In Situ Spectroscopic Study of CO2 ... - ACS Publications

Subscriber access provided by Weizmann Institute of Science

Article

In situ spectroscopic study of CO2 electroreduction at copper electrodes in acetonitrile Marta C. Figueiredo, Isis Ledezma-Yanez, and Marc T.M. Koper ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b02543 • Publication Date (Web): 25 Feb 2016 Downloaded from http://pubs.acs.org on February 25, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

In situ spectroscopic study of CO2 electroreduction at copper electrodes in acetonitrile Marta C. Figueiredo*, Isis Ledezma-Yanez, Marc T. M. Koper* Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands

ABSTRACT

The electrochemical conversion of carbon dioxide (CO2) into valuable compounds is a promising route toward the valorization of this molecule of high environmental impact. Yet, an industrial process involving CO2 electroreduction is still far from reality and requires deep and fundamental studies for a further understanding and better development of the process. In this work we describe in situ spectroelectrochemical studies based on Fourier Transformed Infra-Red (FTIR) and Surface Enhanced Raman Spectroscopy (SERS) of the CO2 reduction in acetonitrile solutions at copper electrodes. The influence of factors such as the water content and the supporting electrolyte on the reaction products were evaluated, and compared to products obtained on metal electrodes other than Cu such as Pt, Pb, Au, Pd and Ag. The results show that at Cu electrodes in acetonitrile containing small amounts of water, the main reaction products from CO2 reduction are carbonate, bicarbonate and CO. The formation of CO was observed at

ACS Paragon Plus Environment

1

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 39

less negative potentials than the formation of (bi)carbonates and the formation of carbonate and bicarbonate species appears to be the result of a reaction with electrochemically generated OHfrom water reduction. In general, our experiments show the sensitivity of the CO2 reduction reaction to the presence of water, even at the residual level.

KEYWORDS . Carbon dioxide, electroreduction, organic solvents, Cu electrodes, acetonitrile.

1. INTRODUCTION: Carbon dioxide (CO2) amounts are increasing continuously in the atmosphere generating an issue of global concern, as CO2 is known to be one of the major contributors to the greenhouse effect 1. Several methods for CO2 sequestration

2

(capture and storage) have been studied and

developed in the last decades. After being captured, CO2 is mainly stored in geological and ocean reservoirs, but this technology still divides opinions around the world, and does not look as an appealing long term solution

3

as the storage capacity of the planet is also limited.

However, CO2 is also a cheap and abundant source of carbon that can be converted into useful and commodity chemicals. In this sense, combining CO2 sequestration with (electro-)chemical methods to convert it into useful fuels or value-added chemicals, for instance by using excess renewable electricity, is probably a more promising option. In the particular case of the electrochemical conversion of CO2, the development of such a technology, that can combine the use of renewable energy sources (like wind or solar) with the use of CO2 feedstocks from sequestration, could have a significant impact on the replacement of petrochemical sources used nowadays

4, 5

. Moreover, CO2 recycling will contribute to a carbon-neutral and sustainable

production of hydrocarbon fuels and/or commodity chemicals for the industry.


2

Page 3 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

The electrochemical reduction of CO2 has, in fact, been studied intensively over the last decades 5-9. It is known that the products depend on many factors such as the solvent, the nature and pH of the supporting electrolyte, the electrode material, and the applied potential 10, 11. In aqueous solutions, possible products from CO2 electroreduction are CO, formic acid and hydrocarbons such as methane, ethane and ethylene

11 12

, . However, in aqueous media, CO2

reduction has the hydrogen evolution as competing reaction, which occurs in the same potential range. Organic solvents show a higher cathodic potential window resulting from the difficulty of breaking the C-H bonds when compared with O-H bonds from water. In this way, higher overpotentials can be applied and more selective CO2 reduction can be obtained because of the suppression of hydrogen evolution. However, efficient CO2 reduction also requires free or easily reducible protons in solution if products other than CO, carbonate and oxalate are envisaged. In addition, it is expected that in the absence of protons, the overpotential for CO2 reduction is high as the only conceivable reaction intermediate is the CO2- radical anion, the formation of which occurs at a very negative potential

13

, although some organic media appear to stabilize this

intermediate13, 14. A more specific advantage of organic solvents is that they exhibit a higher solubility for CO2 than water, and hence the ratio of concentrations of CO2 over proton donor in the solvent can be tuned towards a higher CO2 reduction selectivity. For example, the solubility of CO2 in acetonitrile is about eight times that in water 15. The CO2 electroreduction mechanism in non-aqueous solvents has been reported in literature since the early 1980´s

13, 16, 17

. It has been described that, in organic solvents such as

dimethylformamide (DMF) or dimethyl sulfoxide (DMSO), CO2 activation follows three different pathways: 1) by self-coupling of the anion radical with the formation of oxalate; 2) through oxygen-carbon coupling of CO2•- with CO2, leading to CO and CO32- as products; and,


3

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 39

3) by CO2•- protonation by residual water followed by electron transfer (CO2•- being a possible source of electrons) with formate as main reaction product

14

. In all pathways, the rate

determining step is the formation of the anion radical that takes place at very negative electrode potential (the reduction potential for the formation of CO2•- is approx. -1.9 V vs NHE) 18. The actual path that the reaction follows depends on the material of the working electrode. Metals such as Sn, Pb, In and Hg lead to the formation of oxalate, while Pt, Pd, Au and Cu have carbon monoxide and carbonate as major products 14, 19. Copper is particularly interesting for CO2 electroreduction (both in aqueous and organic media) 12, 20-23 due to its unique ability to produce a significant amount of hydrocarbons. On the other hand, acetonitrile is a good solvent for use in electrochemistry due to its high dielectric constant, potential window and electrochemical stability (in addition to the previously mentioned high solubility of CO2). However, studies using Cu electrodes for CO2 electroreduction in acetonitrile are rather scarce. Ikeda et al. 14 found CO as main product by detecting the products in the gas phase from long-term electrolysis of CO2 at Cu in acetonitrile. They suggested that the CO2 radical anion generated near the electrode surface reacts with CO2 in the vicinity of the surface to form CO and carbonate. Most of the spectroelectrochemical reports concerning CO2 reduction in acetonitrile were performed at Pt electrodes. For these surfaces, the products found by different authors are not in agreement

24, 25

. Using infrared spectroscopy Desilvestro et al.

24

conclude that the observed

bands were due to oxalate formation. On the other hand, the same bands were later attributed by Christensen et al.

25

to carbonate species solvated by water or forming ion pairs with

tetraethylammoniun cations.


