“Once upon a time there was a family living in a small town at the base of a big mountain. On top of that mountain a tremendous dragon, unbeatable by any man, guarded the only known passage to the next valley where a treasure was located.
One day, the two sons of the family decided to try to find the treasure in order to help their family. They started to climb the mountain but soon they realized they had underestimated the ascent. They had too little energy left, ran short on food and decided to return.
On their way back, out of nowhere a Sorcerer appeared in front of them. He claimed he knew a secret passage through the mountains, leading to the valley. Desperate to find the treasure, the two brothers decided to follow the strange man through the secret passage. The road was not easy, but it allowed them to reach the other side of the mountain without having to climb all the way up and having to face the dragon. After a few hours walk through this passage, the boys reached the valley and found the treasure.”
Well… you may not have realized it, but you just had your first lesson in catalysis! This little story explains what a catalyst is. A catalyst offers an alternative path for a reaction, often more complex but much more energetically favorable. The little road through the mountain –revealed by the Sorcerer (catalyst)- offered an alternative, selective and energetically favorable route, enabling the son to get to the valley without having to waste energy on climbing the mountain or even facing the dragon.
By definition, a catalyst is any substance that increases the rate of a reaction without itself being consumed. But if a reaction is thermodynamically unfavorable, a catalyst cannot change it. It acts only on reaction kinetics, not the thermodynamics, and it does so by forming bonds with the reacting molecules, and by allowing these to react to a product. Then the product leaves the catalyst, which unaltered continues to catalyze on the next reaction. In fact, we can describe the catalytic reaction as a cyclic event in which the catalyst participates and is recovered in its original form at the end of the cycle.
But have you ever considered where you can find a catalyst? Some might say at the back of a car, just before the exhaust, that filters the bad gases your car is producing. It is true that the automotive exhaust converter represents an extremely successful application of catalysis. However, there is a wide range of applications in which catalysis can take place. About 85 to 90 % of chemicals produced are made with catalytic processes. On the other hand, what about the enzymes in your body? Enzymes are nature’s catalysts, which allow biological reactions to occur at the rates necessary to maintain life.
In general the field of catalysis is divided in three sub disciplines:
- Heterogeneous Catalysis (solids catalyze reactions of molecules in gas or solution)
- Homogeneous Catalysis (both the catalyst and the reactants are in the same phase)
- Biological Catalysis (enzymes catalyze biological reactions)
The chemical industry of the 20th century could not have developed to its present status on the basis of non-catalytic, stoichiometric reactions alone. Reactions can in general be controlled on the basis of temperature, concentration, pressure and contact time. Raising the temperature and pressure will enable stoichiometric reactions to proceed at a reasonable rate of production, but the reactors in which such conditions can be safely maintained, become progressively more expensive and difficult to make. In addition, there are thermodynamic limitations to the conditions under which products can be formed and without catalysts, many reactions that are common in the chemical industry would not be possible, and many other processes would not be economical. In this way efficient catalysts, in combination with optimized reactor and total plant design, are the key factor in reducing both the investment and operation costs of chemical processes.
|||Parts of the article are reproduced from the book Concepts of Modern Catalysis and Kinetics, Second Edition. I. Chorkendorff, J. W. Niemantsverdriet Copyright © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim|