High CO Adsorption Performance of CuCl-Modified Diatomites by Using the Novel Method “Atomic Implantation”

Abstract

An atomic implantation method was used to modify diatomite with CuCl. The CuCl/diatomite samples were characterized by different techniques, including FTIR, XRD, BET, SEM-TEM, EDX, and CO-TPR. Characterization results revealed the formation of CuCl particles of 50–60 nm highly dispersed on diatomite surface. CO adsorption measurements showed that 2CuCl/diatomite exhibits the highest CO adsorption capacity among all CuCl-modified samples with diatomite. Its CO adsorption capacity of 2.96 mmol/g at 30^°C is 10 times higher than that of unmodified diatomite (0.29 mmol/g). The CO adsorption on CuCl-modified diatomites was found to fit well with the Langmuir–Freundlich model.

This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

1. Introduction

Carbon monoxide (CO) is a toxic gas and metabolic waste, which is produced from the incomplete combustion of carbonaceous substances in coal stoves, thermal power plants, automobile engines, etc. [1–4]. CO poisoning is dangerous to human health because it reacts with hemoglobin in the blood to form carboxy-hemoglobin, which limits the transportation of oxygen in the blood and consequently it may cause the death [5]. Therefore, researchers are now interested in high-efficiency carbon monoxide removal technology. Among many methods for CO removal, adsorption is widely used due to its high efficiency and low-cost treatment. Adsorbents for selective carbon monoxide adsorption such as metal-organic frameworks, zeolite, activated carbons, grapheme, and metal oxides have been reported in the literature [6–15]. Recently, Manh et al. [13] reported that graphene oxide modified with Cu (I) showed much higher CO adsorption capacity than that of unmodified graphene oxide. The formation of π-complexation bond between Cu (I) ions molecules and CO on the adsorbent surface could improve CO adsorption selectivity. Thus, CO adsorption capacity of graphene oxide increased from 0.38 mmol/g to 2.9 mmol/g with CuCl modification. Xue et al. [14] synthesized CuCl-modified activated carbon (CuCl/AC), which showed a high CO adsorption capacity of 45.4 cm³/g, 4 times higher than that of activated carbon (11.5 cm³/g). Doping carbon xerogels with CuCl also found to significantly increase its CO adsorption capacity [15]. He et al. [12] synthesized the CuCl/MCM-41, ZnCl₂/MCM-41, and Zn–Cu (I)/MCM-41 materials for CO adsorption. The author reported that the materials CuCl/MCM-41, ZnCl₂/MCM-41, and Zn–Cu (I)/MCM-41 have CO adsorption capacity of 0.59 mmol/g, 0.26 mmol/g, and 1.82 mmol/g, respectively. Gao et al. [8] investigated the CO adsorption on the CuCl supported AC and showed high CO (3.63 mmol/g) adsorption capacity and high CO/CO₂, CO/CH₄, and CO/N₂ adsorption selectivity. Xie et al. [16] found that modifying zeolite with CuCl increases CO selectivity from a CO and CO₂ mixture at a temperature higher than room temperature. When polystyrene resin with amine groups was modified with CuCl, its CO adsorption capacity increased 5.7 times [17].