4

Page 5 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Despite the efforts on this subject, the detailed requirements for observing the different pathways of the CO2 reduction in non-aqueous solvents are still not fully understood, in particular in the presence of a proton source such as water. The presence of low amounts of water in organic solvents is beneficial or even necessary because the formation of interesting products (such as methane or methanol) requires the presence of a proton source. To throw further light on this issue, we report in this paper a spectroelectrochemical study based on Fourier Transformed Infra-Red (FTIR) and Surface Enhanced Raman Spectroscopy (SERS), of the CO2 electroreduction at Cu electrodes in acetonitrile. Factors such as the water content and the supporting electrolyte on the reaction products will be evaluated, and will be compared to products obtained on metal electrodes other than Cu. We will show that at Cu electrodes, in acetonitrile containing small amounts of water, the main reaction products from CO2 reduction are carbonate, bicarbonate and CO. Under the conditions used in this paper, the formation of CO occurs at less negative potentials than the formation of (bi)carbonates. The formation of carbonate and bicarbonate appears to be the result of a reaction with electrochemically generated OH- from water reduction. Similar results were obtained for electrode materials such as Pt, Au, Ag, Pd and Pb. However, at Pb electrodes, evidence for the formation of oxalate was also found.

2. EXPERIMENTAL: The voltammetric experiments were performed in a three-electrode configuration at room temperature using a Pt coil as counter electrode and an Ag/Ag+ as reference (see ref.

26

). This

reference was prepared with an Ag wire in a solution of 0.1 M AgClO4 (97% from Aldrich) in acetonitrile (99.8% anhydrous from Sigma Aldrich). It should be noticed that there was no further treatment (or drying) of any of the chemicals used in this work, reason why the solutions


5

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 39

of acetonitrile with supporting electrolyte should be considered “wet”. The precise water content of the solutions used was measured by Karl Fisher titration with an Metrohm 381 KF Coulometer using Hydranal Coulomat AG from Fluka and can be found in Table 1. The working electrodes (Cu, Pt, Au and Pb discs) were polished with alumina suspension, rinsed with MilliQ water (18.4 MΩ.cm) and sonicated for 5 min before each experiment. The electrode potential was controlled with a Potentiostat 466 System (Model ER466) from E-DAQ. The acetonitrile solutions containing 0.1 M of supporting electrolyte (Tetraethylammonium tetrafluoroborate ≥99% from Fluka, TEABF4, Tetraethylammonium trifluoromethanesulfonate ≥98% from Aldrich, TEATfO and Sodium trifluoromethanesulfonate 98% from Aldrich, NaTfO) were purged with Ar (6.0, Linde) for

10 min prior the experiments to remove oxygen. For

measurements of CO2 electroreduction, the solution was saturated with CO2 (4.5, Linde) by bubbling

the

gas

for

20

min

through

the

solution.

During

the

electrochemical/spectroelectrochemical measurements the gas flow was kept in the cell atmosphere. Table 1 - Water content of the supporting electrolyte solutions in MeCN.

Solution

Water content (ppm)

0.1 M TEABF4 in MeCN

46

0.1 M TEATfO in MeCN

184

0.1 M NaTfO in MeCN

528

FTIR Spectroscopy (Bruker Vertex 80 V IR spectrophotometer) was used to characterize the products and intermediates of the reaction. A CaF2 prism was used bevelled at 60 ̊. A detailed description of the setup can be found elsewhere27. The spectra were obtained in a thin layer configuration in which the electrode was pressed against the prismatic window, at a controlled


6

Page 7 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

potential for which no Faradaic processes take place. At this potential (which will be indicated for each experiment in the corresponding figure) a spectrum was obtained as background spectrum. Next, the electrode potential was changed and the sample spectrum was obtained. After normalization with the background spectrum (to remove solvent and other spectral interferences) the final spectra are presented as absorbance, according to A=−log (R/R0) where R and R0 are the reflectance corresponding to the single beam spectra obtained at the sample and reference potentials, respectively. In these difference spectra, negative bands (pointing down) correspond to the species that were present on or near the electrode surface at the reference potential and that are “consumed” at the sample potential. Positive bands (pointing up) correspond to the formation of species at the sample potential. All the spectroelectrochemical experiments were performed at room temperature, with an Ag/Ag+ and a platinum coil used as reference and counter electrodes, respectively. Transmission spectra of the solution species were collected using a SeZn window with an incident angle of 60°. One hundred scans were collected with a resolution of 8 cm−1 using ppolarized light. Transmission spectra of 0.1 M of tetraethylammonium bicarbonate (95% from Aldrich) – TEAHCO3, 0.1M tetraethylammonium oxalate – TEAC2O4 (mixture of tetraethylammonium hydroxide 20% in water from Aldrich and oxalic acid ≥99% Sigma Aldrich) and 0.1M tetraethylammonium formate – TEACOOH (mixture of TEAOH and formic acid ≥98,5% from Fluka) were measured with 0.1 M TEABF4 in acetonitrile as background electrolyte. All the FTIR and transmission spectra presented in this work were obtained by averaging 100 scans with 8 cm-1 resolution using p-polarized light. Surface Enhanced Raman Spectroscopy (SERS) measurements were performed with a confocal Raman microscope (LabRam HR, Horiba Yobin Yvon), equipped with a He/Ne laser


7

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 39

(633 nm) and a 50X objective. The setup used is the same as previously reported28. The electrochemical measurements were controlled with a METROHM µAUTOLABIII - Compact Design potentiostat, using a three-electrode electrochemical cell. A Pt wire was used as counter electrode and an Ag/Ag+ in acetonitrile as reference. The Cu surface used as working electrode was roughened in a 1 M KCl solution, with potential steps from -1.05 V vs SCE (10 s) to 0.35 V vs SCE.

3. RESULTS: 3.1. Spectroelectrochemical identification of the reaction products Cyclic voltammetry experiments were performed first to assess the potential window for CO2 electrochemical reduction at Cu electrodes in 0.1 M TEABF4 in acetonitrile. The corresponding cyclic voltammograms (CVs) are presented in Figure 1.

Figure 1. CVs for Cu electrode in Ar atmosphere (dotted black line) and CO2 atmosphere (solid red line) in 0.1 M TEABF4 in CH3CN at 50mV/s.


