Enhanced Hydrolysis of Carbonyl Sulfide in Coking Oven Gas Utilizing an Efficient Ca-Ba-γ-Al₂O₃ Catalyst

Abstract

China possesses a substantial capacity for coke production, resulting in the annual generation of over 100 billion standard cubic meters of the by-product coke oven gas. The comprehensive utilization of this gas has emerged as a matter of significant concern within the coking industry. The removal of carbonyl sulfide (COS) from coke oven gas is crucial for enhancing gas quality, mitigating equipment corrosion, minimizing environmental pollution, elevating the quality of recovered products, and fostering the production of high-quality steel. A novel Ca-Ba-γ-Al₂O₃ catalyst has been devised, employing γ-Al₂O₃ as the catalyst matrix and integrating calcium hydroxide (Ca(OH)₂) alongside barium hydroxide octahydrate (Ba(OH)₂·8H₂O) as the alkaline activating components. The impact of various factors, including reaction temperature, humidity, and the number of activating components loaded, on the hydrolysis efficiency of COS has been meticulously investigated. Furthermore, the catalytic reaction mechanism has been elucidated utilizing advanced characterization techniques such as X-ray diffraction (XRD) and Brunauer–Emmett–Teller (BET) analysis. The outcomes of this research reveal that, under optimal conditions of a reaction temperature of 55 °C and a humidity of 56%, the Ca-Ba-γ-Al₂O₃ catalyst achieves a remarkable COS hydrolysis efficiency of 95.22%.

1. Introduction

Coke production generates a significant quantity of coke oven gas as a by-product, with approximately 300 to 350 cubic meters of coke oven gas produced per ton of dry coal during the coking process. This gas primarily consists of hydrogen and methane, which account for 77% to 94% of its composition. Both hydrogen and methane are high-calorific-value gases, with a calorific value ranging from 16 to 40 MJ/kg, compared to natural gas, which has a calorific value of 38.46 MJ/kg [1]. After removing impurities, the net coke oven gas can be utilized in various applications, including as industrial and city gas, in power generation, hydrogen production, natural gas production, methanol synthesis, and the direct reduction of iron. In recent years, global pig iron production has averaged more than 1.3 billion metric tons annually [2]. Despite China’s dominance in global iron and steel production, accounting for 50% to 60% of the total, the direct reduction in iron is limited, representing less than 10% of the worldwide production. This shortfall is largely attributed to insufficient access to cost-effective gas resources [1,3,4]. Carbonyl sulfide (COS), a toxic substance present in coke oven gas, poses significant risks as it can severely corrode equipment. Furthermore, its atmospheric emissions can convert to sulfur dioxide (SO₂), leading to the formation of sulfate aerosols and contributing to environmental pollution [5,6,7,8,9]. In the petrochemical sector, effective COS removal is crucial to mitigate excessive sulfur levels during subsequent processing and to prevent catalyst poisoning.

Current strategies for the dry removal of COS involve various adsorbents, including metal oxides, alumina, carbon-based carriers, mixed-metal oxides, metal ion-exchanged zeolites, and activated carbon [10,11,12,13,14]. For instance, Kim et al. [15] showed that loading 20 wt% K+ onto activated carbon enhances COS adsorption, achieving a maximum of 56.3 mg/g of adsorbent at 210 °C. Additionally, Qiu et al. [16] modified activated carbon with the help of Cu, Co, and K ion impregnation, and the removal of COS reached 43.34 mg/g adsorbent at 60 °C, 30% humidity, and 1.0% oxygen. Dry desulfurization techniques are primarily employed for fine desulfurization applications, which necessitate meeting standards that require a COS outlet concentration of no greater than 10 parts per million (ppm). These processes typically handle imported gas mixtures with relatively high COS concentrations and are suited for low-sulfur gases. Furthermore, their operation at generally elevated temperatures gives rise to heightened heat consumption and heat losses during the desulfurization process.

