Subscriber access provided by UNIV OF NEW ENGLAND ARMIDALE
Highly Selective Molecular Catalysts for the CO2-to-CO Electrochemical Conversion at Very Low Overpotential. Contrasting Fe vs. Co Quaterpyridine Complexes upon Mechanistic Studies Claudio Cometto, Lingjing Chen, Po-Kam Lo, Zhenguo Guo, Kai-Chung Lau, Elodie Anxolabéhère-Mallart, Claire Fave, Tai-Chu Lau, and Marc Robert ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04412 • Publication Date (Web): 13 Mar 2018 Downloaded from http://pubs.acs.org on March 13, 2018
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 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 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.
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 8 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
Highly Selective Molecular Catalysts for the CO2-to-CO Electrochemical Conversion at Very Low Overpotential. Contrasting Fe vs. Co Quaterpyridine Complexes upon Mechanistic Studies Claudio Cometto,† Lingjing Chen,Ŧ Po-Kam Lo, ‡ Zhenguo Guo,‡ Kai-Chung Lau,‡ Elodie Anxolabéhère-Mallart,† Claire Fave,† Tai-Chu Lau*‡ and Marc Robert*† †
Univ Paris Diderot, Sorbonne Paris Cité, Laboratoire d'Electrochimie Moléculaire, UMR 7591 CNRS, 15 rue Jean-Antoine de Baïf, F-75205 Paris Cedex 13, France. ‡ Department of Chemistry, Institute of Molecular Functional Materials, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong (China). Ŧ School of Environment and Civil Engineering, Dongguan University of Technology, Guangdong, 523808, China.
ABSTRACT: [MII(qpy)(H2O)2]2+ (M = Fe, Co; qpy: 2,2′:6′,2″:6″,2‴-quaterpyridine) complexes efficiently catalyze the electrochemical CO2-to-CO conversion in acetonitrile solution in the presence of weak Brönsted acids. Upon performing cyclic voltammetry studies, controlled-potential electrolysis and spectroelectrochemistry (UV-visible and infrared) experiments together with DFT calculations, catalytic mechanisms were deciphered. Catalysis is characterized by high selectivity for CO production (selectivity > 95 %) in the presence of phenol as proton source. Overpotentials as low as 240 mV and 140 mV for the Fe and Co complexes respectively led to large CO production for several hours. In the former case, the one-electron-reduced species binds to CO2 and CO evolution is observed after further reduction of the intermediate adduct. A deactivation pathway has been identified, due to the formation of a Fe0qpyCO species. With the Co catalyst, no such deactivation occurs and the doubly reduced complex activates CO2. High scan rate cyclic voltammetry allows reaching kinetic conditions, leading to scan rate independent plateau shaped voltammograms from which catalytic rate constant was obtained. The molecular catalyst is very active for CO production (turnover a frequency of 3.3 x 104 s-1 at 0.3 V overpotential), as confirmed by catalytic a Tafel plot showing a comparison with previous catalysts.
1. INTRODUCTION The transition to societies based on renewable energy sources implies the storage of these energies at a massive scale since they are intermittent and non-equally distributed. CO2 could be used as a feedstock for reaching this goal and reduced into various compounds, including fuels (CH3OH, CH4), commodity chemicals or precursors to fuels, like e.g. CO that can further be used in Fischer-Tropsch chemistry.1-3 In other words, the current booming for CO2 reduction studies is mainly motivated by the goal of setting conditions towards a sustainable development and the replacement of fossil fuels with renewable fuels and commodity chemicals. Although many homogenous4-7 and heterogeneous8-10 systems able to catalyze CO2 reduction into some of these products have been recently designed, catalysts being at the same time selective, based on earth abundant elements and showing good stability remain quite rare, slowing the development of device applications and technologies. This calls for designing new catalysts based on rational approaches involving feedback with thorough mechanistic analysis that aim at identifying the factors that control both activity and selectivity during the process. Regarding the conversion of CO2 into CO with molecular catalysts, electrogenerated Fe0 porphyrins are catalysts of choice, showing the best performances.11-12 Another promising class of catalysts is polypyridine-based metal complexes (W, Mo, Re, Mn, Fe, Co, Ni, Ru, Rh, Os).4,13-16 Focusing on Earth
