electrocatalytic reduction of co2 to co by · s1 electrocatalytic reduction of co2 to co by...

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Electrocatalytic reduction of CO2 to CO by

polypyridyl ruthenium complexes

Zuofeng Chen, Chuncheng Chen, David R. Weinberg, Peng Kang, Javier J. Concepcion, Daniel

P. Harrison, Maurice S. Brookhart, and Thomas J. Meyer*

Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, NC, 27599,

USA

Supporting Information

300 400 500 600 700 800

0.0

0.5

1.0

1.5

2.0 (a)

Abs

orba

nce

Wavelength (nm)

300 400 500 600 700 800

0.0

0.5

1.0

1.5

2.0 (b)

Abs

orba

nce

Wavelength (nm)

Figure S1. (a) UV-visible spectra of 50 μM [Ru(tpy)(bpy)(S)]2+ 1 in H2O (red line, as

[Ru(tpy)(bpy)(H2O)]2+), CH3CN (blue line, as [Ru(tpy)(bpy)(NCCH3)]2+), and 10% (v/v)

Electronic Supplementary Material (ESI) for Chemical CommunicationsThis journal is © The Royal Society of Chemistry 2011

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H2O/CH3CN (green line, as [Ru(tpy)(bpy)(NCCH3)]2+). Note the blue and green lines are

overlaid. (b) as in (a) with [Ru(tpy)(Mebim-py)(S)]2+ 2.

0.0 -0.5 -1.0 -1.5 -2.0

0

10

20

30

10 mV/s 20 mV/s 50 mV/s 100 mV/s 200 mV/s 500 mV/s 1000 mV/s 2000 mV/s

(a)

Nor

mal

ized

I (μ

A/(

mV

/s)1

/2)

E (V vs. NHE)

0.0 -0.5 -1.0 -1.5 -2.0

0

20

40

60(b)

10 mV/s 20 mV/s 50 mV/s 100 mV/s 200 mV/s 500 mV/s 1000 mV/s 2000 mV/sN

orm

aliz

ed I

(μA

/(m

V/s

)1/2 )

E (V vs. NHE)

Figure S2. Scan rate normalized (i/υ1/2) cyclic voltammograms of 1 mM 1 (a) and 2 (b) in 0.1 M

nBu4NPF6/CH3CN under CO2 at different scan rates. Insets (without normalization) as in the

main figures but under Ar, and plots of id for the first reduction peak current vs the square root of

the scan rate (υ1/2). Electrode, glassy carbon.

The behavior of complexes 1 and 2 under Ar is governed by Randles-Sevcik equation, id =

0.4463nFA[Ru](nFυDRu/RT)1/2. From the plots of id vs υ1/2 in the insets and Randles-Sevcik

equation, DRu, the catalyst diffusion coefficient, was calculated to be ~5.2 × 10–6 cm2/s.

0 10 20 30 40 500

20406080

100

ν1/2 (mV1/2/s1/2)

i d (

μA)

0 10203040500

20

40

60

80

ν1/2 (mV1/2/s1/2)

i d (μ

A)

0.0 -0.5 -1.0 -1.5-100

-50

0

50

100

150

I (μA

)

E (V vs. NHE)0.0 -0.5 -1.0 -1.5

-100

-50

0

50

100

150

I (μA

)

E (V vs. NHE)


S3

0.0 -0.5 -1.0 -1.5

0

50

100

150

200

0 2 4 6 8 10 120.0

5.0x103

1.0x104

1.5x104

CO2 (%)

i cat2 (

μA2 )

20% CO2

10% CO2

5% CO2

2% CO2

1% CO2

0.5% CO2

0% CO2

(a)

I (μA

)

E (V vs. NHE)

0.0 -0.5 -1.0 -1.5

0

100

200

300

400

500

600

0 5 10 15 20 25 300

1x105

2x105

3x105

CO2 (%)

i cat2 (

μA2 )

20% CO2

10% CO2

5% CO2

2% CO2

1% CO2

0.5% CO2

0% CO2

(b)

I (μA

)

E (V vs. NHE)

Figure S3. Cyclic voltammograms of 1 mM catalyst 1 (a) and 2 (b) in 0.1 M nBu4NPF6/CH3CN

with increasing concentrations of CO2 in CO2/Ar mixtures. Insets show plots of icat2 at the

catalytic peak potential at Ep,c = −1.52 V (a) and −1.46 V (b ) vs [CO2] in CO2/Ar mixtures.

