Silica-supported Pt and Pt-Sn catalysts for the hydrogen assisted 1,2-dichloroethane dechlorination have been investigated. The addition of Sn to Pt suppresses catalyst deactivation and dramatically alters product selectivity. While the main product of the reaction catalyzed by Pt and Pt-Sn with a Pt/Sn atomic ratio greater than or equal to1 is ethane, balanced by ethyl chloride, the catalysts with a Pt/Sn ratio <1 produce ethylene (up to 100% for Pt1Sn3/SiO2; catalyst nomenclature is based on the metal atomic ratio) with ethane as a balance. In situ Mossbauer spectroscopic results show that a considerable fraction of Sn in ethylene-selective Pt1Sn3/SiO2 and Pt1Sn2/SiO2 and in unselective Pt1Sn1/SiO2 catalysts forms Pt-Sn alloys, with the rest of the Sn present as Sn4+ and Sn2+. There is only a Sn-rich Pt-Sn alloy in the Pt1Sn3/SiO2 catalyst after reduction at 220degreesC while both Sn-rich and Pt-rich Pt-Sn alloy species are present in the Pt1Sn1/SiO2 and Pt1Sn2/SiO2. It is suggested that Sn-rich Pt-Sn alloys are responsible for the ethylene formation in the CH2ClCH2Cl + H-2 reaction. The different catalytic performance of Pt1Sn2/SiO2 and Pt1Sn1/SiO2 catalysts is explained by their different microstructures. In the ethylene-selective Pt1Sn2/SiO2, the Sn-rich Pt-Sn alloy phase encompasses the Pt-rich one, forming a cherrylike structure, whereas in the unselective Pt1Sn1/SiO2, both the Pt-rich and Sn-rich alloy phases are exposed to the reaction mixture.