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The Photodissociation of Water

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The Photodissociation of Water The Photodissociation of Water PLATINUM METAL CATALYSTS PROMOTE THE PRODUCTION OF HYDROGEN AND OXYGEN By Anthony Harriman Dab) Faraday Research Laboratory, The Royal Institution. Imndon The development of artificial photosynthesis units capable of mnt'ert- ing solar energy into useful chemical products is still at u very pre- liminary stage, although some progress has been madr. during the lust .fau years. Most work concerns the photodissociation o,f water, and this paper considers the two basic strategies that are being followed. Both 0.f these require the use 0.f a catalyst and, to datp, platinum metal catalysts have proved to be the most efficient. With many laboratories actively investigating photodissociation, rapid progress should ensue. Over the past twenty years there has been a source; water, carbon dioxide or nitrogen. Of steady growth in research effort directed these materials, the large scale consumption of towards the construction of a practical device carbon dioxide involves problems with con- capable of the collection and storage of solar servation of the atmosphere but both the energy. Considerable progress has been made photodissociation of water into hydrogen and during this time and, already, solar energy is oxygen and the photoreduction of nitrogen to used in many countries to heat water for ammonia appear to be promising systems for domestic purposes. Similarly, the space the conversion of solar energy into useful programme has resulted in the development of chemical products. So far, most work has been photovoltaic cells that are able to convert solar concerned with the photodissociation of water energy into electrical energy with efficiencies and the photoreduction of nitrogen has been around 15 per cent. Only cost limits the virtually ignored. widescale use of such cells. Solar energy can Two basic strategies are being followed in the also be converted into chemical potential design of systems for the photodissociation of although, here, development is still at a pre- water into hydrogen and oxygen. The first liminary stage and no practical system exists at approach involves systems that are present. The scope of this field is enormous but, homogeneous, or nearly so, in which a on practical grounds, it seems necessary that photosensitiser is used to promote electron the final chemical product is a gas. This is transfer between suitable electron mediators. because sunlight is a dilute form of energy and The oxidised and reduced forms of these large collection areas are required so that the mediators can be used to liberate oxygen and concentration of the product will always be hydrogen, respectively, from water, provided quite low. Filtration and distillation are too suitable catalysts are present. The second expensive to be off-set by the price of the approach is based upon the irradiation of product. Collection of a gas, provided it is inorganic semiconductors, either evolved at a specific site and not throughout the macroelectrodes or colloidal particles, with light entire collection area, should be straight- of energy higher than the band gap. The forward. In terms of cost and availability, the electrodhole pair which is formed upon such gas must be derived from a cheap, inorganic illumination can migrate separately to the Platinum Metals Rev., 1983,27, (3),102-107 102 surface of the semiconductor where water complete, cyclic dissociation of water still reduction and oxidation can occur. Again, eludes us. In part, this failure is due to the poor catalysts are required to promote gas evolution performance of the catalysts employed and and, in all cases, the most efficient catalysts greatly improved catalysts must be developed involve the platinum group metals. before we can progress much further. Because of its low overpotential (v), platinum Homogeneous Systems is the most commonly used electrode material Here, a photosensitiser (S,), such as a water- for the liberation of hydrogen from water and it soluble metalloporphyrin, is used to reduce an has enjoyed considerable success as a catalyst in electron acceptor (A), such as methyl viologen, model hydrogen producing photosystems. With in aqueous solution. The oxidised form of the a macroelectrode in acidic solution, the Butler- sensitiser is reduced by an electron donor (D) so Volmer equation can be used to relate the over- that the sensitiser is recycled and the reduced potential to the net current density (i, measured form of the electron acceptor is used to liberate in NcmZ). hydrogen from water in the presence of a .. I = 1CA.I. platinum catalyst, see Figure I. In a separate IAN- photosystem, the oxidised form of the donor (I - tr)zFq = i, { exp RT -“p- (D+) is used to oxidise a photosensitiser (SJ, RT such as tris(2,2’-bipyridyl)ruthenium (11), which where tr is the cathodic transfer coefficient is able to oxidise water to oxygen in the pre- (tr = 0.5 for hydrogen evolution) and z is the sence of a suitable catalyst, for example RuO, or charge transfer valence (z = I). The exchange IrOp current density (i,) is a known function of the The two separate photosystems have been concentrations of H+and H,. In acidic solution, well developed, a wide variety of systems are the reoxidation of evolved hydrogen is not too available, and quantum efficiencies for forma- important and the ratedetermining step is tion of hydrogen and oxygen of 60 per cent and discharge of an electron to an adsorbed proton. 