abundant metal-based compounds, Mn(bpy)(CO)3Br (bpy = 2,2’-bipyridine) and derivatives have been characterized by high Faradaic yields for CO formation and good stability over several hours, while the M(bpy)(CO)4 (M = Mo, W) are only active at quite negative potentials (≈ ca. -2.2 V vs SCE). Feand Co bipyridine and terpyridine based catalysts are on contrary suffering from low Faradaic yields for CO production, low selectivity and important H2 evolution as a by-product. Quaterpyridine derivatives (Fe, Co) have been barely investigated, with the exception of a preliminary electrochemical study by Che et al.17 In this paper, 2,2′:6′,2″:6″,2‴quaterpyridine (qpy) was tested as organic ligand with CoII and NiII in acetonitrile as solvent and no added acid. With the nickel catalyst, only CO traces have been observed during the controlled-potential electrolysis (CPE) at -1.7 V vs. SCE. With the cobalt catalyst, selective CO formation with 80% Faradaic yield was obtained at -1.7 V vs. SCE, but a film was rapidly formed on the glassy carbon surface electrode followed by large decrease of CO production. In stark contrast, efficient photochemical CO2-to-CO conversion by [M(qpy)(H2O)2]2+ (M = Fe, Co) (Scheme 1) has been recently achieved under visible-light excitation, with Ru(bpy)32+ as a photosensitizer and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole as a sacrificial reductant in CH3CN/triethanolamine solution.18 CO was the major product with both catalysts (selectivity > 95%), while they also showed excellent stability over the time with turnover numbers as large as 2600. In order to understand
ACS Paragon Plus Environment
a
15
10
10
0
5
-10
0
-20 -0.4 40
-0.6
-0.8
E (V vs SCE) -1.0
c
b
E (V vs SCE)
-5 -0.4 12
30
i (µ A)
20
-0.6
i (µ A)
the origin for high photochemical activity and to set conditions to obtain high electrochemical activity we have used a combination of electrochemical, spectroscopic and DFT studies to shed light on the catalytic mechanisms for CO production. Determining the catalytic pathways led to the identification of optimized electrochemical experimental conditions, giving rise to high catalytic selectivity (96% for the Co catalyst and close to 100% for the Fe catalyst) at very low overpotential (140 240 mV), making this family of compounds one of the most promising class of CO2-electrochemical molecular catalysts.
i (µ A)
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 8
i (µ A)
ACS Catalysis
-0.8
-1.0
-1.2
-1.4
d
8
20 4 10 0 0 -10 -0.4
Scheme 1. Structures of the catalysts. 2.
RESULTS AND DISCUSSION
CO2-to-CO electrochemical [Fe(qpy)(H2O)2] 2+
conversion
with
[Fe(qpy)(H2O)2]2+ (1), noted [FeIIqpy]2+ in the following, gives rise to a first reversible electron transfer to generate FeIqpy under argon (Fig. 1a, black curve). The fast charge transfer reaction is characterized by a standard redox potential E0(FeII/FeI) = -1.01 V vs. SCE while a second electron transfer is observed at more negative potential (E0= -1.22 V vs. SCE, see Fig S1). Upon CO2 saturation of the acetonitrile solution, the first reduction peak is positively shifted by ca. 21 mV at low scan rate and becomes irreversible (Fig. 1a, red curve), indicating fast and irreversible binding of the CO2 to the FeI species upon the following E + C sequence (E: electron transfer, C: chemical reaction): [FeIIqpy]2+ + 1e[FeIqpy]+ + CO2
[FeIqpy]+ [FeIIqpyCO2•−]+ , rate k
(a) (b)
-0.6
-0.8
-1.0
where v is the scan rate and k the binding rate constant between CO2 and the FeI species (reaction (b)). After checking that the experimental values of Ep follow Eq. (1) (30 mV shift of Ep per decade log v), a value of 82 M-1 s-1 was obtained for k (taking into account a saturating concentration of 0.28 M for CO2).20 At ca. -1.2 V vs. SCE, the FeIIqpyCO2•− adduct is irreversibly reduced (Fig. 1b, red curve), which corresponds to C-O bond cleavage, as shown below, and the catalytic reduction of CO2 is otherwise strongly enhanced in the presence of phenol, as shown in Fig 1c (dashed line). Upon backward scan, an oxidation wave appears at ca. -0.71 V vs. SCE, matching the oxidation wave observed upon saturating a solution of the complex with CO (Fig 1b, blue curve).
