# Lesson 7 - Catalysis

What you have learned so far

In lessons 4, 5 and 6 you have discovered how the components, from which the liquid fuel can be made, can be collected and/or produced. You have seen how electrical energy can be generated from solar light energy. Furthermore you have seen how this electrical energy can be used to produce hydrogen from water. Finally you have seen that CO2 can be collected and concentrated. In this lesson you will discover how the liquid fuel can be synthesized from these components (see figure 30), but first we will get you (re)acquainted with activation energy and catalysis.

Activation energy

In lesson 5 the energy-diagram of the reaction of H2 with O2 to form H2O was shown. The chemical energy within the reactants H2 and O2 is higher than the chemical energy within the product H2O. Energy is therefore released during the reaction. Can you remember the observations you did during the indication reaction of hydrogen gas? This reaction did not run spontaneously. You had to ignite the reaction mixture which means that the gas mixture had to be heated to reaction temperature first. In other words: the reaction had to be activated. This is indicated by the small bump, or barrier, in the energy-diagram, just to the right of the reactants (see figure 21). The height of this barrier is called “the activation energy”. This activation energy is caused by the higher energy content of the components, or intermediate products, in this so-called transition state of the reaction. Within this transition state the bonds within the reactants have partially been broken and new bonds have partially been formed.

In this example the bond between the two oxygen atoms of O2 has to be broken before water can be formed. The energy that is needed to break this bond, attributes to the activation energy. During she second part of the reaction, wherein new bonds are formed, energy is released. The released energy from this exothermic reaction is enough to help other molecules to overcome the energy barrier and to make the reaction run by itself. In an endothermic reaction the energy that is released by going from the intermediate to the final products is not high enough to overcome the energy barrier of other molecules and the reaction will stop when no more energy is added from the outside.

Catalysis

As an introduction to this section you can watch the YouTube video on catalysis, made by Southampton University (http://www.youtube.com/watch?v=A_PhvIktMOw). Besides increasing the temperature (adding thermal energy to the reaction mixture) to overcome the activation energy, a catalyst can be used to increase the reaction rate. The catalyst causes other intermediate products to be formed, causing the activation energy to be decreased. Therefore, at equal temperature, more particles will have enough energy to react.

CO2 and water as reactants

Different types of catalysts can result in different reaction paths with different intermediate products and, based on the activation energy of these paths, different products. The selection of the catalyst is therefore an important step in the design of the chemical process. In this module we want to produce fuel from CO2 en water.

Depending on the feedstock, reaction circumstances and type of catalysts, many different types of fuels can be made. Al of these fuels have different properties that influence, among others, the safety precautions that have to be taken when working with these fuels, or the possibilities of the fuels to be used in the different types of engines (which will be discussed in lesson 8. We will focus on the following products:
1. Methane
2. Methanol
3. Ethanol
4. DME (dimethyl ether)
5. Formic acid
6. Alkanes
The alkanes are not a single substance, but is a group of substances. Again, depending on the choice of circumstances and catalyst, there is a wide selection of alkanes that can be produced.

The production of H2 is the most energy-consuming part of the overall solar-tree process. Therefore, the amount of H2 that is needed, in comparison to the amount carbon dioxide, can play in important role in the selection of the fuel.

Other important influences on the fuel selection might be handling, storage and safety aspects.