8

Page 9 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

The CVs show that for potentials more positive than -1.8 V, the voltammetric profile in the presence of CO2 is very similar to that obtained under Ar atmosphere. At -1.8 V, in CO2 atmosphere, negative currents start to be observed indicating the onset potential for CO2 reduction at Cu electrodes. We note that for potentials more positive than -1 V (vs Ag/AgClO4) acetonitrile decomposes on Cu electrodes into cyanide species 29 and for this reason the working potential must be kept below this value. FTIR experiments were first performed for Cu electrodes in 0.1 M TEABF4 under Ar atmosphere to identify any possible product of acetonitrile decomposition at negative potentials (figure 2) that could, eventually, interfere with the identification of the CO2 reduction products. The spectra shown in figure 2 were obtained with a background spectrum taken at -1 V. Several bands can be identified in the spectra obtained at different potentials for the Cu electrode in 0.1 M TEABF4. Bands related with the presence of residual water in the solvent and supporting electrolyte can be observed at 3629/3544 cm-1 (OH stretching) and at 1628 cm-1 (OH bending)

30

acetonitrile

. At 3002/2941 cm-1 the contribution from the CH stretching from TEA+ and 31

can be seen as also confirmed by comparison with the transmission spectra of

acetonitrile and 0.1 M TEABF4 solution shown as inset in figure 2.


9

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 39

Figure 2. FTIR spectra of Cu electrodes in 0.1 M TEABF4 in CH3CN at different indicated potentials, reference potential is – 1 V (vs Ag/AgClO4). Upper panel: Transmission spectra of CH3CN (air as reference) and TEABF4 in CH3CN (CH3CN as reference).

In the CN region the spectra show three bands at 2290, 2258 and 2246 cm-1

32

. The bands at

2290 and 2246 cm-1 are also observed in the transmission spectra from acetonitrile (inset in figure 2) and can be assigned to the CN stretching from acetonitrile free from interactions with anions. These bands are positive in the potential range between -1.2 to -1.6 V, but at -1.8 V the band at 2290 cm-1 becomes negative. At the same potential, the negative band at 2258 cm-1 can


10

Page 11 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

also be observed. When acetonitrile is involved in solvation shells, the CN stretching frequencies increase because coordination increases the σ character of the C-N bond. For this reason, the bands 2258 cm-1 can be attributed to CN groups from acetonitrile that interact with BF4- groups, similarly to what is observed for ClO4- at Pt electrodes 33. With more negative electrode potential there is an accumulation of positive charges (TEA+ cations) in the vicinity of the electrode surface. This is reflected by an increase of the bands at 3000 and 1500-1300 cm-1 related with the stretching and bending vibrations of CH groups of TEA+ cations, respectively, at potentials more positive than -1.6 V. In the same potential range, the bands due to CN groups from acetonitrile free from solvation are also positive, suggesting the accumulation of this species in the thin layer. The negative band at 1109 cm-1 is due to the BF4depletion from the thin layer with more negative electrode potential. At -1.6 V, water reduction starts taking place as evidenced by the negative bands for water bending and stretching (1628 cm-1, 3629/3544 cm-1 respectively). At -1.8 V, the spectra show substantial changes: the water consumption bands become very pronounced and some positive bands (corresponding to the formation of products) start to appear at 1688, 1543, 1487/1393 and 1178 cm-1. These latter bands can be assigned to C=O stretching, NH2 stretching and NH2 bending modes, respectively, suggesting that acetonitrile is decomposing into acetamine. These results are in agreement with previous reports

34

that found

acetamine as the major product of acetonitrile decomposition at negative potentials in the presence of residual amounts of water. The correlation between water reduction and the formation of acetamine observed in Figure 2 supports the mechanism by Foley et al

34

, who

suggested that acetamine is formed by nucleophilic attack of acetonitrile by the OH- ions formed during water reduction.


11

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 39

Having established the surface reactivity of the blank electrolyte in the relevant potential window, FTIR spectroscopy was performed for CO2 reduction at Cu electrodes in the same electrolyte. The spectra are shown in figure 3. The potential was controlled at -1 V when the electrode was pressed against the prismatic window to avoid formation of undesirable species (vide supra). This potential was also used to obtain the reference spectrum.

Figure 3. FTIR spectra for CO2 reduction at Cu electrodes in 0.1 M TEABF4 in CH3CN at the indicated potentials, reference spectra taken at – 1 V (vs Ag/AgClO4). The results in figure 3 show an increasingly negative band at 2341 cm-1 with more negative potential, corresponding to the consumption of CO2. This band can be observed at potentials as positive as -1.4 V but it becomes more significant at -1.8 V, which is the onset potential for CO2


12

Page 13 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

reduction suggested by the CVs in figure 1. Simultaneously with the negative band at 2341 cm-1 several positive bands increase in intensity, corresponding to product formation. The high number of product-related bands presented in the spectra requires careful assignment. There are several features in the carbonyl region (around 1600-1800 cm-1) and C-O region (1300-1500 cm-1). Previous reports on CO2 reduction in acetonitrile conclusive about the obtained products. Desilvestro et al. product while Christensen et al.

25

24

24, 25

have not been

have claimed oxalate as the main

attributed the obtained infra-red bands to carbonate species

forming ion pairs with TEA+ and water. Moreover, it is known that the frequency of C=O bands (from carbonates or oxalates) are very dependent on the charge delocalization of the compounds. Carbonate ions belong to the D3h symmetry and are expected to have 3 active modes in IR (1383, 1065, 887 cm-1). However, when carbonate ion is covalently bound by 1 oxygen atom, the symmetry is lowered and the vibrational modes change 35. In order to assist the assignment of the bands observed during CO2 reduction, transmission spectra of possible products in acetonitrile were collected, as presented in figure 4. Tetraethylammonium salts for carbonate, oxalate and formate were chosen due to the low solvation ability of acetonitrile. The solvation properties of acetonitrile are especially low toward anions due to steric hindrance, making interaction with the partial positive charge of acetonitrile very difficult, and due to electrostatic repulsion by the nitrile π orbitals 36. For this reason, if any solvation or ion pairing exists, it will be with residual water molecules or TEA+ cations. Significantly, the spectra show that (bi)carbonate, oxalate and formate (as tetraethylammonium salts) can be differentiated using FTIR. It is worth mentioning that the vibrational features of these compounds are extremely dependent on the ion pairing of the anion and can significantly differ from spectra obtained in aqueous solution or in acetonitrile solutions without TEA+ cations


13

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

25

Page 14 of 39

. The C=O vibration exhibits two distinguishable features at 1681 and 1646 cm-1 for

(bi)carbonate, but only one at 1586 and 1569 cm-1 for formate and oxalate, respectively. The C-O vibration of formate shows two bands at 1382 and 1348 cm-1, and one for oxalate at 1305 cm-1.

Figure 4. Transmission spectra of TEAHCO3, TEACOOH, TEAC2O4 in 0.1 M TEABF4 in CH3CN with CH3CN as reference, in comparison with the FTIR spectrum for CO2 reduction at a Cu electrode in 0.1 M TEABF4 in CH3CN at -2.4 V, also shown in figure 2.

