Mechanistic Aspects of the Electrochemical Reduction of Carbon

Chapter 35

Mechanistic Aspects of the Electrochemical Reduction of Carbon Monoxide and Methanol to Methane at Ruthenium and Copper Electrodes

Downloaded by PURDUE UNIV on December 2, 2016 | http://pubs.acs.org Publication Date: November 11, 1988 | doi: 10.1021/bk-1988-0378.ch035

David P. Summers and Karl W. Frese, Jr. Materials Research Laboratory, SRI International, 333 Ravenswood Avenue, Menlo Park, CA 94025 Carbon monoxide and methanol are reduced t o methane. The reduction of carbon monoxide a l s o s u f f e r s d e a c t i v a t i o n by a surface species s i m i l a r t o that f o r carbon d i o x i d e reduction but which forms at lower temperatures. The reduction of carbon monoxide does appear to proceed v i a a path s i m i l a r to that which the reduction of carbon dioxide follows. Rates f o r methanol reduction are extremely v a r i a b l e . Methanol reduction, l i k e carbon dioxide reduction, both increases i n rate with decreasing pH u n t i l the surface becomes blocked with s u r f a c e hydrogen and is a l s o d e a c t i v a t e d by increased temperature. For methanol, deactivation does not occur by the formation of the same surface species. Thus the reduction of methanol i s not b e l i e v e d to proceed v i a a mechanism s i m i l a r to that f o r carbon dioxide or carbon monoxide. Copper electrodes a l s o reduce carbon monoxide to methane. The goals of r e p l a c i n g f i n i t e world n a t u r a l gas reserves and producing f u e l s from inorganic sources and s o l a r energy has been a motivating f o r c e f o r studying the e l e c t r o c h e m i c a l reduction of carbon dioxide to methane, e s p e c i a l l y i n l i g h t of increasing carbon dioxide concentrations i n the atmosphere ( 1-3) . E l e c t r o c h e m i c a l routes are a t t r a c t i v e i n that they are low temperature processes and can be coupled to s o l a r energy sources. Carbon monoxide and methanol are both possible intermediates i n the reduction of carbon dioxide to methane and both are formed as side products i n the reduction of carbon dioxide (2-3 and Kim, J . J . ; Summers, D. P.; Frese, K. W., J r . J . E l e c t r o a n a l . Chem. i n p r e s s ) . Also, whether they are intermediates i n carbon dioxide reduction, the reduction of both carbon monoxide and methanol i s l i k e l y to proceed by s i m i l a r pathways as the reduction of carbon dioxide and may have a s i m i l a r intermediates and rate-determining-steps. The electrochemical reduction of carbon monoxide also o f f e r s a route f o r the production of f u e l s from inorganic sources. For example, carbon monoxide i s formed from c o a l i n g a s i f i c a t i o n 0097-6156/88Α)378-0518$06.00/0 ° 1988 American Chemical Society

Soriaga; Electrochemical Surface Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

35.

CO and Methanol Reduction to Methane

S U M M E R S AND F R E S E

519

schemes. Carbon monoxide i s also l i n k e d to carbon dioxide by the water gas s h i f t r e a c t i o n . The reduction of carbon monoxide to methane i n the gas phase has been extensively studied but i t i s only recently that any reports of the electrochemical reduction of carbon monoxide have appeared (5 6 and Kim, J . J.; Summers, D. P.; Frese, K. W., J r . J . E l e c t r o a n a l . Chem. i n press.) There are also only two reports of the electrochemical reduction of methanol to methane r

(ia) The electrochemical reductions of carbon monoxide and methanol to methane (Equations 1 and 2) have p o t e n t i a l s , under standard c o n d i t i o n s , of +0.019 and +0.390 V vs SCE r e s p e c t i v e l y (or a CO + 6 H Downloaded by PURDUE UNIV on December 2, 2016 | http://pubs.acs.org Publication Date: November 11, 1988 | doi: 10.1021/bk-1988-0378.ch035

CH3OH

+

2

+

H

+ 6 e~ -» CH +

+

2

e"

-*

+

4

CH

4

H0

(1)

2

+

(2)

