Surface functionalized TiO2 supported Pd catalysts for solvent-free selective oxidation of benzyl alcohol

• APTES-grafted TiO2 was used as Pd catalyst support in benzyl alcohol oxidation.
• Monolayer attachment promoted small and uniform Pd nanoclusters.
• Excess amounts of APTES resulted in multilayer and reversed NH2 attachment.
• The catalysts contained three types of Pd species: Pd0, PdO, and PdO2.
• The reduction of PdOx by adsorbed benzyl alcohol led to highly active metallic Pd0.

Pd catalysts supported on TiO2 functionalized with various amounts of 3-aminopropyltriethoxysilane (APTES) were prepared using a post-synthesis grafting method combined with electroless deposition of Pd. As revealed by the Fourier transformed infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) results, monolayer APTES grafting was obtained using 0.005 mmol APTES on 1.5 g TiO2 support. Excess amounts of APTES resulted in both multilayer and reversed attachment, which NH2 attached to the TiO2 surface rather than giving free NH2 termination. The catalytic activity in the solvent-free selective oxidation of benzyl alcohol was correlated well with the highest amount of Pd deposited as well as the formation of small and uniform Pd nanoclusters with narrow particle size distribution (average diameter 3.4 nm) on the 1%Pd/TiO2-0.005APTES. Increasing of surface basicity via the hydrolysis of amino groups (NH2) is suggested to enhance the dehydrogenation of benzyl alcohol, and as a consequence the selectivity toward benzaldehyde increased for all the APTES-modified TiO2 supported Pd catalysts. In addition, the combination of metallic Pd0 and PdOx (Pd2+/Pd4+) species gave high catalytic activity in the benzyl alcohol oxidation, emphasizing that the reduction of PdOx species by the adsorbed benzyl alcohol is an essential step to form highly active metallic Pd0 sites.

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