Catalytic Mechanism of Interfacial Water in the Cycloaddition of Quadricyclane and Diethyl Azodicarboxylate.

"On-water" catalysis, the unusual activity of water molecules at the organic solvent-water interface, has been demonstrated in many organic reactions. However, the catalytic mechanism has remained unclear, largely because of the irreproducibility of the organic-water interface under the common stirring condition. Here, the interfacial area was controlled by employing adsorbed water on mesoporous silica nanoparticles as the catalyst. Reliable kinetics of the cycloaddition reaction of quadricyclane and diethyl azodicarboxylate (DEAD) at the toluene-water interface within the nanoparticle pores were measured. Data reveal an Eley-Rideal mechanism, wherein DEAD adsorbs at the toluene-water interface via hydrogen bonds formed with interfacial water, which lower the activation energy of the cycloaddition reaction. The mechanistic insights gained and preparation of surface water in silica pores described herein may facilitate the future design of improved "on-water" catalysts.

Tuning Redox Potential of Gold Nanoparticle Photocatalysts by Light.

Metallic nanoparticle-based photocatalysts have gained a lot of interest in catalyzing oxidation-reduction reactions. In previous studies, the poor performance of these catalysts is partly due to their operation that relies on picosecond-lifetime hot carriers. In this work, electrons that accumulate at a photostationary state, generated by photocharging the catalysts, have a much longer lifetime for catalysis. This approach makes it possible to determine and tune the photoredox potentials of the catalysts. As demonstrated in a model reaction, the photostationary state of the photocatalyzed oxidative etching of colloidal gold nanoparticles using FeCl3 was established under continuous irradiation of different wavelengths. The photoredox potentials of the nanoparticles were then calculated using the Nernst equation. The potentials can be tuned to a range of 1.28 to 1.40 V (vs SHE) under irradiation of different wavelengths in the range of 450 to 517 nm. The effects of particle size or optical power on the photoredox potentials are small compared to the wavelength effect. Control over the photoredox potential of the particles using different excitation wavelengths can potentially be used to tune the activities and selectivities of metallic nanoparticle photocatalysts.