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Electrocatalytic reduction of CO2 to CO by Gd(III) and Dy(III) complexes; and M2O3 nanoparticles (M = Gd and Dy)
Journal of CO2 Utilization 13 (2016) 61–70
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Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Electrocatalytic reduction of CO2 to CO by Gd(III) and Dy(III) complexes; and M2O3 nanoparticles (M = Gd and Dy) Mohammad Taghi Behnamfara , Hassan Hadadzadeha,* , Elaheh Akbarnejadb , Ali Reza Allafchianc , Mohammad Assefia , Neda Khedria a b c
Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran Plasma Physics Research Centre, Science and Research Branch, Islamic Azad University, Tehran, Iran Nanotechnology and Advanced Materials Institute, Isfahan University of Technology, Isfahan 84156-83111, Iran
A R T I C L E I N F O
A B S T R A C T
Article history: Received 6 June 2015 Received in revised form 5 December 2015 Accepted 15 December 2015 Available online xxx
Two Gd(III) and Dy(III) complexes, [M(AyGG)3(H2O)5] (AyGG = Alizarin yellow GG (NaC13H8N3O5)), have been prepared and characterized. The Gd2O3 and Dy2O3 nanoparticles were prepared by the calcination of the Gd(III) and Dy(III) complexes in air at different temperatures. The nanoparticles were characterized by FT-IR, X-ray diffraction analysis (XRD), and field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The voltammograms in the absence and presence of CO2 indicate that the Gd(III) and Dy(III) complexes and their nano-oxides can catalyze the electrochemical reduction of CO2 to CO. The results show that the Dy(III) complex has better electrocatalytic activity than other compounds. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Gd(III) complex Dy(III) complex Nano-oxides CO2 reduction Cyclic voltammetry
1. Introduction Carbon dioxide contributes to the greenhouse effect, which is responsible for the warming of the Earth. The Kyoto Protocol (Japan, 1997) aims to reduce emissions of CO2 and other greenhouse gases [1–3]. Carbon dioxide is emitted through natural processes (such as volcanic activity, respiration, and decomposition) and human activities (such as deforestation, fossil fuels combustion, and industrial processes). The most effective method to reduce CO2 emissions is to reduce fossil fuels consumption. The atmospheric mixing ratio of CO2 is now higher than at any time in at least the last 800,000 years, standing at 385 ppm compared to a pre-industrial high of 280 ppm [3–5]. The current growth rate of 2 ppm per year for CO2 is a serious warning. This rate has caused much worry in the international community. In 2013, CO2 accounted for about 82% of all greenhouse gas emissions from human activities [5]. Therefore, the fixation and conversion of CO2 into a useful chemical feedstock is of potential benefit. Research in the field of electrochemical reduction of CO2 has grown rapidly in the last few decades. This growing research effort is a response by physical scientists and engineers to the increasing amount of CO2 in the atmosphere and the steady growth in global fuel (energy)
* Corresponding author. Fax: +98 31 33912350. E-mail address: [email protected] (H. Hadadzadeh). http://dx.doi.org/10.1016/j.jcou.2015.12.005 2212-9820/ ã 2015 Elsevier Ltd. All rights reserved.
demand [6,7]. Electroreduction of CO2 with metal complexes is a feasible technique for the utilization of CO2 as a C1 source, though the final products usually have been limited to CO and/or HCOOH [8–16]. Metal complexes with CO2 as a ligand are considered to play the key role in the electroreduction and photoreduction of CO2, since the M h1 CO2 bond is easily converted to metal–CO bond through an acid–base reaction in protic media (such as water and alcohols) or through an oxide-transfer to a free CO2 molecule (uncoordinated CO2) in aprotic media (such as CH3CN). CO evolution in the reduction of CO2 is ascribed to reductive cleavage of the resulting metal–CO [17–22]. In recent years, nanocatalysts have received increasing attention and exhibited many lab- and industrial-scale applications [23–26]. When a nanocatalyst participates in an electrochemical reaction, it is called nano-electrocatalysts. Electrocatalysts are a specific form of catalysts that function at electrode surfaces or may be the electrode surface itself. An electrocatalyst can be heterogeneous (such as a platinum surface or nanoparticles) or homogeneous (such as a coordination complex or enzyme (biocatalyst)) [27–30]. The electrocatalyst assists in transferring electrons between the electrode surface and reactant(s), and/or facilitates an intermediate chemical transformation described by an overall half-reaction. Nanoparticles have emerged as sustainable alternatives to conventional materials, as robust, high surface area heterogeneous catalysts and catalyst supports [31,32]. The nanosized particles increase the exposed surface area of the active
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component of the catalyst, thereby enhancing the contact between reactants and catalyst dramatically and mimicking the homogeneous catalysts. However, their insolubility in common reaction solvents renders them easily separable from the reaction mixture like heterogeneous catalysts, which in turn makes the product isolation stage effortless. Also, the activity and selectivity of a nanocatalyst can be manipulated by tailoring chemical and physical properties such as size, shape, composition, and morphology. The scientific challenge is the synthesis of specific size and shape nanocatalysts to allow facile movement of materials in the reacting phase and control over morphology of nanostructures to tailor their physical and chemical properties. However, the rapid advancement of nano-technology made possible the preparation of a variety of nanoparticles with controlled size, shape, morphology, and composition [30–32]. Here, we report the synthesis and characterization of two new mononuclear Gd(III) and Dy(III) complexes, [M(AyGG)3(H2O)5], where AyGG is Alizarin yellow GG (Fig. 1). In addition, a simple method is reported for the preparation of the Gd2O3 and Dy2O3 nanoparticles from these precursor complexes by a facile calcination method. The electrocatalytic reduction of CO2 to CO by the complexes and their nano-oxides is also investigated using cyclic voltammetry (CV) in CH3CN solution. 2. Experimental 2.1. Materials and methods GdCl36H2O, DyCl36H2O, tetra-n-butylammonium hexafluorophosphate, Alizarin yellow GG, and acetonitrile were purchased from Merck. All chemicals and solvents were of high purity and used without any further purification. Elemental analysis (C, H and N) were performed using a Leco, CHNS-932 elemental analyzer. Fourier transform infrared spectra were recorded on an FT-IR JASCO 680-PLUS spectrometer in the region of 4000–400 cm1 using KBr pellets. Electronic absorption spectra were recorded on a JASCO 7580 UV–vis–NIR double-beam spectrophotometer using quartz cells with a path length of 10 mm. XRD analyses were performed using a PHILIPS PW3040/60 diffractometer with CuKa radiation. Morphology of the nanoparticles was observed using a field-emission scanning electron microscope (FE-SEM, HITACHI; S-4160). Transmission electron microscopy (TEM) images were obtained on a Philips CM30 transmission electron microscope with an accelerating voltage of 300 kV. Voltammetric experiments were performed on a SAMA Research Analyzer M-500. All measurements were carried out in a 5 mL cell which was fitted with a Teflon lid incorporating a three-electrode system comprising of a glassy carbon electrode (’ = 2 mm) as the working electrode, a platinum wire as the auxiliary electrode, and a silver wire as the pseudo-reference electrode (the potential values reported vs. SCE). The glassy carbon working electrode surface was freshly cleaned with alumina polish on a micro cloth before each scan and was rinsed with doubly-distilled water between each polishing step. The cyclic voltammograms of [Gd(AyGG)3(H2O)5] and [Dy(AyGG)3(H2O)5] were recorded in acetonitrile using tetran-butylammonium hexafluorophosphate (TBAH) as the supporting
