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Abstract

The faujasite framework structure and the morphological properties of Co²⁺-exchanged NaX zeolite were maintained when samples were treated at a temperature ≤600℃, but were distorted at higher temperature. The cobalt (II) exchange and thermal treatment processes significantly improved the H₂S adsorption capacity of the NaX zeolite and the highest values were 4.24 mg g⁻¹ for 0.10-CoX-600 and 4.42 mg g⁻¹ for 0.15-CoX-600. Moreover, the first-order adsorption kinetic showed a relatively good fit when applied to the experimental data for the H₂S adsorption process.

Keywords

Adsorption kinetics cobalt (II) exchange hydrogen sulfide NaX zeolite thermodynamics

Introduction

Hydrogen sulfide (H₂S) is a very toxic, flammable, and extremely hazardous gas. It has no color, but it has an odor of rotten eggs at less than 1 ppm (Weil et al., 2005). Crude petroleum, natural gas, and hot springs are natural sources that contain a lot of hydrogen sulfide. Moreover, this gas is also formed by the biological degradation of the organic materials for example from human and animal wastes in anaerobic conditions (Weiner, 2012). Industrial activities such as petroleum/natural gas drilling and refining, wastewater treatment, and the use of coke ovens, tanneries, and kraft paper mills can produce H₂S as a by-product. Removal of H₂S from the working atmosphere is required because hydrogen sulfide can irritate the eyes, nose, throat, and lungs and especially because H₂S can stop the breathing center in the brain, which can cause death when this gas is present at high enough concentrations. According to the National Institute for Occupational Safety & Health (NIOSH) of United States, the threshold concentration of H₂S for Worker Exposure Limits is as flowing: NIOSH Recommended exposure limit (10-min ceiling) of 10 ppm; and NIOSH immediately dangerous to life and health (level that interferes with the ability to escape) of 100 ppm (Hydrogen sulfide. NIOSH pocket guide to chemical hazards 1997). In a working environment, H₂S concentrations must be below the lower toxic limit concentration of 10 ppm (Milby et al., 1999).

Currently, many different types of materials are used as adsorbents for the removal of H₂S, such as metal oxides (Garces et al., 2010), activated carbon (Micoli et al., 2014), and zeolites. For example, Cosoli et al. (2008a, 2008b) showed that hydrophilic zeolites such as zeolite A and Faujasite (NaX and NaY) could remove hydrogen sulfide much better than the hydrophobic zeolites, such as MFI. AgY, Cu(I)Y, CuCl/SBA-15, and CuCl/MCM-41 are promising adsorbents that are used to desulfurize natural gas (Crespo et al., 2008; Yang et al., 2003). In addition, Kumar et al. (2011) used Ag (and Cu(I))-exchanged faujasite to adsorb H₂S in the presence of N₂, CO, CO₂, and water vapor. The H₂S adsorption capacity of 13X zeolite was significantly improved after modification by Cu or Zn cations, especially for Cu (II)-exchanged 13X (Micoli et al., 2014). Cobalt (II) ions were used to strongly improve the BET surface area and especially the external surface area and mesopore volume of the 13X zeolite, resulting in an increase in the gas adsorption capacity. This is due to the decrease in the number of extraframework cations, while two monovalent sodium ions are replaced with one divalent cobalt ion. Moreover, Co²⁺ (0.74 Å) cations also occupy less space in the zeolite cavity compared to Na⁺ (0.94 Å) cations (Sebastian et al., 2005). Additionally, the cobalt-containing adsorbent materials, such as cobalt oxide silica membranes (Uhlmann et al., 2011), cobalt (hydr) oxide/graphite oxide composite (Mabayoje et al., 2012), and CoMo/Al₂O₃ (Echard et al., 2001) have a high potential for hydrogen sulfide adsorption. Jeong et al. (2012) investigated the bulk structural and coordination chemistry of thermally treated cobalt (II)-X zeolite. This study showed that the dehydration and the structure changed during thermal treatment, however, the zeolite framework was maintained for thermal treatment at a temperature ≤600℃. Therefore, we need to consider the changes of gas adsorption capacity of the cobalt (II)-exchanged zeolite under thermal treatment.

