Photooxidation of Water by Ferric Hexathiocyanate

: Ferric hexathiocyanate ion is found to oxidize water on irradiation with visible and near U.V. light (quantum yield , 0.2'h. Reaction rates are measured and a simple theory is presented to explain the results.

In recent years the studies on photodissociation of water has received great deal of attention.l,8 The methods proposed for the achievement of these oxidation reduction processes fall into three main categoriesphotoelectrochemical cells %,a semiconductor powder suspensions7,l0 and redox systems with stable sensitizers.4,e Regenerative systems of the third type with stable inorganic sensitizers seems to be one of the most promising methods for conversion and storage of solar energy.' In this work we report our observations on oxidation of water by visible and near U.V. light ( X < 360 nm) in presence of the ferric hexathiocyanate (FH) ion.

Experimental
Ferric chloride solution (-0.01 mol dm-3) is centrifuged to remove the ferric hydroxide suspension and the Fe+ concentration is determined. Aqueous KCNS is mixed with the above until the resulting solution is 1.0 x 10-4 mol dm-3 in Fe3+ and 0.01 mol dm-3 in KCNS. The pH is adjusted by addition of HCI maintaining these concentrations. Photolysis is carried out in a rectangular glass cell (7.5 cm x 6 cm x 3 cm) using a medium pressure mercury lamp (100W). Experiments are also carried out in direct sunlight. In all cases U.V light is L ,< 360 nm filtered off with pyrex glass sheets and the intensity is estimated using a calibrated thermopile. The time variation of the concentration of FH under constant irradiation is determined by colorimetry. To remove photogenerated 0 , the solution is kept purged with oxygen free Na. The oxygen entry from the atmosphere is completely prevented. Absorption spectra and molar extinction coefficients are measured using the Unicamp Sp 500 Series I1 spectrophotometer.

Results
A plot of C/C, (C,, C = concentrations of FH at time t = 0,t) is indicated in Figure 1. There is no evidence for an equilibrium, the reaction proceeds in the forward direction until all ferric ions are reduced. Again.it is found that the plots of In (C/C,) vs t are linear (Figure 2) with slopes independent of temperature and the concentration of KCNS but directly proportional to[OH-] and the irradiation intensity. The reaction occurs when 1 5 550 nm, this corresponds to absorption region of the FH ion (Figure 3). At a given intensity of illumination (A, 360 -550 nm) there is a tendency for the slopes to decrease with the increase of wave length. Unfortunately we did not have facilities to investigate the wave length dependence of the reaction rate.

Theory and Discussion
Experimental results indicate that the reaction obeys simple apparently unimolecular kinetics. It is possible to givea simple theory to explain these observations. The photoactive species in the sorution-is ion generated via, We assume that the primary photochemical act is the formation of excited ions hv Fe (CNS)3-2 Fe*(CNS)3-

1
where E r : extinction coefficient, To = einsteins incident on front face of the cell per sec per unit area. From (5) and (6) we get The plot of In z va pH (Figure 4) is linear in agreement with the theory, the slope this plot gives 9 = 2.1 x 1010 mol-1 dm3 for I. -1.5 x 10-7 einsteins cm-z sec-1-Thus 7, [OH-] (pH = 3) -0.23 % which is the maximum possible quantum yield as the pH cannot be increased further. The reason why z is independent of temperature can be understood. The de-excitation time of FH* is very small compared to its thermal collision time with OH-. Thus the rate of the reaction cannot depend on temperature.

Conclusion
The system is interesting because of the extremely simple kinetics and the adaptability to accurate measurement of the reaction rates. As the quantum yield is quite small, the O2 evolution rate cannot be directly measured. However, the analysis of outgoing gases used for purging reveals the presence of 0,. The analysis of the residual solution also proves that oxygen evolution had taken place. The molarity of KCNS remains constant while ferric is reduced to ferrous, showing that any other material had not oxidized.
Fe3+ ions are known to oxidize water on irradiation with U.V light1'' ( A < 300 nm). In our process which is sensitive to higher wave lengths the active species is FH ion and there is no evidence that Fe3+ participate in theprimaryphotochemical act. The shifting of the equilibrium in (1) towards left by decreasing. the concentration of KCNS does not change the quantum yield of Fez+.
In principle the system is regenerative, when atmospheric oxygen is allowed to combine with the reduced product the ferric complex is regenerated with therelease of energy.