Nanoparticles Dispersed in Activated Carbon Fibers for the Oxygen

Nov 14, 2012 - A novel dispersion of CuCl2 nanoparticles (∼90 nm) in activated carbon microfibers (ACFs) was prepared and used for the production of...
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CuCl2 Nanoparticles Dispersed in Activated Carbon Fibers for the Oxygen Production Step of the Cu−Cl Thermochemical Water Splitting Cycle Bhaskar Bhaduri,† Yogendra Nath Prajapati,† Ashutosh Sharma,†,‡ and Nishith Verma*,†,§ †

Department of Chemical Engineering, ‡Center for Nanosciences, and §Center for Environmental Science and Engineering, Indian Institute of Technology Kanpur, Kanpur-208016, India ABSTRACT: The thermochemical copper−chlorine (Cu−Cl) cycle is a promising method for the production of hydrogen and oxygen. It consists of four steps: chlorination, disproportionation, oxychlorination, and decomposition. In this study, we focused on the last two steps. A novel dispersion of CuCl2 nanoparticles (∼90 nm) in activated carbon microfibers (ACFs) was prepared and used for the production of oxygen through the oxychlorination of CuCl2 with steam, which was followed by the decomposition of the oxychlorinated product. The CuCl2-ACFs were prepared using the wet incipience impregnation method. After calcination of the impregnated ACF, CuCl2 nanoparticles were produced in situ on the ACF. The production rate of O2 was found to be 2.7 × 10−6 mol/g·s, which is significantly higher than the data reported in the literature. The CuCl2-ACFs prepared in this study are a potential candidate for the O2 production step of the thermochemical Cu−Cl cycle.

1. INTRODUCTION Presently, Cu−Cl thermochemical reactions have been suggested for the production of oxygen (O2) and fuel hydrogen (H2) from water. The Cu−Cl cycle is preferred over sulfur− iodine and Cu−SO4 cycles because of the lower reaction temperature (∼550 °C) and its cost-effectiveness. The cycle is safe to operate and does not produce greenhouse gases. Furthermore, the side reactions are negligible.1 There are several variants of the Cu−Cl thermochemical reactions that are used for water splitting.2−5Among them, a four-step Cu−Cl cycle is widely studied. Figure 1 describes the different reactions that are associated with this four-step cycle. As shown, there are three thermal reactions, namely, the exothermic chlorination, the endothermic oxychlorination, and the decomposition, and one electrochemical reaction, namely, the disproportionation of cuprous chloride (CuCl) at 30−80 °C. In the oxychlorination step, cupric chloride (CuCl2) reacts with steam at ∼350 °C to produce copper oxychloride (CuO·CuCl2), which immediately decomposes at approximately 530 °C to generate O2. The CuCl produced from the chlorination and decomposition steps is utilized in the electrolysis reaction to produce CuCl2, which is then recycled to the oxychlorination step. The focus of the present study is on the last two steps: the oxychlorination and subsequent decomposition that produce CuCl and O2. Serban et al. 2 carried out kinetic studies on the decomposition of the synthetic copper oxychloride solid, which is produced using stoichiometric amounts of CuCl2 and CuO, into CuCl and O2 at temperatures between 450 and 550 °C. At 530 °C, the yield of O2 was found to be at a maximum. The overall O2 generation reaction consisted of two steps, namely, the decomposition of CuCl2 into CuCl and Cl2 and the reaction between CuO and Cl2 to produce O2. The first step was shown to be the rate limiting step. Lewis et al.6 experimentally measured the individual reaction rates of the four-step Cu−Cl cycle in a fixed bed reactor. The © 2012 American Chemical Society

