A new approach to solar energy conversion from water photolysis

A new approach to solar energy conversion from water photolysis

TIBTECH- JULY 1989 [Vol. 7] Photoproduction of ammonia A new approach to solar energy conversion from water photolysis Oxidation of water carried ou...

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TIBTECH- JULY 1989 [Vol. 7]

Photoproduction of ammonia

A new approach to solar energy conversion from water photolysis Oxidation of water carried out by plants and green algae is one of the most important biochemical reactions. In photosynthesis electrons flow continuously from water [the primary electron source in the living world despite its rather high redox potential (E'o, pH 7, +0.82V)] and are energized through both photosystems I and II to reduce oxidized carbon, nitrogen and sulfur. Sunlight energy is captured, transduced and finally stored as chemical energy in reduced biomolecules 1. As a direct consequence of the petroleum war in 1973, many scientists began investigating the production of energy-rich compounds (e.g. hydrogen, ammonia, hydrogen peroxide) and studying the mechanisms of water photooxidization in the chloroplasts of green cells. Their main aim was to simulate the natural process through which oxidized compounds are reduced with electrons from water in reactions in which sunlight energy is partially stored in the photoreduced products 2. With this idea in mind, we have designed an artificial photosystem based on ruthenium (II)tris(2,2'bipyridine) as the light-absorbing component. Ruthenium (II)tris(2,2'bipyridine) is a cationic metal complex that is a strong reductant when photoexcited to its metastable triplet state by blue light; the standard redox potential of the ruthenium complex is shifted from +1.26V in the dark to - 0 . 8 6 V in the light (Fig. 1) 3. With a metal dioxide (RuO2, TiO2) as catalyst and methyl viologen as the final electron acceptor, the.system can be made cyclical (Fig. 1). In this way, the electrons originally derived from water

at the redox level of ÷0.82 V, could be driven up and finally stored in the reduced viologen, at the redox level of - 0 . 4 4 V. This is comparable with biological photosynthetic electron transport which stores electrons as reduced ferredoxin at the redox level of - 0 . 4 2 V . The question then is, 'what does one do with the reduced methyl viologen?'

One way of using the stored energy in reduced methyl viologen would be to produce ammonia for fuel or fertilizers. Photosynthetic nitrogen-fixing cyanobacteria (bluegreen algae) accumulate and excrete ammonia when treated with a metabolic inhibitor, L-methioninen,L-sulfoximine, that blocks the incorporation of ammonia into carbon skeletons 4'5. Reduced end camponents of the photosynthetic transport chain (ferredoxin, flavodoxin, or others) have sufficient energy either to reduce dinitrogen to ammonia in a reaction catalysed by nitrogenase, or to reduce nitrate to nitrite and thereafter to ammonia, a sequence catalysed by nitrate and nitrite reductases (Fig. 2). It is important to realize that the reduction of one molecule of N2 to ammonia (although not that of nitrate or nitrite) requires a

- - Fig. 1

E' (V)

--1.0 *Ru(bpy)32+/Ru(bpy)33+ --0.5

J

MV+/MV 2+

0

+0.5

hv (2.12 eV) H20/O 2

+1.0

f

Ru(bpy)32+/Ru(bpy)33+

Water oxidation and methyl viologen reduction as photosensitized by Ru(ll)tris(2,2'-bipyridine) and catalysed by metal dioxides. The process is initiated by the absorption of one blue photon by the reduced photosensitizer (Ru(bpy)32+), which can thus donate an electron to reduce methyl viologen (MV) to its monovalent radical form. The resulting oxidized ruthenium complex (Ru(bpy)33÷) can then be reduced at the redox level of +0.84 V by electrons from water in a reaction catalysed by a metal dioxide (MO2).

