Electrochemical reduction of CO2 to methanol with synthesized Cu2O nanocatalyst: Study of the selectivity

Journal Pre-proof Electrochemical reduction of CO2 to methanol with synthesized Cu2O nanocatalyst: Study of the selectivity Jenasree Hazarika, Mriganka Sekhar Manna PII:

S0013-4686(19)31924-3

DOI:

https://doi.org/10.1016/j.electacta.2019.135053

Reference:

EA 135053

To appear in:

Electrochimica Acta

Received Date: 13 July 2019 Revised Date:

26 September 2019

Accepted Date: 8 October 2019

Please cite this article as: J. Hazarika, M.S. Manna, Electrochemical reduction of CO2 to methanol with synthesized Cu2O nanocatalyst: Study of the selectivity, Electrochimica Acta (2019), doi: https:// doi.org/10.1016/j.electacta.2019.135053. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Cu Nanocatalyst

CO>CH4>C2H4

Selectivity Atmospheric Excess CO2

Electrochemical Reduction

Electric Potential

Selectivity CH3OH>CH4>C2H4 >CO

Cu2O Nanocatalyst

Electrochemical Reduction of CO2 to Methanol with Synthesized Cu2O Nanocatalyst: Study of the Selectivity Jenasree Hazarika and Mriganka Sekhar Manna1 Department of Chemical Engineering, National Institute of Technology Agartala, Tripura 799046, India ABSTRACT The selectivity of a particular product over possible multiple products and their quantitative distribution in the electrochemical reduction of CO2 has been investigated. The generation of products and the said distribution depend on the various methods and materials like catalysts, electrodes, electrolytes, selectively proton permeable membrane used in the reduction process, numbers of available hydrogen ions at the cathodic catalytic surface and its adsorption characteristics on the catalysts surface dispersed on the cathode. Copper (Cu) and cuprous oxide (Cu2O) nanocatalysts have been synthesized and characterized by X-ray Diffraction (XRD), Transmission Electron Microscopy (TEM), and Field Emission Scanning Electron Microscopy (FESEM) for finding the differences in their surface characteristics in order to explain the selectivity of product in their use as catalysts. The efficacy of the reduction process with the synthesized catalysts is studied in the customized electrolytic cell. Methanol is selectively produced by using Cu2O nanocatalyst as a cathode in an aqueous electrolyte, potassium bicarbonate (KHCO3), whereas methane and ethylene are formed predominantly with Cu nanocatalysts. The cathodic potential applied to the electrolytic cell with Cu2O as a catalyst on the cathode is optimized at -2.0 V vs. SHE for both the methanol selectivity and also for the minimization of undesired hydrogen evolution. The maximum faradaic efficiency of 47.5% for

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Corresponding author: Mriganka Sekhar Manna Email:[email protected]

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methanol at a maximum current density of 7.8 mA.cm-2 is obtained with a sustained current density of 5.6 mA.cm-2 at a run time of 50 min.

Keywords: Carbon dioxide, Electrochemical reduction, Nanocatalyst, Selectivity, Methanol

1.0 Introduction Diminution of the problems associated with global warming and uninterrupted supply of energy to the well-being of future generations are two major challenges to be overcome by the scientific community. An extraordinary era of prosperity in the living standard of mankind and socioeconomic growth of the modern society is mostly dependent on the energy liberated through the combustion of fossil fuels that results in the emission of greenhouse gas CO2. On September 2018, the concentration of carbon dioxide (406.99 pm) in the global atmosphere is reported to be exceeded by its upper limit of 350 ppm set by WHO. On the other hand, since finiteness is the inherent nature of fossil fuels, it is depleting gradually but definitely while the natural renewal of fossil fuels requires millions of years [1-3]. Consequently, the search for alternative energy sources is purposefully important. Greenhouse gas, CO2 can be reduced to useful chemicals such as methanol, ethanol, methane, ethylene, formic acid, acetic acid etc. Methanol and derived products are excellent fuels in internal combustion engines, the feedstock for the production of light olefins like ethylene and propylene. Subsequently, useful heavier hydrocarbon products can also be derived. The recycling of a carbon atom by “methanol economy” allows not only mitigation of global warming by reduction of CO2 but also provides us with an inexhaustible source of carbon. Various techniques viz. chemical, thermochemical, electrochemical, biochemical, photochemical, photoelectrochemical and bioelectrochemical etc. can be employed for the conversion of CO2. However, research communities are more interested in the

