*Article* **Valorization of Raw and Calcined Chicken Eggshell for Sulfur Dioxide and Hydrogen Sulfide Removal at Low Temperature**

**Waseem Ahmad <sup>1</sup> , Sumathi Sethupathi 1,\* , Yamuna Munusamy <sup>1</sup> and Ramesh Kanthasamy <sup>2</sup>**


**Abstract:** Chicken eggshell (ES) is a waste from the food industry with a high calcium content produced in substantial quantity with very limited recycling. In this study, eco-friendly sorbents from raw ES and calcined ES were tested for sulfur dioxide (SO<sup>2</sup> ) and hydrogen sulfide (H2S) removal. The raw ES was tested for SO<sup>2</sup> and H2S adsorption at different particle size, with and without the ES membrane layer. Raw ES was then subjected to calcination at different temperatures (800 ◦C to 1100 ◦C) to produce calcium oxide. The effect of relative humidity and reaction temperature of the gases was also tested for raw and calcined ES. Characterization of the raw, calcinated and spent sorbents confirmed that calcined eggshell CES (900 ◦C) showed the best adsorption capacity for both SO<sup>2</sup> (3.53 mg/g) and H2S (2.62 mg/g) gas. Moreover, in the presence of 40% of relative humidity in the inlet gas, the adsorption capacity of SO<sup>2</sup> and H2S gases improved greatly to about 11.68 mg/g and 7.96 mg/g respectively. Characterization of the raw and spent sorbents confirmed that chemisorption plays an important role in the adsorption process for both pollutants. The results indicated that CES can be used as an alternative sorbent for SO<sup>2</sup> and H2S removal.

**Keywords:** chicken eggshell; waste valorization; adsorption; biogas; flue gas

## **1. Introduction**

Sulfur dioxide (SO2) and hydrogen sulfide (H2S) are both considered toxic gases. SO<sup>2</sup> is mainly part of the flue gases while H2S is naturally present in many fossil fuels and quickly oxidizes to SO<sup>2</sup> upon burning. Direct release of these acidic gases to the open air can cause serious environmental repercussions [1,2]. SO<sup>2</sup> can be removed from flue gas in many ways and this process is named as flue gas desulfurization (FGD). The adsorption processes are already used and are well known for SO<sup>2</sup> removal as well as for H2S removal. Common sorbents for SO<sup>2</sup> removal are mostly calcium-based oxide/hydroxide (CaO/Ca(OH)2), zinc oxide-based (ZnO), sodium hydroxide based (NaOH) and ammonia-based [3]. Limestone, slaked lime or a mixture of slaked lime with fly ash is commercially used in FGD systems [4]. There is a lot of room for improvement in the traditional FGD technologies as they consume a large quantity of water and at times CO<sup>2</sup> leakage to the environment [5,6]. Therefore, recently, many alternative sorbents such as red mud and various modified carbonaceous catalysts, have been developed with the aim to reduce the cost, promote principles of circular economy, and improve energy efficiency [7,8].

The removal of H2S, on the other hand, is considered a crucial step in the biogas industry because of its toxic and corrosive nature [9]. Effective utilization of biogas as biomethane is a challenge because of its costly purification steps [10]. Many technologies such as adsorption, alkaline washing (absorption), membrane separation, and cryogenic distillation have been tested to efficiently removes H2S [11]. The most common type of sorbent used for adsorption process is impregnated activated carbon [10]. Activated carbon has been reported to have H2S removal capacities in the range of 150–650 mg/g [12].

**Citation:** Ahmad, W.; Sethupathi, S.; Munusamy, Y.; Kanthasamy, R. Valorization of Raw and Calcined Chicken Eggshell for Sulfur Dioxide and Hydrogen Sulfide Removal at Low Temperature. *Catalysts* **2021**, *11*, 295. https://doi.org/10.3390/catal 11020295

Academic Editor: Daniela Barba

Received: 18 January 2021 Accepted: 19 February 2021 Published: 23 February 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

However, adsorption of SO<sup>2</sup> or H2S using activated carbon generates secondary waste, which is acidic and difficult for landfilling. Apart from activated carbons, various types of waste-based sorbents derived from municipal waste sludge, fly ash, forestry, slaughterhouse, etc. have been also tested for H2S removal [10]. Nevertheless, these sorbents are either not re-generable or has a very low removal efficiency. Thus, recently researchers are focusing on developing new cost-effective and regenerative sorbents for H2S removal.

Chicken eggshell (ES) is a waste product from the food industry and is mostly disposed in landfills in Malaysia. It contains about 90–95% of calcium carbonate in the form of calcite, 1% magnesium carbonate, 1% calcium phosphate and some organic compounds [13,14]. According to Food and Agriculture Organization (FAO) of the United Nations, approximately 70.4 million tons of chicken eggs were produced worldwide in 2015 and the production is estimated to increase to 90 million tons by 2030 [15]. In Malaysia, about 642,600 tonnes of chicken eggs are produced annually which produces approximately 70,686 tonnes of ES waste [16]. Considering the volume of ES waste produced, its reutilization is still very limited and a greater part of it is disposed in landfills. ES has been reported to be used occasionally as a soil conditioner, fertilizer, and additive for animal feed [17]. Recently in the literature, ES waste has been valorized in many innovative applications such as special materials for bone tissue restoration, as a sorbent for metal ions in wastewater treatment and also as a catalyst in different applications [18]. Recently, ES based sorbent was used for CO<sup>2</sup> adsorption and the removal capacity was reported as 10.47 mg/g at 1 bar and 30 ◦C [19]. Sethupathi et al. (2017) had also carried out a preliminary study on the SO<sup>2</sup> removal from the gaseous stream using CES (950 ◦C) and achieved maximum adsorption capacity of 2.15 mg/g [20]. One of the latest literatures reported a carbonized hybrid sorbent (ES and lignin) to remove SO<sup>2</sup> from the air [21]. In this study, the possibility of replacing conventional calcite-based sorbents with raw and calcined chicken eggshell for acidic gases removal at low temperature was appraised. Adsorption experiments were conducted in a lab-scale adsorption rig and SO<sup>2</sup> and H2S were mixed with nitrogen gas with fixed concentration separately. Various characterization techniques like FTIR, XRD, EDX, and FESEM were used to further investigate the adsorption mechanism.

#### **2. Results and Discussions**

#### *2.1. Characterization of Raw and Calcined Eggshell before and after Adsorption Tests*

The morphology of ES sorbents was examined with Field Emission Scanning Electron Microscope (FESEM) images. Figure 1a shows the outer shell of ES and its FESEM images at different magnification. It can be seen that the outer shell of ES has a smooth surface with cracks. Figure 1b is the inner ES with the membrane. FESEM images clearly show fibrous network morphology of protein which is very porous in nature. Figure 1c shows the actual image of powdered (<90 µm) raw eggshell (RES) and FESEM images at different magnification. The porous nature of RES can be clearly seen with the pore hole like structures on the particles. Pore structures of ES sorbents are categorized as Type II as per Brunauer, Deming and Teller classification, stating their characteristics belong to macroporous material, nonporous materials, or materials with open voids.

*Catalysts* **2021**, *11*, x FOR PEER REVIEW 3 of 20

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**Figure 1.** Digital camera and FESEM images of raw eggshell. (**a**) Outer at 300× and 5000× magnification, (**b**) inner at 300× and 5000× magnification, and (**c**) particle size of <90 µm at 10,000× and 30,000× magnification. **Figure 1.** Digital camera and FESEM images of raw eggshell. (**a**) Outer at 300× and 5000× magnification, (**b**) inner at 300× and 5000× magnification, and (**c**) particle size of <90 µm at 10,000× and 30,000× magnification. **Figure 1.** Digital camera and FESEM images of raw eggshell. (**a**) Outer at 300× and 5000× magnification, (**b**) inner at 300× and 5000× magnification, and (**c**) particle size of <90 µm at 10,000× and 30,000× magnification.

Figure 2a,b show FESEM images of (CES 900 °C) and (CES 1100 °C). The images for 900 °C show a stable and structured particle compared to the one calcined at 1100 °C. (CES 900 °C) shows well-arranged particles with smooth surfaces on each particle. (CES 1100 °C) was totally the opposite, particles lose their shapes, and each particle shows intensive surface cracks due to the sintering process. Figure 2a,b show FESEM images of (CES 900 ◦C) and (CES 1100 ◦C). The images for 900 ◦C show a stable and structured particle compared to the one calcined at 1100 ◦C. (CES 900 ◦C) shows well-arranged particles with smooth surfaces on each particle. (CES 1100 ◦C) was totally the opposite, particles lose their shapes, and each particle shows intensive surface cracks due to the sintering process. Figure 2a,b show FESEM images of (CES 900 °C) and (CES 1100 °C). The images for 900 °C show a stable and structured particle compared to the one calcined at 1100 °C. (CES 900 °C) shows well-arranged particles with smooth surfaces on each particle. (CES 1100 °C) was totally the opposite, particles lose their shapes, and each particle shows intensive surface cracks due to the sintering process.

**Figure 2.** FESEM images of calcined eggshell at (**a**) 900 °C and (**b**) 1100 °C with 3000× and 30,000× magnification. **Figure 2.** FESEM images of calcined eggshell at (**a**) 900 ◦C and (**b**) 1100 ◦C with 3000× and 30,000× magnification.

BET surface area values of the CES sorbents in comparison to RES are shown in Table 1. BET surface area of RES was reported as 0.56 m2/g and the readings were increasing as RES was calcined. However, the values decrease when the temperature was further increased up to 1000 ◦C and 1100 ◦C. The highest BET surface area of 6.74 m2/g was recorded at 900 ◦C. The BET surface area of CES was low compared to commercial-grade CaO whose BET surface area is in the range of 11–25 m2/g [22]. BET surface area values of the CES sorbents in comparison to RES are shown in Table 1. BET surface area of RES was reported as 0.56 m2/g and the readings were increasing as RES was calcined. However, the values decrease when the temperature was further increased up to 1000 °C and 1100 °C. The highest BET surface area of 6.74 m2/g was recorded at 900 °C. The BET surface area of CES was low compared to commercial-grade CaO whose BET surface area is in the range of 11–25 m2/g [22].


**Table 1.** BET surface area of calcined eggshell at different temperature. **Table 1.** BET surface area of calcined eggshell at different temperature.

Figure 3 shows the nitrogen-adsorption isotherm for CES. The isotherm is of Type IV. As per IUPAC standard, this kind of isotherm is obtained for a combination of microporous and mesoporous structure which is formed at a higher relative pressure. The low BET surface area of CES could be due to eggshell structure and impurities. Figure 3 shows the nitrogen-adsorption isotherm for CES. The isotherm is of Type IV. As per IUPAC standard, this kind of isotherm is obtained for a combination of microporous and mesoporous structure which is formed at a higher relative pressure. The low BET surface area of CES could be due to eggshell structure and impurities.

**Figure 3. Figure 3.**Pore size distribution and N2 adsorpti Pore size distribution and N2 adsorption-desorption isotherm of (CES 900 on-desorption isotherm of (CES 900 °C). ◦C).

