*Article* **One-Pot Synthesis of Nano CuO-ZnO Modified Hydrochar Derived from Chitosan and Starch for the H2S Conversion**

**Lihua Zang 1,2, Chengxuan Zhou <sup>2</sup> , Liming Dong <sup>1</sup> , Leilei Wang <sup>3</sup> , Jiaming Mao <sup>4</sup> , Xiaomin Lu <sup>5</sup> , Rong Xue <sup>2</sup> and Yunqian Ma 1,2,\***


**Abstract:** A novel kind of hydrochar adsorbent, modified by CuO-ZnO and derived from chitosan or starch, was synthesized for H2S adsorption. The prepared adsorbent was characterized by BET, XRD, EDX, SEM, and XPS. The results showed that the modified hydrochar contained many amino groups as functional groups, and the nanometer metal oxide particles had good dispersion on the surface of the hydrochar. The maximum sulfur capacity reached 28.06 mg/g-adsorbent under the optimized conditions. The amine group significantly reduced the activation energy between H2S and CuO-ZnO conducive to the rapid diffusion of H2S among the lattices. Simultaneously, cationic polyacrylamide as a steric stabilizer could change the formation process of CuO and ZnO nanoparticles, which made the particle size smaller, enabling them to react with H2S sufficiently easily. This modified hydrochar derived from both chitosan and starch could be a promising adsorbent for H2S removal.

**Keywords:** hydrochar; adsorbent; mixed metal oxides; H2S conversion

#### **1. Introduction**

Hydrogen sulfide (H2S), a poisonous, odorous, and corrosive gas, commonly exists in industrial gases such as coal gasification gas, natural gas, and biogas. H2S is harmful to humans and livestock, and it not only brings corrosion to metal pipes and reaction devices in the industrial production process, but also causes catalyst poisoning, which affects product quality [1,2]. H2S is easy to burn, generating SO<sup>2</sup> as a combustion product. Whether through combustion or direct emissions, it can exert a severe impact on the atmospheric environment. Therefore, H2S should be fixed on some materials or removed from the production process and environment.

At present, there are many industrial methods to remove H2S. According to production conditions and desulfurization costs, the methods of H2S removal in industrial processes can be classified into wet flue gas desulfurization (WFGD) and dry flue gas desulfurization (DFGD) [3,4]. The desulfurizer of WFGD is a liquid that absorbs and separates H2S with large processing capacity and mature technology. WFGD has some disadvantages—for example, high energy consumption, secondary pollution, and high regeneration cost. DFGD has mainly been used to remove H2S at low concentrations, and it has the advantages of high H2S removal efficiency and low cost [5]. The commonly used

**Citation:** Zang, L.; Zhou, C.; Dong, L.; Wang, L.; Mao, J.; Lu, X.; Xue, R.; Ma, Y. One-Pot Synthesis of Nano CuO-ZnO Modified Hydrochar Derived from Chitosan and Starch for the H2S Conversion. *Catalysts* **2021**, *11*, 767. https://10.3390/ catal11070767

Academic Editor: Daniela Barba

Received: 18 May 2021 Accepted: 15 June 2021 Published: 24 June 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/).

dry desulfurization methods in industrial processes are the zinc oxide method, iron oxide method, manganese desulfurization method, Claus method, etc. [6–10].

Biochar is defined as a solid, carbon-rich product obtained from biomass through various thermochemical technologies [11–14]. Pyrochar and hydrochar are two kinds of biochar prepared from the pyrolysis and hydrothermal carbonization (HTC) of biomass [15], respectively. The biomass includes straw, sawdust, the dung of herbivores, etc. [16]. Biochar is a promising alternative adsorbent for toxic gas and wastewater treatment [17,18]. Most researchers have focused on pyrochar, but there are a few reports on the application of hydrochar in H2S capture. HTC is an auspicious approach for the use of waste biomass. Compared with pyrochar, hydrochar is suitable for dealing with wet biomass directly, with lower energy consumption [19–21]. Hydrochar also has higher yield and cation exchange capacity, and, during the production process, no PAHs are released. On the surface of hydrochar, it has more oxygen-containing functional groups, which is favorable for H2S capture and oxidation [22,23]. Although hydrochar has such abundant advantages, it has been rarely used to treat gaseous pollutants, especially H2S.

Chitosan, insoluble in water and organic solvents, is a natural macromolecular aminopolysaccharide with a yield second only to cellulose [24]. Moreover, cornstarch is the world's largest source of starch, accounting for approximately 65% of the total amount worldwide [25]. In this work, chitosan and cornstarch were used to synthesize hydrochar, which was modified by CuO-ZnO in a one-pot process. The aim of this work was to explore the formation of CuO-ZnO on hydrochar, the physical and chemical properties of this new material, the desulfurization products, and the mechanisms. This work provides new insights into the development and application of hydrochar products.

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

#### *2.1. Basic Physical and Chemical Properties of Hydrochars*

The results of the EDX analyses of the hydrochars are shown in Figure 1. The mass yield (the ratio of product to the original raw biomass), ultimate analysis by EDX (C, O, N, Zn, Cu), specific surface area, pore volume, and average pore diameter are reported in Table 1. It was found that the type of precursor had a significant effect on the yield of hydrochar (Table 1). The mass yields of the solids recovered changed depending on the content and type of the precursor. With the increase in chitosan content, the yield of hydrochar and nitrogen content increased [26], because the hydrochar yield is related to the solubility of the precursors in water, and the solubility of starch is much higher than that of chitosan [24,25]. Simultaneously, the nitrogen in the hydrochar products mainly came from chitosan, and a small amount of nitrogen came from polyacrylamide; therefore, with the increase in the chitosan content, the nitrogen content increased.

Under the same synthetic conditions, when the ratio of chitosan to starch was 1:1, the specific surface area of hydrochar reached 30.102 m2/g. It was found that, although the added amount of ZnCl<sup>2</sup> and CuCl<sup>2</sup> in the precursor was the same, the detected content of Zn in the product was much lower than that of copper. One possible reason is that the pH value of the filtrate during hydrothermal carbonization kept decreasing, and ZnO could not remain stable under the slightly acidic conditions, while CuO was stable under the acidic conditions.

**Table 1.** The yields, specific surface area, average pore diameter, and elemental composition of the synthesized hydrochars.


Note: Specific surface area (SSA), Pore volume (PV), Average pore diameter (APD).

chars.

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**Figure 1.** EDX images of the hydrochars. **Figure 1.** EDX images of the hydrochars.

**Table 1.** The yields, specific surface area, average pore diameter, and elemental composition of the synthesized hydro-**Hydrochar Yield (%) SSA PV APD Elemental Composition (%) C O Zn Cu N**  S5C5 14.83 30.102 0.071 8.9264 48.49 38.73 0.34 6.68 2.58 S10C0 11.01 12.939 0.041 1.2464 36.42 54.34 0.37 5.57 0.05 S3C7 29.89 16.456 0.069 1.456 38.85 42.07 0.36 6.97 5.93 S7C3 14.44 17.244 0.073 1.6234 43.20 45.37 0.66 5.10 1.18 S0C10 32.37 14.653 0.046 1.093 43.33 39.64 0.41 6.35 7.13 FT-IR spectra of the hydrochars were used to determine the functional groups contained in the sample, and these are shown in Figure 2. The C=C in aromatic groups showed an adsorption peak between 1613 cm−<sup>1</sup> and 1718 cm−<sup>1</sup> . For carbonyl in -COOH and CO-NH, the regions from 1400 cm−<sup>1</sup> to 1500 cm−<sup>1</sup> were ascribed to C-N and C-O groups' stretching vibration. The peak at 1033 cm−<sup>1</sup> was assigned to C-O stretching or O-H bending vibrations. The broad band at 3400 cm−<sup>1</sup> can be assigned to the existence of the N-H structure. Finally, the peaks at 1033 cm−<sup>1</sup> and 1403 cm−<sup>1</sup> were suggested to be C-N and C-O, respectively. According to the elemental analysis and FT-IR analysis, it was proven that amine groups and oxygen-containing groups on the surface of the hydrochar were abundant.

S5C5N 9.7 8.162 0.042 0.891 49.38 36.97 0.42 5.59 2.05 Note: Specific surface area (SSA), Pore volume (PV), Average pore diameter (APD). Under the same synthetic conditions, when the ratio of chitosan to starch was 1:1, the specific surface area of hydrochar reached 30.102 m2/g. It was found that, although the added amount of ZnCl2 and CuCl2 in the precursor was the same, the detected content of Zn in the product was much lower than that of copper. One possible reason is that the pH value of the filtrate during hydrothermal carbonization kept decreasing, and ZnO could not remain stable under the slightly acidic conditions, while CuO was stable under the acidic conditions. The surface morphology of several hydrochars is shown in Figure 3. It was found that with the change in the starch and chitosan content in the precursors, the hydrochars presented different microstructures. Among all the hydrochars without polyacrylamide, the precursor starch mainly showed carbon particles and carbon spheres with diameters from 10 µm to 100 µm, while the precursor chitosan mainly had a porous cellular structure. However, the hydrochar with polyacrylamide was dense, with no regular shape, and consisted of some carbon particles, because both chitosan and cationic polyacrylamide contained positive charges, which could make the distribution of the system more uniform. Furthermore, metal oxide clusters were not observed in any of the photomicrographs. The hydrochar samples with different starch and chitosan content were analyzed by XRD

(in Figure 4), and this showed that the crystallinity of metal oxides in the hydrochar was low. There may be ZnO in S5C5, S5C5N, S10C0, and free Cu in S0C10 due to the reducibility of chitosan [27]. It indicated that the distribution of active metal sites in the hydrochar was relatively uniform or that metal oxides were embedded in carbon spheres or carbon particles [28]. vibrations. The broad band at 3400 cm−1 can be assigned to the existence of the N-H structure. Finally, the peaks at 1033 cm−1 and 1403 cm−1 were suggested to be C-N and C-O, respectively. According to the elemental analysis and FT-IR analysis, it was proven that amine groups and oxygen-containing groups on the surface of the hydrochar were abundant.

an adsorption peak between 1613 cm−1 and 1718 cm−1. For carbonyl in -COOH and CO-NH, the regions from 1400 cm−1 to 1500 cm−1 were ascribed to C-N and C-O groups' stretching vibration. The peak at 1033 cm−1 was assigned to C-O stretching or O-H bending

*Catalysts* **2021**, *11*, x FOR PEER REVIEW 4 of 15

**Figure 2.** FT-IR spectra of the hydrochars. **Figure 2.** FT-IR spectra of the hydrochars.

**Figure 3.** SEM images of different hydrochars. (**a**) S10C0; (**b**) S5C5; (**c**) S5C5N and (**d**) S0C10 before adsorption; (**e**) S5C5 after adsorption. **Figure 3.** SEM images of different hydrochars. (**a**) S10C0; (**b**) S5C5; (**c**) S5C5N and (**d**) S0C10 before adsorption; (**e**) S5C5 after adsorption.

