Abstract
Nanostructured copper (Cu)/nickel- (Ni-) coated nanoporous carbon sheets (NCS) were prepared to improve the toxic acidic vapor (hydrogen chloride, HCl) removal efficiency of NCS using a continuous bimetal electroplating method at various metal content ratios. The surface morphology and nanostructure of Cu/Ni-NCS were observed by scanning electron microscopy and X-ray diffraction, respectively. N2/77 K adsorption isotherms were investigated using the Brunauer-Emmett-Teller equation. HCl vapor removal efficiency was confirmed using two types of detection techniques: a gas detecting tube for low concentrations and gas chromatography for high concentrations. HCl removal efficiency was improved mainly in the copresence of nanostructured Cu/Ni clusters compared to the efficiencies of the as-received and single-metal-plated NCS. In particular, the removal efficiency of Cu/Ni-3 was increased by 270% compared to that of as-received sample, but Cu/Ni-5 showed lower efficiency than Cu/Ni-3, indicating that suitable metal composition on NCS can accelerate HCl removal behaviors of the NCS.
1. Introduction
Hydrogen chloride (HCl) vapor is a by-product of various incineration processes and a serious contributor to atmospheric pollution, such as smog and acid rain. HCl vapor is easily changed to liquid HCl acid, which is very toxic, eroding metals, inducing cancer, and acting as a precursor to dioxin in the burning of garbage (EPA’s permissible exposure limit value is 4.7 ppm). Therefore, there has been a strong push to remove HCl vapor from domestic and industrial combustion processes [1].
Nanoporous carbon sheets (NCS) are promising material for the removal of gas-phase pollutants [2–9], such as nitric oxides () [3, 5], sulfuric oxides () [1, 6, 8, 10], carbon dioxide (CO2), and even HCl [1, 9–12] vapor. Normally, the surface of neat NCS does not have strong functional groups or catalytic active sites due to their origin. Basically, the gas-phase pollutant removal efficiency of the NCS at room temperature is proportional to their specific surface area and micropore volume. The removal efficiency can be enhanced dramatically for several pollutants by adding a small content of active materials, such as functional groups or metal salts on the NCS [1, 2, 10–13].
This is why many researchers examine the surface modification or metal salt impregnation methods on PCs [14–17].
As one of the metal-doping methods onto carbon surfaces, electrolytic metal plating, such as copper, nickel, and silver, of nonconducting porous carbonaceous materials has been studied using a conductive-support-assistance method, such as metallic meshes [13, 16].
In a previous study [4, 13], copper (Cu), silver (Ag), and nickel (Ni) nanoparticles on carbon were found to be so effective in HCl removal because they immediately formed CuCl2, AgCl, and NiCl2, respectively, without any side reactions when they came in contact with HCl vapor. Several single metals plated by electrometal plating have been studied with regard to their efficacy for HCl removal. On the other hand, the effects of bimetallic cluster catalysts on HCl removal have not been reported.
This paper reports the effects of bimetallic catalysts composed of Cu and Ni clusters on porous carbon sheets in the removal of HCl vapor. Bimetallic catalysts were prepared using a continuous metal electroplating technique for nonconducting carbonaceous materials at different current densities.
2. Experiments
2.1. Materials and Methods
The NCS used in this study are nonwoven type sheets (specific surface area is 1350 m2/g and thickness is 3 mm) supplied by Korea ACF Co. Prior to use, the impurities in the PCs were removed via Soxhlet extraction by boiling with acetone at 80°C for 2 h. The PCs were then washed several times with distilled water and dried in a vacuum oven at 120°C for 12 h.
