Electrocatalytic conversion of carbon dioxide to urea

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Received: 19 September 2016 /Revised: 27 December 2016 /Accepted: 14 January 2017 ... mized condition, CO2 is effectively reduced to urea. Fourier .... in a microwave muffle furnace (WM-2 model, 2.45 GHz, 3 kW). ... isotherm in the range of 400 to 700 °C. The sample temperatures ... at low current density (10 mA/cm2).
Ionics DOI 10.1007/s11581-017-1985-1

ORIGINAL PAPER

Electrocatalytic conversion of carbon dioxide to urea on nano-FeTiO3 surface Palanisamy Siva 1 & Periasamy Prabu 1 & Mohanraj Selvam 1 & Subramani Karthik 1 & Venkatachalam Rajendran 1

Received: 19 September 2016 / Revised: 27 December 2016 / Accepted: 14 January 2017 # Springer-Verlag Berlin Heidelberg 2017

Abstract The effect of the electrochemical reduction of carbon dioxide (CO2) on the structure and morphology of two different synthesised nano FeTiO3 composite was examined after a fixed-potential bulk electrolysis process. FeTiO3 was used as the cathode, stainless steel (SS) plate as anode and CO2 saturated NaHCO3 and KNO2 as electrolysis. At optimized condition, CO2 is effectively reduced to urea. Fourier transform infrared spectroscopy (FTIR) used to examine the two different synthesis of nano FeTiO3 surfaces to reveal the carbonate ions and urea species during reduction. Ultraviolet– visible spectroscopy (UV) peaks explain the urea function groups present in electrolyte after the CO2 and NO2 reduction. Charge transfer resistance is using Electrochemical Impedance Spectroscopy (EIS) analysis. A systematic Cyclic Voltammetry (CV) and Linear Sweep Voltammetry (LSV), two different synthesis of FeTiO3 composite, were performed in order to decouple electrochemical reduction processes of CO2 and NO2 to urea in aqueous solutions. Keywords CO2 and NO2 reduction . FeTiO3 . Urea . Cyclic voltammetry (CV) . Linear sweep voltammetry (LSV)

Introduction The present global economy and chemical industry are deeply dependent on fossil fuel resources which have limited global * Periasamy Prabu [email protected]

1

Centre for Nano Science and Technology, KS Rangasamy College of Technology, Tiruchengode, Tamil Nadu 637 215, India

reserve. In addition, the utilization and burning of fossil fuel generates voluminous CO2 gas which is a known greenhouse pollutant. Therefore, sustainable alternatives energy sources need to be developed in order to secure long-term economic growth while mitigating socio-environmental concerns potentially associated with increasing emissions of CO2 [1]. It is already established that CO2 can be converted as a C1 feedstock for the production of synthesis gas (e.g.CO/H2) and various other organic compounds (e.g. methanol, methane, ethylene, formic acid) using suitable electro-catalyst by various chemical and electrochemical routes. An evitable solution to reduce the rising concentration of CO2 in air is the electrochemical reduction of CO2 to carbon based products [2–5]. CO2 conversion using electrochemical catalysis approaches has attracted a great deal of attention among researchers due to the following several reasons: (1) The process is controllable by electrode potentials and reaction temperature, (2) The supporting electrolytes can be fully recycled so that the overall chemical utilization can be minimized to waste water, (3) The electricity used to drive the process can be obtained without generating any new CO2-sources and (4) The electrochemical reaction system is compact, modular, on-demand and easy to scale-up [6]. So far, various copper-based electrocatalysts have been successfully explored which are capable to convert CO2 to hydrocarbons, methanol and multi-carbon oxygenates at a low yield [6–9]. Metallic catalysts such as Au and Ag also offer good selectivity in this regard, but their limited availability and poor durability hinder the practical applications [10–13]. Recently, carbon nanostructure based materials have attracted attention as metal-free electrocatalysts for CO2 reduction because of their low cost, high surface areas and adjustable doping of heteroatom’s (e.g. nitrogen, boron, etc.). The nitrogen (N)-doped carbon nanofibers exhibited negligible over potential (−0.17 V) for CO2 reduction using ionic liquid as an electrolyte and displayed one order of magnitude higher current