electrolyte. Ferrocene (E = 0.665 V vs. NHE) was used as an internal reference at the end of each experiment. The solutions were purged by N2 or CO2 flows during study. All spectral and electrochemical data were collected at ambient temperature. All gas chromatograms were recorded with a Varian STAR 3400 gas chromatograph with He as the carrier gas at a flow rate of 30 mL/min and a temperature of 230 C. After each electrolysis step (at least 3 h at a controlled potential) in the presence of the electrocatalysts ([Gd(AyGG)3(H2O)5], [Dy(AyGG)3(H2O)5], Gd2O3, or Dy2O3), the gas sample was taken using a gas-tight syringe and the gaseous reaction product, i.e. CO, was detected by GC and IR. No detectable quantity of the target product (CO) was found in the headspace of the blank solution (containing the supporting electrolyte (TBAH) + solvent (CH3CN) under CO2) after electrolysis in the absence of the electrocatalysts for 3 h at room temperature. In all samples, no hydrogen (H2) peak was detected by GC. 2.2. Synthesis of [Gd(AyGG)3(H2O)5] [Gd(AyGG)3(H2O)5] was synthesis by the reaction of Alizarin yellow GG and gadolinium(III) chloride hexahydrate in 30 mL water. Alizarin yellow GG (0.93 g, 3 mmol) was dissolved in 15 mL of water under stirring at 80 C until a dark yellow solution resulted. To the solution was then added a solution of GdCl36H2O (0.37 g, 1 mmol) in 15 mL water. The reaction mixture was refluxed for 5 h. The dark orange precipitate was filtered and washed with water and dried in air at room temperature (Scheme 1). Yield: 0.95 g, 86%. Anal. Calc. for GdC39H34N9O20 (MW = 1105.99 g/mol): C, 42.35; H, 3.10; N, 11.40 % Found: C, 42.64; H, 3.21; N, 11.51%. 2.3. Synthesis of [Dy(AyGG)3(H2O)5] Alizarin yellow GG (0.93 g, 3 mmol) was dissolved in 15 mL of water under stirring at 80 C until a dark yellow solution resulted. To the solution was then added a solution of DyCl36H2O (0.38 g, 1 mmol) in 15 mL water. The reaction mixture was refluxed for 12 h. The red–orange precipitate was filtered and washed with water and dried in air at room temperature (Scheme 1). Yield: 0.97 g, 87%. Anal. Calc. for DyC39H34N9O20 (MW = 1112.12 g/mol): C, 42.15; H, 3.08; N, 11.34 % Found: C, 42.23; H, 3.19; N, 11.48%. 2.4. Synthesis of Gd2O3 and Dy2O3 nanoparticles To prepare the nanoparticles, the precursor complexes, [Gd (AyGG)3(H2O)5] and [Dy(AyGG)3(H2O)5], were calcined up to 600 C. The brown gadolinium oxide (Gd2O3) nanoparticles and white dysprosium oxide (Dy2O3) nanoparticles were obtained by subjecting 0.3 mg of the as-prepared Gd(III) and Dy(III) complexes powder to heat treatment in air at given temperatures ranging between 400 and 600 C. An average temperature increase of 20 C every minute was selected before the temperature reached to the target temperature, and after keeping the thermal treatment at the target temperature for 2 h, it was allowed to cool to room temperature naturally. A series of further experiments were carried out to investigate the reaction conditions. The detailed reaction conditions and corresponding results are summarized in Table 1. 3. Results and discussion 3.1. Synthesis and characterization of complexes
Fig. 1. Molecular structure of Alizarin yellow GG (AyGG).
The reaction of Alizarin yellow GG with GdCl36H2O and DyCl36H2O is shown in Scheme 1. The complexes were prepared in good yield. The elemental analyze of the complexes were entirely consistent with their proposed composition. Fig. 2 shows the
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Scheme 1. Synthesis route to [M(AyGG)3(H2O)5], (M = Gd and Dy).
Table 1 The final products obtained under different calcination conditions. Sample no.
Precursor complex
Calcinations temperature ( C)
Calcinations time (h)
Average size of the particles
Morphologies of the final products
1 2 3 4 5 6
Gd(III) Gd(III) Gd(III) Dy(III) Dy(III) Dy(III)
400 500 600 400 500 600
2 2 2 2 2 2
2 mm 20 nm 13 nm 1 mm 20 nm 12 nm
Microparticle Nanoparticle Nanoparticle Microparticle Nanoparticle Nanoparticle
Fig. 2. The optimized structure of [M(AyGG)3(H2O)5] (M = Gd(III) and Dy(III)).
optimized structure of [M(AyGG)3(H2O)5], (M = Gd(III) and Dy(III)). According to the optimized structure, the coordination number of the Gd(III) and Dy(III) complexes is eight. 3.2. UV–vis and FT-IR spectroscopic studies To obtain suitable electronic absorption spectrum, the complexes were dissolved in acetonitrile. The complexes demonstrate three absorption bands in the UV–vis region (200–700 nm) that can be assigned to ligand-centered (p ! p* and n ! p*) transitions. In the electronic absorption spectrum of Alizarin yellow GG ligand (Fig. 3a) the band observed in 260 nm can be assigned to p ! p* transition. The n ! p* transition occur at 430 nm that can be corresponded to the excitation of an electron from one of the unshared pair to the p* orbital. The last transition shows a
significant shift when the ligand coordinated to the lanthanide metals (Fig. 3b and c). This phenomenon can be attributed to contribution of the non-bonding electron pairs of the ligand in the coordination to the lanthanide(III) ions [33]. The orange color observed for Gd(III) complex and Dy(III) complex can be attributed to appearance of some of the n ! p* transitions that overlap with more energetic transitions and thus the sequence of absorption spectrum has been expanded to visible region (Fig. 3). The FT-IR spectra of the pure Alizarin yellow GG, [Gd (AyGG)3(H2O)5], [Dy(AyGG)3(H2O)5], Gd2O3 and Dy2O3 are shown in Fig. 4. In the IR spectra of the complexes, the bands associated with the carbonyl stretching are clearly seen. By comparison of the spectra of [Gd(AyGG)3(H2O)5] and [Dy(AyGG)3(H2O)5] with the free AyGG ligand, it is determined that the bands at 1670 cm1 are carbonyl stretching vibrations in the coordinated AyGG ligands
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morphology and have not any amorphous phase at their structures. The calcination of [Gd(AyGG)3(H2O)5] at 600 C, produces a crystalline phase of gadolinium oxide (Fig. 6), which is in agreement with previous reports [35,37]. As shown in Fig. 5, the XRD pattern of the pure Gd2O3 reveals a cubic phase, which can be indexed to the diffractions from the (2 2 2), (4 0 0), (4 4 0), (6 2 2), (6 6 2), and (8 4 4) planes (JCPDS Card No. 011-0608). From the XRD data for sample no. 3 in Fig. 5, the crystallite size (Dc) of the Gd2O3 nano-oxide can be calculated using the Debey–Scherrer equation [38], Dc ¼
Fig. 3. Electronic spectra of (a) Alizarin yellow GG, (b) [Gd(AyGG)3(H2O)5], and (c) [Dy(AyGG)3(H2O)5]; 1 105 M in CH3CN.
