Facile and surfactant-free synthesis of supported Pd nanoparticles on hydrotalcite for oxidation of benzyl alcohol

We report a facile modified deposition-precipitation method that permits reproducible preparation of a supported Pd catalyst with small particle size and narrow size distribution but without the protection of a surfactant and any additional treatment. The pH value in this technique plays a key role in controlling the size of the Pd nanoparticles as well as the electronic environment of the surface Pd atoms. With the increasing pH (4.0-12.0), the average size of the Pd nanoparticles decreases gradually, meanwhile, the peak area ratio for CO adsorbed on bridge-bonded Pd to that adsorbed on threefold-coordinate Pd increases. Stronger support-metal interaction (electron transfer from Pd-0 to support) is observed at pH values of 7.0 and 10.0. Both the small particle size and the electron-deficient surface metallic Pd contribute to enhancement in the activity for the solvent-free oxidation of benzyl alcohol. Therefore, compared with supported Pd catalysts prepared by sol immobilization, impregnation and deposition-precipitation methods, Pd/hydrotalcite synthesized by this modified deposition-precipitation approach shows a higher TOF value (5330 h(-1)). This enhanced catalytic performance can also be maintained in five cycles. Under the considerations of green chemistry, a number of Pd catalysts were then prepared on alternative supports using this method without the addition of alkali in the preparation process.