Characterization of alumina-supported palladium oxide catalysts used in the oxidative coupling of 4-methylpyridine

A number of palladium and palladium oxide on alumina catalysts for the oxidative coupling of 4-methylpyridine were characterized using X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and X-ray diffraction (XRD) to obtain more information about the properties that govern these reactions. The most active catalysts, the impregnated and precipitated PdO/n-Al2O3(+), were found to have very broad Pd 3d peaks, suggesting the presence of both PdO2 and Pd0, in addition to PdO, on the surface. This could be due to strong metal–support interactions, which result in more electrophilic palladium or a reduced palladium catalyst that is easier to reoxidize, both of which are expected to result in high catalytic activities. As the PdO is completely reduced to Pd metal on spent catalysts (according to XPS and XRD), facilitated reoxidation could be important in this reaction. While reoxidation is a potential deactivation pathway, carbon deposition is also evident in the XPS spectra and this could block active sites. In contrast, both the XPS and the XRD data indicate that Pd leaching into the reaction solution is not a significant deactivation pathway in the PdO/n-Al2O3(+) catalyst system. The nm-sized fine structure observed in the supports of the PdO/γ-Al2O3 and PdO/n-Al2O3(+), together with their measured activities compared to the poorly active PdO/n-Al2O3(−) catalyst with no such fine structure, supports our hypothesis that low-coordination sites on a support can result in strong metal–support interactions and very active catalysts. Highly active crystalline PdO particles with sizes of 5 nm or below may explain the observed lack of correlation between the measured Pd surface area (which should correlate with the PdO surface area) and the catalytic activity.