H0 2

r e v e r s i b l e p o t e n t i a l , under e l e c t r o l y s i s conditions, of -0.161 and + 0.248 V vs SCE r e s p e c t i v e l y at pH 4, 10~ atm methane, 0.1 M CH3OH). S i m i l a r l y , the reduction of carbon dioxide to methane has a p o t e n t i a l , under standard conditions, of -0.061 V vs SCE. At Ru carbon dioxide i s reduced to methane at low overpotentials but with low rates while at copper methane i s formed at high rates but with high overpotentials Q and Kim, J . J . ; Summers, D. P.; Frese, K. W., J r . J . E l e c t r o a n a l . Chem. i n p r e s s . ) . The r e s u l t s presented here show that copper e l e c t r o d e s can reduce carbon monoxide, and ruthenium electrodes can reduce both carbon monoxide and methanol, to methane under conditions s i m i l a r to those f o r the reduction of carbon dioxide to methane. The data also i n d i c a t e that there are s i m i l a r i t i e s i n the mechanism of reduction to methane between carbon dioxide and carbon monoxide. 6

EXPERIMENTAL The e l e c t r o l y s i s of carbon monoxide was conducted under 1 atm carbon monoxide and methanol was e l e c t r o l y z e d under a nitrogen atmosphere. Unless otherwise stated, e l e c t r o l y t e s were aqueous s o l u t i o n s of e i t h e r 0.2 M reagent grade sodium s u l f a t e (Ru electrodes) or 0.5 M Na2HP04 at pH 7.6 (Cu e l e c t r o d e s ) . A l l solutions were made with d i s t i l l e d deionized water ( M i l l i p o r e ) . In the experiments with ruthenium the pH was held constant by the addition of reagent grade s u l f u r i c a c i d using a pH c o n t r o l l e r and a syringe pump except for the data i n Table II i n which the pH v a r i e d from 4 to 5.5. All e l e c t r o l y s i s experiments using Ru electrodes were conducted at ~60°C f o r 5-6 hrs with reagent grade Na S04 unless otherwise noted while a l l Cu experiments were conducted at room temperature and required no pH c o n t r o l . Ru electrodes were prepared as previously described by p l a t i n g Ru metal onto spectroscopic carbon rods, except f o r the electrode used f o r Auger analysis (before and a f t e r carbon dioxide reduction) which was p l a t e d on T i (2.) . Cu electrodes were prepared from Cu f o i l as previously described (Kim, J . J . ; Summers, D. P.; Frese, K. W., J r . J . E l e c t r o a n a l . Chem. i n press.). Each entry i n the tables and figures was obtained on d i f f e r e n t days with the electrode kept i n ordinary laboratory a i r overnight between runs. 2


520

E L E C T R O C H E M I C A L SURFACE SCIENCE

E l e c t r o l y s e s were performed using an Aardvark model PEC-1 p o t e n t i o s t a t and Keithley model 616 d i g i t a l electrometer, and a microcomputer data a c q u i s i t i o n system f o r measuring current as a function of time. A two compartment c e l l was employed to avoid o x i d a t i o n of the carbon d i o x i d e r e d u c t i o n p r o d u c t s . All e l e c t r o l y s e s were c a r r i e d out using a closed system as previously described (2.) . The c i r c u l a t e d gas was bubbled through the solution causing gentle a g i t a t i o n . The temperature was c o n t r o l l e d by placing the e n t i r e system with the exception of the c i r c u l a t i n g pump i n a heated enclosure. In a l l cases e l e c t r o l y t e volumes were 50 ml. Samples were analyzed on a Gowmac model 750 gas chromatograph with a FID detector. Samples were c o l l e c t e d f o r CH analysis from the gas phase over s o l u t i o n . A column of Porapak Q (6ft) followed by Porapak R (3ft) at 50 C was used f o r CH4/CO analysis. Auger spectra were obtained with a Perkin-Elmer PHI Auger spectrometer. Auger samples were removed under p o t e n t i a l control, r i n s e d with water, and allowed to dry before mounting on sample holder.