Although COS is chemically stable, it can undergo redox reactions, decomposition, and hydrolysis under certain conditions [17]. Wet removal techniques leverage the high solubility of COS in amine solutions alongside hydrolysis reactions. Common solvents for this method include diethanolamine (DEA), diisopropanolamine (DIPA), and N-methyldiethanolamine (MDEA) [18]. Zoltán et al. [19] found that the chemical adsorption of COS on n-propylamine-modified porous silica had very little effect on the chemical adsorption capacity of CO₂, and the porous silica material did not undergo irreversible reactions during the adsorption process. Seo et al. [20] used methanol to absorb H₂S and COS in gas mixtures and were able to increase the removal of COS from 80% to 95%. The absorption rate of COS in aqueous amine solutions was determined using a fixed gas–liquid interface reactor. It was found that the absorption performance of COS from aqueous MDEA solution was lower than that reported in the temperature range of 313–353 K, and the mass fraction of alkanolamine bounded at 0.05–0.5 [18]. Zhang et al. [21] prepared a novel absorbent (USD-2) by using MDEA as the base solvent and adding appropriate amounts of five-membered sulfur-containing heterocyclic compounds (SULs) and cycloaliphatic amines (CAs). The removal rate of COS by this absorbent was 42.4% higher than that of pure MDEA. Lammers et al. [22] introduced 25–30 wt% polyhydroxy alcohol (PHA) and glycerol into an aqueous MDEA solution to prepare a novel absorbent, which showed a significantly higher rate of COS uptake and reaction as compared to the aqueous MDEA solution. In addition to the absorption method, COS catalytic hydrolysis also has unique advantages. The hydrolysis reaction of COS occurs at a low rate constant of 0.0011 s⁻¹ at 25 °C without a catalyst [23]. The key to the hydrolysis method for COS removal is the choice of catalyst. Most of the patents use γ-Al₂O₃ as the basic carrier, and the efficiency of the catalyst for COS hydrolysis is improved by loading alkaline oxides. γ-Al₂O₃ is a porous material with a large internal surface area, insoluble in common solvents, such as water, and with very high compressive and heat resistance. γ-Al₂O₃ can be converted to α-Al₂O₃ when the temperature is higher than 1200 °C. Huang et al. [24] investigated the catalytic hydrolysis reaction of γ-Al₂O₃ on COS at 220 °C and found that the presence of singlet S is trapped in the catalyst in the absence of H₂/CO/CO₂ and verified that formic acid does not accumulate on the catalyst during this reaction. Wang et al. [25] prepared La₂O₃ -Co₂O₃-Al₂O₃ and LaCoO₃/γ-Al₂O₃ catalysts by co-precipitation and sol–gel ultrasound impregnation methods and carried out the catalytic hydrolysis of COS, and their results showed that the catalytic efficiency of LaCoO₃/γ-Al₂O₃ was higher than that of La₂O₃-Co₂O₃-Al₂O₃, which was mainly related to the chalcocite crystalline form of the LaCoO₃ γ-Al₂O₃ catalyst. Hence, the selection of appropriate transition metal active components and the development of a catalyst featuring superior performance, utilizing γ-Al₂O₃ as the support, holds paramount importance in the research pertaining to low-temperature COS hydrolysis [26].

The primary objective of this research endeavor is to develop an exceptionally efficient catalyst for the hydrolysis of COS, employing γ-Al₂O₃ of 80–150 mesh size as the supportive matrix. The investigation will delve into the influence of temperature, humidity, and the carrier’s compositional makeup on the effectiveness of the hydrolysis process at relatively low temperatures. Furthermore, this study aims to clarify the underlying reaction mechanism through the application of various characterization techniques.

2. Experimental

2.1. Material Synthesis

The catalysts were synthesized through two distinct methodologies: mechanical grinding and ultrasonic impregnation. Following these synthesis processes, two distinct drying techniques were employed: high-temperature calcination within a muffle furnace and low-temperature drying within an oven. The specific procedures undertaken are outlined as follows.

