Size-Dependent Palladium Species Supported on Phosphomolybdate for the Aerobic Oxidation of Alcohols

It is significant to get insight into the geometrical and electronic effects on heterogeneous catalysts in size-sensitive aerobic oxidation reactions. Herein, the different sized Pd species ranging from single atom (SA) to nanoclusters (NCs) and nanoparticles (NPs) were fabricated onto a cesium-exchanged phosphomolybdate (CsPMo) by using the reduction strategies, resulting in the formation of the corresponding Pd SAs/CsPMo, Pd NCs/CsPMo (1.85 nm), and Pd NPs/CsPMo (4.10 nm). When employed for the selective benzyl alcohol oxidation reaction (BAOR), the Pd NCs/CsPMo exhibited the conversion of 91.0%, the benzaldehyde selectivity of 90.5%, and the reaction rate of 6.81 × 104 h–1, which was superior to those of Pd SAs/CsPMo and Pd NPs/CsPMo and most reported Pd-based catalysts. And the reaction rate was volcanically related to the ratio of Pd0/(Pd0 + Pd2+). Detailed characterizations and DFT calculation studies revealed that the electronic metal–support interaction (EMSI) between Pd and CsPMo was modulated by the sizes of Pd species, inducing the electron transfer from Pd species to CsPMo, and the Pd0 content gradually enhanced with increasing Pd sizes. The moderate EMSI between CsPMo and Pd NCs facilitated the O2 activation for generation of superoxide radicals (•O2–), synergistically promoted BA oxidation. Compared with Pd SAs and NPs, the elevation of the Pd d-band center of Pd NCs to the Fermi level significantly facilitated the charge transfer from the d-band to the unoccupied orbitals of benzyl alcohol (BA), resulting in the adsorption and activation of BA.