Intensity (a.u.)

**Figure 4.** X-Ray diffraction patterns of hydrochar samples.

10 20 30 40 50 60 70 80

2θ (deg.)

S10C0

ZnO PDF#01-1136

S0C10

S5C5N

S5C5

CuO PDF#01-1117

after adsorption.

**Figure 4.** X-Ray diffraction patterns of hydrochar samples.

**Figure 4.** X-Ray diffraction patterns of hydrochar samples.

**Figure 3.** SEM images of different hydrochars. (**a**) S10C0; (**b**) S5C5; (**c**) S5C5N and (**d**) S0C10 before adsorption; (**e**) S5C5

Notably, no metal oxide aggregates were observed in any of the samples. Based on previous results [29,30], we believe that both the molecular weight and concentration of PAM significantly affected the morphology of the end-products. For the synthesis of nanoporous materials at a large scale, the approach was facile and had many potential applications. In addition, it was also applicative for the synthesis of other materials with a high surface area and nanoporous structures. It has been suggested that the addition of polyacrylamide can affect the morphology of the hydrochar. As an important capping agent, PAM has been widely used to synthesize materials with various nanostructures (nanorods, nanowires, nanoplates, nanocubes, etc.). The exact function of PAM on the shape selectivity is not yet fully understood; however, we believe that the selective adsorption of PAM on various crystallographic planes (newly formed CuO, ZnO, or hydrochar particles) suppressed their intrinsic anisotropic growth [30]. With an N-C=O group, PAM was easily attached to the surfaces of these materials and limited the growth of the crystal faces. Selective interactions between PAM and the different surface planes of the CuO or ZnO may greatly influence the growth direction and rate and ultimately result in particles with different shapes [30]. For an oxidation catalyst, its effectiveness can be mainly attributed to the adsorption and desorption of gas molecules from its surface.

#### *2.2. H2S Adsorption Performance*

#### 2.2.1. Effect of Hydrochar Species

In this section, a series of single-factor experiments were carried out to determine the effect of the adsorbent in the desulfurization system. The H2S removal efficiency (%) and breakthrough sulfur capacity were selected as the evaluation index. The ratio of chitosan to starch had a significant influence on H2S removal. The sulfur capacities of hydrochars with different molar ratios of chitosan to starch under 180 ◦C were measured and are shown in Figure 5. It can be observed that the sulfur capacity of S5C5 was higher than that of other hydrochars and the addition of polyacrylamide had a great impact on H2S adsorption.

*2.2. H2S Adsorption Performance*  2.2.1. Effect of Hydrochar Species

sorption.

Notably, no metal oxide aggregates were observed in any of the samples. Based on previous results [29,30], we believe that both the molecular weight and concentration of PAM significantly affected the morphology of the end-products. For the synthesis of nanoporous materials at a large scale, the approach was facile and had many potential applications. In addition, it was also applicative for the synthesis of other materials with a high surface area and nanoporous structures. It has been suggested that the addition of polyacrylamide can affect the morphology of the hydrochar. As an important capping agent, PAM has been widely used to synthesize materials with various nanostructures (nanorods, nanowires, nanoplates, nanocubes, etc.). The exact function of PAM on the shape selectivity is not yet fully understood; however, we believe that the selective adsorption of PAM on various crystallographic planes (newly formed CuO, ZnO, or hydrochar particles) suppressed their intrinsic anisotropic growth [30]. With an N-C=O group, PAM was easily attached to the surfaces of these materials and limited the growth of the crystal faces. Selective interactions between PAM and the different surface planes of the CuO or ZnO may greatly influence the growth direction and rate and ultimately result in particles with different shapes [30]. For an oxidation catalyst, its effectiveness can be mainly attributed to the adsorption and desorption of gas molecules from its surface.

In this section, a series of single-factor experiments were carried out to determine the effect of the adsorbent in the desulfurization system. The H2S removal efficiency (%) and breakthrough sulfur capacity were selected as the evaluation index. The ratio of chitosan to starch had a significant influence on H2S removal. The sulfur capacities of hydrochars with different molar ratios of chitosan to starch under 180 °C were measured and are shown in Figure 5. It can be observed that the sulfur capacity of S5C5 was higher than that of other hydrochars and the addition of polyacrylamide had a great impact on H2S ad-

**Figure 5.** The breakthrough curves of hydrochar S5C5 with different molar ratios of chitosan to starch (T, 230 ◦C; auxiliary agent, cationic PAM; cationic PAM concentration, 2.0 g/L).

#### 2.2.2. Effect of Auxiliary Agents on H2S Removal **Figure 5.** The breakthrough curves of hydrochar S5C5 with different molar ratios of chitosan to starch (T, 230 °C; auxiliary agent, cationic PAM; cationic PAM concentration, 2.0 g/L).

*Catalysts* **2021**, *11*, x FOR PEER REVIEW 7 of 15

Cationic PAM, polyvinylpyrrolidone, and neutral PAM were used as the auxiliary agents in the synthesis of the hydrochar. According to the experimental results, it was found that the addition of polyacrylamide and its concentration can affect the sulfur capacity. The effect of different auxiliary agents on H2S removal by hydrochar S5C5 is shown in Figure 6. Among the three auxiliary agents, the cationic PAM-synthesized hydrochar showed the best performance for H2S removal. In order to explore the best composition of precursors, the PAM concentrations were also optimized, as shown in Figure 7. With the increasing of the PAM concentration from 0.5 g/L to 3.0 g/L, the sulfur capacity decreased. Cationic PAM with a positive charge can attract to and interact with chitosan of a negative charge, leading to a good combination of cationic PAM in hydrochar, but the best amount of cationic PAM depended on the amount of chitosan. 2.2.2. Effect of Auxiliary Agents on H2S Removal Cationic PAM, polyvinylpyrrolidone, and neutral PAM were used as the auxiliary agents in the synthesis of the hydrochar. According to the experimental results, it was found that the addition of polyacrylamide and its concentration can affect the sulfur capacity. The effect of different auxiliary agents on H2S removal by hydrochar S5C5 is shown in Figure 6. Among the three auxiliary agents, the cationic PAM-synthesized hydrochar showed the best performance for H2S removal. In order to explore the best composition of precursors, the PAM concentrations were also optimized, as shown in Figure 7. With the increasing of the PAM concentration from 0.5 g/L to 3.0 g/L, the sulfur capacity decreased. Cationic PAM with a positive charge can attract to and interact with chitosan of a negative charge, leading to a good combination of cationic PAM in hydrochar, but the best amount of cationic PAM depended on the amount of chitosan.

**Figure 6.** The breakthrough curves of hydrochar S5C5 with different auxiliary agents (T, 230 °C; auxiliary agent concentration, 2.0 g/L). **Figure 6.** The breakthrough curves of hydrochar S5C5 with different auxiliary agents (T, 230 ◦C; auxiliary agent concentration, 2.0 g/L).

**Figure 7.** The breakthough curves of hydrochar S5C5 with cationic PAM of different concentrations (T, 230 °C; auxiliary agent, cationic PAM). **Figure 7.** The breakthough curves of hydrochar S5C5 with cationic PAM of different concentrations (T, 230 ◦C; auxiliary agent, cationic PAM).

#### 2.2.3. Effect of Adsorption Temperature on H2S Removal 2.2.3. Effect of Adsorption Temperature on H2S Removal

The effect of adsorption temperature on H2S removal by hydrochar S5C5 is shown in Figure 8. With the desulfurization temperature increasing, the desulfurization ability of hydrochar S5C5 was clearly improved. This result indicated that the desulfurization ability of S5C5 modified by metal oxide was lower than the sorbent derived from the molecular sieve (SBA-15 or MCM-41) with modification or adsorbents with high metal content; however, it was far higher than other types of common active carbon [31–33]. As is known, a low temperature is beneficial for H2S adsorption. Raising the temperature could enhance the molecular mobility and interaction between each reactant to promote H2S adsorption by increasing the reaction rate; however, a higher temperature would be an obstruction to H2S adsorption in an exothermic reaction. The effect of adsorption temperature on H2S removal by hydrochar S5C5 is shown in Figure 8. With the desulfurization temperature increasing, the desulfurization ability of hydrochar S5C5 was clearly improved. This result indicated that the desulfurization ability of S5C5 modified by metal oxide was lower than the sorbent derived from the molecular sieve (SBA-15 or MCM-41) with modification or adsorbents with high metal content; however, it was far higher than other types of common active carbon [31–33]. As is known, a low temperature is beneficial for H2S adsorption. Raising the temperature could enhance the molecular mobility and interaction between each reactant to promote H2S adsorption by increasing the reaction rate; however, a higher temperature would be an obstruction to H2S adsorption in an exothermic reaction. *Catalysts* **2021**, *11*, x FOR PEER REVIEW 9 of 15

**Figure 8.** The breakthough curves of hydrochar S5C5 under different adsorption temperatures (auxiliary agent, cationic PAM; cationic PAM concentration, 0.5 g/L). **Figure 8.** The breakthough curves of hydrochar S5C5 under different adsorption temperatures (auxiliary agent, cationic PAM; cationic PAM concentration, 0.5 g/L).

sulfur capacity of the hydrochar S5C5 is 28.06 mg/g-adsorbent.

adsorption process of H2S on the surface of the hydrochar [33].

*2.3. Adsorption Mechanism* 

shown in Figures 9–11.