γ-Al₂O₃ was combined with a specified quantity of the carrier in a ball milling jar. Quartz balls of various sizes were added to fill half of the jar, followed by deionized water up to three-quarters of the jar’s capacity. Once assembled, the jar was placed in a ball mill, which operated continuously at a speed of 285 r/min for four hours. After milling, the quartz balls were removed, and the catalyst was dried in an oven at 60 °C for approximately 24 h until completely dried. This catalyst was then divided into two groups: one for immediate experimental investigation and the other subjected to high-temperature calcination at 400 °C in a muffle furnace before further research.

In the ultrasonic impregnation method, γ-Al₂O₃ and the quantitative carrier (LiOH, Na₂CO₃, Ni(CH₃COO)₂, Ba(OH)₂·H₂O, Ba(OH)₂·8H₂O, Ca(OH)₂) were placed in a beaker, and deionized water was added until it covered the catalyst. This beaker was then positioned in an ultrasonic mixer, ensuring that the water level in the mixer was above that in the beaker to optimize mixing efficiency. The mixture was subjected to heating and ultrasonic agitation for about two hours while maintaining a moderate temperature to enhance the solubility of the carrier. Following this process, the catalysts were dried in an oven at 60 °C and similarly divided into two groups, repeating the procedures from the first method.

To ensure accurate material preparation, particular attention was given to the properties of γ-Al₂O₃. As a hard, spherical, and insoluble material with a small particle size, it may adhere to the container walls after drying. To counteract potential loss during preparation, the weight of the carrier was adjusted to be 0.001 to 0.002 g above the calculated standard, ensuring the loss rate remained below 1%. The calculation for the loss rate is as follows.

$$\mathrm{Material} \mathrm{loss} \mathrm{rate}={\displaystyle \frac{\mathrm{m}(\mathrm{After} \mathrm{drying} ,\mathrm{the} \mathrm{resdual} \mathrm{catalyst} \mathrm{in} \mathrm{beaker})-\mathrm{m}(\mathrm{Net} \mathrm{beaker})}{\mathrm{m}(\mathrm{Added} \mathrm{chemicals} \mathrm{before} \mathrm{catalyst} \mathrm{preparation})}}\times 100\%$$

(1)

2.2. Experimental Instrument

The instruments employed in the current experiment, along with the fundamental details of the characterization instrument, are presented in

Table 1. Basic information on experimental instruments.

Name	Model Number	Manufacturer
Desktop ultrasonic cleaning machine	PS-30T	Guangdong Foheng Instrument Co., LTD, Foshan, China
Heat collection type constant-temperature heating magnetic stirrer	DF-101S	Zhengzhou Keda Mechanical instrument Equipment Co., LTD, Zhengzhou, China
Carbonyl sulfide detector	APEQ-DCOS-2N	Shenzhen Ampal Technology Co., LTD, Shenzhen, China
Hydrogen sulfide test analyzer	CGM10-70	Shenzhen Angwei Electronics Co., LTD, Shenzhen, China
Hygrograph	AS108D	Guangzhou Aosong Electronics Co., LTD, Guangzhou, China
Box-type resistance furnace	XH4L-16	Zhengzhou Xinhan Instrument Equipment Co., LTD, Zhengzhou, China
Electric oven	101-1A	Beijing Zhongxing Weiye Century Instrument Co., LTD, Beijing, China
Planetary ball mill	KE-0.4L	Qidong Honghong instrument equipment factory, Qidong, China
Mass flow controller	D07	Beijing seven star Huachuang flowmeter Co., LTD, Beijing, China
Rotor flowmeter	LZB-2	Changzhou double ring thermal instrument Co., LTD, Changzhou, China
Electronic analytical balance	FA1604A	Shanghai Jingtian Electronic Instrument Co., LTD, Shanghai, China

and

Table 2. Characterizing instrument information.

