Photoelectrochemical generation of hydrogen using organic pollutants in water as sacrificial electron donors

0360-3199(95)00045-3

In/. J f/whqvn Ewr~~ Vol. 2 I. No. 2. pp. 95 98, 1996 Copyright @ Inwnalional Associalton for Hydrogen Energy Elsevier Science Ltd Printed in Greal Britain. All rights reserved 0360-3 199196 $15.00 + 0.00

PHOTOELECTROCHEMICAL GENERATION OF HYDROGEN ORGANIC POLLUTANTS IN WATER AS SACRIFICIAL ELECTRON

USING DONORS

N. N. RAO and S. DUBE Central

Salt and Marine

Chemicals

Research

Institute,

Gijubhai

Badheka

Marg,

Bhavnagar

364 002, India

Abstract&

This paper describes hydrogen evolution from a photoelectrochemical cell using organic pollutants as sacrificial electron donors. A compartmentalized photoelectrosynthetic cell constituting an illuminated TiOz photoanodeand a Pt cathode has been designed and found to discharge H + as HL at the Pt cathode and simultaneously destroy organic pollutants in the photoanode compartment under short-circuit conditions.

of ‘OH radicals (primary oxidants necessary for the photodegradation of pollutants [4]) via the reduction of 0, in the conduction band appears to be suppressed. Then it is expected that the photoelectrons collected on the Ti substrate can be driven onto a suitable metal cathode via an external circuit where they may be used for the reduction of H’ to H,. We have thus succeeded in constructing a compartmentalized photoelectrosynthetic cell for the discharge of H+ as H, at a cathode while the illuminated TiO, film anode oxidized the organic pollutants under short-circuit conditions. These are two useful reactions with great relevance to energy and the environment. The results of hydrogen evolution, chemical oxygen demand (COD) removed and effects of pH variation on cell performance are discussed in this paper. The conversion of light energy into electrical energy or the use of fuels employing the photoelectrochemical and photocatalytic etfects 15-71 exhibited by illuminated semiconductor/electrolyte junctions have been known for a long time.

INTRODUCTlON

photosensitizer/relay/sacrificial-electronLight-driven donor systems have demonstrated the ability to generate hydrogen, usually in the presence of a platinum catalyst. H,EDTA In these systems, by and large (ethylenediaminetetraacetic acid) and TEOA (triethnanolamine) have been employed as sacrificial electron donor species, which are consumed in an irreversible oxidation reaction. However, the suitability of donors is mainly determined by the redox potentials of the sensitizer in its ground and excited states (the maximum oxidation power of the ground state Ru(bpy)i’ is 1.27 V). Indeed, Konigstein and Bauer [l J reported that the use of some organic compounds such as alcohols, glucose and phenolic waste compounds in place of H,EDTA did not cause liberation of hydrogen from these systems. It is also desirable to use low-cost donors or some industrial waste organic products as sacrificial electron donors in practical systems. This limitation on the choice of sacrificial electron donors imposed by sensitizer molecules seems to have been overcome by using TiO, with which many organic compounds can be oxidized at the edge of its valcncc band (+ 2.6 V NHE at pH 7, E, = 3.0 eV). The technique of using TiO, photocatalysis has been widely examined for treating water containing organic pollutants [2]. We have found that a thin film of TiO, formed on a Ti sheet either by flame or thermal oxidation [S] can also be used for this purpose. However, this Ti/TiO, system could not use the supplied oxygen effectively owing to the accumulation of photoelectrons on the bulk Ti metal in a familiar carrier transport mechanism induced by the potential gradient across the space charge layer. Thus, formation

EXPERIMENTAL A typical cxpcrimental set-up (Fig. 1) constituted TiO, film anode and Pt cathode compartments connected with an agar salt bridge. A film of TiO, formed on Ti sheet (25 x 20 cm’, 0.05 cm thick) by flame (FO, glass blower’s flame) or thermal oxidation (TO) in air (preheated furnace, 5OO’C, 2 h) acted as the photoanode. Such treatments are known to give polycrystalline n-TiO, films [S-l 11. Prior to oxidation, the Ti sheet was moulded into a hollow cylinder of suitable diameter into which the borosilicate thimble of a medium pressure Hg lamp (400 W. peak emission at 365 nm, 5.0 x 10” photons/s) could 95

