Change in the product selectivity for the electrochemical CO2 reduction by adsorption of sulfide ion on metal electrodes

ELSEVIER

Journal of Electroanalytical Chemistry 434 (1'497) 239 243

Short communication

Change in the product selectivity for the electrochemical CO, reduction by adsorption of sulfide ion on metal electrodes K. Hara, A. Tsuneto, A. Kudo, T. Sakata " I~/mrtmem of l'~lectronic Chemisl~3'. hlterdisc~plina~, Graduates School ~f Science and Engineeri.g, Tokyo h~stitute of Technolog); 4259 Nagatsuta, Mi&)ri°ku, ¥okohmna 226, Jalnm Received I Oclol~t 104J6;revi~ed 2 January 1997

Abstrtlel

The et'fee~sof adsorbed sulfide ion (S ~=) formed on metal electrodes in the elec,~ochemicalreduction of CO~ under high pres,~ure were investigated. The selectivity for the reduction products changed markedly with the NazS treatment of metal electrodes, in the case of a Cu electrode, the faradaic efficiency tbr methane Ibrmation decreased rentarkably from 36% to 2%, whereas that for C2 comi~nents (ethylene and ethanol) increased from 13% to 25% due to the Na2S treatment. The faradaic efficiencies for formic acid and hydrogen formation also increased with the treatment. A similar change in the product selectivity was also observed in the electrolysis with addition of Na~S into the aqueous electrolyte. A study of the current-potential curve suggested that adsorbed sulfide ion prevented the formation ol adsorbed CO and thus changed the product selectivity. © 1997 Elsevier Science S,A. geywar~ls: Reduction; Cation dioxide; Sulfide ion; Metal electrodes; Selectivity

1. Introduction Electrochemical CO 2 reduction under I arm on various metal electrodes in aqueous electrolytes has been investigated in the last decade. It was found that the selectivity of formation of the reduction products depends markedly upon the kind of metal [I ~ !4]. The result that the catalytic activity of metals depends markedly upon the location in the periodic table, suggests that the electron configuration of a metal determines the product selectivity. Moreover, it is well-known that the product selectivity changes with the electrolysis conditions such as electrode potential (current density), reaction temperature, and kind of electrolyte. We observed that the product selectivity of CO 2 reduction changed with the CO2 pressure in the cases of group 8 to 10 metal electrodes, which produce hydrogen predominantly by the reduction of water in the electrochemical CO 2 reduction under Iatm [15-18]. The influence of adsorbed species formed on the surface of the electrode is one of the factors that changes the product selectivity, The

*

Corresponding author.

0022-0728/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. Pll S0022-0728(97)00045-4

fi~rmation of adsorbed CO us ~m intermediate of the CO 2

reduction on Ni, Pt, and Cu electrode, was reported from studies with cyclic voltammetry and I-I"IR [19-28]. In the cases of Ni and Pt electrodes, adsorbed CO prevents the electrode reaction due to its stability [29,30], However, adsorbed CO is thought to be an important intermediate for hydrocarbon and alcohol formation in the case of the CO 2 reduction on the Cu electrode. O,t the other hand, it is ~,ell°known that H2S operates as a poisoning reagent in heterogeneous catalytic reactions using Ni and Pt catalysts. This is considered to be caused by the strong ad~rption of sulfide ions on the metal catalyst that prevents adsofp0on of the reactants and hydrogen. In the case of the electrochemical CO~ reduction, the effect of adsorbed specic,~ :~uch a~ ~ulfide ion~ which prevent the electrode reaction0 has hardly been studied. We investigated the effect of rite treatment of metal e!eetroO ~. surface with ~ueous Na~S solution and addition of Na~$ into the e!ectrolyle on the product selectivity of the CO 2 reduction and on th~ cur° rent-potential curve. Based on these resu!ts~ the eft~t o! sulfide ion on the electrochemical CO2 reduction is dee scribed in the present publication.

240

K. Itaru et aL / Juur~ml af Elecmmnalytical Chemistry 434 (1997~ 239-243

2. Experimental

2.4. Measurement of current-potential curve

2. I. Reagents and electrode mawrials

Measurement of current-potential curves was conducted at 25°C under I atm Ar and CO~ in an electrolysis glass cell whose catholyte is separated by a glass filter from the anolyte. Equipment used for measuremem of current-potential curves was a potentiostat-galvanostat (HA-501), an arbitrary function generator (Hokuto, model HB-105), and an x-y recorder (Riken Denshi Co. Ltd., model F-35A). The eleclrolytes were O.! real din-3 KCIO~ aqueous solution and 0.1 mmol dm~°~ Na2S + 0.1 mol dm ~~ KCIO, aqueous solution. Scan rate 50 mV s - t