In this work, we investigate hydrogen sulfide adsorption by thermally treated cobalt (II)-exchanged NaX zeolite. In addition, the thermal stability, surface properties, and crystalline structure of the thermally treated cobalt (II)-exchanged NaX zeolite are also measured by using SEM, XRD, BET, and BJH techniques.

Experimental

Materials and methods

One hundred grams of the NaX zeolite samples was exchanged by cobalt cations in a glass column (with an ID of 4 mm and a length of 40 cm) using a continuous flux of 2 L of Co(NO₃)₂ solution of various concentrations. The pH of the solution was 6.23 and the flow rate was 1 mL/min. The resulting product was washed until free of nitrate and subsequently dried at 120℃ for 12 h. This nitrates removal, which is also the removal of Co²⁺ (excess), Na⁺ cations, is a required process because NaNO₃ (solid) and cobalt oxides, make during thermal treatment process, can fill the pore system of exchanged NaX zeolite leading to a decrease in adsorption capacities. The dried samples were thermally treated at different temperatures (400, 500, 600, and 700℃) for 3 h at a rate of 20℃ min⁻¹. After thermal treatment, the samples were removed from the furnace and allowed to quench at room temperature. Prior to use, the thermally treated samples were stored inside airtight vials to prevent the adsorption of moisture from the atmosphere. The final products were labeled as n-CoX-T, where n (mol L⁻¹) represents the concentration of the Co(NO₃)₂ solution and T represents the heating temperature. The n-CoX-0 is the sample that was left untreated.

Characterization

The adsorption–desorption isotherms were collected in an ASAP 2020 instrument (Micromeritics) at 77.5 K. The obtained data were analyzed using the Brunauer–Emmett–Teller (BET; specific surface area) and Barrett–Joynerr–Halenda (BJH; pore diameter, pore distribution and micro- and mesopore volume) models.

The structure of the samples was investigated by X-ray powder diffraction (XRD) using a PANalytical X’pert PRO system. The diffraction patterns were collected using Cu K_α1 (λ = 1.54 Å) in a 2θ range of 5–65° with a step of 0.02°. The XRD analysis provides information on changes in the crystallinity and mineralogical composition of Co-X after thermal treatment. The percentages of crystallinity of cobalt (II)-treated zeolites were calculated from X-ray diffraction pattern by summation of the heights of 10 major peaks consisting of (1 1 1), (2 2 0), (3 1 1), (3 3 1), (4 4 0), (5 3 3), (6 4 2), (5 5 5), (8 4 0), and (6 6 4) peaks at 6.1, 10.0, 11.7, 15.4, 20.1, 23.3, 26.6, 30.9, 31.9, and 33.6 of 2θ, respectively. These peaks were determined from the diffraction pattern of samples by using the Fd-3 m (227 origins at −3 m) cubic space group. One hundred percent, which is taken from the maximum degree of crystallinity, corresponds to NaX zeolite (Hammoudi et al., 2008).

Scanning electron microscopy (SEM) imaging was studied with a JOEL microscope (model JSM 6330 TF). It can show the change in morphology of NaX zeolite and cobalt (II)-exchanged NaX zeolite under the cobalt (II) exchange process as well as the thermal treatment. Furthermore, the cobalt uptaken was also determined by the atomic absorption spectroscopy (AAS) using an atomic absorption spectrometer with a cobalt cathode lamp and air acetylene flame.

H₂S adsorption experiments

Hydrogen sulfide adsorption was carried out at room temperature using the adsorption system shown in Figure 1. Prior to H₂S adsorption, the zeolite adsorbents were dried at 100℃ for 12 h min in an oven to completely remove moisture. The zeolite was added to a small bottle and placed into the test tank for the adsorption of H₂S. N₂ flow was first passed through the adsorption system to remove air. First, the H₂S flow was passed through the buffer tank to control the H₂S concentration by N₂ flow. When the H₂S concentration reached a value of 100 ppm and remained unchanged after the shut-off valve (V3) was closed, the H₂S flow was passed through the adsorbent for the adsorption process by opening the valve V4. A Honeywell analytical detector was used to determine the H₂S concentration and adsorption time during the adsorption process.

