**1. Introduction**

Green plants and photosynthetic bacteria are responsible for storing solar energy in chemical bonds via photosynthesis. Photosynthesis is not only the major source of food, fuel and oxygen on earth, but it is also the key player in the global carbon cycle by converting 120 gigatonnes of carbon per year.

Conversion of solar energy into chemical energy through utilization of inorganic materials by photocatalytic CO2 reduction; which is also known as *'Artificial Photosynthesis'* is the next challenge for a sustainable development. In the present state-of-the art artificial photosynthesis processes, nature is so far mimicked only to the extent that CO2 is reduced by water to valuable 1- carbon chemicals, not to the multi-carbon equivalents of glucose or cellulose yet. Although mimicking nature is viable by photocatalytic means, enhancing photocatalytic CO2 reduction rates is vital in order to achieve artificial photosynthesis in industrial scales. To illustrate the gap between photosynthetic and photocatalytic rates, we will compare the turnover frequencies of water oxidation process below.

Water oxidation is the key step both in photocatalysis and photosynthesis for being the carbon free hydrogen source and also for providing oxygen for the oxygen consuming organisms. Completion of an S cycle taking place in a Mn4 cluster which is responsible for water oxidation was reported to last for 1.59 ms in order to produce one molecule of oxygen at that one particular site (Haumann et al., 2005). In other words, molecular oxygen is produced in photosynthesis, with a turn over frequency of 630 molecule/site/s. On the other hand, typical rates of photocatalytic synthesis of hydrocarbons are of the order of 30 µmoles/g cat/h, (Ozcan et al., 2007; Uner et al., 2011) which amounts to 1.11\*10-5 molecule/site/s if the typical surface areas of 45 m2/g cat and typical site densities of 1015/cm2 are used. Of course the remarkable rates of 9 µmoles of O2/cm2/s (Kanan & Nocera, 2008), giving a turn over frequency of 5400 molecule/site/s for an oxygen evolving cobalt- phosphate catalyst operating at neutral water is keenly followed by the academic community. Considering the huge gap between photosynthetic and photocatalytic rates reported above, one can easily claim that there is room for further investigation and development in photocatalytic CO2 reduction systems.

It is also important to see the thermodynamic energy demand of the some of the reactions between CO2 and H2O. For this, a number of products are chosen and the standard Gibbs free energy of formation values are listed in Table 1.1 for comparison. The interesting

Artificial Photosynthesis from a Chemical Engineering Perspective 15

Photosynthesis includes a series of photophysical, photochemical and chemical reactions realized by highly sophisticated protein complexes, energy carriers and enzymes. With all the complexity of their mechanisms, reactions involved in photosynthesis are mainly divided into two stages: (i) light dependent reactions including water oxidation and chemical energy generation through electron and proton transport and (ii) light independent reactions including CO2 fixation, reduction and regeneration of ribulose 1,5

The light induced reactions occur in a complex membrane system (thylakoid membrane) via electron transfer through light induced generation of cation- anion radical pairs and intermediate charge carriers such as plastoquinone, plastocyanin and ferrodoxin. Light dependent reactions in green plants follow a Z scheme which was first proposed by Hill & Bendall, 1960 (Figure 2.1). In this scheme, light energy is absorbed by light harvesting molecules and funneled to two special reaction center molecules; P680 and P700 which are acting as major electron donors in PS II and PSI respectively. Electron transport from PSII to PS I is realized by intermediate charge carriers and electron need of P680+ (strong oxidant

Electron transport through thylakoid membrane and water oxidation reactions results in a proton concentration gradient across the thylakoid membrane. Energy created by proton electrochemical potential resulting from this proton gradient is used by ATP synthase to produce ATP from ADP and Pi. The net reaction in light dependent reaction system is the electron transport form a water molecule to a NADP+ molecule with the production of ATP molecules (Figure 2.2).In this complex electron transport system, PS II alone is composed of more than 15 polypeptides and nine different redox components including chlorophylla and

with E0 = 1.1 eV) in PSII is compensated from water molecules (water oxidation).

Fig. 2.1. Z scheme electron transfer in terms of redox potentials (Ke, 2001)

**2.2 Reactions** 

biphosphate (Calvin Cycle).

**2.2.1 Light induced reactions** 

b, pheophytin, plastoquinone.

observation that we make in this table is the following: when compared per mole of hydrocarbon formed, the Gibbs free energies of formation increase with increasing carbon chain length. But when the Gibbs free energy formation values are normalized per mole O2 formed, one can compare the energy demand of the reactions on a common basis. A close examination of the data in the last column reveals the fact that energetically almost all of the reactions are similar. The second conclusion we can arrive at is that once the water splitting reaction is possible, the formed hydrogen can drive the subsequent reduction reactions, almost spontaneously.


Table 1.1. The thermodynamics of the reactions involved in carbon dioxide reduction