Electrode, glassy carbon; scan rate, 100 mV/s.

0

20

40

60

80

100

12001400160018002000

Ru(t)(b)Ru(t-)(b-)Ru(t)(b)+CO2Ru(t-)(b-)+CO2

Wavenumber (cm–1

)

Tra

nsm

ittan

ce (%

)

Figure S4. FT-IR spectra of 1 (black line, as [(tpy)(bpy)RuII(S)]2+ ), doubly reduced 1 (blue line,

as [(tpy–)(bpy–)RuII(S)]0), and doubly reduced 1 followed by addition of CO2 (red line, as

[(tpy)(bpy)RuII(CO2)2–]0). Doubly reduced 1 was generated by electrolysis at a vitreous carbon

electrode with potential held at –1.34 V vs NHE (0.1 M nBu4NPF6, CH3CN) under Ar


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atmosphere. Upon generation of doubly reduced 1, the headspace of the electrolysis cell was

gently purged with CO2. The appearance of a ν(COO–) stretch at 1675 cm−1 in the spectrum

shown in red is consistent with formation of the metallocarboxylate intermediate

[(tpy)(bpy)RuII(CO22–)]0 formed by addition of CO2 to doubly reduced 1. For comparison,

addition of CO2 to a solution of 1, green spectrum, results in no spectral difference.

0.0 -0.5 -1.0 -1.5

0

50

100

150

200

0.0 0.2 0.4 0.6 0.8 1.00

50

100

150

200

250

i cat (

μA)

[1] (mM)0.05 mM0.1 mM0.2 mM

0.5 mM

1 mM(a)

I (μA

)

E (V vs. NHE)

0.0 -0.5 -1.0 -1.5

0

200

400

600

0.0 0.2 0.4 0.6 0.8 1.00

200

400

600

i cat (

μA)

[2] (mM)

1 mM

0.5 mM

0.2 mM

0.1 mM0.05 mM

(b)

I (μA

)

E (V vs. NHE)

Figure S5. Cyclic voltammograms at different concentration of 1 (a) and 2 (b) in 0.1 M

nBu4NPF6/CH3CN under CO2. Insets show plots of icat at the catalytic peak potential at Ep,c= –

1.52 V (a) and –1.46 V (b) vs [Ru]. Electrode, glassy carbon; scan rate, 100 mV/s.

0 200 400 600 8000.0

0.5

1.0

1.5

2.0

Intercept = 0.11

slope = 2.7 * 10-3

1/k ca

t (s)

1/[CO2] (1/M)


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Figure S6. As in Figure 3, plot of 1/kcat vs 1/[CO2]. kcat was calculated from eq 3a. According to

eq 5b, from the intercept (1/kD), kD = 9 s–1 and from the intercept-to-slope ratio (kCO2/k–D), kCO2/k–

D = 40 M–1.

0 50 100 150 2000

100

200

300

400

500(a)

I (μA

)

0% CO2

0.5% CO2

1% CO2

2% CO2

5% CO2

10% CO2

20% CO2

30% CO2

100% CO2

t (s)

0 5 10 15 20 25 300

10000

20000

30000(b)

[CO2]

i cat2

(μA

2 )

0 200 400 600 8000.0

0.2

0.4

0.6

0.8(c)

Intercept = 0.039

slope = 9.2 * 10-4

1/[CO2] (1/M)

1/k ca

t (s)

Figure S7. (a) Controlled potential electrolysis of 1 mM 2 in 0.1 M nBu4NPF6/CH3CN

(unstirred) with increasing concentrations of CO2 in CO2/Ar mixtures at a glassy carbon

electrode (0.071cm2) at –1.46 V. (b) Plot of icat2 (background current subtracted) vs [CO2] in

CO2/Ar mixtures. (c) Plot of 1/kcat vs 1/[CO2]. kcat was calculated from eq 3a. According to eq


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5b, from the intercept (1/kD), kD = 26 s–1 and from the intercept-to-slope ratio (kCO2/k–D), kCO2/k–D

= 43 M–1.