Currently, clay minerals such as halloysite, dolomite, bentonite, Ilmenite, and diatomite have attracted a lot of interest because of their potential application or environmental treatment [18–22]. Clay-based adsorbents for adsorption of gases such as CO, CO₂, CH₄, H₂S, heavy metal ions, and organic pollutants were also investigated. Volzone et al. [23] investigated the adsorption of CO, CO₂, and SO₂ gases on HCl-treated bentonite clay and obtained CO, CO₂, and SO₂ adsorption capacity of 0.1 mmol/g, 0.44 mmol/g, and 1.02 mmol/g, respectively. Benkacem et al. [24] studied the adsorption of C₂H_2, N₂, CH_4, O₂, CO₂, and CO over halloysite, kaolinite, and montmorillonite and found that the adsorption capacities over these adsorbents were low (0.03–0.06 mmol/g). Diatomite is a siliceous sedimentary rock made from the skeletons of aquatic plants [25]. Amorphous silica is the main component of diatomite, and additionally, some metal oxides, clays, and salts are found. Diatomite shell has a high porosity thanks to the presence of a regular pattern of tiny holes. It is has been widely used as drug carriers, filter aid, and adsorbents [25–27]. Bello et al. [28] modified diatomaceous Earth with manganese oxide and found that the MnO-modified diatomite exhibited high removal efficiency of Zn, Cu, Pb, Ni, and Cd. Medjdoubi et al. [29] reported that the local diatomite treated with HCl showed a high dye adsorption capacity of 39.89 mg Janus Green B/g diatomite. Lamastra et al. [30] silanized diatomite surface with bis (triethoxysilylpropyl) disulfide, used as a highly efficient rubber reinforcement filler due to its ability to form chemical bonds with elastic molecules during vulcanization. Yuan et al. [31] reported that the surface functionalization diatomite with APTES increased Cu (II) adsorption efficiency by more than 13 times as compared to that of the original diatomite.

To the best of our knowledge, CO adsorption on diatomite has not been studied. In this paper, we report the treatment of diatomite with HCl, followed by modification with CuCl using an “atomic implantation” method. We performed CO adsorption on modified samples at different temperatures of 20^°C, 30^°C, and 40^°C. We found that the effectiveness of Cu⁺ adsorptive sites plays an important role in the CO adsorption capacity of CuCl-modified diatomite.

2. Materials and Methods

2.1. Acid Treatment of Diatomite

Raw diatomite from Lam Dong Province, Vietnam, was used in this investigation. Acid treatment was done by mixing the diatomite with HCl of 1 M concentration. The ratio of diatomite to HCl was fixed as 1 g diatomite to 20 mL of HCl solution (1 M). The mixture was then magnetically stirred at room temperature for 1 h under reflux condition. The mixture was filtered to separate diatomite from solution. Solid product was washed with distillate water until pH of 7 and then dried in an oven at 100^°C for 5 h.

2.2. CuCl Modification of Diatomite

An atomic implantation method was used to modify diatomite with CuCl (denoted as CuCl/diatomite). The CuCl/diatomite materials were synthesized in a tubular reactor that has two compartments, separated by a film of quartz fibers. A certain amount of diatomite and CuCl was introduced into each compartment. After that, tubular reactor was placed in a quartz reactor and the N₂ gas stream was blown through to expel the oxygen in the reactor. The reaction system was heated to 450°C with a heating rate of 10°C/min. At this temperature, CuCl is dissociated into Cl⁻ and Cu⁺ ions and is fed into a diatomite carrier by a stream of N₂ gas (60 mL/min) to form CuCl/diatomite material (Scheme 1). After one hour of keeping the reaction system at 450°C, the reaction system was cooled down to room temperature.

[figure omitted; refer to PDF]

Diatomite modified with 1, 2, and 3 times CuCl was denoted as follows: 1CuCl/diatomite, 2CuCl/diatomite, and 3CuCl/diatomite.

2.3. Characterization of Diatomite and CuCl/Diatomite Samples

XRD patterns of diatomite and CuCl/diatomite were measured on D8 advance diffractometer (Bruker, Germany). The morphology of CuCl/diatomite was characterized by scanning electron microscopy (SEM, JEOL JSM 6500F) and transmission electron microscopy (JEOL JEM 1400). Transmission electron microscopy studies were performed using a JEOL JEM 1400 at 200 kV acceleration voltage. The composition of CuCl/diatomite samples was determined by energy dispersive spectroscopy (JEOL JED-2300 spectrometer). The FTIR of diatomite and CuCl/diatomite measurement was performed using a Jacos 4700 spectrometer. The Tristar-3000 adsorbent device with N₂ adsorbent was used to determine the surface area, total capillary volume, and capillary diameter of CuCl/diatomite materials. CO-TPR of CuCl/diatomite samples was performed on an AutoChem II 29020 (USA) instrument-micromeritics coupled with a thermal conductor detector (TCD). XPS analysis of CuCl/diatomite was measured using a Thermo ESCALAB spectrometer (USA) employing a monochromic Al Kα source at 1486.6 eV.