The elemental content of RES, CES, and their respective spent sorbents are shown in Table 2. The presence of the sulfur element in the spent RES and CES affirms the adsorption of SO<sup>2</sup> and H2S and the occurrence of chemisorption. The elemental content of RES, CES, and their respective spent sorbents are shown in *Catalysts* **2021**, *11*, x FOR PEER REVIEW 5 of 20 *Catalysts* **2021**, *11*, x FOR PEER REVIEW 5 of 20 *Catalysts* **2021**, *11*, x FOR PEER REVIEW 5 of 20 *Catalysts* **2021**, *11*, x FOR PEER REVIEW 5 of 20

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**Table 2.** The differences in elemental content of raw eggshell and (CES 900 ◦C) before and after adsorption. tion of SO2 and H2S and the occurrence of chemisorption. Table 2. The presence of the sulfur element in the spent RES and CES affirms the adsorption of SO2 and H2S and the occurrence of chemisorption. Table 2. The presence of the sulfur element in the spent RES and CES affirms the adsorption of SO2 and H2S and the occurrence of chemisorption. The elemental content of RES, CES, and their respective spent sorbents are shown in Table 2. The presence of the sulfur element in the spent RES and CES affirms the adsorption of SO2 and H2S and the occurrence of chemisorption. The elemental content of RES, CES, and their respective spent sorbents are shown in Table 2. The presence of the sulfur element in the spent RES and CES affirms the adsorp-

Table 2. The presence of the sulfur element in the spent RES and CES affirms the adsorp-

The elemental content of RES, CES, and their respective spent sorbents are shown in

The elemental content of RES, CES, and their respective spent sorbents are shown in


The Ca content in the CES is more compared to the one in raw eggshell (RES) because, during the calcination, CO2 and other volatile matters are released. It is also evident that the contents of other impurities in ES such as Zn, Mg, Al, and Cu remain almost the same in RES, CES and spent sorbents. This shows that these impurities were not involved in the sorption process. In the spent adsorbent, the content of Ca was reduced. This indicated the conversion of Ca into sulfite complex. The Ca content in the CES is more compared to the one in raw eggshell (RES) because, during the calcination, CO<sup>2</sup> and other volatile matters are released. It is also evident that the contents of other impurities in ES such as Zn, Mg, Al, and Cu remain almost the same in RES, CES and spent sorbents. This shows that these impurities were not involved in the

> The wideband approximately at 3430 cm−1 in the RES is attributed to the stretching of the OH bond [23]. The two well-defined bands at 1413 cm−1 and 874 cm−1 are distinctive to the bending of C–O bond of CaCO3 while the band at 712 cm−1 is related to Ca–O bond [24]. These indicate that RES comprises of calcite [25]. For (CES 900 °C), the well-defined band at approximately 3630 cm−1 corresponds to the vibration of OH bonds probably attached to the surface of CaO [24]. The peak at 1413 cm−1 is sharper in (CES 900 °C) showing a higher percentage of CaO and more prevailing than RES. New peaks were not detected in the spent RES sorbents. Nevertheless, there were changes in the intensity of the peaks. This could be due to the contact of the acidic gases. However, a new peak at 1080 cm−1 was

visible for (CES 900 °C) spent sorbents. This affirms the presence of sulfite [26].

**(a)** 

sorption process. In the spent adsorbent, the content of Ca was reduced. This indicated the conversion of Ca into sulfite complex. sorption process. In the spent adsorbent, the content of Ca was reduced. This indicated the conversion of Ca into sulfite complex.

The Ca content in the CES is more compared to the one in raw eggshell (RES) because, during the calcination, CO2 and other volatile matters are released. It is also evident that the contents of other impurities in ES such as Zn, Mg, Al, and Cu remain almost the same in RES, CES and spent sorbents. This shows that these impurities were not involved in the

S 0.21

Others 0.69

*Catalysts* **2021**, *11*, x FOR PEER REVIEW 6 of 20

FTIR spectra of RES, (CES 900 ◦C) and the spent sorbents are presented in Figure 4. The wideband approximately at 3430 cm−<sup>1</sup> in the RES is attributed to the stretching of the OH bond [23]. The two well-defined bands at 1413 cm−<sup>1</sup> and 874 cm−<sup>1</sup> are distinctive to the bending of C–O bond of CaCO<sup>3</sup> while the band at 712 cm−<sup>1</sup> is related to Ca–O bond [24]. These indicate that RES comprises of calcite [25]. For (CES 900 ◦C), the well-defined band at approximately 3630 cm−<sup>1</sup> corresponds to the vibration of OH bonds probably attached to the surface of CaO [24]. The peak at 1413 cm−<sup>1</sup> is sharper in (CES 900 ◦C) showing a higher percentage of CaO and more prevailing than RES. New peaks were not detected in the spent RES sorbents. Nevertheless, there were changes in the intensity of the peaks. This could be due to the contact of the acidic gases. However, a new peak at 1080 cm−<sup>1</sup> was visible for (CES 900 ◦C) spent sorbents. This affirms the presence of sulfite [26]. FTIR spectra of RES, (CES 900 °C) and the spent sorbents are presented in Figure 4. The wideband approximately at 3430 cm−1 in the RES is attributed to the stretching of the OH bond [23]. The two well-defined bands at 1413 cm−1 and 874 cm−1 are distinctive to the bending of C–O bond of CaCO3 while the band at 712 cm−1 is related to Ca–O bond [24]. These indicate that RES comprises of calcite [25]. For (CES 900 °C), the well-defined band at approximately 3630 cm−1 corresponds to the vibration of OH bonds probably attached to the surface of CaO [24]. The peak at 1413 cm−1 is sharper in (CES 900 °C) showing a higher percentage of CaO and more prevailing than RES. New peaks were not detected in the spent RES sorbents. Nevertheless, there were changes in the intensity of the peaks. This could be due to the contact of the acidic gases. However, a new peak at 1080 cm−1 was visible for (CES 900 °C) spent sorbents. This affirms the presence of sulfite [26].

**Figure 4.** FTIR spectra of (**a**) raw eggshell and (**b**) calcined eggshell (900 °C) before and after adsorption. **Figure 4.** FTIR spectra of (**a**) raw eggshell and (**b**) calcined eggshell (900 ◦C) before and after adsorption.

CES at high temperature and amount of CaO produced.

CaO.

**Table 3.** Proximate analysis of raw and calcined eggshell (900 °C).

and volatile matter of CES were much lesser than RES due to the high-temperature calcination process. The residue for CES was far greater than RES which shows the stability of

**Temperature (°C) Proximate Analysis RES (%) CES (%)**  25–120 Moisture 1.03 0.28 120–450 Volatile content 4.18 8.01 450–800 CO2 43.17 1.63 800–900 Residue (CaO) 51.62 90.08

Figure 5 show X-ray diffraction (XRD) of RES, (CES 900 °C) and the spent sorbents. RES showed a major peak at 2θ = 29.5° which indicates that CaCO3 is a major constituent of the waste ES. In the (CES 900 °C), regular peaks were obtained at 2θ = 32°, 34°, 37.5°, and 54°, showing the conversion of CaCO3 to CaO [23]. It is noted that peak at 2θ = 29.5° is no longer visible in the (CES 900 °C), which implies a complete conversion of CaCO3 to

The proximate analyses of RES and CES are listed in Table 3. The moisture content and volatile matter of CES were much lesser than RES due to the high-temperature calcination process. The residue for CES was far greater than RES which shows the stability of CES at high temperature and amount of CaO produced.


**Table 3.** Proximate analysis of raw and calcined eggshell (900 ◦C).

Figure 5 show X-ray diffraction (XRD) of RES, (CES 900 ◦C) and the spent sorbents. RES showed a major peak at 2θ = 29.5◦ which indicates that CaCO<sup>3</sup> is a major constituent of the waste ES. In the (CES 900 ◦C), regular peaks were obtained at 2θ = 32◦ , 34◦ , 37.5◦ , and 54◦ , showing the conversion of CaCO<sup>3</sup> to CaO [23]. It is noted that peak at 2θ = 29.5◦ is no longer visible in the (CES 900 ◦C), which implies a complete conversion of CaCO<sup>3</sup> to CaO. *Catalysts* **2021**, *11*, x FOR PEER REVIEW 8 of 20

**Figure 5.** *Cont*.

**(b)** 

sorption.

**Figure 5.** XRD pattern of (**a**) raw eggshell and (**b**) calcined eggshell (900 °C) before and after ad-

For the spent RES, it can be seen that there was no significant difference in the crystalline structure after the adsorption of SO2 and H2S. It shows no chemical interaction between the sorbent and the gases. Thus, it can be concluded that for RES the adsorption was merely physical. However, for spent (CES 900 °C), the initial peaks at 2θ = 32°, 34°, **(a)** 

**Figure 5.** XRD pattern of (**a**) raw eggshell and (**b**) calcined eggshell (900 °C) before and after adsorption. **Figure 5.** XRD pattern of (**a**) raw eggshell and (**b**) calcined eggshell (900 ◦C) before and after adsorption.

For the spent RES, it can be seen that there was no significant difference in the crystalline structure after the adsorption of SO2 and H2S. It shows no chemical interaction between the sorbent and the gases. Thus, it can be concluded that for RES the adsorption was merely physical. However, for spent (CES 900 °C), the initial peaks at 2θ = 32°, 34°, For the spent RES, it can be seen that there was no significant difference in the crystalline structure after the adsorption of SO<sup>2</sup> and H2S. It shows no chemical interaction between the sorbent and the gases. Thus, it can be concluded that for RES the adsorption was merely physical. However, for spent (CES 900 ◦C), the initial peaks at 2θ = 32◦ , 34◦ , 37.5◦ , and 54◦ have disappeared and new peaks were formed at 2θ = 17◦ and 29◦ . The peak at 34◦ in the (CES 900 ◦C) corresponds to CaO. This peak reduced in the spent sorbent, indicating the presence of unreacted CaO in the spent sorbent. The small peaks at 2θ = 17◦ , 28◦ , 28◦ , and 2θ = 34◦ , 47◦ , 52◦ , and 54◦ may correspond to CaSO<sup>3</sup> and Ca(OH)<sup>2</sup> respectively for the spent (CES 900 ◦C). These spread-out peaks show that the crystallinity of (CES 900 ◦C) after adsorption has dropped. Similar peaks were reported by others as well [27].

pH values of the RES and CES before and after adsorption are tabulated in Tables 4 and 5 to show the reactivity of the acidic gases on RES and CES. There was a clear increase in pH with the increase in calcination temperature because of the formation of CaO which is basic in nature. Whereas for RES, the sorbents with membrane have slightly higher pH compared to the one without membrane. This is due to the different types of protein in the membrane. It was noticed that for all cases, spent sorbent's pH values dropped one level, indicating successful adsorption of acidic gases.




**Table 5.** Adsorption capacity and pH of calcined eggshell at different temperature.

#### *2.2. Effect of the Eggshell Membrane and Particle Size*

Three different particle sizes (<90 µm, 90–125 µm, and 125–180 µm) of RES with and without membrane were tested for the SO<sup>2</sup> and H2S removal. During this analysis, other parameters such as sorbent dosage (1 g), flow rate (300 mL/min), humidity (0%), gas inlet concentration (300 ppm), pressure (1 bar), and reaction temperature (ambient temperature, 29 ◦C) were all kept constant. Table 4 shows the adsorption capacity and saturation time of SO<sup>2</sup> and H2S adsorption by RES with and without membrane. It was noticed that for both H2S and SO<sup>2</sup> the adsorption capacities were less than 1.1 mg/g on dry basis. The breakthrough point was not detected which indirectly shows that there was no immediate chemical interaction between the gas and sorbent. It is known that CaCO<sup>3</sup> is a highly stable material at ambient conditions. Thus, RES with or without membrane could have similar behavior. RES with membrane recorded higher adsorption capacity and longer saturation time for both H2S and SO<sup>2</sup> gas compared to the one without membrane. According to Tsai et al. (2006), ES membrane comprises of a grid of fibrous proteins which contributes to its large surface areas and these fibrous proteins have higher BET surface area compared to the shell itself [28]. In the literature, the role of ES membrane in the removal of reactive dyes, heavy metals, phenols, and various other substances was reported and in most of the cases, it was reported that the adsorption capacity was better with membrane compared to one without membrane [29,30]. Thus, in this study, it was found that RES with membrane enhanced the sorption of H2S and SO2.