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electrochemical route for the purpose owing to its specific advantages over the others. The inputs to the electrochemical reduction of CO2 are CO2 itself, water and electricity only and the process may possibly be operated at room temperature and atmospheric pressure. The dual purposes of CO2 mitigation and thereby the production of value-added chemicals, specifically fuels may be a potential solution to the above mentioned two challenges, but with the considerable improvement of the existing technology for the purpose. The hydrogen required for recycling of CO2 through “methanol economy" may be obtained from the electrolysis of easily available water but with the expenditure of a substantial amount of energy. In this direction, a catalyst with appropriate surface characteristics must be developed to enable the more efficient technology for the electrochemical conversion of CO2 to fuels and chemicals. Liu et al. have presented the understanding of the electrocatalytic activity for the reduction of CO2 towards the product selectivity with respect to competing for hydrogen evolution using various transition metal catalysts [4]. Adsorption of CO2 on the surface of catalysts is the initial step of electrochemical reduction of CO2 (ERC) and the adsorption efficiency and the selectivity of products as well depends highly on the surface characteristics of the catalysts. The formation of desired products over competing for hydrogen evolution has also been elicited. The design criteria for the formulation of catalysts have been determined for the estimation of activation energy based on density functional theory [4]. The scaling relations to relate transition state energies to the adsorption of carbon monoxide (CO) onto the active sites of the catalysts have also been developed. Adsorption of CO on the catalyst sites is the intermediate step of catalytic electrochemical reduction of CO2 to fuels [4]. In the similar direction, Kuhl et al. have worked on the activity of catalysts and also on the selectivity of products based on the principle of surface chemistry and with the determination of CO binding energy on to the catalysts of different

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transition metals [5]. The role and characteristics of catalysts in the electrochemical conversion of CO2 to chemicals have been enunciated [6, 7]. The theoretical rates of individual product formation by quantitative measurement of partial current for an individual product as a function of applied cathodic potential are also explained [8]. Accordingly, the distribution of products from an electrochemical reduction of CO2 depends on the potential applied to electrodes [6-8]. Frese et al. have observed that methane (CH4) production started at 1.5 to 1.6 V vs. SCE with maximum efficiency at 1.95 V. But the production of CO is favourable over CH4 at less potential around 1.6 to 1.7 V [6]. Ethylene (C2H2) is favourable over both CO and CH4 at the intermediate potential for CO and CH4. At higher potential, undesired hydrogen is evolved instead of production of desired CH3OH with Cu catalyst. M. Le et al. reported the production of CH3OH predominantly with Cu2O catalysts on the cathode with the production rate of 43 µmol cm−2 h−1 and faradaic efficiencies of 38%. They suggested the critical role of Cu2O species in selectivity to CH3OH. More stable Cu2O allowed continuous generation of CH3OH with the reduced rate with time as the copper oxides reduced to metallic Cu in a simultaneous process [9]. On contrary, D. Ren et al. reported the production of multi‐carbon fuels, including n‐propanol and n‐butane, C3–C4 compounds proposing remarkable electrocatalytic conversion behaviour due to the favourable affinity between the reaction intermediates and the catalytic surface of a chloride (Cl)‐induced bi‐phasic cuprous oxide (Cu2O) and metallic copper (Cu) electrode (Cu2OCl) [10]. The selectivity of products and their distribution as a

function of cathodic potential have also been reported by Hara et al. [11]. On the other hand, Nogami et al. have reported that selectivity of production in the catalytic electrochemical reduction of CO2 depends on the concentration of H+ or adsorbed hydrogen atoms at the catalyst surface on cathode and also on the formation of a thin layer of oxides of copper on catalyst surface of cathode by pulse potential techniques [12]. The pH level at the cathodic surface depends on the electrolyte, the membrane, and the electrode in combinatorial activities of all 4

three parameters to transport H+ from the anode to the catalyst surface of the cathode. Hence, the products and their distribution using Cu as a catalyst change as a function of all the variables of an electrochemical reduction process. Earlier, the efficacy of Cu as a catalyst in the electrochemical reduction of CO2 has been extensively investigated by Cook et al. on gas-phase CO2 reduction to hydrocarbons at metal/solid polymer electrolyte [13-15]. Copper deposited onto the polymer electrolyte is 104 times faster due to the higher current conductivity of it for the same overall reaction as compared to the use of other transition metal such as ruthenium. It has also been significantly observed that a total voltage of 0.65 V only is required even with a higher current density of10 mA/cm2 to promote CO2 reduction predominantly to CH4 and C2H4 by using Cu catalyst. Consequently, most of the research works on the electrochemical reduction of CO2 to fuels and other chemicals have been investigated with Cu catalyst in various shapes and sizes in the last two decades [16-18]. Other metals explored for this purpose include Sn, Pt, and Zn etc. Various products formed in the reduction process using Cu nanoparticles are mostly, CH4, C2H4, HCOOH, CO, and undesired H2. The reduction of current density over running time implies the instability of the elemental catalysts such as Cu due to oxidation and the process, in turn, has been ineffective. The selectivity and the product distribution have been reported in terms of faradic efficiency for each individual product. The selectivity of products as a function of applied potentials to both the cathode and anode has been discussed in the light of pH available to cathode surface and consequently the formation of products from adsorbed CO [16]. Formation of CO predominates at a less cathodic negative potential, hydrocarbons and alcohol are favourably produced below -1.3V vs. SHE while the faradic efficiency of CO drops [8]. The understanding of the reduction process is quite difficult as the process involves multiple reactions with mass transfer constraints. The performance depends on the individual effect, and

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interactions of different variables as well, including the electrode material and its surface characteristics, catalysts material and its morphology, reactor configuration, composition and concentration of both the analyte and catholyte, current density, applied potentials to the electrodes, pH of electrolytes and other physical parameters like temperature, pressure etc. Researchers have studied a different combination of parameters in the direction of varied objectives and have done the experiments with a wide range of these parameters. Accordingly, the results reported are still not substantial to generalize the technological know-how of the said process. Although, a set of factorial and parametric experiments to explore the effects of some major variables in the bench-scale continuous reactor have been designed to resolve this problem [17]. Catalyst surface characteristics and the cathodic potential applied to the electrochemical cell have been identified as the two major variables for the product selectivity through the extensive literature survey [16, 19]. Accordingly, in this work, catalyst and applied potentials to the cathode have been selected as the variables to study for the product selectivity. Other parameters like temperature, pressure, the composition of CO2 and reactor configuration are maintained invariably.