[34]. The absorption peaks at 3445 and 1633 cm1 are attributed to the stretching vibration of the O H bond and the bending vibration of H OH from water, respectively. The vibrations of GdO and Dy O bonds appear at 550 and 433 cm1; as well as at 552 and 418 cm1, respectively. In the IR spectra of the nanoparticles, the bands associated with the Gd O and DyO vibrations appear at 546 and 431 cm1; as well as at 557 and 418 cm1, respectively [34–36]. 3.3. XRD analysis The X-ray diffraction patterns of Gd2O3 and Dy2O3 nanoparticles (samples no. 3 and 6) are shown in Fig. 5. The diffraction patterns show that the nanoparticles have a crystalline
Kl bcosu
where b is the breadth of the observed diffraction line at its half maximum intensity, K is the so-called shape factor, which usually takes a value of about 0.9, and l is the wavelength of the X-ray source used in XRD. The average size of the particles of sample no. 3, is 13 nm, which is in agreement with that observed from the FE-SEM and TEM images. The calcination of [Dy(AyGG)3(H2O)5] at 600 C, yields a crystalline phase of dysprosium oxide (Fig. 5), which is in agreement with previous studies [36,39,40]. As shown in Fig. 5, the XRD pattern of the pure Dy2O3 exhibits a cubic bixbyite phase (also known as the C-rare-earth sesquioxide structure), which can be indexed to the diffractions from the o (2 2 2), (4 0 0), (4 4 0), and (6 2 2) planes (JCPDS Card No. 10-0059). There are no other peaks in Fig. 5, showing the high purity and well crystallinity of the nanooxide. The crystallite size (Dc) of the as-prepared Dy2O3, sample no. 6, was calculated to be 12 nm, which is in agreement with the FE-SEM and TEM images.
Fig. 4. FT-IR spectra of (a) Alizarin yellow GG; (b) [M(AyGG)3(H2O)5]; and (c & d) Gd2O3 or Dy2O3 nanoparticles calcined at 400 C and 600 C, respectively.
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Fig. 5. XRD patterns of the Gd2O3 and Dy2O3 nanoparticles calcined at 600 C for 2 h.
Fig. 6. FE-SEM images of the Gd2O3 nanoparticles calcined at 600 C for 2 h.
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Fig. 7. Histogram of the particle size distribution of the Gd2O3 nanoparticles calcined at 600 C for 2 h.
3.4. FE-SEM and TEM studies The FE-SEM technique was used to investigate the morphology of the nanoparticles. Table 1 shows some conditions which have been examined to investigate the morphology of the final products and compare them to each other. Figs. 6 and 8 show the FE-SEM images of the gadolinium and dysprosium nano-oxides (samples no. 3 and 6) calcined at 600 C for 2 h, respectively. At 400 C some parts of the organic ligands are still remaining and the nanoparticles cannot form. The quasi-spherical nanoparticles are observed when the calcination temperature reaches 600 C. The images show that when the temperature reaches 600 C, the particle size becomes smaller. Also the carbonate phase at a temperature of 600 C is gone and the oxide phase is formed (Fig. 4). According to the FE-SEM results, the particle size of Gd2O3 and Dy2O3 (sample no. 3 and 6) is between 8 and 15 nm. This value
Fig. 9. Histogram of the particle size distribution of Dy2O3 nanoparticles calcined at 600 C for 2 h.
is in good agreement with the XRD crystallite size results. Figs. 7 and 9 show the particle size histograms of sample no. 3 and 6. The distribution of the size of nearly spherical particles is peaked at about 12–13 nm. The TEM images of Gd2O3 and Dy2O3 nanoparticles (samples no. 3 and 6) are shown in Figs. 10 and 11. As the images show, the nanoparticles shapes are nearly spherical and have a uniform distribution. Also, they form a chain of beads attached to each other. The size of the nanoparticles obtained from the XRD diffraction patterns and FE-SEM technique are in close agreement with the TEM image which show sizes of 10–20 nm for two samples, gadolinium and dysprosium nano-oxides, calcined at 600 C for 2 h (samples no. 3 and 6).
Fig. 8. FE-SEM images of the Dy2O3 nanoparticles calcined at 600 C for 2 h.
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Fig. 10. TEM images of the Gd2O3 nanoparticles calcined at 600 C for 2 h.
Fig. 11. TEM images of the Dy2O3 nanoparticles calcined at 600 C for 2 h.
To study the electrocatalytic activity of the compounds ([Gd (AyGG)3(H2O)5], [Dy(AyGG)3(H2O)5]), and the nano-oxides
(samples no. 3 and 6) for the reduction of CO2, the cyclic voltammograms of each compounds were taken under a flow of CO2 and the results compared with those obtained under a nitrogen atmosphere. For the complexes, a precatalyst solution was prepared by dissolving 5 mg of complexes in 5 mL of
Fig. 12. Cyclic voltammograms of [Gd(AyGG)3(H2O)5] (103 M) (a) in the absence and (b) presence of CO2 at scan rate of 100 mV/s; in dry acetonitrile; 0.1 M TBAH.
Fig. 13. Cyclic voltammograms of [Dy(AyGG)3(H2O)5] (103 M) (a) in the absence and (b) presence of CO2 at scan rate of 100 mV/s; in dry acetonitrile; 0.1 M TBAH.
3.5. Electrocatalytic activity
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Fig. 15. Cyclic voltammograms of the Dy2O3 nanoparticles, (a) in the absence and (b) presence of CO2 at scan rate of 100 mV/s; in dry acetonitrile; 0.1 M TBAH. Fig. 14. Cyclic voltammograms of the Gd2O3 nanoparticles, (a) in the absence and (b) presence of CO2 at scan rate of 100 mV/s; in dry acetonitrile; 0.1 M TBAH.
Scheme 2. Proposed reaction mechanism for the electrocatalytic reduction of CO2 by the Gd(III) or Dy(III) complex in CH3CN.