Metal wires (from Nilaco; diameter 0.4 to 0.6 mm) were used as the working electrode by sealing with a thermalshrink tu~. The purities of the electrode materials were as follows: Fe, 99.5%; Ni, 99.9%; Pd, 99.95%; Ca, 99.999%: ~ , 99.99%; Pt (from Tanaka Noble Metal Ltd.), 99.99%. Ca electrodes were electropolished in 85% H~PO, and other melal elecmxles were polished with alumina powder (0°05 ram) before the electrolyses. Special reagent grades of KHCO~. KCIO~, and distilled water (from Wako I~we Chemical Industries. Ltd.) were u~ed tbr the preparation of the aqueous eleet~lyte. S~aium sulfide (Na2S) (from ganto Chemical Co.. lnc,) wa~ u~d without any pufificao tion

2.2. Surface treatment with Na:S Surface treatment of the metal electrodes u~d t'or the elec~hemical CO~ reduction was conducted by soaking in 0.Smoldm ~ Na~S aqueous solution for 30 to 60rain (in the case of Cu electrode, 2 to 20 n|in) at room temperatare The surface of Cn and Ag electrodes was changed to gray and black, respectively, after tile treatment. In the cases of other metal electrodes, no apparent change of surface was ob~rved with the Na.,S treatment. The sur~ face tr~tment of the electrode used for measurement of the ¢urrent~l~tcntial curve was conducted by soaking in 0.5 real dm ~ Na~S aqueous solution for 2 rain.

2.3. Elecmdyses and ana6.sis ¢~fthe reductim, /,v,dm:ts Elee.troly~s were conducted under high pressul~e in a ~la~ ~11 ~uipped within ~ stainless steel autoclave as h ~ d e s c r i ~ in a wcvious paler [181, The electrolyte ~a$ ~ st i ~ in this ex~fiment. The call, lyre and anolyt¢ comlmtaments were ~ptwated by a cation exchange memlml~ sheet (Nation ~ 417). The electrolyte was 0~1 moldm ~~ KHCO~ aqueous solution. The reference e ~ was A g ~ l l s a t KCI and a Pt wire was u~d as the counteeei~trode, After ll~ el~trolyte was ~ r a t e d by bubbling CO: for 20 to ~ mitt. a known pressu~ of CO~ was introduced directly into the ~toclaYe, The electrolyses were carried o~| g~lvano~,~|icMly at ~ 7 usit~ a potemiosmt-galvano~ ~ t (HOkuto, ~ 1 H A ~ I ) at~l a ~xmlomb=ampere4!onr melee f ~ m o , model HF~201). ~ potential of the worki~g e I ~ M ~ : was ~o,~cted arm n~easu~d with an iR c~t~m inste~mcmt (Hokum, nu:~¢l H1-203), The ~t~a ~ts s ~ h as hydrocarbons, ethanol, carbon rmm~xide, ~ ~'drogen were quantitatively analyzed usir~g a gas ch~-mrmt~raph, and formic acid was determined by a ~ as has been described in detail in the previous

Dsl.

3. Results and discussion

3. I. Electn~chemical CO~ reduction usbig No:S n~ated metal electrodes The electrochemical reduction of CO~ under 30arm using Na~S treated metal electrifies (Fc, Ni, Pd, Ca, and Zn) was conducted. The faradaic efficiencies fi~r the reduction products ionized on Na~S treated electrodes and those for the normal electrodes are shown for comparison in Table I, The faradaic efficiency for tbrmic acid formation on an Fe electred¢ decreases from 8% to 4.9%, whereas that for CO increase:, from 5,7% to 14% with the Na2S treatment. In the case of Ni electR~e, the faradaic efficiency for fin'mic acid fo,rmation increases from 4.1% m 15% with the Na2S treatment, iLs shown in Table !. In the cam of Pd electrode, the faradaic efficiency for hydrogen formation increams remarkably from 38% to 62~.. whereas those for formic acid and c~t~m monoxide decre~m from 25% to 14% arid from 33'~ to 12%, res~tively, with the Na~S treatment. As simwn in Table l. a drastic change in tile product mlectivity with the No~S treatment was obscrved in the d e c ~ h e m i c a l CO~ reduction on a Cu electrode. In the ca~ of the Na,S treated Cu electrode, the faradaic efficiency of methane formation was only 1.9%, wMreas that in the case of the normal Cu electred¢ was 36%. On the other hand, the faradaie efficiency of formation of the C2 compo~nts (ethylene and ethanol) increa~d from 13% to 25% with the Na,S treatment, as shown in Table I. Like that of th,~ C2 component, the f~rad~ic efficiency of iambic acid formation increased remarkably from I!% to 31% with the Na~S ~atmen~. tn ~!~ecase of Zn electrode, the faradaic efficiency of formic acid |urination decreased from 82% to 52% and that of carbon monoxide increased from 15% to 44% with the Na2S treatment, as shown in Table I. This shows that the product selectivity for the CO_, reduction is changed remarkably by the NaaS treatment of the electrode and the tendency of the change depends on the electrode materi',d.