In the adsorption and oxidation process of H2S, the change in the oxygen functional groups on the surface of the hydrochar plays an important role. The quinone and carbonyl

The adsorption of H2S on the hydrochar consisted of three parts: the first part is the reaction of H2S with the metal active sites, such as CuO and ZnO [34]; the second part is the reaction of H2S with the oxygen-containing functional groups and carbon on the surface of the hydrochar to form C-S bonds and O-S bonds; the third part is the physical

To further investigate the reaction mechanism, the chemical valence states of the element in the whole process were analyzed by XPS. The XPS spectra of S, Cu, and Zn are

C=C bond were broken and combined with H2S to form the S-O bond. This process was endothermic and favored the rising temperature. When the ratio of chitosan to starch is 1:1, the cationic PAM concentration is 0.5 g/L, and the temperature is 230 °C, the maximum

In the adsorption and oxidation process of H2S, the change in the oxygen functional groups on the surface of the hydrochar plays an important role. The quinone and carbonyl groups on the surface of the hydrochar can react with molecular H2S. The C=O bond and C=C bond were broken and combined with H2S to form the S-O bond. This process was endothermic and favored the rising temperature. When the ratio of chitosan to starch is 1:1, the cationic PAM concentration is 0.5 g/L, and the temperature is 230 ◦C, the maximum sulfur capacity of the hydrochar S5C5 is 28.06 mg/g-adsorbent.

#### *2.3. Adsorption Mechanism*

The adsorption of H2S on the hydrochar consisted of three parts: the first part is the reaction of H2S with the metal active sites, such as CuO and ZnO [34]; the second part is the reaction of H2S with the oxygen-containing functional groups and carbon on the surface of the hydrochar to form C-S bonds and O-S bonds; the third part is the physical adsorption process of H2S on the surface of the hydrochar [33].

To further investigate the reaction mechanism, the chemical valence states of the element in the whole process were analyzed by XPS. The XPS spectra of S, Cu, and Zn are shown in Figures 9–11.

The XPS spectrum of S in hydrochar S5C5 after H2S adsorption is shown in Figure 9. The valence state of S was confirmed within the binding energy in the range of 162-172 eV. This showed that S 2p3/2 and S2<sup>−</sup> 2p2/3 appeared at 167.5 eV and 161.7 eV, respectively. The peak at 163.4 eV may vary due to the existence of the structure of C-S [33]. The Cu<sup>+</sup> was confirmed by the Cu 2p3/2 binding energy in the range of 930 eV to 937 eV, and it showed that Cu<sup>+</sup> 2p3/2 appeared at 932.56 eV, and the Cu2+ was assigned to the binding energy of the XPS contribution from 928 to 937 eV with a satellite contribution in the range of 937–947 eV, and it appeared at 934.61 eV. The Zn2+ was confirmed by the Zn 2p3/2 that appeared at 1021.77 eV. Therefore, the existing sulfur, zinc, and copper in the hydrochar S5C5 were CuS, ZnS, Cu2S, sulfur, and a C-S bond. *Catalysts* **2021**, *11*, x FOR PEER REVIEW 10 of 15

**Figure 9.** The XPS spectrum of S in hydrochar S5C5 after H2S adsorption. **Figure 9.** The XPS spectrum of S in hydrochar S5C5 after H2S adsorption.

Cu2p3/2

S 932.56eV

Cu2

80,000

60,000

Intensity (e.u.)

40,000

20,000

**Figure 10.** The XPS spectrum of Cu in hydrochar S5C5 after H2S adsorption.

925 930 935 940 945 950

CuS 934.61eV

CuS satellite

Binding Energy (eV)

3000

4000

5000

Intensity/(a.u.)

6000

7000

155 160 165 170

**Figure 9.** The XPS spectrum of S in hydrochar S5C5 after H2S adsorption.

Binding Energy/(eV)

S2p3/2

S2p3/2

S2p1/2

C-S

CuS

S2p1/2

S2p3/2

S2p1/2

Sulfite

**Figure 10.** The XPS spectrum of Cu in hydrochar S5C5 after H2S adsorption. **Figure 10.** The XPS spectrum of Cu in hydrochar S5C5 after H2S adsorption.

**Figure 11.** The XPS spectrum of Zn in hydrochar S5C5 after H2S adsorption. **Figure 11.** The XPS spectrum of Zn in hydrochar S5C5 after H2S adsorption.

The XPS spectrum of S in hydrochar S5C5 after H2S adsorption is shown in Figure 9. The valence state of S was confirmed within the binding energy in the range of 162-172 During the hydrothermal reaction, CuCl<sup>2</sup> and ZnCl<sup>2</sup> reacted to form CuO and ZnO. The reaction equation can be described as follows:

eV. This showed that S 2p3/2 and S2− 2p2/3 appeared at 167.5 eV and 161.7 eV, respectively.

showed that Cu+ 2p3/2 appeared at 932.56 eV, and the Cu2+ was assigned to the binding energy of the XPS contribution from 928 to 937 eV with a satellite contribution in the range of 937–947 eV, and it appeared at 934.61 eV. The Zn2+ was confirmed by the Zn 2p3/2 that appeared at 1021.77 eV. Therefore, the existing sulfur, zinc, and copper in the hydrochar

During the hydrothermal reaction, CuCl2 and ZnCl2 reacted to form CuO and ZnO.

At the same time, copper (II) oxide has a certain oxidation capacity; H2S can be oxidized partly to elemental sulfur and a fraction of the sulfide ions that have not been oxidized can form Cu2S [35]. Hydrochar, rich in oxygen-containing functional groups, can combine with H2S to form a C-S bond and S-O bond [33,36]. Therefore, the form of sulfur after adsorption can be confirmed to be sulfur and a C-S bond. H2S also can react with oxygen-containing functional groups to form sulfates in the absence of oxygen [33]. There was no oxygen gas to participate in this adsorption, so sulfate did not exist in the product.

CuCl2 + H2O = Cu(OH)2 + HCl (1)

ZnCl2 + H2O = Zn(OH)2 + HCl (2)

Cu(OH)2 = CuO + H2O (3)

Zn(OH)2 = ZnO + H2O (4)

S5C5 were CuS, ZnS, Cu2S, sulfur, and a C-S bond.

The reaction equation can be described as follows:

CuCl<sup>2</sup> + H2O = Cu(OH)<sup>2</sup> + HCl (1)

ZnCl<sup>2</sup> + H2O = Zn(OH)<sup>2</sup> + HCl (2)

$$\text{Cu(OH)}\_{2} = \text{CuO} + \text{H}\_{2}\text{O} \tag{3}$$

$$\text{Zn(OH)}\_{2} = \text{ZnO} + \text{H}\_{2}\text{O} \tag{4}$$

At the same time, copper (II) oxide has a certain oxidation capacity; H2S can be oxidized partly to elemental sulfur and a fraction of the sulfide ions that have not been oxidized can form Cu2S [35]. Hydrochar, rich in oxygen-containing functional groups, can combine with H2S to form a C-S bond and S-O bond [33,36]. Therefore, the form of sulfur after adsorption can be confirmed to be sulfur and a C-S bond. H2S also can react with oxygen-containing functional groups to form sulfates in the absence of oxygen [33]. There was no oxygen gas to participate in this adsorption, so sulfate did not exist in the product.

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

#### *3.1. Materials*

The reagents, all of analytical grade, used in this experiment were purchased directly without further purification. Copper chloride, zinc chloride, cationic polyacrylamides (CPAM), and chitosan (low viscosity, deacetylation >90%) were purchased from Shanghai Macklin Biochemical Co., Ltd (Shanghai, China). Cornstarch (Pharmaceutical grade) was purchased from Shanghai Aladding Biochemical Technology Co., Ltd (Shanghai, China). H2S standard gas of 1% and N<sup>2</sup> of 99.999% were provided by Jinan Deyang Special Gas Co., Ltd (Jinan, China). The solution was prepared using laboratory-made deionized water (18.3 MΩ·cm−<sup>1</sup> ).

#### *3.2. Preparation of Adsorbent*

There were six types of hydrochar synthesized in the experiment. All of them were composed of chitosan and cornstarch, with a cationic polyacrylamide solution (an auxiliary agent) with different dosages. The abbreviation and composition of the synthesized hydrochar samples are shown in Table 2. Taking C5S5 as an example, chitosan (3.60 g), cornstarch (3.60 g), ZnCl<sup>2</sup> (0.84 g), and CuCl<sup>2</sup> (0.84 g) were placed in a mortar, ground evenly with force, and then moved to a glass beaker, following by the addition of 45 mL of cationic polyacrylamide solution (0.5, 1.0, 2.0, and 3.0 g/L). While being treated with ultrasound, the precursors were stirred vigorously until a light blue color appeared. The sample was placed in a hydrothermal reactor and heated up to 230 ◦C for 4 h. The product was washed repeatedly using deionized water until the pH of the rinsed water stabilized. Other hydrochars were synthesized using the same method with different molar ratios of the precursors.


**Table 2.** Abbreviations and compositions of six kinds of hydrochar.

#### *3.3. Characterization of Hydrochar*

The characterization of materials was investigated using a Fourier-transform infrared (FTIR) spectrophotometer (IRAffinity-1s, Shimadzu, Kyoto, Japan). X-ray diffraction (XRD) patterns of hydrochar samples were recorded on an X-ray diffractometer (SmartLab, Rigaku, Tokyo, Japan) and carried out in the 2θ range from 10◦ to 80◦ . The surface morphologies of materials were observed by scanning electron microscope (SEM) apparatus (Regulus 8220, Hitachi, Tokyo, Japan). The element composition and valence state of materials were explored by X-ray photoelectron spectroscopy (XPS) with a multifunctional imaging electron spectrometer (ESCALAB 250XI, Thermo Fisher, Waltham, MA, America). The specific surface areas of materials were measured using the Brunauer–Emmett–Teller (BET) method, and the pore size distribution was calculated using the Barrett–Joymer–Halenda (BJH) method from the isotherm of the adsorption branch with an automatic specific surface area and porosity analyzer (TriStar II 3020, Micromeritics, Norcross, GA, USA). *Catalysts* **2021**, *11*, x FOR PEER REVIEW 13 of 15 *3.4. Batch H2S Adsorption Experiments* 

#### *3.4. Batch H2S Adsorption Experiments* The mixed gas was prepared by blending H2S standard gas with N2, both quantified

The mixed gas was prepared by blending H2S standard gas with N2, both quantified by flow indicators (D08-1F), which were purchased from Beijing Sevenstar Electronics Co., Ltd. (Beijing, China); the concentration of H2S was measured by the gas analyzer (TH-990S) from Wuhan Tianhong Instrument Group. After adsorption, the sulfur capability was calculated by Equation (1): by flow indicators (D08-1F), which were purchased from Beijing Sevenstar Electronics Co., Ltd. (Beijing, China); the concentration of H2S was measured by the gas analyzer (TH-990S) from Wuhan Tianhong Instrument Group. After adsorption, the sulfur capability was calculated by Equation (1):

$$\text{Min (1):}$$

$$\text{H2S removal } efficiency(\%) = \frac{\text{C}\_{\text{in}} \text{ C}\_{\text{out}}}{\text{C}\_{\text{in}}} \times 100\% \tag{5}$$

*<sup>C</sup>in* and *<sup>C</sup>out* (mg·m−<sup>3</sup> ) were the inlet and outlet concentration of H2S in the gas mixture, respectively. A diagram of the test devices for the evaluation of desulfurization performance is shown in Figure 12. *Cin* and *Cout* (mg·m−3) were the inlet and outlet concentration of H2S in the gas mixture, respectively. A diagram of the test devices for the evaluation of desulfurization performance is shown in Figure 12.