Figure 1.

Experimental system for H₂S adsorption.

The reduction rate of the hydrogen sulfide concentration is calculated using the following equation:

r = \frac{C_{n + 1} - C_{n}}{t_{n + 1} - t_{n}}

(1) where

C_{n + 1}, and C_{n}

(ppm) are the concentrations of H₂S at time

t_{n + 1}, and t_{n}

(min).

It was admitted that the equilibrium state was reached when the H₂S concentration did not change anymore reduction rate, r, was less than 0.02. The associated concentration is the equilibrium concentration of H₂S.

The H₂S adsorption capacity of adsorbents, q (mg g⁻¹), is determined as follows:

q = \frac{\frac{V_{1} C_{o} - (V_{1} + V_{2}) C}{24.5} \times M_{H_{2} S} \times 10^{- 3}}{m_{adsorbent}}

(2) where V₁ and V₂ (L) are the volume of the buffer tank and the test tank, respectively; and C_o and C (ppm) are the initial concentration and the equilibrium concentration of H₂S; M (g mol⁻¹) is the molar mass of H₂S; and m_adsorbent (g) is the mass of the adsorbent sample.

Nitrogen gas matrix was used in this work because N₂ is an inert gas and is lighter than H₂S; it was easily desorbed when H₂S adsorption was implemented. Moreover, N₂ is a main component of the atmosphere (∼80%) so the H₂S gas removal from the atmosphere is like from a mixture with N₂. Finally, N₂ is a commonly commercial gas.

Results and discussion

Characterization of the thermally treated cobalt (II)-exchanged NaX zeolite

The effect of cobalt (II) on the crystal structure of NaX and the effect of thermal treatment on the cobalt (II)-exchanged NaX zeolite were examined by using XRD (Figure 2) and the percentage of crystallinity values and unit cell dimensions are listed in Table 1.

Figure 2.

X-ray powder diffraction patterns of NaX and 0.10-CoX thermally treated NaX zeolites.

Table 1.

The structure and textural properties of cobalt (II)-exchanged zeolites: untreated sample (0.1-CoX-0) and treated at 400, 500, 600, and 700℃ for 3 h.

Zeolites	Cobalt uptaken (mg g⁻¹)	Crystallinity (%)	Unit cell parameter a/Å
NaX	00.0000	100	25.026
0.1-CoX-0	85.9627	47.01	24.835
0.1-CoX-400	90.7268	38.28	24.827
0.1-CoX-500	94.6383	37.91	24.859
0.1-CoX-600	96.1746	39.62	24.703
0.1-CoX-700	98.0276	32.06	24.766

Figure 2 shows that the Faujasite framework of NaX zeolite is retained during the cobalt (II) exchange process. However, the important decrease of the percentage of crystallinity was due to the presence of cobalt (II) ions in the NaX zeolite framework, leading to decreased unit cell dimension (Table 1). In the thermal treatment process of the 0.1-CoX(s) zeolite, the framework structure was retained up to 600℃, but it was damaged at T ≥ 700℃, as the (2 2 0) and (3 1 1) peaks at 2θ = 10.03 and 11.7 disappeared in the X-ray diffraction pattern of 0.1-CoX-700 (Figure 2) when the amorphorization occurred. The positions of reflection peaks changed with the treatment temperature. The position of the (111) peak at 2θ = 6.14° (d = 14.45 Å) shifted to lower d (d-spacing is the interatomic spacing in angstroms) values following thermal treatment. The unit cell parameter was calculated for the cubic symmetry as 24.835 Å for the untreated sample, 24.827 Å for the sample treated at 400℃ for 3 h, and 24.703 Å for the sample treated at 600℃ for 3 h. This finding agrees with the studies of Jeong et al. (2012). The decrease in the unit cell parameter was caused by the dehydration (Borissenko et al., 2008). In this study, the dehydration was demonstrated by an increase in the cobalt uptake when the treatment temperature increases (Table 1).