2. Theoretical background An electrochemical cell comprises two electrodes viz. anode and cathode; selectively H+ ions permeable membrane in between the anode and cathode compartments and an electrolyte in both anodic and cathodic compartments. Reduction of CO2 occurs at the cathode whereas oxidation of water takes place at anode on the supply of electrical energy through direct current (DC) source. The anode provides a conducting medium through which free electrons are conducted externally to the cathodic surface where CO2 is reduced. The protons (H+) are transported to cathode from the anode through both the electrolytes and proton permeable membrane. The electrons and

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protons received by the cathode in two different ways react with the CO2 adsorbed on cathodic catalytic surface and form products. CO2 is a stable molecule and a significant amount of energy is required to reduce it into useful products through sequential steps. It has been reported that the formation of CO2 anion radical (CO2 • −) is the rate-determining step in the entire process. The electrical potential for CO2/CO2 • − is -1.9 V vs. SHE. However, the requirement of high energy may be reduced by proton assisted multi-electron reduction process and also by the employment of catalyst for the reduction of the activation energy by altering the transition state of the reaction mechanism. Anode reaction: 

3H O =  O + 6H + 6e ……………………(1)

E = 1.23 V vs. SHE

Cathode reaction: CO + 6H + 6e = CH OH + H O…………..(2)

E = 0.38V vs. SHE

Overall reaction: 

CO + 2H O = CH OH +  O ……………….. (3)

E = 1.61V vs. SHE

Therefore, the required electrical potential to convert one mole of CO2 to generate one mole of the product (methanol) at a pressure of 1 bar and a temperature of 25°C is 1.61 V vs. SHE, whereas the change in Gibbs free energy of the overall reaction is around 932 kJ·mol−1. The higher hydrocarbons like ethanol, propanol etc. are formed if the number of protons available at the cathode is increased. Then the equilibrium potential required for the reduction process also increases. Accordingly, two other representative proton assisted cathodic reactions of electrochemical reduction of CO2 are: CO + 8H + 8e = CH + H O………….......(4)

E = 0.24V vs. SHE

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CO + 2H + 2e = HCOOH+ H O………......(5)

E = 0.225V vs. SHE

The outcome of electrochemical reduction of CO2 (ERC) towards selectivity and distribution of products depends mostly upon the surface characteristics of the catalyst layer on the cathode, the potentials applied to electrodes, the electrolyte used and also on the experimental parameters such as pH, temperature and pressure etc.

3. Experimental 3.1 Materials Surfactant TX-100 (laboratory grade), reducing agent sodium borohydride (NaBH4), cyclohexane (99%) and n-hexanol (98% purity) were procured from Spectrum, India; copper (II) sulfate pentahydrate (98%), potassium hydrogen carbonate (KHCO3) and ethanol (99.5%) were purchased from M/S Biotech, Agartala (India). All the chemicals were used without further purification. An anion exchange membrane, AMI-7001 was procured from Membrane International Incorporation, USA. 3.2 Experimental setup An electrochemical reactor with anode and cathode chambers separated by an ion-exchange membrane was fabricated and employed for the reduction of dissolved carbon dioxide in an electrolyte (KHCO3) under ambient conditions (Fig.1). The reactor consists of two leakage proof closed compartments each of (10×10×10) cm3, separated by the membrane (AMI-7001). The cathode and anode were inserted into the respective compartments and reference electrode (SHE) was inserted into the cathode chamber. SHE was prepared in a glass vessel containing platinized platinum electrode, 1[M] HCl solution and H2 gas (maintained at 1 atm. pressure). The prepared SHE was connected to the electrochemical cell through a salt-bridge to help prevent the intermixing of fluids. The use of salt-bridge did not require the change of pH of the electrolyte. The provision of inlet and outlet

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were provided for the insertion of CO2 to cathode chamber and for collection of gaseous products from the cathode chamber. Both the electrodes were made of stainless steel of dimensions (2 cm × 1 cm × 3 cm) and were cleaned successively by sandpaper and nitric acid of 0.1 M. After cleaning with nitric acid, both the cathode and anode were washed with distilled water for several times. The electrodes were attached with copper wires. The copper wires were passed through hollow glass rod and connected to potentiostat and galvanostat. Nanocatalysts (Cu and Cu2O) were brushed upon the base material as required for the study of their catalytic activity in the electrochemical reduction of CO2. Nanocatalyst of Cu was used on the anode for all cases. Nanocatalysts of Cu and Cu2O were used on the cathode for the study of catalytic activity of Cu and Cu2O, respectively.