As for the effect of particle size, as anticipated, the smallest particle size i.e., <90 µm shows the best results for both RES with and without membrane. The calculated adsorption capacity values followed the following sequence for both gases: 90 µm > 125 µm > 180 µm. Witoon (2011) had stated that the smaller particle size of calcined ES had higher CO<sup>2</sup> capture capacity because it provides a greater exposed surface for the adsorption [23]. Similarly, in this study, smaller particles can offer a greater surface area for gas–solid interactions. The macro-pores and pits are irregularly dispersed over the surface of the RES, which could be one of the factors for low adsorption capacity as evident from the FESEM image. At high temperature, CaCO<sup>3</sup> breaks down to CaO(s) and release CO2. The Ca+2 of CaO is unstable and reacts with SO2(g) replacing oxygen at high temperature. The reaction of SO<sup>2</sup> gas on CaCO<sup>3</sup> can be shown by the following chemical reaction [31];

$$\rm CaCO\_{3(s)} + SO\_{2(g)} \to CaSO\_{3(s)} + CO\_{2(g)}.\tag{1}$$

However, calcium sulfate (CaSO4) is only formed from the reaction between calcium carbonate (CaCO3) and SO<sup>2</sup> gas when the temperature is more than 750 K in presence of oxygen (O2) as illustrated below [31];

$$\text{CaCO}\_{\text{3(s)}} + \text{SO}\_{\text{2(g)}} + \frac{1}{2}\text{O}\_{\text{2(g)}} \rightarrow \text{CaSO}\_{\text{4(s)}} + \text{CO}\_{\text{2(g)}}.\tag{2}$$

As the experiments were carried out in room temperature and O<sup>2</sup> was not induced in this study, the only possible reaction would be the formation of calcium sulfite (CaSO3) rather than CaSO4. For the reaction between CaCO<sup>3</sup> of RES and H2S, the following direct sulfidation reaction is expected [32]:

$$\rm CaCO\_{3(s)} + H\_2S\_{(g)} \to CaS\_{(s)} + CO\_{2(g)} + H\_2O\_{(v)}.\tag{3}$$

However, at room temperature, CaCO<sup>3</sup> is very stable and the chances of the above reactions are very slim. However, for RES only physical adsorption has happened for both SO<sup>2</sup> and H2S which is the main reason for its low adsorption capacities. As physical adsorption is happening, so the particle size and surface porosity played an important role.

#### *2.3. Effect of Calcination Temperature*

The influence of various calcination temperature on RES and their effect on SO<sup>2</sup> and H2S adsorption were tested using dried and powdered (<90 µm) RES with membrane. Other adsorption parameters were kept the same as in section "Effect of the eggshell membrane and particle size". Figure 6a,b shows the breakthrough curves of SO<sup>2</sup> and H2S versus calcination temperature of ES. Among the calcination temperature, 900 ◦C shows a prominent outstanding curve. Breakthrough points were very short however, it has a longer saturation time (84 min and 77 min for SO<sup>2</sup> and H2S, respectively) for both gases.

Table 5 tabulates the adsorption capacity of the CES at different temperatures. (CES 900 ◦C) shows the highest adsorption capacity i.e., 2.63 mg/g and 3.53 mg/g for H2S and SO<sup>2</sup> respectively. High adsorption capacity at 900 ◦C calcination could be due to the complete conversion of calcium carbonate (CaCO3) to calcium oxide (CaO). It is noted that complete conversion takes places around 930 ◦C [32,33]. Thus, at temperature 800 ◦C, lower adsorption capacity was noticed for both gases. Moreover, the 800 ◦C calcined ES was visibly grayish in color compared to the ones calcined at a higher temperature which were whitish confirming the incomplete calcination. At higher temperature i.e., >900 ◦C, the adsorption capacities for both SO<sup>2</sup> and H2S decreased. Moreover, the saturation time was shorter and there was no breakthrough point. The decrease in adsorption capacity is due to the sintering effect. Similar work on SO<sup>2</sup> adsorption by calcined limestone reported that pore size distribution is greatly affected by the calcination temperature.

It was reported that 950 ◦C was the optimized calcination temperature for limestone and at this temperature, the pores of CaO have the least diffusion resistance and highest activity for SO<sup>2</sup> removal [34]. Another similar work mentioned that sintering of CaO derived from pure CaCO<sup>3</sup> starts at 800–900 ◦C and it becomes more severe after 950 ◦C. ES is less pure than the commercial limestone, where for each 100 g of air-dried of ES waste only 88 g are of CaCO<sup>3</sup> [35], which corresponds to an increase in the rate of sintering [36,37]. Also, it was proven that a breakdown of the pores occurs for 1100 ◦C. These statements can be confirmed using BET surface area readings, and FESEM images.

due to the sintering effect. Similar work on SO2 adsorption by calcined limestone reported

that pore size distribution is greatly affected by the calcination temperature.

**Figure 6.** Adsorption breakthrough curves of (**a**) SO2 and (**b**) H2S by eggshell at different calcination. **Figure 6.** Adsorption breakthrough curves of (**a**) SO<sup>2</sup> and (**b**) H2S by eggshell at different calcination.

#### *2.4. Effect of Reaction Temperature and Humidity*

It was reported that 950 °C was the optimized calcination temperature for limestone and at this temperature, the pores of CaO have the least diffusion resistance and highest activity for SO2 removal [34]. Another similar work mentioned that sintering of CaO derived from pure CaCO3 starts at 800–900 °C and it becomes more severe after 950 °C. ES is less pure than the commercial limestone, where for each 100 g of air-dried of ES waste only 88 g are of CaCO3 [35], which corresponds to an increase in the rate of sintering The influence of reaction temperature on SO<sup>2</sup> and H2S adsorption by RES and (CES 900 ◦C) was evaluated using two different reactor temperatures (100 ◦C and 200 ◦C). Other parameters such as sorbent dosage (1 g), flow rate (300 mL/min), gas inlet concentration (300 ppm), and pressure (1 bar) were all kept constant. Table 6 shows the adsorption capacity for the effect of reaction temperature and humidity. Figure 7 shows the breakthrough curve for the effect of reaction temperature on H2S and SO<sup>2</sup> removal. The trend was comparatively better than the ones done at the room temperature (29 ◦C) earlier. However, the impact was very minimal for (CES 900 ◦C). Meanwhile, for RES, the adsorption capacity was doubled when the reactor temperature was increased from 30 ◦C to 200 ◦C. It was claimed that an increase in reaction temperature, increases the chemical interaction of SO<sup>2</sup> and H2S with limestone-based CaCO<sup>3</sup> or CaO or Ca(OH)<sup>2</sup> sorbents [38–40]. The effects

of reactor temperature in the range of room temperature to 200 ◦C is considered low in magnitude. At a temperature below 200 ◦C, only calcium sulfite will be formed during the reaction of SO<sup>2</sup> with CaCO<sup>3</sup> and CaO [41]. Calcium sulfite, however, is stable at around 200 ◦C and only becomes unstable at a temperature above 727 ◦C decomposing to form calcium sulfate and calcium carbide [42]. Though, at a temperature above 100 ◦C the rate of calcium sulfite formation tends to increase [41]. There are limited data on SO<sup>2</sup> and H2S by CaCO<sup>3</sup> and CaO at temperature < 250 ◦C. Most of the studies have been done at elevated temperatures, i.e., >400 ◦C.

**Table 6.** Adsorption capacity of raw and calcined (900 ◦C) eggshell at different reaction temperature and 40% relative humidity.


These analyses illustrated that there are some interactions between SO<sup>2</sup> and H2S with RES and CES even at low temperature. A lower temperature will be favorable for SO<sup>2</sup> and H2S removal because the temperature of flue gas going to the stack is around 150 ◦C and the working temperature of is about 55 ◦C [43,44]. However, at a higher temperature, the reaction could be further enhanced. A study of SO<sup>2</sup> adsorption by lime (80% CaO) reported that the conversion rate gets double when the temperature increases from 400 ◦C to 800 ◦C [45]. Moreover, during the reaction of CaCO<sup>3</sup> with H2S, complete conversion of CaCO<sup>3</sup> to CaS is only feasible if sulfidation is carried out at a temperature above its calcination temperature [46]. Thus, it can be concluded that a more conducive environment is created for the sulfidation of both RES and (CES 900 ◦C) by increasing the reaction temperature up to 200 ◦C. *Catalysts* **2021**, *11*, x FOR PEER REVIEW 13 of 20

**Figure 7.** *Cont*.

**Figure 7.** Breakthrough curves of (**a**) SO2 and (**b**) H2S at different reaction temperature.

The chemical reaction of SO2 and CaCO3 in the presence of RH is as shown below;

Figure 8 shows the breakthrough curves of RES and (CES 900 °C) with a response to the effect of humidity. All other parameters were kept constant. It was noticed that with the addition of 40% relative humidity (RH), the performance of both RES and (CES 900 °C) improved significantly. The adsorption capacity of SO2 and H2S by (CES 900 °C) increased almost triple with humidity. There were significant improvements in the breakthrough time as well as saturation time for both SO2 and H2S. The breakthrough point was clearly noticeable for (CES 900 °C). For RES, a great increase was noticed for SO2, however, for H2S, only small improvement was noticed. Results can be compared from Table 4–6. The presence of humidity in the inlet gas improved the adsorption capacity of SO2 more than the H2S for RES. This could be due to the solubility of SO2. The solubility of SO2 in water is about 16 times more than the solubility of H2 as per the data published by [47].

**(b)** 

**Figure 7.** Breakthrough curves of (**a**) SO2 and (**b**) H2S at different reaction temperature. **Figure 7.** Breakthrough curves of (**a**) SO<sup>2</sup> and (**b**) H2S at different reaction temperature.

Figure 8 shows the breakthrough curves of RES and (CES 900 °C) with a response to the effect of humidity. All other parameters were kept constant. It was noticed that with the addition of 40% relative humidity (RH), the performance of both RES and (CES 900 °C) improved significantly. The adsorption capacity of SO2 and H2S by (CES 900 °C) increased almost triple with humidity. There were significant improvements in the breakthrough time as well as saturation time for both SO2 and H2S. The breakthrough point was clearly noticeable for (CES 900 °C). For RES, a great increase was noticed for SO2, however, for H2S, only small improvement was noticed. Results can be compared from Table 4–6. The presence of humidity in the inlet gas improved the adsorption capacity of SO2 more than the H2S for RES. This could be due to the solubility of SO2. The solubility of SO2 in water is about 16 times more than the solubility of H2 as per the data published by [47]. The chemical reaction of SO2 and CaCO3 in the presence of RH is as shown below; Figure 8 shows the breakthrough curves of RES and (CES 900 ◦C) with a response to the effect of humidity. All other parameters were kept constant. It was noticed that with the addition of 40% relative humidity (RH), the performance of both RES and (CES 900 ◦C) improved significantly. The adsorption capacity of SO<sup>2</sup> and H2S by (CES 900 ◦C) increased almost triple with humidity. There were significant improvements in the breakthrough time as well as saturation time for both SO<sup>2</sup> and H2S. The breakthrough point was clearly noticeable for (CES 900 ◦C). For RES, a great increase was noticed for SO2, however, for H2S, only small improvement was noticed. Results can be compared from Tables 4–6. The presence of humidity in the inlet gas improved the adsorption capacity of SO<sup>2</sup> more than the H2S for RES. This could be due to the solubility of SO2. The solubility of SO<sup>2</sup> in water is about 16 times more than the solubility of H<sup>2</sup> as per the data published by [47]. The chemical reaction of SO<sup>2</sup> and CaCO<sup>3</sup> in the presence of RH is as shown below;

$$\text{CaCO}\_3\text{ (s)} + \text{H}\_2\text{O}\text{ (g)} \rightarrow \text{Ca(OH)(CO}\_3\text{H)}\tag{4}$$