study also investigated the optimization of the cycle efficiency using the engineering software Aspen. In another study, Lewis et al.7 carried out the O2 generation reaction using copper oxychloride, which was synthesized from an equimolar mixture of CuO and CuCl2, at 500 °C, and obtained an overall yield of 85%. Naterer et al.8 carried out an energy balance for the different steps of the Cu−Cl thermochemical cycle. In particular, the authors examined several thermal design aspects of a conceptually designed O2 production reactor for the copper oxychloride decomposition step. The authors also examined several major engineering challenges and probable solutions related to the oxygen production reactor. There is also a theoretical model developed to predict the steam and CuCl2 conversion during the hydrolysis reaction carried out in fluidized bed reactors.9 Lewis et al.10 and Wang et al.11 carried out thermodynamic calculations for the CuCl2-hydrolysis reaction at 390 °C and concluded that the larger steam concentrations result in higher extents of the hydrolysis reaction, which improves the process efficiency and avoids the thermal decomposition of CuCl2 into CuCl and Cl2. Similar energy and thermodynamic analyses on the O2 production step were carried out by Orhan et al.12 Ferrandon et al.13 carried out an experimental study on the hydrolysis of CuCl2, which is the oxychlorination step of the Cu−Cl thermochemical cycle, in a spray reactor. The authors studied the effect of the molar ratio of steam to CuCl2 and found that a ratio of 23:1 achieves an almost 100% conversion of CuCl2 into Cu-oxychloride at atmospheric pressure. Naterer et al.14 investigated the noncatalyzed CuCl2 hydrolysis and copper oxychloride decomposition reactions. The experimental results and the simulations showed that an excess amount of Received: Revised: Accepted: Published: 15633

September 11, 2012 November 9, 2012 November 14, 2012 November 14, 2012 dx.doi.org/10.1021/ie302446p | Ind. Eng. Chem. Res. 2012, 51, 15633−15641

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Figure 1. Schematic of the steps involved in the Cu−Cl water splitting thermochemical cycle. We focused on steps 3 and 4 in this study.

2. MATERIALS AND METHODS 2.1. Materials. The phenolic resin precursor-based ACFs were purchased from Nippon Kynol Inc., Japan. The LR grade cupric chloride dehydrate (CuCl2·2H2O) (purity >97%) was procured from Rankem, India. The GR grade sodium dodecyl sulfate (SDS) was purchased from Merck, Germany. Nitrogen (N2) (purity >99%) and hydrogen (H2) (purity >99%) gases were procured from Sigma Gases, India. 2.2. Pretreatment of ACFs. The ACFs that were received from Nippon Kynol Inc. were pretreated in a 0.05 M HNO3 aqueous solution and then washed several times with deionized water (DI). The washed ACFs were dried in an oven at 120 °C for 12 h. Approximately 1 g of the dried sample was wrapped over the perforated portion of a quartz tubular reactor (ID = 0.635 cm and L = 8 cm) and placed in the vacuum oven at 120 °C for 8 h to remove any entrapped gases. After drying, the samples were cooled at room temperature (∼30 °C). 2.3. Preparation of the ACFs with Dispersed CuCl2 Nanoparticles. The CuCl2 nanoparticles were dispersed in the ACFs through the incipient wetness impregnation method. The impregnating solution was prepared with 0.4 M CuCl2·2H2O as the Cu-precursor and 0.3% (w/w) SDS, an anionic surfactant, in 150 mL of DI water. The impregnating solution was thoroughly stirred for 45 min and then recycled through the perforated tubular reactor wrapped with ACF for 6 h using a peristaltic pump (rotor speed = 138 rpm). Figure 2a shows a schematic representation of the impregnation unit used in this study. After impregnation, the wet CuCl2-ACF samples were dried in static air for 6 h at room temperature and then in the oven for 12 h at 120 °C to remove the surface moisture. The samples were then calcined in a programmable, electric horizontal stainless steel (SS) tubular reactor (ID = 30 mm, L = 0.8 m).