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density compared with the bulk Ag under similar experimental conditions [14]. However, during the electrochemical reaction, nitrogen oxides and their ions are simultaneously formed with CO2 evolution. Therefore, it appears exciting that nitrogen oxides as well as CO2 can be simultaneously removed. Therefore, the electrochemical conversion of CO2 with nitrite ions into a useful material consisting of C–N bonds appears promising and worth of study [6]. The urea molecule [CO (NH2)2], which is one of the simple compounds having C–N bonds, is constructed by a carbonyl group (C = O) and two amino (−NH2) groups. Few metal electrodes have successfully demonstrated the ability to reduce CO2 to urea using Cd catalysts with ~55% efficiency around −1.0 V; then the efficiency decreases with increasing the over potential. The urea formation at Cd catalysts is larger than that at Zn catalysts [15]. In this paper, two different methods, co-precipitate and microwave synthesised nano FeTiO3 particles, were used as the cathodes, SS plate as anode in CO2 saturated NaHCO3 and KNO2 as electrolyte, and at optimised condition, CO2 and NO2 are effectively reduced to urea. In addition, the key parameters of CO2 and NO2 conversion were studied, along with the selectivity and efficiency of the formed products in the presence of control systems.

Experimental section Synthesis of nano-FeTiO3 powders Co-precipitate method Ten milliliters of 1 M iron nitrate precursor was prepared and adjusted the pH 7.0 with 0.5 M NaOH solution. The second precursor solution, i.e. 0.5 M titanium isoproxide solution was prepared in ethanol for 50 ml. Five milliliters of titanium tetra isoproxide was then added into 10 ml of iron nitrate solution with constant magnetic stirring for 1 h. The solution turns turbid and white precipitates were observed, which indicate the co-precipitation of iron titanate. Then, the creamy power solution was dried at 80 °C in hot air oven, and the obtained powder was sintered in a muffle furnace at 150 °C for 2 h; finally, brownish red colour powder was obtained. Microwave method The (Fe + Ti) mixed carbonate precursor was also prepared by co-precipitation route using FeCl2·4H2O and Na2CO3 as raw materials. First, FeCl2·4H2O and Na2CO3 were dissolved in distilled water to prepare FeCl2 (1 mol/L) and Na2CO3 (2 mol/L) aqueous solutions, then TiCl3 was slowly added into ethanol to obtain TiCl3 solution (1 mol/L). TiCl3 solution was mixed with FeCl2 aqueous solution to obtain the TiCl3–FeCl2 mixed solution

(molar ratio of Fe/Ti = 1:1). Finally, excess quantity of Na2CO3 aqueous solution was added to the TiCl3–FeCl2 mixed solution and sonicated for 30 min to obtain (Fe + Ti) mixed carbonate precipitates at room temperature. The co-precipitation synthesis route was carried out under N2 atmosphere to protect Fe2+ ions from oxidation. After filtering, the precipitate was washed with distilled water several times until the pH is about 7 and dried in a vacuum oven at 100 °C for 12 h. The dried powder was calcined in a microwave muffle furnace (WM-2 model, 2.45 GHz, 3 kW). The precursor was kept in an enclosed crucible (3 cm in diameter and 6 cm in height), then it was held for 20 min by selecting an isotherm in the range of 400 to 700 °C. The sample temperatures with microwave heating were determined by a metallic-sheathed Ni (Cr) thermocouple. In order to demonstrate the effect of microwave heating on the calcinations process, the calcinations experiments with conventional heating were also carried out from 600 to 800 °C in a furnace. Materials’ characterization The phase and crystalline nature of the synthesised samples were analysed by X-ray Diffractometer (XRD) (X’-Pert PRO, PANalytical, The Netherlands) using CuKα(λ = 1.5406 Å) as a radiation source. The samples were analysed over the 2θ range of 10 to 80 at room temperature (298 K). The observed peak positions and the relative intensities of the powder pattern were indexed in comparison with the reference diffraction data. The surface morphology of FeTiO3 was determined using a scanning electron microscope coupled with energy dispersive X-ray analysis (SEM-EDAX) (JEOL JSM-6390LV, Japan) at 20 kV with a magnification of ×10,000 at 1 μm scale. Functional group information of the electrolyte was analysed using UV spectroscopy (Cary 8454, Agilent, Alexandra Rd. Singapore), and powder surface was analysed using Fourier transform infrared spectroscopy (Spectrum 100; PerkinElmer, USA). The spectra were collected in the range of 4000–500 cm−1. Electrodes preparation and characterization The Stainless Steel (SS) plate was cut into a small piece of size (16 cm2) and then polished finely used a solvent (acetone). The synthesised nano FeTiO3 powder was mixed with PVDF at a ratio of 85:15 employing small quantity of N-methyl-2pyrrolidoneas a solvent to make a paste. The paste was coated over the SS plate using a doctor blade [16]. The coating was repeated multiple times to achieve a uniform thickness (10 μm) of electrode on SS plate. The FeTiO3 electrode was dried in a hot air oven at 85 °C for 1 h and then used for further studies. Electrolysis was conducted in the Terylene diaphragm cell of 100 ml. The anode was a (SS) net (16 cm2) and cathode was a (FeTiO3) electrode (16 cm2). The distance between the anode and cathode was 2 cm. The electrical power was provided by a laboratory direct current power supply with a current –voltage

