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

Institute of Chemistry, Vietnam Academy of Science and Technology (VAST), 18 Hoang Quoc Viet Street, Cau Giay, Ha Noi, Vietnam Institute of Research and Development, Duy Tan University, Da Nang 550000, Vietnam Faculty of Chemistry, Hanoi National University of Education (HNUE), Hanoi 100000, Vietnam Faculty of Chemical Technology, Hanoi University of Industry (HUI), 298 Minh Khai Bactuliem, Ha No. 100000, Vietnam Center for Research and Technology Transfer, Vietnam Academy of Science and Technology (VAST), 18 Hoang Quoc Viet Street, Cau Giay, Ha Noi, Vietnam


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][2][3][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]. erefore, 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 lowcost 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][7][8][9][10][11][12][13][14][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. e formation of π-complexation bond between Cu (I) ions molecules and CO on the adsorbent surface could improve CO adsorption selectivity.
us, 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 cm 3 /g, 4 times higher than that of activated carbon (11.5 cm 3 /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, ZnCl 2 /MCM-41, and Zn-Cu (I)/MCM-41 materials for CO adsorption. e author reported that the materials CuCl/MCM-41, ZnCl 2 / 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/CO 2 , CO/CH 4 , and CO/ N 2 adsorption selectivity. Xie et al. [16] found that modifying zeolite with CuCl increases CO selectivity from a CO and CO 2 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][19][20][21][22]. Clay-based adsorbents for adsorption of gases such as CO, CO 2 , CH 4 , H 2 S, heavy metal ions, and organic pollutants were also investigated. Volzone et al. [23] investigated the adsorption of CO, CO 2 , and SO 2 gases on HCl-treated bentonite clay and obtained CO, CO 2 , and SO 2 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 C 2 H 2, N 2 , CH 4, O 2 , CO 2 , 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][26][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.

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. e ratio of diatomite to HCl was fixed as 1 g diatomite to 20 mL of HCl solution (1 M). e mixture was then magnetically stirred at room temperature for 1 h under reflux condition. e 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.

CuCl Modification of
Diatomite. An atomic implantation method was used to modify diatomite with CuCl (denoted as CuCl/diatomite). e 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 N 2 gas stream was blown through to expel the oxygen in the reactor. e 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 N 2 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.

Characterization of Diatomite and CuCl/Diatomite
Samples. XRD patterns of diatomite and CuCl/diatomite were measured on D8 advance diffractometer (Bruker, Germany). e 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. e composition of CuCl/diatomite samples was determined by energy dispersive spectroscopy (JEOL JED-2300 spectrometer). e FTIR of diatomite and CuCl/diatomite measurement was performed using a Jacos 4700 spectrometer.
e Tristar-3000 adsorbent device with N 2 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) instrumentmicromeritics coupled with a thermal conductor detector (TCD). XPS analysis of CuCl/diatomite was measured using a ermo ESCALAB spectrometer (USA) employing a monochromic Al Kα source at 1486.6 eV.

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.
e adsorption temperatures were controlled by a glycol water circulating bath.

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.
e 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. e 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. e additional band was observed at 669 cm −1 , which is attached to the stretching vibration of Si-O-Cu groups [37]. e element composition of diatomite and CuCl/diatomites determined by EDX is given in Table 1.
As seen in Table 1 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.
e SEM images of diatomite and CuCl-modified diatomites are illustrated in Figure 4.
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. e 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). e N 2 adsorption-desorption isotherms of all diatomite samples exhibit a type V sorption isotherm, according to the IUPAC classification ( Figure 6) [38]. e hysteresis loop observed is due to the capillary condensation of nitrogen occurred in the mesoporous channels.
Pore volume (V pore ), average pore diameter (D BJH ), and specific surface (S BET ) are listed in Table 2.
As observed in Table 2, surface area of CuCl-modified diatomites decreased with increasing CuCl content. is indicated the deposition of CuCl particles on adsorptive sites of diatomite, causing the decrease of N 2 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 Cu 2+ 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. is 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).
us, CO reduction amount increased from 1.42 cm 3 /g to 3.78 cm 3 /g with increasing Cu content from 8.22 wt% to 21.62 wt%. No CO reduction on diatomite was noted.
XPS was used to investigate the oxidation states of the copper element in the prepared 2CuCl/diatomite adsorbents.
e survey XPS spectrum of 2CuCl/diatomite is presented in Figure 8. In Figure 8 Cu 2p at 936.34 eV [30,41,42], except for the C 1 s at 285.18 eV peak arising from adventitious hydrocarbon in the XPS instrument [42,43]. No characteristic peak of foreign element was observed. In the Si 2p spectrum, the peak of Si 2p at 100.7 eV corresponds to Si atoms of siloxane groups, the main component of diatomite [30]. In Figure 8(c), the binding energy of O 1 s at 534.79 eV was assigned to oxygen atoms of siloxane groups (Si-O-Si) [30]. In Figure 8( [44]. e characteristic peaks of Cu + are more intense than the characteristic peaks of Cu 2+ , and this proves the dominance of Cu + content in the 2CuCl/diatomite sample.

Carbon Monoxide Adsorption on Diatomite and CuCl-Modified
Diatomites. CO adsorption isotherms of diatomite and CuCl-modified diatomite at 30°C are presented in Figure 9.  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). is 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]. e Cu 2+ ions in the sample and CO are linked by the electrostatic bond and unable to form formation of the π-complexation bond [13]. e 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. is may be due to the agglomeration of CuCl particles which caused the low effectiveness of Cu + adsorptive sites and reduction of surface area [14]. us, 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).
e 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 Journal of Chemistry adsorption capacity. In our case, CO adsorption behavior on Cu (I)-modified diatomites was very different. is 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].

Langmuir-Freundlich (L-F).
e L-F model is applied to the adsorption isotherm of CO on CuCl/diatomite samples [46]. e L-F model can be expressed in the following representation:      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.
Experimental data ( Figure 13 and Table 4) show that the CO adsorption fits well with the L-F model. High R 2 values of 0.992-0.997 were obtained for all Cu (I)-modified diatomites. e highest adsorption capacity (q m ) was achieved at 30°C. Among the three samples, the 2CuCl/diatomite sample

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. e loading of CuCl on diatomite was varied by repeating the CuCl incorporation times.
e 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.
e 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.
is 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.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.