**4. Similarities and differences between photosynthesis and artificial photosynthesis**

Analogy between photosynthesis and artificial photosynthesis is in the similar tools and methods utilized in both systems. Collecting solar energy for triggering chemical reactions by chlorophyll pigments packed in thylakoid membrane or by semiconductors; oxidizing water into molecular oxygen and protons and reducing CO2 with transported electrons and H+s are among the similarities of the two systems. However, the gap between the design of the systems and number of reaction sites and intermediate molecules result in more sophisticated and simpler products in photosynthesis ((CH2O)6) and in photocatalysis (CH4 or CH3OH) respectively.

In photosynthesis, there are three major reaction centers in light dependent reactions, regulating electron and proton transport together with the intermediate charge carriers (redox components). In photocatalysis, on the other hand, design of the system is limited to the presence of a pool of charges wandering on the semiconductor/metal surface in an unregulated fashion, increasing the chance of recombination of charge carriers. In addition, realization of oxidation and reduction reactions on the same catalyst surface results in interactions between the surface adsorbates which is proven to be inhibitory on reaction rates as studied with the microkinetic model in Section 3.1.

In photosynthesis, CO2 diffusion from atmosphere to stromal phase in chloroplasts is controlled by stomata activities and permeability of chloroplast membranes. Photosynthetic rate is limited with the CO2 concentration in stromal phase for values lower than a saturation value; i.e., the photosynthetic rate is linearly increasing with CO2 concentration. For CO2 concentrations above the saturation value, photosynthetic rate stays constant, limited by the rate of the enzyme system. Since CO2 concentration in the stromal phase is related to CO2 diffusion, photosynthetic rate is dependent upon diffusion rates.

In photocatalysis, diffusion of dissolved CO2 and other reactants/products to/from the catalyst surface is largely dependent upon the reactor types, reaction media and stirring rates. The photocatalytic experiment parameters are not standardized in literature, resulting in confusion about the proper comparison of the real kinetic data. For photocatalytic tests performed in liquid media, which constitute the majority of the tests reported in literature, presence of gas-liquid-solid interfaces imposes non negligible mass transfer limitations in observed rates. A study performed to reveal the effect of stirring rates on photocatalytic hydrogen evolution rates indicated the importance of boundary layer and gas-liquid equilibrium in liquid phase photocatalytic experiments (Figure 4.1) (Ipek, 2011).

Increase in photo catalytic rates with increasing stirring rates (from 350 rpm to 900 rpm) up to a certain hydrogen concentration could be interpreted as decreased mass transfer limitation effects due to thinning of the boundary layer surrounding the catalyst particles whereas after that concentration, hydrogen seems to accumulate in the gas phase with the same limiting liquid-gas transfer rate. The limitation at the gas-liquid interface could also be inferred from the overlapping hydrogen amounts accumulated in the gas phase regardless of the catalyst amount or the reaction mixture volume (Figure 4.2).

Artificial Photosynthesis from a Chemical Engineering Perspective 31

In PSII of photosynthesis, there are over 15 polypeptides and 9 different redox components responsible for water oxidation and electron transport. Even the oxygen evolving complex is regenerating itself at every 30 minutes in order to sustain its stability. Along with the sophistication of light dependent reactions including numerous intermediate charge carriers, difference in the CO2 reduction mechanism (activating CO2 by fixing it into another chemical) with 13 specific enzymes result in higher photosynthetic rates and more complicated products (such as glucose) in photosynthesis. On the other hand, C-C bond making is still remaining as a challenge in artificial photosynthesis systems. Even with one carbon chemical synthesis, photocatalytic rates are well below photosynthetic rates. To illustrate, CO2 reduction using titanium nanotubes resulted in nearly 1 nmol/m2/s CH4 production rate (Schulte et al., 2010) whereas an avarage photosynthetic rate is 12

Apart from the sophisticated numerous enzymes taking part in carbon dioxide reduction mechanism, the major gap between carbon dioxide reduction rates are suggested to result from undeveloped charge and H transport systems in artificial photosynthesis systems. Recently, regulated electron and hole transport is reported with zeolites increasing the charge separation and water oxidation activity (Dutta & Severance, 2011). Furthermore, H+ transport through oxidation center to the reduction center is reported to be realized with H+ permeable electron conducting membrane (Hou et al., 2011). Design of a reaction system as well as the photocatalyst is of uttermost importance in artificial photosynthesis systems for better activities. As indicated in the microkinetic analysis part, carbon dioxide reduction rates mainly suffer from insufficient H supply to the reduction centers at artificial photosynthesis. In photosynthesis, H transport is highly regulated via the intermediate charge carriers such as NADP+ and via the electrochemical potential difference which may be attributed to the presence of an interstitial membrane. Utilization of such a membrane in photocatalytic systems was first suggested by Kitano et al., 2007b who used the membrane for H transport in water splitting reaction. Similarly, enhanced electron and H transport system should be implemented for carbon dioxide reduction also which would carry the

artificial photosynthesis systems to industrial levels.

