assigned to lattice oxygen O2−, ∼531.8 eV attributed to chemisorbed active oxygen (including O−, such as hydroxyl and carbonate species), and the peak for absorbed molecular water at 533.1 eV,34−37 which arises from adsorbed

after NH3-NTP surface modification. The reason may be that the NH3-NTP treatment introduces nitrogen functional groups on the surface. A slight increase of the relative atomic percentage of C was also observed, while the O content was reduced after plasma modification. Oxygen-containing moieties on the sample surface might react with other active groups and produce certain gases (such as ozone, nitrogen oxides, or carbon dioxide) that escape during the plasma treatment, causing the oxygen attached to C atoms to decrease and thus exposing the C atoms of the surface. The Fe and K content on the surface of the catalysts decreased after plasma modification, which may be the result of metal particles becoming smaller during plasma treatment,31,32 and this changing is more conducive to active components embedded in the Fe/WSAC 2877

DOI: 10.1021/acs.iecr.7b05079 Ind. Eng. Chem. Res. 2018, 57, 2873−2881

Article

Industrial & Engineering Chemistry Research

O γ contents are observed at Dt-1mm and Dt-2mm. Furthermore, the absorbed molecular water content is a very important factor for H2S removal, as it is the medium in which H2S dissociates into HS−, benefiting from the presence of oxygen groups.38−41 The N 1s XPS spectra of the samples are shown in Figure 5B, which were two types of nitrogen functional groups. The peak at 400.1 eV (N1), attributed to amine/amide functionalities, showed no correlation with pyridine- or pyrrolic-like nitrogen atoms, and thus no shoulders were observed in the spectra at binding energies (398−396 eV). The other peak at 407.1 eV (N2) corresponds to nitrate or nitrite species. Plasma surface modification led to significant changes in the distribution of nitrogen species on the surface of the samples (as shown in Table 3). More amine/amide functional groups appeared on the catalyst surface upon NH3-plasma modification process. Ammonia was decomposed into NH2− and NH2− in the plasma state.42−44 It is suggested that NH2− and NH2− react with C−H or C−O bonds to form amine/amide groups. Basic compounds are typically used to modify activated carbon for hydrogen sulfide removal, since a basic environment is beneficial for the catalytic oxidation of hydrogen sulfide.45−47 From Table 3, it is obvious that the N1 intensity increases with the increasing discharge gas gap (or decreasing dielectric thickness). At the highest discharge gas gap (7.5 mm) or lowest dielectric thickness (1.0 mm), the catalyst is exposed to high-discharge plasma, where ammonia is easily ionized to amines and adsorbed on the catalyst surface. According to this analysis, the NH3-NTP treatment could introduce basic amine groups on the Fe/WSAC catalyst surface, thus enhancing the catalytic oxidation capacity of H2S. 3.4. SEM Results. In order to investigate the surface morphology of the samples modified by NH3-NTP under different conditions, SEM was employed. As shown in Figure 6, the samples surface changed after plasma modification at different gas gaps and dielectric thicknesses. Figure 6a shows a

water molecules on the samples that could not be removed after drying for 6 h. The three groups were labeled as Oα, Oβ, and Oγ, respectively. As shown in Figure 5A and Table 3, Table 3. Relative Concentrations of the O 1s and N 1s Peaks O 1s

N 1s

sample

Oα

Oβ

Oγ

N1

N2

Fe/WSAC Dg-3.5mm Dg-5.5mm Dg-7.5mm Dt-1mm Dt-2mm

18.53 22.44 9.38 17.29 12.93 10.98

38.08 34.51 29.65 37.84 28.27 31.09

43.38 43.05 60.97 44.87 58.80 57.93

63.76 79.23 82.74 86.54 86.15 68.69

36.24 20.77 17/26 13.46 13.85 31.31

although the total amount of O 1s decreased, the distribution of oxygen functional groups changed. The content of Oβ presented a slight decrease, and the Oα content was also reduced after plasma modification. Furthermore, the oxygen ratio for Oγ increased significantly after the plasma treatment. The Oα and Oβ content decreased at gas gaps below 5.5 mm and increased at values above 5.5 mm, while the Oγ content displayed the opposite trend. It can be speculated that, at gas gaps below 5.5 mm, certain chemisorbed active oxygen and lattice oxygen species react with radicals, resulting in the cleavage of C−C, C−H, and C−O bonds on Fe/WSAC by high-energy electrons during the plasma modification process. Such oxygen radicals may then rereact with C atoms on the catalyst surface and form CO species. However, at gas gaps above 5.5 mm, excessive interactions exist on the catalyst surface, and the pores of the catalyst could be destroyed. The O2 in these pores may be ionized to form O−, which may then bind other active groups. The samples modified at different dielectric thicknesses should present similar modifications, although variations in the dielectric thickness do not change the electrical and structural nature of the DBD.25 As a result, similar

Figure 6. SEM micrographs of the samples at (a) Dg-3.5mm, (b) Dg-5.5mm, (c) Dg-7.5mm, (d) Dt-1mm, (e) Dt-2mm and (f) the untreated sample. 2878

