Plasmon‐Driven Selective Oxidation of Aromatic Alcohols to Aldehydes in Water with Recyclable Pt/TiO2 Nanocomposites

Selective oxidation of alcohols to aldehydes is a key reaction for the synthesis of fine chemicals, since aldehyde derivatives are widely used in the flavoring, confectionary, and beverage industries. Efficient aerobic oxidation of alcohols has usually been facilitated by catalysts such as palladium, platinum, and copper. With the development of photocatalysis, selective photocatalytic oxidation of aromatic alcohols to aldehydes in water has been reported on nanostructured rutile, anatase, and brookite TiO2 under UV irradiation. [3] One of the great challenges in catalysis is to develop new photocatalysts with a high activity response to visible light, which will allow the utilization of sunlight, an abundant and clean low-cost energy source. Recently, the selective oxidation of alcohols in organic solvent was carried out under visible-light irradiation in a catalyst system containing dye-sensitized TiO2 and 2,2,6,6tetramethylpiperidinyloxyl (TEMPO). Herein, we report an alternative strategy for selective oxidation of aromatic alcohols to aldehydes in water with high selectivity and relatively long-term stability based on surface plasmon resonance of platinum nanoparticles deposited onto TiO2 (Pt/TiO2) film under visible-light irradiation at ambient temperature. Surface plasmon resonance of noble metal nanoparticles has been afforded much attention of late, due to their unique properties and their wide applications in multicolor imaging, photovoltaic cells, chemical sensors, and biosensors. Charge separation has also been realized on a metal nanoparticles/TiO2 nanofilm and successfully applied to photovoltaic cells, photocatalysis, and photolithography. However, plasmon-driven selective conversion of alcohols to aldehydes has, to our knowledge, never been reported. Furthermore, compared to the power catalysts, such as the TEMPO-based system, it is much easier for the film catalyst to separate from the system and undergo recycling. In addition, the oxidation has been carried out in water, which is considerably safer, cheaper, and more environmentally friendly than organic solvents. More importantly, the products can be separated by simple decantation, and the catalyst solution can be recycled. TiO2 films with different particle sizes and crystalline forms (anatase or rutile) were prepared from various TiO2 sols by spin coating followed by sintering in air atmosphere. Rutile TiO2 film was fabricated by calcination of anatase TiO2 at high temperature. The size and crystalline form of the TiO2 films were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The SEM image and XRD pattern of anatase TiO2 STS-21 film sintered at 723 K for 1 h are shown