Hydrogen-assisted 1,2-dichloroethane dechlorination catalyzed by Pt-Sn/SiO2 catalysts of different preparations

The effect of Sri content on the activity and selectivity for hydrogen-assisted 1,2-dichloroethane dechlorination catalyzed by Pt-Sn/SiO2 catalysts prepared via controlled surface reaction (CSR) has been investigated. For catalysts with a Pt/Sn atomic ratio greater than unity, the major and minor reaction products are ethane and ethyl chloride, respectively. As the ratio is decreased, the ethylene selectivity increases from zero to almost 90% at a Pt/Sn ratio of 0.8; concomitantly, the activity expressed as a turnover frequency decreases by a factor of 5. Each Sn-rich catalyst (Pt/Sn less than or equal to 1) exhibits a transient period (5-110 h) during which sites for olefin formation are created. In situ Sn-119 Mossbauer spectroscopic investigations of the Pt-Sn/SiO2 catalyst with a Pt/Sn atomic ratio of 0.5 showed that all Sri in the reduced sample is incorporated into Sn-rich and Pt-rich Pt-Sn alloys; the latter phase predominates. In contrast, a reduced Pt-Sn/SiO2 catalyst with the same overall composition but prepared by co-impregnation contains Sn2+ and Sn4+ ions in addition to the metallic Sri that is incorporated into Pt-Sn alloys. The CSR catalyst exhibits a higher initial selectivity toward ethylene than the co-impregnated catalyst (66 and 5%, respectively, after reduction at 623 K), whereas the steady-state ethylene selectivity is higher for the co-impregnated sample (91 and 75%, respectively). These results are discussed in terms of the different genesis and microstructures of the CSR and co-impregnated Pt-Sn/SiO2) catalysts. (C) 2004 Elsevier Inc. All rights reserved.