By comparing the transmission spectra of the possible products with the spectrum obtained for CO2 reduction at -2.4 V on Cu, we can exclude the preferential formation of oxalate and formate as products of CO2 reduction. The main reaction product under these conditions is the (bi)carbonate species. However, the spectra for TEAHCO3 and the reduction product at - 2.4 V


14

Page 15 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

show some differences in terms of the relative intensity of the several bands attributed to the carbonate species. As all the chemicals (acetonitrile and supporting electrolyte) were used as received and were not dried, the solution of 0.1 M TEABF4 has 46 ppm of water (see Table 1) and its presence will influence the equilibrium between carbonate and bicarbonate species. In order to identify the different contributions of (bi)carbonate species to the observed vibrational bands, a transmission spectrum in the presence of acid (HBF4) was also obtained (figure 5). Under these conditions, the protons in the solution are expected to convert the carbonate species into bicarbonate (and possibly CO2) and hence the bicarbonate bands can be better identified.

Figure 5. Transmission spectra of TEAHCO3 and TEAHCO3 + 0.05 M HBF4 in CH3CN (CH3CN as reference). The results in figure 5 show significant differences in the vibrational spectra in the presence of protons. The bands at 1681, 1646, 1364, 1328 and 1305 cm-1 disappear for the acidified


15

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 39

acetonitrile solution, suggesting that those are due to carbonate species, while the remaining bands must then be attributable to bicarbonate. Based on the spectra in Figs. 3-5, Table 2 summarizes our assignment of the FTIR bands observed during CO2 reduction on Cu electrodes in 0.1 M TEABF4 in acetonitrile. Table 2. Assignment of the FTIR bands observed for CO2 reduction on Cu electrodes in 0.1M TEABF4 in acetonitrile. Wavenumber

Assignment

Compound

Reference

(cm-1) 2341

C=O

CO2

18,19

23

stretching 2245

C-N stretching

acetonitrile

2138

C-O stretching

CO gas

1681

C=O

CO32-

This work

CO32-

This work

HCO3-

This work

stretching 1646

C=O stretching

1607

C=O stretching

1487

C-O stretching

HCO2-

This work

1452

C-O stretching

HCO2-

This work

1388

C-O stretching

HCO2-

This work

1364

C-O stretching

CO32-

This work


16

Page 17 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

1328

C-O stretching

CO32-

This work

1305

C-O stretching

CO32-

This work

1213

C-OH in plane

HCO2-

This work

Acetonitrile

23

bending 1176

N-H bending

decomposition products 1005

B-F

BF4-

26

The existence of different vibrational modes of the C=O or C-O bonds on carbonates and bicarbonates can be explained by the existence of different ion pairs or solvation shells of the anions by TEA+ cation and residual water, as suggested previously

25

. An attempt at the

identification of the bands corresponding to the different ion pairs or solvation shells will be presented in section 3.2, together with the effect of water. After this analysis, and returning to the results shown in figure 3, we can suggest that the reaction produces more carbonates than bicarbonates as the bands at 1681, 1646, 1364 and 1328 cm-1 (attributed to carbonates) show higher intensity than those from bicarbonate. These results also show a strong effect of the local concentration of H+/OH- in the vicinity of the electrode surface under the present conditions. Of course, (bi)carbonate itself is not a reduction product of CO2. As mentioned in the Introduction, in aprotic solvents, CO is considered to be formed together with carbonate during CO2 reduction. However, due to its low solubility in acetonitrile, FTIR has a limited sensitivity for identifying CO (although a small band at 2138 cm-1 observed in the spectra as shown in the inset of figure 3 can be assigned to CO gas). The existence of


17

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 39

adsorbed CO is very difficult to prove based on the FTIR spectra due to the intensity of the bands of (bi)carbonates in solution. Bands due to adsorbed species have a much lower intensity and cannot be clearly identified. In order to obtain direct evidence for the formation of (adsorbed) CO during CO2 reduction, we employed SERS, which can in principle identify molecules adsorbed on the metal surface by surface-enhanced Raman scattering. Such SER spectra obtained for CO2 reduction in 0.1 M TEABF4 at Cu electrodes in acetonitrile are presented in figure 6.

Figure 6. Surface-Enhanced Raman spectra for CO2 reduction on a Cu electrode in 0.1 M TEABF4 in CH3CN. The left panel of Fig.6 shows the region between 200 and 600 cm-1 whereas the right panel shows the region from 1900 to 2200 cm-1. In the left panel, three bands can be distinguished at 283, 382 and 423 cm-1. The first two bands can be attributed to Cu-CO rotation and Cu-CO vibration 37, 38, respectively. However, the third band is more difficult to assign. The Raman shift


18

Page 19 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

for this band corresponds to those observed for CuO 38 but it is not present in the absence of CO2 and the very negative electrode potentials make such a species unrealistic. Perhaps this band should be related with a (bi)carbonate species or with a poisoning species generated during the reaction. It is interesting to notice that the bands at 283 and 382 cm-1 start growing at -1.4 V and their intensity increases with more negative potential. On the other hand, the band at 423 cm-1 has a fairly constant intensity in the same potential range. In the right panel of figure 6, three bands are observed at 2028-1999, 2066-2030, 2102-2087 cm-1 that can be attributed to the C-O stretching vibration of adsorbed CO on Cu

38, 39

. These bands

lower in wavenumber when the electrode potential decreases, as expected for adsorbed CO, and its potential dependence (“electrochemical Stark tuning effect”) can give information about how strongly CO is adsorbed on the Cu surface. The bands at 2036 cm-1 and 2070 cm-1 shift by 29 cm1

/V and by 36 cm-1/V, respectively, and correspond to strongly adsorbed CO. The band at 2102

cm-1 shifts by only 13 cm-1/V and is, most likely, due to physisorbed CO 40. These CO bands can be observed as a single broad band at low potentials (-1.2 V) but separate at -1.4 V into three clearly distinguishable bands. The intensity of the three bands increase until the potential of the electrode reaches -1.8 V. At more negative potentials, the band intensities decrease. We note that the potential dependence of the bands in the range of 250-450 cm-1 and 1750-1990 cm-1 is different, especially at the most negative potentials. While the Cu-CO vibrational intensities appear to increase with negative potential,

the C-O stretching intensities decrease rather

dramatically below – 2.2 V. The absence of bands in the range of 1750-1900 cm-1 corresponding to highly coordinated CO in bridge or hollow sites excludes the shift of CO to different coordination at these negative potentials. We do not have a good explanation for this different


19

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 39

potential dependence, but we do not note that SERS intensities depend on more than coverage alone. 3.2. Effect of the presence of water: It has been reported that the presence of water plays a major role on the products obtained from CO2 electroreduction in organic solvents

11, 14, 15, 19

. In order to evaluate this effect,

controlled amounts of water were added to the solutions of 0.1 M TEABF4 in acetonitrile and the CO2 reduction at Cu electrodes was studied. Cyclic voltammetry was used to detect changes in the onset potential and current density and in situ FTIR was performed to identify the reaction products. Results are presented in figure 7.