The cathode, anode, reference electrode and membrane remained immersed in the aqueous electrolyte. DC power supply was employed for the application of a potential between the electrodes.

[Fig. 1 is to be inserted here]

3.3 Synthesis of Cu nano-catalyst Copper (Cu) nanoparticles were synthesized by mixing of two reverse micro-emulsions (RM-1 and RM-2) [20]. RM-1 was prepared by taking 25 ml of 0.1 M solution of TX-100 in cyclohexane and adding 300 ml of n-hexanol (99%) and 225 ml of a 5%(w/v) aq. solution of CuSO4.5H2O. Similarly, RM-2 was prepared by taking 25 ml of 0.1 M solution of TX-100 in cyclohexane and adding 300 ml of n-hexanol and 225 ml of a 5% (w/v) aq. solution of NaBH4. Both the microemulsions were left stirring for 30 min. so as to obtain optically clear homogeneous dispersions. RM-2 was then added to RM-1 drop-wise with continuous stirring. The resulting solution was left for stirring for another 3 h to allow the growth of Cu nanoparticles to its completion which is termed as “Ostwald ripening”. A nitrogen atmosphere 9

was maintained throughout the process to ensure the complete removal of oxygen thereby preventing the oxidation of the metal. Instantaneous development of brown colour indicated the formation of the copper nanoparticles. 3.4 Synthesis of cuprous oxide (Cu2O) nanocatalyst The following sequential steps were employed for the synthesis of Cu2O nanoparticles. Half (0.5) of a gram of 20 mmol CuSO4.5H2O was dissolved in 30 ml of distilled water at 60oC under stirring for 10 min. Thirty-three ml of 0.5 M NaOH was added dropwise and allowed to react for 30 min. followed by the addition of 2.4 ml of NaBH4. The reaction mixture was maintained at 60ºC and continuous stirring was provided (Fig. 2). The resultant solution was cooled to 25ºC and the solid particles of Cu2O nanoparticles were recovered by centrifugation at 15000 r.p.m. The solids thus recovered were washed twice with distilled water and later once with ethanol. The final product was dried at 60ºC for 2 h. !

2Cu + NaBH + O = Cu O + NaBO + 2H O 

(6)

3.5 Analysis The synthesized nanocatalysts were characterized by XRD using Cu K radiation on a D8 advance (Bruker, Germany) X-ray diffractometer equipped with a secondary graphite monochromator, operating at 40 kV and 40 mA at a step-scan of 0.05˚ 2θ/s. The X-ray patterns were recorded in the 2θ range of 30-90˚. The dried nanocatalyst powder was characterized by FESEM and TEM to know the surface morphology and size ranges of catalyst particles. The product gases except for H2, like CO2, CO, CH4, C2H4 produced at the cathode chamber were analyzed by collecting samples in Tedlar bags. The concentrations of CH4 and C2H4 were subsequently determined in a gas chromatograph (Varian 3600) equipped with a flame-ionization detector (FID) and capillary column (Carboxen-1010 PLOT). On the other hand, concentrations

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of CO and CO2 were measured using thermal conductivity detector (TCD), helium as a carrier gas and capillary column of porapak Q as stationary phase. But, hydrogen gas (H2) was detected in GC

using argon gas as a carrier instead of helium and using same TCD. But as the detection is difficult and not accurate to the required level, the quantification of H2 was accomplished by the method of quantification of H2 based on the methods applied by H. Le et al. [17]. A definite volume (100%) of the gas mixture was injected to the GC and volume of all others gasses except H2 were quantified. The % volume of the H2 in the gas mixture produced in the cathode chamber was the difference of 100% and the sum of all others volume percentages i.e. %volume of H2 is equal to 100% minus sum of %volumes of others gases [17]. The concentration of H2 in the product gas (dry basis)

was estimated from the volume percentage of H2 that was estimated as mentioned above. On the other hand, methanol produced in the cathode chamber was quantified in GC using TG-5MS and FID. 4. Results and discussion 4.1 Characterization of Cu and Cu2O nano-catalysts Synthesis of highly stable, monodispersed, spherical Cu and Cu2O nano-particles from precursor CuSO4.5H2O were easily possible due to the formation of complexes of copper ions with a strong polymeric reducing agent (reduction potential of 1.15 V). The functional groups of reducing agent facilitated the formation of water in oil (W/O) microemulsion. The sizes of the Cu and Cu2O nanoparticles were conveniently controlled by changing the molar ratio of water to surfactant and also by altering the concentration of NaBH4. The stabilizing and protective nature of the surfactant (TX-100) molecules reduced agglomeration and shielded the nanoparticles from unintended oxidation of the atomic Cu nanoparticles. The water is tightly bound to the oxyethylene groups of the polar chain of the surfactant (TX-100) in the microemulsion system. This binding facilitates the much higher local concentration of Cu in water pools of W/O 11

