Dual Pd2+ and Pd0 sites on CeO2 for benzyl alcohol selective oxidation

Abstract

Solvent-free aerobic oxidation of benzyl alcohol is a green approach for synthesizing fine chemicals. Pd-based catalysts are extensively investigated due to their excellent catalytic activity compared to other metal catalysts. Most researchers believe that metallic Pd species (Pd0) provide active centers. However, the critical role of active oxygen and the function of Pd2+ is neglected. Herein, supported Pd clusters on CeO2 (Pd/CeO2) with a stable high Pd2+/(Pd0 + Pd2+) ratio are prepared by atomic layer deposition (ALD). The turnover frequency (TOF) and activity on as-prepared Pd/CeO2 catalysts in solvent-free aerobic oxidation of benzyl alcohol reach 3.50 × 105 h−1 and 1.85 × 105 mol/(molPd·h), respectively, which are much superior to previously reported results. As the Pd loading increases, the TOFs exhibit a volcano relationship with the Pd2+/(Pd0 + Pd2+) ratio, pointing to a dual Pd0 and Pd2+ sites catalyzed mechanism. O2 kinetic isotopic effect and further evidence support a dual-site catalyzed oxygen-promoted β-hydride elimination mechanisms, in which benzyl alcohol is selectively adsorbed to Pd2+ sites and reacts with active oxygen species on nearby Pd0 sites during the reaction.