Fig. 4.3. Schematic illustration of suggested reactor for CO2 reduction

µmol/m2/s.

Fig. 4.1. Effect of stirring rates on photocatalytic hydrogen evolution with methanol as sacrificial agent, with 0.5 wt % Pt/TiO2 , 250 ml deionized water, 2 ml methanol (■) 900 rpm, (●) 350 rpm

Fig. 4.2. Observed hydrogen amount in the gas phase with changing reaction mixture, (■) 62.5 ml (●) 125 ml (▲) 187.5 ml (▼) 250 ml, CH3OH/ H2O: 1/125 (v/v) and Ccat: 1 g/L for each case

Furthermore, temperature sensitivity of the photocatalytic hydrogen evolution reactions indicate presence of diffusion limitations with found 12 and 19.5 kJ/mol apparent activation energies for Pt/TiO2 and Cu/TiO2 reactions respectively (Ipek, 2011).

 900 rpm 350 rpm

0,0 0,5 1,0 1,5 2,0

Fig. 4.1. Effect of stirring rates on photocatalytic hydrogen evolution with methanol as sacrificial agent, with 0.5 wt % Pt/TiO2 , 250 ml deionized water, 2 ml methanol (■) 900 rpm,

> 250 ml 187.5 ml 125 ml 62.5 ml

energies for Pt/TiO2 and Cu/TiO2 reactions respectively (Ipek, 2011).

**Time (h)**

0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

**Time (h)**

Fig. 4.2. Observed hydrogen amount in the gas phase with changing reaction mixture, (■) 62.5 ml (●) 125 ml (▲) 187.5 ml (▼) 250 ml, CH3OH/ H2O: 1/125 (v/v) and Ccat: 1 g/L for

Furthermore, temperature sensitivity of the photocatalytic hydrogen evolution reactions indicate presence of diffusion limitations with found 12 and 19.5 kJ/mol apparent activation

0

100

200

**Micromoles of H2 observed**

300

400

**Produced hydrogen amount (mol H2/gcat)**

(●) 350 rpm

each case

In PSII of photosynthesis, there are over 15 polypeptides and 9 different redox components responsible for water oxidation and electron transport. Even the oxygen evolving complex is regenerating itself at every 30 minutes in order to sustain its stability. Along with the sophistication of light dependent reactions including numerous intermediate charge carriers, difference in the CO2 reduction mechanism (activating CO2 by fixing it into another chemical) with 13 specific enzymes result in higher photosynthetic rates and more complicated products (such as glucose) in photosynthesis. On the other hand, C-C bond making is still remaining as a challenge in artificial photosynthesis systems. Even with one carbon chemical synthesis, photocatalytic rates are well below photosynthetic rates. To illustrate, CO2 reduction using titanium nanotubes resulted in nearly 1 nmol/m2/s CH4 production rate (Schulte et al., 2010) whereas an avarage photosynthetic rate is 12 µmol/m2/s.

Apart from the sophisticated numerous enzymes taking part in carbon dioxide reduction mechanism, the major gap between carbon dioxide reduction rates are suggested to result from undeveloped charge and H transport systems in artificial photosynthesis systems. Recently, regulated electron and hole transport is reported with zeolites increasing the charge separation and water oxidation activity (Dutta & Severance, 2011). Furthermore, H+ transport through oxidation center to the reduction center is reported to be realized with H+ permeable electron conducting membrane (Hou et al., 2011). Design of a reaction system as well as the photocatalyst is of uttermost importance in artificial photosynthesis systems for better activities. As indicated in the microkinetic analysis part, carbon dioxide reduction rates mainly suffer from insufficient H supply to the reduction centers at artificial photosynthesis. In photosynthesis, H transport is highly regulated via the intermediate charge carriers such as NADP+ and via the electrochemical potential difference which may be attributed to the presence of an interstitial membrane. Utilization of such a membrane in photocatalytic systems was first suggested by Kitano et al., 2007b who used the membrane for H transport in water splitting reaction. Similarly, enhanced electron and H transport system should be implemented for carbon dioxide reduction also which would carry the artificial photosynthesis systems to industrial levels.

Fig. 4.3. Schematic illustration of suggested reactor for CO2 reduction

Artificial Photosynthesis from a Chemical Engineering Perspective 33

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Design of a photocatalytic reactor would work in such a way that in one compartment light is harvested by the semiconductor in order to split water to form H+s and by the transfer of produced protons and electrons, CO2 could be reduced with another catalyst, such as copper, in the other compartment. In this way, with the help of proton exchange membranes and separate reaction centers, interaction between reactants or products and therefore reverse reactions could be prevented by supplying Hs to catalytic compartment at the same time. On the other hand, a high conductance electron membrane would prevent charge recombination as illustrated in Figure 4.3.

Such a system has already been proposed by Kitano et al. (2007b) for water splitting reaction. Introducing CO2 in the picture will be the next generation modification of the artificial photosynthesis systems. Further refinement of the design will be possible with more understanding about the rates of chemical conversions and the rates of transport.