For the electroplating of copper (Cu) and nickel (Ni) metals, the NCS were electroplated continuously using a home-made device, as shown in Figure 1. Prior to plating the NCS, each sample was immersed in 10 wt.% HNO3 for 5 min at room temperature in order to remove the impurities on the carbon surfaces and rinsed with distilled water. The purified NCS were activated in an aqueous solution of 0.38 M K2Cr2O7/4.5 M H2SO4 and refluxed in a water bath and maintained at 60°C for 2 h to enhance the interfacial adhesion between the nickel particles and the NCS surfaces. An electroplating device that can plate nanostructured metallic Cu and Ni continuously onto the NCS surfaces was constructed. The metal plating rate was controlled by the current density with a fixed collecting speed of 0.1 m/min [4, 13]. A mechanical sample collecting winder which can collect the sample sheet with fixed tension is placed at the right end. In order to avoid excessive tension load on the sample sheet, each bar (graphite and metals) is free to rotate.
Table 1 lists the formulation of the Cu and Ni electroplating bath and operation conditions [16, 18]. Cu and Ni sulfates were the main salts used in the electroplating solution, and Cu and Ni plates were used as the anode. The Cu and Ni mesh rolls were used as the cathode. The NCS were closely attached to the rolls to enhance the electric conductivity of the sheets, resulting in good Cu and Ni electroplating on the NCS. The current densities were fixed to 30 A/m2 for Cu plating and were in the range of 10 to 50 A/m2 for Ni plating. The samples were named Cu/Ni-1 (10 A/m2), Cu/Ni-2 (20 A/m2), Cu/Ni-3 (30 A/m2), and Cu/Ni-5 (50 A/m2) with the current densities for Ni plating. pH was controlled by diluted sulfuric acid solution with automatic pH meter. The temperature was controlled by heating coils placed under plating baths. The current density of each plating batch was controlled by a power supply which can give fixed electric current. The metal content was measured by atomic absorption spectrophotometry (AAS).
2.2. Surface Nanostructures and Morphologies
To examine the change in the nanostructures of the metal-coated NCS, X-ray diffraction (XRD, Rigaku Model D/MAX-III B) was carried out using a rotation anode with CuKα radiation.
Scanning electron microscopy (SEM, JEOL JSM-840A) was used to observe the surface morphology of the Cu/Ni-coated NCS as well as the distribution of metal particles before and after HCl vapor removal tests. Energy-dispersive X-ray spectrometry (EDS, LINK system AN-10000/85S) was used to observe the formation of metal chlorides after HCl vapor removal tests.
The nitrogen (N2) adsorption isotherms at 77 K were measured using BELSORP-max (BEL Japan). Prior to analysis, the samples were degassed at 573 K for 9 h to obtain a residual pressure of <10−5 mmHg. The amount of N2 adsorbed onto the samples was used to calculate the specific surface area using the Brunauer-Emmett-Teller (BET) equation [19].
2.3. HCl Vapor Removal Tests
Two types of detecting methods were used as a highly accurate HCl removal test. A gas detection tube technique was employed at low concentrations (<30 ppm), whereas a gas chromatograph technique was selected for high concentrations (>30 ppm) [3, 10, 14].
A gas detecting tube (GASTEC No.; 14L, range 1–40 ppm) was used to measure HCl vapor removal efficiency. Before HCl removal experiment, all samples and the reactor were purged with high purity N2 gas (99.9%) at room temperature for 1 h to remove the residual moisture. Approximately 0.1 g of the samples was packed with a cylindrical quartz tube and HCl vapor with a concentration of 1013 ppm and N2 balance was injected at 298 K. The gas flow rate was maintained at 50 mL/min using a mass flow controller. HCl vapor removal efficiency was determined from HCl concentration at the outlet reactor.
A gas chromatograph (DS 6200 model, Donam Co., Korea) with a thermal conductivity detector was used. The samples (0.1 g of each sample) were packed into a cylindrical quartz tube and HCl vapor (1013 ppm of N2 balance) was injected. The gas-feeding rate was maintained at 50 mL/min using a mass flow controller. HCl vapor removal efficiency was determined from HCl concentration at the outlet reactor. Before each analysis, HCl vapor adsorption curves were gained by using 300, 600, and 1000 ppm HCl standard gas.