**Figure 12.** The diagram of test devices for the evaluation of desulfurization performance. (**a**) Pressure reducing valve; (**b**) mass flow controller; (**c**) portable hydrogen sulfide concentration analyzer; **Figure 12.** The diagram of test devices for the evaluation of desulfurization performance. (**a**) Pressure reducing valve; (**b**) mass flow controller; (**c**) portable hydrogen sulfide concentration analyzer; (**d**) tube furnace; (**e**) temperature controller; (**f**) concentrated lye; (**g**) quartz tube; (**h**) three-way valve.

(**d**) tube furnace; (**e**) temperature controller; (**f**) concentrated lye; (**g**) quartz tube; (**h**) three-way valve. To test sulfur capacity, a quartz tube was used and its diameter and height were 6 To test sulfur capacity, a quartz tube was used and its diameter and height were 6 mm and 100 mm, respectively. The adsorption temperature was controlled by a tube furnace. In the tests, a gas mixture containing 3000 ppm (4617 mg/m<sup>3</sup> ) of H2S (nitrogen as balance

when the H2S outlet concentration was higher than 20 mg/m3 by Equation (2).

ൈ ுమௌ

nace. In the tests, a gas mixture containing 3000 ppm (4617 mg/m3) of H2S (nitrogen as balance gas) was passed through the quartz tube filled with adsorbent of 0.5 g, under a gas flow rate of 100 mL/min. The outlet H2S gas was absorbed by KOH solution. The breakthrough sulfur capacity (*Sb*, mg/g) was calculated in the stage from the beginning to

௧

where represents the breakthrough sulfur capacity of sorbents (mg/g), ௌ and ுమௌ are the molar weight of sulfur (32.06 g/mol) and H2S (34.06 g/mol), respectively; is the weight of sorbents; ுమௌ is the H2S gas flow rate; is the reaction time for desulfurization (min), and and ௨௧ are the inlet and outlet concentration of H2S (mg/m3), respectively. When is the saturation adsorption time, Equation (2) was also used to calculate

ቈන ሺ െ ௨௧ሻ

ൈ 10ି (6)

 ൌ ௌ ுమௌ

the max sulfur capacity ().

gas) was passed through the quartz tube filled with adsorbent of 0.5 g, under a gas flow rate of 100 mL/min. The outlet H2S gas was absorbed by KOH solution. The breakthrough sulfur capacity (*S<sup>b</sup>* , mg/g) was calculated in the stage from the beginning to when the H2S outlet concentration was higher than 20 mg/m<sup>3</sup> by Equation (2).

$$S\_b = \frac{M\_S}{M\_{H\_2S}} \times \frac{Q\_{H\_2S}}{m} \left[ \int\_0^t (\mathbb{C}\_{in} - \mathbb{C}\_{out}) dt \right] \times 10^{-6} \tag{6}$$

where *S<sup>b</sup>* represents the breakthrough sulfur capacity of sorbents (mg/g), *M<sup>S</sup>* and *MH*2*<sup>S</sup>* are the molar weight of sulfur (32.06 g/mol) and H2S (34.06 g/mol), respectively; *m* is the weight of sorbents; *QH*2*<sup>S</sup>* is the H2S gas flow rate; *t* is the reaction time for desulfurization (min), and *Cin* and *Cout* are the inlet and outlet concentration of H2S (mg/m<sup>3</sup> ), respectively. When *t* is the saturation adsorption time, Equation (2) was also used to calculate the max sulfur capacity (*Cm*).

#### **4. Conclusions**

For H2S adsorption, this study provides a method for the synthesis of hydrochar, obtained by the hydrothermal reaction of chitosan, starch, cationic polyacrylamide aqueous solution, ZnCl2, and CuCl2. The experimental results showed that the hydrochar contained many amino groups as functional groups, and the nano-scaled metal oxide particles had good dispersion on the surface of the hydrochar. The amine group significantly reduced the activation energy of H2S and CuO-ZnO, which was conducive to the rapid diffusion of H2S among the lattices. At the same time, cationic polyacrylamide as a steric stabilizer can change the formation process of CuO and ZnO nanoparticles, making the particle size smaller and allowing it to react more easily with H2S sufficiently. When the ratio of chitosan to starch is 1:1, the temperature is 230 ◦C, and the cationic PAM concentration is 0.5 g/L, the maximum sulfur capacity of the hydrochar S5C5 is 28.06 mg/g-adsorbent. Therefore, modified hydrochar may be a promising adsorbent for H2S removal.

**Author Contributions:** Conceptualization, Y.M. and L.Z.; methodology, J.M. and C.Z.; software, R.X.; validation, Y.M. and L.Z.; formal analysis, L.W.; resources, L.D. and L.Z.; data curation, J.M. and Y.M.; writing—original draft preparation, J.M., C.Z. and Y.M.; writing—review and editing, X.L.; funding acquisition, L.D. and L.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by the Open Research Fund Program of Key Laboratory of Cleaner Production and Integrated Resource Utilization of China National Light Industry, grant number CP-2020-YB8, Natural Science Foundation of Shandong Province, grant number ZR2020QB199, and Qilu University of Technology (Shandong Academy of Sciences) Youth Doctor Cooperation Fund, grant number 2019BSHZ0028.

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

#### **References**


## *Article* **Selective Catalytic Oxidation of Lean-H2S Gas Stream to Elemental Sulfur at Lower Temperature**

**Daniela Barba \* , Vincenzo Vaiano and Vincenzo Palma**

Department of Industrial Engineering, University of Salerno, Via Giovanni Paolo II, 132, 84084 Fisciano, Italy; vvaiano@unisa.it (V.V.); vpalma@unisa.it (V.P.)

**\*** Correspondence: dbarba@unisa.it

**Abstract:** Ceria-supported vanadium catalysts were studied for H2S removal via partial and selective oxidation reactions at low temperature. The catalysts were characterized by N<sup>2</sup> adsorption at 77 K, Raman spectroscopy, X-ray diffraction techniques, and X-ray fluorescence analysis. X-ray diffraction and Raman analysis showed a good dispersion of the V-species on the support. A preliminary screening of these samples was performed at fixed temperature (T = 327 ◦C) and H2S inlet concentration (10 vol%) in order to study the catalytic performance in terms of H2S conversion and SO<sup>2</sup> selectivity. For the catalyst that exhibited the higher removal efficiency of H2S (92%) together with a lower SO<sup>2</sup> selectivity (4%), the influence of temperature (307–370 ◦C), contact time (0.6–1 s), and H2S inlet concentration (6–15 vol%) was investigated.

**Keywords:** hydrogen sulfide; H2S selective partial oxidation; sulfur; sulfur dioxide; vanadiumbased catalysts

**Citation:** Barba, D.; Vaiano, V.; Palma, V. Selective Catalytic Oxidation of Lean-H2S Gas Stream to Elemental Sulfur at Lower Temperature. *Catalysts* **2021**, *11*, 746. https://doi.org/ 10.3390/catal11060746

Academic Editor: Stefano Cimino

Received: 28 May 2021 Accepted: 16 June 2021 Published: 18 June 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/).

#### **1. Introduction**

Hydrogen sulfide (H2S) is a common gas pollutant, which is harmful to human health with deleterious effects on many industrial catalysts, and represents the main source of acid rains when it is oxidized to sulfur dioxide (SO2) [1]. Many attempts have been focused on H2S removal from gaseous streams due to the worldwide increase in restrictive emission standards. Today, H2S-removal-based processes include wet scrubbing [2], biological methods [3], adsorption [4], and selective catalytic oxidation [5]. Among these purification processes, selective catalytic oxidation seems to be very promising for lean-H2S gas streams, where the concentration of hydrogen sulfide is in the range 0.1–10 vol%.

Typically, lean-H2S gas is characteristic of tail gas treating (<5 wt% H2S), crude petroleum (0.3–0.8 wt% H2S), and natural gas streams (0.03–0.3 wt% H2S), although in this last case the H2S can also reach 30 wt% [6].

The selective catalytic oxidation of H2S into elemental sulfur is one of the treatment methods employed for the removal of H2S from the Claus process tail gas [7,8]. This reaction can be performed above or below the sulfur dew point (180 ◦C) and the processes used are super-Claus, doxosulfreen (Elf-Lurgi), and the mobil direct oxidation process (MODOP) [9]. The super-Claus process, developed in 1985, is continuously being improved and allows achievement of a desulfurization efficiency of ~99.5% at 240 ◦C in the presence of iron- and chromium-based catalysts supported on alumina or silica [10]. In MODOP, the direct oxidation of H2S into elemental sulfur occurs on a TiO2-based catalyst that deactivates in the presence of water [11]. In the super-Claus process, H2S is oxidized without removing water from the tail gas. Metal-oxide-based catalysts, such as Al2O3, TiO2, V2O5, Mn2O3, Fe2O3, and CuO are the most used and investigated for H2S-selective catalytic oxidation [12]. Indeed, vanadium oxides have been investigated as active phases for H2S selective oxidation and are used as bulk V2O<sup>5</sup> [13], mixed with other metals [14], or supported over commercial [15] and mesoporous materials [16].