Method	Manufacturer	Model Number	Test Parameter
BET	Beijing Jingwei Gao Bo Science and Technology Co., LTD, Beijing, China	JW-BK122W	1 atmosphere, 77K, drying for 8 h, 473.15K
XRD	Bruker, Ettlingen, Germany, DE	Bruker D8	Voltage 40 KV, current 40MA, test range 5–90 degrees, Pace 0.02; the target material is Cu target.

, respectively.

2.3. Experimental Process

Figure 1 illustrates the schematic diagram of the COS hydrolysis experiment. COS gas (0.1% concentration) was blended with nitrogen gas in a mixing flask using a mass flow controller to achieve a COS concentration of 200 ppm. This mixed gas was then passed into a single-neck flask containing 100 mL of deionized water to introduce water vapor. An electric thermostatic water bath was utilized to regulate the temperature and maintain the desired humidity level. The outlet gas from the single-neck flask was directed into a reactor, which consisted of an 8 × 200 mm quartz tube, secured in an upright position by an iron frame. The quartz tube was wrapped with temperature-controlled tracer tape to maintain the reaction temperature, while quartz cotton was placed at the bottom of the reactor for stability. Following the reaction, the outlet gas was dried using a drying tube before entering a gas-washing cylinder containing a zinc acetate solution (0.27 mol/L). To ensure the accuracy of the COS analyzer, which detects trace amounts of hydrogen sulfide (H₂S) generated during the hydrolysis of COS, it was necessary to remove H₂S from the gas mixture without altering the COS concentration [27]. The zinc acetate solution effectively captures H₂S while having a negligible impact on COS adsorption. To protect the COS analyzer, an anti-suckback bottle was connected to the gas-washing cylinder to prevent any foamy substances from the reaction from moving back into the pipeline. The COS analyzer was linked to a computer to record data at one-second intervals. Additionally, the tail gas treatment system included an anti-backsiphonage bottle and a wide-mouth container filled with sodium hydroxide (NaOH) solution, which efficiently absorbed both COS and H₂S.

2.4. Evaluation Criteria

The total formula of the COS catalytic hydrolysis reaction is as follows [27]:

$$\mathrm{COS}+{\mathrm{H}}_2\mathrm{O}\rightleftarrows {\mathrm{H}}_2\mathrm{S}+{\mathrm{CO}}_2 \Delta \mathrm{H}=-34\mathrm{kJ}/\mathrm{mol}$$

(2)

COS hydrolysis efficiency is calculated as:

$$\mathsf{\eta }={\displaystyle \frac{\mathrm{c}1-\mathrm{c}0}{\mathrm{c}1}}\times 100\%$$

(3)

According to the above equations, if the hydrolysis reaction proceeds to completion, the molar ratio of hydrogen sulfide (H₂S) to carbonyl sulfide (COS) produced from the hydrolysis reaction is expected to be 1:1. During the experiments, we observed instances where the concentration of COS detected by the analyzer fell to zero, indicating 100% hydrolysis efficiency. However, the amount of COS hydrolyzed, as inferred from the H₂S concentration detected by the H₂S analyzer, did not always reflect this complete hydrolysis efficiency. This discrepancy may be attributed to the adsorption of COS within the catalyst or the slight solubility of COS in water. As quantifying this error is challenging, we presented hydrolysis efficiencies as characterized by both the outlet concentrations of COS and H₂S.

3. Results and Discussion

3.1. Catalyst Support Experiment

Previous studies [28,29] have indicated that both alkaline conditions and the loading of alkali metals play significant roles in the catalytic hydrolysis of COS, suggesting that the reaction likely occurs at alkaline sites. Most patents utilize γ-Al₂O₃ as the primary support material due to its advantageous properties, including superior mechanical strength, heat resistance, a larger specific surface area, and a porous structure that enhances the rate of the hydrolysis reaction. In this work, γ-Al₂O₃ with a mesh size of 80 to 150 was employed as the catalyst support. To assess its effectiveness, we evaluated the hydrolysis efficiency of γ-Al₂O₃ with regard to COS and identified the factors that influence this efficiency prior to the addition of other active components. The impacts of reaction temperature and humidity on the hydrolysis efficiency are illustrated in Figure 2.

