J. Electroanal. Chem, 157 (1983) 179--182 179 Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands

Preliminary note

PHOTO-AIDED REDUCTION OF CARBON DIOXIDE TO CARBON MONOXIDE

ISAO TANIGUCHI*, BENEDICT AURIAN-BLAJENI and JOHN O'M. BOCKRIS

Department of Chemistry, Texas A&M University, College Station, TX 77843 (U.S.A.)

(Received 9th August 1983)

INTRODUCTION

Among raw materials available for the product ion of fuels and/or organic chemicals, only carbon dioxide and water are abundant. The use up of CO2 would reduce climatic effecys of excessive CO2 product ion. In this connection, much at tention has recently been focused on both electrochemical [ 1--6] and photoelectrochemical [7--11] means of reducing CO2. The best metallic elec- t rode found was In in non-aqueous solvents [6]. The best photoelectrochemical system to date is that reported by Bradley et al. [ 10], using tetraazamacrocycles as catalysts for electron transfer from p-Si to CO2. CO2 is unreactive, bu t CO re- acts easily [12]. The effective product ion of CO from CO2 would thus be a signif- icant process. We report here the first example of photoelectrochemical reduc- tion of CO2 to CO at a p-type CdTe electrode at potentials at least 0.7 V less negative than at metal electrodes [4--6].

EXPERIMENTAL

A phosphorus doped single crystal of p-CdTe(100) was obtained from II-VI Incorp., and used after etching for 10 s with an HNO3 + HC1 (1 : 3) solution. The electrode was mounted in a teflon holder with the exposed area of 0.20 cm 2. The ohmic contact was made on the back side with a Ga + In alloy. All chemicals were analytical grade.

Irradiation was performed with an Oriel 8540 Xe.lamp. The light was either monochromatic (~ = 600 nm, filtered' by a Bausch and Lomb monochromator ) or a t tenuated by a neutral density filter. The light intensity, measured with an Eppley radiometer, was 4.1 mW cm -2 in the first case and 12.3 mW cm - 2 in the latter.

Photo-aided controlled potential electrolysis (CPE) was performed using a gas- tight cell under a CO2 atmosphere (cell volume 105 ml, with 40 ml occupied by CO2 gas). The CPE media were N,N-dimethylformamide (DMF) solutions con-

*On leave from the Department of Industrial Chemistry, Kumamoto University, Japan.

0022-0728/83/$03.00 © 1983 Elsevier Sequoia S.A.

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taining 0.1 M te t rabuthylammonium perchlorate (TBAP) and a small amount of water. A Pine potentiostat (RDE 3) was used. A platinum wire and an aqueous SCE were used as the counter and the reference electrode, respectively. After each run, the gas phase was sampled and analyzed gas-chromatographically using a CTR-1 column (Alltech Associates) and a Gow-Mac chromatograph (TCD detector) at ambient temperatures.

RESULTS AND DISCUSSION

The photocurrent (ip)--potential (V) curves under monochromatic light in a DMF + 0.1 M TBAP solution containing 5% water are shown in Fig. 1. The photocurrent due to hydrogen evolution was observed at potentials more nega- tive than ca. -1.7 V vs. SCE under an Ar atmosphere, whereas when CO2 gas was bubbled through the solution the photocurrent increased significantly at less negative potentials. The marked shift in potential of the ip--V curve was ob- served reproducibly by changing the gas, CO2 to Ar and vice versa. Similar re- sults were observed under at tenuated Xe irradiation.

To confirm that the photocurrent is due to CO2 reduction, photo-aided CPE was carried out for up to 24 h or more at -1.6 V vs. SCE. After 1 h irradiation, CO was detected in the gas phase; the percentage found in the gas was ca. 0.3--0.4%. The amount of CO formed increased with the irradiation time and was linear with the quanti ty of electricity passed. The photocurrent (0.35--0.38 mA) was stable for more than 24 h and the photocurrent quantum efficiency calculated for monochromatic light of 600 nm was close to uni ty at -1 .6 V vs. SCE. When the water concentration in DMF was less than 0.2%, the photocurrent decreased gradually with time. The current efficiency for CO pro- duction was ca. 70%. This quanti ty was unaffected by the water concentration in the range 1--10%. The results were similar when CPE was carried out at poten- tials between -1 .2 and -2 .4 V vs. SCE.