Figure 3 presents the SEM images of the NaX zeolite and thermally treated cobalt (II)-exchanged zeolite samples. It can be seen that there are no notable differences in morphology between the NaX zeolite (Figure 3A) and the cobalt (II)-exchanged zeolite (Figure 3B and C). Most of samples show nearly spherically shape crystallites of approximately 1–3 µm in size. Hence, the cobalt (II) exchange does not cause the significant collapse of the crystalline structure of the NaX zeolite. This is consistent with the result of the XRD diffraction. Conversely, the images of the cobalt (II)-exchanged zeolite showed some dark regions (pointed out with arrows in Figure 3C–E) due to the morphous structure caused by thermal treatment at higher temperature. The shape and surface of the particles were retained up to 600℃ but were distorted at higher temperatures as shown in Figure 3E. The surfaces of the particles of cobalt (II) -exchanged zeolite, which was treated at 500℃, are smooth and clear (Figure 3C). Therefore, the most optimal treatment temperature is in the range of 500–600℃. By the way, the morphology of 0.15-CoX-T and 0.2-CoX-T samples was also considered; however, the morphological evolution was similar to 0.1-CoX-T samples.

Figure 3.

SEM images of NaX and 0.10-CoX thermally treated zeolites: NaX sample (A) and the samples treated at 400℃ for 3 h (B), 500℃ for 3 h (C), 600℃ for 3 h (D), and 700℃ for 3 h (E); and image of 0.10-CoX-600 before and after sulfide adsorption (F).

The nitrogen adsorption/desorption isotherms, for each of the NaX and 0.1-CoX-T zeolites treated at various temperatures, are shown in Figure 4(A). According to IUPAC guidelines, the adsorption isotherms are a mixed type I and II isotherms, which show the characteristics of microporous materials. It also exhibits that the micropore filling is observed at a relatively low pressure due to the high adsorption potential or the narrow pore width. In this case, however, a plateau was not achieved at p/p_o ≥ 0.8, obviously indicating expansion of the pore. Consequently, the cobalt (II)-exchanged NaX zeolites displayed a high adsorption potential. The slope of the adsorption isotherm curves in the p/p_o range from 0.1 to 0.8 on Co (II)-exchanged zeolites (Figure 4A) is greater than that on NaX zeolites, i.e., the contribution of the microporous adsorption is less than that in NaX zeolites. Therefore, cobalt (II) exchange into a zeolite makes the zeolite surface becomes more porous and rough, here, the micro- and mesoporosity increase.

Figure 4.

N₂ adsorption (solid symbols)/desorption (open symbol) isotherms (A) and pore size distributions (B) for NaX and thermally treated cobalt (II)-exchanged NaX zeolites.

The adsorption isotherms on the 0.1-CoX-T samples suggest an optimal treatment temperature range of 500–600℃. At a higher temperature (700℃), the thermal treatment of zeolites causes damage to surface texture, resulting in a surface structure that is less porous and rough. The V_meso, S_BET, S_ext, and S_micr of the thermally treated 0.1-CoX-T samples are also derived from the t-plot and are listed in Table 2. In the thermally treated 0.1-CoX-T zeolites, the specific surface areas and micropore surface areas decrease with increasing temperature due to compression of the materials, which is caused by treatment at high temperature. However, the mesopore size and the mesopore volume significantly increase with increasing treatment temperature up to 600℃. These nitrogen adsorption/desorption isotherms exhibit a type H3 hysteresis loop, characteristic of non-rigid aggregate of plate-like particles (slit-shaped pores). This indicates the presence of small mesopores; additionally, the tensile strength failure of the nitrogen meniscus shows capillary condensation in meso- and macropores. A realistic pore size can be obtained by analysis of the pore size distribution derived from the adsorption branch (Figure 4B) and these values are listed in Table 2. Mesopore size increases with increasing temperature because of the texture collapse of zeolite under thermal treatment, leading to an increase in the mesopore volume. The mesopore size of the NaX zeolite decreases substantially after cobalt (II) exchange. It was assumed that the difference in the diameter of metal cations caused this phenomenon.

