Chemical Storage of Solar Energy Using Hydrazones

Hydrazones derived from 4phenoxybutanoic acid were reprepared and their oxidation potentials in Micellar-Ethanol solution were measured using cyclic voltammetric technique. These compounds were used as photosensitizers in three component system containing methyl viologen (MV 2+ ) and Na2EDTA. Different behaviors of the hydrazones were observed as photosensitizers depending on their oxidation potential values and their stability in irradiation solution. Rate constants of methylviologen reduction by the photochemical active hydrazones were recoded and hydrogen was photoproduced using TiO2 as catalyst. 2+ EDTA, MV 2 Photochemical Conversion of Solar Energy, Photosensitizer, Na : s Keyword تٍسَشىا تقاطيى يٗاٍٍَنىا ُضخىا ثاّٗصٗسذٍٖىا ًاذخخساب ًشاب ٔبح ذٍشس شَع ُا٘شّ ءاٍٍَنىا ٌسق – ً٘يعىا تٍيم – ك٘مشم تعٍاج ًلاخسا خٌساح :ثحبىا 8 / 12 / 2010 خٌساح ه٘بق ثحبىا : 11 / 5 / 2013


Introduction
Energy is the most important issue of the 21 st century. About 85% of our energy comes from fossil fuels, a finite resource unevenly distributed beneath the Earth's surface. Reserves of fossil fuels are progressively decreasing, and their continued use produces harmful effects such as pollution that threatens human health and greenhouse gases associated with global warming. Prompt global action to solve the energy crisis is therefore needed [1]. The search for future alternative energy options that are renewable and environmentally friendly is of great importance and solar energy is one of these options [2].
One of the final goals of current research of the photochemical conversion and storage of solar energy is photoproduction of hydrogen, the most abundant element in nature, from water. Many workers postulated and put in order various systems in the way of achieving this target [3,4,5,6,7,8,9,10]. However, the most popular and best studied model system for the photosensitized reduction of water has been the one containing Ru(bpy) 3 +2 (byp=2,2 bipyridine) as the photosensitizer, methyl viologen (1,1 dimethyl -4,4-bipyridinium dication; MV 2+ ) as electron relay, and EDTA as sacrificial electron donor [6,11,12,13,14,15,16,17,18]. Although many other exciting approaches and schemes (eg. Methylporphyrines and some well-known organic dyes) have been attempted and tried as photosensitizers in light induced electron transfer; it is fair to state the other practical system for photolysis of water to hydrogen have yet to be found [15,19,20,21].
Accordingly the increasing interest in the chemistry of hydrazones induced the author aims to prepare new compounds of this class to use them as new photosensitizers.

Experimental
Hydrazones were reprepared according to some previously published [22,23]. UV-visible spectra were recorded on CECIL type CE 599 spectrophotometer. Infrared spectra were recorded on a Perkin -Elmer 983 GIR spectrophotometer. Cyclic voltammetric experiments were carried out using a three-electrodes system (a reference electrode SCE, a platinum wire auxiliary electrode and working electrode, glassy carbon electrode GC). This system was connected with a CV-27 BAS scanning potentiostat supplied with a cell stand type C1B-240 and x-y recorder type Omnigraphic, Houston instrument. Irradiation experiments for sensitization and hydrogen production reactions were performed with a 250 W-Xenone arc lamp (Applied photophysics, LTD). Photochemical reactions were held in 4 mL size quartz cell supplied with 3-cm-neck. Photocatalytic hydrogen production experiments were carried out using a cylindrical 30 mL pyrex cell equipped with a 2 cm 2 diameter quartz irradiation window surrounded by a water-cooling jacket. Hydrogen gas analyses were carried on a Pyeunicam 304 gas chromatography supplied with a carbosieve 5 A column 1/8˝ in diameter, and 3 ft length with thermal conductivity detector, TCD. Column temperature was fixed at 60 °C, injection temperature 80 °C, detection temperature 100 °C and flow rate 20 mL min -1 .

Results and Discussion
Hydrazones (Ia-e, II, and and III) derived from substituted 4-phenoxybutanoic acid was prepared according to [23].   The first peak (Ep 1 ) was attributed to the oxidation of the hydrazones to form the cationic radical (I') via one electron process which is in the anodic reaction of traces of acid could be turned to the formation of the species (II'). The following scheme (1) describes the anodic reaction of these compounds at the first wave [25,26]. The second peak Ep 2 at (1.44 -1.50V) was assigned to the further oxidation of the cationic radical (I') via two electron process followed by the hydrolysis of the intermediate forming the corresponding carbonyl compounds (aldehydes, ketones) and amines by any of the pathways shown below,(scheme 2).

Scheme 2: Oixdation of Cationic Radica (I) Via Two Election Process Follwed by
Hydrolysis Of The Intermediate. Whereas compounds (Id,e) showed, rather positive results in photoreducing methylviologen in the above mentioned micellar solution [7,14]. Table (3) reveals the value of rate constants of photoproducing reaction achieved using compounds(Id &Ie). Whereas Figure (1) shows the spectrum revealing the classical build up of MV +2 radical ion using Id as photosensitizer.  These reactions was found to fit good first order kinetics by plotting in (A∞ -At) against t.
To investigate the effect of particle size on the production of hydrogen, the previous experiments where repeated using (10. 20, 30, 40, 50 mg of 45 mic. Pt/TiO 2 ). Hoverer, the same compounds (Id) in CTAC, showed only 5 mL hydrogen at 20 mg of Pt/TiO 2 after 5 hr irradiation. Figure (3) illustrates the total volume of hydrogen production. The shift of the optimum value for hydrogen production from 50 mg (200µ) to 20 mg (45 µ) may also be ascribed to screening effect caused by finely divided higher amount of Pt/TiO 2 [24]. Fig.(3): Hydrogen production mL/L solution using Hydrazone (Id) as photosensitizer in different loading Pt/TiO 2 (45 µ) at pH=5 The particle size is believed to be strongly correlated with the size of active surface of semiconductor, in other words for a given photocatalyst, the smaller the size of the semiconductor particle, the larger the volume of space charge layer, where electrons and holes were separated sufficiently. Electrons and holes easily reach the surface with their lifetime when the diffusion length (10 4 A o ) for TiO 2 [24] is larger than the radius of the particle.