N. N. RAO and S. DUBE

96

1.6

L = Hila&p MS = Magneticstirrer S = Siliconeseptum M = Manometer SB = Saltbridge

MS 60

120

180

240

300

360

Irradiation time (min) Fig. 1. The compartmentalized PEC cell

be conveniently placed. Only the TiO, layer on the inside wall of the Ti cylinder received illumination. An ohmic contact was established by soldering a copper wire into a bore drilled into the anode. All of the soldered region was protected from electrolyte contact using epoxy resin. Before using the same TiO, electrode for all the tests, it was thoroughly rinsed with distilled water and air dried in an oven at 80°C. The anolyte was either 450 cm3 of double-distilled water or the same quantity of water containing a 2,4-dichlorophenoxyacetic acid (2,4-D, 9.049 x 10m4M; initial COD, 210 ppm)/l: 1 phenol + 2,4-dichlorophenol mixture (2.68 x 10m3M; initial COD, 768 ppm) or 450 cm3 of effluent from a local butanediol (BD) plant (initial COD, 60 ppm). These anolytes (pH 4-6) were constantly exposed to air accessed through a hole in the head space. The cathode compartment, which had a Pt gauge electrode immersed in 50 2), also had facilities for cm3 of O.OlN H,SO,(pH deaeration of catholyte, manometric measurement of gas and withdrawal of gas. The catholyte was thoroughly purged with prepurified N, gas (> 1 h) and was maintained at 30”( +O.S°C). Then the TiO, anode and Pt cathode were short-circuited and the Hg lamp was turned on. Both H, evolved in the cathode compartment and COD [ 123 removed in the anode compartment were determined as a function of irradition time over a period of 5 h. The gas evolved in the cathode compartment was withdrawn using an air-tight syringe and was identified as H, (Shimadzu GC 9A; molecular sieve 5 A, 13 x ; isothermal 5O”C, TCD lOO”C, 80 mA). The volume of the H, determined manometrically was corrected for any

Fig. 2. (a) H, evolution; and (b) COD removal as a function of irradiation time. The anolytes were n---n, double distilled water; x ~ x , 2,4-D pollutant; O--Cl, phenolic mixture; O--O, effluent of the butanediol plant. vapour pressure changes measured simultaneously in an identical cathode compartment containing 50 cm3 of O.OlN H,SO, at 30°C. The volume of H, determined this way agreed within a 5% error with that of gas chromatographic determination. RESULTS

AND DISCUSSION

The yield of H, was more with the TiO,(TO) film electrode and was therefore chosen for further experiments. A maximum of 0.65 cm3 H, was produced in 5 h at a rate of 0.16 cm3 h-’ when distilled water was used as the anolyte (Fig. 2(a)). This represents the simplest case of the photoelectrolysis of water where O,(H,O,) could conceivably form in a corresponding water oxidation reaction. The yield of H, was the highest (1.54 cm’, 0.4 cm3 h-‘) with 2,4-D pollutant. Hydrogen evolved at a rate of 0.3 and 0.2 cm3 h-’ with the phenolic mixture and the effluent of the BD plant, respectively. More than 9&95% of the total yields of H, in each case could be obtained by renewing the pollutants under testing. The rate of production of H, gradually approached plateau, which may be primarily due to the progressive disappearance of pollutants in the anode compartment. Simultaneous COD determinations (Fig. 2(b)) on the anolyte indicated that the rate of COD removal was higher for 2,4-D (15.2 ppm h- I), at a medium level (10 ppm h - ‘) for the phenolic mixture and low (5.6 ppm h-‘) for the effluent of the butanediol plant.

ORGANIC POLLUTANTS AS SACRIFICIAL ELECTRON DONORS

97

Table 1. Electron capture efficiencies for H, evolution using some organic pollutants in the anode compartment Pollutant

2,4-D Phenolic mixture Effluent from the BD plant

EfficiencyJ (%I

COD

4 cm3 h-’

rz( x 10’ mol h - ‘)

ppm h-’

rB( x IO-’ mot h-‘)

0.4 0.3

3.51 2.68

15.2

10.0

0.98 0.84

0.2

1.78

5.60

3.64 3.19 -

* Electron capture rate; t hole capture rate; $ efficiency = r,/r, x 100.