Ionics Fig.1 a XRD patterns of the co-precipitate synthesised FeTiO3. b XRD patterns of the microwave synthesised FeTiO3

monitor. Nitrogen gas was passed into the electrolyte for 30 min in order to remove O2 and CO2 gas from the electrolyte before reaction. One molar of NaHCO3 and 0.2 M KNO2 were chosen as the electrolyte at room temperature and the voltage was 4 V. During the preparative electrolysis, a sample was taken (in 10-min periods). FeTiO3 electrodes could be operated at low current density (10 mA/cm2).

Electrochemical measurements The CV behaviour of FeTiO3 electrode was examined using a three electrode cell set-up. In this set-up, the FeTiO3 metal was used as a working electrode, while platinum and saturated calomel were used as counter and reference electrodes, respectively, for carbonate based electrolyte. CV study was carried out for

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both the co-precipitate method, synthesised-FeTiO3 and microwave synthesised FeTiO3, in fixed scan rate using Autolab equipment (PGSTAT302N, Metrohm Autolab, The Netherlands), and the applied potential window for CV study was −1 to 1 V at 10 mV/s scan rate. Cathode current behaviour was determined from the extrapolated data of the Cyclic Voltammogram test.

Results and discussion The XRD patterns of two different synthesised FeTiO3 powders are shown in Fig. 1a, b; the XRD patterns of coprecipitate method synthesised FeTiO3 illustrates a set of reflections from FeTiO3 at 2θ = 32.16, 35.2 and 48.60corresponding to the (104), (110) and (024) diffraction peaks of the FeTiO3 rhombohedral structure, The XRD patterns of microwave synthesised FeTiO3 are shown in Fig. 1b. A set of reflections from FeTiO3 at 2θ = 24.14°, 33.16°, 35.66° and 49.58° are corresponding to the (012),(104), (110) and (024)

Fig. 2 The SEM images of a and b co-precipitate synthesised FeTiO3. c and d microwave synthesised FeTiO3 at 500 nm and 300 nm

diffraction peaks of the FeTiO3 rhombohedral structure [JSPDF file No. 89–8811]. The XRD patterns clearly explained microwave synthesised FeTiO3 had more crystalline structure than co-precipitate synthesised FeTiO3 powder. The SEM images of co-precipitate synthesised FeTiO3 and microwave synthesised FeTiO3 powders are shown in Fig. 2; images a and b explain the structure morphology of coprecipitate synthesised FeTiO3. It is noticed that particle of the products are 500 and 300 nm size but not uniformly shaped, Images c and d explain the structure morphology of microwave synthesised FeTiO3. It is noticed that particle of the products are 500 and 300 nm size, uniformly shaped and flakes in nature.

Characterization of FeTiO3 FTIR of FeTiO3. The typical FTIR spectra of without electro reduction and after electro reduction FeTiO3 electrodes’ surface are shown in

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Fig. 3a, b, respectively. Figure 3a elucidated a before and after reduction of co-precipitate synthesised FeTiO3. Graph (a) had two characteristic infrared absorption peaks at 1633 and 3362 cm−1. The absorption peak at 1633 cm−1 was due to both stretching and bending motion of NH2 molecule [17]. The absorption peak at 3370 cm−1 was due to symmetric stretching N–H of urea [18]. The graph (b) that elucidates a before reduction FeTiO3 had no clear peak obtained. These functional groups played a key role in depositing on the surface of FeTiO3 of after reduction. FTIR spectra of microwave synthesised FeTiO3 before and after electrochemical reduction are shown in Fig. 3b. Graph (a) illustrates the FTIR spectra after electrochemical reduction of FeTiO3 electrode which possess three characteristic infrared absorption peaks at 1159, 1673 and 3370 cm−1 for urea.