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electrolyte solution (0.1 M TBAH in acetonitrile) to give a concentration of ca. 1 mM. Also for the nanoparticles, a precatalyst was prepared by dispersing 2 mg of the nanoparticles in 1 mL of water and deposited 10 mL of dispersed nanoparticles on the GC electrode surface. TBAH (0.1 M) in acetonitrile was used for electrolyte solution. The voltammetric response of the compounds under two different atmospheres, nitrogen and carbon dioxide, will be briefly discussed here. 3.5.1. Electrochemical studies under N2 atmosphere The cyclic voltammograms of the complexes and nanoparticles at a glassy carbon (GC) electrode are shown in Figs. 12 and 13a. The cyclic voltammetry measurements were performed on an acetonitrile solution of the complexes with 0.1 M TBAH supporting electrolyte at ambient temperature. For the complexes, the voltammograms recorded in N2 present three couples at 0.75, 0.65, and 1.76 V for [Gd(AyGG)3(H2O)5], as well as at 0.78, 0.67, and 2.0 V for [Dy(AyGG)3(H2O)5] vs. SCE, that are assigned to the two redox couples of Alizarin yellow GG, and one redox couple for Gd(III/II) (at 1.76 V) or Dy(III/II) (at 2.0 V). Under the N2 atmosphere, the redox couple of AyGG exhibits equivalent anodic and cathodic waves, with 0.05 V peak separations. The irreversible reduction wave in the more negative potential region is assigned to the reduction of Gd(III/II) and Dy(III/II) couples. For the Gd2O3 nanoparticles (sample no. 3), the voltammogram recorded in N2 shows a voltammetric peak at 2.1 V vs. SCE that is assigned to the irreversible Gd(III/II) reduction couple (Fig. 14a). Also, for the Dy2O3 nanoparticles (sample no. 6), the voltammogram recorded in N2 shows a peak at 2.18 V that can be assigned to the irreversible Dy(III/II) redox couple (Fig. 15a). 3.5.2. Electrochemical studies under CO2 atmosphere Figs. 12b and 13b show the cyclic voltammograms of a solution, purged by CO2 flow, containing 1 mM of the complexes in 0.1 M TBAH/CH3CN at a glassy carbon electrode. Also, Scheme 2 shows the proposed reaction mechanism for the electrocatalytic reduction of CO2 by the Gd(III) and Dy(III) complexes. As can be seen in Figs. 12b and 13b, some significant changes are observed when the N2 atmosphere is replaced by CO2. The signal around 0.8 V can be assigned to the one-electron reduction of the AyGG ligand. In a solution saturated with CO2, the reduction wave (1) is approximately unchanged with respect to an N2-saturated solution (Figs. 12 and 13). The first, second, and third one-electron reductions of the complexes provide three electrons that occupy the empty p* orbitals of the alizarin ligands. These observations are consistent with the alizarin-based reduction to give [MIII(AyGG)3(H2O)5]3 followed by reaction with CO2, step (IV) (Scheme 2). After the substitution of CO2 with H2O, an intramolecular two-electron transfer takes place from the alizarin ligand to the coordinated CO2 ligand to give an intermediate shown as the metallocarboxylate, promoting the reduction of CO2. It may occur via an O2 transfer to CO2 to give a CO32 anion and a CO complex (step (VI)). In fact, the catalytic cycle needs two CO2 molecules. One CO2 is reduced to CO and another CO2 is fixed to a carbonate anion. The carbonate anion (CO32) is stable and cannot convert to CO2 in the experimental condition. The presence of carbonate anion was confirmed by 13C NMR and IR spectroscopy. There is a significant current enhancement for the wave at Ep. c = 1.7 V (wave (2) in Fig. 12b), and Ep.c = 1.65 V (wave (2) in Fig. 13b), which is consistent with the electrocatalytic reduction of CO2 [8]. Figs. 14b and 15b shows the cyclic voltammograms of a precatalyst, purged by CO2 flow, containing 10 mL of the dispersed nanoparticles on the GC electrode surface in 0.1 M TBAH/CH3CN. As can be seen in these figures, some changes are observed when the N2 atmosphere is replaced by CO2. A significant current
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enhancement for the wave at Ep.c = 1.95 V for Gd2O3 and Ep. for Dy2O3, can be assigned to the electrocatalytic reduction of CO2. According to the voltammetry results, the Dy (III) complex, [Dy(AyGG)3(H2O)5] has better electrocatalytic activity than other compounds. For all four electrocatalysts (the complexes and nano-oxides), the formation of CO as the final product was confirmed by GC and IR spectroscopy (2119 and 2172 cm1) after the electrolysis of the reaction mixture for 3 h at room temperature.
c = 2.0 V
4. Conclusions The Gd2O3 and Dy2O3 nanoparticles were prepared by the calcination of two new Gd(III) and Dy(III) complexes at different temperatures up to 600 C in air. All four compounds were used as an electrocatalyst for the reduction of CO2 to CO in CH3CN. The voltammograms in the absence and presence of carbon dioxide indicates that the complexes and the nano-oxide nanoparticles can catalyze the electroreduction of CO2 to CO. In addition, the formation of CO as the final product and carbonate anion as a byproduct was confirmed using several instrumental techniques. The production of CO from the electrocatalytic reduction of CO2 is more thermodynamically and electrochemically favorable than other 2e reduction products. In addition, the final product CO can be utilized to produce H2 gas via water–gas shift reaction (WGSR, CO + H2O ! CO2 + H2) or used as a fuel (CO + 1/2O2 ! CO2 DH = –283.006 kJ/ mol). Finally, according to the experimental results, an electrocatalytic cycle was proposed for the reduction of CO2 in the presence of the Gd(III) and Dy(III) complexes. We hope our new findings can be used to develop new electrocatalysts for electroreduction of CO2 to CO and to reduce this greenhouse gas emission. Acknowledgement We are grateful to the Isfahan University of Technology (IUT) for financial support. References [1] A. Raval, V. Ramanathan, Observational determination of the greenhouse effect, Nature 342 (1989) 758–761, doi:http://dx.doi.org/10.1038/342758a0. [2] W.D. Nordhaus, To slow or not to slow: the economics of the greenhouse effect, Econ. J. 101 (1991) 920–937, doi:http://dx.doi.org/10.2307/2233864. [3] T.L. Frölicher, F. Joos, Reversible and irreversible impacts of greenhouse gas emissions in multi-century projections with the NCAR global coupled carbon cycle-climate model, Clim. Dyn. 35 (2010) 1439–1459, doi:http://dx.doi.org/ 10.1007/s00382-009-0727-0. [4] L.L. Andersen, F.F. Østerstrøm, O.J. Nielsen, M.P.S. Andersen, T.J. Wallington, Atmospheric chemistry of (CF3)2CFOCH3, Chem. Phys. Lett. 607 (2014) 5–9, doi:http://dx.doi.org/10.1016/j.cplett.2014.05.036. [5] International energy agency, CO2 emissions from fuel combustion highlights, IEA Stat. (2013) 158, doi:http://dx.doi.org/10.1787/co2-table-2011-1-en. [6] X. Frogneux, O. Jacquet, T. Cantat, Iron-catalyzed hydrosilylation of CO2: CO2 conversion to formamides and methylamines, Catal. Sci. Technol. 4 (2014) 1529–1533, doi:http://dx.doi.org/10.1039/C4CY00130C. [7] I. Knopf, T. Ono, M. Temprado, D. Tofan, C.C. Cummins, Uptake of one and two molecules of CO2 by the molybdate dianion: a soluble, molecular oxide model system for carbon dioxide fixation, Chem. Sci. 5 (2014) 1772–1776, doi:http:// dx.doi.org/10.1039/c4sc00132j. [8] F.H. Haghighi, H. Hadadzadeh, H. Farrokhpour, N. Serri, K. Abdi, H. Amiri Rudbari, Computational and experimental study on the electrocatalytic reduction of CO2 to CO by a new mononuclear ruthenium(ii) complex, Dalton Trans. 43 (2014) 11317–11332, doi:http://dx.doi.org/10.1039/c4dt00932k. [9] T. Mizukawa, K. Tsuge, H. Nakajima, K. Tanaka, Selective production of acetone in the electrochemical reduction of CO2 catalyzed by a Ru naphthyridine complex, Angew. Chem. Int. Ed. 38 (1999) 362–363. [10] I.M.B. Nielsen, K. Leung, Cobalt-porphyrin catalyzed electrochemical reduction of carbon dioxide in water. 1. A density functional study of intermediates, J. Phys. Chem. A 114 (2010) 10166–10173, doi:http://dx.doi.org/10.1021/ jp101180m.
