A palladium single-atom catalyst toward efficient activation of molecular oxygen for cinnamyl alcohol oxidation

Selective aerobic oxidation of alcohols under mild conditions is of great importance yet challenging, with the activation of molecular oxygen (O2) as a crucial capability of the catalysts. Herein, we demonstrate that an Al2O3-supported Pd single-atom catalyst leads to higher activity and selectivity compared to Pd nanoparticles for the oxidation of cinnamyl alcohol. The Al2O3 support used in this study is rich in coordinately unsaturated Al3+ sites, which are apt for binding to Pd atoms through oxygen bridges and present a distinct metal-support interaction (MSI). The suitable MSI then leads to a unique electronic characteristic of the Pd single atoms, which can be confirmed via X-ray photoelectron spectroscopy, normalized X-ray absorption near-edge structure, and diffuse reflectance Fourier transform infrared spectroscopy. Moreover, this unique electronic state is proposed to be responsible for its high catalytic activity. With the help of in-situ UV-vis spectra and electron spin resonance spectra, a specific alcohol oxidation route with O2 activation mechanism is then identified. Active oxygen species behaving chemically like singlet-O2 are generated from the interaction of O2 with Pd1/Al2O3, and then oxidize the partially dehydrogenated intermediates produced by the adsorbed allylic alcohols and Pd atoms to the desired alkenyl aldehyde. This work provides a promising path for the design and development of high-activity catalysts for aerobic oxidation reactions.