DOI: 10.1021/acs.iecr.7b05079 Ind. Eng. Chem. Res. 2018, 57, 2873−2881

Article

Industrial & Engineering Chemistry Research

689.56, 853.5, 877.1, 1006.6, 1147.7, and 1490 cm−1 were detected, whose intensity changed with the reaction time. These peaks are assigned to sulfur-containing species. The S−H band corresponds to absorbed HS−, and the S−O, SO, SO42−, and SO32− bands arise from the H2S oxidation products. It should be noted that the H2O position shifted and that SO− OH species were observed on the catalyst surface. As mentioned earlier, absorbed water molecules may dissociate H2S into HS− and then oxidize into SO−OH species. Another aspect to note is the existence of SO2−N species. It is suggested that some H2S molecules may react with the nitrogencontaining species and form S−N species, which would then be oxidized to SO2−N. The reaction process on the samples modified by NH3plasma is different from that unmodified. The bands shown in Figure 7B at 2308.5, 1773.8, and 1583.2 cm−1 are assigned to the stretching motion of amino groups. The intensity of the SO2−N band increased after plasma modification. The reason for this observation may be that NH2+, NH3+, and −NH2 species were introduced on the catalyst surface by the ammonia plasma. These functional groups are likely to react with H2S and form S−NH2, which is then oxidized to SO2−N, finally generating SO32− and SO42− species upon further oxidation processes. It is interesting to note that O−S−O and C−O−S species were observed on the catalyst surface at the initial stage of the reaction but no SO−OH. In contrast, HS− was still present on the catalyst surface. HS− species would be preferentially transformed into O−S−O and C−O−S as intermediate products, which would then be further oxidized. As such, they would disappear during the course of the reaction. According to the results, the reaction processes were shown in Figure 8.

smooth surface, and the surface morphology became rougher with the increasing gas gap (as shown Figure 6b and Figure 6c). According to the discharge theory, the gas gap determines the nature of the electron collisions.19 Larger gas gaps cause stronger collisions between the active species and the catalyst surface. Excessive interactions between the active species and the catalyst surface might induce clogging of the pores. Smaller gas gaps would moderately affect the catalyst surface; and the particulates on the catalyst surface were removed, which could account for deposition effects of the plasma.48,49 Figure 6d and Figure 6e show a similar morphology to that in (b) (Dg5.5mm), which may also be due to the fact that they correspond to the same gas gap and the dielectric thickness does not change the electrical and structural nature of the DBD.14 From these results, we suggest that the collision numbers between active particles increase with the increasing gas gap, which includes not only the collision and introduction of atoms but the reaction among C, H, and O atoms on the catalyst surface during the plasma treatment. Those reactions would generate gases such as CO2, H2, or H2O, which would escape in such a high energy environment. It is suggested that the collapse of pore and surface roughening may account for the evolution of gas from inside the catalyst.

4. PRODUCTION ANALYSIS BY IN SITU FTIR RESULTS To clarify the reaction pathways between H2S and the Fe/ WSAC catalysts, in situ FTIR experiments were performed (as shown in Figure 7). All the samples were exposed to a flow of H2S/N2 (500 ppm of H2S and N2 in total) at 333 K for 30 min. As shown in Figure 7A, the peak at 3382.8 cm−1 is assigned to vibrations of surface hydroxyl species (OH). Several peaks at

Figure 8. Reaction process of H2S catalytic oxidation on the surface of plasma-treated catalyst.

The in situ FTIR results revealed that the reaction pathways changed on the catalysts surface modified by NH3-plasma, and the catalytic oxidation ability of catalyst could be enhanced for H2S removal after plasma treatment. The introduction of amino groups on the catalyst surface not only promotes the H2S absorption13 but also is beneficial for its oxidation.15

5. CONCLUSION Plasma surface modification has been proven to be an efficient method to improve the catalytic oxidation capacity of Fe/ WSAC catalysts for H2S removal. The discharge gas gap and dielectric thickness of the DBD were found to have an important role by changing the discharge mode, thus influencing the catalyst surface nature. The hydrogen sulfide removal efficiency was enhanced after plasma treatment. The optimal discharge gas gap and dielectric thickness were 5.5 mm and 1.5 mm, respectively. However, the H2S catalytic oxidation capacity decreased at higher discharge gas gaps (or thicker dielectrics). Higher discharge gas gaps resulted in more ragged sample surfaces, which would be destroyed by the evolved gas

Figure 7. In situ FTIR spectra of (A) untreated and (B) plasmatreated samples in the presence of H2S for 10 min. 2879

DOI: 10.1021/acs.iecr.7b05079 Ind. Eng. Chem. Res. 2018, 57, 2873−2881

Article

Industrial & Engineering Chemistry Research

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generated by the high energy plasma. The number of 0.4−0.75 nm pores increased with increasing discharge gas gap and dielectric thickness, and the number of amine/amide groups also increased after plasma treatment, resulting from the ammonia being ionized into NH2− and NH2−. According to the in situ FTIR spectra, SO2−N species are formed on the catalyst surface after NH3-plasma treatment. The reaction pathways for H2S catalytic oxidation on the Fe/WSAC catalyst surface changed after NH3-plasma treatment, and the introduction of amino groups possibly promotes the catalytic oxidation of H2S. However, the detailed mechanism of plasma surface modification on the catalyst is still difficult to determine. Thus, the nonthermal plasma surface modification mechanism and the H2S catalytic oxidation reaction mechanism on the Fe/WSAC catalyst modified by plasma will be further researched.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-0871-65920507. Fax: +86-0871-65920507. E-mail: [email protected]. ORCID

Kai Li: 0000-0001-5862-4770 Maohong Fan: 0000-0003-1334-7292 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grants 21667015, 51408282, and 51 7 0 8 26 6 ) , Ch i n a S c h o l a r s h i p C o un ci l ( G r a n t s 201508530017, 201608530169, and 201608740011), and the Analysis and Testing Foundation of Kunming University of Science and Technology.

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DOI: 10.1021/acs.iecr.7b05079 Ind. Eng. Chem. Res. 2018, 57, 2873−2881

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DOI: 10.1021/acs.iecr.7b05079 Ind. Eng. Chem. Res. 2018, 57, 2873−2881