The term “phase” refers to the state of matter; there are solid, liquid and gas phases. A heterogeneous catalyst is a participant in a chemical reaction that is not part of the same phase of matter as the actual reactants. For instance, liquids may undergo reaction in the presence of a solid catalyst. While the catalyst speeds the process of a reaction, it itself remains unconsumed by the reactants. Precious or other transition metals are often used as a heterogeneous catalyst, and may be finely divided for increased surface exposure over a substrate or a carrier.
It is the heterogeneous catalyst rather than the homogeneous variety that is most widely used industrially. The exposed catalytic surface provides sites for weak surface bonding of geometrically aligned reactants. This behavior is important, as in, for example, the hydrogenation of carbon-carbon double bonds. It is good to consider an example such as ethylene, which has the chemical structure H2C=CH2. As one molecule of ethylene approaches a bit of catalytic surface, it adsorbs or becomes attached by a lower left hydrogen atom and a lower right hydrogen atom — the upper left and right hydrogen atoms remaining free.
A single molecule of hydrogen, H2 or H‒H, can then add across the double bond, being replaced by a single bond, forming a "saturated" ethane or H3C‒CH3. There are two ways the hydrogen atoms can add across the double bond, however. Either both can add from below the double bond, or one can add below, while the other adds above the double bond at the other side. Addition of both hydrogen atoms to one side of a double bond is called "cis-addition," whereas adding one to the one side and the other to the other is called "trans-addition." Doubtless, the expression "trans-fat" — used to describe an unsaturated fat which has been hydrogenated by trans-addition — will be familiar to the reader.
Catalysts pick up the speed of the reactions they are used for because the reaction pathway is altered by their presence. This changes the transition state and lowers the activation energy required to carry out the reaction. One advantage to such a reaction carried out with a heterogeneous catalyst is the ease of recoverability of the catalyst. Heterogeneous catalysts are especially well-suited for continuous-process chemical reactions, in which material is provided, reacted, removed and replaced, continuously. An example of such a heterogeneous catalyst process from the petroleum industry is the use of pelletized catalytic material in the so-called "moving bed" process.