The most commonly used catalysts in HDS are transition metal sulfides, particularly molybdenum disulfide (MoS2) promoted with cobalt (Co) or nickel (Ni). These catalysts are typically supported on a porous material such as alumina (Al2O3). The Co-Mo/Al2O3 and Ni-Mo/Al2O3 catalysts are highly effective due to their ability to activate hydrogen and facilitate the cleavage of C-S bonds in sulfur-containing molecules.