-4 -0.4
-0.6
-0.8
E (V vs SCE) -1.0
Figure 1. a. CVs of 1 (1 mM) in an acetonitrile (ACN) solution under Ar (black) and CO2 saturation (red). b. CVs of 1 (0.5 mM) in an ACN solution under Ar (black), CO2 (red) and CO (blue) saturation. c. CVs of 1 (1 mM) in a ACN solution upon CO2 saturation (solid line) and in the presence of phenol (PhOH) 120 mM (dashed line). d. CVs of 1 (0.5 mM) in an ACN solution upon CO saturation. Scan rate for all CVs was 0.1 V/s and NBu4PF6 (0.1 M) was used as supporting electrolyte. In that latter case, upon reducing FeII to FeI species, fast reversible binding with CO occurs along reactions (c) + (d) and leads to a shift of the redox potentials towards more anodic values: [FeIIqpy]2+ + 1e[FeIqpy]+ + CO
[FeIqpy]+ [FeIqpyCO]+ , K
(c) (d)
where K is the equilibrium binding constant. The apparent standard redox potential E0app (-0.785 V vs. SCE) of this reversible wave is related to E0(FeII/FeI) through Eq. (2): E 0 = E 0 (FeII / FeI ) +
In pure kinetic conditions, the peak potential location is given by equation (1):19 RT RT RTk CO2 (1) E = E 0 (FeII / FeI ) − 0.78 + ln p F 2F Fv
E (V vs SCE) -1.2 -1.4
app
RT ln(1 + K ) F
(2)
leading to K = 8 x 103. The FeIqpyCO adduct being obtained upon the second reduction wave of 1 under CO2 thus shows that one C-O bond has been cleaved. Remarkably, this FeIqpyCO adduct is reduced at -1.04 V vs. SCE (Fig 1d, dashed line). A re-oxidation peak at around -0.51 V vs. SCE is observed upon backward scan. Although not central to the discussion, it was observed that this oxidation wave becomes more prominent upon raising the Fe complex concentration (from 0.25 to 1 mM, see Fig. S2), suggesting formation of a multimetallic center produced by dimerization or oligomerization in excess of CO occurring upon the reduction of the FeIqpyCO species. To get further insight in the reduction process, spectroelectrochemistry (SEC) was performed at various potentials in CO2 and CO saturated solutions (see Experimental section for details). Under argon, and when the potential was set at -0.9 V vs. SCE, which corresponds to the first reduction process (FeI complex formation), absorption bands were observed 2
ACS Paragon Plus Environment
Page 3 of 8 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
Figure 2. SEC experiments. a. UV-vis. spectra of the one-electron-reduced species of 1 (0.25 mM) at -0.9 V vs SCE under Ar (black) and CO2 (red) atmosphere. b. UV-vis. spectra of 1 (0.25 mM) under CO2 atmosphere at -0.9 V vs. SCE (red dots) and -1.2 V vs. SCE (plain red). c. UV-vis. spectra of 1 (0.25 mM) reduced at -1.2 V vs. SCE under CO2 (red) and at -0.9 V vs. SCE under CO (blue, absorbance values were divided by 10 for easier comparison). d. IR spectra of 1 (1 mM) under CO2 at -1.2 V vs. SCE. e. IR spectra of 1 (1 mM) under CO at -0.95 V vs. SCE. f. experimental spectra (blue line) of the Fe0qpyCO adduct and calculated electronic transitions (black bars); the inset shows the predicted Fe0qpyCO structure calculated by DFT (see SI for details). at 294 and 301 nm along with an asymmetric peak at 334 nm with a shoulder at around 344 nm (Fig. 2a, black). In the visible region, two weak bands (494 nm, 544 nm) were also detected (Fig. 2b, black). Upon CO2 saturation of the solution, the adduct formation between CO2 and FeI produced at -0.9 V vs. SCE was confirmed by small changes of the bands centered respectively at 293 nm, 300 nm, 329 nm and 339 nm (Fig. 2a, red), while an asymmetric band with maximum at 539 nm is observed (Fig. 2b, black). Upon applying -1.2 V vs. SCE at the platinum electrode (foot of the second reduction wave that has been assigned to C-O bond cleavage), new bands at 284 nm, 380 nm, 529 nm, and 827 nm are formed (Fig. 2b and 2c, red). IR spectrum shows an intense band at 1854 cm-1 (Fig. 2d), suggesting the formation of a metalcarbonyl species, in line with C-O bond breaking in the FeIIqpyCO2•− reduced adduct. In a CO-saturated solution of 1, electrode polarization at -0.9 V vs. SCE leads to news bands at 289 nm, 299 nm (sh), 316 nm (sh), 383 nm, 502 nm and 834 nm (Fig. 2c, blue). At this potential, the FeIqpyCO adduct is reduced (foot of the reduction wave as seen in CV (Fig 1d)) and comparison between this spectra and the one recorded at 1.2 V vs. SCE under CO2 both in the UV-vis (Fig. 2c, red) and IR (Fig. 2d and 2e) range strongly suggests that the same Fe0qpyCO species is produced in both cases. DFT and TDDFT calculations further confirmed the formation of this highly reduced Fe0 compound (see Fig. 2f and SI), with good agreement between experimental and simulated spectrum in the UV-vis region, while the computed νCO stretching frequen-