In our previous works, vanadium-based catalysts supported on different metal oxides (CeO2, TiO2, and CuFe2O4) were investigated for H2S removal from biogas by partial and selective oxidation reactions in the temperature range 50–250 ◦C [17]. The optimization of the V2O<sup>5</sup> loading (2.55–50 wt%) was performed on the CeO<sup>2</sup> support at the temperature of 150 ◦C [18]. The 20 wt% V2O5/CeO<sup>2</sup> catalyst showed the best catalytic performance in terms of H2S conversion (99%) and sulfur selectivity (99%) at 150 ◦C, by feeding a very diluted stream containing only 500 ppm of H2S [19]. Structured catalysts starting from a cordierite carrier in the form of a monolith honeycomb were also prepared, characterized, and tested at low temperature and evidenced high activity and very low SO<sup>2</sup> selectivity [20,21]. tion of the V2O5 loading (2.55–50 wt%) was performed on the CeO2 support at the temperature of 150 °C [18]. The 20 wt% V2O5/CeO2 catalyst showed the best catalytic performance in terms of H2S conversion (99%) and sulfur selectivity (99%) at 150 °C, by feeding a very diluted stream containing only 500 ppm of H2S [19]. Structured catalysts starting from a cordierite carrier in the form of a monolith honeycomb were also prepared, characterized, and tested at low temperature and evidenced high activity and very low SO2 selectivity [20,21]. Based on these obtained promising results, in this study, vanadium-based catalysts supported on ceria were prepared, characterized, and tested in the presence of a lean-H2S gas stream containing a H2S concentration higher than 5 vol%, which is a typical con-

for H2S selective oxidation and are used as bulk V2O5 [13], mixed with other metals [14],

In our previous works, vanadium-based catalysts supported on different metal oxides (CeO2, TiO2, and CuFe2O4) were investigated for H2S removal from biogas by partial and selective oxidation reactions in the temperature range 50–250 °C [17]. The optimiza-

*Catalysts* **2021**, *11*, x FOR PEER REVIEW 2 of 13

or supported over commercial [15] and mesoporous materials [16].

Based on these obtained promising results, in this study, vanadium-based catalysts supported on ceria were prepared, characterized, and tested in the presence of a lean-H2S gas stream containing a H2S concentration higher than 5 vol%, which is a typical concentration of the Claus process tail gas. A preliminary screening of the catalysts with different vanadium loadings was carried out at 327 ◦C, in order to identify the catalyst formulation able to maximize the H2S conversion and depress the SO<sup>2</sup> formation in the presence of 10 vol% of H2S. The effect of the main operating parameters, such as temperature, contact time, and H2S inlet concentration, was also investigated. centration of the Claus process tail gas. A preliminary screening of the catalysts with different vanadium loadings was carried out at 327 °C, in order to identify the catalyst formulation able to maximize the H2S conversion and depress the SO2 formation in the presence of 10 vol% of H2S. The effect of the main operating parameters, such as temperature, contact time, and H2S inlet concentration, was also investigated. **2. Results and Discussion** 

#### **2. Results and Discussion** *2.1. Catalytic Activity Test*

#### *2.1. Catalytic Activity Test* First of all, the reaction system was studied in the presence of 10 vol% of H2S at the

First of all, the reaction system was studied in the presence of 10 vol% of H2S at the temperature of 327 ◦C without the catalyst (Figure 1). temperature of 327 °C without the catalyst (Figure 1). Figure 1 shows the behavior of H2S, H2O, and SO2 during 1 h of time on stream.

**Figure 1.** Activity test without catalyst (T = 327 °C, H2S = 10 vol%, residence time = 0.6 s). **Figure 1.** Activity test without catalyst (T = 327 ◦C, H2S = 10 vol%, residence time = 0.6 s).

After the first 5 min, the feed stream was sent to the reactor and the formation of SO2 Figure 1 shows the behavior of H2S, H2O, and SO<sup>2</sup> during 1 h of time on stream.

and water could be observed. The sulfur formation was not detectable because of the removal by the gaseous stream in the sulfur trap. The final H2S conversion was 26%, while the SO2 selectivity was high enough (~39%). The SO3 formation (m/z = 80) was not observed either for the test in the absence of a catalyst or for all the catalytic tests. In Table 1, the values obtained by the test carried out without the catalyst were compared with the ones expected by the thermodynamic equilibrium and with the experimental data achieved with 20 V-CeO2 catalyst. After the first 5 min, the feed stream was sent to the reactor and the formation of SO<sup>2</sup> and water could be observed. The sulfur formation was not detectable because of the removal by the gaseous stream in the sulfur trap. The final H2S conversion was 26%, while the SO<sup>2</sup> selectivity was high enough (~39%). The SO<sup>3</sup> formation (m/z = 80) was not observed either for the test in the absence of a catalyst or for all the catalytic tests. In Table 1, the values obtained by the test carried out without the catalyst were compared with the ones expected by the thermodynamic equilibrium and with the experimental data achieved with 20 V-CeO<sup>2</sup> catalyst.


**Table 1.** Comparison between non-catalytic system, catalytic system, and equilibrium (T = 327 ◦C, H2S = 10 vol%). H2S = 10 vol%). **No Catalyst 20 V-CeO2 Equilibrium No Catalyst 20 V-CeO2 Equilibrium**  H2S Conversion, % 26 (±1.5) 92 (±1.5) 90

**Table 1.** Comparison between non-catalytic system, catalytic system, and equilibrium (T = 327 °C,

**Table 1.** Comparison between non-catalytic system, catalytic system, and equilibrium (T = 327 °C,

*Catalysts* **2021**, *11*, x FOR PEER REVIEW 3 of 13

H2S = 10 vol%).

*Catalysts* **2021**, *11*, x FOR PEER REVIEW 3 of 13

It is evident that the reaction system without the catalyst is very far from the equilibrium conditions; in fact, the expected H2S conversion and SO<sup>2</sup> concentration would be, respectively, 90% and 0.5 vol%. Conversely, the catalytic performance of the 20 V-CeO<sup>2</sup> sample is very close to that expected from the equilibrium, confirming the key role of the catalyst for maximizing the H2S conversion and inhibiting the SO<sup>2</sup> formation. librium conditions; in fact, the expected H2S conversion and SO2 concentration would be, respectively, 90% and 0.5 vol%. Conversely, the catalytic performance of the 20 V-CeO2 sample is very close to that expected from the equilibrium, confirming the key role of the catalyst for maximizing the H2S conversion and inhibiting the SO2 formation. The screening of the vanadium-based catalysts was performed at 327 °C and the respectively, 90% and 0.5 vol%. Conversely, the catalytic performance of the 20 V-CeO2 sample is very close to that expected from the equilibrium, confirming the key role of the catalyst for maximizing the H2S conversion and inhibiting the SO2 formation. The screening of the vanadium-based catalysts was performed at 327 °C and the catalytic activity of the V-CeO2 samples was also compared with the support (CeO2) and

The screening of the vanadium-based catalysts was performed at 327 ◦C and the catalytic activity of the V-CeO<sup>2</sup> samples was also compared with the support (CeO2) and with the bulk V2O5. For each sample, the catalytic performance under steady-state conditions is reported in Figure 2. catalytic activity of the V-CeO2 samples was also compared with the support (CeO2) and with the bulk V2O5. For each sample, the catalytic performance under steady-state conditions is reported in Figure 2. with the bulk V2O5. For each sample, the catalytic performance under steady-state conditions is reported in Figure 2.

**Figure 2.** Catalytic performance of the different catalysts under steady-state conditions (T = 327 °C, H2S = 10 vol%, contact time = 0.6 s). **Figure 2.** Catalytic performance of the different catalysts under steady-state conditions (T = 327 ◦C, H2S = 10 vol%, contact time = 0.6 s). **Figure 2.** Catalytic performance of the different catalysts under steady-state conditions (T = 327 °C, H2S = 10 vol%, contact time = 0.6 s).

The best catalytic performance can be observed for the catalysts having a V2O5 loading of 2.55 and 20 wt%, for which the H2S conversion and SO2 selectivity values are very similar. Although the lowest SO2 selectivity (2.2%) was observed for the 50 V-CeO2 The best catalytic performance can be observed for the catalysts having a V2O<sup>5</sup> loading of 2.55 and 20 wt%, for which the H2S conversion and SO<sup>2</sup> selectivity values are very similar. Although the lowest SO<sup>2</sup> selectivity (2.2%) was observed for the 50 V-CeO<sup>2</sup> sample, it unfortunately showed the lowest H2S conversion (83%). The best catalytic performance can be observed for the catalysts having a V2O5 loading of 2.55 and 20 wt%, for which the H2S conversion and SO2 selectivity values are very similar. Although the lowest SO2 selectivity (2.2%) was observed for the 50 V-CeO2 sample, it unfortunately showed the lowest H2S conversion (83%).

sample, it unfortunately showed the lowest H2S conversion (83%). The influence of the temperature was investigated for the 20 V-CeO2 catalyst and the experimental data for H2S conversion and SO2 selectivity were compared with the equi-The influence of the temperature was investigated for the 20 V-CeO<sup>2</sup> catalyst and the experimental data for H2S conversion and SO<sup>2</sup> selectivity were compared with the equilibrium data (red and blue lines, respectively) (Figure 3). The influence of the temperature was investigated for the 20 V-CeO2 catalyst and the experimental data for H2S conversion and SO2 selectivity were compared with the equilibrium data (red and blue lines, respectively*)* (Figure 3).

**Figure 3.** Temperature effect over 20 V-CeO<sup>2</sup> catalyst on the H2S conversion and SO<sup>2</sup> selectivity (H2S = 10 vol%, contact time = 0.6 s).

As it is possible to observe from Figure 3, the H2S conversion is very close to the equilibrium values (red line) while the SO<sup>2</sup> selectivity is, in the overall investigated temperature range, slightly below the equilibrium calculation (blue line), evidencing that the catalyst is able to inhibit the SO<sup>2</sup> formation. The effect of the H2S concentration, in the range 6–15 vol%, was then evaluated at the temperature of 327 ◦C (Figure 4). As it is possible to observe from Figure 3, the H2S conversion is very close to the equilibrium values *(*red line*)* while the SO2 selectivity is, in the overall investigated temperature range, slightly below the equilibrium calculation (blue line), evidencing that the catalyst is able to inhibit the SO2 formation. The effect of the H2S concentration, in the range 6–15 vol%, was then evaluated at the temperature of 327 °C (Figure 4).