2.4. CO Adsorption Measurements

Before the adsorption measurements, the samples were degassed under vacuum at 150^°C for 4 h. CO adsorption isotherms were measured at 20^°C, 30^°C, and 40^°C using astatic volumetric apparatus (NOVE1000 e, Quantachrome Inc., Boynton Beach, FL, USA). High-purity CO (99.99%) was used for the adsorption measurements. The adsorption temperatures were controlled by a glycol water circulating bath.

3. Results and Discussion

3.1. Structural Characterization of CuCl/Diatomite

XRD patterns of CuCl-modified diatomite samples are presented in Figure 1. In the XRD pattern of diatomite appeared the peak at 21.98^°, which is characteristic for cristobalite of diatomite phase [32, 33]. In XRD patterns of CuCl-modified diatomites, the peaks at 28^°, 33^°, 45^°, and 56^° were assigned to (111), (200), (220), and (311) of CuCl phase, respectively [34]. In addition, the increase in those peak intensities with increasing the CuCl content was observed, indicating the enhancement of crystallinity.

[figure omitted; refer to PDF]

The FTIR spectra of diatomite and CuCl-modified diatomites are presented in Figure 2. In the FTIR spectrum of diatomite appeared to the bending vibration of Si–O–Si groups at 466 cm−1, symmetric stretching vibration at 795 cm−1 of Si–O groups, and asymmetric vibration at 1091 cm−1, of Si–O groups respectively. The band at 3453 cm⁻¹ is attached to the stretching vibration of Si–OH groups [35, 36]. In the FTIR spectra of CuCl-modified diatomites, the bands that are characteristic for diatomite were also observed. The additional band was observed at 669 cm⁻¹, which is attached to the stretching vibration of Si–O–Cu groups [37].

[figure omitted; refer to PDF]

The element composition of diatomite and CuCl/diatomites determined by EDX is given in Table 1.

Table 1

Element composition (wt%) of diatomite and CuCl-modified diatomite determined by EDX.

Element	Diatomite	1CuCl/diatomite	2CuCl/diatomite	3CuCl/diatomite
O	58.18	50.48	43.09	38.62
Si	41.82	36.56	32.45	27.46
Cu	-	8.22	15.80	21.62
Cl	-	4.74	8.66	12.3
Total	100	100	100	100

As seen in Table 1, with increasing the CuCl content into diatomites, O and Si content decreased from 50.48 wt% to 38.62 wt% and 36.56 wt% to 27.46 wt%, respectively, while Cl and Cu content increased from 4.74% wt to 12.3 wt% and 8.22 wt% to 21.61 wt%, respectively. This result can be explained by the fact that O and Si elements were replaced by CuCl.

EDX elemental mapping images of 2CuCl/diatomite are illustrated in Figure 3. As seen in Figure 3(c), nano Cu of 50–60 nm with uniform distribution was observed.

[figure omitted; refer to PDF]

The SEM images of diatomite and CuCl-modified diatomites are illustrated in Figure 4.

[figures omitted; refer to PDF]

As seen in Figure 4(a), diatomite has morphology of cylindrical in shape with a diameter of 8–12 μm and length of 5–10 μm. Diatomite is a porous material with an average pore size of 0.5 μm. The SEM image of CuCl-modified diatomites was similar to that of unmodified diatomite, and only reduction of pores was observed. In order to see better the deposition of CuCl particles on the surface and within the pore system of diatomite, we performed the analysis by using the TEM method. Figure 5 shows the TEM image of 2CuCl/diatomite. As seen in Figure 5, CuCl nanoparticles (dark color) well deposited on the surface and filled up the pores of diatomite, causing the reduction of pore size to 20–30 nm (bright color).

[figure omitted; refer to PDF]

The N₂ adsorption-desorption isotherms of all diatomite samples exhibit a type V sorption isotherm, according to the IUPAC classification (Figure 6) [38]. The hysteresis loop observed is due to the capillary condensation of nitrogen occurred in the mesoporous channels.

