Green Synthesis of Oxide-Supported Pd Nanocatalysts by Laser Methods for Room-Temperature Carbon-Carbon Cross-Coupling Reactions.

This work reports the design and development of a new class of highly active Pd nanocatalysts supported on substoichiometric oxides. These novel catalysts are generated by green laser synthesis methods to generate high-surface-area substoichiometric oxide nanoparticles followed by photoreduction in aqueous solutions to deposit highly active Pd nanocatalysts within the surface defects of the oxides. The laser methods eliminate the use of toxic chemicals, including hazardous solvents and chemical reducing agents, and allow efficient reduction of the Pd ions in aqueous solutions aided by the photogenerated electrons from the semiconductor support. The Pd catalysts incorporated within these oxides exhibit high activity for carbon-carbon bond-forming reactions. The Pd/TiO2 catalyst with 0.3 mol % Pd achieves 100% conversion in the reaction between bromobenzene and benzeneboronic acid to the biphenyl product within 240 minutes at room temperature without any external heating. With a catalyst loading of 0.3 mol % Pd in the microwave-assisted reaction between bromobenzene and benzeneboronic acid at 60 °C, 92 and 83% conversions to the biphenyl product are achieved within 5 min of reaction time using the Pd/TiO2 and Pd/ZnO catalysts, respectively. The results demonstrate a remarkable catalytic activity of the substoichiometric oxide-supported Pd catalysts with turnover frequencies (TOF, h-1) of 24 000, 10 000, and 3200 achieved under mirowave-assisted reactions at 60 °C for the 0.03 mol% Pd of the Pd/TiO2, Pd/ZnO, and Pd/ZrO2 catalysts, respectively. The high activity and good reusability of these nanocatalysts are attributed to the optimum catalyst-support interaction between the small Pd nanoparticles and the surface defects of the substoichiometric oxide support prepared by the laser vaporization-controlled condensation method.