microemulsions thereby formation of pure elemental Cu particles. The diameter of the reverse microemulsion (RM) droplet (water-TX-100-n-hexanol-cyclohexane) increased with a higher ratio of water. The N2 atmosphere maintained in the process of synthesis of Cu nanoparticles played a significant role to prevent the oxidation of Cu nanoparticles. On the other hand, the NaOH solution was added drop wise in the mixture of RM-1 and RM-2 for the synthesis of Cu2O. The blue colour of the CuSO4.5H2O gradually diminished and the mixture turned primarily into yellowish blue, later into green and finally into a brown solution of Cu2O dispersed therein (Fig. 2). Further addition of NaBH4 resulted in blackish brown colour but the optically clear solution. Brown powder of Cu2O was obtained after subsequent processing. The reaction of Cu(II) salt with NaBH4 in the absence of atmospheric O2 produced Cu2O nanoparticles. Effects of various parameters were investigated to optimize the shape and size of the nanoparticles. Factually, this methodology yielded spherical nanoparticles of both Cu and Cu2O which were confirmed by TEM analysis (Fig. 3) [21]. Additionally, the sizes of both types of the synthesized nanoparticles were seen to be varied from 60–120 nm which was confirmed by FESEM analysis (Fig. 4).

[Fig. 2, Fig. 3 & Fig. 4 are to be inserted here]

4.1.1 Effect of concentration of NaBH4 The average diameter of Cu nanoparticles was found to be decreased with the increasing concentration of reducing agent (NaBH4) up to 0.5 M [21]. The stabilization of nanoparticles requires assembling of a minimum number of Cu atoms through successful collisions among themselves and that leads to the growth of these nuclei into stable nanoparticles. Once the nuclei were formed, the growth process superseded the nucleation. The synthesized nanoparticles were

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found to be monodispersed (Fig. 3) and that suggests that most of the nuclei were formed almost at the same time and at a constant rate. The sizes of the nanoparticles were predetermined by the number of the nuclei formed at the early time of the reduction reaction. Contrarily, at a low concentration of reducing agent, only a few nuclei were formed at the early period of the reduction process owing to the very slow rate of reduction of CuSO4.5H2O. The atoms formed later collided with previously formed nuclei and contributed to the formation of comparably larger nanoparticles. However, above the concentration level of 0.5 M of NaBH4, the CuSO4.5H2O was rapidly reduced to unstable nuclei of Cu ions. Hence, beyond this concentration, the nucleation rate was not high enough and the number of nuclei, as well as their sizes, remained constant with the increasing concentration of NaBH4. 4.1.2 Effect of concentration of CuSO4.5H2O The effect of the initial concentration of the CuSO4.5H2O salt on the sizes of the nanoparticles was studied. The average diameters of Cu and Cu2O nanoparticles were not affected below a salt concentration of 0.1 M. Beyond this concentration of salt, the average diameters of both types of nanoparticles increased significantly. Two different reasons for this result may be cited. One explanation was that the relatively lower concentration of the reducing agent (NaBH4) led to the formation of fewer nuclei in the early phase of the reduction. Secondly, the number of atoms formed at the very beginning of the reduction process remained constant due to the high metal ion concentration. The atoms formed in the later period contributed to the growth of the particles and resulted in the formation of larger particles. 4.1.3 Analysis by XRD XRD patterns of the Cu and Cu2O nanoparticles synthesized have been depicted in Fig. 5 and Fig. 6, respectively. Three characteristic diffraction peaks at 2θ of 44.7, 51.6, and 76.4º degree

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for the marked indices of (111), (200) and (311), respectively were observed in XRD spectra of Cu nanoparticles (Fig. 5). Observed three peaks at different 2θ values were compared with the standard powder diffraction card of JCPDS, copper file No. 04–0836 which confirmed the synthesized nanoparticles to be of Cu [22]. The characteristic peaks confirm the formation of face-centred cubic

(FCC) Cu nanoparticles without significant oxides or other impurities. On the other hand, synthesized Cu2O shows its characteristic peaks at 2θ values of 32.5, 35.5, 38.7, 53.5, 58.2, 61.4, 65.8, 67.8º for the marked indices of (110), (002), (111), (020), (202), (113), (022), (113), respectively. The diffraction peak at 38.7° was observed as the confirmation of the four-sided tetrahedral structure of Cu2O (111) crystal lattice [23]. The unassigned diffraction peaks at 32.2 and 35.5° in Fig. 6 were attributed to significant planes (110) and (002) of Cu2O nanoparticles [23]. Other diffraction peaks at 48.6, 53.5, 65.8 and 67.8 were attributed to the various planes of Cu2O e.g. (202), (020), (022) and (113) planes, respectively. However, the comparatively lower intensity diffraction peak at 48.6° was attributed to the crystal plane (202) of CuO that indicated the oxidation of trace amount of Cu2O to CuO even with the use of stabilizing agent [24, 25]. The average particle size of the Cu and Cu2O nanoparticles were calculated from the full width at half maximum (FWHM) of the (111) peaks in the XRD patterns using the Scherer equation [22]. The average particle size of about 34 nm and 41 nm were found out for Cu and Cu2O, respectively. The comparatively larger size of the Cu2O nano-particles is confirmed from the TEM micrograph images (Fig. 3). [Fig. 5 and Fig. 6 are to be inserted here] 4.2 Performance of ERC with the synthesized catalysts The performances of both the catalysts were investigated with identical physicochemical parameters in the same configurations of the reactor illustrated in Section 3.2 and Fig. 1. The temperature and pressure were maintained at 25°C and 1 atm. respectively. The flow rate of pure 14