Table 2.

The surface characteristics and H₂S adsorption constants of NaX zeolites and Co²⁺-exchanged zeolites.

Zeolite adsorbents	S_BET (m²g⁻¹)	S_micro (m²g⁻¹)	S_ext (m²g⁻¹)	D_meso (nm)	V_meso (cm³g⁻¹)	Adsorption time (min)	Adsorption capacity (mg g⁻¹)	k_ads (min⁻¹)	R ²
NaX	388	344	44	16.8	0.130	89	2.70	0.0066	0.92
0.20-CoX-600	273	212	61	11.0	0.170	175	2.93	0.0025	0.89
0.15-CoX-0	413	340	73	9.7	0.145	160	2.15	0.0023	0.77
0.15-CoX-400	395	366	79	9.3	0.172	205	3.47	0.0042	0.95
0.15-CoX-500	390	304	85	9.4	0.188	310	3.60	0.0026	0.89
0.15-CoX-600	382	302	80	9.7	0.176	380	4.42	0.0032	0.94
0.15-CoX-700	340	282	50	13.9	0.178	215	2.77	0.0028	0.85
0.10-CoX-0	421	354	67	10.5	0.143	200	2.34	0.0022	0.93
0.10-CoX-400	376	308	67	10.1	0.155	225	3.10	0.0030	0.91
0.10-CoX-500	362	281	81	10.2	0.193	365	3.97	0.0026	0.94
0.10-CoX-600	263	202	62	10.6	0.187	325	4.24	0.0036	0.94
0.10-CoX-700	241	188	52	14.1	0.175	285	3.96	0.0039	0.96

H₂S adsorption on the thermally treated cobalt (II)-exchanged NaX zeolite

Adsorption kinetic model

In this study, the H₂S adsorption kinetics were assumed to be the first-order kinetic. The first-order kinetic is expressed as

\frac{d (C_{o} - C)}{dt} = k_{ads} C

(3) where

k_{ads}

(min⁻¹) is the rate constant; C_o and C (ppm) are the H₂S initial concentration and at time t (min).

The integrated form of Eq. (3) is

lnC = {lnC}_{o} - k_{ads} t

(4)

The rate constant is determined experimentally by plotting ln C against t. These plots are shown in Figure 6 and the different values of $k_{ads}$ and R² are listed in Table 2 for 0.10-CoX-T and 0.15-CoX-T adsorbents. All R² values for these experimental data were greater than 70%, which indicates a relatively good fit.

Effect of the treatment temperature

Hydrogen sulfide adsorption is shown in Figure 5 for the thermally cobalt (II)-exchanged NaX zeolite adsorbents. The H₂S adsorption capacities and adsorption time for the NaX zeolite, 0.10-CoX-T, and 0.15-CoX-T (T = 0, 400, 500, 600, and 700℃) adsorbents are listed in Table 2. The sulfide adsorption capacities of cobalt (II)-exchanged zeolites were significantly improved by the thermal treatment. Overall, the adsorption capacity, adsorption time, and rate constant increase with increasing the treatment temperature up to 600℃ due to the increase in the V_meso and mesopore size. The H₂S adsorption capacity for the thermally treated cobalt (II)-exchanged zeolite is the largest, when the treatment was carried out at 600℃. The adsorption capacity is 4.241 mg g⁻¹ for 0.10-CoX-600 and 4.423 mg g⁻¹ for 0.15-CoX-600. At the higher temperature (700℃), the sulfide adsorption time and the adsorption capacity were decreased because the mesopore size was very large, resulting in a reduction of H₂S storage into adsorbents. These results are consistent with the results that were obtained from XRD analysis and the SEM and BET methods. The mesopore volumes V_meso for 0.10-CoX-500 and 0.15-CoX-500 are the highest; however, their sulfide adsorption capacity is less than the cobalt (II)-exchanged zeolites that were treated at 600℃.