**(a)** 

$$\text{Ca(OH)(CO\_3H)} + \text{SO}\_2\text{(g)} \rightarrow \text{CaSO}\_3 + \text{H}\_2\text{CO}\_3.\tag{5}$$

Carbonic acid (H2CO3) is a product of this surface reactions, not alike the one without RH which produces carbon dioxide [31]. Between 30% and 85% RH, SO<sup>2</sup> and CaCO<sup>3</sup> reaction are improved significantly, approximately by 5 to 10 fold for single crystal CaCO<sup>3</sup> (calcite) in the presence of moisture [48]. A slight improvement is noticed because of the humidity which made the contact possible between CaCO<sup>3</sup> and the acidic gases [49]. It is known that CaO reaction in the presence of water vapor will form Ca(OH)2. At low temperature, it is expected that only sulfite hemihydrate will be formed when SO<sup>2</sup> is present [41]. The following reaction could occur;

$$\text{Ca(OH)}\_{2} + \text{SO}\_{2} \rightarrow \text{CaSO}\_{3} \cdot 0.5 \text{H}\_{2}\text{O} + 0.5 \text{H}\_{2}\text{O}.\tag{6}$$

The chemisorption process of SO<sup>2</sup> onto the sorbent surface as described Equation (6) chemical reaction increases with increasing RH [41]. It has been reported that moisture can enhance the adsorption capacity of carbonates and oxides for atmospheric gases [31]. For example, in a humid air condition, the deposition velocity of SO<sup>2</sup> gas onto calcite and dolomite increases [31]. Moreover, SO<sup>2</sup> can oxidize to SO3, to form sulfuric acid [50]. Similarly, for H2S, there was a small increase in the adsorption capacity although the gas is not readily soluble in water. This increase is attributed to the contact time between CaO sorbent and water vapor. H2S partly dissolves in water to form a weak acid and CaO would

readily attract a water molecule to convert to a more stable form of calcium hydroxide (CaOH)2. Yet, the chances of additional reaction between CaOH and H2S to form CaS are low as this reaction could only happen at high temperature, i.e., above 900 ◦C [51]. would readily attract a water molecule to convert to a more stable form of calcium hydroxide (CaOH)2. Yet, the chances of additional reaction between CaOH and H2S to form CaS are low as this reaction could only happen at high temperature, i.e., above 900 °C [51].

CaCO3 (s) + H2O (g) → Ca(OH)(CO3H) (4)

Ca(OH)(CO3H) + SO2 (g) → CaSO3 + H2CO3. (5)

Ca(OH)2 + SO2 → CaSO3·0.5H2O + 0.5H2O. (6)

Carbonic acid (H2CO3) is a product of this surface reactions, not alike the one without RH which produces carbon dioxide [31]. Between 30% and 85% RH, SO2 and CaCO3 reaction are improved significantly, approximately by 5 to 10 fold for single crystal CaCO3 (calcite) in the presence of moisture [48]. A slight improvement is noticed because of the humidity which made the contact possible between CaCO3 and the acidic gases [49]. It is known that CaO reaction in the presence of water vapor will form Ca(OH)2. At low temperature, it is expected that only sulfite hemihydrate will be formed when SO2 is present

The chemisorption process of SO2 onto the sorbent surface as described Equation (6) chemical reaction increases with increasing RH [41]. It has been reported that moisture can enhance the adsorption capacity of carbonates and oxides for atmospheric gases [31]. For example, in a humid air condition, the deposition velocity of SO2 gas onto calcite and dolomite increases [31]. Moreover, SO2 can oxidize to SO3, to form sulfuric acid [50]. Similarly, for H2S, there was a small increase in the adsorption capacity although the gas is not readily soluble in water. This increase is attributed to the contact time between CaO sorbent and water vapor. H2S partly dissolves in water to form a weak acid and CaO

*Catalysts* **2021**, *11*, x FOR PEER REVIEW 14 of 20

[41]. The following reaction could occur;

**Figure 8.** Breakthrough curves of SO<sup>2</sup> and H2S with 40% relative humidity.

#### **Figure 8.** Breakthrough curves of SO2 and H2S with 40% relative humidity. *2.5. Comparison Study*

*2.5. Comparison Study*  Table 7 shows a comparison of various sources of Ca-based sorbents and its potential to adsorb SO2 and H2S respectively. It can be seen that the adsorption capacity of the CES is comparable to that of the Ca-based sorbents. Lower adsorption capacity could be due to the low BET surface area, impurities in ES, and the unstable nature of ES. Some of the Ca-sorbents reported in the literature were modified with chemicals which further enhanced the adsorption capacity. No one has reported ES in the form of CaCO3. If it has been tested in the raw form maybe the reported adsorption capacity readings would be much lower compared to ES. Thus, it is expected that if ES is further modified to calcined Table 7 shows a comparison of various sources of Ca-based sorbents and its potential to adsorb SO<sup>2</sup> and H2S respectively. It can be seen that the adsorption capacity of the CES is comparable to that of the Ca-based sorbents. Lower adsorption capacity could be due to the low BET surface area, impurities in ES, and the unstable nature of ES. Some of the Ca-sorbents reported in the literature were modified with chemicals which further enhanced the adsorption capacity. No one has reported ES in the form of CaCO3. If it has been tested in the raw form maybe the reported adsorption capacity readings would be much lower compared to ES. Thus, it is expected that if ES is further modified to calcined ES, it could definitely perform better than the ones reported in the literature. The only one who has done similar work on ES is Witoon (2011) who tried for CO<sup>2</sup> capture at various temperatures by TGA method [23]. The carbonation rate was around 35% at 750 ◦C of calcination temperature. Thus, there is a big potential for calcined ES to be used for pollutant gases removal.


**Table 7.** Comparison of raw and calcined (900 ◦C) eggshell with other Ca-based sorbents for SO<sup>2</sup> and H2S.


**Table 7.** *Cont*.

#### **3. Materials and Methods**

*3.1. Sorbent Preparation*

Chicken eggshell waste (brown in color classified as Grade B and C) was collected from the student's food court in the university campus. It was thoroughly soaked and washed with tap water until a clean eggshell were obtained. Two types of raw eggshell were prepared i.e., with the membrane (RES) and without the membrane. For the samples without the membrane, the membrane was carefully removed after soaking in water. The ES samples were then dried in an UF 110 oven (Memmert, Schwabach, Germany) at 105 ◦C for 24 h to remove the excessive moisture. Finally, the samples were ground to different particle sizes using a MX-GM1011H dry blender (Panasonic, Selangor, Malaysia). The powdered ES samples were then sieved using a WS TYLER RX29 vibrating sieve

(Fisher Scientific, Pittsburgh, PA, USA). Three different particle sizes were isolated (<90 µm, 90–125 µm and 125–180 µm). Calcined ES (CES) samples were prepared by heating the powdered RES at different temperatures (800 ◦C, 900 ◦C, 950 ◦C, 1000 ◦C, 1100 ◦C) for 2 h using a LEF-103S model muffle furnace (LabTech, Debrecen, Hungary). The retention time for calcination was chosen based on preliminary studies.

#### *3.2. Adsorption Tests*

The adsorption tests were performed in a lab-scale adsorption reactor as shown in Figure S1 (Supplementary Material). SO<sup>2</sup> and H2S gas flow from the gas cylinders (2000 ppm, 99% purity) were controlled automatically using SDPROC mass flow controllers and flow meters (Aalborg, New York, NY, USA). The gas flow rates of both gases into the reactor were kept at 300 mL/min throughout the experiments. H2S and SO<sup>2</sup> concentrations were kept constant at 300 ppm by introducing nitrogen gas as balance. Low flowrate and concentration were chosen for the overall safety of the lab. A down-flow fixed bed reactor made of stainless steel with an internal diameter of 9 mm and a height of 180 mm was prepared to fill the sorbents. The reactor was fixed inside an oven (Memmert, Schwabach, Germany) for temperature study. In each run, 1 g of sorbent was placed inside the reactor. The initial and outlet concentrations of H2S and SO<sup>2</sup> gas were measured using Biogas 5000 Portable Gas Analyzer (Geotech, Chelmsford Essex, UK) and Vario-Plus Industrial Gas-Analyzer (MRU Instruments Inc., Humble, TX, USA) respectively. SO<sup>2</sup> and H2S analyzers recorded the gas concentrations every second. The adsorption capacity was calculated from the following equation [58] using the breakthrough curve generated during the experiment:

$$Q = \frac{\text{CoMw } q}{1000w \text{ } Vm} \int\_0^t \left(1 - \frac{\text{C}}{\text{Co}}\right) dt\tag{7}$$

where *Q* is adsorption capacity (mg/ g), *c<sup>o</sup>* is the initial inlet concentration (ppm), *M<sup>w</sup>* is the gas molecular weight (g/mol), *q* is the total flow rate (L/min), *w* is the weight of sorbent (g), *V<sup>m</sup>* is the molar volume (L/mol), *c* is the outlet concentration of the gas (ppm) at time *t* (min). The adsorption capacity calculation was based on an average of 3 repetitions of the adsorption breakthrough curve. The differences between the 3 readings were less than 3%.

#### *3.3. Process Parameters Study*

In the process study, the effects of the relative humidity in the inlet gas and reaction temperature were evaluated for both RES and CES sorbents. Only optimized sorbents (both RES and CES) were selected for the process study. The reactor was fixed inside the oven and the temperature of the oven was varied from room temperature (approximately 30 ◦C) to 200 ◦C to study the effect of temperature on adsorption tests. Whereas the humidity in the inlet gas was created by passing the inlet gas through an airtight conical flask which was submerged in a temperature-controlled water bath before it could enter the reactor at room temperature. Required humidity in the inlet gas was created as the gas passing through the water bath was saturated with water vapor at the set temperature. The temperature of the water bath was set based on the steam tables formulation to create 40% relative humidity in the inlet gas. 40% of relative humidity was selected as it can be generated at room temperature and also to protect the gas sensors in the analyzer.

#### *3.4. Characterization of the Sorbents*

Morphology of the RES and CES were analysed by field emission scanning electron microscope (FESEM), model JEOL JSM-6701F (JEOL, Akishima City, Japan). The energy-dispersive X-ray spectroscopy (EDS) (JEOL, Akishima City, Japan) was employed to detect the specific elements on the surface of the materials. Fourier Transform Infrared (FTIR) Lambda 35 (Perkin Elmer, Waltham, MA, USA) was used to determine the surface functional groups of the sorbents. The spectra were recorded in the spectral range of 400–4000 cm−<sup>1</sup> with a resolution of 4 cm−<sup>1</sup> by mixing a small quantity of the sorbent with potassium bromide. pH was measured by preparing a solution in the ratio of 0.1 g of

sorbent in 20 mL deionized water and stirred for 1.5 hr. A digital pH meter was used (Hanna Instruments, Woonsocket, RI, USA). X-ray diffraction model Lab X XRD-6000 (Shimadzu, Tokyo, Japan) was used to identify the XRD patterns of the sorbents at room temperature at 2θ with a step size of 0.02. The Brunauer–Emmett–Teller (BET) surface area and pore size distribution were calculated using nitrogen adsorption and desorption isotherms conducted at 77 K with micromeritics, ASAP 2020 V4.02 (Micromeritis, Norcross, GA, USA) volumetric gas adsorption instrument. Thermogravimetric analysis (TGA) of RES and CES was done with TGA/DSC 3+ (Mettler Toledo, Ohio, OH, USA) for proximate analysis. Nitrogen gas was used at 20 mL/min at a heating rate of 10 ◦C /min until 900 ◦C. Original images of ES were taken with a smartphone.

