DETAIL ON CATALYSIS

  

Berzelius in 1836 found that there are substance that increase the rate of a chemical reaction without   being consumed. He believed that the function of such substance was to weaken the bonds that hold the atoms in the reacting molecules together. Thus he coined the term catalyst for such substances. The word catalyst is derived from Greek word; kata = wholly and lein = to loosen.

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In most of the cases catalysts increase the rate of chemical reactions, but a number of cases are known in which catalysts are also found to decrease the rate of reactions. Thus a catalyst is defined as a chemical substance that alters the rate of a chemical reaction without itself undergoing any change in mass and its chemical composition.

When potassium chlorate is heated, it melts at about 340°C but no oxygen is evolved. It decomposes to evolve oxygen only around 600°C. However, on the addition of a little amount of manganese dioxide it decomposes even below its melting point producing oxygen. Furthermore, after the decomposition is over, no change is observed in the quantity and composition of manganese dioxide. In this reaction manganese dioxide is a catalyst. Similarly, hydrogen and oxygen at room temperature do not react for years but in the presence of small amount of platinum black, they react at room temperature to give water. In this case, platinum black is a catalyst.

The use of catalyst in industrial production of chemicals minimizes the production cost by lowering the consumption of energy and also by making the reaction fast. Annually, billions of dollars are saved globally by the use of catalyst. More than 95% of the chemical produced in the world use catalyst in one or more steps of the reactions.

Catalysts increase the rate of reaction by providing alternative reaction pathway with lower activation energy so more reactant molecules can cross the activation energy barrier at any given concentration and temperature.

As evident from the definition, a catalyst may increase or decrease the rate of reaction. A catalyst that increases the rate of reaction is known as positive catalyst and the process is positive catalysis. Most of the used catalysts are positive catalysts. Few examples of positive catalysts are mentioned below:

1.      Manganese dioxide as a catalyst accelerates the decomposition of potassium chlorate to liberate oxygen.

2.      Platinum as a catalyst accelerates the decomposition of hydrogen peroxide.

3.      Platinum black as a catalyst accelerates the combination of hydrogen and oxygen to form water.

Similarly, the catalyst that decreases the rate of reaction is known as negative catalyst or inhibitor and the process is known as negative catalysis or inhibition. Negative catalysts are useful to slow down or stop any unwanted reactions. Few examples of negative catalysts are mentioned below:

1.     
The presence of 1-2% ethanol as catalyst, suppresses the oxidation of chloroform with oxygen to give a poisonous gas called phosgene.

              

2.     
The decomposition of hydrogen peroxide is suppressed by adding glycerol to the solution of hydrogen peroxide. Here glycerol acts as negative catalyst.

Criteria or characteristics of catalysts

i.            The mass and chemical composition of catalyst should remain unchanged at the end of the reaction.

ii.            A small amount of catalyst is enough to bring about an appreciable change in the rate of chemical reaction. For example, even 1 mg of fine platinum powder is enough to catalyze the combination of 2.5 liters of a mixture of hydrogen and oxygen to form water. However, there are some reactions that uses significant amount of catalyst.

iii.            For solid catalyst, the use in powder form increases the activity of catalyst by increasing its surface area.

iv.            For a reversible reaction, catalyst does not change the position of equilibrium but helps to attain the equilibrium faster.

v.            Catalysts are specific in action (like a key can open a particular lock). For example, manganese dioxide can

catalyze the decomposition of potassium chlorate but not potassium nitrate or other substances. A catalyst

effective in altering the rate of one reaction may not be necessarily effective in another reaction. The same reactants may give different product with different catalyst.

                

vi.            Catalyst by itself does not initiate a reaction. They only make the reaction fast.

vii.            A catalyst is most active at a particular temperature known as optimum temperature.

he catalysts are divided into two major types: