Influence of the preparation conditions of MgAlCe catalysts on the catalytic hydrolysis of carbonyl sulfide at low temperature

Abstract

MgAlCe hydrotalcite-like compounds (HTLCs) were synthesized by following co-precipitation methods based on different proportions of Al : Ce. Metal oxides derived from HTLCs were tested in the catalytic hydrolysis of carbonyl sulfide (COS) at the relatively low temperature of 50 °C. The removal effect of COS strongly correlated to the proportion of Al : Ce, pH during synthesis and calcination temperature. Hydro-thermal temperatures were applied to improve the crystallinity of the materials and the catalysts were characterized by X-ray diffraction (XRD), thermogravimetry (TG) and Brunauer–Emmett–Teller (BET) methods. These results may contribute to a more full understanding of the effect of preparation conditions on the hydrolysis of COS. The optimum preparation conditions were found to be a 16 : 1 ratio of Al : Ce, a pH of 8 during synthesis, a hydro-thermal temperature of 140 °C, and a calcination temperature of 600 °C. The catalyst prepared under these conditions demonstrated an impressive efficiency (100%) sustained for approximately 80 minutes at a relatively low temperature of 50 °C and space velocity of 5000 h⁻¹.

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Introduction

Carbon monoxide (CO) is a C₁ (one carbon) feed gas known in the chemical industry as a large constituent of yellow phosphorous tail gas. Carbonyl sulfide (COS), however, is a harmful gas which may adversely affect the use of CO,^1,2requiring its removal. Several removal methods exist for COS, including adsorption, reduction, photolysis and oxidation, etc.³ A promising future exists for the hydrolysis method as it involves modest reaction conditions to achieve high removal efficiency.⁴ Simple water vapor in yellow phosphorus tail gas is effective in promoting the occurrence of hydrolysis. Catalytic hydrolysis of COS follows the reaction: COS + H₂O = CO₂ + H₂S.⁵ Development of an efficient catalyst for low temperature hydrolysis is necessary, however, due to the limitations of low reaction temperatures and low COS concentrations.⁶ Most COS hydrolysis catalyst types researched to date^7–9 are based on metal oxides and activated carbons with different promoters, such as alkali metal oxides, metallic oxides or transition metal oxides, etc.^10–12 Wang et al. introduced an Al₂O₃ catalyst with a COS hydrolysis conversion rate of 51.2%.¹³ The catalyst sulphate resistance, however, is poor and the COS removal effect is low. Colin Rhodes et al.¹⁴ investigated rare earth metal oxides for the removal of COS, and discovered that the presence of SO₂ decreased catalyst service life and reduced overall catalyst activity. An excellent catalyst potentially exists in using this method if successful measures can be understood and applied in achieving sulphate resistance for overall improvement in COS hydrolysis activity.

Hydrotalcite-like compounds (HTLCs) were recently discovered when the unique hydrotalcite structure was encountered in nature.¹⁵ When screening for catalysts, the mixed metal oxides originating from HTLCs were found to be a family of anionic clays containing a unique catalyst structure.¹⁶ The general formula of HTLCs is [M_1−x²⁺M_x³⁺(OH)₂]^x+[A_x/nⁿ⁻·yH₂O]^x− (an empirical formula), among which M²⁺ and M³⁺ are metal cations, Aⁿ⁻ is an n-valent anion, and the molar ratio of M²⁺ and M³⁺ is between 2 and 5.¹⁷ The general formula offers various compositions for HTLCs leading to a potentially vast selection of catalysts. There are challenges in seeking functional, efficient catalysts utilizing HTLCs as HTLCs lose crystal water, interlayer anions and hydroxyl groups when calcined at high temperature. The hydrotalcite-like layer structure will, as a result, be partially destroyed, and the inter-surface area will increase. In a related study, Wang et al.¹⁸ found that CoNiAl hydrotalcite-like compounds modified by cerium exhibited an improved COS removal effect, but experienced a relatively low service life and had a difficult preparation process. Ramesh Chitrakar et al.¹⁹ found that the uptake properties of phosphate on a Zr-modified MgFe–LDH(CO₃) (LDH = layered double hydroxide) improved removal efficiency, but also experienced a relatively low service life. Based on previous findings, cost-effective MgAl metals were selected to examine the application of MgAl hydrotalcite-like compounds modified by adding cerium for improving COS hydrolysis activity and increasing service life.

Previous studies revealed that mixed oxides derived from HTLCs produced satisfactory results in the hydrolysis of COS.²⁰ MgAl hydrotalcite modified by adding cerium in varying proportions of Al : Ce is examined in this study. Optimum preparation conditions were determined, including the proportion of Al : Ce, the pH during the synthesis, the hydro-thermal temperatures and calcination temperatures, forming a complete experimental process.

Experimental section

Catalyst preparation

MgAlCe–LDH was synthesized as follows: Al(NO₃)₃·9H₂O, Ce(NO₃)₂·6H₂O and Mg(NO₃)₂·6H₂O were added to distilled water in turn, and stirred continuously (solution A). NaOH and Na₂CO₃ were then added into a separate portion of distilled water with continuous stirring (solution B). The pH of solution A was adjusted through addition of B. The resulting suspension was heated in a water bath for one day. Subsequently, the precipitate was separated by suction filtration and washed with deionized water until the pH was 7. The acquired HTLC samples were denoted as Mg₂Al_xCe_1−x, with x and (1 − x) as values combining the molar ratio of Al : Ce (=2, 4, 8, 16) with (Al + Ce) equal to 1. Finally, it was calcined with oxygen in a roaster for 3 h.

Characterization

An X-ray powder diffraction pattern was obtained using an X-ray diffractometer (XRD) equipped with Ni-filtered Cu K_α radiation (λ = 0.15406 nm) at a rate of 5° min⁻¹ from 2θ = 20° to 80°. The identification of crystalline phases was made by matching with JCPDS files and the crystallinity was calculated using MDI Jade 5.0.

The Brunauer–Emmett–Teller (BET) surface areas and average pore diameter of the catalysts were measured by N₂ adsorption using an Autosorb-1-C instrument. The samples were first outgassed at 573 K for more than 12 h before adsorption isotherms were generated by dosing the catalysts with nitrogen at 77 K.

Thermogravimetry (TG) was carried out using a Shimadzu DTG-60H. The sample was loaded into a platinum pan and heated from room temperature to 600 °C at a heating rate of 10 °C min⁻¹ under a N₂ atmosphere. The sensitivity and precision are 0.001 mg and ±1% respectively. The range of thermography is ±1 to ±1000 μV.

Catalytic performance tests

Only a brief description for the catalyst activity test is included; details are well-described in previous work.²¹ COS (1% COS in N₂) from a gas cylinder was injected with N₂ (99.99%) as the carrier flow, which were then completely mixed in a mixing chamber. A set of mass flow controllers was prepared to adjust the flow rates of air. H₂O was injected from a thermostatic water bath through a controlled saturator, and the relative humidity of the reaction gas was adjusted by changing the temperature of the water bath. The gas containing 470 ppm COS and 2.67% relative humidity passed through the catalyst bed at a rate of 55 ml min⁻¹, with the gas hourly space velocity (GHSV) being 5000 h⁻¹. The temperature of the reactor was regulated at 50 °C over its entire length by a water bath with a circulating pump, with accuracy ±1 °C (Fig. 1). The conversion of COS was determined by analyzing the inlet and the outlet concentration of COS using a HC-6 trace phosphorus sulfur analyzer. In this study, COS conversion was calculated as follows:

COS_inlet (ppm) and COS_outlet (ppm) are the concentrations of COS measured at the inlet and outlet of the reactor, respectively.