cy (1880 cm-1) is in reasonable agreement with the experimental value. All these observations led to the reduction mechanism shown in Scheme 2 where the binding of the reduced FeI species with CO2 is followed by protonation and cleavage of the C-O bond, releasing FeIqpyCO. The exact sequencing of proton and electron transfers to FeCO2 adduct remains to be determined. This FeIqpyCO species may either be easily reduced with another electron to furnish Fe0qpyCO inactive species (blue pathway, Scheme 2) as identified by its UV-vis and IR absorption signature as well as by cyclic voltammetry (Fig. 1d), or it may release a CO molecule and a FeI complex, closing the catalytic cycle (Scheme 2). In line with this competitive mechanism, a significant barrier (14.2 kcal mol-1) was calculated by DFT calculation for CO dissociation from FeIqpyCO (see SI). Controlled-potential electrolysis (CPE) was then performed at a potential of -1.2 V vs. SCE in CO2-saturated solution, corresponding to a very low overpotential of 240 mV, in the presence of 1 M phenol (PhOH). Indeed, E0CO2/CO = -0.96 V vs. SCE in these experimental conditions (see Supporting Information from Reference 12 for a detailed calculation and additional Fig S4).12 This acid was chosen as a source of proton for helping the C-O bond cleavage, being at the same time weak enough (pKa = 27) not to drive the reactivity towards hydrogen evolution. Note that similar results were obtained with 2,2,2trifluoroethanol. CO was produced with over 99% selectivity
3
ACS Paragon Plus Environment
ACS Catalysis adding water to the acetonitrile solution tends to decrease the reduction peak that progressively evolves to a small plateau shape current (see Fig. S3), suggesting that this second reduction wave is preceded by slow H2O ligand loss (C + E mechanism), as already hypothesized by Che et. al.17 When adding PhOH to the solution, the reduction wave of CoIIqpy is slightly shifted towards positive potential while remaining fully reversible (Fig. 3a, dashed), indicating coordination of the acid to the metal center, upon the following E + C sequence:
2+
N
N FeII
N
N
e-
CO2
1+
N
N FeI
N
N
CO
1+
N N Fe0
O
O
where n (= 1 or 2) is the number of phenol molecules in axial coordinating position. The apparent standard redox potential E0app (-0.54 V vs. SCE) of this reversible wave at a concentration of 3 M PhOH is related to E0(CoII/CoI) through Eq. (3):19
N
2e2H+
CO
e1+
N N
(e) (f)
C
N
N FeI
N
H2O
E 0 = E 0 (Co II / Co I ) +
CO
app
Scheme 2. CO2-to-CO reduction mechanism with catalyst 1.
10
conversion
with
[Co(qpy)(H2O)2]2+ (2), noted [CoIIqpy]2+ in the following, gives rise to a first reversible electron transfer to generate CoIqpy under argon (Fig. 3a, black) characterized by a standard redox potential E0(CoII/CoI) = -0.565 V vs. SCE, much more positive than in the case of 1. A second reduction wave (likely centered on the ligand) with E0 = -0.795 V vs. SCE is observed at low scan rate, while increasing the scan rate or
a
100
b
80
5
60 40
0
20 0
-5 0.0 450
-0.2
E (V vs SCE) -0.6
-0.4
c
E (V vs SCE) 0.0 450
300
-0.3
-0.6
-0.9
-1.2
d
300 150
150 0 0
E (V vs SCE)
E (V vs SCE) -150 0.0
CO2-to-CO electrochemical [Co(qpy)(H2O)2] 2+
(3)
with n being the number of phenol molecules involved in reaction (f). Fitting of equation (3) for [PhOH] from 0 M to 3 M led to n = 2 (see SI) and to an apparent equilibrium binding constant K of 0.18 M-2, indicating that phenol has displaced the two water molecules from the coordination sphere. Note that it was not possible to probe this equilibrium by UV-vis spectroelectrochemistry since the reduced CoI species does adsorb onto the working platinum grid electrode. Concomitantly to PhOH binding, the second reduction wave almost disappears and a new reduction wave was further observed at ca. -1.27 V vs. SCE (Fig 3b), corresponding to the one electron reduction of CoIqpy(PhOH)2 that is likely ligand centered. Upon saturating the solution with CO2 in the presence of PhOH, the first reduction peak was not modified (Fig 3a, red), i (µ µ A)
(only traces of H2 were detected from the gas phase and no formate was identified from the liquid phase) and with 48 % Faradaic efficiency (FE), leading to a turnover number (TON) of ca. 8 (TON was calculated relatively to the catalyst concentration, 0.2 mM). In order to enhance CO FE and favor the kinetic competition for CO release from the FeIqpyCO adduct over the reduction to Fe0 carbonyl species (Scheme 2), electrolysis were then performed upon visible light irradiation (λ > 420 nm), leading to ca. 70% FE and a TON of 12 after 4 h. Catalytic selectivity remained close to 100%. When applying a potential of -1.4 V vs. SCE (440 mV overpotential) and upon light irradiation, FE slightly increases to 72% with 19 TON of CO and a current density of 0.15 mA/cm2, again with no loss of selectivity towards CO production. Finally, the exceptional photochemical catalytic activity of 1 (1880 TON with 97% catalytic selectivity and 8.8% apparent quantum efficiency with Ru(bpy)32+ as sensitizer and triethanolamine as sacrificial electron donor)18 could be rationalized by noticing that in purely homogeneous conditions, reduction of the FeIqpyCO adduct with one electron being a bimolecular reaction with the reduced sensitizer (rate constant 2 x 108 M-1 s-1), it does not compete with the unimolecular loss of CO, that is enhanced by continuous irradiation as well as the local heat provided by the illumination. The non heme catalyst 1 is also more efficient in photochemical conditions than the intensively investigated Fe porphyrins because of strong binding to CO2 after only one electron reduction (3 electrons are necessary in the case of porphyrins) and of the more positive potential required to cleave the C-O bond.
