Materials can be defined as solids with specific properties and therefore intended for certain applications. Catalytic Materials (CMs) are thus solids whose specific properties are their activity and their selectivity for a certain chemical reaction. They are also called solid catalysts or heterogeneous catalysts. CMs are complex high-surface area solids, amorphous or crystalline (in most of the cases polycrystalline), generally containing more than one component, with surface compositions and structures different from those of the bulk and susceptible to change during the catalytic process. The development of CMs is strongly related to the development of the science of catalysis and, particularly, to the development of industrial heterogeneous catalysis. Most of CMs are complex systems, amorphous or crystalline, consisting of several components and phases. Traditionally they are classified according to the chemical nature of the active component, in metals, metal oxides, and metal sulfides.