Catalysts can either be in the same phase as the chemical reactants or in a distinct phase.
Catalysts in the same phase are called homogeneous catalysts, while those in different phases are called heterogeneous catalysts.
For example, if we have Pt metal as a catalyst for the reaction of hydrogen gas and ethene gas, then the Pt is a heterogeneous catalyst. However, an enzyme in solution catalyzing a solution phase biochemical reaction is a homogeneous catalyst.
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