Fig. 1 Experimental equipment flow chart. (1) Gas cylinders (N₂, COS and O₂), (2) mass flow meters, (3) mixing tank, (4) water saturator, (5) fixed bed reactor, (6) temperature controller, (7) dilute gas, (8) import sampler and (9) export sampler.

Results and discussion

Influence of Al : Ce on the hydrolysis of COS

Fig. 2 presents the catalytic performances of the catalyst series with varying proportions of Al : Ce, a molar ratio of Mg : (Al + Ce) = 2 and (Al + Ce) = 1. For the Mg₂Al sample, a 98% conversion was obtained and sustained for approximately 60 minutes. Improvement of the COS catalytic efficiency with an alternative catalyst was obviously possible and therefore cerium was introduced. Significant differences appeared in COS hydrolysis with varying proportions of Al : Ce. For the Mg₂Al_xCe_1−x sample, cerium was added and studied. The Mg₂Al_0.94Ce_0.06 sample demonstrated the highest efficiency over the others containing Ce, with 100% conversion obtained and sustained for approximately 80 minutes. As Fig. 2 indicates, first the conversion increased with an increasing proportion of Ce (from Mg₂Al₁Ce₀ to Mg₂Al_0.94Ce_0.06), then it decreased (from Mg₂Al_0.94Ce_0.06 to Mg₂Al_0.70Ce_0.30), to below 100% conversion. A possible explanation may be that the effect of Al : Ce for varying HTLCs and the corresponding catalysts was different, leading to a range in catalytic activity.

Fig. 2 COS conversion over Mg₂Al_xCe_1−x with different metal contents. Reaction conditions: 470 ppm COS, 50 °C, 5000 h⁻¹, a relative humidity of 2.67% and N₂ to balance.

The influence of cerium on the catalytic activity is studied through characterizing typical samples of Mg₂Al_0.94Ce_0.06 and Mg₂Al by XRD. Powder X-ray diffraction patterns of the HTLC precursors (Fig. 3(A)) and the corresponding mixed metal oxides (Fig. 3(B)) were acquired.

Fig. 3 XRD patterns for (A) precursor hydrotalcites and (B) mixed oxides.

As presented in Fig. 3(A), the characteristic diffraction peaks at 2θ = 11°, 23°, 35°, 39°, 47° and 61° correspond to the (003), (006), (009), (015), (018) and (110) reflections, which are signals typical of hydrotalcite-like compounds,^22,23 confirming the formation of the precursor layered double hydroxides composed exclusively of Mg and Al.²⁴ A slightly higher crystallinity was observed for the Ce sample compared to MgAl hydrotalcite. Additionally, alteration of the HTLC structure was minimal in the presence of Ce, indicating that the effect of Ce on the precursors was minimal and that there may be no Ce in the brucite layer; the difference in the ionic radii of octahedral coordinated Ce³⁺ (1.02 Å), resulting in a large distortion of the layers, may provide an explanation.

Fig. 3(B) illustrates the distinct characteristic peaks of crystalline MgO at 2θ = 43°, while the characteristic peaks of Al₂O₃ are nearly imperceptible. The Al³⁺ cation is approximately the same size as Mg²⁺ (ref. 24) thus, the diffraction peak of the Al phase of the catalyst is absent and will present a similar lattice to periclase-like MgO when merged into solid solutions, indistinguishable by XRD. The characteristic peaks of crystalline MgAl₂O₄ are identified at 2θ = 63°. Minor quantities of CeO₂ were obtained simultaneously; these peaks are sharp, indicating that the Ce-containing samples possessed increased crystallinity. Calculated using MDI Jade 5.0, the crystallinity of MgAl was determined to be approximately 37%, while the crystallinity of MgAlCe was determined to be approximately 42%. The degree of crystallinity would, in fact, rise above 90%. The presence of mordenite (unknown peak),²⁵ however, contributed to low crystallinity. It can be explained that increased crystallinity improves catalytic activity and, with the introduction of Ce, though showing minimal effect as a precursor, calcination of HTLCs with mixed oxides occurred and the catalytic activity was enhanced. Catalytic activity enhancement upon introduction of cerium content arose from the differences in structural properties (but not complete reconstruction, due to the presence of minor quantities of CeO₂) and oxidative properties of mixed oxide catalysts.

