Hydrogen Temperature-Programmed Desorption (H2 TPD) of Supported Platinum Catalysts.

Abstract

Hydrogen temperature-programmed desorption (TPD) of supported platinum catalysts, Pt/KLTL, Pt/H-LTL, Pt/K-MAZ, Pt/H-MAZ, Pt/-Al2O3, and Pt/SiO2, was performed after hydrogen reduction at 300, 450, or 650°C. For all catalysts, reversible desorption of chemisorbed hydrogen occurred at approximately 175°C. In addition to chemisarbed H2, at least three peaks corresponding to higher temperature, irreversibly desorbed H2, were observed and assigned to spillover hydrogen. The quantity of spillover hydrogen is related to the number of support hydroxyl groups and the maximum reduction temperature of the catalyst. It is suggested that the irreversibility of the spillover hydrogen results from dehydroxylation of the support at high temperature. The structure of Pt/K-LTL and Pt/H-LTL catalysts has been examined by EXAFS following reduction at 300, 450, and 600°C. After low-temperature reduction, it is proposed that a layer of spillover hydrogen is present between the platinum particle and the support. The distance between the platinum and the support (oxide) surface is ca. 2.6-2.7 Å. During high-temperature reduction, or during TPD, the interfacial hydrogen is irreversibly lost, and the platinum-oxygen distance decreases to 2.2 Å. The catalysts′ turnover frequency (TOF) for propane hydrogenolysis was dependent on both the reduction temperature and the type of support. Platinum on acidic supports and reduced at the lowest temperature displayed the highest TOF. Qualitatively, the catalysts′ hydrogenolysis activity increased with increasing amounts of spillover hydrogen.