Figure 2a illustrates the impact of reaction temperature on the catalytic hydrolysis of COS using γ-Al₂O₃. The data reveals that when the reaction temperature is between 40 °C and 60 °C, the hydrolysis efficiency exhibits an irregular pattern as the temperature increases. Renda et al. [30] also found in the experiment of low-temperature catalytic hydrolysis of COS by Al₂O₃ that COS hydrolysis is always promoted by increasing temperature, but the trend of hydrolysis efficiency is different at 50–70 °C, showing better catalytic hydrolysis performance at 50 °C. This shows that there is indeed an unusual trend when catalyst γ-Al₂O₃ catalyzes COS hydrolysis at a series of low temperatures. Due to different experimental conditions, the optimal catalytic temperature measured in this experiment is 55 °C. In this experiment, a water bath was used to maintain the temperature, which also influenced the system’s humidity. Before conducting the experiments, gases with consistent flow rates and component compositions were introduced into the system, while a hygrometer monitored the humidity at various water bath temperatures to ensure accurate humidity readings. The fluctuations in the hydrolysis efficiency may result from discrepancies between the humidity controlled by the water bath and the actual reaction temperature. Figure 2b displays the effect of humidity on the catalytic hydrolysis efficiency of COS by γ-Al₂O₃. The results indicate that at lower water content, increasing humidity promotes the hydrolysis reaction. However, efficiency does not continuously improve with higher humidity levels. Once the humidity reaches an optimal point, further increases in water vapor content hinder COS hydrolysis. This reduction occurs because the excess water vapor can form a film on the catalyst’s surface, impeding COS from accessing the active sites and significantly reducing the catalyst’s effective surface area, which ultimately decreases hydrolysis efficiency. This observation aligns with the findings by Huang et al. [29], which confirm that increased water vapor content adversely affects COS hydrolysis efficiency.

The relationships between COS concentration and H₂S concentration over time during the catalytic hydrolysis process using γ-Al₂O₃ at a reaction temperature of 55 °C and 56% humidity are shown in Figure 3. The hydrolysis efficiencies calculated from the peak values of the COS–time and H₂S–time relationships were found to be 74.75% and 53.35%, respectively. This significant discrepancy suggests that a considerable amount of COS is not only adsorbed and hydrolyzed but also converted into H₂S. However, both methods suggest that the catalytic hydrolysis of COS using γ-Al₂O₃ alone is insufficient and that additional active components are necessary for improved performance.

3.2. Active Component on the COS Hydrolysis Efficiency

Previous research has demonstrated that alkaline environments, along with the presence of alkali metals and alkali salts, enhance the COS hydrolysis reaction. In this study, several materials were selected to load onto γ-Al₂O₃ carriers for catalyst preparation. The candidate materials included LiOH, Na₂CO₃, Ni(CH₃COO)₂, Ba(OH)₂·H₂O, Ba(OH)₂·8H₂O, and Ca(OH)₂. Five catalysts were formulated by applying 5% of each alternative carrier material onto γ-Al₂O₃. For the experiments, 3 g of each catalyst was placed in the center of a quartz fixed-bed reactor. The drying tube was filled with anhydrous calcium chloride, and the reaction conditions were set with a COS concentration of 200 ppm, a gas flow rate of 200 mL/min, a reaction temperature of 55 °C, and a humidity level of 56%. The results of the hydrolysis are presented in Figure 4.