Furthermore, catalytic hydrolysis of COS is regarded as a base-catalysed reaction,²⁶ and the OH group is the active site for the hydrolysis of COS.²⁷ Therefore, the Boehm titration was used to understand how it is modified by the introduction of different contents of Ce, as well as the relationship between basicity and catalytic activity. From Table 1, it is obvious that Mg₂Al_0.94Ce_0.06 had the highest proportion of OH groups compared to other materials; the OH group mass concentration of Mg₂Al_0.94Ce_0.06 is 2.12 mmol g⁻¹. However, when the Ce content increased, the OH group mass concentration decreased. This indicated that a decrease in the Al : Ce ratio decreases the OH group mass concentration. At the same time, the OH group mass concentration of Mg₂Al is 0.40 mmol g⁻¹, which is also lower than the OH group mass concentration of Mg₂Al_0.94Ce_0.06. This is identical to the results in Fig. 2. Although the lactonic groups are unordered, the OH groups play a vital role in the catalytic hydrolysis of COS.

Table 1 Basicity analysis of the samples

Samples	Lactonic groups (mmol g^−1)	OH groups (mmol g^−1)
Mg2Al	0.96	0.40
Mg2Al0.94Ce0.06	1.12	2.12
Mg2Al0.90Ce0.10	2.28	0.48
Mg2Al0.80Ce0.20	2.20	0.20
Mg2Al0.70Ce0.30	1.84	0.16

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Influence of pH on the hydrolysis of COS

The pH value during synthesis also influences the metal salt co-precipitation system of HTLCs²⁸ as previous studied. The precursors of the catalysts (HTLCs) were prepared in this study at pH = 7, 8, 9, 10 and 11, and the HTLCs were calcined at 600 °C for 3 h. The catalytic activities of COS hydrolysis were tested over these samples at low temperature (50 °C). Results presented in Fig. 4 reveal that the pH during the synthesis of HTLCs is significant for their catalytic hydrolysis activities. Both of the catalysts synthesized at pH 7 and pH 9 exhibited lower activity for the removal of COS compared to the catalyst synthesized at pH 8. 100% removal of COS was observed, and was sustained for approximately 80 minutes for the catalyst synthesized at pH 8. A conversion rate of 99% was obtained for the catalysts synthesized at pH 7 and pH 9, but the conversion decreased rapidly as time passed. The catalytic activity also decreased when the synthesis pH was higher than 9. The conversion of the catalysts synthesized at pH 10 and pH 11 are below 100%. The rationale for this occurrence is obtained by titration of a Mg–Al–Ce metal solution with a NaOH solution and observing the change in pH. Fig. 5 illustrates three platforms observed during the titration. The first platform was due to Al(OH)₃, the second platform was due to Ce(OH)₃, and the third platform was due to the HTLCs, for which the pH value ranged from 7 to 9. According to the principle of coprecipitation,^29,30 the best pH value for the synthesis of HTLCs in this experiment was 8.

Fig. 4 COS conversion over Mg₂Al_0.94Ce_0.06 (Al : Ce = 16/1) synthesized at different pH. Reaction conditions: 470 ppm COS, 50 °C, 5000 h⁻¹, a relative humidity of 2.67% and N₂ to balance.

Fig. 5 The titration curve of the Mg–Al–Ce mixed salt solution.

