CATALYSTS

Catalysts are substances that trigger or speed up chemical reactions (without chemically altering the catalysts in the process). A catalyst combines with a reactant to form an intermediate compound that can more readily react with other reactants.

An example of this is the formation of sulfur trioxide (SO₃), which is an important ingredient for producing sulfuric acid (H₂SO₄). Without a catalyst, sulfur trioxide is made by combining sulfur dioxide (SO₂) with molecular oxygen:

 2SO₂ + O₂ → 2SO₃. 

Because this reaction proceeds very slowly, manufacturers use nitrogen dioxide (NO₂) as a catalyst to speed production of SO₃:

Step One: 

NO₂ (catalyst) + SO₂ → NO + SO₃ (SO₃ is extracted and combined with steam to produce sulfuric acid)

Step Two:

 NO (from Step One) + O₂ → NO₂ (catalyst that is reused in step one)

In the above reactions, nitrogen dioxide (NO₂) acts as a catalyst by combining with sulfur dioxide (SO₂) to form both sulfur trioxide (SO₃) and nitrogen monoxide (NO). The sulfur trioxide is removed from the process (to be used in the production of sulfuric acid). Nitrogen monoxide (NO) is subsequently combined with molecular oxygen (O₂) to produce the original catalyst, nitrogen dioxide (NO₂), which can be continually reused to catalyze sulfur trioxide (SO₃₎.