Mechanistic Study on Aerobic Oxidation of Amine over Intermetallic Pd3Pb: Concerted Promotion Effects by Pb and Support Basicity

A mechanistic study on aerobic oxidation of amine to imine over Pd3Pb/MOx (MOx = SiO2, TiO2, Al2O3, and MgO) intermetallic catalysts was performed to clarify the role of Pb and the support in enhancing catalytic activity. Results from X-ray absorption and photoelectron spectroscopies revealed that formation of the Pd3Pb phase made Pd electron-rich compared to pure Pd, whereas the electronic states of Pd in Pd3Pb/MOx were identical and independent of the nature of the support. Kinetic studies indicated that desorption of imine was promoted by Pb and that adsorption of amine was accelerated by basic sites on the support. Infrared temperature programmed desorption (IR-TPD) experiments demonstrated that desorption of imine was indeed promoted on Pd3Pb compared to Pd. The support effect only appears on Pd3Pb catalysts and not on pure Pd due to change in the rate-determining step from imine desorption to amine adsorption. The combination of Pb and the support basicity provides a unique and highly efficient bifunctional catalysis.