Palladium catalysts supported on various metal oxides were characterized using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and transmission electron microscopy (TEM) to investigate why these catalysts do not show any correlation between the measured Pd surface area and the catalytic activity for the oxidative coupling of 4-methylpyridine to 4,4'-dimethyl-2,2'-bipyridine. The XPS data confirm the classification of n-Al2O3(+), n-MgO and p-SiO2 as non-interacting supports, since the Pd 3d(5/2) binding energy (BE) of palladium on these supports is 336.1 eV, consistent with bulk PdO. In contrast, catalysts supported on p-TiO2, n-ZnO, n-ZrO2, n-ZrO2(CeO2), and n-CeO2 have Pd 3d(5/2) BEs ranging from 336.3 to 337.4 eV, which reveal varying degrees of metal-support interactions. Metal support interactions leading to electron deficient Pd2+ species are likely beneficial for the reaction due to a facilitated C-H insertion step. While both the PdO/p-TiO2 and PdO/n-TiO2 catalysts have a Pd 3d BE of 336.3 eV, their differences in activity can be attributed to (1) the PdO/n-TiO2 catalyst as prepared having a significantly higher number of hydroxyl groups on the surface compared with the PdO/p-TiO2 catalyst, and (2) the p-TiO2 support being crystalline with an anatase phase, while the n-TiO2 support is nearly amorphous. The presence of surface hydroxyl groups before reaction could hinder the first C-H activation step, and an anatase phase of the support can result in more favorable palladium-support interactions compared with an amorphous TiO2. The XPS data also indicates that while Pd-support interactions are beneficial, very strong interactions, such as in the case of CeO2, can lead to migration of the support over Pd, which reduces the Pd surface area and explains the lower than expected activity of the PdO/n-CeO2 catalyst. On some supports in this study leaching may occur during the reaction, but the characterization data indicate that other factors of catalyst deactivation are more important. XRD reveals that the complete reduction of the PdO particles on the surface is very fast compared to the reaction time. This observation explains why reducible supports with mobile oxygen are beneficial in this reaction. These supports can facilitate the reoxidation of palladium due to strong metal-support interactions. Migration of the support over the active palladium species is another deactivation pathway that appears to be more severe than leaching on these catalysts. (C) 2010 Elsevier B.V. All rights reserved.