Fig. 3 a FTIR of the graph (a) after reduction and graph (b) before reduction of co-precipitate synthesised-FeTiO3. b FTIR of the graph (a) after reduction and graph (b) before reduction of microwave synthesised FeTiO3

The absorption peak at 1673 cm−1 was due to both stretching and bending motion of NH2 molecule [17]. The weak band appearing at 1159 cm−1 was due to VCN symmetric stretching vibration. The absorption peak at 3370 cm−1 was due to symmetric stretching N–H of urea [18]. Graph (b) elucidates the FTIR spectra before electrochemical reduction of FeTiO3 electrode without any noticeable evident peaks. Sharp and more urea function groups’ adsorption are on microwave synthesised FeTiO3 than co-precipitate synthesised FeTiO3. Ultraviolet–visible spectroscopy (UV) The UV spectra shown in Fig. 4, for 1 M NaHCO3 and 0.2 M KNO2 solution after electrochemical reduction. This observed bands elucidate the characteristic UV absorption peak at 217

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co-precipitate synthesized FeTiO 3 individually using NaHCO3 as electrolyte.

Cyclic voltammetry (CV)

Fig. 4 UV spectra of graph (a) microwave synthesised and graph (b) coprecipitate synthesised FeTiO3 of CO2 saturated 1 M NaHCO3 and 0.2 M KNO2 solution after reduction

and 355 nm; the peaks conformed the CO2 conversion to urea in 1 M NaHCO3 and 0.2 M KNO2 after reduction by coprecipitate synthesised FeTiO3 and microwave synthesised FeTiO3 electrodes [19]. The peaks absorbed in UV spectra conforms the presence of urea by the reduction of CO−2 and NO−2 simultaneously.

Electrochemical impedance spectroscopy (EIS) The Nyquist plots of Fig. 5 shows the charge transfer resistance analysis of graph (a) microwave synthesised FeTiO3 and graph (b) co-precipitate synthesized FeTiO3 in 1 M NaHCO3. This study clearly represented the microwave synthesis of FeTiO3 had low resistance than

Fig. 5 Nyquist plots for graph (a) microwave synthesised and graph (b) co-precipitate synthesised FeTiO3 in 1 M NaHCO3

The choice of a suitable electrolyte for the study of CO2 reduction on FeTiO3 electrode was quite a complex question because of the known tendency of a number of ions (especially anion) to absorb irreversibly on the surface of FeTiO3 [20]. Therefore, the Cyclic Voltammetry study (CV) of coprecipitate synthesised FeTiO3 and microwave synthesised FeTiO3 was carried out. These two different synthesized FeTiO3 are comparing with each other for cathode electrochemical behaviour. Mercury electrode is using reference electrode and Pt wire is using counter electrode. CV of coprecipitate synthesised FeTiO3 and microwave synthesised FeTiO3 is due to N2 saturated 1 M NaHCO3 and N2, CO2 saturated 1 M NaHCO3 and 0.2 M at 10 mV/s scan rate at room temperature. As shown in Fig. 6, CVof CO2 saturated 1 M NaHCO3 and 0.2 M KNO2 for two different types synthesised FeTiO3. In graph (a), CV for co-precipitate synthesised FeTiO3 reduction peak was observed near to (−0.5 V). In graph (b), CV for microwave synthesised FeTiO3 reduction peak was observed near to (−0.4 V) that should result from the one–electron reduction of CO2 which is attributed to the reduction of CO2 to • CO−2 and NO2 to NO−2 [18]. This anion radical, however, reacts rapidly with the intermediate of the derived product. This suggests that the FeTiO3electrode is able to simultaneously reduce of CO2 and nitrite ions. CV results indicate a favourable effect of Fe doped TiO2 on increasing cathodic reduction at (−0.5 V and −0.4 V) of electrochemical species than Cd, Co (−0.75) [21].

