Reaction-Induced Transformations in Pt―Sn/SiO_2 Catalysts: In Situ ~(119)Sn Moessbauer Study

摘要

Reaction-induced separation of tin-rich surface layers and tin-depleted inner region was observed in metallic particles of Pt―Sn/SiO_2 catalysts in two reactions: (ⅰ) dechlorination of 1,2-dichloroethane at 473 K (modeling catalytic removal of chlorine from hazardous chlorocarbons) and (ⅱ) oxidation of carbon monoxide at room temperature. In the former, a Pt: Sn catalyst (1:2 atomic ratio, 1 wt% metal content), prepared via co-impregnation, showed high selectivity (>80%) toward ethylene (at the expense of ethane), but only after a prolonged (ca. 24 h) period. In situ Moessbauer studies revealed stabilization of a homogeneous Pt―Sn alloy and SnCl_2 after activation in hydrogen; whereas tin-depleted and tin-rich components were separated after a 24-h period. Hence, inhibition of the hydrogenation activity of Pt, by surface tin enrichment and Cl deposition favors high ethylene selectivity. For the oxidation of CO at room temperature, a catalyst with a Pt: Sn atomic ratio of 3 :2 (3 wt% Pt) was prepared by an organometallic (CSR) method using ~(119)Sn(CH_3)_4. Platinum-rich PtSn(1) and tin-rich PtSn(2) components were separated in the Moessbauer spectra of catalyst activated at 570 K. The PtSn(2) component is primarily involved in surface reactions. Both in CO oxidation and the subsequent re-activation in hydrogen at room temperature a reversible PtSn(2)reversible Sn~(4+) interconversion occurred, d ln(A_(77)/A_(300))/dT data indicate the surface location of the involved components.