[figure omitted; refer to PDF]

Pore volume (V_pore), average pore diameter (D_BJH), and specific surface (S_BET) are listed in Table 2.

Table 2

Textural properties of diatomite and CuCl-modified diatomites.

Sample	SBET (m2/g)	V Pore (cm3/g)	DBJH (nm) (average pore diameter)
Diatomite	24.34	0.117	11.7
1CuCl/diatomite	22.83	0.078	7.2
2CuCl/diatomite	18.16	0.068	6.7
3CuCl/diatomite	14.32	0.054	5.7

As observed in Table 2, surface area of CuCl-modified diatomites decreased with increasing CuCl content. This indicated the deposition of CuCl particles on adsorptive sites of diatomite, causing the decrease of N₂ adsorption capacity and consequently decreasing the surface area. Similarly, due to the deposition of CuCl particles on the pore walls of diatomite, volume pore and average pore diameter decreased with increasing the CuCl content.

CO-TPR profiles of diatomite and CuCl/diatomite samples are presented in Figure 7. In the CO-TPR profiles of CuCl-modified diatomites appeared a large peak at 438^°C, which is characteristic for the reaction of Cu⁺ to Cu^° [39, 40]. No peak at 340^°C, which is typical for the reduction of Cu²⁺ to Cu⁺, is noted, indicating the existence of Cu⁺ ions in the CuCl-modified diatomites. Additionally, the peak intensity at 438^°C increased with increasing CuCl content was observed. This proved the CO reduction of Cu⁺ ions in the CuCl-modified diatomites (see Table 3). As seen in Table 3, CO reduction amount increased with increasing CuCl content (Cu⁺ active sites). Thus, CO reduction amount increased from 1.42 cm³/g to 3.78 cm³/g with increasing Cu content from 8.22 wt% to 21.62 wt%. No CO reduction on diatomite was noted.

[figure omitted; refer to PDF]

As observed in Figure 9, CO adsorption capacity of CuCl/diatomites is much higher than that of unmodified diatomite. Especially, 2CuCl/diatomite showed the CO adsorption capacity of 2.96 mmol/g which was 10 times higher than that of diatomite (0.29 mmol/g). This can be attributed to the difference in the nature of CO adsorption. On the surface of diatomite, CO is physically adsorbed via van der Waals force. In contrast, CO is chemically adsorbed on CuCl-modified diatomites via the interaction between Cu⁺ ions and CO to form π-complexation bonds [15]. The Cu²⁺ ions in the sample and CO are linked by the electrostatic bond and unable to form formation of the π-complexation bond [13]. The content of Cu significantly affects CO adsorption capacity. Increasing CuCl content in modified diatomites, CO adsorption capacity was from 1.7 mmol/g to 2.96 mmol/g. However, further increase of CuCl content leads to the decrease of CO adsorption capacity from 2.96 mmol/g to 2.23 mmol/g. This may be due to the agglomeration of CuCl particles which caused the low effectiveness of Cu⁺ adsorptive sites and reduction of surface area [14]. Thus, effectiveness of 2CuCl/diatomite reached the value of 1.198 CO mmol/_{Cu atom} which was nearly twice higher than that of 3CuCl/diatomite (0.66 CO mmol/Cu atom).