20 per cent, respectively, have been obtained Therefore, the above equation can be simplified (I ,z). Tables I and I1 give summaries of the best to developed systems. In all cases listed there, the 0.gF1 i = i, exp - (ii) reversible redox couple D+/D has been replaced -RT -0.gFq with a sacrificial (or irreversible) couple which or i = I.‘ k,C,+ exp -RT (iii) undergoes destruction upon oxidation or reduc- tion. So far, it has not been possible to link where C,, refers to the concentration of protons together two such photosystems so that the in the bulk electrolyte. This simplified equation Platinum Metals Rev., 1983,27, (3) 103 Table I Hydrogen-Evolving Photosystems I3uantun Electron Electron 3fficiencI Sensitiser acceptor donor 16 3H2 I Catalyst per cent bipy,Ru2+ methyl viologen E DTA platinum colloid 26 cysteine PtO, - proflavine E DTA platinum/asbestos - acridine yellow platinum colloid 32 ZnTMPyP4+ platinum colloid 60 none platinum colloid 7 ZnTSPP4- methyl viologen platinum colloid 2 Table II Oxygen-Evolving Photosystems Quantum efficiency Electron z so, Sensitiser acceptor Catalyst per cent bipy,Ru2+ CO(NH,),CI~+ RuO, powder 1.2 RuO, colloid 20 coso, 8 RuOJzeolite 5 RuOj'TiO, 12 MnO, - bi py,R u + s,o,2- RuOJTiO, 24 IrO, 14 bipy3Ru2+ Ti3+ RuO, colloid <1 ZnTMPvP4 zinc tetralN-methvi-4-~vridvll~or~hine ZnTSPP- zinc tesal4-sulphophenyl~porphine relates the net current density to the applied, or will be determined by the redox potential of the available, overpotential, the pH of the bulk electron mediator used, usually methyl viologen solution and the heterogeneous rate constant for which Eo= -0.44 V. Secondly, the limiting for the electron transfer process (kd. At high current density will be set by the rate of charg- overpotential, the currentholtage curve flattens ing the catalyst rather than by the rate of out due to rate determining diffusion of the discharge so that Equation (iv) will apply with electroactive species to the electrode surface. D and C referring to the reduced electron Here, the limiting current density (i,) is given acceptor. We can, therefore, express the by zFDC efficiency of a photosystem in terms of the (iv) 4. = 7 product of the rate of production of the reduced where D IS the diffusion coefficient of the relay and the rate at which this species is species in the medium, C is its concentration reoxidised on the catalyst surface. and 6 is the thickness of the diffusion layer. With methyl viologen as electron acceptor, In order to apply the above equations to our the available overpotential for Equation (iii) can proposed model photosystems, it is necessary to be expressed acknowledge two important points. First, the 1 = (0.44 - 0.059 PHI available overpotential for hydrogen production so that reaction is restricted to acidic solution. Platinum Metals Rea., 1983,27, (3) 104 In order to obtain the maximum rate of mass colloidal platinum particles work extremely well transfer to the electrode surface, it is seen from in model photosystems, they appear to have Equation (iv) that the thickness of the diffusion limited potential for practical applications. layer should be kept as small as possible Similar arguments can be applied to the whereas the total surface area of the catalyst oxygen-evolving photosystems. From should be as large as possible. For a given electrochemical work, it is known that the most amount of platinum, high surface areas are suitable anodes for oxygen liberation are cons- obtained if the catalyst is in the form of tructed from RuO, and IrO, and both materials colloidal particles (radius r nm) rather than as a have been used in model photosystems. So far, planar sheet. For such particles, the most work has centred upon RuO, but it is bimolecular rate constant for interaction known now that this is not a particularly good between a reduced acceptor and a platinum redox catalyst in photochemical systems. particle can be written Although claims exist to the contrary, it is clear k=qnNDr (4 that RuO, is not a selective catalyst for oxygen where N refers to Avogadro's number. If the production but it can be used for hydrogen concentration of particles exceeds the steady- generation; its overpotential in acidic solution is state concentration of the reduced acceptor, not much higher than that of platinum.
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