steam was needed for the complete conversion of CuCl2 into copper oxychloride. In addition, an Aspen Plus simulation predicted that a 100% yield of copper oxychloride could be achieved with a steam to CuCl2 molar ratio of 17 at 370 °C. Ferrandon et al.15 designed and developed an ultrasonic spray reactor that was fitted with an atomizer to carry out the hydrolysis reaction. The authors achieved a 95% conversion into the desired product with a very small amount of CuCl and did not observe any formation of Cl2. Steam and CuCl2 conversion during the hydrolysis reaction carried out in fluidized bed reactors was also predicted by the theoretical model.16 We have prepared a novel dispersion of cupric chloride (CuCl2) nanoparticles in activated carbon microfibers (ACFs) for the production of O2 from steam through the hydrolysis and subsequent decomposition reactions, with an objective of further improving the reaction rate. The reactions were carried out under different gas flow rates, steam flow rates, and reaction temperatures on the prepared CuCl2-ACF samples, which were wrapped over an especially designed perforated tubular reactor. In recent studies, we developed the phenolic resin precursorbased ACFs to be used as an adsorbent, a support for adsorbents and catalysts, and a support for the growth of carbon nanofibers (CNFs) for various adsorption and catalytic reaction applications. The nanoparticles-based material that was prepared in this study yields significantly higher reaction rates than those that have been reported in the literature. Furthermore, the channeling and maldistribution of the gas flow, which are usually prevalent in the fixed or fluidized bed reactors of powdery materials that are currently used for the Cu−Cl thermochemical reactions, are insignificant in the tubular reactor wrapped with ACF mats.17−22 15634

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temperatures in the range of 200−600 °C. The sample was placed on a SS mesh inside the reactor for the calcination. 2.4. Oxychlorination and Oxygen Production by Thermal Decomposition. Figure 3 shows a schematic of the experimental setup that was designed and used for the combined oxychlorination and production of O2 from steam by the CuCl2 nanoparticles dispersed in the ACF. The setup mainly consists of a steam boiler, an inconel tubular reactorshell assembly that is contained within a vertical tubular furnace, an HCl container, and a gas chromatograph (GC, model 5700, Nucon Eng. Co., India) equipped with a thermal conductivity detector (TCD) and a data acquisition system. The temperature of the furnace was adjusted using a PID controller (model PXZ-4, Fuji Electric Co., Japan). The flow rate of N2, which was used as the carrier gas, was controlled using a mass flow controller (model PSFIC-I, Bronkhorst, Netherlands). As depicted in the figure, the tube was perforated and closed at the bottom end. The samples were wrapped over the perforated section of the tube. The mixture of steam and nitrogen flowed down into the tubular reactor (L = 140 mm, i.d. = 14 mm) and then flowed radially outward into the SS shell through the sample wrapped over the tube. In a typical experiment, ∼1 g of the test sample was wrapped over the perforated portion of the tubular reactor. The N2 gas laden with steam was continuously fed to the reactor at a predetermined flow rate. The temperature of the reactor was gradually increased from room temperature (30 °C ± 2 °C) to the oxychlorination reaction temperature (370 °C). The reactor temperature was then held at 370 °C for 30 min and then gradually increased to the decomposition temperature of 530 °C. The reaction lasted approximately 4 h. The liquid product, which contained HCl, trickled down the reactor and was collected in a SS container. The gaseous products and the

Figure 2. Schematic of the experimental setup used for the (a) impregnation and (b) calcination steps.

Figure 2b shows a schematic diagram of the reactor used for the calcination. The calcination was carried out in a N2 atmosphere (flow rate = 0.1 standard liters per min (slpm)) at different

Figure 3. Schematic of the experimental setup used for the production of oxygen from steam using CuCl2-ACF. 15635

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Figure 4. Schematic of the preparation of ACFs with dispersed CuCl2 nanoparticles for oxygen production from steam.

1.547178 Å) radiation. Prior to the analysis, finely powdered samples were placed on a cleaned glass plate in a thin layer. A 2θ range of 10−70° at a step size of 0.5° was used during the analysis. A Tensor 27 (Bruker, Germany) FTIR was used to ascertain the functional groups present in the samples within the wavelength range of 700−4000 cm−1 in the attenuated total reflectance (ATR) mode with the help of the germanium (Ge) crystal. The spectra and automatic data acquisition were obtained using the software supplied with the instrument. During the analysis, the sample chamber was continuously purged with inert N2 to minimize the effect of atmospheric CO2 and moisture. Prior to the analysis, the background spectrum was recorded using air as the background medium. The resolution was set to 4 cm−1. A total of 100 scans were recorded for each sample. The atomic absorption spectroscopic (AAS) analysis was carried out using a Varian AA240 instrument (Varian, Germany) and an air−acetylene flame. Field-emission SEM (Supra 40 VP, Zeiss, Germany) was used to ascertain the surface morphology of the samples.

unconverted steam were cooled in a water-chilled condenser. The condensate was returned to the liquid container, and the gaseous mixture was sent to the GC for the detection of O2.