The cell produced no hydrogen in the dark. The ability of the cell to produce H, under illumination was also verified under the following conditions: (i) no chemical bias, i.e., ApH = 0 in E = -0.059 x ApH where ApH is the difference between the pH of the catholyte and anolyte; (ii) ApH = 0, but with 2,4-D pollutant in the anode compartment; (iii) ApH = 2.5, i.e., E = -0.150 V and (iv) ApH = 8, i.e., E = -0.472 V. Hydrogen did not form under conditions (i) and (ii). A total of 0.1 cm3 H, was produced at a rate of 0.018 cm3 h-’ in 5 h under condition (iii). Hydrogen evolved at a rate of 0.26 cm3 h-i producing 1.24 cm3 H, in 5 h under condition (iv). It is evident that both illumination and a slight chemical bias to the tune of 0.150 V are essential for the formation of hydrogen. Comparison of the rate of H, evolution (0.018 cm3 h-‘) under condition (iii) with the rate (0.40 cm3 h- ‘) obtained using 2,4-D pollutant (pH = 4.5, ApH = 2.5) reveals that the use of the 2,4-D pollutant lead to a 20-fold enhancement in the rate of H, evolution. Furthermore, comparison of the rate of H, evolution (0.26 cm3 h- ‘) under a chemical bias of 0.472 V (i.e. under condition (iv)) with the rate attained (0.40 cm3 h - ‘) using the 2,4-D pollutant reveals that better H, evolution rates could be obtained using a smaller chemical bias and the 2,4-D pollutant together. This clearly demonstrates the sacrificial nature of the organic pollutants which lead to enhanced hole-capture rates at the TiOJsolution interface and in turn increase the electron-capture rate at the Pt cathode producing more hydrogen. The dissolved oxygen present in the catholyte strongly limited the yield of H,. This may be due to the preferential reduction [ 131 of 0,. In the presence of 0, the O,/OH- level becomes an electron acceptor level ( + 0.8 V vs SCE at pH 2) which is 1.2 V below the Ht/H2 level. As both the photoanode and Pt cathode are shortcircuited in the PEC cell under consideration, the Fermi level (Et) is the same for both the electrodes in the dark as well as under illumination. Upon illumination, the E, in the TiO, moves to a more negative potential (with a consequent rise in the E, of the Pt cathode). Our experiments indicate that this shift, in itself, is insufficient to cause discharge of H’ at the cathode, and H, evolution would require an additional bias of 0.150 V derivable by maintaining a certain pH difference between anode and cathode compartments.

The efficiency of the cell with respect to H, evolution is calculated to be 3.64 and 3.2% for the cases of the 2,4-D pollutant and the phenolic mixture respectively (Table 1). The estimation of efficiency involved the calculation of electron- and hole-capture rates (r, and r,,, respectively) corresponding to the rates of H, evolution and COD removal as per stoichiometric equations (l), (2) and (3). In the presence of these pollutants in the anolytes, parallel consumption of holes in the reaction of the photooxidation of water is considered to be insignificant 2H’ +2e-

--) H,

(1)

Cl,C,H,OCH,COOH

+ 15h +( OH)

+ 8C0, + 2HCl-t C,H,OH

+ C,H,Cl,OH -

2H,O + 15H+

(2)

+ 24h+( ‘OH)

12C0, + 2HCl+

2H,O + 28H+

(3)

CONCLUSIONS Organic pollutants can act as sacrificial electron donors in PEC cells which may be used to generate hydrogen and to destroy water soluble organic pollutants simultaneously, both of which are useful reactions. This system appears to possess some practical interest as it combines energy and environment-related reactions. Acknowledyement~--Theauthors are grateful to Professors P. Natarajan (Director, CSMCRI, Bhavnagar) and V. Krishnan (II%., Bangalore, India) for useful discussions. REFERENCES 1. C. Konigstein and R. Bauer, Proc. Indian Acad. Sri. (Chem. Sri.) 105, 353 (1993). 2. 0. Legrini, E. Oliveros and A. M. Braun, C/rem. Rev. 93, 671 (1993). 3. N. N. Rao and S. Dube, Unpublished results. 4. N. Serpone, D. Lawless, R. Terzian and D. Meisel, Alectrochemistry in Colloids und Dispersions (R. A. Mackay and J. Texter, eds), p. 399. VCH Publishers, New York (1992). 5. M. Gratzel and K. Kalyanasundaram, Curr. Sci. 66, 706 (1994).

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N. N. RAO and S. DUBE

6. K. S. C. Babu, 0. N. Srivastava and G. V. Subba Rao, Curr. Sri. 66, 7 I5 ( 1994). 7. N. N. Rao and P. Natarajan. Curr. Sci. 66, 742 (1994). 8. S. N. Steven and A. J. Bard, J. Am. Chem. Sot. 99. 4667 (1977). 9. D. Haneman and P. Holmes, Sol. Energy Muter. I. 233 (1979). 10. G. A. Hope, G. G. Morgan and A. R. McKenzie, Aus. J. C%em. 34, 713 (1981).

11. G. Prasad, N. N. Rao and 0. N. Srivastava, Prog. Hydrogen Energy (R. P. Dahiya ed.), p. 31. D. Reidel (1987). 12. Srundurd Test Method ,fbr Chemicul Oxygen Demund of Wuter, Annual book of ASTM Standards, 1980, part 31, Designation : D-l 252-78, pp. 665-669. 13. L. M. Rouse, Mat. Rex. Bull. 13, 861 (1978). 14. J. G. Mavorides, D. 1. Tchernev, J. A. Kafalas and D. F. Kolesar, Mat. Rex Bull IO, 1023 (1975).