~) 1989, Elsevier Science Publishers Ltd (UK) 0167 - 9430/89/$02.00

TIBTECH - JULY 1989 [Vol. 7]

Fig. 2 Biochemical reactions leading to ammonia production. Certain photosynthetic organisms can produce the ammonia required for their own subsistence either by reducing dinitrogen or nitrate and nitrite.

nitrogenase 6e

large input of energy, at least 12 molecules of ATP 6. The major drawback of such biological ammonia-producing systems is that ammonia is lethal to the cells above a certain concentration. This is why much attention has been paid to chemical photosystems that simulate the natural process. However, wholly artificial systems have not been very successful. A combination of chemical and biological components 7 could provide a more promising approach. For ammonia photoproduction, a semisynthetic photosystem combining chemical water photolysis (Fig. 1) with enzymatic reduction of either dinitrogen or nitrate (Fig. 2) could have some merit. A compound such as methyl viologen could link the artificial and biological reactions. The application of these semisynthetic photosystems in the design of solar bioreactors could have substantial practical impact, especially if photosensitive components and []

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N2

li,

2 NH4 +

--0.28 V

nitrate reductase

nitrite reductase

2e

6e

NO 3-

-__

the enzymes were immobilized on inert supports. References

1 Losada, M., Herv~s, M. and Ortega, J.M. (1987) in Inorganic Nitrogen Metabolism (Ullrich, W. R., Aparicio, P. J., Syrett, P. J. and Castillo, F., eds), pp. 3-15, Springer-Verlag 2 Grassi, G. and Hall, D. O. (eds) (1988) Photocatalytic Production of EnergyRich Compounds, Elsevier Applied Science 3 De la Rosa, M. A., Roncel, M., Navarro, J. A. and De la Rosa, F. F. (1986) Era Solar 21, 33-53 4 Ramos, J. L., Guerrero, M.G. and []

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(~ 1989, Elsevier Science Publishers Ltd (UK)

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characterizing the dispersion state of air bubbles at various locations in the reactor under actual process conditions. An ultrasound pulse transmission technique enables such measurements to be made. M e a s u r i n g the interfacial area

Transfer of oxygen from the gas to the liquid phase is often limited by the liquid diffusion film adjacent to the gas-liquid interface. If so, then the rate of oxygen flow from the bubble to the liquid is proportional to the surface area of the bubble. Similarly, the specific oxygen transfer rate (OTR: the rate per unit volume) is proportional to the specific interfacial area a (the bubble surface area per unit volume). Many parameters determine OTR, though a is the only one which can be altered greatly without profoundly changing the fermentation conditions in other

0167 - 9430/89/$02.00

NH4+

+0.33 V

+0.42 V

Characterizing bubbles in bioreactors by ultrasound In many important chemical processes like hydrogenation, chlorination or fermentation, mass has to be transferred from the gas phase, in bubbles, to the liquid phase. In aerobic bioprocesses, the oxygen transfer rate is a crucial parameter: it determines not only the productivity of the bioreactor but also profoundly affects product yields and production rates. Oxygen-depleted zones in the reactor may irreversibly damage biomass or force it to modify its metabolism. Preventing oxygendepleted zones entails maintaining excellent aeration and high oxygen transfer rates in the whole reactor volume, and this is especially difficult in large scale bioreactors and viscous, non-newtonian liquids like fermentation broths. Systematic improvement of aeration in bioreactors requires a way of

P

NO 2-

Losada, M. (1982) Appl. Environ. Microbiol. 44, 1013-1019 5 Ramos, J. L., Guerrero, M.G. and Losada, M. (1984) Appl. Environ. Microbio]. 48, 114-118 6 Postgate, J. R. (1982) The Fundamentals of Nitrogen Fixation, Cambridge University Press 7 Willner, I. (1989) The Spectrum 2, 10-11 M I G U E L A. DE L A ROSA

Instituto de Bioquimica Vegetal y Fotosintesis, Universidad de Sevilla y CSIC, Apartado 1113, 41080Sevilla, Spain. []

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ways (Fig. 1): a increases with the gas volume fraction c and inversely with the Sauter mean diameter 1 of the bubbles Dsm (Box 1). The gas volume fraction c can be increased to 30-40% (although this reduces the working volume in the reactor), by increasing the air flow and the residence time of the bubbles in the reactor. Ds~ can be reduced to less than I mm with mechanical agitation or by preventing bubble coalescence. Theoretically, therefore, a could be as high as 2000 m -1 but, in practice, it rarely exceeds 400 m -1.

Existing measuring methods There are already measuring methods which provide local values of a and Dsm. Photography is the most straightforward: bubbles on photographs of the dispersion are measured and counted. In the light transmission method 2, the reduction in intensity of a light beam transmitted through the bubble dispersion is related to the interfacial area of the bubbles. Both the photo-