Catalytic behaviour during Artificial Photosynthesis

Authors Avatar

Catalysis with Respect to Artificial Photosynthesis

Extended Essay

Field: Chemistry

Research Question: What characteristics are important to the development of an artificial catalyst for artificial photosynthesis?

Author: Eamon O’Connor

Table of Contents

  1. Introduction
  1. Introduction to artificial photosynthesis
  2. Summary of Catalysis
  1. Structure of the natural enzyme
  1. Photosystem II summary
  2. Structure of the manganese cubane core
  3. Role of Ca
  4. Role of ligands
  1. Reaction mechanism of water oxidation
  1. S-state cycle
  2. Oxidation states of manganese
  3. Summary of certain proposed mechanisms
  4. Questions regarding to substrate binding
  1. Important Characteristics
  1. Characteristics of manganese
  2. Other characteristics
  1. Conclusion


Natural fossil fuels, the primary source of energy relied upon by our society, run the risk of being entirely consumed within the next 50 years. Without energy, the industrial sector falters. For this reason, we have begun to concentrate on alternative sources of energy. However, most other energy sources would not be able to be used as our primary one. Solar panels cannot produce enough energy to for our industrial needs as they are very inefficient. Nuclear power creates toxic waste which damages the environment, and wind turbines are inefficient and unreliable as they only work intermittently. At the same time, our planet is suffering from nearly irreversible climate change widely believed to be caused by the emission of carbon gases.

The amount of solar energy that hits the earth in a year is roughly 1000 times greater than the amount of energy consumed by the entire human race in that same amount of time. At the moment, plants are the only beings on our planet able to utilize this to create organic chemical energy. This process is called photosynthesis and is represented by the equation:

6CO2(g) + 12H2O(l) + photons → C6H12O6(aq) + 6O2(g) + 6H2O(l). As we can see, this process also consumes CO2, the suspected cause of global climate change and releases oxygen. Therefore artificial photosynthesis is emerging as a theoretical solution for both of these problems.

However, photosynthesis is not a simple process. The chemical reaction which forms C6H12O6 (glucose) is uphill, meaning it requires energy to drive the reaction. On top of this, the reaction requires a constant stream of electrons and protons. Water is among the most abundant compounds on our planet and was therefore chosen as the source of these subatomic particles. The photolysis of water can be represented by the half equation: 2H2O (l) → 4e- + 4H+ + O2 (g) . Unfortunately, two obstacles are present in the path to splitting water: its stability and its inertness. The bond energy of O-H is 110 kcal. Because of this stability, energy is required to effect the desired transformation. It is also extremely inert and therefore requires an effective catalyst. Therefore, scientists’ intent on developing an artificial system for photochemically splitting water must develop two parts, an effective photon harvester to transform the sun’s energy into usable chemical energy and an effective artificial catalyst to accelerate the rate of oxygen evolution, and to lower the minimum amount of required energy for this reaction to occur.

Catalysis is the process in which the rate of a chemical reaction is accelerated by means of a substance known as a catalyst.  A catalyst helps achieve this accelerated rate by providing an alternative reaction mechanism involving a lower activation energy. Hence, the use of a catalyst will result in more molecular collisions with the energy required to reach the transition state and complete the reaction. Alternatively, catalysis can help reactions run at a lower temperature, or with less required energy, as in the case of photosynthesis.

Catalysts are classified into two categories: homogeneous catalysts and heterogeneous catalysts. The latter is in a different phase then the reactants. Heterogeneous catalysts create an intermediate compound with a reactant through adsorption on one of its active sites. Some sort of interaction occurs at the active site that results in increased reactivity of the reactant depending on how the adsorption takes place. For example, in the synthesis of nitrogen and hydrogen to ammonia, the reacting gases adsorb onto the active sites of finely divided iron particles. These intermediate compounds result in a weakening of the bonds of the reacting molecules. Hence, the strong bonds present in N3 are weakened and hydrogen and nitrogen combine with decreased activation energy.  A homogeneous catalyst is a catalyst in the same phase as the reactants. It is consumed in the process and therefore changes the reaction mechanism, but is reproduced somewhere along the reaction.

Join now!

Catalysts are present in living organisms as enzymes. Enzymes are biomolecules composed of many proteins that increase the rate of chemical reactions. Nearly all processes in biological cells require enzymes to occur at significant rates. Enzymes differ from other catalysts in that they are much more specific. They are also extremely efficient, for example, carbonic anhydrase increases the rate of its natural reaction by 107 times. Enzymes, however, function only in very specific conditions and are very fragile if removed from their natural environment. Enzymes function like heterogeneous catalysts in that they form an intermediate compound with the reactants.

Structure ...

This is a preview of the whole essay