#### **4. Conclusions**

RES and CES sorbents were tested for SO<sup>2</sup> and H2S adsorption. It was found that RES with membrane and having the smallest particle size i.e., <90 µm showed the best adsorption capacity for both SO<sup>2</sup> and H2S. (CES 900 ◦C) showed the best adsorption capacity among the other calcination temperature and RES. It can be concluded that physical adsorption was dominant over the chemical adsorption for both RES and CES sorbents. The characterization study shows the existences of sulfur element in the spent adsorbents which further verifies the adsorption of SO<sup>2</sup> and H2S by CES. The presence of the relative humidity in the inlet gas and increasing reaction temperature improved the performance of both RES and CES sorbents. (CES 900 ◦C) showed a greater adsorption capacity compared to RES with the addition of humidity. The best adsorption capacity of SO<sup>2</sup> and H2S was recorded as 11.68 mg/g and 7.96 mg/g respectively using (CES 900 ◦C) with 40% RH. These results indicate that chicken eggshell have great potential to be used as sorbents upon modification for the removal of pollutant gases such as SO<sup>2</sup> and H2S from contaminated air.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/2073-434 4/11/2/295/s1, Figure S1: Schematic diagram of adsorption experimental setup.

**Author Contributions:** W.A. and S.S. contributed equally. Conceptualization, S.S. and Y.M.; methodology, R.K.; validation, R.K., Y.M., and S.S.; formal analysis, W.A. and S.S.; investigation, W.A. and S.S.; resources, S.S. and Y.M.; data curation, W.A. and R.K.; writing—original draft preparation, W.A.; writing—review and editing, W.A., S.S., R.K., and Y.M.; visualization, W.A. and Y.M.; supervision, S.S.; project administration, S.S. and Y.M.; funding acquisition, S.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Universiti Tunku Abdul Rahman Research Fund number UTARRF/2017-C1/S07 and the APC was funded by Universiti Tunku Abdul Rahman under Financial Support for Journal Paper Publication Scheme and the authors.

**Data Availability Statement:** Data is contained within the article or Supplementary Material.

**Acknowledgments:** The authors gratefully acknowledge the financial support received from Universiti Tunku Abdul Rahman.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


## *Article* **Tailoring Properties of Metal-Free Catalysts for the Highly Efficient Desulfurization of Sour Gases under Harsh Conditions**

**Cuong Duong-Viet 1,2, Jean-Mario Nhut <sup>1</sup> , Tri Truong-Huu <sup>3</sup> , Giulia Tuci <sup>4</sup> , Lam Nguyen-Dinh <sup>3</sup> , Charlotte Pham <sup>5</sup> , Giuliano Giambastiani 1,4,\* and Cuong Pham-Huu 1,\***


**Abstract:** Carbon-based nanomaterials, particularly in the form of N-doped networks, are receiving the attention of the catalysis community as effective metal-free systems for a relatively wide range of industrially relevant transformations. Among them, they have drawn attention as highly valuable and durable catalysts for the selective hydrogen sulfide oxidation to elemental sulfur in the treatment of natural gas. In this contribution, we report the outstanding performance of N-C/SiC based composites obtained by the surface coating of a non-oxide ceramic with a mesoporous N-doped carbon phase, starting from commercially available and cheap food-grade components. Our study points out on the importance of controlling the chemical and morphological properties of the N-C phase to get more effective and robust catalysts suitable to operate H2S removal from sour (acid) gases under severe desulfurization conditions (high GHSVs and concentrations of aromatics as sour gas stream contaminants). We firstly discuss the optimization of the SiC impregnation/thermal treatment sequences for the N-C phase growth as well as on the role of aromatic contaminants in concentrations as high as 4 vol.% on the catalyst performance and its stability on run. A long-term desulfurization process (up to 720 h), in the presence of intermittent toluene rates (as aromatic contaminant) and variable operative temperatures, has been used to validate the excellent performance of our optimized N-C2/SiC catalyst as well as to rationalize its unique stability and coke-resistance on run.

**Keywords:** mesoporous N-doped carbon coating; silicon carbide composites; gas-tail desulfurization treatment; BTX contaminants; elemental sulfur

#### **1. Introduction**

Natural gas (NG) is certainly the cleanest fossil fuel employed for energy purposes. At odds with other natural sources (i.e., petroleum and charcoal) [1], NG holds important environmental merits because it can produce more heat and light energy by mass while keeping its environmental impact significantly lower in terms of carbon footprint and other pollutants that contribute to smog and unhealthy air. The primary constituent of NG is methane (CH4) but, depending on its origin, it may also contain higher hydrocarbons, including aromatics and sulfur-containing compounds, N2, CO2, He, H2S and noble gases to various extents [2]. In this regard, a complex but effective sequence of thermal and catalytic transformations have been implemented for NG processing as to remove undesired compounds and preventing the diffusion of corrosive and potentially hazardous substances for human health and environment. In particular, organic and inorganic S-compounds removal from natural gas is a mandatory step before any gas manipulation/processing [3–5]. Indeed,

**Citation:** Duong-Viet, C.; Nhut, J.-M.; Truong-Huu, T.; Tuci, G.; Nguyen-Dinh, L.; Pham, C.; Giambastiani, G.; Pham-Huu, C. Tailoring Properties of Metal-Free Catalysts for the Highly Efficient Desulfurization of Sour Gases under Harsh Conditions. *Catalysts* **2021**, *11*, 226. https://doi.org/10.3390/ catal11020226

Academic Editor: Daniela Barba Received: 19 January 2021 Accepted: 4 February 2021 Published: 9 February 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

their presence besides being highly risky for the toxicity of selected species (e.g., H2S) can led to undesired phenomena such as the fouling or even the permanent deactivation of catalysts employed in the gas processing. Although current catalytic technologies allow a selective and almost complete (up to 99.9%) hydrogen sulfide conversion into elemental sulfur [6], they still suffer from technical limitations linked to a progressive catalyst deactivation when desulfurization process is operated under harsh reaction conditions (gas hourly space velocity (GHSV) close to those employed in industrial plants), or in the presence of contaminants such as heavy hydrocarbons and aromatics (i.e., benzene, toluene and xylene (BTX)) [2] that are commonly present in untreated natural gas streams. Such impurities deeply impact the performance, stability, and lifetime of catalysts and detrimentally burden on the overall process economy balance. BTX (C ≥ 7) in particular can lead, throughout the desulfurization process, to the formation of carbonaceous or heavy carbon-sulfur deposits [7] whose incomplete removal translates into catalyst fouling phenomena with the subsequent alteration of its performance or—in worst cases—to the complete catalyst deactivation. Reduction of BTX concentration in acid gas streams is therefore a mandatory step before that gaseous streams reach the catalysts surface, thus increasing the process complexity as well as that of the reactor setup. Complementary and equally valuable technological options such as the use of either amine or solvent enrichment units (Acid Gas Enrichment units—AGE) [8] or the use of activated carbon beds [9] housed upstream of the catalytic desulfurization (SRU) unit, have successfully been implemented for BTX fractions removal.

Whatever the option at work, regeneration treatments of AGE or activated carbon units are periodically needed along with the downstream treatment of BTX wastes. These phases are costly and can cause the change of feed conditions in the desulfurization plant or even the complete feed shut down.

There are little doubts on the relevance of the selective H2S oxidation to elemental sulfur from both an environmental and commercial viewpoint [10] (~75 Mt/year of elemental sulfur are globally produced from oil and gas processing units). However, the search for robust, highly effective and selective catalysts for the process remains a challenging area of research for the chemistry and engineering community engaged in the field. In particular, the development of robust and durable catalysts suitable to operate the selective H2S oxidation under variable acid concentrations and showing an excellent resistance to aromatics deactivation is a highly challenging task and thus a widely investigated subject of research.

The last years have witnessed a growing interest of catalysis community towards the use of carbon nanomaterials as pure C-networks or light-heterodoped matrices (i.e., nanotubes (CNTs), nanofibers (CNFs) or thin-carbon film deposits, including N-doped counterparts) as metal-active phase supports or single-phase, metal-free catalysts for a wide series of chemical and electrochemical transformations [11,12]. Metal-free systems of this type have successfully been scrutinized as selective and efficient catalytic materials for a number of industrially relevant oxidation [13–18] and reduction [19–22] processes or as valuable promoters of other challenging catalytic transformations [23,24]. Nanocarbonbased materials, particularly in the form of N-doped networks, have already been exploited as effective metal-free systems for the selective hydrogen sulfide oxidation to elemental sulfur from natural gas tails [16,25–33]. The unique features of these single-phase systems (e.g., the absence of a metal active-phase, the prevalent basic surface character of the N-doped samples together with reduced production cost and environmental impact) have largely contributed to overcome the drawbacks classically encountered with metal nanoparticles-based catalysts. The absence of metal nanoparticles rules out unwanted sintering and leaching phenomena of the active phase occurring on heterogeneous catalysts typically operating under harsh reaction conditions. Moreover, their basic surface character can largely prevent the occurrence of cracking side-processes responsible for the rapid catalyst fouling with subsequent permanent alteration of its performance. This aspect is of particular relevance for catalytic materials operating in the presence of variable

concentrations of aromatics whose ultimate and detrimental effects are well known for metal-based catalysts of the state-of-the-art.

In recent years, our group has proposed a versatile technology for the coating of 3D-shaped materials (including open-cell foam structures with variable void fractions), with thin and highly N-enriched mesoporous carbon deposits [15,26,34]. Accordingly, selected 3D hosting matrices underwent successive soaking/impregnation cycles using an aqueous solution of cheap and food-grade components, followed by controlled drying/calcination/annealing steps. Such an approach to the N-C coating of macroscopically shaped networks, besides reducing the formation of toxic by-products typically encountered with more conventional Chemical-Vapor-Deposition (CVD)-based synthetic schemes, offers a versatile and straightforward tool to the easy upscale of challenging metal-free catalytic materials.

The combination of this coating technology with a non-oxide ceramic support (i.e., silicon carbide—SiC) has generated ideal catalyst candidates suitable to operate the selective H2S desulfurization efficiently under relatively high acid gas concentrations and in the presence of high contents of aromatics as contaminants (0.5 vol.% ≤ (H2S) ≤ 2 vol.%; 1000 ppm ≤ (tol) ≤ 20,000 ppm) [35,36]. SiC as a support for the N-C active phase was selected in light of its high chemical inertness and stability under acidic/oxidative or basic environments along with its excellent mechanical resistance that made it the ideal choice for the target process. Moreover, its medium thermal conductivity [37] prevents or reduces the generation of local temperature gradients (hot spots) while operating highly exothermic transformations [27,38,39]. This additional feature, not available with more classical oxide-based ceramics, contributes to the ultimate catalyst stability and its lifetime on stream.

