In heterogeneous catalysis, bulk material catalysts are used to convert gaseous or liquid reactants. On an industrial scale, fixed bed reactors are generally used for these types of reactions. The actual catalyst–i.e. the active catalytic substance–may be used alone or on a carrier. Carriers are used in situations where high demands are placed on the mechanical strength of the catalyst, the active catalytic substance must be present in a thin layer or there is a need to conserve valuable catalyst substances. A variety of materials are used to create catalyst carriers, during which ceramic catalyst carriers are an important group of carrier materials in heterogeneous catalysis.