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Contents lists available at ScienceDirect
Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Electrocatalytic reduction of CO2 to CO by Gd(III) and Dy(III) complexes; and M2O3 nanoparticles (M = Gd and Dy) Mohammad Taghi Behnamfara , Hassan Hadadzadeha,* , Elaheh Akbarnejadb , Ali Reza Allafchianc , Mohammad Assefia , Neda Khedria a b c
Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran Plasma Physics Research Centre, Science and Research Branch, Islamic Azad University, Tehran, Iran Nanotechnology and Advanced Materials Institute, Isfahan University of Technology, Isfahan 84156-83111, Iran
A R T I C L E I N F O
A B S T R A C T
Article history: Received 6 June 2015 Received in revised form 5 December 2015 Accepted 15 December 2015 Available online xxx
Two Gd(III) and Dy(III) complexes, [M(AyGG)3(H2O)5] (AyGG = Alizarin yellow GG (NaC13H8N3O5)), have been prepared and characterized. The Gd2O3 and Dy2O3 nanoparticles were prepared by the calcination of the Gd(III) and Dy(III) complexes in air at different temperatures. The nanoparticles were characterized by FT-IR, X-ray diffraction analysis (XRD), and field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The voltammograms in the absence and presence of CO2 indicate that the Gd(III) and Dy(III) complexes and their nano-oxides can catalyze the electrochemical reduction of CO2 to CO. The results show that the Dy(III) complex has better electrocatalytic activity than other compounds. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: Gd(III) complex Dy(III) complex Nano-oxides CO2 reduction Cyclic voltammetry
1. Introduction Carbon dioxide contributes to the greenhouse effect, which is responsible for the warming of the Earth. The Kyoto Protocol (Japan, 1997) aims to reduce emissions of CO2 and other greenhouse gases [1–3]. Carbon dioxide is emitted through natural processes (such as volcanic activity, respiration, and decomposition) and human activities (such as deforestation, fossil fuels combustion, and industrial processes). The most effective method to reduce CO2 emissions is to reduce fossil fuels consumption. The atmospheric mixing ratio of CO2 is now higher than at any time in at least the last 800,000 years, standing at 385 ppm compared to a pre-industrial high of 280 ppm [3–5]. The current growth rate of 2 ppm per year for CO2 is a serious warning. This rate has caused much worry in the international community. In 2013, CO2 accounted for about 82% of all greenhouse gas emissions from human activities [5]. Therefore, the fixation and conversion of CO2 into a useful chemical feedstock is of potential benefit. Research in the field of electrochemical reduction of CO2 has grown rapidly in the last few decades. This growing research effort is a response by physical scientists and engineers to the increasing amount of CO2 in the atmosphere and the steady growth in global fuel (energy)
* Corresponding author. Fax: +98 31 33912350. E-mail address: [email protected] (H. Hadadzadeh). http://dx.doi.org/10.1016/j.jcou.2015.12.005 2212-9820/ ã 2015 Elsevier Ltd. All rights reserved.
demand [6,7]. Electroreduction of CO2 with metal complexes is a feasible technique for the utilization of CO2 as a C1 source, though the final products usually have been limited to CO and/or HCOOH [8–16]. Metal complexes with CO2 as a ligand are considered to play the key role in the electroreduction and photoreduction of CO2, since the M h1 CO2 bond is easily converted to metal–CO bond through an acid–base reaction in protic media (such as water and alcohols) or through an oxide-transfer to a free CO2 molecule (uncoordinated CO2) in aprotic media (such as CH3CN). CO evolution in the reduction of CO2 is ascribed to reductive cleavage of the resulting metal–CO [17–22]. In recent years, nanocatalysts have received increasing attention and exhibited many lab- and industrial-scale applications [23–26]. When a nanocatalyst participates in an electrochemical reaction, it is called nano-electrocatalysts. Electrocatalysts are a specific form of catalysts that function at electrode surfaces or may be the electrode surface itself. An electrocatalyst can be heterogeneous (such as a platinum surface or nanoparticles) or homogeneous (such as a coordination complex or enzyme (biocatalyst)) [27–30]. The electrocatalyst assists in transferring electrons between the electrode surface and reactant(s), and/or facilitates an intermediate chemical transformation described by an overall half-reaction. Nanoparticles have emerged as sustainable alternatives to conventional materials, as robust, high surface area heterogeneous catalysts and catalyst supports [31,32]. The nanosized particles increase the exposed surface area of the active
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component of the catalyst, thereby enhancing the contact between reactants and catalyst dramatically and mimicking the homogeneous catalysts. However, their insolubility in common reaction solvents renders them easily separable from the reaction mixture like heterogeneous catalysts, which in turn makes the product isolation stage effortless. Also, the activity and selectivity of a nanocatalyst can be manipulated by tailoring chemical and physical properties such as size, shape, composition, and morphology. The scientific challenge is the synthesis of specific size and shape nanocatalysts to allow facile movement of materials in the reacting phase and control over morphology of nanostructures to tailor their physical and chemical properties. However, the rapid advancement of nano-technology made possible the preparation of a variety of nanoparticles with controlled size, shape, morphology, and composition [30–32]. Here, we report the synthesis and characterization of two new mononuclear Gd(III) and Dy(III) complexes, [M(AyGG)3(H2O)5], where AyGG is Alizarin yellow GG (Fig. 1). In addition, a simple method is reported for the preparation of the Gd2O3 and Dy2O3 nanoparticles from these precursor complexes by a facile calcination method. The electrocatalytic reduction of CO2 to CO by the complexes and their nano-oxides is also investigated using cyclic voltammetry (CV) in CH3CN solution. 2. Experimental 2.1. Materials and methods GdCl36H2O, DyCl36H2O, tetra-n-butylammonium hexafluorophosphate, Alizarin yellow GG, and acetonitrile were purchased from Merck. All chemicals and solvents were of high purity and used without any further purification. Elemental analysis (C, H and N) were performed using a Leco, CHNS-932 elemental analyzer. Fourier transform infrared spectra were recorded on an FT-IR JASCO 680-PLUS spectrometer in the region of 4000–400 cm1 using KBr pellets. Electronic absorption spectra were recorded on a JASCO 7580 UV–vis–NIR double-beam spectrophotometer using quartz cells with a path length of 10 mm. XRD analyses were performed using a PHILIPS PW3040/60 diffractometer with CuKa radiation. Morphology of the nanoparticles was observed using a field-emission scanning electron microscope (FE-SEM, HITACHI; S-4160). Transmission electron microscopy (TEM) images were obtained on a Philips CM30 transmission electron microscope with an accelerating voltage of 300 kV. Voltammetric experiments were performed on a SAMA Research Analyzer M-500. All measurements were carried out in a 5 mL cell which was fitted with a Teflon lid incorporating a three-electrode system comprising of a glassy carbon electrode (’ = 2 mm) as the working electrode, a platinum wire as the auxiliary electrode, and a silver wire as the pseudo-reference electrode (the potential values reported vs. SCE). The glassy carbon working electrode surface was freshly cleaned with alumina polish on a micro cloth before each scan and was rinsed with doubly-distilled water between each polishing step. The cyclic voltammograms of [Gd(AyGG)3(H2O)5] and [Dy(AyGG)3(H2O)5] were recorded in acetonitrile using tetran-butylammonium hexafluorophosphate (TBAH) as the supporting
electrolyte. Ferrocene (E = 0.665 V vs. NHE) was used as an internal reference at the end of each experiment. The solutions were purged by N2 or CO2 flows during study. All spectral and electrochemical data were collected at ambient temperature. All gas chromatograms were recorded with a Varian STAR 3400 gas chromatograph with He as the carrier gas at a flow rate of 30 mL/min and a temperature of 230 C. After each electrolysis step (at least 3 h at a controlled potential) in the presence of the electrocatalysts ([Gd(AyGG)3(H2O)5], [Dy(AyGG)3(H2O)5], Gd2O3, or Dy2O3), the gas sample was taken using a gas-tight syringe and the gaseous reaction product, i.e. CO, was detected by GC and IR. No detectable quantity of the target product (CO) was found in the headspace of the blank solution (containing the supporting electrolyte (TBAH) + solvent (CH3CN) under CO2) after electrolysis in the absence of the electrocatalysts for 3 h at room temperature. In all samples, no hydrogen (H2) peak was detected by GC. 2.2. Synthesis of [Gd(AyGG)3(H2O)5] [Gd(AyGG)3(H2O)5] was synthesis by the reaction of Alizarin yellow GG and gadolinium(III) chloride hexahydrate in 30 mL water. Alizarin yellow GG (0.93 g, 3 mmol) was dissolved in 15 mL of water under stirring at 80 C until a dark yellow solution resulted. To the solution was then added a solution of GdCl36H2O (0.37 g, 1 mmol) in 15 mL water. The reaction mixture was refluxed for 5 h. The dark orange precipitate was filtered and washed with water and dried in air at room temperature (Scheme 1). Yield: 0.95 g, 86%. Anal. Calc. for GdC39H34N9O20 (MW = 1105.99 g/mol): C, 42.35; H, 3.10; N, 11.40 % Found: C, 42.64; H, 3.21; N, 11.51%. 2.3. Synthesis of [Dy(AyGG)3(H2O)5] Alizarin yellow GG (0.93 g, 3 mmol) was dissolved in 15 mL of water under stirring at 80 C until a dark yellow solution resulted. To the solution was then added a solution of DyCl36H2O (0.38 g, 1 mmol) in 15 mL water. The reaction mixture was refluxed for 12 h. The red–orange precipitate was filtered and washed with water and dried in air at room temperature (Scheme 1). Yield: 0.97 g, 87%. Anal. Calc. for DyC39H34N9O20 (MW = 1112.12 g/mol): C, 42.15; H, 3.08; N, 11.34 % Found: C, 42.23; H, 3.19; N, 11.48%. 2.4. Synthesis of Gd2O3 and Dy2O3 nanoparticles To prepare the nanoparticles, the precursor complexes, [Gd (AyGG)3(H2O)5] and [Dy(AyGG)3(H2O)5], were calcined up to 600 C. The brown gadolinium oxide (Gd2O3) nanoparticles and white dysprosium oxide (Dy2O3) nanoparticles were obtained by subjecting 0.3 mg of the as-prepared Gd(III) and Dy(III) complexes powder to heat treatment in air at given temperatures ranging between 400 and 600 C. An average temperature increase of 20 C every minute was selected before the temperature reached to the target temperature, and after keeping the thermal treatment at the target temperature for 2 h, it was allowed to cool to room temperature naturally. A series of further experiments were carried out to investigate the reaction conditions. The detailed reaction conditions and corresponding results are summarized in Table 1. 3. Results and discussion 3.1. Synthesis and characterization of complexes
Fig. 1. Molecular structure of Alizarin yellow GG (AyGG).
The reaction of Alizarin yellow GG with GdCl36H2O and DyCl36H2O is shown in Scheme 1. The complexes were prepared in good yield. The elemental analyze of the complexes were entirely consistent with their proposed composition. Fig. 2 shows the
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Scheme 1. Synthesis route to [M(AyGG)3(H2O)5], (M = Gd and Dy).
Table 1 The final products obtained under different calcination conditions. Sample no.
Precursor complex
Calcinations temperature ( C)
Calcinations time (h)
Average size of the particles
Morphologies of the final products
1 2 3 4 5 6
Gd(III) Gd(III) Gd(III) Dy(III) Dy(III) Dy(III)
400 500 600 400 500 600
2 2 2 2 2 2
2 mm 20 nm 13 nm 1 mm 20 nm 12 nm
Microparticle Nanoparticle Nanoparticle Microparticle Nanoparticle Nanoparticle
Fig. 2. The optimized structure of [M(AyGG)3(H2O)5] (M = Gd(III) and Dy(III)).
optimized structure of [M(AyGG)3(H2O)5], (M = Gd(III) and Dy(III)). According to the optimized structure, the coordination number of the Gd(III) and Dy(III) complexes is eight. 3.2. UV–vis and FT-IR spectroscopic studies To obtain suitable electronic absorption spectrum, the complexes were dissolved in acetonitrile. The complexes demonstrate three absorption bands in the UV–vis region (200–700 nm) that can be assigned to ligand-centered (p ! p* and n ! p*) transitions. In the electronic absorption spectrum of Alizarin yellow GG ligand (Fig. 3a) the band observed in 260 nm can be assigned to p ! p* transition. The n ! p* transition occur at 430 nm that can be corresponded to the excitation of an electron from one of the unshared pair to the p* orbital. The last transition shows a
significant shift when the ligand coordinated to the lanthanide metals (Fig. 3b and c). This phenomenon can be attributed to contribution of the non-bonding electron pairs of the ligand in the coordination to the lanthanide(III) ions [33]. The orange color observed for Gd(III) complex and Dy(III) complex can be attributed to appearance of some of the n ! p* transitions that overlap with more energetic transitions and thus the sequence of absorption spectrum has been expanded to visible region (Fig. 3). The FT-IR spectra of the pure Alizarin yellow GG, [Gd (AyGG)3(H2O)5], [Dy(AyGG)3(H2O)5], Gd2O3 and Dy2O3 are shown in Fig. 4. In the IR spectra of the complexes, the bands associated with the carbonyl stretching are clearly seen. By comparison of the spectra of [Gd(AyGG)3(H2O)5] and [Dy(AyGG)3(H2O)5] with the free AyGG ligand, it is determined that the bands at 1670 cm1 are carbonyl stretching vibrations in the coordinated AyGG ligands
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morphology and have not any amorphous phase at their structures. The calcination of [Gd(AyGG)3(H2O)5] at 600 C, produces a crystalline phase of gadolinium oxide (Fig. 6), which is in agreement with previous reports [35,37]. As shown in Fig. 5, the XRD pattern of the pure Gd2O3 reveals a cubic phase, which can be indexed to the diffractions from the (2 2 2), (4 0 0), (4 4 0), (6 2 2), (6 6 2), and (8 4 4) planes (JCPDS Card No. 011-0608). From the XRD data for sample no. 3 in Fig. 5, the crystallite size (Dc) of the Gd2O3 nano-oxide can be calculated using the Debey–Scherrer equation [38], Dc ¼
Fig. 3. Electronic spectra of (a) Alizarin yellow GG, (b) [Gd(AyGG)3(H2O)5], and (c) [Dy(AyGG)3(H2O)5]; 1 105 M in CH3CN.
[34]. The absorption peaks at 3445 and 1633 cm1 are attributed to the stretching vibration of the O H bond and the bending vibration of H OH from water, respectively. The vibrations of GdO and Dy O bonds appear at 550 and 433 cm1; as well as at 552 and 418 cm1, respectively. In the IR spectra of the nanoparticles, the bands associated with the Gd O and DyO vibrations appear at 546 and 431 cm1; as well as at 557 and 418 cm1, respectively [34–36]. 3.3. XRD analysis The X-ray diffraction patterns of Gd2O3 and Dy2O3 nanoparticles (samples no. 3 and 6) are shown in Fig. 5. The diffraction patterns show that the nanoparticles have a crystalline
Kl bcosu
where b is the breadth of the observed diffraction line at its half maximum intensity, K is the so-called shape factor, which usually takes a value of about 0.9, and l is the wavelength of the X-ray source used in XRD. The average size of the particles of sample no. 3, is 13 nm, which is in agreement with that observed from the FE-SEM and TEM images. The calcination of [Dy(AyGG)3(H2O)5] at 600 C, yields a crystalline phase of dysprosium oxide (Fig. 5), which is in agreement with previous studies [36,39,40]. As shown in Fig. 5, the XRD pattern of the pure Dy2O3 exhibits a cubic bixbyite phase (also known as the C-rare-earth sesquioxide structure), which can be indexed to the diffractions from the o (2 2 2), (4 0 0), (4 4 0), and (6 2 2) planes (JCPDS Card No. 10-0059). There are no other peaks in Fig. 5, showing the high purity and well crystallinity of the nanooxide. The crystallite size (Dc) of the as-prepared Dy2O3, sample no. 6, was calculated to be 12 nm, which is in agreement with the FE-SEM and TEM images.
Fig. 4. FT-IR spectra of (a) Alizarin yellow GG; (b) [M(AyGG)3(H2O)5]; and (c & d) Gd2O3 or Dy2O3 nanoparticles calcined at 400 C and 600 C, respectively.
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Fig. 5. XRD patterns of the Gd2O3 and Dy2O3 nanoparticles calcined at 600 C for 2 h.
Fig. 6. FE-SEM images of the Gd2O3 nanoparticles calcined at 600 C for 2 h.
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Fig. 7. Histogram of the particle size distribution of the Gd2O3 nanoparticles calcined at 600 C for 2 h.
3.4. FE-SEM and TEM studies The FE-SEM technique was used to investigate the morphology of the nanoparticles. Table 1 shows some conditions which have been examined to investigate the morphology of the final products and compare them to each other. Figs. 6 and 8 show the FE-SEM images of the gadolinium and dysprosium nano-oxides (samples no. 3 and 6) calcined at 600 C for 2 h, respectively. At 400 C some parts of the organic ligands are still remaining and the nanoparticles cannot form. The quasi-spherical nanoparticles are observed when the calcination temperature reaches 600 C. The images show that when the temperature reaches 600 C, the particle size becomes smaller. Also the carbonate phase at a temperature of 600 C is gone and the oxide phase is formed (Fig. 4). According to the FE-SEM results, the particle size of Gd2O3 and Dy2O3 (sample no. 3 and 6) is between 8 and 15 nm. This value
Fig. 9. Histogram of the particle size distribution of Dy2O3 nanoparticles calcined at 600 C for 2 h.
is in good agreement with the XRD crystallite size results. Figs. 7 and 9 show the particle size histograms of sample no. 3 and 6. The distribution of the size of nearly spherical particles is peaked at about 12–13 nm. The TEM images of Gd2O3 and Dy2O3 nanoparticles (samples no. 3 and 6) are shown in Figs. 10 and 11. As the images show, the nanoparticles shapes are nearly spherical and have a uniform distribution. Also, they form a chain of beads attached to each other. The size of the nanoparticles obtained from the XRD diffraction patterns and FE-SEM technique are in close agreement with the TEM image which show sizes of 10–20 nm for two samples, gadolinium and dysprosium nano-oxides, calcined at 600 C for 2 h (samples no. 3 and 6).
Fig. 8. FE-SEM images of the Dy2O3 nanoparticles calcined at 600 C for 2 h.
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Fig. 10. TEM images of the Gd2O3 nanoparticles calcined at 600 C for 2 h.
Fig. 11. TEM images of the Dy2O3 nanoparticles calcined at 600 C for 2 h.
To study the electrocatalytic activity of the compounds ([Gd (AyGG)3(H2O)5], [Dy(AyGG)3(H2O)5]), and the nano-oxides
(samples no. 3 and 6) for the reduction of CO2, the cyclic voltammograms of each compounds were taken under a flow of CO2 and the results compared with those obtained under a nitrogen atmosphere. For the complexes, a precatalyst solution was prepared by dissolving 5 mg of complexes in 5 mL of
Fig. 12. Cyclic voltammograms of [Gd(AyGG)3(H2O)5] (103 M) (a) in the absence and (b) presence of CO2 at scan rate of 100 mV/s; in dry acetonitrile; 0.1 M TBAH.
Fig. 13. Cyclic voltammograms of [Dy(AyGG)3(H2O)5] (103 M) (a) in the absence and (b) presence of CO2 at scan rate of 100 mV/s; in dry acetonitrile; 0.1 M TBAH.
3.5. Electrocatalytic activity
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Fig. 15. Cyclic voltammograms of the Dy2O3 nanoparticles, (a) in the absence and (b) presence of CO2 at scan rate of 100 mV/s; in dry acetonitrile; 0.1 M TBAH. Fig. 14. Cyclic voltammograms of the Gd2O3 nanoparticles, (a) in the absence and (b) presence of CO2 at scan rate of 100 mV/s; in dry acetonitrile; 0.1 M TBAH.
Scheme 2. Proposed reaction mechanism for the electrocatalytic reduction of CO2 by the Gd(III) or Dy(III) complex in CH3CN.