RT n ln(1 + K [ PhOH ] ) F
i (µ µ A)
N
N
[CoIqpy]+ [CoIqpy(PhOH)n]+
i (µ µ A)
N
[CoIIqpy]2+ + 1e[CoIqpy]+ + n PhOH
N FeII
i (µ µ A)
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 8
-0.3
-0.6
-0.9
-1.2
0.0
-0.3
-0.6
-0.9
-1.2
-1.5
Figure 3. a. CVs of 2 in an ACN solution under Ar (black), under Ar (dashed) and CO2 (red) + 3 M phenol (PhOH). b. CV of 2 in an ACN solution under Ar +1.5 M PhOH. c. CVs of 2 in an ACN solution + 3 M PhOH under Ar (black) and CO2 (red). d. CVs of 2 in an ACN solution + 3 M PhOH under CO2 at increasing scan rates (5 V/s, 60 V/s, 175 V/s, 250 V/s), using a 1-mm diameter glassy carbon electrode. Scan rate for all CVs was 0.1 V/s (except in d) and NBu4PF6 (0.1 M) was used as supporting electrolyte. Concentration of 2 was 0.5 mM. 4
ACS Paragon Plus Environment
Page 5 of 8 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 -1 Log TOF (s )
N N
a
N
Fe
a
4
b c d d' f g
2
e
0
h
N
Cl N
6 N
N N
b -2 -4 -6
c
-8
Scheme 3. CO2-to-CO reduction mechanism with Co catalyst 2.
-10
Overpotential (V)
I
indicating that Co species does not bind significantly to CO2. At more negative potentials, an intense catalytic wave with a ca. 40 times current amplification was observed at the level of the reduction wave of CoIqpy(PhOH)2 (Fig 3c, red). CPE was then performed at -1.1 V vs. SCE in the presence of 3 M PhOH, corresponding to 140 mV overpotential. CO was produced with 96% catalytic selectivity (a small amount of H2 (4%) was obtained as only by-product) and 94% FE. A TON of 17 in CO was obtained after 3 hours. By applying a slightly more negative potential (-1.3 V vs. SCE, 340 mV overpotential), the same selectivity for CO was obtained with 87% FE with a TON of 64 after 8 hours electrolysis (0.68 mA/cm2). A linear scan recorded at the end of the electrolysis showed that the solution retains the same initial catalytic activity (Fig. S6). These observations point towards the reduction mechanism shown in Scheme 3. Remarkably, at high scan rates (> 150 V/s) in the presence of 3 M phenol, a catalytic plateau independent from the scan rate is obtained, indicative of pure kinetic conditions resulting from mutual compensation between the catalyst diffusion and fast catalytic rate (iplateau = 330 µA, half-wave potential E1/2 = -1.245 V vs. SCE, see Fig. 3d). At such high scan rates, the voltammetry responses are devoided from the secondary phenomena (e.g. partial inhibition of the surface by the reaction product, CO) that led to peak shape responses at lower scan rates, thanks to the smaller charges being passed through the electrode surface.11,12,21,22 To assess that CVs responses were free from adsorption phenomena, it was carefully checked that the catalytic plateau was linearly dependent on the catalyst concentration (from 0.5 to 1.2 mM) and that the first non-catalytic reduction wave was under diffusion control with peak current being proportional to the square root of the scan rate (Fig. S7). The catalytic plateau current is given by equation (4) corresponding to the mechanism sketched in Scheme 3 (two electrons reduction of 2 followed by CO2 binding and C-O bond cleavage upon protonation):23 0 i plateau = 2 FS × Ccat × Dcat × kcat
(4)
while for the one electron diffusion current of the catalyst equation (5) applies: 0 i0peak = 0.446 × FS × Ccat × Dcat ×
Fv RT
(5)
0.2
d
0.4
0.6
0.8
1.0
1.2
1.4
N
H3CCN
e
N
Ru
N
N N
g
f
h
Figure 4. Benchmarking of the main molecular catalysts (a,12 c12, d,13 e,24 f,25 g,26 h27) of the CO2-to-CO electrochemical conversion in N,N-dimethyl formamide or ACN by means of their catalytic Tafel plots (Log TOF as a function of the overpotential η = E0CO2/CO – E). d’ dotted lines: data for complex d in the presence of 0.1 M Mg2+.13 : TOF value for 2 obtained from electrolysis data (see text and SI). Using the ratio iplateau/i0peak avoids determining the electrode surface S and the diffusion coefficient Dcat of the catalyst:
kcat
i plateau = 0 i peak
2
Fv 1 × × (2 × 2.24) 2 RT
(6)
Applying equation (6) finally led to kcat = 3.3 x 104 s-1. 2 could then be benchmarked towards the most efficient molecular catalysts for the CO2-to-CO electrochemical conversion12,13,2427 by positioning on a catalytic Tafel plot that relates the turnover frequency (TOF) of each given catalyst towards the overpotential η (= E - E0CO2/CO) applied at the electrode: TOF =
kcat F 0 F 1 + exp η − E1/2 × exp − E RT RT CO2 /CO
(7)