241

g. Hara e~ al. / J o m , m l of Electroanalylical ChemistO, 434 ~1997) 2~ ;~ 24.~ ..

Table I Electrochemical reduction of CO: under high pressu, e on Na~S treated electrodes Ek:ctmde

Na~S treatment

Current densky/mAcm -'~

Fe

Yes No Yes No Yes No Yes No Yes No

80 80 80 80 80 I00 60 60 80 80

Ni Pd Cu Zn

Faradaic efficiency/% HCOO-

CO

CH4

C2 component ~

H~

Total

4,9 8.0 15 4. I 14 25 3!

14 5.7 1.0 nb 12 33 2.1 8.1 44

1.4 1.5 i.2 0.5 ! trace 0.35 1.0 36 trace

O. 16 O. 13 0.13 trace trace trace 25 13 trace

15

n

n

7I 72 72 76 62 38 23 14 3.0 0.4

91 87 9O 81 88 96 83 83 99 98

II

52 82

Tile electrolyses wel~ conducted galvano,~tulically without stiffing the el¢c|t'oly|~. Conditions: ~l~c|tx,lytc O, I reel Ont ~ KHCO~; CO~ pl~essu;'c 30arm; passed chargr~ 200~. 300C~ E|hyleno and ¢|hauoL Not delectod,

,¢~2, EIt~t'tlvJ¢lwmical ( t , redtwtion ola Na ,S treated Cu

HectrtMe As shown above, the product selectivity for CO~ rcduclion on a Cu electrode changed markedly with the Na,S treatment, in order to clarify the details of this effect, the dependence of the faradaic efficieaeies of the reduction products formed on the Na~S treatment Cu electrode on the current density were investigated. Fig. 1 shows the

curl~nt density dependence of the faradaic efficiencies for methane, C2 component, carbon monoxide, and formic acid formed on Cu and Na2S treated Cu electrodes at con:;tant current density electrolyscs of 30, 60, and 163 mA cm =2 As shown in this figure, the product selec=

50

1.0 mA ca~';~

40 s

30 2O

E 0.0

,0.8

-1.6

IAI

b 't

'2

f

30

÷

20 0

I

Ir !

|

10

1.0 mA cm-2

I

0

.....

20

60

100

140

Ni

t80

Current Density I mA c m °°~ Fig. I. Dependence of faradaic efficiencies for the reduction pr¢~ducts formed on a Cu and an Na2S treated Cu electrode on the curren! density under 30arm CO 2 in O.I moid,n -~ KHCOs aqueous solution: (al (O) CH4, (El) C2 component formed on Cu electrode, (O) CIt,,, ( B ) C2 component formed on Na2S treated Cu clectlod¢; (b) (A) CO, (Q) HCOO- formed on Cu electrode. (A) CO. (O) HCOO- formed on Na2S treated Cu electrode.

ol;6

,

`0;B

0.0

Potential / V va A~/AgGI Fig. 2. Cyclic w)llammogram~ o, Ni electrode; (a) 0. I moldm ~ KCIO4 aqueous solution, (b) 1.0 mmol d i n ~ Na ~S + 0. ! reel dm° ~ KCIO4 aqu~= ous solution; ( m - - - - ) under Ar, ( ) under I arm CO 2. Scan rate 5 0 m r s -=.

"~=4.n

K. Itara et al, / Jour~ud of Electroanah'tical.

i .I ,6

i 1,0mA.2 ,0,8

o.0

b

i

0

Pd • 1,6

.0,~

00

P o ~ t ~ l / V v~ Ag/AgCZ Fi~. 3. Gycltc ~oltammograms on Pd e ~ t n ~ ¢ : (a) O. I m o l d m ~ K('IO~ aqtacoa~ ~.qtttio,. (b) 1.0 mmol dm ~ Na ~S + 0. I nmldm ~ ~ KCIO~ aqueo ous ~olution; ( ~ ~ ~ ) under Ar, ( - - - - - ) onder 1 arm CO~, Scan

fivity changed markedly with the Na,S treaunem of the declro,Je al each current density. The faradaic efficiency of methane fom~ation tk~rea~d drastically with Ihe Na~S treatment regardless of currem dens, v (e,g. 48% to 4.1% at 163 mA em -~ ), In contrast, ;he fiu~daic ,A'ficiency fi~r C2 ¢ompo~tcnl formation increased from 4,7% to 24% iu i ~ ¢a~ of the el~trolysis at I ~ m A c m - ~, 'PI~ farad,ale e~:ieacy ~ ibm fie acid fon~tion also increa:~d with the Na~S It~atttrent regardless of current density, a~ shown in

~.I.