Figure 7. A) FTIR spectra of CO2 reduction at a Cu electrode at -2.4 V (vs Ag/AgClO4) in 0.1 M TEABF4 in CH3CN with different amounts of water, reference spectra taken at – 1 V (vs


20

Page 21 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Ag/AgClO4) B) Cyclic voltammograms for CO2 reduction at a Cu electrode with different amounts of water, scan rate 50mV/s. The results show that, independently of the amount of water added (from 0 to 2M), no bands related with oxalate (1569 and 1305 cm-1) and/or formate (1586, 1382, 1348 cm-1) are observed in the FTIR spectra. Increasing of the amount of water leads to bands of higher intensity (especially the negative band of CO2 at 2341 cm-1) and also higher reduction currents in the CVs, together with a small lowering of the onset potential for reduction. Concerning the spectroscopic results, the most important differences with increasing the amount of water are observed in the OH bending region (3700-3500 cm-1) where, not surprisingly, the negative bands increase due to the higher rates of water reduction. In fact, water reduction can be the principal reason for the higher reductions currents observed in the CVs. Regarding the CO2-related bands, an increase of the band at 1607 cm-1 (bicarbonate, table 2, figure 5) and a decrease in the number of bands in the 1400-1200 cm-1 region is observed. More water will increase the availability of protons making the presence of HCO3- species more likely, as observed by the increase of the bands at 1607 and 1388 cm-1, that were attributed to bicarbonate species. On the other hand, bands at 1364, 1328 and 1305 cm-1 merge into a single band at 1339 cm-1. These three bands have been attributed to carbonate (solvated or forming ion pairs with different species like H2O or TEA+) and the fact that they merge into one band suggests that water is now preferred for solvation of the carbonate species. In order to shed light onto the nature of the solvation or ion pairing of the species that give rise to the different bands in the (bi)carbonate region, experiments with D2O were performed. The comparison between the spectra obtained for CO2 reduction at Cu electrodes at -2.4 V in the presence of 100 mM of H2O or 100 mM of D2O are presented in figure 8.


21

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 39

The results show that two bands shift when D2O is added to the system instead of H2O. The band at 1646 cm-1, previously attributed to carbonate, moves 21 cm-1 to lower wavenumbers in D2O. The band at 1364 cm-1, also assigned to carbonate, shifts to 1350 cm-1 in D2O. Based on these results, we suggest that the bands for carbonate at 1646 and 1364 cm-1 are related with anions solvated by water molecules, while the bands at 1681, 1328 and 1305 cm-1 originate from carbonate anions free from any solvating water, probably forming ion pairs with TEA cations. Similar results were found by Christensen et al.

25

for Pt electrodes who found that the high

number of bands from carbonates are due to different solvation shells of the anion. However, Christensen et al. did not distinguish between carbonate and bicarbonate ions.

Figure 8. FTIR spectra of CO2 reduction at a Cu electrode at -2.4 V (vs Ag/AgClO4) in 0.1 M TEABF4 in CH3CN with 100 mM of water and 100 mM of D2O, reference spectra taken at – 1 V (vs Ag/AgClO4).


22

Page 23 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

3.3.Effect of supporting electrolyte nature and its water content Another factor of influence on the reaction mechanism and product distribution is the nature of the supporting electrolyte 15, 41. In this study, salts with different anions (TEABF4 and TEATfO) and cations (TEATfO and NaTfO) were used as supporting electrolytes. The obtained results combining CV and FTIR are presented in figure 9.

Figure 9. A) FTIR spectra of CO2 reduction at a Cu electrode at -2.4 V (vs Ag/AgClO4) in 0.1 M X (X= TEATfO; TEABF4; NaTfO) and transmission spectra of Na2CO3 in MeCN reference spectrum taken at – 1 V (vs Ag/AgClO4), B) Cyclic voltammograms for Cu electrode in CO2 atmosphere in 0.1 M X in CH3CN at 50mV/s (positive scan – solid line; negative scan – dotted line).


23

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

It has been reported

15, 42

Page 24 of 39

that product formation from CO2 reduction in organic solvents,

including acetonitrile, requires the presence of TEA+ cations, that act as electron mediators with the reaction being suppressed when they are replaced by, for example, Li+. In these studies 15, 42, it was reported that tetralkylammonium salts promote the formation of CO from CO2 due to the stabilization of CO2.- by the cation. Evidence for the dependence of the CO production on the stabilization of the CO2 anion radical was also found in studies performed in ionic liquids 43. On the other hand, ion pairs formed between Li+ and the radical anion was considered to be too strong, forming species that is so stable in solution that it cannot be further reduced. However, recent work

41

has suggested that in electrolytes containing alkali metal cations, the CO2

reduction is inhibited by the competitive reduction of the M+ to M0, deactivating the electrode surface for other reactions. Therefore, we decided to evaluate the effect of the cation, by replacing the TEA+ by Na+. The results presented in figure 9 show that the CO2 reduction activity indeed depends sensitively on the nature of the cation and the anion of the supporting electrolyte. According to the voltammetric results (right panel in figure 9), TEATfO is the supporting electrolyte that shows the highest currents, but with a more negative onset (-2 V) than in TEABF4 electrolyte (1.8 V). The supporting electrolyte with Na+ shows very low reduction currents (although higher currents are obtained on the negative sweep) and the onset potential is very negative, around -2.3 V. These results are in agreement with previous reports 41 on the inhibitive effect of alkali metal cations, which has been suggested to be due to the passivation of the electrode surface with films formed by the alkali cation and reaction products. However, the total suppression of the reduction reaction as claimed by ref.

15

was not observed in our experiment. This effect can be

explained by the high amount of water in the Na solutions (ca. 500 ppm, see Table 1).


24

Page 25 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Considering the two TEA salts, it is worth noticing that, compared with TEABF4, the spectra for TEATfO reveal higher amounts of residual water as evidenced by the more pronounced OH stretching FTIR bands (3700-3500 cm-1) as expected from the KF measurements (see Table 1). The higher amount of water can, in this case, be the reason for the higher activity in TEATfO electrolytes. The FTIR results (left panel of figure 9) show the same bands in the spectra for the two TEA+ salts suggesting that the same reaction products are obtained. Other bands presented in the spectra, related to water and acetonitrile, are similar to those obtained in figure 2 and have been explained before. In addition to these bands, the spectra for the TfO salt shows three bands at 1271 (SO3 asymmetric stretching), 1224 (CF3 symmetric stretching) and 1154 cm-1 (CF3 asymmetric stretching) due to the depletion of the anion from the thin layer 44. The presence of water in the supporting electrolyte does not lead to different reaction paths nor does it inhibit the electrocatalytic effect of the Cu electrodes (as previously shown for CO2 reduction at Pt

15

in acetonitrile). Its major influence is on the different relative amount of

carbonate and bicarbonate.