**Figure 3.** Temperature effect over 20 V-CeO2 catalyst on the H2S conversion and SO2 selectivity

*Catalysts* **2021**, *11*, x FOR PEER REVIEW 4 of 13

(H2S = 10 vol%, contact time = 0.6 s).

**Figure 4.** Influence of the H2S inlet concentration over 20 V-CeO2 catalyst on the H2S conversion and SO2 selectivity (O2/H2S = 0.5, T = 327 °C, contact time = 0.6 s). **Figure 4.** Influence of the H2S inlet concentration over 20 V-CeO<sup>2</sup> catalyst on the H2S conversion and SO<sup>2</sup> selectivity (O2/H2S = 0.5, T = 327 ◦C, contact time = 0.6 s).

The highest value of H2S conversion and the lowest SO2 selectivity were observed when the H2S inlet concentration was 10 vol%. In the presence of a feed stream more concentrated in H2S (15 vol%), the conversion was drastically reduced to 75% and the SO2 concentration was about 1.5 vol%; in this case, the selectivity increase is of one magnitude order (14%) with respect to the other obtained values. In Table 2, the equilibrium data are The highest value of H2S conversion and the lowest SO<sup>2</sup> selectivity were observed when the H2S inlet concentration was 10 vol%. In the presence of a feed stream more concentrated in H2S (15 vol%), the conversion was drastically reduced to 75% and the SO<sup>2</sup> concentration was about 1.5 vol%; in this case, the selectivity increase is of one magnitude order (14%) with respect to the other obtained values. In Table 2, the equilibrium data are compared with those obtained experimentally at different H2S inlet concentrations.

compared with those obtained experimentally at different H2S inlet concentrations. **Table 2.** Equilibrium and experimental data by varying the H2S inlet concentration (T = 327 °C, **Table 2.** Equilibrium and experimental data by varying the H2S inlet concentration (T = 327 ◦C, contact time = 0.6 s).


15 75 (±1.5) 90 14 (±2) 9 Based on the data listed in Table 2, it is possible to see that the reaction system deviates from the equilibrium values especially in presence of 15 vol% of H2S.

Based on the data listed in Table 2, it is possible to see that the reaction system deviates from the equilibrium values especially in presence of 15 vol% of H2S. The influence of the contact time on the catalytic performance is reported in Figure The influence of the contact time on the catalytic performance is reported in Figure 5. For comparison, the equilibrium data for both H2S conversion and SO<sup>2</sup> selectivity at the temperature of 327 ◦C are also shown.

5. For comparison, the equilibrium data for both H2S conversion and SO2 selectivity at the temperature of 327 °C are also shown. The catalytic performance resulted in little affected from the variation of the contact time. In particular, it is noteworthy to evidence that the H2S conversion is quite close to the equilibrium values, while the SO<sup>2</sup> selectivity is in all cases below the equilibrium value.

**Figure 5.** Effect of the contact time over 20 V-CeO2 catalyst on the H2S conversion and SO2 selectivity (T = 327 °C, H2S = 10 vol%). **Figure 5.** Effect of the contact time over 20 V-CeO<sup>2</sup> catalyst on the H2S conversion and SO<sup>2</sup> selectivity (T = 327 ◦C, H2S = 10 vol%).

#### *2.2. Catalyst Characterization*

The catalytic performance resulted in little affected from the variation of the contact time. In particular, it is noteworthy to evidence that the H2S conversion is quite close to the equilibrium values, while the SO2 selectivity is in all cases below the equilibrium The nominal and measured vanadium oxide content of the catalysts before the activation step is reported in Table 3.

value. **Table 3.** Theoretical and measured vanadium content of the catalysts before the sulfuration.


**Table 3.** Theoretical and measured vanadium content of the catalysts before the sulfuration. **Sample V2O5 Nominal wt% % V2O5 Measured wt%**  The results reported evidence that the nominal V2O<sup>5</sup> loading is very close to the measured loading.

2.55 V-CeO2 2.55 2.7 The specific surface areas of the fresh and used catalysts are reported in Table 4.


20 V-CeO2 20 22 50 V-CeO2 50 51 **Table 4.** Specific surface area (SSA, m2/g) of the fresh and used catalysts.

**Table 4.** Specific surface area (SSA, m2/g) of the fresh and used catalysts. **Sample CeO2 V2O5 2.55 V-CeO2 20 V-CeO2 50 V-CeO2 Fresh** 29 8 25 22 20 **Used** 17 2 17 4 14 The lowest SSA was observed for the sample that was not supported (V2O5). In particular, the value of bulk V2O<sup>5</sup> (8 m2/g) decreased more than 50% after the catalytic test. For the fresh V-CeO<sup>2</sup> catalysts, the values of SSA were slightly lower than the CeO<sup>2</sup> support (~30 m2/g). After the catalytic activity tests, the SSA decrease was likely due to the sulfur deposition on the catalyst surface. This aspect was more evident for the 20 V-CeO<sup>2</sup> sample (SSA = 4 m2/g) and was confirmed by XRD and Raman characterizations.

The lowest SSA was observed for the sample that was not supported (V2O5). In particular, the value of bulk V2O5 (8 m2/g) decreased more than 50% after the catalytic test. For the fresh V-CeO2 catalysts, the values of SSA were slightly lower than the CeO2 support (~30 m2/g). After the catalytic activity tests, the SSA decrease was likely due to the sulfur deposition on the catalyst surface. This aspect was more evident for the 20 Raman spectra of the support and fresh catalysts are shown in Figure 6. The Raman spectrum for pure CeO<sup>2</sup> shows the main band at 460 cm−<sup>1</sup> , ascribable to ceria in the typical cubic crystal structure of fluorite-type cerium oxide [22,23]. The 2.55 V-CeO<sup>2</sup> sample shows that such Raman band slightly shifted to 465 cm−<sup>1</sup> , while in the case of the catalysts with the highest V loading this band shifted up to 454 cm−<sup>1</sup> . A more detailed discussion of these results is reported in the Supplementary Materials (Figure S1).

V-CeO2 sample (SSA = 4 m2/g) and was confirmed by XRD and Raman characterizations. Raman spectra of the support and fresh catalysts are shown in Figure 6. The Raman

shows that such Raman band slightly shifted to 465 cm–1, while in the case of the catalysts

these results is reported in the Supplementary Materials (Figure S1).

these results is reported in the Supplementary Materials (Figure S1).

*Catalysts* **2021**, *11*, x FOR PEER REVIEW 6 of 13

**Figure 6.** Raman spectra of CeO2, V2O5, and 2.55, 20, 50 V-CeO2 fresh catalysts. **Figure 6.** Raman spectra of CeO<sup>2</sup> , V2O<sup>5</sup> , and 2.55, 20, 50 V-CeO<sup>2</sup> fresh catalysts. The XRD spectra of CeO2 and the fresh catalysts are shown in Figure 7. All the cat-

The XRD spectra of CeO2 and the fresh catalysts are shown in Figure 7. All the catalysts exhibit the characteristic peaks of CeO2 at 28.3°, 32.8°, 47.3°, 56.1°, 58°, and 69°, corresponding to diffraction planes indexed as (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), and (4 0 0), respectively [24]. These patterns are ascribable to the typical cubic crystal structure of fluorite-type cerium oxide [25]. No additional reflections attributable to V2O5 are detectable, evidencing that the sulfuration of the catalysts completely occurred [26]. Furthermore, there were no peaks detected that related to typical vanadium sulfides (VS2, The XRD spectra of CeO<sup>2</sup> and the fresh catalysts are shown in Figure 7. All the catalysts exhibit the characteristic peaks of CeO<sup>2</sup> at 28.3◦ , 32.8◦ , 47.3◦ , 56.1◦ , 58◦ , and 69◦ , corresponding to diffraction planes indexed as (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), and (4 0 0), respectively [24]. These patterns are ascribable to the typical cubic crystal structure of fluorite-type cerium oxide [25]. No additional reflections attributable to V2O<sup>5</sup> are detectable, evidencing that the sulfuration of the catalysts completely occurred [26]. Furthermore, there were no peaks detected that related to typical vanadium sulfides (VS2, VS4, V2S3, V3S4) that might have formed following the sulfuration treatment [27]. alysts exhibit the characteristic peaks of CeO2 at 28.3°, 32.8°, 47.3°, 56.1°, 58°, and 69°, corresponding to diffraction planes indexed as (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), and (4 0 0), respectively [24]. These patterns are ascribable to the typical cubic crystal structure of fluorite-type cerium oxide [25]. No additional reflections attributable to V2O5 are detectable, evidencing that the sulfuration of the catalysts completely occurred [26]. Furthermore, there were no peaks detected that related to typical vanadium sulfides (VS2, VS4, V2S3, V3S4) that might have formed following the sulfuration treatment [27].

VS4, V2S3, V3S4) that might have formed following the sulfuration treatment [27].

with the highest V loading this band shifted up to 454 cm–1. A more detailed discussion of

with the highest V loading this band shifted up to 454 cm–1. A more detailed discussion of

**Figure 7.** XRD spectra of CeO2 and 2.55, 20, 50 V-CeO2 fresh catalysts. **Figure 7.** XRD spectra of CeO2 and 2.55, 20, 50 V-CeO2 fresh catalysts. **Figure 7.** XRD spectra of CeO<sup>2</sup> and 2.55, 20, 50 V-CeO<sup>2</sup> fresh catalysts.