Chemi.;'t..a'..434 (1997) 239-243

potential shift was small in the case of the Cu electrode. This negative shift might be due to the prevention of the hydrogen formation by CO adsorbed on these electrodes formed by the reduction of CO:, as has already been suggested by Hori and coworkers [24,29,30]. in the case with Na~S, the negative shift was also observed in the cases of Ni and Pd electrodes, however, the degree of the shift decreased largely compared to that observed in the absence of Na2S. This suggests that the surface coverage of adsorbed CO on these electrodes decreases due to the adsorption of sulfide ion. Moreover, in the cases of PI and Cu electrodes, the current curves under l arm CO~= were shifted towa~ the positive direction, compared to that under I arm At. as shown in Figs. 4 and 5. suggesting no tormation of adsorl~d CO on PI and Cu electrodes due Io the influence of sulfide to,, When the current curves obtained under i arm Ar wid~ and without Na:S are cnmpared with each other, a small negative shift with Na~S was observed, compared to that without Na:S at all elecm~les. This negative shift might be due to the prevention of the hydrogen Ibnuation on metal el~trodes due to the formation of adsorbed sulfide ion, By comparison between the values of the negative shift by adsod~ed CO aud sulfide ion, it is suggested tha~ the suppression effecl of hydrogen evolution by adsorbed

a

<

E ,Qa

•,1.6

3,3, Change in lhe curr~m=~tential curves with addititm

~f Ya:S Next, the ¢ ~ s in the ¢urrent=polential ¢un, es o1~ t a i r ~ in v ~ous metal d¢¢tro~s wifll ~ addition of Na:S intn the electroly|¢ were investigated, The a~e~zsuremen~ wcr~ ~ ; e d in O,i tool din ~ KCiO~ ~ueou~ ~]utio~ ~ l with addition of 0,1 mn~! d m ~ of Na,S ~r l arm Ar and CO~. Figs. 2-5 show the cu~ve~t~ a f i ~ ~eves o b ~ a i ~ t~n Ni, Pal, P~, and Cu electrodes, respectively, without the addition of Na:S (a) and with

+

b

,F

F

i

I

O

1,0 macro-2

' -1,6

Pt 0.0

,,0,8 =

In ih~ absence of Na~S, the current curves obtained on Ni, I~L ~ Pt e l e c ~ under l aim CO, were shifted ~w=xl the negative direction up to 0 2 V~ compared to Ihose under l atra At, As shown in Fig. 5, this negative

0,0

Pt

II

Potential ! V ~s Ag/AgCl Fig. 4. Cyclic voltammograms or= PI electrode: (a) 0. I moldm -~ KCIO4 aqueous solution, (b) 1.0 mac| d m - 3 Na z S + 0. I tool rim- ~ KCIO4 aqueous solution: ( - - - - - - ) under Ar, ( - - . ) under I arm CO 2. Scan rzle 50 mV s - i

K. Hara el al. / Journal of Elec¢r~amz(vtic¢d Chemist~3' 434 ~: ~ 7 ~

~ {~ .

243

?,~

References ÷

fit !

! I i

-1.6

o0,8

0.0

+

7 I

0

1,0 m A ¢ m ~

Cu -1,6

~0.8

0.0

Potential / V vs Ag/AgCI I:i~o 5, Cyclic volt~mmu~gr~m~son Cp electrode: (~i) O, I moldm- ~ KCIO.~ aqueous solution, tb) I.()mmohlm ~~ N~l~S + O, I moldm " ~ KCIOa aqueOtis sollllion', ( ................ 1 ul~der Ar. ( ) under I arm CO~, Scan rate 50mVs ~ .

sulfide ion is smaller than that by adsorbed CO for Ni, Pd, and Pt electrodes, as shown in Figs. 2-4. It has already been reported that the hydrogen ibrmution on a Pd electrode is p~'evented with the addition of Na2S [31]. Moreover, the currem~-potential curves using the Na~S treated metal electrodes (Ni, Pd, Pt, and Ca) were also measured. These showed that the effect of the Na:S treatment of the electrode was similar to that of the addition of Na~S into the electrolyte~

Acknowledgements This work was supported by the Grant-in-Aid for Scieno tific Research No. 04241106 from the Ministry of Education, Science and Culture.

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