Figure 5.

H₂S adsorption on different zeolite adsorbents: (A) 0.1-CoX-T and (B) 0.15-CoX-T thermally treated cobalt (II)-exchanged NaX zeolite.

Figure 6.

The first-order H₂S adsorption kinetic model of thermally treated cobalt (II)-exchanged NaX zeolite: (A) 0.1-CoX-T and (B) 0.15-CoX-T.

In addition to the hydrogen sulfide physisorption, the H₂S chemisorption also occurred on Co²⁺-exchanged NaX zeolite by an acid–base reaction. Jeong et al. (2012) revealed that during thermal treatment at a temperature ≤600℃, the Co²⁺ in exchange site I is coordinated by six framework oxygens (i.e., Co(Ox)₆²⁺), and the Co²⁺ in sites I’ an II is Co(Ox)₃H₂O²⁺ or Co(Ox)₃²⁺. Therefore, it can be assumed that the cobalt (II)-exchanged zeolites reacted with H₂S according to the following equation:

H_{2} S + [Co (Ox)_{6}]^{2 +} X^{2 -} \to {CoS}_{↓ black} + H_{2} X + Ox

The black solid CoS can be observed in the image of Co(II)-exchanged zeolite before and after H₂S adsorption (Figure 3F). The mesopore sizes of 0.10-CoX-600 and 0.15-CoX-600 are greater than those of the 0.10-CoX-500 and 0.15-CoX-500 zeolites. Consequently, the reaction occurred more easily, leading to an increase of H₂S adsorption capacity.

Effect of the concentration of Co(NO₃)₂ solution

The cobalt (II) exchanges were carried out in Co(NO₃)₂ solutions with different concentrations: 0.1, 0.15, and 0.2 mol L⁻¹. The sulfide adsorption time and the adsorption capacity of the thermally treated Co²⁺-exchanged zeolite are higher than those of the raw NaX zeolite due to an increase in V_meso after modification by cobalt (II) exchange and thermal treatment (Table 2). In general, the H₂S adsorption capacity of the thermally treated cobalt (II)-exchanged zeolite decreases with an increase of the initial concentration of the Co(NO₃)₂ solution. The R² values for 0.10-CoX-T are higher than the values for 0.15- and 0.20-CoX-T. Moreover, all these values are greater than 91%, which indicates a better fit to the first-order adsorption kinetics model used to analyze the experimental data. According to these results, the H₂S adsorption on 0.10-CoX-T zeolites can be a first-order reaction.

Conclusion

XRD and SEM analyses showed that the framework structure of cobalt (II)-exchanged NaX zeolite crystals and their morphological properties are retained for treatment temperatures up to 600℃ and distorted at a temperature ≥700℃. The nitrogen adsorption isotherm is a mixed type I/II isotherm with a type H3 hysteresis loop for all zeolite samples. The mesopore size and the mesopore volume increase with increasing treatment temperature up to 600℃.

H₂S adsorption on the thermally treated cobalt (II)-exchanged NaX zeolite occurs via both physisorption and chemisorption. The first-order adsorption kinetics showed a relatively good fit to the experimental data for the H₂S adsorption process.

The modification, which includes both cobalt (II) exchange and thermal treatment processes, notably improved the H₂S adsorption capacity of the NaX zeolite. The H₂S adsorption capacity increased with increasing treatment temperature up to 600℃ and the highest values were 4.24 mg g⁻¹ for 0.10-CoX-600 and 4.42 mg g⁻¹ for 0.15-CoX-600.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work is supported by the Ministry of Science and Technology of Taiwan (Grant no: MOST 103-2221-E-151-055).

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