In Figure 8, the Raman spectra of the fresh samples (CeO2 and V-CeO2 catalysts) are compared with the used catalysts. The used CeO2, equally to the fresh one, has the characteristic Raman peak perfectly centered at 460 cm–1 (Figure 8a) [22,23]. A slight shift of this Raman band up to 465 cm–1, 457 cm–1, and 454 cm–1 (Figure 8b–d) is detectable for 2.55 V-CeO2, 20 V-CeO2, and 50 V-CeO2 fresh catalysts, respectively, as already previously observed (Figure 6). A detailed discussion of the Raman results is reported in the Sup-In Figure 8, the Raman spectra of the fresh samples (CeO2 and V-CeO2 catalysts) are compared with the used catalysts. The used CeO2, equally to the fresh one, has the characteristic Raman peak perfectly centered at 460 cm–1 (Figure 8a) [22,23]. A slight shift of this Raman band up to 465 cm–1, 457 cm–1, and 454 cm–1 (Figure 8b–d) is detectable for 2.55 V-CeO2, 20 V-CeO2, and 50 V-CeO2 fresh catalysts, respectively, as already previously observed (Figure 6). A detailed discussion of the Raman results is reported in the Sup-In Figure 8, the Raman spectra of the fresh samples (CeO<sup>2</sup> and V-CeO<sup>2</sup> catalysts) are compared with the used catalysts. The used CeO2, equally to the fresh one, has the characteristic Raman peak perfectly centered at 460 cm−<sup>1</sup> (Figure 8a) [22,23]. A slight shift of this Raman band up to 465 cm−<sup>1</sup> , 457 cm−<sup>1</sup> , and 454 cm−<sup>1</sup> (Figure 8b–d) is detectable for 2.55 V-CeO2, 20 V-CeO2, and 50 V-CeO<sup>2</sup> fresh catalysts, respectively, as already previously observed (Figure 6). A detailed discussion of the Raman results is reported in the Supplementary Materials (Figure S2).

plementary Materials (Figure S2).

plementary Materials (Figure S2).

**Figure 8.** Raman spectra of fresh and used catalysts: CeO2 (**a**), 2.55 V-CeO2 (**b**), 20 V-CeO2 (**c**), 50 V-CeO2 (**d**). **Figure 8.** Raman spectra of fresh and used catalysts: CeO<sup>2</sup> (**a**), 2.55 V-CeO<sup>2</sup> (**b**), 20 V-CeO<sup>2</sup> (**c**), 50 V-CeO<sup>2</sup> (**d**). **Figure 8.** Raman spectra of fresh and used catalysts: CeO2 (**a**), 2.55 V-CeO2 (**b**), 20 V-CeO2 (**c**), 50 V-CeO2 (**d**).

Furthermore, the absence of any characteristic bands of the vibrational modes of crystalline V2O5 [28] and V = O stretching vibration ascribable to monovanadate species (VO43-) denotes that the sulfuration of the catalysts occurred completely [16]. The Raman spectrum of the bulk V2O5 after the catalytic test is reported in Figure 9. Furthermore, the absence of any characteristic bands of the vibrational modes of crystalline V2O<sup>5</sup> [28] and V = O stretching vibration ascribable to monovanadate species (VO<sup>4</sup> <sup>3</sup>−) denotes that the sulfuration of the catalysts occurred completely [16]. The Raman spectrum of the bulk V2O<sup>5</sup> after the catalytic test is reported in Figure 9. Furthermore, the absence of any characteristic bands of the vibrational modes of crystalline V2O5 [28] and V = O stretching vibration ascribable to monovanadate species (VO43-) denotes that the sulfuration of the catalysts occurred completely [16]. The Raman spectrum of the bulk V2O5 after the catalytic test is reported in Figure 9.

**Figure 9.** Raman spectra of V2O5 used. **Figure 9.** Raman spectra of V2O<sup>5</sup> used.

**Figure 9.** Raman spectra of V2O5 used. The Raman bands at 140, 192, 282, 405, 688, and 993 cm–1 are characteristic of the vanadium sulfide in VS2 form, as reported in the literature [29]. In particular, all the signals correspond to the rocking combination and stretching vibrations of V–S bonds or The Raman bands at 140, 192, 282, 405, 688, and 993 cm–1 are characteristic of the vanadium sulfide in VS2 form, as reported in the literature [29]. In particular, all the signals correspond to the rocking combination and stretching vibrations of V–S bonds or their combination [30]. Moreover, no bands related to the formation of vanadyl sulfate The Raman bands at 140, 192, 282, 405, 688, and 993 cm−<sup>1</sup> are characteristic of the vanadium sulfide in VS<sup>2</sup> form, as reported in the literature [29]. In particular, all the signals correspond to the rocking combination and stretching vibrations of V–S bonds or their combination [30]. Moreover, no bands related to the formation of vanadyl sulfate (984 cm−<sup>1</sup> and 1060 cm−<sup>1</sup> ) were observed [31].

their combination [30]. Moreover, no bands related to the formation of vanadyl sulfate (984 cm–1 and 1060 cm–1) were observed [31]. (984 cm–1 and 1060 cm–1) were observed [31]. In Figure 10, the XRD patterns of the fresh samples are compared with the used In Figure 10, the XRD patterns of the fresh samples are compared with the used ones. There are no differences between the XRD spectra of the fresh/used bulk CeO2; for the used

ones. There are no differences between the XRD spectra of the fresh/used bulk CeO2; for the used sample less intensity of the peaks is observed, which is likely due to the sulfur

sample less intensity of the peaks is observed, which is likely due to the sulfur deposition (Figure 10a). For the used 2.55 V-CeO<sup>2</sup> catalyst, in addition to the characteristic peaks of the CeO<sup>2</sup> fresh sample, a signal is visible at 2θ = 23◦ due to the sulfur formation [32], as also confirmed from Raman analysis (Figure 10b). The spectra of the used 20 V-CeO<sup>2</sup> catalyst (Figure 10c) are different, where other peaks attributable to the sulfur are observable at 2θ = 23◦ , 24◦ , 26◦ , 27◦ , and 28◦ [32]. For the 50 V-CeO<sup>2</sup> catalyst, the XRD spectrum of the fresh sample is perfectly stackable with that of the used sample (Figure 10d) because all the peaks are ascribable only to the CeO<sup>2</sup> support. deposition (Figure 10a). For the used 2.55 V-CeO2 catalyst, in addition to the characteristic peaks of the CeO2 fresh sample, a signal is visible at 2θ = 23° due to the sulfur formation [32], as also confirmed from Raman analysis (Figure 10b). The spectra of the used 20 V-CeO2 catalyst (Figure 10c) are different, where other peaks attributable to the sulfur are observable at 2θ = 23°, 24°, 26°, 27°, and 28° [32]. For the 50 V-CeO2 catalyst, the XRD spectrum of the fresh sample is perfectly stackable with that of the used sample (Figure 10d) because all the peaks are ascribable only to the CeO2 support.

*Catalysts* **2021**, *11*, x FOR PEER REVIEW 8 of 13

**Figure 10.** XRD spectra of fresh and used catalysts: CeO2 (**a**), 2.55 V-CeO2 (**b**), 20 V-CeO2 (**c**), 50 V-CeO2 (**d**). **Figure 10.** XRD spectra of fresh and used catalysts: CeO<sup>2</sup> (**a**), 2.55 V-CeO<sup>2</sup> (**b**), 20 V-CeO<sup>2</sup> (**c**), 50 V-CeO<sup>2</sup> (**d**).

The average crystallite size of ceria for the different catalysts, calculated with the Scherrer equation, are listed in Table 5. The average crystallite size of ceria for the different catalysts, calculated with the Scherrer equation, are listed in Table 5.

**Table 5.** Average crystallite size (<L>, nm) of the fresh and used catalysts.


50 V-CeO2 24 25

As it is possible to observe from Table 5, the increase of the V-loading for the different catalysts has involved an increase in the crystallite size of the CeO2, as reported in the literature for supported vanadium catalysts [25]. The average crystallite size of bulk CeO2 before the catalytic tests was 13 nm; it increased to 24 nm for the catalyst having the highest V-content (50 V-CeO2). Relatively to the catalysts, there is a negligible variation of As it is possible to observe from Table 5, the increase of the V-loading for the different catalysts has involved an increase in the crystallite size of the CeO2, as reported in the literature for supported vanadium catalysts [25]. The average crystallite size of bulk CeO<sup>2</sup> before the catalytic tests was 13 nm; it increased to 24 nm for the catalyst having the highest V-content (50 V-CeO2). Relatively to the catalysts, there is a negligible variation of the ceria average crystallite size between fresh and used samples.

support; the greater segregation of the CeO2 after the catalytic activity tests involved the increase of the crystallite dimension (21 nm). The segregation of the CeO2 crystallite may

The only significant variation between fresh and used samples was obtained for the support; the greater segregation of the CeO<sup>2</sup> after the catalytic activity tests involved the increase of the crystallite dimension (21 nm). The segregation of the CeO<sup>2</sup> crystallite may be due to the high SO<sup>2</sup> formation observed on the support in the absence of the active phase; in fact, among the catalysts, the highest value of SO<sup>2</sup> selectivity (~10%) was obtained for the CeO<sup>2</sup> at 327 ◦C as previously reported in Figure 2. The reaction temperature could favor the formation of sulfate species and also the oxygen in the ceria lattice could facilitate the CeO<sup>2</sup> sulfuration [33]; therefore, the reaction between CeO<sup>2</sup> and SO<sup>2</sup> could occur, leading to the formation of cerium sulfate Ce(SO4)2, which is stable at high temperature and decomposes between 722 and 843 ◦C to CeO<sup>2</sup> [34].

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

#### *3.1. Catalyst Preparation and Characterization*

The preparation of vanadium-based catalysts supported on ceria with different loading of active phase (2.55, 20, and 50 wt% V2O<sup>5</sup> nominal loading) was described in detail in our previous work [19]. All the reactants were provided by Sigma Aldrich. After the calcination, the sulfuration procedure was carried out in a quartz reactor containing the catalyst to be sulfurized. In particular, the activation step was realized by feeding a gaseous stream containing N<sup>2</sup> and H2S at 20 vol%, by increasing the temperature from ambient temperature up to 200 ◦C with a heating rate of 10 ◦C/min for 1 h. Finally, the catalysts were reduced to the size 38–180 µm. For simplicity, the catalysts are named in the paper as follows: 2.55 V-CeO2, 20 V-CeO2, 50 V-CeO2, where "2.55" means the nominal V loading (wt%) expressed as V2O5. The sulfurized samples before the testing are named "fresh", while they are named "used" after the catalytic activity test.

