Reduction of PdO/Al2O3 in Liquid Cyclohexane Followed In Situ by ATR-IR, High-Energy XRD, and XAS

Supported Pd-based catalysts used in heterogeneously catalyzed liquid phase processes often undergo an activation step through reduction of the oxide phase or of the oxidic passivation layer prior to reaction. An oxidized Pd/Al2O3 catalyst was reduced by using H-2-saturated cyclohexane at 70 degrees C, and reduction was followed in real time by attenuated total reflection infrared spectroscopy (ATR-IR), high-energy X-ray diffraction (XRD), and quick scanning X-ray absorption spectroscopy (XAS) to obtain the perspective from different materials properties. We show that dynamic Pd speciation changes can be monitored during reduction in the liquid phase. XRD and XAS indicated that the isothermal PdO reduction passes through an intermediate Pd-0 phase toward the formation of a well-defined Pd hydride phase. Based on the different sensitivities of the two techniques, XRD suggested that the process ends with a dominant Pd hydride phase, while XAS revealed an equal proportion of Pd-0 and Pd hydride. The time-resolved XAS data suggested that the isothermal reduction of PdO in cyclohexane to Pd-0 occurs through a nucleation mechanism and that Pd-0 is rapidly converted into the hydride phase according to a contracting sphere model. The identification and the presence of a mixed Pd-0/PdHx phase appears crucial in view of the relevance of these species in for instance hydrogenation reactions.