It can be concluded from Figure 4 that Ni(CH₃COO)₂ and Na₂CO₃ can create a base environment, but the catalytic effect is much less than that of the alkali metal compounds. Among the tested alkali metal compounds, Ca(OH)₂ and Ba(OH)₂·8H₂O proved to be the most effective. Consequently, these two compounds were selected as active components for the preparation of Ca-Ba-γ-Al₂O₃ catalysts in subsequent experiments. Regarding catalyst preparation, the initial assessment encompassed both the mechanical grinding technique and the high-temperature calcination process conducted within a muffle furnace. However, multiple failed attempts revealed that these methods led to clogging in the fixed-bed reactor, causing equipment malfunctions. To address this issue, ultrasonic impregnation was adopted as the mixing method, while a low-temperature, long-duration drying process in a vacuum constant-temperature oven was selected, particularly because Ca(OH)₂ is thermally unstable.

The experimental results for the catalytic hydrolysis of COS using catalysts with 5% additions of Ca(OH)₂ and Ba(OH)₂·8H₂O are shown in Figure 5. As can be seen from Figure 5, the hydrolysis efficiency of COS was 89.29% when the 3 g carrier was loaded with 5% Ca(OH)₂ and 86.17% when it was loaded with 5% Ba(OH)₂·8H₂O. The loading effect of Ba(OH)₂·8H₂O was slightly higher than that of Ba(OH)₂·H₂O. This disparity could be attributed to the octahydrate’s higher content of dissociable OH⁻ ions.

3.3. Preparation of Ca-Ba-γ-Al₂O₃ Catalyst

Figure 6 shows the performance of the Ca-Ba-γ-Al₂O₃ catalyst in the catalytic hydrolysis of COS. To prepare the Ca-Ba-γ-Al₂O₃ catalysts, it was essential to first assess the effect of varying amounts of Ca(OH)₂ on the hydrolysis efficiency of catalytic hydrolysis of COS. In this experiment, five catalysts were prepared with 1%, 2%, 3%, 4%, and 5% additions of Ca(OH)₂, and the resulting COS hydrolysis efficiencies, as measured by the H₂S detection analyzer, are illustrated in Figure 6a.

As shown in Figure 6a, the hydrolysis efficiency of COS increased with the increase carrier at ≤3% of Ca(OH)₂, but the hydrolysis efficiencies at 4% and 5% were lower than that at 3% of carrier, which indicated that it was not more effective with the increase in carrier, and the solubility of Ca(OH)₂, an inorganic compound that is slightly soluble in water, decreased with the temperature, and the solubility was only 1.76 g/L at 10 °C. In the ultrasonic impregnation step of the catalyst preparation, it was necessary to put ice in the ultrasonic water tank to maintain the low temperature and add more deionized water into the beaker to maintain the low temperature and the solubility of the catalyst. The solubility of Ca(OH)₂ decreases with the increase in temperature, and the solubility is only 1.76 g/L at 10 °C. In the ultrasonic impregnation step of catalyst preparation, the solubility of Ca(OH)₂ was improved by putting ice cubes in the ultrasonic water tank to maintain the low temperature and adding more deionized water into the beaker. The higher the addition amount, the lower the uniformity of impregnation during the preparation of catalysts, which may be the reason for the lower efficiency of catalytic hydrolysis of COS.