4

Results and Discussion Carbon Monoxide Reduction at Ruthenium. Carbon monoxide can be reduced to both methane and methanol under c o n d i t i o n s n e a r l y i d e n t i c a l to those f o r the reduction of carbon dioxide (Table I, A l l experiments, using one electrode, are presented i n the order they were performed). The rate and faradaic e f f i c i e n c y of methane formation from carbon monoxide appears lower than from carbon d i o x i d e . For comparison, at 60 °C, -0.545 V vs SCE, pH 4, and 60 °C a t y p i c a l rate f o r carbon dioxide reductions i s 1.5 χ 10~ mol cm"^ h r " with a faradaic e f f i c i e n c y of 20-30 %. Carbon monoxide i s 40 times less soluble i n water than carbon dioxide (at 1 atm) . However the surface coverage w i l l depend on the p a r t i a l pressure which i s the same (1 atm) f o r both carbon monoxide and carbon d i o x i d e . A d d i t i o n a l l y , carbon monoxide probably adsorbs more strongly than carbon dioxide and so should have a higher coverage than carbon d i o x i d e at the same p a r t i a l pressure. Indeed a c l e a r anodic s t r i p p i n g peak can be seen at +0.15 V vs SCE (pH 3) f o r carbon monoxide adsorbed on the Ru surface at 1 atm (60 °C) while exposure to carbon dioxide produces no peak. Since these rates are too slow to be d i f f u s i o n controlled, transport l i m i t a t i o n cannot account f o r the differences i n rate. It i s not known i f the lower rate from carbon monoxide i s simply due to the blockage of surface hydrogen s i t e s necessary f o r hydrogénation of intermediates to methane or i f there i s a fundamental d i f f e r e n c e i n rate. The rate of methane formation from carbon dioxide decreases (along with the t o t a l current) when carbon monoxide i s added (Table II) consistent with the blocking of the surface by more-slowly-reduced carbon monoxide. 7

1

When the temperature i s raised to 75 °C a decrease i n the rate of carbon monoxide reduction i s observed with a p a r a l l e l decrease i n the faradaic e f f i c i e n c y . When the electrode i s used a second time f o r carbon monoxide reduction at 60 °C, a f t e r i t was used f o r e l e c t r o l y s i s at 75 °C, (last entry i n Table I) i t shows considerable deactivation. The reduction of carbon dioxide also shows a s i m i l a r



j b

-2

-0.54 -0.54 -0.54 -0.54

Ε 0 cm ) (V vs SCE)

845 538 101 141

(UA 60 60 75 60

Τ (°C)

c

1.3 2.1 5.8 N.D.

Eff (%) -2

d

6.6 7.0 3.7 N.D.

Rate (mol cm

Methane 8

c

Methanol

1

d

2

8

N.M. ~5 -15 N.M.

N.M. -25 -14 N.M.

1

Eff R a t e χ 10 χ 10 h r " ) (%) (mol cm" h r " )

a

Rate and Faradaic E f f i c i e n c y of C H 4 and C H 3 O H formation from CO at Electroplated Ru Electrodes

A l l e l e c t r o l y s e s were i n 0.2 M reagent grade Na2S04. Average current density based on geometrical area. Faradaic e f f i c i e n c y f o r methane formation. Average rate of methane formation.

3 3 4 4

6.3 20.3 18.3 *6.0

a) b) c) d)

PH

Time (hr)

Table I.


522


Table I I . The E f f e c t of CO on the Electrochemical Reduction of to Methane at Electroplated Ru Electrodes

CO2

a

CO added (ml)

c

(UA

cm )

2

107 72 79 63

0 35 0 50

Eff (%)

8

Rate χ 10 (mol cm" hr" )

-2

1

d

18.6 14.7 20.2 12.4

9.3 4.9 7.5 3.6

a) A l l e l e c t r o l y s e s times are 5-6 hrs i n 0.2 M reagent grade Na2S0 at 60°C and -0.545 V vs SCE with an i n i t i a l pH of 4. CO was added d i s p l a c i n g an equal volume of CO2 from the 1.3 1 CO2 reservoir (see experimental). b) Average current density based on geometrical area. c) Rate of methane formation. d) Faradaic e f f i c i e n c y for methane formation.