3. SURFACE CHARACTERIZATION The prepared ACF samples with dispersed CuCl2 nanoparticles were characterized using several spectroscopic and analytical techniques, including atomic absorption spectroscopy (AAS), X-ray diffraction (XRD), BET surface area and pore size distribution (PSD) analysis, temperature programmed desorption (TPD), Fourier transform infrared (FT-IR) spectroscopy, and scanning electron microscopy (SEM). The surface area, the pore volume, and the adsorption and desorption isotherms of the calcined samples were measured based on the amount of N2 that was adsorbed and desorbed at 77 K using the Autosorb-1C instrument (Model: AS1-C, Quantachrome, USA). Prior to the analysis, the samples were degassed at 200 °C under vacuum. The total pore volume of the samples was estimated on the basis of the amount of N2 adsorbed at a relative pressure near unity (0.9994). The TPD of the samples was carried out using the same instrument that was used for the BET surface area measurements. The TPD was performed in a U-shaped quartz tube using ammonia (NH3) as the adsorbate molecule. Approximately 0.5 g of the test sample was first degassed for 1 h at 150 °C under a helium (He) flow of 10 mL/min to remove the surface moisture and other volatile impurities from the sample. After degassing, NH3 (99.9% pure) was introduced into the sample at a flow rate of 8 mL/min and a temperature of ∼40 °C for 30 min. After adsorption, the system was continuously purged with helium gas at 10 mL/min. The temperature of the sample was simultaneously raised from 40 to 550 °C at a rate of 10 °C/min. The initial TCD signal was set to 0 mV, and the TCD current was set to 150 mA. The amount of NH3 released was recorded by the TCD. Powder XRD analysis was carried out using a Seifert X-ray generator (ISO Debye flex 2002, Germany) for the Cu Kα (k =

4. RESULTS AND DISCUSSION 4.1. Role of Surfactant. SDS (sodium dodecyl sulfate), an anionic surfactant, was added to the impregnating Cu-salt solution to facilitate the monodispersion of CuCl2 in the solution. SDS is an organosulfate compound in which the sulfate functional group (SO42−) is attached to the carbon tail. Figure 4 shows a schematic of the SDS molecule and the preparation of the dispersion of CuCl2 nanoparticles in the ACF using SDS, for the end-application. During the impregnation, the hydrophilic parts of the SDS molecules bind with the positively charged Cu2+ ions that are generated upon the dissolution of the cupric chloride salts in the solution. This prevents the agglomeration of the Cu2+ ions and, thus, results in a monodispersion of CuCl2. Consequently, the Cu2+ ions and the SDS molecules are transferred to the ACF surface 15636

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temperature and is negligible in samples calcined at 600 °C. In these samples, however, the peak for metallic Cu is at a maximum. As shown later in the paper, the XRD data (decrease in the CuCl2-phase with increasing calcinations temperatures) are consistent with those obtained from the EDX analysis. The average grain size was calculated from the width of the peaks in the XRD plot using Scherrer’s formula. The grain size of the materials calcined at various temperatures ranges from 14 to 90 nm. Furthermore, the material calcined at 500 °C had a grain with an average size in the range of 14−78 nm. 4.4. BET Measurement. The N2 adsorption and desorption isotherms of the CuCl2-ACFs that were calcined at temperatures ranging between 200 and 600 °C are shown in Figure 6.

in relatively large amounts from the bulk solution.22 During the calcination step, the SDS decomposes and the nanoparticles of CuCl2 remain on the ACF. As discussed later in the manuscript, the use of SDS significantly increases the metal loading on the ACF surface. 4.2. AAS Analysis. Table 1 describes the Cu loading on the impregnated ACF with and without the use of surfactant. From Table 1. AAS Analysis of the Cu Contents in the Prepared Material sample