Blank experiments were also carried out. No CO was detected after CPE for more than 24 h at various potentials between -1 .6 and -2 .5 V vs. SCE either under Ar with irradiation or under CO2 in the dark. No CO was detected after anodic oxidation of the solutions at 1.9 V vs. SCE for more than 24 h at a Pt electrode.

For CO formation, two reactions [3,4], CO2 + 2 H + + 2 e -~ CO + H20 and 2 CO2 + 2 e -~ CO + CO32-, have been considered. In the present experiment, both of these reactions would take place, because when the water concentration in DMF was changed in the range 0.2--10%, the change in the onset potential for COs reduction was'smaller (ca. 0.15 V) than that for hydrogen evolution (ca. 0.5 V) observed under an Ar atmosphere, but not negligible.

Tetrabutylammonium ion plays an important:irole in providing a suitable en- vironment for CO2 reduction at the electrode due to its hydrophobic nature, be- cause when LiC104 was used as a supporting electrolyte the photocurrent for COs reduction was smaller (initially ca. 1/3--1/2 and further decreased with time to less than 1/10 of that obtained in the presence of TBAP) and unstable.

We compared the performance of the illuminated p-CdTe electrode with that of an In electrode which is the best metal electrode known to date for reduction of COs in non-aqueous media [6]. The result is shown in Fig. 1. From Fig. 1 the

181

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// ;/ - 0 . 5 under A r ~

I /://~under CO~ at In / . I /Z/

0.0 ~ Z j ~ - - - - . . . . . . . . . . . . . . . . . . . . . ~ ....... Dark

, I , , , , _11 .5 , , , I , , I -1.0 -2.0 -2 5

Potential/V vs SCE

Fig. 1. Cur ren t - -po ten t i a l curves at p-CdTe e lec t rode in a DMF + 0.1 M TBAP so lu t ion con- ta ining 5% water unde r i r radia t ion wi th m o n o c h r o m a t i c light o f 600 nm, co mp a red wi th t he cu r ren t - -po ten t i a l curve in a CO~ a t m o s p h e r e at an In e lec t rode . E lec t rode area: 0.20 c m 2 (p-CdTe) and 1.0 cm 2 (In). Potent ia l scan ra te : 0.1 V s - ' .

energy saving is evident. If the point P in Fig. 1 is considered one can see that the fraction of energy saved by using an illuminated p-CdTe electrode is ( ~ a r k _

• ht ark VI~Te)/~ , i.e., about 30% of the electrical cost for this reaction could be saved.

The following conclusions may be drawn: (I) This is the first evidence for activity by means of p-CdTe for the photoelec-

trochemical reduction of CO2. (2) p-CdTe is a better electrode (i.e., less electrical energy used) in photoelec-

trochemical cells than any known alternative electrode system.

ACKNOWLEDGEMENTS

We are grateful for financial support from the Gas Research Insti tute and to Dr. Kevin Christ of this organization for stimulating discussion.

REFERENCES

1 P.G. Russell, N. Kovac, S. Srinivasan and M. Steinberg, J. Electrochem. Soc., 124 (1977) 1329 and references therein•

2 R. Williams, R,S. Crandall and A. Bloom, Appl. Phys. Lett., 33 (1978) 381. 3 B. Fisher and R. Eisenberg, J. Am. Chem. Soc., 102 (1980) 7361. 4 C. Amatore and J.M. Save'ant, J. Am. Chem. Soc., 103 (1981)5021 and references

therein. 5 M. Tezuka, T. Yazima, A. Tsuchiya, Y. Matsumoto, Y. Uchida and M. Hidai, J. Am.

Chem. Soc., 104 (1982) 6834.

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6 K. Ito, S. Ikeda, T. Iida and A. Nomura, Denki Kagaku, 50 (1982) 483. 7 M. Halmann, Nature, 275 (1978) 115. 8 T. Inoue, A. Fujishima, S. Konishi and K. Honda, Nature, 277 (1979) 637. 9 B. Aurian-Blajeni, M. Halmann and J. Manassen, Solar Energy, 23 (1980) 165 and

references therein. 10 M.G. Bradley, T. Tysak, D.J. Graves and N.A. Vlachopoulos, J. Chem. Soc. Chem.

Commun., (1983) 349. 11 B. Aurian-Blajeni, I. Taniguchi and J. O'M. Bockris, J. Electroanal. Chem., 149 (1983)

291. 12 J. Falbe, Carbon Monoxide in Organic Synthesis, Springer, Berlin, New York, 1970.