Figure 6b reveals that the maximum hydrolysis efficiencies obtained from the COS–time and H₂S–time relationships were 90.38% and 73%, respectively. While the addition of Ca(OH)₂ significantly enhanced hydrolysis efficiency, the removal rate of 90.38% was still considered low. Consequently, Ba(OH)₂·8H₂O was added on top of the existing 3% Ca(OH)₂ to further enhance the hydrolysis efficiency. Subsequently, five additional catalysts were prepared with varying contents of Ba(OH)₂·8H₂O at 1%, 2%, 3%, 4%, and 5%, after combining these with γ-Al₂O₃ and the initial 3% Ca(OH)₂. The effects of the Ba(OH)₂·8H₂O addition on the catalytic hydrolysis of COS, along with the exit COS concentration over time at a reaction temperature of 55 °C and 56% humidity, are depicted in Figure 6c. It demonstrates that the hydrolysis efficiency of the catalysts prepared with Ba(OH)₂·8H₂O was consistently higher than that of the catalyst containing only 3% Ca(OH)₂. The optimal combination was found in the catalyst with 3% Ca(OH)₂ and 2% Ba(OH)₂·8H₂O, achieving a hydrolysis efficiency of 95.22%. In the experiment of COS hydrolysis by Cao et al. [31], the modified γ-Al₂O₃ catalyst with 5% NaOH was prepared, and the efficiency of COS hydrolysis was 91%, which was slightly lower than that of Ca-Ba-γ-Al₂O₃ catalyst prepared in this experiment, as illustrated in the concentration–time relationship for COS efficiency. A comparative analysis of the catalytic effects of the single addition of 3% Ca(OH)₂ versus the Ca-Ba-γ-Al₂O₃ catalyst is shown in Figure 6d.

3.4. Characterization Results and Discussion

The experimental results indicate that γ-Al₂O₃ exhibits a noticeable catalytic effect on the hydrolysis reaction of carbonyl sulfide (COS), with significantly improved catalytic efficiency after the introduction of Ca(OH)₂ and Ba(OH)₂·8H₂O. To further investigate the structural changes in the catalyst before and after the experiments, two characterization methods (X-ray diffraction (XRD) and Brunauer–Emmett–Teller (BET)) surface area analysis were employed. These methods assessed various structural properties of the catalysts, including specific surface area, pore volume, and pore size. The BET characterization results for the two catalysts are summarized in

Table 3. Catalyst structural properties.

Sample	Single Point BET Specific Surface Area (m2g−1)	Pore Volume (cm3g−1)	Aperture (nm)
Pure γ-Al2O3	57.494	0.162	12.765
Ca-Ba-γ-Al2O3	50.646	0.161	14.416

.

From the BET analysis, it is evident that loading the carrier material with the two compounds resulted in a slight decrease in both surface area and pore volume, while the pore diameter showed improvement. The minor decline in surface area and pore volume of γ-Al₂O₃ after incorporating the active components may be attributed to the attachment of Ca and Ba compounds to the carrier. The limited change in these properties suggests that structural factors alone do not account for the enhanced performance of the catalysts.

The XRD characterization results for γ-Al₂O₃ and Ca-Ba-γ-Al₂O₃, both before and after the COS catalytic hydrolysis experiment, are displayed in Figure 7.

Figure 7 illustrates that the peak intensities and positions remain largely consistent before and after the catalytic reaction, with no new substances observed post-reaction. The presence of Al(OH)₃ both pre- and post-experiment suggests that the γ-Al₂O₃ purchased was not of sufficient purity and may have been contaminated with traces of Al(OH)₃. Should the outcomes of this experiment prove applicable to pilot testing in chemical industries, γ-Al₂O₃ with a purity of at least 99% could be chosen as the carrier. This figure also confirms that no new substances are formed in the Ca-Ba-γ-Al₂O₃ catalyst before or after the reaction. The detected Al(OH)₃ indicates the presence of initial impurities in the carrier. Moreover, the presence of Ca in the characterization results does not imply that Ca(OH)₂ was present before and after the reaction. Instead, it suggests that a minor fraction of free Ca²⁺ may have combined with other ions during the loading process. Notably, neither Ca(OH)₂ nor Ba(OH)₂·8H₂O were identified in the XRD analyses, likely due to the relatively small quantity added compared to the mass of γ-Al₂O₃. This finding also suggests that the ultrasonic impregnation method employed leads to more homogeneous mixing and effective loading.