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electrolyte solution (0.1 M TBAH in acetonitrile) to give a concentration of ca. 1 mM. Also for the nanoparticles, a precatalyst was prepared by dispersing 2 mg of the nanoparticles in 1 mL of water and deposited 10 mL of dispersed nanoparticles on the GC electrode surface. TBAH (0.1 M) in acetonitrile was used for electrolyte solution. The voltammetric response of the compounds under two different atmospheres, nitrogen and carbon dioxide, will be briefly discussed here. 3.5.1. Electrochemical studies under N2 atmosphere The cyclic voltammograms of the complexes and nanoparticles at a glassy carbon (GC) electrode are shown in Figs. 12 and 13a. The cyclic voltammetry measurements were performed on an acetonitrile solution of the complexes with 0.1 M TBAH supporting electrolyte at ambient temperature. For the complexes, the voltammograms recorded in N2 present three couples at 0.75, 0.65, and 1.76 V for [Gd(AyGG)3(H2O)5], as well as at 0.78, 0.67, and 2.0 V for [Dy(AyGG)3(H2O)5] vs. SCE, that are assigned to the two redox couples of Alizarin yellow GG, and one redox couple for Gd(III/II) (at 1.76 V) or Dy(III/II) (at 2.0 V). Under the N2 atmosphere, the redox couple of AyGG exhibits equivalent anodic and cathodic waves, with 0.05 V peak separations. The irreversible reduction wave in the more negative potential region is assigned to the reduction of Gd(III/II) and Dy(III/II) couples. For the Gd2O3 nanoparticles (sample no. 3), the voltammogram recorded in N2 shows a voltammetric peak at 2.1 V vs. SCE that is assigned to the irreversible Gd(III/II) reduction couple (Fig. 14a). Also, for the Dy2O3 nanoparticles (sample no. 6), the voltammogram recorded in N2 shows a peak at 2.18 V that can be assigned to the irreversible Dy(III/II) redox couple (Fig. 15a). 3.5.2. Electrochemical studies under CO2 atmosphere Figs. 12b and 13b show the cyclic voltammograms of a solution, purged by CO2 flow, containing 1 mM of the complexes in 0.1 M TBAH/CH3CN at a glassy carbon electrode. Also, Scheme 2 shows the proposed reaction mechanism for the electrocatalytic reduction of CO2 by the Gd(III) and Dy(III) complexes. As can be seen in Figs. 12b and 13b, some significant changes are observed when the N2 atmosphere is replaced by CO2. The signal around 0.8 V can be assigned to the one-electron reduction of the AyGG ligand. In a solution saturated with CO2, the reduction wave (1) is approximately unchanged with respect to an N2-saturated solution (Figs. 12 and 13). The first, second, and third one-electron reductions of the complexes provide three electrons that occupy the empty p* orbitals of the alizarin ligands. These observations are consistent with the alizarin-based reduction to give [MIII(AyGG)3(H2O)5]3 followed by reaction with CO2, step (IV) (Scheme 2). After the substitution of CO2 with H2O, an intramolecular two-electron transfer takes place from the alizarin ligand to the coordinated CO2 ligand to give an intermediate shown as the metallocarboxylate, promoting the reduction of CO2. It may occur via an O2 transfer to CO2 to give a CO32 anion and a CO complex (step (VI)). In fact, the catalytic cycle needs two CO2 molecules. One CO2 is reduced to CO and another CO2 is fixed to a carbonate anion. The carbonate anion (CO32) is stable and cannot convert to CO2 in the experimental condition. The presence of carbonate anion was confirmed by 13C NMR and IR spectroscopy. There is a significant current enhancement for the wave at Ep. c = 1.7 V (wave (2) in Fig. 12b), and Ep.c = 1.65 V (wave (2) in Fig. 13b), which is consistent with the electrocatalytic reduction of CO2 [8]. Figs. 14b and 15b shows the cyclic voltammograms of a precatalyst, purged by CO2 flow, containing 10 mL of the dispersed nanoparticles on the GC electrode surface in 0.1 M TBAH/CH3CN. As can be seen in these figures, some changes are observed when the N2 atmosphere is replaced by CO2. A significant current
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enhancement for the wave at Ep.c = 1.95 V for Gd2O3 and Ep. for Dy2O3, can be assigned to the electrocatalytic reduction of CO2. According to the voltammetry results, the Dy (III) complex, [Dy(AyGG)3(H2O)5] has better electrocatalytic activity than other compounds. For all four electrocatalysts (the complexes and nano-oxides), the formation of CO as the final product was confirmed by GC and IR spectroscopy (2119 and 2172 cm1) after the electrolysis of the reaction mixture for 3 h at room temperature.
c = 2.0 V
4. Conclusions The Gd2O3 and Dy2O3 nanoparticles were prepared by the calcination of two new Gd(III) and Dy(III) complexes at different temperatures up to 600 C in air. All four compounds were used as an electrocatalyst for the reduction of CO2 to CO in CH3CN. The voltammograms in the absence and presence of carbon dioxide indicates that the complexes and the nano-oxide nanoparticles can catalyze the electroreduction of CO2 to CO. In addition, the formation of CO as the final product and carbonate anion as a byproduct was confirmed using several instrumental techniques. The production of CO from the electrocatalytic reduction of CO2 is more thermodynamically and electrochemically favorable than other 2e reduction products. In addition, the final product CO can be utilized to produce H2 gas via water–gas shift reaction (WGSR, CO + H2O ! CO2 + H2) or used as a fuel (CO + 1/2O2 ! CO2 DH = –283.006 kJ/ mol). Finally, according to the experimental results, an electrocatalytic cycle was proposed for the reduction of CO2 in the presence of the Gd(III) and Dy(III) complexes. We hope our new findings can be used to develop new electrocatalysts for electroreduction of CO2 to CO and to reduce this greenhouse gas emission. Acknowledgement We are grateful to the Isfahan University of Technology (IUT) for financial support. References [1] A. Raval, V. Ramanathan, Observational determination of the greenhouse effect, Nature 342 (1989) 758–761, doi:http://dx.doi.org/10.1038/342758a0. [2] W.D. Nordhaus, To slow or not to slow: the economics of the greenhouse effect, Econ. J. 101 (1991) 920–937, doi:http://dx.doi.org/10.2307/2233864. [3] T.L. Frölicher, F. Joos, Reversible and irreversible impacts of greenhouse gas emissions in multi-century projections with the NCAR global coupled carbon cycle-climate model, Clim. Dyn. 35 (2010) 1439–1459, doi:http://dx.doi.org/ 10.1007/s00382-009-0727-0. [4] L.L. Andersen, F.F. Østerstrøm, O.J. Nielsen, M.P.S. Andersen, T.J. Wallington, Atmospheric chemistry of (CF3)2CFOCH3, Chem. Phys. Lett. 607 (2014) 5–9, doi:http://dx.doi.org/10.1016/j.cplett.2014.05.036. [5] International energy agency, CO2 emissions from fuel combustion highlights, IEA Stat. (2013) 158, doi:http://dx.doi.org/10.1787/co2-table-2011-1-en. [6] X. Frogneux, O. Jacquet, T. Cantat, Iron-catalyzed hydrosilylation of CO2: CO2 conversion to formamides and methylamines, Catal. Sci. Technol. 4 (2014) 1529–1533, doi:http://dx.doi.org/10.1039/C4CY00130C. [7] I. Knopf, T. Ono, M. Temprado, D. Tofan, C.C. Cummins, Uptake of one and two molecules of CO2 by the molybdate dianion: a soluble, molecular oxide model system for carbon dioxide fixation, Chem. Sci. 5 (2014) 1772–1776, doi:http:// dx.doi.org/10.1039/c4sc00132j. [8] F.H. Haghighi, H. Hadadzadeh, H. Farrokhpour, N. Serri, K. Abdi, H. Amiri Rudbari, Computational and experimental study on the electrocatalytic reduction of CO2 to CO by a new mononuclear ruthenium(ii) complex, Dalton Trans. 43 (2014) 11317–11332, doi:http://dx.doi.org/10.1039/c4dt00932k. [9] T. Mizukawa, K. Tsuge, H. Nakajima, K. Tanaka, Selective production of acetone in the electrochemical reduction of CO2 catalyzed by a Ru naphthyridine complex, Angew. Chem. Int. Ed. 38 (1999) 362–363. [10] I.M.B. Nielsen, K. Leung, Cobalt-porphyrin catalyzed electrochemical reduction of carbon dioxide in water. 1. A density functional study of intermediates, J. Phys. Chem. A 114 (2010) 10166–10173, doi:http://dx.doi.org/10.1021/ jp101180m.
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