CO2 fed to the cathode chamber was at 3.0×10-6 m3/s. The electrochemical reduction of CO2 was conducted for 50 min. 4.2.1 Dissimilar distribution of products with Cu and Cu2O The reduction mechanism of CO2 was investigated by analyzing the faradaic efficiency of individual products. The products and their distribution were studied at different constant negative cathodic potentials by the estimation of the faradaic efficiencies. Faradaic efficiency measures the efficiency of potential utilization for the formation of products in the electrochemical reduction of CO2. ERC was carried out at different applied cathodic potential in the range of -2.8 to -1.4 V vs. SHE by using Cu nanoparticles as both cathode and anode [2]. The faradaic efficiency of the individual products produced in ERC was determined at various negative cathodic potentials. Carbon monoxide was found as the major product up to a negative potential of -2.2 V vs. SHE, but with maximum faradaic efficiency (30%) at -2.6 V vs. SHE (Fig. 7). The employment of overpotential to the electrochemical cell has an important role in both the selectivity and distribution of products in the electrochemical reduction of CO2. Two electronproducts such as H2, CO are produced at low overpotential whereas CH4, C2H4 are generally produced at comparatively higher overpotential [26]. With the reduction of negative cathodic potential, both of CH4 and C2H4 were obtained up to -1.6 V vs. SHE with maximum efficiency of 22% and 15% for CH4 and C2H4, respectively but with maximum efficiency at the same cathodic potential of -2.0 V vs. SHE. The slightly higher production rate of CH4 than C2H4 at the same potential of -2.0 V vs. SHE can be explained by the pulsed method developed by G. Nogami et al. which reported that the optimum anodic bias for the generation of C2H4 was slightly more anodic than that of CH4 [27]. The production of for CH4 and C2H4 with the reduction in negative potential over -2.0 V vs. SHE reduced due to a faster rate of production by reduction of CO2 and

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adsorption of amorphous carbon on the catalyst (Cu) surface [16]. On the other hand, a substantial amount of H2 evolution was observed due to the fact that some percentage of charge (H+) passed through the cationic membrane of the electrochemical cell was utilized for hydrogen evolution rather than the reduction of CO2. The intermediate species such as carbon monoxide radical blocked the catalyst active sites on the electrode surface which hindered the reduction reaction of fresh CO2 on to the catalyst surface which resulted in the low conversion of CO2. Moreover, the structural change of Cu prevented the adsorption of amorphous graphite thus increasing the production rate of methane instead of CO at a later time. The structural change of Cu is referred to as the formation Cu2O by the oxidation of Cu that takes place at a very slow rate with time. The formation of a very thin layer of Cu2O on Cu cathode surface can be confirmed by applying pulsed potential technique [12, 28]. Moreover, the available surface area of Cu electrodes for the conversion process continuously decreases due to the formation of Cu2O [16]. The other synthesized catalyst Cu2O was employed on the cathode surface with all other parameters keeping intact for the investigation of the differences in both the selectivity and distribution of products. The formation of CO became almost zero after the negative cathodic potential of -2.2 V vs. SHE. Instead, production of CH3OH was increasing very rapidly beyond -2.4 V vs. SHE and maximum production of methanol were found at -2.0 V vs. SHE with the faradaic efficiency of 47% (Fig. 8). The selective production of CH3OH as a major product over CH4, C2H2 and CO and also the minimization of undesired H2 evolution with the employment of Cu2O catalysts on the cathode surface were in line with the results of very recent works on the identical topic [23, 29-32]. P. Hirunsit et al. reported the production of CH3OH along with CH4 over C2+ hydrocarbons in electro-reduction of CO2 on copper-based alloys stepped surfaces [32]. The tetrahedral structure of the Cu2O (111) crystal lattice was found favourable for CH3OH selectivity [33]. Productions of CH4 and C2H4 were found to be less compared to the

previous case where Cu nanocatalysts were used (Fig. 7). The products selectivity in this study of electrochemical reduction of CO2 was found different from the results reported by Sargent et 16

al. [29]. They reported that the copper(I) i.e. Cu2O produced the C2+ hydrocarbons such as C2H5OH, C2H2 etc. instead of C1+ molecules like CH4 and CH3OH. They used a thin layer of copper nitride (copper-on-nitride) on the cathode and reported that the use of copper nitride contributed to the reduction of CO dimerization energy barrier which was a rate-limiting step in CO2 reduction to multicarbon (C2+) products. The methanol production in high proportion in this study can be attributed to the use of pure Cu2O as a catalyst which was not supported by nitride for reduction of the energy barrier to CO dimerization to C+2 products. Moreover, the change of the reaction pathway with the use of Cu2O associated with other parameters was more favourable to the production of CH3OH as a major component. On the other hand, the product distribution changed with the applied potentials to electrodes due to change in the supply of the quantitative requirement of H+ ions and more potential facilitates the supply of more H+ ions to the catalyst surface on cathode for the production of CH3OH [12]. [Fig. 7 & Fig. 8 are to be inserted here]

