The Interaction of palladium with alumina and titanium oxide supports

Abstract

In a series of recent electron microscopy studies attempts were made to relate the phenomenological consequences of strong metal-support interactions (SMSI), such as suppressed chemisorption capacity, with morphological changes in either the support or supported metal particles. In the present investigation, a combination of high-resolution transmission electron microscopy and hydrogen chemisorption measurements were employed to address the sintering behavior of Pd supported on Al 2O 3 and TiO 2 following reduction at temperatures up to 800 °C. Palladium was chosen, since it has the unique ability to both adsorb and absorb (β-hydride formation) hydrogen. This behavior was anticipated to provide an added dimension for characterizing SMSI systems. Palladium on alumina was selected as a control system. At reduction temperatures in excess of 700 °C, Pd particles sinter on both Al 2O 3 and TiO 2 supports. The extent of sintering is, however, more extensive in the case of the TiO 2 support. The quantities of hydrogen adsorbed and absorbed by Pd TiO 2 decreased in parallel with increasing reduction temperature, reaching essentially zero uptakes at reduction temperatures above 500–600 °C. The amount of H 2 adsorbed by Pd Al 2O 3 decreased about 60% upon increasing the reduction temperature from 500 to 700 °C, in agreement with the extent of sintering detected by TEM which occurs within this temperature range. The quantity of H 2 absorbed by Pd Al 2O 3 , however, did not significantly decrease after reduction at 700 °C. The anomalous behavior of Pd TiO 2 is rationalized in terms of a model whereby reduction of TiO 2 to Ti 4O 7 leads to the simultaneous formation of mobile titanium suboxides, which are free to migrate onto the metal particle surface. The process ultimately results in the decoration of the Pd particles by titanium suboxide species. In this coated state, Pd is no longer capable of either adsorbing or absorbing hydrogen.