4

d e a c t i v a t i o n though, i n t h i s case, a decrease i n the rate i s not seen u n t i l higher temperatures, ~90 °C (2) . There are s i m i l a r i t i e s between d e a c t i v a t i o n of Ru by carbon dioxide and by carbon monoxide. The Auger electron spectrum of the surface of the electrode (Ru on Ti) deactivated by carbon dioxide reduction at 90 °C i s very d i f f e r e n t from that taken before e l e c t r o l y s i s (Figure 1). The primary signals f o r Ru and C overlap but by comparing the s i z e of the secondary Ru peaks at s l i g h t l y lower energy i t can be seen that the spectrum taken before deactivation shows mostly a Ru s i g n a l while the spectrum taken a f t e r deactivation shows mostly a C s i g n a l and v i r t u a l l y no Ru s i g n a l . If the surface i s subjected to A r sputtering a l l s i g n a l s attenuate with respect to those f o r Ru i n d i c a t i n g that we are observing a surface carbon species. This has been interpreted as the formation of e i t h e r g r a p h i t i c carbon or d e a c t i v a t i n g C H species on the electrode surface (2) . The Auger e l e c t r o n spectrum of the surface of the electrode deactivated by carbon monoxide reduction at 75 °C has a spectrum nearly i d e n t i c a l to that of the electrode deactivated during carbon dioxide reduction, showing the presence of enough surface carbon to almost t o t a l l y block the Ru s i g n a l (Figure 2) . Ar sputtering restores the spectrum of a clean Ru surface, again i n d i c a t i n g surface carbon. This implies that the reduction of carbon monoxide proceeds v i a d i s s o c i a t i o n to surface carbon atoms just as the reduction of carbon dioxide does. However, the temperature of deactivation i s lower f o r carbon monoxide than f o r carbon dioxide i n d i c a t i n g that i t i s easier to s p l i t the carbon monoxide to carbon. +

n

m

+

Methanol Reduction at Ruthenium. The reduction of methanol to methane does occur as shown by the data i n Table I I I . The data f o r each electrode are presented i n the order that they were c o l l e c t e d . Rates can be higher f o r methanol reduction compared to carbon dioxide reduction though faradaic e f f i c i e n c i e s are lower. Unlike carbon dioxide reduction, the rate of methane formation i s extremely


35.


S U M M E R S AND FRESE

After Carbon Dioxide Reduction at 60 °C

| 1 1 I

523

After Carbon Dioxide Reduction at 90 °C

• S

I

Ru

LU R

u

Ru

•σ


Ru (+trace C) C (+trace Ru) 1 300

200

1

1 200

1 300

Electron Energy (eV)

Figure 1. Auger e l e c t r o n spectrum of the surface of a Ru electrode before and a f t e r deactivation by reduction of carbon dioxide at higher temperatures (-90 °C i n 0.2 M Na2S0 at pH 4 and -0.545 V vs SCE). 4

4\rwii Ν

Ru

LU p

Ru

After Reduction of Carbon Monoxide at 75 °C

K

»

Ru (+ trace C) 1 200

300

After Reduction of Methanol at 90 °C C (+ trace Ru) 1 200

300 Electron Energy (eV)

Figure 2. Auger e l e c t r o n spectrum of the surface of two Ru electrodes a f t e r deactivation by reduction of carbon monoxide and methanol at higher temperatures (75 and 90 °C respectively i n 0.2 M Na S0 at pH 4 and -0.545 V vs SCE ). The presence of Κ on the surface must r e s u l t from the adsorption of K ions present as an impurity i n the e l e c t r o l y t e . 2

4

+


524


Table I I I . Rate and Faradaic E f f i c i e n c y of C H 4 Formation from at Electroplated Ru Electrodes