Carbon monoxide adsorption isotherms of 1CuCl/diatomite, 2CuCl/diatomite, and 3CuCl/diatomite are shown in Figures 10–12. For all CuCl/modified diatomite samples (Figures 10–12), CO adsorption capacity increased with increasing the adsorption temperature from 20^°C to 30^°C. Further increase of adsorption temperature from 30^°C to 40^°C led to the decrease of the CO adsorption capacity. CO adsorption capacity on 1CuCl/diatomite at 20^°C, 30^°C, and 40^°C was 1.55 mmol/g, 1.74 mmol/g, and 1.42 mmol/g, respectively. CO adsorption capacity on 2CuCl/diatomite at 20^°C, 30^°C, and 40^°C was 2.63 mmol/g, 2.96 mmol/g, and 2.47 mmol/g, respectively. The above saturated CO adsorption capacity of 3CuCl/diatomite at 20^°C, 30^°C, and 40^°C was 1.72 mmol/g, 2.23 mmol/g, and 1.59 mmol/g, respectively. Because carbon monoxide (CO) adsorption is exothermic (ΔH < 0) and it is thermodynamically restricted at high temperature, one would expect that increasing adsorption temperature will lead to a decrease in the adsorption capacity. In our case, CO adsorption behavior on Cu (I)-modified diatomites was very different. This can be rationalized as follows: when CO adsorption temperature increased from 20^°C to 30^°C, the diffusion to the Cu⁺ adsorptive sites is promoted, resulting in the enhance of CO adsorption capacity. However, further increase of adsorption temperature leads to the decrease of carbon monoxide adsorption capacity because at high adsorption temperature (40^°C), CO desorption is favored, resulting in the decrease of carbon monoxide adsorption capacity [45].

[figure omitted; refer to PDF][figure omitted; refer to PDF][figure omitted; refer to PDF]

3.3. Langmuir–Freundlich (L-F)

The L-F model is applied to the adsorption isotherm of CO on CuCl/diatomite samples [46]. The L-F model can be expressed in the following representation: $q=q_{m}b.p^{1/n}/1+b.p^{1/n}$ where q_m is the saturation adsorbed amount, q is the adsorbed amount, p is the pressure, n is the corresponding deviation from the Langmuir isotherms, and b is the adsorption affinity.

Figure 13 shows the relationship between ln (q) (as a function) and ln (p) at 2CuCl/diatomite different temperatures. Langmuir–Freundlich fitting parameters of carbon monoxide isotherms on CuCl-modified diatomites are listed in Table 4.

[figure omitted; refer to PDF]

Table 4

Langmuir–Freundlich fitting parameters of carbon monoxide isotherms on CuCl-modified diatomites.

Sample	Temperature (°C)	qm (mmol/g)	b	N	R2
1CuCl/diatomite	20	1.55	0.203	1.066	0.996
	30	1.74	0.214	1.083	0.999
	40	1.42	0.263	1.086	0.998
2CuCl/diatomite	20	2.63	0.206	1.011	0.995
	30	2.96	0.257	1.024	0.999
	40	2.48	0.260	1.046	0.998
3CuCl/diatomite	20	1.72	0.245	1.015	0.998
	30	2.23	0.235	1.021	0.998
	40	1.59	0.243	1.024	0.997

Experimental data (Figure 13 and Table 4) show that the CO adsorption fits well with the L-F model. High R² values of 0.992–0.997 were obtained for all Cu (I)-modified diatomites. The highest adsorption capacity (q_m) was achieved at 30^°C. Among the three samples, the 2CuCl/diatomite sample exhibited the highest carbon monoxide (CO) adsorption capacity of 2.96 mmol/g.

4. Conclusion

Modification of diatomite with CuCl was achieved by using a novel “atomic implantation” method in which CuCl was incorporated into diatomite in a vapor phase. The loading of CuCl on diatomite was varied by repeating the CuCl incorporation times.

The results are obtained from characterization using FTIR, XRD, BET, SEM-TEM, EDX, and CO-TPR. XRD revealed the presence of CuCl nanoparticles with a particle size of 50–60 nm. The CuCl particles were uniformly distributed on the surface of diatomites. CO adsorption on diatomite and CuCl-modified diatomites was evaluated. We found that modified diatomites exhibited much higher CO adsorption capacities compared to that of unmodified sample.

Among CuCl-modified diatomites, the 2CuCl/diatomite sample showed the highest carbon monoxide adsorption capacity of 2.96 mmol/g at 30^°C which was 10 times higher than that of unmodified diatomite. This result can be explained on basis of the effectiveness of Cu⁺ adsorption sites.

Our adsorption data showed that carbon monoxide adsorption on CuCl/diatomites fits well with the Langmuir–Freundlich model.

Acknowledgments

This research was funded by the Vietnam Academy of Science and Technology-(VAST) under grant number “TĐPCCC.01/18–20 and TĐPCCC.03/18–20.”

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