This contribution takes advantage from our recent findings in the area of metalfree catalysts for the highly efficient and selective H2S desulfurization and it aims at pointing out the importance of controlling the morphological and chemical properties of the N-C phase to make a step forward in the direction of catalytic materials featured by improved desulfurization performance and resistance towards deactivation/fouling phenomena. We have recently demonstrated the excellent performance of a N-C/SiC catalyst in the presence of relatively high concentrations of toluene (up to 20,000 ppm, 2 vol.%) as contaminant in a desulfurization process operated under relatively harsh reaction conditions (GHSV up to 2400 h−<sup>1</sup> ). We have also demonstrated the existence of a beneficial "solvent effect" played by toluene on the process selectivity as a distinctive feature of these metal-free catalytic materials. With these catalysts, the toluene present in the gas stream was found to facilitate the desorption and removal of elemental sulfur from the catalyst surface thus reducing the sulfur residence time in contact with the N-C network and preserving the process from the occurrence of undesired over-oxidation paths at the catalyst surface. Hereafter we demonstrate how a more appropriate control of the soaking/thermal treatment cycles in the N-C phase growth holds beneficial effects on the chemical and morphological properties of the latter as well as on the ultimate materials performance in the direct H2S oxidation from sour gases [H2S = 0.3 vol.%; O2/H2S = 2.5; GHSV up to 3200 h−<sup>1</sup> ) containing aromatic contaminants (i.e., toluene) at concentrations as high as 40,000 ppm (4 vol.%). It should be pointed out that such a toluene concentration is markedly higher compared to that traditionally encountered in sour gas stream (i.e., 1200–2100 ppm) and it has been deliberately employed to validate the remarkable resistance of our metal-free catalyst towards BTX deactivation phenomena. To address these goals a new impregnation/thermal sequence for the N-C phase growth on SiC extrudates is proposed and a comparison with that previously reported by us has been used to shed light on the ideal chemical and morphological properties of the N-C phase for the desulfurization process to occur. In addition, N-C/SiC performance has been compared with that of the benchmark Fe2O3/SiO<sup>2</sup> catalyst [40] for the sake of completeness.

#### **2. Results and Discussion**

#### *2.1. Catalysts Characterization and Properties of N-C<sup>2</sup> /SiC and N-C<sup>4</sup> /SiC*

The SiC coating with the N-C phase was accomplished following two alternative sequences of the solid support soaking in a standardized impregnation solution and successive thermal treatments (see Section 3 for details). Figure 1 provides a visual sketch of the operative sequences employed for the preparation of N-C2/SiC and N-C4/SiC composites, respectively. At odds with N-C4/SiC, it can be inferred that preparation of N-C2/SiC follows a simplified synthetic path that includes only two impregnation/drying steps (green cycles) before undergoing annealing treatment at 900 ◦C for 2 h under inert atmosphere (red cycle) without passing any intermediate calcination step (orange cycle in N-C4/SiC sequence).

**Figure 1.** Quick visual representation of the two alternative synthetic paths used for the preparation of N-C4/SiC (upper black arrow) and N-C2/SiC (down black arrow) composites, respectively. Green cycles refer to the impregnation/drying steps, orange cycles refer to the material calcination at 400 ◦C for 2 h and red cycles refer to the material annealing at 900 ◦C for 2 h under Argon atmosphere.

Although a complete N-C4/SiC characterization and its H2S desulfurization properties have previously been detailed by us elsewhere, [26,34,36] their comparison with those of N-C <sup>2</sup>/SiC is necessary to better highlight the improved properties and catalytic performance of the latter. Both composites (Figure 2A) were analyzed in terms of specific surface area (SSA), total pore volume and pore-size distribution by N<sup>2</sup> physisorption at the liquid N<sup>2</sup> temperature (77 K).

As Table 1 shows, the specific surface areas (SSA) of the two composites are very close each other and they are almost twice than that measured on the plain SiC support. Both composites present Type IV isothermal profiles (Figure 2A) with distinctive H<sup>2</sup> hysteresis loops in the 0.45–1.0 P/P<sup>0</sup> range, typical of mesoporous networks featured by complex pore structures of ill-defined shape [41]. Sample N-C4/SiC presents a more pronounced hysteresis loop that is ascribed to the presence of a large extent of smaller mesopores and a lower mean pore-size distribution (Table 1) that facilitate the occurrence of capillary condensation phenomena. This datum is in line with the pore-volume distribution curves recorded on the three samples at comparison (Figure 2B). From the inspection of these profiles, it can be deduced a reduced content of small mesopores (in the 2–5 nm range) in N-C2/SiC compared to its counterpart N-C4/SiC, together with the presence of larger mesopores (prevalently in the 20-60 nm range) available on the former only. As far as N-C phase mass content is concerned, thermogravimetric analysis (TGA) on air (50 mL min−<sup>1</sup> ) on the two samples (Figure 2D) showed only negligible differences on the content of organic deposit (6.7 wt.% on N-C2/SiC and 6.9 wt.% on N-C4/SiC) whatever the catalyst preparation sequence adopted (Figure 1). Both profiles evidence a distinctive weight loss

at 622 and 613 ◦C for N-C2/SiC and N-C4/SiC, respectively, where the derivative of the thermogravimetric curves (DTG) holds its maximum value.

**Figure 2.** (**A**) N<sup>2</sup> adsorption-desorption isotherm linear plot of SiC (light grey curve), N-C2/SiC (blue curve) and N-C4/SiC samples (red curve) recorded at 77 K along with (**B**) the respective pore-size distributions measured (BJH method). (**C**) Highresolution XPS N 1s core level region of N-C2/SiC (up) and N-C4/SiC (down) at comparison, along with the relative curves fittings. (**D**) Thermogravimetric/derivative of the thermogravimetric curves (TG/DTG) profiles of N-C2/SiC (left-side hand) and N-C4/SiC (right-side hand) at comparison. Weight loss is measured arbitrarily in the 200–700 ◦C temperature range. Operative conditions: Air, 50 mL/min; heating rate: From 40 to 900 ◦C at 10 ◦C/min.

The XPS analysis of the materials at comparison confirmed the same composition providing largely superimposable survey profiles (Figure S3). The high-resolution N 1s analysis of the two samples has pointed out a slightly changed composition of the N-configurations available. Table 1 lists the relative % of the different N-species in the samples. N 1s peaks deconvolution (Figure 2C) accounts for three main components centered at 398.4 ± 0.2 eV (N-pyridinic—blue line), 399.7 ± 0.2 eV (N-pyrrolic—green line) and 401.2 ± 0.2 eV (N-quaternary—orange line), along with an additional shoulder (more pronounced in N-C2/SiC) at higher binding energies (403.0 <sup>±</sup> 0.3 eV—red line) and ascribed to the presence of N-oxidized species.

As Table 1 shows, the relative % of N-species obtained by the two alternative impregnation/thermal sequences (Figure 1) give rise to a redistribution of the N-configurations available at the materials surface. In particular, reducing the number of impregnation/drying steps and omitting the material calcination at 400 ◦C for 2 h, the percentage of pyrrole moieties is nearly doubled, that of N-oxide species increases, while that of basic N-pyridine sites decreases appreciably.


**Table 1.** Selected chemico-physical and morphological properties of catalysts and precursors.

*<sup>a</sup>* Brunauer-Emmett-Teller (BET) specific surface area (SSA) measured at T = 77 K. *<sup>b</sup>* Total pore volume determined using the adsorption branch of N<sup>2</sup> isotherm at *P*/*P*<sup>0</sup> = 0.98. *<sup>c</sup>* Determined by BJH desorption average pore width (4V/A). *<sup>d</sup>* measured by acid-base titration. *<sup>e</sup>* Determined by XPS analysis. *<sup>f</sup>* Determined by high resolution XPS N 1s core region and its relative peak deconvolution. *<sup>g</sup>* The pH value of an aqueous SiC dispersion lies close to pH 6.6 whereas the pH value of aqueous N-Cx/SiC dispersions (x = 2, 4) ranks close to 9.4. Data reported for elemental analysis (EA) and acid-base titration are calculated as the mean values over three independent runs. *n.d.* = not determined.

> At odds with this trend, a quantitative estimation of basic sites available at the surface of both catalysts carried out by acid-base titration (see Section 3) has unveiled the higher basic character of N-C2/SiC (0.63 mmol g−<sup>1</sup> ) respect to N-C4/SiC (0.45 mmol g−<sup>1</sup> ). This discrepancy can be justified by the higher N-content of the N-C phase in N-C2/SiC. Indeed, according to the N-C wt.% measured by TGA on the two samples (see Table 1 and Figure 2D) and the N wt.% measured by EA (Table 1), the N wt.% content normalized to the weight of N-C coating was calculated in 22 N wt.% and 31 N wt.% for N-C4/SiC and N-C2/SiC, respectively. In spite of a reduction in the percentage of basic pyridine sites for N-C2/SiC, its markedly higher N-content reasonably accounts for its higher basic surface character [26,42].

#### *2.2. Desulfurization Performance of N-C<sup>2</sup> /SiC and N-C<sup>4</sup> /SiC in the Presence of a Relatively High Vol.% of Toluene as Acid Gas Contaminant*

The catalysts screening in the sour gas desulfurization starts from the awareness that aromatic contaminants like toluene hold a positive "solvent effect" on the catalytic outcomes of these metal-free systems [36]. Indeed, they favor a faster removal of sulfur deposits from the material mesopores, thus preventing the occurrence of undesired overoxidation paths. In a recent desulfurization report with N-C4/SiC, we have already shown the remarkably high resistance of this metal-free catalyst towards deactivation/fouling in the presence of toluene as the acid gas stream contaminant up to 5000 ppm. We also claimed an increase of the elemental sulfur rate up to 30% compared to selectivity values recorded for the same metal-free catalyst operated under identical—but toluene-free conditions [36]. The comparative study between N-C4/SiC and N-C2/SiC points out on the importance of controlling the morphology and chemical composition of the N-C phase in order to get more robust, selective and efficient desulfurization catalysts suitable to operate the process under severe operative conditions. In this study, toluene was selected again as a model aromatic contaminant in sour gases [43] and the performance of two metal-free systems were compared for the sake of completeness with that of the benchmark Fe2O3/SiO<sup>2</sup> catalyst under the same conditions.

Aimed at stressing the relevance of morphological and chemical surface properties of N-C active phase in the process, we deliberately selected harsh operative conditions since the beginning of the desulfurization reaction. The process was then followed for a relatively long time (>200 h) and until the three samples clearly followed distinct catalytic paths. As a first trial, a mixture of H2S (0.3 vol.%; O2-to-H2S ratio = 2.5; steam: 10 vol.%) and toluene (40,000 ppm; 4 vol.%) was passed downward the catalyst bed (6 g, Vcat ~7.5 cm<sup>3</sup> for N-C4/SiC and N-C2/SiC and 6 g, Vcat ~5.8 cm<sup>3</sup> for Fe2O3/SiO2) heated at 210 ◦C and with a GHSV of 3200 h−<sup>1</sup> (STP) (Figure 3A).

**Figure 3.** (**A**) Desulfurization performance on N-C2/SiC, N-C4/SiC and Fe2O3/SiO<sup>2</sup> catalysts of an acid gas stream ([H2S] = 0.3 vol.%) in the presence of 40,000 ppm of toluene (4 vol.%) as contaminant in the stream. Catalysis details: 6 g (Vcat ~ 7.5 cm<sup>3</sup> for N-C4/SiC and N-C2/SiC) or 6 g (Vcat ~ 5.8 cm<sup>3</sup> for Fe2O3/SiO<sup>2</sup> ); O<sup>2</sup> -to-H2S ratio = 2.5, [H2O] = 10 vol.%, He (balance); reaction temperature = 210 ◦C, GHSV (STP) = 3200 h−<sup>1</sup> . (**B**) Desulfurization performance on N-C2/SiC at variable toluene concentrations (10,000, 20,000 and 40,000 ppm or 1, 2 and 4 vol.%).

The long-term desulfurization process (up to 220 h) in the presence of 40,000 ppm of toluene in the stream served to highlight the excellent sulfur selectivity (S<sup>S</sup> up to ~94% with N-C2/SiC at the steady-state-conditions) as well as the remarkably high coke resistance of both metal-free catalysts under severe and prolonged operative conditions. As Figure 3A shows, all catalysts ensure a quantitative H2S conversion (100%) within the first 10 h on stream. Afterwards, H2S conversion (XH2S) decreases appreciably whatever the nature of the catalyst employed, with the benchmark Fe2O3/SiO<sup>2</sup> showing the much faster deactivation rate compared to its metal-free counterparts. Under these conditions, the iron-based catalyst shows a quantitative selectivity towards elemental sulfur although the high toluene content in the gas stream rapidly compromises its H2S conversion capacity that falls below 50% just after 85 h on reaction. In spite of a slightly lower sulfur selectivity (S<sup>S</sup> in the 93–95% range), N-C2/SiC and N-C4/SiC show a markedly higher deactivation resistance and follow distinct deactivation paths. The N-C4/SiC starts an appreciable deactivation only after 25 h on stream. Afterwards, XH2S gradually but constantly decreases down to 65% (after ~200 h on reaction). Noteworthily, N-C2/SiC shows a more rapid deactivation in the first hours on stream that however reaches a H2S conversion plateau around 160 h that is almost constantly maintained even after 220 h. As far as sulfur selectivity is concerned, both metal-free catalysts constantly rank above 90%. According to our previous report, the positive S<sup>S</sup> increase is ascribable to a co-solvent action played by toluene. Indeed, it facilitates the dissolution of sulfur deposits thus reducing their contact time with the N-C active phase and hence limiting the occurrence of undesired over-oxidation paths. The higher stability of N-C<sup>2</sup> active phase must be searched in the minor but critical chemical and morphological differences with N-C<sup>4</sup> phase and hence within the simplified impregnation/thermal sequence for the synthesis of N-C2/SiC compared to N-C4/SiC. It can be inferred that a larger mean pore size distribution in N-C<sup>2</sup> (Table 1, entry 2 vs. 3), a lower percentage of small mesopores in favor of larger ones (Figure 2B) together with a higher basic surface character of the sample (Table 1) account for its improved catalytic performance. Indeed, larger mesopores reduce the occurrence of pore clogging phenomena, ensure a more effective reagents access to the catalyst active phase and allow a more effective toluene scrubbing action towards the formed sulfur deposits. At the same time, a higher basic surface character creates the ideal microenvironment for the generation of local H2S gradients and reduces the occurrence of cracking side-processes responsible for undesired "catalyst coking" [15,44–46]. The ability of N-C2/SiC to stabilize on a relatively high H2S conversion values is attributed to the simultaneous occurrence of all these phenomena. Selectivity values up to 94% at the steady-state conditions together with a XH2S constantly lying on 68% denote an efficient and selective desulfurization process where the toluene constant rate in the stream no longer affects the catalyst performance. On the other hand, it

positively influences the process selectivity and dynamically controls the accumulation of sulfur that might compromise the catalyst performance, particularly on long-term desulfurization runs. All of this evidence taken together underline the importance of controlling the chemical and morphological properties of the N-C phase through a rational optimization of the SiC impregnation/thermal treatment sequences thus allowing a tuning of the active phase surface properties as a function of its downstream application.

Expectedly, the lower the toluene content in the stream the slower the N-C2/SiC deactivation rate under desulfurization conditions. This trend is confirmed by the catalyst deactivation measured with N-C2/SiC in the presence of variable toluene concentrations (from 1 vol.% to 4 vol.%) within a 115 h desulfurization run (Figure 3B). At the same time, selectivity follows a similar and positive trend irrespective from the toluene content but reaching appreciably higher values when the concentration of the latter increases.

As an additional trial, N-C2/SiC desulfurization capacity was investigated as a function of the reaction temperature and in the presence of variable percentages of toluene as contaminant. To this aim, N-C2/SiC was initially conditioned at 210 ◦C using a toluene-free sour gas stream and the process was constantly monitored throughout about 50 h. During this time, the catalyst showed a quantitative H2S conversion (XH2S) and a sulfur selectivity (S<sup>S</sup> %) laying around 55–60% (Figure 4). The addition of toluene (4 vol.%) to the acid gas stream leads to the rapid increase of the process selectivity (up to 96%) while XH2S gradually stabilizes to a constant plateau value. An increase of the catalyst temperature to 230 ◦C results into a rapid XH2S increase that reaches values close to 80% for gradually dropping down to 72% after additional 100 h on stream.

**Figure 4.** Effect of the temperature on the desulfurization performance of N-C2/SiC using an acid gas stream ([H2S] = 0.3 vol.%) in the presence of 40,000 ppm of toluene (4 vol.%) as contaminant. The first 47 h on stream were operated under toluene-free conditions. Catalysis details: 6 g (Vcat ~ 7.5 cm<sup>3</sup> of N-C2/SiC); O<sup>2</sup> -to-H2S ratio = 2.5, [H2O] = 10 vol.%, He (balance); GHSV (STP) = 3200 h−<sup>1</sup> . Catalyst regeneration (last 150 h on stream) occurs upon switching-off the toluene content from the acid gas stream.

As far as sulfur selectivity is concerned, the reaction temperature and XH2S increase affect only marginally its mean value that slightly reduces from 96% to 95%. Similarly, an additional temperature increase from 230 to 250 ◦C gives rise to a further increase of H2S conversion values that grow over 85% while S<sup>S</sup> does no longer reduce appreciably. As Figure 4 shows, the XH2S decreases again and finally stabilizes around 78% after additional 120 h on stream together with a mean S<sup>S</sup> value of 93% that highlight the unique desulfurization properties of this metal-free catalyst while stressing again its robustness and durability under quite unconventional conditions. It should be noticed that N-C2/SiC still maintains a remarkably high XH2S and S<sup>S</sup> even after more than a 3 weeks (550 h) H2S desulfurization run operated under continuum mode and severe operative conditions.

Notably, switching-off toluene from the acid gas stream, translates into a gradual but complete recovery of the pristine catalyst performance (Figure 4, Regeneration section). This positive trend in the absence of harsher oxidative conditions, led us to consider the action of toluene on the performance of our metal-free catalyst as that of an "interfering solvent" rather than a source of carbon for the growth of coke deposits. Indeed, the toluene confinement into the pores of the catalyst active phase alters the performance of the latter reversibly without causing any real catalyst coking and thus any irreversible deactivation. If the use of steam is known to be functional to the process by creating a thin water film on N-C surface that favors the diffusion of hydrophilic H2S molecules into the catalyst pores, [28] the co-existence with a hydrophobic co-solvent (e.g., toluene) will translate into a depletion of reagents uptake and their diffusion towards the catalyst active sites. Anyhow, once toluene molecules are gradually desorbed from the pores and channels of the N-C network by using a toluene-free sour gas stream, the catalyst recovers its original performance.

The excellent coke resistance of N-C2/SiC catalyst in the presence of relatively high concentrations of aromatics in the gas stream (40,000 ppm), has been confirmed by the analysis of the recovered N-C2/SiC (spent catalyst) after 720 h on reaction. Figure 5A refers to the TGA analysis of the freshly prepared N-C2/SiC (left-side hand) with its spent (right-side hand) counterpart put at comparison.

**Figure 5.** (**A**) TG/DTG profiles of the freshly prepared N-C2/SiC (left-side hand) and its exhaust counterpart (right-side hand) at comparison. Weight loss is measured arbitrarily in the 200–700 ◦C temperature range and it corresponded to: 6.7 wt. loss % on the fresh N-C2/SiC and 7.6 wt. loss % on the spent N-C2/SiC. Operative conditions: Air, 50 mL/min; heating rate: from 40 to 900 ◦C at 10 ◦C/min. (**B**) SEM micrograph of N-C2/SiC after its recovery at the end of a long-term catalytic run of 720 h. White arrows indicate residual sulfur deposits marked all around by yellow dashed lines.

The spent sample presents a minor shoulder featured by a maximum weight loss centered around 365 ◦C that accounts for about 0.9% of the overall weigh loss after the complete N-C active phase burning. Although we cannot definitively rule out the generation to a certain extent of low-melting coke deposits, we believe that such a little shoulder in the TGA profile of the exhaust sample is reasonably ascribable to the oxidation of sulfur residues. In spite of a temperature in the last part of the catalytic run (250 ◦C) that is higher than the sulfur dewpoint, some residues still remain available on the catalyst surface. Indeed, the SEM analysis of the recovered N-C2/SiC (Figure 5B) clearly shows their presence in the form of patchy islands, whose generation is attributed to the residual catalyst activity during its cooling phase.

#### **3. Experimental Section**

#### *3.1. Materials and Methods*

<sup>β</sup>-SiC supports [extrudates (3 <sup>×</sup> 1 mm; *<sup>h</sup>* <sup>×</sup> <sup>∅</sup>), V = ~0.002 cm<sup>3</sup> , SSA measured by N<sup>2</sup> physisorption (at 77 K) of 30 <sup>±</sup> 1 m<sup>2</sup> <sup>g</sup> −1 ] were provided by SICAT SARL (www.sicatcatalyst.com), thoroughly washed with deionized water in order to remove powdery fractions, hence dried at 130 ◦C overnight before use. Ammonium carbonate ((NH4)2CO3, MW: 96.09 g mol−<sup>1</sup> ; Lot: A0356079), D-glucose 100% (C6H12O6, MW: 180.16 g mol−<sup>1</sup> ) and citric acid (C6H8O<sup>7</sup> anhydrous, <sup>≥</sup>99.5%, MW: 192.12 g mol−<sup>1</sup> ) were

provided by ACROS OrganicTM, MYPROTEINTM and VWR Chemicals, respectively. Unless otherwise stated, all reagents and solvents were used as provided by commercial suppliers without any further purification/treatment. N-C4/SiC sample was prepared following the literature procedure previously reported by us [26,34,36] (and described in brief hereafter for the sake of completeness). N-C2/SiC composite was prepared from the same soaking water solution of food-grade components, using a simplified impregnation/thermal sequence (vide infra). Scanning Electron Microscopy (SEM) was carried out on an UHR-SEM Gaia 3 FIB/SEM (TESCAN, Brno-Kohoutovice, Czech Republic). A 10 kV electron beam was used for SEM imaging operated in high-vacuum mode, using BSE and SE detectors. N<sup>2</sup> adsorption-desorption measurements were carried out on a Micromeritics® (Milan, Italy) sorptometer at the liquid N<sup>2</sup> temperature and relative pressures between 0.06 and 0.99 P/P0. Each sample was outgassed at 250 ◦C under ultra-high vacuum for 8 h prior analysis in order to desorb moisture and adsorbed volatile species. The X-ray Photoelectron Spectroscopy (XPS) was carried out in an ultrahigh vacuum (UHV) spectrometer (Prevac, Rogów, Poland) equipped with a CLAM4 (MCD) hemispherical electron analyzer. The Al Kα line (1486.6 eV) of a dual anode X-ray source was used as incident radiation. Survey and high-resolution spectra were recorded in constant pass energy mode (100 and 20 eV, respectively). The CASA XPS program with a Gaussian-Lorentzian mix function and Shirley background subtraction was employed to deconvolute XPS spectra. Elemental analyses were performed on a Thermo FlashEA 1112 Series CHNS-O analyzer (Thermo Fisher Scientific, Waltham, MA, USA) and elemental average values were calculated over three independent runs. Powder Diffraction (PXRD) measurements were carried out on a Bruker D-8 Advance diffractometer (Bruker, Billerica, MA, USA) quipped with a Vantec detector (Cu Kα radiation) working at 40 kV and 40 mA. X-ray diffractogram was recorded in the 10–80◦ 2θ region at room temperature in air. Iron loading for the benchmark Fe2O3/SiO<sup>2</sup> was fixed by Inductively Coupled Plasma Atomic Emission spectrophotometry (ICP-AES) after sample acidic mineralization, using an Optima 2000 Perkin Elmer Inductively Coupled Plasma (ICP) Dual Vision instrument (Perkin Elmer Italia, Milan, Italy). Thermogravimetric analyses were performed on air (50 mL min−<sup>1</sup> ) from 40 to 900 ◦C (heating rate: 10 ◦C min−<sup>1</sup> ) on an EXSTAR Seiko 6200 analyser (Riga, Latvia). Acid-base titration was accomplished using the following procedure [47–49]: 10 mg of N-Cx/SiC (x = 2 or 4) were suspended in 7 mL of a standardized HCl solution (3 <sup>×</sup> <sup>10</sup>−<sup>3</sup> M, standardized with Na2CO<sup>3</sup> as primary standard) and stirred at room temperature for 48 h. After that, three aliquots of the solution were titrated with a standardized NaOH solution (2.5 <sup>×</sup> <sup>10</sup>−<sup>3</sup> M). The basic sites loading was finally calculated as the average value over the three independent titration runs.