CH3OH

a

experiment

[CH3OH]

pH


electrode #1 1 75 2 75 3 75 4 75 electrode #2 1 75 2 1 3 500 4 200 5 200 6 200 7 200 electrode #3 1 1 2 75 3 75 4 75 a) b) c) d)

b

j Ε Τ (LA cm" ) (V vs SCE) (°C) 2

(mM)

c

d

8

Eff Rate χ 10 (%) (mol cm" h r " ) 2

4 4 4 4

255 220 408 147

-0.,54 -0..54 -0.,54 -0..54

60 70-75 90 60

5.,0 6.2 5.,0 0.,8

23. 6 25. 5 38.,0 2.2

4 4 4 4 5.2 3 1.7

255 178 165 121 60 312 929

-0..54 -0..54 -0..54 -0,.54 -0,.61 -0,.48 -0 .41

60 60 60 60 60 60 60

1..9 1..8 1..6 1,.6 0,.5 1,.8 2 .5

9..0 5..9 4..8 3..6 0,.5 10..3 2,.5

4 3 4 4

485 907 146 296

-0 .54 -0 .54 -0 .54 -0 .70

60 60 22 60

0 .3 0 .3 0 .3 0 .8

2 .5 4 .6 0 .7 4 .7

1

A l l e l e c t r o l y s e s were i n 0.2 M reagent grade Na2S04« Average current density based on geometrical area. Faradaic e f f i c i e n c y f o r methane formation. Average rate of methane formation.

v a r i a b l e from one electrode to the next. However the a c t i v i t y of each electrode i s r e l a t i v e l y constant from experiment to experiment. This i n d i c a t e s that the mechanism f o r methanol reduction i s s e n s i t i v e to some, as yet unknown, surface condition that does not a f f e c t the reduction of carbon dioxide which shows much more reproducible rates. Like the reduction of carbon dioxide to methane, the rate of reduction of methanol to methane increases with temperature (see experiments 1-4, e l e c t r o d e #1 i n Table III) (2.) . Unlike the reduction of carbon dioxide, the f a r a d a i c e f f i c i e n c y does not increase i n d i c a t i n g that the formation of methane has an a c t i v a t i o n energy s i m i l a r to the competing process ( H 2 formation). Like carbon monoxide and carbon dioxide reductions, the r e a c t i o n deactivates when run at excessive temperature. However, i n the case of methanol reduction, even at 90 °C the rate i s s t i l l i n c r e a s i n g i n d i c a t i n g that methanol reduction i s the least prone to deactivation. There i s some deactivation since e l e c t r o l y s i s at 60 °C, a f t e r e l e c t r o l y s i s at 90 °C, leads to s i g n i f i c a n t l y reduced rates (Table III) . An electrode deactivated during methanol reduction does not show the presence of a large amount of surface carbon (Figure 2) i n d i c a t i n g a d i f f e r e n t deactivating species. The lack of surface carbon and the higher temperature of deactivation implies that, contrary to carbon monoxide and carbon dioxide reduction, methanol reduction does not involve d i s s o c i a t i o n to surface carbon. At 60° C the reduction of


35.

SUMMERS AND FRESE

525


methanol also shows a slow deactivation from one experiment to the next (see below) that i s not seen f o r carbon dioxide reductions at the same temperature (2.) . Species such as COH j are possible poisons ac


(£) . The f i r s t four experiments with electrode #2 (Table III) were run at d i f f e r e n t methanol concentrations. Dropping the concent r a t i o n from 75 mM to 1 mM and from 500 mM to 200 mM does lead to a decrease i n the rate of reduction, but less than would be expected f o r such a large change i n concentration. Also, there i s a steady drop i n the rate across a l l the experiments i n d i c a t i n g a slow deactivation of the electrode so that not a l l of the decrease i s due to concentration changes. In increasing the concentration from 1 mM to 500 mM there i s even a small decrease i n rate. Thus there does not appear to be a strong dependence on methanol concentration implicating the importance of a surface chemical step. The influence of pH i s more prominent than e i t h e r the e f f e c t s of d e a c t i v a t i o n or methanol concentration. The data (experiments 4-7 on electrode #2, Table III) show that there i s an optimum pH f o r methane formation from methanol with the rate i n c r e a s i n g with greater a c i d i t y u n t i l a maximum at pH ~3 i s reached whereupon the rate begins to decrease (Figure 3) . This i s nearly i d e n t i c a l to r e s u l t s seen (Figure 4) f o r carbon dioxide reduction to methane (2.) . This i s interpreted as i n d i c a t i n g that the reaction proceeds at a f a s t e r rate as the hydrogen coverage increases due to f a s t e r hydrogénation of surface intermediates, but that excessive surface hydrogen coverage blocks carbonaceous intermediates (hence the decrease at lower pH). Again the importance of a surface chemical step i s implied. Carbon Monoxide Reduction at Copper. At copper electrodes carbon monoxide i s thought to be an intermediate i n the reduction of carbon d i o x i d e and i s formed as the major product with n i t r i c a c i d pretreated electrodes (2. and Kim, J . J . ; Summers, D. P.; Frese, K. W., J r . J . E l e c t r o a n a l . Chem. i n press.). As the data i n Table IV i n d i c a t e s , methane can be formed by carbon monoxide reduction at