In this equation, E1/2 is the half-plateau wave potential, and is often equal to the standard redox potential of the active form of the catalyst. Turnover frequency for 2 was also calculated from the electrolysis current (see SI and Fig. S8) and gave a value in excellent agreement with CVs data (TOF = 533 s-1 at 140 mV overpotential, black square in Fig. 4). It can be seen from Fig. 4 that 2 is an excellent catalyst even at very low overpotential (left part of the diagram), better than the most active Mn complexes so far reported,13 and being only surpassed by the Fe tetraphenyl porphyrin bearing four trimethylammonio groups in ortho position of each phenyl (complex a, Fig. 4).12 Such excellent intrinsic properties as CO2-toCO catalyst provide a rational view on its high photochemical
5
ACS Paragon Plus Environment
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
catalytic activity (2660 TON with 98% catalytic selectivity and 2.1% apparent quantum efficiency with Ru(bpy)32+ as sensitizer and triethanol amine as sacrificial electron donor).18
3.
CONCLUSIONS
Proton-assisted electrochemical CO2 reduction to CO in acetonitrile solution by [MII(qpy)(H2O)2]2+ (M = Fe, Co) is characterized by high selectivity (above 95% for both complex) and low overpotential (CO production could be achieved at 140 mV and 240 mV overpotential for the Co and Fe catalyst respectively). Upon combining cyclic voltammetry, controlled-potential electrolysis, IR and UV-visible spectroelectrochemistry as well as complementary DFT calculations, the catalytic mechanisms have been deciphered. In the case of 1, the one electron reduced species FeIqpy is able to efficiently bind CO2 while further one-electron reduction of the FeIIqpyCO2•− adduct leads to C-O bond cleavage in the presence of a weak acid. FeIqpyCO is engaged in a competition between reduction to Fe0qpyCO, ending in catalyst deactivation. CO release could be boosted upon visible light illumination of the solution, with Faradaic efficiency up to 70%. These intrinsic properties are at the origin of the high activity towards photochemical conversion of CO2 into CO. In the case of the Co complex 2, the two-electron reduced species activates CO2 in the presence of phenol. High scan rate cyclic voltammetry allows reaching pure kinetic conditions characterized by plateau shape CV from which the catalytic rate constant was obtained. It led to a maximum turnover frequency of 3.3 x 104 s-1 at overpotential as low as 300 mV, making 2 one of the most active molecular catalyst so far reported, closely matching performance of the highly efficient Fe0 porphyrins. While employing earth-abundant metal centers, this new, promising family of quaterpyridine complexes could be further tuned by introducing various substituents on the pyridine rings so as to modulate the reactivity. The flat, conjugated structure makes them good candidates to be adsorbed onto various conductive or semi-conductive materials in order to prepare hybrid catalytic systems. Combined to the very low overpotential of the complexes, such hybrid systems may be promising to efficiently and selectively catalyze CO2 in pure aqueous conditions. Studies along these lines will be soon reported. Experimental part Chemicals Acetonitrile (Acros, >99.9%) and supporting electrolyte NBu4PF6 (Fluka, purriss.) were used as received. Compounds 1 and 2 have been synthesized as previously reported.18 Phenol and 2,2,2-trifluoroethanol were purchased from Sigma Aldrich and used without further purification. Electrochemistry and Spectroscopic Analysis All the experiments have been performed by using dry acetonitrile as solvent and NBu4PF6 as supporting electrolyte. Before each experiment the solution was purged with Ar or CO2 or CO for 20 minutes. Cyclic Voltammetry. The working electrode was a 3 mmdiameter glassy carbon (Tokai) disk carefully polished using diamond paste of various size (from 15 to 1 µm), and ultrason-
ically rinsed in absolute ethanol and dried before use. The counter-electrode was a platinum wire and the reference electrode was an aqueous SCE electrode. All experiments were carried out under argon or carbon dioxide or carbon monoxide atmosphere at 20 °C, the double-wall jacketed cell being thermostated by circulation of water. Cyclic voltammograms were obtained by use of a Metrohm AUTOLAB instrument. Ohmic drop was compensated using the positive feedback compensation implemented in the instrument. For high speed cyclic voltammetry, a 1 mm-diameter glassy carbon was used as working electrode and a home-made potentiostat was used for ohmic drop compensation. Controlled Potential Preparative Scale Electrolysis. Electrolyzes were performed using a Princeton Applied Research (PARSTAT 