3.4.Effect of the electrode material Finally, to assess the effect of the electrode material on the product distribution of CO2 reduction in “wet” solutions of acetonitrile, Pb, Ag, Pt, Pd and Au electrodes were also investigated. The FTIR results (spectra taken at the negative end of the potential window) for the different metals are presented in figure 10. The results plotted in figure 10, show that in the presence of small amounts of water, the different metals show similar products for CO2 reduction. All the spectra show bands in the


25

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 39

region 1680-1605 and 1450-1250 corresponding to the C=O and C-O vibrations in bicarbonates and bicarbonates forming ion pairs with water or TEA+ cations. The only exception is the Pb electrode that in addition to the bands for carbonate also shows the characteristic bands for oxalate formation (1574 and 1300 cm-1, see table 2).

Figure 10. FTIR spectra of CO2 reduction at different electrode materials in 0.1 M TEABF4 reference spectra taken at – 1 V (vs Ag/Ag+), (spectra taken at the indicated potential).


26

Page 27 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

4. General Discussion The results described in this paper show that CO2 reduction in acetonitrile electrolytes are highly sensitive to even trace amounts of water (~46 ppm for 0.1 M TEABF4 in MeCN) , to the extent that the products obtained and most likely, also the reaction path through which they are formed, are significantly affected by the presence of water. There are two main effects of the presence of water in acetonitrile electrolytes. The first effect in “wet” acetonitrile solutions is related with the potential window for reduction. As observed in figure 2, the presence of water leads to the decomposition of the acetonitrile solution into acetamide at negative potentials 34. For these reasons, the negative limit for Cu in acetonitrile solutions must be kept above -2.5 V

34

, to avoid solvent decomposition.

This limit comes in addition to a positive limit of ca. -1.0 V due to the formation of undesirable CN species 29. The second effect is on the CO2 reduction itself. We observed that the onset of (bi)carbonate formation is correlated with the onset potential for water reduction, suggesting some relation between both processes. In dry organic solvents such as DMF or DMSO, the rate determining step of the CO2 reduction is the formation of the radical anion, involving the transfer of 1 electron from the electrode to CO2 13. Subsequently, the coupling of two radical anions leads to the formation of oxalate, while the formation of CO and carbonate involves a reaction between CO2 and its radical anion

13, 17

. In the presence of water, the radical anion can be protonated to

form formate. However, in our results, the formation of (bi)carbonates seems to be related with the onset of water reduction, while the CO formation was observed at potentials less negative than those required for (bi)carbonate formation. We note that the formation of (bi)carbonates does not happen in “wet” acetonitrile alone (figure S1, Supporting information); the formation of


27

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 39

(bi)carbonates requires the application of a negative potential most likely related to the formation of OH- species. Other techniques were used for the identification of other possible products such as ethanol and ethylene. The samples from long term electrolysis were analyzed by HPLC (as described in the supporting information) and NMR (results not shown) and no additional products were found. It has been reported previously that on some electrode materials the formation of glycolate can take place for CO2 reduction in organic solvents in the presence of water and quaternary ammonium salts

17

. Glycolate is supposed to be produced from the partial reduction of oxalate

and it has a well-defined IR spectrum (see figure S2, supporting information). Oxalate can indeed be reduced in the same potential window as CO2 at Cu electrodes (Figure S3, Supporting information) having glycolate as main product as noticed by the appearance of two pronounced bands in the spectra at 1555 and 1386 cm-1. A careful look at the results plotted in figure 3, shows that no bands other than those identified as (bi)carbonates are present in the FTIR spectra for CO2 reduction in “wet” acetonitrile solutions, and therefore we exclude the formation of significant amounts of oxalate in this system. According to the Amatore-Savéant mechanism

13

the formation of carbonate should involve

the formation of an equimolar amount of CO. As explained previously, the solubility of CO in acetonitrile is very low and its identification with FTIR very difficult. Moreover, the bands observed in figure 3 that can be cautiously attributed to CO gas, are in the same frequency region as CuCN, which are known to be formed by decomposition of acetonitrile (figure 2). However, the results from SERS (figure 6) show the presence of CO on the electrode surface prior to the formation of (bi)carbonates. The most likely explanation for these results is that under our experimental conditions (wet solvent) the two products are formed in different pathways.


28

Page 29 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Formation of CO at less negative potentials is most likely due to the CO2 reduction by the residual water. On the other hand, the formation of (bi)carbonates will be due to the reaction of CO2 with the electrogenerated OH- species from water reduction. It is known that OH- will react with CO2 to form carbonates so it is not surprising that the same occurs in the “wet” organic solvents. However, the amounts of water in this case are low (below 50 ppm for TEABF4 solutions, see Table 1) and the effect of water on the reaction path is significant. The formation of CO and carbonates happens independent from each other in contrast to what has been described previously in the case of dry organic solvents. Our results show that special attention should be given when interpreting voltammetry and product distribution in organic electrolyte solutions because even a small amount of water is able to change the mechanism of product formation. This interpretation of our results is also supported by the results related to the water and electrolyte effects shown in figures 7 and 9, respectively. Increasing amounts of water in the solution causes an increase in the reduction currents (figure 7B) and in the negative band due to CO2 consumption (figure 7A). In the carbonate region, the presence of water also results in significant changes, but no products other than (bi)carbonates are observed. Higher amounts of reduced water lead to an enhanced formation of carbonates. Taken together, these results favor the assumption that the electro-generated OH- is responsible for converting CO2 to (bi)carbonates. A similar conclusion can be drawn on the effect of the supporting electrolyte (figure 9). Rather than an influence of the salt itself, the results suggest that is the residual water content associated with the salt (that is significantly higher for TEATfO, ~180 ppm) that leads to more significant water reduction and hence to an enhanced conversion of CO2 to carbonates.