To better understand the crystallinity of materials synthesized at different pH values, the XRD spectra of the materials synthesized at different pH values were obtained. Fig. 6 shows that the catalyst has a higher crystallinity when synthesized at pH 8 than at other pH values. The catalyst synthesized at a pH value of 8 had two obvious sharp peaks and few scattering peaks, which indicated the improvement in crystallinity of the material. The crystallinity of catalysts synthesized at other pH values was basically identical; what is more, all of them have more scattering peaks than for the catalyst synthesized at pH 8. Synthesis at pH 8 gave a high crystallinity, contributing to high catalytic activity. This is consistent with the experiment showing that synthesis at pH 8 gave a catalyst with higher activity than those synthesised at pH 7, 9, 10 and 11. But as the crystallinity is low for all of them, this may be due to the influence of the scattering peaks. In order to improve the crystallinity of the materials, the effect of different hydro-thermal temperatures during synthesis on the crystallinity was tested.

Fig. 6 XRD patterns of samples synthesized at different pH values.

Influence of hydro-thermal temperatures on crystallinity

The influence of hydro-thermal temperature during synthesis on crystallinity was tested to improve the crystallinity of the materials. Fig. 7 indicates that increasing temperatures may be conducive to improving the catalytic efficiency of COS hydrolysis in this work. It is obvious that hydro-thermal temperatures 140 °C and 175 °C gave catalysts with high conversion of COS; 100% catalytic activity was obtained and sustained for approximately 80 minutes. But the catalytic activity was observed to decline rapidly for catalysts synthesized at lower hydro-thermal temperatures. Hydro-thermal temperatures under 105 °C gave catalysts with lower catalytic activity compared to those synthesized at hydro-thermal temperatures above 105 °C, possibly due to low crystallinity. Excellent catalytic activity is exhibited when the hydro-thermal temperature during synthesis was 140 °C, as the curvilinear trend of the catalytic activity stabilizes and declines slowly, with the efficiency initially at 100% and sustained for 80 minutes. The COS conversion rate for the catalyst synthesized at 175 °C remains virtually unchanged, indicating 140 °C as the superior hydro-thermal synthesis temperature.

Fig. 7 COS conversion over Mg₂Al_0.94Ce_0.06 (Al : Ce = 16 : 1) synthesized at different hydro-thermal temperatures. Reaction conditions: 470 ppm COS, 50 °C, 5000 h⁻¹, a relative humidity of 2.67% and N₂ to balance.

XRD patterns of samples synthesized at various hydro-thermal temperatures were also characterized. Fig. 8 indicates that the crystallinity of catalysts synthesized at 140 °C and 175 °C are superior to those synthesized at 35, 70 and 105 °C. The characteristic peaks of crystalline Mg₆Al₂(OH)₁₈ could be easily identified at 2θ = 28°, 43° and 63°. Crystalline Mg₂Al(OH)₇ could be easily identified for synthesis temperatures below 105 °C, but is barely identified for when hydro-thermal temperatures were 140 and 175 °C. Mg₆Al₂(OH)₁₈ may be more stable than Mg₂Al(OH)₇, allowing the crystalline form of Mg₂Al(OH)₇ to disappear when the hydro-thermal temperatures during synthesis are 140 and 175 °C. The hydro-thermal temperature range may be divided into two parts: hydro-thermal temperatures under 105 °C produce patterns that exhibit low crystallinity as a result of unknown peaks or scattering peaks, while the unknown peak essentially disappears for hydro-thermal temperatures above 140 °C. A higher crystallinity is obtained during synthesis at a hydro-thermal temperature of 140 °C compared to hydro-thermal temperatures below 140 °C, explaining why a hydro-thermal temperature of 140 °C afforded a catalyst exhibiting higher efficiency than hydro-thermal temperatures below 140 °C. An increase in hydro-thermal temperature during synthesis to 175 °C results in a nearly imperceptible change in crystallinity, indicating 140 °C as the superior hydro-thermal temperature for synthesis of the catalyst, consistent with other experimental results.

