A catalyst is a chemical compound that acts to help promote or speed up the rate of a chemical reaction. The catalyst itself is not incorporated into the product, it simply allows the reactants to chemically react with one another in a timely fashion to make the product or products. Metals, particularly transition metals in the center of the periodic table, are commonly used catalysts. Iron (Fe on the periodic table) is a catalyst for some important industrial chemical reactions. One major advantage it has is that it is plentiful and inexpensive, thus making the use of it on the metric ton scale feasible. Many other metals are not nearly as inexpensive. One disadvantage to iron is than over time it will oxidize in the presence of air to form rust (iron oxide), which is generally a useless form of iron.