Fig. 6 Graph (a) Cyclic voltammetry of microwave synthesised and graph (b) co-precipitate synthesised FeTiO3 in CO2 saturated 1 M NaHCO3 and 0.2 M KNO2 at scan rate 10 mV/s

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Linear sweep voltammetry (LSV) The Linear Sweep Voltammetry is one of the electrochemical activity studies for CO2 and NO2 reduction. Figure 7 clearly elucidated the co-precipitate synthesised FeTiO3 and microwave synthesised FeTiO3 reduction of CO2 and NO2; high efficiency reduction peaks were observed between −0.5 to −0.4 V. As shown, graph (a), elicited the microwave synthesised FeTiO3. This graph (a) clearly shows reduction peak occur near to −0.4 V. Graph (b) elicited the co-precipitate synthesised FeTiO3. This graph (b), co-precipitate synthesised FeTiO3 clearly gives reduction peak occur near to −0.5 V. As shown in Fig. 8 is a tafel plot for Linear Sweep Voltammetry of co-precipitate synthesised FeTiO 3 and microwave synthesised FeTiO3. This figure clearly gives evidence by co-precipitate synthesised FeTiO3 reduction peak occur near to −0.5 V and microwave synthesised FeTiO3 reduction peak occur near to −0.4 V. The above evidence proves that the microwave synthesised FeTiO3 slightly shifted to positive site than co-precipitate synthesised FeTiO3. Urea formation of gas-diffusion electrode We have analysed simultaneous reductions of CO2 and nitride anions using FeTiO3 electrode and confirmed that CO formed in CO2 reduction cannot combine with ammonia formed in nitride reduction [22]. Carbon dioxide and nitride ions would be reduced to the CO-like and ammonia-like precursors on the catalysts, respectively, while these precursors were not detected. A proposed reduction mechanism at the gas diffusion electrode in simultaneous reduction from CO2− and NO2− reduction to urea formation [22–24].

Conclusions The electrolyte experiments were carried out in an undivided cell under mild conditions to avoid the addition of toxic

Fig. 7 Linear sweep voltammetry of graph (a) microwave synthesised and graph (b) co-precipitate synthesised FeTiO3 in CO2 saturated 1 M NaHCO3 and 0.2 M KNO2 at scan rate 10 mV/s

Fig. 8 Tafel plot for linear sweep voltammetry gramme of graph (a) microwave synthesised and graph (b) co-precipitated synthesised FeTiO3

solvents and catalysts. The electrical power was provided by a laboratory direct current power supply with a currentvoltage monitor. FeTiO3 electrodes could be operated at low current density 10 (mA/cm2) achieved using electrode for CO2 and NO2 reduction. XRD indicates that the microwave synthesised FeTiO3 has given good crystallite than co-precipitated-FeTiO3.SEM images indicate surface morphology of two different types of synthesis FeTiO3; good uniform structure morphology is present in microwave synthesised FeTiO3 than in co-precipitated-FeTiO3. FTIR has given the adsorbed functional groups of urea on two different types of synthesis FeTiO3 after the electrochemical reduction; more and sharp peaks were obtained in microwave synthesised FeTiO3 surface than co-precipitated-FeTiO3.UV spectra conform the presence of urea by the reduction of CO−2 and NO−2 simultaneously in electrolytes after the reduction. A systematic electro chemical study on two different synthesised FeTiO3 surfaces was performed in order to decouple the electrochemical processes of CO2 and the water electrochemical reduction taking place at these surfaces. EIS analysis conform the microwave synthesis of FeTiO3 had low charge transfer resistance than co-precipitate synthesised FeTiO3 individually using NaHCO3 as electrolyte. CV results indicate a favourable effect of Fe doped TiO2 on increasing cathodic reduction (−0.5 to −0.4 V) of electrochemical species than Cd, Co (−0.75). Linear Sweep Voltammetry of co-precipitated FeTiO3 reduction peak occur near to −0.5 V and microwave synthesised FeTiO3 reduction peak occur near to −0.4 V. Microwave synthesised-uniform structure FeTiO3 shows slightly high cathodic behaviour than co-precipitate synthesised-not uniform structure FeTiO3 for CO2− and NO2− reduction. The working potential, charge passed on the yields, was investigated.

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