Fig. 8 XRD patterns of samples synthesised at different hydro-thermal temperatures.

Influence of calcination temperatures on the hydrolysis of COS

Fig. 9 depicts the catalytic activities of the hydrotalcite-like compounds calcined at temperatures of 200, 300, 400, 500, 600 and 700 °C. Uncalcined samples exhibit a poor catalytic hydrolysis activity; under 40%. While catalytic activities increased with increasing calcination temperatures around the ranges of 200–600 °C, the efficiency reaches 100% when calcination is carried out at 400 °C and 500 °C, but is only sustained for a short time. The catalyst exhibited enhanced hydrolysis activity when the calcination temperature was 600 °C, reaching 100% and sustained for approximately 80 minutes. A rapid decrease is experienced, however, when the calcination temperature was higher than 600 °C. Explanations for this may be: (1) hydrotalcite-like compounds lost some anions and surface water at low calcination temperatures, but the layered double hydroxide structure was not destroyed; (2) the LDH structure was destroyed when HTLCs were calcined at high temperature, acquiring a rising surface area and leading to an improvement in catalytic activities and (3) calcination temperatures higher than 600 °C contributed to the destruction of the pore structure, reducing the number of active sites³¹ and contributing to a low COS conversion (catalytic hydrolysis of COS is regarded as a base-catalyzed reaction, and the OH⁻ groups are the active sites for the hydrolysis of COS). The catalyst exhibited superior catalytic activity with a calcination temperature of 600 °C. Efficiency of the catalytic activity decreased quickly when the calcination temperature exceeded 600 °C. Furthermore, in order to explain these phenomena, TG and BET analysis was carried out

Fig. 9 COS conversion over Mg₂Al_0.94Ce_0.06 calcined at different temperatures. Reaction conditions: 470 ppm COS, 50 °C, 5000 h⁻¹, a relative humidity of 2.67% and N₂ to balance.

Thermal decomposition (Fig. 10) can be divided into three stages. Removal of surface water and physically adsorbed water occurred during the first stage (<200 °C) and the loss of mass accounted for 14.15% of the total, but the LDH structure is not destroyed at this stage. During the second stage (200–400 °C), weight loss occurred mainly due to disruption of the hydrotalcite layer structure, while CO₃²⁻ in the interlayer was released in the form of CO₂. The third stage (400–540 °C) leads to the collapse of the laminate structure as loss of the crystalline water in the layer occurred, forming mixed oxides. Weight loss was not evident above 540 °C (Table 2 summarizes each stage of weight loss). The effective temperature from TG may be 540 °C, but the effective temperature from the experiment was 600 °C. The calcination temperature was generally higher (50–100 °C) than the temperature of hydrotalcite-structure collapse, indicating 600 °C as the superior calcination temperature.³² The TG is consistent with the catalytic activity of Mg₂Al_0.94Ce_0.06. In addition, in connection with Fig. 3, the d-spacing of the (003) reflection of these precursors at 2θ = 11°, which corresponds to the thickness between layers, is 2.34–2.37 nm.^33,34 The d-spacing of the (110) reflection at 2θ = 61° is related to the average metal–metal distance in the layer, which for these precursors is 0.3 nm; these results are in good agreement with those reported by Serrano-Lotina et al.³⁵ By the information provided from XRD (Fig. 3), the structure of the hydrotalcite-like precursors and the catalyst after calcination is visually described in Scheme 1.

Fig. 10 TG curve of the Mg₂Al_0.94Ce_0.06 hydrotalcite precursor.

Table 2 TG data for the catalyst precursor

Sample	Total weight loss rate (%)	The first stage of weight loss		The second stage of weight loss		The third stage of weight loss
		Weight loss rate (%)	Maximum weight loss temperature (°C)	Weight loss rate (%)	Maximum weight loss temperature (°C)	Weight loss rate (%)	Maximum weight loss temperature (°C)
Mg2Al0.94Ce0.06	42.68	14.15	200	21.95	400	6.58	540

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Scheme 1 A schematic of the structure of the hydrotalcite-like precursors and the catalyst.