The catalysts were characterized by nitrogen adsorption at 77 K, Raman spectroscopy and X-ray diffraction. The specific surface area was evaluated with a Costech Sorptometer 1040 (Costech International, Firenze, Italy) by using N<sup>2</sup> and He, respectively, as adsorptive and carrier gas. The powder catalysts were treated at 150 ◦C for 30 min in a He flow prior to testing. A BET method multipoint analysis based on N<sup>2</sup> adsorption/desorption isotherms at 77 K was used to evaluate the specific surface area of the fresh and used catalysts. X-ray diffraction (XRD) was performed using a Brucker D2 Phaser (Germany) using CuKa radiation (λ = 1.5401 A◦ ). Laser Raman spectra of the catalysts were obtained in air with a Dispersive MicroRaman (Invia, Renishaw, Italy), equipped with a 514 nm diode-laser, in the range of 100–2000 cm−<sup>1</sup> Raman shift. The V-content of the fresh catalysts (expressed as V2O<sup>5</sup> wt%) was evaluated by X-ray fluorescence (XRF) spectra by using an ARL QUANT'X EDXRF spectrometer (ThermoFisher Scientific, Italy).

#### *3.2. Experimental Apparatus*

The catalytic activity tests were performed in the laboratory plant schematized in Figure 11.

The laboratory plant is made of three sections: feed, reaction, and analysis sections. The feed stream containing H2S, O2, and N<sup>2</sup> is sent by a three-way valve to the reactor, or in bypass position to the analyzer to verify the composition. All gases came from SOL S.p.A with a purity degree of 99.999% for N2, O2, and SO2, and 99.5% for H2S.

The reaction system comprises a furnace, a reactor, and a sulfur abatement trap. The quartz-made reactor, consisting of a tube of 300 mm length and an internal diameter of 19 mm, is housed in a vertical furnace heated with silicon carbide (SiC)-based resistances. At the bottom of the reactor are a reactant inlet and a thermocouple sheet concentric to the reactor. The catalytic bed is placed in the isothermal zone of the reactor and the temperature is measured continuously by a K-type thermocouple. In the head of the reactor is welded a trap for the sulfur abatement, which is made of an expansion vessel that allows the sulfur to liquefy, involving its separation by the gaseous stream. This trap is maintained at the temperature of 250 ◦C.

**Figure 11.** Scheme of the apparatus plant. **Figure 11.** Scheme of the apparatus plant.

All the lines downstream of the reactor were heated at the temperature of 170 ◦C to avoid sulfur solidification and possible clogging of the mass spectrometer capillary and to maintain the water in the gas phase for the analysis. The analysis of the gaseous stream (H2S, O2, N2, SO2, SO3) was performed with the mass spectrometer quadrupole (Hiden HPR-20) (Warrington, United Kingdom).

be due to the high SO2 formation observed on the support in the absence of the active phase; in fact, among the catalysts, the highest value of SO2 selectivity (~10%) was obtained for the CeO2 at 327 °C as previously reported in Figure 2. The reaction temperature could favor the formation of sulfate species and also the oxygen in the ceria lattice could facilitate the CeO2 sulfuration [33]; therefore, the reaction between CeO2 and SO2 could occur, leading to the formation of cerium sulfate Ce(SO4)2, which is stable at high tem-

The preparation of vanadium-based catalysts supported on ceria with different loading of active phase (2.55, 20, and 50 wt% V2O5 nominal loading) was described in detail in our previous work [19]. All the reactants were provided by Sigma Aldrich. After the calcination, the sulfuration procedure was carried out in a quartz reactor containing the catalyst to be sulfurized. In particular, the activation step was realized by feeding a gaseous stream containing N2 and H2S at 20 vol%, by increasing the temperature from ambient temperature up to 200 °C with a heating rate of 10 °C/min for 1 h. Finally, the catalysts were reduced to the size 38–180 µm. For simplicity, the catalysts are named in the paper as follows: 2.55 V-CeO2, 20 V-CeO2, 50 V-CeO2, where "2.55" means the nominal V loading (wt%) expressed as V2O5. The sulfurized samples before the testing are

The catalysts were characterized by nitrogen adsorption at 77 K, Raman spectroscopy and X-ray diffraction. The specific surface area was evaluated with a Costech Sorptometer 1040 (Costech International, Firenze, Italy) by using N2 and He, respectively, as adsorptive and carrier gas. The powder catalysts were treated at 150 °C for 30 min in a He flow prior to testing. A BET method multipoint analysis based on N2 adsorption/desorption isotherms at 77 K was used to evaluate the specific surface area of the fresh and used catalysts. X-ray diffraction (XRD) was performed using a Brucker D2 Phaser (Germany) using CuKa radiation (λ = 1.5401 A°). Laser Raman spectra of the catalysts were obtained in air with a Dispersive MicroRaman (Invia, Renishaw, Italy), equipped with a 514 nm diode-laser, in the range of 100–2000 cm−1 Raman shift. The V-content of the fresh catalysts (expressed as V2O5 wt%) was evaluated by X-ray fluorescence (XRF) spectra by using an ARL QUANT'X EDXRF spectrometer (ThermoFisher

The catalytic activity tests were performed in the laboratory plant schematized in

perature and decomposes between 722 and 843 °C to CeO2 [34].

named "fresh", while they are named "used" after the catalytic activity test.

**3. Materials and Methods** 

Scientific, Italy).

Figure 11.

*3.2. Experimental Apparatus* 

*3.1. Catalyst Preparation and Characterization* 

Finally, the abatement of unconverted H2S was realized by adsorption on activated carbons loaded in a special vessel having a capacity of 10 Lt. Furthermore, the entire apparatus plant was housed under the hood and isolated from the external environment in order to avoid gas leakage.

The operating conditions of the catalytic activity tests are listed in Table 6.


**Table 6.** Operating conditions.

H2S conversion (x H2S) and the SO<sup>2</sup> selectivity (s SO2) were calculated by using the following relationship (Equations (1) and (2)), by considering the gas phase volume change to be negligible:

$$\text{At H}\_2\text{S}\_\prime\prime\%= (\text{(H}\_2\text{S}^{\text{IN}} - \text{H}\_2\text{S}^{\text{OUT}})/\text{H}\_2\text{S}^{\text{IN}}) \cdot 100\tag{1}$$

$$\text{'s SO}\_2\text{, }\%=\text{(SO}\_2^{\text{OUT}}/\text{(H}\_2\text{S}^{\text{IN}}-\text{H}\_2\text{S}^{\text{OUT}})) \cdot 100\tag{2}$$

For the equilibrium calculation, the *GasEq* program was used, software (0.7.0.9 version, Chris Morley) based on the minimization of Gibbs free energy, which is able to calculate the equilibrium product composition of an ideal gaseous mixture when there are a lot of simultaneous reactions. The thermodynamic analysis was carried out considering the following chemical species that could be present at equilibrium: H2S, O2, SO2, S2, S6, S8, H2O, and nitrogen.

#### Calibration Procedure

The calibration procedure is required in order to measure the concentration of all the species that could be in the gas stream for analysis and, for this reason, it must be performed prior to carrying out experimental tests. However, it could be necessary to repeat

the calibration every time the process conditions are changed (e.g., after the replacement of the capillary, the filaments, change of pressure chamber value) or when the signal seems to be affected by derivative effects. The measurements could be affected by interference due to the presence of ions of different molecules having the same m/z ratio. Each molecule has a matrix of interference, which defines the "weight" of the disturbance of other molecules on the partial pressure of the molecule in the phase of calibration. The partial pressure obtained, net of the relative interference, must be corrected by a response factor, thus returning the actual partial pressure of each molecule in the stream analyzed. At this point, it is possible to calculate the correct concentration of each component. The calibration procedure is characterized by different steps:


After the calibration, which is carried out in a by-pass position, the feed stream can be sent to the reactor. The reactor, before each test, is purged with nitrogen to avoid humidity and/or impurity and is heated up to the reaction temperature at which the feed stream is sent.

Similarly, at the end of the activity test, the reactor is cooled down with nitrogen to room temperature.

#### **4. Conclusions**

The H2S selective oxidation reaction to sulfur and water was investigated over vanadium-sulfide-based catalysts supported on CeO2. The catalysts were prepared with different vanadium loading and were characterized before and after the catalytic tests with different techniques. X-ray diffraction and Raman analysis showed a good dispersion of the V-species on the support because the V-sulfide presence was not detected on the different catalysts. The only vanadium sulfide in VS<sup>2</sup> form was observed for the bulk V2O<sup>5</sup> after the catalytic tests. Furthermore, the presence of the sulfur was observed especially over the used catalysts at lower V-loading by Raman and SSA analysis.

From the preliminary screening of the catalysts performed at 327 ◦C, the higher catalytic activity was observed over the 2.55 V-CeO<sup>2</sup> and 20 V-CeO<sup>2</sup> catalysts, with H2S conversion, respectively, of 90% and 92%, and SO<sup>2</sup> selectivity of ~4%. No SO<sup>3</sup> formation and catalyst deactivation phenomena by the sulfur deposition were observed. The effect of the temperature, contact time, and H2S inlet concentration was studied over 20 V-CeO<sup>2</sup> catalysts. By increasing the H2S inlet concentration (up to 15 vol%), the conversion decreased from 86% to 75% with an SO<sup>2</sup> concentration of about 1.5 vol%. The effect of the contact time was almost negligible on the H2S conversion and SO<sup>2</sup> selectivity, while the temperature had a significant influence. In the range of temperatures investigated (300–370 ◦C), the H2S conversion was very close to the equilibrium values while the SO<sup>2</sup> selectivity was below the equilibrium calculation, evidencing that the catalyst is effectively able to inhibit SO<sup>2</sup> formation.

Based on the obtained results, the ceria-supported vanadium catalysts could be considered good candidates to carry out the selective oxidation of H2S to sulfur by an H2S-lean gas stream (e.g., natural gas, Claus process tail gas) at very low temperature.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/ 10.3390/catal11060746/s1: Figure S1: Raman Spectra of CeO2, V2O5 and 2.55, 20, 50 V-CeO2 fresh catalysts; Figure S2: Raman Spectra of fresh and used catalysts CeO2 (a), 2.55 V-CeO2 (b), 20 V-CeO2 (c), 50 V-CeO2 (d).

**Author Contributions:** Conceptualization, D.B. and V.V.; methodology, D.B.; software, D.B.; validation, D.B. and V.V.; formal analysis, V.P.; investigation, D.B.; resources, V.V. and V.P.; data curation, V.P.; writing—original draft preparation, D.B.; writing—review and editing, V.V.; visualization, V.P. and V.V; supervision, V.P.; project administration, D.B.; funding acquisition, V.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

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

#### **References**

