Effects of Potassium on Silica-Supported Pt and Pt/Sn Catalysts for Isobutane Dehydrogenation

Article abstract of DOI:10.1006/jcat.1995.1322

Isobutane conversion was studied over silica-supported Pt, Pt/Sn, and Pt/Sn/K catalysts at temperatures from 673 to 773 K.The addition of tin to Pt/SiO2 suppresses isobutane isomerization and hydrogenolysis reactions, and decreases to a smaller extent the activity for isobutane dehydrogenation.The addition of potassium to Pt/Sn/SiO2 promotes isobutane dehydrogenation, increases the dehydrogenation selectivity, and improves resistance of the catalyst to deactivation.The additions of tin and potassium lead to the formation of smaller ensembles of surface platinum atoms, and the hydrogenolysis, isomerization, and coking reactions are thereby suppressed.Microcalorimetric measurements of hydrogen and carbon monoxide adsorption indicate that the addition of potassium to Pt/SiO2 and Pt/Sn/SiO2 does not significantly alter the heats of adsorption of these molecules, whereas potassium increases the saturation extent of hydrogen adsorption and decreases the extent of carbon monoxide adsorption.The rate of isobutane dehydrogenation becomes zero order with respect to hydrogen pressure over Pt/SiO2 at hydrogen pressures lower than 75 Torr, and dissociative adsorption of isobutane appears to control the dehydrogenation rate.The dehydrogenation reaction is negative order with respect to hydrogen over both Pt/Sn/SiO2 and Pt/Sn/K/SiO2 catalysts, indicating a higher coverage of hydrogen at the active sites.The increased dehydrogenation rate over Pt/Sn/K/SiO2 may be attributed to the increased number of sites available for removal of hydrogen from adsorbed isobutane and perhaps also to the stabilization of molecularly adsorbed isobutane.