The catalysts following calcination temperatures of 300, 400, 600 and 700 °C were characterized by BET. Surface area, pore volume and average pore diameter of the catalysts are listed in Table 3. Normally, catalytic activity is related to the BET surface area, however, the surface area increased with increasing calcination temperatures as the surface area after calcination at 700 °C was greater than after calcination at 600 °C. The surface area may not act as a main factor leading to increased activity in COS catalytic hydrolysis. Total pore volume and the average pore diameter are related to the activity of COS catalytic hydrolysis. The total pore volume after calcination at 600 °C was 0.69 cm³ g⁻¹, larger than the total pore volume after calcination at 300, 400 and 700 °C. However, the average pore diameter after calcination at 600 °C is lower than for calcination temperatures of 300, 400 and 700 °C. The efficiency of COS catalytic hydrolysis is inversely related to average pore diameter, as indicated when a higher average pore diameter aligns with lower activity of COS catalytic hydrolysis. The surface area is not linked with catalytic activity; however, the total pore volume is sufficient, as supported by results of the pore size distributions presented in Fig. 11. As shown in Fig. 11, all pores of the samples were smaller than 90 nm when the calcination temperature reached 600 °C; the volume of the pores at a range of 15–30 nm was significantly larger than that of those formed at calcination temperatures of 300, 400 and 700 °C. The pore size behavior suggests that mesoporosity may potentially influence COS hydrolysis. N₂ adsorption–desorption isotherms of the catalysts (type IV for all samples), representing typical mesoporous materials are presented in Fig. 12, and align with the pore distributions.

Table 3 Calcination temperature and corresponding surface area of the samples

Calcination temperature (°C)	Surface area (m^2 g^−1)	Total pore volume (cm^3 g^−1)	Average pore diameter (nm)
300	88.41	0.51	22.71
400	110.65	0.60	21.34
600	116.26	0.69	20.50
700	119.68	0.60	22.29

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Fig. 11 Pore size distribution isotherms for the catalysts.

Fig. 12 N₂ adsorption–desorption isotherms for the catalysts.

Conclusion

The experimental studies indicate that catalyst preparation conditions had considerable influence on the catalytic hydrolysis of COS and that corresponding mixed oxides derived from hydrotalcite-like precursors with Al : Ce equal to 16 were also highly influential. Varying proportions of Al : Ce significantly influenced the catalytic hydrolysis of COS. The pH during synthesis also acts as an important factor in the formation of the hydrotalcite-like compounds, with an optimum pH of 8. Individual metal salts precipitate at a suitable pH as many metal ions co-precipitate. Hydro-thermal temperature during synthesis plays a pivotal role in crystal formation with an optimum hydro-thermal temperature of 140 °C. When the calcination temperature was 600 °C, the catalyst exhibited superior catalytic activity. Lower calcination temperature prevented the LDH structure from being destroyed, while a calcination temperature higher than 600 °C led to pore structure damage, reduction of the number of active sites, and a lower COS conversion. In these optimum conditions, the catalytic effect of COS reached 100% and was sustained for approximately 80 minutes at a low temperature (50 °C) and a space velocity of 5000 h⁻¹. The deactivation of the catalyst may be because of the formation of sulfur and sulfate on the catalytic surface. Further studies focused on the reaction mechanism for the catalytic hydrolysis of COS are in progress.

Acknowledgements

We are indebted to the National Natural Science Foundation of China (21367016), the Scientific Research Foundation for the Introduction of Talent in Yunnan Province (kksy201222146), National Program on Key Basic Research Project of China (973 Program, 2014CB643404), the National Natural Science Foundation of China (51104073) and National Natural Science Foundation (51408282).

Notes and references

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