Transformation of Pd/SiO2 into palladium silicide during reduction at 450 and 500°C

This work provides evidence that reduction of Pd/SiO2 catalysts in dihydrogen at temperatures as low as 450 and 500 degreesC may already lead to a considerable interaction between palladium and silica, identified by the formation of palladium silicides (Pd4Si and Pd3Si). This result seems significant in view of the frequent usage of this temperature level in activation and pretreatment of Pd/SiO2 catalysts. Application of a high flow of a reducing gas mixture and different reduction time periods indicates that an efficient removal of water vapor from the catalyst zone during hydrogen pretreatment facilitates reduction of silica to silicon, which, in turn, interacts with nearby palladium species. Silation of palladium in Pd/SiO2 catalysts during a prolonged reduction at 450 and 500 degreesC has a considerable effect on the catalytic properties in the reaction of 2,2-dimethylbutane with dihydrogen. Incorporation of silicon into palladium lowers the catalytic activity, decreases the activation energy, increases the selectivity toward isomerization (at the expense of hydrogenolysis), and also changes the type of 2,2-dimethylbutane interaction with the surface of catalyst (from agamma to agamma' mode). (C) 2003 Elsevier Inc. All rights reserved.