29

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 39

Concerning the effect of the electrode material on the reaction in “wet” acetonitrile, the results show that (bi)carbonates are the main reaction products for all the materials investigated, with the exception of Pb. In agreement with previous reports

13, 17

, Pb forms mainly oxalates, also in

“wet” conditions. Our results show that, similarly to Cu, Pd, Pt, Ag and Au form (bi)carbonates at potentials at which water reduction starts taking place. A significantly higher overpotential needs to be applied in order to observe oxalate formation on Pb. At such negative electrode potentials, oxalate was never spectroscopically observed on the other electrode materials (results not shown). The potential window and the onset potential for (bi)carbonate formation on the different materials (Figure S4, supporting information) is correlated with their tendencies toward water reduction. The lower the potential at which water starts being reduced (as observed by the negative bands at 3600-3500 cm-1 in Figure 10), the sooner the bands for (bi)carbonates are observed. It can also be noticed that the main differences between the spectra for different metals are related with the ratio between carbonates and bicarbonates. Pt, Pd and Ag show almost no FTIR band at 1605 cm-1, associated with the formation of bicarbonates, while for example for Au, the band is much more pronounced. These results can be related with the local pH in the thin layer. The more water is reduced the more OH- will be present in the vicinity of the electrode. This will increase the pH near the electrode and increase the conversion rate of bicarbonates into carbonates. It should be mentioned that, for comparison with the results obtained with spectroscopy, some chromatography measurements were also performed (see Figures S5 and S6 in Supporting information). Samples collected online during the CV measurement 45 were taken and compared with standards of formate, carbonate and oxalate analyzed by ion chromatography. These results


30

Page 31 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

did not show any formate or oxalate (figure S5). However, samples obtained after 30 min of electrolysis analyzed by HPLC (figure S6) revealed a small peak for formic acid at very cathodic potential (-2.5 V). These results show that the formation of other products from CO2 reduction, apart from (bi)carbonates and CO, occurs only at very high overpotentials requiring long electrolysis times to be observed. The use of high overpotentials in order to achieve other products than (bi)carbonates is, however, limited by the potential window where the decomposition of acetonitrile can be avoided that, in turn, depends on the water content of the solutions. We cannot exclude the formation of other products at high overpotentials, but in the potential window accessible under these conditions their amount is so small that it remains under the detection limit of the techniques used in this paper. Therefore, carbon monoxide and (bi)carbonates can be considered the main products from CO2 reduction in “wet” acetonitrile solutions, but as mentioned they are formed in different pathways. Although no significant amounts of product other than (bi)carbonates are observed, the CV shows a significant reduction current for the solution saturated with CO2 compared to the blank voltammetry (see Figure 1). It would be intuitive to expect that, if the only electrochemical process happening is water reduction, this difference should not be so accentuated. Since no other products were detected in this potential range, we can tentatively suggest that these currents are caused by the CO2 reduction to CO (that was observed also at lower potentials). From these results, we suggest that “wet” acetonitrile behaves differently from “dry” organic solvents. Even the presence of residual amounts of water (coming from the solvent itself and the supporting electrolyte salts, 46 ppm) is enough to influence the reaction paths by the interaction with water reduction (reaction 1). The electrochemically generated OH- converts CO2 to


31

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 39

(bi)carbonates (reaction 2 and 3). A similar mechanism was suggested for protic solvents such as methanol46. 2 H2O + 2e- H2 + 2 OH- (1) CO2 + OH- ↔HCO3- (2) HCO3- + OH- ↔ CO32- + H2O (3) On the other hand, the formation of CO at less negative potentials is probably due to the CO2 reduction, also by residual water in the solution (reaction 4). CO2 + H2O + 2e- CO + 2 OH- (4) Therefore, the formation of CO and (bi)carbonate during CO2 reduction is decoupled from each other by the influence of residual water, in contrast to the classic Amatore-Savéant mechanism in which the formation of CO and carbonate happen in the same reaction, which applies to rigorously dry solvents. It can therefore be suggested that for CO2 reduction, “wet” acetonitrile behaves as a “weak” protic solvent.

5. Conclusions In this paper, spectroelectrochemical studies of CO2 reduction in acetonitrile solutions with small amounts of water were reported. Under these conditions, the observability of the AmatoreSavéant pathway is suppressed, even if high overpotentials are applied (in this case, due to the presence of water, acetonitrile decomposition is the dominant process). As in dry solvents, carbon monoxide and (bi)carbonates were found as major products (no products such as ethanol, methanol, ethylene, etc. where detected in our experiments), but in the presence of water they are formed in a different pathway than in a totally water-free solution 13. CO formation takes place at an onset potential more positive (already at -1.2 V) than the onset of the formation of (bi)carbonates (-1.8 V). On the other hand, the formation of (bi)carbonates was shown to be


32

Page 33 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

concurrent with the water reduction and consequently, to be dependent on the amount of water in the solution. The reason for this difference can be attributed to the decrease on the potential window available for the reduction process, due to solvent decomposition in presence of water. This decrease of the potential window does not allow applied overpotentials as high as those required for the formation of the CO2 anion radical which is the first and rate determining step for the formation of products in aprotic solvents. The exact composition of the supporting electrolyte also showed to be less significant than the residual water content of the salt used. Salts with higher reduction currents in the cyclic voltammograms and corresponding higher rates for CO2 consumption and bicarbonates formation, where those with higher residual water content. Similar results to Cu were obtained for Pt, Pb, Au, Pd and Ag electrodes. For all electrode materials, (bi)carbonates were observed as products when water reduction takes place. Only for Pb electrodes oxalate was detected at very high overpotentials. These were mainly due to the larger cathodic potential window of Pb in acetonitrile that allowed a higher overpotential to be applied. Based on the results, we can suggest that the formation of CO and (bi)carbonate during CO2 reduction is decoupled from each other by the influence of residual water. The formation of CO at less negative potentials is consequence of the CO2 reduction by residual water in the solution (reaction 4) through an electrochemical reaction while the formation of bicarbonates occurs due to the CO2 conversion by the electrogenerated OH- species (reaction 2) formed from water reduction (reaction 1). The results show that the mechanism for CO2 reduction is highly sensitive to the presence of water, even at the residual level (from solvent and supporting electrolyte), and that under these conditions, acetonitrile behaves as a weak protic solvent.


33

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 39

Supporting Information. Supplementary material including transmission spectra for CO2 in acetonitrile solutions with water, oxalate electroreduction, ion chromatography and HPLC results is provided as supporting information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors * Marta C. Figueiredo [email protected] * Marc T. M. Koper [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The authors acknowledge Bayer MaterialScience/Covestro and the Netherlands Organization for Scientific Research (NOW) for funding of this project. L. Jongbloed (University of Amsterdam) for her help with the Karl-Fisher titration measurements is acknowledged.


34

Page 35 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

REFERENCES 1.

Pacala, S.; Socolow, R. Science 2004, 305, 968-972.

2.

Rau, G. Environ. Sci. Technol. 2010, 45, 1088-1092.

3.

Lackner, K. Science 2003, 300, 1677-1678.