Table IV. Rate and Faradaic E f f i c i e n c y of CH

4

Formation from CO at

Cu Electrodes Electrolyte (0.2 M)

pH

Na HP0 Na HP0 Na HP0 Na HP0 Na HP0 KHC0

6..1 6..1 7..3 7,.4 7,.3 9 .5

2

4

2

4

2

4

2

4

2

3

4

a

j (niA cm" ) 2

5 14 5 10 10 26

Ε (V vs SCE) -1..53 -1..72 -1..8 -1..83 -2,.0 -1,.98

Eff (%)

b

3..64 0..54 0..61 0..91 2,.1 1,.5

a. Average current density based on geometrical b. Faradaic e f f i c i e n c y f o r methane formation. c. Average rate of methane formation.

c

6

Rate χ 10 (mol cm" h r " ) 2

1..13 0..46 0..19 0,.56 1,.33 2,.43

area.


1



526

Figure 4. The e f f e c t of pH on the average rate of methane formation from carbon dioxide. In 0.2 M Na2S0 at 60 °C and at constant over p o t e n t i a l (-0.545 V vs SCE at pH 4). 4



35. SUMMERS AND FRESE

CO and Methanol Reduction to Methane527

copper electrodes under similar conditions to those at which methane is formed from carbon dioxide. Just as copper gives higher current densities but poorer overvoltages for carbon dioxide reduction than ruthenium, the reduction of carbon monoxide at copper electrodes shows higher rates and larger overvoltages than at ruthenium electrodes. Again, as with ruthenium electrodes, the reduction of carbon monoxide is slower than the reduction of carbon dioxide along with lower faradaic efficiencies (the major other product is hydrogen) (Kim, J . J . ; Summers, D. P.; Frese, K. W., Jr. J • Electroanal. Chem. in press.). Total currents are also lower, consistent with the blocking of the surface with carbon monoxide. However, in this potential region the reduction of carbon dioxide is diffusion controlled and the ratio of the rates of carbon dioxide reduction and carbon monoxide reduction (30-60 in favor of carbon dioxide) is within experimental error of the ratio of solubilities (-40 in favor of carbon dioxide). Thus lower solubility can not be dismissed as the source of lower rates for carbon monoxide reduction. Acknowledgments The Authors acknowledge the support of the Gas Research Institute. Literature Cited 1) Catalytic Activation of Carbon Dioxide; W. M. Ayers, ed.; ACS Symposium Series No. 363; American Chemical Society: Washington, DC, 1988 2) Summers, D. P.; Frese, K. W., Jr. Langmiur 1988,4,51. 3) Hori, Y.; Kikuchi, K.; Murata, Α.; Suzuki, S. Chem. Lett. 1986,897. 4) Cooke, R. L . ; MacDuff, R. C.; Sammells, A. F. J. Electrochem. Soc. 1987,134,1873. 5) Summers, D. P.; Frese, K. W., Jr. J. Electrochem. Soc. 1988,135,264. 6) Hori, Y.; Murato, Α.; Takahashi, R.; Suzuki, S. J. Amer. Chem. Soc. 1987,109,5022. 7) Stenin, V. F.; Podlovchenko, Β. I. Elektrokhimiya 1967,3,481. 8) Yasil'ev, Y. B.; Bagotsky, V. S. J. Appl. Electrochem. 1986,16,703. 9) Summers, D. P.; Frese, K. W., Jr. SRI Annual Report, April 30, 1987, GRI Contract No. 5083-260-0922, SRI Project PYU 7142. RECEIVED May 17, 1988


Mechanistic Aspects of the Electrochemical Reduction of Carbon

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