4.2.2 Effect of cathodic potential on the current density The consistency of the ERC process, specifically the performance of the cathodic catalyst is measured on the basis of a change of current density with running time. Invariability in the current density is the measure of the appropriateness of the catalyst employed for the reduction process. Methanol was considered as the target product due to its large fuel value and it was selectively produced with Cu2O catalyst (Fig. 8). Therefore, Cu2O was employed for the present experiment and current density was measured for negative cathodic potentials of -2.2 V, -2.0 V and -1.8 V vs. SHE with all other physicochemical parameters remained as before. Reduction in current density was found from 4.9 mA.cm-2 at 5 min. to 1.9 mA.cm-2 at 50 min. for negative cathodic potential of -2.2 V, from 6.23 mA.cm-2 at 5 min. to 5.31 mA.cm-2 at 50 min. for the 17

negative cathodic potential of -2.0 V and from 7.17 mA.cm-2 at 5 min to 2.58 mA.cm-2 at 50 min. for the negative cathodic potential of -1.8 V (Fig. 9). The various losses occurring on the cathode surface include activation loss, ohmic loss etc. Poisoning by the intermediate species like CO formed on the cathode surface is another reason for the reduction in current density with time. Current density diminished initially up to 10 min. at a higher rate and later almost linearly at a lower rate for all cathodic potentials. The result implies that all the reasons mentioned for the reduction of the current increased steadily with time. The % losses of the current during the reduction process for individual cathodic potentials are important for the selection of the cathodic potential to be applied for the ERC process. It was found that losses were 61.2%, 14.8% and 35.1% for -2.2 V, -2.0 V and -1.8 V, respectively with the lowest for -2.0 V. Therefore, cathodic potential of -2.0 V is preferable in comparison with -1.8 V with respect to the higher stability of current density for processing time of at least 50 min. due to the behavioural change of surface characteristics of the cathodic catalyst at negatively lower potential (-1.8 V). On the other hand, the current density for -2.0 V was always higher for that of -2.2 V. Therefore, subsequent experiments were conducted for the cathodic potential of -2.0 V vs. SHE. In addition, the losses were found comparatively lesser for Cu2O nanocatalyst used at the cathodic surface as compared to the experiments where Cu nanocatalyst was used. This result explained that the Cu catalyst was oxidized and eroded with time, but the effect has been minimized by replacement of Cu by Cu2O. Carbon monoxide formed from CO2 as an intermediate was adsorbed on the Cu electrode interfering with hydrogen formation at cathode [8]. Available ions (H+) to catalyst surface on cathode were more utilized for the production of CH3OH instead of hydrogen evolution with Cu2O as a catalyst instead of Cu [12]. Rate of the reaction was higher at lower

18

negative cathodic potential and the charge passed through the electrochemical cell was utilized for the hydrogen evolution rather than carbon dioxide reduction. [Fig. 9 is to be inserted here] 4.2.3 Methanol selectivity with Cu2O catalyst Maximum individual faradaic efficiency (49%) was found for methanol with Cu2O as a catalyst on the surface of the cathode at the negative cathodic potential of -2.0 V vs. SHE. Though methanol was found as the major product, CO, CH4, C2H4, and undesired H2 were also produced in varying proportions depending on the surface characteristics of the two catalysts i.e. Cu and Cu2O nanoparticles. Each of the experiments of electrochemical reduction was studied for 50 min. following the fact that the lowest reduction in current density was found as only 14.8% (for the cathodic potential of -2.0 V vs. SHE) during this duration (50 min.) of the reduction process (Fig. 9). Various products formed during 50 min. of the reduction process was quantified and results were presented in Fig. 10. Production of CH4 found maximum (28µmol/L) followed by CO2 (22 µmol/L), C2H4 (21µmol/L) and CH3OH (5 µmol/L) among the major products with the Cu nanoparticles as a catalyst on the cathode. A substantial amount of undesired H2 (29 µmol/L) was also produced. The total production of desired products (i.e. excluding H2) was found as 76 µmol/L. The distribution of the product is equivalent to that obtained from the study conducted with Cu nanoparticles by Lee et al. [16] except the production of a small amount of CH3OH in their study. On the other hand, CH3OH production was facilitated (48.5 µmol/L) and the evolution of H2 (12 µmol/L) was minimized while Cu2O nanoparticles were used as a catalyst on the cathode. The other products were distributed as CH4 (20 µmol/L), C2H4 (14 µmol/L), and CO (5 µmol/L). The total production of desired hydrocarbons was found as 87.6 µmol/L with Cu2O as compared to76.1 µmol/L with Cu as a catalyst. Cu2O is a more stable state of copper,