4.

Whipple, D. ; Kenis, P. J. Phys. Chem. Lett. 2010, 1, 3451-3458.

5.

Jhong, H.; Ma, S.; Kenis, P. Curr. Opin. Chem. Eng. 2013, 2, 191-199.

6.

Qiao, J.; Liu, Y.; Hong, F.; Zhang, J. Chem. Soc. Rev. 2014, 43, 631-675.

7.

Lim, R.; Xie, M.; Sk, M.; Lee, J; Fisher, A.; Wang, X.; Lim, K. Catal. Today 2014, 233,

169-180. 8.

Kuhl, K.; Hatsukade, T.; Cave, E.; Abram, D.; Kibsgaard, J.; Jaramillo, T. J. Am. Chem.

Soc. 2014, 136, 14107-14113. 9.

Ren, D.; Deng, Y.; Handoko, A.; Chen, C.; Malkhandi, S.; Yeo, B. ACS Catal. 2015, 5,

2814-2821. 10. Kortlever, R.; Shen, J.; Schouten, K.; Calle-Vallejo, F.; Koper, M. J. Phys. Chem. Lett. 2015, 6, 4073-4082. 11. Hori, Y., Electrochemical CO2 Reduction on Metal Electrodes. In Modern Aspects of Electrochemistry, Vayenas, C.; White, R.; Gamboa-Aldeco, M., Eds. Springer New York: 2008; Vol. 42, pp 89-189. 12. Kuhl, K.; Cave, E.; Abram, D.; Jaramillo, T. Energy Environ. Sci. 2012, 5, 7050-7059.


35

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 39

13. Amatore, C.; Saveant, J. M. J. Am. Chem. Soc. 1981, 103, 5021-5023. 14. Ikeda, S.; Takagi, T.; Ito, K. Bull. Chem. Soc. Jpn 1987, 60, 2517-2522. 15. Tomita, Y.; Teruya, S.; Koga, O.; Hori, Y. J. Electrochem. Soc. 2000, 147, 4164-4167. 16. Gressin, J., Nadjo, D., Savéant, J.. Nouv. J. Chim. 1979, 3, 545-554. 17. Gennaro, A.; Isse, A.; Severin, M..; Vianello, E.; Bhugun, I.; Saveant, J. J. Chem. Soc. Faraday Trans. 1996, 92, 3963-3968. 18. Lamy, E.; Nadjo, L.; Saveant, J. J. Electroanal. Chem Interfacial Electrochem. 1977, 78, 403-407. 19. Ito, K.; Ikeda, S.; Yamauchi, N.; Iida, T.; Takagi, T. Bull. Chem. Soc. Jpn 1985, 58, 3027-3028. 20. Hori, Y.; Murata, A.; Takahashi, R. J. Chem. Soc. Faraday Trans. 1 1989, 85, 23092326. 21. Hori, Y.; Kikuchi, K.; Murata, A.; Suzuki, S. Chem. Lett. 1986, 15, 897-898. 22. Peterson, A.; Abild-Pedersen, F.; Studt, F.; Rossmeisl, J.; Norskov, J. Energy Environ. Sci. 2010, 3, 1311-1315. 23. Schouten, K.; Kwon, Y.; van der Ham, C.; Qin, Z.; Koper, M. Chem. Sci. 2011, 2, 19021909. 24. Desilvestro, J.; Pons, S. J. Electroanal. Chem Interfacial Electrochem. 1989, 267, 207220.


36

Page 37 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

25. Christensen, P.; Hamnett, A.; Muir, A.; Freeman, N. J. Electroanal. Chem Interfacial Electrochem. 1990, 288, 197-215. 26. Ledezma-Yanez, I.; Díaz-Morales, O.; Figueiredo, M.; Koper, M. ChemElectroChem 2015, 2, 1612-1622. 27. García, G.; Rodríguez, P.; Rosca, V.; Koper, M. Langmuir 2009, 25, 13661-13666. 28. Lai, S.; Kleyn, S.; Rosca, V.; Koper, M. J. Phys. Chem. C 2008, 112, 19080-19087. 29. Mernagh, T.; Cooney, R. J. Electroanal. Chem Interfacial Electrochem. 1984, 177, 139148. 30. Kitadai, N.; Sawai, T.; Tonoue, R.; Nakashima, S.; Katsura, M.; Fukushi, K. J. Solution Chem. 2014, 43, 1055-1077. 31. Irish, D.; Hill, I..; Archambault, P.; Atkinson, G. J. Solution Chem. 1985, 14, 221-243. 32. Marinković, N.; Hecht, M.; Loring, J.; Fawcett, W. Electrochim. Acta 1996, 41, 641-651. 33. Suárez-Herrera, M.; Costa-Figueiredo, M.; Feliu, J. Langmuir 2012, 28, 5286-5294. 34. Foley, J.; Korzeniewski, C.; Pons, S. Can. J. Chem. 1988, 66, 201-206. 35. Gatehouse, B.; Livingstone, S.; Nyholm, R. J. Chem. Soc. 1958, 3137-3142. 36. McCarthy, B.; Martin, D.; Rountree, E.; Ullman, A..; Dempsey, J. Inorg. Chem. 2014, 53, 8350-8361. 37. Akemann, W.; Otto, A. Surf. Sci. 1993, 287–288, 104-109.


37

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 39

38. Smith, B.; Irish, D.; Kedzierzawski, P.; Augustynski, J. J. Electrochem. Soc. 1997, 144, 4288-4296. 39. Oda, I.; Ogasawara, H.; Ito, M. Langmuir 1996, 12, 1094-1097. 40. Schmitt, K.; Gewirth, A. J. Phys. Chem. C 2014, 118, 17567-17576. 41. Setterfield-Price, B.; Dryfe, R. J. Electroanal. Chem. 2014, 730, 48-58. 42. Taniguchi, I.; Aurian-Blajeni, B.; Bockris, J. J. Electroanal. Chem Interfacial Electrochem. 1984, 161, 385-388. 43. Sun, L.; Ramesha, G.; Kamat, P.; Brennecke, J. Langmuir 2014, 30, 6302-6308. 44. Liu, Z.; El Abedin, S.; Endres, F. ChemPhysChem 2015, 16, 970-977. 45. Yang, J.; Kwon, Y.; Duca, M.; Koper, M. Anal. Chem. 2013, 85, 7645-7649. 46. Ortiz, R.; Márquez, O. P.; Márquez, J.; Gutiérrez, C. J. Electroanal. Chem. 1995, 390, 99-107.


38

Page 39 of 39

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

120x63mm (150 x 150 DPI)


In Situ Spectroscopic Study of CO2 ... - ACS Publications

Recommend Documents