0 1 2 3 4 50

50

100

150

3 00 40 0 50 0 6 0 0 7 0 0Wavelength (nm)

Nor

mal

ized

abs

orba

nce

(a)

I (μA

)

t (h)

0 5 10 15 20 25 30

0

10

20

30

40

50

60(b)

1 2 3

01

2

3

45

t (min)

Inte

nsi

ty (

a.u

.)

O2

N2

CO

CO2

t (min)

Inte

nsity

(a.

u.)

Figure S8. (a) Controlled potential electrolysis of 1 mM 1 at –1.52 V in CO2 saturated 0.1 M

nBu4NPF6/CH3CN at a glassy carbon electrode (0.071 cm2). The solution was stirred during

electrolysis. Inset shows UV-visible spectra of 1 before (blue line) and after (red line)

electrolysis and after addition of acid at the end of the electrolysis period (green line). Note the

slight decrease at 455 nm and increase at 485-600 nm in the red line consistent with possible

anation in the shift of the lowest energy MLCT absorption to lower energy. (b) The

corresponding gas chromatogram after electrolysis. The inset shows a magnified view in the

initial 3 min.

Over extended electrolysis periods, as HCO3−/CO3

2− builds up in solution, the electrolysis

current gradually falls due to the buildup of a precipitate on the electrode with the solution color

gradually turning to purple. These observations are consistent with precipitation of the catalyst as

the CO32− salt and coordination of HCO3

−/CO32− to give the carbonate or bicarbonate complexes

possibly as [Ru(tpy)(bpy)(OC(O)O)]0 or [Ru(tpy)(bpy)(OC(O)OH)]+. Addition of dilute HClO4


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or HNO3 regenerates the UV-visible spectrum of the catalyst quantitatively, Figure S8a inset,

and restores electrocatalytic activity.

Figure S9. (a) 13C NMR spectrum showing resonances for CO32−/HCO3

− and HCOO− as co-

products after electrolysis under CO2 (Note that normal CO2 and not 13C enriched CO2 was used

in the13C NMR experiment.). In a control experiment, there was no evidence in 13C spectra for

CO32−/HCO3

− or HCOO− resonances before electrolysis under the same experimental conditions.

(b) 1H NMR spectrum showing the HCOO− resonance obtained after electrolysis under CO2. A

CO32−/HCO3

−

HCOO−

(a)

HCOO−

(b)


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vitreous carbon electrode was used for the electrolysis at −1.52 V vs NHE in solution 1 mM in 1

in 0.1 M nBu4NPF6/CH3CN.

0.0 -0.5 -1.0 -1.5

0

50

100

150

200

250I (

μA)

E (V vs. NHE)

Figure S10. Cyclic voltammograms of 1 mM [Ru(tpy)(bpy)(Cl)]+ (Inset shows its structure) in

0.1 M nBu4NPF6/CH3CN under Ar (red line) and CO2 (blue line). The dotted lines are the CVs of

[Ru(tpy)(bpy)(NCCH3)]2+ under the same conditions for comparison. Electrode, glassy carbon;

scan rate, 100 mV/s. Under Ar, Ep,c for the first tpy reduction of [Ru(tpy)(bpy)(Cl)]+ is shifted by

−0.18 V relative to [Ru(tpy)(bpy)(NCCH3)]2+ largely due to the decrease in charge. Following

tpy reduction, the bpy-based reduction occurs at the same potential as in

[Ru(tpy)(bpy)(NCCH3)]2+ pointing to loss of Cl− and substitution by CH3CN solvent.

+

N

N

NClN

NRu


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0.0 -0.5 -1.0 -1.5

0

40

80

120

I (μA

)

E (V vs. NHE)

Figure S11. Cyclic voltammograms of 1 mM [Ru(Mebimpy)(bpy)(S)]2+ (Inset shows its

structure) in 0.1 M nBu4NPF6/CH3CN under Ar (red line) and CO2 (blue line, 10 successive scan

cycles). Electrode, glassy carbon; scan rate, 100 mV/s.

