Mechanisms of Catalysis - The Surface Adsorption Theory
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The surface adsorption theory is used to describe most of what type of catalysis?
Heterogeneous
What is absorption?
When one substance moves into another
What is an example of absorption?
Water moving into a sponge
What is adsorption?
The accumulation of substances only at the surface of another substance
Give an example of adsorption.
Gas molecules accumulating on the surface of a solid
Give an example of a reaction which involves adsorption
The reaction between hydrogen and oxygen to form water in the presence of a platinum catalyst
How many main steps are there to adsorption?
Three
What are the three main steps of adsorption called?
• Adsorption Stage • Reaction On The Surface • Desorption Stage
Using the example of the reaction between H and O to form H2O in the presence of a Platinum catalyst, describe what occurs in the first step of Adsorption.
• The H and O molecules diffuse towards the surface and settle on the surface of the platinum • The O and H molecules are held on the surface of the platinum by temporary bonds formed between these molecules and the platinum • The vacant d orbitals of platinum help form temporary bonds with the molecules i.e. the molecules are chemisorbed on the platinum
Define chemisorbed
Chemically attached
What holds the O and H molecules to the surface of the platinum?
Temporary bonds formed using the vacant d orbitals of platinum hold the molecules to the surface
Why does platinum have vacant d orbitals?
As it is a transition metal
Using the example of the reaction between H and O to form H2O in the presence of a Platinum catalyst, describe what occurs in the second step of Adsorption.
There is a high concentration of O and H molecules adsorbed to the surface of the platinum which makes it more likely that molecules will collide with each other These collisions result in bonds being broken and new ones being formed (chemical reactions occur to form water molecules)
Why is collision more likely to occur on the surface of a catalyst?
As there is a high concentration of reactants adsorbed to it
What helps the chemical reactions which occur when two reactants collide at a catalyst ?
The covalent bonds holding the atoms together are weakened by the formation of new bonds between the reactants and the surface of the catalyst
Using the example of the reaction between H and O to form H2O in the presence of a Platinum catalyst, describe what occurs in the third step of Adsorption.
The products (water molecules) leave the surface of the catalyst and the process begins again