Au-Pd@g-C3N4 as an Efficient Photocatalyst for Visible-Light Oxidation of Benzene to Phenol: Experimental and Mechanistic Study

In the present study, a mesoporous photocatalyst based on Au-Pd nanoparticles incorporated into g-C3N4 was prepared by a coassembly method using melamine as the carbon and nitrogen source, polyvinyl pyrrolidone as the dispersing agent, and pulse laser ablation in liquid technique for preparing gold nanoparticles and subsequent decoration with Pd nanoparticles. At the final stage, Au-Pd/g-C3N4 nano-photocatalyst was obtained via low-ramping pyrolysis in an argon atmosphere. The activity of the catalyst was related to its structure, which was characterized by high-resolution transmission electron microscopy, field-emission scanning electron microscopy, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, and Brunauer-Emmett-Teller analysis. The results demonstrated that the Au-Pd-containing catalyst exhibited superior performance compared to its counterparts containing monometallic nanoparticles. The influence of variables such as reaction temperature, time of irradiation, amount of hydrogen peroxide, and amount of metal nanoparticles was investigated. Under optimized conditions, the Au-Pd/g-C3N4 photocatalyst showed benzene conversion of 26% at a phenol selectivity of 100%, giving no dihydroxylated byproducts. The catalyst was highly stable and recyclable, thus showing promise for the direct conversion of benzene to phenol. Time-dependent density functional theory (TD-DFT) calculations describe the activation of the oxidant by charge transferring from the metal clusters to the graphitized carbon nitride support and explain why the Au-Pd/g-C3N4 composite (rather than Au/g-C3N4) has superior efficiency in promoting the benzene-to-phenol conversion. The same DFT calculations showed that the Pd/g-C3N4 composite cannot catalyze the same processes.