Cu loading (g/g of ACF)

Cu loading (%)

CuCl2-ACFs (with SDS) CuCl2-ACFs (without SDS)

0.47 0.25

32 21

the table, it can be observed that the addition of SDS to the impregnating solution significantly increases the loading of Cu from 21% (w/w) to 32% (w/w). Any excess concentration of either CuCl2 salt or SDS resulted in a blockage of the substrate pores; this phenomenon was corroborated by SEM images and is not discussed here for brevity.22,23 4.3. XRD Data. The XRD analysis of the calcined samples was carried out to ascertain the different phases and the crystal size of the dispersed CuCl2 in ACFs. Figure 5 shows the XRD

Figure 6. N2 adsorption/desorption isotherms of CuCl2 dispersed ACFs calcined at (a) 200 °C, (b) 300 °C, (c) 400 °C, (d) 500 °C, and (e) 600 °C.

As shown, the amounts (volume) of N2 adsorbed significantly increase in the relative pressure range of 0−0.01 and then gradually level off. The isotherms of this type belong to the type I category and reflect the monolayer coverage of N2 over the surface. The figure also shows that the adsorption capacity of N2 increases with increasing calcination temperature, which is indicative of the development and enlargement of the micropores. The BET surface area of the CuCl2-ACF samples that were calcined at different temperatures was measured using the multipoint BET method and found to be 744 m2/g and 1341 m2/g at 200 and 600 °C, respectively. Therefore, it may be concluded that the calcination opens up the pores, thereby increasing the internal surface area as well as the active sites for the reaction. 4.5. SEM and Energy-Dispersive X-ray (EDX) Analysis. The SEM and EDX analysis of the calcined samples was carried out to ascertain the surface morphology (dispersion of the nanoparticles) of the prepared CuCl2-ACF samples. Figure 7 shows the SEM images of the nonpretreated ACFs and the CuCl2-ACFs samples that were calcined at 500 °C. For each sample, the images are shown at small (5 KX) and large (200 KX) magnifications. The scale of the images is in the micrometer range for the small magnifications and in the nanometer range for the higher magnifications. The samples were coated with gold to improve the thermal conductivity of the materials and to obtain good images. Figure 7a, b shows the representative SEM images of the nonpretreated ACFs at small (5 KX) and high magnifications (200 KX). As observed in Figure 7a, the external surface of the

Figure 5. XRD spectra of CuCl2-ACFs calcined at (a) 200 °C, (b) 300 °C, (c) 400 °C, (d) 500 °C, and (e) 600 °C.

spectra of the ACF samples with dispersed CuCl2 that were calcined at various temperatures ranging from 200 to 600 °C. As shown in the figure, a broad diffraction peak was observed at an approximate 2θ angle of 22° in all the samples; this diffraction peak was attributed to the d002 plane, which is characteristic of a disordered (amorphous) carbonaceous interlayer of coke-like substances in the ACF.24 Five different characteristic peaks were also observed at the 2θ angles of 16.2°, 32.46°, 39.81°, 43.4°, and 50.47°, which correspond to the crystallographic indices of the (110), (201), (221), (040), and (311) planes, respectively. The observed peaks were compared to the XRD patterns described in the literature,25 and the peaks at the first three 2θ values were attributed to the CuCl2 phase. The additional peaks at the 2θ angles of 43.4° and 50.47°, which were observed in the samples calcined at temperatures greater than 300 °C, correspond to metallic Cu that is formed by the partial decomposition of the Cucontaining compounds in the ACF. Consequently, the intensity of the CuCl2 peaks decreases with increasing calcination 15637

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Figure 7. SEM images of (a, b) as-received ACFs; (c, d) CuCl2 dispersed ACFs calcined at 500 °C; and EDX spectra of (e) as-received ACF and (f) CuCl2 dispersed ACFs calcined at 500 °C.

nonpretreated ACF is smooth. At high magnification, the pores can be observed on the surface. These pores include a combination of micropores (average pore size