From Nature.com,
Heterogeneous catalysis is a type of catalysis in which the catalyst occupies a different phase from the reactants and products. This may refer to the physical phase — solid, liquid or gas — but also to immiscible fluids. Heterogeneous catalysts can be more easily recycled than homogeneous, but characterization of the catalyst and optimization of properties can be more difficult.
About 90% of all industrial processes involve heterogeneous catalysis. The catalyst is a solid while the reactants are in gas or in the liquid phase. Compared to the homogeneous catalysis, heterogeneous catalysts are easier to prepare, handle and separate from the reaction mixture. Usually they consist of nano-sized powders supported on different substrates, such as Alumina, Zirconia or Carbons. The catalyst is often a metal. The support-metal system can be tuned to enhance the catalytic efficiency and /or the selectivity. The catalytic process is run inside a batch or continuous flow reactor, where the rate is determined, apart from the nature of the catalytically active surface, by external parameters like temperature, partial pressures and flow rate.