Adsorption and Co-adsorption of Ethylene and Carbon Monoxide on Silica-Supported Monodisperse Pt Nanoparticles; Volumetric Adsorption and Infrared Spectroscopy Studies

Abstract

The adsorption of carbon monoxide and ethylene, and their sequential adsorption, was studied over a series of Pt/SBA-15 catalysts with monodisperse particle sizes ranging from 1.7 to 7.1 nm by diffuse-reflectance infrared spectroscopy and chemisorption. Gas adsorption was dependent on the Pt particle size, temperature, and sequence of gas exposure. Adsorption of CO at room temperature on Pt/SBA-15 gives rise to a spectroscopic feature assigned to the C-O stretch: î(CO) ) 2075 cm-1 (1.9 nm); 2079 cm-1 (2.9 nm); 2082 cm-1 (3.6 nm); and 2090 cm-1 (7.1 nm). The intensity of the signal decreased in a sigmoidal fashion with increasing temperature, thereby providingsemiquantitative surface coverage information. Adsorption of ethylene on Pt/SBA-15 gave rise to spectroscopicfeatures at 1340, 1420, and 1500 cm-1 assigned to ethylidyne, di-ó-bonded ethylene, and ð-bonded ethylene, respectively. The ratio of these surface species is highly dependent on the Pt particle size. At room temperature, Ptparticles stabilize ethylidyne as well as di-ó- and ð-bonded ethylene; however, thylidyne predominated on the surfaces of larger particles. Ethylidyne was the only identifiable species at 403 K, with its formation being more facile on larger particles. Co-adsorption experiments reveal that the composition of the surface layer is dependent on the order of exposure to gases. Exposure of a C2H4-covered Pt surface to CO resulted in an50% decrease in chemisorbedCO compared to a fresh Pt surface. The î(CO) appeared at 2050 cm-1 on Pt/SBA-15 pretreated with C2H4 at room temperature. The di-ó-bonded and ð-bonded species are the most susceptible to displacement from the surface by CO. The formation of ethylidyne appeared to be less sensitive to the presence of adsorbed carbon monoxide, especially on larger particles. Upon exposure of C2H4 to a CO-covered Pt surface, little irreversible uptake occurred due to nearly 100% site blocking. These results demonstrate that carbon monoxide competes directly with ethylene for surface sites,which will have direct implications on the poisoning of the heterogeneously catalyzed conversion of hydrocarbons.