3. Results and Discussion
3.1. Cu/Ni Bimetal Plating
Figure 2 shows the metal content obtained from the AAS results for the Cu/Ni-coated NCS (Cu content was fixed at 3.7 wt.%). Ni content on the NCS surfaces increased steadily to 6.12 wt.% with increasing current density. The specific content ratio of Ni in the bimetallic cluster increased from 28% at Cu/Ni-1 to 62% at Cu/Ni-5. Cu/Ni-3 showed a similar content ratio of both metals. As mentioned above, the collecting speed for metal plating in the continuous electroplating process was fixed. Therefore, Ni plating rate is obviously dependent on the input current density. Ni content of Cu/Ni-2 was 0.99 wt.% higher than that of Cu/Ni-1. Ni content of Cu/Ni-3 was 1.12 wt.% higher than that of Cu/Ni-2. A similar behavior was also observed at Cu/Ni-5 (1.28 wt.% per 10 A/m2). This increase suggests that the increase in Ni content enhances the specific conductivity of the metal-coated NCS, resulting in an acceleration of the metal plating rate.
Figure 3 exhibits the mechanism of metal deposition on carbon surfaces. In the initial plating state, newly coated nickel nanoparticles are placed separately or near copper nanoparticles due to the higher metal-metal interaction when compared to carbon-metal interaction. In the medium state, nickel nanoparticles make clusters with copper nanoparticles because metal-metal interaction is much higher than metal-carbon interaction. This sate can be seen Ni/Cu copresence state. In the final state, rich nickel nanoparticles mainly cover the surfaces of copper nanoparticles, so that effective removal behaviors by Ni/Cu copresence can be diminished. This schematic can have good correlation with XRD data.
3.2. Surface Morphology and Nanostructure
Figure 4 shows the XRD patterns of the Cu/Ni-coated NCS. Cu peaks were observed at 43° 2θ (111) and 49° 2θ (200), and Ni peaks were observed at 44° 2θ (111) and 51° 2θ (200). The intensity of Cu peaks was almost regular due to the fixed current density under the plating condition. Ni peaks at 44° and 51° 2θ increased proportionally with increasing current density for Ni plating, indicating that the content of Ni in Cu/Ni clusters had increased steadily. These results show good agreement with Figures 2 and 3.
The crystalline size of Cu and Ni particles on the NCS was calculated as a function of the coating time from the XRD results using Scherrer equation [20]:where is the crystalline size (nm), is the Scherrer constant (=0.9), is the X-ray wave length (CuKα = 0.154 nm), is the Bragg angle, and is full width at half max.
Table 2 shows XRD results and crystal size of Cu(111) and Ni(111) with Ni content. It was found that Cu nanoparticles showed very uniform size of 10.5 nm at each sample. However, the crystal size of Ni(111) was decreased with current density, indicating that surface resistivity of Cu-precoated NCS affected Ni deposition. In our previous work [16], high current density for metal plating can cause fine metal particles due to the change of surface resistivity.
SEM analysis was used to observe the morphology of Cu/Ni clusters introduced on the NCS before and after HCl removal tests (Figure 5). The nickel particles grew gradually and formed island-like metallic clusters that were well coated over the surface, as shown in Figure 5(b), whereas the surface of the as-received NCS was quite clean.
(a)
(b)
(c)
(d)
Figures 5(c) and 5(d) show SEM image and EDS result of Cu/Ni-3 sample after HCl test, respectively. The size of Cu/Ni clusters increased after HCl removal. This suggests that Cu/Ni clusters reacted chemically with HCl vapor and might form metal chlorides. Figure 5(d) can be obvious evidence for the formation of metal chlorides after HCl removal test. As shown in Figure 5(d), Cu, Ni, and Cl atoms were observed together, meaning that and were newly formed by a reaction with HCl vapor [4, 13, 17].