0.0 -0.5 -1.0 -1.5

0

20

40

60

I (μA

)

E (V vs. NHE)

Figure S12. Cyclic voltammograms of 1 mM 1 in 0.1 M nBu4NPF6/PC under Ar (red line) and

CO2 (blue line, 25 successive scan cycles). Electrode, glassy carbon; scan rate, 100 mV/s.

2+

N

N

N

N

N

NN

Ru

S


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DFT calculations: The geometric optimization and electronic structures were obtained at the

ub3lyp/lanl2dz level with the Gaussian 03 (G03) program package.1

Figure DFT1. Singly occupied molecular orbital (SOMO) (left) and total spin density (TSD)

(right) for 1e− reduced [Ru(tpy)(bpy)(NCCH3)]+. Both SOMO and TSD are mainly spread over

the tpy ring, indicating that the first reduction occurs at the tpy ligand.

Figure DFT2. HOMO-1(left), HOMO (middle) orbitals, and TSD (right) for the 2e− reduced

complex 3[Ru(tpy)(bpy)(CH3CN)]0. The 2e− reduced [Ru(tpy)(bpy)(CH3CN)]0 complex has two

possible electronic configurations: (1)The two added electrons are paired and both are distributed

over the tpy ring, i.e., the complex is in the singlet state, 1[Ru(tpy)(bpy)(CH3CN)]0. (2) The


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second electronis added to the bpy ring, i.e., the complex is in the triplet state,

3[Ru(tpy)(bpy)(CH3CN)]0. The calculated energy for the electron-paired singlet complex is ~85.1

kcal/mol higher than the triplet favoring the triplet as the ground state. The electronic structures

here indicate that, upon two electron reduction, the electrons enter π* orbitals of the tpy ring and

the bpy ring, respectively. Accordingly, the spin density of 3[Ru(tpy)(bpy)(CH3CN)]0 is

distributed on both tpy and bpy rings.

Figure DFT3. HOMO for [Ru(tpy)(bpy)(CO2)]0. In contrast to [Ru(tpy)(bpy)(NCCH3)]

0

complex, the singlet state of the 2e− reduced [Ru(tpy)(bpy)(CO2)]0 complex is more stable than

the triplet by ~9 kcal/mol. The HOMO for [Ru(tpy)(bpy)(CO2)]0 is located at the coordinated

CO2, consistent with the formulation [RuII(tpy)(bpy)(CO22−)]0 and the internal 2e− transfer

reaction shown in eq 1c with internal electron from the tpy and bpy ligands to the coordinated

CO2 when CH3CN is substituted by CO2 in the 2e− reduced complex.


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Negative positive

Figure DFT4. The electrostatic potential mapped on the surface of molecular electron density

(isodensity, 0.004 e−3) of twice reduced Ru-NCCH3 (left) and Ru-CO2 (right) complexes.

Electrons are distributed on tpy and bpy rings (red area) of Ru-NCCH3 (left), but when the

CH3CN ligand is replaced by CO2 (right), the electrostatic potential became negative on the CO2

ligand, indicating the electron move from the tpy and bpy rings to CO2 after the substitution of

CH3CN by CO2.

Reference

(1) Gaussian 03 Revision E01. Frisch, M. J.; Trucks, G. W.; Schlegel, H.B.; Scuseria, G. E.;

Robb, M. A.; Cheeseman, J. R.; Montgomery, Jr., J.A.; Vreven, T.; Kudin, K. N.; Burant, J. C.;

Millam, J. M.; Iyengar, S. S.;Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.;

Rega,N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.;Fukuda, R.;


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Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao,O.; Nakai, H.; Klene, M.; Li, X.;

Knox, J. E.; Hratchian, H. P.; Cross, J.B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.;

Stratmann, R.E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.;Ayala, P.

Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J.J.; Zakrzewski, V. G.; Dapprich, S.;

Daniels, A. D.; Strain, M. C.;Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;

Foresman,J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;Stefanov, B. B.;

Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,

M. A.; Peng, C. Y.;Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,W.;

Wong, M. W.; Gonzalez, C.; and Pople, J. A.; Gaussian, Inc.,Wallingford CT, 2004.


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