Dimethyl Disulfide (DMDS) is the most commonly used chemical for sulfiding hydrotreating and hydrocracking catalysts.  These hydroprocessing catalysts contain metal oxides that must be converted to the active metal sulfide before they will promote desulfurization and denitrification reactions on hydrocarbon feeds.  The activation process, often called presulfiding, begins with the introduction of DMDS into a reactor pressurized with hydrogen at a temperature of 370F or higher.  Under these conditions and when exposed to a hydrotreating catalyst, DMDS rapidly decomposes, forming the H2S required to convert the oxides to metal sulfides. Note that if no catalyst is present, thermal decomposition of DMDS to H2S will not occur until the temperature exceeds 1000°F.