2273) potentiostat. Experiments were carried out in a two-compartment cell with a glassy carbon plate as working electrode (the volume of the solution was 3 mL and active surface area was 1.8 cm2). The reference electrode was an aqueous SCE electrode and the counter electrode was a platinum wire positioned in a bridge separated from the cathodic compartment by a ceramic frit, containing a 2% H2O + 0.1 M NBu4PF6 CH3CN solution. The electrolysis solution was purged with CO2 during 20 min prior to electrolysis. The ohmic drop between working electrode and reference electrodes was minimized by dipping the former one directly in the solution and positioning it close the working electrode. Continuous irradiation during the controlled-potentialelectrolysis was provided by a Newport LCS-100 solar simulator (1 sun irradiation). Wavelengths above 420 nm were selected by using a Schott GG420 long pass filter, whereas IR and low UV were cut off by a 2 cm long glass OS cell filled with deionized water. All CPE experiments have been performed under stirring. Gas Chromatography. Analyses from the gas evolved in the headspace during electrolysis were performed with an Agilent Technologies 7820A GC system equipped with a thermal conductivity detector. CO and H2 production were quantitatively assessed using a CP-CarboPlot P7 capillary column (27.46 m in length and 25 µm internal diameter). Temperature was held at 150 °C for the detector and 34 °C for the oven. The carrier gas was argon flowing at 9.5 mL/min at constant pressure of 0.5 bars. Injection was performed via a 250-µL gas-tight (Hamilton). These conditions allowed for separation of both H2, O2, N2, CO, and CO2. Calibration curves for H2 and CO were determined separately by injecting known quantities of pure gas. For the formic acid analysis, 500 µl sample of the solution was diluted into 9 ml of H2O, filtered and analyzed with a Dionex ICS-1100 Ionic Chromatography System equipped with a IonPac AS15 column (KOH 20 mM as eluent). UV-visible Spectroelectrochemistry Experiments were performed with a home-made setup,28 equipped with a quartz cell (light path length = 0.5 mm or 2 mm). The working electrode was a platinum grid placed directly on the beam, while a Pt wire was used as counter electrode in a bridge separated by a ceramic frit. An aqueous SCE electrode was used as reference. A Metrohm AUTOLAB instrument was used for the electrochemical part, while a UV/vis Varian Cary 60 Spectrophotom6
ACS Paragon Plus Environment
Page 6 of 8
Page 7 of 8 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 eter has been employed for recording the UV-visible spectra (1 spectrum every minute). A 1 mM (or 0.25 mM) solution of 1 (in ca. 5 ml) was used during each experiment and the temperature was kept constant at 20 °C by a thermostat. During experiment, a constant gas flow maintained the saturation of the solution with either Ar, CO2 or CO.
5. 6. 7. 8.
Infrared Spectroelectrochemistry Experiments were performed with a home-made Teflon cell, equipped with two KBr windows (radiation path length 0.5 mm).29 The working electrode was a platinum grid, placed on the infrared beam. An aqueous Ag/AgCl electrode was used as reference (ESCE - EAg/AgCl = 47 mV) and a platinum grid was employed as counter electrode. A Metrohm AUTOLAB instrument was used for the electrochemical part, while a Perkin Elmer 2000 FT-IR Spectrometer was employed for recording the infrared spectra (1 spectrum every minute, 6 scans). A 1 mM solution of 1 (in ca. 5 ml) was used during each experiment, while the solution was preliminary purged with the appropriate gas for 20 minutes and the cell then loaded with the liquid and sealed.
9. 10. 11. 12. 13. 14.
15.
AUTHOR INFORMATION 16. 17.
Corresponding Author *
[email protected]. *
[email protected] 18.
Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
19.
ASSOCIATED CONTENT
20.
Supporting Information. Additional data (CV, CPE, catalytic rate constant estimation for 2 from electrolysis data), DFT calculations. This material is available free of charge via the Internet at http://pubs.acs.org.
21.
22.
ACKNOWLEDGMENT The work described in this paper was supported by a Strategic Research Grant from City University of Hong Kong (7004819) to KC Lau, as well as by Hong Kong University Grants Committee Area of Excellence Scheme (AoE/P-03-08), the National Science Foundation of China (No. 21703034) and the French National Agency for Research (ANR-16-CE05-0010-01). PhD fellowship to C. C. from Université Sorbonne Paris Cité (USPC) is gratefully acknowledged.