#### *3.2. General Procedure for the Preparation of N-C<sup>2</sup> /SiC and N-C<sup>4</sup> /SiC Catalysts*

N-C2/SiC and N-C4/SiC catalysts were prepared from the same water soaking solution for the SiC support but following different impregnation/thermal treatment sequences. The impregnation solution was prepared by dissolving at room temperature 3 g of Dglucose and 4.5 g of citric acid in 20 mL of ultrapure Milli-Q water. Afterwards, 3.46 g of ammonium carbonate were added to the stirred solution during which an effervescence due to CO<sup>2</sup> evolution starts (CAUTION! CO<sup>2</sup> effervescence needs to be carefully controlled during this phase by portioning the amount of (NH4)2CO<sup>3</sup> added over time). The as-prepared solution was used for the soaking/impregnation of 20 g of SiC extrudates whatever the nature of the target composite prepared (N-C2/SiC or N-C4/SiC) [34]. N-C <sup>4</sup>/SiC was obtained following previously reported procedures [36]. Accordingly, SiC was soaked twice in the above water solution and excess of water—remaining after each impregnation step—was gently evaporated at 40 ◦C for 3 h. Afterwards, the solid was dried at 130 ◦C overnight before being calcined in air at 400 ◦C for 2 h (heating rate 2 ◦C min−<sup>1</sup> ). The as-obtained composite underwent identical impregnation/thermal treatment sequence at the end of which the sample was annealed at 900 ◦C (heating rate: 10 ◦C min−<sup>1</sup> ) for 2 h

under inert (Ar) atmosphere. As a result, a N-doped and mesoporous C-graphitic coat at the SiC outer surface was formed.

As far as N-C2/SiC is concerned, it was obtained by soaking SiC twice in the impregnation solution, evaporating the excess of water at 40 ◦C for 2 h, drying the sample at 130 ◦C overnight before moving it directly to the annealing phase at 900 ◦C under inert atmosphere.

Fe2O3/SiO<sup>2</sup> (2.6 wt.% Fe) was prepared according to literature data [40]. To this aim, 10 g of SiO<sup>2</sup> powder were treated by incipient wetness impregnation with an aqueous solution of iron nitrate (Fe(NO3)3·9H2O, 2.23 g; MW: 404,00 g mol−<sup>1</sup> ) in 10 mL of ultrapure Milli-Q water. The resulting solid was dried at 130 ◦C overnight before being calcined in air at 350 ◦C (heating rate: 5 ◦C min−<sup>1</sup> ) and maintained at the target temperature for additional 2 h before being used as such in catalysis. The final iron loading was measured by ICP-AES analysis and it was fixed to 2.6 wt.%. The XRD spectrum of the iron catalyst (Figure S1) was in accord with related literature reports [50].

Although the comparison of a metal-based catalyst with a metal-free one might appear as a meaningless exercise, Fe2O3/SiO<sup>2</sup> is a common benchmark system for the H2S desulfurization, and its employment under conditions identical to those (hard) operated with the metal-free composites provides a clear-cut evidence of the superior performance and stability of the latter. Moreover, the direct H2S oxidation to elemental sulfur in the presence of aromatics as contaminants in the gaseous stream is almost absent in the literature. In addition, SiO<sup>2</sup> as the metal active phase support was properly selected and compared with SiC, the latter being naturally coated by a thin layer of SiOxCy/SiO<sup>2</sup> once exposed to air at room temperature [27].

### *3.3. Selective H2S Desulfurization of Sour Gases to Elemental Sulfur*

The H2S oxidation process can be described by Equations (1)–(3) reported below [51,52]. For catalytic trials, 6 g of N-C2/SiC or N-C4/SiC (Vcat ~7.5 cm<sup>3</sup> ) were loaded on a silica wool pad, housed in a Pyrex tubular (∅ID: 16 mm) reactor housed in a vertical electrical furnace, and the catalytic reactions were operated isothermally under atmospheric pressure. A graphical representation of the desulfurization scheme is provided on Figure S2.

$$\text{2H}\_2\text{S} + \text{O}\_2 \rightarrow 2\text{S} + 2\text{H}\_2\text{O} \qquad\qquad\qquad\qquad\Delta\text{H} = -187 \text{ kJ/mol}\tag{1}$$

$$\text{S} + \text{O}\_2 \rightarrow \text{SO}\_2 \tag{2}$$

$$\text{H} + \text{O}\_2 \rightarrow \text{O}\_2 + \text{O}\_2 \tag{3}$$

$$2\text{H}\_2\text{S} + 3\text{O}\_2 \to 2\text{SO}\_2 + 2\text{H}\_2\text{O} \qquad \qquad \Delta\text{H} = -518 \text{ kJ/mol} \tag{3}$$

The temperature of the furnace was controlled by a K-type thermocouple and a Minicor regulator. The reactants gas mixture [H2S (0.3 vol.%), O2/H2S = 2.5, H2O (10 vol.%) in inert He as carrier (balance)] was passed downward through the catalyst bed, being gas flow rates monitored through Brooks 5850TR mass flow controllers. Steam (10 vol.%) was ensured by bubbling the inert carrier in a saturator containing hot water at 61 ◦C. CAUTION! *H2S is a colourless, flammable, highly toxic gas. It must be handled—including its solutions—rigorously under a fume-hood and with all necessary precautions, especially a specific leak detector installed close to the operating setup*. The gas hourly space velocity (GHSV) was fixed at 3200 h−<sup>1</sup> (corresponding to 400 mL min−<sup>1</sup> or 4000 mL gcat <sup>−</sup><sup>1</sup> h −1 ) and the O2-to-H2S molar ratio was kept constant to 2.5. The influence of toluene on the catalyst performance was investigated by feeding the reactants stream with toluene vapors at concentrations comprised between 1 vol.% (10,000 ppm) up to 4.0 vol.% (40,000 ppm). Toluene was fed-up in the stream by flowing He through a saturator containing pure toluene constantly maintained at 40 ◦C. To this purpose an independent line of He (Figure S2) was used to feed-up toluene in the reagents stream and its target concentration was adjusted by regulating the flow of the carrier. The amount of toluene passing through the catalyst was double checked by measuring the real liquid volume of toluene vaporized per day of experiment. All catalytic runs were carried out in continuous mode. Hence, most of the formed elemental sulfur was vaporized (because of the high partial pressure of sulfur

at the target reaction temperatures) and condensed alongside with steam at the reactor outlet in a trap maintained at room temperature. The analysis of the inlet and outlet gases was performed on-line using a Varian CP-3800 gas chromatograph (GC) equipped with a Chrompack CP SilicaPLOT capillary column and a thermal catharometer detector (TCD) for the detection of O2, H2S, H2O, and SO<sup>2</sup> (down to 30 ppm). H2S and SO<sup>2</sup> concentrations were recalculated on the basis of the corrected flow after steam condensation in a trap (Figure S2). All connecting lines were wrapped with thermal tapes maintained at 140 ◦C in order to prevent any condensation phenomena.

#### **4. Conclusions**

To summarize, the optimization of the SiC impregnation/thermal treatment sequences for the control of the surface chemistry and morphology of a highly N-rich carbon phase coating has been proposed. The new sequence for the N-C2/SiC preparation has pointed out the importance of controlling the chemico-physical properties of the N-C phase as to get more efficient, selective and stable metal-free catalysts to be employed in the selective H2S oxidation of sour gas streams and in the presence of aromatic contaminants concentrations as high as 40,000 ppm (4 vol.%). In the study, we have demonstrated how larger mesopores at the N-C active phase along with its higher basic surface character hold largely beneficial effects on the catalyst performance and its stability on stream. While the former reduces the occurrence of pore clogging phenomena, ensures a more effective reagents access to the catalyst active phase and allows a more effective scrubbing/removal action of the sulfur deposits by the toluene, the latter creates the ideal microenvironment for the generation of local H2S gradients and reduces the occurrence of cracking side-processes responsible for the "catalyst coking". Most importantly, harsh H2S desulfurization conditions in the presence of an intermittent toluene rate (from 0 to 4 vol.% and again down to 0 vol.%) in the sour gas stream has allowed to better elucidate the effect of aromatics on the performance and long-term stability of these metal-free desulfurization catalysts. Our results have pointed out that metal-free catalysts of this type suffer only marginally of irreversible deactivation caused by the generation of coke deposits. On the other hand, the reduced XH2S efficiency in the presence of toluene can be reasonably ascribed to a competitive pore-filling by the toluene as the steam co-solvent. While steam facilitates H2S diffusion into the pores and channels of the N-C active phase, the hydrophobic toluene can detrimentally compete with the reagent uptake on the catalyst active phase. However, once toluene molecules are gradually desorbed from the pores and channels of the N-C network (i.e., purging the catalyst under a toluene-free sour gas stream), the latter recovers its original performance.

Overall, three catalytic system at comparison (N-C2/SiC, N-C4/SiC and Fe2O3/SiO2) have served to highlight the role of metal-free catalysts and their surface chemico-physical properties on their H2S desulfurization performance in the presence of aromatics as contaminants. As shown in Figure 3A, while the iron-based catalyst rapidly deactivates because of the fouling of its active-phase (catalyst coking), the two metal-free systems behave differently as a function of their chemical and morphological properties. Under these conditions, the higher the basic surface properties and the higher the density of larger mesopores in the material, the higher the catalyst stability and durability on run.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/2073-434 4/11/2/226/s1, Figure S1: XRD profile of Fe2O3/SiO<sup>2</sup> catalyst (Fe 2.6 wt.%), Figure S2: Schematic representation of a desulfurization apparatus, Figure S3: XPS survey spectra of N-C2/SiC and N-C4/SiC.

**Author Contributions:** Conceptualization, C.P.-H. and G.G.; methodology, C.D.-V., C.P.H., J.-M.N., C.P. and G.G.; validation, C.D.-V., C.P.H., J.-M.N. and G.G.; formal analysis, C.D.-V., T.T.-H, G.T. and L.N.-D.; investigation, C.D.-V., C.P.H., J.-M.N., G.T. and G.G.; resources, C.P.-H., G.G., C.P. and L.N.-D.; data curation, C.D.-V., C.P.-H., G.T. and G.G.; writing—original draft preparation, G.G. and C.P.-H.; writing—review and editing, G.G., G.T., C.P., L.N.-D. and C.P.-H.; supervision, C.P.-H., J.-M.N., G.G. and L.N.-D.; funding acquisition, C.P.-H., G.G. and L.N.-D. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the TRAINER project (Catalysts for Transition to Renewable Energy Future) of the "Make our Planet Great Again" program (Ref. ANR-17-MPGA-0017), the PRIN 2017 Project Multi-e (20179337R7) "Multielectron transfer for the conversion of small molecules: an enabling technology for the chemical use of renewable energy" and the Vietnam National Foundation for Science and Technology Development (NAFOSTED; grant number 104.05-2017.336). This research was also supported by SATT-Conectus through the DECORATE project.

**Data Availability Statement:** Data available on request.

**Acknowledgments:** SICAT SARL (www.sicatcatalyst.com) is gratefully acknowledged for providing SiC pellet samples.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


## *Article*