19

consequently is less vulnerable to be corroded in the reaction conditions in comparison to Cu. Moreover, the intermediate species such as CO could have blocked the active sites of Cu on the cathode surface which hindered the fresh carbon dioxide to be contacted on to the surface of the catalyst and resulted in the low conversion of CO2. The lesser amount of desired products with Cu catalyst may be due to the fact that the charge (H+) passed through the electrochemical cell was utilized partially for hydrogen evolution rather than CO2 reduction for the reduction mechanism with Cu catalysts. Kinetics of reaction will be higher at lower negative cathodic potentials and the charge passed through the electrochemical cell was utilized for the evolution of H2 rather than reduction of CO2. M. Le et al. reported the production of CH3OH predominantly with Cu2O catalysts on the cathode with a production rate of 43 µmol cm−2 h−1 and faradaic efficiencies of 38%. They suggested the critical role of Cu2O species in selectivity to CH3OH. More stable Cu2O allowed continuous generation of CH3OH with the reduced rate with time as the copper oxides reduced to metallic Cu during the reduction time of 50 min. [9]. The Cu2O catalyst is likely to be reduced to Cu metal when cathodic potential more negative than -2V vs. SHE is applied. Therefore, the stability of Cu2O nanoparticle as a catalyst in electrical reduction

process was evaluated experimentally. The result was explained based on the possible reduction in the rate of production of CH3OH over time. The reduction in CH3OH was found gradually decreasing at almost the same rate, but slightly more rate at later times (Fig. 11). Up to 20 min. of process, the production rate was found almost constant at an average value of 8.0 ×10-4 µmol.cm-2.s-1. The average rate of CH3OH formation is comparable with the result of a similar study conducted by J. Albo et al. [31]. Thereafter, it became slightly slower and at the time of 50 min. it became 6.4 ×10-4 µmol.cm-2.s-1. The % reduction in the production of CH3OH was found as 21% during the catalytic reduction process of 50 min. The reduction of current density with

20

running time also implied the instability of Cu2O catalyst (Fig. 9). The instability of catalysts was found for a possible reduction of Cu2O to CuO and Cu [9]. [Fig. 10 and Fig. 11 are to be inserted here]

5. Conclusion The size of the Cu and Cu2O nanoparticles were controlled by changing the molar ratio of water to a surfactant or by altering the concentration of the reactants in an attempt for their synthesis prior to their employment in the electrochemical conversion of CO2 to usable products. Characterization by FESEM, TEM, and XRD confirmed the synthesis of pure Cu and Cu2O, their sizes and narrow size distribution. Cu2O was found more stable in comparison to Cu in terms of both current density, amount of desired product formation and restriction to H2 evolution. Methanol was the major product with very less H2 evolution when Cu2O catalyst was used; whereas both CH4 and C2H4 were major products with more amount of H2 evolution when Cu catalyst was used. The cathodic potential of -2.0 V vs. SHE was optimized for both methanol selectivity and minimization of undesired H2 evolution. Nanocatalyst of Cu2O was found to be more stable and better catalyst for CH3OH selectivity in comparison with that of elemental Cu.

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Caption of the figures: Figures

Captions

Fig. 1

Schematic of the experimental setup

Fig. 2

Cu2O nanoparticles formed at different stages of processing time

Fig. 3

TEM micrographs showing the particle size distribution: (a) Cu nanoparticles and (b) Cu2O nano-particles

Fig. 4

FESEM micrograph showing the particle size distribution of Cu2O nanoparticles

Fig. 5

XRD pattern of Cu nanoparticles

Fig. 6

XRD pattern of Cu2O nanoparticles

Fig. 7

Effect of cathodic potential on the selectivity of products using Cu nanocatalyst on the cathode

Fig. 8

Effect of cathodic potential on the selectivity of products using Cu2O nanocatalyst on the cathode

Fig. 9

Reduction in current density over time of electrochemical process for various cathodic potentials

Fig. 10

Quantification of products with both the nanocatalysts (Cu and Cu2O): variation in distribution of products due to the selectivity of catalysts

Fig. 11

Change in the rate of methanol production over the time of catalytic reduction of CO2

25

List of figures:

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Fig. 6

Fig. 7 35 Carbon monoxide

Faradaic efficieny (%)

30

Ethylene Methane

25 20 15 10 5 0 -3

-2.8

-2.6 -2.4 -2.2 -2 -1.8 -1.6 Cathodic potential, E (V vs. SHE)

-1.4

Fig. 8 50 Methanol Methane Ethylene Carbon monoxide

45

Faradaic efficiency (%)

40 35 30 25 20 15 10 5 0 -3

-2.8

-2.6 -2.4 -2.2 -2 -1.8 -1.6 Cathodic potential, E (V vs. SHE)

-1.4

Fig. 9 7.5 -2.2 V (vs SHE) -2.0 V (vs. SHE) -1.8 V (vs. SHE)

Current density (mA.cm-2)

7 6.5 6 5.5 5 4.5 4 3.5 0

10

20

30 Time (min.)

40

50

Fig. 10

Formation of products (µmol/l)

55 50

Cu

45

Cu2O

40 35 30 25 20 15 10 5 0 Carbon Ethylene Methane Methanol Hydrogen monoxide

Fig. 11

Rate of methanol production (µmol.cm-2. s-1 × 10-4

8.4 8.2 8 7.8 7.6 7.4 7.2 7 6.8 6.6 6.4 6.2 0

5

10 15 20 25 30 35 40 45 50 55 Time of reduction process (min.)

Research highlights: 

Study of the product selectivity in the electrochemical reduction of CO2



Determination of surface characteristics and stability of synthesized Cu and Cu2O nanocatalysts



Study of the role of catalyst surface and stability for product distribution



Study of the role of cathodic potential for product distribution