Experimental studies and Monte Carlo simulations of the state of the surface during oxidation reactions on supported platinum catalysts.

摘要

The state of the catalytic surface during oxidation reactions on supported Pt catalysts has been investigated by kinetic studies and in-situ infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies. The results of this work show the effect of the preparation method, the pretreatment, and most significantly of the reaction atmosphere on the CO oxidation activity. Experimental data are compared with theoretical results obtained with Dynamic Monte Carlo (DMC) simulations. Kinetic results indicate that, under oxidizing conditions, the specific rate of CO oxidation on Pt/SiO2 catalysts increases with increasing particle size. DMC simulation results suggest that these differences are due to the variation on the activity of the various types of sites present in a crystallite. IR results clearly show that the active surface of Pt/SiO2 catalysts during CO oxidation is the metallic Pt.; The presence of residual Cl on Pt/Al2O3 and Pt/SiO 2 catalysts affects their activity during CO, methane and ethane oxidation. On the Cl-containing catalyst, Pt-Cl as well as Pt-Pt and Pt-O bonds were detected using EXAFS. This catalyst, however, does not chemisorb CO, indicating that there are no reduced surface Pt atoms. IR results show that chlorine significantly reduces the amount of CO adsorbed on metallic Pt sites. The mechanism of chlorine poisoning occurs mainly by site blocking.; The state of the working surface of Pt/SiO2 and Pt/Al 2O3 catalysts were also studied in the presence of sulfur. The activity results show that the light-off temperature for CO oxidation increases with ex-situ H2S addition. In-situ IR results show a shift of the linear CO band indicating a change in the bonding of adsorbed CO due to the presence of sulfur. Continuous addition of 20 PPM of SO 2 to the reactant mixture also increases the light-off temperature. Activity and IR results demonstrate that alumina acts as a sulfur storage reservoir. This delays the initial sulfur deactivation but after extended time on stream in presence of SO2 the activity of the silica- and alumina-supported catalysts is similar. The results indicate that sulfur poisoning is not only due to site blocking but also because of modification of the Pt-CO bonding.