23. 24.
25. 26.
REFERENCES 1. 2. 3.
4.
Jhong, H.-R. M.; Ma, S.; Kenis, P. J. Curr. Opin. Chem. Eng. 2013, 2, 191-199. Aresta, M.; Dibenedetto, A.; Angelini, A. Chem. Rev. 2014, 114, 1709-1742. Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Nørskov, J. K.; Jaramillo, T. F. Science 2017, 355, eaad4998. Qiao, J.; Liu, Y.; Hong, F.; Zhang, Chem. Soc. Rev. 2014, 43, 631-675.
27.
28. 29.
Takeda, H.; Cometto, C.; Ishitani, O.; Robert, M. ACS Catal. 2017, 7, 70-88. Loewen, N. D.; Neelakantan, T. V.; Berben, L. A. Acc. Chem. Res. 2017, 50, 2362-2370. Francke, R.; Schille, B.; Roemelt, M. Chem. Rev., ASAP, DOI: 10.1021/acs.chemrev.7b00459. Liu, X.; Xiao, J.; Peng, H.; Hong, X.; Chan, K.; Norskov, J. K. Nat. Commun. 2017, 8:15438. Khezri, B.; Fisher, A. C.; Pumera, M. J. Mat. Chem A 2017, 5, 8230-8246. He, J.; Johnson, N. J.; Huang, A.; Berlinguette, C. ChemSusChem 2018, 11, 48-57. Costentin, C.; Robert, M.; Savéant, J.-M. Acc. Chem. Res. 2015, 48, 2996-3006. Azcarate, I.; Costentin, C.; Robert, M.; Savéant, J.-M. J. Am. Chem. Soc. 2016, 138, 16639-16644. Sampson, M. D.; Kubiak, C. P. J. Am. Chem. Soc. 2016, 138, 1386−1393. Reuillard, B.; Ly, K. H.; Rosser, T. E.; Kuehnel, M. F.; Zebger, I.; Reisner, E. J. Am. Chem. Soc. 2017, 139, 14425-14435. Elgrishi, N; Chambers, M. B.; Wang, X.; Fontecave, M. Chem. Soc. Rev. 2017, 46, 761-796. Nakada, A.; Ishitani, O. ACS Catal., 2018, 8, 354–363. Lam, K.-M.; Wong, K.-Y.; Yang, S.-M.; Che, C.-M. J. Chem. Soc., Dalton Trans. 1995, 1103-1107. Guo, Z.; Cheng, S.; Cometto, C.; Anxolabéhère-Mallart, E.; Ng, S. M.; Ko, C. C.; Liu, G.; Chen, L.; Robert, M.; Lau, T. C. J. Am. Chem. Soc. 2016, 138, 9413-9416. Savéant, J.-M. Elements of molecular and biomolecular electrochemistry: an electrochemical approach to electron transfer chemistry; John Wiley & Sons: Hoboken, NJ, 2006, Chap. 2, pp 80-91. Bhugun, I.; Lexa, D.; Savéant, J.-M. Anal. Chem., 1994, 66, 3994-3996. Costentin, C.; Drouet, S.; Robert, M.; Saveant, J.-M. J. Am. Chem. Soc. 2012, 134, 11235-11242.; 2012, 134, 19949. Azcarate, I.; Costentin, C.; Robert, M.; Savéant, J.-M. J. Phys. Chem. C 2016, 120, 28951-28960. Costentin, C.; Saveant, J.-M. ChemElectroChem 2014, 1, 1226-1236. Raebiger, J. W.; Turner, J. W.; Noll, B. C.; Curtis, C. J.; Miedaner, A.; Cox, B.; DuBois, D. L. Organometallics 2006, 25, 3345-3351. Wong, K.-Y.; Chung, W.-H.; Lau, C.-P. J. Electroanal. Chem. 1998, 453, 161-169. Smieja, J. M.; Sampson, M. D.; Grice, K. A.; Benson, E. E.; Froehlich, J. D.; Kubiak, C. P. Inorg. Chem. 2013, 52, 2484-2491. Chen, Z.; Chen, C.; Weinberg, D. R.; Kang, P.; Concepcion, J. J.; Harrison, D. P.; Brookhart, M. S.; Meyer, T. J. Chem.Comm. 2011, 47, 12607-12609. Gueutin, C.; Lexa, D. Electroanalysis 1996, 8, 10291033. El-Kasmi, A.; Lexa, D.; Maillard, P.; Momenteau M.; Savéant, J-M. J. Am. Chem. Soc. 1991, 113, 1586-1595.
7
ACS Paragon Plus Environment
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 8
SYNOPSIS TOC (Word Style “SN_Synopsis_TOC”).
8
ACS Paragon Plus Environment