3.5. Analysis of COS Hydrolysis Mechanism

Early studies [32,33,34,35] have suggested that during the COS-catalyzed hydrolysis reaction, the catalyst surface adsorbs water (H₂O) molecules from the vapor phase. The dissociation of these water molecules generates hydroxide ions (OH⁻), which then cover the catalyst surface. Under specific conditions, COS interacts with these OH⁻ ions to produce thiocarbonates, indicating that the alkaline center acts as the active site for the hydrolysis reaction, as illustrated in the equations below. The reaction mechanism we propose is depicted in Figure 8, which shows that the reactions occur both in the flask and within the water film on the catalyst surface.

$${\mathrm{COS}+\mathrm{OH}}^{-}\stackrel{{\mathrm{Al}}_2{\mathrm{O}}_3}{\to }{{\mathrm{HSCO}}_2}^{-}$$

(4)

$${{\mathrm{HSCO}}_2}^{-}\stackrel{{\mathrm{OH}}^{-}{,\mathrm{H}}_2\mathrm{O}}{\to }{\mathrm{H}}_2\mathrm{S}+{\mathrm{CO}}_2$$

(5)

Liu et al. [32] prepared α-Al₂O₃ and γ-Al₂O₃ catalysts by high-temperature calcination and used them to catalyze the hydrolysis of COS. They found that the thiocarbonate ions formed by the reaction of COS with OH⁻ would generate bisulfite ions and bicarbonate ions with the participation of O₂, and these two ions would generate CO₂ with the participation of O₂. Furthermore, the presence of oxygen vacancies within the catalyst could also contribute significantly to the facilitation of the reaction. This is attributed to the fact that the oxygen vacancies in perovskite-based mixed-metal oxides enhance the adsorption of O₂ molecules, thereby augmenting the catalyst’s activity [36,37]. Zhang et al. [33] found that the effect of water vapor on the hydrolysis reaction of COS was not the more, the better. The water film formed by too many H₂O molecules dispersed the alkaline center, resulting in COS molecules not being easily aggregated towards the alkaline center. The alkaline center is critical to the COS hydrolysis reaction, with OH⁻ ions playing a vital role in forming and decomposing intermediates during the process. Our experimental results indicate that relying solely on OH⁻ ions dissociated from the surrounding water vapor is inadequate. The introduction of Ca(OH)₂ and Ba(OH)₂·8H₂O significantly enhances the hydrolysis efficacy, as these compounds provide a greater supply of OH⁻ compared to other alkali metal compounds. However, excessive loading can be detrimental, as higher concentrations may reduce the available reaction sites on the catalyst. Additionally, H₂S produced from the hydrolysis of COS can oxidize to form sulfate and sulfur monomers in the presence of oxygen. Accumulation of sulfur monomers on the catalyst surface leads to sulfur-induced catalyst deactivation. Interestingly, our XRD characterization of the catalyst before and after the reaction did not detect any elemental sulfur, suggesting that the catalyst exhibits good resistance to sulfur poisoning, thereby enhancing its operational lifespan.

4. Conclusions

This study focused on the preparation of a Ca-Ba-γ-Al₂O₃ catalyst for the hydrolysis of carbonyl sulfide (COS) by loading Ca(OH)₂ and Ba(OH)₂·8H₂O onto γ-Al₂O₃ using an ultrasonic impregnation method. The catalyst was subsequently dried in a low-temperature oven over an extended period. The mechanism of COS hydrolysis was elucidated through catalyst characterization results. The optimal conditions for COS hydrolysis were determined to be at a temperature of 55 °C and a humidity level of 56%. Experimental trials aimed at identifying the optimal proportions of the catalyst components revealed that incorporating 3% Ca(OH)₂ and 2% Ba(OH)₂·8H₂O into the Ca-Ba-γ-Al₂O₃ catalyst achieved the highest catalytic efficiency of 95.22%. Further characterization indicated that the surface area, pore volume, and pore diameter of the Ca-Ba-γ-Al₂O₃ catalyst were enhanced compared to previously used catalysts. Additionally, XRD analysis did not detect any new substances or sulfur elements, underscoring the catalyst’s excellent resistance to poisoning and its extended operational lifespan.