3.3. Hydrogen Chloride Vapor Removal
Figure 6 shows HCl vapor removal efficiency of Cu/Ni-coated NCS. The as-received NCS showed good HCl removal efficiency for a removal time up to 20 min and then HCl concentration at the outlet reactor increased rapidly. This suggests that HCl removal behavior of the as-received NCS occurred mainly via physisorption mechanism due to the sharp breakthrough curve (K1). On the other hand, the gradient between 25 and 35 min (K3) decreased slightly, meaning that a mild chemical reaction could have occurred by the PCs themselves.
In the case of Cu/Ni-coated NCS, all Cu/Ni samples showed a significantly higher HCl removal efficiency than the as-received NCS, and the total removal time (up to breakthrough concentration) was proportional to the metal content. On the other hand, the removal efficiency of Cu/Ni-5 sample was lower. This can be explained by the fact that excessive metal plating of over Ni 6.1 wt.% (total 9.8 wt.%) can cause a decrease in HCl removal efficiency because of the severe pore filling behavior.
Interestingly, the gradient of the removal efficiency curves for Cu/Ni-2,3,5 has “K2” region, and the “K3” gradient is significantly lower than that of the as-received and Cu/Ni-1 PCs, even though the “K1” gradient was almost the same in all samples in this work. These results clearly mean that Cu/Ni introduction enhanced the physisorption capacity of the samples dramatically, and the chemisorption capacity also increased. Moreover, the patterns of the removal efficiency curve between Cu/Ni-3 and 5 were quite similar, indicating that the chemisorption behaviors of the two samples were similar, but the excessive metal content of Cu/Ni-5 caused a decrease in the physisorption capacity of the PCs.
Figure 7 shows the relationship between the specific surface area and breakthrough time of the samples. The specific surface area of all Cu/Ni samples ranged from 900 to 1,000 m2/g (10% margin of error). On the other hand, the breakthrough time increased by 250% at Cu/Ni-3 compared to the as-received NCS. This suggests that the physisorption capacity of the NCS is controlled not only by the large specific surface area but also by the removal effects of the metal cluster loading [21–25].
To observe the effect of the total metal content on HCl removal capacity, single Cu or Ni-coated samples with a similar total metal content of Cu/Ni-3 and -5 were prepared, and their HCl removal results are listed in Table 3. The single Cu or Ni-plated NCS showed much lower breakthrough time at a similar total metal content than that of Cu/Ni-3 sample. This suggests that the copresence of Cu/Ni on the NCS can effectively improve HCl vapor removal ability. It is probably due to the synergetic effects on HCl dissociation by the presence of bimetal particles on NCS surfaces.
In order to confirm reusability of Cu/Ni/PCs, the same HCl removal tests were repeated five times, and results were shown in Table 4. Before the reusability tests, all samples were degassed for 4 h at 120°C. It was found that as-received sample showed almost constant breakthrough time. However, the breakthrough time of Cu/Ni-3 decreased rapidly and then showed stable state after 3rd repeating tests. This result indicates that the as-received sample removes HCl gas mainly by physisorption mechanism, but Cu/Ni-3 has mixed removal mechanisms, such as physisorption/chemisorption.
4. Conclusions
Cu/Ni-coated NCS with various content ratios were prepared using a continuous electroplating technique as a function of the inputted current densities. Ni content increased with increasing current density, and HCl removal capacity was in proportion to Ni content up to Cu/Ni-3 sample. HCl removal behavior of Cu/Ni-coated samples was improved effectively by the enhanced physisorption and chemisorption to form metal chlorides. A comparison of the single metal loading showed that the copresence of Cu and Ni on the PCs resulted in excellent HCl removal abilities compared to single Cu or Ni samples with a similar metal content.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgments
This study was supported by a grant from the “Carbon Valley R&D Project (Project no. R0002651) and Material & Component Technology Development Project (Project no. 10050391)” funded by the Ministry of Trade, Industry & Energy (MOTIE), Republic of Korea.