In heterogeneous catalysis, the substance is considered to be a catalyst if, in its absence, the reaction does not occur, or it occurs much slower. A special case of heterogeneous catalysis, electrocatalysis involves oxidation or reduction by electrons transfer across the interface between the electrode and electrolyte, while photocatalysis uses light absorbed either by the catalyst or by the reactant during the reaction. The identification of active sites in heterogeneous catalysis was and still is a challenging task, due to the fact that not all sites on the surface have the same activity. In heterogeneous catalysis, the porous solids or fine powders with large specific surface areas offer different types of sites for catalytic reactions, provided that reactants have access to the centers located inside the pores. Base chemicals like butadiene, isoprene, styrene, and vinyl chloride monomers, alcohols, ethers, ketones and aldehydes, ethylene oxide, are just a few of the compounds produced using heterogeneous catalysis.